Harmonic interference is one of the most common issues encountered during power supply operations. Engineers frequently need to upgrade Power Factor Correction (PFC) circuits to enhance the performance of their power products. But how can they accurately identify and measure each harmonic frequency and value when the power supply is in operation?
For precision electronic equipment, external interference is a major concern. It can lead to catastrophic failures. In reality, such interference is widespread. For example, in industrial environments, the power grid is often subjected to harmonic currents from various sources, which can significantly affect sensitive electronic devices connected to the same network. So, what exactly causes this type of interference?
In the power grid, there are numerous non-linear loads—such as medium-frequency furnaces, inverters, DC motor drives, and electronic ballasts—that cause large fluctuations in current. These devices inject harmonic currents into the grid, leading to voltage distortion. This distortion can interfere with PLCs, CNC machines, computers, and other precision instruments, causing them to malfunction.
It’s important to understand that the harmonic current generated by a non-linear load doesn’t directly affect other devices. Instead, the real issue arises when these harmonics create harmonic voltages due to the impedance of the grid. The figure below illustrates this phenomenon:
[Image: Harmonic Voltage Generation]
Here, Device 1 generates harmonic currents that are injected into the grid. Since the grid has an impedance (Z), including transformer impedance (Z0), line impedances (Z1 and Z2), the total impedance is Z = Z0 + Z1 + Z2. When Device 1 injects harmonic current (In), it creates a corresponding harmonic voltage (Un) across the grid impedance. This harmonic voltage appears at the input of Device 2, potentially causing disturbances if it exceeds the device's tolerance level. Most electronic equipment can handle a Total Harmonic Distortion (THD) of less than 5%.
[Image: Harmonic Impact on Equipment]
In practice, Device 1 could be a device with highly variable current, like a medium-frequency furnace or a DC drive, while Device 2 might be a PLC, CNC machine, computer, or a precision measuring instrument. The effects of harmonics on such devices include:
- Malfunctions in digital control systems and PLCs
- Reduced accuracy in signal acquisition and measurement systems
- Motor vibration and overheating
From the above explanation, it becomes clear whether a harmonic source will interfere with other equipment depends on two factors: the level of voltage distortion at the input of the affected device and the device's ability to resist harmonic interference.
When the same harmonic current is produced by the source, the farther the distance from the transformer, the higher the grid impedance, and thus the greater the voltage distortion. This increases the likelihood of interference for devices connected to the same power grid. Additionally, different devices have varying levels of immunity to voltage distortion. Therefore, even if one device is affected, it doesn't necessarily mean all devices in the same location will be impacted.
To address this, it's essential to use a power analyzer to test whether the harmonic current from non-linear loads exceeds the limits set by relevant standards. Zhiyuan PA’s full-range power analyzers support the widely used IEC61000-4-7 harmonic test standard. For precision equipment requiring high power quality, testing the power supply’s anti-harmonic capability is crucial. The PA8000 certified power analyzer features a powerful FFT function that can analyze the energy at each frequency point with a minimum resolution of 0.1Hz, allowing users to view detailed harmonic data.
[Image: PA8000 FFT Analysis]
In actual testing, it's also necessary to evaluate the interharmonic index of the power input. Currently, only the PA8000, PA6000H, and PA5000H models support this feature. The PA5000H, purchased by a leading Chinese testing organization, is widely recognized as the best solution for power supply immunity testing in the industry.
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