There is a polarity relationship between the primary side and the secondary side of the power transformer coil. If there are several coils or several power transformers combined, it is necessary to know their polarity before they can be used correctly. For a power transformer with two coils, if the potential induced in it at any instant has the same direction, it is called the same polarity or minus polarity, otherwise it is plus polarity. The connection group of power transformers is one of the important parameters of power transformers, and it is an important condition for parallel operation of power transformers, which needs to be measured in many cases.
1. Purpose and significance of power transformer polarity group and voltage ratio test
Under the condition of no-load operation of the power transformer, the ratio of the voltage of the high-voltage winding to the voltage of the low-voltage winding is called the transformation ratio of the power transformer:
K=U 1 /U 2
The voltage ratio is generally calculated according to the line voltage, which is an important performance index of the power transformer. The purpose of measuring the transformation ratio of the power transformer is:
1. Ensure that the voltage ratio of each tap of the winding is within the technically allowable range;
2. Check the correctness of the winding turns;
3. Determine whether the leads of each tap of the winding and the tap changer are connected correctly.
2. Test method for polarity group and voltage ratio of power transformer
1. DC method to determine the polarity of power transformers
There are DC method and AC method for measuring the polarity of power transformer windings.
DC method: Connect a dry battery to the high-voltage terminal of the power transformer, connect a milliammeter or microammeter to the secondary side of the power transformer, and observe the swing direction of the needle when the battery switch is closed during the experiment. Determine the polarity.
2. Determine the group of power transformers by DC method;
3. Use the ZSBC-VI automatic transformation ratio group tester to measure the transformation ratio.
3. Precautions and Analysis of Results
1. When determining the polarity by the DC method, the test process should be repeated several times to avoid the wrong conclusion due to the fast swing of the watch hand.
2. When measuring groups, choose a DC millivoltmeter, microammeter or multimeter with a higher voltage and a small range for a power transformer with a large transformation ratio; choose a lower voltage and a higher voltage for a small transformation ratio. A high range millivoltmeter, microammeter or multimeter.
3. The transformation ratio of the power transformer should be measured under each tap. When more than one coil has a tap, it can be measured at all the tap positions of each coil in turn, while the corresponding tapped coil should be measured. connected to the rated tap.
4. For those with load voltage regulating device, electric operating device must be used to change and tap.
5. During the whole measurement process, special attention should be paid to the fact that the power transformers A and a cannot be reversed, otherwise the high voltage will enter the bridge body.
6. When the test voltage is gradually increased, the power supply should be turned off for inspection when the voltmeter rapidly rises to full scale.
7. For the measured results, the voltage ratio sequence of each corresponding tap should be the same as that on the nameplate; the allowable deviation of the rated tap voltage ratio is , and the deviation of other taps should be within the impedance value of the power transformer, but not more than