Design and simulation of different harmonic mitigation techniques

PROJECT MENTOR: Mr. AKHILESH ARVIND NIMJE

SUBMITTED BY:

PRANAV SIDDHARTH 807072NISHITH MAYANK 807066

HARISH V 807037K VIKASH 907225

KUMAR PARTHJEET 807047

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CONTENTS:

OBJECTIVE Pg-3

INTRODUCTION Pg-3

STATEMENT OF PROBLEM Pg-3

LITERATURE REVIEW Pg-4

HARMONICS MITIGATION AND FILTER Pg-6

PASSIVE FILTER Pg-7

TUNED FILTER DESIGN Pg-8

HARMONIC MEASUREMENT Pg-10

WORK ALREADY DONE Pg-11

REFFERENCE Pg-11

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OBJECTIVETo study the harmonic filtering techniques used for mitigation on harmonics in modern power system.

INTRODUCTION The increased use of nonlinear electronic equipment has become a concern in most utility power systems. Nonlinear loads draw current discontinuously during the cycle of the input voltage waveform and produce low power factors when harmonics are taken into account. This increases line current and can limit the available capacity of branch circuits. In addition, harmonic currents can cause heating in utility and facility transformers. Modern personal computers and other information technology equipments utilize “switching regulators” or switch mode power supplies, to convert utility AC power to regulated DC power. These switching regulators and switch mode power supplies generate high third and fifth harmonic current. If the equipments are not properly designed or rated, equipment will often malfunction when harmonics are present in an electrical system and that equipment can be personal computer in business environment or an ultrasonic imaging machine in a hospital. To eliminate this harmful effect, in depth study of power system analysis is required. In this project, study of power quality and detailed analysis of harmonics is performed. This project will look at causes and effects of harmonics in power systems. In depth analysis is performed and mathematical model and software simulation for passive and active harmonic filter is developed.

Normally, power systems are designed to operate at frequencies of 50 or 60Hz. Although certain types of loads produce current and voltage signal with frequencies that are integer multiples of the 50 or 60 Hz fundamental frequency. These higher frequencies are called electrical pollution that is known as power system harmonics. Harmonics analysis involved the calculation of system parameters. In this project how the capacitor bank parameters contribute to develop the harmonics and recalculating the value of the capacitor bank can help resolve the system harmonics. Hence, harmonics analysis of a power system forms an important aspect of a reliable system design.

Statement of Problem

This project involves in designing the harmonic filter to eliminate the harmonic from the system. In the first phase passive filter and in the next phase active filter is designed and simulated via many simulation software available.

In passive filter design a single tuned shunt notch filter is to be designed. For active filter ADAPTIVE HYSTERESIS CURRENT CONTROLLER form will be used..

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Passive filter designThe harmonic analysis of power system involves the calculation of power factor, frequency responses, capacitor bank size, filter reactor size, evaluating filter duty requirements, fundamental duty requirements, harmonic duty requirements, harmonic currents and voltage parameters. Calculation of the peak voltage, RMS voltage, RMS currents and kVAR values then compared with the standard limitations.

LITERATURE REVIEW

Introduction In linear circuits current is directly proportional to the voltage. However, in nonlinear circuits current is not proportional to the applied voltage. Figure 1 shows this concept by applying voltage to a nonlinear resistor where the voltage and current vary as shown in the curve. As we can see the voltage is perfectly sinusoidal but the resultant current waveform is distorted. Now as we increase the voltage by just few percentages may cause the current to double the value and takes the different shapes. This is the source of most harmonics in a power system. The distorted waveform can be a sum of sinusoidal signals. When the waveform is identical, it can be shown as a sum of pure sine waves where the frequency of each sinusoid is an integer multiple of the fundamental frequency of the distorted wave. This multiple is called a harmonic of fundamental. The sum of the sinusoidal is called the Fourier series. Figure 2 shows Fourier series of a distorted waveform. Here the fundamental frequency is the frequency of the power system. That is 50 Hz and the multiples that are 100Hz, 150Hz, 200Hz, 250Hz called second, third, fourth and fifth harmonics respectively. The combine waveform shows the result of adding the harmonics on to the fundamental.

Figure 1.

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Figure 2.

Harmonics Generation

There are different types of loads that generate harmonics in power systems. The linear time-invariant loads are designed such a way so that the sinusoidal voltage results in a sinusoidal flow of current. These loads have constant steady-state impedances during the applied sinusoidal voltage. When the voltage is increased, the current increases in direct proportion. The transformers and rotation machines are the examples of this kind of loads when operated in normal condition. In nonlinear load, the applied sinusoidal voltage does not result in a sinusoidal flow of current. These loads are not constant impedances during the entire cycle of the applied sinusoidal voltage. For example, wind and solar power generation.In utility distribution feeders and industrial plant power systems, the main tendency is for the harmonic currents to flow from the harmonic producing load to the power system source. This is shown in Figure 3. The impedance of the power system is normally the lowest impedance seen by the harmonic currents. That means the bulk of the current flows in to the source. The source of harmonics can be located by using this general tendency of the harmonic current flow. The power quality meters can be used to measure the harmonic currents in each branch starting at the beginning of the circuit and trace the harmonics to the source.

Figure 3

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Effect of Harmonics

Harmonics practically effect to every equipment in the power system. The effect of voltage distortion is divided in three major categories, the thermal stress, the dielectric stress and load disruptions. Heating effects: Harmonic current flowing in the circuits cause heating effects in the conductors. Especially eddy current losses are proportional to the square of the frequency. Some harmonics, notably the 5th, are negative sequence or backward rotating and it can increase losses by inducing even higher frequency currents in machine rotors. Interference: Harmonics can cause interference to communications systems, protections systems and signaling circuits due to electromagnetic induction or to the flow of the ground currents. Resonance: Harmonics generated in one part of circuit may increase the resonance effects in another part of the circuit. Some resonance can be dangerous if the magnification is large because of high circuit Q-factor or low damping. Even harmonics: Even harmonics may cause asymmetrical magnification and can lead to saturation. Some more adverse effects of harmonics listed as follows:

Malfunction in electronics devices and computer equipments

Errors in measurements

Lamp flicker when harmonic pulses involved

Blowing out of small auxiliary devices like fluorescent lamp capacitors.

Harmonics Mitigation and Filters The harmonics is becoming a bigger concern now a day with the increase nonlinear load in the power system. There are multiple ways to control the harmonics as follow: - Find the nonlinear load and reduce the harmonic current - Add filter to remove the harmonic current or block the harmonic current from entering to the system - Modify the system frequency response to avoid harmful interaction with harmonic current.

There are two approaches to mitigate harmonic problems in order to improve the power quality problems. The first approach is called load conditioning. It means that to ensurethe equipment must be less sensitive to power disturbances, allowing the operation under significant voltage or current distortion. Secondly, is to install line-conditioning systemsthat suppress the power system disturbances. The second approaches are very interesting where the passive and active power filters are connected to line system either in series orshunt configurations.

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Passive filters have been most commonly used to limit the flow of harmonic currents in distribution systems. They are usually custom designed for the application. However, their performance is limited to a few harmonics and they can introduce resonance in the power system. Also, a separate filter is necessary for each harmonic frequency.

Among the different new filters to improve harmonic problem is active power filter. The idea of using active power filter is to compensate for current and voltage disturbances in power distribution system but their practical development was made possible with the good control strategy in reducing total harmonic distortion as well as with cost reduction. It also not introduces resonance that can move a harmonic problem from one frequency to another. Through power electronics, the active filter produces current or voltage components, which cancel the harmonic components of the nonlinear loads supply lines, respectively.

PASSIVE FILTERS

Passive filters are inductance, capacitance, and resistance elements configured and tuned to control harmonics. Passive filtering techniques that make use of

Single or double-tuned filters providing low impedance path to harmonic currents at certain frequencies or

high or band-pass filters (damped filters) that can filter harmonics over a certain frequency bandwidth.

Passive filters are relatively inexpensive compared with other methods for eliminating harmonic distortion. However, they have the disadvantage of potentially interacting adversely with the power system, and it is important to check all possible system interactions when they are designed.

Passive filters work efficiently when they are located closer to harmonic generators

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( nonlinear loads). The resonant frequency must be safely away from any significant harmonic or other frequency component that may be produced by the load. Filters are commonly tuned slightly lower than the harmonic frequency for safety.

Passive filter design must take into account expected growth in harmonic current sources or load reconfiguration because it can otherwise be exposed to overloading, which can rapidly develop into extreme overheating and thermal breakdown. Passive filters always provide reactive compensation to a degree dictated by the voltampere size and voltage of the capacitor bank used, they can in fact be designed for the double purpose of providing the filtering action and compensating power factor to the desired level. If more than one filter is used — for example, sets of 5th and 7th or 11th and 13th branches — it will be important to remember that all of them will provide a certain amount of reactive compensation.

Passive filter is a series combination of an inductance and a capacitance. In reality, in the absence of a physically designed resistor, there will always be a series resistance, which is the intrinsic resistance of the series reactor sometimes used as a means to avoid filter overheating. All harmonic currents whose frequency coincides with that of the tuned filter will find a low impedance path through the filter.

Tuned filter design

Tuning a capacitor to a certain harmonic requires an additional reactor at the tuned harmonic.

The capacitor should withstand the total voltage across its terminals.

The reactive power absorbed by the reactor is

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The reactive power delivered by the capacitor is

A series tuned filter is designed to trap a certain harmonic by adding a reactor to an existing capacitor. Design steps of the following single-tuned series filter is as follows.

Determine the value of the capacitance, QC to improve the power factor and to eliminate any penalty by the electric power company. Power factor compensation is generally applied to raise power factor to around 0.98 or higher.

Evaluate the capacitor reactance at fundamental frequency

Calculate the reactor size providing the resonance,

Calculate the reactor resistance for a specified quality factor, Q,

The characteristic reactance

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Filter impedance

The following points summarize the relevant quality factor aspects in single-tuned filters: Typically, the resistance of a single-tuned harmonic filter is the intrinsic resistance

of the reactor. However, R can be favorably used to vary the quality factor of the filter and

provide a way to control the amount of desired harmonic current through it. A large Q value implies a prominent valley at the resonant (tuning) frequency of a

filter and therefore the trapping of the largest amount of harmonic frequency. The best reduction of harmonic distortion will be achieved with large Q value

filters. However, care should be exercised in assessing harmonic currents of frequencies other than the one for which the filter is tuned because they will also find a reduced impedance path

Lower quality factor filters could be used in situations in which harmonic distortion barely exceeds the limits and a small filtering action is all that is needed to bring it into compliance.

Harmonic measurementTotal harmonic distortionThe total harmonic distortion, or THD, of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. Lesser THD allows the components in a equipment to produce a more accurate reproduction.THD is defined as the root mean square (r.m.s.) of the harmonics expressed asa percentage of the fundamental component, i.e.

Where, Vn is the single frequency r.m.s. voltage at harmonic n, N is the maximum harmonic order to be considered and V1 is the fundamental line to neutral r.m.s. voltage.

Current distortion levels can also be characterized by a THD value but it can be misleading when the fundamental load current is low. A high THD value for inputCurrent may not be of significant concern if the load is light, since the magnitude ofthe harmonic current is low, even though its relative distortion to the fundamental

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frequency is high. To avoid such ambiguity a total demand distortion (TDD) factor is

used instead, defined as:

WORK ALREADY DONE

Till now we have designed a single tuned notch shunt passive filter for 5th , 7th and 11th

harmonics. We are presently trying to simulate the filter on a single phase bus with the help of simpower toolbox of MATLAB software. The simulation involvescalculation of total harmonic distortion of current and voltage with and without filter and FFT analysis of frequency response.

M ATLAB Code :%single tuned filter design calculation clc;clear all;close all;voltage= input('Enter the line volatage in kv');load= input('enter the load in KVA');powerf= input('enter the power factor');nwpowerf= input('enter the desired power factor');h=input('enter the harmonic present');reactive_demand=load.*sin(acos(powerf)); %present reactive power demandnew_reactive_demand=load.*sin(acos(nwpowerf)); %final reactive power reqcomp=(reactive_demand - new_reactive_demand); %the compensation from filterxfilter=(voltage.^2).*1000/reqcomp; %filter impedanceif h==5 %reduced harmonics for design tolerance h=4.7elseif h==7 h=6.6elseif h==11 h=10.5elseif h==13 h=12.4 endxc=xfilter*(h^2)/(h^2-1); %xc is the capitance reactance(ohms)xl=xc/(h^2); %inductive reactance(ohms)

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c=1/(100*3.14*xc); %capacitance faradl=xl/(100*3.14); %inductance henryxfund= abs(xl-xc); %impedance at fundamental frequency 50 hz

Active filterTo reduce the harmonics conventionally passive L–C filters were used and also capacitors were employed to improve the power factor of the ac loads. But the passive filters have several drawbacks like fixed compensation, large size and resonance problem. To mitigate the harmonics problem active power (APF) filters or active power line conditioners.

Figure 4 INTRODUCTION:To cancel the harmonics and compensate the reactive power APF is a viable solution. The concept is to use an inverter to inject currents or voltages harmonic components to cancel the load harmonic components. The more usual configuration is a shunt APF to inject current harmonics into the point of common coupling (PCC). The APF can be installed in a low voltage power system to compensate one or more loads; thus, it avoids the propagation of current harmonics in the system. As APF compensate the reactive power and cancel the harmonics, it is also called as active power line conditioners (APLC). The three main aspects of an active power conditioner are:

The configuration of power converter (the topology of the filter) The control strategy (the calculation of APLC control reference signals) The control method used (how the power inverter follows the control reference)

The main component in the APF is the control unit. The control unit is mainly divided into two parts as follows.

Harmonic extraction technique Current modulator

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Harmonic Extraction:Harmonic extraction is the process in which, reference current is generated by using the distorted waveform. Here we have used sine multiplication theorem for active part of current calculation.

Current Modulator (Gate control signal):Current modulator is mainly used to provide the gate pulse to the active power filter (Inverter). There are many techniques used for giving the gating signals to PWM VSI such as sinusoidal PWM, triangular PWM, hysteresis current controller etc. here we have used adaptive hysteresis controller for the gate control signal.

CONFIGURATION OF ACTIVE POWER FILTERS:APF’s can be classified based on converter type, topology, and the number of phases.The converter type is mainly two types.

Voltage source inverter (VSI) Current source inverter (CSI)

The topology of active power filter is classified in to three types. Series active power filters Shunt active power filters Hybrid active power filters(both active and passive filter)

Finally based on the phases the APF is of mainly two types. Two-wire (single phase) system. Three or four-wire three-phase system.

APF SYSTEM STUDIED: Single phase source A nonlinear load A voltage source PWM inverter. Controller(reference current generator and gating signal generator)

Control strategy

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Figure 5Figure 5 shows the basic block diagram of the control strategy being used here. The integrator block is used to generate the fundamental active component of current Is

* which is then subtracted with the load current to generate the reference current for the inverter. The next part involves a adaptive hysteresis controller which compares the output of the inverter with the reference and orders the switching of inverters to minimize the error between them.

REFERENCE CURRENT GENERATIONThe equations below describe the method of generation of reference current for the inverter. It explains the working of the integrator block for generation of the reference current.Let us supposeIl = load currentVs= source voltage ω= fundamental frequency (50hz) Load current can be written as :

I L=∑n=1

∝

I nsin (nωt+θn)

Multiplying both sides with sin (ωt) and integrating over a period of 2π we get;

∫0

2 π

I L sin ωt dωt = ∫0

2 π

I1 sin (ωt+θ1 ) sinωt d (ωt ) + ∫0

2 π

∑n=2

∝

sin (nωt+θn) sin nωt d (ωt )

∫0

2 π

I L sin ωt d (ωt )=I 1∫0

2 π

sin2 ωt cosθ1+cos ωt sin ωt sin θ1 dωt

= I 1¿

= I 1cosθ1∫0

2 π1−cos2 ωt

2dωt

= I 1

2cos θ1[2 π ]

= I 1cosθ1 π

Therefore gain of the integrator needed = 1π

The term when multiplied with sin (ωt) gives the active component of the load current at fundamental frequency.I s

¿ = I 1cosθ1 sinωt=Active current I F

¿ = I s¿−I L gives the reference current to be generated by the inverter.

= −I 1sin θ1 cosωt –∑n=2

∝

I n sin(nωt+θn)

This reference current contains the reactive current and the harmonics present, thus the active power filter can improve the power factor by sending reactive power and thus the source current becomes in phase to the voltage phasor giving unity power factor.

Hysteresis current control loop

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The hysteresis band current controller for active power filter generates the switching pattern of the inverter. There are various current control methods proposed for such active power filter configurations, but in terms of quick current controllability and easy implementation hysteresis current control method has the highest rate among other current control methods. Hysteresis band current controller has properties like robustness, excellent dynamics and fastest control with minimum hardware. In the case of positive input current, if the error current e(t) between the desired reference current iref(t) and the actual source current iactual(t) exceeds the upper hysteresis band limit (+h), the upper switch of the inverter arm is become OFF and the lower switch is become ON.

Figure 6ADAPTIVE HYSTERESIS CURRENT CONTROLLERIn spite of several advantages, the basic hysteresis technique exhibits several undesirable features, such as uneven switching frequency that causes acoustic noise and difficulty in designing input filters. The hysteresis band current controller is composed of a hysteresis around the reference line current. In equation the reference line current of APF is referred to as iref, and measured line current of the APF is referred to as ‘i’. The difference between i and iref is referred to as δThe switching logic for a half bridge VSI inverter is formulated as follows:If δ >HB upper switch is ON If δ <-HB upper switch is OFF

Figure 7Figure 7 shows the adaptive hysteresis band HB and –HB around the error and the switching pattern generated using the logic devised above.

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Figure 8The above figure 8 shows the basic connection of the half bridge inverter to the line at point of common coupling using an inductor with suitably chosen value so as to have a linear rise in current.Using KVL in the circuit given in figure we can write

V f ( t )−Ld if

dt=V s(t)

d if

dt=1

L(V f (t )−V s (t ) )

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Figure 9Using the geometry of the above figure 9 we can write the following equations where I F

+¿∧ IF−¿arerising∧falling current due¿ the inductor ¿

diF+¿

dt=

1L

¿¿ - V s ( t )¿ - (1)

dif−¿

dt¿ = 1

L (−V dc

2−V s)

dif−¿

dt=−1

L (V dc

2+V s)¿ - (2)

From the figure ACDC

=tanα=di f+¿

dt¿

AC = dif+¿

dt∗t1 ¿

BCDC

=tan β=d if

¿

dt

BC = d if

¿

dt∗t1

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AC – BC

= AB = 2 HB = dif+¿

dt∗t1 ¿ -

d if¿

dt∗t1 - (3)

Similarly

- 2HB = dif−¿

dt∗t 2¿ -

d if¿

dt∗t 2 - (4)

Let f = 1

t1+t 2

=switching frequency

Equation (3) + (4) = ¿ - (5)

¿ - d if

¿

dt( t1+t 2) = 0

Equation (3) – (4) = 4HB = ¿ - d if

¿

dt( t2+t 1) = 0 - (6)

From (5)

dif+¿

dtt1+d

if−¿

dtt 2¿¿ =

d if¿

dtUsing (1) and (2)

1L [(V dc

2−V s)t 1−(V dc

2+V s)t 2] =

d if¿

dt1f

1L [ V dc

2( t1−t 2 )−V s ( t1+t 2 )] =

d if¿

dt1f

¿Similarly in equation (6)

4HB = 1L (V dc

2−V s)t 1+

1L (V dc

2+V s)t 2+

d if¿

dt(t 2−t 1¿

4HB = 1L [ V dc

2 f+V s( −2

V dc f (Ld if

¿

dt+V s))] +

d if¿

dt [−2

V dc(1f

d if¿

dt+V s)]

4HB = 1L [ V dc

2 f+

4V s LL V dc f

d i f¿

dt−

2 V s2 L

L2 V dc f−( d if

¿

dt )2

2 LV dc f ]

This equation gives the instantaneous values of hysteresis band determined by the values of

Vs ,Vdc , ( d if¿

dt ) and the switching frequency of the inverter chosen.

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HB = V dc

8 Lf−[( d if

¿

dt )+ V s

L ]2

L

2V dc f

Figure 10 Simulink model of the adaptive hysteresis model design.

Logical circuit for S1 and S2 firing

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Figure 11The above logical circuit shows the implementation of switching circuit.

WORKING:The inputs taken are reference current If

* , filter current If and the adaptive hysteresis band generated instantaneously using the derived formula. Their difference gives the error current which is to be checked with the upper and lower hysteresis band. So the error signal is passed through a differentiator to check the slope of error. If going positive the error current is compared with upper hysteresis and while if going negative the error current is compared with the lower hysteresis band. The output of both the comparator is fed to a SR flip flop. If the value of error is greater than the upper hysteresis band then S=1 R=0 thus Q=1 and hence upper switch of half bridge inverter S2 is OFF and Lower switch of half bridge inverter S1 is ON and vice versa.

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REFERENCE

1. Power System Harmonics: fundamentals, analysis, and filter design , George J.Wakileh, Springer VerlagPress, 2001

2. Power System Harmonics, J.Arrilaga, D.A.Bradley, P.S.Jodger, John Willey and Sons, 1985.

3. Power Quality in Electrical Systems, Alexander Kusko, Marc T.Thompson, McGraw Hill P.C., 2007

4. Harmonics and Power Systems, Francisco C. De La Rosa, CRC Press, 20065. Performance for Passive and Active Power Filter in Reducing Harmonics in the

Distribution System

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