Partial Discharge Solutions | Dynamic Ratings Case Study
Lightning Arc Flash

Partial discharge (PD) can pose a significant threat to high voltage electrical equipment and they are also typically the first indicator that an electrical asset is degrading. If left undetected or if reparative actions are not taken, partial discharges can lead to costly electrical failures.

Partial discharge occurs inside insulation in assets such as a transformer, turbine generator, motor, switchgear or cable, and usually happens when one part of the insulation cannot withstand electric stress, thereby flashing over. At this time, the gasses are ionized and the voltage drops quickly. Then the current pulse equalizes the remaining charge in the remaining insulation.

It’s a rapid process that negatively affects the asset’s life, which is why asset management condition based monitoring reporting is so important to the lifecycle of all utilities equipment.

Below, we’ll be examining a case study demonstrating how online partial discharge monitoring identifies degraded bushings on a 345kV gas circuit breaker, and why partial discharge should be included in the overall comprehensive monitoring of assets.

We’ll look at how asset managers can evaluate partial discharge activity over time and make informed strategic decisions regarding the timely repair or replacement of the equipment before an unexpected outage occurs. We’ll also be showcasing our suite of tools and our best digital asset management software solutions available to asset managers who want to protect their equipment.

Where Does Partial Discharge Occur?

Typically, there are only a handful of sources of partial discharge: operating voltage, voltage induced by main magnetic and stray flux, and impairment of the insulation properties due to moisture ingress and particle contaminants. In addition, human error can cause partial discharge in the case of poor installation or manufacturing defects. Some causes are environmental such as temperature and humidity. Over time, partial discharge can also occur due to aging of the insulation.

Luckily, online condition monitoring of high voltage electrical equipment is an effective way to identify degrading electrical assets at an early stage.

PD is Often the First Sign of Asset Degradation

Utilizing a comprehensive approach allows utilities to implement technologies to identify and prevent nearly all failure modes typically found in their high voltage electrical assets.

While technologies like DGA and online bushing monitoring have been used for years to identify degrading high voltage electrical assets, implementation of online partial discharge monitoring is not as common. However, since partial discharge is typically the first indicator that an electrical asset is degrading, it makes sense to include partial discharge monitoring as part of a comprehensive approach to monitoring high voltage assets.

Asset Manager working on equipment.
Dynamic Ratings Comprehensive Transformer Monitoring System

Thankfully, technologies exist which allow the detection of partial discharges in energized electrical equipment. While PD cannot be measured directly, its energy by-products such as electrical transients, electromagnetic emissions, sound, light, and heat can be.

Of these, the monitoring of electrical transients (that is, electrical partial discharge monitoring) is the easiest and most cost effective method to implement continuously online. Electrical partial discharge monitoring also offers the ability to utilize a single sensor for both bushing monitoring as well as online partial discharge monitoring.

With electrical PD measurement (IEC 60270), the apparent charge measures in pC which is the integrated current pulse caused by a PD, which flows through the test circuit. The conventional method allows a precise calibration but requires a sufficiently high signal-to-noise ratio (SNR) in the measurement circuit to easily resolve the PD signal in question.

Because partial discharge (PD) activity is often present well in advance of insulation failure, it provides the most evident indication of defects and deterioration in transformers.

The Right Tools to Solve the Issue

Because online condition monitoring of high voltage electrical equipment is an effective way to identify degrading electrical assets at an early stage, a comprehensive approach including a full suite of products is necessary.

Rogowski Coils

Rogowski coils are used as sensors in partial discharge (PD) detection systems to monitor high-frequency current pulses generated by partial discharge activity in electrical equipment. These coils are particularly effective at detecting internal PD noise within the insulation of high-voltage equipment, such as transformers, cables, and switchgear. However, they can also pick up external noise sources that may interfere with the PD signal.

The Dynamic Ratings Rogowski Coil allows for advanced noise rejection based on pulse polarity when used in conjunction with a BAU+ Sensor. With this added level of filtering, a transformer monitor can utilize lower frequencies to provide deeper “visibility” of PD within the windings.

Rogowski Coil

Our Rogowski Coil allows for advanced noise rejection based on pulse polarity when used in conjunction with a BAU+ Sensor.

Rogowski Coil benefits include:

  • The use of materials proven to resist UV deterioration.
  • An easy to install and non-intrusive design including a buckle strap provided to allow for quick installation
  • Availability in multiple sizes to fit a wide variety of bushing diameters
  • IP67 connections well-suited for harsh environments

Rogowski Coil sensors are selected by length to encircle the circumference of a bushing’s porcelain base and are fitted to the base of the bushing below the bottom skirt/shed. There are several size options available for client  convenience, as well.

BAU+ Sensor installed on a bushing

BAU+ Sensor

The Bushing Adapter Unit (BAU+) sensor serves multiple purposes which eliminates the need to purchase a unique sensor for each application. It provides:

  • Bushing signals for bushing insulation characteristics
  • Partial discharge in bushings and transformers
  • Additional circuitry for noise filtering using Rogowski coils

The BAU+ sensor also has a robust, environmentally-hardened design, rated to IP-67 specifications. These sensors are ideally suited for use in power transformers, isophase bus ducts, and HVCTs.

Coupling Capacitor Sensor

The Coupling Capacitor is a highly sensitive partial discharge (PD) sensor used to decouple PD from the monitored conductor. Coupling Capacitors are installed as close to the winding as possible for maximum sensitivity. Applications include generators, switchgear, motors, Iso-phase bus, and transformers.

The Epoxy Resin is Specifically Designed for High Voltage Insulator Applications

The epoxy resin is specifically designed for high-voltage insulator applications, and this material provides excellent insulation properties, mechanical strength and superior resistance to chemicals including concentrated acids. It also has superior arc resistance as compared to standard electrical grade epoxy materials and meets UL 94/V-O requirements.

Coupling Capacitor

How We Help Our Clients with Partial Discharge

Looking directly at this case study, we can see that as part of a comprehensive online monitoring program, an electrical utility installed key gas DGA, bushing, and partial discharge condition based monitoring on their generator step up transformers.

High Voltage Insulators

The bushing and PD system consisted of sensors installed in the test taps of the high voltage bushings, Rogowski coils installed around the porcelain skirt closest to the bushing flange, and coupling capacitors connected to the bolted connections of the flexible links connected to the LV bushings. Coupling capacitors were used to monitor the LV winding of the transformer since the LV bushings did not have test taps.

Shortly after the monitor was installed, the customer began receiving alarms due to high levels of partial discharge intensity. The partial discharge intensity from all sensors was acceptable except for periodic spikes associated with activity on channel 2 of the monitor which exceeded the warning and alarm thresholds. The Phase Resolved Data plots of the partial discharges on the HV side of the transformer identified the type of partial discharge that is occurring.

In addition, since the patterns were being detected by sensors on different phases and because there were discharges appearing on various parts of the waveform, this indicated that the discharges were coming from multiple sources.

Since the analysis identified that the discharges were coming from outside the transformer, Dynamic Ratings recommended that the customer scan the high voltage yard with a corona camera.

Using the corona camera, the customer was able to identify multiple sources of partial discharge. The most concerning problem was an issue that was identified with the bushings on the high voltage SF6 breaker protecting the transformer.

Large amounts of partial discharge were emanating from the bushing cap and a visual inspection of the bushing cap identified corrosion problems on this bushing as well as 12 other GCB bushings in the plant.

As a result of the partial discharge monitoring and the follow up corona camera inspections, the customer contacted the breaker manufacturer to perform a visual inspection of the HV gas circuit breakers.

The manufacturer then recommended the replacement of 12 bushings on the GCB’s protecting the transformers.

Corona Camera Capture
PD Monitoring System Installation

Concluding Thoughts

One of the benefits of the Dynamic Ratings Online Partial Discharge Monitor system is that, unlike with traditional coupling capacitors that are installed outside the generator, we use RTDs that are embedded within the windings as antennae – which means we can see down into the windings themselves.

Partial discharge (PD) activity is usually present well in advance of insulation failure, and 24 hour online monitoring provides the most reliable indicator of defects and deterioration in  turbines, motors, transformers, cable and switchgear. The benefit of this continuous 24/7 monitoring is you can observe the condition of the transformer at operating temperature.

If you’re interested in a continuous monitoring system for both generators and transformers clocking in at the same price as a test set, our team of specialists can take your call and guide you to the best asset performance management software for your specific needs.

Author: Tyler Willis, Dynamic Ratings