The transformer bushing CT is installed on the transformer and the other end is immersed in the transformer oil, and the winding on one side of the CT is connected with the transformer winding, so it is difficult to carry out the test, if the traditional transformer tester is used, the bushing CT must be removed and hoisted down from the transformer before it can be carried out, the general test process needs to be coordinated with the maintenance class and the high test class, and the large equipment such as the crane is required, and the transformer bushing CT hoisting process is prone to safety accidents. With the increase of system capacity, the CT current is getting larger and larger, up to tens of thousands of amperes, and the on-site current is also very difficult, Wuhan Halma power CTP series transformer comprehensive tester can perfectly solve the above problems, using the voltage method to measure the transformation ratio, small size, light weight, simple and convenient, well received by the majority of users.
The principle of the test
The ratio of the AC voltage on the core of the secondary winding and the amplitude of the induced voltage on the primary side is theoretically equal to the turn ratio, compared with the direct method of passing a large current on the primary side, this variable ratio test method does not require a large current, and has the characteristics of small capacity, safety and reliability of the test equipment.
The equivalent circuit diagram of the conversion ratio of the voltage method measurement casing CT is shown in Figure 1 below.
Fig.1. Equivalent circuit diagram of the conversion ratio of the voltage method to measure the CT of the casing.
Wherein: U1 is the induced voltage on the primary side of the casing CT;
u2'It is converted to the secondary voltage of the casing CT on the primary side;
R1 and X1 are the resistance and reactance of the primary coil of the casing CT;
r2'、x2'It is the resistance and reactance of the secondary coil of the casing CT;
RM and XM are the excitation resistance and reactance of the casing CT;
IE is the excitation current of the casing CT.
When the conversion ratio of the casing CT is measured by the voltage method, the primary coil is open, the magnetic flux density is very high, and it is easy to be saturated, and the following equation can be obtained from the equivalent diagram:
Generally, from the empirical value, the resistance and reactance of the secondary coil of the casing CT are less than 1, and the excitation current of the casing CT is small about 10mA, so
The part is very small and basically negligible, so it has to be.
Variable ratio of cannula CT
Test wiring
We do the test of phase A of the transformer bushing, the output current terminals S1 and S2 of the instrument and the mining voltage terminals M1 and M2 are shorted at the other end of the test line and then connected to the two ends of a certain winding of phase A of the bushing CT, and then the end of the primary line P1 is connected to the primary output terminal of the bushing CT (that is, the terminal of the transformer output lead), and the other side is connected to the neutral point CT, and the reliable grounding after the short circuit of the non-experimental phase B phase and C phase and the neutral point position, the test wiring diagram is shown in Figure 2
Fig.2. Wiring diagram of the CT ratio test of the casing.
Tests and analysis of results
After connecting the wire, according to the parameter setting on the CT nameplate, test the cannula CT a 0The 5-level metering winding starts to run the test for about 50 seconds, and the device automatically stops the test after automatically completing the test items such as excitation characteristics, error curve, and variable ratio polarity, and prompts to save the test report.
Fig.3 The results of cannula CT test.
Looking at the test results, the ratio is 1200:09979, a difference of 021% is very accurate, the same method and steps to complete the excitation error curve and ratio polarity test of phase B and phase C, and its accuracy and test effect are very perfect.