Product Introduction

According to the requirements of the IEEE 519 standard, the total harmonic distortion rate (THD) must be less than 5%; use AHF to cover the 2-50th harmonic compensation to meet the IEEE 519 standard.

The AHF uses an external Transformer CT to detect the current or load current in real time. The AHF based on 3-level topology .The DSP+FPGA controller extracts the base sine wave current and each harmonic current of the load through the fast Fourier transform, and obtains active, reactive, and negative sequence data through the dq rotating coordinates. Then, the IGBT power converter generates a compensated current with same amplitude of the harmonic wave, but reverse polarity(harmonic, reactive, unbalanced), and compensated current is injected into the grid to achieve the power quality improvement function on the power grid side to eliminate the harmonic waves in the system.

AHF control principle diagram

AHF system topology (NPC three-level architecture)

AHF operation principle diagram

List of AHF Technical Parameters

Harm of harmonics and causes（References: Fuji Electric）

There are lots of electrical devices that have nonlinear operating characteristics even when applied voltage is sinusoidal in nature, the current drawn by the device is non-sinusoidal in nature. These nonlinear devices used in power distribution circuits create nonlinear currents and which subsequently causes voltage distortions. These nonlinear currents and voltages have been generally referred to as harmonic currents and voltages. These harmonics if disregarded or undetected may cause harmonic resonant conditions, which could present system operating problems resulting in complaints from customers and reduced life of power equipment as well as degraded efficiency and performance. Harmonic currents and voltages can cause many unfavourable effects on the power system itself and the connected loads. Malfunctioning of electronic equipment, capacitor failure, transformer and neutral conductor overheating, excessive heating in rotating machinery are some of these effects.

how do AHF work?(References: Eaton)

Harmonics come from the nonlinear load. Active harmonic filters, also called harmonic correction units, are parallel devices that act like a noise cancellation system and inject equal and opposite frequencies to mitigate harmonics. The filters can also provide additional current to correct the power factor. So, what’s left coming from the source flowing back to the utility is only a nice, clean current that is in phase.

For example, if we run four 6-pulse variable frequency drives at the same time, we have a harmonic spectrum of 5th and 7th and 11th and 13th harmonics. The waveform will show a significant amount of harmonic current in the total harmonic distortion. When the active harmonic filter is turned on, it will inject equal and opposite harmonics to cancel what’s there. The waveform now will be clean and in phase. If we go back and look at the harmonic spectrum, the current distortion is very low. 

Adding active harmonic filters can be a good harmonic solution for power systems. Though more costly than other options, if you have multiple drives running all the time and multiple drives as backup, harmonic filters would be a reliable way to capture any and all of the harmonics coming from the loads. However, one important thing to know from a system design standpoint is that your drive should have either a DC choke or an AC line reactor to minimize the harmonics coming out of the drives.

Active harmonic filters typically come in 50, 75, 100, 200 and 300 ampere units that you can parallel. Another benefit to using active harmonic filters is that you cannot overload them, because once they put out the maximum harmonic current and power factor correction that they can, they stop producing at that level—whether it’s 100 or 120 amperes, whatever the case may be.

Using active harmonic filters on your power system provides a system-level solution for internal and external harmonic protection.