The DC resistance test of transformer windings is an indispensable test item after the transformer is handed over, overhauled and tap changer changed. Under normal circumstances, traditional methods (bridge method and voltage drop method) are used to measure transformer windings and high power The DC resistance of inductive equipment is a time-consuming and labor-intensive work. In order to change this situation, shorten the measurement time and reduce the workload of testers.

Formula one

Through the AC load current in the rated voltage of the transformer winding, the nominal magnetic flux density of the iron core BN, at this time, the magnetic permeability of the core will be low. When measuring and controlling DC input resistance, it is necessary to make the magnetic density in the iron core greater than Bn, so that the iron core conductivity becomes lower, so as to reduce the time constant of the system circuit and the back electromotive force dLi/dt, and shorten the time for stable social development. Therefore, the DC current when measuring DC resistance should be at least:

I=k√ 2i0In+ 100

Where k: constant> 1

i0: AC rated frequency, percentage of no-load current under rated voltage

In: rated current of the winding under test (A)

The constant √2 is equivalent to the magnitude of the DC current of the AC current. When the factor k is greater than 1, the magnetic density in the core> Bn,

Decrease the permeability μ of the iron core during DC resistance measurement.

Formula two

When the internal windings of the transformer are connected in star (Y), the line current can be equal to the phase current. From the above formula, the current that should be applied when measuring the DC input resistance of the system is:

IY=1.41 ki0In: 100

Formula three

When the transformer windings are delta (D) coupled, the load current of the line current converter is 3 times the phase current, and the DC current is distributed by measuring the ratio of the DC resistance] and the total current of 1/3 and 2/3. So the current we should add to measure the control DC resistance is:

ID=1.41x3/2+1/√3 ki0Inx100=1.22 ki0In÷ 100

When k is 3~10, that is, when the excitation ampere-turn when measuring the DC resistance is 3-10 times of the no-load current ampere-turn, you can make

The magnetic density in the core is greater than Bn and is close to saturation, that is, the DC current when measuring the DC resistance is equal to 2%-10% of the rated current.

If the DC current is too large and the measurement time is too long, the resistance changes due to the heating temperature of the winding, which will increase the measurement error.

The above formulas can accurately select an appropriate current value.