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Silica gel breather is used in a transformer to provide dry atmospheric air to the conservator when transformer breathes. The breather consists of an inner container and outer container. The inner container contains silica gel, which absorbs moisture. An oil bath in provided at the bottom of breather so that the silica gel will not be in direct contact with the atmosphere. Also it will trap dust and dirt entering the breather. Dry silica gel will be deep blue in colour. After it gets saturated with moisture it will turn into white pink. The change of colour silica gel can be viewed externally through transparent viewer provided on the breather. When the silica gel is saturated with moisture it must be replaced or regenerated or recharged. Silica gel is recharged by heating it to a temperature of 250º F to 300º F till the deep blue colour of silica gel is got back.

a) Temperature rises not exceed 90 ºC. 

b) Withstand short ckt stresses. 

c) Take care of thermal expansion. 

a) To avoid relay mal-operation due to CT saturation

b) Better sensitivity is got. 

c) High pickup and TMS avoided in IDMT earth fault relay.  

I – Trips 6.6 kV breakers only. It gives primary protection for 6.6 kV bus bars. 

I₁ – Trips the both HT and LT breakers. It acts as a backup to ref and also acts as backup to bus bar earthfault relay. 

Voltage regulation is the difference between no load voltage and full load voltage by no load voltage.

Voltage regulation = No load voltage – Full load voltage / No load voltage. 

Voltage regulation is mentioned in % (percentage).

% Voltage regulation = No load voltage – Full load voltage * 100 / No load voltage. 

Efficiency of a transformer is the ratio of output in watts and input in watts.

Efficiency = Output in watts / Input in watts. 

% Efficiency = Output in watts * 100 / Input in watts. 

% Efficiency = Output in watts * 100 / Output in watts + losses. 

To have smooth commutation in generation in between stator and rotor. 

Incase of a heat exchanger tube failure the water should not go inside the transformer. For this purpose the oil pressure is kept above the water pressure. 

a) To facilitate interchange. 

b) To have momentary parallel during changeover. Protections

a) Door interlock to trip HT and LT breakers. 

b) LT breaker can on only after HT breaker is in on position. 

c) Instantaneous O/C and inverse O/C (50 + 51). 

d) Instantaneous E/F (50N). 

e) IDMT E/F and restricted E/F (51N + 64). 

f) Winding temp high trip (140°C trip and 130°C alarm) 

Canned rotor pumps.

In reclaiming process the oil treated to remove all its impurities like acidity, sludge, sediments, moisture etc. The treated oil will be in par with the new oil. In reconditioning process (filtering of oil) only moisture and suspended impurities and sediments are removed. 

Oil is used removal of heat produced in the transformer and also as insulating medium. 

During tap changing action the load current has to be shifted from one tap to another tap. In case HV wining the load current will be less. Hence lesser arcing will take place.

When the voltage is increased and the frequency is reduced the transformer will draw high magnetising current. This will result in higher core loss and subsequent heating of core and ultimate failure of transformer. Hence over fluxing protection is provided for the transformer. 

a) Buchholz relay 

b) Explosion vent or relief valve 

c) Gas operated relay for on load tap changers.

UT and SUT are getting paralleled at 6.6 kV bus. Hence they should have voltage of same phase relationship. This is achieved by assigning different vector group to the transformers. 

a) There are 5 factors are there. 

b) Density

c) Coefficient of thermal expansion

d) Viscosity

e) Specific heat

f) Thermal conductivity.  

Zn O (zinc oxide) types.  

Ensure the differential temperature between top and bottom of transformer is minimum and Effect of ambient air temperature is minimum. 

Since GT is provided with overfluxing protection, it is adequate to protect main generator / UT also. Main generator can withstand higher degree of overfluxing. If a generator CB is used, separate overfluxing protection is essential for main generator. 

ONAN – Natural cooling – up to 15 MVA. 

ONAF – Air forced radiators cooling – 10 to 100 MVA depending on availability of area. 

OFWF = oil forced and water forced used in more than 100 MVA. 

Yd11 

Dy1 

Yy0

a) To limit the earth fault current for equipment safety else, high short ckt forces dislocate in windings/bus bars etc, 

b) Over voltage due to arcing ground reduced 

c) Permits earth fault protection (not possible in ungrounded system) 

a) Less transient over voltage due to arcing grounds. 

b) Voltage of phases are limited to phase to ground voltage. (No neutral shifting) 

c) Allows graded insulation of transformer (low cost) d) Fast E/F protection. 

a) To reduce burning and melting in faulted switchgear or machine. 

b) To reduce mechanical stresses in equipment. 

c) To reduce the electrical hazards by stray ground fault currents in the ground return path. 

d) To reduce momentary line voltage dip due to ground fault. 

e) The current is limited to 400a, that is ¼ ^th of the load current to reduce the size of the screen in 6.6 kV XLPE (cross-linked polyethene). Therefore the cost of the cable decreases. 

No, due to switchgear limitation. 

11 kV was rejected in view of the high insulation cost with 11 kV motors. 3.3 kV was rejected, since max motor size with 3.3 kV bus is limited to 2 MW. But we are having the motors having rating more than 2 MW, which cannot suit to 3.3 kV bus. 6.6 kV bus we can start upto 5 mw size motor. 

Star connected circuit, which has an isolated neutral there can be no zero sequence components. Since the zero sequence components are by definition in time phase with another their sum can not be zero at the junction point as per kirchoff’s law. It follows that there are limitations upon the phase loading of a bank of transformers connected in star – star unless the neutral points are connected to the source of power in such a manner that the zero sequence components of current have a return path or unless the transformer are provided with tertiary winding. 

IS – 1866 FOR MAINTENANCE AND SUPERVISION OF OIL 

IS – 10593 FOR GAS ANALYSIS 

IS – 1886 FOR INSTALLATION AND MAINTENANCE

Oil fire point = 170 ºC easy catching of fire. 

To remove water, sediments, sludge etc. 

On reweaving oil test results. 

Draining of oil for maintenance 

Topping up of transformer oil

To keep required dryness/improve dryness of the transformer insulation, internal part of transformer. When transformer operates, due to pressure head between top and bottom small quantity of oil flows through filters (absorbent material activated alumina grade g-80 removes the moisture from oil). Absorbent material remove slag, acids, peroxides, ionic impurities from oil, which otherwise accelerate against of oil. Absorbent unit is reactivated at regular intervals.

The header breaker ensures oil pressure greater than that of water pressure always. Therefore there is no leak of water into oil.

Infrared camera.  

Flexible separator avoids direct contact with atmosphere. Efficient barrier between oil and air. Ensures the protection against water vapour, suppression of gas bubbles formation in the oil. 

To accommodate the change in volume of oil during increase in temperature. 

This is done to limit the earth fault current. 

400a limited by 10 ohms resistor.  

As per is, the rating single secondary power transformer is limited to 25MVA (6.6 kV) or 40 MVA (11 kV), in order to limit the 3 phase symmetrical fault level with in 26-40 kA (contribution from grid and local machines) 

The LV sides of the two transformers are earthed through the resistance. This will limit the flow of current in case of LV earth fault. Hence the differential protection may not act for a LV earth fault. Hence ref protection is provided.

Covers from 230 kV bushing to 6.6 kV breaker end. 

a) 3 wire to 4 wire conversion since neutral is required for lighting load. 

b) Prevents transfer of E/F currents 

c) Reduces the fault level on secondary side and permits use of small-sized cables / CB’s / fuses.

Supply is maintained even with fault on one line 

Less interference to communication lines because of absence of zero sequence currents.

No fire hazard. 

It can be mounted indoor. 

For true measurement of IR value of HV to earth of a transformer, connect line to HV, earth to transformer tank and guard to LV. Therefore leakage current from HV to LV is not included.  

During internal fault, the internal pressure rise is relieved by the expelling out of oil through Pressure relief device /explosion vent. However the Pressure relief device closes back when the pressure drops. Hence the oil exposure to atmosphere is minimised, thus saving large quantity of costly transformer oil from oxidation and moisture absorption. Fire hazard due to transformer oil does not exist after the closure of Pressure relief device.

a) To reduce insulation cost of tap changer. 

b) But reactance changeover the tap range increases. 

Because our plant works on base load always. 

Dy11 is the vector group of distribution transformer.

When the tap changing takes place arc is struck between the contacts. Due to this the oil inside the tap changer will be highly carbonised. If both oil get mixed up the quality of transformer tank oil will come down. This is not advisable. Hence both oils are kept separately.