After completing the transformation ratio test, the three-phase error is within ± 0.5%, and many engineers believe that the transformer winding is fine and can be put into operation. But there is a kind of fault that can deceive the transformation ratio test - that is, the situation where the number of turns in the inter turn short circuit is extremely small. At this point, the excitation current becomes a key supplement to the judgment.

Why can't we measure the short circuit ratio with fewer turns

The transformation ratio depends on the ratio of effective turns. If one winding has 1000 turns and a short circuit occurs in 2 turns, the effective number of turns becomes 998 turns, and the change in transformation ratio is only 0.2%, which is completely within the instrument error range and the standard allowable deviation. There is no abnormality at all from the transformation ratio value. But these two short circuits will form a low resistance closed circuit, generating a large circulating current under the drive of alternating magnetic flux, and the excitation state of the iron core will change significantly accordingly.

How does the excitation current reflect this issue

HZTTR80A measures the excitation current while measuring the transformation ratio, with a range of 0-2000mA and a resolution of 0.001mA. The three-phase excitation current of a normal transformer of the same model and capacity should be basically balanced, with differences within a reasonable range. If the excitation current of a certain phase is significantly high or the imbalance between the three phases is abnormal, it is often a signal of a short circuit inside the winding of that phase, which needs further inspection.

Specific judgment method for excitation current

The instruction manual clearly states that if the number of short-circuit turns is small, the fault may not be detected solely through the ratio test. The problem can be detected by testing the excitation current. In practical operation, comparing horizontally with historical or factory data of similar transformers is the most effective method; When there is no historical data, the relative comparison between the three phases can also provide reference - if the difference in three-phase excitation current exceeds 30%, it is worth being cautious.

The combination of two indicators is necessary for completeness

Looking at the transformation ratio alone, one cannot see a short circuit with fewer turns; Looking at the excitation current alone, it is difficult to determine whether the absolute value is normal without reference data. Only by combining the two can a comprehensive evaluation of the status of transformer windings be made. The HZTTR80A integrates two measurements into one test, and after measuring the transformation ratio, the excitation current value is displayed on the screen without the need for additional wiring or equipment replacement. This is its practicality as a fully functional model.