POWER QUALITY BASICS: UNDERVOLTAGE

Monday, March 21, 2011

Undervoltage is classified as a Long-duration Voltage Variation phenomena, which is one of the general classification of power quality problems mentioned in the second post of the Power Quality Basics series of this site. Long-duration voltage variation is commonly defined as the root-mean-square (RMS) value deviations at power frequencies for longer than one (1) minute. It is important to note the duration of one minute or more as this differentiates undervoltage from short-duration voltage variations such as voltage sags.
Undervoltage Waveform
Undervoltage Waveform
Undervoltage is described by IEEE 1159 as the decrease in the AC voltage (RMS), typically to 80% - 90% of nominal, at the power frequency for a period of time greater than 1 minute. Undervoltage generally results from low distribution voltage because of heavily loaded circuits that lead to considerable voltage drop, switching on a large load or group of loads, or a capacitor bank switching off. 
voltage drop in utility lines

Undervoltage can expose electrical devices to problems such as overheating, malfunction, premature failure and shut down, especially for motors (i.e. refrigerators, dryers and air conditioners). Common symptoms of undervoltage include: motors run hotter than normal and fail prematurely, dim incandescent lighting and batteries fail to recharge properly.

In connection, IEEE discourages the use of the term “brownout” and should be avoided in future power quality activities to prevent confusion. Brownout is sometimes used to describe sustained periods of low power frequency voltage initiated as a specific utility dispatch strategy to reduce delivered power. Basically, the disturbance described by brownout has the same meaning as that of undervoltage. However, there is no formal definition for brownout and it is not as clear as the term undervoltage.

Undervoltage problems may be alleviated by:

1.      Reducing the system impedance - increase the size of the transformer, reduce the line length, add series capacitors or increase the size of line conductors.
2.   Improving the voltage profile - adjust transformers to the correct tap setting (for manual tap changers) or install voltage regulators or automatic on-load tap changers. Voltage regulators include the mechanical tap changing voltage regulators, electronic tap switching voltage regulators and the ferroresonant transformers.
3.    Reducing the line current – de-load the feeder or circuit by transferring some loads to other substations or load centers, add shunt capacitors or static VAR compensators, or upgrade the line to the next voltage level.

The choice of appropriate solution shall be based on the effectiveness of the mitigating device considering its benefit-cost factor.

Synopsis:
Magnitude: 0.8 to 0.9 pu (typical)
Source: Utility or facility
Duration: More than 1 minute
Symptoms: Malfunction or premature equipment failure and overheating of motors
Occurrence: Medium to high
References:
Dugan, R., McGranaghan, M., Santoso, S., and Beaty, H.W. (2004). Electrical Power Systems Quality (2nd ed.). 
IEEE 1159-1995. Recommended Practice For Monitoring Electric Power Quality.
Leng, O.S. (2001). Simulating Power Quality Problems.
Utility Systems Technologies, Inc. (2009). Power Quality Basics

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I am an Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality, Diagnostic Testing and Protective Relaying. I have been working in an electric distribution utility for more than a decade. I handle PQ studies, power system analysis, diagnostic testing, protective relaying and capital budgeting for company projects.