CAPACITY FACTOR

Fig。 7。  EENS for different wind farms with different capacity factors

Fig。 8。  Ratio of EENS change between 4% and 20% wind turbine FOR

It can be seen that the increase of wind turbine outage rate has limited impact on system reliability if wind capacity factor is low。 The impact of wind turbine outage may become significant in the cases of high wind capacity factor。 The results of reliability assessment may be unrealistic if wind profile does not incorporate wind turbine availability。

Note that the availability of wind turbines is affected not only by the wind turbine outage, but also by the outage of collector system and the auxiliary facilities in the wind farm。 The outage rate of collector system in the wind farm is normally small since underground cable is often used。 On the other hand, the impact of collector system outage may be not small since a collector feeder outage will result in all wind turbines on the same feeder unavailable。 Test result of the impact of collector system outage is not presented in this paper because of lack of outage data of collector system。 Still, the joint capacity state model of wind generator and transmission  line,

which has been presented in Section II。C, can be used  to create the capacity state probability model of a wind farm considering collector system outage。

H。Target capacity for transmission upgrade

Assume a 570 MW wind farm is interconnected with the system via the plan of interconnection shown in Fig。 1。 Three scenarios of transmission upgrades are compared with different transmission capacity。 Normally the upgrade is decided by deterministic criteria widely used in transmission system planning。 In the first scenario, the capacity of the transmission upgrade is based on the full capacity of the wind farm, which is assumed to be 570 MW as two lines in service and 285 MW when one line in service。 A remedial action scheme (RAS) of tripping wind generation may be needed when only one line is in service。 The second scenario is that the transmission upgrade can deliver 80% of the nameplate capacity of the wind farm, which is 456 MW as two lines in service and 228 MW as one line in service。 In the third scenario, the transmission upgrade is based on 60% of wind capacity, which are 342 MW and 171 MW as two lines in service and one line in service, respectively。

Probabilistic reliability models proposed in Section II。C are used in system reliability assessment。 Line outage data are given in Table II, which is the typical outage data of 230 kV lines used in some utilities in North America。 Further assume that the length of the transmission lines is 200 miles to reflect the long distance of the wind interconnection。 The outage rate of each line can be obtained by (6) and the capacity states of the two-line transmission system can be calculated using the method discussed in Section II。C。 The capacity states of the transmission system with two parallel transmission lines are shown in Table III。

First assume a 570 MW wind farm with 40% wind capacity factor is interconnected。 The EENS of three scenarios of transmission upgrades are illustrated in Table IV。 In the first column of Table IV, “Full capacity” means the target capacity of transmission upgrade is based on the full capacity of the wind farm; “80% capacity” and “60% capacity” mean the target capacity is 80% of wind farm capacity and 60% of wind farm capacity, respectively; “plus outage” means the line outage is modeled。 Also listed in Table IV is the ratio of the consumed wind energy and the total consumed energy。 It can be seen from Table IV that the availability of transmission capacity does impact the system reliability and the consumed wind energy, but not significant。 The outage of transmission lines between the wind farm and the main grid has slight impact on the system reliability and the utilization of wind energy。