A New Reactive Power Control Method of Intermittent Distributed Generators for Conservation Voltage Reduction

Abstract

This dissertation proposes a network-model-based conservation voltage reduction (CVR) method considering distributed generators (DGs) and a novel reactive power controller for intermittent DGs to achieve reliable and economical CVR.

CVR is a concept linked to reductions in energy consumption through reduced voltage amounts supplied to customers. The network-model-based CVR method implements CVR in a centralized manner and determines the references for volt/var control devices based on an optimization scheme. The impacts of DGs on the network-model-based CVR method were analyzed. The result showing that the inaccuracies of the DG model and intermittent active power can degrade the CVR effect. Moreover, intermittent active power can increase the operation cost of a distribution system by causing frequent switching operations of tap changes and shunt capacitors. Because the CVR function is periodically executed with a certain time interval in a distribution management system (DMS), these problems cannot be solved by using only the CVR function. In other words, both the CVR function and the local control system should be utilized.

This dissertation proposes unified models for a three-wire voltage source converter (VSC), which is widely used in DG interface systems, for an unbalanced steady-state analysis. In a steady state, the active and reactive power supplied by a DG depends on the terminal voltage, output filter, voltage and current sensor positions, and power control strategy. Considering these factors, two equivalent circuit models consisting of an equivalent three-phase current source (ETCS) and some of filter impedances are proposed. In addition, the current output of each ETCS is formulated as a function of the active and reactive power reference, the terminal voltage, and the impedances of the output filter. With unified models, 35 different three-wire VSC types can be represented. The accuracy of the models are verified by comparing the results of a power flow study to results obtained from a PSCAD simulation.

In order to reduce the adverse effects of the intermittent DG on CVR, this dissertation proposes a new reactive power control method which it terms the two-segment active power versus reactive power (Q–P) droop method. The reactive power is controlled based only on the measured active power

thus, the proposed method corresponds to the general interconnection requirements for DGs. The relationship between the active and reactive power is represented by two line segments given by the active and reactive power references and the lower and upper droop constants. If the active power is lower than its reference, the lower droop determines the reactive power. Otherwise, the upper droop constant does. The droop constants are not fixed

they are determined by the CVR function in the DMS.

This dissertation proposes a network-model-based CVR method. In order to prevent frequent switching operations, the expected value of the active power for an intermittent DG, which changes slowly over time, is used to formulate the optimization problem. By solving the optimization problem, the references for the volt/var control devices are determined. Finally, the droop constants are determined based on the optimal state. In order to reduce the computational burden, the droop constants are calculated with the sensitivities. The droop constants are initially estimated under the assumption that only the active power output of a DG changes. Later, the droop constants are corrected considering the active power variations of multiple DGs.

In a case study using the modified IEEE 123-node test feeder with three DGs, it is shown that the effect of CVR is improved with the proposed models and methods. Specifically, with the proposed reactive power control method for an intermittent DG, the CVR effect can be remarkably improved.