Voltage stability and distance protection zone3

In this work, an investigation of the role played by line distance protection zone3 towards voltage instability resulting into a major system blackout of large power network was carried out. This is with a view of finding out the scenarios that can lead to voltage collapse with all the three zones of the distance protection relays activated. The motivation for doing so is that; recent large power blackouts generated lots of controversial debates in Europe and in the United States and the concept of voltage collapse is still a subject of research under investigation. In addition, there is a disagreement over the exact nature of the negative forces that triggered the big blackouts. This raised several questions as “who to be blamed”. Among several others distance protection zone3 has been blamed for the majority of voltage collapse incidence that took place in 2003. The study begins with developing and solving the load flow of the simple IEEE14-bus test system. To create a scenario similar to the actual blackout in 2003 Sweden/Denmark a detailed dynamic simulation of standard CIGRE Nordic32-bus network which closely resembles Swedish network is carried out and the critical lines that led to cascaded tripping were identified using PSS/E software. As a short term solution to voltage collapse, a mitigation scheme incorporating underfrequency relays and undervoltage relays is implemented. According to the findings of this work, within 2.52 seconds with respect to the triggering event all the critical lines tripped in cascade and 370 milliseconds is the time lag observed between the commencements of the cascaded trippings to the last event. As an additional example from the simulation result, the reactive power losses increases for the same loading condition by 47% during the pre-fault condition when tap changers are blocked and raised to 52% when tap changers are in operation. It was observed that zone3 distance relay operation should not be entirely blamed for the cascaded trippings because even when zone3 and zone1 were blocked, the voltage collapse still occurred in zone2, since at that critical condition the apparent impedance seen by the distance relay traverse through all the three zones. Moreover, it is shown that switching out of major transmission lines is associated with large increase in power losses and if these losses are determined using simulation software prior to granting a planned outage and adequate reactive compensation is provided, then the need for load shedding and risk of voltage collapse could be reduced