Insulation Resistance (IR) Values - Index
1.
IR values for Electrical Apparatus & Systems
4. IR Value for Electric Motor
5. IR Value for Electrical
cable and wiring
6. IR Value for Transmission /
Distribution Line
8. IR Value for Substation
Equipment
9. IR Value for Domestic /
Industrial Wiring
Required Precautions
|
Max.
Voltage Rating of Equipment |
Megger
Size |
Min.
IR Value |
|
250 Volts |
500
Volts |
25
MΩ |
|
500 Volts |
1,000
Volts |
100
MΩ |
|
5 KV |
2,500
Volts |
1,000
MΩ |
|
8 KV |
2,500
Volts |
2,000
MΩ |
|
15 KV |
2,500
Volts |
5,000
MΩ |
|
25 KV |
5,000
Volts |
20,000
MΩ |
|
35 KV |
15,000
Volts |
100,000
MΩ |
|
46 KV |
15,000
Volts |
100,000
MΩ |
|
69 KV |
15,000
Volts |
100,100
MΩ |
<
1KV = 1MΩ minimum
>1KV
= 1 MΩ / 1KV
As
per IE Rules-1956
At a
pressure of 1000 V applied between each live conductor and earth for a period
of one minute the insulation resistance of
HV installations shall be at least 1 Mega ohm or as specified by the Bureau of
Indian Standards.
Medium
and Low Voltage Installations- At a pressure of 500 V applied between each live
conductor and earth for a period of one minute, the insulation resistance of
medium and low voltage installations shall be at least 1 Mega ohm or as
specified by the Bureau of Indian Standards] from time to time.
As
per CBIP specifications, the acceptable values are 2 Mega ohms per KV
Insulation
resistance tests are made to determine insulation resistance from individual
windings to ground or between individual windings. Insulation resistance tests
are commonly measured directly in megohms or may be calculated from
measurements of applied voltage and leakage current.
The recommended practice in measuring insulation resistance is to always ground the tank (and the core). Short circuit each winding of the transformer at the bushing terminals. Resistance measurements are then made between each winding and all other windings
Insulation resistance testing: HV – Earth and HV – LV
Transformer windings are never left
floating for insulation resistance measurements. Solidly grounded winding must
have the ground removed in order to measure the insulation resistance of the
winding grounded. If the ground cannot be removed, as in the case of some windings
with solidly grounded neutrals, the insulation resistance of the winding cannot
be measured. Treat it as part of the grounded section of the circuit.
We
need to test winding to winding and winding to ground (E). For three phase
transformers, we need to test winding (L1, L2, L3) with substitute Earthing for
Delta transformer or winding (L1, L2, L3) with earthing (E) and neutral (N) for
wye transformers.
|
IR
Value for Transformer (Ref:
A Guide to Transformer Maintenance by. JJ.
Kelly. S.D Myer) |
|
|
Transformer |
Formula |
|
1 Phase Transformer |
IR Value (MΩ) = C X E / (√KVA) |
|
3 Phase Transformer (Star) |
IR Value (MΩ) = C X E (P - n) / (√KVA) |
|
3 Phase Transformer (Delta) |
IR Value (MΩ) = C X E (P - P) / (√KVA) |
|
Where C= 1.5 for oil filled T/C with Oil
Tank, 30 for Oil filled T/C without Oil Tank or Dry Type T/C |
|
Temperature
correction Factor (Base 20°C):
|
Temperature
correction Factor |
||
|
°C |
°F |
Correction
Factor |
|
0 |
32 |
0.25 |
|
5 |
41 |
0.36 |
|
10 |
50 |
0.50 |
|
15 |
59 |
0.720 |
|
20 |
68 |
1.00 |
|
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= (1.5 x 20000) / √1600 =
16000 / 40 = 750 MΩ at 20°C
·
IR Value at LV Side= (1.5 x 400) / √1600 =
320 / 40 = 15 MΩ at 20°C
·
IR Value at 30°C = 15 x 1.98 = 29.7 MΩ
Insulation
Resistance of Transformer Coil
|
Transformer
Coil Voltage |
Megger
Size |
Min.
IR Value Liquid Filled T/C |
Min.
IR Value Dry Type T/C |
|
0 – 600 V |
1
KV |
100
MΩ |
500
MΩ |
|
600 V to 5 KV |
2.5
KV |
1,000
MΩ |
5,000
MΩ |
|
5 KV to 15 KV |
5
KV |
5,000
MΩ |
25,000
MΩ |
|
15 KV to 69 KV |
5
KV |
10,000
MΩ |
50,000
MΩ |
Insulation
Resistance of Transformer Coil
|
Voltage |
Test
Voltage (DC) LV
Side |
Test
Voltage (DC) HV
Side |
Min.
IR Value |
|
415 V |
500
V |
2.5
KV |
100
MΩ |
|
Up to 6.6 KV |
500
V |
2.5
KV |
200
MΩ |
|
6.6 KV to 11 KV |
1000
V |
2.5
KV |
400
MΩ |
|
11 KV to 33 KV |
1000
V |
5
KV |
500
MΩ |
|
33 KV to 66 KV |
1000
V |
5
KV |
600
MΩ |
|
66 KV to 132 KV |
1000
V |
5
KV |
600
MΩ |
|
132 KV to 220 KV |
1000
V |
5
KV |
650
MΩ |
Steps for
measuring the IR of Transformer:
·
Shut down the transformer and disconnect the
jumpers and lightning arrestors.
·
Discharge the winding capacitance.
·
Thoroughly clean all bushings.
·
Short circuit the windings.
·
Guard the terminals to eliminate surface
leakage over terminal bushings.
·
Record the temperature.
·
Connect the test leads (avoid joints).
·
Apply the test voltage and note the reading.
The IR. Value at 60 seconds after application of the test voltage is referred
to as the Insulation Resistance of the transformer at the test temperature.
·
The transformer Neutral bushing is to be
disconnected from earth during the test.
·
All LV surge diverter earth connections are
to be disconnected during the test.
·
Due to the inductive characteristics of
transformers, the insulation resistance reading shall not be taken until the
test current stabilizes.
·
Avoid meggering when the transformer is under
vacuum.
Test conditions of Transformer for IR Test (Not Less than 200 MΩ)
Two
winding transformers
1. (HV
+ LV) – GND
2. HV –
(LV + GND)
3. LV –
(HV + GND)
Three
winding transformers
1. HV –
(LV + TV + GND)
2. LV –
(HV + TV + GND)
3. (HV +
LV + TV) – GND
4. TV –
(HV + LV + GND)
Auto
transformer (Two windings)
1. (HV +
LV) - GND
Auto
transformer (Three windings)
1. (HV +
LV) – (TV + GND)
2. (HV +
LV + TV) – GND
3. TV –
(HV + LV + GND)
For
any installation, the insulation resistance measured shall not be less than:
·
HV – Earth 200 MΩ
·
LV – Earth 100 MΩ
·
HV – LV 200 MΩ
Factors affecting on IR value of Transformer
The
IR value of transformers are influenced by
·
Surface condition of the terminal bushing
·
Quality of oil
·
Quality of winding insulation
·
Temperature of oil
·
Duration of application and value of test
voltage
·
IR between HV and LV as well as windings to
earth.
·
Minimum IR value for Tap changer is 1000 ohm
per volt service voltage.
For
electric motor, we used a insulation tester to measure the resistance of motor
winding with earthing (E).
·
For rated voltage below 1KV, measured with a
500VDC Megger.
·
For rated voltage above 1KV, measured with a
1000VDC Megger.
·
In accordance with IEEE 43, clause 9.3, the
following formula should be applied.
·
Min IR value (For Rotating Machine) = (Rated
voltage (v) / 1000) + 1
Insulation resistance
(IR) value for electric motor
|
As
per IEEE 43 standard 1974, 2000 |
|
|
IR
Value in MΩ |
|
|
IR (Min) = kV+1 |
For most windings made
before about 1970, all field windings, and others not described below |
|
IR (Min) = 100 MΩ |
For most dc armature and
ac windings built after about 1970 (form wound coils) |
|
IR (Min) = 5 MΩ |
For most machines with
random – wound stator coils and form-wound coils rated below 1kV |
Example
- 1: For 11KV, Three Phase Motor.
·
IR Value = 11+1=12 MΩ but as per IEEE43 It
should be 100MΩ
Example
- 2: For 415V, Three Phase Motor.
·
IR Value = 0.415+1=1.41 MΩ but as per IEEE43
It should be 5MΩ
·
As per IS 732 Min IR Value of Motor =
(20XVoltage(p-p/(1000+2XKW)
IR Value of Motor as per NETA ATS 2007. Section 7.15.1
|
Motor
Name Plate
(V) |
Test
Voltage |
Min.
IR Value |
|
250 V |
500
V DC |
25
MΩ |
|
600 V |
1000
V DC |
100
MΩ |
|
1000 V |
1000
V DC |
100
MΩ |
|
2500 V |
1000
V DC |
500
MΩ |
|
5000 V |
2500
V DC |
1000
MΩ |
|
8000 V |
2500
V DC |
2000
MΩ |
|
15000 V |
2500
V DC |
5000
MΩ |
|
25000 V |
5000
V DC |
20000
MΩ |
|
34500 V |
15000
V DC |
100000
MΩ |
IR Value of Submersible Motor:
|
As
per IEEE 43 standard 1974, 2000 |
|
|
IR
Value in MΩ |
|
|
New Motor |
20
MΩ |
|
A used motor which can
be reinstalled |
10
MΩ |
|
Motor Installed in Well
(with Cable) |
|
|
New Motor |
2
MΩ |
|
A used motor which can
be reinstalled |
0.5 MΩ |
For
insulation testing, we need to disconnect from panel or equipment and keep them
isolated from power supply. The wiring and
cables need to test for each other ( phase to phase ) with
a ground ( E ) cable. The Insulated Power Cable Engineers Association (IPCEA)
provides the formula to determine minimum insulation resistance values.
R = K x Log 10 (D/d)
R =
IR Value in MΩs per 1000 feet (305 meters) of cable.
K =
Insulation material constant. (Varnished, Cambric =2460, Thermoplastic, Polyethylene
= 50000, Composite Polyethylene=30000)
D = Outside diameter of conductor insulation for single conductor
wire and cable (D = d + 2c + 2b diameter of single conductor cable)
d – Diameter of conductor
c – Thickness of conductor insulation
b – Thickness of jacket insulation
HV test on new XLPE cable (As per ETSA Standard)
|
Application |
Test
Voltage |
Min.
IR Value |
|
New cables – Sheath |
1
KV DC |
100
MΩ |
|
New cables – Insulation |
10
KV DC |
1000
MΩ |
|
After repairs – Sheath |
1
KV DC |
10
MΩ |
|
After repairs – Insulation |
5
KV DC |
1000
MΩ |
11kV and 33kV Cables between Cores and Earth (As per ETSA
Standard)
|
Application |
Test
Voltage |
Min.
IR Value |
|
11kV New cables – Sheath |
5
KV DC |
1000
MΩ |
|
11kV After repairs – Sheath |
5
KV DC |
100
MΩ |
|
33kV no TF’s connected |
5
KV DC |
1000
MΩ |
|
33kV with TF’s connected |
5
KV DC |
15
MΩ |
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