Power Quality

Q Many power users need high availability power supplies. Why can’t the distribution network operators provide them?

A The electricity distribution network in most areas is characterised by being quite complex and also mature. It consists of overhead lines that are subject to damage in extreme weather, underground cables that can be damaged by construction work, and switchgear and transformers that may be quite old. Additionally, it is used and abused by a mixture of users, some operating heavy equipment, others running sensitive equipment. In turn, the distribution network takes power from the grid, which is subject to many similar issues.

Improving the availability of power from the network is possible, but would be difficult and expensive - and it would still not be good enough for some users. The cost of additional infrastructure would have to be reflected in the price of electricity, so all consumers would pay for a benefit that is of real value to only a small number of users.

Some Regulators have already expressed the view that those who require ‘high quality’ have to pay for the benefit - meaning that if a process or operation requires better quality than that available from the local infrastructure, then the beneficiary should finance the additional plant required. This is sensible up to a point, but, ultimately, industry and commerce will choose business locations where the network infrastructure is better, just as the road and rail communications are taken into account now.

At present, there is insufficient data available on the quality of power at different locations for a reliable assessment to be made.

Q My datacentre has a UPS. Isn’t that enough protection?

A How important is it that your data integrity is maintained and that your users have instant access to it?

A (static) UPS has a finite energy store so that, in the event of a supply failure it will provide power for a short time. This available time can be used in several ways depending on the needs of the organisation.

It can be used (and most frequently is used) to perform an orderly shut down of the network and servers. This protects transactions and ensures integrity of the data. However, once triggered, the process allows no new access to data - i.e., your users have no service. Service is only restored after the power supply has been re-established and the servers and network devices re-booted. If the operation is losing money by the minute, this is unlikely to be acceptable!

Another approach is to use the limited time available to start up an auxiliary generator to take over the supply from the UPS. This allows the operation to continue as if nothing had happened - assuming of course that the generator and fuel supply are properly maintained and work as expected.

Alternatively the time can be used to transfer transactions to another site, either an active peer site or a passive standby site. Again, as far as users are concerned, nothing has changed.

Whatever approach is taken it is most important that the standby power available is used carefully. Only essential equipment should be connected to the UPS system - servers, communications, network devices, essential PCs - no coffee machines, laser printers, etc.!

Q I am installing two ‘fully rated’ transformers on my critical site. Should I use one transformer and maintain one as a cold standby, or should I use both simultaneously?

A There are advantages and disadvantages to either approach.

If you use both transformers simultaneously then you know they are both functional. There is a risk that the total load grows beyond the capacity of each individual transformer so that if the load is transferred to just one transformer it may be overloaded.

If you use only transformer A and keep B as a cold standby you run the risk that B, or some of the infrastructure supporting B, may fail when put into service. On the other hand, you can easily check that the single unit is not overloaded. You need to be aware that harmonics cause excess losses in transformers, so simply measuring the load and comparing it to the nameplate is not enough. You either need to know the harmonic profile of your load and correct for it, or carefully monitor the transformer temperature.

Whichever redundant arrangement you select, you need to put in place maintenance procedures to guard against these potential problems.

Q What is the difference between a voltage regulator and a Dynamic Voltage Restorer (DVR)?

A Both are used to mitigate the effects of voltage dips. Dips are characterised by the depth - the retained voltage - and the duration. Short and deep dips are best served by a DVR while long and shallow dips are the province of the voltage regulator.

A voltage regulator has no energy store. It has a transformer secondary winding in series with the supply. When the input voltage moves outside the tolerance band the primary of that transformer is driven to boost, or in anti-phase to reduce, the voltage appropriately. Because the load voltage is kept constant, the power to the load is constant so, when the input voltage falls, the input current increases. The current capability of the supply and the device itself limits the working range to about +/-30% of nominal voltage.

A DVR has an energy store, so requires no additional input power (in the short term) to boost the voltage during a dip. A DVR can correct a dip to 0% retained voltage. But the DVR has a limited energy store and so is suitable for short-term effects only - it cannot correct for long term under voltage, for example. Also, the store has to be recharged between events so it is not suitable multiple dips are expected frequently. Typically, DVRs use super capacitors, large secondary batteries or high-speed flywheels as energy stores.

Unsurprisingly, DVRs are more expensive than voltage regulators.

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

A 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.

Q My system suffers from voltage dips that appear to originate outside my installation, i.e. they are present at the point of common coupling. I suspect that a neighbour involved in metal fabrication is the source. What can be done to resolve the issue?

A 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.