International Journal of Applied Engineering Research ISSN 0973-4562 Volume 9, Number 21 (2014) pp. 10059-10071 © Research India Publications http://www.ripublication.com
DSTATCOM with Sine PWM based Cascaded H-Bridge Converter for Reactive Power Compensation 1
1
Krishna Kumar. S and 2Chandramohan.S
research scholar, DEEE, College of Engineering, Anna University, Chennai, India
[email protected] 2 Professor, department of Electrical Engineering, College of Engineering, Anna University, Chennai, India,
[email protected]
Abstract Reactive power compensation plays a major role in reducing distribution losses by improving power factor. A three phase three wire DSTATCOM consisting of an H-bridge Voltage Source Converter (VSC) is proposed for power factor improvement in the distribution systems. The synchronous reference frame theory is used for the control of VSC of the DSTATCOM. The DSTATCOM is controlled to compensate reactive power and eliminate harmonic currents. Sinusoidal PWM method is used for generating the PWM pattern of switching pulses for cascaded H-bridge converter. The performance of the proposed DSTATCOM system is validated through simulations using MATLAB software with its Simulink and Power System block set toolboxes. Keywords: Power Quality, DSTATCOM, H bridge VSC, reactive power compensation, sine PWM
1. Introduction In the recent years, there has been a considerable interest in power quality. This is mainly due to the increase in non linear loads such as power electronic converter based adjustable speed drives, electronic ballasts etc., which have deteriorated the power quality. The power quality problems mainly include high reactive power burden, harmonic currents, load unbalance and excessive neutral current apart from voltage sag and swell [1-2]. Therefore, reactive power compensation of non-linear and/or poor power factor loads and load balancing is an important issue in the modern power distribution systems. The distribution static compensator (DSTATCOM) has been used extensively for reactive power compensation, load balancing and harmonic mitigation in the distribution systems [3]. The main function of a DSTATCOM is to
Paper Code: 27117-IJAER
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inject or absorb reactive power from the grid for improving power factor and voltage regulation. With change in control approach, the DSTATCOM can also be used as an active filter and a dynamic uninterruptable power source. It may be noted that the active filter in this context does the work of filtering the lower order harmonics apart from reactive power compensation. DSTATCOM is a Voltage Source Converter (VSC) based Flexible AC Transmission Systems (FACTS) device. When operated in a current control mode, it can improve the quality of power by eliminating harmonic content of load, balancing source currents for unbalanced loads apart from mitigating the poor load power factor [4]-[6]. As the power rating of DSTATCOM increases, high voltage switching devices have to be used to reduce its current rating. High voltage switching devices are comparatively costlier and cannot be operated with high switching frequency. To overcome these problems, several new inverter topologies have been used in high voltage FACTS, custom power equipment and industrial drives. Multilevel inverter has drawn attention of many researchers. Multilevel converters based on neutral point clamped philosophy and cascaded H-Bridge converters are widely used for high power conditioning applications [7]. In this paper, a cascaded H bridge inverter based DSTATCOM has been proposed. The implementation of H-bridge Cascaded converters for DSTATCOM applications leads to reduced injection of harmonics and low cost. The major advantages of the Hbridge cascaded converters are scalable power rating, modularity, and cost effectiveness. The output voltage of the cascaded H-bridge converter is the summation of the output voltage of the individual H-bridges [8]. By connecting a number of H-bridge converters in series, the output voltage of the converter can be increased. To achieve the quality of output waveform in the cascaded H bridge converters same as its individual counterpart, the switching frequency of the converters can be decreased. The decreased switching frequency results in reduction of switching losses as well. Sine PWM technique is used for generation of PWM pattern of switching pulse. The effectiveness of reactive power compensation by DSTATCOM has been verified through simulation by using SimPower System of MATLAB/ Simulink. The paper has been organized as eight sections. In Section II, operating principle of DSTATCOM is summarized. The third section illustrates the working principles of H Bridge VSC, fourth about the Sine PWM generation, fifth section deals with the control methodology for power factor compensation and sixth section describes the performance evaluation of reactive power compensation and the seventh section gives the conclusion.
2. Proposed DSTATCOM The schematic diagram of the proposed three leg VSC based three phase three wire DSTATCOM is shown in Figure 1. The loads are connected at the Point of Common Coupling (PCC). The DSTATCOM can be operated either for power factor correction or to regulate the voltage at the PCC to the reference value. It is well known that the PCC is the point at which the different loads and compensating devices are connected. The source current is controlled to be in-phase with the PCC voltage, when it is operated in the power factor correction mode. The source power has two components,
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viz., instantaneous active power and instantaneous reactive power. Similarly, the load power and inverter power also comprise of active and reactive components. For full reactive power compensation of the load, the inverter has to supply reactive power of the same magnitude, but of opposite sign. Thus, in such a case, the reactive power drawn from the source is zero. Hence, if reactive power supplied by the source is monitored and controlled in closed-loop to maintain it at zero, then the desired objective of full VAR compensation or unity power factor (UPF) can be achieved.
Figure 1 schematic diagram of the proposed DSTATCOM
Figure 2 phasor diagram for UPF operation
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As shown in figure 2 reactive component of the load current Il is exactly out of phase with DSTATCOM current Ish for UPF operation. In the phasor diagram Vt represents the terminal voltage at PCC and Is is the source current and Rs, Xs are the line parameters of the system and θ is the angle between terminal voltage Vt and source current.
3.Cascaded H-Bridge Converter A cascaded H-bridge converter consists of a series of single-phase full-bridge inverters per phase. The general function of the cascaded H-bridge converter is to synthesize a desired voltage from several dc sources. Figure 3 shows single-phase structure of the cascaded inverter with two dc sources. One separate dc source is connected to each single phase full-bridge inverter. For DSTATCOM applications, the DC link capacitor voltage of each H-bridge (Vdc) can be maintained at desired level by a closed loop control to draw power from the grid. By connecting the dc source to the ac output side by different combinations of the four switches S1, S2, S3 and S4 each inverter level can generate three different voltage outputs, +Vdc, 0 and-Vdc as shown in figure 4. To obtain +Vdc switches S1 and S4 are turned on and turning on of switches S2 and S3 yields-Vdc. By turning on of S1 and S2 or S3 and S4 the output voltage is 0. The ac output of each of the full bridge inverters is connected in series such that the synthesized voltage waveform is the sum of the individual inverter outputs. The number of output phase voltage levels m in a Cascaded Inverter is defined by m=2s+1; where s is the number of dc sources. For instance, two H bridges in series yields five level of voltage in the output waveform as shown in figure 4.
Figure 3 single phase structure of a multilevel cascaded H bridge inverter of DSTATCOM
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Figure 4 output voltage waveform for a 5 level H-bridge inverter.
A three phase cascaded H-bridge converter consists of three numbers of series connected H bridge inverters, connected in star or delta. The star connected cascaded H-bridge converters are popular because of higher output voltage and better waveform quality compared to delta connected scheme with same numbers of converter units per phase. Figure 4 shows the output voltage waveform for 5 level H bridge inverter and gating pulse requirement of five level output.
4. Generation of Sinusoidal PWM Pulses The most popular method of generating the PWM pattern of switching pulses for cascaded H-bridge converter is the Sinusoidal PWM method. In this method, the PWM pulses are generated by comparing the sinusoidal modulating signal with triangular carrier wave. The gating signals for a pair of devices S1 &S4 of an H-Bridge converter are obtained by comparing the sinusoidal modulating signal with triangular carrier wave. For another pair of devices S2, S3 gating signals are obtained by comparing the inverse of the sinusoidal modulating signal with triangular carrier wave. The frequency of modulating signal is kept at 50 Hz and that of carrier signal is kept at 1080 Hz.
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5. Control of DSTATCOM The reactive power control at PCC is obtained by taking feedback of the system line current. The synchronous reference frame theory is used for the control of VSC of the DSTATCOM. A block diagram of the control algorithm is shown in Figure 5. The load currents (iLa, iLb, iLc), the PCC voltages (vta, vtb, vtc) and DC bus voltage (Vdc) of DSTATCOM are sensed as feedback signals. The load currents in the three phases are converted into the dq0 frame using the Park’s transformation. A three phase PLL (Phase Locked Loop) is used to synchronise these signals with the PCC voltage. The DC bus voltage is measured and compared with a reference voltage.The error between the reference DC voltage and the sensed DC bus voltage of DSTATCOM is given to a PI (proportional-integral) controller which in turn gives the reference voltage in d axis Vdref.
Figure 5 control algorithm for H bridge inverter based DSTATCOM.
The three phase inverter currents (isha, ishb, ishc) are converted into equivalent direct axis and quadrature axis component currents (id, iq) by using abc to dqo transformation. In order to maintain the reactive power drawn from the source as zero, the output currents of the cascaded H-bridge inverter are controlled in such a way that the inverter supplies the required reactive power for the load. Thus for a unity power factor at source end, the load reactive power sets the reference for inverter control which sets iq reference as iqload. The reactive current supplied by the inverter iq is compared with the reference value iqrefr to obtain the error in reactive current for full compensation. This error signal is passed through a PI controller block to obtain the reference voltage signal Vqref. The voltages Vdref and Vqref are fed to the dqo-abc
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transformation block which act as reference voltages Via, Vib, Vic for PWM signal generators of cascaded H bridge converter. These signals are compared with a triangular carrier wave to obtain PWM signals for the switches employed in each of the Cascaded H-bridge inverter phases, respectively.
6.Simulation results and analysis The power, control circuit of DSTATCOM has been modeled and simulated using SimPower System of MATLAB/ Simulink. The power system network has been modeled as an AC source feeding inductive load. The simulink model of DSTATCOM controlled power system taken up for study is shown in figure 6. As depicted in figure 7 each phase of the DSTATCOM consists of two H-Bridge inverter connected in series. Figure 8 and 9 show the control circuit for balancing the dc link voltage and figure 9 shows reactive power compensation respectively. The DSTATCOM is controlled in such a way that the source current Is is always in phase with the PCC voltage Vt. To accomplish this, the load current is continuously monitored and the H Bridge VSC has to supply or absorb reactive power based on the nature of the load.
Figure 6 simulink model of DSTATCOM controlled three phase distribution system.
Simulation results have been obtained to validate that the DSTATCOM employed with Sine PWM switching pattern technique operates in UPF mode. From figure 10, it has been found that the load current Il lags behind the load voltage Vl by 90o as the load is inductive. The DSTATCOM draws a leading current Ish of 90o with respect to the load voltage Vl as illustrated in figure 12. Hence it is observed that net reactive power drawn by the load from the source is zero. As a result, the PCC voltage and source current are in phase with each other irrespective of the nature of the load as portrayed in figure 11. Owing to this reason, the source operates at unity power factor where as the load exhibits a lagging power factor of 0.73. Figures 13 and 14 show the real and reactive power of load and source respectively. It has been ascertained from the figures 14 and 15 that the real power of the load and source remains the same. The
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reactive power drawn by the load from the source is zero as depicted in figure 14. At PCC the power factor is unity as depicted in figure 15. It reveals that the entire reactive power demand of load is supplied by DSTATCOM. The table I summarizes the performance of power system controlled by DSTATCOM. Figures 16 and 17 show the output voltage of DSTATCOM and the gating pulses of switches employed in cascaded H bridge inverters. The gating pulses for switching are derived from the sine PWM technique.
Figure 7 H Bridge circuit of individual phases of DSTATCOM
Figure 8 control circuit for balancing dc link voltage
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Figure 9 control circuit for reactive power compensation 400
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Figure 10 load voltage and load current voltage current
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Figure 11 PCC voltage and source current
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Voltage Current
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Figure 13 load active and reactive power 5
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Figure 14 active and reactive powers at PCC
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Load Power factor
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Figure 15 load and source power factor 600
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Figure 16 output voltage of H bridge inverter
Figure 17 switching pulse generation for one switch
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Krishna Kumar. S and Chandramohan.S Table I power system performance with DSTATCOM System 400 volts (rms) three phase 50Hz ac supply parameters load source Real power 75 KW 75KW Reactive power 80 KVAR 0 Power factor 0.728 (Lag) 0.99
VII. Conclusion In this paper, the performance of DSTATCOM employed with H-bridge VSC has been demonstrated for reactive power compensation in a three-phase, three-wire distribution system using MATLAB/ SIMULINK. The simulation results exhibits that the DSTATCOM cater the entire reactive power needs of the load. Owing to this reason, with proposed DSTATCOM in the network, the power factor at PCC is unity. At the outset, it has been concluded that the DSTATCOM built with cascaded H bridge converter is a better option for the power factor correction in the distribution system.
Appendix A System parameters Rated source voltage: 400 volts (rms value). Supply frequency: 50 Hz Source parameters Rs = 0.605; Ls = 3.85 mH Compensator parameters Vdc = 500 V, Cdc = 1000µF, PI controller for dc capacitor voltage balancing: Kp= 0.05; Ki =.01 PI controller for Reactive power compensation of DSTATCOM: Kp= 2.5; Ki =.01 Inductive load: 1+ j 0.942
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