Non-Linear Loads effects in Electrical Design

[elec master]

1- Problems generated from using Non-linear Loads:

The Non-Linear loads will generate harmonics in the electrical distribution network and these harmonics will create:

– Large load currents in the neutral wires of a 3 phase system. Theoretically the neutral current can be up to the sum of all 3 phases therefore causing overheating of the neutral wires. Since only the phase wires are protected by circuit breakers of fuses, this can result in a potential fire hazard.

– Overheating of standard electrical supply transformers which shortens the life of a transformer and will eventually destroy it. When a transformer fails, the cost of lost productivity during the emergency repair far exceeds the replacement cost of the transformer itself.

– High voltage distortion exceeding IEEE Standard 1100-1992 “Recommended Practice for Powering and Grounding Sensitive Electronic Equipment” and manufacturer’s equipment specifications.

– High current distortion and excessive current draw on branch circuits exceeding IEEE Standard 1100-1992 “Recommended Practice for Powering and Grounding Sensitive Electronic Equipment” and manufacturer’s equipment specifications.

– High neutral-to-ground voltage often greater than 2 volts exceeding IEEE Standard 1100-1992 “Recommended Practice for Powering and Grounding Sensitive Electronic Equipment.”

– High voltage and current distortions exceeding IEEE Std. 519-1992 “Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems.”
Poor power factor conditions that result in monthly utility penalty fees for major users (factories, manufacturing, and industrial) with a power factor less than 0.9.

– Resonance that produces over-current surges. In comparison, this is equivalent to continuous audio feedback through a PA system. This results in destroyed capacitors and their fuses and damaged surge suppressors which will cause an electrical system shutdown.

– False tripping of branch circuit breakers.

– high frequency harmonics can be induced into phone lines and data cabling. The end result is noisy phone lines and unexplained data lose or data corruption in your LAN or WAN.

– Heat generation in special facilities such as call centers or data centers due to the large concentration of monitors and PCs so, The air computer room (CRAC) or building air conditioning system will run longer or harder, therefore requiring more energy to maintain the desired temperature.

2- Electrical design recommendations for electrical distribution systems having harmonics

– Use double-size neutral wires or separate neutrals for each phase.
– Specify a separate full-size insulated ground wire rather than relying on the conduit alone as a return ground path.
– On a branch circuit use an isolated ground wire for sensitive electronic and computer equipment.
– Segregate sensitive electronic and computer loads on separate branch circuits all the way back to the electrical panel.
– Run a separate branch circuit for every 10 Amps of load.
– Install a comprehensive exterior copper ground ring and multiple deep driven ground rods as part of the grounding system to achieve 5 ohms or less resistance to earth ground.
Oversize phase wires to minimize voltage drop on branch circuits.
– Shorten the distance on branch circuits from the power panel to minimize voltage drop.
– Oversize all local power sources (generators – UPS) to overcome harmonics effects on them.

Notes:
– these recommendations only keep the electrical distribution systems safe and do not eliminate or cancel high levels of harmonics.
– Actual circuit measurements of current for nonlinear loads should be made using only true RMS measuring ammeter instruments. Averaging ammeters produce inaccurate values if used to measure nonlinear loads.

to read more, please follow the link in below:

http://alihassanelashmawy.blogspot.com/2012/03/electrical-load-classification-and_07.html

[Spir Georges GHALI]

Dear ;

I have some commitments that are :
– Where there’s harmonics in a network, the phases’ currents will be increased accordingly, and the Neutral current will be composed of :
– The Vectorial Sum of the 3 phases’ currents.
– The Sum of the 3rd harmonics that means “ 3, 9, 15, 21, … ”
So, the Neutral current will be equal or bigger than the phase current depending on the values of the 3rd harmonics, but when we haven’t the Neutral current will be only the Vectorial sum of 3 phases’ currents that’s small.

– When we have harmonics in a network, and depending on the harmonics’ values and types, we will decide the sizes of the Neutral lines, but not using always “ Double-size ” of these lines.

– When we do the study of any project, as we have all necessary information about the loads, we should know if there are harmonics or not, if yes, what the kind of them to find the best solution at the beginning. Either for Phases or Neutral sizes cables.

– As recommendations, You said about the Neutral lines “ …. or separate neutrals for each phase ”, please explain how you can separate the neutral if you have a machine generating harmonics that should be supplied by “ 3 P + N ” ???? We can use your solution only for Lighting Circuits supplied by “ 3 P + N ” but not for other Single Phase Circuits. Normally, we supply single each circuit by Phase + Neutral, and accordingly the IEC… standard it’s not permitted to use the same neutral for many Single Phase Circuits.

[Author]

Posts