Electricity + Control October 2018

EARTHING + LIGHTNING PROTECTION

Full and partial GIS substations A totally GIS station with GIS arresters is quite different from other substations and surge protec- tion here is often specifically engineered. Arrester rating is often quite high in voltage to handle sig- nificant energy during any surge. If a station is par- tially air-insulated and exposed to lightning, surge protection is clearly necessary. But if the station is partially air-insulated, yet inside a building, surge protection becomes of lesser concern. When all inputs and outputs are underground, surge protec- tion design studies should be carried out to deter- mine if and where to install arresters. Generator stations Generator stations are the most difficult to protect because generator withstand voltage for very fast front surges is actually lower than the BIL. If a sta- tion is a small unit where incoming lines are only a few hundred metres from the generator, special protection is necessary. Not only can surges get through the step-up transformer, they can be gen- erated at low voltages (e.g. 15 kV) by the breakers in the system. Very fast rising surges and high en- ergy surges can be found on the generator bus. This is the place where surge capacitors are as important as surge arresters. The primary reason to use station class arresters at substations lies in their fault current withstand capability. Since fault current in a substation often comes from several directions, available fault cur- rent can easily reach above the 20 kA limit of dis- tribution arresters. The default arrester is therefore station class. Distribution class The distribution class arrester can be used in sub- stations where fault current is below 20 kA. Here, the residual voltage of the distribution class arrest- er can often satisfy the station’s performance re- quirements. Moreover, for stations below 260 kV, the energy handling capability of a distribution ar- rester is typically also adequate. Types of arresters used in substations Station class

with breakers, if these are used to clear faults, it is possible that during such a fault-clearing event a second surge can impinge on the line side bushing while the breaker is in the open state. Now, the ar- rester at the transformer cannot protect the bush- ing and voltage doubling can occur with subse- quent bushing flashover. To protect the breaker’s line side bushing, an arrester should be installed at the line entrance. Distribution substations Distribution substations can become quite compli- cated, with many types of equipment that need protection. The line side of the power transformer, for example, should always be protected. Howev- er if the low side cannot see a surge, arresters on that side are not necessary. The exception to this rule occurs when a breaker is located on the low side. In this case, it is recommended that arresters be installed between the sensitive windings and the potentially hazardous fast surge that a breaker can create. At distribution feeder exits where lightning surges can enter the station, arresters should al- ways be applied to mitigate these surges that will invariably arrive sooner or later. If these arresters are close enough to the LV side of the power trans- former, they can also be used as protection for the transformer. Another condition that often arises in distri- bution stations is the use of neutral grounding resistors (NGR) to limit the fault current into the system. When NGRs are used, arrester Uc and MCOV ratings need to be increased to account for the potentially long and high amplitude overvoltag- es that occur. Indoor air-insulated substations A problem that can arise with this type of station is visibility of the arresters that are often buried within a cabinet, making easy access to them difficult. This only becomes a concern should the need arise to verify the arrester’s health using an infrared camera or by other means. It is therefore suggested that arrester enclosures have at least one side that is not fully enclosed and allows ar- resters to be evaluated if the need arises. Anoth- er issue that arises is long cable runs with open points. Voltage doubling issues arise in this con- figuration and arresters at the open points should be considered.

Author: Jonathan Woodworth is the Co-Con- vener of the IEC and notable arrester expert.

32 Electricity + Control

OCTOBER 2018