ZnO Gapless Lightning Arrester or Surge Arrester
An electrical surge can be occurred in an electrical power transmission system due to various reasons. Surge in an electrical system originated mainly due to lightning impulses and switching impulses. Electrical surge produces a large transient over voltage in the electrical network and system. The shape of the transient over voltage has a steeply rising front with slowly decaying tail as shown in the figure below. This steep voltage wave travels through the electrical network and causes over voltage stresses on all the electrical insulators and equipment come under its travelling path.
That is why all electrical equipment and insulators of power system must be protected against electrical surges. The method of protecting system from surge is normally referred as surge protection. The main equipment commonly used for this purpose is lightning arrester or surge arrester. There are two types of surges one comes externally from atmosphere such as atmospheric lightning. Second type is originated from electrical system itself, such as switching surges. When an electrically charged cloud comes nearby an electrical transmission line, the cloud induces electrical charges in the line. When the charged cloud is suddenly discharged, through lightning, the induced charge in the transmission line is no longer confined static.
It starts travelling and originate dynamic transient over voltage. This transient over voltage travels towards both load and source side, on the transmission line because of distributed line inductance and stray capacitance. This surge voltage travels with speed of light. At the end of the transmission line, as the surge impedance changes, the surge voltage wave reflected back. This forward and backward travelling of surge voltage wave continues until the energy of the surge or impulse is attenuated by line resistance. This phenomenon causes voltage stress on the transmission system many times greater than normal rated voltage of the system. Hence, surge protection scheme must be provided to the electrical power transmission system to make reliable and healthy system. Lightning arrester is one of the main components to to protect the system from surge.
As we said earlier, that the electrical surge also can be generated from the system itself. Actually during switching operation there may be a chance of current chopping. If during normal operation, if electrical isolator is opened on load. Sudden open circuit is occurred in the system. In addition to these, the basic arc-quenching techniques of SF6 circuit breaker and vacuum circuit breaker may give rise to current chopping and multiple re-ignition sometimes. As we know that sudden current chopping give rise to the di/dt. [di/dt = rate of change of current with respect to time].
As the electrical load is generally inductive, there is a transient voltage, expressed by L(di/dt) where L is the inductance of load of system. This voltage is induced across the opening contacts, and travels towards load and reflects in similar manner of lightning impulse. lightning arrestor or surge arrester are provided at the end of the transmission line to withstand the surge voltage. Generally oil field electrical power transformer, electrical switchgear, cables, electrical transmission lines, distribution lines are quite capable for withstanding these switching impulse voltages, as their insulation level is quite high to withstand these over voltages. But, generator, electric motor, dry type transformers and electric arc furnaces etc. can not withstand large switching impulse voltages. As essentially this types of equipment do not have very high level of insulation. To protect this equipment from surges, lightning arrester is must. In electrical sub-station, arresters are mainly used at the entrance of any feeders and also they are used at both rides of electrical power transformers as transformer is also considered as inductive load and very costly equipment. In modern era, gap less ZnO or zinc oxide surge arresters are mainly used for surge protection. Let us discuss zinc oxide type gap less arresters.