Substation Equipments

Substation equipments

Transformer: transformers in distribution s, step down the primary voltage to lower values suitable for distribution purpose. Three phase oil cooled in delta/star are used. Sometimes 3-winding transformers are also used so that a different supply voltage may be obtained, from tertiary.

Circuit breakers: whenever a fault occurs, the relays sense the fault and send trip signal to the circuit breaker. The circuit breakers disconnect and isolate the faulty section. Oil circuit breakers, air blast circuit breakers, and sulphur hexafluoride circuit breakers are used in power systems

Isolators: whenever maintenance or repair work is  to be carried out on an equipment in a substation , it is disconnected from the supply by isolators. They are normally operated on no load. Isolators are generally interlocked with circuit breakers and earthing switches.

CT/PT: a current transformer steps down the line current for the purpose of relaying and metering. Generally single turn primary current transformers are used. The line or conductor passes through the core and acts as primary. The secondary, having large no. of turns, is wound on the core. The instrument or relay is connected to the secondary. The secondary rated current is either 1A or 5A. Normally 1A is used for metering and 5A is used for protection.

A voltage or potential transformer is used to step down the system voltage to a low value for metering and relaying. It may be an iron core type or capacitive voltage transformer (CVT), the CVT being more commonly used. The rated secondary voltages are 110V or 230V line to line.

Bus bar: the incoming and outgoing circuits are connected to bus bars. These may be either rigid aluminium tubular bus bars supported on post insulators or flexible ACSR stranded conductors bus bars supported from two end by strain insulators.

Protective relays: wherever a fault occurs the protective relay operates and sends a trip signal to circuit breakers. The relays are housed in panel in the control room.

Time graded over current protection is used for feeders. Distance protection and carrier current protection are used for transmission line. Differential protection is used for transformer protection.

Surge arrester: surge arrestor protects the substation equipment from lighting and switching surges. Present day power system invariably uses valve type arrestors.

Earthing switch: the earthing switch is connected between the line conductor and earth and is mounted on the frame of of isolator. Normally it is in open position. When the line is disconnected the earthing switch is closed to discharge the trapped charges to earth.

Station earthing system: the function of station earthing system is to provide a low resistance path for flow of earth fault current for safety of equipment and personnel.

Shunt capacitors: they supply the reactive power to the loads and thus help in improving the power factor. They are switched on during pick load conditions and switched off during opp-peak condition. Shunt capacitor banks are connected in star or delta across the tertiary winding of the transformers.

Station battery and charging current: a substation needs DC supply for protection and control purposes. This supply is obtained from batteries. 110Vdc supply is used for medium size substations while 240V supply is used for large substations. The batteries are equipped with charging equipment.

Typical Limiting Values of Sub Station Electrical Equipments.

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.