Electrical Load Classification and Types – Part Three


In the previous topic, Electrical Load Classification and Types – Part Two
 , I listed all the Electrical Load Classification according to Load Nature (1) and today I will list the Electrical Load Classification according to Load Nature (2) and according to load function.

You can review the Electrical Load Classification and Types list represented in the previous topic; Electrical Load Classification and Types – Part One for more information and good following.



Second: According To Load Nature-2

  1. Linear Electrical Load.
  2. Non-Linear Electrical Load.

1- Linear Electrical Load



A load where the wave shape of the steady-state current will follow the wave shape of the applied voltage.

Examples of Linear Loads:

Power Factor Improvement Capacitors, Indescent Lamps, Heaters.

Characteristics of Linear Loads: (see fig.1)

  • In AC circuits, linear loads’ voltage and current waveforms are sinusoidal, so The current at any time is proportional to voltage and these loads does not change the shape of the waveform of the current, but may change the relative timing (phase) between voltage and current. 
  • Linear loads’ impedance remains fixed with changing the applied voltage. The fixed impedance means that the current drawn by the linear load will be sinusoidal as like the voltage and the current at any time will be proportional to voltage. 
  • Linear loads don’t produce any new frequency (harmonics) or change the applied frequency. 

fig (1): Characteristics of Linear Loads




2- Non-Linear Electrical Load


Nonlinear Electrical Load is a load where the wave shape of the steady-state current does not follow the wave shape of the applied voltage.

Examples for Non-Linear Loads:



Electronic equipment, electronic/electric-discharge lighting, adjustable-speed drive systems, and similar equipment.

Characteristics of Non-Linear Loads:
(see fig.2) 

  • Non-linear loads change the shape of the current waveform from a sine wave to some other form. 
  • Non-linear loads create harmonic currents in addition to the original (fundamental frequency) AC current causing distortion of the current waveform leads to distortion of the voltage waveform. Under these conditions, the voltage waveform is no longer proportional to the current. 
  • Non-linear loads’ impedance changes with the applied voltage. The changing impedance means that the current drawn by the non-linear load will not be sinusoidal even when it is connected to a sinusoidal voltage. These non-sinusoidal currents contain harmonic currents that interact with the impedance of the power distribution system to create voltage distortion that can affect both the distribution system equipment and the loads connected to it. 


fig (2): Characteristics of Non-Linear Loads


The following table will summarize the major differences between Linear and Non-linear Loads as follows:






LINER LOADS
NON-LINER LOADS
Ohms Law
Ohms law is applicable
Ohms law is not applicable
Crest Factor
Crest Factor= 1Peak/1 RMS =  √2=1.41
Crest Factor could be 3 to 4
Power Factor
Power factor  = Watts/ (V X I) = Cos Ø
Power factor = Watts/ (V X I)  ≠ Cos Ø =  Displacement factor X Distortion factor
Harmonics
Load current does not contain harmonics.
Load current contains all ODD harmonics.
Load Category
Could be inductive or capacitive.
Can’t be categorized. As leading or lagging Loads.
Load Type
Resistive, Inductive or capacitive
Usually an equipment with Diode and Capacitor.
Neutral Current
Zero neutral current if 1 Ph. loads are equally balanced on 3Ph. Mains (Vector sum of line current)

Neutral current could be 2.7 times the line current even if 1Ph. loads are equally balanced on 3 Ph. Mains

Inrush Current
May not demand high inrush currents while starting.

Essentially very high inrush current (20 time of I Normal) is drawn while starting for approx. One cycle.

        

Non-Linear Loads effects in Electrical Design 


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. 

3- Treatment for harmonics problems


Harmonic treatment can be performed by two methods:
  1. Filtering. 
  2. Cancellation. 

1- Filtering
A harmonic filter consists of a capacitor bank and an induction coil. The filter is designed or tuned to the predetermined non-linear load and to filter a predetermined harmonic frequency range. Usually this frequency range only accounts for one harmonic frequency. This application is mostly used when specified for a UPS or variable frequency drive motor in a manufacturing plant.


2- Cancellation
Harmonic cancellation is performed with harmonic canceling transformers also known as phase-shifting transformersA harmonic canceling transformer is a relatively new power quality product for mitigating harmonic problems in electrical distribution systems. This type of transformer has patented built-in electromagnetics technology designed to remove high neutral current and the most harmful harmonics from the 3rd through 21st. The technique used in these transformers is call "low zero phase sequencing and phase shifting". These transformers can be used to treat existing harmonics in buildings or facilities. This same application can be designed into new construction to prevent future harmonics problems.



In the next topic, I will continue explaining Other Types of Electrical Loads. So, please keep following. 



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