“You insured your assets, now ensure asset availability”

AbstractWe are all familiar with the issue of gassing in transformers and the potential hazards of the combustible gasses that form within the transformer. One of the main causes of gassing is arcing or electrical discharges occurring inside the transformer. These electrical discharges are called Partial Discharge (PD). This arcing causes the paper insulation to gradual decomposition with eventual breakdown. This breakdown of insulation can lead to a failure of the unit resulting in explosion, fire or other damage to the transformer. Virginia Transformer produces a high-quality product that minimizes the causes of Partial Discharge.

Are you losing your sleep over Partial Discharge on the transformer that is serving your industrial plant?

If you are an asset manager at an industrial plant, then we know all about your Power Transformer Gassing nightmares. One of the main causes of transformer gassing is Partial Discharge (PD). Once started, PD can cause more gassing and eventual failure of the transformer. A transformer suffering Partial Discharge means it is deteriorating every minute.

It is common in the electrical industry to monitor the creation of the gases, as an indication of a breakdown in insulation occurring. Investigative steps can then be performed, to find the cause of the electrical discharges that are causing the breakdown of the insulation materials. One of the causes of electrical discharge is small pockets or voids of space within the solid insulation of a transformer, sharp points of metal burrs, or loose pieces of metal dust within a transformer. Partial Continuous Deterioration of a Transformer is termed Partial Discharge (PD).

To insure the quality of the transformer, the Partial Discharge test is a final test performed at the Virginia Transformer (VTC) factory after manufacturing of a transformer. This Partial Discharge (PD) test provides a measurement of how well the quality construction of the transformer will prevent the creation of arcing/electrical discharges within the transformer. Due to the quality of their engineering and production processes, VTC is a leader in the industry for low Partial Discharge rates that provide our industrial and utility clients assurance of the high-quality transformers produced at Virginia Transformer (VTC).

What is a Partial Discharge (PD) Partial Discharge is a partial voltage breakdown within a series of insulating elements between two electrodes of different potential. Partial discharge is defined as incomplete discharges in gaseous, liquid or solid insulation which is locally electrically overstressed, resulting in discharge inception. PD is measured in Pico Coulomb (pC).

How does PD affect a transformer?The occurrence of partial discharges during transformer testing or later in service indicates the existence of a dielectric problem which as it progresses, may eventually cause catastrophic failure. Partial Discharge will initially cause gradual deterioration of paper insulation and as it progresses will result in total insulation failure. In other words, PD will reduce the life of a transformer.

How much life reduction will PD causeIt is difficult to quantify loss of life in years, but general transformer experience shows that once PD exceeds a certain level, a transformer will fail within a few years. A 40-year life transformer may last less than half its life.

What is the cause of PDThe simple answer is POOR manufacturing QUALITY!The poor-quality manifests in many ways:

Poor connection between conductors. Loose nuts & bolts. High moisture content (high PPM & poor BDV) in the overall insulation & oil. High moisture content or foreign metallic particles in wood used for cable rack & supports. Sharp points in winding braze, crimps etc. Inadequate settling time - not allowing all the insulation to fully soak with oil. - a process defect Foreign particles that may include insulation dust, metallic dust, or items left inside by mistake.

What we do to minimize PD levels?Flawless manufacturing is the key to lower PD. Here are some examples of paying attention to details:

Extreme care while clamping Core Coil Assembly (CCA) not allowing any damaged thread - slivers to fall on cables/windings.

Thorough internal/external cleaning of the tank.

Not leaving behind loose foreign material inside the tank.

Proper tightness of the LTC, bushings & PA connections.

Internal cleaning before lid welding. Putting an adequate number of clamps on

the lid which will prevent the weld slag entering inside the tank during the lid welding. Zero pressure test to verify no leaks on lid weld area.

Applying vacuum & checking leak rate. Well trained operators; quality check to

eliminate sharp points

What are PD Levels at VTC-GT?Yes, you guessed it right – the lowest average PD in the industry.We follow the mantra of robust design and flawless execution to achieve average PD levels of typically 100 pC compared to IEEE Standard of 500 pC.At these levels of PD along with the overall you can expect 60 years of life from our industrial grade transformers.You do not have to lose your sleep over PD!