Assessment of hydrogenerators capacity to provide the system service of voltage control in the secondary band

In the context of qualification of the hydrogenerator groups from Romania for providing system services, tests were carried out, aiming to: assess their up-to-date and actual capacity to provide the system service of voltage control and to determine on the diagram of reactive power versus active power two zones (for inductive and capacitive reactive power), defined as the secondary band of voltage control, where producing or absorbing reactive energy incurs higher costs and high wear of the generator. In these zones the reactive energy produced or absorbed on System Operator's demand is paid. The performed tests revealed more types of generators behavior. In order to find explanations for the generators behaviors, an extensive program of tests, followed by theoretical computations was performed. The results of the investigations are presented in the paper.