Active Filters - the ultimate solution

Mike Thornton, ABB’s Power Quality Products Manager for the UK, explains how active filter technology can combat network harmonic issues
Recently, some new banking offices required a major power network upgrade. Naturally, the building designers were well aware that they needed to protect PCs and other vital equipment against power outages and other network problems. So they specified multiple transformer infeeds and UPS systems. In addition, backup generators were installed in case the main utility power supply should be interrupted. Finally, power factor correction (PFC) capacitor banks were installed to maintain the power factor at an optimum level.

The designers believed they had done a good job. However, when the installation was energized the following problems occurred:
PCs located some distance from the UPS-systems could not boot-up. Investigation showed that this was due to a high harmonic voltage distortion being present on the line. In addition, due to the unbalanced load current, a neutral current flowed, which together with the harmonic current in the neutral created too high a neutral to earth voltage, which also caused problems for some PCs.
When running in generator backup mode, the harmonic voltage distortion added to the harmonics generated by the UPS systems caused the feeding voltage to become destabilized and the generators tripped out frequently.
To solve these problems, ABB active harmonic filters were installed both upstream and downstream of the UPS systems.
What are harmonics?
An harmonic frequency is simply a frequen­cy that is a multiple of the fundamental frequency. For example, a 250 Hz waveform superimposed on a 50 Hz network is the 5th harmonic. The first effect of harmonic pollution is to increase the RMS (root mean square) and peak value of the distorted waveform.
This increase in RMS value leads to the increased heating of electrical equipment. Furthermore, circuit breakers may trip due to higher thermal or instantaneous levels. Additionally, fuses may blow and power factor correction capacitors may be damaged. The winding and iron losses of motors increases and they may experience perturbing shaft currents. Sensitive electronic equipment control and communications equipment may also be disrupted.
In installations with a neutral, zero-phase sequence harmonics may give rise to excessive neutral currents. This is because they are in phase in the first three phases of the power system and sum together in the neutral. Excessive neutral currents are often found at locations where many single phase loads (PCs, light dimmers, printers etc) are in service.
As well as affecting local systems, harmonics may also disturb equipment in other plants. In order to limit this disturbance, maximum permissible distortions are defined in standards and recommendations such as the ENA Engineering Recommendation G5/4-1 and BS EN 61000.
Where do harmonics come from?
Variable speed drives (VSDs) that allow the accurate control of motor speed and power are increasingly popular as they allow flexible control of processes and improved energy efficiency. However standard drives produce harmonics.
Most UPS systems used to provide clean power for sensitive loads themselves produce harmonics at their input side and will generate some background distortion at their output side if loaded with harmonic producing loads.
PCs and laptops.
Modern lighting systems.
In short, most ordinary loads connected to the supply generate some form of harmonics. If too many of these loads are present in an installation, then problems will start to occur.
How does an active filter work?
The basic concept of the active filter is very simple. If you add two currents, identical in magnitude and frequency, but exactly opposite in phase so that the peak in one coincides with a trough in the other – then they cancel each other out. ABB’s PQF concept does this by continuously monitoring the line current in real time (at 40 ms intervals).
The PQF utilizes closed-loop mea­surement for greater accuracy and can be programmed to filter up to 20 individual harmonics for three-phase systems (15 harmonics in four-wire systems) from the 2nd to the 50th harmonic. It can filter the selected harmonics either until their magnitudes are close to zero (maximum filtering) or until their magnitudes reach a pre-set level (filtering to curve).
Latest PQFS Rating
The PQFS range has been extended to include a 120 A-rated version suited for commercial, residential and light industrial applications. The new version widens the offering for ABB’s customers and the hardware across the full range has been modified for improved protection and ease of maintenance.

Introducing QCap

ABB has over 70 years of expertise in capacitor technologies. This has been incorporated in the development of a range of low-voltage capacitors purpose built to deliver reactive power to address power quality issues in industrial and commercial installations.

Main benefits include:
- Reducing or eliminating expensive utility penalties for low power factor
- Reducing power losses in cables and transformers
- Increasing power transmission capacity in cables
-Increasing available transformer capacity
- Improving voltage stabilization in long cables
The latest addition to the range is the QCap standardized cylindrical design with key features including:
- Dry type design
- Safe sealing and overpressure disconnection system
- ABB in-house metalized film giving excellent dielectric properties
- Optimized thermal dissipation
- Long life
- Highly reliable
- Easy to mount in a capacitor bank
- Complies with international standards, CE marked
- Low loss design
ABB's PQF range of active harmonic filters