Megger test | Transformer insulation resistance test

Introduction of Transformer

A transformer is a static device which helps in the transformation of electric power from one circuit to another circuit at same frequency. The voltage can be raised or lowered in a circuit, but with a proportional increase or decrease in the current ratings.The principle of operation of a transformer is mutual inductance between two circuits which is linked by a common magnetic flux.

A basic transformer consists of two coils that are electrically separate but are magnetically linked through a path of reluctance.Various tests are done on a transformer to know its condition. The primary incentive of these tests is to make sure the transformer meet manufacturing specifications.

Transformers are an integral part of the power grid. Their reliability directly impacts the reliability of the grid. The failure of this critical asset can handicap the grid and increase its volatility. Because replacing a high voltage transformer requires planning for many reasons, including long manufacturing lead times that can exceed a full year, it is widely accepted that asset management, particularly of transformers, is a beneficial contribution to the operation of the grid.

What is The Megger Test?

Megger testing is an electrical test of insulation performance in an electrical apparatus. The Megger Test helps us to check the quality of insulation resistance of an electrical system, higher the insulation resistance value higher is the quality of insulation. The device used to perform the megger test is called Megger.

Why do we Megger a transformer?

Deterioration of the transformer insulation resistance is one of the most common causes of failure of the transformer: a failed transformer is a costly replacement in an electrical system with the potential for lengthy downtime.The megger test or insulation resistance test is carried out to ensure the healthiness of the overall insulation system of an electrical power transformer.

Step of Doing of Meggar Test of Transformer

1. Disconnect the Transformer from the power supply.
2. Remove the jumpers, connections of Lighting Arresters and neutral terminals 
3. Discharge the winding Capacitance.
4. Thoroughly clean all the bushings
5. Short circuit the Low Voltage and High Voltage bushing studs  as shown in figure below.\\
6. Connect the Megger leads to HV bushing studs and earth point of transformer tanks to calculate the insulation resistance between HV windings and earth as shown in figure. Note down the reading as HV – Earth. Take three to four readind by repeating the step three to four times.\\
7. Connect the Megger leads to LV bushing studs and earth point of transformer tanks to calculate the insulation resistance between LV windings and earth as shown in figure. Note down the reading as LV – Earth. Take three to four readings by repeating the step three to four times.\\
8. Connect the Megger leads to LV and HV bushing studs to measure insulation resistance between the LV and HV windings as shown in figure . Note down the reading as HV -LV. Take three to four readings by repeating the step three to four times.\

Observation Table for Megger Test

S.No	Observation	Reading 1	Reading 2	Reading 3	Average
1	HV – Earth
2	LV – Earth
3	HV – LV

Selection of Megger(or IR Tester)

Voltage Level	Megger
650KV	500V (D.C.)
1.1KV	1KV (D.C.)
3.3KV	2.5KV (D.C.)
66KV and Above	5KV (D.C.)

IR Value of Transformer

Voltage Level	Min IR Value
415V	100MΩ
Upto 6.6 KV	200MΩ
6.6KV to 11KV	400MΩ
11KV to 33KV	500MΩ
33KV to 66KV	600MΩ
66KV to 132KV	600MΩ
132KV to 220KV	650MΩ

IR Value of Transformer as Per Formula

1. Single phase Transformer \\ IR Value in MΩ = \frac{C \times E}{\sqrt{KVA}}
2. Three Phase Transformer (Star Connection) \\ IR Value in MΩ = \frac{C \times E(P-n)}{ \sqrt{KVA}}
3. Three Phase Transformer (Delta Connection) \\ IR Value in MΩ = \frac{C \times E(P-P)}{ \sqrt{KVA}}

Temperature Correction Factor

0C	0F	Correction Factor
0	32	0.25
5	41	0.36
10	50	0.5
15	59	0.720
20	68	1
30	86	1.98
40	104	3.95
50	122	7.85

Example: For 1600KVA, 20KV/400V, Three Phase Transformer\\IR Value at HV Side = \frac{1.5\times 20000}{\sqrt{1600}} = 750 MΩ \;at \;20^{0}C\\IR Value at LV Side = \frac{1.5\times 400}{\sqrt{1600}} = 15 MΩ \;at \;20^{0}C\\IR value at 30^{0}C= 1.98 \times 15= 29.7 MΩ

Read Also

1. why we don’t use wooden core in a transformer
2. Why Transformer is Not Connected to DC Source
3. Distribution Substation | 11 KV/0.433 KV