Switching Equipment

CIGRE Study Committee (SC) A3 is responsible for the collection of information, technical evaluation of power system studies, and technical analyses of both AC and DC substation equipment from distribution through transmission voltages which are not explicitly dealt with by other SCs. SC A3 covers all AC and DC switching devices, surge arresters, instrument transformers, insulators, bushings, capacitors, fault current limiters, shunt and series capacitor banks, and diagnostic lifetime management and monitoring techniques. This scope is well suited to the various technical needs of utilities that require technical and sustainable solutions for emerging problems and challenges in changing network conditions.

Electricity is supplied to a large number of households, offices, and factories every day. Its availability has increased over the last 100 years since electricity has begun to be supplied in the late 1800s, and nowadays it is considered to be an essential commodity. It is a versatile and clean source of energy; it is rather cheap and “always available.” The purpose of a power system is to transport and distribute the electrical energy generated in the power generation plants to the consumers in a safe and reliable way, no matter how far the power generation plants are located from the load. In most cases, alternating current (AC) technology is used for electrical energy transportation, and in a minority of the applications such as a point-to-point international connection and long-distance, large-capacity transportation, direct current (DC) is preferred. The advantage of an AC power system lies in the fact that the voltage can easily be brought to a higher level in order to reduce losses during energy transportation. Figure 2.1 shows a typical AC power system including power generators, power transformers, and substation equipment such as circuit breakers.

Opening and closing operations of mechanical circuit breakers normally generate arc discharge phenomena between contacts. In the 1940s, Mayr, Cassie, and Browne (CIGRE WG 13.01; CIGRE Working Group 13.01; Cassie 1939; Mayr 1943) expressed arc behavior using dynamic arc equations with a couple of arc parameters and showed that they reproduce interrupting phenomena analytically in conjunction with circuit equations of power systems. Intensive investigations to explore superior interrupting media were conducted by EPRI in the United States (EPRI Report, EL- 284 1977; EPRI Report, EL-1455 1980; EPRI Report, EL-2620 1982) and revealed physical properties of many potential interrupting media. Furthermore, a practical computer thermo-fluid dynamics simulation has made it possible to analyze the arc interrupting behavior in detail by using more precise arc models (Herman and Ragaller 1977; Kuwahara et al. 1983; Smeets and Kertesz 2000).

A circuit breaker should be capable of making, carrying, and interrupting the current under both normal and abnormal conditions especially in case of short circuit or fault occurrence. Several short-circuit conditions including a single-phase grounded, three-phase grounded, and phase-to-phase ungrounded fault are expected to occur in power systems. When a circuit breaker interrupt the short-circuit current, different switching phenomena are observed depending on the conditions.

High-voltage (HV) circuit breakers are an indispensable piece of equipment in the power system which perform several functions to make, carry, and break a variety of the AC currents expected in operating and protecting the grid. For instance, a circuit breaker (CB) must cope with the following types of normal current carrying stresses under normal conditions:

Sulfur hexafluoride (SF₆) gas was recognized as an insulating medium having superior dielectric characteristics in the 1920s, and investigations into its interrupting capability started in the 1950s. Industrial production of SF₆ gas started in the United States in 1947, and the application for gas-insulated switchgear with SF₆ was first attempted in 1953. A manufacturer in the United States, Westinghouse, produced the first SF₆ gas circuit breaker in the world in 1959. The current interrupting principle has been improved from double-pressure type to single pressure puffer type with various configurations.

Vacuum circuit breakers are generally operated with an operating mechanism with smaller operating energy as compared with those of other types of circuit breakers, because the vacuum interrupter employs disc-shaped “butt” contacts instead of finger-shaped contacts often used for SF₆ gas interrupter.

A power plant is required to supply the energy produced by power generators in as stable and efficient method as possible. A circuit breaker is used to connect the generator to the network and to separate the generator from the network. Figure 8.1 shows two different layouts of a power plant. The circuit breaker which connects and disconnects the generators in the power plant is located either at the high-voltage (HV) side of the step-up transformer (HV synchronization of the generator with HV circuit breaker) or at the medium voltage (MV) side between the generator and the step-up transformer (MV synchronization of the generator with MV circuit breaker). In the latter case, the MV circuit breaker is called a generator circuit breaker because special duties and capabilities are required.

A disconnecting switch (disconnector) is a mechanical switching device which energizes and de-energizes parts of an electrical circuit. Earthing switch is a mechanical switching device which earths parts of an electrical circuit.

High-voltage dielectric withstand performance testing with equipment utilizes the phenomena in electrical insulation under the influence of electric fields changing with the power frequency. When electrical insulation is stressed in an electric field, ionization may cause electrical discharges initiating from one electrode of high potential to the one of low potential or vice versa. It may also cause a high current rise and lose the dielectric withstand capability to separate different potentials of the equipment.

Interruption and switching performance tests with switching equipment are usually performed with one of the following objectives:

Historically, modeling of arc behavior advanced the development of large capacity circuit breakers. A great step forward in understanding arc-circuit interaction was made in 1939 when A. M. Cassie (Cassie 1939; Cassie and Mason 1956) published the paper with his well-known equation for the dynamics of the arc (arc model) and then, in 1943, O. Mayr (1943) followed with the supplement that takes care of the time interval around current zero. Much work was done afterward to refine the mathematics of these arc models and to confirm their physical validity through practical measurements (CIGRE Working Group 13.01 1988, 1993).

Fault current limiters (FCLs) are special power system devices used to mitigate and lower high short-circuit currents to much more manageable levels for existing protection equipment like circuit breakers (CBs). FCLs are generally installed with the goal to lower the available fault current to a level the CB is capable of interrupting safely prior to the CB beginning its opening operation. The FCL limits the fault current to a preset level based on the particular system configuration and system conditions. In the past, engineers were forced to either upgrade existing equipment to protect the system from the increased short-circuit capacity (if replacements can safely interrupt the new short-circuit level), or they must find a way to lower the short-circuit rating such as bus splitting, increase impedance of transformer, etc. The FCL allows the engineer to apply the chosen technology to avoid a full system upgrade of protection equipment like circuit breakers, which can be rather costly.

Controlled switching systems (CSSs) have become an economical solution and are commonly used to reduce switching surges for various switching applications (CIGRE TF 13.01 1995, 1996). Recent developments of transformer switching taking account of the residual flux can effectively mitigate severe inrush currents and temporary overvoltages that may lead to false operation of protective relays and degradation in power quality. CSS combined with metal oxide surge arresters can reduce undesirable overvoltages caused by energization of a long transmission line and contributes to the optimization of insulation coordination. The limited number of applications for line switching in service up to 2005 may arise from initial difficulties due to insufficient technical considerations; however, the applications recently increase with a controller with highly processing capability. IEC62271–302 Technical Report titled “High voltage alternating current circuit breakers with intentionally non-simultaneous pole operation” was published to standardize the testing procedures required for CSS based on the recommended evaluation tests by CIGRE WG A3.07 (CIGRE WG13.07 1999, 2001). The CIGRE guide emphasizes the importance of compensation for the variations of the operating time because a CSS requires accurate operation consistency during the lifetime of circuit breaker. Variations of the operating times due to external variables such as ambient temperature, control voltage, and mechanical energy of the drives can be compensated by the controller using dependencies evaluated according to the testing requirements.

Surge arresters are installed in substations and in transmission lines with the purpose of limiting both lightning- and switching-induced overvoltages to a specified protection level, which is, in principle, below the withstand voltage of the equipment in order to protect it from excessive overvoltages. The ideal surge arrester would have a nonlinear voltage and current characteristic that starts to conduct at a specified voltage level (switch-on), keeping a certain margin above its rated voltage, holds the specified voltage level without variation for the duration of the overvoltage for expected lifetime, and then ceases to conduct as soon as the voltage across the surge arrester returns to a value below the specified voltage level (switch-off). Therefore, surge arresters are fundamentally required to absorb the energy that is associated with the overvoltages.

The application of HVDC transmission has been expanding due to the rapid progress of power electronics technology driven by increasing needs for connection of offshore or remote wind farms and/or large hydropower generators. HVDC transmission brings several technical benefits such as less system stability problems in a DC system compared with AC transmission and low energy loss for long-distance transmission. In the near future, multiterminal HVDC transmission connected to offshore wind power generation will be constructed. Figure 16.1 shows the future multiterminal HVDC transmission planned in Europe (Hafner et al. 2011) and the future ASEAN international networks combined with HVAC and HVDC transmissions.

Life management of equipment such as circuit breakers covers all periods of the life of a group of equipment (specification, development, manufacturing, testing, acceptance, erection, on-site commissioning, inspection, maintenance, diagnostics and monitoring, refurbishment, dismantling and disposal, and all necessary administrative actions). But more specifically speaking, the term “life management” is related to the decision-making process with respect to the equipment’s (e.g., a circuit breaker) residual life. The term “residual life” refers to the equipment’s remaining technical life. Relevant moments in the life of equipment in the case of a circuit breaker are given in the definition in Fig. 17.1.

Electrical power systems have not changed in essence since their first appearance more than a hundred years ago. In the design and construction of power transformers, motors, generators, cables, and transmission lines, it is better to speak of evolution rather than revolution. But a lot of advanced technologies and techniques are applied in today’s power system.