Tuesday, March 15, 2011

Power Quality Problems, Disturbances or Phenomena are terms used to describe voltage or current deviations from its ideal waveform. These disturbances can cause failure of loads or equipment, which is typical since electric power is generated hundreds of kilometers away from the end-users’ location. Electric power has to travel such long distances from the generating plants through transmission and distribution lines, which include several transformations that cumulatively deteriorate its quality even before reaching the end-users. As a consequence, utilities have to install devices and equipment called power conditioners in order to improve the quality of power delivered to the customer loads. 

In addition, to further enhance the quality of power to desired levels and/or avoid contributing power quality problems back to the system, some customers install mitigating devices at their site. This is an effective scheme considering that the common sources of power quality phenomena in order of frequency of occurrence are from the end-users. Please refer to the list below:
·         End-User Equipment
·         End-User Electrical System and Grounding
·         Weather phenomena such as lightning, wind and/or rain
·         Electric distribution utility system
·         Transmission grid
·         Electric power generation system

Primarily, the reason for categorizing the power quality or electromagnetic phenomena is that there are different ways to solve them depending on the particular variation that is of concern and classifying will also help in implementing the proper approach and analysis.

Here’s a short list of the types of power quality phenomena according to IEEE 1159-1995:

a. Short Duration Variations

b. Long Duration Variations

d. Waveform Distortion 

Ø  DC Offset
Ø  Harmonics
Ø  Interharmonics
Ø  Notching
Ø  Noise

g. Power Frequency Variation

Categories and Characteristics of Power Quality Phenomena
Categories and Characteristics of Power System Electromagnetic Phenomena
On the other hand, the IEC has published a table of electromagnetic phenomena with its typical sources and causes. Please take note that the IEC, as posted in the first part of this series, is currently using the term electromagnetic compatibility as it has a strong overlap with power quality - although not exactly the same.

Below is the list of principal phenomena causing electromagnetic disturbances according to the IEC:

a. Conducted low-frequency phenomena

Ø  Harmonics
Ø  Interharmonics
Ø  Signal systems (power line carrier)
Ø  Voltage fluctuations (flicker)
Ø  Voltage dips and interruptions
Ø  Voltage imbalance (unbalance)
Ø  Power frequency variations
Ø  Induced low-frequency voltages
Ø  DC in AC networks

b. Radiated low-frequency phenomena

Ø  Magnetic fields
Ø  Electric fields

c. Conducted high-frequency phenomena

Ø  Induced continuous-wave (CW) voltages or currents
Ø  Unidirectional transients
Ø  Oscillatory transients

d. Radiated high-frequency phenomena

Ø  Magnetic fields
Ø  Electric fields
Ø  Electromagnetic fields
Ø  Continuous waves
Ø  Transients

f. Nuclear electromagnetic pulse (NEMP)

The connection between the two standards is that the IEEE 1159 contains a few additional terms related to the IEC terminology:

·         Voltage sag is used in the power quality community as synonym to the IEC voltage dip.
·         Short duration variation is used to refer to voltage dips and short interruptions.
·         Voltage swell is introduced as an inverse to voltage sag or dip.
·         Long duration variation has been added to deal with the limits set by ANSI C84.1.
·         Noise has been added to deal with broadband conducted phenomena.
·       Waveform distortion is used as a container category for the IEC harmonics, interharmonics, and dc in ac networks phenomena as well as an additional phenomenon from IEEE 519-1992 called notching.

A detailed discussion for each of these power quality problems will be tackled in the next posts.

Bollen, M. (2000). Understanding Power Quality Problems: Voltage Sags and Interruptions.
Dugan, R., McGranaghan, M., Santoso, S., and Beaty, H.W. (2004). Electrical Power Systems Quality (2nd ed.).
IEC 61000-6-5. Generic Standards – Immunity For Power Station and Substation Environments.
IEEE 1159-1995. Recommended Practice For Monitoring Electric Power Quality.


Eng.Elgendy said...

Thank you so much I found it very helpful :)

yepyep said...

Glad to be of help...

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About Me

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I am a Professional 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.