Research On AC/DC Power System Restoration Strategies

In recent years, power systems have increased in both size and complexity due to the rapid growth of interconnections and widespread application of long distance HVDC transmission. Meanwhile, with development of power market and extensive operation of economic dispatch, power systems tend to be operated closer and closer to their stability limits. These factors have issued big challenges to system secure operation so that bulk power systems are more vulnerable to potential disturbances and even blackouts. In order to minimize adverse impact on the public, the system needs to be restored to normal operation as quickly and reliably as possible in case of a complete or partial blackout. This dissertation mainly deals with the voltage and frequency control problems during power system restoration process. The main work is organized as follows:Modeling and simulation issues pertaining to system restoration process are addressed, which covers various stages of restoration including the static, transient and dynamic behavior of power system during restoration. Models for cold load pickup and generation ramping process of generators are developed. Simulation studies are carried out for energization of transmission lines and transformers, start-up of auxiliary motors and generation ramping process, which are indispensable for development of power system restoration plan and real system restoration.A sensitivity analysis based method is presented for control of power frequency overvoltages caused by energization of lightly loaded transmissions lines during early restoration stages. The efficient control variables are identified by means of sensitivity analysis to ensure that the minimum shifts are needed for control of overvoltages. Efficiency Coefficient (EC) is defined to take into account both voltage sensitivity to control variables and other factors including equipment limitations, system operation constraints and current adjusting margins as well. The control variables selected according to their ECs guarantee that no new voltage violations will be generated during the control process.The harmonic overvoltages originating from switching of the nonlinear devices such as transformers are of great concern during early stages of power system restoration. Based on detailed analysis on the cause of harmonic overvoltages, control of harmonic overvoltages is accomplished through control of the transformer inrush current and improvement of the system frequency response characteristics. Sensitivity analysis is performed to find the most efficient loads for improvement of the system frequency response characteristics so that the harmonic overvoltages can be controlled with the minimum loads and operations. Weighting factors are introduced to take into consideration the harmonic characteristics of the transformer inrush current so as to get sensitivity of the harmonic overvoltages to system load with enough accuracy. In order to reduce the large amount of simulation time, the energy of the signal is applied to predict the tendency of the voltage waveform and an algorithm is developed to terminate the electromagnetic transient simulation in a minimum amount of time, which enables fast assessment and efficient control of the harmonic overvoltages.The optimal load restoration model considering cold load pickup is formulated as an optimization problem subjected to system operation constraints including frequency, voltage and generation output, so that the maximum load may be picked up while maintaining reasonable frequency and voltage profiles. With IPLAN programming language provided by PSS/E, particle swarm optimization algorithm is applied to solve the proposed optimization problem. In addition, penalty is introduced to handle system operation constraints. The proposed method is able to determine the load positions and amounts that can be picked up without disturbing system security and stability.Application of voltage source converter (VSC) based HVDC transmission during power system restoration is investigated. VSC based HVDC (VSC-HVDC) technology is proposed to supply the auxiliary motors of the steam plant that is to be hot restarted by the adjacent system or black-start generator. Based on the transient model of VSC in dq0 reference frame, a simulation model of VSC-HVDC system for start-up of the auxiliary motors is developed and different control strategies are adopted for controllers at rectifier and inverter sides respectively. Simulation is performed to verify the effectiveness of the proposed VSC-HVDC scheme for start-up of large induction motors such as boiler feed pump and induced draft fan motors. The results show that VSC-HVDC is able to provide effective reactive power support for start-up of induction motors and voltage drop at the auxiliary bus can maintain reasonable profiles during start-up process of induction motors as well, which are indispensable for successful start-up of the auxiliary motors.A restoration strategy is proposed for China Southern Power Grid (CSPG) when some generating units have to be tripped following severe DC system contingencies. In case the tripped units go through a shutdown and cold start cycle for long duration, this dissertation presents a control strategy that power generation of some running units should be reduced during this process to permit the tripped units to be resynchronized to the system without changing the power level at the sending end, which enables the tripped units to be restored to load rapidly without disturbing system stability. With user models developed in PSS/E, simulations are performed on CSPG to investigate the system behavior during reloading process of those thermal units which have been tripped following DC block fault. The results show that power generation at the sending end can be maintained at a reasonable steady level when output of some running units are appropriately reduced during the reloading process of the thermal units, thus having little impact on the system stability, which demonstrates the feasibility and effectiveness of the proposed scheme.