With the development of control technology, the modulation signals of voltage and current are widely used. If the signal has high harmonic content, the traditional active power measurement method will be difficult to accurately measure. This paper introduces the active power measurement principle of power analyzer combined with the measurement application in the field of inverter.

1、 The most common method of active power measurement

1. Phase method

The phase difference of voltage and current is measured by the phase measuring circuit, and then according to the active power calculation formula P = uicos of sinusoidal circuit φ Calculate the active power.

Since the active power calculation formula P = uicos φ This method is derived from the sinusoidal circuit technology, and it is only applicable to the active power measurement of sinusoidal circuit.

In addition, because the phase measurement circuit usually uses zero crossing detection method, and there will inevitably be a certain burr near the zero point of AC, so the phase measurement accuracy is low. The accuracy of power measurement is also low at low power factor.

2. Analog multiplier method

An analog multiplier is used to obtain the product of voltage and current to obtain the instantaneous power, and then the instantaneous power is integrated in a fixed time to obtain the average value of the instantaneous power, that is, the active power. This method can be used to measure the active power of any waveform electric quantity.

2、 Measurement principle of power analyzer

Taking power analyzer pa8000 as an example, the basic principle of measurement is as follows:

The power analyzer samples current and voltage signals

In each measurement channel of power analyzer, the input current or voltage signal is sampled, and the sampled data is calculated according to a specific formula.

Where u (n) is the voltage signal data (instantaneous data) collected in the update period, I (n) is the current signal data (instantaneous data) collected in the update period, and U (n) and I (n) are the sampling data at the same time.

3、 Application of active power measurement method in frequency converter

The main circuit of the frequency converter is generally composed of “AC-DC-AC”. There is a large capacitor connected in the rectifier circuit, and the waveform of the input current is not a sine wave; In the inverter output circuit, when the output voltage signal is adjusted by the PWM carrier signal, that is, the input and output are not standard sine waves, and there are more higher harmonic content.

Typical input waveform of frequency converter is as follows:

The traditional calculation formula for the measured input power is as follows:

P=Urms*Irms*cos φ

Where p is the active power; Urms is the effective value of voltage; IRMS is the effective value of current; φ Is the angle between voltage and current.

However, the input current of the frequency converter contains high-order harmonics, so it is difficult to measure the phase angle. Pa8000 power analyzer adopts the digital method, that is, analog multiplier method, to obtain the average value of the instantaneous power, that is, the active power, by integrating the instantaneous power in a fixed time with the product of the instantaneous voltage and current obtained in the specified effective period

Similarly, for the output side of the frequency converter, the voltage is a carrier signal with adjustable frequency and contains more higher harmonics, as shown in the figure below:

Editor in charge: GT