Understanding the Principles of Active Harmonic Filters (AHF)

Understanding the Principles of Active Harmonic Filters (AHF)

Active Harmonic Filters (AHF) are devices used to improve power quality. They reduce harmonic distortions and compensate for reactive power in electrical systems. The main idea behind AHF is to detect and counteract unwanted harmonic currents and reactive power. This helps make the power system more efficient and stable. Let’s look at how AHF works step by step.

1. Current Detection and Signal Processing

First, the AHF uses external current transformers (CTs) to detect the load current. These transformers are placed in the system to monitor the current. The detected current is then sent to the main control chip of the AHF.

Inside the control chip, the load current is separated into three parts: active current, reactive current, and harmonic current. The active current is the part that does useful work. The reactive current is related to energy stored in inductive or capacitive elements. The harmonic current is the unwanted distortion caused by non-linear loads like variable frequency drives or rectifiers.

After separating the current, the control chip sends the data to a liquid crystal display (LCD). This allows operators to see the system’s performance in real-time.

2. Control Algorithm and PWM Signal Generation

Next, the control chip calculates the required compensating current. It uses the detected reactive and harmonic currents to do this. The goal is to create a current that is equal in size but opposite in direction to the unwanted currents. This is called "reverse compensation."

To achieve this, the AHF uses two control loops: a voltage loop and a current loop. The voltage loop keeps the DC bus voltage stable. The current loop generates the correct compensating current. The control chip then produces Pulse Width Modulation (PWM) signals. These signals are used to control the IGBT switches.

3. IGBT Switching and Energy Interaction

The PWM signals are sent to the IGBT driver circuit. The driver circuit amplifies the signals and sends them to the IGBTs. The IGBTs act as fast switches. They turn on and off quickly to control the flow of current between the DC bus capacitor and the AC power grid.

During this process, the AHF injects compensating currents into the power grid. These currents are opposite to the harmonic and reactive currents in the load. This cancels out the unwanted currents. As a result, harmonic distortion is reduced, and the power factor of the system improves. The power factor shows how effectively electrical power is being used.

4. Feedback Control and Output Monitoring

To ensure accurate operation, the AHF uses a feedback control system. It has internal Hall effect sensors that monitor the output current of the AHF. The sensed current is sent back to the control chip. The control chip compares it with the desired compensating current. If there is any difference, the control chip adjusts the output to correct it.

The output current data is also shown on the LCD screen. This lets operators monitor the AHF’s performance in real-time. They can make adjustments if needed.

5. Pulse Width Modulation (PWM) and Current Waveform Control

Pulse Width Modulation (PWM) is a key part of AHF operation. PWM controls the IGBT switches by varying the width of the pulses in a pulse train. This controls the average voltage or current delivered to the load. In the AHF, PWM is used to shape the output current waveform. The goal is to make the output current match the desired compensating current.

By adjusting the PWM signals, the AHF can create a current waveform that is opposite to the harmonic and reactive currents in the load. This cancels out the unwanted currents, improving power quality.

YT Electric is the biggest OEM manufacturer of low-voltage AHF and SVG with more than 15 years' experience. All products hold certifications for ISO9001, CE, and CQC standards, and type test reports support them.

For more information on how our SVGs can help manage harmonics and improve power quality:

contact us at david@yt-electric.com