Using the grid-side converters of renewable energies for ancillary services

The modern versions of the converters used for renewables (e.g. variable speed wind turbine generator) are connected to the lines (grid) through a both sides PWM controlled dc link voltage source inverter. The fast current control of the line currents by the grid-side converter makes possible to compensate the deviation of the line currents from their fundamental component. This possibility can be used to compensate the lower order current harmonics in the line currents (generated by nonlinear loads on the lines) and to balance the negative sequence current component (caused by asymmetrical load). Besides, advanced control strategies make possible to control the reactive power of the lines, resulting in power factor correction. Since all the mentioned subjects of compensations require at most pulsating active power with zero mean value, the listed compensations are possible even at the absence of the renewable energy source (e.g. at no-wind condition). All the investigated ancillary services improve the power quality of lines (grid) where the renewable energy converter (e.g. wind turbine generator) is connected to. A key element of the system is the sensing of the component to be compensated. A space-vector based sensing method is proposed and investigated. The implementation issues of the presented control methods are detailed. The methods are investigated and validated by simulations of practical examples.