Electrical Switching Transients

Basic Concepts and Simple Switching Transients

The purpose of a power system is to transport and distribute the generated electrical energy from power plants to the consumers in a safe and reliable way. Conductors are used to transport the current, transformers are used to bring the electrical energy to the appropriate voltage level, and generators are used to take care of the conversion of mechanical energy into electrical energy. When we speak of electricity, we think of current ﬂowing through the conductors from generator to the consumers.

When load break switches, circuit breakers, disconnectors, or fuses operate, a switching action takes place in the network and parts of the power system are separated from or connected to each other. The switching action can be either be closing or opening operation. Fuses can perform opening operations only.

After a closing operation, transient currents will ﬂow through the system, and after an opening operation, when a power frequency current is interrupted, a transient recovery voltage (TRV) will appear across the terminals of the interrupting device. The conﬁguration of the network as seen from the terminals of the switching device determines amplitude, frequency, and shape of the current and voltage oscillations.

When capacitor banks for voltage regulation are placed in a substation, the switching devices interrupt a mainly capacitive load when operating under normal load conditions. The current and voltage are approximately 90° out of phase and the current is leading the voltage. When a large transformer is disconnected in a normal load situation, current and voltage are approximately 90° out of phase but the current is lagging. Closing a switch or circuit breaker into a dominantly capacitive or inductive network results in inrush currents which can cause problems for the protection system.

Interrupting Capacitive Currents

Power systems contain lumped capacitors such as capacitor banks for voltage regulation or power factor improvement and capacitors that are part of ﬁlter banks to ﬁlter out higher harmonics. Cable networks on the distribution level, likewise form a mainly capacitive load for the switching devices. An example are long subsea cables. Capacitive switching requires special attention since after current interruption, the capacitive load contains an electrical charge and can cause a dielectric re-ignition of the switching device. When this process repeats, the interruption of capacitive currents causes high over-voltages.

Capacitive Inrush Currents

Capacitor banks for load factor improvement or for ﬁltering out higher harmonics have to be switched in and out of service regularly. The interruption of a capacitive current can cause dielectric problems for the switching device, but when a capacitor bank is taken into service, large inrush currents can ﬂow through the substation.

Interrupting Small Inductive Loads

When an interrupting device interrupts a small inductive current, the current can be interrupted at a short arcing time. Interrupting devices such as high-voltage circuit breakers are designed to clear a large short-circuit current in milliseconds so that the cooling of the arc maintained by a small current is easy. The gap between the arcing contacts, after current interruption, is rather small and the capability to withstand dielectric breakdown is relatively low. Small inductive currents occur when unloaded transformers are taken in and out of service, motors are disconnected, or electric furnaces are switched.

Transformer Inrush Currents

Power transformers bring the electrical energy to higher voltage levels to reduce losses when transporting electrical energy over long distances. At the distribution level they transform the voltage down to the required level for the consumer. A switching operation carried out in a substation always involves transformer switching. When switching off a transformer under load, the load determines the power factor and the switching device interrupts the load current, which is normally not a problem for the switch or circuit breaker, creating no overvoltage in the system. When a part of the system is energised by connecting it to the rest of the system by a closing operation of a switch or breaker, the transformer can cause high inrush currents. The nonlinear behaviour of the transformer core is the cause of this. An air-core reactor switched on to compensate for cable charging currents does not cause inrush currents. When a power transformer is energised under no-load condition, the magnetising current necessary to maintain the magnetic ﬂux in the core is in general only a few percent of the nominal rated load current.

Short-Line Faults

A fault is called a short-line fault when the short-circuit, usually a single line-to-ground fault, occurs on a high-voltage transmission line, a few hundred meters to a few kilometres from the breaker terminals. A very steep triangular-shaped oscillation immediately after current zero puts stress on the still-hot arc channel and can easily cause a thermal breakdown.

In the 1950s, it was observed that minimum-oil circuit breakers, in particular, exploded after clearing short-circuit currents with a current level below the nominal rated short-circuit breaking capability. In addition, air-blast breakers occasionally failed to interrupt short-circuit currents considerably smaller than the current values they were designed and tested for. Careful analysis has revealed that in many cases the fault occurred on a high-voltage transmission line, a few kilometres from the circuit breaker terminals.

The short-line fault was discovered, and several years later the short-line fault test appeared in the standards. Presently, the short-line fault test is considered to be one of the most severe short-circuit duties for high-voltage circuit breakers. The short-line fault test creates, after current interruption, a very steep voltage oscillation at the line side of the breaker. This puts a high stress on the still-hot arc channel, and this can cause a thermal breakdown of the arc channel. The arcing time is prolonged, and when the same happens at the next current zero, the breaker fails to interrupt the short-circuit current.

The short-line fault is of special importance for SF6 circuit breakers. SF6 circuit breakers are especially very sensitive for short-line faults occurring only a few hundred meters to a few kilometee from the breaker terminals. Air-blast breakers are, in particular, stressed by faults occurring further away, several kilometres from the breaker terminals.