Rapid variation of voltage, even over a relatively small range, can cause lights to flicker and can cause certain types of equipment to malfunction. These issues need to be investigated on a case by case basis as the causes and solutions are many and varied.
With voltage fluctuations the voltage does not necessarily vary through too great a range, but the variation in the voltage occurs quickly and often repetitively. Sources of such problems can for example be arc welders, or motors with repetitive start patterns. The solutions to mitigate such problems very tremendously and are dependant on what is causing the problem and therefore need to be investigated on a case by case basis.
Voltage dips is the aspect of power quality that is responsible for untold production stoppages and losses. Can be tricky to solve, but typically something can be done to better the situation.
Particular note should be given to this problem as it is typically the most troublesome and costly power quality problem in many areas, particularly where long rural supply lines are involved.
A voltage dip is a reduction in voltage for a short time. Different definitions are used but one definition is that a voltage dip is a reduction in voltage of between 10 percent and 90 percent of the nominal voltage, and it persists for any time period from 5 milliseconds (half a cycle) to 1 minute. (A voltage drop of greater than 90 percent is considered to be a supply interruption). Dips can be caused by the customer’s equipment on site (or by equipment on nearby sites) such as direct on-line starting of motors that are large in relation to the supply capability. But equally, dips can be caused on the utility’s network and thereby affect a high number of customers. The most common cause on the utility side is when there are electrical faults on the utility’s network and the dip is experienced for the small period of time that it takes for the fault to be cleared.
Such faults can for example be a digger striking an underground cable, but more commonly are caused by such things as wind blown debris striking overhead power lines during a storm. It depends on the depth and duration of the dip, but voltage dips originating on the utility network are notoriously difficult to compensate for when production processes are being interrupted. One valuable technique is to ensure that all the computerised control equipment and other control circuits in an industrial plant can “ride through” the typical dips. This is often a relatively low cost solution and there are many cases where the power hungry production process will ride through a dip so long as the control equipment can do so. This is unfortunately not always the case and certain sensitive industries (such as semiconductor manufacture and steel mills) invest large quantities of money in sophisticated techniques and equipment to mitigate against voltage dips on the supply.
It must be noted that often there is nothing practical that the utility can do to reduce the number of supply side dips.
However, there are many occasions when they can take action, such as refining protection settings or simply doing better vegetation control near overhead lines. So if you are experiencing problems with voltage dips it is worth a discussion with the local electrical utility. In such a case it is best to be armed with a complete record of all dips you have experienced including the depth and duration of each dip – such information would be obtained from a comprehensive power quality survey.
Voltages swells are not particularly common and are not discussed as a separate section, suffice to say though that where such issues exist they are identified by a standard power quality survey.
Ordinarily not a problem, but when it is an issue serious consequences can result. Any voltage unbalance issues are immediately identified as part of a standard power quality audit.
Typically medium and large commercial and industrial customers are supplied by means of three phase electricity. In an ideal world the measured value of all three phase voltages would be the same. Due primarily to unbalanced loads on the utility’s electrical network the three phase voltages experience different levels of regulation and voltage drop which causes them to be unequal at the customers’ point of supply. Small variations are of little consequence; however there are certain types of load that require that the imbalance be less than about 2 percent. The most notable load in this category is the three phase induction motor, as such motors overheat quite dramatically if the three phase supply voltage has significant imbalance present. Depending on their design, three phase rectifiers can also mal-operate or fail if the supply voltage is significantly unbalanced.
The type of equipment affected is often found in medium to large production facilities so the negative consequences can be quite significant.
This is a very important aspect of power quality and relates to distortion of the voltage and current waveforms. Beyond certain levels harmonic distortion can cause all manner of problems for customers and utility networks.
Harmonic distortion that is causing problems can ordinarily be identified by a standard power quality survey, however, this area can get quite complex especially if transient harmonic resonance phenomena are present. The ideal power supply waveform is sinusoidal in shape. Modern electronic and power-electronic equipment however has a current waveform that is very different to a sine wave. This tends to distort the voltage waveform and so the effect can be spread to many users. If the harmonic distortion is high negative effects can arise. Such effects can include; increased neutral current, cables, transformers and other equipment running hotter then normal, various electronic controllers misbehaving, etc.
Serious negative effects of high harmonics are not particularly prevalent but do certainly exist. As harmonic generating loads are on the increase the subject is very topical and the effects need to be monitored closely. In those areas where harmonic distortion is a problem it should be identified and rectified early so that damage, energy losses, or malfunction can be prevented.
There are various categories of transients, the most common and most troublesome is the high voltage transient spike. These have the potential to damage or destroy equipment, but fortunately if done correctly preventative solutions are relatively inexpensive and easy to apply.
Overvoltage transients are also commonly referred to as “spikes” which is a less technically correct name, but is a more easily understood. Transients can exist for as short a time as a few microseconds but can reach very high values. There are several causes, but the most common cause tends to be lightening strikes on or near an electricity distribution network. Transient over-voltages are typically controlled by good insulation coordination and effective surge suppression at various points in the network.
Additional surge protection can also be installed at the customers main switch-board and on the terminals of various equipment, should the need arise. (Oscillatory transients are not discussed here as an individual topic as they tend not to be particularly harmful, but can be a bit of an issue where utilities switch large capacitors in and out of circuit.)
The term “electrical noise” is very broad and is used in several contexts. What is normally meant is that there are electrical signals getting into circuits where they are unwanted.
Noise occurs on both power as well as signal and data circuits, but generally speaking it is more often a problem in signal or data circuits. Signal and data circuits are particularly vulnerable to noise because they operate at fast speeds and at very low voltages.
Noise does however occur on power circuits and this is the type of noise that we are interested in under the general heading of power quality. Frequency of electrical noise signals on power conductors can be in the range of 2.5 kHz up to many megahertz. In the range of 2.5 kHz through to 150 kHz the allowable limits are not well defined by regulation, but where there is a problem reactors and low frequency filters can often by employed to reduce the noise, it is a tricky area though as low frequency filters tend to be bulky and quite costly. In the range of 150 kHz to 1 GHz the allowable limits are well regulated and many quality appliances have line filters on the input to ensure adequate immunity to this range of noise. Filters in this range tend to be quite compact and economic and standard stock units can normally be fitted to the power supply if there are issues.
Typically such interference is generated by equipment on the customer’s site or on a nearby site (although spikes can come from a long way away and sometimes these are also included in the broad category of noise.) Persistent noise on the power supply is not a wide spread problem but the reader needs to know of its existence. Due to the high frequencies involved specialist equipment such as a high speed digital oscilloscope is needed to investigate thoroughly and would not ordinarily be included in a power quality survey unless high frequency interference was suspected to be a problem. If it is suspected to be a problem a practical option is often to straight away install a good EMI filter on the input mains, as this is typically cheaper than employing very specialist equipment and investigators.
A word of warning though; EMI filters are very good at filtering out noise in the range of about 100 kHz up into many megahertz, but they do nothing to correct normal harmonic distortion and do little if any correction in the frequency range of tenss of kHz. Some EMI filters have a degree of surge protection built in, but if lightening surges are likely to be an issue then a dedicated surge protection device would likely be warranted also.
As a last point, the reader should also to be aware that, particularly in the upper frequency range, that frequencies are also transmitted through open space in the same way as radio waves and that an appliance experiencing problems could be due to electro magnetic frequencies transmitted in this way, and the interference may not be present on the power supply at all. (Remember the old days of how the portable battery operated AM radio would sound when a hair-drier or vacuum cleaner was being used).
Working with the local electrical utility this problem can normally be resolved reasonably easily, particularly if you are armed with accurate voltage profile recordings from a power quality investigation.
Traditionally this was normally just a function of the tap-setting at the local utility transformer. Working with the local electrical utility this problem can normally be resolved reasonably easily. With imbedded generation increasing significantly (i.e. rooftop solar) it is however becoming more common that an overvoltage at the installation can be caused by the local generation. Such cases need to be evaluated individually so that the most appropriate solution for each case can be found.
Long duration over-voltages and under-voltages are also normally associated with the tap-setting of the utility’s local distribution transformer (as modified by local generation and load). But it is worth noting that such effects can also be caused by utility equipment higher up in the network malfunctioning, or by the utility implementing an emergency configuration change due to the failure of some piece of equipment. In either case it must be followed up with the utility, but you can rest assured that the utility will normally give your query much more prompt attention if you show them accurate record of exactly what your voltage profile has been (as obtained from a power quality survey). If you present the utility with evidence that something is wrong they are obliged to act.
The utility will often propose to upgrade their network to solve this issue, which can be very expensive for the customer. However, lower cost non-utility solutions are often available and these should therefore be investigated in detail.
The cause of this can be complex, but it is often a factor of the low voltage network between the utility’s transformer and the customer not being strong enough to supply the customer/s particular load pattern. One way to address this problem is by upgrading the low voltage cables or in some cases moving the transformer closer to the load, or installing another transformer to shorten the length of low voltage cabling. Such upgrades can be costly and utilities often require a significant financial contribution from the customer/s. There are however mitigation measures that the customer can sometimes implement to reduce the problems they are experiencing.
