The electrical surge is the transient overvoltage occurring in the electrical power system and network. We recognize it with suddenly rising and slowly decaying voltage. We standardize it with the standard impulse voltage. The standard impulse voltage is 1.2/50 microsecond impulse voltage waveform. All the pieces of electrical equipment may suffer from external or internal electrical transient overvoltages.
External Transient Overvoltage or External Surge
Most commonly the atmospheric lightning causes the external electrical surge. During discharging of charge from clouds to earth through arcing during lightning, there may be an induction effect on the nearby transmission line. And due to this induction, a transient overvoltage may occur in the transmission line. The transient disturbance then propagates with the speed of light towards end substations. The distributed inductance and stray capacitance of the transmission line facilitate this propagation. But at the end of the transmission line, due to sudden change in search impedance, the surge reflects back and starts traveling in the opposite direction. In this way, the surge travels back and forth until the entire surge energy becomes minimum due to line resistance. The phenomenon causes the system-voltage much higher than its rated value.
Internal Transient Overvoltage or Internal Surge
Electrical surges may also occur due to switching operations is in the substation. On many occasions, a circuit breaker cannot chop the current exactly during its zero crossings. Due to the presence of inductance, there may be a transient overvoltage occurring in the system. We refer to these surges as switching surges. The switching surge is one type of internal electrical surges. The equation given below can explain the cause of switching surges or overvoltages in the system.
Surge Protection Devices
The surge protection devices connected in the system protect the system from such overvoltages.
A surge protection device cannot prevent the occurrence of surges in the system. Instead, it reduces the level of overvoltages to bring the voltages within the voltage withstanding limit of the system.
Functions of Surge Protection Devices in Power System
If we make the insulation level of all pieces of substation equipment so high that it can withstand all types of surge overvoltages occurring in the system then there is no need for surge protection devices. But it is not practically possible and also it is not economical at all. The insulation of the system is based on voltage-time characteristics instead of only on its voltage characteristics. It means if overvoltages do not continue for a long time, there may be no harm in the system. Because in these cases, the normal insulation system can withstand the overvoltages. Surge protection devices limit the occurrences as well as the duration of surges. In other words, these devices help to reduce the insulation cost of the system.
Development of Surge Protection Devices
Previously horn gap or spark gap type surge protection devices used in the power system. The response time of spark gap surge protection devices was quite long and also it was affected by the ambient condition. This is the reason this type of device could not protect effectively the substation equipment from surges. Then thyrite pile type surge arrester with a spark gap comes into the picture. The gap prevented the flow of power frequency through the surge arrester. Next, the silicon carbide type surge arresters come to the picture. The amount of leakage current in such arresters was quite high in the normal operating condition. For extra-high voltage applications, this type of arrester requires parallel resistance to distribute the voltage uniformly. The high response time of the series gap and high residential voltage are two main disadvantages of series gap silicon carbide lightning arresters.
On the other hand, some metal oxides have very nonlinear v-i characteristics. The nonlinearity of v-i characteristics makes the metal oxides as the most popular materials for making surge arrester.
Modern Surge Protection Devices
In the modern era, there are mainly two types of surge protection devices mostly used in power systems.
- Gapless Zinc Oxide Surge Arrester.
- Capacitance – Resistance Surge Suppressor.
Gapless Zinc Oxide Surge Arrester
The main construction of a gapless zinc oxide surge arrester is a column of zinc oxide blocks. There is no gap available between the blocks. Hence, we refer to it as the gapless surge arrester. The response time of this arrester is very small in the range of nanoseconds. Whenever there is a transient overvoltage appearing in the system without any time delay it absorbs the surge energy to decay the effect of the surge. Actually, during normal operating conditions, it provides infinite resistance between phase and earth of the system. But during the surge, the resistance of zinc oxide becomes almost nil to provide a short circuit path to the surge energy to flow to the earth through the arrester. Once the surge decays the arrester regains its normal high resistance to withstand normal system voltage.
Capacitance – Resistance Surge Suppressor
The capacitance – resistance surge suppressor is commonly known as the surge capacitor. The device balances the inductance of the system. This is a simple series combination of capacitance and resistance connected between line and earth. The device changes the impedance of the surge path and therefore reduces the magnitude of the surge. Instead of absorbing the surge energy, this device prevents the formation of the surge during switching operation.