Measurements of the error of current and voltage sensors. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Measurements of the error of current and voltage sensors. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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It is difficult to measure the error of current sensors (less than 1%), and even more so the non-linearity of 0,1% by the usual method by measuring the input and output signals with standard measuring instruments.

To measure the error, it is necessary to measure the input and output signals with an error of less than 0,1%, and to measure the non-linearity, less than 0,01%.

A method is proposed for measuring the error directly without measuring the input and output signals (by comparing the normalized input and output signals).

Consider the measurement of the error using the example of a 1000 A current sensor with a current output (LT 1000-SJ / SP58 accuracy class 0,2). Sensor transformation ratio K=1/5000, i.e. with an input current of 1000 A, the output current is 0,2 A. We wind a winding of 500 turns on the sensor through the busbar hole (Fig. 1, where 1 is the winding, 2 is the busbar hole, 3 is the current sensor, 4 is the power source, 5 - voltmeter Shch300, R1 - rheostat 10 Ohm, R2 electric resistance coil P321 1Ohm ± 0,01%, R3 - electrical resistance coil P321 - 0,1 Ohm ± 0,01%), which is equivalent to a stranded bus.

Using source 4, let's pass a current of 2 A through the winding (total current 1000 A). The input current is controlled by the voltage drop (200 mV) on the electrical resistance measuring coil P321 - 0,1 Ohm ± 0,01% (R3). The output current is controlled by the voltage drop (200 mV) on the measuring coil of electrical resistance P321 - 1 Ohm ± 0,01% (R2).

The absolute error of the sensor, equal to the difference between the voltage drops across the precision resistors R2 and R3, is measured with a voltmeter 5. The measurement error practically does not depend on the error of the input current setting and the error of the voltmeter 5, the error of the voltmeter and the input current setting is 10%. The measurement error is determined by precision resistors R2 and R3 and is 0,02%.

The product of the sensor transformation ratio (K) and the number of turns (W) must be a multiple of 10, because electric resistance coils are produced with ratings 1⋅10n (where n = ±1, ±2, ±3, etc.).

It is advisable to implement the winding using a 50-core cable (Fig. 2, where X1 is a GRPM61 socket; X2 is a GRPM61 plug; X3, X4 is a 35,5-28 lug), passing the cable through the bus window 10 times.

The error measurement scheme for this case is shown in Fig. 3, where 1 is a cable (see Fig. 2), 2 is a busbar hole, 3 is a current sensor, 4 is a power source, 5 is a Sch300 voltmeter, R1 is a 10 Ohm rheostat , R2 electrical resistance coil R321 - 1 Ohm ± 0,01%, R3 - electrical resistance coil R321 0,1 Ohm ± 0,01%. To exclude the influence of the magnetic field of the return wire, a magnetic shield can be put on the sensor, but, as measurements have shown, it can be neglected. The only drawback of the method is the lack of technology.

Figure 4 shows a diagram for measuring the error without a cable, where 1 is a bus, 2 is a hole for a bus, 3 is a sensor, 4 is a power source, 5 is a Shch300 voltmeter, R1 is a 1000 A shunt, R2 is a 0,2 A shunt Shunts are used instead of a resistance coil. The measurement error is determined by the error of the shunts R1, R2 and does not depend on the error of the measuring device and the error of setting the input current.

Figure 5 shows the error measurement scheme for sensors with a potential output (output voltage 10 V at an input current of 1000 A), where 1 is a bus, 2 is a bus hole, 3 is a sensor, 4 is a power source, 5 is a Sch300 voltmeter, R1 - shunt for 1000 A, R2 resistance box P33 (13233 Ohm), R3 - measuring coil of electrical resistance P321 100 Ohm ± 0,01%. The voltage at the shunt R1 is compared with the voltage at the resistance coil R3, which forms an output voltage divider with the resistance box P33 (R2). The measurement error is determined by the error of the shunt R1 and the resistance box R2. The resistance coil error of 0,01% can be neglected.

For most sensors, including LT 1000-SJ/SP58 accuracy class 0,2, the output signal delay is no more than 1 µs; measurements by the proposed method can be carried out at direct and alternating current with a frequency of 50 Hz.

Author: A. Aldokhin

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