In little more than ten years, electricity power quality has grown from obscurity to a major issue.Electronic converters and power electronics gave birth to numerous new applications, offering unmatched comfort, flexibility and efficiency to the customers. However, their proliferation during the last decade is creating a growing concern and generates more and more problems: not only these electronic loads pollute the AC distribution system with harmonic currents, but they also appear to be very sensitive to the voltage distortion.
Today, recent advances in power electronic technology are providing an unprecedented capability for conditioning and compensating harmonic distortion generated by the non-linear loads.
Among the different technical solutions, a shunt – current injection mode – active harmonic conditioner is evaluated, and detailed site measurements are presented as confirmation of the unsurpassed performances. This new innovative active conditioner appears to be the easiest of use, the most flexible, the most efficient and cost effective one.
A large proportion of the industrial, commercial and domestic load is now non-linear and the distortion level on the low-voltage distribution network has become a serious concern. The potential problems that could be caused by excessive harmonic voltage on the supply network were recognised long ago, and procedures and standards put in place to limit the distortion. This has been very successful in that problems experienced by customers are nearly always due to conditions within their own site and only rarely imported from the network. If this situation is to be maintained, then consumers must limit the harmonic current they draw. Consequently, customers must ensure that harmonic filtration is provided, where necessary, to achieve this.
Generically speaking, there are three methods available, each with particular advantages and disadvantages. They are:
◆ Passive filters
◆ Transformer solutions - isolation, zig-zag, vector grouping
◆ Active filters .
The examples used here relate to the commercial version produced by MGE UPS Systems Limited and sold under the trade name ‘SineWave’.
Harmonic mitigation equipment may be provided either to satisfy the electricity supplier (i.e. to meet the requirements of G5/4 or local equivalent) or to deal with the problems arising from the harmonic currents within the site. The position and selection of the equipment will be dependent on the particular circumstances and will usually require a detailed harmonic survey.
Where information technology (IT) equipment is in use, all odd harmonics will be present leading to problems such as the overloading of neutrals by triple-N (i.e. the odd multiples of three) harmonics. Such problems can be eased by good design practice - by rating the cables correctly at installation time - but, often, changes in building function and layout mean that these problems arise long after the building has been commissioned. The problem is compounded by the fact that office accommodation is frequently reorganised, so that circuits that were once relatively ‘clean’ become heavily polluted. In other words, the harmonic culture of the building changes as new equipment is added and existing equipment relocated. These changes are usually planned without regard to the effect that they may have on the electrical infrastructure.
Replacing cables in a working building can be very expensive and far too disruptive to contemplate, so other mitigation methods are required. Passive filters are possible, but it is quite difficult to design an efficient third harmonic passive shunt filter. Any passive filter will deal only with harmonic frequencies it was designed for, so individual filters will be required for other troublesome frequencies. In any case, as the harmonic culture changes, passive filters may have to be replaced or supplemented. Zig-zag transformers and delta wound isolation transformers are effective against triple N harmonics but have no effect on other harmonics. In this type of application, the active harmonic conditioner is a very good solution.
Topologies of active harmonic conditioners
The idea of the active harmonic conditioner is relatively old, however the lack of an effective technique at a competitive price slowed its development for a number of years. Today, the widespread availability of insulated gate bipolar transistors (IGBT) and digital signal processors (DSP) have made the AHC a practical solution.
The concept of the AHC is simple; power electronics is used to generate the harmonic currents required by the non-linear loads so that the normal supply is required to provide only the fundamental current.
The load current is measured by a current transformer, the output of which is analysed by a DSP to determine the harmonic profile. This information is used by the current generator to produce exactly the harmonic current required by the load on the next cycle of the fundamental waveform. In practice, the harmonic current required from the supply is reduced by about 90 %.
Because the AHC relies on the measurement from the current transformer, it adapts rapidly to changes in the load harmonics. Since the analysis and generation processes are controlled by software it is a simple matter to programme the device to remove only certain harmonics in order to provide maximum benefit within the rating of the device.
A number of different topologies have been proposed and some of them are described below. For each topology, there are issues of required components ratings and method of rating the overall conditioner for the loads to be compensated.
Series conditioners
This type of conditioner, connected in series in the distribution network, compensates both the harmonic currents generated by the load and the voltage distortion already present on the supply system. This solution is technically similar to a line conditioner and must be sized for the total load rating.
Parallel conditioners
Also called shunt conditioners, they are connected in parallel with the AC line and need to be sized only for the harmonic power (harmonic current) drawn by the non-linear load(s). This type is described in detail later.
Hybrid conditioners
This solution, combining an active conditioner and a passive filter, may be either of the series or parallel type. In certain cases, it may be a cost-effective solution. The passive filter carries out basic filtering (5th order, for example) and the active conditioner, due to its precise and dynamic technique, covers the other harmonic orders.
Operating principle of the parallel active harmonic conditioner
The active conditioner is connected in parallel with the supply, and constantly injects harmonic currents that precisely correspond to the harmonic components drawn by the load. The result is that the current supplied by the power source remains sinusoidal.
The entire low-frequency harmonic spectrum, from the second to the twenty fifth harmonic, is supported.
If the harmonic currents drawn by the load are greater than the rating of the AHC, the conditioner automatically limits its output current to its maximum rating; the conditioner cannot be overloaded and will continue to correct up to the maximum current rating. Any excess harmonic current will be drawn from the supply; the AHC can run permanently in this state without damage.
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