My process control system is not reliable and I suspect a voltage dip problem. Where do I start?

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Answer by David Chapman

The steps in identifying the problem are:

• Monitor at the supply to one or more of the affected devices. One problem is that the monitor threshold settings need to be set carefully so that all interesting events are captured, but the smaller, uninteresting events are not. This can take some trial and error to get right, but it improves the quality of data that you collect and is worthwhile. Alternatively, choose a tool that applies the thresholds retrospectively - these capture all the data, but let you choose what you view. Often, the simple transient capture functions found on hand-held power analysers are useful in the early stages - they are simple to use, the results are easy to interpret and they are easily moved around the installation.
• Assuming that the first stage identifies that you do indeed have a voltage dip problem, you now have to find the source.
  Move the analyser back to the origin of the supply, i.e. the point of common coupling (PCC) and monitor there. Monitor the current in each phase as well to check for increased current correlating to voltage dips (although it may be difficult to identify them at this measurement position). If the voltage dips are less frequent and have a higher retained voltage, and if there are identifiable correlated current increases, then the dips are caused by equipment in your own installation. Move forward, monitoring the voltage dips at each distribution point together with the current on each sub-circuit, and the source of the problem should be revealed. You can also take a more pragmatic approach and test circuits feeding heavy or cyclic loads first - suspect photocopiers and laser printers, lifts and hoists, heating and ventilating equipment, presses, arc furnaces…
• Once you have found the problem, the solution is simple. The disturbing load must be wired directly to the PCC - lowest impedance point in the system - so that it has the least effect on voltage
• If the voltage dip performance at the PCC is similar to that at the load, then it is more likely that the source of the dips is outside your installation. Now you have the evidence to talk to your Distribution Network Operator.

20/12/2006