Eliminating zero-missing phenomenon in long, high voltage, underground cables

Abstract: The maximum length of an high voltage underground cable (HV UGC) cable is oftenconstrained by the criterion that the cable cannot have more than 50% reactivepower compensation. If this limit is exceeded the current in the circuit breaker maynot have a zero crossing after energization, which is referred to as the zero missingphenomenon. This is problematic if a fault occur shortly after energization. Inthe past 10 years, different methods have been proposed which would allow greaterreactive power compensation. These methods either prevent the zero missing phenomenon(preventive methods) or provide a way to open the circuit breaker if afault occurs (handling methods).A new 200 km, 220 kV line has been proposed in Iceland referred to as Sprengisandslína.One proposed option is to build it as an OHL-UGC-OHL line as the voltagecriteria is not fulfilled if Sprengisandslína is built as an UGC with a 50% reactivepower compensation. The aim of this thesis is to see whether the zero missing phenomenoncan be avoided by implementing countermeasures - this gives a preliminaryresults whether Sprengisandslína can be built as an UGC. In this thesis the four differentpreventive methods are analyzed with a transient study for Sprengisandslína:(1) Pre-insertion resistor, (2) simultaneous synchronized switching, (3) synchronizedswitching where the cable is energized before the shunt reactor, and (4) synchronizedswitching where the shunt reactor is energized before the cable.Preliminary steady state studies were performed to determine the minimum numberof shunt reactors needed to fulfill the voltage criteria. The results showed thatthe minimum number needed were three assuming they are all of equal size locatedevenly along the cable (one at each end and one in the middle). Additionally, it isnecessary to see whether the generators would become underexcited if the cable isiiienergized with 100% reactive power compensation as it can reduce the lifetime ofthe generators. The results showed that the generators did not become underexcited.The countermeasure of synchronized switching where the shunt reactor is energizedbefore the cable and the countermeasure of simultaneous synchronized switchingwere shown to eliminate the zero-missing phenomenon when the cable was energizedwith 100% reactive power compensation. Synchronized switching where theshunt reactor is energized before the cable was seen to have lower switching overvoltages,21% higher than the nominal value, and the lower inrush current of 2.38kA. However, the method of simultaneous synchronized switching is cheaper andthe switching overvoltages and inrush current were within an acceptable margin(switching overvoltages were 35:9% higher than the nominal value and the inrushcurrent was 4.01 kA).The results of the study indicate that Sprengisandslína can be energized as an UGCwith 100% reactive power compensation if either the countermeasure of simultaneoussynchronized switching or synchronized switching where the shunt reactor isenergized before the cable are used. However, a detailed frequency study must beperformed before either of the countermeasures can be recommended.