Development of Different Control techniques for Shunt Active Power Filter
Kranthi Kumar Vanukuru
Research Scholar, Dept. of EEE
KLEF, Deemed to be University.

Guntur Dt. – 522 502, India.

[email protected] Pakkiraiah
Associate Professor, Dept. of EEE
KLEF, Deemed to be University.

Guntur Dt. – 522 502, India.

[email protected] – This paper deals with the development of different control methods for power quality improvement in shunt active power filter. Apart from other familiar existence techniques, four different methods are developed, first constant instantaneous power control, second sinusoidal current control technique using phase locked loop control, third Synchronous Reference Frame theory (DQ), and fourth Fryze current control technique. For each techniques different operating conditions are discussed. Only resistive-inductive load with three phase three wire system has been considered. Using MATLAB SimulinkTM simulation results are obtained with wave forms for different parameters. All THD data in tabular form also obtained and discussed. By performing this simulation, it comes out that Synchronous Reference Frame method gives good results.
Keywords – Constant Instantaneous Power Method (CIP), Synchronous Reference Frame (DQ), Shunt Active Power Filter (SAPF), Sinusoidal Current Control Strategy with Phase Locked Loop (SCC), Total Harmonic Distortion (THD), Point of Common Coupling (PCC), Voltage Source Inverter (VSI), Positive Sequence Detector (PSD).

The vast use of semiconductor devices are the main causes of power distortion. Since from 1970’s inventions of thyristors has been changed the way of current control 12. These devices control the current with atomic level and they are called atomic switching power devices. By virtue of this property, these devices give very fast control response. Very fast ON and OFF response makes the line current distorted and eventually distorted voltage. These distortion increases the harmonics 3 and reduced the power quality. It has been known that 5th and 7th order harmonics are more significant in power system 3. Fourier Transformation Theory is very useful for analysis of harmonics3 4. One phenomena that is “harmonic propagation” 1 which is a result of series and/or parallel harmonic resonance between line inductors and shunt capacitors in power system. So ‘harmonics damping’ has done with series active filter and other phenomena ‘harmonic elimination’ 1 has been carried out with shunt active power filter.

Now a day devices like TV, computers, mobile charger, LED bulb, regulator of fan, arc furnace, Adjustable speed drive (ASD), Power electronic converter etc. are used at homes and private companies. In early days effect of these devices on power quality not properly known. But mal- operation of protecting devices such as relays and circuit breakers open the new search area of power quality.

So these kind of switching devices generate nonlinear current 3 and is called non-linear load. Before SAPF, passive filters were used. But because of more disadvantages 23, usage of active power filter becomes popular. Since After remarkable work done by Hirofumi Akagi, the new way of control with SAPF have been opened up. He suggested the methods by which the flow of current can be controlled in such a way that source current will become near to pure sinusoidal 1.

There after lots of other methods have been proposed by authors 125679. These techniques are for reducing harmonics, voltage regulation, correction of power factor etc. New techniques have been developed by engineers using Neural networks and fuzzy logic. In this paper we considering three different techniques. And there is wide classification of SAPF based on different topology, based on supply type and based on connection 2.

Fig.1 shows fundamental block diagram of SAPF with source, nonlinear load. Where isa, isb, isc are source currents, ila, ilb, ilc are line currents and ica, icb, icc are compensating currents.

Fig.1. Block diagram of Shunt Active Power Filter
The reference current calculation block represents any of three methods mentioned above. In this block inputs are line voltages and line currents which are used to generate the reference current I*C. The error signal obtained by comparing I*C and IC is fed to PWM converter. The output of PWM converter are six gate signals which in turn applied to VSI. The VSI circulates the compensating current ‘IC’ through capacitor.
In VSI these switching process generates switching losses also. Those switching losses are fed by p? loss , which has been generated by comparing actual VDC and reference V*DC voltage. In between them there is PI controller which track the IC current. For generating gate pulses, Hysteresis band current control method has selected.
These pulses are fed into six switches of VSI. ON and OFF process of six switches allow to flow the IC current. So the PI controller track to change the gate pulses until IC becomes equal to I*C.

Four techniques are considered here, one is Constant Instantaneous Power (CIP), second is Sinusoidal Current Control (SCC), third one is Synchronous Reference Frame Theory (DQ), and fourth one is Generalized Fryze Current Control.

Constant Instantaneous Power Technique (CIP)
This technique used Clarke transformation 8 for converting a, b, c three phase axis values to alpha-beta two phase axis.

Fig.2. Block diagram of CIP method
The key thing for separation these all things is that we can eliminate any power out of four (p, p,q, q). So we are flowing the current in such a way that controls the different power.

In CIP technique we selected p and whole q (q+ q) for elimination so that there are no harmonics in source current because all oscillating part of power has been eliminated. But only in balanced supply case. In unbalanced and/or disturbed supply voltage it will give bad result as we will see in simulation result. All the discussion of above has simply given in fig.2.
B.Sinusoidal Current Control Technique with Phase Locked Loop (SCC)
This method is modification of CIP method. This method is very useful when there is distortion and /or unbalance from supply side. Positive sequence detection block determine sequence and give only positive sequence voltage as output.

1685925131572000Fundamental positive sequence diagram(PSD) is given in fig.4. Here supply voltages transformed into alpha beta axis. And currents are fed into PLL block and that currents are used for calculation of instantaneous power p` and q`. By applying these powers to two low pass butterworth filters with cut off frequencies 50Hz. Which gives p?? and q ?. These powers and alpha- beta currents are fed to alpha-beta voltage calculation block with equation that shown in fi.4.

Fig.3. Block diagram of SCC strategy
so we have finally pure sine wave which have positive sequence.

Fig.4. Block diagram of PSD
Here in PLL circuit block PI controller decides w. then integrator block gives wt which gives two 90-degree phase shift alpha and beta currents. In Sinusoidal Current Control technique, the key operation is inside the PSD and PLL blocks. In distortion and/or unbalance supply condition this method gives best result that we will discuss in simulation result later.

Synchronous Reference Frame Theory (DQ)
In the proposed controller with the existing phase (considered ? phase), other orthogonal fictitious phase is created which is considered as ? phase. This orthogonal ? phase generation has been done with the quarter cycle time delay method. Generation of ?-? quadrature phases resembles the Clarke’s transformation in three phase theory. Accordingly this ? and ? instantaneous values are transformed into synchronous reference frame (SRF) based d-q components. Here Clarke and then Park transformation is implemented where the reference angle is obtained through single phase PLL. In order to obtain the fundamental positive sequence component of distorted PCC voltage due to presence of nonlinear load, the variable dc value along d axis due to presence of nonlinear load has been kept constant with the application of Moving-Average Filter (MAF)

Fig.5. Block diagram of DQ
GeneralizedFryzeCurrentsMinimization Technique (Fryze)
Plus point of this method is that it does not require Clarke transformation. So it is very simple compared with previous two methods. It directly calculates instantaneous voltage and current for each phase.

Fig.6 represent diagram for Fryze method. In that also Gloss has added same as p? loss in above two methods which supply the switching losses for VSI. Reference current are fed into PWM block to generate gate pulses. Fryze methods draws less value of RMS current for same power compared with other two methods. But it has some higher THD in source current that we will see in simulation result. So we can say that this is the second best method for out of three.

Fig.6. Block diagram of FRYZE current control strategy
Above discussed all three methods are performed in MATLAB 2016a software for different operating condition.

MATLAB/Simulink of SAPF

System Data for simulation
Vsupply (L-L) 400 V
Load (RL) R=150 ohm, L=0.5mH
Frequency 50Hz
Smoothing reactor 8mH with 0.02 ohm resistor
DC capacitance 1000 micro F
Reference DC voltage 600 V
Load current (peak) ~ 3A (peak)
Line parameter(RL) R=0.1 ohm, L=2mH
Technique % THD
Constant Instantaneous Power 3.39
Sinusoidal Current Control 3.26
Fryze Current Control 3.16
Synchronous Reference Frame 3.12
Simulation for Sinusoidal Current Control technique, Constant Instantaneous Power technique and Fryze Current Control technique have been prepared for the nonlinear load. The performance of all the techniques have been evaluated under same operating conditions. The performance of Shunt Active Power Filter using different techniques under unbalance conditions (both in terms of phase and magnitude) have been evaluated.

From the Simulation result, it has been observed that all the techniques are useful operating techniques, which satisfies the IEEE standards. However Synchronous Reference Frame theory gives best performance out of all the four techniques discussed.

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