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 the Basic block diagram of SAPF. When the Rectifier type load is connected to the three phase supply, harmonics are induced in rectifier load and it affects source current. SAPF is used to supress the harmonics in source current by injecting compensating currents from VSI. Switching pulses of a VSI are given by the HCC by comparing the actual and reference compensating currents. Reference currents are generated from different control strategies by providing source voltage and load current. Switching losses are generated by PI controller by comparing the actual and reference Vdc values.

Fig.1. Block diagram of Shunt Active Power Filter

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 Generalized Fryze Current Control theory, and fourth one is Synchronous Reference Frame (DQ) theory.

Constant Instantaneous Power Technique (CIP)

The p-q theory consists of alpha beta zero transformation. It converts 3-phase voltages and currents to the corresponding alpha beta zero reference frame. These phase voltages and currents on the alpha beta zero axis will define the instantaneous active (p) and reactive (q) power. The reference compensating currents can be obtained from active (p) and reactive (q) power. The reference compensating currents are compared with the actual compensating current generated at the inverter output and the error signals obtained are used to generate the gating pulses for the 3-phase inverter by HCC as shown in Fig.2.

Fig.2. Block diagram of CIP method

B.Sinusoidal Current Control Technique with Phase Locked Loop (SCC)

This method is modification of CIP method. Positive sequence voltages are given as CIP source voltage input, which are generated from the positive sequence detector as shown in Fig.3. In Fig.4, The control strategy of PSD is given, which improves the system performance. Phase Locked Loop (PLL) is shown in Fig.5. PLL calculates the I?/, I?/ for getting instantaneous power p and q from source voltages.

Fig.3. Block diagram of SCC strategy

Fig.4. Block diagram of PSD

Fig.5. Block diagram of PLL

Generalized Fryze Currents Minimization Technique (Fryze)

The proposed FCC is presented in Fig.6. Power is calculated from the source voltages and load currents. P loss is added to the calculated power and multiplied with the source voltages in order to get the active currents. The difference between the actual currents and load currents gives the compensating current values.

Fig.6. Block diagram of Fryze

Synchronous Reference Frame (DQ) method.

DQ method is shown in Fig.7. Compensating currents are calculated by comparing the actual and reference currents. Reference currents are generated from the direct and quadrature axis load currents. Low pass filter is used to remove the fluctuation in current.

Fig.7. Block diagram of DQ method.

IV.SIMULATION RESULTS

All the four methods considered in this paper are developed in MATLAB 2015a software. A balanced three phase source supplies power to the non-linear load. Non-linear load consists of bridge rectifier along with RL load. Basic active power filter is connected to the line through a circuit breaker. In the APF, switching signals for VSI are generated by the CIP method, SCC method, FCC method, and DQ method separately. Fig.8 shows the working model of the proposed method.

MATLAB/Simulink of SAPF

Fig.8. MATLAB/Simulink model.

System Parameters

A three phase, 400 V, 50 Hz source is given to the load consists of a bride rectifier and RL load, R=150 ?, and L=0.5 mH. Smoothing reactor is having 8 mH inductance with 0.02 ? resistance. The value of DC capacitance is 1500 ?F, and the Reference DC voltage is 600 V. Line parameters are, 0.1 ? resistance and 2 mH inductance. The peak value of load current is approximately 3 ampere.

Simulation waveforms

(i) Source current without APF: Source current waveform is affected by the non-linear load when it is operated without APF. Fig.9 (a) shows the distortion in Is affected by the IL, (b) shows the harmonic content in Source current

Fig.9 (a) Load current (IL) and Source current (Is) (b) % THD of Source current (Is)

(ii) Power Factor (PF):

TABLE I.PF without APF and with APF by using CIP control method:

Without APF With APF

(iii) Source Current (Is) and Compensating Current (Ic):

TABLE II:Source current and Compensating current for Different Techniques:

Sl. No. Method Source Current Compensating Current

01 CIP

02 SCC

03 FCC

04 DQ

TABLE III.%THD FOR DIFFERENT TECHNIQUES

Technique % THD

Constant Instantaneous Power (CIP) 3.39

Sinusoidal Current Control (SCC) 3.26

Fryze Current Control (FCC) 3.16

Synchronous Reference Frame (DQ) 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.

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