Typical Limiting Values of Electrical Substation Equipments
MS Chaudhry
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.