AVO New Zealand

Does the CAT fit?

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Market Development Manager

A convenient method for accurately determining the insulation condition in twisted copper cable pairs is an essential requirement in the CATV and telecommunications sectors, as defective insulation invariably leads to performance impairments in services such as video, voice, data and VOIP.

Among the most common causes of problems are insulation that has become weak or brittle, causing cracks or otherwise exposing copper within the cable, and the ingress of water into the cable. Insulation checks will identify these problems quickly and positively.

A little care is, however, necessary when choosing an insulation test set, since types that have been specifically optimised for use in CATV and telecommunications applications will provide valuable time savings as well as offering enhanced operating convenience. A particularly useful feature is a dual display that shows two test parameters simultaneously. The primary display shows the main parameter being measured – usually the insulation resistance – while the secondary display shows a related parameter such as leakage current. Users save valuable time by measuring these two parameters with a single test.

In the best instruments, the dual display system is rendered even more useful by the provision of both analogue and digital readouts for the primary parameter. In many applications, the response of the analogue readout provides the instrument user with a very rapid indication of the insulation condition.

Infinity and beyond
Another important feature to consider when choosing an insulation test set is the maximum value of insulation resistance for which it can provide a meaningful reading. Many types simply show values above, say, 1 GΩ as infinity. If a developing fault on a cable has reduced the insulation resistance from, for example, 15 GΩ to 5 GΩ, a tester of this type will provide no indication of the problem, as both readings will be shown as infinity.

The latest testers can, however, provide meaningful readings up to at least 50 GΩ. This means that incipient faults can be detected much sooner, allowing more time for action to be taken to avert a complete failure. Insulation test sets for CATV and telecommunications applications should have provision for adjusting the test voltage over a wide range so that the voltage can always be accurately matched to the application. Facilities for testing continuity are also important, as this is a frequent requirement when testing installations of all types. To ensure operator safety when testing at higher voltage levels, a CAT IV 600 V rating in line with IEC 61010 is desirable.

Finally, to allow convenient use in the field, the instrument should be rugged yet lightweight and compact, and it should have facilities for attaching it to a tool belt. Accessories should be expected to include a carrying bag with shoulder strap and a bed-of-nails test lead set.

Selecting the right CATV instrument
Several insulation test sets satisfy all of the requirements that have been discussed here. These are the MIT410TC, which offers insulation test voltages from 250 V to 500 V and measurements up to 50 GΩ, and the MIT430TC, which offers test voltages from 50 V to 1,000 V and measurements up to 200 GΩ. In addition, the MIT430TC includes memory, on screen recall of stored results and Bluetooth communications for downloading. Both products are supplied with bed of nails test leads for telecom applications and a convenient soft carry case with shoulder strap.

Transformer ratio testing is an invaluable and widely used diagnostic technique that can detect problems of many types. Applying this technique to phase-shifting transformers, however, requires extra care to determine the true ratio.

Introduction
The turns ratio of a transformer should remain constant throughout the life of the unit. In reality, however, the insulation surrounding the windings of a transformer can deteriorate or become damaged for many reasons including high voltage transients, contamination, exposure to fault conditions and shipping damage. If the damage is sufficiently severe, it is possible that it will lead to shorted turns in the windings, which will effectively change the turns ratio of the transformer. This change can most easily be detected by turns ratio testing.

In principle, turns ratio testing is straight-forward, relying as it does on the direct relationship that exists between the turns ratio and the voltage ratio of windings within a transformer. Indeed, in ordinary single-phase transformers and in some types of three-phase transformer, the turns ratio and voltage ratio will be the same although, as we’ll see shortly, things are not always this simple.

From the foregoing, it will be clear that it is change in turns ratio that is important and this is invariably determined by comparing the measured ratio with the ratio stated on the transformer nameplate. The extent of deviation from the nameplate ratio is a direct indicator of winding deterioration. A transformer will tolerate a limited amount of deterioration, but deterioration is nevertheless a certain precursor of problems to come. For this reason, ANSI Standard C57.12, for example, specifies that the measured turns ratio must be within 0.5% of the nameplate figure.

Transformer turns ratio testers (TTRs) have been widely available for some years, they can successfully be used to test transformers with the many vector groups that are defined in national and international standards. The number of such vector groups should not be underestimated – in the principal ANSI, IEC and Australian standards alone have more than 130 vector groups are defined. Currently available TTRs are also capable of handling these and many other variations, including autotransformers.

Vector notation and transformer configurations
Figure 1 is a typical phasor diagram showing angular displacements. It follows the European convention where the leading phase is at 12 o’clock. In the standard HV practice in the UK, the red phase is designated as the HV leading phase. It is vital to have correct phasing and rotation throughout any open or closed ring-configured HV or LV three-phase system, this ensures the system will always be ‘phase true’.

Fig. 1 Typical Phasor daigram

Figures 2, 3 and 4 show standard transformer configurations and there associated designations. Note that D represents a delta-connected winding, Y a star-connected winding and Z a zigzag winding. Upper case letters are used for primary windings, and lower case for secondary windings. The number represents phase shift in multiples of 30º – for example 0 corresponds with 0º phase shift and 2 corresponds with 60º phase shift.

Fig. 2 Delta-Star configuration Dy1

Fig.3 Star-Star configuration Yy0

Fig. 4 Delta - Zig Zag configuration Dz1

Ratio Testing Methods

Voltage measurement method
Megger TTRs determine transformer turns ratio using the IEEE C57.12.90 method. The test set applies a test voltage to the transformer primary windings and measures the voltage at the corresponding secondary winding. The voltage ratio is displayed with accuracy better than 0.1%, after comparison with the expected ratio.

Measured ratio
The measured voltage ratio is conventionally taken as the measured turns ratio. The target ratio – that is, the expected voltage ratio – is calculated from the transformer nameplate ratio by applying a factor that relates to the configuration of the transformer. The value of the factor used in the calculation depends on the vector configuration of the transformer and, in some cases, the actual test set up.

Voltage ratio and nameplate ratio
For single-phase transformers and standard three-phase Yy and Dd transformers, the turns ratio/voltage ratio is the same as the nameplate ratio. However, for Yd or zigzag transformer configurations without an accessible neutral, the voltage ratio will be different from the nameplate ratio. Examples of the calculation factors that need to be applied for various configurations are given in Table 1. To obtain the turns ratio/voltage ratio, the nameplate ratio should be multiplied by the factor shown.

Table 1 – Factors relating nameplate ratio to voltage ratio

Turns ratio and voltage ratio
For most transformer configurations, the turns ratio is the same as the voltage ratio. However, for some configurations without accessible neutral connections, the turns ratio is different from the voltage ratio. In these cases, where the turns ratio is needed, it must be calculated by applying a factor to the measured voltage ratio. Some typical factors are given in Table 2. To obtain the turns ratio, the measured voltage ratio must be multiplied by the factor shown.

Table 2 – Factors relating turns ratio to voltage ratio

Phase-shifting transformers
A phase-shifting transformer (PST), which is sometimes also referred to as a tilted transformer, differs from a conventional transformer in that it generates an “abnormal” phase angle (phase shift) between its primary and secondary terminals. This is achieved by introducing a boost voltage with a phase angle perpen-dicular to the line voltage between two phases, thereby creating a phase shift. This influences real power flow, thereby improving transmission efficiency and reducing the transfer of harmonics via the transformer. Installing a PST can therefore reduce the load on trans-mission lines, and also provide opportunities for controlling power flow.

PSTs are not a new concept; they have been in limited use in special rectification applications for decades. The increasing use of harmonic generating rectification systems, particularly by the rail network, and the growing popularity of switch-mode power supplies are, however, creating a need for ways to reduce harmonics in the supply network. This has led to a big increase in the numbers of PSTs being used in supply networks worldwide.

Testing PSTs is not as straightforward as testing ordinary transformers because the windings are very asymmetrical compared to the standard Dd, Yy and Dz transformer vector groups. PSTs use partial windings of two phases in differing proportions to achieve the required amount of phase shift from the primary to secondary.

Nominal phase shift angles are typically 2.5º, 5.0º, 7.5º, 10º, 12.5º, or 15.0º, with either a positive or negative shift (tilt) with respect to the zero degree point.

The extra work involved in manufacturing a PST relates to the extra winding end terminations that have to be interconnected while ensuring that the insulation is maintained so that it can withstand the required over-potential test voltages.

To keep both the phase shift angles and voltages correct, each tap change provided on the transformer needs two taps, one for voltage variation required, the other to compensate for the phase angle shift. This is because tap changing not only results in a turns ratio change and, therefore, a voltage change, but also a phase angle change.

Summary
Phase shifting transformers provide a very useful way of improving power system operation. They are wound in many different ways to achieve the required phase shift or tilt, and achieving high accuracy of the chosen tap on the windings and overall voltage transformation ratio is often not feasible. ANSI Standard C57.12 specifies that turns ratio should be no more than 0.5% in error, and that this figure should not be exceeded when testing the ratio of any winding on the transformer.

The calculations involved during turns ratio testing on PSTs are complex and require that the phase shift angles are known for each individual tap position – these angles can, for example, vary from 6.5º to more than 9.0º on a nominal 7.5º PST. Provided this information is available, however, an exact ratio for every winding can be determined and verified against the manufacturer’s nameplate ratio to a high degree of accuracy

Insulation or high pot test. What’s the difference?

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Applications Engineer

Two common types of electrical testing are often easily confused - insulation testing and “high-potting”. Both are similar in their method, but critically differ in degree and aim. This two-part article, the second part of which will appear in the November 2011 email edition of Electrical Tester, demonstrates how an understanding of the basis for the two tests is paramount to their respective applications.

Insulation testers are also known by the generic name megohmmeters. High-pots derive their name from “high potential”, a reference to their test voltages, and are also dielectric breakdown testers. Both types have two terminals by which a high voltage is applied across the test sample. Some current will flow through the insulation of the test item, and is measured by the tester’s circuitry. Usually, the current flow is small - nano-Amp - but increases as insulating material ages and becomes worn or is damaged by catastrophic events like voltage spikes or a sudden influx of contaminants, as in flooding.

As current flow is the inverse of insulation quality, the tester uses this information to make its determination. In insulation testing, the test voltage is a means by which a measurement is taken and displayed. In high-potting, voltage is more of an end, just to see if the insulation can take it. The two methods overlap quite a bit in their use, technique, and design of instrumentation, so it is a good idea to be familiar with both.

The prime goal of insulation testing is to provide a measurement. Megohmmeters are always DC testers. The high-voltage output is direct current. The tester measures the para-meters of voltage and current, and via Ohm’s Law, calculates the resistance of the insulation on the test item. Because the purpose of insulation is to seriously hinder current flow, there isn’t much of it before the test item has broken down. Consequently, the megohmmeter must be capable of measuring very small currents, but conversely, can be severely limited in its maximum output.

Accordingly, insulation testers are typically limited to only a few milli-amps of test current. This limitation serves four purposes. First, it makes the testers comparatively safe. Even though they commonly output up to 15 kV and sometimes higher, insulation testers are not dangerous to the extent that their voltages may imply.

That does not mean that safety can be treated with a cavalier attitude, however. The item being tested can certainly become lethal. Capacitive items can store large static charges that remain after conclusion of the test and pose a serious hazard. While insulation tests are always performed offline, the test item can become energised either through human error or an “event”, and accidents can occur. Fortunately, quality megohmmeters have redundant safety features that protect the operator as much as is reasonably possible.

Secondly, current limitation facilitates the implementation of convenient on-board power sources, most notably batteries. Common AA batteries can output a thousand volts in a handheld instrument through current limitation, and sealed lead-acids can provide higher voltages without having to go to line power.

Thirdly, current limitation protects the test item in the event of breakdown. Many failed test items can be restored to service by simple maintenance like cleaning and drying. But this would not be so simple if breakdown current had pinholed or in some other way physically damaged the insulation. With limited current, when resistance becomes too low, something has to yield, and that is the test voltage.

Accordingly, when no more current flow can be accommodated, the voltage collapses. A good quality instrument should exhibit sharp rise of test voltage up to a resistance value commensurate with “good” insulation; i.e., around 1 megohm for 1 kV tests, 5 megohms for 5 kV, and so on. Below these values, insulation isn’t truly insulation any more, but above these values, full selected test voltage should be maintained so that the item is being tested to proper specifications. Some testers of poor quality exhibit a slow rise that doesn’t reach selected voltage until well into the megohm range. Such models are not providing a true 1 kV test or whatever the specified voltage is, and should be avoided.

Finally, current limitation serves one of the most valuable purposes of insulation testing: predictive/preventive maintenance. It would be counter-productive to have the recommended testing procedure stressing insulation to the point of reducing its life. Therefore the limited power behind an insulation test allows it to be performed again and again over the life of a given piece of equipment while extending the life, not abbreviating it.

So the immediate aim of an insulation test is to provide a measurement. It is then incumbent upon the operator to make proper use of this information. Consequently, insulation testers should be operated by adequately trained personnel. Among other things, the operator must be familiar with the circuit that is being tested, know what lead hookup is testing what element of the item under test, and know how to interpret results. This is no easy task, considering that “good” insulation might be anything from a few megohms to several tera-ohms. The operator also must know how to interpret pointer travel and dancing digits, and be familiar with various industry-standard test procedures that serve the purposes of testing the same insulation in different ways, saving test time, or providing built-in “good/bad” interpretation. In short, an insulation tester demands considerable operator involvement.

This article will be continued in the November 2011 email edition of Electrical Tester. The second part will detail high-pot testing.

All at sea with SS Shieldhall

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UK Distribution Sales Manager

A new multifunction installation tester from Megger is now playing a key role in the maintenance of the electrical systems aboard SS Shieldhall, the UK’s largest working steamship. Although primarily designed for use on ac electrical installations in domestic and commercial properties, this versatile test set is also proving its worth on the vessel’s extensive dc systems.

Originally built in 1955 to carry sludge down the River Clyde and dump it out to sea, the SS Shieldhall has been extensively refurbished and is now in regular use as a passenger-carrying vessel. It is operated by The Solent Steam Packet Limited, a charitable organisation.Reliability is essential for the vessel to fulfil its demanding present-day role, which means that the electrical systems – many of which are now over half a century old – must be tested regularly and kept in tip-top condition. The responsibility for this falls to Maurice Dibsdall, Senior Electrical Officer of the SS Shieldhall, who has spent more than five decades working on ships’ electrical systems, including those of the P&O flagship vessel, The Canberra.

“Throughout my career, I’ve used Megger test kit,” said Maurice Dibsdall, “and I have absolutely no doubt that it’s the best there is. So when it came to replacing Shieldhall’s test equipment, I knew exactly what I wanted!” Having received its new tester, the SS Shieldhall team wasted no time in putting it to good use for tasks that including checking the insulation resistance of the ship’s main dc switchboard and faultfinding on the dc faceplate starters used by the vessel’s oil pumps.

“The tester is versatile and easy to use,” said Maurice. “It’s compact, convenient and lightweight, yet it’s very robust, which is important as it will have a tough life aboard Shieldhall. We’ve already found it to be exceptionally useful, and we’re continuing to find new applications for it almost daily.”

SS Shieldhall is based at Southampton Docks, where it is berthed courtesy of Associated British Ports. The 1,792 tonne vessel is 82 metres long and has two triple-expansion main engines, fed from two single-ended three-furnace oil fired Scotch boilers. When in passenger carrying service, SS Shieldhall has a crew of 32, and can accommodate a maximum of 150 passengers.

To find out more about the SS Shieldhall, visit www.ss-shieldhall.co.uk