Transformers Insulation Resistance Measurement

This test is performed at or above rated voltage to determine if there are low resistance paths to ground or between winding to winding as a result of winding insulation deterioration. The test measurement values are affected by variables such as temperature, humidity, test voltage, and size of transformer.
This test should be conducted before and after repair or when maintenance is performed. The test data should be recorded for future comparative purposes. The test values should be normalized to 20°C for comparison purposes. The general rule of thumb that is used for acceptable values for safe energization is 1 MΩ per 1000 V of applied test voltage plus 1 MΩ. Sample resistance values of good insulation systems are shown in Table. The test procedures are as follows:

Typical Insulation Resistance Values for Power and Distribution Transformers


1. Do not disconnect the ground connection to the transformer tank and core. Make sure that the transformer tank and core are grounded.
2. Disconnect all high-voltage, low-voltage, and neutral connections, lightning arresters, fan systems, meters, or any low-voltage control systems that are connected to the transformer winding.
3. Before beginning the test, jumper together all high-voltage bushings, making sure that the jumpers are clear of all metal and grounded parts. Also jumper together all low-voltage and neutral bushings, making sure jumpers are clear of all metal and grounded parts.
4. Use a megohmmeter with a minimum scale of 20,000 MΩ.
5. Resistance measurements are then made between each set of windings and ground. The windings that are to be measured must have its ground removed in order to measure its insulation resistance.
6. Megohmmeter reading should be maintained for a period of 1 min.
Make the following readings for two-winding transformers:
a. High-voltage winding to low-voltage winding and to ground
b. High-voltage winding to ground
c. Low-voltage winding to high-voltage winding and to ground
d. Low-voltage winding to ground
e. High-voltage winding to low-voltage winding

The connections for these tests are shown in Figures 1.1a through e and 1.2a through e for single-phase and three-phase transformers, respectively.

Megohmmeter readings should be recorded along with the test temperature (°C). The readings should be corrected to 20°C by the correction factors shown in Table. If the corrected fi eld test values are one-half or more of the factory insulation readings or 1000 MΩ, whichever is less, the transformer insulation system is considered safe for a hi-pot test.

Figure 1.1 Test connections for insulation resistance of a single-phase transformer. Note: In figure (e) reverse the L and E leads to measure from high-winding to low-winding. 

For three-winding transformers, test should be made as follows:
High to low, tertiary and ground (H-LTG)
• Tertiary to high, low and ground (T-HLG)
• Low to high, tertiary and ground (L-HTG)
• High, low, and tertiary to ground (HLT-G)
• High and tertiary to low and ground (HT-LG)
• Low and tertiary to high and ground (LT-HG)
• High and low to tertiary and ground (HL-TG)

Do not make the megohm test of the transformer winding without the transformer iquid because the values of insulation resistance in air will be much less than in the liquid. Also, do not make the insulation resistance test of the transformer when it is under vacuum because of the possibility of  flashover to ground.
The test connections shown in Figure 1.1a, c, and e are most frequently used. The test connections in Figure 1.1b and d give more precise results.
The readings obtained in the connections in Figure 1.1a and b are practically equal to readings in test connections in Figure 1.1c and d, respectively.

DC voltage testing of electrical equipment

FIGURE 1.2
Test connections for insulation resistance of a three-phase transformer: (a) connection for high winding to low winding to ground; (b) connection for high winding to ground and low winding guarded; (c) connection for low winding to high winding to ground; (d) connection for low winding to ground and high winding guarded; and (e) connection for high winding to low winding.

Acceptable insulation resistance values for dry and compound-filled transformers should be comparable to those for Class A rotating machinery, although no standard minimum values are available.
Oil-filled transformers or voltage regulators present a special problem in that the condition of the oil has a marked influence on the insulation resistance of the windings.
In the absence of more reliable data the following formula is suggested:

where
IR is the minimum 1 min 500 V DC insulation resistance in megohms from winding to ground, with other winding or windings guarded, or from winding to winding with core guarded
C is a constant for 20°C measurements
E is the voltage rating of winding under test
kVA is the rated capacity of winding under test

This formula is intended for single-phase transformers. If the transformers under test is one of the three-phase type, and the three individual windings are being tested as one, then E is the voltage rating of one of the single-phase windings (phase to phase for delta connected units and phase to neutral or star connected units) kVA is the rated capacity of the completed three-phase winding under test.