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.

POWER INTERRUPTION : POWER QUALITY PROBLEM?

INTERRUPTION POWER QUALITY PROBLEMS

An interruption occurs when the supply voltage or load current decreases to less than 0.1 pu for a period of time not exceeding 1 min. Interruptions can be the result of power system faults, equipment

failures, and control malfunctions.

The interruptions are measured by their duration since the voltage magnitude is always less than 10 percent of nominal. The duration of an interruption due to a fault on the utility system is determined by the operating time of utility protective devices.

Instantaneous reclosing generally will limit the interruption caused by a nonpermanent fault to less than 30 cycles. Delayed reclosing of the protective device may cause a momentary or temporary interruption.

The duration of an interruption due to equipment malfunctions or loose connections can be irregular. Some interruptions may be preceded by a voltage sag when these interruptions are due to faults on the source system.

The voltage sag occurs between the time a fault initiates and the protective device operates. Figure 2.5 shows such a momentary interruption during which voltage on one phase sags to about 20 percent for about 3 cycles and then drops to zero for about 1.8 s until the recloser closes back in.

The difference between long (or sustained) interruption and interruption is that in the former the supply is restored manually, but during the latter the supply is restored automatically. Interruption is usually measured by its duration.

For example, according to the European standard EN-50160 [24]:

- A momentary interruption is between 30 cycles and 2 seconds;

- A temporary interruption is between 2 seconds and 2 minutes; and

- A sustained interruption is longer than 2 minutes.

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.