Dr Jonathan Hiscock, managing director of Fundamentals Ltd gives us an insight into why tapchangers are so important in the power grid, how their performance can be enhanced, their reliability extended and what to expect if they’re not properly maintained.
The purpose of a tapchanger is to safely switch between the tapped windings of a power transformer in order to modify its output voltage. Tapchangers are critical for the performance of the grid. They are used to keep voltage levels within statutory limits; something network operators are obliged to do and which is an increasingly challenging task as the dynamics of the grid changes with the realisation of the low carbon economy.
There are two generic types of tapchanger, on-load tapchangers (OLTC) and deenergised (also known as off-circuit) tapchangers (DETC).The main difference is the OLTC can be operated without interrupting the supply of power, whereas an off-circuit tapchanger must be deenergised and isolated from the energy network in order to be operated which of course interrupts the supply of power.
Most network power transformers are equipped with OLTCs since they commonly supply many customers and experience regular system voltage variations which need to be corrected. Off-circuit tapchangers are typically found on secondary or more local transformers where fewer customers are supplied and voltage variations are more seldom.
The tapchanger is arguably one of the most vulnerable parts of the transformer since it is the only moving part, and like any mechanical device requires regular maintenance to ensure optimum performance. OLTCs are in continuous use and experience arcing between contacts during every tapchanger operation. This causes degradation of insulating oil and contact wear which over time can compromise the integrity of the tapchanger.
A typical OLTC has a recommended service interval (time between maintenances) of ten years or less. It is critical to ensure that service requirements of OLTCs are observed in order to maintain the health of the transformer. Service of OLTCs is quite often overlooked or not done properly (an oil change is not maintenance!) which can increase the risk of failure.
Tapchanger failure can ultimately lead to total failure of the transformer itself which can be extremely costly. It is estimated that up to a third of transformer failures are caused by tapchanger failure. The cost of replacing a transformer is around two orders of magnitude higher than a scheduled tapchanger maintenance or repair, so it does not make much sense to try to save money by avoiding regular service.
Considering the typical service life of a tapchanger is less than ten years, (depending on the tapchanger specification) maintenance should be performed by the Original Equipment Manufacturer (OEM) in order to ensure technical updates are carried out and that all parts and accessories are produced and manufactured to original designs and specifications and fitted correctly.
Many tapchanger problems can be related to the external motor-drive mechanism and control scheme. These are often overlooked and can result in incomplete tapchanger operations and tapchanger ‘run aways’ (where tapchangers run to end positions) with extreme high or low voltage conditions. Typically, with these issues, control schemes are ‘locked’ in a fixed tap position, voltage control is disabled and grid performance compromised.
These aren’t strictly failures, but they do lead to some down-time and extra repair costs and are ultimately caused by a lack of maintenance. Even where maintenance of the tapchanger has taken place, the drive mechanism and control scheme are often overlooked.. Tapchanger drive mechanisms and control schemes can be upgraded and modernised to improve reliability before such issues arise and should be considered as part of tapchanger servicing.
Is servicing a disruptive process?
Normal tapchanger service can tan take one to two days for a technician to complete, depending on the scope of the work. In order to work on a tapchanger, the transformer must be switched out and isolated. This does not normally disrupt power flows to customers since transformers are operated in parallel for security of supply and load will be picked up by other transformers in service. However, servicing is disruptive in respect of the need to remove insulating oil and gain access to the internals of the tapchanger so that inspections, measurements and replacements of parts can take place.
Occasionally, a spare tapchanger, often referred to as a ‘traveller’, is kept on site where more extensive maintenance is required due to high amounts of tapchanger operations and/or heavy loads (e.g. arc furnace transformers). To minimise down-time, the spare tapchanger is fitted in place of the one to be serviced so operation can be resumed very quickly. The serviced tapchanger then becomes the spare, ready for the next outage.
It is very difficult to determine the condition of the tapchanger without an internal inspection. However, with good records and regular maintenance tapchanger condition can be monitored. Even then, loading conditions can change due to the demands of the network which can lead to increased tapchanger duty.
The current drawn by the tapchanger-drive motor can give an indication to the mechanical integrity of the tapchanger and can therefore be monitored. However, it will not give any information about the type of problem which might exist. Dissolved Gas Analysis (DGA) is often used to detect gases which are indicators of internal faults in oil-filled transformers.
On-line DGA units can be fitted in order to continuously monitor transformer health. This can also be applied to tapchangers, but the various diverter/selector configurations need to be taken into account. The tapchanger diverter is the part which switches current and therefore expected to contain particular gases, whereas the tapchanger selector determines the transformer tapped winding connection and does not. Although absolute gas concentration levels will not give any particular health indication, the trending of the levels will indicate if problems are emerging and prompt an internal inspection.