Standards for Monitoring Power Quality | Electricity

Standards are very important in the area of power quality monitoring. Power quality levels must be defined consistently and characterized using the same methods if they are going to be compared from one site to another and from one system to another. IEEE 1159 is the IEEE.

Working Group that coordinates the development of power quality monitoring standards. The existing IEEE 1159 provides general guidelines and definitions for monitoring power quality, and there are three separate task forces that are working on more specific guidelines and requirements. Much of this work is being coordinated with IEC activities so that the monitoring requirements can be more consistent internationally. This article describes the most important IEEE and IEC standards. These standards are a moving target as there are many groups working on new developments.

Standard # 1. IEEE 1159: Guide for Power Quality Monitoring:

IEEE Standard 115915 was developed to provide general guidelines for power quality measurements and to provide standard definitions for the different categories of power quality problems. These definitions were provided previously and provide the basis for a common language in describing power quality phenomena.

Power quality monitoring equipment can use this language to correctly differentiate between different power quality variations and disturbances. After publication of the basic monitoring guidelines, working groups were established for development of more advanced guides for power quality monitoring. Three working groups were established. Progress can be tracked at the IEEE 1159.

The IEEE 1159.1 Working Group is developing guidelines for instrumentation requirements associated with different types of power quality phenomena. These requirements address issues like sampling rate requirements, synchronization, A/D sampling accuracy, and number of cycles to sample.

The IEEE 1159.2 Working Group is developing guidelines for characterizing different power quality phenomena. This includes definition of important characteristics that may relate to the impacts of the power quality variations (such as minimum magnitude, duration, phase shift, and number of phases for voltage sags). Example waveforms have been collected illustrating the importance of different characteristics of the power quality variations.

Recently, the work of the IEEE 1159.1 and 1159.2 Working Groups has been combined into a single task force and is being coordinated with the development on an international standard for characterizing power quality variations with monitoring equipment—IEC 61000-4-3016.

The IEEE 1159.3 Working Group is defining an interchange format that can be used to exchange power quality monitoring information between different applications. IEEE developed the COMTRADE format for exchanging waveform data between fault recorders and other applications, such as relay testing equipment. A more complete data interchange format is needed for power quality data, which can include harmonic spectra, rms envelopes, characterized power quality data, and statistical power quality data, as well as steady-state and disturbance waveforms.

The new Power Quality Data Interchange Format (PQDIF) has been defined, and the standard is being balloted at the time of this writing. The common data interchange format will allow software developers to develop applications for analyzing power quality events and problems independently from the manufacturers of the actual power quality monitoring equipment.

Standard # 2. IEC 61000-4-30: Testing and Measurement Techniques—Power Quality Measurement Methods:

IEC standards for monitoring power quality phenomena are provided in a series of documents with the numbers 61000-4-xx. The individual standards in this series cover specific requirements for each type of power quality variation or concern. For instance, IEC 61000-4-7 provides the specifications for monitoring harmonic distortion levels.

IEC 61000-4-15 provides the specifications for monitoring flicker. The overall requirements for characterizing power quality phenomena are summarized in a new standard that is just being completed within IEC (61000-4-30). This new standard refers to the appropriate individual standards (like 61000-4-7 and 61000-4-15) for detailed specifications where appropriate.

This standard provides detailed requirements for the measurement procedures and the accuracy requirements of the measurements. Not all monitoring equipment will be able to meet the exact requirements of this standard.

As a result, two classes of measurement equipment have been defined which can both be considered compliant with the procedures of IEC 61000-4-30:

a. Class A performance is for measurements where very precise accuracy is required. Two instruments that comply with the requirements of class A should give the same results (within the specified levels of accuracy) for any of the types of power quality variations considered.

b. These instruments could be appropriate for laboratories or for special applications where highly precise results are required.

c. Class B performance still indicates that the recommended procedures for characterizing power quality variations are used but that the exact accuracy requirements may not be met. These instruments are appropriate for most system power quality monitoring (surveys, troubleshooting, characterizing performance, etc.).

The concept of aggregation is also introduced in this standard. Aggregation is used so that multiple measurements that are associated with essentially the same event are not counted multiple times. For example, multiple voltage sags caused by reclosing operations should only be counted as a single event for evaluating the impact on customers and the number of problem events on the system. Three different aggregation intervals are defined in IEC 61000-4-30: 3s, 10 min, and 2 h.

The work in IEC 61000-4-30 is also becoming the basis for updates and enhancements to the IEEE power quality monitoring standards (IEEE 1159 series). This is part of the general trend toward internationalizing power quality standards.

These aggregation periods are also very important intervals for characterizing steady-state power quality variations like voltage magnitude, unbalance, harmonics, and flicker. All these quantities are described statistically using 10-min values as the most important quantity. Note that the basic measurement period for the steady-state power quality parameters is 200 ms.

This permits characterization of interharmonics in 5-Hz bins, and it provides some smoothing of very fast changes that should not be considered part of steady-state power quality performance. Of course, voltage sags and transients are characterized with actual waveforms and rms versus time plots.