For well-shielded transmission lines, the return condition, near the substation, is of prime importance in determining the location and number of surge arresters needed to coordinate the isolation of the substation against HV lightning surges.
Hubbell’s best transmission protecta-lite line arrester line has been simplified with fewer parts and less risk of parts wear. Several steel components have been replaced with a copper bracelet. The strap makes installation easier and reduces wear points. Vibration and wear are greater on transmission line arresters because they are more exposed to wind and vibration.
The risk of a back flashover can be reduced by keeping tower foot impedances to a minimum, especially near the substation (first five to seven turns ).
The terminal tower is usually bonded to the substation earth mat and has a very low earth impedance ( 1 ohm ).
However, the ” gaping ” procedure on the first three or four turns where the distance between lines is reduced to reduce incoming surges will increase the risk of shutting down in ‘Backflashover.
Location of surge arresters
Considering the system when the transmission line is directly connected to a 420 kV GIS (gas-insulated switchgear), a computer model can be created to take into account the parameters described previously.
A transient study would reveal the level of lightning current needed to cause a backlash.
Then according to the number of lines flashing 100 km/year calculated for the transmission line and using the probability curve for the amplitude of the lightning current, a return time for this running current can be evaluated ( Le. 1 to 400 years, 1 to 10 years, etc. ) in, for example, the first mile of the line.
The voltage that then arrives at the substation can be evaluated and compared to the LIWL ( Lightning Impulse Withstand Level ) for the substation equipment.
The open-circuit breaker condition should be investigated here because if the line circuit breaker is open, the overvoltage will “ double ” at the open terminal. Different levels of running current can be simulated at different locations of the tower and the resulting surges from the substation can be evaluated.
If it is considered that the LIWL of the substation will be exceeded or if the margin between the calculated overvoltage levels and the calculated pressure drop level is insufficient to generate an acceptable risk, then HV lightning surge protection must be applied.
The rating of MOA ( Metal Oxide Surgearresters ) will have been evaluated from TOV ( Temporary Surge ), and from the manufacturer’s data, a surge arrester model can be included in the system model. The repetition of the various studies reveals the level of protection of the surge arrester and makes it possible to assess the safety factor for this system configuration.
IEC 60071 recommends a safety factor of 1.25 for equipment at 420 kV (safety factor = LIWL / protection level).
The surge suppressor current calculated for this condition should be the “worst-case” and can therefore be used to estimate the surge arrester-rated discharge current requirement ( 5kA, 10kA, or 20kA ).
( IEC 60091-1 is the international standard for surge arresters , and an accompanying guide with detailed information on the use of surge arresters is available.
To fully utilize the MOA Protection Level, the surge arrester should be placed as close as possible to the equipment to be protected.
In the case of the circuit breaker open, this distance can be between 10 and 20 m.
Depending on the rate of rising of the surge voltage, a voltage greater than the residual voltage at the location of the surge arrester will be detected across the open circuit breaker. This should be taken into account when evaluating the overvoltage of the substation.
the overvoltage profile of the SIG (Gas Insulated Switchgear) with the line circuit breaker closed. It shows that additional surge arresters may be needed due to the distances involved in the configuration of the substation.
It follows that surge arresters have a ‘ protection length ‘ which is sensitive to the rate of rising of the incoming surge voltage, which should be taken into account when evaluating the lighting surge on equipment remote from the surge arrester.