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Corona Imaging

See the Invisible

August 28th, 2017

It felt like a fine spring morning in eastern Oregon as my colleague and I rolled up the power plant driveway, although the date on my Blackberry indicated mid-February. After some friendly greetings and hand-shaking with maintenance personnel, we signed in at the control room and viewed the not too lengthy or boring safety video. Then it was time to don our hard hats and head toward the substation – all half-million volts of it.

It was dry and sunny as our work boots crunched against the coarse substation gravel rocks. The massive bushings and insulators did not appear to have much, if any, condensation from the night before as we gazed across an acre of GSU transformers, SF6 dead-tank breakers and other equipment. Ideal conditions for a corona image survey. After powering up our new corona camera and making a few minor adjustments, we began doing what no other person had ever done before in this substation – we were viewing an invisible phenomenon known as corona. What we and plant personnel saw during the next few hours that morning would better our understanding of high voltage engineering in a manner that no text book possibly could. We saw corona from insulators that required cleaning, sharp edges from hardware connections, poor bus work corona suppression, and dangerous insulator corona caused by cooling tower residue deposits. As the prevailing wind puffed another cloud from the cooling tower onto the westernmost insulators, we could hear the corona crackling intensify as the camera revealed near flashover conditions. “When it sounds like this, I tell my guys to get out of the substation,” our customer stated. Not a bad idea, I thought. Next, we followed our customer to a corner of the substation were we examined a flash-damaged insulator laying in the gravel, then observed the additional weather sheds added recently to the in-service insulators nearest the cooling tower. For the most part, the increased surface creep age of the modified insulators were doing their job by eliminating most of the corona. However, as can be seen in

Figurine Image 1


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CABLE INSULATION ASSESSMENT OF A SMALL M-V DISTRIBUTION SYSTEM USING ONLINE PARTIAL DISCHARGE TECHNIQUES

August 22th, 2017

Once a method used only for laboratory testing purposes, partial discharge (PD) testing has evolved into an effective field testing technique. The high background noise environment typically present in most locations throughout the U.S. initially presented challenges to obtaining trustworthy test results, especially for cable systems. However, the very good noise removal capability of today’s instruments, coupled with greater field application experience gathered over the last 20 years, have greatly enhanced test result reliability.
 


CASE STUDY: LOCATION OF GENERATOR PD SOURCES USING MULTIPLE SENSORS

August 7th, 2017

Generator health is one of the single most critical elements of a power-producing operation's ability to maintain a reliable revenue stream. These generators typically operate at medium voltage levels; thus, a high percentage of overall failures are attributed to insulation defects, as is often the case in any medium voltage equipment.

Statistics from IEEE and EPRI studies indicate that approximately 37% of generator failures can be attributed to stator insulation failure. Stator insulation deterioration can be tracked by regular partial discharge (PD) testing or continuous monitoring methods. The importance of monitoring the generator's insulation for PD activity — along with monitoring other common mode failure components such as bearing condition — is a widely accepted practice in North America. This article will briefly address the general application of multiple generator partial discharge sensors and then discuss a recent event where the sensors predicted an impending failure.

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Coupling Capacitor Image


TRACKING DOWN TEV SWITCHGEAR SIGNALS USING TIME OF FLIGHT TECHNIQUES

July 31th, 2017

Transient earth voltage or TEV is a term coined by a pioneering British scientist in the 1970s. If the signals it identifies were first discovered in the United States, they would have more likely been called transient ground voltage (TGV) as engineers in Europe typically refer to our ground as earth.

TEV signals are generated by partial discharge activity, which is essentially a partial insulation failure. TEV signals are high-frequency voltage transients or impulses on the ground. They also have the unique feature of being synchronous – more about this later. In the case of switchgear assemblies, these signals propagate or ride along the external surface of the grounded switchgear enclosure. As shown in Figure 1, their magnitudes are greater on the enclosure panels that are closest to the discharge source.

This is how it works: Partial insulation failure creates a spark or partial breakdown that emits a radio frequency signal – think of the flawed insulation area as a small radio transmitter within the switchgear. For outdoor or open structure insulation, these radiated signals can produce signals that interfere with AM radio receivers, sometimes causing that sudden annoying static on a vehicle's radio as it is driven under an overhead distribution line. Partial discharge activity occurring inside of switchgear or other electrical assemblies acts in the same manner.

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TEV Signals


Picnics And Partial Discharge

May 30th, 2017

Partial Discharge (PD) activity creates a sound often described as that of sizzling bacon. The sound is only applicable to surface PD activity, as sounds emitted from internal discharges are hopelessly trapped within the insulation. Other types of PD signals do escape, as we will discuss later. The sound produced by surface PD is not always similar to sizzling bacon, but the analogy is generally a good one for describing these surface PD events.

While barbecuing one recent weekend, steaks sizzling nicely on the grill, my thoughts randomly drifted to partial discharges. It occurred to me that the process of cooking meat over fire — at least when I am acting chef — shares some similarities to the stages of surface insulation damage that partial discharge causes. When meat is cooking, it exhibits physical, chemical, audible, and even aromatic changes. Similarly, when an insulation's local surface electrical field is compromised, partial discharge activity occurs and creates changes like that of cooking meat. The physical changes can include visible signs of surface tracking, chemical alteration of the insulation, and reaction with atmosphere. Audible sounds are more easily detected with ultrasonic detectors than with the unaided human ear. Aromatic changes, such as the generation of ozone and acidic byproducts which create a unique sharp smell, can also be detected.

As meat continues through the cooking process, the amplitude and frequency of the sizzling events increase — similar to partial discharge events during surface insulation breakdown. Typically, when surface PD is initiated, ultrasonic signals are all that can be detected. During the next stages of the cooking process, the meat could create a sudden fire, which is similar to a complete surface flashover of the electrical insulation. Fire or flashover can cause permanent damage to meat or insulation.

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Advanced Surface PD


Electrical Equipment & Water Damage

May 8th, 2017

Post-Tropical Cyclone Sandy, otherwise known as Hurricane Sandy, made landfall just south of Atlantic City, N.J., in late October 2012. The storm caused billions of dollars in damages and left millions without power. By the time this article goes to press, nearly all damaged facilities will be back up and running and performing at full operational capacity. Most flooded electrical assemblies and components will have been replaced or field reconditioned. But just because the lights are on doesn’t mean there aren’t underlying issues that linger behind the scenes. The reality is hidden flaws may have developed as a result of this natural disaster that — if left undetected — could lead to future failures.

Other than NEMA’s free guide to “Evaluating Water-Damaged Electrical Equipment,” available for download on the organization’s website, there isn’t a great deal of information that addresses remedial measures related to flood-damaged electrical equipment. There’s even less, if any, information related to potential ongoing damage that can reduce equipment life and cause unexpected failures.

This article will examine the consequences of flood damage in regard to the development of short- and long-term flaws based upon observations obtained from previous experience as well as take a look at appropriate test methods that can ensure long-term reliability of electrical equipment.

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Electrical Equipment & Water Damage


How PD Technology Can Make You the Hero of the Day

May 1st, 2017

The electrical field testing business can be very trying. Often our work involves long, lonely hours. These hours may be far from home and sometimes in difficult environments or circumstances, or perhaps be under ambitious time constraints. For numerous reasons our best-made plans may require modifications and adjustments along the way, making quick thinking, knowledge of new technologies, improvising, and changing courses necessary to meet customer demands.

So why do we continue to endure these challenges? Reward. While monetary reward is appreciated, the satisfaction of knowing that you did a good job is what brings us back time and time again. And, every once in a while, there is that especially gratifying reward when we are actually the hero of the day.

Recently, I was in the field to perform cable partial discharge testing at a campus-like facility, consisting of an extensive 15kV class underground distribution system. This is a task I have performed hundreds of times, but this day, I was armed with the latest, most advanced cable partial-discharge testing technology. Having thoroughly studied this new technology in advance and conducted some dry run experiments in the lab, I was cautiously optimistic about how the day would go. Given that this new technology was not well known to my customer, I was also conscious of the fact that they would be looking over my shoulder throughout the day, evaluating both my work and the new PD testing equipment.

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How PD Technology



We have BIG news!

April 25th, 2017

News that many have been waiting for and many have asked for. As you may or may not know, not only are we consistently out in the field, but we are often on boards for various electrical associations, have published publications and are sought after consultants on large electrical testing issues and projects.

We are proud to announce that we will be launching our very own Halco Testing Services (HTS) Blog where we will publish prior electrical testing publications, latest electrical testing news and informative articles relevant to the electrical testing field.

Are you an electrical contractor? Electrical engineer or perhaps a building manager? Are there certain topics you would like to know more about? If so, feel free to send us an email with your request at lucero@halco.net

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