Shunt Active Filter Project Report

ABSTRACT A Shunt Active Power Filter(APF) is a device that is connected in parallel to group of loads.APF cancels the reactive and harmonic currents drawn by the load so as to make supply current sinusoidal. Active Power Filter play a vital role in present day liberalized energy market. Active Power Filter are explored for executing different power conditioning function simultaneously along with harmonic elimination due to increase in nonlinear and unbalanced load, at the point of common coupling. The aim of present dissertation is to study different control strategies for Active Power Filter. More importantly to study instantaneous power theory based Shunt Active Power Filter which is predominantly used in present scenario. The shunt active power filter is investigated through Matlab/Simulink simulation under different load conditions. Simulation results are discussed in depth. Then the design issues of Active Power Filter for different load conditions are also discussed.

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CHAPTER 6 SIMULATION DESIGN The p-q theory based shunt APF is implemented for Harmonic compensation and power factor correction. Logic utilized for shunt APF is discussed in chapter 5 and is summarized in fig.5.2 6.1 Specification of the design: Simulation is performed on 2 types of Three phase Balanced Non –Linear Load as fallows: System Parameters Source Voltage

220

System Frequency

60 Hz

(line- line)

APF Dc-link voltage Dc side capacitance

800V C

1100

Ac side inductance

3.75mH

Ac side resistance

0.01 Ω

(Rating of APF is generally decided by peak voltage and RMS Current)

Load 1 Thyristor Rectifier (of rating 4 KVA)supplying to DC motor equivalent of 2.5KW AC side inductance

1mH

AC side resistance

0.01 Ω 2

DC side Resistance

18 Ω

DC side Inductance

85mH

Load2 Diode rectifier (of rating around 3KVA) supplying to purely resistive load AC side inductance

NA

AC side resistance

NA

DC side Resistance

18 Ω

DC side Inductance

NA

(NOTE: Rating of APF is generally decided by peak voltage and RMS Current[14] APF rating for Load1 is

will result in rating of .Thus in practical cases can be assumed to be around 1-1.5KVA}.

3

Vc

4

[Vsa]

Vsb

Vsc

Isa

Isb

Isc

[Vsb]

[Vsc]

[isa]

[isb]

[isc]

i -

i -

[isc]

[isb]

[isa]

p

Vbeta

Valpha

Low Pass FIlter

In2

[Vdc]

Out1

butter

PI controller

In1

800

Current measurement

Voltage measurement

+

i + -

Clarke Transformation

Vsa

[Vsc]

[Vsb]

+ -

v

[Vsa]

220 V rms L-L 3-phase Source + v

Vb

+ v

+

+ i -

Va

+ i -

pow ergui

Isalpha

Ia*

current measurement1

g22

g21

g12

g11

ploss

pdc

Vbeta

Valpha

Isbeta

Ic*

[isc]

[isb]

[isa]

Inverse Transformation

Isbeta

Ib*

C2

C1

B2

B1

A2

A1

Hysterisis Band Current Controller

Isc

Isb

Isa

Isc*

Isb*

Isa*

c

b

a

Vdc

Compensator

g32

Isalf a

Load Current measurement

non-linear load

C

B

A

g31

Rc

Lc

i -

i -

+

+

i -

+

+ i -

Continuous

Capacitor voltage

Goto7 [Vdc]

Fig 6.1 p-q theory based control block diagram of three-phase shunt APF system.

6.2 Clark Transformation: is done in accordance with section 4.2.2

1

a

Vsa 2

b

Vsb 3

c

alpha Vbeta

beta

Ibeta

Vsc

Subsystem1

3

p Valpha

p

Ialpha

Subsystem5 4

a

Isa 5

b

alpha

Isb 6

c

Isc

1 Valpha

beta

Subsystem2 2 Vbeta

Fig 6.2 Block Diagram for Clark Transformation and p calculation

1

1

a

2

-K-

b

-K-

K=-1/2 3 c

Sum of Elements

K=sqrt(2/3)

1 alpha

-KK=-1/2

1 -KSum of K=sqrt(2/3) Elements1

-1

Fig 6.3 Clark transformation block diagram for both 5

2 beta

6.3 Calculation of According p-q theory real and imaginary power can be separated into two parts: Real power: Imaginary power:

(from eq)

and

are average power due to component

and

are oscillating power due to components

And

respectively respectively.

will produces a purely sinusoidal waveform. But in order to

achieve unity power factor APF must compensate for

from component

. Thus,

will produce purely sinusoidal waveform with unity power factor. Thus, inverse transformation phase.

can deduced from

will produce reference current

which is filtered out using low pass filter from p.

1 1

Vbeta 2 Ibeta

p Product

3 Valpha 4

Product1

Ialpha

Fig 6.4 Block diagram for calculation of p

6

for each

Fig 6.5

from using Low Pass filter

6.4 DC-Bus Voltage Control Under a loss free situation, the shunt APF need not provide any active power to cancel the reactive and harmonic currents from the load. These currents show up as reactive power. Thus, it is indeed possible to make the DC-bus capacitor delivers the reactive power demanded by the proposed shunt APF. As the reactive power comes from the DC-bus capacitor and this reactive energy transfers between the load and the DC-bus capacitor (charging anddischarging of the DC-bus capacitor), the average DC-bus voltage can be maintained at a prescribed value. However, due to switching loss, capacitor leakage current, etc., the distribution source must provide not only the active power required by the load but also the additional power required by the VSI to maintain the DC-bus voltage constant. Unless these losses are regulated, the DC-bus voltage will drop steadily. A PI controller used to control the DC-bus voltage is shown in Figure6.6. Its transfer function can be represented as

where is the proportional constant that determines the dynamic response of the DC-bus voltage control, and is the integration constant that determines its settling time. 1 PID

Vdc 2 constant

Subtract

PID Controller

1 ploss

Fig 6.6 PI controller for DC-bus voltage control (Note: ) It can be noted that if and are large, the DC-bus voltage regulation is dominant, and the steady-state DC-bus voltage error is low. On the hand, if and are small, the real 7

power unbalance give little effect to the transient performance. Therefore, the proper selection of and is essentially important to satisfy above mentioned two control performances.

6.5Reference Current Calculation: Reference Currents are calculated from inverse clark transformation.

3

1

pdc

Isalfa

Divide Product 4 1

ploss

Valpha Product1 2 Vbeta Product2

2 Product3

Divide1

Fig 6.7 Block diagram for calculation of

1 Isalpha

-KGain

1 Ia* -KGain1

2 Ib*

2 Isbeta

-K-

-K-

Gain2

Gain3

3 Subtract

Fig 6.8 Reference Current calculation 8

Ic*

Isbeta

6.6 Hysteresis Band Current Controller: It is introduced in chapter 3 section 3.5.2

4

1

Isa Subtract2

A1

Relay3

1

NOT

Isa* Logical Operator 5

2 A2

3

Isb Subtract1

2

Relay1

B1

Isb* NOT Logical Operator1 6

4 B2

5

Isc Subtract3

Relay2

C1

3 NOT

Isc*

Logical Operator2

6 C2

Fig 6.9 Hysteresis Band Current Controller Actual source currents (

) are compared with the reference currents

generated by the control algorithm in the hysteresis-band current controller. Three hysteresis-band current controllers generate the switching pattern of the VSI. The switching logic is formulated as follows If

<(

HB) higher switch is OFF and lower switch is ON for leg “A” (QA=1)

If

>(

+ HB) higher switch is ON and lower switch is OFF for leg “A” (QA=0).

The switching functions of QB and QC for legs „„B‟‟ and „„C‟‟ are determined similarly, using corresponding reference and measured currents and hysteresis bandwidth (HB). The hysteresis-band current control is the fastest control method with minimum hardware and software but variable switching frequency is its main drawback

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6.7 Compensator: Switching is done according to gating signals from Hysteresis Band Current Controller.

1

3

m

2

m

2

1

g

1

2 1

g11

g

m g

Capacitor Voltage is continuously measured and fed to PI controller as explained earlier.

C

g21

5

m

2

m

2

g

1

g12

1

2

g

2

c

m

3

b

1

2

a

g

1

g31

4

g22

6

g32

Fig 6.10 Compensator

10

+ v -

Voltage Measurement3

1 Vdc

6.8 Non-Linear Loads Case:1

Thyristor Converter Supplying to DC motor equivalent

DC motor equivalent circuit

Rlac Llac

g +

+

A

i -

Id B C

Thyristor Converter Synchronization Voltages

1

A

2

+ v -

alpha_deg

Vab

AB

+ v -

BC CA

Vbc

B

+ v -

3

C

Vca

pulses

0

Block

Synchronized 6-Pulse Generator

PI Curent Regulator 5 1 s

100 Id_Refence 90

Fig6.11 Block Diagram for Thyristor Converter controlled DC motor Using PI controller DC motor current value is maintained at 20 Amps. PI controller varies alpha of thyristor until motor current matches reference current. Pulse width is takes as

11

.

case2:Diode Rectifier supplying to pure resistive load

1

Diode

Diode2

Diode4

Diode1

Diode3

Diode5

A

2

B

3

C

Fig 6.12 Block diagram for Diode rectifier supplying to pure Resistive Load A pure resistive load is taken in order to APF performance. As in this load phase current varies in abrupt manner on the contrary to RL load where load phase current is smooth varying curve.

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CHAPTER 7 SIMULATION RESULTS 7.1 Case 1: Thyristor converter supplying to DC motor Equivalent(R-L Type Load)

FiFig 7.1 Source Voltages and Load Currents with APF(Case 1)

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Fig 7.2 Harmonic Analysis of Load Current with APF(Case 1)

Fig 7.3 Reference Current 14

(Case 1)

Fig 7.4 Source Current with APF(Case 1)

Fig 7.5 Compensating Current and Load Current(Case 1)

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Fig 7.6 Source Voltage and Source Current with APF(Case 1)

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Fig7.7 Harmonic Analysis of Source Current (Case 1)

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Fig7.8 DC Capacitor Voltage for three-phase APF(Case 1)

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7.2 Case 2: Diode Rectifier supplying to pure resistive

Fig 7.9 load Source Voltage & Load Current with APF

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Fig 7.10 Harmonic Analysis of Load Current

20

Fig 7.11 Source Current after Compensation(Case 2)

Fig 7.12 Compensating Current and Load Current(Case 2)

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Fig 7.13 Source Voltages and Source Current(Case 2)

22

Fig 7.14 Harmonic analysis of Source Current(Case 2)

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Fig 7.15 DC Capacitor voltage for three-phase APF(Case 2)

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7.3 Simulation Result Discussion: As the source current and voltage are in phase,also the source current is almost sinusoidal(very low THD) it can be said that source is providing only active power required by the circuit. In instantaneous power theory view, source current is providing only average real power component( ) while remaining components i.e real oscillating power( ), imaginary average power( ) and imaginary oscillating power( ), is being provided by Shunt APF.(see Discussion in section 6.3 ) From source currents and THD in case1 (RL load) and case 2 (purely resistive load )it can be said that the effectiveness of the active filter in compensating for harmonic components of the load current depends on the specific load current waveform involved. Two different waveforms may have the same rms harmonic content but the active filter may do a better job of compensating for one of the waveforms because of the waveshapes involved. Source current has very less THD in case of RL load compared to purely resistive one. Thus it can be inferred performance of shunt APF with RL load is much better than purely resistive load. In general, the current waveform of an ac regulator with resistive load is an example of the waveshape that poses the severest challenge for an active filter. The problem is the high di/dt that is required of the filter to compensate for the high di/dt at turn on of the regulator. The problem is most severe when the regulator is turned on with a firing angle close to 90 degrees because this is when the available driving voltage stored on the dc capacitor is at a minimum. The output di/dt capability can be raised either by increasing the dc voltage setting or by reducing the size of the interfacing inductance. The limiting factor for increasing the dc voltage is the voltage withstand capability of the IGBT devices. The limiting factors for reducing the interfacing inductance include the IGBT di/dt withstand capability, control requirements, the interface passive filter requirement, and overall system stability. If the interfacing inductance becomes too small, the dc voltage cannot be kept constant for normal operation. From harmonics analysis of Source Current it can be seen due to uneven switching of compensator large number of interharmonics are introduced. But,it should be noted that 25

those components have very less magnitude.(Maximum magnitude of interharmonic is 0.11 % in case 1) Using PI Controller DC capacitor is maintained at reference value. It was seen that Settling time improved drastically using PI controller. It is worth to also to note that p-q based APF can be used for complete harmonic elimination not selective harmonic elimination.

7.4 Future Scope As p-q theory can be implemented in three-phase with excellent results in terms of THD, transient response, reference current generation. The work on extending use of p-q theory in single phase APF is being done[13]. Switching required in APF is very high in order of 10 kHz. Resulting in appreciable amount of power. Thus, one can further work on to reduce switching frequency and to switching losses. One can also work on linear control technique to replace hysteresis band controller .So, that irregular switching in compensator can be removed. Study of Control system of APF is also a possibility n order to get lesser steady state error and improved settling time. Most importantly to study various APF techniques and comparing them in terms of dynamic response, performance under various type of load, total harmonic compensation is to be done.

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CHAPTER 8 CONCLUSION The validity in terms of eliminating p-q theory in terms of eliminating harmonics and power factor improvement is confirmed from low THD source current which is in phase with source voltage. But p-q theory utilizes large number of sensors and reference current calculation block. Large number of calculation in p-q theory demands higher processing power. Resulting in utility to be complex and expensive. The p-q theory base APF is predominantly utilized in three phase circuits thus can not be used at remote single phase customer. As a result, Harmonics are present in large part of system. From source currents of the both cases (i.e. RL Load and purely resistive load) it can be inferred that APF is most effective when the load current waveform does not have abrupt changes. The overall filtering effectiveness depends significantly on the types of loads being compensated. As a result, it is very effective for most voltage source inverter-type loads, even when the distortion is high. From comparing reference current and source waveforms it can be concluded that hysteresis band current controller done the compensation at the cost of high switching frequency. Which can result in high switching losses in practical high power APF applications. PI controller performance is also validated from the DC-bus capacitor voltage which shows decreased settling time. In theoretical view p-q theory has some shortcomings which need to be addressed. Like mathematical expression of instantaneous power does not fallow power conservation and real and imaginary power needed to be more accurately defined as zero sequence instantaneous power can not be defined by the theory. In practical approach also it can be noted that p-q theory is incapable of providing selective harmonic elimination and specific power factor compensation.

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References 1.H. Akagi, Y. Kanazawa and A. Nabae, "Generalized Theory of Instantaneous Reactive Power and Its Applications," Transactions of he lEE-Japan, Part B, vol. 103, no.7, 1983, pp. 483-490 2.Power Quality C.Sankaran 3.H. Akagi. “New trends in active filters for power conditioning”, IEEE Trans. on Industry Applications, vol. 32, pp. 1312-1322, (1996). 4. Das, J. C. Passive Filters – Potentialities and Limitations. IEEE Trans. On Industry Applications. 2004. 40(1): 232-241. 5. Power Electronics Handbook CRC PRESS 6. El-Habrouk, M., Darwish, M. K., and Mehta, P. Active Power Filters: A Review. Proc. IEE Electric Power Applications. 2000. 147(5): 403-413. 7. Characteristics of Three Phase Active Power Filter using Extension pq Theory. Proceedings of the IEEE International Symposium on Industrial Electronics (ISIE). July 7-11, 1997. Guimaraes, Portugal: IEEE. 1997. 302-307. 8.Chen, C. L., Chen, E. L., and Huang, C. L. An Active Filter for Unbalanced Three-Phase System using Synchronous Detection Method. Proceedings of the Power Electronics Specialist Conference (PESC). June 20-25, 1994. Taipei, Taiwan: IEEE. 1994. 1451-1455. 9.Chen, D. –H. and Xie, S. –J. Review of Control Strategies Applied to Active Power Filters. Proceedings of the IEEE International Conference on Electric Utility Deregulation, Restructuring and Power Technologies (DRPT). April 5-8, 2004. Hong Kong: IEEE. 2004. 666-670. 10.Textook of “Modern Power Electronics and AC Drives”, B.K.Bose 11.Instantaneous p-q Power Theory for Compensating Non-sinusoidal Systems E. H. Watanabe, Senior Member, IEEE, H. Akagi, Fellow, IEEE and M. Aredes, Member, IEEE

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12.Instanteneous Power Theory and applications to power conditioning, IEEE Press, H. Akagi, E. H. Watanabe, M. Aredes. 13. M. Tarafdar Haque “SINGLE-PHASE PQ THEORY”, IEEE Trans. 14 “Active filter design and specification for control of harmonics in industrial and commercial facilities”, Mark McGranaghan Electrotek Concepts, Inc.

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