Capacity factor Average capacity of study  periodMaximum capacity

in power systems reliability evaluations。 Similar to  [10], EENS is used in this paper as the reliability index。 It has been verified that the reliability assessment results using EENS in this paper are consistent with the results using hourly   LOLE。

Only the results using EENS are presented。 According to   [6],

In order to calculate the joint states of a wind farm, two capacity state tables need to be created first。 One is the state table of available wind。 It can be represented as a pair of

available  wind capacity and its probability ( Cwnd , Pwnd ),

EENS can be calculated as。

(1)

for j = 1 to s; s is the number of wind  capacity states。 The wind capacity availability and its probability can be obtained from the historical or field-measured wind profiles。 The other state table is for the capacity of available wind turbines and its probability。 The state table for the wind turbine capacity can be  calculated  using  the  forced  outage  rate  (FOR)  of wind

turbines。 The method is as same as used for conventional generator  [10]。  Then  the  probability  of  a  given  wind farm

farm is connected to the system through two transmission lines。  The  wind  generation  and  its  associated  transmission

output Ck

can be obtained by:

upgrades  can  be  modeled  as  a  system  in  series connection

from the reliability assessment standpoint。

where, n is the number of states of wind turbine availability;

wtg

Pi is  the  probability of  the  wind  turbine  availability at

state i; U(•) is a step function。

From equation (3), it can be seen that the probability of wind capacity is the weight of the probability of wind turbine availability in calculating the probability of the joint capacity。 Then a large wind capacity factor implies that the wind turbine availability has large impact on the probability of the joint capacity, especially, under high wind conditions。 Therefore, it is necessary to evaluate and revise the wind profile by incorporating   the   wind   turbine   availability   when    wind

Fig。 1。  An example of wind farm interconnection

The series connection system is shown in Fig。 2。 The input and output of the series system are wind capacity and the delivered power to the main grid, respectively。 The capacity of the wind farm, transmission lines and the output of the composite system are denoted by x, y, and z, respectively。 In this series connection system, the output z is the minimum of x and y, i。e。

capacity factor is large。

C。Joint capacity of wind generation and transmission

z min(x, y)

Probabilities   of   x   and   y   are   denoted   by Px

Interconnecting wind generation to the grid may need long distance lines since the wind resources are normally located at

respectively; hence the probability of the composite system output  Pz   can be obtained by

remote   areas。   The   transmission   path   between   the  wind

P (z) P (z) P (y) P (z)P (x)

generation and the main grid may consist of one or multiple transmission   lines   and   reactive   power   support   devices。