30917947 Reactive Power Compensation Using Capacitor Banks

8/6/2019 30917947 Reactive Power Compensation Using Capacitor Banks

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INTRODUCTION

In this chapter we are going to discuss about power system in short and about A.P

TRANSCO and its role in maintaining power in state from buying and selling the power.

1.1 INTRODUCTION TO POWER SYSTEM

Electrical power is a little bit like the air one breathes. One doesn't really think

about it until it is missing. Power is just "there," meeting ones daily needs, constantly. It

is only during a power failure, when one walks into a dark room and instinctively hits the

useless light switch, that one realizes how important power is in our daily life. Without it,

life can get somewhat cumbersome.

Electric Energy is the most popular form of energy, because it can be transported

easily at high efficiency and reasonable cost. The power system of today is a complex

interconnected network as shown in fig. 1.

1Figure 1 Power System interconnected


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A Power System can be subdivided into four major parts:

i. Generation.

ii. Transmission and Sub transmission.

iii. Distribution.

iv. Loads.

Power is generated at generating stations, usually located away from the actual

users. The generated voltage is then stepped up to a higher voltage for transmission,

as transmission losses are lower at higher voltages. The transmitted electric power is then

stepped down at grid stations.

The modern distribution system begins as the primary circuit, leaves the sub-

station and ends as the secondary service enters the customer's meter socket. First, the

energy leaves the sub-station in a primary circuit, usually with all three phases.

The most common type of primary is known as a wye configuration.The wye

configuration includes 3 phases and a neutral (represented by the center of the "Y".) The

neutral is grounded both at the substation and at every power pole. The primary and

secondary (low voltage) neutrals are bonded (connected) together to provide a path to

blow the primary fuse if any fault occurs that allows primary voltage to enter the

secondary lines. An example of this type of fault would be a primary phase falling across

the secondary lines. Another example would be some type of fault in the transformer

itself.

The other type of primary configuration is known as delta. This method is older

and less common. In delta there is only a single voltage, between two phases (phase to

phase), while in wye there are two voltages, between two phases and between a phase

and neutral (phase to neutral). Wye primary is safer because if one phase becomes2


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grounded, that is, makes connection to the ground through a person, tree, or other object,

it should trip out the fused cutout similar to a household circuit breaker tripping. In delta,

if a phase makes connection to ground it will continue to function normally. It takes two

or three phases to make connection to ground before the fused cutouts will open the

circuit. The voltage for this configuration is usually 4800 volts.

Transformers are sometimes used to step down from 7200 or 7600 volts to 4800

volts or to step up from 4800 volts to 7200 or 7600 volts. When the voltage is stepped up,

a neutral is created by bonding one leg of the 7200/7600 side to ground. This is

commonly used to power single phase underground services or whole housing

developments that are built in 4800 volt delta distribution areas. Step downs are used in

areas that have been upgraded to a 7200/12500Y or 7600/13200Y and the power

company chooses to leave a section as a 4800 volt setup. Sometimes power companies

choose to leave sections of a distribution grid as 4800 volts because this setup is less

likely to trip fuses or reclosers in heavily wooded areas where trees come into contact

with lines.

For power to be useful in a home or business, it comes off the transmission grid

and is stepped-down to the distribution grid. This may happen in several phases. The

place where the conversion from "transmission" to "distribution" occurs is in a power

substation. A power substation typically does two or three things:

i. It has transformers that step transmission voltages down to distribution voltages

ii. It has a "bus" that can split the distribution power off in multiple directions.

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iii. It often has circuit breakers and switches so that the substation can be

disconnected from the transmission grid or separate distribution lines can be

disconnected from the substation when necessary.

It often has circuit breakers and switches so that the substation can be

disconnected from the transmission grid or separate distribution lines can be disconnected

from the substation when necessary. The primary distribution lines are usually in the

range of 4 to 34.5 KV and supply load in well defined geographical area. Some small

industrial customers are served directly by the primary feeders.

1.3 APTRANSCO

Government of Andhra Pradesh enacted the AP Electricity REFORMS ACT in

1998.As a sequel the APSEB was unbundled into Andhra Pradesh Power Generation

Corporation Limited (APGENCO) & Transmission Corporation of Andhra Pradesh

Limited (APTRANSCO) on 01.02.99. APTRANSCO was further unbundled w.e.f.

01.04.2000 into "Transmission Corporation" and four "Distribution Companies"

(DISCOMS).

a.)CURRENT ROLE

From Feb 1999 to June 2005 APTRANSCO remained as Single buyer in the state

-purchasing power from various Generators and selling it to DISCOMs in accordance

with the terms and conditions of the individual PPAs at Bulk Supply Tariff (BST) rates.

Subsequently, in accordance with the Third Transfer Scheme notified by Go AP,

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APTRANSCO has ceased to do power trading and has retained with powers of

controlling system operations of Power Transmission.

1.4 CONCLUSION

In this chapter we discussed about the power system and role of A.P TRANSCO

in the state of A.P.

In next chapter we are going to discuss about the salient features of

A.PTRANSCO.

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INTRODUCTION

In this chapter, we are going to discuss about the salient feature of A.P

TRANSCO/A.PGENCO/DISCOMS.

The object of reform and restructure of power sector in the state is to create

conditions for sustainable development of the sector through promoting competition,

efficiency, transparency and attracting the much needed private finances into power

sector. The ultimate goal of the reform program is to ensure that power will be supplied

under the most efficient conditions in terms of cost and quantity to support the economic

development of the state and power sector ceases to be a burden on the States budget and

eventually becomes a net generator of resources.

A key element of the reform process is that the government will withdraw from

its earlier role as a regulator of the industry and will be limiting its role to one of policy

formulation and providing directions.

In accordance with Reform Policy, the Government of A.P entacted the A.P

Electricity Reforms Act 1998 and made effective from 1.2.1999. Transmission

Corporation of A.P Ltd (APTRANSCO and APGENCO) were incorporated under

Companies Act, 1956. The assets, liabilities and personnel were allocated to these

companies. Distribution companies have been incorporated under Companies Act as

subsidiaries to distribution to APTRANSCO and the assets, liabilities and personnel have

been allocated to distribution companies through notification of a second transfer scheme

by the Govt. on 31.3.2000.

The Government of A.P established the A.P Electricity Regulatory Commission

(APERC) as per the provision of the act and the Commission started functioning from

3.4.1999. Regular licenses have been issued to APTRANSCO by APERC for

Transmission and Bulk supply and Distribution and Retail supply from 31.1.2000. The

commission has been issuing yearly Tariff orders since then based on Annual Revenue

Requirement (ARR) and tariff proposals of these companies.

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2.2 SALIENT FEATURES OF A.P TRANSCO/A.PGENCO/DISCOMS

Table 2.2 (a) features of A.P power system

PARAMETER UNITS 2008-09

(UPTO

MARCH

09)

31.03.09

(PROVL)

2009-10

(UPTO

MARCH

10)

31.03.10

(PROVL)

Energy generated (cumulative) MU - - -

1. Thermal MU - 23325.67 - 24180.38

2. Hydel MU - 7785 - 5510.46

3. Wind MU - - - -

Total MU - 31110.67 - 29690.84

Energy purchased and imported

(includingothers energy handled)

MU - 36511.56 - 45075.68

Energy available for use (2+3) MU - 67622.23 - 74766.52

Maximum demand during the year

(at generation terminal) MW

ME - 9997

(27-03-

2009)

- 10880

(21-03-2010)

PercpaitaConsumption (includes

captive generation)

KWH - 746 - -

APTRANSCO LINE (EHT) - - - - -

400kv CKM 21.44 3008.20 24 3032.79

220kv CKM 265.88 1250.25 19068 12693.18

132kv CKM 233.02 14938.57 164.88 15103.45

DISCOMS Lines # - - - - -

33kv Km 1421.78 38628 1230 39858

11kv Km 19521.82 248670 10596 259266

LT km 10166.53 527852 4212 532064

TOTAL - 26630.14 845599.15 6418.17 862017.32

Table 2.2 (b) load generation and sharing of A.P with other state

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2.3 CONCLUSION

In this chapter, we discussed about the salient features of

A.PTRANSCO / A.PGENCO / DISCOMS.

In next chapter we are going to discuss about the need for compensation and types

of compensations used.

3.1 INTRODUCTION

In this chapter, reactive power compensation, mainly in transmission systems

installed at substations is discussed. Reactive power compensation in power systems can

be either shunt or series.

Except in a very few special situations, electrical energy is generated, transmitted,

distributed, and utilized as alternating current (AC). However, alternating current has

several distinct disadvantages. One of these is the necessity of reactive power that needs

to be supplied along with active power. Reactive power can be leading or lagging. While

it is the active power that contributes to the energy consumed, or transmitted, reactive

power does not contribute to the energy. Reactive power is an inherent part of the total

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power. Reactive power is either generated or consumed in almost every component of

the system, generation, transmission, and distribution and eventually by the loads. The

impedance of a branch of a circuit in an AC system consists of two components,

resistance and reactance. Reactance can be either inductive or capacitive, which

contribute to reactive power in the circuit. Most of the loads are inductive, and must be

supplied with lagging reactive power. It is economical to supply this reactive power

closer to the load in the distribution system.

3.2 TYPES OF COMPENSATION

Shunt and series reactive compensation using capacitors has been 3 widely

recognized and powerful methods to combat the problems of voltage drops, power losses,

and voltage flicker in power distribution networks. The importance of compensation

schemes has gone up in recent years due to the increased awareness on energy

conservation and quality of supply on the part of the Power Utility as well as power

consumers. This amplifies on the advantages that accrue from using shunt and series

capacitor compensation. It also tries to answer the twin questions of how much to

compensate and where to locate the compensation capacitors.

i.) SHUNT CAPACITOR COMPENSATION

Since most loads are inductive and consume lagging reactive power, the

compensation required is usually supplied by leading reactive power. Shunt

compensation of reactive power can be employed either at load level, substation level, or

at transmission level. It can be capacitive (leading) or inductive (lagging) reactive power,

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although in most cases as explained before, compensation is capacitive. The most

common form of leading reactive power compensation is by connecting shunt capacitors

to the line.

Fig. 3.2(i) represents an A.C generator supplying a load through a line of series

impedance (R+jX) ohms, fig.3.2(ii) shows the phasor diagram when the line is delivering

a complex power of (P+jQ) VA and Fig. 3.2(iii) shows the phasor diagram when the line

is delivering a complex power of (P+jO) VA i.e. with the load fully compensated. A

thorough examination of these phasor diagrams will reveal the following facts which are

higher by a factor of2

Cos

1

compared to the minimum power loss attainable in the

system.

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Figure 3.2 (i) represents an A.C generator supplying a load through a line of

series.

Figure.3.2 (ii) shows the phasor diagram when the line is delivering a complex

power of (P+jQ)

Figure. 3.2 (iii) shows the phasor diagram when the line is delivering a complex

power of (P+jQ)

The loading on generator, transformers, line etc is decided by the current flow.

i. The higher current flow in the case of uncompensated load necessitated by the

reactive demand results in a tie up of capacity in this equipment by a factor of

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Cos

1i.e. compensating the load to UPF will release a capacity of (load VA

rating X Cos ) in all these equipment.

ii. The sending-end voltage to be maintained for a specified receiving-end voltage is

higher in the case of uncompensated load. The line has bad regulation with

uncompensated load.

iii. The sending-end power factor is less in the case of an uncompensated one. This is

due to the higher reactive absorption taking place in the line reactance.

iv. The excitation requirements on the generator are severe in the case of

uncompensated load. Under this condition, the generator is required to maintain a

higher terminal voltage with a greater current flowing in the armature at a lower

lagging power factor compared to the situation with the same load fully

compensated. It is entirely possible that the required excitation is much beyond

the maximum excitation current capacity of the machine and in that case further

voltage drop at receiving-end will take place due to the inability of the generator

to maintain the required sending-end voltage. It is also clear that the increased

excitation requirement results in considerable increase in losses in the excitation

system.

It is abundantly clear from the above that compensating a lagging load by using

shunt capacitors will result in

i. Lesser power loss everywhere upto the location of capacitor and hence a more

efficient system.

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ii. Releasing of tied-up capacity in all the system equipments thereby enabling a

postponement of the capital intensive capacity enhancement programs to a later

date.

iii. Increased life of equipments due to optimum loading on them.

iv. Lesser voltage drops in the system and better regulation.

v. Less strain on the excitation system of generators and lesser excitation losses.

vi. Increase in the ability of the generators to meet the system peak demand thanks to

the released capacity and lesser power losses.

Shunt capacitive compensation delivers maximum benefit when employed right

across the load. And employing compensation in HT & LT distribution network is the

closest one can get to the load in a power network. However, various considerations like

ease of operation end control, economy achievable by lumping shunt compensation at

EHV stations etc will tend to shift a portion of shunt compensation to EHV & HV

substations. Power utilities in most countries employ about 60% capacitors on feeders,

30% capacitors on the substation buses and the remaining 10% on the transmission

system. Application of capacitors on the LT side is not usually resorted to by the utilities.

Just as a lagging system power factor is detrimental to the system on various

counts, a leading system pf is also undesirable. It tends to result in over-voltages, higher

losses, lesser capacity utilization, and reduced stability margin in the generators. The

reduced stability margin makes a leading power factor operation of the system much

more undesirable than the lagging p.f operation. This fact has to be given due to

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consideration in designing shunt compensation in view of changing reactive load levels in

a power network.

Shunt compensation is successful in reducing voltage drop and power loss

problems in the network under steady load conditions. But the voltage dips produced by

DOL starting of large motors, motors driving sharply fluctuating or periodically varying

loads, arc furnaces, welding units etc can not be improved by shunt capacitors since it

would require a rapidly varying compensation level. The voltage dips, especially in the

case of a low short circuit capacity system can result in annoying lamp-flicker, dropping

out of motor contactors due to U/V pick up, stalling of loaded motors etc. and fixed or

switched shunt capacitors are powerless against these voltage dips. But thyristor

controlled Static VAR compensators with a fast response will be able to alleviate the

voltage dip problem effectively.

a.) SHUNT CAPACITORS

Shunt capacitors are employed at substation level for the following reasons:

i. Voltage regulation: The main reason that shunt capacitors are installed at

substations is to control the voltage within required levels. Load varies over the

day, with very low load from midnight to early morning and peak values

occurring in the evening between 4 PM and 7 PM. Shape of the load curve also

varies from weekday to weekend, with weekend load typically low. As the load

varies, voltage at the substation bus and at the load bus varies. Since the load

power factor is always lagging, a shunt connected capacitor bank at the substation

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can raise voltage when the load is high. The shunt capacitor banks can be

permanently connected to the bus (fixed capacitor bank) or can be switched as

needed. Switching can be based on time, if load variation is predictable, or can be

based on voltage, power factor, or line current.

ii. Reducing power losses: Compensating the load lagging power factor with the

bus connected shunt capacitor bank improves the power factor and reduces

current flow through the transmission lines, transformers, generators, etc. This

will reduce power losses (I2R losses) in this equipment.

iii. Increased utilization of equipment: Shunt compensation with capacitor banks

reduces KVA loading of lines, transformers, and generators, which means with

compensation they can be used for delivering more power without overloading

the equipment.

Reactive power compensation in a power system is of two typesshunt and series.

Shunt compensation can be installed near the load, in a distribution substation, along the

distribution feeder, or in a transmission substation. Each application has different

purposes. Shunt reactive compensation can be inductive or capacitive. At load level, at

the distribution substation, and along the distribution feeder, compensation is usually

capacitive. In a transmission substation, both inductive and capacitve reactive

compensation are installed.

b.) SHUNT CAPACITOR INSTALLATION TYPES:

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The capacitor installation types and types of control for switched capacitor are

best understood by considering a long feeder supplying a concentrated load at feeder end.

This is usually a valid approximation for some of the city feeders, which emanate from

substations, located 4 to 8 Kms away from the heart of the city.

Absolute minimum power loss in this case will result when the concentrated load

is compensated to up by locating capacitors across the load or nearby on the feeder. But

the optimum value of compensation can be arrived at only by considering a cost benefit

analysis.

Figure 3.2 (iv) long distribution feeder supplying a concentrated load

It is evident from fig. 3.2 (v) that it will require a continuously variable capacitor

to keep the compensation at economically optimum level throughout the day. However,

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this can only be approximated by switched capacitor banks. Usually one fixed capacitor

and two or three switched units will be employed to match the compensation to the

reactive demand of the load over a day. The value of fixed capacitor is decided by

minimum reactive demand as shown in Fig 3.2 (v)

Figure. 3.2 (v) reactive demand

Automatic control of switching is required for capacitors located at the load end

or on the feeder. Automatic switching is done usually by a time switch or voltage

controlled switch as shown in Fig 3.2(v). The time switch is used to switch on the

capacitor bank required to meet the day time reactive load and another capacitor bank

switched on by a low voltage signal during evening peak along with the other two banks

will maintain the required compensation during night peak hours.

ii) SERIES CAPACITOR COMPENSATION

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Shunt compensation essentially reduces the current flow everywhere upto the

point where capacitors are located and all other advantages follow from this fact. But

series compensation acts directly on the series reactance of the line. It reduces the transfer

reactance between supply point and the load and thereby reduces the voltage drop. Series

capacitor can be thought of as a voltage regulator, which adds a voltage proportional to

the load current and there by improves the load voltage.

Figure 3.2 (vi) Aerial view of 500-kV series capacitor installation

Series compensation is employed in EHV lines to

i. Improve the power transfer capability

ii. Improve voltage regulation

iii. Improve the load sharing between parallel lines.

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Economic factors along with the possible occurrence of sub-synchronous

resonance in the system will decide the extent of compensation employed.

Series capacitors, with their inherent ability to add a voltage proportional to load

current, will be the ideal solution for handling the voltage dip problem brought about by

motor starting, arc furnaces, welders etc. And, usually the application of series

compensation in distribution system is limited to this due to the complex protection

required for the capacitors and the consequent high cost. Also, some problems like self-

excitation of motors during starting, ferro resonance, steady hunting of synchronous

motors etc discourages wide spread use of series compensation in distribution systems.

3.3 ECONOMIC JUSTIFICATION FOR USE OF CAPACITORS:

Increase in benefits for 1KVAR of additional compensation decrease rapidly as

the system power factor reaches close to unity. This fact prompts an economic analysis to

arrive at the optimum compensation level. Different economic criteria can be used for

this purpose. The annual financial benefit obtained by using capacitors can be compared

against the annual equivalent of the total cost involved in the capacitor installation. The

decision also can be based on the number of years it will take to recover the cost involved

in the Capacitor installation. A more sophisticated method would be able to calculate the

present value of future benefits and compare it against the present cost of capacitor

installation.

When reactive power is provided only by generators, each system component

(generators, transformers, transmission and distribution lines, switch gear and protective

equipment etc) has to be increased in size accordingly. Capacitors reduce losses and

loading in all these equipments, thereby effecting savings through powerless reduction

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and increase in generator, line and substation capacity for additional load. Depending on

the initial power factor, capacitor installations can release at least 30% additional

capacity in generators, lines and transformers. Also they can increase the distribution

feeder load capability by about 30% in the case of feeders which were limited by voltage

drop considerations earlier. Improvement in system voltage profile will usually result in

increased power consumption thereby enhancing the revenue from energy sales.

Thus, the following benefits are to be considered in an economic analysis of

compensation requirements.

a) Benefits due to released generation capacity.

b) Benefits due to released transmission capacity.

c) Benefits due to released distribution substation capacity.

d) Benefits due to reduced energy loss.

e) Benefits due to reduced voltage drop.

f) Benefits due to released feeder capacity.

g) Financial Benefits due to voltage improvement.

Capacitors in distribution system will indeed release generation and transmission

capacities. But when individual distribution feeder compensation is in question, the value

of released capacities in generation and transmission system is likely to be too small to

warrant inclusion in economic analysis. Moreover, due to the tightly inter-connected

nature of the system, the exact benefit due to capacity release in these areas is quite

difficult to compute. Capacity release in generation and transmission system is probably

more relevant in compensation studies at transmission and sub-transmission levels and

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hence are left out from the economic analysis of capacitor application in distribution

systems.

a.) BENEFITS DUE TO RELEASED DISTRIBUTION SUBSTATIONCAPACITY:

The released distribution substation capacity due to installation of capacitors

which deliver Qc MVARs of compensation at peak load conditions may be shown to be

equal to

c2c

c

2/1

2c

22cc S1

S

SinQ

S

CosQ1S

+

=

In general and SinQS cc when10

SQ CC

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30917947 Reactive Power Compensation Using Capacitor Banks

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