TEMPERATURE-RISE TEST

   
 
PURPOSE OF THE MEASUREMENT
   
 

The purpose is to check that the temperature rises of the active part of the transformer: oil, windings, core, do not exceed the limits agreed on or by the standards.

The test system is different whether we have to carry out the tempeature rise test on one transformer liquid filled (oil or silicone liquid) or dry-type. In fact in the first case we use the short-circuit method while in the second case we use the back-to-back method.
   
 

PERFORMANCE OF TEST ON LIQUID FILLED TRANSFORMERS
 
 
 

As told above, the test is performed by using the short-circuit method. The temperature rise of windings is determined by the resistance method. The supply and measuring facilities as well as the measuring circuit are the same as in the load losses measurement and in the resistance measurement. In addition thermometers are used for the measurement of the temperature of the oil, cooling on different points of the tank and the ambient temperature. The test is performed as follows:.
   
 
Cold resistance measurement
  The resistance and the corresponding oil temperature are measured. Resistances are measured between line terminals e.g., A-B. The winding temperature is the same as oil temperature.
   
 
Determination of oil temperature rise
  The power to be supplied to the transformer is the sum of the no-load losses and the load losses on the nominal tapping on which the temperature-rise test is to be generally performed. With this power the transformer is warmed up to thermal equilibrium. The supply values and the temperatures of different points are recorded at suitable time intervals. The oil temperature rise above the cooling medium temperature can be calculated from the steady state conditions.
   
 
Determination of windings temperature rise
 

Without interupting the supply the current is reduced to rated current for min. 1 h. The supply and the temperatures are recorded as above. When the current has been cut off the hot-resistance measurement is performed. The test connection is changed for carrying out the resistance measurement and after the inductive effects have disappeared the resistance -time-curves are measured for a suitable period of time (zero time is the instant of switching off the supply). The resistance is measured between the same line teminals as in the cold resistance measurement.

The resistance of the windings at shut-down are obtained by extrapolating the resistance-time-curves to the instant of switching off. The temperature rises of the windings above the oil temperature are calculated on the basis of the "hot" and "cold" resistance values and the oil temperature. The temperature rises of the windings above the cooling medium temperature are found by adding the temperature to the before mentioned winding temperature rises.

The ambient temperature is measured by means of three sensors Pt 100 of a digital thermometer, which are placed at different points around the transformer at a distance defined by the standards approximately half-way up the transformer.

Other sensors Pt 100 of the some thermometer are used for:
- Top oil temeprature on an oil filled thermometer pocket on the tank cover.
- Temperatures of oil coming in and going out of the cooling radiators.
   
 
RESULT
 

The temperature rises are calculated as follows:
   
 
Oil temperature rise
  The temperature rise of top oil is:
   
 
(10.1)
PN = rated losses Pk75°C + P0
Pm = power supplied during the test
X = exponent according to the standard = 0.8 (ONAN distribution trafo)
= final temperature rise of oil by temperature rise oil-time curve
   
  The average temperature rise of the oil is:
   
 
(10.2)
= oil temperature going into the radiators
= oil temperature coming from the radiators
= average ambient temperature
   
 
Windings temperature rise
  The average oil temperature before the hot-resistance measurement is:
   
 
(10.3)
= top oil temperature
   
  the average winding temperature is:
   
 
(10.4)
R1 = cold resistance
R2 = hot resistance
km = 235 for copper
km = 225 for aluminium
= the average oil temperature during cold resistance measurement
   
  The average temperature gradient g of the winding above the oil temperature is:
   
 
(10.5)
IN = winding rated current
Im = test current
y = exponent according to the standard = 1.6 (ONAN)
   
  The average winding temperature rise above the ambient temperature is:
   
 
(10.6)
   
 
Result
  The report indicates:
- cold resistance values and the corresponding oil temperature
- oil and cooling system temperatures and the corresponding losses
- hot resistances at shut-down
- temperature rises calculated from the measuring results.
   
   
 

TEMPERATURE RISE TEST FOR DRY TYPE TRANSFORMERS
ACCORDING TO IEC 726
   
 
PURPOSE OF THE MEASUREMENT
   
 

The purpose is to check that the temperature rise of the active part of the transformer windings and core do not exceed the agreed limits those stated by the standards.
   
 

PERFORMANCE TESTS ON DRY TYPE TRANSFORMERS
 
 
 

The temperature rise test on this kind of transformers is performed by using the simulated load method. This method is used either for an enclosed and non-enclosed dry-type unit air natural cooled or forced cooled. Temperature tests are made by utilising the rises obtained from two tests, one with no-load loss only, and one with load losses only, i.e., short-circuit run.
The no-load test, at rated voltage (fig. 4.1), is continued until steady-state conditions are obtained; individual winding temperature rises, is determined via measurement of winding resistance after shutdown.
   
 

Fig. 4.1
Circuit to temperature rise test (no-load losses at nominal voltage)
G1 supply regulator, T1 transformer to be tested, T3 current transformers, P1 - P2 - P3 digital wattmeter.
   
  The short-circuit runs with rated current (fig. 3.1) flowing in one winding and the other winding short-circuited is stated immediately following the no-load run, and continued until steady-state conditions are obtained; individual winding temperature rises, is determined via measurement of winding resistance after shutdown.
   
 


Fig. 3.1
Circuit to temperature rise test (short-circuit at nominal current)
G1 supply regulator, T1 transformer to be tested, T3 current transformers, P1 - P2 - P3 digital wattmeter.
   
  The resistances of the windings at shut-down are obtained by extrapolating the resistance-time-curves to the instant of swithing off.
The calculation method of the windings temperature is:
   
 
= average ambient temperature
R2 = hot resistance
R1 = cold resistance		 km = 235 for copper
km = 225 for aluminium
   
  The total winding rise, , of each winding, with rated current in the winding and normal excitation of the core, is calculated by the following formula:
   
 
Winding temperature-rise correction for reduced current
   
  When the input test current (It) is below the rated value of current (In), but not less than 90% (In), of the temperature rises, of the windings, shall be measured by the resistance method when steady-state conditions have been reached, and corrected to rated load conditions, by the formula:
   
 
The value of q shall be taken as:
AN transformers: 1.6
AF transformers: 1.8
Determination of constant temperature conditions
   
  The ultimate temperature rise is reached when the temperature rise becomes constant; this is considered to have been achieved when the temperature rise does not vary by more than 2% of the permissible temperature rise per hour or 2 K per hour, whichever is smaller.
During the test, the ambient temperature is measured by means of three pt100 probes. Other pt100 probes are used for checking the temperature in centre of top yoke, on the LV windings up and bottom.
   
 
RESULTS
   
  The report indicates:
- cold resistance value and the corresponding ambient temperature
- core and windings temperatures and their corresponding losses
- temperature rises calculated from the measuring results.
   
 
Performance of test on dry-type transformers
  The temperature rise test on this kind of transformers is performed by using the back-to-back method when two similar transformers are available. The two transformers one of which is the unit under test, are connected in parallel as shown on the following fig. 10.1.
One side of the transformers (generally the H.V. side) is excited at the rated voltage of the transformer under test. By means of an injected voltage by a special regulator, the rated current is made to flow in the transformer under test.
   
 


Fig. 10.1
The circuit for temperature rise test on dry-type transformer
G supply regulator, T1 transformer under test, T2 similar transformer, T3 - T5 current transformers, T4 voltage transformer, P1 wattmeter, P2 - P4 ammeters, P3 voltmeters (r.s.m. value).
   
  During the test the ambient temperature is measured by means of three sensors Pt 100 of a digital themometer which are placed at different points around the transformer under test. Other sensors of the same thermometer are used fo checking the temperature in centre of top yoke and as close as practicable to the innermost low-voltage winding conducors at the top of the winding. After the transformer has got to steady state conditions the hot measurement of windings resistances is carried out. The resistances of the windings at shut-down are obtained by extrapolating the resistance-time-curves to the instant of switching off. The calculation method of the windings temperature is:
   
 
(10.7)
R1 = cold resistance
R2 = hot resistance
km = 235 for copper
km = 225 for aluminium
= the average oil temperature during cold resistance measurement