Stability Analysis Of Hubei Power Grid With UHVDC Integration And Its Countermeasures

With the rapid expansion of ultra high-voltage direct current(UHVDC) and ultra high-voltage alternate current(UHVAC) projects, China will gradually form the national interconnected power grid which can realize west-to-east and north-to-south power transmission. Nowadays, UHVDC transmission has been widely used for large-capacity electricity transmission from thousands of kilometers away because of its unique advantages on technology and economy. However, DC transmission needs the support of AC system, and the structure of receiving-end grid will be more complex with the DC integration, especially the multi-infeed HVDC system. HVDC fault will have some negative impact on the stability of the AC power grid and the interaction of AC-DC system increased the complexity of the problem. At the same time, along with the growing scale of interconnected power grid and widely use of fast excitation in large generator unit, the dynamic stability problem of the system is increasingly outstanding. Taking Hubei power grid with MX UHVDC integration as background, this paper is to study the UHVDC connection mode, the interaction of AC-DC system and dynamic stability of interconnected system for the safe of power system.Receiving-end system strength has great influence in power and voltage stability of the system and short circuit ratio(SCR) is characterized as an effective index of receiving-end system strength. In this paper, the short circuit ratio of UHVDC with hierarchical connection or single connection to 500 kV grid or 1000 kV grid is studied and compared. Influence factors of short circuit ratio of single infeed HVDC system with hierarchical connection mode is analyzed. Based on the MX UHVDC project, different access modes are analyzed by the strength of receiving-end system and power flow distribution to give the proper connection mode.With different construction sequences of UHVAC and UHVDC projects, interaction of AC-DC system is analyzed and transient stability is compared with MX integration under different system operation modes. A reasonable UHVAC and UHVDC project construction schedule is given. Because of the large transmission power of MX UHVDC, when MX UHVDC bipolar latching fault occurs, receiving-end system will appear a serious shortage of active power,causing a threat to the system stability. Therefore, the transient angle, voltage and frequency stability and the power distribution of electromagnetic loop are analyzed and solutions are put forward to ensure the grid security.The inter-area oscillation mode of interconnected system after MX access is studied and the interconnected system damping ratio under different transmission power of MX UHVDC is calculated. Optimal configuration of PSS parameters and the DC modulation are presented to improve the dynamic stability of interconnected power system,which can effectively restrain the power oscillation of UHVAC connection lines after disturbance.