Control and Protection of Wind Power Plants with VSC-HVDC Connection

Wind power plants are the fastest growing source of renewable energy. The European Union expects to generate 230 GW wind power, in which the offshore wind power is expected to contribute 40 GW. Offshore wind power plants have better wind velocity profile leading to a higher energy yield. Europe has a huge potential of offshore wind energy, which is a green and sustainable resource. All these have led to the development of offshore wind power plants.
However, overall cost of the offshore installation, operation, and maintenance are higher than those of the onshore wind power plants. Therefore, the plant size needs to be higher such that the unit cost of energy can be lowered. An overall increase in operating efficiency would further reduce the cost of energy, thereby increasing the viability of the project. Multi-MW variable speed wind turbine generators, of unit sizes between 3-10 MW, have been developed so as to take advantage of the lower cost per MW of installed wind power capacity. The current trend is that these large units will comprise of multi-pole, low-speed synchronous generators equipped with full scale converters. VSC-HVDC cable transmission is a favourable option for a large and remote offshore wind power plant, which needs a long distance cable connection to the onshore power grid. It has lower power losses, higher transmission efficiency, and fast control of both the active and the reactive power.
This dissertation presents a test system for the simulation analysis of different operational and control aspects of a potential wind power plant with VSC-HVDC connection to the onshore grid. The test system is modelled in the PSCAD/EMTDC environment for the time domain electromagnetic simulation. In such a system, the offshore terminal of VSC-HVDC is controlled to establish the reference voltage waveform in the offshore grid. The ac voltage controller in the offshore VSC-HVDC terminal has been improved by utilizing the measured active and reactive power-flows to determine the feed-forward terms for the current references in the dq-axes.
HVDC transmission decouples the offshore grid frequency from the onshore grid frequency. Three different methods have been evaluated here for relaying the onshore grid frequency to the offshore grid, such that the wind power plant can participate in the grid frequency control. One of the schemes does not involve communication, while the other two depend upon communication of onshore frequency signal. Similarly, three different methods have been evaluated and compared for the fault ride through behaviour of this system.
The current control capability of the converters in the offshore wind power plant grid can be utilized to enhance the fault time behaviour of the whole system. A novel approach has been proposed to allow a calculated amount of negative sequence current injection from the VSC-HVDC converters as well as the full scale converters in the wind turbine generators. The proposed approach is demonstrated to have lower power oscillations, and hence, lower dc voltage overshoots in the VSC-HVDC system.
On the protection side, the coordination of over-current relays has been analysed in the new environment. A simple yet reliable scheme utilizing the well-known over-current relay characteristics has been presented for the detection of faults and the determination of faulted feeder in the offshore grid. It is demonstrated that the communication capability of modern relays can help avoid the potential cases of over-reach.
The test system is modelled for real time simulation in RSCAD/RTDS platform, such that the physical relays could be connected to it. The performance of the proposed relay coordination scheme has been tested using an industrial relay. Moreover, since RTDS simulation allows continuous simulation of the system in real time, multiple events can be simulated. Simulation studies have been carried out for the fault detection, circuit breaker tripping, and system recovery after fault clearance.