Grid connection of offshore wind power plants and transmission system constraints

摘要

Historically, the transmission system of Denmark has been built upon central (thermal based)power plants. The locations of the central power plants have been selected and optimised from thelogistic, economic and system considerations. The optimised location in the system provides theability to comply with power production plans, balance the consumption and agreed powerexchange with the neighbouring systems, deliver ancillary system services and maintain securityof supply in normal system operation and in outages of lines or other generation units. The lineoutages should not isolate the central power plants from the consumption centres. In a systemcontingency forcing one power plant to reduce the factual production, another power plant shalldeliver immediate power reserves to balance the system. Securing immediate power reservesrelies on that a required amount of the power reserves is (always) available for delivery on-timeand located on the right side of the system regarding the severest contingencies.At present, the Danish system transforms into that with a significant share of dispersed generationand large offshore wind power plants (OWPP), whereas the central power plants are taken out ofoperation. This system transformation means that the criteria on the system-optimised locationand availability (by amount and on-time delivery) of immediate power reserves will notnecessarily be fulfilled. The large OWPP are connected through long, tens kilometres, submarineand land cable systems to the nearest suitable onshore substations, being costly grid-connectionsolutions. The grid-connection optimisation focus has moved from the overall systemoptimisation onto the cost-benefit evaluation of the grid-connection equipment, i.e. cable systemand substation, and the transmission system adequacy in vicinity of the grid-connection point.Commonly, the transmission system adequacy is evaluated from probabilistic load-flow cases innormal and (n-1) contingency operation to secure the power transport from the OWPP to the localsystem. Evaluation of extreme operation conditions or securing of immediate power reserves inthe entire system have not been part of such grid studies or decision making on the OWPP gridconnectionpoint.The commissioning and grid-connection of the 600 MW OWPP at Kriegers Flak in the Baltic Seawill take place by 2017/2018, at changed conditions in the East Danish transmission system. Theconditions will change because the announced decommission of central power plants will leave Eastern Denmark with a severe lack of immediate power reserves. Recognizing such changes inthe system conditions, the common evaluation procedure for the grid-connection solution hasbeen adjusted. The procedure still comprises the common part dealing with a cost-benefitevaluation and selection of a suitable grid-connection point from onshore substations, a cablesystem technology and trace optimisation and the local system adequacy studies. The procedurehas been extended with the technical and socioeconomic evaluation of establishment andmaintaining of immediate power reserves for the pre-selected connection-point candidates. Thesocioeconomic evaluation includes that such immediate power reserves shall be maintainedthroughout the life-time of the established grid-connection solution of the OWPP. When appliedfor the grid-connection evaluation of the OWPP at Kriegers Flak, the revised procedure haschanged the total socioeconomic picture of the cost estimates for some pre-selected substations.Specifically for the OWPP at Kriegers Flak, the grid-connection to Bjæverskov is the cheapestsolution when disregarding the extra cost of immediate power reserves. When accounting for theneeds and cost of such immediate power reserves, the grid-connection to Ishøj becomes thecheapest one, whereas the grid-connection to Bjæverskov gets about 50% more expensivecompared to Ishøj.The experience from the grid-connection evaluation accounting for the system constraints andneeds of immediate power reserves, gained from the OWPP at Kriegers Flak study, will beimplemented into the main procedures for evaluation of the grid-connection solutions of futureOWPP in Denmark.