Control and protection of multi-terminal DC transmission systems based on voltage-source converters.

摘要

This thesis deals with the control and protection of Multi-Terminal DC (MTDC) transmission systems based on Voltage-Source Converters (VSCs). The MTDC system enables several AC systems to be integrated by way of a DC network. Potential applications of the MTDC system are: off-shore wind farms, city center feeds.; The complexity of the combined AC/DC circuits and their cross-coupling make it difficult to adjust the VSC control parameters to eliminate sustained oscillations and instability which appear frequently. The control part of the thesis is a systematic investigation which addresses the problems.; Firstly, the mechanism of sustained oscillations is discovered to be cross-converter resonance between the AC and DC circuits of a VSC system. Secondly, the unstable modes have been identified by establishing the small signal perturbation model of the entire MTDC system and applying eigenvalue analysis. The general philosophy adopted in the solution consists of reducing complexity by partitioning the MTDC system into several independent subsystems by applying DC voltage decoupling and AC current-tracking techniques at each VSC station. This eliminates the cross-converter resonance. It is found that the addition of damping filters stabilize the MTDC.; Because AC fault protection is standard know-how, the thesis focuses on protection against faults in the DC network of MTDC system. After gaining knowledge on DC fault characteristics, four protection strategies (which have different costs to be balanced against restoration times) are developed. The location of the faulted DC line and its isolation (so that the remainder of the DC network can resume service) is a most challenging task. The Handshaking Method is invented to handle this problem. Selective fault detection schemes, which ensure, for example, that AC faults are not mistaken for DC faults, are also developed. During a DC fault, the DC capacitors can become unbalanced and a method of balancing the DC capacitors during the restoration stage is developed.; Digital simulations have been used to demonstrate the feasibility of the control and protection strategies.