Abstract:
Two new applications for power electronics based voltage source converters (VSC), and their controllers, in power systems are proposed and studied in this thesis. One of them is to use a voltage source converter based high voltage direct current (VSC-HVDC) sub-transmission as a 'controlled' highway exchanging power flow between zones for reinforcing a city distribution networks. The other is to use a VSC based device as a signal injection device for load identification in power systems. In the first proposed application, a VSC-HVDC sub-transmission scheme is proposed for reinforcing an urban sub-transmission network. The proposed scheme has the advantages that power can be easily exchanged among the different distribution zones, the direction of the AC power flow in the distribution network can be controlled, and thereby. the utilisation of the existing transmission assets can be increased. Moreover, the environmental impact can be reduced since an underground power DC cable can be used for the power transmission. A proposed VSC-HVDC transmission system is extensively studied in this thesis. ·. Drooping control, a control strategy where rectifiers in such a system can be paralleled and independently controlled is proposed and studied. Using this technique, the rectifiers can be i made to proportionately share the changes in the load demand according to a pre-specified ratio. Simultaneously, the direct current (DC) bus voltage and the reactive power taken by the rectifiers can be controlled independently. Local control can also be achieved in a VSC-HVDC system using the proposed drooping control strategy. To use the drooping control strategy in the case where the line resistance cannot be neglected, the power flow equation of the proposed HVDC transmission system is derived. Whenever the line resistance is included, the derived power flow equation can be used to determine the corresponding no load DC bus terminal voltages that relate the pre-specified ratios of the load power among the rectifiers. Using the no load DC bus voltages, the rectifiers can still be made to proportionately share the changes in the load demand according to a pre-specified ratio and independently controlled in the case where the line resistance cannot be neglected. Inverter system control strategies normally depend on the requirements of the power systems. In · the inverter system control design for the proposed VSC-HVDC system, the control objective is set to achieve a constant load alternating current (AC) voltage, since a passive load has been used in the study and there is no other power supply connected in parallel with the load. In a stability analyses study, the state space models for both the rectifier control system and the inverter control system are derived respectively. The system matrices of both systems are used to analyse the stability of the individual control system. Through the stability analyses suitable values of the plant parameters and the controller parameters can be decided against the· desired system performance. The operations of two VSC-HVDC systems are simulated using EMTDC/PSCAD power system simulation software. The simulation results show that the proposed drooping control scheme can successfully control both the DC bus voltage to be substantially constant and the load power allocation to be the pre-specified ratio between the rectifiers in both situations with and without line resistance. The simulation results also show that the inverter controller can very tightly control the load voltage which hardly changes from the set point during the load disturbance in both situations. In the second proposed application, a new approach for system identification in power systems is proposed. Using an input signal injection strategy, a VSC based device is proposed and designed to inject the required perturbation signals. A correlation technique with a pseudo random binary sequence (PRBS) input signal is adopted as a modelling tool. Two control functions are derived to implement the required amplitude or frequency variation of load voltage for system identification. Using the derived input signal injection method, one of the input signal parameters (amplitude and frequency) is controlled to vary with a required pattern while the other can be controlled to be a constant and vice versa. Simulation studies using EMTDC/PSCAD power system simulation software show that the above variations are satisfactorily fulfilled. An example for P-V modelling has been presented in the thesis. The identified load is a double cage induction motor available in PSCAD library. A linearised model of this double cage induction motor is derived to compare with the simulation result. The simulation shows that they substantially agree with each other.