Mike Thornton of ABB explains how active filtering technology can provide an effective solution to the growing problem of harmonic pollution.
AC electricity supplies with a pure sine wave are now almost a thing of the past. Of course, harmonic pollution is nothing new – the concept has been with us for many years, in large industrial complexes such as the chemical and steel industries, which employed large mercury arc similar rectifiers. The problem, in recent years, has grown considerably due to the proliferation of non-linear devices such as high power inverter drives and static UPS equipment, and has crept down to the building level thanks to the ubiquitous PC, printer and fluorescent light.
An immediate reaction might be to think there is no problem as long as your equipment continues to function correctly. But even when all appears well, un-filtered harmonics can be the ‘invisible killer’ in your system, causing nuisance tripping, mysterious fuse blowing and overheating of cables and transformers, considerably shortening their service life.
A 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, 350 Hz is the 7th and so on. The first effect of harmonic pollution is to increase the RMS and peak value of the distorted waveform. It is possible for a distorted waveform containing harmonics up to the 25th harmonic to have a peak value of more than twice the pure waveform, and an RMS value that is 10 per cent higher.
This increase in RMS value is what 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 may be damaged, and equipment using the supply voltage as a reference may not be able to synchronise, and either apply the wrong firing pulses to switching elements or switch off. Interference with electronic communications equipment may occur.
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 summate in the neutral. Excessive neutral currents are often found at locations where many single phase loads (PCs, faxes, dimmers etc) are in service.
A further motivation for taking action against harmonics is that as well as affecting local systems, they may also disturb equipment in other plants. In order to limit this disturbance, maximum permissible distortion levels have been defined in standards and recommendations such as the recently revised ENA Engineering Recommendation G5/4-1 and BS EN 61000.
All UK consumers have an agreement to connect with their DNO (distribution network operator) and part of any new agreement includes a requirement to meet the conditions of G5/4 -1. Failure to meet these conditions could carry the threat of possible disconnection from the supply should action not be taken to reduce the levels of harmonic distortion generated.
Tackling the problem
Methods employed to limit distortions in the supply network can take a variety of forms. One simple approach has been to move the point of common coupling (PCC), since a load causing a problem at 415V may be trouble-free at 11kV. Phase shifting transformers have also been employed, especially in drive systems where they can eliminate specific harmonic frequencies.
Another common solution is offered by the use of detuned passive filters consisting of a series circuit of reactors and capacitors connected in parallel on the system, with the cut-off frequency fixed at a value just below the lowest order harmonic of concern - typically 210 Hz.
Historically, the primary function of detuned passive filters was to protect the capacitors in PFC (power factor correction) equipment. Whenever standard PFC equipment is applied to an inductive network, there will always be a frequency at which the capacitors are in parallel resonance with the supply. Where harmonic currents are present, this parallel resonant circuit will cause amplification of those currents on the system and in turn contributing to the premature dielectric failure of the PFC capacitors. The passive filter is a simple, cost-effective solution preventing the magnification of the harmonic currents at frequencies above the tuned frequency, whilst contributing to the reduction of harmonic current generated and providing reactive power compensation required in the pursuit of reduced electricity costs.
However, the passive filter does have limitations and its effectiveness in harmonic attenuation relies upon, in general, the availability of lagging vars, the PFC applied and the transformer rating. Where there are a number of harmonic frequencies that need to be reduced to specific levels to meet G5/4-1 requirements, the effectiveness of the passive filter is limited by being tuned to a single frequency.
In order to overcome the problems associated with traditional passive filters, ABB has, over the past seven years, developed and launched the PQF (Power Quality Filter) range of active filters for low voltage applications – see Figure 1. 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 by 180° so that the peak in one coincides with a trough in the other - then they cancel each other out. The PQF, which utilises advances in DSP (digital signal processing) and power electronic switches ie the insulated gate bipolar transistor (IGBT), does this by continuously monitoring the line current in real time (at 40ms intervals) to determine which harmonics are present and then actively generating a harmonic current spectrum with exactly the opposite phase to the components that are selected to be filtered. The two out of phase harmonic signals effectively cancel each other out so that the supply transformer sees a clean sine wave – see Figure 2.
The PQF family covers a wide range of RMS current ratings from 30A up to 3600A with direct connection at voltages up to 690V. Higher voltages are possible, up to approximately 30kV, using special couplings. A key advantage of the PQF design is that it provides a compact solution with a small installation footprint that can be positioned for global or local compensation.
A 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).
All PQF filters have a selectable facility of load current balancing across the phases, between phases and neutral (PQFS) and reactive power compensation. However power factor correction is not one of its main priorities, and since reactive power consumes the filter’s current capability we recommend that, where possible, a de-tuned capacitor bank is used to provide the power factor correction as well as some harmonic cancellation. This then allows the use of a smaller active filter, resulting in lower overall costs.
ABB PQF active filters have been installed in a wide range of industrial and non-industrial applications world-wide including: electrolysis equipment; induction heating; printing press operations; variable speed drives; commercial buildings; banks; dealer floors; call centres.
A typical installation of an active filter was carried out recently at South West Water’s foreshore pumping station in the traditional Cornish fishing village of Padstow. Western Power Distribution, the local DNO, was concerned the pumping station’s drives could give rise to unacceptable harmonic distribution on the local network. So ABB was called in to investigate and implement a solution.
The traces from Padstow in Figure 3 provide a dramatic illustration of active filtering at work. In trace 1 we see the unfiltered 400V AC three-phase supply with serious distortion of the current waveform – instead of two crossover points per cycle there are six. Trace 2 shows what happened when the 70A ABB active filter type PQFL was energized – there is a considerable improvement to the waveform and a 60 per cent reduction in current distortion.
As well as carrying out on site test analysis to solve existing harmonic problems, incorporating active filtration at the project design stage can prevent them from occurring in the first place
There is in fact, a considerable portfolio of harmonic information already in existence regarding the type and magnitude of the harmonic spectrum that can arise in a wide range of load situations. ABB has developed a specific harmonic system design program, and by utilizing this portfolio of harmonic data and experience gained over many years, cost- effective solutions can be incorporated at the design stage.
Harmonics and how to deal with the effects of harmonic distortion has often been seen as a ‘black art’. But thanks to the advent of active filtering technology that reputation is disappearing rapidly as it emerges as a scientific, cost-effective approach to improving network reliability, preventing unexplained outages and prolonging the service life of electrical installations
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