Abstract:  It may be unstable that isolated island operation mode occurs passively in the delivering side of UHVDC system, which belongs to a weak system. At present, the recommended solutions are blocking DC link, cutting off AC filters and cutting off generators. The coordination problem of the above strategies caused by action sequence remains to be studied. ±800 kV Yan-Huai UHVDC is taken as the research object. Its max transmission power is 4 300 MW when Hu-Guan line Ⅰ is uncharged. The isolated island operation process is calculated through the electromagnetic transient simulation. It is found that once the isolated island operation mode occurs, the voltage and power will fluctuate disorderly. The frequency falls into the operation range of low-frequency protection. Different time series of solutions action is analyzed. If the strategy of blocking DC link is earlier than cutting off generators, twice of rated voltage will attack the delivering-side power grid immediately. If blocking DC by low-voltage protection after cutting off generators, power shortage impact on the receiving side will continue for 2.00 s and the frequency reduces 0.2 Hz. It is suggested that when receiving the signal of isolated island, control and protection system blocks DC after a certain delay. So generators will be cut off firstly and overvoltage is eliminated significantly. Cooperative control system can execute the command of power modulation in time.

Key words: Yan-Huai UHVDC, isolated island, secure and stable control system, overvoltage