Saturday, September 17, 2016

Methods for reducing Harmonics in System

There are so many methods available by which harmonics can be reduced in system and power quality can be improved. There are following methods of reducing the harmonics in system:-
(i)                  DC choke
(ii)                By reducing harmonics at loads
(iii)               By using filters

Now let’s discuss about them:-

(i)                 DC choke:-

VFD’s are main source of harmonics generation in a plant. DC choke is used in VFD’s to reduce harmonics. DC choking is an inductance in series with semiconductor bridge circuit. DC choke provides reduction in 5th and 7th Harmonics. DC choke is comparable to an equivalent AC-side line reactor, although the %Total Harmonic Distortion (THD) is somewhat less.

(ii)               By reducing Harmonics levels at Loads and Source:-

In electrical Transmission and distribution system there are lot of equipment’s connected in distribution so there will be harmonics getting in system . So it will become even more important to minimize the harmonics.

If there are harmonics get generated in plant i.e. at source end then it is always best to counter these harmonics at source end. As stated earlier in a plant source of harmonics are VFD’s so harmonics can be reduced installing a Delta-Star Transformer in parallel with Delta- Delta Transformer. This is done to convert two synchronized 6-Pulse VFD’s to a 12-Pulse application. This is done as to reduce 5th and 7th harmonics as there will phase shift of 30 degree in 12 pulse applications instead to 60 degree in 6-pulse application. Delta connected Transformer block the zero sequence harmonics so triple Harmonics are eliminated.

(iii)             By using filters:-
Filters are used to minimize the harmonics levels in distribution systems.  These are used in Plants also. There are following types of filters.

(a)    Passive
(b)   Active
Now let’s discuss about these 2 types of filters

(a)   Passive filters:-
 These types of filters are generally inductors and capacitors. These filters are used to block Harmonics. These are even used to shunt the Harmonics to ground. For a particular type particular application these filters are designed accordingly.

We know that as frequency increase impedance of an inductor increases on the other hand impedance of capacitor deceases with increased frequency.

There are some disadvantages associated with these filters are that these become ineffective if harmonics changes due to variation in loads.

In AC drives usually line reactors and Transformers are used to reduce the harmonics. If DC bus choke is not used then inductor of suitable capacity is used to reduce harmonics level to significant levels.
Thus passive filter may contains series/ Shunt capacitor/ Inductor network and a series inductor or Transformer. Usually now days in electrical system passive filters are integral part of drive system.

(b)   Active filters:-
Unlike passive filters in which harmnocis are blocked or shunted away , active filters are used to condition the power lines and also known as power line conditioners. In active filters these are condition monitors which sense the harmonic currents electronically and generate the counter waveform corresponding to harmonic current generated. So by this way harmonics currents are cancelled out.

Active filters are used to reduce the harmonics levels as per limited specified by IEEE. So by using these filters power factor of system get improved.

Working of Active filters:-
Active filters are installed in parallel to the loads so as to offset the generated harmonics. In electrical systems Current Transducers are used to provide the control logic according to current waveform to counter harmonics. In Three phase system 2 Current transducers are sufficient for these filters working.

Active filters remove the fundamental frequency from this waveform. The resulting waveform after removal of fundamental frequency is inverted and used to direct the firing of the IGBTs. This inverted waveform is injected back into the ac line.

This will leads to cancellation of cancellation of current Harmonics in electrical systems.

As we know that voltage harmonics are due to current harmonics so since we have redcued current harmonics voltage harmonics are also get reduced.

There are following components of Active filters:-
(a)    Power semiconductors
(b)   DC link capacitors and Bus Bars
(c)    Fuses.

Power semiconductor consists of IGBT pulse width modulation (PWM) at an appropriate switching speed. An internal filter in active filters blocks this frequency from entering the ac lines and decouples the active harmonic filter from the rest of the system so no harmful interaction occurs.
In active filters Rated output current= square root of the sum of the squares of the harmonic and reactive currents at the bus.

If total harmonics > Rating of an active harmonic filter, then additional units can be installed in parallel.

For knowing what are Harmonics and effects on Electrical Systems:-

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