Voltage Responsive Distribution Networks: Comparing Autonomous and Centralized Solutions

Power system voltage control has traditionally been the responsibility of transmission-connected reactive power resources. Accordingly, high penetration levels of distributed generation present new challenges for reactive power management. Simply stipulating voltage control operation for distributed generators will not generally deliver voltage-responsive reactive power flows at the transmission system level, due mainly to the voltage-isolating effects of tap-changing bulk supply transformers. Additionally, the resistance of distribution system conductors establishes an unhelpful interaction between active power flows and voltage magnitudes. This work uses optimal power flow techniques to explore two ways to overcome these challenges. Most innovatively, a methodology is presented to optimally select static voltage control settings for distributed generators and transformers, such that they will provide an autonomous voltage-responsive behavior without supervisory control systems. In this scheme, distributed generators are exposed to transmission voltage fluctuations as far as is feasible, by blocking the tapping of the bulk supply transformer when operating within an optimally-determined range of transmission voltages. Comparatively, an active control scheme is presented, where reactive power and tap positions are dispatched period-to-period to support the transmission system voltage. Comparing these approaches suggests the level of smart-grids investment required to effectively harness the reactive power available from distributed generation.