Voltage Quality Standards - KPI

The term voltage quality (or power quality) is an umbrella concept for a variety of disturbances within a power system. The quality of delivered electricity is hard to describe and quantify. The quality is mainly determined through the quality of the voltage waveform as it is impossible to control the currents drawn through customer loads. Voltage quality is not just the responsibility of the network operator but also, in certain respects, depends on customers and producers.

Commonly, voltage quality covers a range of factors involving interruptions. There are various technical standards for voltage quality criteria, but in the end the quality is directly and indirectly determined through the ability of customer equipment to perform properly. Therefore customer awareness about power quality is highly subjective. A good definition of voltage quality should thus incorporate the impact of the (lack of) quality on the customer. In practice technical parameters such as frequency, voltage level or harmonics are used to imply the voltage quality. While definitions are not fully consistent in standards, the most relevant quality phenomena are the same.

For power systems these phenomena are given below:

• Frequency variations;
• Fluctuations of voltage magnitude;
• Short-duration voltage variations (dips, swells and short interruptions);
• Long-duration voltage variations (over- or under-voltages);
• Transients (temporarily transient over voltages);
• Unbalance; and
• Interruptions.