POWER QUALITY BASICS: VOLTAGE FLUCTUATIONS AND FLICKER

Saturday, September 10, 2011

Voltage Fluctuations are described by IEEE as systematic variations of the voltage waveform envelope, or a series of random voltage changes, the magnitude of which falls between the voltage limits set by ANSI C84.1. Generally, the variations range from 0.1% to 7% of nominal voltage with frequencies less than 25 Hz. Subsequently, the most important effect of this power quality problem is the variation in the light output of various lighting sources, commonly termed as FlickerThis is the impression of instability of the visual sensation brought about by a light stimulus, whose luminance fluctuates with time.

Voltage Fluctuations (Flicker)
Voltage Fluctuations
Voltage fluctuation and light flicker are technically two distinct terms, but have been erroneously referred to the same meaning. Aggravating the confusion is the use of the expression “voltage flicker”, which does not actually exist, even though it is often heard. In fact, IEEE has cautioned on the incorrect usage of these terms.

Nevertheless, voltage fluctuation and flicker are closely related to each other. This is because flicker is derived from the impact of voltage fluctuation on lighting intensity due to large loads that have rapidly changing active and reactive power demand. In fact, voltage variations as low as 0.5% could result in perceptible light flicker if the frequencies are in the range of 6 to 8 Hz.
Voltage Fluctuations and Light Flicker
Voltage Fluctuations and Light Flicker
In other words, voltage fluctuation is the response of the power system to fast changing loads. On the other hand, light flicker is the response of the lighting system to such load variations as observed by the human eye.

Moreover, International standards have been developed for characterizing the voltage fluctuations based on the potential effects on lighting and the human perception of the lighting variations.

Sources and Causes

Equipment or devices that exhibit continuous, rapid load current variations (mainly in the reactive component) can cause voltage fluctuations and light flicker. Normally, these loads have a high rate of change of power with respect to the short-circuit capacity at the point of common coupling. Examples of these loads include:

  • Electric arc furnaces
  • Static frequency converters
  • Cycloconverters
  • Rolling mill drives
  • Main winders
  • Large motors (starting)

Similarly, small power loads such as welders, power regulators, boilers, cranes and elevators, to name a few, may cause voltage fluctuation and flicker depending on the electrical system where they are connected.

Other causes include, but not limited to the following:

  • Capacitor switching, transformer on-load tap changers (OLTC), step voltage regulators and other devices that alter the inductive component of the source impedance.
  • Variations in generation capacity, particularly intermittent types (e.g. wind turbines).
  • Low frequency voltage interharmonics.

Furthermore, loose connections may also result to voltage fluctuations and flicker. Lightly loaded loose connections may cause flickers for longer periods as compared to heavily loaded ones that quickly burn out.

Effects

Flicker is considered the most significant effect of voltage fluctuation because it can affect the production environment by causing personnel fatigue and lower work concentration levels. In addition, voltage fluctuations may subject electrical and electronic equipment to detrimental effects that may disrupt production processes with considerable financial costs.

Other effects of voltage fluctuation include the following:

  • Nuisance tripping due to misoperation of relays and contactors.
  • Unwanted triggering of UPS units to switch to battery mode.
  • Problems with some sensitive electronic equipment, which require a constant voltage (i.e. medical laboratories).
Measurements

Presently, the basic parameters that determine voltage fluctuations are the short-term flicker severity (PST) and long-term flicker severity (PLT) index. These factors refer to voltage fluctuation effects on lighting and their influence on humans.

Flicker measurements are primarily performed to evaluate the supply quality by comparing the existing flicker level at the measurement point to the published standard limits. The second is to gauge the emission levels of equipment before it is introduced to the market - a type test for certification purposes.

The IEC flickermeter is the standard for measuring light flicker. Recently, IEEE has adopted the flickermeter method after making necessary modifications for the IEC standard to become applicable to the 120 V electrical systems in the US. Today, many monitoring equipment manufacturers have already implemented the flickermeter design specified in IEC 61000-4-15 and IEEE 1453-2004.

CLICK HERE TO READ MORE ABOUT IEC FLICKERMETER

Synopsis:

Magnitude: 0.1% to 7% (typical)
Spectral Content: Less than 25 Hz
Duration: Intermittent
Source: Loads that exhibit continuous, rapid variations in load current magnitude
Symptoms: Light flicker and malfunction of electrical equipment and devices
Occurrence: Low to medium

References:
Baggini, A. (2008). Handbook of Power Quality
Hanzelka, Z. and Bien, A. (2006). Voltage Disturbances - Flicker
IEEE 1159-1995. Recommended Practice For Monitoring Electric Power Quality

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I am an Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality and Protective Relaying. I have been working in an electric distribution utility for about seven years now as a Planning & Design Engineer. I handle PQ studies, power system analysis and capital budgeting for company projects.