The continuity of supply (CoS) measuring reliability and the voltage quality are important issues in the electricity distribution sector. In general, the quality of service is related to the commercial relations with customers and technical perspectives. According to the American Public Power Association (APPA), reliability, from a system engineering perspective, is the ability of an electric system to perform its functions under normal and extreme circumstances.
For most countries the main issues related to the technical aspects of quality of service can be grouped in two main fields of power quality:
* Continuity of supply (CoS) or supply quality is measured by means of interruptions in electricity supply identifying the events during which the voltage at the supply terminals of a network user drops to zero or nearly zero. These interruptions of supply are generally described by two quality dimensions, the number of interruptions and their duration. On system level, most common continuity indices related to long interruptions are SAIDI (System Average Interruption Duration Index), SAIFI (System Average Interruption Frequency Index), CADI (Customer Average Interruption Duration Index), and ENS (Energy Not Supplied). Most countries use separate classifications for planned (notified) and unplanned interruptions, and there is not a single standard if exceptional events (usually weather events) are included (or not) in the calculation of SAIDI and SAIFI.
* Product or voltage quality covers a subset of possible variation of voltage characteristics from the desired values (excluding interruptions) such as: supply voltage variations; rapid voltage change; voltage swells; flickers; voltage unbalance; harmonic voltage distortions; transient overvoltage; and mains signaling voltage. For example, delivery of high-quality, flicker-free power are important considerations for industrial or commercial customers due to equipment damage and data loss. Large industrial customers that are energy intensive can suffer significant financial losses when voltage dips occur at their sites, but this is less relevant in residential customers. Measuring the voltage quality however can be a complex task due to technical difficulties to select the proper indicator and establish the limits. Standard EN50160 establishes the limits set for voltage disturbances.
To face these challenges, due to a managerial decision or encouraged by the current regulation, the search for high levels of service must be present in the following pillars: (i) quality indicators: selecting indicators to describe their performance; (ii) performance standards: level of quality that company is expected to supply; and (iii) financial incentives to void penalization for the performance below the standard.
