Main and Auxiliary Substation Equipment : General Technical Requirements (Articles)

1. INTRODUCTION

The chapter briefly outlines the general technical requirements of the important equipment generally installed in EHV sub-stations.

2 CIRCUIT BREAKERS

Circuit Breaker is a switching device capable of making, carrying and breaking currents under normal circuit conditions and also making, carrying for a specified time breaking currents under short circuit conditions. Circuit breakers of the types indicated below are being presently used in India.

Table- 1

36 kV	- Minimum oil, Vacuum and Sulphur hexa fluoride (SF6)
72.5 kV	- Minimum oil, Air blast and Sulphur hexa fluoride (SF6)
145 kV and 245	- Minimum oil, Air blast and Sulphur hexa fluoride (SF6)
420 kV	- Minimum oil, Air blast and Sulphur hexa fluoride (SF6)
800 kV	- Sulphur hexa fluoride (SF6)

(a) Rated Operating Sequence (Duty Cycle)

The operating sequence denotes the sequence of Opening and Closing operation which the breaker can perform. The, operating mechanism experiences severe mechanical stresses during the auto re-closure duty. The circuit breaker should be able to perform the operating sequence as below.

(i) O-t-CO-T-CO

CO - Closing followed by opening

t - 0.3 Sec. for rapid or auto re-closures T - 3 minutes

(ii) CO-t - CO where t = 15 sec. for circuit breaker not to be used for auto-reclosure

Table 2

Rated voltage (kV)	Rated short circuit breaking current (kA)	Rated normal current (Amp.)
36	8	630
	12.5	630		1250
	16	630		1250	1600
	25			1250	1600		2500
	40			1250	1600		2500
72.5	12.5		800	1250
	16		800	1250
	20			1250	1600	2000
	31.5				1600	2000
145	12.5		800	1250
	20			1250	1600	2000
	25			1250	1600	2000
	31.5			1250	1600	2000
	40				1600	2000		3150
245	20			1250
245	31.5			1250	1600	2000
	40				1600	2000		3150
420	31.5				1600	2000
	40				1600	2000		3150
	50					2000		3150	4000
	63							3150	4000
800	40					2000		3150

(b) Total Break Time (As per IEC: 62271-100)

72.5 KV	60 ms to 100 ms
145 Kv	60 ms to 100 ms
245 kV	Not exceeding 60 ms
420 kV	Not exceeding 40 ms
800 kV	Not exceeding 40 ms

Pre-insertion resistor, if required shall normally have following values. However, precise value shall be decided based on transient over voltage studies.

420 kV	300-450 ohms
800 kV	300-400 ohms

(c) Operating Mechanism

The circuit breaker may be operated by anyone of the following operating mechanisms or a combination of them:

(a) Pneumatically operated mechanism

(b) Spring operated mechanism

3. DISCONNECT SWITCHES/ISOLATORS AND EARTHING SWITCHES

Disconnect switches are mechanical devices which provide in their open positions, isolating distances meeting the specified requirements. A disconnect switch can open and close a circuit when either a negligible current has to be broken or made or when no significant change in voltage across the terminals of each pole of the disconnect switch occurs. It can also carry currents under normal circuit conditions and carry for a specified time the short circuit currents. Disconnect switches are used for transfer of load from one bus to another and also to isolate equipment for maintenance.

The location of disconnect switches in substations affects not only the substation layouts but maintenance of the disconnect contacts also. In some substations, the disconnect switches are mounted at high positions. Although such substations occupy smaller areas, the maintenance of disconnect switches in such substations is more difficult and time consuming.

Earthing switch is a mechanical switching device for earthing parts of a circuit, capable of withstanding for a specified time short-circuit currents, but not required to carry normal rated currents of the circuit.

Various types of disconnect switches presently being used are given below:

36 kV	Horizontal Double Break
72.5 kV	Horizontal Double Break/ Center Break
145 kV	Horizontal Double Break/ Center Break
245 kV	Horizontal Double Break/ Center Break
420 kV	Horizontal Center Break/Pantograph, Double Break
800 kV	J Vertical Break

4. INSTRUMENT TRANSFORMERS

Instrument transformer is device used to transfer the current and voltage in the primary system to values suitable for the necessary instruments, meters, protective relays etc. They also serve the purpose of isolating the primary system from the secondary system.

Current transformer may be either of the bushing type or wound type. The bushing types are normally accommodated within the turret of main transformer and the wound types are invariably separately mounted. The location of the current transformer with respect to associated circuit breaker has an

important bearing upon the protection scheme as well as layout of substation.

The voltage transformer may be either of the electro- magnetic type or the capacitor type. The electro-magnetic type VTs are commonly used where higher accuracy is required as in the case of revenue metering. For other applications capacitor type is preferred particularly at voltages above 132 kV due to lower cost and it also serves the purpose of a coupling capacitor for the carrier equipment. For ground fault relaying, an additional core or a winding is required in the VTs which can be connected in open delta. The voltage transformers are connected on the feeder side of the circuit breaker. However, another set of voltage transformer is normally required on the bus-bars for synchronization.

Typical ratings for instrument transformers normally used are given below: (a) Current Transformer

1	Nominal system voltage	765 kV	400 kV	220 kV	132 kV	66 kV		33 kV
2.	Highest system voltage	800 kV	420 kV	245 kV	145 kV	72.5 kV		36 kV
3.	Frequency	50 Hz	50 Hz	50 Hz	50 Hz	50 Hz		50 Hz
4.	Basic insulation level (kV peak)	2100	1425	1050	650	330		170
5.	Power frequency withstand strength	830	630	460	275	140		70
6.	Rated primary current	3000- 2000- 1000 A	2000- 1000- 500 A	800A/ 600A	800A/ 600A	400A/ 200A/		800A/ 400A/ 200A
7.	Rated burden for metering	20 VA	20 VA	20 VA	20 VA	20 VA		20 VA
8.	Rated short time current for 1 sec.	40kA	40 kA	40 kA	31.5 kA	31.5 kA		25 kA
9.	Secondary current amps.	1	1	1	1	1		1
10.	No. of cores	5	5	5	3	3		3
11.	Maximum temperature rise over design ambient temp	As per IEC : 60044-1
12.	Type of insulation	Class A
13.	Instrument safety factor	<10	<10	<10	<10	<10	<10		<10
14.	Class of accuracy (a) Metering Core (b)Protection Core	0.2 3P	0.2 3P	0.2 3P	0.2 3P	0.2 3P	0.2 3P		0.2 3P

(b) Voltage Transformers

1.	Type	Single phase, oil filled, Natural oil cooled
2.	Nominal system voltage	220 kV	132 kV	66 kV	33 kV
3.	Highest system voltage	245 kV	145 kV	72.5 kV	36 kV
4.	Insulation leve (a) Rated one min. Power Frequency withstand Voltage kV (rms) HV Terminal to earth	460	275	140	70
	(b) Impulse withstand voltage (1.2/50 micro sec. wave shape) kV (Peak)	1050	650	325	170
5.	Over voltage factor (a) Continuous (b) 30 sec.	1.2 1.5	1.2 1.5	1.2 1.5	1.2 1.5
6.	No. of secy. winding	Three	Three	Three	Three
7.	Voltage ratio	220 kV/ 3	132kV/3	66 kV/ 3	33 kV/ 3
7.	Voltage ratio	110 V/ 3	110 V/ 3	110 V/ 3	110 V/ 3
8.	Rated burden (not less than) (a) Core I (Metering) (b) Core II (Protection) (c) Core III (Open Delta	100/50 VA 100/50VA 100/50 VA	100/50VA 100/50VA 100/50VA	100/50VA 100/50VA 100/50VA	100/50VA 100/50VA 100/50VA
9.	Connection	Y/Y/open delta
10.	Class of accuracy
	(a) Core I (Metering)	0.2 3P 3P	0.2 3P 3P	0.2 3P 3P	0.2 3P 3P
	(b) Core II (Protection)
	(c) Core III (Open Delta)

(c) Capacitor Voltage Transformer

Voltage	765 kV	400 kV	220 kV	132 kV
Transformation ratio	765√ 3 kV 110/√3V	400√3 kV 110/√3V	220√3 kV 110/√3V	132√3 kV 110/√3V
No. of secondary winding	3	3	3	3
Voltage factor	1.2 Continuous & 1.5 for 30 seconds
Rated capacitance	4400/8800 pF	4400 PF/8800 pF	4400 pF	4400 pF
Rated burden	50 VA	100 V A/50 VA	IOOVA/50 VA	100 V A/50 VA
Insulation Level (a) Rated one minute power frequency with stand voltage kV (rms)	830	630	460	275
(b) Impulse withstand voltage (1.2/50) micro second wowe shaped kV (Peak)	2100 1550	1425 1050	1050 -	650 -
(e) Switching Impulse withstand voltage (250/2500 Microv micro secs	1550	1050	-	-
Class of accuracy (a) Core I (Metoring) (b) Core 11(Protection) (e) Core III (open Delta)	0.2 3P 3P	0.2 3P 3P	0.2 3P 3P	0.2 3P 3P

5. TRANSFORMERS

General technical requirements of the transformers presently being used are given below:

33 kV Power Transformers

Three Phase Rating MV A	Voltage Ratio	Cooling
1.0	33/11	ONAN
1.6	33/11	ONAN
3.15	33/11	ONAN
4.0	33/11	ONAN
5.0	33/11	ONAN
6.3	33/11	ONAN
8.0	33/11	ONAN
10.0	33/11	ONAN

Vector Group: Dyll

66 kV Power Transformers

Three Phase Rating MV A	Voltage Ratio	Cooling
6.3	66/11	ONAN/ONAF
8.0	66/11	ONAN/ONAF
10.0	66/11	ONAN/ONAF
12.5	66/11	ONAN/ONAF
20.0	66/11	ONAN/ONAF

Vectcr Group: YyO

145 kV Power Transformers

Three Phase Rating MVA	Voltage Ratio	Impedance Voltage (Percent)	Cooling
Two Winding
20	132/33	10	ONAN/ONAF
40	132/33	10	ONAN/ONAF

Vector Group: YNynO or YNdl1

245 kV Power Transformers

(A)Two Winding
			ONAN/OF AF
50	220/66 kV	12.5	or ONAN/ODAF
			ONAN/OF AF
			or ONAN/ODAF
	220/66 kV	12.5	ONAN/OFAF
100			or ONAN/ODAF
100	220/33 kV	15.0

(B) Interconnecting Auto Transformers
35,50	220/33	10	ONAN/ONAF ONAN/ONAF ONAN/ONAF/OF AF Or ONAN/ONAF/ODAF ONAN/ONAF/ODAF or ONAN/ONAF/ODAF ONAN/ONAF/OF AF or ONAN/ONAF/ODAF
50	220/132	10
100	220/1 32	12.5
160	220/132	12.5
200	220/1 32	12.5

Vector Group: YNaodl1

Auto Transformers (420 kV voltage level) (Constant Percentage Impedance)

Three-Phase HV/IV/LV	Voltage Ratio	Tapping Range percent	Per Cent	Impedance	Voltage	Cooling
MVA			HV-IV	HV-LV	IV-LV
100/1 00/33.3	400/132/33	+ 10% to -10% 16 steps of 1.25%	12.5	27	12	ONAN/ONAF
200/200/66.7	400/132/33	+10% to -10% 16 steps of 1.25%	12.5	36	22	ONAN/ONAF Or ONAN/ONAF
250/250/83.3	400/220/33	+10% to -10% 16 steps of 1.25%	12.5	45	30	ONAN/ONAF Or ONAN/ONAF
315/315/1 05	400/220/33	+10% to -10% 16 steps of 1.25%	12.5	45	30	ONAN/ONAF Or ONAN/ONAF
500/500/166.7	400/220/33	+10% to -10% 16 steps of 1.25%	12.5	45	30	ONAN/ONAF Or ONAN/ONAF
630/630/210	400/220/33	+ 10% to -10% 16 steps of 1.25%	12.5	45	30	ONAN/ONAF Or ONAN/ONAF

Vector Group: YNaodll

Auto Transformers (800 kV voltage level)

Ratings
Three phase rating HV/IV/LV MVA	Voltage Ratio kV	Tapping range (Percent)	Percent	Impedanc e	Voltage	Cooling
Three phase rating HV/IV/LV MVA	Voltage Ratio kV	Tapping range (Percent)	HV/IV	HV/LV	IV/LV
315/315/105	765/220/33	+4.5% -7.5% 24 steps	12.5	40	25	ONAN/OFAF or ONAN ODAF or ODAF
630/630/210	765/400/33	-do-	12.5	60	40	-do-
750/750/250	-do-	-do-	-do-	-do-	-do-	-do-
1000/1 000/333.3	-do-	-do-	14.0	65	45	-do-
1500/1500/500	-do-	-do-	-do- tolerance	-do- ±10%	-do- ±15%	-do- ±15%

Vector Group: YNaodll

6 PROTECTION AGAINST LIGHTNING

A substation has to be shielded against direct lightning strokes either by provision of overhead shield wire/earthwire or spikes (masts).

Typical technical parameters adopted for surge arrestors are as

follows:

Sl.No.	Item	765 kV	400 kV	220 kV	132 kV	66 kV
1.	System voltage kV	765	400	220	132	66
2	Highest system voltage kV	800	420	245	145	72.5
3.	Rated voltage Arrestor kV	624	390/360/336	198/216	120	60
4.	Nominal discharg current	20kA	----------------10kA-----------
5.	Class	Class 5	Class 3	Class 3	Class 3
6.	Pressure relief class	--------------------A--------------------
	Pressure relief class	--------------------A--------------------

7. INSULATORS

The creepage distances for the different pollution levels are provided according to the following table:

Pollution level	Creepage distance (mm/kV)
Light	16
Medium	20
Heavy	25
Very Heavy	31

For determining the creepage distance requirement, the highest line-to-line voltage of the system forms the basis.

The following types of insulators are normally used:

(A) Support Insulators:

(i) Cap and pin type

(ii) Solidcore type

(iii) Polycone type

(B) Strain Insulators:

(i) Disc insulators

(ii) Long rod porcelain insulators

(iii) Polymer insulators

8. PROTECTION

(A) Line Protection

(i) 400 kV Lines

Generally two independent high speed main protection schemes called Main-I and Main-II with atleast one of them being carrier aided non-switched three zone distance protection are adopted. The other protection may be a phase segregated current differential (this may require digital communication) phase comparison, directional comparison type or a carrier aided non-switched distance protection. Further, if Main-I

and Main II are both distance protection schemes, then they should be preferably of different type. However, they need not necessarily be of different make. Both the protections should be suitable for single and three phase tripping. In addition to the above following shall also be provided:

(i) Two stage over-voltage protection.

(ii) Auto reclose relay suitable for I ph/3 ph reclosure.

(iii) Sensitive IDMT directional Overcurrent E/F relay.

(ii) 220 k V Lines

There should be atleast one carrier aided non-switched three zone distance protection scheme. In addition to this another non-switched/switched distance scheme or directional over current and earth fault relays should be provided as back up. Main protection should be suitable for single and three phase tripping. Additionally, auto-reclose relay suitable for I ph/3 ph (with dead line charging and synchro check facility) reclosure shall be provided. In case of both line protections being Distance Protections, IDMT type E/F relay shall also be provided additionally.

(B) Bus bar Protection

Bus bar protection is required to be provided for high speed sensitive clearance of bus bar faults by tripping all the circuit breakers connected to faulty bus.

(C) Transformer Protection

Generally following protective and monitoring equipment for transformers of 400 kV and 220 kV class are provided:

(i) Transformer differential protection

(ii) Overfluxing protection

(iii) Restricted earth-fault protection

(iv) Back-up directional O/C + E/F protection on HV side

(v) Back-up directional O/C + E/F protection on LV side

(vi) Protection and monitors built in to Transformer (Buchholz relay, Winding and Oil

Temperature Indicators, Oil Level Indicator, OLTC Oil Surge Relay and Pressure

Relief Device)

(vii) Protection for tertiary winding

(viii) Overload alarm

(ix) Circulating current Differential Protection (Inter-turn phase fault)

(D) Local Breaker Back-up Protection

In the event of any circuit breaker failing to trip on receipt of trip command from protection relays, all circuit breakers connected to the bus section to which the faulty circuit breaker is connected are required to be tripped with minimum possible delay through LBB protection.

All protections need to be tested periodically for functional operation and record of testing should be provided in the substation for future records.

9. CLEARANCES

Minimum clearances required for substation upto 800 kV voltage level are as follows:

Highest system voltage (kV)	Basic Insulation level (kVp)	Switching impulse voltage (kVp)	Minimum clearances $		Sectional clearances (mm)
			Between Phase And Earth (mm)	Between Phases (mm)
36	170	-	320	320	2800
72.5	325	-	630	630	3000
145	550 650	-	1100 1300	1100 1300	4000 4000
245	950 1050	-	1900 2100	1900 2100	4500 5000
420	1425	1050	3400*	-	6500
		(Ph-E) 1575 (Ph-Ph)	-	4200**
800	2100	1550 (Ph-E) 2550 (Ph-Ph)	6400*	9400**	10300

* Based on Rod-structure air gap.

** Based on Rod-Conductor air gap.

$ These values of air clearances are the minimum values dictated by electrical consideration and do not include any addition for construction tolerances, effect of short circuits, wind effects and safety of personnel, etc.

10. Earthing

Provision of adequate earthing system in a substation is extremely important for safety of the operating personnel as well as for proper system operation and performance of the protection devices. The primary requirements of a good earthing system in a substation are:

(a) The impedance to ground should be as low as possible. In the substations with high fault levels, it should not exceed 1 ohm and in the substations with low fault levels it should not exceed 5 ohms.

(b) The step and touch potentials should be within safe limits.

To meet these requirements, an earthing system comprising an earthing mat buried at a suitable depth below ground, supplemented with ground rods at suitable points is provided in the substation. The non-current-carrying parts of all the equipment in the substation and neutral of the transformer are connected to that earthing mat so as to ensure that under fault conditions, none of these parts is at a potential higher than that of the earthig mat. The ground rods are helpful in maintaining low value of resistance which is particularly important for installations with high system earth fault currents.

All substations should have provision for earthing the following:

(a) The neutral points of equipment in each separate system. There should be independent earth for the different systems. Each of these earthed points should be interconnected with the station earthing mat.

(b) Equipment framework and other non-current carrying parts.

(d) Surge arresters: These should have independent earthing which should in turn be connected to the station grounding grid or earthmat.

Switchyard areas are usually covered with about 10 cm of gravel or crushed rock which increases the safety of personnel against shocks, prevents the spread of oil splashes and aids in weed control. This entails the provision of service roads for movement of vehicles required for carrying the equipment from the switchyard to service bay and back.

Bare stranded copper conductor or copper strip found extensive application in the construction of earth mat in the past. However on account of high cost of copper and the need to economies in the use of copper, current practice in the country is to use mild steel conductor for earth mat.

11. Fire Fighting System

All substations should be equipped with fire fighting systems conforming to the requirements given in IS: 1646-1982 and Fire Protection Manual Part-I issued by Tariff Advisory Committee of Insurance Companies.

The more valuable equipment or areas forming concentrated fire risk should be covered by special fire protective systems. In this class are:

(a) Transformers, both indoor and outdoor;

(b) Oil-filled reactors;

(d) Oil tanks and oil pumps;

(e) Oil, grease and paint stores and

(f) Synchronous condensers.

Although the replacement of bulk-oil and minimum oil circuit breakers by vacuum type and SF6 gas circuit breakers has reduced the risk of fires in electrical installations, considerable risk still exists on account of transformers, reactors, cables etc. which contain combustible insulating materials. It is therefore necessary to provide efficient Fire Protection Systems in the Electrical Installations. Fire Protection System consists of the following:

(i) Fire 'Prevention

(ii) Fire Detection & Annunciation

(iii) Fire Extinguishing

(i) Fire Prevention

Fire prevention is of utmost importance and should be given its due if risk of occurrence of fires has to be eliminated/minimized. The safety and preventive measures applicable for substations as recommended by the relevant authorities must be strictly followed while planning the substations.

All fire fighting equipment and system should be properly maintained. Regular mock drills should be conducted and sub station staff made aware of importance of fire protection and imparted training in proper use of the fire fighting equipment provided in substation I control room.

(ii) Fire Detection and Annunciation

Fire detection if carried out at the incipient stage can help in timely containment and extinguishing of the fire speedily. Detection can either be done visually by the personnel present in vicinity of the site of occurrence or automatically with the use of detectors operating on the principles of fixed temperature resistance variation, differential thermal expansion, rate of rise of temperature, presence of smoke, gas, flame etc. Fire detectors of the following type are usually used:

(i) Ionization type

(ii) Smoke type

(iii) Photoelectric type

(iv) Bimetal type

(v) Linear heat Detection type/Quartzoid bulb type

(iii) Fire Extinguishing

The Fire Extinguishing Systems used for fire protection of the various equipments /building in substations are the following:

(i) Hydrant System

(ii) High Velocity Water Spray System

(iii) Portable Fire Extinguishers

(iv) Fire Buckets.

(a) Hydrant System

This type of Fire Protection System is provided for Buildings.

The system consists of a network of laid MS Pipes fed from storage tank and water hydrant outlets provided at suitable locations. Fire fighting canvas pipes are provided in appropriate cabinets near the hydrants which can be accessed by breaking the glass of the storage unit. The canvas pipes are connected to the hydrants and water can be sprayed on the fire after opening the valve of the hydrant.

(b) High Velocity Water (HVW) Spray System

This type of Fire Protection System is provided for the following types of equipment:

(i) Power Transformers, both auto and multi-winding

(ii) Shunt Reactors

This system is designed on the assumption that one reactor/transformer is on fire at a time. For this assumption, the largest piece of equipment forms the basis.

(c) Portable Fire Extinguishers

The portable fire extinguishers are strategically placed in the control room as well as the switched for easy accessibility and are used for extinguishing small fires or fires in a restricted area.

The following types of portable fire extinguishers are normally used.

(i) Chemical Foam type

(ii) Mechanical Foam type

(iii) Dry Powder cartridge type

(iv) Carbon Dioxide type.

Fire Buckets

These are specially fabricated buckets which filled with river sand and kept in the substation on stands. These buckets are provided with an additional handle on the side so that the sand can be easily sprayed on the fire.

These buckets are used for extinguishing fires on the ground.

Water Supplies

Water for fire fighting purposes should be supplied from the water storage tanks meant exclusively for the purpose. The aggregate storage capacity of these tanks should be equal to the sum of the following:

(i) One-hour pumping capacity of Hydrant System or 135 cum whichever is more

(ii) Half-an-hour water requirement for single largest risk covered by HVW Spray System.

Instrumentation and Control

HVW Spray System should include suitable instrumentation and necessary controls to make the system efficient and reliable. There should be local control panels for each of the pumps individually as also for the operation of deluge valve of the HVW Spray System. There should be a common control panel for the Jockey Pump and Air Compressors. Main annunciation panel should be provided in the control room with provision for repeating some annunciation from the pump house.

A diesel engine operated water pump is also provided for back-up in case electrically operated motor-pumps fail due to interruption in electricity supply.

Filtration/Hot Oil Circulation

· Connect bottom filter valve of tank to inlet point of filter machine.

· Connect top filter valve of tank to outlet of vacuum filter machine and start oil circulation

· The filter outlet temperature should be limited to 60 ~ 70⁰ C.

· Continue filtration for 4 cycles whole oil should be circulated 4 times.

· Oil circuit should include a vacuum chamber in which oil drawn from the transformer is sprayed and the moisture and gases are released from the oil are extracted by the vacuum pump.

· Oil drawn from transformer is passed through a filter press before being admitted to the vacuum chamber to remove impurities.

· A minimum capacity of 6000 litres per hour is recommended for the circulation equipment.

· Cooler connection at inlet shall be kept closed to minimize loss of temperature during circulation. Outlet valve shall be kept open to allow expansion of oil inside the cooler.

· Coolers also shall be included in the hot oil circulation towards the end of the process.

· Drain the oil by simultaneously admitting dry air or nitrogen gas from the top. This is to avoid winding insulation coming in contact with moisture.

· Apply vacuum of 1.0 torr or better and maintain for 12 Hrs. (1 mm of Hg)

· Inject oil under vacuum upto a level of approximately half of the conservator.

· Repeat vacuum/hot oil circulation cycle till required dryness is obtained. The oil temperature shall not increased more than 75⁰ C in any case.

· Normally 3 or 4 cycles of hot oil circulation and evacuation will be sufficient to obtain the required dryness for the insulation.

· Dryness of insulation is determined by measuring insulation resistance of transformer winding.

· Insulation resistance between each pair of windings and also between windings and earth shall be measured by using a 2000 V megger. Readings shall be comparable with the factory test results.

· Direct heating of transformer is not recommended for drying out at site.

· Oil samples shall be tested for moisture content, (below 20/15/10 ppm for 145/220/400 KV class respectively). Break down voltage (more than 60 KV at 2.5 mm gap). Resistivity ( > 10¹² ohm meter) before final oil filling.

· Do not measure insulation resistance when the transformer is under vacuum.

Note : As the temperature of oil rises the megger value drops down upto minimum value and after remaining some hours at minimum value when it starts rising again then it should be understood as the circulation/filtration is complete.

TEMPERATURE OF OIL (⁰C)

Variation of Insulation Resistance with Temperature

Dielectric Strength of Insulating Oil (12.5mm dia. Spheres, 2.5 mm gap)

No.	Nominal Voltage of Transformer	Dielectric Strength of Insulating Oil (KV)
1.	145 KV class and above	More than 50
2.	72.5 KV class to less than 145 KV	More than 40
3.	Less than 72.5 KV class	More than 30

Acid Content of Insulating Oil (By neutralization)

No.	Judgment	Acid Content of Oil (mg KOH/g)
1.	Good	Less than 0.2
2.	Replace or do filtrations	0.3 ~ 0.5
3.	Replace immediately	Above 0.5

Resistivity of Insulating Oil

No.	Judgment	Resistivity of oil at 90⁰ C (Ω - cm)
1.	Good	More than 0.1 x 10 ¹²
2.	Fair	1 x 10 ¹¹ to 0.1 x 10 ¹²
3.	Poor	Less than 0.1 x 10 ¹¹

Water Content

No.	Nominal Voltage of Transformer	Water Content (ppm)
1.	145 KV class and above	20 ppm max.
2.	Below 145 KV class	40 ppm max.

Dielectric Dissipation Factor

No.	Nominal Voltage of Transformer	At 90⁰ C, 40 ~ 60 Hz
1.	145 KV class and above	0.2 max.
2.	Below 145 KV class	1.0 max.

Oil Handling Capacity Rating of Filtering Machine

No.	Rating of T/F	Capacity of oil in T/F Kilo Litre	Oil handling Capacity of machine Ltr./hr
1.	5/8 MVA 33/11 KV	3/5	1000 Ltr/hr.
2.	20/40 MVA 132/33 KV	18/20	4000 Ltr/hr.
3.	100/160 MVA 220/132 KV	45/50	6000 Ltr/hr.
4.	240/315 MVA 400/220 KV	70/90	6000 Ltr/hr.