Practical applications of active power filters (V) [Part 1/2: Active harmonic filters (AHF) for testing of electrical equipment]

Active harmonic filters

Active harmonic filters (AHF for short) have been around since the beginning of the 1990s. Description of their topology and operating principle can be found as far back as 1990. They were developed as a customised design of shunt active power filters (APF for short) to take care of the increasing harmonic problems in the electric power system caused by the widespread use of nonlinear equipment like variable speed drives (VSD for short) or switched-mode power supplies (SMPS for short) that conventional passive solutions like reactors, passive harmonic filters (PHF for short), K-factor transformers and isolation transformers, or conventional active solutions like active front ends (AFE for short) could not handle.

AHFs can be applied to small, medium or large installations in a wide range of segments. They have many low and high voltage potential applications where their use offers many benefits including equipment using variable speed drives, arcing devices, equipment using switched-mode power supplies, lighting devices, solar inverters, wind turbine generators and saturable or rotating devices, to name a few.

Functions

AHFs eliminate waveform distortions from the loads like harmonics, interharmonics and notching, by injecting in real-time in the electric power system the distorted current of same magnitude but opposite in phase. They can also work as harmonic generators for harmonic injection testing purposes.

Modern AHFs can take care of several power quality problems and support the development of clean energy by combining different control functions in a single device.

Connection

An AHF is a power electronics-based shunt compensation device connected in parallel with the equipment generating the power quality problems or that has issues to comply with grid code and energy efficiency requirements. The AHF behaves as a controlled current source providing any kind of current waveform (in terms of phase, amplitude and frequency) in real time (typical reaction time is under 50 microseconds and typical overall response time is under 100 microseconds).

AHFs can be connected to the electric power system as 3-wire or 4-wire devices:

• 3-wire AHFs are typically used for industrial and generation applications where there are VSDs or other nonlinear generators and loads present.
• 4-wire AHFs are typically used for applications in facilities where there are nonlinear loads like switched-mode power supplies and information technology equipment. They can filter triplen harmonics in the neutral conductors.

The most common operating voltage range for AHFs is 200 V up to 690 V as they are built using low voltage IGBT switches. It is possible to connect them to higher voltages using a suitable step-up transformer.

AHFs for testing of electrical equipment

Controlled and selectable real-time harmonic current injection can be used for validating the performance of different components of the electric power system. Harmonic current injection tests are also on many occasions a mandatory requirement during manufacturing process tests, factory acceptance tests (FAT for short) or site acceptance tests (SAT for short) of electrical equipment.

Active harmonic filters can work as real-time three-phase generators of harmonic currents. They can be used for testing electrical equipment in a realistic environment with site-like conditions. For example, an active harmonic filter can simultaneously load the tested equipment with fundamental current plus specified harmonic currents, which can also be modified in respect of amplitude.

Requirements

Background

A semiconductor fabrication plant is suffering from harmonic problems at a production line. The plant owner decided to install active harmonic filters to take care of those problems. The owner also requested as part of the site acceptance tests for the active harmonic filters that they would be tested in realistic harmonic loading conditions but, for safety reasons, without connecting them to the machinery of the production line. This means that the solution for testing the installed active harmonic filters should be able to:

• Simultaneously load the tested active harmonic filters with fundamental current plus specified harmonic currents.
• Offer the possibility to select the harmonic orders of the generated currents.
• Offer the possibility to control the amplitude of the generated currents.

System description

The electric power system of the production line has several variable speed drives installed in the production machinery that are generating high harmonic currents causing disturbances in the installation and affecting the operation of sensitive equipment.

Active harmonic filters installed together with the problematic production machinery were the chosen solution to mitigate the harmonics at the production line and also to comply with local harmonic distortion limits.

Solution

Analysis

The collected measurement data from the power quality analyser indicated the magnitude and order of the harmonics present in the system. The production machinery has variable speed drives, so the harmonics present are mainly of the 5th, 7th, 11th and 13th orders.

Proposed solution

The harmonic generation functionality of active harmonic filters have several potential applications where its use offers many benefits.

• Factory or site testing of power quality improvement solutions like active harmonic filters, hybrid var compensators and passive harmonic filters.
• Harmonic injection testing of electrical equipment like protection relays, transformers, circuit breakers, motors and variable speed drives.
• Testing laboratories at R&D facilities.
• Education and formation courses at technical universities, polytechnics and training institutions.

In this case, the proposed solution was to temporary install an AHF rated 415 V 75 A in parallel with the installed active harmonic filter to be able to conduct the site acceptance test as required by the customer.

Active harmonic filters can simultaneously load the tested equipment with fundamental current plus specified harmonic currents that can also be modified in amplitude.

Conclusions

Active harmonic filters are flexible devices that can provide customers with lots of different functionalities. They can work as harmonic generators for validating the performance of different components of the electric power system.

The main benefits of active harmonic filters used for harmonic current injection testing of electrical equipment can be summarised as:

• Flexible connection and simple operation compared with conventional harmonic generators.
• They allow manufacturing, factory or site testing of new or existing electrical equipment.
• Compact size and simple installation.

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About the author:

Pedro Esteban is a versatile, multicultural and highly accomplished marketing, communications, sales and business development leader who holds since 2002 a broad global experience in sustainable energy transition including renewable energy, energy efficiency and energy storage. Author of over a hundred technical publications, he delivers numerous presentations each year at major international trade shows and conferences. He has been a leading expert at several management positions at General Electric, Alstom Grid and Areva T&D, and he is currently working at Merus Power Plc.