Voltage Regulation Strength at DSO/TSO Interfaces: Swedish Case Studies

Reactive power provision from distribution systems to support transmission systems' voltages has been identified in the literature as viable alternative in cases where access to transmission-connected reactive power resources, such as synchronous generators, is diminished. Distribution systems' potential to provide reactive power is usually described through capability charts at DSO/TSO interface giving feasible values of active and reactive power exchange at the interface for different operating conditions of the distribution system. This paper discusses that such description should be complemented with information on short- to medium-term reactive power response of the distribution system to the voltage changes at the DSO/TSO interface. For this purpose, a quantity called voltage regulation strength at DSO/TSO interface is introduced. A method on how it can be calculated is presented and its application is illustrated on case studies of Swedish distribution systems.