Study Of Voltage And Frequency Control For Island Power Systems With Voltage-Dependent Loads

With the rapid modern economic-social development, island power systems have been built to meet the need of increasing customer demand, industry production, and sustainable energy integration, which are important parts of smart grids in the future. However, due to the simple configuration, limited controls, and no support from large-scale public power grids, the stability of island power systems has become a challenging issue that needs to be addressed. In order to keep the stability of island power systems, it is essential to design effective and applicable control schemes.This thesis focuses on the voltage and frequency control of island power systems with voltage-dependent loads. The voltage-dependent load represents a load whose power response depends more on voltage than frequency. Such loads widely exist in industries and customer demand, such as aluminum electrolysis loads, electric heats, lighting loads, etc. In this thesis, an actual island power system for aluminum production is used as an example to illustrate voltage and frequency control schemes for island power systems with voltage-dependent loads. These shemes are useful and helpful to maintain the stability of actual island power systems. The details are given as follows.(1) A dynamic voltage control for island power systems with voltage-dependent loads in a WAMS (Wide-Area Measurement System)-based framework is proposed. The control is able to maintain stable post-fault voltages and improve system fast voltage dynamics. A reduced and equivalent system model has been established using wide-area measurements. By applying Pade approximation, the time delay of a WAMS is compensated. And then, the voltage control problem is formulated as a linear quadratic regulator (LQR) problem. Furthermore, a simple voltage control scheme is induced form the LQR theory. The WSCC 9-bus system as well as the actual island power system are used to verify the effectiveness of the proposed scheme.(2) A coordinated static voltge control for island power sytems is proposed. The control can coordinate the short-term voltage controls and long-tenn voltage controls to optimize the system reactive power dispatch and improve the generator fast reactive power control capability. Firstly, a static voltage control model is established with the objective of minimizing grid losses and voltage deviations, and increasing the generator reactive power reserves. And then, the control problem is formulated as a programming problem including continuous variables and discrete variables. To solve the programming problem, an improved nich genetic algorithm (INGA) is proposed to obtain the voltage control scheme. IEEE 57-bus system is used to verify the well performance of the ING A and the actual island power system is employed to validate the effectiveness of the proposed scheme.(3) An emengency voltage and frequency control for island power systems with voltage-dependent loads is proposed. This control is aimed at designing a generator excitation voltage control scheme for emergency frequency regulation, which is able to regulate the load voltages and their power demand to maintain stable frequency when the systems have few or no active power reserves. A frequency and excitation voltage control model for the actual island power system is established. Then, by applying the output regulation theory, feedback control signals with the frequency deviations and the power imbalances are used to obtain a practical feasible control scheme whose structure is similar to that of primary and secondary frequency control. Furthermore, by taking into account the constraints of generator voltage controls and load bus voltages, two useful conditions are found to limit the power imbalances. Both the WSCC 9-bus system and the actual island power system are used to validate the effectiveness of the proposed scheme.(4) A coordinated frequency control for island power systems with high DFIG-based wind power integration is proposed to enhance the fast system frequency response and even prevent the system freqnency collapses. The DFIG operates in the deloaded mode using the rotor speed control and pitch angle control. In this mode, DFIG can maintain a certain power reserves and anticipate the fast system freqnency control. From a viewpoint of power systems, an effective DFIG model is used, and then a coordinated frequency control with thermal generators and DFIGs is established. By using the nonlinear output regulation method, a coordinated scheme with the rotor speeds of DFIGs and generator governors in low wind speeds or a shceme with the pitch angles of DFIGs and generator governors in high wind speeds are developed. The effectiveness of proposed control scheme is verified on the actual island power sytem with high wind power integration.