test on transformer
Post on 14-Apr-2018
243 Views
Preview:
TRANSCRIPT
-
7/30/2019 Test on Transformer
1/13
Test on transformerTesting of Transformers The structure of the circuit
equivalent of a practical transformer is developed earlier.
The performance parameters of interest can be obtained by
solving that circuit for any load conditions. The equivalent
circuit parameters are available to the designer of the
transformers from the various expressions that he uses for
designing the transformers. But for a user these are notavailable most of the times. Also when a transformer is
rewound with different primary and secondary windings the
equivalent circuit also changes. In order to get the
equivalent circuit parameters test methods are heavily
depended upon. From the analysis of the equivalent circuit
one can determine the electrical parameters. But if thetemperature rise of the transformer is required, then test
method is the most dependable one.
There are several tests that can be done on the
transformer :-
1.Winding resistance test2. Polarity Test
3 .Open Circuit Test
4 .Short Circuit Test
5. Dvdf Test
6. Hv Test
7. T.T.R Test
-
7/30/2019 Test on Transformer
2/13
1. Winding resistance test:-
Winding resistance measurements in transformers are
of fundamental importance for the following purposes:- Calculations of the I2R component of conductor
losses.- Calculation of winding temperature at the end of a
temperature test cycle.- As a base for assessing possible damage in the field.
Transformers are subject to vibration. Problems orfaults occur due to poor design, assembly, handing,poor environments, overloading or poor maintenance.Measuring the resistance of the windings assures that
the connections are correct and the resistance
measurements indicates that there are no severemismatches or opens. Many transformers have tapsbuilt into them. These taps allow ratio to be increased
or decreased by fractions of a percent. Any of theratio changes involve a mechanical movement of a
contact from one position to another. These tapchanges will also be checked during a winding
resistance test.Regardless of the configuration either wye or delta,
the measurements are made phase to phase andcomparisons are made to determine if the readings
are comparable. If all readings are within one percentof each other, then they are acceptable. Keep in mindthat the purpose of the test is to check for gross
differences between the windings and for opens in theconnections. The tests are not made to duplicate the
-
7/30/2019 Test on Transformer
3/13
readings of the manufactured device which was testedin the factory under controlled conditions and perhapsat other temperatures
2. Polarity Test:-
transformer polarity test are used two know about the
poalrity of a transformers .for these purpose to
transformer are used and his low voltage winding and
high voltage winding are coonected in parallel to each
other the secondary windings of a transformer which
has commonly high voltage winding fuse are
connected to his secondary side of a transformer a manpoint to be noted here that a wire used for fuse are so
small rating similarly when the polarity of
autotransformer same no current will be flow as a
reason the dont blow. otherwise when the polarity are
different the current will be flow and fuse will be blow
out
-
7/30/2019 Test on Transformer
4/13
3. Open Circuit Test
Method :-
The secondary of the transformer is left open-circuited. A
wattmeter is connected to the primary. An ammeter is
connected in series with the primary winding. A voltmeter
is optional since the applied voltage is same as the
voltmeter reading. Rated voltage is applied at primary.
If the applied voltage is normal voltage then normal fluxwill be set up. As the Iron loss is a function of applied
voltage, normal iron loss will occur. Hence the iron loss is
maximum at rated voltage. This maximum iron loss is
measured using the wattmeter. Since the impedance of the
series winding of the transformer is very small compared to
that of the excitation branch, all of the input voltage isdropped across the excitation branch. Thus the wattmeter
measures only the iron loss. It should be noted that the iron
losses consist of the hysteresis loss and the eddy current
loss. This test only measures the combined loss. Although
the hysteresis loss is less than the eddy current loss, it is not
negligible. The two losses can be separated by driving the
transformer from a variable frequency source since thehysteresis loss varies linearly with supply frequency and the
eddy current loss varies with the square.
Since the secondary of the transformer is open, the
primary draws only no load current which will have
some copper loss. This no load current is very small
and because the copper loss in the primary is
-
7/30/2019 Test on Transformer
5/13
proportional to the square of this current, it is
negligible. There is no copper loss in the secondary
because there is no secondary current.
Current, voltage and power are measured at the
primary winding to ascertain the admittance and powe
factor angle.
Another method of determining the series impedance
of a real transformer is the short circuit test.
Calculation:-
The current is very small.
If is the wattmeter reading then,
The above equation can be rewritten as,
http://en.wikipedia.org/wiki/File:Open_circuit_test.png -
7/30/2019 Test on Transformer
6/13
Thus,
Impedance:-
By using the above equations, and can be
calculated as,
Thus,
or
Admittance:-
The admittance is the inverse of impedance. Therefore,
The conductance can be calculated as,
-
7/30/2019 Test on Transformer
7/13
Hence the suspectance,
or
Here,
is the wattmeter reading
is the applied rated voltage
is the no load current
is the magnetizing component of no load current
is the core loss component of no load current
is the exciting impedance
is the exciting admittance
4.Short Circuit Test:-
-
7/30/2019 Test on Transformer
8/13
The test is conducted on the high voltage (HV) side of
the transformer where the low voltage (LV) side or thesecondary is short circuited. The supply voltage
required to circulate rated current through the
transformer is usually very small and is of the order of
a few percent of the nominal voltage and this voltage
is applied across primary. The core losses are very
small because applied voltage is only a few percentage
of the nominal voltage and hence can be neglected.Thus the wattmeter reading measures only the full load
copper loss.
Procedure:-
http://en.wikipedia.org/wiki/File:Short_Circuit_test.jpg -
7/30/2019 Test on Transformer
9/13
For carrying short circuit test on power transformer:-
1. Isolate the power transformer from service.
2. Remove HV/LV jumps and disconnect neutralfrom earth/ground.
3. Short LV phases and connect these short circuited
terminals to neutral
4. Energise HV side by LV supply.
5. Measure current in neutral, LV line voltages, HV
voltage and HV line currents
Calculations:-
is the Full load copper loss
is the applied voltage
is the rated current
is the resistance as viewed from the primary
is the total impedance as viewed from the primaryis the reactance as viewed from the primary
-
7/30/2019 Test on Transformer
10/13
Analysis:-
If neutral current is near to zero transformer windings
are operational.
If neutral current is higher or equal to line current
between LV phase one of the winding is open.
5. Dvdf Test:-
-
7/30/2019 Test on Transformer
11/13
The DVDF (double voltage double frequency) test is
an overvoltage withstand test. It is performed byapplying a higher than nominal voltage (2 times) to the
transformer for a relatively short period of time
(usually 1 minute). Since a prolonged (significant)
overvoltage will cause the transformer to overexcite,
the volts per hertz are kept constant by applying a
higher than nominal frequency (2 times in this case).
-
7/30/2019 Test on Transformer
12/13
6. Hv Test:-
Seperate source voltage withstand test (High Voltagetests on HV & LV). This test checks the insulation
property between Primary to earth, Secondary to earth
and between Primary & Secondary.
HV high voltage test:-LV winding connected together
and earthed. HV winding connected together and given28 KV ( for 11KV transformer) for 1 minute.
LV high Voltage test :- HV winding connected
together and earthed. LV winding connected together
and given 3 KV for 1 minute.
Equipment used:-High Voltage tester ( 100 KV &3KV).
-
7/30/2019 Test on Transformer
13/13
top related