My system suffers from voltage dips that appear to originate outside my installation. What can be done to resolve the issue?

Answer by David Chapman

The correct way to proceed is via the Distribution Network Operator (DNO). It is the responsibility of the DNO to ensure that no consumer causes interference to another. Of course - every consumer affects and is affected by all other local consumers to some extent, so whether the interference is excessive or not is a matter of judgement.

In the real world, problems such as this can only be solved by co-operation starting from an assessment of the real nature of the problem.

This type of problem is common where several small factory units are fed from a single transformer. Consumers are simply ‘tapped off’ a single feeder cable, so that a problem load, especially if it is located at or near the remote end, will, affect other users. One obvious solution is to up-size the feeder cable and/or transformer to reduce the system impedance (= increase the fault level) or to run a dedicated cable from the transformer (or an additional transformer) to the problem facility. Both these solutions are expensive and, of course, someone has to pay.

If the supply is adequate for normal running, but suffers problems when starting large equipment - like a sheet metal guillotine, for example, where a flywheel has to accelerated up to speed, or when particular equipment is in use, such as a spot welder, other solutions may be more acceptable.

Starting currents are large but last for only a few seconds or tens of seconds. ‘Soft starters’ are available that reduce the acceleration rate so that the starting current is reduced in magnitude but increased in duration. This has an impact on the duty cycle of the driven equipment that may or may not be acceptable.

The impact of cyclic loads, such as spot welders, can be mitigated by the use of a static VAR compensator that corrects power factor ‘on the fly’ and reduce the impact on the system.