TYPES OF WAVEFORM DISTORTION BASIC INFORMATION AND TUTORIALS

Waveform distortion is defined as a steady-state deviation from an ideal sine wave of power frequency principally characterized by the spectral content of the deviation.

There are five primary types of waveform distortion:

■ DC offset

■ Harmonics

■ Interharmonics

■ Notching

■ Noise

DC offset. The presence of a dc voltage or current in an ac power system is termed dc offset. This can occur as the result of a geomagnetic disturbance or asymmetry of electronic power converters. Incandescent light bulb life extenders, for example, may consist of diodes that reduce the rms voltage supplied to the light bulb by half-wave rectification.

Direct current in ac networks can have a detrimental effect by biasing transformer cores so they saturate in normal operation. This causes additional heating and loss of transformer life. Direct current may also cause the electrolytic erosion of grounding electrodes and other connectors.

Harmonics. Harmonics are sinusoidal voltages or currents having frequencies that are integer multiples of the frequency at which the supply system is designed to operate (termed the fundamental frequency; usually 50 or 60 Hz). Periodically distorted waveforms can be decomposed into a sum of the fundamental frequency and the harmonics.

Harmonic distortion originates in the nonlinear characteristics of devices and loads on the power system. Harmonic distortion levels are described by the complete harmonic spectrum with magnitudes and phase angles of each individual harmonic component. It is also common to use a single quantity, the total harmonic distortion (THD), as a measure of the effective value of harmonic distortion.

Current distortion levels can be characterized by a THD value, as previously described, but this can often be misleading. For example, many adjustable-speed drives will exhibit high THD values for the input current when they are operating at very light loads. This is not necessarily a significant concern because the magnitude of harmonic current is low, even though its relative distortion is high.

To handle this concern for characterizing harmonic currents in a consistent fashion, IEEE Standard 519-1992 defines another term, the total demand distortion (TDD). This term is the same as the total harmonic distortion except that the distortion is expressed as a percent of some rated load current rather than as a percent of the fundamental current magnitude at the instant of measurement. IEEE Standard 519-1992 provides guidelines for harmonic current and voltage distortionlevels on distribution and transmission circuits.

Interharmonics. Voltages or currents having frequency components that are not integer multiples of the frequency at which the supply system is designed to operate (e.g., 50 or 60 Hz) are called interharmonics. They can appear as discrete frequencies or as a wideband spectrum. Interharmonics can be found in networks of all voltage classes.

The main sources of interharmonic waveform distortion are static frequency converters, cycloconverters, induction furnaces, and arcing devices. Power line carrier signals can also be considered as interharmonics. There is now a better understanding of the origins and effects of interharmonic distortion.

It is generally the result of frequency conversion and is often not constant; it varies with load. Such interharmonic currents can excite quite severe resonances on the power system as the varying interharmonic frequency becomes coincident with natural frequencies of the system. They have been shown to affect power-line-carrier signaling and induce visual flicker in fluorescent and other arc lighting as well as in computer display devices.

Notching. Notching is a periodic voltage disturbance caused by the normal operation of power electronic devices when current is commutated from one phase to another. Since notching occurs continuously, it can be characterized through the harmonic spectrum of the affected voltage.

However, it is generally treated as a special case. The frequency components associated with notching can be quite high and may not be readily characterized with measurement equipment normally used for harmonic analysis.

Noise. Noise is defined as unwanted electrical signals with broadband spectral content lower than 200 kHz superimposed upon the power system voltage or current in phase conductors, or found on neutral conductors or signal lines.

Noise in power systems can be caused by power electronic devices, control circuits, arcing equipment, loads with solid-state rectifiers, and switching power supplies. Noise problems are often exacerbated by improper grounding that fails to conduct noise away from the power system.

Basically, noise consists of any unwanted distortion of the power signal that cannot be classified as harmonic distortion or transients. Noise disturbs electronic devices such as microcomputer and programmable controllers. The problem can be mitigated by using filters, isolation transformers, and line conditioners.

EXAMPLES OF POOR POWER QUALITY

POOR POWER QUALITY EXAMPLES

Poor power quality is usually identified in the “powering” part of the definition, namely in the deviations in the voltage waveform from the ideal. A set of waveforms for typical power disturbances is shown in Figure 1.5. These waveforms are either (a) observed, (b) calculated, or (c) generated by test equipment.

The following are some examples of poor power quality and descriptions of poor power-quality “events.” Throughout, we shall paraphrase the IEEE definitions.

■ A voltage sag (also called a “dip”9) is a brief decrease in the rms linevoltage of 10 to 90 percent of the nominal line-voltage. The duration of a sag is 0.5 cycle to 1 minute [1.44–1.50]. Common sources of sags are the starting of large induction motors and utility faults.

■ A voltage swell is the converse to the sag. A swell is a brief increase in the rms line-voltage of 110 to 180 percent of the nominal line-voltage for a duration of 0.5 cycle to 1 minute. Sources of voltage swells are line
faults and incorrect tap settings in tap changers in substations.

■ An impulsive transient is a brief, unidirectional variation in voltage, current, or both on a power line. The most common causes of impulsive transients are lightning strikes, switching of inductive loads, or switching in the power distribution system. These transients can result in equipment shutdown or damage if the disturbance level is high enough. The effects of transients can be mitigated by the use of transient voltage suppressors such as Zener diodes and MOVs (metal-oxide varistors).

■ An oscillatory transient is a brief, bidirectional variation in voltage, current, or both on a power line. These can occur due to the switching of power factor correction capacitors, or transformer ferroresonance.

■ An interruption is defined as a reduction in line-voltage or current to less than 10 percent of the nominal, not exceeding 60 seconds in length.

■ Another common power-quality event is “notching,” which can be created by rectifiers that have finite line inductance. The notches show up due to an effect known as “current commutation.”

■ Voltage fluctuations are relatively small (less than 5 percent) variations in the rms line-voltage. These variations can be caused by cycloconverters, arc furnaces, and other systems that draw current not in synchronization with the line frequency [1.51–1.61]. Such fluctuations can result in variations in the lighting intensity due to an effect known as “flicker” which is visible to the end user.

■ A voltage “imbalance” is a variation in the amplitudes of three-phase voltages, relative to one another.

VOLTAGE SWELLS : POWER QUALITY PROBLEM

WHAT ARE VOLTAGE SWELLS?

A“swell” is the converse of the sag, and is a brief increase in the rms line voltage. A swell is defined as an increase to between 1.1 and 1.8 pu in rms voltage or current at the power frequency for durations from 0.5 cycle to 1 min.

As with sags, swells are usually associated with system fault conditions, but they are not as common as voltage sags. One way that a swell can occur is from the temporary voltage rise on the unfaulted phases during an SLG fault.

Figure 2.8 illustrates a voltage swell caused by an SLG fault. Swells can also be caused by switching off a large load or energizing a large capacitor bank.

Swells are characterized by their magnitude (rms value) and duration. The severity of a voltage swell during a fault condition is a function of the fault location, system impedance, and grounding. On an ungrounded system, with an infinite zero-sequence impedance, the line-to-ground voltages on the ungrounded phases will be 1.73 pu during an SLG fault condition.

Close to the substation on a grounded system, there will be little or no voltage rise on the unfaulted phases because the substation transformer is usually connected delta-wye, providing a low-impedance zero-sequence path for the fault current.

Faults at different points along four-wire, multigrounded feeders will have varying degrees of voltage swells on the unfaulted phases. A 15 percent swell, like that shown in Fig. 2.8, is common on U.S. Utility feeders.

Swells are not as common as sags and their main causes are

- switching off of a large load,

- energizing a capacitor bank, or

- voltage increase of the unfaulted phases during a single line-to-ground fault [10].

In some cases the term "momentary overvoltage" is used as a synonym for the term swell. As in the case of sags, UPS or power conditioners are typical solutions to limit the effect of swell.

SHORT DURATION VOLTAGE VARIATION : POWER QUALITY ISSUES

A voltage sag (dip) is defined as a decrease in the root-mean-square (rms) voltage at the power frequency for periods ranging from a half cycle to a minute.11 It is caused by voltage drops due to fault currents or starting of large motors. Sags may trigger shutdown of process controllers or computer system crashes.

A voltage swell is defined as an increase up to a level between 1.1 and 1.8 pu in rms voltage at the power frequency for periods ranging from a half cycle to a minute.

An interruption occurs when the supply voltage decreases to less than 0.1 pu for a period of time not exceeding 1 min. Interruptions can be caused by faults, control malfunctions, or equipment failures.

All these types of disturbances, such as voltage sags, voltage swells, and interruptions, can be classified into three types, depending on their duration.

a. Instantaneous: 0.5–30 cycles

b. Momentary: 30 cycles–3 s

c. Temporary: 3 s–1 min

It is helpful to distinguish the term outage used in reliability terminology from sustained interruption when the supply voltage is zero for longer than 1 min.

Outage refers to the state of a component in a system that has failed to function as expected and is used to quantify reliability statistics regarding continuity of service, whereas sustained interruptions as used in monitoring power quality to indicate the absence of voltage for long periods of time.