5 Ways to Suppress Inverter Harmonics

The fundamental cause of harmonic generation in low-voltage power supply systems is due to the nonlinear characteristics of certain equipment and loads in the power grid (such as high-power rectifier equipment, thyristor rectifier equipment, frequency converters, electronic ballasts, and computers). Because these electronic rectifiers have commutation problems, and their loads are non-linear loads. That is, the waveform distortion caused by the non-linear (proportional) relationship between the applied voltage and the generated current.

Generation of inverter harmonics

Due to the extensive use of nonlinear power electronic components such as thyristors in the inverter. No matter which rectification method is used, the way the inverter draws energy from the grid is not a continuous sine wave, but asks for current from the grid in a pulsating and intermittent way. This pulsating current and the impedance of the grid form a superposition of pulsating voltage drop. On the voltage of the power grid, the voltage is distorted, and the Fourier series analysis shows that this aperiodic sine wave current is composed of the fundamental wave with the same frequency and the harmonics with a frequency greater than the fundamental wave frequency. Harmonics will affect and interfere with the power supply system, loads and other equipment in the line.

5 ways to adjust the harmonics of the inverter:

1. Improve the internal resistance of the power supply;

When the power supply capacity is smaller than the inverter capacity, the internal impedance value is relatively larger, and the harmonic content is smaller; therefore, when selecting the inverter power supply transformer, it is best to choose a transformer with large short-circuit impedance.

2. Assemble the reactor;

Installing a reactor is actually to increase the internal impedance of the inverter power supply from the outside. Install a reactor on the AC side of the inverter or the DC side of the inverter or install them at the same time to suppress harmonic current.

3. Use the multi-phase operation mode of the inverter;

Usually the rectifier part of the inverter is a 6-pulse rectifier, so the generated harmonics are large. If two transformers combined with Y-Δ and Δ-Δ whose phase angles are different from each other by 30° constitute a rectifier equivalent to 12 pulses, the harmonic current can be reduced and harmonic suppression can be achieved.

4. Use appropriate harmonic filters;

• Passive power filter:

Passive filter, also known as LC filter, is a filter circuit composed of a combination of inductors, capacitors and resistors, which can filter out one or more harmonics. The most common and easy-to-use passive filter structure is to combine the inductor In series with a capacitor, it can form a low impedance bypass for the major sub-harmonics (3, 5, 7)

• Active power filter:

The basic principle is to detect the harmonic current from the compensation object, and the compensation device generates a compensation current spectrum with the same magnitude and opposite polarity as the harmonic current to cancel the harmonics generated by the original line harmonic source, so that the The grid current contains only the fundamental component.

At present, in terms of specific harmonic control, a complementary and mixed use of passive filters (LC filters) and active filters has emerged, giving full play to the simple structure, easy implementation, low cost of LC filters, and active power filtering. It has the advantages of good compensation performance and overcomes the disadvantages of large capacity and high cost of active power filters. The combination of the two can make the whole system obtain good performance.

5. Control the carrier ratio value of the inverter;

Increasing the carrier ratio of the inverter can reduce the weak harmonics.

The generation of inverter harmonics will cause great harm to the system, and reasonable measures should be taken to ensure the normal operation of the equipment system.

Xichi Electric provides the following three devices for harmonic control for different users to choose according to their needs.

Active Power Filter

Low-voltage hybrid filter compensation device

Harmonic protector