Active, Reactive, Apparent and Complex Power. Simple explanation with formulas.(1)Real Power: (P)Alternative words used for Real Power (Actual Power, True Power, Watt-full Power, Useful Power, Real Power, and Active Power)In a DC Circuit, power supply to the DC load is simply the product of Voltage across the load and Current flowing through it i.e., P = V I. because in DC Circuits, there is no concept of phase angle between current and voltage. In other words, there is noPower factorin DC Circuits.But the situation is Sinusoidal or AC Circuits is more complex because of phase difference between Current and Voltage. Therefore average value of power (Real Power) is P = VI Cos is in fact supplied to the load.In AC circuits, When circuit is pure resistive, then the same formula used for power as used in DC as P = V I.You may also read aboutPower Formulas in DC, AC Single Phase and and AC Three Phase Circuits.Real Power formulas:P = V I(In DC circuits)P = VI Cos(in Single phase AC Circuits)P = 3 VLILCosor(in Three Phase AC Circuits)P = 3 VPhIPhCosP = (S2 Q2)orP = (VA2 VAR2) orReal or True power = (Apparent Power2 Reactive Power2) orkW = (kVA2 kVAR2)(2)Reactive Power: (Q)Also known as (Use-less Power, Watt less Power)The powers that continuously bounce back and forth between source and load is known as reactive Power (Q)Power merely absorbed and returned in load due to its reactive properties is referred to asreactive powerThe unit of Active or Real power is Watt where 1W = 1V x 1 A.Reactive power represent that the energy is first stored and then released in the form of magnetic field or electrostatic field in case of inductor and capacitor respectively.Reactive power is given by Q = V I Sin which can be positive (+ve) for inductive, negative (-Ve) for capacitive load.The unit of reactive power is Volt-Ampere reactive. I.e. VAR where 1 VAR = 1V x 1A.In more simple words, in Inductor or Capacitor, how much magnetic or electric field made by 1A x 1V is called the unit of reactive power.Reactive power formulas:Q = V I SinReactive Power= (Apparent Power2 True power2)VAR = (VA2 P2)kVAR = (kVA2 kW2)(3)Apparent Power: (S)The product of voltage and current if and only if the phase angle differences between current and voltage are ignored.Total power in an AC circuit, both dissipated and absorbed/returned is referred to asapparent powerThe combination of reactive power and true power is calledapparent powerIn an AC circuit, the product of the r.m.s voltage and the r.m.s current is calledapparent power.It is the product of Voltage and Current without phase angleThe unit of Apparent power (S) VA i.e. 1VA = 1V x 1A.When the circuit is pure resistive, then apparent power is equal to real or true power, but in inductive or capacitive circuit, (when Reactances exist) then apparent power is greater than real or true power.Apparent power formulas:S = V IApparent Power = (True power2+ Reactive Power2)kVA = kW2+ kVAR2AlsoNote that;Resistor absorbs the real power and dissipates in the form of heat and light.Inductor absorbs the reactive power and dissipates in the form of magnetic fieldCapacitor absorbs the reactive power and dissipates in the form of electric or electrostatic filedThese all quantities trigonometrically related to each other as shown in below figure.Click image to enlarge

For more Clearance andexplanation., i used Lays Chips and Beer Analogy for Real or True Power, Reactive Power , Apparent power andpower factorLays Chips Analogy of Real or True Power, Reactive Power, Apparent power &power factor

Advantages of Power factor improvement and Correction:Following are the merits and benefits ofimprovedPower factor;1. Increase in efficiency of system and devices2. Low Voltage Drop3. Reduction in size of a conductor and cable which reduces cost of the Cooper4. An Increase in available power5. Line Losses (Copper Losses) I2R is reduced6. Appropriate Size of Electrical Machines (Transformer, Generators etc)7. Eliminate thepenalty of low power factorfrom the Electric Supply Company8. Low kWh (Kilo Watt per hour)9. Saving in the power bill10. Better usage of power system, lines and generators etc11. Saving in energy as well as rating and the cost of the electrical devices and equipment is reduced

Methods for Power Factor ImprovementThe following devices and equipments are used forPower FactorImprovement.1. Static Capacitor2. Synchronous Condenser3. Phase Advancer1. Static CapacitorWe know that most of the industries and power system loads are inductive that take lagging current which decrease the system power factor(See Disadvantages of Low Power factor). For Power factor improvement purpose, Static capacitors are connected in parallel with those devices which work on low power factor.These static capacitors provides leading current which neutralize (totally or approximately) the lagging inductive component of load current (i.e. leading component neutralize or eliminate the lagging component of load current) thus power factor of the load circuit is improved. These capacitors are installed inVicinity of large inductive load e.g Induction motors and transformers etc, and improve the load circuit power factor to improve the system or devises efficiency.

Suppose,here is a single phase inductive load which is taking lagging current (I) and the load power factor is Cos as shown in fig-1.In fig-2, a Capacitor (C) has been connected in parallel with load. Now a current (Ic) is flowing through Capacitor which lead 90 from the supply voltage ( Note that Capacitor provides leading Current i.e., In a pure capacitive circuit, Current leading90from the supply Voltage, in other words, Voltage are90lagging from Current). The load current is (I). The Vectors combination of (I) and (Ic) is (I) which is lagging from voltage at2as shown in fig 3.It can be seen from fig 3 that angle of 2< 1i.e.angle of2is less than from angle of2. Therefore Cos2is less than from Cos1(Cos2> Cos1). Hence the load power factor is improved by capacitor.Also note that after the power factor improvement, the circuit current would be less than from the low power factor circuit current. Also, before and after the power factor improvement, the active component of current would be same in that circuit because capacitor eliminates only there-active componentof current. Also, theActive power (in Watts)would be same after and before power factor improvement.Advantages: Capacitor bank offers several advantages over other methods of power factor improvement. Losses are low in static capacitors There is no moving part, therefore need low maintenance Itcan workinnormalairconditions (i.e. ordinary atmospheric conditions) Do not require a foundation for installation They are lightweight so it is can be easy to installedDisadvantages: The age of static capacitor bank is less (8 10 years) With changing load, we have to ON or OFF thecapacitorbank, which causes switching surges onthe system If the rated voltage increases, then it causesdamageit Once the capacitors spoiled, then repairing is costly2. Synchronous CondenserWhen a Synchronous motor operates at No-Load and over-exited then its called a synchronous Condenser. Whenever a Synchronous motor is over-exited then it provides leading current and works like a capacitor. When a synchronous condenser is connected across supply voltage (in parallel) then it draws leading current and partially eliminates the re-active component and this way, power factor is improved. Generally, synchronous condenser is used to improve the power factor in large industries.Advantages: Long life (almost 25 years) High Reliability Step-less adjustment of power factor. No generation of harmonics of maintenance The faults can be removed easily Its not affected by harmonics.Require Low maintenance (only periodic bearing greasing is necessary)Disadvantages: It is expensive (maintenance cost is also high) and therefore mostly used by large power users. An auxiliary device has to be used for this operation because synchronous motor has no self starting torque It produces noise3. Phase AdvancerPhase advancer is a simple AC exciter which is connected on the main shaft of the motor and operates with the motors rotor circuit for power factor improvement. Phase advancer is used to improve the power factor of induction motor in industries. As the stator windings of induction motor takes lagging current90 out of phase with Voltage, therefore the power factor of induction motor is low. If the exciting ampere-turns are excited by external AC source, then there would be no effect of exciting current on stator windings. Therefore the power factor of induction motor will be improved. This process is done by Phase advancer.Advantages: Lagging kVAR (Reactive component of Power or reactive power) drawn by the motor is sufficiently reduced because the exciting ampere turns are supplied at slip frequency (fs). The phase advancer can beeasilyused where the use of synchronous motors isUnacceptableDisadvantage: Using Phase advancer is not economical for motors below 200 H.P. (about 150kW)

POWER FACTOR BASIC

1).The Cosine of angle between Current and Voltage is called Power Factor.P = VI Cos ORCos = P / V I ORCos = kW / kVACos = True Power/ Apparent Power2).The ratio between resistance and Impedance is Called Power Factor.Cos = R/Z3).The ratio between Actual Power andApparent Poweris called power factor.Cos = kW / Kva

This is going to be a long post, be ready (I'll try my best to explain it in less technical terms as possible) -

First, let us consider what reactive power is mathematically and then we'll see what it means practically.

Active power = Voltage * component of current in phase with the voltage i.e. V*I cos(theta)

Reactive power = Voltage * component of current 90 degrees out of phase with voltage i.e. V*I sin(theta)

Where theta = angle between Voltage and Current.

This was mathematical explanation which we find in most textbooks butthat doesn't tell a thing about what actually reactive power is.

So, let us now jump into practical considerations and i'll try to explain what reactive power is and why it is said that it does not do any useful work with practical examples.

Let us consider a Transformer.As you might be knowing, both the windings of a transformer are not connected electrically (they have insulation between them) but still electric power flows from one winding to another.Electric power cannot flow through air or any insulation under normal conditions, then how the hell this happens ?This is the point where reactive power comes into play.

Before explaining the role of reactive power, let me explain about transformer working in short.In transformer, current flows in primary winding and sets up magnetic flux. This magnetic flux links with secondary winding and then current is induced in secondary winding and this is how we get current on secondary side.

Now let us comeback to the role of reactive power.So, from where do you think this magnetic flux responsible for transformer operation came from ? and the answer isREACTIVE COMPONENTof current.Reactive power acts as a BRIDGE between primary and secondary windings.It creates a constant bridge over which active power travels and move on to do some useful work.

I think you still might not have got what actually reactive power.

So let me explain it one more time in layman's terms using simple analogy -

Consider there is a small river dividing two cities and you need to build bridge over it to connect both the cities. After building bridge, you also need to construct a house on the other side of the river.

For the whole work including bridge and house, you've got only 100 wooden planks of which 10 are used to create bridge over the river. The workers will use this bridge to cross the river and transport items and construct a house on the other side. Here the main work is to build the house on the other side for which we've created a bridge to move from one point to another. In this case, are the wooden planks used in constructing the bridge directly contributing to the building of house ? NO. We've used 10 wooden planks for bridge and that cannot be used for building the house.

And this - THE BRIDGE is what reactive power does.100 wooden planks is the total complex power of which some amount is used to create bridge (10 planks) and remaining active power (90 planks) does the useful work. This is why we say reactive power does not do useful work.

Now you might be thinkingOK, THIS WAS THE CASE WITH A TRANSFORMER BUT WHAT ABOUT OTHER THINGS ? EVERYTHING DOES NOT HAVE ELECTRICAL ISOLATION LIKE A TRANSFORMER BUT STILL REACTIVE POWER FLOWS ?Yes, you are 100% correct.

Reactive power assists the flow of electromagnetic energy.

To explain it further it will require some technical details and will take another long post.

I think you've got a basic idea of what a reactive power does.In AC system power is a complex quantity. It consist of real part(active power) and the imaginary part(reactive part).

active power=V*IcosA reactive power=V*IsinA where,A=angle between the voltage and the current.(In case of DC we have sinA=O,hence no reactive power).REACTIVE POWER.It is that power that cannot be converted to another form for example light,heat or to run a motor. but it is very much essential for the transfer of real power through transmission lines. It is that power which oscillates between the source and the load.

We always in practice, used to reduce reactive power to improve system efficiency .This are acceptable at some level. If system is purely resistively or capacitance it make cause some problem in Electrical system. Alternating systems supply or consume two kind of power: real power and reactive power. Real power accomplishes useful work while reactive power supports the voltage that must be controlled for system reliability. so the reactive power affects voltages throughout the system.IMPORTANCE OF REACTIVE POWER:

Voltage control in an electrical power system is important for proper operation for electrical power equipment to prevent damage such as overheating of generators and motors, to reduce transmission losses and to maintain the ability of the system to withstand and prevent voltage collapse. Active power is the energy supplied to run a motor, heat a home, or illuminate an electric light bulb. Reactive power provides the important function of regulating voltage. If voltage on the system is not high enough, active power cannot be supplied. Reactive power is used to provide the voltage levels necessary for active power to do useful work. Reactive power is essential to move active power through the transmission and distribution system to the customer .Reactive power is required to maintain the voltage to deliver active power (watts) through transmission lines. Motor loads and other loads require reactive power to convert the flow of electrons into useful work. When there is not enough reactive power, the voltage sags down and it is not possible to push the power demanded by loads through the lines.

Electric poweris the rate at which electric energy is transferred by an electric circuit. It is transformed to other forms of power when electric charges move through an electric potential difference, which occurs in electrical components in electric circuits.

In AC circuits, the electrical components which are inductors and capacitors might go under a periodic change in the direction of energy flow. This in turn gives rise toActive & Reactive power.

The portion of power that, averaged over a complete cycle of the AC waveform, results in net transfer of energy in one direction is known asactive power(real power). The portion of power due to stored energy, which returns to the source in each cycle, is known asreactive power.

When we pass an electric current through a wire, it obviously produces magnetic field around it. When this field alternates between opposite peak values both in time and space, an induced voltage is produced in any of the conductors lying in the path of this field. This particular field can also react with any other magnetic field established by any other conductor and a mechanical force is created between the two conductors. This alternating field produced by an alternating current is the basis for enabling us to use electric power extensively. A DC current with a steady, non alternating field, does not offer this advantage.The alternating flux posed a lot of problems and one of them being thereactive power. It is the power required to establish & maintain an AC fluctuating magnetic flux, without which no energy transfer can take place. Against this we haveactive powerwhich delivers power in an electrical, mechanical, thermal or any form we want.Reactive power is a necessity. Without which the system won't function properly yet it is the one posing a major problem. The problem area linked with the AC fields are reactances, arcs, surges, resonances, skin effect, hunting torque. To overcome the above problems, CAPACITOR is an effective tool.

When a voltage is initially placed across the coil, a magnetic field builds up, and it takes a period of time for the current to reach full value. This causes the current to lag behind the voltage in phase; hence, these devices are said to be sources oflaggingreactive power.A capacitor is an AC device that stores energy in the form of an electric field. When current is driven through the capacitor, it takes a period of time for a charge to build up to produce the full voltage difference. On an AC network, the voltage across a capacitor is constantly changing the capacitor will oppose this change, causing the voltage to lag behind the current. In other words, the current leads the voltage in phase; hence, these devices are said to be sources ofleadingreactive power.Electric generators supply reactive power (in addition to active power) that is consumed by customer load.

Purpose of Reactive Power Synchronous generators, SVC and various types of other Distributed energy resource (DER) equipment are used to maintain voltages throughout the transmission system. Injecting reactive power into the system raises voltages, and absorbing reactive power lowers voltages. Voltage-support requirements are a function of the locations and magnitudes of generator outputs and customer loads and of the configuration of the DER transmission system. These requirements can differ substantially from location to location and can change rapidly as the location and magnitude of generation and load change. At very low levels of system load, transmission lines act as capacitors and increase voltages. At high levels of load, however, transmission lines absorb reactive power and thereby lower voltages. Most transmission-system equipment (e.g., capacitors, inductors, and tap-changing transformers) is static but can be switched to respond to changes in voltage-support requirements

System operation has three objectives when managing reactive power and voltages 1. First, it must maintain adequate voltages throughout the transmission and distribution system for both current and contingency conditions.2. Second, it seeks to minimize congestion of real-power flows.3. Third, it seeks to minimize real-power losses.Written 25 Mar. 203 views.UpvoteDownvoteCommentShareSuvra Pattanayak,a M.tech student6upvotes byAnnadana Dhanunjaya,Febin Sunny,Birend Pratap Singh,Vishnu Kumar Das,(more)Reactive power is the electrical power that oscillate between the magnetic field of an inductor and the electric filed of the capacitor. Reactive Power Cannot Converts to non-electrical power e.g. heat, light & torque. Instantaneous reactive power equal to the multiplication instantaneous current, instantaneous voltage and the sinusoidal of phase difference of current & voltage. It only present in ac power if there is any phase difference between instantaneous current and instantaneous voltage.During complete cycle average reactive power is zero.Active power is the energy supplied to run a motor, heat a home, or illuminate an electric light bulb.Reactive power provides the important function of regulating voltage. If voltage on the system is nothigh enough, active power cannot be supplied. Reactive power is used to provide the voltage levels necessary for active power to do useful work. Reactive power is essential to move active power through the transmission and distribution system to the customer .Reactive power is required to maintain the voltage to deliver active power (watts) through transmision lines. When there is not enough reactive power, the voltage sags down and it is not possible to push the power demanded by loads through the lines.Written 18 Nov, 2014. 378 views.UpvoteDownvoteComment1ShareAnonymous2upvotes byAnshuman UpadhyayandGautham AshokBefore explaining the details of Reactive power in a Transmission Line , We need to understand the Transmission Line .I will try to explain the bit about why there is a need of reactive power in Power Transmission Line. With Consecutive inputs and Edits I will try to Provide as much Information as Possible.

Power Transmission Line can be Modeled as Distributed series Line Inductance(s) and Distributed Shunt Capacitance.

If there is DC Supply Then After the transient period the series reactor gets short-circuited by getting the required energy for creating the magnetic field inside it. But In the case of AC System the amount of energy required to do so is fulfilled by reactive power.

Some of the Questions need to be answered before getting into the details of Reactive Power.1. What will happen when the current flows through Inductance ?2. How and Why the energy stored in Reactor and Capacitor in known as Reactive Power ?

The reason this power is known as reactive power cause This power only amounts to create the Field Inside a Capacitor or in a Reactor. This Reactive power does not get wasted Like the Real Power Loss in a resistor.

Power in a resistor :

So The Power Becomes Positive always and The average power amounts to the power loss.

ImgSRC:Electronic ComponentsWhile In case of L or C :

The current Lags or Leads which create

for ideal Inductor the power is shown. This power when averaged for a full cycle of Voltage Its value becomes zero.

In this case Taking positive cycle of power you can say this was during the creation of the field or while the reactor is charging . For different Instant you can analyse the circuit and find your conclusion.In this curve thesteady state operationis shown.Observations : ( When mentioned increase or decrease I meant magnitude )V = + ve & I= + ve then P = + ve( Reactor Field Charging | Current Increasing )V = - ve & I= + ve then P = - ve( Reactor Field Discharging | Current Decreasing )V = - ve & I= - ve then P = + ve( Reactor Field Charging | Current Increasing | similar to first case only in -ve Direction )V = + ve & I= - ve then P = - ve( Reactor Field Discharging | Current Decreasing | similar to first case only in -ve Direction )

Same case be Done for Capacitor

ImgSRC:John and Marion Hearfield

The inductor and capacitor does not dissipate energy instead they store is temporarily and return the same back to the source.

So When the current and voltage are in phase Then the amount of power average is equals to active power as averaged power multiplied with period is the amount of energy dissipated or utilized to do useful work.While the quadrature phase component does not contribute for work done instead it helps to store the energy.I hope the whole explanation will make you understand a little bit about the reactive power.Will Edit if further explanation needed.Written 17 Jan. 169 views.UpvoteDownvoteComments1+ShareAnand Prasad GuptaReactive power is present in a system containing reactive (inductive or capacitive) components and can be either produced or consumed by different load/generation elements. Though "imaginary", the reactive power has great physical significance and is essential to the operation of the electrical system as a whole. While the real power P is used to supply the energy required to perform actual work (such as running a motor), the reactive power regulates the voltage in the system. If the reactive power is too low, inductive loads such as transformers will be unable to maintain voltages necessary for the generation of electromagnetic fields, leading to a "voltage collapse" that create blackouts . Transmission line impedances also make it necessary to provide reactive power to maintain voltage levels necessary for active power to flow through. Therefore reactive power is essential to move active power through transmission and distribution systems to the customer. However if reactive power in a system is too high, there is increased heat loss in transmission lines and loads as the current flowing through the system is much higher, creating a potentially hazardous breakdown situation. The power factor of a load tells us what fraction of the apparent power is in the form of real power and performs actual work. A high power factor is desirable since it minimizes the amount of reactive power needed by the load, reducing heat losses and maximizing efficiency.Written 8 Apr. 121 views.UpvoteDownvoteCommentShareHarsh Dhiman,Intense Gamer, Electrical Engneer, Fo...(more)2upvotes byArul RajandHardeep SinghAlso known as pseudo power this power is greatly affected by the voltage profile of the system. It is desired that reactive power of any system remains as low as possible. As this affects the power factor of the system and also the tariffs of generating company. For a better voltage regulation and high system security reactive power compensation is done. FACT devices are most commonly used to cater this purpose.Written 10 Apr. 136 views.UpvoteDownvoteCommentShareRon LeavittHere is a good article about VAR.

Can reactive power be utilized?What do you mean ? None of your AC loads will work if you dont supply reactive power !. Reactive power is required to establish the magnetic and/or electric fields so that energy conversion can take place. All the AC motors require reactive power and the transformers and most of the static loads also require reactive power.Written 21 Jul, 2014. 253 views.UpvoteDownvoteCommentShareSwapnil Shah,Studying has become my favorite timepass1upvote bySandeep KumarObviously. Reactive power is like a built up. Just as if you want a water to flow from pipe You need to fill the pipe first. The filled water is not actively used, But without it there will be no flow.

How is reactive power produced? What are the effects of reactive power in grid?Reactive power is chiefly produced from the windings of the electrical generators in the power grid. External events, such as solar winds and X-ray bursts from the sun can induce electrical currents on the grid which manifest themselves as reactive power.

the different units are used to differentiate between the power types, even if they represent the same physical unit.in other words:kW: what you can usekVAr: losskVA: the total capacity available

Reactive power is also be produced as a side effect of unbalanced loads. Large buildings often employ 3 phase power to counteract this to some extent and thus minimise the induced currents on the 'return' line. Big drives: electrified railways (including trams) induce losses due to the fact that the wire above is mechanically tensioned and further worsened by the uneven distance of the locomotives.

instead of having a nice 3-corner star (a), the vectorial sum of which is 0, you get this mess (b), where there is going to be a deficit somewhere:Animated

kVAr, orkiloVoltAmpere reactive, on power lines are pure losses. This manifests itself by increasing the apparent power of the line, while you are still transmitting the same in-phase 'real' power. All this loss means that the line transformers, which are often weak links in terms of power limitations, can deliver much less compared to their rated maximum. If the total current passes the maximum current safe for the winding, the insulation will melt and it will short the transformer. In case of a power grid where MVA are involved, this will make for a very spectacular, loud explosion. Very expensive to replace and could leave whole areas without power for months.

This is what happens in general terms: The V-I get out of phase whenever there is a load that's not purely resistive (almost no loads are purely resistive)

The other problem is due to grid frequency control (either 50 or 60Hz). Getting a generator to output its power within >99.5% of this level is tricky. Here is an example of both reactive power (vehicles that pull from the sides do not use their full force, and only wear down their tyres more) and frequency control: the wheel itself wobbles as the cars struggle to stay in sync to avoid the bumps(uneven electricity usage) in the road

It is thus crucial to monitor both reactive and active power and limit the current on the systems where this happens.

Update 1 (March 2014):

New electrical machines are very well balanced, with a generational power quality factor of >99% in regular conditions. If the grid operator dispatch does not manage the consumer load imbalance, the power out of phase does go back to the generator, further limiting maximum generation capabilities.

Update 2:suboptimal power transfer due to impedance mismatch is what causes reactive power in general today. This is a difficult problem, because load is always variable, and the constant modulation of today's electronics impose a very high demand on the grid regarding fed -back harmonics (as a result of switching). To someone measuring at the transformer neutral point, this will look as a noisy, (DC) offset spectra.Updated 19 Aug, 2014. 1,404 views. Asked to answer bySaswata Sarkar.UpvoteDownvoteCommentShareRobert Wagner,Uses electricity daily4upvotes byRazvan Baba,Saswata Sarkar,Soumendra Kumar Sahoo, andAnthony ChuahReactive power is current flowing out of synch with voltage (on the AC waveform). It is produced by capacitive and inductive elements in the network, both end-user devices and network components. Effect on the network is reduced power delivery. Reactive power must be compensated for to prevent degradation of power delivery and, in worse cases, current flowing backwards to the plant.Written 25 Sep, 2013. 682 views.UpvoteDownvoteCommentShare

What is reactive power? What is the use of reactive power?Reactive power occurs in an AC circuits when the voltage and current are not in phase. Its unit is VAR(Volt Ampere Reactive).

It is the part of Apparent power(S) which is the combination of the Real (P)and reactive power(Q). While active power is the energy supplied to run a motor, heat a home, or illuminate an electric light bulb, reactive power provides the important function of regulating voltage. If voltage on the system is not high enough, active power cannot be supplied. Reactive power is used to provide the voltage levels necessary for active power to do useful work. Reactive power is essential to move active power through the transmission and distribution system to the customer.

Uses - It is mainly used to regulate the system voltage. It helps in increasing the efficiency of the system.

More reference :Page on electrical-engineering-portal.comWritten 24 Feb. 104 views.UpvoteDownvoteCommentSharePrasad KulkarniReactive Power is that power which is constantly oscillating between load and source. You can think it as a carrier.

Load needs some power. That power is given to the load in bursts, if it is non-resistive.

You have a resistor. Whatever change in voltage is immediately reflected in current. Hence, the entire power transfer takes place throughout the cycle.

Suppose you have a non-resistive load - like inductive or capacitive. Changes in voltage/current are not instantaneous. There will be some gap between source voltage change and load current/voltage.

Load needs power and this power is sent in bursts. What happens is, source sends some power to load. Load utilizes some part of it and sends the remaining back to the source. If it does not do so, source does not have a 'carrier' for power that load needs to consume. So, a part of the power keeps shuttling between load and source, while another part keeps getting consumed in the load.

The part that keeps getting consumed is 'real power' or 'active power', as we see its manifestation as torque of a motor or heating of a coil. The part that keeps shuttling is the 'reactive power'.

The more non-resistive the load, the more is the shuttling power requirement, as greater is the lag for load.

Hope this clears up.

The other things like units, etc are cleared up by Angia RajarajeswariWritten 9 Mar. 849 views.UpvoteDownvoteCommentShareWhat is reactive power? What is its physical significance and how is reactive power compensation performed?For making you understand i"ll take an example :consider if you are having 1000 $ and one of your friends comes to you and borrows 500 $ .After 1/2 hour he returns your money. But after another 1/2 hour he again comes and asks for 500 $ and you give him.Suppose he comes to you again within another 1/2 to return that money. But after another 1/2 hour he comes again to borrow and you give him. (suppose if you have to give, whenever he asks).Suppose this cycle repeats forever.what will be the result? - you won't be able to use those 500 $ anywhere else because your friend can come any time to ask money and you have to facilitate him.means your 500 $ are totally wasted.If you compare this to power system scenario .1000$ = total available capacity of a power plant500$( that you are giving to your friend) = total reactive power supplied by power system,only remaining 500 $ is useful & that is.. = active power.Actually this reactive power is never used but it travels forward and backward between the generation side and load side (like your 500$ are travelling b/w you and your friend but no one is actually using those 500$ ) .This is a big headache for the power plants to supply this reactive power ( like this is headache for you to give/take 500 $ thousands times in a day from/to your friend) because power plant capacity is wasted (like here you wasted 500 $).You can't neglect demand of the reactive power from the consumer end otherwise it will cause to change the voltage of the power system on large scale. This can cause under voltage (in case when supplied reactive power is less than the demanded) or Over voltage (when supplied reactive power is more than demanded).Actually we can't expect AC power systems without reactive power demand/supply. Reactive power demand occurs because of the presence of inductive/capacitive loads( however mostly are inductive ; 99%).However, we can compensate the demand using compensation (FACTS devices). It helps us to avoid wasting power plant capacity.

PS: I have answered the same for a question before.What is real, reactive and apparent power?Written 26 Nov, 2014. 194 views.UpvoteDownvoteCommentShareAnkush Rajput2upvotes bySuhas HoysalandSherif AzmyREACTIVE POWER ASSISTS THE FLOW OF ENERGY IN AC CIRCUITS.

Below is the explanation in layman's terms -

Consider there is a small river dividing two cities and you need to build bridge over it to connect both the cities. After building bridge, you also need to construct a house on the other side of the river.

For the whole work including bridge and house, you've got only 100 wooden planks of which 10 are used to create bridge over the river. The workers will use this bridge to cross the river and transport items and construct a house on the other side. Here the main work is to build the house on the other side for which we've created a bridge to move from one point to another. In this case, are the wooden planks used in constructing the bridge directly contributing to the building of house ? NO. We've used 10 wooden planks for bridge and that cannot be used for building the house.

And this - THE BRIDGE is what reactive power does.100 wooden planks is the total complex power of which some amount is used to create bridge (10 planks) and remaining active power (90 planks) does the useful work. This is why we say reactive power does not do useful work.

In transmission lines, reactive power maintains the voltage level of the line so that active power can flow to do useful work and this is why we compensate reactive power by external means. It is very much neccessary for the line to operate, for active power to flow.

And for how it is done, a capacitor (to supply reactive power to increase voltage profile) or an inductor (to absorb reactive power to decrease voltage profile) is connected in parallel to transmission line.Nowadays, power electronic compensators are used for automatic and varying degree of compensation.

What is the use of the reactive power when it is only real power that is utilized for useful purposes by the load?I'll add a little concise answer to the list of better ones already answered here.

One of the best ways that I've come across of explaining power factor while considering reactive power is the beer mug analogy.

Quoting from this pageThe Art of Explaining Power FactorYoull notice that a portion of the beer in your glass above is froth reactive power. When you tilt the glass to get the beer flowing into your mouth, the froth jumps in first creating a delivery delay (or lag) in beer-to-mouth timing. Reactive power does the same: creating a lag in power-to-equipment timing. The more froth, the more lag and interference and the lower (worse) the power factor ratio.If the true power (beer) is 800 watts and the apparent power (glass capacity) is 1000 watts, you will find your power factor by dividing 800 by 1000 equaling 80%. Theless beer and more frothyou have in your glass, the lower and worse your power factor will get as demonstrated below:

Basically, pf = kW/kVA, or kW/(kW + kVAR) [= (real beer)/(real beer + froth)]

To sum it up, reactive power is a practical real-life constraint that we face with electrical systems. Just like froth in a mug of beer, it is unavoidable. So if we want to pour a standard volume of beer in a mug, the mug has to be designed so as to accommodate the froth as well. If we don't, we'll have a mess on our hands.

There is no "use" as such of reactive power just like the beer froth. It's not the real stuff. Reactive power increases machine ratings and decreases overall efficiency. The more the reactive power, the more will be the power loss in the system, which would lead to problems like machine failures, heat-ups, losses in the distribution system, etc, not to mention the financial penalty that would be thrown at you by the utility for poor pf. So reactive power will always have to be considered in electrical design and optimization.Written 11 Jan, 2013. 858 views.UpvoteDownvoteComments2ShareMounica Arroju,Undergrad in Electrical Engineering27upvotes byPrashant Venkatasubban,Sandeep Chillakuri,Rajyalakshmi Kola,Neha Khawas,(more)Reactive power and Active power are two components of power generated because of the time phase lag/lead between the voltage and current. That is, if the voltage and current go up and down at the same time, only active power is transmitted. But when there is a time shift between the voltage and the current, both active and reactive power are transmitted.

The average of the active power is the net energy transmitted. But the average of reactive power is zero, no net energy is transmitted; the energy due to reactive power flowing in one direction is equal to the energy flowing in the other direction. We can say that the reactive power keeps oscillating from the source to load and from load to the source in the lines.

Also, we can look at it as if the reactive power is carrying the active power from the source to the load and then goes back to the source to collect more active power to supply to the load again. The following analogy might help u in understanding this concept:Suppose I want to fill a water tank with water, one bucket at a time. Only way is to climb a ladder, carrying a bucket of water and pouring the water into the tank. Once I fill up the tank, then I have to go down the ladder to get more water. In this one cycle of going up the ladder and coming down I have done some work or the energy required to go up is more than the energy required for coming down.If I had climbed the ladder with an empty bucket, and I had come down with the same bucket I am not doing any work. The energy for upward and downward motion is the same. Though I have not done any work worth paying for- I require some energy.That is, the energy that it takes to go up and down a ladder carrying nothing either way requires reactive power, but no real power. The energy that it takes to go up a ladder carrying something and come down without carrying anything requires both real power and reactive power.The analogy can be extended for explaining 3 phase system if If we put 3 ladders going up to the tank and 3 people climb up in sequence such that there is always a steady flow.

Source:Explaining Reactive Power - 4 further analogies(refer to this link for further analogies)

So, what causes reactive power losses? Strictly speaking, reactive power itself is not a loss to the system. But, reactive power causes higher current to be drawn for the same load. This we can measure through the power factor (ratio of active power to the apparent power) of the load.

Lets say, there is a motor whose rating is 100KVA, 230V. Ideally, if this motor operates at a power factor of 1, it should take a current of 434.7 Amperes.But actually it operates at a power factor of 0.8 (say). Then it will take a current of 543.48 Amps. This extra current of 108 Amps is going into magnetizing the motor because of the presence of reactive power which finally is not useful work to us. The extra current causes higher copper losses in the wires. These are what we call reactive power losses.

There is another favourite site of mine which explains all the basic confusions in the field of Electrical Engineering:Electricity F.A.Q.Written 21 Dec, 2012. 1,548 views. Asked to answer bySandeep Chillakuri.UpvoteDownvoteComments4+ShareMohammad Ibrahim10upvotes byMikael Bengtsson,Rob Ert,Soumendra Kumar Sahoo,Kalyan Reddy,(more)Well, the current drawn by the motors composed of magnetizing current and actual working current

The magnetizing current is inevitable to produce the magnetic field to allow the motor to run.

in the above wheelbarrow, Think of magnetizing current as the lifting force you have to do in order to push the barrow.

another analogy for reactive power is the horse and the boat analogy .

The fact that the horse is not walking straight in front of the boat, does not influence the work it has to do to pull the boat. But without compensation by the rudder, the boat will be pulled towards the bank of the canal.Consequences: The turned rudder leads to extra losses The fact that the rope is pulling at the flank of thehorse and not straight behind it, limits the horsescapacity to deliver work

I hope this helps

the following link is pdf which shows 4 analogies of reactive powerPage on Sari-energy.orgWritten 21 Dec, 2012. 2,089 views.UpvoteDownvoteComment1ShareAnkit Goel,small fish in an ocean.1upvote bySaswata Sarkarthe answers given by Mounica and Ibrahim are sufficient enough to know about the reactive and active component of current. But this totally depends on the type of components you have in a circuit.. Basically load can be sub categorized as resistive, capacitive and inductive.and it is very important to know that only the active component of current is responsible for any work done by the motor or any machine or any equipment that utilizes electricity. If the circuit is resistive only, then only the active component would be present in the circuit, similarly if only capacitive and inductive loads are there then only the reactive current would be flowing. reactive current is the current that flows into the capacitors and inductors.

since the question asks for the use of reactive part I would say that since the machines about which the question talks about needs both the electric fields and active power (to do work) so inductor and capacitors along with resistors are used. Electric fields are produced by inductors and capacitors (not going to details as in how both are used together and why magnetization of these machines are required), hence the reactive component comes into play and is responsible to produce the electric field in the said load (capacitor and inductor).

so its not a matter of choice whether one wants reactive component of current or not, rather its the necessity as in whether the machine needs electric field or not. if not then no reactive component.Written 27 Dec, 2012. 595 views.UpvoteDownvoteCommentShareDivya Malika,Electrical Engineer4upvotes byKrishna Kumar,Anil Thapa,Manoj Patil, andGaurav KumarSo many gave good answers, I'll add a few points in cut short.

Reactive power provides the important function of regulating voltage. For example assume two persons traveling on a Bi-cycle. One is cycling and the other just sits behind him. If the 'second person' suddenly got moved/bends aside while traveling (same as that of power fluctuations), then the first person will automatically bends in the opposite direction of second person to balance the bi-cycle. Here 1st person - Reactive Power & 2nd person - Active Power.

So, it is not possible to push the power demanded by loads without reactive power. That is why few people call it as compensating power.

Disclaimer: The above example is to show up the very purpose of reactive power. Two persons traveling on a bicycle is a mandate assumption in the above case.Written 15 Jul, 2014. 1,155 views.UpvoteDownvoteComments2+ShareSuvra Pattanayak,a M.tech studentReactive power is the electrical power that oscillate between the magnetic field of an inductor and the electric filed of the capacitor. Reactive Power Cannot Converts to non-electrical power e.g. heat, light & torque. Instantaneous reactive power equal to the multiplication instantaneous current, instantaneous voltage and the sinusoidal of phase difference of current & voltage. It only present in ac power if there is any phase difference between instantaneous current and instantaneous voltage.During complete cycle average reactive power is zero.Active power is the energy supplied to run a motor, heat a home, or illuminate an electric light bulb.Reactive power provides the important function of regulating voltage. If voltage on the system is nothigh enough, active power cannot be supplied. Reactive power is used to provide the voltage levels necessary for active power to do useful work. Reactive power is essential to move active power through the transmission and distribution system to the customer .Reactive power is required to maintain the voltage to deliver active power (watts) through transmision lines. When there is not enough reactive power, the voltage sags down and it is not possible to push the power demanded by loads through the lines.Written 19 Nov, 2014. 315 views.UpvoteCapacitor Bank | Reactive Power CompensationUnder Electrical TransmissionThe demand of active power is expressing Kilo watt (kw) or mega watt (mw). This power should be supplied from electrical generating station. All the arrangements in electrical pomes system are done to meet up this basic requirement. Although in alternating power system, reactive power always comes in to picture. This reactive power is expressed in Kilo VAR or Mega VAR.. The demand of this reactive power is mainly originated from inductive load connected to the system. These inductive loads are generally electromagnetic circuit of electric motors, electrical transformers, inductance of transmission and distribution networks, induction furnaces, fluorescent lightings etc. This reactive power should be properly compensated otherwise, the ratio of actual power consumed by the load, to the total power i.e. vector sum of active and reactive power, of the system becomes quite less. This ratio is alternatively known as electrical power factor, and fewer ratios indicates poor power factor of the system. If the power factor of the system is poor, the ampere burden of the transmission, distribution network, transformers, alternators and other equipments connected to the system, becomes high for required active power. And hence reactive power compensation becomes so important. This is commonly done by capacitor bank. Let's explain in details, we know that active power is expressed =vIcos where,cos is the power factor of the system. Hence, if this power factor has got less valve, the corresponding current (I) increases for same active power P. As the current of the system increases, the ohmic loss of the system increases. Ohmic loss means, generated electrical power is lost as unwanted heat originated in the system. The cross-section of the conducting parts of the system may also have to be increased for carrying extra ampere burden, which is also not economical in the commercial point of view. Another major disadvantage, is poor voltage regulation of the system, which mainly caused due to poor power factor. The equipments used to compensate reactive power. There are mainly two equipments used for this purpose. (1) synchronous condensers (2) Static capacitors or Capacitor Bank synchronous condensers, can produce reactive power and the production of reactive power can be regulated. Due to this regulating advantage, the synchronous condensers are very suitable for correcting power factor of the system, but this equipment is quite expensive compared to static capacitors. That is why synchronous condensers, are justified to use only for voltage regulation of very high voltage transmission system. The regulation in static capacitors can also be achieved to some extend by split the total capacitor bank in 3 sectors of ratio 1: 2:2. This division enables the capacitor to run in 1, 2, 1+2=3,2+2=4, 1+2+2=5 steps. If still further steps are required, the division may be made in the ratio 1:2:3 or 1:2:4. These divisions make the static capacitor bank more expensive but still the cost is much lower them synchronous condensers. It is found that maximum benefit from compensating equipments can be achieved when they are connected to the individual load side. This is practically and economically possible only by using small rated capacitors with individual load not by using synchronous condensers. Static capacitor Bank Static capacitor can further be subdivided in to two categories, (a) Shunt capacitors (b) Series capacitor These categories are mainly based on the methods of connecting capacitor bank with the system. Among these two categories, shunt capacitors are more commonly used in the power system of all voltage levels. There are some specific advantages of using shunt capacitors such as, a) It reduces line current of the system. b) It improves voltage level of the load. c) It also reduces system Losses. d) It improves power factor of the source current. e) It reduces load of the alternator. f) It reduces capital investment per mega watt of the Load. All the above mentioned benefits come from the fact, that the effect of capacitor reduces reactive current flowing through the whole system. Shunt capacitor draws almost fixed amount of leading current which is superimposed on the load current and consequently reduces reactive components of the load and hence improves the power factor of the system. series capacitor on the other hand has no control over flow of current. As these are connected in series with load , the load current always passes through the series capacitor bank. Actually, the capacitive reactance of series capacitor neutralizes the inductive reactance of the line hence, reduces, effective reactance of the line. Thereby, voltage regulation of the system is improved. But series capacitor bank has a major disadvantage. During faulty condition, the voltage across the capacitor maybe raised up to 15 times more than its rated value. Thus series capacitor must have sophisticated and elaborate protective equipments. Because of this, use of-series capacitor is confined in the extra high voltage system only. Shunt CapacitorConstruction of Shunt CapacitorThe active parts of capacitor unit are composed by two aluminum foils separated by impregnated papers. The thickness of the papers may vary from 8 microns to 24 microns depending upon the voltage level of the system. The thickness of the aluminum foil is in the order of 7 microns. For low voltage applications, there may be one layer of impregnated paper of suitable thickness between the foils but for higher voltage applications more than one layer of impregnated papers are placed between the aluminum foil to avoid unwanted circulation of short circuit current between the foil due to presence of conducting matters in the papers. The capacitor sections are wound into rolls thereafter they are flattened out, compressed into packs, enclosed in multiple layers of heavy paper insulations and inserted into the containers. When the lid had been welded to the container, the capacitor unit is dried and integrated in large autoclaves by a combination of heat and vacuum. After the paper is completely dried and all gases removed from the insulation the capacitor tank is filled with impregnant degassed at the same vacuum. In the early stages of development, it was generally mineral insulating oil which was used as impregnant. This has now been replaced by most of the manufacturers with synthetic liquids of chlorinated diphenyl group bearing different trade names. Mineral insulating oil has very low electric conductivity and very high dielectric strength. But it has however some drawbacks such as, a) It has low dielectric constant. b) The voltage distribution in the mineral oil is not uniform. c) It is very inflammable. d) It is subjected to oxidation. With the synthetic impregnant it is quite possible to manufacture smaller capacitor unit with higher voltage rating. The voltage rating of the capacitor unit is restricted within certain limits because on low voltage the cost per kilo VAR goes high. For high voltage applications, numbers of capacitor units are connected in series and parallel combination to form a capacitor bank for required voltage and Kilo VAR ratings. For example when 5.1 Mega VAR capacitor bank is to be commissioned in an 11 KV system, each unit of the bank is made of 11 KV rated. In this installation, per phase requirement of Mega VAR is 5.1/3=1.7. In this installation, there should be only one capacitor unit connected in series and 17 of such units are connected in parallel to meet up the mega VAR requirement of one phase. For three phase system three such groups of capacitor unit are connected together in star or delta form. Lets show another example for better understanding. When a bank of 5.4 Mega VAR is to be installed at 33 KV 3 phase system. There shall be three capacitor units connected in series and six of such series combinations are connected in parallel to meet up 1.8 Mega VAR demand of per phase. The same capacitor units can be used for 132 KV systems too. For that Series and parallel combinations of the basic capacitor units will be assembled as per mega VAR requirement.

Importance of Reactive Power forSystemMARCH 21, 201136 COMMENTSIntroduction: We always in practice to reduce reactive power to improve system efficiency .This are acceptable at some level. If system is purely resistively or capacitance it make cause some problem in Electrical system. Alternating systems supply or consume two kind of power: real power and reactive power. Real power accomplishes useful work while reactive power supports the voltage that must be controlled for system reliability. Reactive power has a profound effect on the security of power systems because it affects voltages throughout the system. Find important discussion regarding importance about Reactive Power and how it is useful to maintain System voltage healthyImportance of Reactive Power: Voltage control in an electrical power system is important for proper operation for electrical power equipment to prevent damage such as overheating of generators and motors, to reduce transmission losses and to maintain the ability of the system to withstand and prevent voltage collapse. Decreasing reactive power causing voltage to fall while increasing it causing voltage to rise. A voltage collapse may be occurs when the system try to serve much more load than the voltage can support. When reactive power supply lower voltage, as voltage drops current must increase to maintain power supplied, causing system to consume more reactive power and the voltage drops further . If the current increase too much, transmission lines go off line, overloading other lines and potentially causing cascading failures. If the voltage drops too low, some generators will disconnect automatically to protect themselves. Voltage collapse occurs when an increase in load or less generation or transmission facilities causes dropping voltage, which causes a further reduction in reactive power from capacitor and line charging, and still there further voltage reductions. If voltage reduction continues, these will cause additional elements to trip, leading further reduction in voltage and loss of the load. The result in these entire progressive and uncontrollable declines in voltage is that the system unable to provide the reactive power required supplying the reactive power demandsNecessary to Control of Voltage and Reactive Power: Voltage control and reactive power management are two aspects of a single activity that both supports reliability and facilitates commercial transactions across transmission networks. On an alternating current (AC) power system,voltage is controlled by managing production and absorption of reactive power. There are three reasons why it is necessary to manage reactive power and control voltage. First, both customer and power system equipment are designed to operate within a range of voltages, usually within5% of the nominal voltage. At low voltages, many types of equipment perform poorly, light bulbs provide less illumination, induction motors can overheat and be damaged, and some electronic equipment will not operate at. High voltages can damage equipment and shorten their lifetimes. Second, reactive power consumes transmission and generation resources. To maximize the amount of real power that can be transferred across a congested transmission interface, reactive power flows must be minimized. Similarly, reactive power production can limit a generators real power capability. Third, moving reactive power on the transmission system incurs real power losses. Both capacity and energy must be supplied to replace these losses. Voltage control is complicated by two additional factors. First, the transmission system itself is a nonlinear consumer of reactive power, depending on system loading. At very light loading the system generates reactive power that must be absorbed, while at heavy loading the system consumes a large amount of reactive power that must be replaced. The systems reactive power requirements also depend on the generation and transmission configuration. Consequently, system reactive requirements vary in time as load levels and load and generation patterns change. The bulk power system is composed of many pieces of equipment, any one of which can fail at any time. Therefore, the system is designed to withstand the loss of any single piece of equipment and to continue operating without impacting any customers. That is, the system is designed to withstand a single contingency. The loss of a generator or a major transmission line can have the compounding effect of reducing the reactive supply and, at the same time, reconfiguring flows such that the system is consuming additional reactive power. At least a portion of the reactive supply must be capable of responding quickly to changing reactive power demands and to maintain acceptable voltages throughout the system. Thus, just as an electrical system requires real power reserves to respond to contingencies, so too it must maintain reactive-power reserves. Loads can also be both real and reactive. The reactive portion of the load could be served from the transmission system. Reactive loads incur more voltage drop and reactive losses in the transmission system than do similar size (MVA) real loads. System operation has three objectives when managing reactive power and voltages. First, it must maintain adequate voltages throughout the transmission and distribution system for both current and contingency conditions. Second, it seeks to minimize congestion of real power flows. Third, it seeks to minimize real power losses.Basic concept of Reactive Power1)Why We Need Reactive Power: Active power is the energy supplied to run a motor, heat a home, or illuminate an electric light bulb. Reactive power provides the important function of regulating voltage. If voltage on the system is not high enough, active power cannot be supplied. Reactive power is used to provide the voltage levels necessary for active power to do useful work. Reactive power is essential to move active power through the transmission and distribution system to the customer .Reactive power is required to maintain the voltage to deliver active power (watts) through transmission lines. Motor loads and other loads require reactive power to convert the flow of electrons into useful work. When there is not enough reactive power, the voltage sags down and it is not possible to push the power demanded by loads through the lines.2)Reactive Power is a Byproduct of AC Systems Transformers, Transmission lines, and motors require reactive power. Electric motors need reactive power to produce magnetic fields for their operation. Transformers and transmission lines introduce inductance as well as resistance1. Both oppose the flow of current2. Must raise the voltage higher to push the power through the inductance of the lines3. Unless capacitance is introduced to offset inductance3)How Voltages Controlled by Reactive Power: Voltages are controlled by providing sufficient reactive power control margin to supply needs through1. Shunt capacitor and reactor compensations2. Dynamic compensation3. Proper voltage schedule of generation. Voltages are controlled by predicting and correcting reactive power demand from loads4)Reactive Power and Power Factor Reactive power is present when the voltage and current are not in phase1. One waveform leads the other2. Phase angle not equal to 03. Power factor less than unity Measured in volt-ampere reactive (VAR) Produced when the current waveform leads voltage waveform (Leading power factor) Vice verse, consumed when the current waveform lags voltage (lagging power factor)5)Reactive Power Limitations: Reactive power does not travel very far. Usually necessary to produce it close to the location where it is needed A supplier/source close to the location of the need is in a much better position to provide reactive power versus one that is located far from the location of the need Reactive power supplies are closely tied to the ability to deliver real or active power.Reactive Power Caused Absence of Electricity -A Blackout The quality of the electrical energy supply can be evaluated basing on a number of parameters. However, the most important will be always the presence of electrical energy and the number and duration of interrupts. When consumption of electrical energy is high, the demand on inductive reactive power increases at the same proportion. In this moment, the transmission lines (that are well loaded) introduce an extra inductive reactive power. The local sources of capacitive reactive power become insufficient. It is necessary to deliver more of the reactive power from generators of power plants. It might happen that they are already fully loaded and the reactive power will have to be delivered from more distant places. Transmission of reactive power will load more the lines, which in turn will introduce more reactive power. The voltage on customer side will decrease further. Local control of voltage by means of auto transformers will lead to increase of current (to get the same power) and this in turn will increase voltage drops in lines. In one moment this process can go like avalanche reducing voltage to zero. In mean time most of the generators in power plants will switch off due to unacceptably low voltage what of course will deteriorate the situation. Insufficient reactive power leading to voltage collapse has been a causal factor in major blackouts in the worldwide. Voltage collapse occurred in United States in the blackout of July 2, 1996, and August10, 1996 on the West Coast While August 14, 2003, blackout in the United States and Canada was not due to a voltage collapse as that term has traditionally used by power system engineers, the task force final report said that Insufficient reactive power was an issue in the blackoutand the report also overestimation of dynamics reactive output of system generation as common factor among major outages in the United States. Demand for reactive power was unusually high because of a large volume of long-distance transmissions streaming through Ohio to areas, including Canada, than needed to import power to meet local demand. But the supply of reactive power was low because some plants were out of service and, possibly, because other plants were not producing enough of it.Problem of Reactive Power: Though reactive power is needed to run many electrical devices, it can cause harmful effects on appliances and other motorized loads, as well as electrical infrastructure. Since the current flowing through electrical system is higher than that necessary to do the required work, excess power dissipates in the form of heat as the reactive current flows through resistive components like wires, switches and transformers. Keep in mind that whenever energy is expended, you pay. It makes no difference whether the energy is expended in the form of heat or useful work. We can determine how much reactive power electrical devices use by measuring their power factor, the ratio between real power and true power. A power factor of 1 (i.e. 100%) ideally means that all electrical power is applied towards real work. Homes typically have overall power factors in the range of 70% to 85%, depending upon which appliances may be running. Newer homes with the latest in energy efficient appliances can have an overall power factor of 90%. Electric companies correct for power factor around industrial complexes, or they will request the offending customer to do so, or they will charge for reactive power. Electric companies are not worried about residential service because the impact on their distribution grid is not as severe as in heavily industrialized areas. However, it is true that power factor correction assists the electric company by reducing demand for electricity, thereby allowing them to satisfy service needs elsewhere. Power factor correction will not raise your electric bill or do harm to your electrical devices. The technology has been successfully applied throughout industry for years. When sized properly, power factor correction will enhance the electrical efficiency and longevity of inductive loads. Power factor correction can have adverse side effects (e.g. harmonics) on sensitive industrialized equipment if not handled by knowledgeable, experienced professionals. Power factor correction on residential dwellings is limited to the capacity of the electrical panel (200 amp max) and does not over compensate household inductive loads. By increasing the efficiency of electrical systems, energy demand and its environmental impact is lessenedEffects of Reactive Power in Various elements of Power System:1)Generation: An electric power generators primary function is to convert fuel into electric power. Almost all generators also have considerable control over their terminal voltage and reactive-power output. The ability of generator to provide reactive support depends on its real power production. Like most electric equipment, generators are limited by their current carrying capability. Near rated voltage, this capability becomes an MVA limit for the armature of the generator rather than a MW limitation. Production of reactive power involves increasing the magnetic field to raise the generators terminal voltage. Increasing the magnetic field requires increasing the current in the rotating field winding. Absorption of reactive power is limited by the magnetic-flux pattern in the stator, which results in excessive heating of the stator-end iron, the core-end heating limit. The synchronizing torque is also reduced when absorbing large amounts of reactive power, which can also limit generator capability to reduce the chance of losing synchronization with the system. The generator prime mover (e.g., the steam turbine) is usually designed with less capacity than the electric generator, resulting in the prime-mover limit. The designers recognize that thegenerator will be producing reactive power and supporting system voltage most of the time.Providing a prime mover capable of delivering all the mechanical power the generator can convert to electricity when it is neither producing nor absorbing reactive power would result in under utilization of the prime mover. To produce or absorb additional VARs beyond these limits would require a reduction in the real power output of the unit. Control over the reactive output and the terminal voltage of the generator is provided by adjusting the DC current in the generators rotating field .Control can be automatic, continuous, and fast. The inherent characteristics of the generator help maintain system voltage. At any given field setting, the generator has a specific terminal voltage it is attempting to hold. If the system voltage declines, the generator will inject reactive power into the power system, tending to raise system voltage. If the system voltage rises, the reactive output of the generator will drop, and ultimately reactive power will flow into the generator, tending to lower system voltage. The voltage regulator will accentuate this behavior by driving the field current in the appropriate direction to obtain the desired system voltage.2)Synchronous Condensers: Every synchronous machine (motor or generator) with a controllable field has the reactive power capabilities discussed above. Synchronous motors are occasionally used to provide dynamic voltage support to the power system as they provide mechanical power to their load. Some combustion turbines and hydro units are designed to allow the generator to operate without its mechanical power source simply to provide the reactive power capability to the power system when the real power generation is unavailable or not needed. Synchronous machines that are designed exclusively to provide reactive support are called synchronous condensers. Synchronous condensers have all of the response speed and controllability advantages of generators without the need to construct the rest of the power plant (e.g., fuel-handling equipment and boilers). Because they are rotating machines with moving parts and auxiliary systems, they may require significantly more maintenance than static alternatives. They also consume real power equal to about 3% of the machines reactive-power rating.3)Capacitors & Inductors: Capacitors and inductors (which are sometimes called reactors) are passive devices that generate or absorb reactive power. They accomplish this without significant real power losses or operating expense. The output of capacitors and inductors is proportional to the square of the voltage. Thus, a capacitor bank (or inductor) rated at 100 MVAR will produce (or absorb) only 90 MVAR when the voltage dips to 0.95 pu but it will produce (or absorb) 110 MVAR when the voltage rises to 1.05 pu. This relationship is helpful when inductors are employed to hold voltages down. The inductor absorbs more when voltages are highest and the device is needed most. The relationship is unfortunate for the more common case where capacitors are employed to support voltages. In the extreme case, voltages fall, and capacitors contribute less, resulting in a further degradation in voltage and even less support from the capacitors; ultimately, voltage collapses and outages occur. Inductors are discrete devices designed to absorb a specific amount of reactive power at a specific voltage. They can be switched on or off but offer no variable control. Capacitor banks are composed of individual capacitor cans, typically 200 kVAR or less each. The cans are connected in series and parallel to obtain the desired capacitor bank voltage and capacity rating. Like inductors, capacitor banks are discrete devices but they are often configured with several steps to provide a limited amount of variable control which makes it a disadvantage compared to synchronous motor.4)Static VAR Compensators : (SVCs) An SVC combines conventional capacitors and inductors with fast switching capability. Switching takes place in the sub cycle timeframe (i.e. in less than 1/60 of a second), providing a continuous range of control. The range can be designed to span from absorbing to generating reactive power. Consequently, the controls can be designed to provide very fast and effective reactive support and voltage control. Because SVCs use capacitors, they suffer from the same degradation in reactive capability as voltage drops. They also do not have the short term overload capability of generators and synchronous condensers. SVC applications usually require harmonic filters to reduce the amount of harmonics injected into the power system.5)Static Synchronous Compensators : (STATCOMs) The STATCOM is a solid-state shunt device that generates or absorbs reactive power and is one member of a family of devices known as flexible AC transmission system. The STATCOM is similar to the SVC in response speed, control capabilities, and the use of power electronics. Rather than using conventional capacitors and inductors combined with fast switches, however, the STATCOM uses power electronics to synthesize the reactive power output. Consequently, output capability is generally symmetric, providing as much capability for production as absorption. The solid-state nature of the STATCOM means that, similar to the SVC, the controls can be designed to provide very fast and effective voltage control. While not having the short-term overload capability of generators and synchronous condensers, STATCOM capacity does not suffer as seriously as SVCs and capacitors do from degraded voltage. STATCOMs are current limited so their MVAR capability responds linearly to voltage as opposed to the voltage squared relationship of SVCs and capacitors. This attribute greatly increases the usefulness of STATCOMs in preventing voltage collapse.6)Distributed Generation: Distributing generation resources throughout the power system can have a beneficial effect if the generation has the ability to supply reactive power. Without this ability to control reactive power output, performance of the transmission and distribution system can be degraded. Induction generators were an attractive choice for small, grid-connected generation, primarily because they are relatively inexpensive. They do not require synchronizing and have mechanical characteristics that are appealing for some applications (wind, for example). They also absorb reactive power rather than generate it, and are not controllable. If the output from the generator fluctuates (as wind does), the reactive demand of the generator fluctuates as well, compounding voltage-control problems for the transmission system. Induction generators can be compensated with static capacitors, but this strategy does not address the fluctuation problem or provide controlled voltage support. Many distributed generation resources are now being coupled to the grid through solid-state power electronics to allow the prime movers speed to vary independently of the power-system frequency. For wind, this use of solid-state electronics can improve the energy capture. For gas-fired micro turbines, power electronics equipment allows them to operate at very high speeds. Photovoltaics generate direct current and require inverters to couple them to the power system. Energy-storage devices (e.g., batteries, flywheels, and superconducting magnetic-energy storage devices) are often distributed as well and require solid-state inverters to interface with the grid. This increased use of a solid-state interface between the devices and the power system has the added benefit of providing full reactive-power control, similar to that of a STATCOM. In fact, most devices do not have to be providing active power for the full range of reactive control to be available. The generation prime mover, e.g. turbine, can be out of service while the reactive component is fully functional. This technological development (solid-state power electronics) has turned a potential problem into a benefit, allowing distributed resources to contribute to voltage control.7)Transmission Side: Unavoidable consequence of loads operation is presence of reactive power, associated with phase shifting between voltage and current. Some portion of this power is compensated on customer side, while the rest is loading the network. The supply contracts do not require a cos equal to one. The reactive power is also used by the transmission lines owner for controlling the voltages. Reactive component of current adds to the loads current and increases the voltage drops across network impedance.Adjusting the reactive power flow the operator change voltage drops in lines and in this way the voltage at customer connection point. The voltage on customer side depends on everything what happens on the way from generator to customer loads. All nodes, connection points of other transmission lines, distribution station and other equipment contribute to reactive power flow. A transmission line itself is also a source of reactive power. A line that is open on the other end (without load) is like a capacitor and is a source of capacitive (leading) reactive power. The lengthwise inductances without current are not magnetized and do not introduce any reactive components. On the other hand, when a line is conducting high current, the contribution of the lengthwise inductances is prevalent and the line itself becomes a source of inductive (lagging) reactive power. For each line can be calculated a characteristic value of power flow. If the transmitted power is more than pre define Value, the line will introduce additionally inductive reactive power, and if it is below pre define Value, the line will introduce capacitive reactive power. The pre define Value depends on the voltage: for 400 kV line is about 32% of the nominal transmission power, for 220 kV line is about 28% and for 110 kV line is about 22%. The percentage will vary accordingly to construction parameters. The reactive power introduced by the lines themselves is really a nuisance for the transmission system operator. In the night, when the demand is low it is necessary to connect parallel reactors for consuming the additional capacitive reactive power of the lines. Sometimes it is necessary to switch off a low-loaded line (what definitely affect the system reliability). In peak hours not only the customer loads cause big voltage drops but also the inductive reactive power of the lines adds to the total power flow and causes further voltage drops. The voltage and reactive power control has some limitations. A big part of reactive power is generated in power plant unites. The generators can deliver smoothly adjustable leading and lagging reactive power without any fuel costs. However, the reactive power occupies the generation capacity and reduces the active power production. Furthermore, it is not worth to transmit reactive power for long distance (because of active power losses). Control provided on the way in transmission line, connation nodes, distribution station and other points requires installation of capacitors or\and reactors. They are often used with transformer tap changing system. The range of voltage control depends on their size. The control may consist e.g. in setting the transformer voltage higher and then reducing it by reactive currents flow. If the transformer voltage reaches the highest value and all capacitors are in operation, the voltage on customer side cannot be further increase. On the other hand when a reduction is required the limit is set by maximal reactive power of reactors and the lowest tap of transformer.Assessment Practices to control Voltage & Reactive Power: Transmission and Distribution planners must determine in advance the required type and location of reactive correction.1)Static vs. Dynamic Voltage Support The type of reactive compensation required is based on the time needed for voltage recovery. Static Compensation is ideal for second and minute responses. (Capacitors, reactors, tap changes). Dynamic Compensation is ideal for instantaneous responses. (condensers, generators) A proper balance of static and dynamic voltage support is needed to maintain voltage levels within an acceptable range.2)Reactive Reserves during Varying Operating Conditions The system capacitors, reactors, and condensers should be operated to supply the normal reactive load. As the load increases or following a contingency, additional capacitors should be switched on or reactors removed to maintain acceptable system voltages. The reactive capability of the generators should be largely reserved for contingencies on the EHV system or to support voltages during extreme system operating conditions. Load shedding schemes must be implemented if a desired voltage is unattainable threw reactive power reserves3)Voltage Coordination The reactive sources must be coordinated to ensure that adequate voltages are maintained everywhere on the interconnected system during all possible system conditions. Maintaining acceptable system voltages involves the coordination of sources and sinks which include:1. Plant voltage schedules2. Transformer tap settings3. Reactive device settings4. Load shedding schemes. The consequences of uncoordinated of above operations would include:1. Increased reactive power losses2. A reduction in reactive margin available for contingencies and extreme light load conditions3. Excessive switching of shunt capacitors or reactors4. Increased probability of voltage collapse conditions. Plant Voltage Schedule :Each power plant is requested to maintain a particular voltage on the system bus to which the plant is connected. The assigned schedule will permit the generating unit to typically operate:1. In the middle of its reactive capability range during normal conditions2. At the high end of its reactive capability range during contingencies3. Under excited or absorb reactive power under extreme light load conditions. Transformer Tap Settings :Transformer taps must be coordinated with each other and with nearby generating station voltage schedules. The transformer taps should be selected so that secondary voltages remain below equipment limits during light load conditions. Reactive Device Settings :Capacitors on the low voltage networks should be set to switch on to maintain voltages during peak and contingency conditions. And Off when no longer required supporting voltage levels. Load Shedding Schemes:Load shedding schemes must be implemented as a last resort to maintain acceptable voltages.4)Voltage and Reactive Power Control Requires the coordination work of all Transmission and Distribution disciplines. Transmission needs to:1. Forecast the reactive demand and required reserve margin2. Plan, engineer, and install the required type and location of reactive correction3. Maintain reactive devices for proper compensation4. Maintain meters to ensure accurate data5. Recommend the proper load shedding scheme if necessary. Distribution needs to:1. Fully compensate distribution loads before Transmission reactive compensation is considered2. Maintain reactive devices for proper compensation3. Maintain meters to ensure accurate data4. Install and test automatic under voltage load shedding schemesReferences:1. Samir Aganovi,2. Zoran Gaji,3. Grzegorz Blajszczak- Warsaw, Poland,4. Gianfranco Chicco5. Robert P. OConnell-Williams Power Company6. Harry L. Terhune-American Transmission Company,7. Abraham Lomi, Fernando Alvarado, Blagoy Borissov, Laurence D. Kirsch8. Robert Thomas,9. OAK RIDGE NATIONAL LABORATORYReactive power: what it is; why it is important06 Mar 2012Jeff Palermo0load,There is a global trend toward adding dynamic reactive devices to serve urban load areas. A number of interrelated factors contribute to this increasing use of dynamic reactive devices, such as the following:> Many urban areas are experiencing a growth in load levels. These urban areas also see less generation close to the load center. Either legacy generating units have been retiring or they are operating less, because they are less efficient than newer generators. Losing these generators reduces the amount of local power generation and dynamic var support available in the urban areas. As a result, urban areas often rely on increased power imports to serve their load.> Increased power imports tend to reduce local transmission voltages, which, in turn, reduce the effectiveness of various devices that support the local voltages. In the extreme, this could become a kind of voltagespiral of death.> In many cases, there is enough transmission thermal capability to deliver the needed imports, but there are increasingly frequent periods where voltage/var factors limit system operation.> Public opposition to new transmission lines is also common.> In many deregulated markets, there are limited means to require or encourage generator construction or to schedule power outside the normal generation dispatch market. In other markets, requiring generators to operate outside normal economic dispatch can be very costly.These urban areas, however, often face an underappreciated risk associated with the addition of these complex dynamic reactive devices.Reactive power and why it is importantFundamentally, electrical power is developed, delivered, and consumed as voltage and current. In a simple direct current (DC) device (like a flashlight), the power (the brightness of the bulb) is the voltage times the current, which is measured in watts. Watts are a measure of the ability of a device to perform useful work. This is what is calledreal(and sometimesactive) power because it can produce useful (real) work.In alternating current (AC) systemslike all modern power systemsthings become a bit more complicated. Common customer load devicesespecially motorshave inherent characteristics that shift the relationship between current and voltage. This shift is measured in varsfrom the French term Volt-Ampere reactive. Since much of the power system load is motors in factories, businesses, and homes, this becomes a significant issue for utilities. Transformers and transmission and distribution lines and cables also have characteristics that contribute to this shift.Increasing var load reduces the ability of the system to deliver real power and perform useful work. In extreme cases, a high var load can shift the voltage and current so much that it reduces the power systems delivery capability so that almost no active power can be delivered. There can also be other undesirable effects like low voltages and increased equipment heating and system losses.While reactive power does not provide useful work, it is essential for AC transmission and distribution systems, motors, and many other types of customer loads. For motor loads, sufficient var levels are needed to avoid voltage sags that inhibit the conversion and flow of watts to meet load demand. Therefore, actual power systems require both real and reactive power to function properly.Compensating for reactive loadsThe shift of motors and other reactive loads can be offset using compensation devices. Reactive compensation commonly comes from three types of devices:1)Capacitorsare the largest source of compensating reactive power and are commonly used throughout the power system.2)Synchronous condensersare a type of rotating machinelike a generatorbut they do not produce real power, only reactive power. There are also other devices that use high-power electronics to rapidly control reactive power from large banks of capacitors.3)Conventional generators, in addition to supplying real power, are an important source of reactive power.Reactive power loads must be supplied either