In this article we will discuss briefly about the megger insulation test of transformer. We will try to understand what is megger test and what is the standard megger test procedure. Result of megger testing for transformer are also discussed.
Megger insulation test (A type of insulation test) is used to check the condition of insulation of electrical equipment. It is used to check insulation level or strength in HV and LV electrical devices such as motors, transformers, generators, cables etc. In simple words, megger testing finds out whether there is any breakdown in insulation or not. Megger insulation test not only to checks the breakdown but also spots weakened areas or partial breakdown of insulation.
Working Principle of Megger testing
Working principle of megger test is based on ohm’s law. Megger produce high DC voltage across the those two points between which insulation is to be find out. As a result current flows and using Ohms’ law, resistance across two terminals can be determined. The value of the resistance shows the insulation level or insulation strength. The megger insulation test is explained with the help of transformer example.
Voltage limits and Insulation level for transformer
Ideally the insulation between two electrically isolated conductors should be infinite. However in real world there is always some finite resistance value between two isolated conductors. If the manual or any other assistance document is not available, following values should be used. These values are based on ANSI/NETA ATS-2009 standard that provides the test voltages and accepted insulation strength for different electrical components.
|RATED VOLTAGE( V)||TEST VOLTAGE(DC V)||INSULATION STRENGTH(MΩ)|
|25000 and above||15,000||100,00|
Standard Megger test procedure (100 KVA T/F)
Let’s take 100KVA transformer as an example as shown in figure For normal working transformer, both windings must be electicaly isolated from each other and from ground. So Megger test is done to check
- Insulation between HV and LV winding
- Insulation between HV and body
- Insulation between LV and body
To check the insulation level between HV and LV winding, put one terminal on HV winding and one on LV winding. Apply the correct value of voltage (voltage values shown in above table) and check the reading of the megger. Megger test equipment will show resistance between the HV and LV winding. The value of resistance will show the insulation strength between HV and LV winding.
- If resistance value < 100Ω, it means there is some conducting path between two windings. Therefore transformer must be checked and repaired before installation.
- If 100Ω <resistance value < 10MΩ, there may be some insulating material touching the two windings. It can be a piece of wood or any other insulating material. Therefore the transformer again should be checked and verified before installation.
- If insulation value >10ΩM, it means the two windings are well separated from each other and the transformer pass the insulation test for HV and LV winding.
The insulation value between HV and LV winding for 100KVA transfer shown in figure 1 is 180 MΩ which shows that LV and HV are well separated from each other.
NOTE. This test does not verify intra-winding insulation i-e insulation between consecutive windings. TTR test is performed to check short circuit inside windings.
Next is to find the insulation level between HV and ground. Put one terminal of megger on HV winding and other on the body of the transformer (Note that body of transformer is always grounded). Repeat the procedure same as mentioned above. Check the value of the resistance. The value should be MΩ or GΩ. If not, check the transformer. The value found for 100KVA transformer was 192MΩ.
Lastly check the insulation between LV winding and ground and verify that insulation value lies in the range of MΩ and GΩ. The insulation level determined for 100KVA transformer was 187MΩ.
What is the difference between HiPot and Megger insulation test
Hipot stands for high potential. As name suggests high potential is applied in this test. Similar to Megger tesr, it is also a type of insulation test. Besides these similarities, there are some differences as well.
- Megger testing illustrates a general picture of the insulation strength while Hi-Pot spots the weak points in insulation.
- Megger test gives output in terms of resistance that is Ohms, Kilo ohms, Mega Ohms etc while Hi-Pot testing gives output in terms leakage current through the insulation.
- Voltage applied during Megger insulation test is between 600V to 2500V for duration of 1 min, while in Hi-Pot test, voltage uptill 15000V is applied.
If you have any question please ask in comments.