There are issues with the meter that uses variable frequency measurement, resulting in lower values for active power
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A three-phase four-wire meter equipped with a current transformer is installed on the feed pump; the current transformer is placed at the input side of the frequency converter. The meter shows a voltage of 235 V, a current of 74 A, and a power factor of 0.999, resulting in an active power of around 37 kW. Using the formula 1.732*1.732*235 V*74 A*0.999, the calculated value is 52 kW. However, the meter only displays around 37 kW – what is the reason for this 15 kW difference? If it is the frequency conversion that affects the measurements, then it should affect the accuracy of measuring voltage, current, and power factor, rather than the calculation results, right? In the same control cabinet, there is another pump that does not have an inverter installed. The meter shows a voltage of 235 V, a current of 54 A, and a power factor of 0.81; the active power is around 30–31 kW. These values match the calculation 1.732*1.732*235 V*54 A*0.81 = 30.8 kW. The meter was brand new, and it was confirmed that the meter was fine through replacement testing; once it was switched to the power-frequency pump meter, everything worked properly.
The inverter adjusts the current waveform, resulting in a non-sinusoidal current shape. Traditional meters are based on sine waveforms for measurement; therefore, in the case of non-sine waveforms, the measurement results of such meters may be inaccurate. Since the inverter alters the current waveform, in the calculation of active power, the current value used by the wattmeter is the average or effective current rather than the actual peak current. This is why there is a difference between the power calculated using the formula and the power shown by the meter. In fact, the impact of inverters on voltage and power factor can also lead to inaccurate measurement results. However, this effect is relatively small and usually does not result in significant errors. Therefore, based on the information you provided, it can be inferred that the impact of the inverter on the meter readings stems mainly from the non-sinusoidal nature of the current waveform. For pumps that do not have variable frequency drives, their current waveform remains sinusoidal, so the readings taken by the meter are relatively accurate. .
Initially, meters from Jiangsu Linyang were used, with a power factor of nearly 1. Later, Chint/Winson meters were installed; the active power remained the same, but the power factor was around 0.75. In this case, the active power calculated using the formula 1.732 * voltage * current * power factor matches the value displayed on the meter. I looked up some information later and found an article online; it indeed was the effect of harmonics from variable frequency. Lin Yang’s calculation method does not take into account the reactive power caused by harmonics, which is why the power factor is high. I took another look at the manual for the Hiti clamp-on power meter; there are two ways to measure the power factor: PF/Q/S calculation mode, which allows selection of the calculation method for power factor (PF), reactive power (Q), and apparent power (S). Generally, operations on the effective value are used for tasks such as confirming transformer capacity, but fundamental wave operations are employed when measuring the power factor or reactive power related to electricity costs. RMS calculation uses the RMS values of voltage/current to compute power factor, reactive power, and apparent power. Power factor PF (RMS power factor), reactive power Q (calculated based on RMS values), apparent power S (calculated based on RMS values). Fundamental wave calculation: The power factor, reactive power, and apparent power are calculated using the fundamental waves of voltage/current. Power factor DPF (displacement power factor), Reactive power Q (fundamental reactive power), Apparent power S (fundamental apparent power)