Megger Testing of Transformers: Earth Terminals & Insulation Check

In summary, Megger tests are used to test the insulation of transformers. Be aware that on large transformers these areas can have substantial capacitance and they can hold a possibly lethal charge for days after the test. Discharging the capacitance by the use of a shorting bar is important.
  • #1
Ron_04
14
0
hi, in megger test of transformers which terminals we have to Earth and why? How this test is used to check insulation?
 
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  • #2
Megger tests on transformers are to test the insulation of the transformer.

So you would measure between windings, and between windings and the metal core of the transformer.

Be aware that on large transformers these areas can have substantial capacitance and they can hold a possibly lethal charge for days after the test. So, it is important to discharge this capacitance by the use of a shorting bar.
 
  • #3
vk6kro said:
Megger tests on transformers are to test the insulation of the transformer.

So you would measure between windings, and between windings and the metal core of the transformer.

Be aware that on large transformers these areas can have substantial capacitance and they can hold a possibly lethal charge for days after the test. So, it is important to discharge this capacitance by the use of a shorting bar.

thnx for the reply but i want to know the exact process to check the insulation and the theory behind it?...please help
 
  • #4
A transformer should not conduct at all between windings or from any winding to ground.

So, depending on the type on Megger, you attach the Megger to the windings you want to test the insulation between and then push a button to produce a high voltage.

If the insulation is OK then the meter will indicate that there is no conduction or that there is infinite resistance between the windings.

You release the button and wait for the voltage to be dissipated or you may have to short out the leads to make the transformer safe.

Never touch both leads of a Megger at the same time, even if you believe it is safe. They can deliver a vicious shock.

I have used hand-held meggers and they are mostly simple to use. Really old ones used to have a handle which you had to wind to get a high voltage. More recent ones just have a DC-DC converter in them which generates the high voltage.

Mostly, you get a choice of voltage to test with. This voltage might be 500 V, 1000 V or 2000 V.
It is possible to do damage with this feature if you select too high a voltage when testing components that are not rated for such a high voltage, but for testing transformers you could test on 1000 volts safely.
 
  • #5
vk6kro said:
A transformer should not conduct at all between windings or from any winding to ground.

So, depending on the type on Megger, you attach the Megger to the windings you want to test the insulation between and then push a button to produce a high voltage.

If the insulation is OK then the meter will indicate that there is no conduction or that there is infinite resistance between the windings.

You release the button and wait for the voltage to be dissipated or you may have to short out the leads to make the transformer safe.

Never touch both leads of a Megger at the same time, even if you believe it is safe. They can deliver a vicious shock.

I have used hand-held meggers and they are mostly simple to use. Really old ones used to have a handle which you had to wind to get a high voltage. More recent ones just have a DC-DC converter in them which generates the high voltage.

Mostly, you get a choice of voltage to test with. This voltage might be 500 V, 1000 V or 2000 V.
It is possible to do damage with this feature if you select too high a voltage when testing components that are not rated for such a high voltage, but for testing transformers you could test on 1000 volts safely.
thank u very much for ur xplanation...i have only one doubt tht is which part of the transformer has to be grounded while performin the test and y?
 
  • #6
You may have misunderstood this.

The frame of most transformers is grounded in normal use, so testing the transformer windings for leakage to frame involves grounding one side of the Megger.
 
  • #7
vk6kro said:
You may have misunderstood this.

The frame of most transformers is grounded in normal use, so testing the transformer windings for leakage to frame involves grounding one side of the Megger.

ok i got it thank u very much i truly appreciate ur help
 

1. What is Megger testing and why is it important for transformers?

Megger testing is a method of measuring the insulation resistance of electrical equipment, including transformers. It is important because it helps to detect any faults or weaknesses in the insulation, which can lead to equipment failure or safety hazards.

2. How is Megger testing performed on transformer earth terminals?

Megger testing on transformer earth terminals involves connecting the Megger instrument between the earth terminal and the transformer body, and measuring the resistance between these two points. This helps to ensure that the earth terminal is properly connected and has a low enough resistance to provide adequate grounding for the transformer.

3. What is the purpose of insulation check in Megger testing of transformers?

The purpose of insulation check in Megger testing is to measure the resistance between the transformer windings and the ground. This helps to determine the condition of the insulation and detect any potential defects or moisture that could compromise the performance and lifespan of the transformer.

4. How often should Megger testing of transformer earth terminals and insulation be performed?

The frequency of Megger testing depends on the specific industry standards and regulations, as well as the age and condition of the transformer. In general, it is recommended to perform Megger testing at least once a year, or more frequently for older or heavily used transformers.

5. Can Megger testing of transformers be done while the equipment is energized?

No, Megger testing should not be performed while the transformer is energized, as it can damage the Megger instrument and pose a safety risk. The transformer should be de-energized and properly isolated before performing Megger testing.

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