System Protection Schemes in Eastern Denmark

The Ph.D. project investigates different aspects of voltage stability in the power system of Eastern Denmark taking into account the large amount of wind power. In the project, a simple System Protection Scheme (SPS) against voltage instability in Eastern Denmark is developed. The SPS design is based on static and dynamic simulation analyses using a large-scale model¹ that considers a number of realistic power system conditions. The southern part of the 132-kV system is prone to voltage stability problems due to reactive power deficit in the area and the long distance to the large generating units. It was observed that the addition of large amounts of wind power to a relatively weak power system without reinforcements may cause voltage instability in the power system. The maximum power transfer in the heavily loaded system with two large off-shore wind farms is approached at about 70% of the wind generation capacity in Eastern Denmark. The restricted reactive power transfer from the 132-kV main system is the key indicator of voltage instability. The high load situation with high wind generation is considered a worst-case scenario in relation to serious problems with reactive power. Line outages in the southern part of the 132-kV system introduce further stress in the power system, eventually leading to a voltage collapse. The local System Protection Scheme against voltage collapse is designed as a response-based scheme, which is dependent on local indication of reactive and active power flow in relevant 132 kV lines, violation of SVC rating and low voltages at selected 132-kV buses. As supplementary input, the SPS includes phase angle measurements from two separated locations in the 132-kV system. The phase angle recordings between the remote points can be used instead of measurement of active power in the tie-lines. The power transfer in the 132-kV system is improved by additional reactive power support in the system using voltage control devices and/or SPS control actions: adjustment of adequate SVC setpoint, switching of additional capacitive shunts, start-up of Masnedø gas turbine etc. Stigsnæs power plant could possibly improve the reactive power support in emergency situations, as it is the closest power plant in the southern part of the system. In general, rescheduling power plants and voltage regulation at remote generators (MVAr adjustment) are not considered the most effective measures, because they are associated with large reactive power losses in the transmission system. Ordered reduction of wind generation is considered an effective measure to maintain voltage stability in the system. Reactive power in the system is released due to tripping of a significant amount of wind turbines based on induction generators. On the other hand, the wind turbine rejection is associated with loss of active power that has to be compensated using immediate reserves. To avoid unnecessary disconnection of wind turbines, fast fault clearance time is the main factor. Phasor Measurement Units placed at strategic points are evaluated as an efficient tool for power system monitoring of important 400 kV and 132 kV transmission corridors in Eastern Denmark. The first PMU is connected to a 400 kV bus near Asnæs power plant, the largest generating unit in Eastern Denmark. The PMU in Radsted (RAD) is connected to a central bus in the southern part of the 132-kV system, which is close to a large concentration of Wind turbines. The third PMU at Hovegård (HVE) is selected, because the bus terminates the interconnection to Sweden. Hovegård is geographically located near a number of power plants and the load centre in the Copenhagen area. The PMU at HVE is considered as a reference for the recorded phase angles at Asnæs and Radsted. Real-time phasor measurements are utilized for tracking power system dynamics during a number of severe power system disturbances that are characteristic for the Eastern Danish power system, such as wind farm rejection, cascading line outages and power oscillations. E.g. Nordic inter-area oscillation modes and damping were easily detected from phasor data during the outage of the 400 kV tie-line between Eastern Denmark and Sweden. It is concluded that recording of power oscillation frequencies is more convenient than computation of eigenvalues using a detailed dynamic model for the Nordic power system. One case study demonstrates that the recorded power system response is consistent with the simulation results. The application of synchronized phasor measurements for, e.g., validation of power system models used in stability studies, is seen as having a great potential in the future. It was evaluated that PMU applications are mostly suitable in large interconnected networks, where abnormal dynamic changes in phase angles, power flows, frequency etc. can be easily observed. Great potential in the future is seen in advanced System Protection Schemes (SPS), where PMUs give precise input about the actual system state. In the future, the PMU units could enable the system operator to detect catastrophic events in due time and issue remedial orders in the power system. In that way, the power system capability could be extended beyond normal limits.