Load modulation at two locations for damping of electro-mechanical oscillations in a multimachine system

Modulation of customer loads for increased power system damping is suggested as an alternative to control of HVDC links and FACTS devices. The loads are modeled as real power controlled by local bus frequency, which is equivalent to viscous damping in a mechanical system. The qualitative aspects of the interaction between two load modulation controllers are studied using linearized models of two test systems with three and twenty-three machines respectively. Root locus plots show that transfer function zeros limit damping. Tuning a controller for maximum relative damping is equivalent to impedance matching. This is used to explain the zero dynamics and why a second controller may not be able to contribute to added damping. Time simulations of a nonlinear system model verifies the final design of the controllers for the large test system.