ÇUKUROVA UNIVERSITY INSTITUTE OF NATURAL AND APPLIED ...
ÇUKUROVA UNIVERSITY INSTITUTE OF NATURAL AND APPLIED ... ÇUKUROVA UNIVERSITY INSTITUTE OF NATURAL AND APPLIED ...
6. CONCLUSION Mustafa İNCİ 6. CONCLUSION The most severe power quality problems in electrical systems are called as voltage sag and swell. Dynamic Voltage Restorer (DVR) is an effective solution to solve these power quality problems. Dynamic voltage restorer is a series connected device located between sensitive/nonlinear load and grid in system, it both detects voltage sag/swell problems and injects controlled voltage to system. In this study, multilevel inverter based DVR with DC-DC converter is modeled using PSCAD/EMTDC. The proposed DVR is designed for medium voltage level (11 kV) system. DVR in a three phase system is designed to protect 1 MVA nonlinear load. The voltage sags with phase 12◦ jump are generated by controlled short circuit impedance in the grid. The basic elements and trends in literature are described in Chapter 3. The design parameters for a DVR are given in Chapter 4. Control strategies are also explained in this chapter. Two voltage injection strategies (Presag, Inphase) are implemented and tested. When phase jump occurs in system, In-Phase and Pre-sag methods show similar performance. If phase between source-side and load-side consist of phase jump, Pre-Sag compensation method represents better performance than in-phase compensation method. SRF based control technique is used to detect and extract the PQ disturbances in system. Also, EPLL and SOGI-PLL are used to detect and extract the voltage sag and swell. SOGI-PLL is a new method to extract voltage magnitude and phase angle simultaneously. The comparison results of EPLL, SRF and SOGI-PLL are presented in simulation results. The accuracy and dynamic operation of dynamic voltage restorers is an important issue. In available literature, there are two voltage control systems used in DVR applications: open loop and closed loop. Error signals are obtained by using closed loop and open loop voltage control strategies in proposed study. It is observed that rms characteristics in closed loop system have more smooth shape than injected voltage in open loop system. 109
6. CONCLUSION Mustafa İNCİ The inverter circuit in DVR is used to inject controlled voltage and maintain the desired output voltage. The most common inverter topologies in literature are the two- or three-level three-phase converter used in DVR. For high power applications, the use of two-level voltage converters becomes difficult to perform because of switch ratings and efficiencies. To prevent this condition, the symmetrical five level diode-clamp inverter is selected in proposed DVR. Symmetrical five level diodeclamped inverter consists of two single phase three level diode clamped inverter for each phases. In this wise, it can compensate balanced and unbalanced voltage sag/swell. It has advantages compared with cascade and diode clamped mutilevel inverters such as reduction the quantity, size and dimension of dc-link capacitors with lower cost. Sinusoidal Pulse Width Modulation (SPWM) based control scheme is chosen for the proposed multilevel inverter, two carrier based modulation technique for each diode clamped inverter has been presented and explained in this thesis. DC link voltage is an important issue when voltage sag occurs. To keep dc link voltage constant and to compensate deep and long duration voltage sag, DC-DC converter is employed. Full-Bridge isolated DC-DC converter is used in proposed DVR. The controller design procedures of the full bridge isolated dc/dc converter are presented. The simulation results show its effectiveness in DC link capacitor and keep it constant. In Chapter 6, the simulation results of proposed DVR are presented. System is constructed in PSCAD/EMTDC. Firstly, simulation is performed for different sag detection methods, voltage control strategies and voltage injection techniques. Secondly, simulation results are initiated for different voltage sag cases using open loop and closed loop voltage control methods. Source-side, load-side, injected voltages and DC-link voltages are given in simulation results. The proposed system shows that sag compensation is restored successfully. Voltage sags are the most important power quality problems in industrial areas. DVR is the most effective solution to compensate the disturbances. To reduce cost and improve performance of DVR, studies continue in the following topics. 110
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6. CONCLUSION Mustafa İNCİ<br />
6. CONCLUSION<br />
The most severe power quality problems in electrical systems are called as<br />
voltage sag and swell. Dynamic Voltage Restorer (DVR) is an effective solution to<br />
solve these power quality problems. Dynamic voltage restorer is a series connected<br />
device located between sensitive/nonlinear load and grid in system, it both detects<br />
voltage sag/swell problems and injects controlled voltage to system.<br />
In this study, multilevel inverter based DVR with DC-DC converter is<br />
modeled using PSCAD/EMTDC. The proposed DVR is designed for medium<br />
voltage level (11 kV) system. DVR in a three phase system is designed to protect 1<br />
MVA nonlinear load. The voltage sags with phase 12◦ jump are generated by<br />
controlled short circuit impedance in the grid.<br />
The basic elements and trends in literature are described in Chapter 3. The<br />
design parameters for a DVR are given in Chapter 4. Control strategies are also<br />
explained in this chapter. Two voltage injection strategies (Presag, Inphase) are<br />
implemented and tested. When phase jump occurs in system, In-Phase and Pre-sag<br />
methods show similar performance. If phase between source-side and load-side<br />
consist of phase jump, Pre-Sag compensation method represents better performance<br />
than in-phase compensation method.<br />
SRF based control technique is used to detect and extract the PQ disturbances<br />
in system. Also, EPLL and SOGI-PLL are used to detect and extract the voltage sag<br />
and swell. SOGI-PLL is a new method to extract voltage magnitude and phase angle<br />
simultaneously. The comparison results of EPLL, SRF and SOGI-PLL are presented<br />
in simulation results.<br />
The accuracy and dynamic operation of dynamic voltage restorers is an<br />
important issue. In available literature, there are two voltage control systems used in<br />
DVR applications: open loop and closed loop. Error signals are obtained by using<br />
closed loop and open loop voltage control strategies in proposed study. It is<br />
observed that rms characteristics in closed loop system have more smooth shape than<br />
injected voltage in open loop system.<br />
109
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