Overvoltage Impact on Internal Insulation Systems of Transformers in Electrical Networks with Vacuum Circuit Breakers

Abstract

Vacuum circuit breakers are increasingly used as switching apparatus in electric power systems. The vacuum circuit breakers (VCBs) have very good operating properties. VCBs are characterized by specific physical phenomena that affect overvoltage exposure of the insulation systems of other devices. The most important phenomena are the ability to chop the current before the natural zero crossing, the ability to switch off high-frequency currents, and the rapid increase in dielectric strength recovery. One of the devices connected directly to vacuum circuit breakers is the distribution transformer. Overvoltages generated in electrical systems during switching off the transformers are a source of internal overvoltages in the windings. The analysis of the exposure of transformers operating in electrical networks equipped with vacuum circuit breakers is of great importance because of the impact on the insulation systems of switching overvoltages (SO). These types of overvoltages can be characterized by high maximum values and atypical waveforms, depending on the phenomena in the circuit breaker chambers, system configuration, parameters of electrical devices, and overvoltage protection. Overvoltages that stress the internal insulation systems are the result of the windings response to overvoltages at transformer terminals. This article presents an analysis of overvoltages that stress the transformer insulation systems, which occur while switching off transformers in systems with vacuum circuit breakers. The analysis was based on the results of laboratory measurements of switching overvoltages at transformer terminals and inside the winding, in a model medium-voltage electrical network with a vacuum circuit breaker.

Study on the Effect of Helium on the Dielectric Strength of Medium-Voltage Vacuum Interrupters

“…p e f f = C He × p reading (10) Below a pressure value of 3.00 × 10 −1 Pa, the pressure reading is equal to the actual value, where the heads operate in ionization mode (Penning). Above a pressure value equal to 3.0 × 10 1 Pa, the actual pressure value should be read from the characteristics provided by the vacuum gauge manufacturer.…”

“…Vacuum insulation, on the other hand, despite the fact that it is not harmful to the environment, has several significant disadvantages that make it difficult to use reliably. These include significant values of cut-off currents and the problem of overvoltages [10], as well as the difficulty of maintaining the integrity of equipment despite its large size.…”

A novel overvoltage protection scheme for power electronics switching under differential surge conditions

“…Florkowski et al [17] have explained over-voltages that strain transformer insulation methods happen when switching off transformers in systems with vacuum circuit breakers. Laboratory tests of switching overvoltages at transformer terminals and within the winding in a model medium-voltage electrical network with a vacuum circuit breaker were used in their study.…”

A Special Issue: Electric Machinery and Transformers

“…Florkowski, Jakub Furgał, Maciej Kuniewski, and Piotr Paj ąk [5]. Vacuum circuit breakers may cause overvoltages in distribution transformers when interrupting a current.…”