What current is generally selected for a transformer DC resistance tester?

The DC resistance test of transformer windings is an indispensable test item for transformers after handover, overhaul, and change of tap changer. Under normal circumstances, measuring the DC resistance of transformer windings and high-power inductive equipment using traditional methods is a time-consuming and labor-intensive task. To change this situation, shorten measurement time, and reduce the workload of testers, a transformer DC resistance tester can be used.

The transformer DC resistance tester adopts a new power supply technology, which has the characteristics of stable performance, fast measurement, compact size, convenient use, high measurement accuracy, and good data repeatability. It is a device for measuring the DC resistance of transformer windings and high-power inductance equipment.

At the nominal magnetic flux density BN of the iron core, the magnetic permeability of the core will be low through the AC load current at the rated voltage of the transformer winding. When measuring and controlling the DC input resistance, it is necessary to make the magnetic density of the iron core greater than Bn, so as to reduce the system circuit's time constant and back electromotive force dLi/dt, and shorten the time for social stability and development.

Therefore, when measuring the DC resistance, the DC current should be at least:

I=k √ 2i0In+100

In the formula, k: constant>1

I0: AC rated frequency, percentage of no-load current at rated voltage

In: Rated current of the measured winding (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 of the iron core is greater than Bn,

Measure the magnetic permeability of the iron core during DC resistance measurement μ Reduce.

When the internal winding of a transformer is connected in a star (Y) shape, the line current can be equal to the phase current. From the above equation, it can be concluded that the current to be applied when measuring the DC input resistance of a measurement system is:

IY=1.41 ki0In: 100

The transformer winding is delta (D) coupled, and the load current of the line current converter is three times the phase current √, and the DC current is measured as 1/3 of the DC resistance and 2/3 of the total current distribution. So when we measure and control the DC resistance, we should apply the following current:

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

When k is taken as 3-10, that is, when the excitation ampere turn when measuring the DC resistance is 3-10 times the no-load current ampere turn, it can be made

The magnetic density of the iron core is close to saturation when it is greater than Bn, that is, the DC current measured 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 will change due to the increase in winding heating temperature, which will increase the measurement error.

Weshine Electric Manufacturing Co., Ltd.