Practical applications of active power filters (III) [Part 2/5: Active harmonic filters (AHF) for information technology 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 information technology equipment

Achieving high power quality and energy efficiency in today's energy-constrained world is a huge challenge. This is especially true for critical process facilities like banks. Power quality problems can set out within a fraction of a second and can bring huge costs in downtime. Banks rely heavily on the quality and reliability of supplied power for the success of their operations. They are facilities with a large number of information technology (IT for short) equipment like computers, servers, printers, wired or mobile phones and other IT peripherals that have substantial power requirements.

Banks need to ensure that uptime of their processes is maximised but they are highly susceptible to even the most common power quality problems. The main consequences of poor power quality in banks can be summarised as:

• Revenue losses: Poor power quality and unreliable power supply can cause losses in terms of lost data, resources and productivity. The entire infrastructure and the equipment critical to the functioning of these facilities are also at risk.
• Failing customer service: Customer service is bound to go down with poor power quality. This ultimately brings down the level of customer satisfaction which in turn affects the client base of a particular facility.
• Service interruptions and downtime: Poor power quality and related unexpected problems cause interruption in the smooth functioning of the facility and downtime as well.

Requirements

Background

A bank was suffering from severe harmonic distortion that was affecting its operations caused mainly by computers, monitors, projectors, printers, scanners and servers. They needed to secure the operations by eliminating harmonic distortions in order to maintain the quality and integrity of their business processes. The power factor of the facility was also not good, and this was bringing extra costs in their electricity bills.

System description

The loads of the bank are divided between three distribution boards where the IT equipment is connected and balanced between the three-phases and the neutral.

Solution

Analysis

To be able to dimension a solution it is necessary to collect power quality measurement data from the different distribution boards of the bank over a period of time by using a power quality analyser.

The measurement results of the different distribution boards were very similar. As an example, the existing values at one of the distribution boards before applying a solution were:

• THDi: 25%.
• Power factor: 0.85 (ind.).

Proposed solution

Based on the analysis of the measurements, it was possible to dimension a solution for the bank that would comply with owner’s requirements of reducing the amount of harmonics to be able to comply with a THDi under 5% and at the same time improving the power factor of the whole facility. It was decided to use an active harmonic filter rated 120 A installed on each distribution board of the bank.

Because of the nature of the loads and the application, and the need for real-time harmonic filtering, using conventional solutions like passive harmonic filters or capacitor banks was not an option.

Based on the values monitored, the following functions are proposed for the AHF.

The results of the measurements done at one of the distribution boards after applying the solution show:

• Harmonics THDi reduced from 25% to below 5%.
• Power factor improved from 0.85 to 0.99.

Conclusions

Critical process facilities like banks have evolved to become large power consumers. Their supporting infrastructure, such as cooling and power distribution, consumes big quantities of. electric power, and their IT equipment generates power quality and energy efficiency problems that affect their secure and reliable operation. Therefore, the power quality and energy efficiency of these facilities are important topics that should be addressed preferably at project design stage or if not possible, then during the operation stage.

The installation of active harmonic filters in banks brings several benefits:

• Reduction of harmonic distortion.
• Power factor improvement achieving a reduction in kVA maximum demand, in losses and in the electricity bill.
• Facility’s electrical equipment lifetime increased.
• Load current reduction.
• Increased utilisation capacities for power cables and power devices.

If you would like to receive any of my publications on the topic or to explore how #ActivePowerFilters can benefit your application, feel free to reach me at [email protected].?

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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.