Consequences of offshore wind farms on voltage stability at the transmission network level

Offshore and onshore wind farms play an increasing role in electricity production. In a future scenario of the Dutch government approximately one third of the generating capacity is based on wind farms. In this paper the consequences of a wind farm based on the Doubly Fed Induction Generator (DFIG) technology and connected via a High Voltage Direct Current (HVDC) link to the power system are simulated and investigated. The grid side converter of the HVDC link controls the grid voltage. A test power system is used which becomes, under certain conditions, voltage-unstable. Two cases are considered. In the first case the wind farm is installed in addition to the existing conventional generators. For this case it is investigated at which power rating the wind farm can prevent a classical voltage instability problem due to line tripping. In the second case, the wind farm substitutes part of the conventional generation. For this case it is investigated under which conditions the system can become voltage-unstable due to a decrease in wind speed. Different power ratings for the wind farm and severities of the dip in wind speed are investigated. Furthermore general considerations which should be taken into account for emergency control are presented. For the case where the wind farm is installed in addition to the conventional generation, a classical instability can be prevented with a wind farm of 200 turbines (with a total power rating of 500 MW, equivalent to of the local generation and of the local load). For the case where the wind farm replaces part of the conventional generation, a severe dip in wind speed (75 % of the rated speed) can introduce voltage instability. This problem only occurs for large wind farms, in the test system for a farm of 300 turbines (with a total power rating of 750 MW, equivalent to of the local generation and of the local load). 36 % 8 % 54 % 12 %.