  To implement controllable loads for frequency regulation in transmission
networks in a practical way, the control scheme needs to be granulated down at
least to subtransmission networks since loads in transmission networks are
usually the aggregation of lower voltage networks. However, not only frequency
but also bus voltage will be affected by active power changes in
subtransmission networks due to a higher R/X ratio of transmission lines.
Further, the costs for loads participating in frequency and voltage regulation
should also be considered. In this paper, a control scheme is proposed for
electric spring (ES) aggregators which consist of back-to-back ESs and
exponential type of noncritical loads in subtransmission networks. A
distributed optimization which aims to minimize the costs and implements both
frequency and voltage regulation is adopted for ES aggregators to obtain new
active and reactive power setpoints by sharing information with neighbors.
Power consumption of each ES aggregator is then adjusted accordingly to conduct
frequency and voltage regulation simultaneously. Simulation results show that
ES aggregators are able to achieve required active power response and regulate
frequency cooperatively, and meanwhile maintain bus voltages within the
acceptable range with minimized costs under the proposed control scheme.
