Power Quality

Degradation in Power Quality (PQ)

One of the properties of electricity is that some of its characteristics depend not only on the electricity producer/distributor but also on the equipment manufacturers and the customer. The large number of players combined with the use of terminology and definitions which may sometimes be imprecise partly explain why this subject area is so complex.

This “Cahier Technique” aims to facilitate exchanges on this topic between specialists and non-specialists, as well as customers, manufacturers, installers, designers and distributors. The clear terminology used should help avoid confusion. It describes the main phenomena causing degradation in Power Quality (PQ), their origins, the consequences for equipment and the main solutions. It offers a methodology for measuring the PQ in accordance with differing aims. Illustrated with practical examples for the implementation of solutions, it shows that only by observing best practice and by applying strict methodology (diagnostics, research, solutions, implementation and preventive maintenance) can users obtain the right quality of power supply for their requirements.

Contractual application

Within the context of a deregulated market, contractual relations may exist not only between the electricity supplier and the end user, but also between the power production company and transmission company or between the transmission company and distribution company. A contractual arrangement requires that terms are defined jointly and mutually agreed upon by all parties. The parameters for measuring quality must therefore be defined and the values compared with predefined, i.e. contractual limits. This arrangement frequently requires the processing of significant quantities of data.

Corrective maintenance

Even where best practice is observed (singleline diagram, choice of protective devices and neutral point connection, application of appropriate solutions) right from the design phase, malfunctions may occur during operation:

• Disturbances may have been ignored or under-estimated.
• The installation may have changed (new loads and/or modification).

Troubleshooting is generally required as a consequence of problems of this nature. The aim is frequently to get results as quickly as possible, which may lead to premature or unfounded conclusions. Portable measurement systems (for limited periods) or fixed apparatus (for continuous monitoring) make it easier to carry out installation diagnostics (detection and archiving of disturbances and triggering of alarms).

Optimising the operation of electrical installations

To achieve productivity gains (operational economies and/or reduction of operating costs) correct operation of processes and sound energy management are required, both of which are factors dependent on PQ.

Operating, maintenance and management personnel of service sector and industrial sites all aim for a PQ which matches their requirements. Complementary software tools to ensure control-command and continuous monitoring of the installation are thus required.

Statistical surveys

Such research requires a statistical approach on the basis of wide-ranging results from surveys generally carried out by the operators of transmission and distribution power systems.

• Benchmark the general performances of a power system These can be used, for example, to:
  • Plan and target preventive actions by mapping disturbance levels on a network. This helps reduce operating costs and improve control of disturbance. An abnormal situation with respect to an average level can be detected and correlated with the addition of new loads. Research can also be carried out into seasonal trends or excessive demand.
  • Compare the PQ of various distribution companies in different geographical areas. Potential customers may request details of the reliability of the electricity supply before installing a new plant.
• Benchmark performances at individual points on the power system These can be used to:
  • Determine the electromagnetic environment in which a future installation or a new piece of equipment may have to operate. Preventive measures may then be taken to improve the distribution power system and/or desensitise the customer power system.
  • Specify and verify the performance levels undertaken by the electricity supplier as part of the contract. This information on the electricity quality are of particular strategic importance for electricity companies who are seeking to improve competitiveness, satisfaction of needs and customer loyalty in the context of liberalising energy markets.

Electromagnetic compatibility (EMC)

Electromagnetic compatibility is the ability of an equipment or system to fonction satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to anything in that environment (IEC 60050-161).

AUTHOR: Schneider Electric expert | Philippe FERRACCI