Integration of islanded very large scale onshore wind power with LCC HVDC and a small STATCOM A novel control method

Wind power development is booming globally, and large scale onshore wind power is under development in China, the Americas and India. Compared with AC transmission, HVDC technology can due to lower losses be attractive if the distance from generation to consumption is long. VSC (Voltage Source Converter) HVDC is rapidly maturing as a technology, and offers some significant advantages compared to traditional LCC (Line Commutated Converter) HVDC, such as independent control of active and reactive power. This independent control enables VSC HVDC to operate in very weak/islanded AC systems, however LCC HVDC still has a considerably higher DC current capability than VSC HVDC. Onshore wind conditions require both very high DC current capability, and control flexibility due to very weak/islanded AC systems. Therefore, there is good reason to investigate new solutions to transmit large scale onshore wind power via LCC HVDC, which complements existing VSC and LCC HVDC technology. In previous works by the authors, a proof of concept for black start using LCC HVDC was presented [1], which may enable initial startup of an islanded AC network. Once an islanded AC system has been started up, it must be operated in an optimal way. Thus in this paper, a novel control method that enables LCC HVDC combined with a small STATCOM to transmit the rated HVDC power capacity from an islanded large scale PMSG (Permanent Magnet Synchronous Generator) wind farm AC system, is presented. As known from micro grids [2], an islanded AC system with full converter power sources changes the fundamental behavior of the AC system; the AC voltage amplitude depends on the active power balance instead of the reactive power balance; the AC voltage frequency depends on the reactive power balance instead of the active power balance. Considering these non-conventional relations, an LCC HVDC rectifier is used to control the AC voltage amplitude, and a small STATCOM is used to control the AC voltage frequency. A typical ± 800 kV, 8GW bipolar UHVDC system is combined with a large scale onshore PMSG wind farm, and time domain simulations for initial islanding from a supporting AC network, AC faults and DC faults are performed. It is shown that system stability can be achieved with the presented control method. However, the key issue is the AC overvoltage that occurs when the LCC rectifier cannot transmit active power from the islanded AC system, namely DC line faults and LCC inverter AC faults. KeywordsHVDC transmission; Power system control; Reactive power control; Wind energy integration