1 Abstract--With the increasing use of High-Voltage Cables, which have different electric characteristics from Overhead Lines, phenomenon like current zero-missing start to appear more often on the transmission systems. Methods to prevent zero- missing phenomenon are still being studied and compared to see which countermeasure works the best. Technically the best way to avoid zero-missing phenomenon produces very high switching overvoltages, making the operator to choose to either avoid the zero-missing phenomenon or to minimize the switching transients. This paper presents a method of determining an optimal value of the resistance of the pre-insertion resistor that results in minimizing both the zero-missing phenomenon and switching overvoltages simultaneously. Index Terms--Cables, Circuit Breaker, Pre-Insertion Resistor, Shunt Reactor, Switching Transients, Level-crossing problems I. INTRODUCTION he increase of public pressure regarding the devastation of areas of natural beauty by Overhead Lines (OHL), drove to increase demand for the substitution of the OHL by underground cables. The Danish Transmission System Operator (Energinet.dk) is studying a possible expansion of the high-voltage system using AC underground cables instead of OHL. One of the problems that must be studied when using systems with underground cable is zero-missing phenomenon. Electrically the main difference between an OHL and an Underground Cable is the much higher capacitance of the last ones, which can be dozens of times superior to the capacitance of OHL [2]. Because of this high capacitance, it is required to connect This work was supported in part by the Danish Transmission System Operator, Energinet.dk. F. F. da Silva is a PhD student at the Institute of Energy Technology, Aalborg University, 9220 Aalborg, Denmark (e-mail:ffs@iet.aau.dk). C. L. Bak is with the Institute of Energy Technology, Aalborg University, 9220 Aalborg, Demark (e-mail: clb@iet.aau.dk). U. S. Gudmundsdottir is a PhD student at the Institute of Energy Technology, Aalborg University, 9220 Aalborg, Denmark (e-mail: usg@iet.aau.dk). W. Wiechowski is a Senior System Analyst at the Planning Department of Energinet.dk (e-mail: wwi@energinet.dk) M. R. Knardrupgård is with the Planning Department of Energinet.dk (e- mail: mra@energinet.dk) shunt reactor(s) in parallel to the cables, in order to compensate the reactive power generated by them. If the shunt reactors are compensating all the reactive power generated by the cable, the AC component of the current in the cable has opposite phase angle to AC component of the current into the shunt reactors, and therefore they cancel out each other. As the transient of the shunt’s reactor current contain both AC and DC components, during transient conditions (as for instance energization of a cable compensated with shunt reactors) under ideal conditions only DC component would remain. During energizing, the cable has no load and it is open in the far end. As the resistance of the system (cables+shunt reactors) is very small, it may take several seconds for the DC component to be damped. As the current does not cross zero during those seconds, it is not possible to open the circuit breaker without risking damaging it, unless it is prepared to interrupt DC currents or currents with several amperes [3][4]. If in the meanwhile, a single-phase or two-phase fault occurs on the cable, the circuit breaker will be able to open the faulted phases but not the healthy phase(s) because of the lack of zero-crossings of the current. This can leads to a damage of the circuit breaker. A method that can be used to damp the DC component in just half of cycle, is to use circuit breakers equipped with pre- insertion resistors. II. ZERO-MISSING PHENOMENON AND SWITCHING OVERVOLTAGES An easy way to understand zero-missing phenomenon is to analyze the transient behaviour of an inductor in parallel with a capacitor of equal impedance. In this situation the currents in each one of the elements have equal amplitude and are in phase opposition. But in transient conditions the current in the inductor also has a DC offset, whose value depends on the voltage value at the connection moment. There is a 90º difference between the phases of the current and the voltage in an inductor, therefore if the inductor is connected in a voltage peak the current should be zero. If it is connected when the voltage is zero the current should have a peak value. As the current in the inductor must maintain its continuity, and it was zero before the connection of the Use of a Pre-Insertion Resistor to Minimize Zero-Missing Phenomenon and Switching Overvoltages F. Faria da Silva, C. L. Bak, U. S. Guðmundsdóttir, W. Wiechowski and M. R. Knardrupgård T 978-1-4244-4241-6/09/$25.00 ©2009 IEEE