Temperature rising test of transformer is usually to take according to the relevant provisions after insulation , loss, voltage ratio and DC resistance tests. The purpose is to ensure that temperature rising of each part of transformer accord with the relevant standards, so as to provide reliable basis for the long-term safe operation of transformer.
Here are several common methods for transformer temperature rising test.
1.Direct load method
Transformer temperature rising test [Heat cycle test system] by direct load method, in the two side of the transformer is connected with the subjects with appropriate load (such as electric stove, water resistor, inductor or capacitor). Rated voltage is applied in one side, and then adjust the load which current is equal to the rated current of the connection, as shown in figure 1.
Figure 1 Test connection of direct load method
Tx - test transformer
Z - load
PA - ammeter
PV - voltmeter
When the ambient temperature is 20±5℃ or the cooling water temperature is 20~25 ℃, the rated current is allowed to be used as the experimental condition instead of correction. The voltage applied to the primary side must be equal to the rated voltage of the distributor, and the deviation shall be no more than ± 2%.
The test condition of the direct load method is consistent with the operating conditions. The measurement results are accurate and reliable, and the method should be used as much as possible. But the power capacity required for the test is greater than the sample capacity, and is not easy to find the appropriate load, so it is suitable for small capacity transformer and transformer temperature rise test, on-site test can be used the following methods.
1) Load with water
According to the size of the load, we design different volume of the pool. A polar plate with adjustable distance is suspended in the pool, through adjusting the distance between the polar plate to adjust the load, and if necessary, water can add some salt. In this way, the temperature rise test of the converter transformer along with the rectifier is very good.
2) Use shifting coil voltage regulator as a load
As long as the regulator changes its usage and wiring, it can become a variable reactance reactor with stepless resistance, which is an ideal load. The following examples are given to illustrate how to use a single-phase shifter as an example. The principle of using a shift regulator as a variable reactor is shown in figure 2.
Figure 2 schematic diagram of shifting coil voltage regulator as a variable reactor
Tx - test transformer
TR - shift coil voltage regulator
In the diagram, the load side of the test sample is connected with the two side of the shift coil voltage regulator. When the shifting coil moves up and down, its impedance changes accordingly and becomes a variable reactor.
When the three-phase shift coil voltage regulator acts as a single-phase load, the two phase parallel connection and three-phase parallel connection can be selected according to the voltage and current of the load. Of course, the single phase shift coil voltage regulator composed of three units can also be used in three-phase operation.
2.Circulating current method
When the sample size is larger, the water resistance test is more difficult, so the circulating current method of temperature rising test is relatively simple, the auxiliary equipment is less, but need a test and auxiliary transformer with same capacity.
When using the circulating current method to conduct the transformer temperature rising test, the wiring is shown in Figure 3.
Figure 3: test wiring of current cycling method
TX - test transformer
T - auxiliary transformer
TR - voltage regulator
During the experiment, two sets of transformer with the same ratio and wiring group (TX and T) are in the same line ends in parallel. Tap changer is used to adjust one or two high voltage side of transformer. The voltage difference is equal to the two tap transformer impedance voltage and current in the test. If the impedance voltage of the two transformers is 5%and 5% or 4.5% and 5.5%, the voltage difference between the taps shall be 10%.
During test, the rated voltage of the transformer TX and the auxiliary transformer T is applied at one time, and the voltage difference between AA ',' BB 'and' CC 'is detected by Voltmeter before the secondary connection. If the wiring is correct, the difference should be equal to the voltage difference of the tap; on the contrary, it is approximately equal to the two line voltage of TX (T). Verify that the connection is correct, disconnect the power supply and connect the secondary line. Then, the rated voltage is applied to the test transformer at once, and the circulating current during the test shall be measured. The value shall be equal to or near the rated current. When the temperature rise test is carried out, the capacity of the power supply can be estimated by pressing
ST≥（I01%＋UkI%）Sn＋（I02%+Uk2%）S
ST- capacity of test power supply (kVA)
I01% - percentage of no-load current of the transformer tested
UkI%- percentage of the impedance voltage of the transformer tested
Sn - rated capacity of the transformer tested (kVA)
I02% - auxiliary transformer percentage of no-load current at test voltage;
Uk2% - percentage of the impedance voltage of the auxiliary transformer at the test current;
S - rated capacity of auxiliary transformers (kVA).
During calculation, to make the test transformer is busy in the positive tap, and the auxiliary transformer should be changed into a negative tap. In the wiring test according to figure 3, when the input transformer U3~ is rated voltage, and the power is switched on, the transformer is in a full load state.
The transformer may be over loaded when the tap gear is not properly chosen. Therefore, it is better to use a three-phase voltage regulator to carry out zero voltage rise, so that the current of the transformer is raised from zero to the rated current. The capacity of the regulator is the capacity of the test power supply.
3).Test with system load
When the transformer is located in the power station, the generator can be used to test . To adjust the excitation of the generator to make the transformer load full and reach the rated current. This method is suitable for testing high voltage and large capacity transformers on site.
4.Mutual loading method
When the temperature rising test of transformer is carried out by mutual load method, the connection is shown in Figure 4
Figure 4: test wiring of mutual loading method
TV - voltage transformer
TA - current transformer
At this point, three transformers and two test power sources are needed, and the tested transformer TX is connected in parallel with the same end of the same side of the auxiliary transformer T that supplies no-load losses. The rated voltage of the rated frequency is supplied by the power U3~, so that the no-load loss under rated voltage is generated in the tested transformer, and the short-circuit loss under rated current is regulated by adjusting the U'3~.
Auxiliary transformer voltage and wiring group should be the same as the test transformer, whose capacity is greater than or equal to the capacity of the transformer tested. Usually we use the same specifications products. A transformer T supplied with short-circuit loss, whose current should be greater than or equal to the rated current of the transformer under test, and whose voltage is greater than or equal to the sum of the impedance voltage of the transformer and the auxiliary transformer.
In the test, before the high voltage side of the TX and T is not connected, a power U3~ equal to the rated voltage of the test transformer is added, and the voltage between AA ',' BB 'and' CC 'is measured by a voltmeter. When the wiring is correct, the indication value should be close to zero. Conversely, it is approximately equal to the secondary line voltage of TX (T'), which indicates that the phase of the two transformers (TX and T) is not correct. When the connection is correct, the power U3- is disconnected so that the secondary time of the TX and the T can be connected. At this point, if the U3- is connected, there is no current for the secondary time, and the transformer in the test is in no load state. The U'3~ is then increased from zero, causing the T to generate secondary cycles of current and to adjust the U '3~ to make the transformer current up to the rated current. In this way, both voltage and current meet the requirements of the test.
Test conditions for mutual load method of test and consistent operating condition of the transformer, which provides accurate and reliable data and test power supply capacity is greatly reduced (loss when spent T time), can be estimated by that.
ST≥（I01%＋ I02%）Sn＋（UkI%＋Uk2%）Sn
ST - capacity of test power supply (kVA)
I01%、 I02% - percentage of no-load current of transformer and auxiliary transformer
UkI%、Uk2% - percentage of short-circuit voltage for tested transformers and auxiliary transformers;
Sn - rated capacity of the tested transformer (kVA).
When the power supply of different three-phase power U3 "-" and "U3-" can not be synchronized, the frequency difference between the two should be no more than 2~4Hz., so as to eliminate the swing of meter.
The secondary T of the short-circuit loss auxiliary transformer used in the mutual load method must have 6 high voltage outlet tubes. During test, there must be a certain distance between transformer and auxiliary transformer in order to avoid the influence of heat radiation and cause test error.
5.Short circuit method
The connection of the temperature rising test with the short circuit method is shown in Figure 5. The test can be carried out as follows.
Figure 5 short-circuit test wiring diagram
(1) To determine the upper oil temperature rise of the transformer. Adjust the applied voltage so that the power added to the test transformer is equal to the sum of no-load loss and short-circuit loss, and then test after the loss which is equivalent to the operating condition. The current calculation with equivalent loss is applied.
IT≈√(Pk85＋P0)／Pk85×In
IT - test current of equivalent loss (A);
Pk85 - short circuit loss (kW) at rated current at 85℃ for transformer tested;
P0 - no-load loss of transformer under rated voltage (kW);
In - rated current of the tested transformer (A).
To test the current IT is the transformer with equivalent loss, should be regularly measuring transformer top oil temperature, radiator (or tank), and oil cooler (forced oil circulation transformer) of the import and export of oil temperature and cooling water temperature. When the temperature is stable, the temperature of each part and environment is measured and the upper oil temperature is calculated.
(2) Determine winding temperature rising. The voltage of the input transformer is equal to the short-circuit loss, and the temperature of each part of the system is measured at the same time as the item (1). The temperature rise of the winding is calculated until the stable temperature of each part is measured.
(3) Determine the core temperature rise. The short circuit of the transformer is removed, and the no-load temperature rise test under rated frequency and rated voltage is done according to the connection of Figure 6 and figure 7.
Figure 6 measuring diagram of single phase transformer loss
a. small current under no-load test
b. semi indirect measurement
c.indirect measurement wiring wiring
PF - frequency meter
PA - ammeter
PV - voltmeter;
PW - power meter
TV - voltage transformer
TA - current transformer
Figure 7: double power meter measuring three-phase transformer loss wiring diagram
a. direct measurement
b.semi indirect measurement
c. indirect measurement
PA1 - A phase current
PA2 - B phase current
PA3 - C phase current
The measured temperature is the same as item(1), until the temperature is stable, the temperature of the core and the environment are measured, and the temperature rise of the core is calculated. The difference between the applied voltage and the rated voltage shall not exceed 2% during the test.
For large capacity transformer, in item(1) after the experiment, subjects should be the actual load of transformer with equivalent load and equivalent, measuring the temperature of upper oil, and compared with the measurement of equivalent load of the top oil temperature, to determine the operating temperature limit. Then, the power supply is removed, the DC resistance of the winding is measured, the average temperature is converted, and the average temperature rise of the winding is determined.
