TWENTIES for IEEE ISGT Gothenburg 2010

Fig 2 example of hvdc offshore wind network for a

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Fig. 2. Example of HVDC offshore wind network For a HVDC offshore wind network, such as the example in Fig. 2, 4 offshore wind farms are connected at Buses 2, 3, 6 and 7, respectively, with HVDC links. If there is a fault, say, on line 1-2, the fault is a Zone 2 fault for relay C. The voltage at Bus 3 is expected to fall. One can calculate the voltage of Bus 3 using the Z-bus method [16]. If the voltage is below the PCC threshold for tripping of wind turbines, offshore wind farm at Bus 3 may trip at Zone 2 operating time (The operating time of the relay for Zone 2 fault is about 30 cycles [17]). However, HVDC provides a control for offshore wind farms. HVDC control can raise the voltage of Bus 3 quickly (The adjusting time of HVDC is in the order of ms [18]) and maintain the Bus 3 voltage above the tripping threshold. The procedure of the coordinated protection system is as follows: (a) Detect the location of fault for a faulted HVDC offshore wind network; (b) Calculate voltages at the buses with HVDC offshore wind farm connections near the fault; (c) Compare the bus voltage bus V and the threshold voltage of tripping wind farm th V ; If bus V > th V , retune; if bus V < th V , grid side VSC based (d) HVDC will send reactive power to raise bus V until bus V th V ; (e) Return. Otherwise, the use of special protection schemes is recommended for line 1-2 [19] . b) Defence System Integration of offshore wind farms will increase the complexity of the interconnected system. Then inherent intermittence of wind and the large-scale and long-distance power transmission lead to new sources of vulnerability. It is therefore necessary to design a defense system to reduce the threat against the security of integrated offshore wind-AC mainland grid. The defense system includes two major functions: vulnerability assessment and self-healing. c) Vulnerability Assessment HVDC offshore wind network is assessed with possible sources of vulnerability, such as natural events, and failures in protection/control facilities. To determine the cascaded sequence of events leading to a catastrophic outage, a wide range of scenarios and contingencies of the HVDC offshore wind network need to be simulated. These studies should incorporate system analysis tool and relay simulation methods. Some specific scenarios are shown in Table 1. Based on the results of the simulation, the vulnerability index in terms of severity and likelihood is calculated. TABLE I E XAMPLE EVENTS OF POSSIBLE HVDC OFF - SHORE WIND NETWORKS No. Events of HVDC Offshore Wind Network 1 DC monopolar blocking fault or DC bipolar blocking fault 2 AC mainland grid fault 3 Offshore side transformer fault 4 Wind farm bus fault d) “Self-Healing” Strategy Based on the vulnerability assessment of each event of the HVDC offshore wind network, control actions are needed to steer the system to a secure and less vulnerability operating condition. For a HVDC offshore wind network, there are few connection points to the AC grid, the power injection of a large offshore wind power may create overloading conditions on transmission lines. This issue can be addressed using wind generation curtailment and/or load shedding. For a multi-
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