TWENTIES for IEEE ISGT Gothenburg 2010

For a multi terminal dc offshore wind farm dc

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generation curtailment and/or load shedding. For a multi- terminal DC offshore wind farm, DC modulation based VSC converters can shift the power flow pattern to mitigate the overload without having to curtail generation or shed load. The “self-healing” capability identifies the optimal sequence of control actions that can reduce vulnerability, in the preventive or corrective self-healing mode [20]. C. Control centre operations While fast automatic actions, such as protection and defence systems, are needed to cope with rapidly unfolding phenomena such as faults or short-term instability, general operation of the offshore VPP connected to the mainland grid will be monitored and controlled in adapted control centres. It is therefore necessary to revise all the typical control centre functions, tools, and procedures, in order to adapt them to the new needs brought about by the multi-terminal VPPs. In particular, the control centre Energy Management System (EMS) applications have to be upgraded, to extend the monitoring, analysis, control, and display functions. The first focus regards monitoring. The VPP must provide to the control centre measurements and information such as the power generated by the wind farms, the power flows through the HVDC grid, converter operation status, and VPP alarms. In turn, the control centre must communicate with the local,
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7 master control of the VPP for normal and emergency operation functions 1 . Normal operation provides the VPP operation settings (e.g. power dispatch strategy, power limits for security reasons, frequency and voltage regulation modes, etc.). Emergency actions consist of fast power reduction (calling for generation trip by the master control) or possibly power shift among converters. The AC grid security analysis tools have to account for the contingencies coming from, or affecting, the HVDC grid, and the effects of manoeuvres and changes in the DC side on the AC stability. Under normal control, security assessment functions may evaluate the secure operating region of the HVDC injections, hence the security margins with respect to the actual condition. Conventional generation re-dispatch, used as a preventive control strategy to maintain or restore security, can be complemented by power injection shifting among the terminals of the multi-terminal VPP. While the former action implies resorting to the balancing markets, the latter is virtually for free thus it could have priority. During emergency control, overload relief may be obtained by an HVDC injection variation. Again, in case of power shift from one converter to another, great care must be paid in order not to jeopardise system security elsewhere, because of the modified flow configuration. To this aim, the power system model considered in security assessment functions must include all the areas in which the HVDC converters are located, which may result into a very large region. This requirement is particularly critical for online applications, because of the dimensionality and jurisdiction issues.
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