Field Test of Energy Meters and Current Transformers with Clamp Multimeters
Portable clamp multimeters enable simple, convenient and accurate on-site detection of current transformer (CT) transformation ratios and energy meter errors.
Without disconnecting any original wiring or affecting power supply, the transformation ratio of current transformers and the accuracy of the revolutions per minute of energy meters can be quickly measured. This is a technical on-site troubleshooting method used in power supply business inspections.
Working principle: On-site testing using the AC current gear of a portable clamp multimeter. First, measure the primary load current and the secondary current converted by the CT. Then perform simple calculations based on the measured data to immediately obtain the CT transformation ratio and the energy meter error coefficient.
Formula for calculating CT transformation ratio:\(n=I1/I2\)Where:n - CT transformation ratioI1 - Measured primary load currentI2 - Measured CT secondary current
Example 1: Calculation of CT transformation ratioA metering box has a rated CT ratio of 150/5. A clamp multimeter measures a load current of 90 A and a secondary current of 2 A. Calculate the CT transformation ratio.
① Calculate the actual CT transformation ratio:Substitute into the formula: \(n'=I1/I2=90/2=45\)
The marked rated CT ratio is \(n=150/5=30\). However, the actual CT transformation ratio measured on site is 45.It can be seen that the original marked ratio of 150/5 may be incorrect, or there may be an error in metering wiring, which requires further inspection.
Example 2: Calculation of energy meter disk revolutionsA three-phase four-wire metering energy meter has a rated secondary current of 5A, rated voltage of 220 V, a meter constant of 1200 revolutions/kW·h, and a CT ratio of 100/5. A clamp meter measures a primary current of 70 A on one phase. The energy meter is observed to rotate 38 revolutions in one minute on site. Calculate the theoretical number of revolutions of this meter. (Power factor = 0.8)
