Local Dynamic Reactive Power for Correction of System Voltage Problems

Electricity distribution systems are experiencing outages due to a phenomenon known as local voltage collapse. Local voltage collapse is occurring in part because modern air-conditioner compressor motors are much more susceptible to stalling during a voltage dip than older motors. These motors can stall in less than three cycles (0.05 s) when a fault, for example, on the subtransmission system, causes voltage to sag to 70 to 60% of nominal. The reasons for this susceptibility are discussed in the report. During the local voltage collapse, voltages are depressed for a period of perhaps 1 or 2 minutes. There is a concern that these local events are interacting together over larger areas and may present a challenge to system reliability. An effective method of preventing local voltage collapse is the use of voltage regulation from distributed energy resources (DER) that can supply or absorb reactive power. DER, when properly controlled, can provide a rapid correction to voltage dips and prevent motor stall. This report discusses the phenomenon and causes of local voltage collapse as well as the control methodology that the authors have developed to counter voltage sag. The problem is growing because of the use of low-inertia, high-efficiency A/C compressor motors and because the use of electric air-conditioning is growing and becoming a larger percentage of system load. A method for local dynamic voltage regulation is discussed which uses reactive power injection or absorption from local DER. This method is independent and rapid and will not interfere with conventional utility system voltage control. The results of simulations of this method are provided. The method has also been tested at the ORNL’s Distributed Energy Communications and Control (DECC) Laboratory using its research inverter and synchronous condenser. These systems at the DECC Lab are interconnected with an actual distribution system, the ORNL distribution system, which is fed from the Tennessee Valley Authority’s 161 kV subtransmission backbone. The test results are also provided and discussed. The simulations and testing show that local voltage control from DER can prevent local voltage collapse. Our inverter-based DER can start to respond in 100 μs or less to the start of a transient voltage change; our rotating-based DER is slower but still can start to respond in milliseconds. The results also show that the control can reach a new steady-state operating (reference) point so quickly, within 0.5 seconds, that it does not interfere with conventional utility methods. With regard to air-conditioning stall, although the 0.5 s steady-state response may not be fast enough, our controls will result in immediate response by the DER. With enough reactive power output from multiple DER, they could keep the voltage raised high enough and long enough to prevent stalling. As indicated in the report, not all air-conditioner compressors will begin to stall at once; therefore, DER should be able to at least mitigate a complete stalling of the air-conditioners even if the voltage sags below the stall voltage for some units.