Power quality and electromagnetic compatibility
Introduction
The term power quality seeks to quantify the condition of the electrical supply. It not only relates largely to voltage but also deals with current and it is largely the corrupt- ing effect of current disturbances upon voltage. Power quality can be quantified by a very broad range of parameters, some of which have been recognized and studied for as long as electrical power has been utilized. However, the advent of the term itself is more modern and it has created a useful vehicle for discussing and quantifying all factors that can describe supply quality.
Power quality is yet another means of analysing and expressing electromagnetic compatibility (EMC), but in terms of the frequency spectrum, power quality characterizes mainly low-frequency phenomena. Perhaps because of this and because of the manner in which it affects electrical equipment, power quality has largely been dealt with by engineers with electrical power experience rather than those with an EMC expertise. In reality, resolving power problems can benefit from all available expertise, particularly since power quality disorders and higher frequency emissions can produce similar effects.
In 1989, the European Community defined the supply of electricity as a product, and it is therefore closely related to the provisions and protection of the EMC Directive (89/336/EEC), but in drawing a comparison between electricity and other manufactured product it is essential to recall a significant difference. Electricity is probably unique in being a product which is manufactured, delivered and used at the same time. An electricity manufacturer cannot institute a batch testing process for example and pull substandard products out of the supply chain. By the time electricity is tested it will have been delivered and used by the customer whether it was of good quality or not.
What determines power quality
Arguably, power quality is not an absolute quantity. Many users of electrical power would consider their electrical supply to be of adequate quality, provided the output of the process it is used to supply is of a high standard. For example, if the desktop computer functions without interruption, temporary resets or voltage-related component failures, then the quality of its electrical supply would be considered perfectly accept- able. Industry would not consider a supply voltage with repeated temporary 100 ms dips to be of poor quality if it did not have an affect on the manufacturing processes.
So power quality is a perception driven by the tolerance (susceptibility being the more usual EMC parlance) of the electrical devices that depend upon it. Therefore, the power quality parameters that have evolved to describe acceptable limits are largely an expression of equipment tolerance.
Importantly, although utilities may declare the power quality limits, their attempts to meet this may not be achieved at all times and there is a finite probability that power quality will sometimes fall outside these limits. If the number of times that a supply falls outside the declared power quality limits is more than can be economically tolerated due to the adverse effects upon plant and equipment, then mitigating measures should be taken.
Supply characteristics
The complexity introduced by the length of supply distribution circuits, environmental factors, loading characteristics and the natural degradation of the distribution equipment means that power quality can differ significantly with location.
In general, power quality tends to degrade progressively, the greater the distance is from the source of generation. This is because the supply impedance is lowest near the terminals of the generating systems or where interconnections exist between several sources, and from such points the supply impedance increases along lines and cables, and through transformers. In addition, the predominantly radial nature of power distribution means that load current fluctuations tend to have a more pronounced effect closer to where the load is concentrated.
The power quality at the Point of Common Coupling (PCC) with the public elec- tricity supply network defines the quality of the electricity supply to the consumer. For many years the electricity supply industry in the UK has applied planning limits and set standards through engineering recommendations, such as G5/4 (references 14A and 14B) for harmonics. In recent years CENELEC has created EN 50160 (reference 14C) which defines the standards of power quality that is common in Europe for medium-voltage (less than 35 kV) and low-voltage (less than 1 kV) systems. EN 50160 effectively defines the electrical supply environment that European consumers can expect to experience rather than setting the standards for supply. It has usefully added to the awareness of power quality by encompassing within one document the broad spectrum of power quality parameters defined in Table 14.1. It has not set tighter standards, but merely characterized those which already exist.
In order to set immunity levels for equipment, it is first necessary to define the electrical environment. This is defined by IEC 61000-2-2 (reference 14D) which sets the compatibility levels for low-frequency conducted disturbances on low-voltage net- works. These levels are less stringent than EN 50160. Therefore, not surprisingly, the margin between voltage characteristic levels (as defined by EN 50160) with the end-user equipment immunity (as defined by IEC 61000-6-1 and IEC 61000-6-2) shows that the level of immunity for most power quality phenomena would be insufficient to protect end-user equipment adequately. In practice, the actual level of disturbance is likely to be equal or less than the immunity levels defined by generic standards, and the likelihood of a disturbance affecting the compliant end-user equipment is greatly reduced.