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Typical Limiting Values of Electrical Substation Equipments

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:

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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.

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