Power Quality Monitoring
The reliability of an electrical in a facility supply must be known and resilience of the process to variations must be understood. In reality, of course, electricity is very different from any other product – it is generated far from the point of use, is fed to the grid together with the output of many other generators and arrives at the point of use via several transformers and many kilometres of overhead and possibly underground cabling. Where the industry has been privatised, these network assets will be owned, managed and maintained by a number of different organisations. Assuring the quality of delivered power at the point of use is no easy task – and there is no way that sub-standard electricity can be withdrawn from the supply chain or rejected by the customer.
From the consumers’ point of view the problem is even more difficult. There are some limited statistics available on the quality of delivery power, but the acceptable quality level as perceived by the supplier (and the industry regulator) may be very different from that required, or perhaps desired, by the consumer. The most obvious power defects are complete interruption (which may last from a few seconds to several hours) and voltage dips or sags where the voltage drops to a lower value for a short duration. Naturally, long power interruptions are a problem for all users, but many operations are very sensitive to even very short interruptions. Examples of sensitive operation are: Continuous process operations, where short interruptions can disrupt the synchronisation of the machinery and result in large volumes of semi-processed product. A typical example is the paper making industry where the clean-up operation is long & expensive. Multi-stage batch operations, where an interruption during one process can destroy the value of previous operations. An example of this type is the semiconductor industry, where the production of a wafer requires a few dozen processes over several days and the failure of a single process is catastrophic. Data processing, where the value of the transaction is high but the cost of processing is low, such as share and foreign exchange dealing. The inability to trade can result in large losses that far exceed the cost of the operation. These are examples of the most sensitive industries, but it is surprising how many apparently mundane operations have quite critical power supply requirements. Example include large retail units with computerised point of sale and stock control equipment and manufacturing plant with distributed control. So, what do we mean by ‘power quality’? A perfect power supply would be one that is always available, always within voltage and frequency tolerances, and has a pure noise-free sinusoidal wave shape. Just how much deviation from perfection can be tolerated depends on the user’s application, the type of equipment installed and his view of his requirements. Power quality defects – the deviations from perfection – fall into five categories:-
Harmonic distortion Blackouts Under or over voltage Dips (or sags) and surges Transients
Each of these power quality problems has a different cause. Some problems are a result of the shared infrastructure. For example, a fault on the network may cause a dip that will affect some customers and the higher the level of the fault, the greater the number affected, or a problem on one customer’s site may cause a transient that affects all other customer on the same subsystem. Other problems, such as harmonics, arise within the customer’s own installation and may or may not propagate onto the network and so affect other customers. Harmonic problems can be dealt with by a combination of good design practice and well proven reduction equipment. As the majority of voltage dips and interruptions originate in the transmission and distribution system and are the responsibility of the supplier, harmonic problems are almost always the responsibility of the consumer. It is harmonic currents that cause problems in installations and when these currents flow back into the supply impedance at the point of common coupling, a harmonic voltage is developed. This voltage distortion, or at least some components of it, are distributed around the system and are combined with the background harmonic voltage distortion present in any transmission system (due to the non-linearity of transformers for example). By limiting the harmonic current the level of voltage distortion on the supply is kept within acceptable limits. Transient disturbances are high frequency events with durations much less than one cycle of the supply. Causes include switching or lightning strikes on the network and switching of reactive loads on the consumer’s site or on sited on the same circuit. Transients can have magnitudes of several thousand volts and so can cause serious damage to both the installation and the equipment connected to it. Nevertheless, non-damaging transients can still cause severe disruption due to data corruption. The generation and influence of transients is greatly reduced and the efficacy of suppression techniques greatly enhanced where a good high integrity earthing system has been provided. Such an earthing system will have multiple ground connection and multiple paths to earth from ant point, so ensuring high integrity and low impedance over a wide frequency band. Ensuring good power quality requires good initial design, effective correction equipment, co-operation with the supplier, frequent monitoring and good maintenance. In other words, it requires a holistic approach and a good understanding of the principles and practice of power quality improvement.