Harmonic interference is one of the most common issues that occur during power supply operations. Engineers frequently need to enhance the PFC (Power Factor Correction) circuit in their power products to mitigate such disturbances. But how can they accurately identify each harmonic frequency and its corresponding value while the system is running?
For precision electronic equipment, external interference poses a serious threat—often leading to catastrophic failures. In reality, such interference is widespread. For example, in industrial environments, the power grid is constantly affected by harmonic currents generated by various devices, which can severely impact sensitive electronics connected to the same network. The question is: how do these harmonics form and what makes them so disruptive?
In power systems, there are numerous non-linear loads, such as medium-frequency furnaces, inverters, DC motor drives, and electronic ballasts. These devices cause abrupt changes in current, injecting harmonic currents into the grid. These harmonic currents lead to voltage distortions, which can interfere with PLCs, CNC machines, computers, and other precision instruments, causing malfunctions or unstable operation.
It's important to understand that the harmonic current generated by a non-linear load itself doesn’t directly affect other devices. What causes the disturbance is the harmonic voltage that forms across the grid’s impedance due to the harmonic current. This phenomenon is illustrated in the following diagram:
[Image: Harmonic Voltage Generation]
Here, Device 1 is a source of harmonic current. When it operates, it injects harmonic current (In) into the grid. Since the grid has an inherent impedance (Z), which includes the transformer impedance (Z0), line impedances (Z1 and Z2), the total impedance is Z = Z0 + Z1 + Z2. As a result, a harmonic voltage (Un) is created at this impedance. This voltage appears at the input of Device 2, and if it exceeds the tolerance level of the device, it can cause interference. Most electronic devices are designed to handle a Total Harmonic Distortion (THD) of less than 5%.
[Image: Harmonic Voltage Distribution]
In practice, Device 1 might be a high-current device like a medium-frequency furnace, inverter, or DC drive, while Device 2 could be a PLC, CNC machine, computer, or a precision measurement instrument. The negative impacts of harmonics on these devices include:
- Malfunctioning of digital control systems, PLCs, and CNC machines
- Reduced accuracy in signal acquisition and measuring systems
- Motor vibration and overheating
From the above principles, we can determine whether a harmonic source will interfere with other devices on the same power grid. It depends on the level of voltage distortion at the power input of the device and the device’s ability to resist such distortion.
When the same harmonic current is produced, the farther the distance from the transformer, the higher the grid impedance, leading to greater voltage distortion and more potential for interference. Additionally, different devices have varying levels of immunity to harmonic distortion. Therefore, even if one device is affected, it doesn't mean all devices in the same network will be disturbed.
To address this, it's essential to use a power analyzer to test whether the harmonic current emitted by non-linear loads exceeds standard limits. 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 power analyzer, certified for high-accuracy measurements, offers powerful FFT analysis with a resolution as fine as 0.1Hz, allowing users to clearly view harmonic data across all frequencies.
[Image: PA8000 FFT Analysis]
In real-world testing, it's also important 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 ideal for power supply immunity testing and is considered the best solution in the industry.
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