DIgSILENT

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March 01, 2006

Wind Power Integration Study for NEMMCO / Australia completed

In February 2005, the Australian National Grid Operator NEMMCO engaged DIgSILENT to carry out power system stability studies to assess the impact of high levels of wind generation on the South Australian power transmission system. This follows indications that more than 1000 MW of wind generation capacity may be connected to the South Australian grid within the next few years. This was a concern as the South Australian load varies between a forecast peak demand of 3378 MW for summer of 2005/06 and a minimum demand as low a 900MW.

The scope of work was to review how large amounts of wind generation could affect the stability performance of the power system, in order to determine:

  • What future operational and power system security issues could arise;
  • In broad terms, the extent of these impacts; and
  • Whether there may be a fundamental limit to the amount of wind generation that could be supported.

The scope of the work was limited to power system stability only i.e. issues that could affect wide areas of the South Australian power system.While localised issues were noted, it was considered that these need to be managed on a case by case basis.

The work was aimed at determining:

  • What are the critical stability mechanisms that could significantly impact operation of the South Australian transmission system;
  • Whether the existing stability mechanisms change fundamentally or new stability mechanisms emerge;
  • What is the trend for increased wind installation, in order to understand what level of wind generation may cause significant changes to network limits or other operational difficulties, and whether there is a fundamental limit to installed wind capacity in South Australia.

The scope was limited to:

  • An assessment of transmission system issues only that is, impacts that can affect a wide area. While localised impacts must be dealt with, this was considered to be the subject of other, more detailed studies;
  • The assessment of power system stability. Thermal limitations were not studied;
  • A limited (but broad) number of scenarios and faults were studied. This is in line with the basic aims of understanding stability mechanisms and how they may change.

Simulations were conducted for high and low load cases for 0 MW; 400 MW; 800 MW and 1200 MW of wind generation respectively. The capability to transfer power between Victoria and South Australia can be used as a measure for the impact of wind generation on power system stability in South Australia. The transfer capability of the base case (no installed wind generation) for each major contingency is compared with the transfer capability for each of the 400MW, 800MW and 1200MW wind scenarios for the same contingencies. An increase in the transfer capability, and the extent of that increase, is a measure of the improvement in stability, whereas a decrease is a reduction in stability.

In the development of the wind scenarios, it has been assumed that wind generation will displace the existing scheduled generators. That is, when a wind farm is generating power, the most expensive scheduled generator(s) in South Australia will reduce their output by an equivalent amount and, if necessary, will disconnect from the power system. In the scenarios, the installed wind farms are assumed to be operating at their maximum generation. Hence, in the 400MW, 800MW and 1200MW wind scenarios scheduled generation of those amounts are assumed to be disconnected from the power system.

This approach could only be applied provided:

  • There was sufficient scheduled generation to be displaced;
  • A minimum amount of scheduled generation remained on line.

Approximately 400-500MW of scheduled generation was assumed to remain on line, operating at minimum load. This takes into account:

  • Some generators remain on line at minimum load in spite of low market costs, due to the cost of restarting the unit; and
  • An expectation that some generators may stay on line to protect their commercial positions under contract.

While it is more likely that South Australia would be exporting power to Victoria during high wind and access generation conditions, it was still necessary to determine the operating envelope of South Australian import limits.
For the 800MW and 1200MW wind scenarios and low load in South Australia therefore, import into South Australia is limited or no power import into South Australia will occur. For these cases, therefore, South Australian demand was increased to accommodate the minimum scheduled generation, the installed wind generation and the imported power demand was increased in the low load cases to approximately 1600MW and 2000MW, respectively, representing conditions which would be classified as medium, rather than low, demand.

The study also involved the following activities:

  • Conversion of various cases of PSS/E NEMMCO network models into PowerFactory and benchmarking of load flows.
  • Modelling and benchmarking of NEMMCO dynamic models of conventional generator controllers.
  • Preparation of models for existing and proposed new wind farms in South Australia. For existing wind farms, the actual manufacturer specific dynamic models were used.
  • Dynamic simulation of contingencies to assess the impact of various levels of wind generation in the South Australian grid on existing stability limits.
  • Modal analysis to asses the impact of additional wind generation on stability of critical NEM inter-connectors.
  • Evaluate long term stability limits with the aid of PV curves.

An executive summary report has been published by NEMMCO and is available from
http://www.nemmco.com.au/dispatchandpricing/260-0012.pdf.

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