Power Quality Problems IEEE

IEEE defined power quality disturbances into seven categories based on wave shape:

1. Transients
2. Interruptions
3. Sag/ Under voltage
4. Swell/ Overvoltage
5. Waveform distortion
6. Voltage fluctuations
7. Frequency variations

1. Transients

Potentially the most damaging type of power disturbance, transients fall into two subcategories:

1. Impulsive
2. Oscillatory

2. Interruptions

An interruption is defined as the complete loss of supply voltage or load current. Depending on its duration, an interruption is categorized as instantaneous, momentary, temporary, or sustained.

3. Sag/ Undervoltage

A sag is a reduction of AC voltage at a given frequency for the duration of 0.5 cycles to 1 minute. Sags are usually caused by system faults and are also often the result of switching on loads with heavy startup currents. more »

4. Swell/ Overvoltage

A swell is the reverse form of a sag, having an increase in AC voltage for a duration of 0.5 cycles to 1 minute. For swells, high-impedance neutral connections, sudden (especially large) load reductions, and a single-phase fault on a three-phase system are common sources. more »

5. Waveform Distortion

There are five primary types of waveform distortion:

1. DC offset
2. Harmonics
3. Interharmonics
4. Notching
5. Noise

6. Voltage Fluctuations

A voltage fluctuation is a systematic variation of the voltage waveform or a series of random voltage changes, of small dimensions, namely 95 to 105% of nominal at a low frequency, generally below 25 Hz.

7. Frequency Variations

Frequency variation is extremely rare in stable utility power systems, especially systems interconnected via a power grid. Where sites have dedicated standby generators or poor power infrastructure, frequency variation is more common especially if the generator is heavily loaded.

General Classes of Power Quality Problems

The terminology presented here reflects recent U.S. and international efforts to standardize definitions of power quality terms. The IEEE Standards Coordinating Committee 22 (IEEE SCC22) has led the main effort in the United States to coordinate power quality standards. It has responsibilities across several societies of the IEEE, principally the Industry Applications Society and the Power Engineering Society. It coordinates with international efforts through liaisons with the IEC and the Congress Internationale des Grand Réseaux Électriques a Haute Tension (CIGRE; in English, International Conference on Large High-Voltage Electric Systems).

U.S. power industry efforts to develop recommended practices for monitoring electric power quality have added a few terms to the IEC terminology. Sag is used as a synonym for the IEC term dip. The category short-duration variations is used to refer to voltage dips and short interruptions. The term swell is introduced as an inverse to sag (dip). The category long-duration variation has been added to deal with American National Standards Institute (ANSI) C84.1 limits. The category noise has been added to deal with broadband-conducted phenomena. The category waveform distortion is used as a container category for the IEC harmonics, interharmonics, and DC in ac networks phenomena as well as an additional phenomenon from IEEE Standard 519-1992, Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, called notching.

The categories and their descriptions are important to be able to classify measurement results and to describe electromagnetic phenomena that can cause power quality problems.

Power Quality Solutions

There are four ways to solve power quality problems:

Design equipment and electrical systems to prevent electrical disturbances from causing equipment or systems to malfunction. Manufacturers of sensitive equipment can reduce or eliminate the effect of power quality problems by designing their equipment to be less sensitive to disturbances. They can add some devices to their equipment according to the situation, for instance, a capacitor to provide temporary energy storage when the voltage sags are too low. They can also alter their equipment to desensitize it to power quality problems for example; they can design special K factor transformers that tolerate harmonics.
Analyze the symptoms of power quality problems to determine their causes and solutions. It is important to determine the source and type of power quality problems, the type of power quality problem and its cause often determine the solution.
Identify the medium that is transmitting the electrical disturbances and reduce or eliminate the effect of that medium.
Treat the symptoms of power quality problems by use of power conditioning equipment. It provides essential protection against disturbances. Power conditioning equipment includes devices that reduce or eliminate the effect of a power quality disturbance. It can be used to condition the source, the transmitter, or the receiver of the power quality problems. The equipment can be divided into ten categories, surge suppressors, noise filters, isolation transformers, low-voltage line reactors, various line voltage regulators, motor-generator sets, dual feeders with static transfer, uninterruptible power supplies, harmonic filters, and Dynamic voltage restorer (DVR).