Novel Control Strategies For Dynamic Reactive Power Compensation

The insufficient reactive power compensation is causing ever-increasing detrimental effects for the modern electrical power system. There are many available methodologies to regulate reactive power flow in electric power system, including synchronous condensors, thyristor switched capacitors (TSCs) and static synchronous compensators (STATCOMs). This paper focuses on the study of single phase TSC, three single-phase STATCOM and three-phase four-wire STATCOM.Firstly, the new TSC scheme based on detuned resonant LC filter banks is studies in this paper. The reactive power detection method, capacitor bank switching control strategies are presented consecutively, which avoids the switching oscillation and abnormal switching problems related to the conventional TSC control schemes. The validity of the proposed switching strategies is verified by the simulation results obtained by Matlab/Simulink. The experimental results further confirm the simulation results, which demonstrate that surge-free capacitor switching is achieved, while reactive power compensation and harmonic currents mitigation can be realized simultaneously.The TSCs have the drawbacks of slow dynamic response and poor accuracy for reactive power compensation. These shortcomings are overcome by STATCOMs which can provide continuous reactive power compensation, which has the capability to inject or absorb reactive currents. In this paper, two types of STATCOM topologies, namely, three single-phase STATCOM and three-phase four-wire STATCOM are investigated.In order to overcome the deficiencies of the poor harmonic detection precision and the unsatisfactory performance of the conventional double closed-loop proportional-integral controllers, a new current-loop controller is proposed for the three single-phase STATCOMs, which combines the conventional PI regulators and the resonant controllers. Extensive simulation results under various load disturbance scenarios demonstrate that the proposed control strategy has excellent performances with improved harmonic rejection capability and perfect grid-side currents.As for the three-phase four-wire STATCOM topologies, the three-dimensional pulse-width modulation scheme is adopted and the mathematical model in stationaryα-β-0 coordinates is derived. Besides, the perfect harmonic compensation (PHC) strategy is proposed to ensure stable operation of the STATCOM under grid voltage unbalance and/or harmonics. The decoupled double synchronous reference frame phase-locked loop (DDSRF-PLL) is utilized for phase synchronization of the STATCOM. By decoupling the vector of grid voltages into positive- and negative-sequence components, fast positive-sequence component estimation for the grid voltages can be achieved, even under severe grid voltage unbalance and/harmonics. The simulation results demonstrate that the three-phase four-wire STATCOM with DDSRF-PLL can provide accurate compensation performance for the unbalanced load by compensating the zero-sequence and negative-sequence components of load currents, and also provide satisfactory performance under grid voltages unbalance and/or harmonics.Finally, a cost-effective soluation for reactive compensation is proposed by the combined TSC and STATCOM methodology. The coordinated operation of these two devices is realized by properly selection of the reference compensation currents for individual device. The validity and effectiveness of the proposed compensation scheme is verified by the simulation results obtained from Matlab/Simulink.