On this page you can find permissible limits for different equipment’s which are part of a power distribution electrical substation. Also where available reference to the international standard is also available in last column. Typical values are provided for transformers, reactors, circuit beakers, current transformers, capacitors, isolators, surge arrestors, batteries and other miscellaneous equipment’s.
1. Transformer / Reactor
| Sr. No. |
Equipment / test data |
Permissible limits |
Reference |
| A) |
Transformer oil |
|
|
| a) BDV |
|
|
| -At the time of first charging |
600 kV (Gap – 2.5 mm) – Minimum |
IS – 1866 |
| -During O&M |
50 kV (Gap – 2.5 mm) – Minimum |
IS – 1867 |
| b) Moisture content |
|
IS – 1868 |
| -At the time of first charging |
15 PPM (Max.) |
IS – 1869 |
| -During O&M |
25 PPM (Max.) |
IS – 1870 |
| c) Resistivity at 90 degree C |
0.1-1012 Ohm-CM (Min.) |
IS – 1871 |
| d) Acidity |
0.2 mg KOH/gm (Max.) |
IS – 1872 |
| e) IFT at 27 degree C |
0.018 N/M (Min.) |
IS – 1873 |
| f) Tan delta at 90 degree C |
0.20 (Max.) |
IS – 1874 |
| g) Flash point |
126 Deg. C (Min.) |
IS – 1875 |
| B) |
Vibration level for reactors |
200 Microns (Peak to Peak) |
IS – 1876 |
|
60 Microns (Average) |
IS – 1877 |
| C) |
Tan delta for bushing at 20 Deg. C |
0.007* |
IEC – 137 |
| D) |
Capacitance for bushing |
+ 5% variation |
IEC – 138 |
| E) |
IR value for winding |
1000 M-Ohm By 5.0/10.0 kV Megger |
IEC – 139 |
| F) |
Tan delta for windings at 20 Deg. C |
0.007* |
IEEE/C57.12.90.1980 |
| G) |
Contact resistance of bushing terminal connectors |
10 M. Ohm / Connector |
NGC.UK Recommendations |
| H) |
Turret Neutral CT ratio errors |
39 |
IS – 2705 |
2. Circuit Breakers
| Sr. No. |
Equipment / test data |
Permissible limits |
Reference |
| A) |
Dew point of SF6 gas |
Dew point values as per Annexure – II |
|
| B) |
Dew point of operating air |
-45 Deg. C at ATM. Pressure |
|
| C) |
CB Operating timings |
400 kV 220KV |
|
| a) Closing time (Max.) |
150 MS 200MS |
|
| b) Trip time (Max.) |
25 MS 35MS |
|
| c) Close/trip time, Pole discrepancy |
|
|
| – Phase to Phase (Max.) |
3.33 MS 3.33MS |
|
| -Break to break (Max.) of same phase |
2.5 MS 2.5MS |
|
| D) |
PIR time |
|
|
| BHEL make |
12-16 MS |
Manufacturers Recommendations |
|
|
|
| ABB make |
8-12 MS |
Manufacturers Recommendations |
| NGEF make |
8-12 MS |
Manufacturers Recommendations |
| M&G make |
8-12 MS |
Manufacturers Recommendations |
| TELK make |
8-12 MS |
Manufacturers Recommendations |
| ABB make (HVDC) |
8-12 MS |
Manufacturers Recommendations |
| E) |
PIR opening time prior to opening of main contacts (ABB, CGL, NGEF make CBs) |
5 MS (Min.) at rated pressure |
Manufacturers Recommendations |
| F) |
Pir and main contacts overlap time [BHEL, M&G, ABB (imported) make CBs] |
6 MS (Min.) at rated pressure |
Manufacturers Recommendations |
| G) |
Tan delta of grading capacitors |
0.007 at 20 Deg. C |
|
| H) |
Capacitance of grading capacitors |
Within + 10% / -5% of the rated value |
IEC 359 |
| I) |
Contact resistance of CB |
150 M. Ohm |
|
| J) |
Contact resistance of CB terminal connector |
10 M. Ohm per connector |
NGC, UK recommendations |
| K) |
IR value: |
|
|
| 1. Phase – earth |
1000 M Ohm (Min.) by 5.0 / 10.0 kV Megger |
|
| 2. Across open contacts |
1000 M Ohm (Min.) by 5.0/10.0 kV Megger |
|
| 3. Control cables |
50 M Ohm (Min.) by 0.5 kV Megger |
|
| L) |
Pressure switch settings |
|
|
| -SF6 gas pressure switches |
Within + 0.1 Bar of set value |
|
| -Operating air pr. Switches |
Within + 0.1 Bar of set value |
|
| -Operating oil pr. Switches |
Within + 0.1 Bar of set value |
|
| M) |
BDV of oil used for MOCB |
|
|
| -At the time of filling |
40 kV at 2.5 mm Gap (Min.) |
Mfgs. Recommendation |
| -During O&M |
20 kV at 2.5 mm Gap. (Min.) |
Mfgs. Recommendation |
3. Current Transformer
| Sr. No. |
Equipment / test data |
Permissible limits |
Reference |
| A) |
IR value |
|
|
| 1. Primary – earth |
1000 M – Ohm (Min.) by 5.0/10.0 kV Megger |
|
| 2. Secondary – earth |
50 M – Ohm (Min.) by 0.5 kV Megger |
|
| 3. Control cables |
50 M-Ohm (Min.) by 0.5 kV Megger |
|
| B) |
Tan delta value |
0.007* at 20 Deg. C |
|
| C) |
Terminal Connector |
10 M-Ohm per connector |
NGC, UK Recommendations |
| D) |
CT ratio errors |
+ 3% -Protection cores |
IS – 2705 |
| D) |
CT ratio errors |
+ 1% -Metering cores |
IS – 2706 |
4. Capacitive Voltage
| Sr. No. |
Equipment / test data |
Permissible limits |
Reference |
| A) |
Tan Delta |
0.007* at 20 Deg. C |
|
| B) |
Capacitance |
Within +10%/-5% of the rated value |
IEC – 358 |
| C |
Contact resistance of terminal connector |
10 M-Ohm per connector |
NGC, UK Recommendations |
| D) |
IR Value |
IR Value |
|
| 1. Primary – earth |
1000 M – Ohm (Min.) by 5.0/10.0 kV Megger |
|
| 2. Secondary – earth |
50 M – Ohm (Min.) by 0.5 kV Megger |
|
| 3. Control cables |
50 M-Ohm (Min.) by 0.5 kV Megger |
|
| E) |
EMU tank oil parameters |
EMU tank oil parameters |
|
| a) BDV (Min.) |
30 kV (Gap. –2.5 mm) |
IS – 1866 |
| b) Moisture content (Max.) |
35 ppm |
-do- |
| c) Resistivity at 90 Deg. C |
0.1 – 1012 Ohm. – CM |
-do- |
| d) Acidity |
0.5 mg kOH /gm (Max.) |
-do- |
| e) IFT at 27 Deg. C |
0.018 N/M (Min.) |
-do- |
| f) Tan delta at 90 Deg. C |
1.0 Max. |
-do- |
| g) Flash point |
125 Deg. C (Min.) |
-do- |
| F) |
CVT voltage ratio errors |
+ 5% protection cores |
IEEE/C93.1.1990 |
| F) |
CVT voltage ratio errors |
+ 0.5% metering cores |
IEC 186 |
5. Isolators
| Sr. No. |
Equipment / test data |
Permissible limits |
Reference |
| A) |
Contact resistance |
300 M-Ohm. (Max.) |
|
| B) |
Contact resistance of terminal connector |
10 M – Ohm per connector |
NGC, UK Recommendations |
| C) |
IR value |
|
|
| 1. Phase – earth |
1000 M – Ohm (Min.) by 5.0/10.0 kV Megger |
NGC, UK Recommendations |
| 2. Across open contacts |
1000 M – Ohm (Min.) by 5.0/10.0 kV Megger |
NGC, UK Recommendations |
| 3. Control cables |
50 M-Ohm (Min.) by 0.5 kV Megger |
|
6. Surge Arrester
| Sr. No. |
Equipment / test data |
Permissible limits |
Reference |
| A) |
Leakage current |
500 M-Amp. (Resistive) |
Hitachi, Japan Recom. |
| B) |
IR value |
1000 M-Ohm. (Min.) |
Hitachi, Japan Recom. |
7. Miscellaneous
| Sr. No. |
Equipment / test data |
Permissible limits |
Reference |
| A) |
Station earth resistance |
1.0 Ohm (Max.) |
|
| B) |
Thermo vision scanning |
|
|
| Temp. up to 15 Deg. C (above ambient) |
Normal |
|
| Temp. above 15-50 Deg. C (above ambient) |
Alert |
|
| Temp. above 50 Deg. C (above ambient) |
To be immediately attended |
|
| C) |
Terminal connectors – Contact resistance |
10 M-Ohm per connector |
HGC, UK Recommendations |
| D) |
IR values |
|
|
| 1. All electrical motors |
50 M-Ohm (Min.) by 0.5 kV Megger |
|
| 2. Control cables |
50 M-Ohm (Min) by 0.5 kV Megger |
|
| 3. Lt. Transformers |
100 M.-Ohm (Min.) by Megger |
|
| 4. Lt. Switchgears |
100 M – Ohm (Min.) by 0.5 kV Megger |
|
8. Batteries
| Sr. No. |
Equipment / test data |
Permissible limits |
Reference |
| A) |
Terminal connector resistance |
10 M – Ohm + 20% |
ANSI/IEEE – 450 1989 |
| B) |
Specific gravity |
1200 + 5 GM/L at 27 Deg. C |
|
Temperature Correction Factor for Tan Delta Measurement
| Sr. No. |
Oil temperature Deg. C |
Correction factor(K) |
| 1 |
10 |
0.8 |
| 2 |
15 |
0.9 |
| 3 |
20 |
1.0 |
| 4 |
25 |
1.12 |
| 5 |
30 |
1.25 |
| 6 |
35 |
1.40 |
| 7 |
40 |
1.55 |
| 8 |
45 |
1.75 |
| 9 |
50 |
1.95 |
| 10 |
55 |
2.18 |
| 11 |
60 |
2.42 |
| 12 |
65 |
2.70 |
| 13 |
70 |
3.00 |
If Tan Delta of bushing/winding/CVT/CT is measured at oil temperature T Deg. C. Then Tan Delta at 20 Deg. C shall be as given below:
Tan Delta at 20 Deg. C = Tan Delta at Temp T Deg. C / Factor K.
Dew Point Limits for SF6 Gas in EHV Circuit Breakers
| Sr. No |
Make of C.B |
Dew point at rated Pr. Deg. C |
Corresponding dew point at Atmospheric .Pr. |
Remarks |
| 1 |
BHEL |
-15 |
-36 |
At the time of commissioning |
| -7 |
-29 |
During O&M |
| -5 |
-27 |
Critical |
| 2 |
M&G |
— |
-39 |
At the time of commissioning |
|
-32 |
During O&M |
| 3 |
CGL |
-15 |
-35 |
At the time of commissioning |
| -10 |
-31 |
During O&M |
| 4 |
ABB |
-15 |
-35 |
At the time of commissioning |
| -5 |
-26 |
During O&M |
| 5 |
NGEF |
-15 |
-36 |
At the time of commissioning |
| -7 |
-29 |
During O&M |
Note: Dew point of SF6 gas varies with pressure at which measurement is carried out. So it is to be ensured that if measurement is done at pressure other than atmospheric pressure, it needs to be converted to the atmospheric pressure.
