09,22,2026 15 views
1、 Measure the DC resistance of the transformer winding
The diameter of the primary winding of a voltage transformer is relatively small, which can easily lead to faults such as wire breakage, short circuit, or turn to turn breakdown. However, this rarely occurs in the secondary winding due to the thicker wire. Therefore, during handover and overhaul, the DC resistance of the primary winding of the voltage transformer should be measured. The DC resistance of the primary winding of various types of voltage transformers ranges from several hundred ohms to several thousand ohms. Generally, a DC resistance tester is used for measurement, and the measurement results should not show significant changes from the data measured by the manufacturer or before.
Sometimes, in order to determine whether there is poor contact at the primary winding joint of a current transformer, it is necessary to use voltage drop method and double arm bridge to measure the DC resistance of the primary winding; Sometimes, in order to determine the position of the tap of the bushing type current transformer, a transformer DC resistance tester is also used to measure the DC resistance of the winding.
2、 Polarity test
The polarity of current transformers and voltage transformers is very important. Incorrect polarity judgment can lead to incorrect indication of measuring instruments, and more seriously, cause directional relay protection to malfunction. The polarity between the primary and secondary windings of the transformer is reduced. The polarity test method is the same as that of power transformers, generally using the DC method, but it can also be measured using our company's CTP-1000B variable frequency transformer comprehensive tester. During the experiment, it should be noted that the power supply should be added to the transformer for primary measurement; The measuring instrument is connected to the secondary side of the transformer.
3、 Variable ratio test
The regulations stipulate that the transformation ratio of each tap of the transformer should be checked and there should be no significant difference compared to the nameplate.
1. Inspection of current transformer ratio
Check the transformation ratio of the current transformer and compare it with the labeled current transformer. The test wiring is shown in Figure 1-1. During the experiment, the tested current transformer is connected in series with the standard current transformer for primary measurement, and a 0.5-level ammeter is connected to each secondary side. A suitable current is supplied to the primary side through a voltage regulator and a current booster. When the current reaches the rated current value of the transformer (or multiple points are selected within the range of 30% to 70% of the rated current), the readings of both ammeters are recorded simultaneously. The actual transformation ratio of the tested current transformer is:
K=KNIN/I
The transformation ratio error is
△ K=[(K-KxN)/KxN]×
In the above equation, KN and IN represent the transformation ratio and secondary current value of the standard current transformer;
K. I - Transformation ratio and secondary current value of the tested current transformer;
KxN - rated transformation ratio of the tested current transformer.
During the experiment, attention should be paid to short-circuit the secondary winding of the non tested current transformer to prevent open circuit; It is advisable to choose a standard current transformer with the same transformation ratio as the tested current transformer. If the transformation ratio is correct, the reading on the secondary winding ammeter should also be the same.
2. Ratio inspection of voltage transformers
For voltage transformers with a transformation ratio within the measurement range of the transformation ratio bridge, the CTP-1000B variable frequency transformer tester can be directly used to measure their transformation ratio. For voltage transformers with large transformation ratios, the double voltmeter method can be used to check their transformation ratios, or the method shown in Figure 1-2 can be used to compare them with standard voltage transformers. When measuring the transformation ratio of a voltage transformer using the method shown in Figure 1-2, it should be noted that voltage is generally applied to the high voltage side through a regulator and a test transformer, and measured on the secondary side.
Wiring diagram for voltage transformer ratio inspection test
Figure 1-2 Wiring diagram for voltage transformer ratio inspection test
T1- single-phase voltage regulation; T2- Test Transformer;
TVN - Standard Voltage Transformer; TVX - Tested Voltage Transformer
4、 Excitation characteristic test of transformer
The excitation characteristics of a transformer refer to an open circuit in the primary winding of the transformer. The relationship curve between the excitation current on the secondary side and the applied voltage is actually the magnetization curve of the iron core. The main purpose of the excitation characteristic test of the transformer is to check the quality of the iron core of the transformer, and to determine whether there are defects such as inter turn short circuits in the winding of the transformer by identifying the saturation degree of the magnetization curve. Given the frequent occurrence of iron core resonance overvoltage and poor quality of voltage transformers in the system, it is required to conduct no-load excitation characteristic tests on voltage transformers.
1. Voltage current characteristic test of current transformer
The experimental wiring is shown in Figure 1-3. Before the test, the secondary winding leads and grounding wires of the current transformer should be removed. During the test, an open circuit should be measured and voltage should be applied from the secondary side. For ease of reading, several current points can be selected in advance and the corresponding voltage values can be read point by point. The current or voltage applied shall not exceed the specifications of the manufacturer's technical conditions. When the current increases but the voltage changes little, it indicates that the iron core is saturated and the test should be stopped. After the experiment, draw the volt ampere characteristic curve based on the experimental data.
Wiring diagram for volt ampere characteristic test of current transformer
Figure 1-3 Wiring diagram for volt ampere characteristic test of current transformer
The current customs only conduct volt ampere characteristic tests on secondary windings that have requirements for relay protection. The measured volt ampere characteristic curve should not show a significant decrease in voltage compared to past or factory volt ampere characteristic curves. If there is a significant decrease, check for inter turn short circuits in the secondary winding.
2. No load excitation characteristic test of voltage transformer
The wiring for the no-load excitation characteristic test of the voltage transformer is shown in Figure 1-4. During on-site testing, the high voltage side of the voltage transformer is open circuited, and the low voltage side is supplied with the rated voltage. The no-load current and no-load loss are read.
Wiring diagram for no-load excitation characteristic test of voltage transformer
Figure 1-4 Wiring diagram for no-load excitation characteristic test of voltage transformer
The no-load excitation characteristic test of voltage transformers can be conducted together with the power frequency induction withstand voltage test. During the experiment, several sets of no-load losses and no-load current values are first read when the voltage rises to the rated voltage. After the voltage rises to 0.3 times the rated voltage and can withstand for 40 seconds, it is then lowered to the rated voltage or below, and several sets of no-load losses and no-load current values are re read.
The measured excitation characteristic curve or no-load current value at rated voltage should have no significant difference compared to the characteristics of past or similar voltage transformers. When conducting an induction withstand voltage test at 1.3 times the rated voltage, there should be no significant difference in the no-load current and no-load loss before and after the withstand voltage, otherwise the cause should be identified.
In general, the excitation curve measurement points are 20%, 50%, 80%, and 120% of the rated voltage. For voltage transformers with a neutral point directly grounded (N terminal grounded), the highest measurement point for voltage transformers with a voltage level of 35kV and below is 190%; The maximum measurement point for voltage transformers with a voltage level of 66kV and above is 150%. According to the regulations, there should be no significant difference between the no-load current and the factory value at rated voltage; The no-load current of a neutral point non effectively grounded system voltage transformer at a voltage of 1.9UN/√ 3 should not exceed the maximum allowable current; The no-load current of the voltage transformer in the neutral point grounding system at a voltage of 1.5UN/√ 3 should not exceed the maximum allowable current.
