The current interruption process in vacuum analysis of the currents and voltages of current-zero measurements

Abstract

The circuit breaker helps protecting vulnerable equipment in a power network from hazardous short-circuit currents by isolating a fault, when it occurs. They perform this task by extinguishing a plasma arc that appears as soon as the breaker's contacts separate, and through which the short-circuit current flows. In an AC network, the current's value runs periodically through zero, and each current zero provides the breaker with an opportunity to quench the arc, because here, its energy input is temporarily zero. Due to the inductive nature of most short-circuit networks, the voltage tends to rise immediately to its maximum value after the current interruption. This complicates the current interruption process for breakers, because just after they have been loaded with the arc, they have to cope with this recovery voltage as well. To ensure their reliability, new circuit breakers are subjected to tests with artificially generated short-circuit currents and recovery voltages, with values that are appropriate for the network in which they are intended to use. These tests follow strict rules, recorded in standards such as the IEC 62271-100, about the size and shape of the current and voltage waveforms. Specialised institutes, such as the High Power Laboratory at KEMA, perform such tests and hand out certificates to breakers that pass all tests. The certification process usually provides little more information than that the breaker passed a test, or not, and it would be beneficial for both the certification institute, and the breaker's manufacturer, to obtain more information form the tests. Such analysis on SF6 breakers has already taken place with success in the past, and this work applies it to vacuum circuit breakers. Vacuum circuit breakers (VCBs) are the most widely used type of breakers to protect distribution level networks, with operating voltages of up to 72.5 kV. This thesis analyses the electrical signals from short-circuit interruptions in vacuum, to detect trends and indicators on the breaker's performance. For this purpose, it describes the test circuits and the measuring techniques, used to obtain the electrical behaviour of the vacuum circuit breaker just after current zero. This includes the efforts to reduce the distortion from the strong electric and magnetic fields that inevitably involve a short-circuit test. After its extinction, the vacuum arc leaves residual plasma behind, which provides a conducting path through which a post-arc current can flow. Since the post-arc current is the most distinctive electrical signal in a vacuum current interruption, the analysis mainly focusses on this phenomenon. The residual plasma decays within microseconds, thereby finishing the breaker's transition from a near perfect conductor to a near perfect insulator. The thesis pays special attention to the measuring equipment that was used to track these fast changes in the signals (sub-microsecond), and its large dynamic ranges (from kilo amperes to tenths of amperes, and from volts to kilo volts). In addition to the post-arc current research, the thesis analyses the VCBs reignition behaviour. Since VCBs are created to prevent reignition, they had to be subjected to much higher currents and voltages than their rated values, to force them to reignite. These results, and the results from the post-arc current research, provide new insight in the current quenching mechanism in vacuum. Finally, this thesis also pays attention to the interaction between the electrical circuit and the VCB after current zero. To this end, it describes how existing models are extended with theories and insights that emerged from this research. The result has been implemented as a function block in Matlab's SimPowerSystems, which facilitates the incorporation of the model in different electrical circuits.Electrical Engineering, Mathematics and Computer Scienc