engleski

The insulation of medium voltage networks with isolated neutral

In an isolated neutral system, the neutral has no intentional connection to ground and the system is connected to ground through the line-to-ground capacitances. For a single-line-to-ground fault on these systems, the only path for ground current to flow is through the distributed line-to-ground capacitance of the surrounding system and of the two remaining un-faulted phases of the faulted circuit. For these systems, two major ground fault current magnitude-limiting factors are the zero-sequence line-to-ground capacitance and fault resistance. Since the voltage triangle is relatively undisturbed, these systems can remain operational during sustained, low-magnitude faults. In transient arcing faults, at zero-current points, the fault can extinguish permanently or reignite again, as, for example, in intermittent faults, mainly based on the arc voltage (magnitude and its rate of rising) and the arc current magnitude. At higher magnitudes of fault current, faults are less likely to self-extinguish at the fault current natural zero-crossing because of the high transient recovery voltage. The isolated neutral point voltage gives rise to the overvoltage phenomenon, which increases the risk of a double line to earth fault. Intermittent arcing faults are particularly dangerous because they cause malfunctioning of protective relays and overvoltages. In isolated medium voltage cable network transient overvoltages may reach 3 p.u. just after the fault occurrence while the steady-state overvoltages may reaches 2.0 p.u. These overvoltages affect insulation system of the equipment in MV network leading to insulation degradation. When tracking a fault in ungrounded network, although certain operators merely identify the faulty feeder, accurate determination of the location of this fault is recommended (e.g. damaged cable or insulation fault in a device) to put it back into operation as quickly as possible. The synchronous demodulation principle can be used by measuring, first, the injection current flowing in the feeders (by the toroid sensors) and, second, the injection voltage. Insulation monitor device (IMD) applies either DC or a low frequency AC voltage between the network and the earth. In the case of AC, the device measures the current flowing back via network insulation impedance and calculates the voltage-current shift. It is then possible to determine the resistive and capacitive components of this current and thus relate the threshold to the resistive component only. Development of this tracking method is encouraged by application of digital techniques to the management of electrical power distribution. This report includes specifications for MV equipment to withstand first fault (such as reinforcement of insulation of motors, transformers, cables, etc.) and international experience about MV fault location using intelligent fault location devices (IFL) or similar systems.

medium voltage networks ; isolated neutral ; intermittent arcing faults ; ground fault detection and localization

This study is conducted for Électricité de France (EDF) R&D.