hi,

i have gathered a lot of confusion.

say i am using bridge converter on 3 phase line. how does it create harmonics in the power supply of other equipments? how do the harmonics move in the reverse direction to the current flow, ie the current flow is in the direction towards the load applied to the converter? how do harmonics move towards the power supply!

i hope i have put the question properly.

one more thing, does the reactive power move backwards to the power supply? when i put a vfd to a 3 phase supply,in which portion of the electrical circuit it produces harmonics? is there a connection between harmonics and reactive power?

 

Start off with a simple system - a single-phase half-wave rectifier with resistive load.

Apply a sine wave of voltage to the anode of the resistor. The current that will flow will be the positive half-cycle only, so can be treated as a sine wave at the same frequency as and in phase with the source voltage together with a whole lot of harmonics. This will be the current throughout the series circuit.

Since the element that "causes" the distortion is the diode, it is valid to treat it as the source of the harmonics

I'm not sure what you mean by " harmonics move in the reverse direction to the current flow, ie the current flow is in the direction towards the load applied to the converter". The phase of harmonics relative to the fundamental current will depend on the element causing the distortion.

Since the power fed to the resistive load is the product of RMS voltage and RMS of the fundamental component of current only, the distortion will introduce a power factor effect. This is NOT related to any phase angle effects but is a different phenomenon entirely.

The RMS value of the combined half-wave current is Ip/2, and the RMS of the fundamental component of the half-wave current is Ip/[2sqrt(2)].

So the apparent power = Vp/[sqrt(2)] x Ip/2 = Vp x Ip/[2 sqrt(2)] while the actual power = Vp/[sqrt(2)] x Ip/[2sqrt(2)] = Vp x Ip/4.

The harmonic power factor for the half-wave rectifier is apparent power/actual power = 1/sqrt(2).

With more complex rectifiers, the analysis is more complex again. If you have phase-controlled rectifiers, with a delay between the zero crossing of voltage and the current being turned on, there is then also a phase shift between the voltage sine wave and the fundamental current, so you get a "displacement power factor" (cos phi) as well.

I find it useful to consider the rectifier (or any other distorting element) as the "source" of the harmonics, and the associated reactive power, as the best way of dealing with them by filtering is to apply the filter elements as close to the rectifier as possible.

Cheers,
Bruce.

 

The harmonics are generated by very narrow conduction angles that draw high current for a short time which distorts the power waveform. Any time you distort the waveform you are adding harmonics. And the harmonics don't have to travel. Greatly simplified for out purposes here, what happens at one end of a wire is happening at the other end. This is a little less true for the higher harmonics, but holds well for the lower. Inductors are sold to help mitigate this problem by limiting the rate of rise in current which smooths out the waveform at the line end while the voltage does even wilder things on the load end. This is the simple explanation. I'm sure our PE's can provide the exact network analysis and even models if called upon, but I think this is what you are asking about. And it's the voltage that causes problems.

Regards
cww

 

you people must be PHD's. thanks a lot for helping a humble engineer
as i see it, let's take a hypothetical situation,

i have a 3phase supply and the same is connected to lighting works, a motor A as well as a rectifier (these are connected to same 3 phase source, none of these three are in series). the rectifier is used to drive another motor B.

the rectifier is the source of harmonics. now the thing that is most prone to harmonics is motor B.

motor A and lighting are also prone to harmonics (due to the rectifier) but to a lesser extent than motor B. i can use inductors before motor B (place inductors between the rectifier and motor B). should i use inductors for lighting and motor A?

 

"the harmonics move in opposite direction"

say i take the positive direction as 3 phase power supply to rectifier and to motor B. if harmonics move from rectifier towards power supply also, that is back direction, and so it can go to motor A and lighting.
as you can see i draw very clear hypothetical diagrams

 

You are correct in that harmonic effects will be more intense close to the rectifier, and can be reduced by adding filtering elements. However, the behaviour of the circuit is very dependent on the type of rectifier used.

If you want to reduce the effects of harmonics on the upstream circuits, the best place for filtering is immediately on the supply side of the rectifier circuit. Don't forget that harmonics can radiate as well as travel by conduction so it is good practice to keep any circuits carrying harmonics as short as possible.

 

Distorted current waveforms (usually in power converters) can produce distortion in voltage waveforms present in the supply bus bar. If the current have high crest factor, this can cause problems in your supply transformer, also transient voltage drops in a conductor not designed to carry peak currents above the nominal values.

Solution may range from installing filters at the converter input (close to the equipment). Isolate the supply bars (dedicated bar to the converter). To change the transformer or change the rectifier to one who has input power factor correction.