Development and discussion of transformer DC resistance tester

1 Introduction

　 The measurement of DC resistance of transformer winding is a simple and important test item in transformer experiment. By measuring the DC resistance of winding, the welding quality of the wire inside the winding and the welding quality of the lead wire and the winding can be checked, whether the wire used in the winding meets the design requirements, whether the tap changer (on-load voltage regulator OLT) lead wire and the current-carrying parts such as the bushing are in good contact, whether the three-phase resistance is balanced, etc. It is of great significance, and research and discussion in this regard is worthwhile.
2 Introduction to basic principles
　 In order to facilitate display, the DC resistance of the transformer winding should be converted into voltage or current, so the resistance-voltage conversion method is used for measurement. The traditional resistance measurement method uses the bridge measurement method, and its output voltage U0 changes with the change of the measured resistance Rx. This method has certain advantages in some places. For example, when using strain gauges to measure pressure, it is easier to achieve temperature compensation〔2〕. But there are also some disadvantages, such as using a balanced bridge to measure the DC resistance of the transformer winding, the circuit is complicated; if an unbalanced bridge is used for measurement, U0 and Rx have a nonlinear relationship.
   According to Ohm's law I＝U／R, we know that R＝U／I. As long as the current I is fixed, it is easy to get the linear transformation relationship between resistance and voltage. When the current I is fixed, a constant current source can be used to supply power to the measured resistor Rx, and then the voltage Ux at both ends of it can be measured to convert the value of the measured resistor Rx. Therefore, a constant current source is used to supply power to the transformer winding and measure its terminal voltage to measure the DC resistance of the transformer winding. Figure 1 shows the system principle diagram of measuring the DC resistance of the transformer. This method can be used to measure the DC resistance of the transformer. The basic principle of measuring the DC resistance of the transformer is introduced below: 2.1 Implementation of constant current source 　 In Figure 2, A1 is an adder, A2 and VT1~VT2 form an inverting amplifier (where T1~T2 are composite tubes used to amplify the output current of A2, and the maximum output current can be 3 amperes of reference current), and A3 is a voltage follower. The constant current source can ensure constant output current after multiple ripple filtering. The constant current source uses integrated operational amplifier ICL7650 (or CF7650). It is a high-precision, low-drift, dynamic zero-calibration CMOS type chopper monolithic integrated operational amplifier successfully developed by Intersil in the early 1980s, known as the fourth generation operational amplifier. Its input offset voltage Uos is only one thousandth of the general-purpose operational amplifier F007, Uos = 1.0μV, the offset voltage temperature drift is 0.01μV/℃, the monthly drift is less than 100pA, and it has a very high common-mode rejection ratio (≥130dB), the open-loop gain Ad＞140dB, and the fluctuation of the input voltage Uin has little effect on the amplifier output, so the constant current source is stable and reliable, and its drift and noise indicators can be greatly improved〔4〕. (The power supply is positive and negative ±12V.)     For the convenience of analysis, let R1 = R, and simply analyze its working principle as follows: The output voltage U1 of adder A1: 　 U1 = -(Uin + U4) (1) According to the general op amp analysis method for op amp A2: 　 U2 = -U1 = Uin + U4 (2) The output voltage U4 of follower A3: 　            U4 = U3 (3) U1~U4 are marked in the figure.    The current IN on the standard resistor RN is: Combining (1)~(4): 　　            Uin = U2-U3 (5)　  　Because op amp A3 constitutes a follower circuit, its input impedance is very high (≥1012Ω), so the current IN on the standard resistor RN all flows to the measured resistor Rx, that is,    from the above formula, it can be seen that the constant current Ix and the input voltage Uin are in a one-to-one linear relationship. Then, the voltage Ux across the measured resistor Rx is Ux = Ix·Rx, so ,
                    


                   









           



     

             

Then the value of Ux is the value of the measured resistor Rx. To ensure the technical indicators of the constant current source, R1 is generally selected as a resistor with higher precision, such as a 0.1% precision resistor, and the resistance value is 10K.
2.2 Low-pass filter
　 As shown in Figure 1, the INA128 instrument amplifier used in the measurement link has an amplification factor of 1, so it also amplifies the ripple voltage output by the previous link by 1 times. It is connected to the inverting amplifier link later. If the ripple is not filtered out, the ripple will be further amplified, which will inevitably affect the measurement accuracy and stability of small resistance values. For this reason, a voltage-controlled voltage source low-pass filter is used〔1〕, as shown in Figure 3, R1＝R2＝100K, C1＝0.22μF, C2＝0.42μF, its gain is 1, and the same phase mode is adopted, and its cut-off frequency fc≤5Hz.
2.3 High-precision inverting amplifier〔5〕
　 Since there are 6 gears in the direct resistance tester, namely 20mΩ, 200mΩ, 2Ω, 20Ω, 200Ω, and 2000Ω, the currents required to pass through each gear are 100A, 10A, 1A, 0.1A, 0.01A, and 0.001A. Since the switching power supply is 5V/10A, in the design, if 3A is selected as the maximum output current, to obtain 10A, it is necessary to amplify 10/3 times, and to obtain 100A, it is necessary to amplify 100/3 times. It can be seen that the test circuits of 20mΩ and 200mΩ are exactly the same except for the 10-fold fixed amplification circuit link. Therefore, this 10-fold amplification relationship must be particularly accurate and reliable. This design makes the circuit compact, small in size, more beautiful in shape, easy to use, and low in cost.
   Applying the network analysis theory to Figure 4, we can get: From the ordinary inverting amplifier theory, we know that: Since the previous link uses a low-pass filter, fc≤5Hz, A1 and A2 op amps use dual op amps, such as LF358, then A1 (f) ≌ A2 (f). Take 　 If Rf＝10R1, then F＝1/10, and select a high-precision resistor to ensure a more accurate 10-fold magnification relationship. Therefore, it can ensure that the transformer DC resistance tester can achieve accurate measurement of DC resistance in multiple ranges. 3 Main technical indicators    ·Constant current source stability: 　(1) 0.1A＜0.05%, 0.01A＜0.07%, 0.001A＜0.09%   (2) 1.0A＜0.1%, 3A＜0.2%   (3) Temperature drift＜0.05%／℃ (using 12V DC fan)    ·Range: 20mΩ, 200mΩ, 2Ω, 20Ω, 200Ω, 2000Ω     ·Output voltage: full scale is 2.000V 　 ·Warm-up time: ≤1 minute (preheat with 20mΩ or 200mΩ)    ·Ambient temperature: 0～60℃    ·Power supply: 220V±10%, 50Hz, power consumption is about 30W.
                  　　　
              

               

　                  

　　　　　　　　　　

1 Li Qiyan. Analog signal processing circuit. Internal data of Huazhong University of Science and Technology, 1999
2 Liu Yingchun. Sensor principle design and application. Changsha: National University of Defense Technology Press, 1989
3 Kong Youlin. Integrated operational amplifier and its application. Beijing: People's Post and Telecommunications Press, 1988
4 Intersil Corporation. Intersil Corporation Product Manual, 1999
5 Chen Senjin. Collection of operational amplifier application circuits. China Metrology Press, 1989