Relaying problems in the connection of a dispersed generating plant to the 132 kV network

The problem of power swings as a consequence of transient performance of generators after fault removal is well known and easily faced if previously taken into account in the planning stage of a transmission network where large power plant are likely to be connected. On the contrary, this may result in a problem when power swings occur as a consequence of the connection of a power plant on a network planned to operate only supplying loads. False tripping of line distance relays is to be expected with the consequent risk of network separation in unexpected points, thus increasing the risk of network blackout. The paper illustrated the benefits obtained from a stability study in the setting of line distance relays previously installed on a 132 kV network, where a new 140 MW combined cycle power plant was set up. The simulation of several disturbances allowed the setting of the existing distance relays without adding any power-swing blocking function. The plant has been in operation for two years: few disturbances on the network have been experienced so far and no unwanted tripping of distance relays occurred.