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Partial Discharge Testing: What Is It & How It Works

Whether it’s one of your rotating equipment or electric motor, the chances of partial discharge (PD) occurring are pretty likely. Over the years, a large percentage of disruptive breakdowns in high voltage (HV) machinery and medium voltage (MV) are mostly related to partial discharge.

When it is not detected and dealt with early on – equipment failure might even occur. And this will eventually take a toll on your business, jeopardising its operations which may even affect productions and profits.

Once you notice a little spark as you are doing your daily or weekly checks, you will know that it’s time for your engineer to check it out. If it’s your electric motor facing the problem, an electric motor overhauling might be necessary to determine whether partial discharge testing is required.

Now that you see how partial discharge can negatively impact your business – how does it work?

What is partial discharge?

Also known as an electric discharge, partial discharge (PD) is a localised dielectric breakdown (DB) which does not entirely bridge the space between two conducting electrodes under high voltage (HV) stress. Usually, partial discharge can happen due to defects in the high voltage insulation such as:

  • Equipment installation
  • Overstressed operations
  • Ageing and degradation
  • In-service damage

Additionally, it can also happen within a solid, liquid or gaseous insulating medium. For partial discharge in a gaseous state, it means that corona activity is occurring as well.

How to detect partial discharge

There are mainly two ways to test for partial discharge. Off-line testing is more favourable as the equipment is turnt off and testing will occur from an external source without any noise interference. However, this would mean that regular business operations cannot go on for some time.

On the other hand, online measurements happen when the equipment is energised or kept running. Online or continuous measurements will still continue while the equipment is operating. But as said previously, the noise which will occur while the equipment is running will cause inaccurate measurements. So, before starting the measurements – higher filter frequencies have to be used and there should be sufficient distance.

Besides the two varying types of partial discharge testing – the equipment size also plays a part. Different monitors and sensor have to be used to test for any partial discharge. For instance, larger machinery will use Stator Slot Couplers (SSCs) while small to medium-sized equipment will use Epoxy-Mica Capacitor (EMC) sensors for testing. At the same time, diagnostic software is applied together with the monitors and sensors to really understand the state of the insulation.

Why partial discharge testing is important

There are many ways for your equipment to experience failure. It could be due to a leakage in your lubrication system or misaligned shafts in rotating equipment. Either way, services such as switchgear repairs or electric motor rewinds have to be called in to fix the issue.

Similarly, partial discharge will also cause your equipment to deteriorate and end up breaking down. Which is why partial discharge testing is necessary whenever you spot the signs such as sparks occurring in your equipment. Remember, before the situation gets worse – it’s always best you take action to prevent a complete equipment failure from happening.