A new method for measuring the DC resistance of transformer windings during the transient process is proposed, which is simple, rapid, and accurate. Simulation tests were conducted, and an intelligent instrument implementation scheme with a microcontroller as the core is presented.
1 Introduction
After manufacturing or overhaul, during handover and preventive tests, as well as in the determination of average winding temperature rise and fault diagnosis, it is necessary to measure the DC resistance of transformer windings to analyze and judge the characteristics of power transformers [1]. Especially in preventive tests, test personnel hope to quickly measure the DC resistance of windings and approach the DC resistance value under actual operating conditions. However, in actual measurement, due to the large inductance and small resistance of transformer windings, the inherent time constant t=L/R is large. Using conventional bridge methods or DC voltage drop methods often takes a long time to reach equilibrium, thus failing to achieve rapid measurement [2]. Although methods such as circuit mutation with increased loop resistance and short-circuit demagnetization have been adopted to accelerate the time for circuit current to reach a stable value, they have only improved somewhat and cannot fundamentally achieve rapid measurement. A new rapid measurement method has now been developed, which is simple, reliable, and easy to expand. It is introduced below for reference.
2 Basic Principle of the New Method
A power transformer winding can be equivalent to a series circuit of an inductance and a resistance R. The basic measurement circuit for winding DC resistance is shown in Figure 1. When switch K is closed, the loop voltage equation is: E=L（di/dt）+iR. Considering the non-ideal nature of the switch, let the moment when the voltage E is applied after closing the switch be t=0, at which time the transient
From equation (5), it can be seen that when measuring the DC resistance of transformer windings, regardless of the circuit state, it is only necessary to sequentially measure i(t1), i(t2), and i(t3) in equation (5). Given the known power supply electromotive force E, the value of R can be obtained. Therefore, the measurement time can be arbitrarily short, achieving rapid measurement of the DC resistance R of power transformer windings.
The outstanding advantage of this method is that it fundamentally eliminates the influence of the circuit's steady state on measurement, enabling maximum rapid measurement. This is unattainable by other measurement methods, as it overcomes the issues of large power supply capacity required by traditional methods and the impact of additional measurement components such as series resistors and capacitors on measurement accuracy.
3 Simulation Test of the New Method
Based on the above principle, a simulation test was conducted using a Y/D (Z) type power frequency test transformer and an HT-1712F type DC regulated power supply. In the test, a sampling resistor connected in series in the test circuit was used to obtain the current signal. The power supply voltage was set to 2.02V, and the sampling resistor R5 was 3.11Ω. A TDS-220 oscilloscope was used to record the test waveform (as shown in Figure 2).
In the formula: US1, US2, and US3 are the voltages across the sampling resistor corresponding to i(t1), i(t2), and i(t3) in equation (5), respectively. Using the data obtained from the oscilloscope for calculation and processing, the relationship between the A value and time can be obtained, as shown in Figure 3.
Obviously, i=Us/Rs. Comparing equation (6) with equation (5), we can obtain: R=Rs•E•A……(7)
That is, the A value and the R value differ only by a coefficient. Comparing Figure 3 with Figure 2, it can be seen that after about 0.4s, when the power supply voltage reaches stability, the A value remains almost constant. The average value of A in this segment is 0.525 (V-1), and thus from equation (7), R=3.298Ω is obtained. After removing the sampling resistor, it is 0.188Ω (which also includes contact resistance). This result is very close to the value (0.180Ω) measured with a CA10 type rapid tester, demonstrating the correctness and feasibility of equation (5). From Figure 3, it can be seen that the measurement time is less than 1s, achieving the goal of rapid measurement.
Although the simulation test used a power frequency test transformer, which differs from actual power transformers in characteristics, it can reflect the fundamental process. In practice, the DC resistance of power transformer windings is at the mΩ level, and small changes in the sampling resistor value may cause significant errors in the measured DC resistance, affecting measurement accuracy. Therefore, in actual measurement, current sensors with high accuracy and wide bandwidth should be used, and sampling resistors should be avoided as much as possible. If the current signal is too small, an amplifier should be added before the sample-and-hold circuit to amplify the signal to meet measurement requirements.
4 Specific Implementation of the New Method
In summary, the new method involves sequentially measuring the current values in equation (5), performing certain calculations, and also conducting some analysis and comparison to improve measurement accuracy.
A microcontroller contains timers/counters and can perform a certain amount of calculation, fully meeting the above requirements. An intelligent instrument made with a microcontroller also has the advantages of simplicity, reliability, and easy expansion. Figure 4 shows an intelligent instrument with a microcontroller as the core to implement the new method for rapid measurement of DC resistance.
In the figure, the data processing part of the microcontroller mainly completes reading and storing data from the A/D converter (i.e., the current values in equation (5)), calculating and storing results using equation (5), analyzing and comparing the calculated data, averaging valid data to obtain the final result, and converting the final result to BCD code for display. The control part of the microcontroller mainly controls the timing of the sample-and-hold circuit and the A/D converter,
