NEURO-FUZZY BASED POWER QUALITY IMPROVEMENTS IN A THREE PHASE FOUR WIRE DISTRIBUTION SYSTEM USING DSTATCOM

Transcription

1 NEURO-FUZZY BASED POWER QUALITY IMPROVEMENTS IN A THREE PHASE FOUR WIRE DISTRIBUTION SYSTEM USING DSTATCOM E.Babu 1,R.Subramanian 2 1, Department of Electrical and electronics engg 2 Department of Electrical and electronics engg 1 [email protected], 2 [email protected] Abstract- This project presents reactive power compensation in a distribution power system. DSTATCOM (distribution static compensator) is a reactive power compensator for power quality improvement in a three phase four wire distribution system. A voltage source converter (VSC) based DSTATCOM with a zig-zag transformer is used for the compensation of reactive power for voltage regulation along with load balancing, elimination of harmonic currents, and also the neutral current compensation at the point of common coupling. The neuro-fuzzy based system is used to control scheme of the VSC. The zig-zag transformer is used for providing a path to the zero sequence current in a three-phase four-wire distribution system. The performance of the proposed DSTATCOM system is verified through simulations using MATLAB software with its Simulink. Keywords- Distribution static compensator (DSTATCOM), voltage-source converter (VSC), zigzag transformer and neutral current compensation. I. INTRODUCTION The present day ac power distribution systems are suffering from severe power quality problems. These power quality problems include high reactive power burden, harmonics currents, load unbalance and excessive neutral current etc. The power quality at the point of common coupling (PCC) with the utility grid is governed by the various standards and the IEEE- 519 standard is widely accepted. Some remedies to these power quality problems are reported in the literature. A group of controllers together called Custom Power Devices, which includes the DSTATCOM (distribution static compensator), DVR (dynamic voltage restorer) and UPQC (unified power quality conditioner). The DSTATCOM is a shunt connected device, which takes the care of the power quality problems in the currents. Three-phase four-wire distribution systems are used to supply single-phase low voltage loads such as computer loads, lighting 953

2 ballasts, small rating adjustable speeds drives (ASD) in air conditioners, fans, refrigerators and other domestic, commercial appliances etc. The most of the connected loads are generally nonlinear loads. The harmonics in currents and unbalance in load increases the neutral line currents. The excessive neutral current which has both harmonic and fundamental component of load currents may over load the neutral conductor of three-phase four-wire distribution system. A simple three phase four wire system circuit is shown in fig. 1. There are many topologies reported in the literature for threephase four-wire DSTATCOM such as a three single-phase VSC, VSC with four leg, three leg VSC with capacitors and three leg VSC with a zig-zag transformer and three-leg VSC with neutral terminal at the positive or negative terminal of dc bus. The application of a zig-zag transformer for reduction of the neutral current is having an advantage due to passive compensation, rugged and less complex over the active compensation techniques. DSTATCOM for a three-phase four-wire system. There are instantaneous reactive power theory (p q theory), synchronous reference frame theory, power balance theory, etc. A fuzzy based control of an active filter is proposed in the literature for harmonic elimination in the supply currents. In this paper, a Fuzzy logic control of the three-phase four-wire DSTATCOM is proposed with a new method control for the voltage regulation at the PCC. II. PROPOSED DSTATCOM The three-phase four-wire DSTATCOM is used for reactive power compensation and harmonic current compensation along with load balancing and neutral current compensation. Fig. 2. Shows the power circuit of proposed VSC Fig. 1. Three phase four wire system circuit based DSTATCOM along with a zig-zag transformer connected in the three-phase four-wire distribution system. This Zig-Zag transformer can supply the path for the zero-sequence current. Fig. 3. Shows the phasor diagram of zig-zag transformer. There are many theories available for the generation of reference source currents for the control of VSC of 954

3 Fig. 2. Schematics of three phase four wire systems with DSTATCOM. From phasor diagram, it can be found that the voltage across the transformer s winding is of the phase voltage of the threephase four-wire distribution power system. Fig. 3. Zig-Zag transformer phasor diagram. 955

4 The linear and non-linear, balanced and unbalanced loads are connected at the point of common coupling. The DSTATCOM consists of a 3 leg pulse width modulated (PWM) voltage source converter (VSC) using six insulated gate bipolar transistors (IGBTs), and dc capacitors. The T- connected transformer connected at the load terminal provides a circulating path for zero sequence harmonic and fundamental currents. The DSTATCOM provides harmonics elimination, neutral current compensation and load balancing along with power factor correction or line voltage regulation. The compensator current is used to compensate the reactive power component of the load current. The DSTATCOM injects a current Ic, such that the load current, Is and source voltage, Vs. III.CONTROL OF DSTATCOM The block diagram of the control scheme is shown in Fig. 4. The fundamental real and reactive power components of load current are extracted in each phase, and they are considered as the reference source currents (isa, isb, isc). The load currents (ila, ilb, ilc) and the PCC voltages (vsa, vsb, vsc) are extracted. The extraction of the weights corresponding to the fundamental active components of the load current (wpa, wpb, wpc) and the weights corresponding to the fundamental reactive components of the load current (wqa, wqb, wqc). The averages of the respective weights are Wp and Wq are updated at each sample time. For an ac system, the source voltages and load currents have harmonic components along with fundamental components where V1 and Vn are the peaks of the fundamental and harmonic components of the voltage. Similarly, I1 and In are the peaks of the fundamental and harmonic components of the current. The unit template for three phases can be represented as u a = U sin ῳt u b = U sin (ῳt-120) u c = U sin (ῳt-240) x a = U cos ῳt x b = U cos (ῳt-120) x c = U cos (ῳt-240) Fig. 4. Extraction of fundamental real and reactive current in a three-phase system. 956

5 where U = 1. The fundamental load current can be decomposed as I1 = Ip + Iq where Ip and Iq are the active and reactive power components of the load current, respectively. The estimates of the fundamental active power and fundamental reactive power components of the load current for a single phase are obtained by estimating the respective weights corresponding to the fundamental active components of the load current (wpa, wpb, wpc) and the weights corresponding to the fundamental reactive components of the load current (wqa, wqb, wqc). The average weight corresponding to the active and reactive components of the load is shown as wp =(wpa + wpb + wpc)/3 wq =(wqa + wqb + wqc)/3 The reference source currents are obtained as the sum of active and reactive power currents as i*sa = ipa + iqa i*sb = ipa + iqa i*sc = ipa + iqa. The reference source currents in three phases are used for the control of the three-leg VSC. The sensed and reference source currents are compared, and the error is used to generate the gating signals. IV.FUZZY LOGIC SYSTEMS Fuzzy logic systems are one of the main developments and successes of fuzzy sets and fuzzy logic. It is a rule-base system that implements a nonlinear mapping between its inputs and outputs. A fuzzy logic system is characterized by four modules. Fuzzifier Defuzzifier Inference engine Rule base A schematic representation of a FLS is presented in Fig. 5. The operation of a FLS is based on the rules contained in the rule base. Fuzzification Two proportional integral (PI) controllers are used for controlling the dc bus voltage and ac terminal voltage. A self supporting dc bus is realized using a PI controller over the sensed (vdc) and reference values (v dc) of the dc bus voltages of DSTATCOM. This PI controller estimates the loss component of the source current (wloss), and hence, this is added with the wp. The second PI controller is used over the amplitude of PCC voltage (VS) and reference values (V S ), and it estimates the reactive component of the DSTATCOM current (wqr), and hence, this is added with the wq. Now, the real components of the reference source currents are computed as ipa = wp*ua ipb = wp*ub ipc = wp*uc. Similarly, the reactive components of the reference source currents are obtained as iqa = wq*xa iqb = wq*xb iqc = wq*xc. Measure the values of input variables Perform a scale mapping that transforms the range of values of input variables into corresponding universe of discourse. Function of the fuzzification that converts input into suitable linguistic values. Knowledge Base Fig.5. Structure of fuzzy based system 957

6 base. It consists of the data base and linguistic control rule The database provides a necessary definitions, which are used to define linguistic control rules and fuzzy data, manipulation in an, FLC. The rule base characterizes of the control goals and control policy of the domain experts by means of set of linguistic control rules. Decision Making Logic Defuzzification It has a capability of simulating human decision making based on fuzzy concepts and of inferring fuzzy control actions employing fuzzy implication and the rules of inference in fuzzy logic. A scale mapping which converts a range of values of input variables into corresponding universe of discourse. The defuzzification which yields a non-fuzzy, control action from an inferred fuzzy control action. V. MATLAB-BASED MODELING OF THE SYSTEM The three leg VSC and the zig-zag transformer based DSTATCOM for a three-phase four wire system are modeled and simulated using MATLAB and its Simulink and SimPower System toolboxes. The DSTATCOM system shown in Fig. 2 is modeled in MATLAB, and its developed model is shown in Fig.6. The multi winding transformer model available in the SPS is used for modeling the zig-zag transformer. The ripple filter has connected to the VSC of the DSTATCOM for filtering the ripple in the PCC voltage. The control algorithm for the DSTATCOM is also modeled in MATLAB/SIMULINK. The reference source currents are derived from the sensed PCC voltages (vsa, vsb, vsc), load currents (ila, ilb, ilc), and the dc bus voltage of DSTATCOM (vdc). Fig. 7 shows the Simulink model of control model of distribution system. Fig. 6. Simulink model for three phase four wire system with DSTATCOM 958

7 Fig. 7. Simulink model of control model of distribution system A pulse width modulation (PWM) current controller is used over the reference and sensed compensator currents to generate the gating signals for the IGBTs of the VSC of the DSTATCOM. transformer. The DC bus voltage of the capacitor (vdc) of the VSC of DSTATCOM is regulated by the controller, and the dc voltage has maintained near the reference dc voltage under varying load disturbances. VI. SIMULATION RESULTS AND DISCUSSION The performance of the three phase VSC based DSTATCOM and the zig-zag transformer for PCC voltage regulations, along with the neutral current compensation and load balancing of a three phase four wire linear load, is shown in Fig. 8. At 0.10 s, a three phase linear load is changed to two phase load and again to single phase load at 0.15 s. The PCC voltages (vs), balanced source currents (is), load currents (ila, ilb, ilc), compensator currents (ic), load neutral current (iln), transformer neutral current (icn) and dc bus voltage (vdc) are demonstrated under change of load conditions. It s observed that the amplitude of PCC voltage (VS) is regulated to the reference amplitude by the required reactive power compensation and that the source neutral current (isn) is maintained at nearly zero because of the zig-zag The performance of DSTATCOM for PCC voltage regulation and harmonic eliminations, along with neutral current compensation, is shown in Fig. 9. At 0.10 s, a three-phase nonlinear load is changed to two phase load and again to single phase load at 0.15 s. These loads are applied again. The PCC voltages (vs), balanced source currents (is), load currents (ila, ilb, ilc), compensator currents (ic), load neutral current (iln), transformer neutral current (icn) and dc bus voltage (vdc) are demonstrated under varying nonlinear loads. The PCC voltage 959

8 Fig. 8. Performance of three-phase three-leg VSC and a zig-zag transformer for neutral current compensation, load balancing, and voltage regulations. is regulated to the reference amplitude value. It s observed that the load is drawing a non sinusoidal and unbalanced current, but the source currents are sinusoidal and balanced due to the proper compensation currents injected by DSTATCOM. Fig. 9. Performance of three-phase three-leg VSC and a zig-zag transformer for neutral current compensation, harmonics compensation, and voltage regulation. CONCLUSION The analysis and simulation of a new three phase four wire DSTATCOM consisting of a three leg VSC with a zig-zag transformer controlled with fuzzy logic have been carried out, and its performance have been demonstrated for neutral current elimination along with reactive power compensation, harmonic eliminations, and load balancing. The VSC is controlled using the fuzzy logic controller. The voltage regulation and power factor correction modes of operation of the DSTATCOM have been observed. The dc bus voltage of the DSTATCOM has been regulated to the reference dc bus voltage under wide varying loads. 960

9 The zig-zag transformer has been found to be effective for compensating the zero-sequence fundamental and harmonic neutral currents. The kilovolt ampere rating of transformers in the zig-zag configuration is much less compared to that of a similar star delta transformer. It has been found that the control technique is simple to implement, is fast in response, and gives nearly zero phase shift in the estimated reference currents. in distribution system, IEEE Trans., Ind. Electron, vol. 54, no. 1, pp , Feb B. Singh, P. Jayaprakash, S. Kumar and D. P. Kothari, Implementation of neural network controlled three leg VSC and a transformer as three phase four wire DSTATCOM, IEEE Trans. Ind. Electron., vol. 47, no. 4, Jul-Aug REFERENCES 1. R. C. Dugan, M. F. McGranaghan, and H. W. Beaty, Electric Power Systems Quality, 2nd ed. New York, Mc Graw Hill, K. R. Padiyar, FACTS Controllers in Transmission and Distribution. New Delhi, India, New Age Int., J. C. Montano and P. S. Revuelta, Strategies of instantaneous compensation for three phase four wire circuits, IEEE Power Eng. Rev., vol. 22, no. 6, p. 63, Jun H. L. Jou, J. C. Wu, K.. D. Wu, W. J. Chiang, and Y. H. Chen, Analysis of zig-zag transformer applying in the three-phase four-wire distribution power system, IEEE Trans., Power Del., vol. 20, no. 2, pp., , Apr H.-L. Jou, K.-D. Wu, J.-C. Wu, and W.-J. Chiang, A three phase four wire power filter comprising a three phase three wire active filter and a zig-zag transformer, IEEE Trans., Power Electron., vol. 23, no. 1, pp., , Jan M. I. Milanés, E. R. Cadaval, and F. B. González, Comparison of control strategies for shunt active power filters in three phase four wire system, IEEE Trans., Power Electron., vol. 22, no. 1, pp , Jan B. Singh, V. Verma, and J. Solanki, Neural networks based selective compensation current quality problems 961