Power electronic loads as providers of reactive power ancillary service to the grid: Analytical and experimental study

Reactive power injection has been the common practice to stabilize and support the voltage in electric power systems and its provision was traditionally considered to be part of the duties of the units dispatching active power. This article proposes the provision of reactive power by electric loads which are interfaced to the grid by active rectifiers that enable them to inject reactive current to support the electric grid voltage locally. The reactive power characteristic at the point of load connection in a distribution system was analytically obtained and experimentally verified in a reduced scale laboratory model. The obtained reactive power curves are used to determine the required rating of the converter interfacing the load, for consideration of stability and operability margins, effect on power factor, impact of distributed compensation and for operation after transients. Relevance of the curves is also discussed for low voltage ride through (LVRT) operation of the converters interfacing the loads. Influence of the type of line (X/R >1, X/R=1, X/R<1) is discussed as well as the sensitivity of the reactive power compensation to the resistance and reactance of the line impedance. The required rating of the converter for voltage compensation in steady state conditions is found to be high when a single load is considered, but can be significantly reduced and become reasonable when distributed compensation by several loads is assumed along the line. Although the analytical investigation considered only ideal controllers, behavior of the system during experiments suggests that the control of the converter is a critical aspect of the stability problem especially during and after transient events and that the operability limits can be highly influenced by the controller stability. This suggests that special attention be given to the structure and tuning of controllers for stable operation of converters that are simultaneously absorbing power and injecting re- active current for voltage compensation.