Tips for locating harmonic interference during power supply operation

Harmonic interference is the most common during the operation of the power supply. Engineers often need to upgrade the PFC circuit to improve their own power products. How to locate each harmonic value and frequency when the power supply is running? .

For precision electronic equipment, the most feared is the impact from external interference, which is often fatal. In fact, external interference is ubiquitous. For example, in an industrial field, the power grid is constantly being hit by harmonic currents, which also causes serious interference to the precision electronic equipment in the power network. So how is this interference formed?

In the power network, there are many non-linear loads, such as medium frequency furnaces, inverters, DC motor drives, electronic ballasts and other devices with severe changes in operating current, which will inject harmonic current into the grid. The voltage distortion caused by such harmonic currents is likely to cause interference to PLCs, CNC machine tools, computers, precision instruments, etc., and abnormal operation occurs.

It should be remembered that the harmonic current emitted by the nonlinear load to the grid itself does not affect other equipment. The effect of the harmonics we see on other equipment is that the harmonic current generates harmonic voltage through the impedance of the grid. of. The explanation for this phenomenon is shown in the following figure:

Here, device 1 is a device that generates harmonic currents, which injects harmonic current into the grid during operation. Since the grid has a certain impedance, the impedance of the grid includes the impedance Z0 of the transformer, the impedances Z1 and Z2 of the line, and the total impedance is Z = (Z0 + Z1 + Z2). When the device 1 injects harmonic current into the grid (denoted as In), a harmonic voltage (denoted as Un) is generated at the impedance Z of the grid, so that the harmonic voltage Un appears at the power input of the device 2. If the harmonic voltage exceeds the level that the device 2 can withstand, the device 2 will be disturbed by this harmonic voltage. The harmonic distortion allowed by general electronic equipment is UTHD < 5%.

In reality, the equipment 1 is often a device with a drastic change in operating currents such as an intermediate frequency furnace, a frequency converter, and a DC motor driver. The equipment 2 is often a PLC, a numerically controlled machine tool, a computer, a precision measuring instrument, and the like. The adverse effects of harmonics on other equipment are mainly reflected in the following aspects:

Digital control equipment, PLC, CNC machine tools, etc., malfunctions;

The accuracy of signal acquisition systems, measuring instruments, etc. is reduced;

The motor is shaking and overheating.

It can be known from the above principle whether the harmonic source load will cause interference to the electronic equipment on the same power grid, mainly depending on the magnitude of the voltage harmonic distortion of the input line of the power line of the electronic device and the anti-interference ability of the power supply of the electronic device.

When the harmonic source load produces the same harmonic current, the farther the distance from the transformer is, the larger the corresponding grid impedance is, the larger the voltage distortion is, and the easier it is to form the electronic equipment on the same grid. interference. The ability of different electronic devices to resist distortion voltage also has advantages and disadvantages. In the same power supply network, an electronic device may be interfered, which does not mean that all electronic devices will be interfered at this location.

Therefore, for non-linear loads, a power analyzer is required to test whether its harmonic current exceeds the limits specified by the relevant standards. The full range of power analyzers from Zhiyuan PA supports the most common IEC61000-4-7 harmonic test standard in the world. For precision equipment with high power quality requirements, it is required to test the anti-harmonic capability of its power supply. Among them, PA8000 certified power analyzer powerful FFT measurement function can analyze the energy of each frequency point, the minimum resolution is 0.1Hz, through this function you can view the data of each harmonic.

In the actual test, it is necessary to test the interharmonic index of the power input. Currently, only the PA8000 certified power analyzer and the PA6000H and PA5000H support this function. The PA5000H purchased by an authoritative testing organization in China is used for power supply immunity test and is the most suitable solution in the industry.