Constraints Relieved by Reactive Following

 The Active-Power, Reactive-Power and Voltage Constraints within the wind power plants (WPPs) or Solar Power Plants (SPPs).

There are many dynamic constraints that occur within a wind power plant or solar power plant. We present just some of the constraints which prevent the wind plant from providing the ranges of active power, reactive power and voltages at the point of interconnected to the transmission system.

Collector Impedance

In the case of wind generation plants with long collector lines, going from to the furthest out wind turbine generators (WTG) from the Main Station transformer, there can be over voltage challenges at the WTG terminals due to the voltage rise.  Such challenges occur at the FAR wind turbines since they are the farthest from the point from the interconnect.

Looking from the WTG to the POI, there is a change in voltage due to the resistance and reactance of the WTG’s generator step up transformer from a lower voltage to usually up to 34.5 kV. The voltage rise at the WTG may be most severe at maximum generation which includes a maximum angle.


An automatic tap changer installed on the main station transformer, which is not operating properly may cause a reactive constraint at the wind power plant since it is unavailable to control the voltage for the collector system. On Load Tap Changers(OLTC) located on the station Transformer are mechanical devices which need maintenance and do fail from time to time. OLTCs change the winding ratio of the station transformer in small increments in an  “attempt” to control the voltage on the 34.5 kV side of the transformer to regulate the voltage of the distributed generation plant.  Generally such devices are slow.

Depending on the design of the wind plant, and the volatility of the wind, an OLTC may operate a number of times per day, and may fall behind or just fail. This may be bad news for the power plant.

The constraint at the Main Plant Transformer (MPT) occurs when the tap is out of the desired position and the wind turbines go into local reactive control and start reactive limiting causing a deficiency of reactive power at the high side of the MPT.  In some cases, instability may occur within the plant depending on the capability of the wind turbines.  At this point, the plant may not supply the reactive power at the point of interconnect, or worse wind turbines go off line and there is a loss of production.

MPT Impedance & Constraints

The impedance through the Main Station Transformers creates a reactive constraint, where the series impedance contributes to a consumption of capacitive reactive power and causes a rise in voltage on the low side of the MPT and cause the dynamic reactive power sources to limit their reactive power.

Constraints due to Generator Step Up (GSU) Transformer

The constraint caused by excessive voltage at the mains of the WTG or solar inverter is due to active and reactive power levels of the generators within the plant; as the power changes, the reactive production and consumption changes, this in turn cause the voltage to change on the collector system. Even though the voltage will not be the same throughout the collector system,  some equipment will see excessive voltage and will reduce its contribution of reactive power when the voltage goes out of normal range, for example, 95% or 105% percent at the mains of the low side of the GSU.

Reactive Following along with dynamic studies with PSCAD provide the necessary relief to WPP or SPP from its own internal Active-Power, Reactive-Power and Voltage constraints which prevent it from dynamically meeting its simultaneous Active Power, Reactive Power and Voltage requirements at the Point of interconnection.


Copyright Thomas Wilkins et al.  2017, All Rights Reserved