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3. FUNDAMENTALS <strong>OF</strong> DVR Mustafa İNCİ 3.2.1. Operation of DVR Dynamic voltage restorers (DVRs) are considered effective custom power devices for mitigating the impacts of upstream voltage disturbances on sensitive loads. The disturbances include voltage distortions and/or sudden changes in loadterminal voltage, in the form of sags or swells. By injecting a compensating voltage in series with the sensitive load terminal voltage during the disturbances, a DVR can maintain the load voltage at the desired amplitude and waveform. In the course of the compensation, however, a DVR will inevitably inject (absorb) a certain amount of active power to (from) the external system. The amount of the power exchange will dictate the severity of the sag/swell that can be ridden through by the load and the rating of the energy storage device required to undertake the task. In fact, by choosing an appropriate amplitude and phase angle of the DVR output injection voltage, one can control the injected/absorbed power such that compensation with zero or minimum power injection can be realised. This means either the minimum power-rating energy storage device can be incorporated into the design or the maximum load ride-through can be achieved with the given energy-storage capacity (Li et al., 2007). The intention is only to protect one consumer or a group of consumers with value added power. Applying a DVR in the medium or low voltage distribution system would often be possible and a radial grid structure is the only type of system considered here. In Europe three wire systems are common in the medium voltage systems and four wires in low voltage systems. In both systems the main purpose is to inject synchronous voltages during symmetrical faults and in some cases inject an inverse voltage component during non-symmetrical faults (Oğuz et al., 2004). A typical DVR connected system circuit at medium voltage (MV) distribution network is shown in Figure 3.2. The DVR essentially consists of a series connected injection transformer, a voltage source inverter (VSI), inverter output filter and an energy storage device connected to the dc link. The high voltage (HV) power system upstream to the DVR is represented by an equivalent voltage source which is transformed to MV level by a step-down transformer. The basic operation principle 15
3. FUNDAMENTALS <strong>OF</strong> DVR Mustafa İNCİ of the DVR is to inject an appropriate voltage in series with the supply through injection transformer when a PCC voltage sag is detected. MV loads or low voltage (LV) loads connected downstream after another step-down transformer are thus protected from the PCC voltage sag (Li et al., 2007). Figure 3.2. DVR connected in a medium voltage level power system Implemented at medium voltage level, the DVR can be used for high power applications or to protect a group of MV or LV consumers who would expect reduced costs per MVA by operating at MV level. But this implementation at medium voltage level also subjects the DVR to more frequent faults in the downstream load side. Large fault currents will flow through the DVR during a downstream fault before the opening of a circuit breaker. This large fault current will cause PCC voltage drop, which would affect the MV or LV loads on the other feeders connected to PCC (Figure 3.2). Furthermore, if not controlled properly, DVR might also contribute to the PCC voltage sag in the process of compensating the missing voltage, thus further worsening the fault situation. During downstream faults, passive control methods are often used to protect the DVR by enabling a bypass circuit (usually a slow mechanical bypass together with a fast solid-state switch), while allowing a large fault current to flow which could cause PCC voltage to drop. Compared to the passive protection of DVR, active control of DVR during a downstream fault makes the additional protection circuits unnecessary and the implementation easy. But the disadvantage is the requirement of higher compensation capacity from the DVR (Li et al., 2007). 16
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Page 3 and 4: ABSTRACT MSc THESIS MODELING AND AN
Page 5 and 6: ACKNOWLEDGEMENTS First and foremost
Page 7 and 8: 3.2.2.1. Inverters ................
Page 9 and 10: VII
Page 12 and 13: LIST OF FIGURES PAGES Figure 1.1. D
Page 14 and 15: Figure 5.3. Injected voltages by us
Page 16 and 17: LIST OF SYMBOLS C : DC Link Capacit
Page 18 and 19: LIST OF ABBREVATIONS A AC APF ASD C
Page 20: 1. INTRODUCTION Mustafa İNCİ 1. I
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Page 25 and 26: 2. POWER QUALITY Mustafa İNCİ 2.1
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4. MODELING OF PROPOSED DVR Mustafa
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5. SIMULATION RESULTS AND CASE STUD
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6. CONCLUSION Mustafa İNCİ 6. CON
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6. CONCLUSION Mustafa İNCİ • En
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DELFINO, B., FORNARI, F., PROCOPIO,
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KARIMI-GHARTEMANI M., IRAVANI M.R.
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OĞUZ, G., 2004. Performance Of A D
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TESTA A., AKRAM M.F., BURCH R., CAR
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BIOGRAPHY Mustafa İNCİ was born i
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APPENDIX 123
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LIST OF PUBLICATIONS International
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