Wind energy has remained the world's fastest growing renewable source for the last seven years, and this trend is expected to continue with falling costs of wind turbines and the urgent international need to tackle CO2 emissions to prevent climate change.

Over the past few years, ERA has been actively involved in many different wind energy projects, providing electrical engineering expertise to help solve some of the unique technical problems that can arise with this form of power generation.

The wind farm consists of 30 turbines with a total capacity of 60MW and is connected to the Distribution Network Operator (DNO) at 33kV. The electrical system consists of substantial 33kV submarine cables, with a total cable system length of 42.1km, resulting in a net cable capacitive charge of 4.7MVAr. The issue of concern to the operator was the potential level of capacitive currents within the wind farm electrical systems. If these were not correctly estimated, they could present major operational problems at the 33kV substation.

For example, ERA has reviewed the 33kV electrical power system of the North Hoyle Offshore Wind Farm for SP PowerSystems Ltd and npower renewables. North Hoyle Offshore Wind Farm was the first large-scale wind farm to be constructed in UK coastal waters. It is located off the North Wales coast and officially began generating in November 2003.

To solve the problem, ERA undertook a fast transient (transient overvoltage) study of the complex power system interactions that could occur and their impact on the performance of the associated 33kV substation equipment.The objective was to assess the capability and verify the suitability of the 33kV switchgear with respect to switching operations and fault clearance duty.

The study investigated a range of switching conditions that the 33kV switchgear would be subjected to, including energisation, de-energisation and fault interruption. The results of the studies were then compared with the appropriate switchgear characteristics to assess their capabilities under those conditions.

The ERA study provided important information to the wind farm operator about the 33kV power system and assurance of the suitability of the installed equipment.This ensured that the DNO connection agreement was accurately defined and any necessary procedures were in place to ensure the correct operation of the DNO equipment under all operating modes of the wind farm.

ERA recommends a range of studies to confirm the suitability of equipment within power system substations and to ensure the safety of personnel, including:
	steady state calculations to check voltage profiles and equipment loading
	fault calculations to determine switchgear fault ratings
	protection settings
	transient calculations to check system stability when disturbances occur
	fast transient studies to assess the impact of switching operations
It is important to include these studies early in a design project so that the following engineering considerations can be taken into account:
	appropriate coordination of insulation systems
	suitability of equipment prior to ordering
	planning the physical layout of a substation
	preparation of connection agreements

ERA's electrical engineering services include consultancy and power studies using classical software or the ElectroMagnetic Transients Program (EMTP). ERA has also developed Double-Fed Induction Machine (DFIM) wind turbine models for power system analysis and supplied ERA's power system analysis software (ERACS) to wind farm designers and operators.