Specification Checklist: Limiting Values for Substation Components

July 8, 2025

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Specification Checklist: Limiting Values for Substation Components

It is essential to have a thorough understanding of the allowable test limits for substation electrical equipment and to adhere to them in order to ensure that the operation is both reliable and safe.

This organized post that follows provides a description of standard test parameters and permitted values for a wide range of substation components.

These parameters and values are based on national & international standards.

1). Transformers/Reactors

1.a). Transformer Oil Quality Parameters

Test Parameter	Condition	Limit	Reference
BDV (Breakdown Voltage)	First Charging	≥ 60 kV (2.5 mm gap)	IS 1866
	During O&M	≥ 50 kV (2.5 mm gap)	IS 1866
Moisture Content	First Charging	≤ 15 ppm	IS 1866
	During O&M	≤ 25 ppm	IS 1866
Resistivity @ 90°C	–	≥ 0.1 x 10¹² Ohm-cm	IS 1866
Acidity	–	≤ 0.2 mg KOH/g	IS 1866
Interfacial Tension (IFT) @ 27°C	–	≥ 0.018 N/m	IS 1866
Tan Delta @ 90°C	–	≤ 0.20	IS 1866
Flash Point	–	≥ 126°C	IS 1866

1.b). Reactor Vibration Levels

Parameter	Limit	Reference
Peak-to-Peak	≤ 200 microns	IS 1876
Average	≤ 60 microns	IS 1877

1.c). Other Transformer Parameters

Parameter	Limit	Reference
Tan Delta (Bushing @ 20°C)	≤ 0.007	IEC 137
Capacitance Variation (Bushing)	±5%	IEC 138
IR Value (Winding)	≥ 1000 MΩ @ 5/10 kV	IEC 139
Tan Delta (Winding @ 20°C)	≤ 0.007	IEEE C57.12.90
Contact Resistance (Bushing Terminal)	≤ 10 µΩ per connector	NGC UK
CT Ratio Error (Neutral CT)	≤ 39	IS 2705

2). Circuit Breakers

2.a). SF₆ & Air Systems

Parameter	Condition	Limit	Reference
Dew Point (SF₆ Gas)	See detailed table below	Varies	–
Dew Point (Operating Air)	–	≤ -45°C @ atmospheric pressure	–

2.b). Circuit Breaker Timing (Operating)

Voltage Level	Closing Time	Trip Time	Phase Discrepancy	Break-Break Time
400 kV	≤ 150 ms	≤ 25 ms	≤ 3.33 ms	≤ 2.5 ms
220 kV	≤ 200 ms	≤ 35 ms	≤ 3.33 ms	≤ 2.5 ms

2.c). Pre-Insertion Resistor (PIR) Timings

Make	PIR Time (ms)	Reference
BHEL	12–16	OEM
ABB, NGEF, M&G, TELK	8–12	OEM
ABB (HVDC)	8–12	OEM

2.d). Contact Characteristics

Parameter	Limit	Reference
PIR-to-Main Contact Overlap	≥ 6 ms	OEM
PIR Opening Time Before Main	≥ 5 ms	OEM
Tan Delta (Grading Capacitor @ 20°C)	≤ 0.007	IEC 359
Capacitance Variation	-2	IEC 359
CB Contact Resistance	≤ 150 µΩ	–
Terminal Connector Resistance	≤ 10 µΩ per connector	NGC UK

2.e). Insulation Resistance (IR)

Test Point	Limit	Reference
Phase to Earth	≥ 1000 MΩ @ 5/10 kV	–
Across Contacts	≥ 1000 MΩ @ 5/10 kV	–
Control Cables	≥ 50 MΩ @ 0.5 kV	–

2.f). Pressure Switch Settings

It is essential that all pressure switches, including SF₆, air, and oil, remain within a range of ±0.1 bar of their predetermined values.

2.g). MOCB Oil BDV

Condition	Limit	Reference
At Filling	≥ 40 kV (2.5 mm gap)	OEM
During O&M	≥ 20 kV (2.5 mm gap)	OEM

3). Current Transformers (CTs)

Parameter	Limit	Reference
IR (Primary to Earth)	≥ 1000 MΩ @ 5/10 kV	–
IR (Secondary to Earth)	≥ 50 MΩ @ 0.5 kV	–
IR (Control Cables)	≥ 50 MΩ @ 0.5 kV	–
Tan Delta (@ 20°C)	≤ 0.007	–
Connector Resistance	≤ 10 µΩ per connector	NGC UK
CT Ratio Error – Protection	±3%	IS 2705
CT Ratio Error – Metering	±1%	IS 2706

4). Capacitive Voltage Transformers (CVTs)

Parameter	Limit	Reference
Tan Delta (@ 20°C)	≤ 0.007	IEC 358
Capacitance Variation	-2	IEC 358
Connector Resistance	≤ 10 µΩ per connector	NGC UK
IR (Primary to Earth)	≥ 1000 MΩ @ 5/10 kV	–
IR (Secondary to Earth)	≥ 50 MΩ @ 0.5 kV	–

4.a). EMU Tank Oil Parameters

Parameter	Limit	Reference
BDV	≥ 30 kV (2.5 mm gap)	IS 1866
Moisture Content	≤ 35 ppm	IS 1866
Resistivity @ 90°C	≥ 0.1 x 10¹² Ohm-cm	IS 1866
Acidity	≤ 0.5 mg KOH/g	IS 1866
IFT @ 27°C	≥ 0.018 N/m	IS 1866
Tan Delta @ 90°C	≤ 1.0	IS 1866
Flash Point	≥ 125°C	IS 1866

4.b). CVT Ratio Errors

Core Type	Tolerance	Reference
Protection	±5%	IEEE C93.1
Metering	±0.5%	IEC 186

5). Isolators

Parameter	Limit	Reference
Contact Resistance	≤ 300 µΩ	–
Terminal Connector Resistance	≤ 10 µΩ per connector	NGC UK
IR (Phase to Earth)	≥ 1000 MΩ @ 5/10 kV	–
IR (Across Contacts)	≥ 1000 MΩ @ 5/10 kV	–
IR (Control Cables)	≥ 50 MΩ @ 0.5 kV	–

6). Surge Arresters

Parameter	Limit	Reference
Leakage Current (Resistive)	≤ 500 µA	Hitachi
IR Value	≥ 1000 MΩ	Hitachi

7). Miscellaneous Parameters

7.a). Earthing

The station earth resistance equals or less than 1 ohm.

7.b). Thermal Scanning

Temperature Rise Above Ambient	Condition
≤ 15°C	Normal
15°C – 50°C	Alert
> 50°C	Immediate Attention Required

7.c). General IR Values

Equipment	Minimum IR
Electric Motors	50 MΩ
Control Cables	50 MΩ
LT Transformers	100 MΩ
LT Switchgear	100 MΩ

8). Batteries

Parameter	Limit	Reference
Terminal Connector Resistance	≤ 10 µΩ + 20%	IEEE 450
Specific Gravity	1200 ± 5 gm/L @ 27°C	–

Temperature Correction for Tan Delta Measurements

Utilize the following formula in order to adjust Tan Delta to 20 degrees Celsius:

Corrected Tan Delta (20°C) = Measured Tan Delta/Correction Factor (K)

Temperature (°C)	K Factor
10	0.8
15	0.9
20	1
25	1.12
30	1.25
35	1.4
40	1.55
45	1.75
50	1.95
55	2.18
60	2.42
65	2.7
70	3

SF₆ Dew Point Limits (Atmospheric Pressure)

Make	Rated Pressure Dew Point	Atmospheric Pressure Dew Point	Remarks
BHEL	-15°C	-36°C	Commissioning
	-7°C	-29°C	O&M
	-5°C	-27°C	Critical
M&G	–	-39°C	Commissioning
	–	-32°C	O&M
CGL	-15°C	-35°C	Commissioning
	-10°C	-31°C	O&M
ABB	-15°C	-35°C	Commissioning
	-5°C	-26°C	O&M
NGEF	-15°C	-36°C	Commissioning
	-7°C	-29°C	O&M

Note: Take note that the dew point varies depending on the pressure that is being measured. If readings are taken at the rated gas pressure, make sure they are adjusted to reflect the pressure of the atmosphere.

Summary

Substation equipment limiting values and test requirements must be followed to ensure electrical asset dependability, safety, and longevity. Regular testing, diagnostics & maintenance based on these standard parameters assist in determining faults, avoid equipment failure, and reduce unplanned outages.

National & international standards including IS, IEC, IEEE, and OEM establish essential test values for transformers, circuit breakers, CTs, CVTs, isolators, surge arresters, and auxiliary systems like batteries and earthing. Understanding and utilizing these values during commissioning, maintenance, and condition monitoring optimizes substation performance.

A planned approach to monitoring these characteristics improves power transmission and distribution network stability, efficiency, asset management, and regulatory compliance.