The do’s and don’ts of insulation power factor testing – part 2
Power factor (PF) and dissipation factor (DF) tests are widely used to assess the condition of the insulation in transformers and other electrical assets. The first article in this short series (which is still available online) took a brief look at the theoretical background for these tests. This instalment moves on to look at practical aspects, using an easily accessible Do’s and Don’ts format.
Safety is a fundamental concern for all types of electrical testing, so we make no apology for dealing with it first. This section will look at some of the safety issues specifically associated with PF and DF measurements, but it’s important to make it clear that the list presented here is NOT exhaustive! There are many aspects of safety – an example is working at height on ladders – that are relevant, but we don’t have room to cover. So, when carrying out any form of testing, consider safety carefully, follow safe working practices and, if in doubt, ASK the site manager or your health and safety representative!
- DO hold an informal meeting before each test job. Talk to your colleagues before starting work. Discussing and talking about the task from a safety perspective will naturally raise safety awareness. These informal meetings challenge those involved to think about hazards that have not previously been identified. Colleagues may point out a risk they are aware of through personal experience, but isn’t familiar to other team members.
- DO check grounding. The asset under test must be correctly and securely grounded. With modern test equipment, the test instrument’s ground lead connects to the asset’s ground connection. The test instrument’s supply voltage ground will automatically be compared with the asset ground by a comparative ground circuit relay to make sure that the resistance between the two is sufficiently low. If the resistance is not low enough, the relay will inhibit testing.
- DO inspect test equipment and leads before use. If there are any signs of damage or if indications of excessive wear, do not use the affected equipment and/or leads.
- DO connect the instrument ground lead first. Always connect the ground lead to the test instrument first, before making any other connections, and remove it last, after testing is complete.
- DO use a grounding stick. Develop the good habit of touching each terminal with a grounding stick before moving or disconnecting test leads. This will ensure that any stored charge that may be present is discharged safely.
- DO be aware of induced voltages. Always remember that energising a transformer winding with an AC source will induce voltages into the transformer’s other windings. Take care not to become part of the circuit on one of these other windings! For example, if you put a wrench in your back pocket and then inadvertently catch the wrench on a winding shorting lead behind you, you will surely be zapped! So stay aware of the hazards around you.
- DON’T ever take safety for granted. It’s easy to become complacent about safety, especially when carrying out familiar tasks. But taking safety for granted is a swift route to accidents.
- DON’T forget that only qualified persons can perform testing. Only qualified persons are permitted to perform tasks such as testing. This is for the very good reason that, in becoming qualified, they will have undergone extensive safety training.
- DON’T forget that improper use of test equipment can create electrocution and arc flash hazards. No matter how well designed and constructed the test equipment is, if it is used incorrectly, there will be a risk of shock, electrocution and arc flash. Modern test equipment is designed to promote safety – there’s never been a fatal accident associated with a Megger PF test set, for example – but no one should ever cut corners and hoping to “get away with it.”
- DON’T cheat the comparative ground circuit relay. The comparative ground circuit relay, the operation of which was described earlier, is there for a reason – to help make sure you stay safe. If it inhibits testing, don’t cheat it, however great the temptation. Instead, investigate the problem with the ground connections and fix it properly.
- DON’T get distracted. It only takes a moment or two’s distraction for an accident to happen. Stay focussed on the job in hand.
GENERAL INSULATION KNOWLEDGE
When insulation is the patient undergoing diagnostic tests, there are a few things it’s good to know!
General Insulation Knowledge Do’s
- DO memorise the state in which insulation performs best. It’s worthwhile remembering that insulation performs best when it’s clean, dry and relatively void-free. Insulation must also be used only within its designed temperature range – if it isn’t, early failure is almost inevitable.
- DO be wary of the enemies of insulation. The principal enemies of insulation are heat, moisture and oxygen. They often arise from external sources, but remember that they can also be produced as natural by-products of the insulation ageing process. In addition, remember that losses produce heat and heat can increase losses, producing even more heat in a process that can ultimately lead to thermal runaway and dielectric failure.
- DO remember insulation fails when stresses exceed withstand capability. As the diagram shows, manufacturers design insulation systems so that there is a very comfortable margin between anticipated stresses and the insulation’s withstand capabilities. But many insulating materials, particularly those used in transformers, are organic – paper, oil and pressboard – so they will inevitably deteriorate over time and eventually reach the end of their lives. This means that the withstand capability of the insulation falls as it ages. Stresses on the insulation, however, may well increase over time. An unanticipated increase in loading, for example, may lead to more heating. In such circumstances, it is possible that a point will be reached where the stresses exceed the insulation’s withstand capability, leading to dielectric failure. This point is shown as an explosion on the diagram, and this is not accidental. Dielectric failures can result in explosions and fires – the impressive ones that find their way onto social media for all the wrong reasons.
General Insulation Knowledge Don’ts
- Don’t be a bystander! Dielectric failure is dangerous, disruptive and costly – but, with proper testing and maintenance, YOU can help to stop it happening. Testing allows you to determine if, for example, a transformer is wet so that you can take remedial action by drying it out or reconditioning the oil. You can influence external stresses by reducing the load on a transformer so that it operates at a lower temperature. You can think carefully about how wet you will allow your transformers to operate, bearing in mind that water accelerates ageing. And you can improve your transformers’ preservation systems by promptly repairing leaking gaskets and seals.
The next instalment in this series of articles, which will appear in future issue of Electrical Tester, will look at the terminology associated with PF and DF testing, and at general testing knowledge as it applies to these test techniques.