Power transmission lines and associated equipment in the substations are subjected to overvoltages or surge voltage, which are generally due to lightning strikes. A lightning strike is an electrical discharge in the form of a huge spark that may occur in air, between the clouds, between a cloud and earth, or between the different charges on the same cloud.
When lightning discharges on a transmission line it may have a current in the range of 10kA to 100kA which induces a voltage of the order of 1 million volts in the form of the traveling wave.
Hence it is necessary to provide protection for the equipment at stations or substations against the traveling waves caused by lightning. Such protection can be provided by a protective device called Lightning Arrester. Let us see the ratings and characteristics of a lightning arrester to be considered while selecting.
Ratings of Lightning Arrester :
Depending upon the following factors the rating of lightning arrester is selected.
Rated Voltage :
Rated voltage of lightning arrester can be defined as the maximum allowable RMS value of the power frequency voltage which the lightning arrester can withstand across its phase to earth terminals and can carry the flow of current after the breakdown had taken place without damage to itself. The rated voltage that appears between the phases and earth on the occurrence of an earth fault in one phase can be calculated by knowing the system's maximum voltage and coefficient of earthing.
Coefficient of Earthing :
The coefficient of earthing can be defined as the ratio of the highest RMS voltage of healthy line to ground to the normal line-to-line RMS voltage multiplied by 100.
Rated Discharge Current :
It is defined as the discharge current having a prescribed peak value and wave shape which is used to differentiate an arrester with respect to its durability and protective characteristics.
Power frequency Sparkover Voltage :
Power frequency sparkover voltage can be defined as the least value of sparkover voltage which is kept at about 2-5 times the rated voltage of lightning arrester, to avoid frequent sparkover due to internal overvoltage of inadequate magnitude which may damage the systems.
Maximum Impulse Sparkover Voltage :
The maximum impulse sparkover voltage is defined as the voltage at which the lightning arrester sparkover occurs frequently for every impulse overvoltage. This shows that the rated voltage of the lightning arrester is either equal to or greater than the sparkover voltage. And it helps in protecting the basic insulation levels against lightning surges.
Residual or Discharge Voltage :
Residual voltage can be defined as the highest value of voltage that appears across the terminals of the lightning arrester during the discharge of surge currents. The residual voltages are kept at a fixed value for currents ranging from 5,000 to 10,000 amps. During higher discharge currents, the rise in residual voltage is smaller than the BIL of the system to be protected.
Maximum Discharge Currents :
Maximum discharge current can be defined as the highest value of discharge current that can pass through the lightning arrester without causing any damage to it. The value of the maximum discharge current of lightning arresters used in power stations is about 100kA and for the lightning arresters used in distribution systems is 65kA.
Characteristics of Lightning Arrester :
An ideal lightning arrester or surge diverter should possess the following essential characteristics.- It should not draw any current at normal power frequency voltage i.e., during the normal operation.
- It should break down very quickly when the abnormal transient voltage above its break down value appears so that a low impedance path to the ground can be provided.
- When the breakdown has taken place, the lightning arrester must be capable of withstanding the discharge current without getting damaged itself.
- It must be capable of interrupting the power frequency follow-up current after the surge is discharged to the ground.
Selection of Lightning Arrester Location :
The surge diverters or lightning arresters are located very close to the equipment to be protected because of the following reasons,- The lightning surges occurring near the terminals of the equipment may enter the circuit before the lightning arrester operates.
- For each operation of the lightning arrester, the voltage wave corresponding to the sparkover voltage may be reflected towards the equipment, so that the voltage across the terminals of equipment rises to the sum of incident voltage and the reflected voltage which will be many times greater than the equipment's rated voltage value.
- The terminal voltage V of the equipment will also be affected by the inductance Z of the leads of the lightning arrester, the residual voltage VR of the arrester, and the rate of change of surge current dis/dt,i.e., V = L(dis/dt) + VR
The above problem can be avoided to some extent by using a shunt capacitor at the equipment terminals, which reduces the steepness of the wavefront and also the rate of change of surge current. But, it results in additional costs and maintenance of capacitors. Hence lightning arresters are located very close to the equipment to be protected.
