AVO New Zealand

Toil and Trouble? Making oil tester selection simple! Paul Swinerd, Product Manager

When selecting an oil test set, bear in mind how crucial the electrode gap is. Many users find themselves struggling to set accurate electrode gaps and then having to regularly check them as they have a tendency to change. Look for a tester with an easy to use thumb wheel adjustment that includes a gap locking mechanism. The electrode gap has to be very accurate, so there’s no substitute for good quality, accurate gauges that are specially coated to protect the electrodes from wear.

Specially tested vessels that are not made of expensive glass and are impervious to chemical attack are now available. These vessels are not so prone to breakages, and the lower cost makes buying additional vessels for use in the lab much more bearable.

Select an oil test set that is easy to clean, with no sharp corners for dirt to collect and a test chamber drain for when you accidentally spill oil. A vessel with a baffle and an oil fill level will exclude air, and enable you to comply with ASTM standards, and all without the usual oil spills	Most oil test sets can automatically run a test sequence, but not all of them are so easy to use. Look for a tester with a bright, easy to read display that is easy to use. If you are testing to IEC60156, you should specify an instrument with a built in temperature sensor so that the temperature is automatically added to your test report.	Whether you save electronically to USB for analysis on a PC or print directly from the instrument can be important. You should specify this at the time of ordering. Do not feel compelled to pay for functions you do not need; if you do not want a built-in printer, stipulate this.

MOISTURE CONTENT DETECTION IN OIL IMMERSED CURRENT TRANSFORMERS – PART II

As discussed in the last edition of Electrical Tester, current transformers belong to the group of electrical apparatus dedicated for the protection of electrical systems and measurement of control parameters, although they are not protected themselves.

Every time the Technical Support Group carries out a Substation Best Practices Seminar, one of the questions we ask the audience is: “How often do you test your CTs?” and the answer is usually close to never.

Let’s summarize several examples of testing CTs and determining the moisture concentration of the units.

Factory samples
In North and South America brand new CTs were tested, some in the factory and others at the end-user’s facility. Let’s look at the shape of the curves obtained and the subsequent analysis. Figure 1 shows the data acquired from a brand new CT tested in the factor, and it can clearly be seen that the S shape curve is similar to those obtained from power transformers.

Figure 1. Sample Unit 1 - Factory Test of Oil Immersed CT	Figure 2. Moisture Analysis on Sample Unit 1

The information gathered during the test was sent to MODS for analysis. The results are shown in Figure 2.

Something to note and take into account when analyzing moisture concentration on CTs is that there are no spacers physically in the unit and, therefore, the Y% value is set to zero. Because this is brand new oil, the conductivity was found to be very low but the moisture concentration (2.4%) is higher than would normally be seen on power transformers. In a similar procedure, Sample Unit 2 was tested after passing all acceptance factory tests and the results are shown below in Figure 3.

Figure 3. Sample Unit 2 - Factory Test of Oil Immersed CT	Figure 4. Moisture Analysis on Sample Unit 2

As can be seen, the shape of the curve is mainly influenced by the cellulose part of the insulation and the numbers obtained after analysis using MODS confirm similar geometry, similar quality of the liquid insulation but a lower moisture concentration compared with the previous specimen. The results are as follows:

Figure 5. Sample Unit 3 - Field Test	Figure 6. Moisture Analysis on Sample Unit 3

It is not, of course, possible to include all the different tests used to validate the application on CTs, but it is the intention of this article to look at the most informative results, so as to provide the reader with enough guidelines to recognize and understand the results of the DFR technique as applied to CT testing.

Sample units 1 and 2 are from the same manufacturer and have a similar voltage rating. They were tested under factory conditions.

For the next example, Sample Unit 3 was acquired by the end-user and stored as a spare. The end-user requested that the unit should be tested to verify the quality of the insulation of the CT prior to bringing it on-line.

The shape of the curve is similar to those observed previously, but there are not enough data points for a more accurate analysis. The results from the moisture analysis software are:

Now let’s look in the opposite direction. An operational CT (Sample Unit 4) with over 5 years of service was contaminated by opening it in the field and changing the seal on one of the high current terminals. This operation allowed ingress of moisture, leading to the results shown below.

Figure 7. Aged CT Sample Unit 4 - Field test	Figure 8. Aged CT Sample Unit 4 - Moisture Analysis

Looking in detail at the shape of the curve, it is clear that the effect of the contamination is that the power factor value at lower frequencies tends to unity, specifically in the region where solid insulation has the largest influence.

There are two important things to highlight from the data presented in Figure 8. The first is a similar geometrical characteristic with a lower X% (barriers) due to possible insulation degradation. Second is a high moisture concentration together with high oil conductivity. This means that the unit was not only contaminated with external moisture but also carbonic byproducts were dissolved in the oil and therefore increased its conductivity. This unit was removed from service and sent to dry-out and internal inspection.

Figure 9. CT Factory Test using IDAX 300

In summary, experience of using DFR (Dielectric Frequency Response) testing on oil-immersed current transformers has provided the following guidelines:

Classification
A classification guideline based on analysis of measurements on numerous current transformers in various conditions has been established. Significantly aged units had moisture contents of more than 4% (sometimes considerably higher). The following table could be used as a reference/benchmark when classifying current transformers (head-type) by moisture content.

Taming Cable Fault
Allen Joyce, Product Manager

In almost every country of the world, ageing power transmission and distribution infrastructure means that faults in underground cables are becoming increasingly common. Such faults typically result in disruption and costly service interruptions, so speedy rectification is essential. Unfortunately, faults in power cables are notoriously hard to locate.

A completely new cable fault location system is now showing great promise in its initial field trials. One of the key features of the new unit is a novel power supply that uses phase-angle modulation technology. This eliminates the need for variable transformers and ballast chokes, thereby saving space, weight and money.

System control is accomplished with a purpose built programmable controller, and dual-colour output status lights, using LEDs that are easily visible in daylight, provide safe and unambiguous operation. The real key to the instrument’s versatility and success however, is a high performance time domain reflectometer (TDR) that uses highly intuitive single knob control (jog-dial) and provides clear text-based information. The TDR has an auto-cursor function to find the end-of-cable (EOC), and subsequently to find the cable fault location when using arc reflection or arc reflection plus.

While prototypes of the new design had delivered good results in preliminary testing, there is no substitute for verifying the performance of a cable fault locator under field conditions. Accordingly, arrangements were made with one of the largest electric utilities in the US to work with technicians in one of its underground cable repair groups. It was agreed that the technicians would evaluate the new test set by using it for a day as an aid to locating some particularly challenging cable faults.

The first fault was on the site of a small factory warehouse. After the new instrument was set up, it was initially used to carry out DC tests to confirm that there was indeed a fault present on the suspect cable. The instrument was then used to determine a distance to the fault, working with the arc reflection method. This yielded a fault distance of 67 metres. Two technicians who were familiar with the cable layout on the site then moved to approximately this distance from the tester and, when the cable was subsequently “thumped” using the voltage impulse capability of the instrument. They found that they could hear the thump almost immediately. The time from the initial test to locating the fault was less than five minutes. Since this was an instrument trial, the accuracy of the distance measurement was confirmed by repeating the test on the other two phases of the cable. In both cases, the result obtained was almost identical with the first measurement.

The second fault was rather more challenging in that many attempts with a different tester had already been made to find it, all of them without success. After DC verification, the new the arc reflection method was used. Perhaps unsurprisingly, given the elusive nature of the fault, the initial traces were indeterminate. One of the features of the new cable fault locator, is the arc reflection plus method, whereby it captures anywhere from 32 to 1,024 traces automatically, depending on range. When these additional traces were examined, it was found that one of them did indeed show two possible fault locations.

One of these was at a distance of 200 metres and, by chance, there was a traffic warning sign posted at a distance of about 1 metre from this point.

Using this as an impromptu stethoscope and thumping the cable, confirmed that the fault was close by. A pin pointer was then used to further refine the location, which proved to be almost exactly where the new instrument had predicted.

The results of these field tests were eminently satisfactory, but the real accolades came from the electric utility’s technicians, who are experienced users of cable fault location equipment. They commented that the new instrument was far more capable than anything they had previously used and, unlike many other products, it provided useful data on difficult faults, rather than “just sitting there and doing nothing”.

Cable faults, as previously noted are costly and disruptive, hence a fast and dependable method of fault location is highly desirable. In this challenging field, no instrument is ever going to deliver 100% perfect results, but the new instrument discussed in this article gets much closer to that ideal than its predecessors. This innovative and effective instrument is currently undergoing final refinements in line with suggestions received from the technicians involved in the trials described here and in other trials, and it will shortly be launched as Megger’s new PFL22M test set. At last, the tyranny of the cable fault has been tamed!

Remember “It’s your Fault” –
it’s your fault if you don’t find it
and it’s your fault if you do!

Programma’s Provenance

Today’s Megger organisation has at its core the many businesses that have over the years, joined the Megger family. One of the most recent to join is Programma, a comparatively young company that nevertheless has an interesting history.
Programma owes its existence to two friends, Roger Haraldsson and Christer Österlind, who in 1976 decided that the time had come to start their own business. Their first venture, which gave their company its name, was to develop and produce electronic programmers for washing machines, which were intended to replace the electromechanical devices that were being used at the time. The idea was sound, but the response from appliance manufacturers was unenthusiastic – they preferred to develop their own products.

Roger Haraldsson’s brother Sverker was
however a switchgear designer who had
considerable experience of testing protective relay systems. He had built a small relay test set for his own use and on learning of his brother’s plight, he suggested that Programma might try to produce this as a commercially available unit.

Original Sverker 1974

Interestingly, it was anticipated at this time that the company might be able to sell around 20 units. This was a rather substantial under-estimate – to date, more than 20,000 of the test sets, which was given the name SVERKER after Roger’s brother, have been sold. Admittedly, there are now many versions, but they are all based on the original design and concepts.

The new instrument was soon adopted by ASEA and branded as the ASEA SVERKER. This development had two important consequences for the fledgling Programma business. The first was that considerable interest was received from overseas markets, which highlighted the export potential of the product, and the second was that many requests were received for other items of switchgear test equipment that would, like the SVERKER, be readily portable and easy to use.

Programma responded enthusiastically, and this led to rapid growth and a move to new 4,000 m2 premises in Täby Kyrkby. The name SVERKER became well known and was seen as representing quality and smart design in test equipment. When new products were developed therefore, it was decided that like IKEA, the company would continue to give them distinctly Swedish names.

The names chosen were taken from Nordic Viking mythology and include, for example, Freja, Oden, Vidar and Egil. This naming convention proved successful for Programma, with customers in power, utility and service companies perceiving the Nordic names as synonymous with test equipment that is portable, robust and straightforward in operation.

Programma continued to expand – between its foundation in 1976 and 2001 when Roger Haraldsson decided to sell the company, it had grown by an average of more than 25% each year. Programma was sold to General Electric but after six years, was again offered for sale after GE decided to rationalise its business portfolio.

This time Programma was bought by Megger, and an immediate change of strategy followed the purchase. In particular, development and production, which had started to be transferred overseas, were brought back home to Sweden.

These changes served as a catalyst that once again helped the Täby based business to move into strong growth which has been sustained to this day.