Design and Development of Advanced Control strategies for Power Quality Enhancement at Distribution Level

Abstract

In recent times, power quality (PQ) issues such as current and voltage harmonics, voltage sag/swell, voltage unbalances have become the important causes for malfunctioning and degradation of the quality of power. Poor power quality severely affects on electrical equipment and finally results in significant economic losses. Hence, installation of the custom power devices to improve the power quality issues becomes an important consideration. Therefore, this thesis considers the enhancement of power quality for power distribution systems by utilizing unified power quality conditioner (UPQC). An UPQC can adequately handle several power quality problems such as load current harmonics, supply voltage distortions, voltage sags/swells and voltage unbalance. Therefore, the main focus behind this thesis is to develop advanced control strategies that improve the compensation capability of the UPQC so that power quality issues of distribution network are efficiently improved. Firstly, the current harmonics are considered and are compensated by using the shunt active power filter (SAPF). Therefore, two control strategies such as Hysteresis current control (HCC) and Sliding Mode Control (SMC) based control algorithms are implemented to compensate current harmonics in the power distribution network. Furthermore, both the current control techniques utilize the Coulon oscillator based PLL (CO-PLL) for extraction of positive sequence signal from the supply voltage and generate the three-phase reference currents by employing PI-controller based DC-link voltage regulation. The performances of both current control techniques for SAPF are evaluated under different source voltage conditions such as balanced, unbalanced and non-sinusoidal. The SAPF effectively compensates currents harmonic, however, it is unable to compensate voltage related problems. To overcome this drawback, this thesis considers the UPQC, which comprises with shunt APF and series APF, can be utilized to compensate both current and voltage related problems. The research on UPQC is carried out progressively by considering different advanced control strategies. Each progress in the research enhances the compensation capabilities of the previous UPQC control system. The simulation and realtime Opal-RT studies are carried out to verify the operating performance of each design concept of UPQC.
At first, operating principle and design of UPQC is presented and then a novel control algorithm is introduced with the aid of nonlinear DC-link voltage controller such as nonlinear variable gain fuzzy (NVGF) controller and nonlinear sliding mode controller (NLSMC) with modified synchronous reference frame (SRF) control strategy for improvement of both current and voltage compensation performance of the UPQC. However, existence of large settling time in dc voltage leads to poor dynamic performance of NVGF control technique and hence current harmonics, voltage distortions and voltage disturbance such as voltage sag/swell as well as voltage unbalance compensation capability of this technique is not quite effective in comparison to the NLSMC technique. Moreover, NLSMC is very sensitive to model mismatch and noise. It is quite sluggish in rejecting long drifting grid disturbances. Hence, a suitable control strategy has to be developed in UPQC, which has improved DC-link voltage regulation as well as tracking performance through load and grid perturbations. To overcome this drawback a resistive optimization technique (ROT) incorporated with enhanced phase-locked loop (EPLL) based NVGF hysteresis control strategy and an optimum active power (OAP) technique combined with enhanced phase-locked loop (EPLL) based fuzzy sliding mode (FSM) pulse-width modulation (PWM) control strategy for UPQC have been discussed. ROT-NVGF and OAP-FSMC based UPQC control strategies are adaptive as well as robust and able to mitigate the PQ problems satisfactorily during all dynamic conditions of power system perturbation. However, performances of these controllers are not effective when there is a variation occurring either in the nonlinear load parameter or supply voltage parameter. Thus, UPQC may not be able to compensate PQ problems satisfactorily. Considering aforesaid problems, this thesis proposes a command generator tracker (CGT) based direct adaptive control (DAC) applied to a three-phase three-wire UPQC to improve the current and voltage harmonics, sag/swell and voltage unbalance in the power system distribution network. CGT is a model reference control law for a linear timeinvariant system with known coefficients and is formulated for the generation of reference signal for both shunt and series inverter. The main advantage of the proposed control algorithm is that no online extraction is needed to perceive the UPQC parameters. Moreover,
IV the adaptive control law is designed to track a linear reference model to reduce the tracking error between model reference output and measured signal to be controlled. Additionally, this proposed algorithm adaptively regulates the DC-link capacitor voltage without utilizing additional controller circuit. As a result, the proposed algorithm provides more robustness, flexibility and adaptability in all operating conditions of the power system network. At last, model reference robust adaptive control (MRRAC) technique is proposed for single phase UPQC system. This control strategy is designed with the purpose of achieving high stability, high disturbance rejection and high level of harmonics cancellation. From simulation results, it is not only found to be robust against PI-controller, but also satisfactory THD results have been achieved in UPQC system. This has motivated to develop a prototype experimental set up in the Laboratory using FPGA (Field Programmable Gate Array) based NI (National Instruments) cRIO-9014. From both the simulation and experimentation, it is observed that the proposed MRRAC approach to design a UPQC system is found to be more effective as compared to the conventional PI-controller.