What is Megger Test? How to do Megger Test/Insulation Resistance for Transformer?

What is Megger Test?

A Megger test, which stands for “megohmmeter,” is an electrical test used to determine the resistance, insulation integrity (insulation resistance), & condition of electrical wire, cables, motors, and other electrical components. It includes applying a high voltage, often 500 (or) 1000 volts DC, to the equipment under test in order to measure resistance between different sites and evaluate the insulation’s resistance to current leakage.

The test facilitates in the identification of potential problems such as insulation breakdown (or) degradation, moisture infiltration, or other concerns that may endanger the safety and functionality of the electrical equipment. Megger tests are routinely used to ensure safety and reliability during routine maintenance (or) before energizing any electrical installations.

Working Principle of Megger Testing

The megger test’s operation is based on Ohm’s law. Megger generates a strong DC voltage between the two points where insulation is to be tested. As a result, current flows and resistance between two terminals are easily calculated using Ohm’s law. The resistance value indicates the insulation level (or) insulation strength.

What is the acceptable Megger Reading for a Transformer?

It is possible to state the rule as follows: the insulation resistance should be about one (1Ω) megohm for every one thousand volts (1,000 volts) of working voltage, with a minimum value of one (1Ω) megohm.

What is IR in Transformer?

Insulation resistance testing is simply one of the most important tests that should be performed on a transformer. The winding insulation integrity is tested using insulation resistance (IR) test, but at extremely high voltages.

What is Insulation Resistance (IR) value for Transformer?

At room temperature (20 °C), it is generally advised that the insulation resistance be a minimum of 1 MΩ for every kV of winding rating.  However, when the operating temperature rises, the recommended resistance rises significantly. For each additional 10 °C, it more than doubles.

IR Value of Transformers

Transformers are one of the costliest equipment used in distribution and transmission process of power. The transformer has to go through different testing procedures periodically to prove that the transformer works properly.

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 transformer resistance testing will reveal a lot of details about the transformer by measuring the winding resistance of a transformer from one HV transformer bushing to another. The megger test or insulation test is carried out to ensure the healthiness of the overall insulation system of an electrical power transformer.

To find the insulation resistance between individual windings or from one winding to the ground, insulation resistance tests are conducted. Insulation resistance tests can be computed using measurements of

• Applied voltage &
• Leakage current,

or they can be measured directly in megohms.

When measuring insulating resistance, it is always recommended to ground the tank (as well as the core). At the bushing terminals, short circuit each winding of the transformer. After then, resistance readings are taken between each winding & the ground windings.

For the purpose of measuring insulating resistance, windings never remain floating. The ground must be removed from a winding that is firmly grounded.

in order to gauge the grounded winding’s insulation resistance. The winding insulation resistance cannot be measured if the ground cannot be removed, as in the case of some windings with neutrals that are firmly grounded.

Consider it to be a component of the circuit’s grounded segment.

Testing

• Winding to Winding &
• Winding to Ground (E)

is necessary.

The required minimum value for IR is given by the empirical relation below, with mega ohms (MΩ) serving as the unit. The value measurements give us a concept of the transformer’s insulation strength and whether or not it has deteriorated.

IR_min (in MΩ) = kV + 1

Where,

kV – Rated Service Voltage (in kV). 

We must test the windings (L₁, L₂, L₃) of three-phase transformers using a replacement.

Earthing for delta transformers (or) winding (L₁, L₂, L₃) for wye transformers with neutral (N) and earthing (E).

Where,

C – 1.5 for oil-filled T/C with the oil tank,

C – 30 for oil-filled T/C without oil tank, or Dry Type T/C.

Regulations (or) standards followed for megger testing of transformer

• IEEE 43: Insulation resistance testing guidelines for rotating machinery & transformers.

Procedure of Megger Test/Insulation Resistance Test of Transformer

Step-1: Disconnect the lightning arrestors and jumpers and turn off the transformer.

Step-2: Release the capacitance of the coil.

Step-3: Make sure every bushing is clean.

Step-4: Windings must be short-circuited.

Step-5: To prevent surface leakage over terminal bushings, protect the terminals.

Step-6: Take note of the temperature.

Step-7: Join the test leads (do not join them).

Step-8: When the test voltage is applied, record the reading. The IR.

Step-9: The transformer’s insulation resistance at the test temperature is defined as the value at sixty seconds after the test voltage is applied.

Step-10: During the test, the transformer’s neutral bushing needs to be cut off from the earth.

Step-11: During the test, all LV (Low Voltage) surge diverter earth connections must be unplugged.

Step-12: The insulation resistance reading should not be taken until the test current stabilizes due to the inductive nature of transformers.

Step-13: Whenever the transformer is under vacuum, do not megger.

 In procedure of megger test to be inlcuded

Transformer Coil Insulation Resistance

Transformer Test Connections for IR Testing (Not Less < than 200 MΩ)

Transformer with Two Windings

• (HV + LV) – GND
• HV – (LV + GND)
• LV – (HV + GND)

Transformer with Three Windings

• HV – (LV + TV + GND)
• LV – (HV + TV + GND)
• (HV + LV + TV) – GND
• TV – (HV + LV + GND)

Where,

HV – High Voltage Side

LV – Low Voltage Side

GND – Ground

Auto Transformer with Two Windings

• (HV + LV) – GND

Auto Transformer with Three Windings

• (HV + LV) – (TV + GND)
• (HV + LV + TV) – GND
• TV – (HV + LV + GND)

The measured insulating resistance for any installation cannot be less than:

• (HV – Earth) – 200 MΩ
• (LV – Earth) – 100 MΩ
• (HV – LV) – 200 MΩ

Note:

In three-phase transformers, it is not necessary to do insulation resistance tests on each phase.

Because all of the windings on the HV side are internally connected (interconnected) together to form either a star or a delta, and all of the windings on the LV side are internally connected (interconnected) together to form either a star or a delta, IR values are measured between the windings collectively.

Factors affecting the Transformer’s IR value

Transformers’ IR values are affected by

• Condition of the terminal bushing’s surface
• Oil’s quality
• Winding insulation’s quality
• Oil’s temperature
• The duration of application & test voltage value

Difference Between Megger Insulation Test and HIPOT Test

Megger Insulation Test vs HIPOT Test

What is HIPOT Test?

High Potential test, commonly referred to as a Dielectric Withstand Test, is abbreviated as HIPOT test. It is essentially a test that provides high voltage to equipment to determine the electrical insulation’s ability to survive voltage transients and ensure that the insulation is not inadequate.

Regulations (or) standards followed for megger testing of transformer and their types

• ANSI C57: This series contains a number of transformer-related standards, some of which may obliquely address insulation resistance testing.

Polarization Index Test in Transformer

At the end of 60 and 600 seconds, insulation resistance will be measured and expressed in mega-ohms.

The “POLARISATION INDEX” is the ratio of the second reading measurement to the first reading.

The popular polarization index (PI) test is the simplest application of the time resistance test for solid insulation, requiring only two readings accompanied by a simple division:

The one-minute reading is divided by the ten-minute reading to obtain a ratio.

The result is a pure number that can ordinarily be regarded temperature independent because the thermal mass of the electrical equipment being evaluated is usually so large that the total cooling that occurs during the 10 minutes test is minimal.

A low ratio, in general, indicates little change and thus poor insulation, whereas a large ratio implies the opposite. The use of typical PI values is frequent in research, making this test relatively easy to use.

A test such as the Polarization Index is especially beneficial because it can be done on even the most powerful machines and produces a self-contained assessment based on related readings instead of absolute numbers.

However, no P.I. may be determined with a limited-range tester because “infinity” is not a number.

Advanced testers attain the tera ohm range and so do not deviate from the graph.

The most expensive and cutting-edge capital equipment may be easily tested to produce reproducible data for recording & subsequent trend analysis.

How do you Test Insulation Resistance in a Transformer?

The megohmmeter is attached across the transformer’s windings insulation for this test. A test voltage is supplied for a set amount of time, often 60 seconds & reading is determined.

Only if the winding temperature is higher than the dew point1 should the spot reading test be performed.

Procedure for Megger Testing

• Step-by-Step Procedure: Megger Testing for Transformer Insulation Resistance

Calculation to do Megger Test/Insulation Resistance for Transformer

The Megger test for the transformer (IR) insulation resistance measures current flow, applies a high DC voltage (500V or 1000V) across windings, and calculates resistance using Ohm’s Law. Resistance shows insulation quality; higher resistance implies better insulation.

• Insulation Resistance in Excel: Simplifying Calculations for Safer System