Unlike the conventional power generation plants that generates stable and more predictable amount of power, wind power generator output power is, more or less, unpredictable due to the intermittent wind speed which is the primary source of energy that is being converted by the wind turbine to electricity. Such variation of output power of wind farm may affect the stability and security of the transmission system.
There are various methods that can be applied to study the impact of wind farm on the transmission grid. A power flow study must be conducted to determine the condition of the transmission system, specially the voltages at different locations and line loading of the wire, during extreme conditions like off peak and peak load periods. Voltage Sensitivity Analysis must also used to determine the sensitivity of the system from changes in real and reactive power injection in various nodes. And finally, a probabilistic power flow simulation must be conducted to determine the effect of random variation of load consumption and generation (due to Outage Rate, FOR, for conventional generator, and due to the random fluctuation of output power of wind farms) to the transmission system. Probabilistic Power flow simulation is a more applicable methodology for assessing the transmission system since most of the components of the power system behaves in a stochastic manner.
The following are the recommended steps to analyze the effect of connecting a wind farm in the system;
Power Flow Study
The main objective of the power flow study is to determine the line or branch power flow in the transmission lines and the voltage profile during peak and off-peak load of the system when the wind farm output power is at maximum and minimum.
Voltage Sensitivity Analysis
The voltage sensitivity analysis is used to analytically determine the per unit change in voltage for every per unit change in real and reactive power. It is simply a measure of how sensitive the voltage on particular bus with respect to the change in power and reactive power on the other bus at a certain load condition. Using voltage sensitivity analysis, it can be determine which nodes in the grid are the most sensitive to the variation of power (active and reactive) of the wind farm.
The voltage sensitivity analysis is based on the sensitivity matrix that was derived from the Jacobian Matrix and Newton-Rhapson power flow equations;
The bus voltage sensitivity is equal to derived from the above equations.
Probabilistic Load flow
The probabilistic load flow outputs the probability distribution function (PDF) of the voltage fluctuation in every location of the grid due to the stochastic variation of output power from the generator, most specially, from wind farm.
It is also a good consideration to study the effect of the power output fluctuation of the wind farm to the system frequency and dynamic and transient stability of the transmission system. However, this study will require a detailed data of all the generators of the system which is very hard to gather. It will also require a sophisticated software to perform the analysis.