Control And Protection Of VSC Based HVDC System Under AC System Fault Conditions

The control and protection of VSC-HVDC under ac system fault conditions directly affect its practical operations. Thus to research the control and protection is of evident application value and practical significance. In this dissertation, the control and protection of VSC-HVDC and its related technologies under ac system fault conditions are investigated by means of theorical analyses and the simulation validations. The main contents and the related conclusions of this dissertation are as follows:(1) The mathematical models of VSC-HVDC under ideal conditions and unbalanced ac voltage conditions are studied and developed respectively. Under the ideal conditions, the high frequency mathematic models and the low frequency dynamic models of the VSC converter in the three-phase static coordinates are developed. Based on these studies, the low frequency dynamic models of the VSC converter inα-βand d-q coordinates are established respectively. Using the concept of the instantaneous symmetrical components, the low frequency dynamic mathematical models of the VSC converter under unbalanced ac voltage conditions are developed inα-βcoordinates and d-q synchronous reference frame. The active power balance relation between the ac and dc side of the VSC converter are analyzed under both ideal and unbalanced conditions. The results show that under ac system asymmetrical fault conditions the double frequency fluctuation will appear in the dc voltage and the active power transmitted by the converter. The negative components that result from the asymmetrical faults will increase the losses of the reactor of the converter.(2) A real-time detecting method for the synchronous phase and the instantaneous symmetrical components is investigated. It is an important part of the control and protection of VSC-HVDC under ac system fault conditions. Based on the concept of space vector, a real-time detecting circuit for the synchronous phase and the symmetrical components is designed. In this circuit the component of the positive sequence voltage in q axis is taken as the input signal of the PLL. Thus the symmetrical components can be detected quickly and the effects of the negative components on the performance of the PLL can be eliminated. A real-time detecting circuit for the synchronous phase and the symmetrical components based on synchronous reference frame is also designed. It is a frequency adaptive close loop detecting system composed of the symmetrical components and the synchronous phase detecting circuits. So it is applicable to the ac systems with the variable frequency.(3) The control strategies of VSC-HVDC under ideal conditions are studied. The theory of the dual closed loop control is applied to design the controller of VSC-HVDC, and the control system of the VSC converter based on a series PI regulator is analyzed and designed. This controller is of clear physical meaning and simple structure. Using the theory of the feedback linearization of nonlinear system, another controller, a current decoupled controller based on input-output variable feedback linearization is designed. So the controller of the VSC is got simplified and the performances of the controller are improved. In the two kinds of controllers mentioned above, an inner current loop control structure is adopted to realize the current limit control of the VSC converter and avoid the over current and over voltage of the VSC converter. A controller for VSC-HVDC supplying passive network is designed and the performances of this controller have been validated under multi-conditions such as variable frequency, voltage regulating and passive load disturbance. At the same time, the start-up control of VSC-HVDC is investigated and a compound start-up control strategy is presented. It can effectively suppress the over current that arise from the start-up of the VSC.(4) The control strategy of the VSC-HVDC system is studied when the AC fault occurs. First, a VSC-HVDC control system with a double current control loop is designed aiming at restraining the negative sequence current. The controller can restrain the negative sequence current caused by the asymmetrical faults, with favorable control performance and current limitation. Second, an unbalanced control strategy of VSC-HVDC is brought forward, where the negative sequence voltage can be compensated in real time. Not only the negative sequence current is effectively restrained, but also the controller structure is simplified and the controller's characteristics are improved. Third, a control strategy for balancing the active power is proposed for systems with high-demanded DC voltage control such as BTB-VSC-HVDC. The double frequency fluctuation of the active power and the DC voltage caused by the asymmetrical faults are effectively restrained; accordingly influences of the negative sequence components on the AC system of the other side caused by one side's asymmetrical faults are decreased. Finally, the influences of different AC sides' faults of VSC-HVDC on the DC voltage control of the VSC converter are analyzed, and the corresponding countermeasures are suggested. The results show that, the DC voltage is controlled and current limitation of VSC converters is realized by automatic switching of the converter's control modes, and the VSC-HVDC system's ability of operating continuously and safely is improved accordingly. The two control systems designed, aiming at negative sequence current limitation and DC voltage fluctuation restraint of the VSC converter respectively, are analyzed and compared. Conclusions are drawn that, for systems with high-demanded DC voltage control such as BTB-VSC-HVDC, the control system aiming at restraining the DC voltage fluctuation of VSC converter can be applied, while the control system aiming at limiting the negative sequence current is more suitable in the VSC-HVDC system with the power transmitted via~-DC lines. In comparison, the unbalanced control of the VSC based on the negative sequence voltage compensation is of simple structures and favorable control performance, and is more applicable in real projects consequently.(5) The control strategy of the VSC-MTDC system is studied. A 5-terminal VSC-MTDC simulation case is created with PSCAD/EMTDC, and the master-salve single-point DC voltage control strategy is proposed and verified. For the system with such a control mode, the active power adjustment and DC voltage control are both with good rigidities, but the reliability is poor, and operation mode is not flexible. Next, a multi-point DC voltage control strategy is proposed based on the DC voltage error control. This approach can realize automatic switching of the control modes according to the system's performance, so that control of DC voltage and active power balance can still be realized after the loss of the converter which is in DC voltage control mode. As a result, both the reliability and the flexibility of the multi-terminal HVDC system are improved.(6) A power supply system based on VSC-MTDC is designed. The power controller based on the DC voltage error control and the VSC converter's unbalanced control strategy based on the negative sequence voltage real-time compensation are adopted in the power supply system. When supplying the important loads with the power supply system, the influences of the disturbances and faults on power supplying to important loads can be restrained effectively, the power supply quality is very high, and the integrated control of power quality can be realized.