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] – Extensive use of the Non-linear elements like power electronic devices, circuits with inductor and capacitor are effecting the quality of power. All non-linear loads induces harmonics into the system. Shunt active power filters (SAPF) are used to improve the power quality by reducing the harmonics under specified levels along with improving the power factor. 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 by using the Instantaneous active reactive power theory, second sinusoidal current control method by using phase locked loop control, third Fryze current control with a positive sequence detector, and fourth Synchronous Reference Frame theory (DQ). Bridge rectifier with RL load is considered. SAPF controlled by mentioned controlling techniques were implemented by using MATLAB/Simulink environment. Simulation results are obtained with wave forms for different controlling techniques for SAPF. Total harmonic distortion (THD) data of all four cases are tabulated and discussed. By analyzing all the simulation results, it comes out that Synchronous Reference Frame method gives good results.

Keywords – Shunt Active Power Filter (SAPF), Constant Instantaneous Power Method (CIP), Sinusoidal Current Control Strategy with Phase Locked Loop (SCC), Synchronous Reference Frame (DQ), Positive Sequence Detector (PSD).

Introduction

The utilization of power electronic devices has been enormously increased now a days, major industries with different equipment, loads in the distribution system creates lot of disturbances to the current waveforms, causing distortion in the waveform. Non-linear loads causes disturbances to the other users and causes interference in Telecommunication networks. Compensation control scheme was developed to limit the effect of non-linear loads 1. Rapid technical advances in power electronics and micro-electronics such as uninterrupted power supplies, personal computers, and various other consumer electronics draw highly distorted current from the grid. A large harmonic currents flows in phases in case three phase loads, and flows through neutral wire in case of single phase loads. A constant-hysteresis, tolerance-band controller is developed to protect the other users from the adverse effects of these loads 2. Shunt active power filter is designed to achieve reactive power compensation to bring the power factor near to the unity. Feedback control law with analytically tuned PI controller is used in APF 3. Various controlling techniques are developed for APF, mathematical modelling of synchronous reference frame approach is proposed to eliminate harmonics and to compensate reactive power of non-linear load in three phase systems 4. Measuring the all variables for power quality analysis at each every point is not economical. Therefore, an efficient algorithm has been prepared to make the system observable. So that the overview of the power quality can be observed by managers of utility companies. Algorithm evaluate the power in distribution network and make available for the power quality process 5. Harmonics caused by the non-linear loads has an effect on the other users, efficiency of the transmission lines and responsible for malfunctioning of the protective relays. Kalman filter and the artificial neural network algorithm was developed for the protection of the transmission line 6.

There are various control algorithms for shunt active power filter to reduce the harmonics in non-linear loads. Synchronous source current detection method, instantaneous active reactive power theory, I cos Ø theory and power balance theories are discussed and analysed the results of all the four methodologies 7. PQ theory was developed for APF for reactive power compensation and current harmonic reduction. It improves the efficiency of the system and brings the power factor nearer to the unity. APF with PQ theory will also eliminates the oscillations in active power. Thereby, end user will get constant instantaneous active power 8. Shunt active power filter with sinusoidal source current control strategy has been discussed. This method balances the voltage fluctuations and voltage imbalances in the power supply 9. Fryze current control method for active power filter is one of the efficient method of control. A positive sequence detector was introduced in this method. The reference currents are generated by using the Fryze current minimization algorithm and hysteresis current controller generates the switching signal for voltage source inverter 10. A comparative study of three control methods for APF was discussed. From the obtained results, it has been observed that the current harmonics are reduced considerably to bring the value down to meet the IEEE standards. Now, in this paper, one more technique ‘synchronous reference frame’ (DQ) is introduced. Results shows that this method is effectively reduces the harmonic level in the given distribution system.

II.BASIC OPERATION OF SHUNT ACTIVE POWER FILTER

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.

III.TECHNIQUES FOR SHUNT ACTIVE POWER FILTER

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

IV.SIMULATION RESULTS

Above discussed all three methods are performed in MATLAB 2016a software for different operating condition.

MATLAB/Simulink of SAPF

System Data for simulation

TABLE I.PARAMETERS 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

TABLE II.%THD FOR DIFFERENT TECHNIQUES

Technique % THD

Constant Instantaneous Power 3.39

Sinusoidal Current Control 3.26

Fryze Current Control 3.16

Synchronous Reference Frame 3.12

V.CONCLUSION

A non-linear load is supplied by the three phase power supply and shunt active power filter is used. Constant instantaneous power method, sinusoidal current control method, Fryze current control technique, and synchronous reference frame methods for active power filter are developed in MATLAB/Simulink environment. The shunt active power filter effectively eliminates the unwanted harmonic content introduced by the non-linear load in the distribution systems.

Source current and compensating currents for all methods are plotted. It is observed that each and every technique is effective in reducing the harmonic content. Results shows that the % THD is under 5 % in all the control methodologies.

From the analysis, it shows that % THD is equal to 3.12 % in synchronous reference frame method, which is superior to the remaining three methods. Finally the proposed control methods for shunt active power filter smoothens the current waveform, voltage imbalance, reduces the % THD of the source current, improves the power factor to the unity, and overall efficiency of the distribution system is improved.REFERENCES

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