POWER QUALITY BASICS: OVERVOLTAGE

Thursday, March 17, 2011

Overvoltage is classified as a Long-duration Voltage Variation phenomena, which is one of the general categories 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.

Overvoltage is defined by the IEEE as an increase in the AC voltage (RMS), typically to 110% - 120% of nominal, at the power frequency for duration longer than 1 minute. It is often the result of high distribution voltage due to incorrect tap settings on transformers, switching off a large load, or excessive correction for voltage drop on the transmission and distribution systems such as energizing several capacitor banks. These occur mainly because either the voltage controls are inadequate or the system is too weak for voltage regulation. Possible effect could be hardware failure in the equipment due to overheating. 
Overvoltage Waveform
Overvoltage
Electrical and electronic devices are designed to operate at a prescribed voltage range (rated voltage) in order to achieve specified levels of efficiency, performance, reliability and safety. Subjecting electrical or electronic devices to overvoltage can lead to problems such as overheating, malfunction, premature failure, shut down and shorter operating life. This is particularly true for electronic devices (includes appliances with sensitive electronics), which run hotter than normal and fail prematurely. Also, overvoltage protection leads to equipment shutdown. Furthermore, a printed circuit board can be expected to have a shorter life when operated above its rated voltage for long periods of time.

On the contrary, some equipment such as motors, transformers and power supplies and may benefit from voltage levels slightly above the nameplate voltage, as long that it is within the voltage limits for the equipment. The reason for this is that the increased voltage decreases current flow in the device resulting to lower copper losses. Thus, efficiency improves and the operating temperature decreases. The challenge becomes one of determining and maintaining a voltage level that maximizes efficiency for certain devices without adversely affecting the life or operation of other devices.

Generally, overvoltage can be mitigated by:

1.      Adjusting transformers to the correct tap setting (for manual tap changers) or installing voltage regulators or automatic on-load tap changers to improve the voltage profile. Voltage regulators include the servo-mechanical tap switching voltage regulators, electronic tap switching voltage regulators and the ferroresonant transformers.

2.   Manually or automatically switching off excess capacitor banks during light load or off peak hours.

The chosen solution shall be evaluated based on its effectiveness and with consideration to the benefits and costs.

Synopsis:

Magnitude: 1.1 to 1.2 pu (typical)
Source: Utility or facility
Duration: More than 1 minute
Symptoms: Malfunction/premature equipment failure particularly electronics and printed circuit boards
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.