Engineers who conduct transformer tests know that DC resistance testing is the second most important item in preventive testing, second only to dissolved gas chromatography analysis. It is almost indispensable for every major overhaul and handover acceptance. But many people only know "to test" and are not very clear about the logic of testing and what problems can be found. Explain this matter clearly today.

What does a DC resistance tester do

The principle is simple, which is Ohm's Law - apply a known magnitude of DC current to the transformer winding, measure the voltage drop across the winding, and calculate the DC resistance of the winding using R=U/I. HZ-3110B adopts a four wire system (Kelvin connection) for measurement, with the current terminal (I+/I -) and voltage sampling terminal (V+/V -) separated, eliminating the influence of contact resistance and lead resistance. The measurement accuracy reaches ± (0.2%+2 characters), and the minimum resolution is 0.1 µ Ω.

What problems can be identified

DC resistance testing can effectively detect the following types of faults:

Winding welding quality: If the welding points of the winding wires have poor contact, the DC resistance will be high, and the imbalance rate between the three phases will exceed the standard, which is the most direct manifestation of welding problems.

Interturn short circuit: When a turn to turn short circuit occurs in a certain phase winding, the short circuited turn is bypassed, the effective number of turns decreases, the DC resistance of the phase is low, and the three-phase imbalance rate is abnormal.

Contact status of tap changer: The tap changer contacts are oxidized or have insufficient contact area, resulting in increased contact resistance and higher DC resistance measurement values for the corresponding gear. By measuring step by step, it can be discovered which tap position has contact issues.

Breakage or breakage of lead wires: If multiple wires are wound together and some of them are broken, the DC resistance will be high; If the lead wire is completely broken, it cannot be measured directly.

Verification of actual position of tap changer: By measuring the DC resistance value of the current gear and comparing it with the theoretical value, it can be confirmed whether the actual gear of the tap changer is consistent with the indicated position.

Why not use alternating current testing

DC resistance is extremely sensitive to the above-mentioned defects, while in AC measurement, the impedance of inductance is much greater than that of DC resistance, masking subtle changes in resistance. Only with direct current can we "see through" the inductance and directly measure the pure resistance value of the winding.