Search

Premium Membership ♕

Limited Time Offer: Save 15% on PRO Plan with discount code: LRN15 and study specialized LV/MV/HV technical articles and studies.

Home / Technical Articles / Definitions of Abnormal Voltage Conditions
Definitions of Abnormal Voltage Conditions
Definitions of Abnormal Voltage Conditions (Sag, Swell, Surge and Interruption)

Sag

A sag is a temporary reduction in the normal AC voltage.

A momentary sag is a variation, which lasts for a period of 0.5 cycle to about 2 s usually the result of a short circuit somewhere in the power system. Instances of longer duration of low voltage are called sustained sags (see Figure 1).

Sag - momentary and sustained
Figure 1 – Sag – momentary and sustained

Swell

Swell is the opposite of sag and refers to the increase of power frequency voltage. A momentary swell lasts from 0.5 cycles to 2 s. A sustained swell lasts for longer periods (see Figure 2).

Swell - momentary and sustained
Figure 2 – Swell – momentary and sustained

Surge

Surge is a sub-cycle disturbance lasting for a duration of less than half a cycle and mostly less than a millisecond. The earlier terminology was transient or spikes.

The decay is usually oscillatory. Surges generally occur due to atmospheric disturbances such as lightning or due to switching of large transformers, inductors or capacitors (see Figures 3a and b for examples).

Surge voltage with oscillatory decay
Figure 3a – Surge voltage with oscillatory decay

Surge caused by lightning
Figure 3b – Surge caused by lightning

Interruption

Interruption means the complete loss of voltage. A momentary interruption lasts from half-cycle period to less than 2 s. Longer interruptions are called sustained interruption.

Momentary interruption is usually the result of a line outage with the supply being restored automatically from another source or by auto-reclosing operation. Refer Figure 4 for illustration. An interruption can be instantaneous or of slowly decaying type.

Examples of supply interruption
Figure 4 – Examples of supply interruption

In Figure 4, the one at the top shows the RMS voltage value during a momentary interruption. The figure on the lower left depicts the waveform of a sustained interruption where the voltage drops to zero almost instantaneously.

The waveform on the lower right shows an interruption where the voltage decays slowly.

Resource: Grounding-Bonding-Shielding-and-Surge-Protection – G. Vijayaraghavan, B.Eng (Hons) Consulting Engineer, Chennai, India

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information
author-pic

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

2 Comments


  1. chandra shekhar
    Aug 18, 2017

    I am very thankful to you.Please send me all information about power quality


  2. FELIX Jr.
    Feb 01, 2015

    DATE: 01-FEB-2015

    THIS IS A GOOD ARTICLE ABOUT DIFFERENT VOLTAGE ABNORMALITIES IN THE ELECTRICAL POWER SYSTEM OPERATIONS WHICH ARE CAUSED BY DISTURBANCES IN THE SYSTEM BY EITHER THE LOADS OR SWITCHING OPERATION. HOWEVER, EFFECTS OF FROM VACUUM CIRCUIT BREAKER OR CONTACTORS OR SF6 CKT. BREAKER SWITCHING OPERATIONS SHOULD ALSO BEEN EMPHASIZED ON IMPACT TO THE VOLTAGE PROFILES AT THE SOURCE BUS AND AT LOAD SIDE ENDS WITH RESPECT TO SURGE EVENTS.
    THANK YOU.

Leave a Comment

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

  +  four  =  five

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

Subscribe to Weekly Newsletter

Subscribe to our Weekly Digest newsletter and receive free updates on new technical articles, video courses and guides (PDF).
EEP Academy Courses - A hand crafted cutting-edge electrical engineering knowledge