AVO New Zealand

Published by Megger | May 2011

Contents

Testing High-Voltage Bushings

Earth testing without the stake – Part 2

Megger in Royal Service

Testing power for air traffic control is critical

Testing High-Voltage Bushings

Introduction
Bushings provide an insulated path for energized conductors to enter grounded electrical power apparatus. Bushings are a critical part of the electrical system that transforms and switches ac voltages ranging from a few hundred volts to several thousand volts. Although a bushing may be thought of as somewhat of a simple device, its deterioration could have severe consequences. Most modern HV bushings have a power factor/capacitance test tap which permits dissipation factor testing of the bushing while it is in place on the apparatus without disconnecting any leads to the bushing.  The figure below shows a typical test connection between a DELTA4000 test set and a bushing in a UST test.

Bushing troubles
Operating records show that about 90 percent of all preventable bushing failures are caused by moisture entering the bushing through leaky gaskets or other openings. Close periodic inspection to find leaks and make repairs as needed, will prevent most outages due to bushing failures.

The table below lists common causes of bushing troubles and inspection methods used for detection. [Schurman, D.: Testing and maintenance of high voltage bushings, Western Area Power Administration, 1999].

Examples of Bushing Tests
Power/dissipation factor & capacitance test C1
The voltage or test tap allows for testing the main bushing insulation while it is in place in the apparatus without disconnecting any leads from the bushing. The main insulation is the condenser core between the center conductor and the tap layer. The test is conducted in the UST test mode which eliminates the losses going to grounded portions of the bushing. The UST method measures only the bushing and is not appreciably affected by conditions external to the bushing.

Test procedure
For all power factor testing, recording more information at the time of testing will ensure the best comparison of results at the next routine test. Test data should be compared to the nameplate data. If nameplate or factory readings are not available, compare the results of prior tests on the same bushing and results of similar tests on similar bushings.

Interpretation of capacitance and dissipation factor measurements on a bushing requires a knowledge of the bushing construction since each type bushing has its own peculiar characteristics. For example, an increase in dissipation factor in an oil-filled bushing may indicate that the oil is contaminated, whereas an increase in both dissipation factor and capacitance indicates that the contamination is likely to be water. For a condenser type bushing which has shorted layers, the capacitance value will increase, whereas the dissipation factor value may be the same in comparison with previous tests.

All measurements should be temperature corrected to a base temperature, usually 20°C. The insulation temperature for a bushing mounted to a transformer is normally assumed to be the average of ambient temperature and the top oil temperature of the transformer. Temperature correction can be done using manufacturer’s correction tables or preferably by using the intelligent temperature correction (ITC) method available in Delta4000 test sets, whereby the temperature correction for the individual bushing is accurately estimated.

Test results
General guidelines for evaluating the C1 power and dissipation factor test data are as follows:
• Between nameplate tan delta and up to twice nameplate tan delta - bushing acceptable
• Between twice nameplate tan delta and up to three times nameplate tan delta - monitor bushing closely
• Above three times nameplate tan delta - replace bushing
• General guidelines for evaluating the C1 capacitance data are as follows:
• Nameplate capacitance < ± 5% - bushing acceptable
• Nameplate capacitance ± 5% to ± 10% - monitor bushing closely
• Nameplate capacitance > ± 10% – replace bushing

Changes in C1 test data are usually contamination issues caused by moisture ingress, oil contamination or breakdown and short-circuited condenser layers.

Power and dissipation factor & capacitance test C2
The C2 test measures only the insulation between the tap and ground and is not appreciably affected by connections to the bushing center conductor. The tap is energized to a pre-determined test voltage and measured to ground in the grounded specimen test (GST) mode.

Always refer to nameplate data or manufacturer’s literature on the bushing for tap test voltages. Please note that the power factor tap is normally designed to withstand only about 500 V while a capacitance tap may have a normal rating of 2.5 to 5 kV. Before applying a test voltage to the tap, the maximum safe test voltage must be known and observed. Typical test voltages for potential taps are between 0.5kV and 2kV.

Hot collar test
For bushings not equipped with either a test tap or a voltage tap, the only field measurement which can be performed is the hot collar test. The dielectric losses through the various sections of any bushing or pothead can be investigated by means of the test which generates localized high-voltage stresses. This is accomplished by using a conductive hot collar band designed to fit closely to the porcelain surface, usually directly under the top petticoat, and applying a high voltage to the band. The center conductor of the bushing is grounded. The test provides a measurement of the losses in the section directly beneath the collar and is especially effective in detecting conditions such as voids in compound filled bushings or moisture penetration since the insulation can be subjected to a higher voltage gradient than can be obtained with the normal bushing tests.

The hot collar measurements are made by normal GST-GND test method and the bushing need not be disconnected from other components or circuits. Make sure that the collar band is drawn tightly around the porcelain bushing to ensure a good contact and eliminate possible partial discharge problems at the interface.

Test results
General guidelines for evaluating the hot collar data are as follows:
• Watts-loss values less than 100 mW - bushing acceptable
• Watts-loss values of 100 mW or more – bushing unacceptable (contamination)
• Current values within 10% of similar bushings – bushing acceptable
• Current values less than 10% of similar bushings - bushing unacceptable (low level of liquid or compound)

If watt-loss values are in the unacceptable range, cleaning may be necessary on the exposed insulation surface of the bushing. Effects of surface leakage can be minimized by cleaning and drying the porcelain surface.

Earth testing without the stake – Part 2 Paul Swinerd, Product Manager
The previous part of this article provided an introduction to the stakeless measurement of earth resistance and looked at some applications where this method is particularly suitable. This second and concluding part looks at further applications, potential sources of error, and the benefits of stakeless testing. If you would like to see the first part, please email us at This e-mail address is being protected from spambots. You need JavaScript enabled to view it
The stakeless measuring technique is well suited to testing earth electrodes installed within primary cross-connection points, which are sometime called street cabinets or flexibility points (Figure12). These electrodes typically need to have a resistance below 25 Ω. In this application there may only be two parallel earth paths in series with the electrode. However, provided that the stakeless method gives a result below 25 Ω, then the resistance of the electrode itself must certainly be below 25 Ω. Figure 12 – Street cabinet   A very similar application is cable TV street cabinets Figure 13 shows the stakeless technique being used at a remote switching site. In this application, the object of the test is not to measure the earth resistance but to verify earth connections. By recording the test results and trending these over time, it is possible to identify the onset of problems such as corrosion. Figure 13 – remote switching site Cellular sites/microwave and radio towers are another good application for stakeless earth resistance testing. Figure 14 shows a typical four-leg tower. Each leg has been individually earthed and connected to a buried copper ring. As with the remote switching site, this test is used to verify an electrical connection, and is not a true measurement of earth resistance. Figure 14 - Cellular sites/microwave and radio tower Telephone pedestal electrodes can also be tested using the stakeless method. Cable sheaths are all connected to an earth bar, which in turn is connected to earth electrode. The instrument clamp can be placed around the cable connecting the ground bar to the electrode to perform a test. If access is difficult a temporary extension cable can be fitted to accommodate the clamp. Figure 15 - Telephone pedestal Switchyard and substation earths are yet another good application for stakeless testing. The method is ideal for checking connections to earth mats, but caution needs to be exercised over possible interference from induced ground currents.	Substation and switchyard metal fencing connections to earth mats can be easily checked for continuity using the stakeless method. Stakeless testing is very useful to transformer test engineers. Pad-mounted transformer earths can be readily verified. However sometimes there are a number of connections to the same electrode. In such cases, it may be necessary to clamp around the electrode itself, below the connections. Figure 16 – Pad mounted transformers Potential sources of error When applied correctly and a good quality instrument is used, the stakeless method gives very reliable measurements. Nevertheless, it is well to be aware of factors that may introduce errors. Among these are: Poor understanding of the circuit under test Remember the two rules of stakeless testing: • There must be a loop resistance to measure. • The earth path must be included in the circuit unless the test is being carried out solely to verify a connection. Dirt trapped in the clamp head • Dirt trapped between the closing faces of the clamp will modify the magnetic circuit. The result will be false low readings and, in some cases, this could result in a poor electrode being measured as good. • Many instruments use interlocking laminations (teeth). These trap dirt and are difficult to clean. They are also easily damaged. Damaged teeth will produce inaccurate measurements and may even render the instrument unusable. Noise currents affecting measurement • In noisy environments, high noise currents may be flowing in the electrode under test. This can cause readings to fluctuate making them difficult to interpret or, if the noise current is too high, it can make measurement impossible. To avoid these problems, clamp-type earth resistance testers with good noise immunity should be used. The benefits of stake-less earth resistance testing • Tests can be carried out without disconnecting the earth electrode from the system. This method is, therefore, safer and less time consuming. • Loop testing includes bonding and grounding connections. Because of this, it identifies poor continuity anywhere in circuit • There is no need to drive auxiliary test spikes into the ground, so testing can be carried out easily in locations with hard ground or concrete surfaces. There are also time savings as there is no need to run out test leads. • Clamp-type earth electrode resistance testers can also be used to measure earth current. If an electrode is to be disconnected, the instrument can be used prior to disconnection to measure the current flowing in it, thereby confirming whether or not it is safe to proceed. It is worth noting however, that the results from stakeless measurements will rarely be the same as those obtained with a three-pole instrument, as the stakeless test is technically a loop resistance measurement. In applications with only one or a small number of return earth paths the measurement may be higher than the expected electrode resistance limit. In this case the stakeless method is still a useful tool to identify changes over time. The benefits of Megger instruments The latest Megger digital clamp-type earth resistance testers, models DET14C and DET24C, offer a number of additional benefits. These include: • Elliptical clamp with a slim profile, which facilitates access to earth straps and electrodes in pits. • Large clamp capacity, allowing tapes up to 50 mm wide as well as electrodes and cables up to 39 mm in diameter to be easily accommodated. • Low maintenance flat jaw faces, with no interlocking teeth that easily become bent and damaged. • CATIV 600 V safety rating in line with IEC 61010. This is the highest safety rating currently available for an instrument of this type. • Auto-current measurement safety feature that provides an instant warning if current exceeds a user-set limit. • Automatic noise filter function that reduces the effect of noise current in electrically noisy environments such as substations. The most common reason given by users for not being able to use the stakeless method is poor access. Often cable or tape sizes are too large for the clamp. Until now 50 mm wide earth tapes could not easily be tested. To circumvent this problem, some users resorted to cutting the tape and welding in a round cable to make their earth clamp testers usable. This time-consuming procedure is not required with the Megger DET14C and DET24C clamps as their elliptical heads can accommodate 50 mm tapes with ease.

Megger in Royal Service

When the former Royal Yacht Britannia rolled down the slipway at John Brown’s Clyde bank shipyard on 16th April 1953, this prestigious vessel incorporated the very best engineering technology then available. And that included rudder indicators supplied by Evershed & Vignoles, the company that originated the Megger brand name.

Rudder indicators were not new product for the company, as it had first introduced them in 1893, and they had quickly established themselves as the standard pattern for the Royal Navy.

Surprisingly perhaps, those early indicators were essentially a development of the movements used in Megger ohmmeters of the period.

These comprise a soft iron needle freely pivoted at the centre of two coils, the axes of which are at right angles. The position taken up by the needle depends on the ratio of the currents in the coils. In a rudder indicator, this instrument is used with a transmitter, which is mechanically coupled to the rudder head which alters the resistance in the coil circuits as the rudder moves.

This arrangement has the benefit that the indication is unaffected by fluctuations in the supply voltage, and that it is impossible for the indicator to get out of step with the
transmitter.

For the Royal Yacht Britannia, however, the latest Syntorque Rudder Indicator was used, which shared these same benefits but also embodied a more robust movement with enhanced damping.

We are grateful to the IET Archive for providing access to the material used in preparing this item, and for permission to reproduce the illustrations.

The Syntorque transmitter comprised a toroidal resistance tapped at three equal intervals and connected by three wires to the three-phase stator winding of the polarised rudder indicator. A low-voltage DC supply was connected to two contacts spaced 180º apart that rotated on the toroid in line with the movements of the rudder. The resulting currents flowing in the indicator windings set up a field corresponding with the position of the contacts, and this acted on the polarised rotor, causing it to move in sympathy with the contact rotation. The pointer responded almost instantly.

The instrument installed on the Royal Yacht was provided with large clear scales extending from 35º port to 35º starboard. The scale was illuminated to make the instrument equally easy to use by day or night.

The Royal Yacht remained in service for 44 years, a tribute to the quality of the engineering used in its construction. There’s no confirmation of course, that the Syntorque rudder indicator remained in service throughout the whole life of the vessel but given the elegant design and robust construction of Megger products then and now, it’s certainly a possibility!

Testing power
for air traffic control
is critical
Tony Wills, Applications Engineer

The safety of aircraft around the globe depends on the work of air traffic controllers – work that they can only carry out with the aid of input from complex radar installations. If those installations stop working, the air traffic controllers are instantly deprived of the information that they need to guide flights safely, and the consequences can be disastrous.

It’s no surprise, therefore, that the radar systems used in air traffic control applications feature a high level of redundancy and that they are invariably fed from sophisticated uninterruptible power supply (UPS) systems. The performance of even the most sophisticated UPS, however, is no better than that of the batteries upon which it depends.

Modern storage batteries are, of course, very reliable but there is still only one way to make sure that when they are called upon to deliver power, they are ready and able to do so. And that is by testing them regularly. Naturally, in an application as critical as air traffic control, that testing has to be carried out to the highest standards.

It is well known that the most dependable and revealing method of checking the performance of a battery installation is to carry out a full discharge test. In air traffic control applications however, this method has several drawbacks.

The first of these is that, as the name suggests, the battery has to be taken out of service and discharged, during the test. Therefore, unless elaborate and usually costly measures are in place to provide an alternative back-up supply if the mains supply should fail at the point during the test when the battery is fully discharged the UPS will be unable to operate, and the radar system will go off line.

It is worth noting that some discharge test sets, notably those in the Megger Torkel range, provide an option where the batteries can remain in service and are only discharged down to 20% of their capacity during the test, rather than being fully discharged. It has been found that this has very little effect on the accuracy of the results provided by the test.

Nevertheless, a battery that is called to go on line with only 20% of its charge remaining will be exhausted very quickly, which is usually unacceptable in air traffic control applications.

Discharge testing also has another critical drawback – it is very time consuming. Testing a large battery installation may take many hours or even days. This not only makes the testing inconvenient but also costly to perform.

There is, however, an alternative to discharge testing in the form of battery impedance testing. For this type of test, an AC voltage is applied to the battery and the resulting current flow measured so that the battery impedance can be calculated.

While impedance measurements do not provide a direct measure of battery capacity in the same way that discharge testing does, extensive research confirmed by practical experience has shown that there is a very good correlation between battery impedance and capacity.

The battery capacity is to a very good approximation, inversely proportional to its impedance. It must however be stressed that the correlation between impedance and capacity is not 100% - the only way to determine battery capacity with 100% certainty is, as has already been suggested, to carry out a full discharge test. Nevertheless, impedance testing is a very useful way of performing routine periodic tests on batteries, and of locating weak cells.

The advantages of impedance testing are clear. Testing can be carried out without taking the battery out of service and even with large battery banks, the time required for testing is unlikely to be much longer than 30 minutes when a modern test set such as the Megger BITE3 is used. In addition this type of testing does not affect the charge held by the battery.

In the case of batteries used in critical applications like air traffic control, the recommended approach to testing and condition monitoring is to perform a discharge test once per year as, despite the inconvenience involved, this provides an excellent baseline. Between discharge tests, the battery performance can be checked regularly by impedance testing. This combination of test techniques provides the best possible assurance that the battery remains healthy at all times.

It is worth noting however, that loss of capacity is not the only problem that can affect a battery installation. In practice, faults are just as likely to relate to poor connections and, in particular, inter-cell straps that have been loosened by the heating and cooling cycles that occur during charging and discharging. This type of problem can however, be located relatively easily using a low-resistance ohmmeter.

Often less easy to locate are earth faults on floating battery strings. Typically resulting from either water ingress or the spillage of electrolyte, these faults can be very hard to localise using ordinary resistance measurements.

For this reason, dedicated instruments have been developed that work by injecting an AC signal into the battery installation, which can then be tracked using a clamp-type probe on the battery feeder cables. Since the amplitude of the signal seen by the probe is inversely proportional to the fault impedance on the feeder, this arrangement allows earth faults to be located quickly and easily.

Batteries play a crucial role in ensuring that air traffic control systems around the world can continue to operate reliably and safely even if there is a failure of the mains supply at the radar stations on which they rely. It is imperative, therefore, that those batteries should be maintained in good condition, with potential problems detected and remedied before they can develop into outright failures.

The key to effective routine and preventative maintenance is to test the batteries regularly and, as has been discussed in this article, modern instruments and test techniques are making this testing easier and more convenient than ever to perform. It’s no exaggeration to say that the latest battery testers are helping to ensure that we always have the power to keep our skies safe!