Reactive Power Ancillary Service by Constant Power Loads in Distributed AC Systems

Reactive power is one of the power system ancillary services and can be supplied from either static or dynamic VAR sources. Provision of reactive power in the proximity of load is clearly beneficial for the efficiency of the overall system. Reactive power provision as an ancillary service by the load itself could greatly increase the margin to voltage collapse and, therefore, influence the stability of the power system. Constant power loads (CPLs), interfaced by active rectifiers, have the potential for providing reactive power to the ac distributed system they are connected to. These types of loads with negative resistance characteristic are, at the same time, the most susceptible to voltage stability problems under abnormal operating conditions. This paper first investigates the effect that a large share of CPLs will have on the voltage stability of distributed ac systems during voltage dips. It then investigates the influence of introducing reactive current control by the active rectifiers on the overall voltage stability of the system. The critical clearing time of the system is selected as a measure of the transient stability limit for investigating and comparing the cases of CPLs with and without reactive current control. Centralized reactive compensation by a STATCOM is used as reference to compare with distributed compensation by the CPLs. Results show that the total required distributed injection of reactive current is lower compared to the rating of a centralized STATCOM. The influence of reactive compensation on the total attainable CPL current is investigated analytically as well as through simulations. It is observed that an optimal reactive current injection by CPL reduces the total current to a minimum value.