Published by Megger July 2014 ELECTRICAL TESTER · ELECTRICAL TESTER ... Transformer test systems ... system, or transmitted in any form or by any means,

1 www.megger.com ELECTRICAL TESTER - July 2014

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Published by Megger July 2014

ELECTRICALTESTERA better way for transformer condition assessment

Figure 1 – Comparison of three transformers in very different conditions, but with the same tan delta.

Matz Ohlen - Director - Transformer test systems

Accurate information about the condition of insulation in a transformer is essential for effective asset management and accurate risk assessment. Information about moisture content limits the loading of the transformer accelerates the aging of the paper insulation, thus reducing transformer life. Dielectric frequency response measurement provides a cost- and time-effective method for determining transformer health, including the moisture content of the oil/paper insulation system.

Traditionally insulation condition is assessed by measuring tan delta/power factor at the mains frequency of 50 or 60 Hz. This approach is however of limited value. To see why, consider for example three power transformers that have the same tan delta/power factor at mains frequency. Further examination may, however, reveal that one of the transformers is wet with good oil and needs to be dried out, another is dry but has oil that needs reconditioning or regeneration and the third transformer is just in normal service-aged condition. The mains frequency tan delta test provides no way of distinguishing between these three different cases. Figure 1 clearly illustrates how misleading tan delta results can be.

In contrast, a dielectric frequency response (DFR/FDS) diagnostic test will provide accurate and conclusive information about the condition of the transformer insulation. Such data is essential in prioritising the

maintenance of transformer fleets and making sure assets will reach and exceed their expected service lives. See Figure 1.

High moisture in the transformer insulation gives rise to a number of well-known issues, including:

� Reduced load capability – The load capability is limited because of the decreased bubbling inception temperature

� Reduced dielectric strength – The dielectric strength of the oil is decreased, along with the partial-discharge inception voltage

� Increased ageing – High temperatures and moisture dramatically accelerate ageing that lowers the mechanical strength of the cellulose insulation

For good life management of transformers, the moisture content of the insulation must be kept at the lowest possible level. Transformers are dried during the manufacturing process until measurements or standard practices yield moisture content in the cellulose-based insulation of typically less than 1%. After the initial drying, however, the moisture content of the insulation system will continually increase.

There are several sources of water that drive this increase, including:

� Residual moisture not removed during the factory dryout process

�Moisture on the insulation surface during assembly and/or maintenance

�Moisture ingress from the atmosphere (gasket leaks, breathing during load cycles, site erection process)

� Ageing and decomposition of cellulose and oil

It is important to understand that the moisture in the transformer mainly resides in the solid insulation: typically 99% of the moisture in the transformer resides in the cellulose-based insulation. Knowing the moisture content is essential for transformer owners whether this information is used to verify that the transformer is dry as part of the commissioning process or to assess the condition of an older unit to determine if there is a need for preventive or reactive measures.

Assessing moisture content in transformer insulation based on oil sample tests is unreliable as the water migrates between the solid insulation and oil as the temperature and/or transformer load changes. An oil sample for moisture analysis has to be taken at relatively high temperature and when the transformer is in equilibrium. Unfortunately, this is a rare state for transformers, and the result is that most assessments based on oil sampling are unreliable. Experience has shown that this method tends to overestimate the amount of water in the insulation.

The DFR/FDS method is in contrast a very reliable method providing a high degree of accuracy in assessing the moisture content in the paper insulation, since it derives the

moisture content in paper or pressboard from the measured dielectric properties.

The curve of dissipation factor plotted against frequency has the typical shape shown in Figure 2. With increasing temperature the curve shifts towards the higher frequencies. Moisture influences the low and the high frequency areas. The linear, middle section of the curve reflects oil conductivity. Insulation geometry determines the knee points, which are located to the left and right side of the steep gradient.

With DFR/FDR instruments like the Megger IDAX moisture determination is based on a comparison of the transformer’s measured response to a modelled dielectric response. The insulation model used is the internationally recognized X-Y model described in e.g. CIGRE TB 254 and 414. The insulation duct between the LV and HV windings of the transformer is reduced to a single capacitor where the dielectric materials consist of cellulose and oil.

The single capacitor can be analytically described. A matching algorithm compares the model data with the measured data and adjusts the dielectric response of the single capacitor until a best fit with the measured response from the transformer is achieved. From the modelled response, the moisture in the cellulose material and oil conductivity/dissipation factor is determined as well as temperature dependence of the dissipation

Out of the ordinary!p8

The right choice for low resistance p3

Umbilical success p4

..turn page

2 ELECTRICAL TESTER - July 2014 www.megger.com

Contents

Editor Nick Hilditch.

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A better way for transformer condition assessment ............................................p1-2 Matz Ohlen, Director - Transformer test systems

Earth spikes in the trenches ....................p2 Nick Hilditch editor, Electrical Tester

Make the right choice for low resistance p2

Standards for transformer testing .........p2

Making the right choice for low resistance ...................................................................p2 Tony Wills, Technical support group

Phase verification in power networks ...p4 Damon Mount, UK power sales manager

TDR delivers umbilical success ................p4 Ian Jary

Developments in protection relay testing . ...................................................................p5 Stan Thompson, product manager,

Cable fault location without the complexity ...............................................p5 Henning Oetjen, product manager, cable test

Time domain reflectometry: techniques and examples ...........................................p6 Peter Herpertz, product manager, power

Dynamic measurements on load tap-changers ...................................................p7 Matz Ohlen, Director, transformer test systems

Lithium battery breakthrough ...............p7 Georg Halfar, Marketing communications manager, Germany

Q and A - Harmonics ................................p8

An award that’s out of the ordinary ......p8 David Danner, Distribution sales manager, USA

The birth of insulation testing ...............p8 Nick Hilditch editor, Electrical Tester

continued from page 1

When you have finished with this magazine please recycle it.

Anyone who has done much work in the field of earth electrode resistance testing will know at least a little about driving earth spikes into the ground, and concepts like the concentric earth shells that surround the electrodes. They may be slightly surprised, however, to discover that those who were unfortunate enough to be fighting in the trenches a century ago during the First World War also had good reason to be familiar with these ideas.

They weren’t of course, particularly interested in making earth resistance measurements – they had somewhat more pressing matters on their minds. But they were interested in finding a solution to a challenging problem, that of how to communicate reliably with each other and with their commanders. To appreciate the magnitude of this challenge, it’s important to remember that at the time of the First World War, radio was primitive.

Spark transmitters were still in use and valves (vacuum tubes) were in their infancy. Lee de Forest had invented the triode, the first electronic amplifying device, just eight years before the war began, and the types available during the war were not only fragile but also temperamental and limited in performance. Solid-state devices, other than

point-contact (cat’s whisker) diodes, weren’t even a glimmer on the horizon.

For these reasons, the main method of communication in the trenches was by wired telephones. These had, however, a rather serious and obvious weakness – the telephone wires were very easily damaged by shellfire so communications would frequently be interrupted, especially in battle situations where they were most needed.

One solution that proved surprisingly effective and was, as result used extensively, was the delivery of messages by carrier pigeons. Despite its benefits, however, this method is not particularly well suited to use during heavy artillery bombardments or for providing minute-by-minute situation updates. So another method was pressed into service – ground telegraphy.

Essentially, this involves driving two earth stakes into the ground as far apart as possible at the transmitting station, and connecting them to a circuit that includes a buzzer, a battery and a telegraph key (a type of switch that can be rapidly and repeatedly opened and closed). At the receiving station, two similar stakes are driven into the ground and are connected to a sensitive pair of headphones.

Via the earth stakes at the transmitting station, the buzzer injects rough AC currents into the ground. The receiving earth stakes pick up a tiny fraction of these currents which, according to reports published at the time, produced a signal that could be clearly heard in the

headphones. With this arrangement, someone operating the telegraph key at the transmitting station could send a message in Morse code to the receiving station.

The ground telegraph had big advantages over other communication methods: it needed no wires between the transmitting and receiving station, and it needed neither delicate valves nor bulky power-hungry spark transmitters. One of its potential disadvantages, however, was that in principle all that the opposing forces had to do to listen in on the messages was to deploy a few earth stakes of their own. A partial solution was apparently found by using tuned buzzers, but the reports of the day are unsurprisingly rather cagey about providing much in the way of detail.

It seems that the ground telegraph achieved significant success, and it is reported as being widely used especially in the later phases of the war. If you’d like to find out more, the January 1918 issue of the American publication “Electrical Experimenter” contains an interesting article “Ground Telegraphy in War” and can be found on line at http://electricalexperimenter.com/n9electricalexperi05gern.pdf.

And next time you’re driving in those ground stakes to carry out an earth electrode resistance test, you might like to spare a thought for your predecessors who were doing essentially the same thing but for a rather different reason in the hellish conditions of the World War I battlefields.

Earth spikes in the trenches

factor. Only the insulation temperature (oil/winding temperature) needs to be entered as an input parameter.

Several international standards, guides and reports give guidelines for insulation assessment in terms of power frequency tan delta/power factor at 20°C, moisture content in solid insulation and conductivity/dissipation factor of oil. Insulation assessment using DFR/FDS techniques provides results and categorization for tan delta/power factor of the whole insulation system at 20°C, moisture in cellulose and oil conductivity/dissipation factor at 25°C, giving the user a detailed description of the insulation condition.

Guidelines for oil conductivity/dissipation factor at 25°C are given in IEEE C57.152-2013. For power frequency tan delta/power factor at 20°C the assessment limits are based on industry practice and the guidelines in CIGRE TB 445 and IEEE C57.152-2013.

In summary, DFR/FDS analysis of transformer insulation is a timesaving method based on the use of multi-frequency techniques to give the shortest possible measurement time. It offers temperature independent results and, when a wide frequency range and the latest modelling

methods are used, it gives results that are accurate and reliable. Further, the DFR/FDS test method has been validated by more than

Figure 2 – Dissipation factor plotted against frequency for a power transformer

ten years of practical experience, and is now included in international standards and guides for transformer diagnostics.

Nick Hilditch, editor, Electrical Tester



TESTERStandards for transformer testing

Power transformers are costly and, in many cases, critical assets. This means that accurate information about their current condition is exceptionally useful, as it allows maintenance to be rationally planned and also allows incipient faults to be identified and remedied before they develop into outright failures.

In the past, however, building a comprehensive picture of the condition of a transformer involved carrying out a whole battery of tests and, even then, some types of potential problem could not be detected without actually dismantling the transformer, which is a very costly, time consuming and inconvenient process. It was to address these shortcomings of conventional test methods that sweep frequency response analysis (SFRA) testing was developed.

Electrically, a transformer is made up of multiple capacitances, inductances and resistances. It is in effect a very complex circuit that produces a unique “fingerprint” when test signals are injected over a range of frequencies and the results plotted as a frequency response curve.

Movement of the windings, which can be caused by electrical overloads, mechanical shocks etc will therefore alter the capacitances (and/or inductances) and change the shape of the frequency response curve. The SFRA test technique for transformers is based on comparisons between measured curves, which allow variations to be detected. An SFRA test involves multiple sweeps on a particular transformer and reveals whether the mechanical or electrical integrity of the transformer has been compromised.

SFRA testing has now been in use for well over a decade, and during that time it has amply proved its worth as an efficient and effective means of identifying faults and potential faults in power transformers. Nevertheless, there are still many engineers and technicians who see SFRA testing as a new and relatively untried technique. This view has, in part at least, been based on the perception that SFRA techniques are rarely referenced in national and international standards and guides.

Once, of course, this was true – because of the nature of the standardisation process, there is always a significant ‘waiting period” before major standards and guides catch up with any new form of technology no matter how useful it may be. In the case of SFRA testing, however, this waiting period is now over, and the technique is making an appearance in the latest editions of many international standards and guides that relate to transformer testing, including

� CIGRE TB 342, “Mechanical-Condition Assessment of Transformer Windings” (2008)

� CIGRE TB 445, “Guide for Transformer Maintenance” (2011)

� IEEE PC57.149, “Guide for the Application and Interpretation of Frequency Response Analysis for Oil Immersed Transformers” (2012)

� IEC 60076-18, “Power transformers Part 18: Measurement of frequency response” (2012)

� IEEE C57.152, “IEEE Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators, and Reactors” (2013)

Given this wide recognition of SFRA testing, potential users need have no further qualms about the validity or usefulness of this technique. SFRA testing is no longer an unknown application but a recognized, described, standardized and recommended method for transformer diagnostics.

Low resistance measurements have many and diverse applications, ranging from checking the condition of circuit breaker contacts to verifying the integrity of welded joints. Low resistance test sets are, however, available in an almost bewildering range of types; this article offers some useful advice on choosing the right test set for the application in hand.

Low resistance measurements are generally considered to be those that involve resistance values of less than one ohm. There are many reasons why such low values cannot be reliably measured with an ordinary ohmmeter – the resolution will almost certainly be too low to provide accurate results, the resistance of the test piece may well be swamped by the resistance of the test leads, the results will be distorted by the tiny voltages generated at the junctions of dissimilar metals in the test object, and so on.

To overcome these problems, special-purpose low resistance ohmmeters have been developed. There are many types and sizes, but most share certain features. These include the use of four-terminal connections, where the current to the test object is delivered by one set of cables, and the voltage across the test object is measured with another set, an arrangement that eliminates the effect of test lead resistance.

Most types of low-resistance also have facilities for bipolar testing, which means, in effect, that two tests are performed with the polarity of the test current reversed between them. The readings are averaged to produce a result that is unaffected by thermal and contact voltages that may be present in the test object. And another feature that’s common to low resistance test sets is that their test currents are much higher than those of ordinary ohmmeters – currents from 10 A up to as much as 600 A are in common use. These high currents are needed because many types of test object behave differently with high and low test currents.

An example is the main contacts in a large circuit breaker. With a low test current, these sometimes show a comparatively high resistance, usually because of minor contamination, which can lead to the circuit breaker being judged as not fit for service. If, however, the contact resistance is measured at a higher current, comparable with the

normal operating current of the breaker, the contamination will often break down and the contact resistance will be shown to be perfectly acceptable.

Having examined the features they share, let’s now look at how various types of modern low resistance test set differ. Traditionally, because of the high currents they need to deliver, these instruments have been large and comparatively heavy but, within the last couple of years, a new and very different type has become available in the form of the Megger MOM2 micro-ohmmeter, which uses patented technology.

The MOM2 is small, light handheld instrument that, despite its diminutive design, can deliver test currents as high as 220 A. The secret of this innovative device is that it incorporates an ultra-capacitor that has a capacity of several farads. Between tests, this is charged by a battery-operated inverter circuit and, during the tests it is discharged into the object under test. The test current and voltage are continuously and synchronously sampled throughout the test period, and the results automatically averaged to obtain the final value. The results are accurate and reliable, and this test method complies fully with IEEE and IEC standards for tests on a wide range of equipment.

The benefits of such a small and convenient instrument are clear – it can be taken and used where using a conventional instrument would be problematic, such as at the top of a ladder and tests can be carried out almost anywhere, as there’s no need for access to mains power.

This particular instrument also has another big benefit, in that it supports Dual Ground testing. Typically used when carrying out tests on circuit breakers in substations, Dual Ground testing allows the tests to be performed with both sides of the breaker grounded. This means that the potentially lethal hazards associated with voltages induced into ungrounded equipment are completely eliminated.

These compact new instruments are clearly invaluable in many applications, such as the testing of circuit breaker contacts, busbar joints and high-current connections, but are they the complete answer for every application? The answer has to be no. One thing that ultra-capacitor based instruments can’t do is to maintain the test current for more than a few seconds.

Often this doesn’t matter, but there are some applications where prolonged testing is needed, either because of the nature of the test or because that’s what the applicable test standards specify. An example is checking joints between the rails in railway systems, where low resistance is essential because of the very high traction currents drawn by electric trains. In this application, extended tests are always carried out with high currents.

In other applications, it may be useful for the application of the test current to be prolonged so that it heats the object under test. This is sometimes the case with busbar systems, where testing is best performed with the busbars heated to their normal operating temperature. It may also be useful to monitor how the resistance of the test object changes over a period of hours, in which case an instrument with continuous current capability is clearly essential.

For these types of applications, conventional test sets are needed, but even then the latest instruments are lighter and more compact than their predecessors. Versatile 200 A test sets are now available that weigh no more than 15 kg, making them relatively easy to handle although, admittedly, not at the top of a ladder.

The best of these instruments allow the test current to be adjusted to match the application, as the highest current is not always the most appropriate choice. Sometimes it may be desirable not to heat the test object unnecessarily, in which case a relatively modest test current will need to be used. Also, it may be useful to explore the behaviour of the test object with low currents as a preliminary to proceeding to higher current tests.

Another key feature in the best instruments is the availability of multiple testing modes. Continuous mode, as the name suggests, is ideal for users who need to monitor the resistance of the test object over a period of time, while with normal mode, after the test has been initiated, the current ramps up to the preset value, remains steady for a short time – typically two seconds – and then ramps down again to zero. This is well suited to everyday testing.

Some testers additionally offer a useful and time saving automatic mode. With this, after setting the desired test current, the user connects only the current leads and then presses the instrument’s “test” button. The test doesn’t actually start, however, until the potential leads are connected and it stops as soon as they are disconnected.

This mode is invaluable when testing multiple joints in a busbar as the current leads can be connected to the ends of the busbar and left in place, while the user applies the potential leads across each joint in turn. The instrument automatically carries out a test on each joint, and records the results in its internal memory.

As we have seen, there is no single low-resistance ohmmeter that is the ideal solution for every application, although many are very versatile. Hopefully, the guidance given in this article will help potential users to pick the instrument that’s best suited to their needs. Nevertheless, expert advice is always useful and is readily available from Megger, one of the few companies that can be truly impartial as its product range includes low-resistance test sets of almost every type.

Making the right choice for low resistance

Tony Wills - Technical support group

Matz Ohlen - Director - transformer test systems



TESTER

Incorrect phasing in power networks can have devastating and even deadly consequences. Yet phasing has, until now, been surprisingly hard to verify. This has changed with the introduction of an innovative phase verification system that makes use of the ultra-precise time-base provided by the signals from GPS satellites.

Phasing is often far from readily apparent. Sometimes there is no phase identification available, phase identification can be incorrect due to line transposition, crossover in joints or there could just be incorrect labelling of conductors or terminations. This can open the door to misunderstandings amongst staff, and dangerous phasing errors when adding new equipment, connecting or re-connecting a network.

The consequences of incorrect phasing can be catastrophic. If a link is inadvertently made between two wrongly phased supplies in a power network, the instantaneous currents that flow are likely to be enormous. In a properly designed system the protection will operate very quickly, but in the very short time before this happens or if the protection fails, the energy produced by the fault current could damage or destroy equipment, and possibly even cause an explosion.

It’s worth noting that phasing mistakes can lie dormant for months or years. Often open ring distribution networks incorporate switches. One of these switches can remain open in normal circumstances, but could be closed to reconfigure the network if there a need for planned maintenance or correcting a fault. Closure of such a switch will temporarily connect two supply sources that normally operate as two radial feeds into a closed ring. If the phasing is incorrect – possibly because of changes that have been made to the network since it was originally commissioned – the results can be dire.

The solution to all of these problems is to use a dependable method of phase verification. Before discussing this, however, it’s important to emphasise one key point: phase verification is NOT the same as checking phase rotation. It’s perfectly possible to have two supplies that are phase displaced with respect to each other, yet have the same phase rotation. Phase rotation testing is no substitute for phase verification; for safe network operation, it is essential to be able to determine the ABSOLUTE phasing at every point in the network.

Absolute phasing can only be determined when measurements are made with respect to a known reference phase. Unless the system is very small, this means that a two-part instrument is needed to make the measurement – one part to act as the base station that provides the reference information, and the other to perform the measurement at the point in the network where it is required to measure the absolute phase.

This is the concept used in a novel phase verification system that is now available. It comprises two identical devices. One is configured as the base station and is connected to a low-voltage reference phase while the other is configured as the phase-measuring device, which, for convenience, can be referred to as the mobile device, as it will normally be used at a location remote from the base station.

An ultra-precise timebase for synchronisation of the devices is established using signals from the GPS satellites that are more usually associated with satellite navigation systems. Synchronisation data is passed between the devices via the mobile phone network using a GSM module that can operate in standard data mode (circuit switched data, or ‘CSD’) as well as in GPRS mode.

If the voltage for the mobile device is up to 400 V, you can connect directly to the phase that you need to identify. You can use the same connections when capacitive measuring taps on switchgear are available.For higher voltages, you could use a high-voltage sensor with wireless data transmission to the measuring device with an insulated pole approved for use at the appropriate voltage.

If the base station is not connected to L1 as the reference phase, the corresponding correction angle of +120º or -120º must be entered into the mobile device. Also, depending on the application, there may be transformers with the same or different vector groups between the base station and the measuring device. These vector group shifts, which will lead to a particular phase shift, can then be entered into the mobile device in order to obtain the correct absolute phase indication.

To cater for the widest possible range of applications scenarios, this innovative phase verification system offers four operating modes.

NET mode NET mode is applicable when a low-voltage supply, such as a mains socket, is available at the location where the phase measurement is to be performed. In this case, the mobile device is connected to the low-voltage supply and a one-time synchronisation process is carried out with the base station. The mobile device determines the phasing of the low-voltage supply and uses this the local reference phase for all measurements carried out at this location.

The low-voltage source must remain connected to the mobile device throughout the entire measurement process.

The major advantage of this operating mode is that GPS and GSM reception are only needed for a short period while the one-time synchronisation process is carried out. After that, all measurements are carried out

with respect to the local reference phase, which means that the measurements can be performed very quickly.

NO NET mode

Where no local low-voltage supply is available – for example, when carrying out measurements on overhead lines – the mobile device operates from its internal rechargeable battery. If immediate phase display during measurement is needed in such cases, the mobile device must be continuously synchronised with the base unit via GSM, and GPS reception must be available.

NO NET/NO GSM mode

There are a few locations where no local low-voltage supply is available and there is also no GSM reception. To make measurements in these locations, NO NET/NO GSM mode is used. In this mode, when the measurement is being made, the mobile device stores only the GPS time signals and the voltage zero crossings. When the mobile device is subsequently moved to a location where GSM reception is available, post-synchronisation is performed – the absolute phase identifiers are determined and stored in a measurement file for later analysis. Post-synchronisation can be performed at any time up to 10 days after the measurement data is recorded.

LOCAL mode

In this mode, only the mobile device is used. It is connected to a local supply with a known reference phase, and all measurements are made with respect to local reference phase. No synchronisation with the base station is needed.

The novel phase verification system described in this article allows safe, fast, and accurate phase identification at all voltage levels. The range of operating modes available makes the system convenient and easy to operate in virtually every application. The use of this system helps to eliminate the serious safety hazards associated with incorrect phasing in a network, as well as ensuring that documentation and any future additions will be correct. The ultimate benefits are, therefore, significant overall improvements in network reliability, efficiency and safety.

Phase verification in power networks

A fault in the umbilical cable on a prototype seafloor drill seemed at first as if it would be a difficult and costly problem to rectify. By using a time domain reflectometer (TDR) however, freelance remote operated vehicle (ROV) pilot and technician Ian Jary was able to locate the fault quickly, accurately and conveniently, which meant that the time needed to make the repair was much less than expected, as was the cost of the remedial work.

ROV expert Ian Jary was working for Gregg Marine, a part of US-based Gregg Drilling, on the new and innovative Gregg Marine Seafloor Drill (SFD), which was at the time undergoing trials in the Indian Ocean approximately 120 miles off the west coast of Australia.

The Gregg SFD has been developed to provide an efficient and exceptionally cost-effective alternative to ship-based drilling rigs for carrying out seabed sampling and core penetration testing at depths up to 150 m. The SFD is readily transportable as, when it is disassembled, it fits into three 20-foot sea containers and it can also be used for carrying out seafloor surveys at depths up to 3,000 m.

During the Indian Ocean trials, the SFD was remotely operated from multipurpose platform supply and support vessel Greatship Mamta via a 3,000 m long hybrid fibre optic/copper umbilical. At an early stage of the trails, problems were experienced with the fibre-optic cores of the umbilical, but these were quickly rectified. Later, however, several of the copper conductors showed a dead short.

The usual procedure for location a fault of this type is to use the half-split method, which involves repeatedly cutting sections of the umbilical in half until a short section containing the fault is found. After this the sections, excluding the one with the fault, are spliced back together. This process is slow, costly and often results in an umbilical with an undesirably large number of splices.

In search of a better solution, Ian Jary suggested that it might be possible to locate the fault using a Megger TDR500 time-domain reflectometer (TDR). This compact handheld instrument launches short pulses into copper cables and, if there is a fault on the cable, this reflects some of the energy from the pulses back to the instrument. By measuring the time between the launch of the pulse and the arrival of the reflection, the TDR can accurately calculate the distance to the fault.

Unfortunately, Ian Jary had one small problem. While he had access to a TDR500, he had no operating instructions! An email to the Megger technical support department elicited a quick response in the form of a pdf instruction manual, and TDR testing on the faulty umbilical was soon underway. In a very short time, the instrument indicated that the fault was 270 m from the end of the umbilical and subsequent dissection of the cable at this point confirmed that the measurement was accurate to within less than a metre.

“In the world of marine exploration, time is not just money, it’s a lot of money” said Ian Jary, “so the speed and ease with which we were able to locate the umbilical fault using the TDR translated into savings that certainly run into thousands of dollars, if not tens of thousands. And the prompt response we received when we needed a new copy of the operating instructions was also noteworthy. I have absolutely no reservations in recommending this invaluable little instrument to anyone who works in the offshore sector. By finding just one fault it will pay for itself many times over.”

TDR delivers umbilical success

Damon Mount - power sales manager

Keith Wilson, electrical engineer



TESTER

Recent developments have transformed the field of protection relay testing, with the latest multi-phase test sets in particular offering features and benefits far beyond those provided by even their recent predecessors. But exactly what can users expect from these new test sets? Let’s take a look.

FlexibilityTest sets with fixed configurations deny their users future proofing. In contrast, those that offer flexible configurations mean that future changes in requirements can be accommodated easily, conveniently and at modest cost. Flexible multi-phase test sets are now available that allow users to choose up to four voltage/current generators (VIGENs) to provide four voltages and four currents, or to combine the four VIGENs with one double-current generator to provide four voltages and six currents simultaneously, or even to combine three VIGENs with a single-channel voltage source (VGEN) to provide four voltages and three currents.Other options available with some of the most versatile test sets include using five VIGENs with both of their channels configured as current outputs to provide a total of 10 current channels. This allows the latest multi-phase differential protection relays to be tested conveniently.In addition, the ability to interconnect multiple units together, along with the convertible voltage/current channels, can provide up to 18 currents for testing multi-phase bus differential protection schemes.Flexibility not only means that users can configure their test sets so that they match their exact requirements, but also that they can initially purchase a unit with a modest specification safe in the knowledge that they can upgrade it to a full four- or even five-channel unit in the future, should they need to do so.

Constant power outputThe latest high-power voltage-current amplifiers deliver maximum compliance voltage to the load constantly during the test, and range changing is done automatically under load. This ensures better test results and saves time, as the outputs don’t have to be turned off to change ranges, as was typically the case in older instruments.In many cases, constant power output eliminates the need to parallel- or series-connect channels when testing high-burden relays. Outputs are regulated and, therefore, they are unaffected by changes in load impedance which, for example, occur with saturating loads like inverse and extremely inverse time characteristic electromechanical overcurrent relays.

Higher output currentBest-in-class current amplifiers now provide 30 A at 200 VArms per phase continuous and up to 60 A for 1.5 seconds. Six of these amplifiers can be paralleled to provide 180 A at 1,200 VA or up to 360 A at 1,800 VA for 1.5 seconds.

Low range with higher accuracyThe availability of a 1 A range provides

enhanced accuracy at low test currents. Low current ranges typically also have reduced switching noise, which improves regulation and accuracy with test currents as low as a few tens of milliamps.

Constant power output voltage amplifiersThe new voltage amplifiers provide higher current output at low test voltages, from 30 V to 150 V. This is important for users who want to test a complete panel of relays as a single operation, as this is often impossible with older test equipment that has voltage amplifiers with a limited VA rating. The new amplifiers also solve the problem of testing old-style electromechanical distance relays that can only be tested using a high-power voltage source.

Compact construction and light weightThe best of modern test sets are very much smaller and lighter than their predecessors. Specifiers and intending purchasers should, however, exercise a little caution in this respect. There are compact, lightweight units available that have achieved their diminutive dimensions and weight by sacrificing performance, particularly in terms of output power. These units are best avoided as, for most users, their limitations outweigh their benefits.

Large, high-resolution colour displayAn important innovation is the introduction of large, high-resolution touch-screen colour user interface systems that provide intuitive automatic, semi-automatic and manual control of the test set. In leading instruments, the screens are complemented by a context-aware control knob that allows easy and convenient adjustments to be made to key parameters related to the particular test being performed.

Automatic ramp, pulse ramp and pulse ramp binary search capabilitiesThe best test sets have all of these capabilities and they also allow users to use the control knob to ramp outputs up and down manually. When using the automatic ramp functions, users can easily adjust the ramp increment to suit their requirements.

Click-on-fault impedance relay test screenAnother key feature found in the latest instruments is a click-on-fault test screen facility for easy and convenient testing of impedance relays. Typically, all the user needs to do is to select from a menu the generic operating characteristics of the relay to be tested, or to select its exact operating characteristics from the instrument’s relay library.

Click-on-fault impedance relay libraryAn integral relay library means that users simply pick from a list of predefined operating characteristics for popular relays from manufacturers such as ABB, AREVA, General Electric and SEL. They then enter the relay settings and select the type of fault desired – line-to-earth, line-to-line or three-phase. The test will then progress down a line as a ramp, pulse ramp of pulse ramp binary search, looking for the relay to operate. With the shot test method, users can also select a test point within and outside the characteristic to quickly check the relay under test.

Real-time click-on-fault impedance test screenThis feature allows users to see, on the screen, the operating characteristic of the relay under test with the test pointer moving in the impedance plane to indicate exactly where the test impedance is, in real time. When the relay operates, a coloured test point is displayed showing where the relay operated – the test point is shown in red if the relay failed the test, green if it passed. The user can also see the test vectors changing in real time, or see positive-, negative- and zero-sequence vector values. When the test is complete, the test results are displayed for each defined test point with pass/fail indication.

Binary inputs and outputsUser-configurable binary inputs to monitor contacts and high-current binary outputs are invaluable, and in leading instruments they can even be allocated user-defined names, such as Trip1 or Recl1, to make them even easier to work with.

Overcurrent testing with IEC/IEEE curve algorithms and relay-specific curves

The test set evaluates timing tests by automatically comparing actual and theoretical operating times. The user doesn’t need to refer to the manufacturer’s time curve to determine error, which means that this feature is a big time saver. The test set displays the time curve for the relay under test, plots the test results automatically with the time curve, and determines pass/fail.Dynamic sequencing test capabilityThis feature provides multi-state dynamic testing to include trip and reclose timings tests for, typically, up to 15 states, including lockout. Each binary input and output can be set with each state along with voltage, current amplitudes and phase angles. With each state, the user may input values of voltage, current, phase angle, frequency and phase angle, as well as setting the binary input sensing. With some instruments, three-pole tripping as well as single-pole can be simulated.

Simplified testing of three phase transformer differential relaysIn order to correctly test new three phase transformer differential relays, three compensations must be taken into consideration by the relay; current magnitude compensation, phase angle compensation and the zero sequence current compensation. By performing transformer differential tests for each sequence component verifies that the differential protection will be stable for all symmetrical and non-symmetrical external faults, and through-load conditions. In addition, tests will also confirm that the differential relay will trip for any internal fault. The most modern test systems can not only provide the six currents required for testing two winding transformer differential relays, but they also should include both ANSI and IEC transformer models to accurately calculate the required test currents and phase angles relationships that get applied to the relay. From the transformer model, the test system should be able to take into consideration the transformer vector groups, be able to eliminate zero sequence when testing single phase to earth faults. Using a touch screen to select test lines, it should also be able to perform dual slope characteristic tests using one of the seven different Biasing equations, and one of the four different slope characteristics that the modern microprocessor based relays use today.

Manually defined harmonic waveformsA recently introduced feature on top-of-the-range test sets, this allows users to seasily create harmonic waveforms consisting of a fundamental plus superimposed second, third and fourth harmonic waveforms. Individual harmonic amplitudes or phase angles can be easily ramped as required. This feature provides harmonic restraint testing with a single current channel and is also useful for checking the performance of power quality meters.With the latest relay test sets offering so many new and useful features, now is a very good time for those involved with protection system testing to take a long hard look at the test equipment they are currently using. Undeniably, a new test set represents a significant investment, but if the new test equipment prevents just one significant power outage it will, for most users, have paid for itself many times over.

Stan Thompson - Product manager

Developments in protection relay testing

Cable fault location without the complexity

Locating faults on underground power cables is a task that’s usually considered difficult, complicated, potentially dangerous and, in short, best left to the experts. However it is becoming more and more common to employ all available field personnel – essentially, whoever happens to be available on a given shift – to perform cable fault location. This approach means that fault location is now often carried out by field personnel who perform the task only a few times each year. This change means that test equipment suppliers must now provide equipment that, in as many situations as possible, will allow users to locate faults successfully even if they are not fault location experts. Until recently, this challenge had never been fully addressed, but this situation has changed with Megger’s introduction of a new

generation of intelligent cable fault locators, based on the E-TRAY platform.

Just like conventional instruments, the new units incorporate a “thumper” that injects a surge current at high voltage into the faulty cable, and in combination with a time-domain reflectometer (TDR), they measure the distance to the fault. The difference with the new E-TRAY platform is that the units are not only much easier to use, they also feature an automate test sequence for the fault location process and, crucially, provide interpretation of the test results.

The E-TRAY user interface is operated by a turn-and-click rotary pushbutton that allows the user to automatically proof-test, pre-locate and pinpoint faults. No adjustments or settings are required and, at every stage, the unit suggests the next logical step in the testing sequence. The test set adjusts the surge voltage

automatically to minimise the stress on the cable and, before the testing starts, it carries out safety checks to protect the user from the risks that could result from incorrect or faulty earth connections.

Even the interpretation of the TDR results – a task that’s often considered to require a large amount of experience and expertise – is eliminated in many situations through the use of the smart software being an integral part of the E-TRAY platform. After the initial test sequence has been completed, the smart software analyses the TDR results and presents them to the user in a clear and readily understandable form. It is, however, worth noting that the user can, at any time, overwrite the automatic setting of test parameters, the automatic sequence of the process and/or the automatic interpretation of the test results.

This high level of automation makes the new intelligent cable fault location test sets, of which the new Megger Smart Thump ST16 is a leading example, an ideal choice for use by users who are experienced in high-voltage systems, but are not specialists in cable fault location. That doesn’t mean, however, that these test sets are of limited use to more experienced operators, as they offer the full complement of typical fault locating features and options..The new E-TRAY intelligent test sets have a wide range of capabilities and are suitable for use on low and medium-voltage distribution networks with system voltages up to 35 kV. They are capable of working with circuit lengths from typically 100 m to 5,000 m, and are intended for use with cables having solid dielectric insulation like EPR or XLPE. Typical users who will find them of interest include power utility companies, street lighting maintenance providers, private network operators, airports, ports, military bases, large industrial organisations and high-voltage electrical contractors.

Henning Oetjen - Product manager



TESTER

Time domain reflectometry: techniques and examples

One of the best ways of enhancing the evaluation of results produced using time domain reflectometers (TDRs) is to make comparative measurements, as comparisons invariably show the details of faults much more easily and clearly. If the equipment used has facilities for storing measurements, it is even possible to compare a single-core cable with itself, by comparing the current measurement with a stored trace. Between two measurements on the same cable, the fault can be enhanced by manipulating it – for example, by burning or by using various Arc Reflection Methods (ARM).

Typical procedure for making a reflection measurement

1. Determine the fault resistance by using an ohmmeter or an insulation tester. This insulation tester must have a measurement range of less than 1 kΩ in order to resolve resistances between 10 and 1000 Ω. Connect the reflectometer to the faulty cable and set the typical propagation velocity for the cable.

2. Select the measurement range so that the entire cable length is visible at the start of the measurement. If necessary, check on a good, fault-free core.

Important rule: Always ensure that the cable end is visible!3. Where possible, set the compensation so

that a horizontal trace is visible at the start of the reflectogram. The remaining small visible positive and negative deflections should be relatively symmetrical. Avoid saturation (Clipping).

4. Start the measurement with a large pulse width, and once all details including the end are recognisable, reduce the pulse width, until these details are just still visible. By doing this, the measurement will be done with the highest resolution and accuracy and will provide the highest level of detail.

5. Enhance the fault position by using the gain setting. Reduce the display range if necessary.

6. Measure the fault location. The exact measurement of the fault position is made by using the vertical cursor. Set this to the foot point of the fault reflection, which is the position where the fault reflection just starts and where it causes the first deviation from the normal horizontal behaviour of the trace. This is not necessarily a deviation from the horizontal line – it can also start within an existing reflection.

7. If possible, a comparison of good and faulty traces should be made. The deviation (splitting) of the reflectograms at the fault location, which is usually readily apparent, can be measured very easily, resulting in a high level of accuracy and easier fault recognition.

MEASURING METHODS

Direct reflection measurement

Line comparisonA fault-free, good core is necessary for line comparison. Alternate switching between the good and faulty lines reveals a difference between the reflectograms, which indicates the fault position.

Intermittent fault location (IFL) modeIFL Mode is intended for the localisation of intermittent faults. These are faults that appear only sporadically, in certain situations, and can usually not be reproduced in normal operation. During IFL operation, a reflectogram is continuously recorded. This records all changes irrespective of their appearance. With no fault present, the changes will only affect a small area (envelope) around the normal trace, and will only thicken it. As soon as a significant fault occurs, however, a clear change (splitting) of the envelope will be seen as shown in Fig 3 at around 1.35 km. The operator can wait for something to happen, or force the fault to change by moving or shaking the cable

Differential measurementWhen the differential method is used, the good and faulty lines are connected simultaneously via a differential transformer to the reflectometer. In this mode, one line is measured in the normal way. When measuring the comparison line, the polarity of all reflections is changed by the differential transformer. This means that only real differences are shown in differential mode. Faults of the same size or completely interrupted cables cannot be seen since no difference exists.

Note: When using the differential method, always ensure that the test leads are correctly connected. Interchanging the leads changes the polarity of the reflection.

AveragingInductive couplings or other external influences as radio stations etc. generate noise and disturbances in the trace. These disturbances can be eliminated by using the averaging mode to average multiple (up to 200 or more) measurements.

Examples of parallel faults with various resistances (negative reflection)

The closer the value of fault resistance is to the cable impedance, the less visible it becomes. When both values are identical, no reflection can be seen. This exact effect is used in the communication area for the resistive termination of circuits. (LAN, antenna circuits and other data lines). If the terminating resistor is equal to the cable impedance, no signal is reflected. The quality of a termination can be tested and verified with a reflectometer.

Examples of longitudinal faults with various resistances (positive reflection)For fault location it is essential always to bear in mind that an easily visible open reflection is not necessarily the end of the cable – it can also be a break or cut in the cable. A simple test is to

Fig 1: Reflection measurement - 8 km cable

Fig. 2: L2 positive reflection – Cable end, joint visible. L1 negative reflection – Short circuit in joint

Fig. 3: Temporary short circuit at cable end

Fig. 4: Cable joint, parallel fault R = 0 Ω, end of cable

Fig. 6: Joint, longitudinal fault R = 100 Ω Fig. 7: Joint, longitudinal fault R = infinite, open cable end

Fig. 5: Joint, parallel fault R = 100 Ω

Peter Herpertz - Product manager



TESTER

short the end and check if this is visible. If it is

not visible, the reflection is from an interruption

in the cable.

This is the final part of a five-part series by

Peter Herpertz covering cable fault location. If

you’ve missed either of the previous four parts,

you’ll find them in the back issues of Electrical

Tester, starting with the July 2013 edition. Back

issues are available on line at

www.megger.com/electricaltester

Dynamic measurements on load tap-changers

The dynamic resistance measurement (DRM) technique for circuit breaker diagnostics was invented by Megger and is used extensively on high voltage circuit breakers to determine the condition and deterioration of arcing contacts inside the breaker. DRM is also an advanced diagnostic tool for load tap-changers to detect problems related to transition times and resistance values.

A newly introduced accessory power unit for use with Megger circuit breaker analysers (CBA) provides three 1 A or one 5 A constant-current outputs, as well as facilities for safely discharging the transformer windings via the current and voltage leads. In conjunction with a circuit breaker analyser, this power supply forms a complete and convenient set up for dynamic voltage and dynamic resistance testing on load-tap changers.

The example in Figure 1 shows resistance change/transition time results produced using this equipment on one tap changer of a 20 MVA transformer. The transition times and resistor values can be readily determined from these results.

A new test method/application – dynamic voltage measurement – has recently been added for performing dynamic measurements on load tap-changers. Dynamic voltage measurement can be used for verification of continuity and contact/resistor switching times. Depending on the settings used and test current it can also give an indication of diverter resistor values.

Dynamic voltage measurements are preferably done on the opposite side of the transformer to the load tap changer – that is, on the LV side if the load-tap changer is on the HV side – recording the inductive/induced voltage. A typical arrangement is shown in Figure 2.

The main advantage of this method is its simplicity. It is only necessary to connect a voltage measurement input to one (or three) windings, inject the test current and record LV voltage during the tap transition. With some circuit breaker analysers, such as the Megger Egil, no computer is needed – the voltage

versus time curves can be recorded and printed without additional equipment.

To perform the test, the power supply is connected to inject about 0.1-1% of rated current for the winding (typically 1or 5A). The voltage on the secondary is measured during the tap transition. The measured voltage will reflect the inductive voltage generated inside

the tap-changer when the diverter circuitry is switched. Measurements can be performed one tap at a time or over the complete operation over all taps.

An example of a dynamic voltage measurement for one tap transition is shown in Figure 3. As will be seen, the individual, parallel and total resistor transition times can easily be identified.

Figure 1 – DRM test on a load-tap changer in good condition, using

a constant current source. Blue – voltage/

resistance. Red – test current. H1 1-2 tap

change with LV shorted.

Figure 3 Measured dynamic voltage on the LV side of a transformer during a tap transition. (Tap changer on HV side). Total resistor transition time 49 ms. 6 MVA transformer, Dyn11, 21/11 kV, test current 4 A.

Figure 2 - Set up for dynamic voltage measurement.

In this day and age, if you start reading about current battery technologies it won’t be long before you see lithium mentioned. That’s because in recent years lithium-ion (Li-ion) batteries have come to be recognised as an important breakthrough in the field of portable energy sources.

Compared with other types of primary and secondary cell, Li-ion batteries have very high energy density (the amount of energy stored per unit volume) and specific energy (the amount

Lithium battery breakthrough

Georg Halfar - Marketing communications

manager, Germany

Matz Ohlen - Director - transformer test systems

of energy stored per unit mass). Also, their cell voltage is high. Just as important in many applications, however, are their long shelf life before they start to deteriorate, and their ability to retain charge for long periods.

These characteristics make Li-ion batteries ideal for use in emergency equipment, such as a flashlight, that is often left untouched for long periods and then expected to work instantly on demand. In fact, the same expectations often apply to test instruments, which sit on the shelf for months but are then required to be ready for immediate use to solve a problem or to help in locating a fault. Li-ion batteries are therefore an excellent option for powering portable test equipment. In some other common applications

the most important advantage of Li-ion batteries is their low weight. It is for just this reason that they are almost invariably the battery of choice for electric and hybrid vehicles. The more an electric vehicle weighs, the more energy it takes to move it and the less efficient it becomes. In addition, the extra weight increases wear and tear on the tyres and on the vehicle’s mechanical components in general. It’s easy, therefore, to see the attraction of Li-ion batteries for use in vehicles.

Electric and hybrid vehicles – and the batteries they use – have attracted a lot of attention in recent times, but the number of electric vehicles currently in use worldwide is still very small compared with the number of conventionally fuelled vehicles. This will almost certainly change in the near future. In the UK, for example, it has been suggested that somewhere between 8% and 30% of cars sold in 2020 will incorporate either hybrid or pure battery drive systems.

The main reason that this estimate is so wide is that much depends on future progress in battery technology. The high price of today’s electric vehicles is mainly due to the relatively high cost of the Li-ion batteries used in them. Much research is, however, being done into ways of reducing this cost, and it is reasonable to expect that this will start to bear fruit in the not-too-distant future.

Until then, we have to accept that currently available Li-ion batteries, despite their many advantages over batteries based on older technologies, are still only just adequate in vehicle applications. They are costly and need to deliver even higher energy density and efficiency so that the vehicles can achieve adequate range between charging stations.

Li-ion batteries are, however, still the most promising option for use in electric vehicles and are currently the prime focus of most battery research. This will, however, need to cover not only efficiency and cost concerns but also safety.

The unavoidable downside to cramming so much energy into a small package is the potential for that energy to cause problems if it is incorrectly released. There have already been reported instances of this occurring with Li-ion mobile phone and laptop batteries leading, in the worst cases, to fires. Vehicle batteries will be much larger and will store much more energy, so safety is clearly an area that will need to be given very careful attention. Li-ion batteries are undeniably a breakthrough. They make possible our tiny yet versatile mobile phones, our compact laptops with long run times and test instruments that are ready to use straight off the shelf whenever we need them. And it’s a racing certainty that they’ll be playing an even more significant role in our lives in the very near future.

Open end

Load at the end

Parallel resistor

Shortcircuit

Resistance

Joint / Splice

Change to lower impedance

T - Joint / Splice

Interrupt

Examples of typical responses in reflection measurements


Insulation testing, in all its many guises, is something that is almost taken for granted today, at least in everyday applications. But it wasn’t always so. Back in the early decades of the twentieth century, insulation testing was new technology and a lot of work was being done to try to understand how insulating materials behaved and how test results should be interpreted.

It will be no surprise that one of the leading workers in this field was Sidney Evershed, the inventor of the Megger, which was the world’s first practical insulation tester. After years of painstaking research, Evershed presented some of his findings in an important paper that he read before the Institution of Electrical Engineers (the IEE, now the IET) in London on 27th November 1913.

His paper was no brief summary – with the appendices and discussion, it runs to 32 closely printed pages and contains 30 figures! Despite its age, the paper still makes interesting reading. On the very first page, for example, Evershed makes clear his disdain for flash testing with a footnote that reads “The flash test as applied to some costly piece of electrical apparatus must have been inspired originally by something akin to the heroism of the savage.”

The paper also makes clear the deficiencies of the insulating materials that were in common use in the early years of the last century.

Q&AThe industry’s recognised information tool

ELECTRICALTESTER

Q: What is meant by the order and sequence of harmonics?

A: Harmonics are voltages and currents at frequencies that are integer multiples of the supply frequency. The order of the harmonic is simply a way of stating which multiple applies to a particular harmonic. For example, with a 50 Hz supply system, the frequency of a second-order harmonic is 100 Hz, that of a third order harmonic is 150 Hz and so on. The sequence of harmonics in three-phase systems – and, in principle, in any other type of multiphase system – defines whether the particular harmonic tends to produce a rotating magnetic field that adds to or opposes the rotating field produced by the fundamental. Positive-sequence harmonics add to the field of the fundamental, while negative-sequence harmonics oppose it. Zero-sequence harmonics create a stationary field that doesn’t rotate, but they do increase overall current demand and

generate heat. Harmonics with orders divisible by three always generate a zero-sequence field and are often referred to as triplens.

Q: Why are harmonics so troublesome?

A: The biggest problem is that they cause heating. Triplens in particular produce currents in the neutral conductor that don’t cancel out, even if the load is nominally balanced across the three phases. This leads to heating of the neutral, which can be a significant issue as in many three-phase installations, the neutral conductor is not fully rated – that is, it has a lower current carrying capacity than the phase conductors. Also, in equipment like a transformer or a motor that has a magnetic core, even relatively small harmonic currents can produce a lot of heat, because the heating effect increases with frequency. Other problems that can be caused by harmonics include, but are by no means

limited to, buzzing in transformers and motors and flickering of lights.

Q: What can be done to reduce the impact of harmonics?

A: The best solutions involve tackling harmonics at the source, typically by fitting chokes or filters to the equipment that’s producing the harmonics. As always, however, to control something effectively, it’s necessary to be able to measure it and for this a power quality analyser is needed. A good instrument will detect and measure harmonics up to at least the 50th multiple of the power frequency, and will provide information about the amplitude of individual harmonics as well as the total harmonic distortion of the supply. This means that the sources of harmonics can be readily determined, and the effectiveness of harmonic mitigation measures quickly assessed.

Most readers of Electrical Tester, at least in the UK, will be familiar with the “IET Code of Practice for In-Service Inspection and Testing of Electrical Equipment”, (PAT testing) and many readers will also be aware that Megger offers a range of products that facilitate this type of testing.

What’s a little more surprising, however, is that one of these products – the PAT450 professional PAT tester – has been recognised as Top Product of the Year in the prestigious Electrical Products & Solutions awards. Let’s be clear, though, that the surprise doesn’t relate directly to the product, which is widely recognised as being a leader in its class – it’s more a matter of geography!

Electrical Product & Solutions is, you see, published in the USA and, in the USA, PAT testing is largely unknown. Is this award, therefore, an indication that things are about to change? Not really! There are many reasons why PAT testing, or an equivalent, is much less needed in the USA than it is in Europe and countries like Australia and New Zealand.

First of all, most portable appliances in the USA are powered from 120 V supplies, rather than the 220/240 V supplies used in many other countries. This means that the risk of receiving a serious electric shock from a faulty appliance is significantly reduced. In addition, two-pole plugs and sockets tend to be the norm, so earth bond tests are irrelevant. For these and many other reasons, the USA is unlikely to have a big requirement for PAT testing any time soon.

So why did the PAT450 win the award? Well, as the citation states, it’s “ideal for portable appliance testing in locations like service workshops, hotels, schools, hospitals, prisons, long-term care facilities, construction sites, offices, restaurants and sports facilities. It features 10 A and 25 A bond tests with a high duty cycle designed for fast testing, easy operation and all-day use.” And, although it’s not specifically mentioned, it can store up to 10,000 test results.

But that still doesn’t explain its appeal to American users. The mystery of the award is, however, finally solved in the citation’s last sentence, “Because the new PAT450 conforms to the latest European regulations and standards, it enables US companies shipping electrical products internationally to ensure that their exported products will pass

Harmonics in power systems are a major concern as they are becoming increasingly prevalent and they can give rise

to serious problems that include excessive heating and premature equipment failure. While there is good general

awareness of these issues, many questions still arise, not least in relation to terminology. Here are answers to a few of

the most common of these questions.

Q: Why is there a tendency for harmonic levels in supply systems to be greater now than in the past?

A: Harmonics are, for the most part, produced by non-linear loads. Linear loads, such as incandescent light bulbs and electrical heating elements draw current throughout the whole cycle of the supply waveform. Non-linear loads, such as the power supplies used in computers and televisions and the variable speed drives widely much used in industry, draw current for only part of the supply cycle. Because of this, they tend to distort the supply waveform, which is the same thing as saying that they generate harmonics. Since non-linear loads are becoming increasingly common, the level of harmonics in many supply systems is also increasing.

import testing.” In a nutshell, if you’re an exporter of electrical goods and you have CE marking in mind, the PAT450 is just what you need. That’s why it’s a winner across the pond as well as in Europe!

In the introductory part of the paper, Evershed provides some test results for “lighting circuits” – no further details are specified, but this could easily have been a domestic lighting installation.

These results (see Figure 1) show that on a dry day, the insulation resistance of the installation when tested at 500 V was almost exactly 1 MΩ. This in itself is worrying enough, but on a wet day the tester couldn’t deliver enough current to raise the output voltage above 200 V and the insulation resistance had fallen to a truly alarming value of just under 400 kΩ! Maybe in that era there was something to be said for gas lighting, after all.

In spite of the shockingly low resistance values obtained in this and other tests, the main thrust of Evershed’s argument throughout the paper is that the insulating materials themselves are almost perfect.

For example, when discussing a sample of micanite cloth – a material that was then widely used as an insulator for windings – which had been very thoroughly dried, he quotes an initial insulation resistance after drying of 1,300,000 MΩ (1.3 TΩ). After 24 hours of exposure to the air this had already fallen to 200,000 MΩ and after 90 days had dropped to 1,000 MΩ. That’s still not bad, but it does represent a decrease in resistance by a factor of 1,300!

As an aside, it’s interesting to note that more than a century ago, Evershed was already able to make measurements in the teraohm range, with test voltages of just 500 V. And he mentions the use of a guard wire. Teraohm measurements and guard terminals are clearly not just the province of modern instruments.

But if the insulators were almost perfect, why was the insulation resistance measured in practical situations often so low? Evershed’s conclusion was that moisture was the culprit. When carrying out an insulation test, he concluded, it was the resistance of the moisture on and in the test object that was being measured – the intrinsic resistance of the insulator itself made no significant contribution to the results obtained.

To support this conclusion, he carried out tests on a wide range of materials from drawing paper to engine oil and demonstrated that, in every case, if the materials were well dried, they exhibited very high insulation resistance and obeyed Ohm’s law, showing little variation in resistance with applied voltage. If even the slightest trace of moisture was introduced, however, the insulation resistance dropped dramatically and became non-ohmic, falling as the test voltage increased.

There’s much more of interest in this seminal paper and we may well revisit it in future issues of Electrical Tester. In the meantime, let’s finish with another perhaps unintentionally amusing comment from Evershed. Speaking of his attempts to devise a formula describing how the measured resistance of a damp insulator varies with the applied voltage, he notes that “nature generally declines the Procrustean bed of formula”. So much more interesting, I think you’ll agree, than simply admitting he couldn’t quite get it right!

An award that’s out of the ordinary!

The birth of insulation resistance testing

Figure 1: Diagram reproduced by kind permission of the archive department of the IET

David Danner, Distribution sales manager, USA

Nick Hilditch, editor, Electrical Tester

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