System stability of large wind power networks: A Danish study case

The article describes the considerations and the results of the investigation on short-term voltage stability carried out on a large wind power network model that is similar to a part of the Danish power grid. In the investigated power network, around 50% of the electricity consumption is covered by wind turbines and local combined heat and power (CHP) units.

A distinction is made between local wind turbines that may trip and those in a large wind farm subject to the Grid Specifications of the transmission system operator (TSO) that must ride through the grid faults. When a short-circuit fault occurs in the transmission network, the main concerns are: (i) a risk of uncontrollable voltage sags that may result in voltage instability, (ii) a risk of significant power loss due to tripping of local units and possibly (iii) a risk of over-voltage at the periphery of the transmission network.

Significant presence of induction generator based wind turbines is the main reason of a risk of voltage instability manifesting itself by temporarily uncontrollable voltage decay in a part of the power grid and being caused by significant reactive power absorption of the induction generators. However, voltage instability does not necessarily develop to voltage collapse, because local wind turbines will trip at abnormal grid operation, for example, at registering excessive under-voltage.

This ‘eliminates’ the problem of reactive power absorption in the power grid and leads to the voltage reestablishment. However, protective disconnection introduces the new issues such as (i) establishing power reserves and (ii) protecting the periphery of the transmission network from excessive over-voltage.