Friday, September 9, 2016

Harmonics and its effects on electrical systems

Harmonics in electrical systems:-
Before discussing about consequences of Harmonics you must know what are harmonics:-

Harmonics are electric current and voltages that causes power quality problems. These are generated due to electronic equipment’s which draws non linear loads with abrupt pulses. These pulses causes distorted current and voltages causes of which will be discussed in this article.
Harmonics are classified according of level of harmonics these are multiple of frequencies such as
2nd level, 3rd level, 4th level, and upto 11th levels
Where 2nd level means Harmonics with frequency of 100 HZ
3rd level means Harmonics with frequency level of 150 HZ
And similar for other levels
Now 2nd level, 5th level, 8th level, and 11th level will have –ve sequence of currents
3rd level, 6th level and 9th level will have zero sequence of currents
4th level, 7th level, 10th levels will have +ve sequence of currents

Harmonics are now integral part of electricity. Harmonics is also known as pollution in electricity. Now days while power availability is not a big concern so intention now shifts towards quality of electiricity.
There is an increase in harmonics levels in electricity there are following few reasons for harmonics in electricity:-
1.       Increase in non-linear loads such as power electronics equipment’s. Such as VFD, SMPS etc where power is chopped and used as per requirements at low or other frequencies.

2.       There are various other devices which leads to Harmonics in electrical systems such as :-
(a)    Arcing devices
(b)   Ferromagnetic devices
(c)    Appliances
(d)   Electronic switching power converters
There will be following effects due to harmonics in electrical systems:-
(a)    Overheating of electrical equipment’s as more eddy currents are produced due to these harmonic levels.
(b)   Due to overheating and other losses this will leads to reduction in life of equipment
(c)    These harmonics will leads to premature failure of equipment’s
(d)   Harmonics will leads to malfunctioning of equipment’s
(e)   Harmonics will leads to more losses in systems
(f)     Harmonics will leads to interference in communication signals
(g)    Harmonics will leads to Nuisance tripping of circuit breaker and blowing off the fuses.
(h)   Harmonics will leads to more vibrations in motor and other equipment’s.
(i)      This will leads to flickering of computers.
There are so many effects of Harmonics on various equipments
When there are harmonics in systems than we can’t be able to get 1 Power factor as there are so many reactive powers associated with these harmonics which will not get compensated using capacitor banks. Even when we try to compensate the same by installation of more and more capacitors then we lose a hafety amount on installation of capacitors and also this will leads to higher losses due to capacitor banks.

Earlier we known that Power factor=  Voltage X Current X Power factor
                                                                         Voltage X Current

Now days due to these Harmonics there are Distortion power factors which will be calculated as below:-
Distortion power factor due to Harmonics

Where THDi Stands for Total harmonics distortion in currents
And THDv Stands for Total Harmonics distortion in voltage.

Now Total Power factor is given as = Power factor X Distortion power factor

As Distortion power factor will never will be unity so Total power factor of system will never will be unity.
There are following effects of harmonics on Transformers and Motors:-
1.       There will be more eddy current loses in Transformers.
2.       Harmonics will leads to more losses due to skin effect.
3.       Harmonics will leads to overheating of coils and that will cause premature failure of Transformers
4.       Harmonics will leads to more Hysteresis and eddy current losses in motors as Hysteresis losses are directly proportional to frequency and eddy current losses are directly proportional to square of frequency.
5.       There will be more winding eddy current losses, High frequency Rotor and stator losses, and tooth pulsations in motors due to harmonics

6.       More harmonics will be leads to more I2R losses in both Transformers and motors
7.       There will be increased over temperature due to these losses and leads to premature failure
There are following effects of harmonics on Cables and Capacitors:-

1.       There will be Higher Proximity effects and Skin effects in cables
2.       There will be additional losses in cables
3.       These will cause additional overheating of cables
4.       This will decrease insulation level of cables
5.       Harmonics will cause increased Resistance
6.       Harmonics will cause derating of cables
7.       Harmonics will leads to higher neutral currents in cables and leads to unbalanced loads on cables.
8.       Harmonics will leads to resonance with 7th Harmonics in capacitor banks which will leads to lower capacitive reactance.
9.       Harmonics will leads to more electricity bill due to increased losses due to harmonics.
10. Harmonics will leads to more KVA demand as stated earlier due to increased losses.

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