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 – Shunt

Active Power Filter, harmonics, Synchronous Reference Frame Algorithm,

Hysteresis Current Control

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.

1 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)

2.

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.

III. 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

IV. 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

Dugan.C.Roger,

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

second edition McGraw-Hill, 2002, USA

1

Joao 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.

2

Deepathi Joseph, “P-Q Theory for Shunt Active Filter

using Ramp Comparator” IEEE transaction on International conference on Power,

Energy and Control. 2013.

3

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.

4

Alberto 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.

5

Mohammad 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.

6

Diyun 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.

7

Leszek 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.

VI. 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, in 2002. 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.