Voltage Fluctuations

From Wind wiki

Voltage fluctuations are the changes that occur in the root mean square (RMS) value of the voltage produced in a time span of minutes or more. As stated by national standards, the allowable variations in nominal voltage is 230/400 V for 50Hz systems. At this rate, the user’s terminal voltage must not differ more than ±10% from the rated voltage.

Wind turbines cause voltage fluctuation because of the different energy content of the wind with which the turbine interacts. On the grid, it is mostly caused by the varying load and power production units. When wind power is introduced, fluctuation starts to occur because wind turbine power production varies, a phenomenon caused not only by wind variations. It may also be due to an emergency stop at a start under high wind conditions where power goes from full to zero or vice versa.

There are several ways used to calculate voltage fluctuations. Some companies use computer programs available to predict voltage fluctuations caused by load variations. Another method is by calculating the voltage variation caused by the grid impedance Z, the active power P and reactive power Q, as in the following formula:

(formula here)

The formula above uses a simple impedance model. U1 stands for the fixed voltage at the end of the power system and U2 for the voltage at the point of common connection, PCC. The PCC voltage can be expressed as follows:

U2 = √a+ √(a^2 )- b

Where

U_(1 )^2 a = - (RP + XQ) 2

b = (P2 + Q2) Z2