Study Of Frequency Stability Of HVDC Sending System With Hydro Power Unit In Islanded Mode

With the development of the DC transmission technology and the implementation of the nationwide power network project in China, HVDC project exerts an increasingly crucial role in electricity transmission link. However, due to line maintenance or line fault which results in disconnecting between rectification station and power grid, as well as to transmit electricity of some specific power plants which disconnect to power grid, sending-ends of DC transmission systems will form a relatively independent system with some power plants, which is called islanded system. When HVDC system operates in islanded mode, frequency stability of the system is hard to maintain. Besides, if the system is not well controlled, low frequency oscillation of the hydraulic turbine will probably be caused by coupling effect with turbulence in prime motor of hydraulic turbine and DC system. It is said that water pressure pulsation caused by vortex strip in turbine draft tube will result in frequency vibration in islanded system.Firstly, this paper has established the hydraulic system, which including governor model. Then, it designs a resonance function to simulate the actual water pressure in turbine draft tube and set up a new hydraulic turbine model considering the draft tube water pressure pulsation as well as the elastic water column, which is a big improvement to the original linear turbine model. At the same time, it gives the pole figure of the new hydraulic pulse function. According to the figure, the original hydraulic turbine model may miss unstable poles, but the new resonance function has put forward higher requirement to the DC frequency limiter in structure and parameters.Secondly, in order to verify the accuracy of the hydraulic turbine model as well as to analyze the influences of hydraulic system parameter and governor parameter on frequency oscillation of the HVDC system, of which sending-end is in islanded mode, simulation model of such HVDC system containing four hydraulic generators has been built under the PSCAD/EMTDC electromagnetic transient simulation environment. The simulation results illustrate that hydraulic startup time Tw, governor parameters KD have greater influence on frequency oscillation of the system, while elastic time Te has less influence.At last, to avoid reforming the structure of hydraulic turbine, DC frequency limiter which has been improved is proposed to restrain from frequency oscillation of islanded sending-end of the HVDC system caused by the draft tube water pressure pulsation. By adjusting the governor parameters, making the dead zone of governor coordinated with the dead zone of DC limiter, the frequency of island to about 50Hz.