Transformers
1. What is the colour of silica gel in dry stage and when saturated with moisture in a
breather?
Dry stage – Deep blue.
Saturated stage – Whitish pink.
2. What is the static pressure of diaphragm in the transformer explosion vent?
5 psi.
3. In buchholz relay how the top and bottom mercury switches are connected?
Top mercury switch is connected for alarm and bottom switch for trip.
4. What is the normal value of moisture content allowed in transformer oil?
<10 ppm.
5. What is the vector group of distribution transformer?
Dy11.
6. What are the losses in a power transformer and mention how these losses can be
minimised?
There are two losses in a transformer.
a. Iron losses.
b. Copper losses.
Iron losses – Iron losses constitutes of two losses.
a. Eddy current losses, these are due to the induced emf in the core, which
constitutes a current in the core. These will heats up the core.
Eddy current losses can be minimised by using laminated core immersed in
varnish. This provides a high resistance between the laminations and thus eddy
current in reduced.
b. Hysterisis losses, these are due to the magnetic reversal of current by which there
is friction between molecules of core and heat is generated.
Hysterisis losses can be minimised by selecting proper magnetic material, like
silicon steel.
Copper losses – these losses are due to the resistance of the winding, which is equal
to I2rt (calories). These losses are depends on load. That is the losses are
increased to the square of the load current
I – current through winding.
r – resistance of winding.
t – time duration.
7. Define the percentage voltage regulation and efficiency of a power transformer.
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.
8. Mention the important parts of a power transformer and their purposes.
Conservator:
This allows for shrinkage and swelling of transformer oil. When the oil
is heated up it swells and rises to the conservator. When cools down it goes back to
main tank. Conservator reduces sludge formation of oil because only the oil surface
in conservator is exposed to atmosphere where oil in the main tank is not exposed to
atmosphere.
Breather:
It provides dry sir to conservator when transformer breathes. That is when
there is shrinkage of oil atmospheric air enters conservator through breather. The
moisture is absorbed in breather by silica gel.
Buchholz relay (gas operated):
If there is an initial fault, heating up of core, high
resistance joints heating up by conduction through insulation and supports. There is
heating up of oil, which breaks down and gases are released. This gas actuates the
mechanics in the relay, There by closing contacts of mercury switches for alarm.
Also if there is a short circuit, the buchholz relay will trip the transformer. Also if
there is any leakage of oil through bushing etc and oil level comes down the relay
will give alarm and also will trip the transformer if transformer oil level comes down
the point. Gases can be taken from the relay to identify nature of fault.
Explosion vent: It provided on transformer main tank, provided with two Bakelite
diaphragm which break when the pressure exceeds 5 psi in the transformer tank and
relieve the pressure.
Core:
To provide low reluctance path for the magnetic lines of force. It carries both
the HV and LV windings.
HV Winding:
High voltage is given to HV winding and low voltage is taken from
the LV winding.
LV Winding:
Low voltage is given to LV winding and high voltage is taken from the
HV winding.
Cooling tubes:
These are provided to cool the transformer oil so that the heat of oil
will be given to the atmosphere.
HT bushing:
Carries the HV terminals.
LT bushing:
Carries the LV terminals.
Tap changer: this is provided so that we can get the required voltage out put. There
are two types of tap changer. Online tap changer and off line tap changer.
9. Mention the properties of transformer oil.
Properties of transformer oil are,
a) Colour – pale yellow.
b) Moisture content - <10 ppm.
c) Acidity (KOH/gramme of oil) – <0.1.
d) Dielectric strength – limit value is 45 kV and preferable value is 60 kV.
e) Flash point - 141º C.
f) Inter surface tension – 30 to 40 dynes / cm or 0.3 to 0.4 Newton.
g) Resistivity –
10. How explosion vent works?
Explosion vent is provided on the transformer tank to relieve pressure if the pressure
in the transformer exceeds 5 psi. It is swan neck shaped having two Bakelite
diaphragms. One at top and another at bottom. These break if the static pressure
increases to 5 psi. Wire meshes are provided below the bottom diaphragm and above
the top diaphragm. When there is any breakage due to excess pressure the bottom
wire mesh prevents broken pieces from entering transformer tank and the wire mesh
provided above the top diaphragm protects the diaphragm from any external damage.
There is an oil level indicator provided above the bottom diaphragm. It indicates the
level of oil in the vent if the bottom diaphragm ruptures.
A ruptured diaphragm must be immediately replaced. Also we should check the top
diaphragm for any external damage.
11. Explain the operation of silica gel breather.
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.
12. Explain with diagram the operation of on load tap changer.
Tap changer (ON LOAD type) can be used to increase or decrease transformer
output voltage without break in the voltage to the load.
The tap changer consists of a diverter switch. The odd taps are taken on one side and
even on another side as shown in figure.
The diverter switch is provided so that there will be no break in the supply to the
load and also no cut of transition resistance when the tap changing is achieved.
Transition resistors are provided to limit the current when the windings are shortcircuited
by the diverter switch.
Operation: In the above figure, the voltage at tap 2 is 406 V. the position of diverter
switch is shown. It short-circuited with transition resistance.
We want to increase voltage to 420 V at tap 5. When we begin to change the tap the
diverter switch connects 2 transition resistance and when the tap changeover is
achieved that is when the tap changer reaches tap 5, the diverter switch short circuits
transition resistance and thus the resistance is eliminated. The diverter switch
switching time is very high. This is to reduce arcing, which can decompose the oil.
The whole assembly is immersed in oil. OLTC is connected to HV side, because in
LV side current handled will be more. But in HT side current to be handled is lesser
than LV side.
13. Explain the procedure for finding out the vector group of a transformer
Procedure: Take the nameplate details. See from nameplate what group the
transformer belongs. Suppose nameplate says that transformer belongs to Dy11.
Take the IR value between
a) HV and LV with body grounded. That is between A2 – a2, B2 – b2, C2 – c2.
b) HV and body with LV grounded and
c) LV and body with HV grounded. A2 c2 a2
C2 B2 b2
Connect A2 to a2 and give low voltage (415 V) to HV side. Measure voltage between
a. C2 – b2 410 V (example).
b. C2 – c2 395 V (example).
c. B2 – b2 395 V (example).
d. B2 – c2 395 V (example).
Draw the diagram of Dy11 and check that the readings got are correct.
A2
a2
30º
b2
c2
According to the fig. C2 B2
C2 c2 = B2 b2 = B2 c2.
And C2 b2 will be greater than C2 c2 , B2 b2, B2 c2.
That is C2 b2 >> C c2.
If these conditions are satisfied then that transformer belongs to vector group Dy11.
A2
/ a2
For Dy1 transformer
B2 c2 will be greater than B2 b2, C2 c2, C2 b2.
That is B2 b2 >> B b2. c2
C2
b2 B2
For Yy0 transformer A2/a2
B2 c2 = C2 b2
B b2 = C c2
B b2 and C c2 will be lesser than B2 b2 and C2 b2.
C2/c2 B2/b2
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1. What is the colour of silica gel in dry stage and when saturated with moisture in a
breather?
Dry stage – Deep blue.
Saturated stage – Whitish pink.
2. What is the static pressure of diaphragm in the transformer explosion vent?
5 psi.
3. In buchholz relay how the top and bottom mercury switches are connected?
Top mercury switch is connected for alarm and bottom switch for trip.
4. What is the normal value of moisture content allowed in transformer oil?
<10 ppm.
5. What is the vector group of distribution transformer?
Dy11.
6. What are the losses in a power transformer and mention how these losses can be
minimised?
There are two losses in a transformer.
a. Iron losses.
b. Copper losses.
Iron losses – Iron losses constitutes of two losses.
a. Eddy current losses, these are due to the induced emf in the core, which
constitutes a current in the core. These will heats up the core.
Eddy current losses can be minimised by using laminated core immersed in
varnish. This provides a high resistance between the laminations and thus eddy
current in reduced.
b. Hysterisis losses, these are due to the magnetic reversal of current by which there
is friction between molecules of core and heat is generated.
Hysterisis losses can be minimised by selecting proper magnetic material, like
silicon steel.
Copper losses – these losses are due to the resistance of the winding, which is equal
to I2rt (calories). These losses are depends on load. That is the losses are
increased to the square of the load current
I – current through winding.
r – resistance of winding.
t – time duration.
7. Define the percentage voltage regulation and efficiency of a power transformer.
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.
8. Mention the important parts of a power transformer and their purposes.
Conservator:
This allows for shrinkage and swelling of transformer oil. When the oil
is heated up it swells and rises to the conservator. When cools down it goes back to
main tank. Conservator reduces sludge formation of oil because only the oil surface
in conservator is exposed to atmosphere where oil in the main tank is not exposed to
atmosphere.
Breather:
It provides dry sir to conservator when transformer breathes. That is when
there is shrinkage of oil atmospheric air enters conservator through breather. The
moisture is absorbed in breather by silica gel.
Buchholz relay (gas operated):
If there is an initial fault, heating up of core, high
resistance joints heating up by conduction through insulation and supports. There is
heating up of oil, which breaks down and gases are released. This gas actuates the
mechanics in the relay, There by closing contacts of mercury switches for alarm.
Also if there is a short circuit, the buchholz relay will trip the transformer. Also if
there is any leakage of oil through bushing etc and oil level comes down the relay
will give alarm and also will trip the transformer if transformer oil level comes down
the point. Gases can be taken from the relay to identify nature of fault.
Explosion vent: It provided on transformer main tank, provided with two Bakelite
diaphragm which break when the pressure exceeds 5 psi in the transformer tank and
relieve the pressure.
Core:
To provide low reluctance path for the magnetic lines of force. It carries both
the HV and LV windings.
HV Winding:
High voltage is given to HV winding and low voltage is taken from
the LV winding.
LV Winding:
Low voltage is given to LV winding and high voltage is taken from the
HV winding.
Cooling tubes:
These are provided to cool the transformer oil so that the heat of oil
will be given to the atmosphere.
HT bushing:
Carries the HV terminals.
LT bushing:
Carries the LV terminals.
Tap changer: this is provided so that we can get the required voltage out put. There
are two types of tap changer. Online tap changer and off line tap changer.
9. Mention the properties of transformer oil.
Properties of transformer oil are,
a) Colour – pale yellow.
b) Moisture content - <10 ppm.
c) Acidity (KOH/gramme of oil) – <0.1.
d) Dielectric strength – limit value is 45 kV and preferable value is 60 kV.
e) Flash point - 141º C.
f) Inter surface tension – 30 to 40 dynes / cm or 0.3 to 0.4 Newton.
g) Resistivity –
10. How explosion vent works?
Explosion vent is provided on the transformer tank to relieve pressure if the pressure
in the transformer exceeds 5 psi. It is swan neck shaped having two Bakelite
diaphragms. One at top and another at bottom. These break if the static pressure
increases to 5 psi. Wire meshes are provided below the bottom diaphragm and above
the top diaphragm. When there is any breakage due to excess pressure the bottom
wire mesh prevents broken pieces from entering transformer tank and the wire mesh
provided above the top diaphragm protects the diaphragm from any external damage.
There is an oil level indicator provided above the bottom diaphragm. It indicates the
level of oil in the vent if the bottom diaphragm ruptures.
A ruptured diaphragm must be immediately replaced. Also we should check the top
diaphragm for any external damage.
11. Explain the operation of silica gel breather.
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.
12. Explain with diagram the operation of on load tap changer.
Tap changer (ON LOAD type) can be used to increase or decrease transformer
output voltage without break in the voltage to the load.
The tap changer consists of a diverter switch. The odd taps are taken on one side and
even on another side as shown in figure.
The diverter switch is provided so that there will be no break in the supply to the
load and also no cut of transition resistance when the tap changing is achieved.
Transition resistors are provided to limit the current when the windings are shortcircuited
by the diverter switch.
Operation: In the above figure, the voltage at tap 2 is 406 V. the position of diverter
switch is shown. It short-circuited with transition resistance.
We want to increase voltage to 420 V at tap 5. When we begin to change the tap the
diverter switch connects 2 transition resistance and when the tap changeover is
achieved that is when the tap changer reaches tap 5, the diverter switch short circuits
transition resistance and thus the resistance is eliminated. The diverter switch
switching time is very high. This is to reduce arcing, which can decompose the oil.
The whole assembly is immersed in oil. OLTC is connected to HV side, because in
LV side current handled will be more. But in HT side current to be handled is lesser
than LV side.
13. Explain the procedure for finding out the vector group of a transformer
Procedure: Take the nameplate details. See from nameplate what group the
transformer belongs. Suppose nameplate says that transformer belongs to Dy11.
Take the IR value between
a) HV and LV with body grounded. That is between A2 – a2, B2 – b2, C2 – c2.
b) HV and body with LV grounded and
c) LV and body with HV grounded. A2 c2 a2
C2 B2 b2
Connect A2 to a2 and give low voltage (415 V) to HV side. Measure voltage between
a. C2 – b2 410 V (example).
b. C2 – c2 395 V (example).
c. B2 – b2 395 V (example).
d. B2 – c2 395 V (example).
Draw the diagram of Dy11 and check that the readings got are correct.
A2
a2
30º
b2
c2
According to the fig. C2 B2
C2 c2 = B2 b2 = B2 c2.
And C2 b2 will be greater than C2 c2 , B2 b2, B2 c2.
That is C2 b2 >> C c2.
If these conditions are satisfied then that transformer belongs to vector group Dy11.
A2
/ a2
For Dy1 transformer
B2 c2 will be greater than B2 b2, C2 c2, C2 b2.
That is B2 b2 >> B b2. c2
C2
b2 B2
For Yy0 transformer A2/a2
B2 c2 = C2 b2
B b2 = C c2
B b2 and C c2 will be lesser than B2 b2 and C2 b2.
C2/c2 B2/b2
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