분산전원에서 전력품질 개선을 위한 UPQC 제어기법 : Advanced Control Strategies for UPQC to Improve Power Quality of Power Distribution Systems
TRINH QUOC NAM
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Recently, power quality problems such as current and voltage harmonics, voltage sag, voltage unbalance, etc have become the major reasons causing the degradation or malfunction of the electrical equipment. Poor power quality may cause various serious impacts on electrical loads and ultimately results in huge economic losses. Therefore, installing extra power filters or custom power devices to deal with power quality problems become a mandatory requirement. This thesis deal with the improvement of power quality for power distribution systems by using unified power quality conditioner (UPQC). A UPQC can sufficiently deal with various power quality issues such as supply voltage distortions, voltage sags, voltage unbalance, and load current harmonics. The main purpose of researches in this thesis is to develop advanced control strategies that enhance the compensation performance of the UPQC so that power quality of distribution system is effectively improved. Firstly, the current and voltage harmonics are considered and compensated by using the UPQC. The final control target is to maintain the load voltage and the supply current sinusoidal despite the distorted supply voltage and the use of nonlinear load. In order to compensate current harmonics caused by nonlinear loads, an advanced control strategy for shunt APF is introduced. The proposed current controller, operated without load current measurement and harmonic detectors, is designed in the synchronous (d-q) reference frame, composed of a proportional-integral (PI) and five vector PI (VPI) controllers tuned at 6n (n = 1...5) of the fundamental frequency. The source current is effectively compensated to be almost pure sinusoidal with a low total harmonic distortion despite the high distortion of the nonlinear load current. The shunt APF sufficiently compensates harmonic currents, but it is unable to deal with voltage distortion problems. To overcome this limitation, the UPQC consisting of series APF and the shunt APF is employed. The proposed control strategy is also designed in the d-q reference frame and developed in both the series APF and shunt APF of the UPQC. In the series APF, a PI plus a resonant controller tuned at sixth of the fundamental frequency are performed to compensate harmonics in the distorted source voltage. Meanwhile, a PI plus three resonant controllers tuned at 6n(n=1, 2, 3) of the fundamental frequency are designed in the shunt APF to mitigate harmonic currents produced by nonlinear loads. The control performance of the UPQC is significantly improved thanks to the effective design of resonant controllers. The load voltage and supply current are effectively compensated to be almost pure sinusoidal regardless of the high distortion of the supply voltage and the load current. The resonant control strategy is effective but the digital implementation of resonant controller is quite complex due to trigonometric functions. In addition, we regularly need to use a large number of resonant controllers to compensate all selected harmonic components. In order to overcome the difficulty and complexity in digital implementation of multiple resonant controllers, this thesis introduces a simplified and effective UPQC control system by using a single repetitive controller (RC) instead of multiple resonant controllers. In this thesis, the conventional RC is modified suitable for the UPQC application: time delay is reduced by six times and the frequency adaptive function is added. In proposed control algorithm, resonant controllers in both the voltage and current controller are replaced by a RC. One single RC designed with the delay time of one-sixth of the fundamental period can sufficiently replace a bank of resonant controllers tuned at 6n (n=1, 2, 3...) of the fundamental frequency. As a consequence, the control strategy can maintain a good compensation performance of the UPQC with a simplified controller configuration. In addition, a frequency-adaptive scheme is proposed to ensure a robust operation and an excellent performance of the UPQC under the grid frequency deviations. In addition to the current and voltage harmonics problems, this thesis takes into account the voltage sag/swell and voltage unbalance at the supply side. Under such conditions, to maintain the load voltage undisturbed from voltage sag/swell and unbalance, an appropriate compensation algorithm should be developed in the series APF of the UPQC. The voltage sag/swell and unbalance are compensated by adopting a resonant controller tuned at fundamental frequency in the voltage control scheme of the series APF. Meanwhile, the current controller in the shunt APF is maintained as previous control algorithms to deal with current harmonic problems. As a result, the load voltage is maintained undisturbed at the nominal value regardless of the severe conditions at the supply voltage and various kind of power quality problems are effectively compensated with a unified control algorithm.