Investigation and improvement of transient response of DVR at medium voltage level

Dynamic Voltage Restorer (DVR) is proposed for use in the power distribution network to protect sensitive loads from sudden sags in grid voltages. An area of interest for DVR research is the damping of transient LC oscillations initiated at the start of a voltage sag and at the recovery instant from a voltage sag. Nonlinear loads, with harmonic currents around the DVR LC filter resonance frequency, would continuously excite resonance oscillations. For compensating voltage sags and damping high frequency oscillations simultaneously, an investigation of the transient response of the DVR has first been carried out, with the inclusion of the source of LC resonance and factors that affect this resonance. Possible control schemes and their effects on the oscillation attenuation are also studied. Such studied control schemes are: single voltage loop control, voltage feedback plus reference feedforward control, double-loop control with an outer voltage loop and inner current loop. Subsequently an effective and simple resonance damping method is proposed by employing closed-loop (single-loop or multi-loop) control with an embedded two-step Posicast controller. Finally, the transient responses for proposed control methods have been extensively tested on a 10 kV laboratory DVR system with different loading conditions. It is shown that with a linear load, the proposed damping methods are effective for transient response improvement, while for a nonlinear load, the methods would improve both the transient and steady state performance.