DIgSILENT

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March 11, 2005

DIgSILENT Comments to dena Study "Aspects of Grid Integration of Onshore and Offshore Wind Power in Germany for the Time Frame until 2015"

DIgSILENT has been contracted by dena, Berlin to asses and review the dena grid study mainly under the focus of network integration aspects which have been analysed and elaborated by the consortium formed by major German utilities, the University of Cologne (EWI) and DEWI.

In the meanwhile, the study report of the consortium has been released by dena together with an Executive Summary produced by the steering committee. Respective links for downloading of pdf-files are to be found on the dena website. Decision was made not to publish the DIgSILENT review allowing for speculations on its content possibly not being in favour of one or the other side.

However, DIgSILENT declares that misuse of its report by either party on basis of black copies circulating via various sources is not inline with their interest to provide independent and professional expertise for utilities, governmental agencies and the wind power industry. As dena has the exclusive right to publish DIgSILENT report it is very much suggested to release the DIgSILENT review to prevent any further speculations on either side.

Independent on the review prepared for dena, DIgSILENT would like to respond to requests for comments based on the published dena report as follows:

Main Study Results
Grid Expansion Requirements

Study Results

For the integration of up to some 35 GW wind power in the German grid (year 2015) a total of 851 km new HV overhead lines must be installed and a total of 392 km must be upgraded to allow for (n-1) reliability based evacuation of bulk power to the main load centres in Germany. As main wind generation is located in the north where lower load density is observed, such transmission expansion is a must.

Scenarios and Assumptions

The grid expansion is being identified on basis of a number of assumptions and constraints agreed by the Steering Committee:

  • The peak wind scenarios assumes a uniform generation factor of 0.9 resulting in a peak wind power generation of some 32 GW (excl. other renewables);
  • Absolute priority of renewable generation according to EEG;
  • Conventional generator dispatch according to "Merit Order" without consideration of network constraints.
  • Maximum loading of transmission lines under (n-1) conditions: 100% at 30 °C and 0.6 m/s wind speed (according to DIN EN 50182);
  • Application of deterministic load flow procedures and algorithms via the definition of worst case scenarios (high/no-wind and max./min. load)

DIgSILENT Comments

The identified transmission system expansion and system upgrade requirements are transparent. In principal, they fall into 2 categories:

  1. Transmission system expansion needs that have been identified for maximum wind and minimum load conditions are required to evacuate EEG priority wind power. All identified overhead lines will serve fulfilling legal requirements imposed by the German government for implementing renewable power priorisation. Delay in identified transmission expansion will lead to limitation in EEG priority wind power generation capabilities.
  2. Bottlenecks in case of maximum wind and maximum load conditions are basically caused by optimal dispatch requirements for conventional power plants resulting in a TC conflict between EEG priority wind and conventional power transfer. These bottlenecks are relieved via the identified transmission expansions. Delays in the identified transmission expansions can be relieved by less economic conventional generation dispatch during high-load and high-wind hours or by a forced reduction of wind generation, depending on the legal situation.
System Stability Aspects

Study Results

The study identifies non-compliance of the German grid with UCTE requirements already for grid conditions since 2003. Voltage sags caused by 3phase busbar faults with subsequent trip of conventional generation as well as bulk wind power generation units may lead to loss of generation far beyond the 3000 MW limit established by UCTE as a critical contingency limit. Most decisive factor here is the disconnection of wind power generation units according to inadequate grid Connection Conditions. Missing low-voltage ride-through and voltage support / control capabilities are major reasons for the voltage sag problem. Expected re-powering of old wind generators and new, updated Connection Conditions in place since 2003/2004 will reduce the critical situation slightly but until 2015 still some 3000 MW will be lost in case voltage sags caused by critical faults combined with less short circuit power contribution of wind generators.

Due to increased line loadings and less short circuit contribution, the system is operated closer to transient stability limits resulting in slower voltage recovery. However, the study results indicate that transient stability and voltage recovery remains in acceptable ranges not endangering system security.

Scenarios and Modelling Assumptions

The consortium analysed a number of 4 basic scenarios and grid conditions for the years 2007, 2010 and 2015. For each scenario 6 different critical fault sequences are studied. For the year 2003, only one scenario is analysed. Contingencies consider mainly 3phase faults cleared within 150 ms. Additional scenarios are detailing on the impact of neighbouring countries, mainly Denmark where similar grid connection conditions will lead to further loss of generation in case of faults at the German grid in the vicinity of Denmark.

System modelling covered the European transmissions system represented by a total of 6700 buses, 600 synchronous generation units / power plants and some 400 wind parks modelled as lumped and aggregated generators with mixed technologies such as ASG, DFIG and SG. Wind generator models were represented according to benchmark models approved by DIgSILENT.

Detailed voltage recovery analysis was made comparing the different wind generation technologies with synchronous generators. The impact of SVC on voltage recovery support was analysed in detail.

DIgSILENT Comments

Inadequate Connection Conditions for wind generators and other renewables imposed by the German utilities in the past years are causing the German/European system to be operated in alert status under certain conditions. Main reasons are lag of voltage / reactive power support in case of voltage sags, unclear requirements on voltage control principles and strategies for dispersed generation, fast disconnection of generation units in case of grid faults (e.g. some 800 MW photovoltaic systems in the German grid will disconnect at 49.8 Hz) and missing contribution of DFIG and SG units to system inertia.

The loss of generation identified and subsequent spread of voltage sag regions with further trips of units are obviously too optimistic as other dispersed generation in Germany of some additional 6-7 GW were not taken into account. In addition, effects from renewables in neighbouring European countries were not adequately modelled.

Conclusions

The study is a first and important step to identify needs for future transmission system expansions to cope with the drastic increase of onshore and offshore wind power generation until 2015. Urgently required transmission corridor upgrades have been identified to allow for expected growth of EEG priority wind power.

A serious voltage sag problem caused by inadequate wind generator Connection Conditions will have to be solved on a top urgent basis. There are a number of options and alternatives ranging from wind generator control and protection upgrades, installation of rotating reactive power sources (synchronous compensation) to improve voltage control and short circuit power contribution and operational measures regarding commitment and dispatch of conventional power plants.

Of similar priority is the development of adequate Connection Conditions for wind generators and other renewable energy sources. It must be guaranteed that any new installation will comply with today's and future needs.

Wind turbine manufacturers will have to do their utmost to comply with adequate Connection Conditions to ensure future wind power expansion plans.

DIgSILENT GmbH / Gomaringen
March 11th, 2005

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