Electricity + Control October 2018

EARTHING + LIGHTNING PROTECTION

For example, shield wires are situated above the equipment and around the perimeter such that they intercept any strike and transfer it to earth. Still, even though a direct strike is averted, the possibility exists that an induced surge will make its way to equipment. Where insulation withstand voltage is greater than 350 kV, it is not likely that flashover will occur from an induced surge. But if insulation withstand voltage is less than 350 kV arresters are needed to protect equipment from induced surges. If masts are used, these are situated strategi- cally so that all direct strikes will hit them instead of the equipment or bus. The only way for a potentially damaging light- ning surge to challenge a substation is by entering on an incoming line. If the incoming line is shield- ed, there needs to be a backflash not too many spans from the substation to a phase conductor for the surge to pose a problem. When a backflash occurs at a tower, it produces a very fast rising surge. But as the surge travels down the line, the corona on the line reduces the travelling wave- front steepness such that by the time it reaches the station it has become manageable. But if the backflash occurs only a span or two away, the wave-front steepness of the entering surge is extremely high and will significantly stress insulators in the substation – even when arrest- ers are installed. Fortunately, substation arresters seldom see more than 10-15 kA of current from a backflash on an incoming line. As a result they are not thermally stressed by lightning, only electrical- ly stressed. Switching surges on incoming lines Switching surges on incoming lines are the most common type of surge to reach within a substation and can travel hundreds of kilometres due to their low frequency relative to lightning. When these types of surges enter, the whole primary side of the substation sees the same rise in voltage and arresters at the transformer can protect the whole station. Switching surge energy dissipated by the arrester on systems with voltage greater than

240 kV can at times be significant and challenge the arresters’ energy handling capability.

Transformer windings and all oil/ paper type insulation cannot recover from a disruptive discharge and are therefore considered non- self-restoring insulation.

Switching surges created within substations Switching surges are also generated within the station from the operation of breakers or other switching devices. For both external and inter- nal switching surges, the point on the ac wave where the device operates has a profound effect on surge amplitude. For example, when switched at peak voltage, a surge of two to three times initial voltage can be generated. In some cases, a breaker may pre-strike or restrike during oper- ation. For internally generated surges, the surge wave-front can be very fast rising and will pose a serious threat to nearby insulation. For this type of switching surge, special mitigation with surge capacitors and arresters may be necessary. If a capacitor bank is switched within a substation, a significant surge can be generated that will need arrester mitigation. Types of substations and special protec- tion considerations Transmission substations There are several different types of transmission substations, e.g. those with and without trans- formers, combination transmission and distribu- tion stations, those that have large voltage control equipment such as series capacitor banks, etc. In all cases, arresters are applied to mitigate lightning and switching surges that may occur. In stations equipped only with breakers but no transform- ers, arresters may or may not be installed. For example, they may not be necessary if there are no possible significant end points or open points. Moreover, if line impedance remains unchanged, arresters are not required. In transmission stations where there is either a step-up or step-down transformer, arresters should be used on both high side and low side of the transformer if surges can enter from either direction. If breakers are present, line entrance ar- resters should be considered for protection if the breaker is open at any time. At critical substations

Electricity + Control

OCTOBER 2018

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