Transformer Insulation Test Using Megger
For each and every electrical system, we should do the insulation testing before its commission. To do it, we use a machine called megger. But here I’m not going to discuss the working principle of megger. I’ pretty sure that you will have a question about why we need an insulation test in electrical systems. The simplest answer is it is for the safety of both equipment and human. Is there an insulation failure, there will be fault currents in exposed conductive parts and that can cause hazard situations.
There is a standard way of doing an insulation test for transformers.
1. Remove the connection of the transformer if it is already connected to the supply or load.
2. After that according to the voltage ratings of the windings, we should select the test voltages of megger. The megger in figure 01 has the maximum value of 5000V and minimum of 250V. The basic selection criteria are the value we set in the megger is larger than to the rated voltage of transformer terminals but not too much. But in most cases, the rated voltage of HV winding is greater than our megger max voltage. Since we can’t apply a higher voltage than its rated to the transformer in a normal environment. So, what we do is, just apply our maximum. Yes, I know what you think. You are testing your insulation using a voltage which is less than the rated voltage. But since we can’t access that much of voltage at the site, we have to satisfy with our maximum voltage. If your satisfaction level is high, what you should do is bring a test laboratory to the site of the transformer of bringing your transformer to the test laboratory. if not use a megger which has a higher maximum voltage.
3. Then we should figure out the winding specifications of the transformer. According to the configuration, we should do the test all terminals by connecting the ground cable to the earth and between primary and secondary terminals.
Result analyzing
In the test, we get the result as ‘O.L’ means it is open loop. Which means the insulation is perfect. But in some cases, there are small insulation problems which are negligible or not. In this case, we will get a value reading from the megger. in such situations, the assessment criteria are our megger reading should be greater than the calculated resistance R where R is 1 megaohm per 1KV. The total KV value is counted as the sum of terminal voltage and the megger voltage we have used (Both in KV).
Additional Note,
In this method, we can measure the Dielectric Absorption Ratio (DAR) too without any concern. Before we talk about DAR, we should understand the Dielectric Absorption Current. t is the polarising current that is drawn by the insulation system (dielectric) to align the dipoles within the dielectric with the applied electric field. This current draw a high current initially but then gradually drops off as the dipoles in the dipole become increasingly polarised.
In this test, the testing device, the megger is applied and IR measurements are taken after 30 seconds and 60 seconds DAR is calculated as:
DAR = R_60/R_30
Where R60 and R30 are the IR test measurements at 30 and 60 seconds respectively (Ω A general guide to interpreting the DAR test results are as follows:
If DAR is less than 1.25, the transformer is questionable.
If DAR between 1.25 and 1.6, the transformer is Adequate.
If it is greater than 1.6, our transformer is good.