Carrier signal-based protection to prevent dispersed generation islanding on mv systems

Dispersed generation connected to distribution system can supply unintentional system islands, isolated from the remainder of the network. Since these events pose an actual risk to safety and equipment they must be quickly detected and eliminated. Unfortunately, passive protections based on traditional measures (under/over-voltage, frequency) demonstrate, in particular system operating condition, the possibility of failing the island detection (non-detection zones) [1]. In this paper the feasibility of a relay protection based on Distribution Line Carrier (in the following DLC) signal is investigated for MV distribution systems. The proposal consists in the continuous injection, through a suitable coupling device, of a phase-to-phase superimposed carrier high frequency (a kind of guard signal) at the busbar of HV/MV substation. In this way the signal can be transmitted at any section of the feeder so that any line tripping event will thus be immediately detected by each dispersed generator at the common coupled point, independently of the actual power flowing in the feeder before the loss of main supply. It has been implemented the high frequency models for overhead lines with ground return and cable lines (Carson's and Wedephol's theories) and has been simulated the transmission signal in different system configurations. It has been studied the dependence of signal attenuation on the type of line, the number of lines entering a substation and on the presence of compensation capacitor banks installed at the primary substation bus. Simulations allow us to predict standing wave patterns and the possibility of assessing, with the presence of more receivers, an “optimal mismatching” for the high frequency transmission. Moreover two measurement campaigns have been performed on real MV networks in order to validate the theoretical results.