Simulation of Three Phase Cascaded H Bridge Inverter for Power Conditioning Using Solar Photovoltaic System

Transcription

1 Simulation of Three Phase Cascaded H Bridge Inverter for Power Conditioning Using Solar Photovoltaic System 1 G.Balasundaram, 2 Dr.S.Arumugam, 3 C.Dinakaran 1 Research Scholar - Department of EEE, St. Peter s University, Avadi, Chennai, Tamilnadu, India. 2.Principal - Department of EEE, GRT Engineering College, Tirutani, Tamilnadu, India. 3.Assistant Professor - Department of EEE, Sri Venkateswara College of Engineering & Technology, Chittoor, A.P, India. Abstract This paper focuses on the implementation of the voltage balancing problem of DC capacitors in cascade inverters for power quality conditioners. Model for charge and discharge processes of DC capacitor is established based on the point of output pulses of H-bridge inverters. In analyzing model, a novel capacitor voltage balancing method is proposed. The capacitor voltage balance scheme is achieved by shifting the switching patterns (PSPWM). The developed model has been applied to power quality conditioner and simulation results at each capacitor are observed. It proposes a novel capacitor voltage balance control technique with closed loop harmonic compensation for the cascaded multilevel inverter used in high Active Power Filter (APF). This method maintains capacitor voltage balance and good filtering performances which suitable for Power filter based on more level cascaded inverters. The performance of the proposed system is verified by Simulation using MATLAB/SIMULINK Environment. Keywords: Cascaded H Bridge Inverter, Power Quality Conditioner, Multilevel Inverter, PWM Inverter, PV System, Total Harmonic Distortion (THD). 1. Introduction Power quality problems are more due to voltage unbalances and non linear equipments. To provide high power quality at the point of common coupling (PCC) of a distribution system, power quality conditioner, including voltage regulation, reactive power and harmonic compensation is widely used and researched in the power engineering field nowadays [1]. But due to the limitation of voltage capability of the present power devices, it is very difficult to handle the nonlinear loads in high voltage grid using the traditional power quality conditioner with two-level inverter recently and multilevel technology has become an effective and practical solution for high-voltage as well as high-power application field. As described in many literatures, using multilevel technology reduces voltage stress on switches, shape of output waveform will be improved and the rate of voltage and power can be increased [2]. Due to advancement in power electronics technology, Active Power Filters (APF) continues to attract considerable attention. APF technology is the most efficient way to compensate for reactive power and neglect lower harmonics generated by nonlinear loads [3]. Harmonic currents on the power systems can distort the line voltage and lead to several adverse effects including equipment overheating, the malfunction of solid-state equipment and interference with communication systems. Some commercial APF units are currently available. Now a days a single power semiconductor switch is directly to medium voltage grids (2.3, 3.3, 4.16, or 6.9 kv). In all multilevel technology, cascade multilevel inverters have become the major circuits used in power quality conditioners at present [4]. When the cascade multilevel inverter is applied to the power quality conditioner, each of the cascaded H-bridge inverters is equipped with an isolated DC capacitor without any power source. To make the equipment work properly, each DC capacitor voltage must be maintained high enough and balance [5] [6]. 2. Multilevel Inverter Concept Multilevel inverters divide the main dc supply voltage into several smaller dc sources which are used to synthesize an ac voltage into a staircase, or stepped, approximation of the desired sinusoidal waveform. Using multiple levels, the multilevel inverter can yield operating characteristics such as high voltages, high power levels, and high efficiency without use of transformers. The multilevel inverter combines individual dc sources at specified times to yield a sinusoidal closeness by using more steps to synthesize the sinusoidal waveform, the waveform approaches desired sinusoidal and the total harmonic distortion approaches zero. Fig. 1: Multilevel Inverters classification More than the years many different Multilevel Converter topologies have been reported. They can be classify in two main groups, as shown in Fig. 1, depending on the number of independent dc sources used in their structure. The most known and conventional topologies are the Neutral Point Clamped (NPC) or diode clamped, the Flying Capacitor (FC) or capacitor clamped, and the Cascaded H-Bridge (CHB). The Cascaded H-Bridge usually fed by equal dc voltage sources have also been modified by introduces an asymmetry in the dc voltage sources. This customized version is known as the asymmetric or hybrid Cascaded H-Bridge. It generates significantly more output voltage levels than other topology using less semiconductors and capacitors. 3187

2 3. Cascaded Multilevel Inverter A cascaded multilevel inverter consists of series single phase H-bridge inverter units. The general function of this multilevel inverter is to synthesize a desired voltage from several separate dc sources which may be obtained from batteries, fuel cells, or solar cells. The ac terminal voltages of different level inverters are connected in series. Unlike the diode-clamp or flying-capacitors inverter, the cascaded inverter does not require any voltage-clamping diodes or voltage-balancing capacitors as shown in Fig. 2. The phase output voltage is synthesized by the sum of four inverter outputs, van=va1+va2+va3+va4. Each inverter level can generate three different voltage outputs, +, 0, and -, by connecting the dc source to the ac output side by different combinations of the four switches,sa1,sa2,sa3,and Sa4. Fig. 2: Three Phase- Three leg Cascaded H-bridge 4. PWM Techniques Figure 3 shows the Pulse width modulation (PWM) control strategies development concerns the development of techniques to reduce the total harmonic distortion (THD). It is generally recognized that increasing the switching frequency of the PWM pattern reduces the lower-frequency harmonics by moving the switching frequency carrier harmonic and associated sideband harmonics further away from the fundamental frequency component. While this increased switching frequency reduces harmonics, resulting in a lower THD by which high quality output voltage waveforms of desired fundamental R.M.S value and frequency which are as close as possible to sinusoidal wave shape can be obtained. The quality of the output waveform will develop with increase in switching frequency. Higher switching frequency can be employed for low and medium power inverters, whereas, for high power and medium voltage applications and switching frequency is of the order of 1 khz. Fig. 3: PWM carrier technique (triangular carrier) To compare the performance of phase and levelshifted modulation schemes, it is assumed that the average switching frequency of the solid-state devices is the same for both schemes. One of the most important problems in controlling a VSI with variable amplitude and frequency of the output voltage is to obtain an output waveform as much as possible of sinusoidal shape employing simple control techniques. Harmonics caused by non-sinusoidal voltage feeding involve power losses, electromagnetic interference (EMI), and pulsating torques in ac motor drives. Harmonic reduction can then be closely related to the performance of an inverter with any switching strategy, Line and Phase voltages are shown in Fig. 4 & 5. Fig. 4: Line Voltages 3188

3 Figure 6 shows that each phase of this equipment consists of two H-bridge inverter units., are DC flying capacitor voltages of the two H-bridge units of phase respectively. The H-bridge inverter is built-up of four switches, each switch with its freewheeling diode. The states of T1 and T3, T2 and T4 are complementary. The switch T1 is closed, while T3 is opened at every time instant. The logic configuration of switches of the H-bridge inverter can produce four switching states. The H-bridge inverter is able to provide the three different output voltage values. The general function of this cascade multilevel inverter is to synthesize a desired voltage from output voltage of each H-bridge inverter of each phase. 6. Power Filter Topologies 5. Power Quality Fig. 5: Phase Voltages Power system is designed to withstand outages by using lightning arresters, breakers and disconnect switches, and redundancy. The main concern was to prevent the frequency of the power system from deviating 60 Hz during outages. More devices were utilized to maintain the reliability of the power system. For example, if an outage of a major transmission line caused a large load to be dropped, there was involvement about the generator running away and the frequency increasing above acceptable limits. Then, the whole power system would collapse. Large dynamic breakers consisting of many stainless steel wires were utilized to keep the generators from spinning out of control. The block diagram of cascaded multi-level inverter with power system is shown in Figure 6. In cascaded multilevel inverter is connected at input supply of the system. Two comparators are used in the firing circuit, one is for 3-phase sine-wave reference signal and the other is for phase shifted carriers. By using of this combination total 24 pulses is generated, according this 24 switches are required to generate these pulses. The easiest method of harmonic filtering is with passive filters. It uses reactive storage components, namely capacitors and inductors. Among the more commonly used passive filters are the shunt-tuned LC filters and the shunt low-pass LC filters and its Advantages such as simplicity, reliability, efficiency, and low cost. The main disadvantages are the resonances introduced into the ac supply and filter effectiveness. These drawbacks are overcome with the use of active power filters. A. Shunt Active Filters The shunt-connected active power filter with a selfcontrolled dc bus has a topology similar to that of a static compensator (STATCOM) used for reactive power compensation in power transmission systems. Fig. 7: Shunt Active power filter topology Fig. 6: Schematic diagram of power quality conditioner with cascade multilevel inverter Figure 7 shows the Shunt active power filters are compensate load current harmonics by injecting and opposite harmonic compensating current. In this case the shunt active power filter operates as a current source injecting the harmonic components generated by the load but phase-shifted by 180. Figure 8 shows how the active filter works to compensate the load harmonic currents. 3189

4 C. Series Shunt Active Filters The series-shunt active filter is a combination of the series active filter and the shunt active filter as shown in Figure 11. The shunt active filter is located at the load side and can be used to compensate for the load harmonics. On the other hand, the series portion is at the source side and can act as a harmonic blocking filter. This topology has been called the Unified Power Quality conditioner. The power supplied or absorbed by the shunt portion is the power required by the series compensator. Fig. 8: Filter current generated to compensate load harmonics B. Series Active Filters The series-connected filter protects inadequate supply-voltage quality. This type of approach is especially recommended for compensation of voltage unbalances, voltage sags from the ac supply and low-power applications. It represents an economically attractive alternative to Uninterrupted Power Supply, since no energy storage (battery) is necessary and the overall rating of the components is smaller as shown in Figure 9. The series filter works to compensate the voltage harmonics on the load side as shown in Figure 10. Fig. 11: Unified power quality conditioner D. New Topologies using Multilevel Inverter Multilevel inverters are being investigated and recently used for active filter topologies. Figure 12 shows a shunt active power filter implemented with a three-level inverter. Three-level inverters are becoming very popular today for most inverter applications, such as machine drives and power factor compensators. This feature helps to reduce the harmonics generated by the filter itself. Fig. 9: Series active power filter topology with shunt passive filters Fig. 10: Filter voltage generation to compensate voltage disturbances. Fig. 12: Shunt active power filter using a three-level inverter 3190

5 7. Results The system has been tested and simulation results are shown in this section. This model has been implemented using MATLAB/SIMULINK environment with SIMPOWER system toolbox. Fig. 15: Simulation block diagram of whole system Fig. 13: Simulation diagram of power quality conditioner with cascaded multilevel inverter Fig. 16: Simulation block diagram of overall active power system Fig. 14: Simulation diagram of cascaded H-bridge five level inverter Fig. 17: Simulation Diagram of individual voltage Control loop 3191

6 Fig. 18: Input Voltage Fig. 19: Input Current Fig. 20: Load Voltage Fig. 21: Load Current 8. Conclusion This paper has been implemented for power quality conditioning using cascaded multilevel inverter. The cascaded multilevel inverter has the superior advantages over diodeclamped, capacitor-clamped multilevel inverter topologies. The modulation techniques of level PWM and PSPWM are compared. DC capacitor voltage balancing problem is examine for the power quality conditioner with cascaded multilevel inverter. The issues of voltage balance due to output pulses of H Bridge inverter and proposed new method for balancing of voltages is verified by simulation results using MATLAB/SIMULINK environment. The simulation results of cascaded multi level inverter as a power quality with PSPWM are analyzed with respect to DC capacitor voltage with control loops and without control loop. It can be conclude that using individual voltage balance loop the system will effective performance and percentage of THD source current is 2.46%, load current is 28.92% with PSPWM Technique is obtained. References [1] F.Z. Peng, H. Akagi, and A. Nabae, A new approach to harmonic compensation in power systems, a combined system of shunt passive and series active filter, IEEE Trans. Ind. Appl., vol. IA-26, pp , Nov/Dec [2] M.D. Manjrekar and T.A. Lipo, A hybrid multilevel inverter topology for drive applications, in Proc. IEEE Applied Power Electronics Conf., 1998, pp [3] J. Dixon and L. Morán, Multilevel inverter, based on multi-stage connection of three-level converters, scaled in power of three, in Proc. IEEE 2002 Industrial Electronics Conf., IECON-02, Sevilla, Spain, 5 8 Nov [4] R. S. Kanchan, et.al. Space vector PWM signal generation for multilevel inverters using only the sampled amplitudes of reference phase voltages, IEE Proc-Electr. Power Appl., Vol.152, No.2, March 2005, pp [5] Fang Zheng Peng, Jih-Sheng Lai, John McKeever and James VanCOevering, A Multilevel Voltage-Source Inverter with Separate DC Sources for for Static Var Generation. IEEE Trans Industry Applications, 1996,32(5): [6] F Z Peng, J S Lai, J W McKeever, and J VanCoevering, A multilevel voltage-source inverter with separate dc sources for static var generation. IEEE Trans Industry Applications,1996, 32(5): Fig. 22: Total Harmonic Distortion 3192