How To Conduct A Wind Farm Impedance Test?
Across the world, wind farms have become the preferred choice for the provision of renewable energy supply. Historically the first known wind turbine was constructed by Professor James Blyth of Anderson's College Glasgow (now Strathclyde University) in Scotland in 1887, the following year the first known US turbine was created by inventor Charles Bush.
In December 1980 US Wind Power installed the first wind farm on the shoulder of Crotched Mountain in New Hampshire. The size of the generators at that time, as well as the size of the farms were very small by today’s standards, with 30 Kilowatts generators and consisting of just 20 Turbines.
When the 11 Turbines of the world’s first offshore windfarm were constructed and commissioned at Vindeby in Denmark by Orsted in 1991, not many believed it was possible or practical to operate wind turbines at sea, but today that is very common all over the world. Today the world over both and offshore wind farms have become commonplace.
The size of modern generators can vary, reaching a generator capacity of 8 MW and output of 66 kV. These turbines are then interconnected via cables with an umbilical cable coming ashore (in the case of offshore farms) to the grid connection point. Then the connection to the grid will be through grid transmission lines. The length of the transmission cables will be determined by the rated capacity and the distance of the farm to the grid.
The protection is therefore carried out using Distance Relays, these relays monitor:
- Impedance
- Reactance
- Admittance
- MHO
Therefore, the correct operation of these relays is dependent upon knowledge of the impedance and reactance of the circuit under protection. That includes the cables themselves (having an impedance and reactance proportional to their length) and the impedance of the switchgear in use. To ensure the correct operation of these relays and the protection of the system as a whole, accurate measurements are vitally important instead of relying on calculations.
An ideal instrument to measure the system would be a Line Impedance Measurement Tester, such as the Z-OVR manufactured by T&R Test Equipment Limited.
The Z-OVR is a Cable Impedance Measurement Test Set. It is made up of a control unit and a load unit. There are 3 output ranges 75V 80A, 150V 40A and 300V 20A. The idea is to select the V range to inject the maximum current into the cable. The resultant magnitude of the voltage and phase angle across the line is measured. The Current, Voltage, Phase Angle and Impedance (Z & X) is then shown.
The unit was initially designed for overhead and underground lines; as such Z-OVR is ideal for measurements within the Wind Power Industry when taking Distance Relay measurements.
So What’s The Method When Testing Line Impedance?
Connect the tester locally whilst at the far end linking the 3 phases and earth together.
(See the diagram below)
The Impedance Test Setup Procedure would be conducted as follows…
- Test A to B
- Test B to C
- Test A to C
- Lastly test AB &C in parallel to earth
The information gained would then allow the engineer to determine the Impedance of A, B and C thus giving the engineer the correct information to setup the distance relays correctly and avoiding guesswork!
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