Where a transformer needs to be stored (even on a temporary basis) the applicable specification is National Grid TS 2.20 which requires that a National Spare Transformer be stored in a reinforced concrete Oil Retaining Area that is sufficient “to provide an effective life in excess of 40 years”. The oil retaining area must incorporate a drainage system that allows for water to escape without allowing oil to seep into the water course. The cost of this permanent solution is of the order of £750k and takes around 8 months (from design) to implement following the decision to build it.
The proposed system constructs the Oil Retaining Area (tank) from a geo-membrane sheet, held together with pre-stressed aluminium sections. The geo-membrane is designed to resist weathering, ultraviolet light and rain as well as many chemical substances (e.g. acids, bases and hydrocarbons). It has a high resistance to tearing, piercing and abrasion. Rain water is filtered through a Filtrelec Petro-Pipe, so it does not need to be connected to an interceptor. The filters simply allow rainwater to pass through but block in the presence of hydrocarbons. In the worst case (low probability) scenario where the transformer has emptied all its oil into the bund, the Petro-Pipe has blocked and it is raining, a siphon allows the rain water sitting at the bottom of the tank to drain whilst leaving the oil in the bund. The system is covered by a 12 month warranty.
The supplier’s existing temporary storage tank designs are not large enough to meet National Grid specifications for oil capture and containment. It is anticipated that the proposed design will use two separate tanks, one to store the transformer and another to store the cooler banks, the two tanks will be connected by piping hence creating a sufficiently large storage capacity.
Landulph Substation has been identified as an appropriate trial site as there was already a scheme in place to construct a permanent bund to store a “Solar Spare”. (The plan is then to redeploy the transformer when the GIC risk has passed).
Objectives
The project will deliver a proven method for the deployment of a temporary oil containment storage facility (temporary is defined as up to 5 years) that will give National Grid a greater degree of flexibility in the delivery of transformer replacement schemes.
It will provide a means to mitigate temporarily heightened risks at a site by strategically deploying a National Spare Transformer to a site where it would not normally be stored. The identified risks are (i) the failure of a transformer during a geomagnetically induced currents (GIC) peak; and (ii) system outage planning or other constraints that mean we may not be able to replace an asset that has reached a state requiring replacement in the desired timeframe.
