Abstract- with the arrival of

semiconductor and power electronic devices and their easier controllability has

caused wide use of nonlinear loads. But the use of power electronic devices is

responsible for harmonic and reactive power disturbances. These harmonics creates

the disturbance in normal operation, excessive heating in the equipments etc.

so it is necessary to eliminate these harmonics problems. So importance

is being given to the development of Active Power Filters to solve these

problems to improve power quality among which shunt active power filter is used

to eliminate voltage and load current harmonics and for reactive power

compensation. The shunt active power filters have been developed based on

Synchronous Reference Frame Algorithm Method. Synchronous Reference Frame (SRF)

Algorithm is used to extract the harmonics components. Hysteresis band current

control (HBCC) technique is used for the generation of firing pulses to the

inverter. This system is simulated using MATLAB and results are observed.

Keywords

– Harmonics, Hysteresis

current control, Shunt Active Power Filter, Synchronous Reference

Frame Algorithm

I INTRODUCTION

Now days,

power system uses large number of power electronic devices to control the power

system equipments. However power electronic based equipments which includes

adjustable speed motor drives, electronic power supplies, electronic ballasts

are responsible for the rise in power quality related issues.1. These

nonlinear loads appear to be important sources of harmonic distortion in a

power distribution system. These harmonics reduces the quality of power, low

efficiency, low power factor. Hence to overcome these problems of harmonics

passive filters have been used. But due to some disadvantages, namely it will

introduce system resonances that can move a harmonic problem from one frequency

to another, it is difficult to design the filters to avoid leading power factor

operation for some load conditions. To overcome these disadvantages, active

power filter have been developed.2 The Active Power Filter (APF) based on

power electronics technology is a viable solution for power conditioning to

suppress the harmonics in the power system. With recent developments in power

electronic switches, the Active Power Filters (APFs) have been applied to

mitigate the problems created by non-linear loads. One of the most commonly

used active filters is the Shunt Active Filter (SAF) which is used to eliminate

the unwanted harmonics and compensate reactive power consumed by non-linear

loads 3.

The Shunt Active Power Filter is connected in parallel with the line through a coupling

inductor. Its main power circuit

consists of a three phase three-leg

current controlled

voltage source inverter with a DC link capacitor. An active power filter operates

by generating a compensating current with 180 degree phase opposition and injects it back to the line so

as to cancel out the current harmonics introduced by the nonlinear load. This will thus

suppress the harmonic content present in the line and make the current waveform sinusoidal. So the process comprises of detecting

the harmonic component present in the line current,

generating the reference current, producing the switching pulses for the power circuit,

generating a compensating current

and injecting it back to the line

4-7.

Figure.1 Three phase

shunt active power filter

II. SHUNT

ACTIVE POWER FILTER

Shunt active power filters

are widely used in power system to compensate reactive power and current

harmonics. It can also play the role of static VAR generator in the power

system for improving and stabilizing the voltage profile. Shunt active filter

compensate current harmonic by injecting complementary current that of produced

by nonlinear load. shunt active filter acts as a current source by introducing

the harmonic components created by the load. Consequently, the current harmonic

component present in the load current got cancelled and the source current

remains sinusoidal. By the use of proper control scheme, APF can also improve

system power factor. However the performance of SAPF largely depend on the

control strategy which is responsible for generating complementary harmonic

current to cancel out the current harmonics present in the load current. There

are several control strategies like, Instantaneous power theory based on

symmetrical components, Generalized Instantaneous reactive power theory,

Synchronous reference frame theory(SRF), Synchronous detection method(SDM),

etc. In this paper, SRF theory is used to generate the reference signals

applied to current control algorithm.

III SYNCHRONOUS REFERENCE FRAME ALGORITHM

Number of control

strategies being used for the determination of reference currents in shunt

active power filters namely Instantaneous Reactive Power Theory (p-q theory),

sliding mode control strategy, Unity Power Factor method, One Cycle Control,

Fast Fourier Technique etc. Here, SRF theory is used to evaluate the

three-phase reference 3currents(ica*, icb*, icc*)

by the active power used filters by targeting the source (ica, icb,

icc) current Fig.2 shows the block diagram which explains

three-phase SRF-theory, used for harmonic component extraction.

Figure.2 Reference Frame Transformation

Figure.3 Block diagram of SRF based

algorithm

In this

method, the source currents (ia, ib, ic) are

first detected and transformed into two-phase stationary frame (??-0) from the three-phase stationary frame (a-b-c), as per equation

(1).

(1)

Here two directand inverse parks transformation is used which

allows the evaluation of specific harmonic component of the input signals and a

low pass filtering stage LPF. Now, the two phase current quantities i?

and i? of stationary ??-axes are transformed into two-phase

synchronous (or rotating) frame (d-q-axes) using equation (2), where Cos? and

Sin? represents the synchronous unit vectors which can be generated using phase-locked

loop system (PLL).

(2)

The d-q currents thus obtained comprises of AC and DC parts.

The fundamental component of current is represented by the fixed DC

part and the AC part represents the harmonic component. This harmonic component

can be easily extracted using a high pass filter (HPF), as implemented in Fig

2. The d-axis current is a combination of active fundamental current (id

dc) and the load harmonic current (ih). The fundamental component of

current rotates in synchronism with the rotating frame and thus can be

considered as dc. By filtering id, the current is obtained, which

represents the fundamental component of the load current in the synchronous frame.

Thus, the AC component idh can be obtained by subtracting id

dc part from the total d-axis current (id), which leaves behind the

harmonic component present in the load current. In the rotating frame the

q-axis current (iq) represents the sum of the fundamental reactive

load currents and part of the load harmonic currents. So the q-axis current can

be totally used to calculate the reference compensation currents.Now inverse

transformation is performed to transform the currents from two phase

synchronous frame d-q into two-phase stationary frame ?-? as per equation (3).

(3)

Finally the current from two phase stationary frame ??0 is

transformed back into three-phase stationary frame abc as per equation (4) and

the compensation reference currents ica*, icb* and icc*

are obtained.

(4)

Where,

(5)

IV HYSTERISIS BAND

CURRENT CONTROL

The hysteresis band current control (HBCC)

technique is used for pulse generation in current controlled VSIs. The control

method offers good stability, gives a very fast response, provides good

accuracy and has got a simple operation. The HBCC technique employed in an

active power filter for the control of line current is shown in Figure 4. It

consists of a hysteresis band surrounding the generated error current. The

current error is obtained by subtracting the actual filter current from the

reference current. The reference current used here is obtained by the SRF

method as discussed earlier which is represented as Iabc*. The actual filter

current is represented as If abc. The error signal is then fed to

the relay with the desired hysteresis band to obtain the switching pulses for

the inverter.

Figure.4 Hysteresis Band Current Controller

The operation of APF depends on the sequence of pulse generated

by the controller. Figure 5 shows the simulation diagram of the hysteresis

current controller. A band is set above and below the generated error signal.

Whenever this signal crosses the upper band, the output voltage changes so as

to decrease the input current and whenever the signal crosses the lower band,

the output voltage changes to increase the input current. Accordingly switching

signals are generated.

Figure.5 Simulation diagram of hysteresis

current control

The switching signals thus generated are fed to the power

circuit which comprises of a three phase three leg VSI with a DC link capacitor

across it. Based on these switching signals the inverter generates compensating

current in phase opposition to the line current. The compensating current is

injected back into the power line at the PCC and thus suppressing the current

harmonics present in the line. The overall simulation block diagram is shown in

Figure 6.

Figure.6 Overall simulation diagram.

V. SIMULATION RESULTS AND DISCUSSION

After simulation of three phase transmission line having non

linear load with SRF based shunt active filter the harmonic

current is compensated within a permissible limits of IEEE standard. In this

the source current waveform without filter in a-phase is shown in Figure 7.

when filter is not connected in the system the harmonics are produces due to

non linear load. These harmonics distort the source current as shown in

figure.7. Also if the THD is cheked, then Total Harmonic Distortion (THD) spectrum

in the system without filter is shown in Figure.7, which indicate a THD of 15.59% These compensating

current is produced by the filter when we are injecting this compensating

current we get the source current with minimum harmonics. The source current

after the injection of compensating current is shown in Figure 8. The THD with

active power filter included is observed to be 3.77% which is within the allowable harmonic

limit. Figure.8 shows the THD spectrum with active power filter in the circuit.

Figure.7 Source current and THD spectrum without SAF

Figure.8 Source current and THD Spectrum with SAF

VI. CONCLUSIONS

The SAPF explained

in this paper compensate the line current harmonics generated due to the

nonlinear loads in the system. HBCC technique used for the switching pulse

generation was found to be effective and its validity is proved based on

simulation results. Thus SRF based SAPF has been proved to be effective to keep

the harmonic content in power lines within the permissible limit of IEEE

standards i.e. THD is 3.77%.

References

1Dugan.C.Roger,

M.F.McGranaghan, Santoso and H.W.Beaty, “Electrical Power Systems Quality”,

second edition McGraw-Hill, 2002, USA

2Joao

Afonso,Mauricio Aredes,Edson Watanabe, Julio martins “Shunt active filter for

power quality improvement.” International conference UIE 2000- Electricity for

a sustainable Urban Development , Lisboa, potugal, 1-4 Novembro 2000 pp

683-691.

3Deepathi

Joseph, “P-Q Theory for Shunt Active Filter using Ramp Comparator” IEEE transaction

on International conference on Power, Energy and Control. 2013.

4

Preeti Yadav, Swati Maurya, “Single phase shunt active power filter for

harmonic filtering” International Journal of Emerging Technology

and Advanced Engineering, Volume 4, Issue 4, April 2014.

5Alberto

Pigazo, “A

Recursive Park Transformation to Improve the Performance of Synchronous

Reference Frame Controllers in Shunt Active Power Filters” IEEE Transactions On Power

Electronics, Vol. 24, No. 9, September 2009.

6Mohammad Monfared, “A New

Synchronous Reference Frame-Based Method for Single-Phase Shunt Active Power

Filters” Journal of Power Electronics,

Vol. 13, No. 4, July 2013.

7Diyun WU, “Design and Performance of a Shunt Active

Power Filter for Three phase Four-wire System” 2009 3rd International

Conference on Power Electronics Systems and Applications.

8Leszek S. Czarnecki,

“Instantaneous Reactive power p-q theory and Power properties of 3-phase

system”, IEEE Transactions on Power

Delivery, Vol. 21, No. 1, pp.362-367, Jan. 2006.

ACKNOWLEGMENT

Ms. Dipeeka P. Sawant received his B.E degree in Electrical Engg. from Pune University,

in 2012. Now

she is doing

M.E. in Electrical Power System from Yadavrao Tasgaonkar Institute of Engg. And

Technology

Bhivpuri Road , Karjat.

Ms. Pranita P. Chavan received his B.E degree in Electrical

Engg. from Mumbai University, in2002.And M.E

degree from Pune University in Electrical Power System in 2004 . Now she is working as Assistant

Professor In Yadavrao Tasgaonkar Institute of Engg. And

Technology Bhivpuri Road , Karjat. She has

Total Experience spans of over 11 years.