Link

The largest power engineering event in Mexico, the 2013 IEEE Mexico chapter conference is scheduled for July 7th through July 13th in Acapulco, Mexico.

Applications engineers Matz Ohlen and Nils Eng Wäcklen from Megger Sweden and Dr. Diego Robalino from Megger US will be making two presentations during the conference.

Presentation Abstract:

A NEW TECHNIQUE FOR DYNAMIC RESISTANCE MEASUREMENTS ON-LOAD TAP CHANGERS

The transformer is a critical substation asset that should operate reliably and safely. Based on studies and surveys done by power industry operators, most transformer failures start because of accessory failures such as bushings and tap changers. Like transformer testing, it is equally important to diagnose and test external or internal accessories for any fault or malfunction before it is too late.

Dynamic Resistance Measurement (DRM) has been used for circuit-breaker diagnostics for about 30 years. It is an interesting technique that can also be used to verify the switching operation of load tap changers (LTC). DRM, together with static winding resistance measurements per tap, can be a valuable method to validate the LTC operation and diagnose its condition.

Existing methods and techniques for dynamic measurements on load tap-changers are based on measuring current and/or voltage on the primary side of the transformer and short-circuiting the secondary side to minimize the inductance in the circuit. Static resistance measurements per tap are performed in a separate test sequence with the secondary side open.

A new technique (patent pending) involves combining current measurement with voltage measurements on both primary and secondary sides of the transformer. Then, the tester uses the transformer parameters to calculate inductive and resistive voltages to be able to calculate the dynamic resistance during a tap change. The new technique does not need a short-circuited primary side to reduce the inductance, and static and dynamic resistance can be measured in the same sequence thus saving total test time.

The presentation will focus on the DRM application and how it can be used for tap-changer diagnostics. Measurements have been performed on both non-mounted tap-changers (no-oil condition) and after being mounted inside a transformer (with oil). Different test setups have also been evaluated, including the new technique. The results from the tests will be presented and discussed.

Presentation Abstract:

TRANSFORMER ADVANCED DIAGNOSTICS TECHNIQUES BY FREQUENCY RESPONSE

Frequency response techniques (developed to better understand the overall condition of power transformers) were introduced more than 20 years ago. Scientists, researchers and utility operators have shown great interest in the development and application of frequency response techniques and, as a result, several documents have been published summarizing the research work and field tests all over the world. Frequency response techniques are practical non-intrusive and non-destructive diagnostic methods that allow investigation and help uncover solutions to problems that were once only suspected by other testing techniques. Good understanding of the fundamentals and objectives of each technique are paramount for manufacturers, operators and researchers.

Dielectric Frequency Response (DFR) also known as Frequency Domain Spectroscopy (FDS) is an advanced application of the well-known dissipation factor (tand) insulation test. The difference is the wide frequency “spectrum” used to evaluate the condition of the insulation system, along with being able to discriminate between the moisture concentration in the solid insulation versus the contamination of the liquid insulation. DFR provides several benefits to the industry, not only as a diagnostics technique but also as a quality assurance and quality control tool. DFR has evolved, and, simultaneously, the instrumentation utilized in the field has evolved parallel to overcome the field challenges such as: testing time constraints and the effect of electromagnetic noise in the substations.

Sweep Frequency Response Analysis (SFRA) is an advanced diagnostic technique based on measurements of the frequency dependency of the electrical responses (transfer functions) of transformer windings to low voltage applied signals. The primary objective of the SFRA technique is to detect winding deformation via changes to capacitance or inductance distributions. The sensitivity of the instrumentation and the accuracy of results are of extreme importance for end-users, and a thorough but simple guideline to evaluate the condition of the instrumentation and the correctness of connections and grounding must be discussed. Moreover, validation of results considering reinforcing conclusions with results from other electrical testing techniques have been included to support field decision making.

It is critical for the technical community to gather valuable information during testing using DFR and SFRA techniques. Users recognize that major transformer failures and/or catastrophic failures may be prevented if the fundamentals, execution and interpretation of these techniques are well known and implemented. Throughout this presentation, authors will discuss main objectives, fundamentals, best testing practices and interpretation of results of the frequency response methods developed for advanced condition assessment of power transformers.

Working with distributor Oropeza Ingenerios, Megger will showcase the latest in power testing equipment software enhancements and handheld instruments.  Megger application engineers in the areas of substation maintenance and protection will be at the exhibition to answer any questions.

To schedule an appointment during this event please contact Washington Cabrera at 1-800-723-2861, extension 7382.

Event Alert: Beckwith Electric Protection Seminar

Scheduled for July 30th in Clearwater Florida, the Beckwith Electric Protection Seminar provides an in-depth study of generator, transformer, feeder, and distributed generation interconnection protection, as well as motor bus transfer and automatic synchronizing. There are two tracks (power plant protection and distribution protection) and you can sign up for the either option.  Megger will host a exhibition space that display latest in relay and protection test equipment including the MPRT8445 and SMRT36.

Contact Wayne Armstrong to schedule an appointment during the event at 1-877-723-2861, extension 7394.

Locating Battery Ground Faults with a BGL

The BGL is used to trace and locate ground faults on a live DC system, without sectionalizing any circuit.

STEP ¹ Turn on the BGL. This instrument is battery operated or can be used also connected to the power supply. After turning on the unit the LED “In Progress” will illuminate for a while. If it is being operated from the internal battery the LED “Overrange” will illuminate as well when the charge is over 90%.

STEP ² Test Lead Connection. Connect the current sensor and output lead set to the instrument.

 

STEP ³ Test Leads Connection to Battery System. The output test lead set should be connected to the battery system in the following order:

• Connect the black and green ground wires to earth ground for both a ground and safety.
• Red clip to positive or negative battery bus bar. A small spark may occur when connecting to battery terminals and the LED “Warning > 30VDC” may turn on.
• Clamp the current sensor around the red cable from the output lead set.

STEP ⁴ Measure total resistance to ground. After connecting the clamp, set the “Function” switch to resistance position, then wait for VALID LED to illuminate.

Verify you can measure the ground fault and note the value.

With the current sensor around the red cable of the lead set, the instrument will measure the total parallel resistance to ground of the battery system. This includes all leakage paths from positive and negative side. If this resistance is in excess of 10 Kohms, the chances of finding any problem is limited.

STEP ⁵ Measure resistance of each branch. If the total resistance measured in step 4 is below 10 Kohms the next step is to measure the resistance of each branch of the system to trace down on which is the fault or faults. Troubleshoot the lowest resistance branch first.

To measure the resistance of each branch disconnect the clamp from the red cable of the output lead set and clamp it to the first branch to be measured, wait for “Valid” LED to illuminate and take note of the resistance to ground of this branch. Repeat this for all branches and compare them to determine which branch has the lowest resistance.

Once a branch has been identified further measurements should be done downstream of it to locate the specific point of fault in the branch.

STEP ⁶ Terminating measurements. Once a particular panel has been measured and you need to move to another panel or you have completed the test, the instrument needs to be disconnected. To remove the test leads follow the sequence below:

• Remove the clamp from the branch
• Disconnect the red clip from battery bus bar. If the “Warning > 30VDC”LED illuminates, connect the red clip to ground until the led turns off.
• Disconnect the black clip from ground.
• Disconnect the green clip from ground.
• Turn off the unit and disconnect it from the power supply if it is being used.

THE BGL

Simplify fault location. This instrument was developed to detect, track and locate ground faults on battery systems – without resorting to sectionalizing! Model BGL tracks and locates ground faults on live or dead battery systems. To save hours of unnecessary troubleshooting, the BGL readily differentiates between the resistive fault currents and capacitive charging currents. This feature allows the instrument to detect and track leakage paths, even in the presence of surge-suppression capacitors.

Request more information

BGL Datasheet

Free Battery Testing Guide

NERC compliance for battery testing

The majority of NERC standards violations since enforcement began, are for PRC-005-2. The NERC reliability standard PRC-005-2 is the standard for protection systems maintenance and testing. This standard gives the minimum maintenance requirements for protective relays, DC supplies, DC control circuits, current and voltage sensing devices, stationary battery backup strings and associated telecommunications equipment.

The NERC PRC-005-2 requirements for battery maintenance include the following.

The following chart breaks down the application of Megger’s BITE series for NERC compliance testing:

* Requires hydrometer kit

Browse Megger’s battery test equipment

10 ways grounding can be positive

As a kid, being grounded is a negative. We don’t get to watch our favorite show on TV or visit a movie theater to see “The Avengers” with our friends. As an adult in the power industry, being grounded becomes quite the positive. In fact, being grounded is one way to keep that favorite TV on and that favorite movie playing by preventing a short-circuit catastrophe within the electrical system. A good protection system relies on low-resistance electrical paths to assist relays when there’s a fault in play. In simple terms: grounding this stuff help keeps your TV on.

Electrical ground or putting a good overall grounding system in place—one that protects both your personnel and your infrastructure—takes a bit of forward thinking. You’ve got to examine weather issues, safety issues, mathematics and dirt. Yep, just like when you were a kid, you get to go back to playing in the dirt.

While this is, by no means, an extensive or complete step-by-step guide to electrical grounding and earth resistance testing, do keep these ten items in mind.

1. For comprehensive coverage, it’s all about the soil. We tend to think of dirt as a rather basic, single element. The truth is, there are all kinds of dirt: rocky, clay, sandy. And, just as each of those categories can impact growing plants in the dirt, it can also impact electrical resistance. Generally speaking, all soil has some electrical property, just as your body does. But, on the whole, dirt’s a rather poor conductor—at least, it’s not as good as, say, copper wire. Still, get yourself a big enough path, and even dirt can be a decent conductor. And, helpfully, it’s everywhere. Unfortunately, there’s a lot of geography in this step. You don’t need to know just the type of soil, you need to know stratification, moisture, temperature and chemical composition to figure out the resistant factor and your needs. Never was playing in the dirt so darn complicated. (See table.)

2. Send people who know stuff. Knowledge is the most effective tool for all types of field work, and it becomes that much more valuable in applications where variables are as large and uncontrollable as they are in ground testing. Bottom line: This is important. So, don’t send a rookie. Don’t think, “Eh. It’s not that difficult. How much trouble can he get into?” The fact is: Grounding isn’t as simple as banging a metal pole into the earth for testing. And electricity is dangerous. Send the pro. He can bring the rookie.

3. Do a dry run. Assess the site and recent conditions. Take a look around before testing. Get the lay of the land and take a look at weather cycles as well. This will help you make an educated decision as to where test results may fall in the min/max testing cycle. Then, proceed accordingly. At the least, arrangements should be made to retest at a suspected worst time.

4. Pay attention to schedules when it comes to maintenance. If a maintenance schedule is established, be judicious about the interval. Those are important. Don’t let those times lag and maintenance fall by the wayside. For most electrical maintenance, a regular schedule—for instance, annually or semi-annually—is the order of the day.

5. Forget schedules when it comes to ground testing. Testing at regular intervals will result in readings being taken under the same general weather conditions year in and year out. If these are optimal times of year, a false sense of security can develop. Instead, test at irregular intervals, like 5, 7, or 11 months, so that all times of year and all weather conditions will be evaluated. A “worst case” will be recognized and, if necessary, the grid can be expanded or improved so that there will be no unpleasant surprises—no shocks, if you will. If you don’t know what your ground is like in every season, you’re doing yourself a disservice.

6. Don’t fear the winter. Testing in every season means that, likely, you’ll get to work when your fingers are freezing, when there’s a storm watch or, even, when it snows. But, being methodical and thorough will prevent the cold from impacting your tests. First, the steps: the test rods must be driven through the frost layer. Then, the ground tester must establish a minimum amount of current through the soil in order to meet its measurement parameters and to sense the voltage drop across the measured resistance. (Modern testers include indicators that will warn the operator if these parameters are not being met.) Additional measures must then be taken, such as driving deeper rods, to bring the test setup within specifications. (Pouring hot water provides only a marginal temporary advantage and may backfire by freezing solid around the probe and making it nearly impossible to remove.) But, once an adequate setup is accomplished, testing under snow is just as reliable as at any other time.

7. Sometimes snow can be an advantage. We know it seems unlikely. But, when snow falls early in the season, before the first major frost, it may thermally insulate the ground and limit frost penetration to more workable depths, say 6-8 inches. If snow has been plowed or drifted away, frost penetrates deeper and test results may be rendered less consistent. Testing under snow can actually be more reliable. Just shovel away an area large enough to drive the test rods.

8. Don’t be a wet weather hero. No one is likely to want to perform a ground test in a driving rain or with lightning about, even if miles away. Well, no one sane. Besides, Benjamin Franklin already did that dangerous bit of testing for electricity; no need to repeat this. Don’t even think about it. So, dangerous conditions are to be avoided because of the risk to the operator. Dangerous voltages developing on the power lines can also be transmitted through the grounding system and will appear at the terminals of a tester if a test is in progress.

9. Consider the instrument. But aside from those extreme Benjamin Franklin circumstances, ground testing can be performed on moist or rainy days. The sudden appearance of a slight shower shouldn’t set you on a dead run to the truck as long as you’re working with the correct instrument for your environment. The determining factor here is the IP rating of the instrument.

10. Mind your “I”s and “P”s. This IP rating should be available in the instrument’s specifications and is commonly referred to as Ingress Protection. IP ratings were established by the International Electrotechnical Commission (IEC) in Standard #529, and provide a means of evaluating the effectiveness of an instrument’s casework in keeping out dirt and moisture. The IP rating consists of two numbers and both cases, the higher the better. The first number indicates how well the instrument is sealed against particle invasion, with six being “dust tight.”

Quarries and mines are particularly bad environments in this regard, while a steady wind in a dusty environment can also pose a hazard to the instrument. The second number in the rating refers to moisture ingress, with eight, the highest rating, translating to “continuous immersion.” Since ground tests are not performed under water, this would be overkill. But note the IP rating and obtain an instrument that is adequate to the rigors of your fieldwork.

Armed with soil and weather knowledge, experience and a good instrument, the skilled technician will finish that ground test in no time flat—well, unless he has to shovel some snow first.

Browse Megger ground testing equipment