Development of Control techniques for SAPF for Power Quality Enhancement
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) function is to improve the quality of power by reducing the harmonics under specified levels along with improving the power factor. In this paper, the development of various control methods for improving the power quality 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 method, 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 MATLAB/Simulink environment. Results are observed 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, Synchronous Reference Frame method shows better performance
Keywords – Shunt Active Power Filter (SAPF), Constant Instantaneous Power Method (CIP), Sinusoidal Current Control Strategy (SCC), Synchronous Reference Frame (DQ).
The utilization of power electronic equipment 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. An efficient 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 Q in the 3-phase system 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 SAPF to maintain power quality 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 performance of the system and brings the PF nearer to unity. APF with PQ theory will also eliminates the oscillations in active power. Thereby, end user will get constant instantaneous active power 8. SAPF with sinusoidal source current control technique has been discussed. This method balances the voltage fluctuations and voltage imbalances in the power supply 9. Fryze current control method for SAPF is one of the efficient method of control. A PSD technique was introduced in this method. The compensating currents are generated by using the Fryze current reduction algorithm and hysteresis current controller gives the switching signal for VSI 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.WORKING PRINCIPLE OF SAPF
Shunt Active Power filter is explained with the help of the diagram shown in Fig.1. 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 by infusing 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. Basic Structure of SAPF
III.SAPF CONTROLLING METHODS
There are so many controlling techniques for SAPF. In this paper, four methods are taken into consideration, at first CIP method is considered, secondly SCC method, third method is FCC theory and fourth one is Synchronous Reference Frame (DQ) theory.
CIP Method for SAPF
In this method, alpha beta zero transformation is applied to source voltages (Vs) and load currents (Is). It transforms the given three variables to the corresponding alpha beta zero reference frame. Instantaneous Active power (P) and Reactive power (Q) is being calculated by the transformed voltages and currents. VSI getting gating pulses with the comparison of actual compensating currents and reference compensating currents. These gating signals are given through HCC as depicted in Fig.2.
Fig.2. CIP method of Control
B.SCC Method for SAPF
A slight modification of CIP method yields SCC method of control. Positive sequence voltages are given as CIP source voltage input, which are generated from the positive sequence detector. PSD operation is explained in Fig.3. In Fig.4, The control strategy of PSD is given, which improves the system performance. Phase Locked Loop (PLL) is depicted in Fig.5. PLL calculates I?/, I?/ for getting instantaneous power p and q from source voltages.
Fig.3. SCC controlling technique.
Fig.4. PSD Operation with PLL
Fig.5. Block diagram of PLL
Fryze Controlling technique
The proposed FCC is presented in Fig.6. Power (Pe) is calculated from the Vs and IL. The calculated power oscillations are reduced by LPF. Pe is added to Ploss and multiplied with source voltages in order to get the compensating currents. The difference between the generated currents and load currents gives the compensating current values.
Fig.6. Fryze method of control
DQ Control for SAPF.
DQ method is a Synchronous reference frame method, and is given 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.
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. The specified load consists of bridge circuit along with RL load. Basic SAPF is attached to the line through a circuit breaker. In the APF, VSI getting controlling signals 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.
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.
(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) Generated waveforms of IL and 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:Is and Ic Waveforms obtained from the Simulink model for different methods:
Sl. No. Method Source Current Compensating Current
TABLE III.%THD FOR DIFFERENT TECHNIQUES
Technique % THD
CIP method of control 3.39
SCC method of control 3.26
FCC method of control 3.16
Synchronous Reference Frame (DQ) 3.12
A non-linear load is fed from three phase supply and SAPF is used for maintaining Power quality. CIP method of control, SCC method of control, FCC technique, and SRF methods for active power filter are developed in MATLAB/Simulink environment. The SAPF effectively eliminates the unwanted harmonic content generated from non-linear loads, sudden disturbances, and automated systems.
Source currents, 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 SAPF smoothens the current waveform, voltage imbalance, reduces the distortion of Is, improves the power factor to unity, and overall efficiency of the distribution system is improved.
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