A brief discussion on the measurement principle of power analyzer

With the development of control technology, voltage and current modulation signals are more widely used. If the signal has a high harmonic content, the traditional active power measurement method will be difficult to measure accurately. This article is based on the active power measurement principle of the power analyzer and combines it with the measurement application in the field of frequency converters to briefly introduce it.

1. The most commonly used active power measurement method

1. Phase method

The phase difference between voltage and current is measured by the phase measurement circuit, and then the active power is calculated according to the active power calculation formula of the sinusoidal circuit P=UIcosφ.

Since the active power calculation formula P=UIcosφ is derived from sinusoidal circuit technology, this method is only applicable to the active power measurement of sinusoidal circuits.

In addition, since the phase measurement circuit usually adopts the zero-crossing detection method, and there will inevitably be certain burrs near the zero point of the AC power, the phase measurement accuracy is low. The power measurement accuracy is also low at low power factors.

2. Analog multiplier method

The product of voltage and current is obtained by using an analog multiplier to obtain instantaneous power, and then the instantaneous power is integrated over a fixed time to obtain the average value of the instantaneous power, that is, the active power. This method is suitable for active power measurement of arbitrary waveform electrical quantity.

2. Basic measurement principles of power analyzer

Taking the power analyzer PA8000 as an example, the basic principle of measurement is as follows:

Power analyzer samples current and voltage signals

Each measurement channel of the power analyzer samples the input current or voltage signal and calculates the result based on a specific formula for the sampled data.

Where u(n) is the voltage signal data (instantaneous data) collected during the update period, i(n) is the current signal data (instantaneous data) collected during the update period, and u(n) and i(n) are the sampled data at the same time.

3. Application of active power measurement method in inverter

The main circuit of the frequency converter is generally composed of "AC-DC-AC". A large capacitor is connected in the rectifier circuit, and the waveform of the input current is not a sine wave. In the inverter output circuit, the output voltage signal is a waveform adjusted by the PWM carrier signal, that is, the input and output are not standard sine waves, and there are more high-order harmonics.

The typical input waveform of the inverter is as follows:

The traditional calculation formula for the measured input power is:

P=Urms*Irms*cosφ

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 inverter contains high-order harmonics, and it is difficult to measure the phase angle. The PA8000 power analyzer uses a digital method, that is, an analog multiplier method, to obtain the product of the instantaneous voltage and current within a specified effective period, and then integrate the instantaneous power for a fixed time to obtain the average value of the instantaneous power, that is, the active power, as shown in the following figure:

Similarly, on the output side of the inverter, the voltage is a carrier signal with adjustable frequency, which contains many high-order harmonics, as shown in the following figure:

PA8000 Power Analyzer