Standard requirements for Insulating Oil

Characteristics	Equipment Voltage	Permissible Limits
Dielectric Strength	145 kV and above	30 kV (Min)
Break down voltage	Below 72.5 kV	40 kV (Min)
	72.5kV and <145kV	50 kV (Min)
Water content	Below 145 kV	35 ppm (Max)
	145 kV and above	25 ppm (Max)
Specific resistance (Ohm-cm) at 90 deg C	All voltages	0.1 x1012 ohm-cm (Min)
Dielectric dissipation factor (tan d) at 90°C	All voltages	0.005 (Max:)
Neutralisation value (Total acidity)	All voltages	0.4 mg KOH/g (Max)
Interfacial tension at 27 deg C	All voltages	0.018 N/M (Min)
Flash point	All voltages	Min.-125°C Max -15 °C decrease of initial value
Sediment or precipitable sludge	All voltages	No sediment or precipitable sludge should be detectable
Dissolved gas analysis	145 kV and above	Refer IS 10593-1993 & DGA study chart

Requirements of Transformer Installations in India

Outdoor

1. Minimum clearance between supplier's and consumer's 11 kV structures shall be 3 m. If it is 5 m or less they should be braced together both at top and middle.

2. Minimum size of RSJ poles shall be 175 x 85 mm. Channels shall be of minimum 75 x 50 mm.

3. Phase to phase and phase to earth clearances at AB switch shall be minimum 915 mm and 610 mm respectively.

4. Minimum size of 11 kV jumpers shall be Number '0' SWG copper.

5. Maximum length of unsupported 11 kV jumpers shall be 2.44 m for solid conductors and 1.5 m for stranded conductors.

6. Minimum safety working clearance of 2.6 m shall be maintained for bare conductors and live parts.

7. Lightning arrestors shall be provided between AB switch and D.O. / H.G. fuse.

8. Group control AB switch shall be provided for multiple transformer installations.

9. The minimum clearance from ground to the 11 kV bushings of the transformers shall be 2.6 m

10. AB switch operating handle shall be fixed at 1 m to 1.25 m from ground level.

11. 11 kV AB switch shall be three insulator type.

12. Fencing with a height of 2.5 m shall be provided, keeping a minimum clearance of 1.5 m from the transformer body.

13. Minimum clearance between 11 kV up and down jumpers shall be 75 cm.

14. Outdoor transformer is not permitted in corporation areas and at the premises of high-rise apartment buildings. However outdoor transformers with cable end boxes at both sides and indoor breaker/SFU at primary side may be permitted judiciously.

15. Indoor transformer is recommended for rating above 630 kVA

16. If more than one transformer, having more than 2000 litres of oil, are installed side by side, baffle walls of minimum 4 hr fire rating shall be provided in between transformers.

Indoor

1. Only dry type transformers shall be installed inside residential/commercial buildings (Rule 64(2)(e)(iv)).

Oil filled transformers are permitted outside residential and commercial buildings subject to the following conditions.

(a) Only indoor type transformers are permitted in Corporation areas and at the premises of residential buildings.

(b) There shall be free access for fire engines to the transformer stations.

(c) The building in which transformer is installed shall have minimum clear distance of 1.5 m from the residential/commercial buildings.

(d) The substation building shall be of weatherproof and fire proof construction. Walls shall be of 355 mm thick brick or 230 mm thick RCC. If there is any window/door opening towards the residential/commercial building, there shall be a fire resistant wall between the substation building and residential/commercial building keeping a minimum clear distance of 1.5 m from both the buildings.

(e) Transformers may be installed outdoor, keeping a minimum distance of 6 m from commercial buildings in areas other than corporation areas.

(f) If the transformer is installed outdoor with in 6m from commercial buildings, there shall be a fire resistant wall in between the transformer and the building keeping a minimum distance of 1.5m both from the building and the transformer.

2. Only dry type transformer shall be installed in cellars and above ground floor. Indian Electricity Rules, 1956 (Rule 64(2) (f) (iii))

3. ABSF unit/VCB shall be provided at primary side. The 11 kV ABSF unit shall have CPRI test certificate and brand certification. At hospital premises breaker shall be recommended.

4. Minimum size of cable at 11 kV side shall be 150 sq.mm XLPE (Aluminium). But size may be 70 sq.mm XLPE (Aluminium) if back up fuse is provided.

5. Fire separation walls shall be provided in between oil filled transformers. Thickness of the wall shall be minimum 30 cm (Brick) or 20 cm (RCC)

6. Oil filled transformers shall be segregated from HT and MV switchgears using fire separation wall

7. Oil drainage facility/soak pits shall be insisted for oil filled transformers if quantity of oil is more than 2000 litres.

8. All-round clearance shall be provided for indoor transformers. It shall be minimum 1.25m if walls are provided at 4 sides.

9. Transformers shall be installed at the periphery of the building for getting easy access and adequate ventilation.

10. Doors and windows of transformer room shall be of non-inflammable materials.

11. Minimum 50 cm headroom clearance shall be provided for the transformer.

12.

a). If dry type transformers are installed above ground level, emergency switching off facility shall be provided at ground floor.

b). Structural fitness certificate from a competent structural engineer shall also be furnished.

13. Minimum phase to phase and phase to earth clearance of bus bars in 11 kV panel shall be 130 mm and 80 mm respectively.

14. Minimum 1.0 metre clear space shall be available in front of HT panel in breaker drawn out position.

Voltage classifications

Electrical engineering terminology is rife with references to such voltages as MV (medium voltage), LV (low voltage) and HV (high voltage). It is often confusing and frequently misleading for new engineers to assess which is which. The problem is made more complicated by the fact there is no unanimous agreement among standard-making bodies regarding this classification. It is typical for an engineer in India to consider its own domestic network (240/415 V) as medium voltage (MV) while that in the U.S. (110 V) as LV. This is far from the truth. Any voltage less than 1 kV is LV according to all institutions making standards. The following table illustrates various categories in the table below.

Authority	Document/ clause ref.	LV	MV	HV
IEC	IEC 60269-1:2006 IEC 62271-200:2003	Less than 1 kV		1 kV to 52 kV
IEEE	IEEE 141:1983 Clause 3.1.1.2	Less than 1 kV	1 kV to 100 kV	100 kV to 230 kV
NEMA	NEMA C84.1:1995 Clause 2.1.3	Less than 1 kV	1 kV to 100 kV	100 kV to 230 kV
India	IS 10118-1:1982 Clause 5.1	Less than 1 kV		Above 1 kV
Shell	DEP 33.64.10.10 Clause 1.3.2	Less than 1 kV	1 kV to 36 kV	Above 36 kV

Capacitors for Domestic Equipments

Running capacitors for domestic equipment

Capacitor (MFD)	Equipment
2.00	Table fan
2.25	Table fan
2.50	Ceiling fan
3.00	Table fan
3.15	Exhaust fan
3.50	Exhaust fan
4.00	Exhaust fan
6.00	Grinder motor
8.00	Washing machine
10.00	Motor 0.375 kW (½ HP)
12.50	Motor 0.375 kW (½ HP)
15.00	Motor 0.375 kW (½ HP)
18.00	Motor 0.75 kW (1 HP)
20.00	Motor 0.75 kW (1 HP)
25.00	Motor 1.125 kW (1½ HP)
30.00	Motor 1.125 kW (1½ HP)
36.00	Motor 1.125 kW (1½ HP) and AC

Starting capacitors for motors

Capacitor	Equipment
40/60	Motor 0.18 kW (¼ HP)
60/80	Motor 0.25 kW (1/3 HP)
80/100	Motor 0.375 kW (½ HP)
100/120	Motor 0.55 kW (3/4 HP)
120/150	Motor 0.75 kW (1 HP)
150/200	Motor 1.125 kW (1½  HP)
200/250	Motor 1.5 kW (2 HP)

Motor Starting Schemes

1. Direct-on-line Starting (DOL)

This is an ideal type of starting, and is simple and economical. In this type the full voltage is applied across the stator windings. The torque developed by the motor is maximum. The acceleration is fast and the heat of starting is low (See figure 1). For heavy rotating masses, with large moments of inertia, this is an ideal switching method. The only limitation is the initial heavy inrush current, which may cause severe voltage disturbances to nearby feeders (due to a large voltage drop in the supply line). With this in mind, even local electricity authorities sometimes restrict the use of DOL starting beyond a certain rating, say, 10 hp, for small installations. For large installations, this condition may be of little significance as, in most cases, the power from the electricity authorities may be available on HT 11-33-110 kV. When it is so, a transformer is provided at the receiving end for the distribution of power on the LT side thus eliminating the cause of a line disturbance due to a voltage dip. On the HT side, the effect of a voltage dip, caused by small motors, is of little significance. However, the method of DOL switching for larger motors is recommended only where the supply source has enough capacity to feed the starting kVA of the motor, with a voltage dip of not more than 5% on the LT side. That's why in large industries, nearly all of the motors are started in DOL.

For a large LT motor, say, 300 hp and above, there is no economical alternative other than DOL starting. One can, however, employ delayed action coupling with DOL starting to start the motor lightly and quickly. Soft starting through a solid-state device or a liquid electrolyte are costly propositions. Autotransformer starting is also expensive due to the cost of its incoming and outgoing control gears and the cost of the autotransformer itself. This is also the case with Star/Delta  starting. Moreover, Auto transformer and Y/Delta startings are reduced voltage startings and influence the starting torque of the motor. It is possible that the reduced starting torque may not be adequate to drive the load successfully and within the thermal withstand time of the motor. Unlike in smaller ratings, where it is easy and economical to obtain a high starting torque characteristic, in large motors, achieving a high torque say, 200% and above, may be difficult and uneconomical.
 

With the availability of large contactors up to 1000 A and breakers up to 6400 A, DOL starting can be used for LT motors of any size, say, even up to 1000 hp. The use of such large LT motors is, however, very rare, and is generally not recommended. Since large electrical installations are normally fed from an HT network, whether it is an industry, residential housing, an office or a commercial complex, DOL switching. even when large LT motors are used, may not pose a problem or cause any significant disturbance in the HT distribution network. The power and control-circuit diagram of a DOL started machine is given.

It is, however, advisable to employ only HT motors, in large ratings. HT motors have little alternative to be switched other than a DOL or a soft starting. Y/D switching in an HT motor is neither advisable nor possible for its windings are normally wound in a star formation to reduce the winding's design voltage and economize on the cost of insulation. Autotransformer switching is possible, but not used, to avoid a condition of open transient during a changeover from one step to another and for economic reasons as noted above. Moreover, on a DOL in HT, the starting inrush current is not very high as a result of low full-load current. For example, a 300 hp LT motor having a rated full-load current (FLC) of 415 A on a DOL will have a starting inrush of approximately 2500 A, whereas a 3.3 kV motor will have an FLC of only 45 A and starting inrush on a DOL of only 275 A or so. An HT motor of 3.3, 6.6 or 11 kV will thus create no disturbance to the HT distribution network on DOL starting.

2. Reduced Voltage Starting

When the power distribution network is available on LT, and the motors are connected on such a system, it becomes desirable to reduce the starting inrush currents, for motors beyond a certain rating, say, 10 hp, to avoid a large dip in the system voltage and an adverse influence on other loads connected on the same system. Sometimes it may also be a statutory requirement of the local electricity authorities for a consumer to limit the starting inrush of motor currents beyond a certain hp to protect the system from disturbances. This requirement can be fulfilled by adopting a reduced voltage starting. Sometimes the load itself may call for a soft start and a smoother acceleration and a reduced voltage switching may become essential. A few common methods to achieve a reduced voltage start are described below.

2.1 Star/Delta (Y/Δ) starting

The use of this starting aims at limiting the starting inrush current, which is now only one third that of DOL starting. This type of starting is, however, suitable for only light loads, in view of a lower torque, now developed by the motor, which is also one third that of a DOL. If load conditions are severe, it is likely that at certain points on the speed-torque curve of the motor the torque available may fall short of the load torque and the motor may stall. See the curves in Figure 3,  showing the variation in current and torque in the star and delta positions and the severe reduction in the accelerating torque. When employing such a switching method precautions should be taken or provision made in the starter to ensure that the windings are switched ON to delta only when the motor has run to almost its full speed. Otherwise it may again give almost a full kick as on a DOL and defeat the purpose of employing a star-delta switching. Figure 3 explains this. The power and control circuit diagrams for a typical fully automatic starter is given in Fig. 4

Limitations
 

1. Due to the greatly reduced starting torque of the motor, the accelerating torque also reduces even more sharply and severely (Figure 3). Even if this reduced accelerating torque is adequate to accelerate the load, it may take far too long to attain the rated speed. It may even exceed the thermal withstand capacity of the motor and be detrimental to the life of the motor. One considers this aspect when selecting this type of switching. To achieve the required performance, it is essential that at every point on the motor speed-torque curve the minimum available accelerating torque is 15-20% of its rated torque. In addition, the starting time must also be less than the thermal withstand time of the motor.

2. This type of switching is limited to only LT system for HT motors are normally wound in star. However, in special cases, HT motors can also be designed for delta at a higher cost to the insulation system which may also call for a larger frame size. Also a provision can be made in the switching device to avoid the condition of an open transient during the changeover from Y to Δ.

3. This type of switching requires six cable leads to the motor as against three for other types of switching to accomplish the changeover of motor windings from Y to Δ.
 

2.2 Auto Transformer Starting

For smoother acceleration and to achieve a still lower starting current than above, this type of switching, although more expensive, may be employed. In this case also the starting current and the torque are reduced in a square proportion of the tapping of the autotransformer. The normal tapping of an autotransformer are 40%, 60% and 80%. At 40% tapping the starting current and the starting torque will be only 16% that of DOL values. At 40% tapping, therefore, the switching becomes highly vulnerable as a result of greatly reduced torque and necessitates a proper selection of motor. The control circuit of a fully automatic autotransformer starter is given in Fig. 5

Since the transformer will be in the circuit for only 15 to 20 seconds, the approximate short-time rating of the transformer can be considered to be 10-15% of its continuous rating. The manufacturer of the auto transformer would be a better judge to suggest the most appropriate rating of the transformer, based on the tapping and starting period of the motor.

2.3 Soft Starting

Soft starting minimizes the starting mechanical and thermal stressed shocks on the machine and the motor. It results in reduced maintenance cost, fewer breakdowns and hence longer operating life for both. Reduced starting current is an added advantage. In a solid-state static switching device the voltage can be varied smoothly to any required value from high to low or low to high without creating a condition of open transient. For HT motors particularly and large LT motors generally, it provides a more recommended alternative over an autotransformer or a Y/Δ starting.