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Effect of Harmonics

Effect of Harmonics on Power Distribution Systems

If improperly designed or rated, electrical equipment may malfunction when voltage and current harmonics are present in the electrical system.

Harmonics have existed in power systems for many years. In the past, most electrical equipment used balanced loads referred to as linear loads (loads where the voltage and current follow one another without any distortion to their pure sine waves). The current drawn by the load is proportional to the voltage and impedance, and follows the envelope of the voltage waveform. Examples of linear loads are constant speed induction and synchronous motors, resistive heaters, and incandescent lamps.

The rapid increase in electronic device technology such as diodes, thyristors, variable frequency drives, electronic ballasts, battery chargers, and switching mode power supplies cause industrial loads to become non-linear. The non-linear load connected to the power system distribution will generate current and voltage harmonics. The current and voltage have waveforms that are non-sinusoidal, containing distortion, whereby the 60 Hz waveform has numerous additional waveforms superimposed upon it, creating multiple frequencies within the normal 60 Hz sine wave. The multiple frequencies are harmonics of the fundamental frequency. For example, if the fundamental frequency is 60 Hz, then the 2nd harmonic is 120 Hz, the 3rd is 180 Hz, and so on.

The graph above shows how a fundamental frequency and several harmonics combine to produce a resultant waveform. Source: www.hersheyenergy.com

To quantify the distortion, the term total harmonic distortion (THD) is used. The THD value is the effective value of all the current harmonics added together, compared with the value of the fundamental current. Normally, current distortions produce voltage distortions. However, when there is a stiff sinusoidal voltage source (when there is a low impedance path from the power source, which has sufficient capacity so that loads placed upon it will not affect the voltage), one need not be concerned about current distortions producing voltage distortions.

Power systems designed to function at the fundamental frequency, which is 60 Hz in the United States, are prone to unsatisfactory operation in the presence of harmonics.

· There is an increasing use of variable frequency drives (VFD) that power electric motors. The voltages and currents emanating from a VFD that go to a motor are rich in harmonic frequency components.
· The harmful effects of harmonic voltages and currents on transformer performance often go unnoticed until an actual failure occurs.
· Many industrial and commercial electrical systems have capacitors installed to offset the effect of low power factor. Since capacitive reactance is inversely proportional to frequency, unfiltered harmonic currents in the power system find their way into capacitor banks. These banks act like a sink, attracting harmonic currents, thereby becoming overloaded.
· The flow of a normal 60 Hz current in a cable produces resistance losses, and current distortion introduces additional losses in the conductor. Because of both the fundamental and the harmonic currents that can flow in a conductor, it is important to make sure a cable is rated for the proper current flow.

Often, the operation of electrical equipment may seem normal, but under a certain combination of conditions, the impact of harmonics is enhanced, with damaging results.

Once you have recognized that harmonics are in an electrical system, the next step is to carry out tests to determine the magnitude and type of harmonics. Harmonic analyzers are effective instruments for determining the wave shapes of voltage and current, and measuring the respective frequency spectrum.

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Contactor Rating IEC utilization categories - Type of application

Contactor Rating 

IEC utilization categories

Utilization Category        Type of Application

AC-1      Non-inductive or slightly inductive loads, example: resistive furnaces, heaters

AC-2      Slip-ring motors: switching off

AC-3      Squirrel-cage motors: starting, switches off motors during running time

AC-4      Squirrel-cage motors: starting, plugging, inching, Jogging

AC-5a    Switching of discharge lamps

AC-5b    Switching of incandescent lamps

AC-6a    Switching of transfomers

AC-6b    Switching of capacitor banks

AC-7a    Slightly inductive loads in household appliances: examples: mixers, blenders

AC-7b    Motor-loads for household appliances: examples: fans, central vacuum

AC-8a    Hermetic refrigerant compressor motor control with manual resetting overloads

AC-8b    Hermetic refrigerant compressor motor control with automatic resetting overloads

AC-12    Control of resisitive loads and solid state loads with opto-coupler isolation

AC-13    Control of solid state loads with transformer isolation

AC-14    Control of small electromagnetic loads

AC-15    Control of A.C. electromagnetic loads

AC-20    Connecting and disconnecting under no-load conditions

AC-21    Switching of resistive loads, including moderate overloads

AC-22    Switching of mixed resistive and inductive loads, including moderate overloads

AC-23    Switching of motor loads or other highly inductive loads

A             Protection of circuits, with no rated short-time withstand current

B             Protection of circuits, with a rated short-time withstand current

DC-1      Non Inductive or slightly inductive loads, resistance furnaces, heaters

DC-3      Shunt-motors, starting, plugging(1), inching(2), dynamic breaking of motors

DC-5      Series-motors, starting, plugging(1), inching(2), dynamic breaking of motors

DC-6      Switching of incandescent lamps

DC-12    Control of resistive loads and solid state loads with opto-coupler isolation

DC-13    Control of D.C. electromagnetics

DC-14    Control of D.C. electromagnetic loads having economy resistors in the circuit

DC-20    Connecting and disconnecting under no-load conditions

DC-21    Switching of resistive loads, including moderate overloads

DC-22    Switching of mixed resistive and inductive loads, including moderate overloads (i.e. shunt motors)

DC-23    Switching of highly inductive loads (i.e. series motors)

Elevator rating

CSA B44.1/ASM 19.2.1. On load rating

CSA B44.1/ASM 19.2.2. On unload rating

Thermal overload Class

Tripping curves as standardized in classes acc to IEC 60947-4

Class10A : 2-10Sec

Class10    : 4-10Sec

Class20    : 6-20Sec

Class30    : 9-30Sec

Contactor IEC / NEMA

Nema 00 = Iec 25a or 7.5hp 600vac.

Nema 0 = Iec 28a or 10hp 600vac.

Nema 1 = Iec 45a or 20hp 600vac.

Nema 2 = Iec 80a or 50hp 600vac.

Nema 3 = Iec 105a or 75hp 600vac.

Nema 4 = Iec 230a or 125hp 600vac.

Nema 5 = Iec 350a or 250hp 600vac.

Nema 6 = Iec 650a or 500hp 600vac.

Nema 7 = Iec 900a or 700hp 600vac.

Nema 8 = Iec 1650a or 1150hp 600vac

Contactor application Type

Type A

Disconect / Fuse / Contactor / Overload

 Type B

Disconect / Circuit breaker / Contactor / Overload

 Type C  

Circuit breaker Thermo-Magnetic inverse-time has disconect and protection include

Circuit Breaker / contactor / Overload

 Type D

Circuit breaker Instantaneous trip  magnetic only /  has disconect and protection include

Circuit Breaker / contactor / Overload

 Type E  

Manual motor starter with type E adaptor

Remplace disconect/fuse or breaker/controle manual/overload

 Type F 

Manual motor starter with type E adaptor  / contactor 

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