Filter for an inverter circuit

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chaitu68

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Hello all,

I need to design an output filter for the inverter circuit. The DC input voltage is around 20kV DC and the input current is 6000Amps. I want a sinusoidal voltage at the inverter output to be 20kV AC. I do not have much knowledge in this Power Electronics area as Im much into Power system Engineering. Can you kindly help me designing the filter i.e., help me in getting the values of L and C for the filters by providing me any general equations to suppress the harmonics at the output of the inverter.

Your help is very highly appreciated.

Regards,
Chaitanya
 

Designing a LC filter actually isn't a power electronics problem (although a power electronics application is involved).
It's a basic AC network problem, only fundamental electrical engineering knowledge is required to manage it.
 

Thanks for your swift reply...I understand what you said..But can u provide me the general equations to calculate the L,C values for the filter to suppress the harmonics for the Inverter Output.
 

what cut-off frequency of your filter do you want? fix the C value to something you can buy and then design for L. that is your first iteration.

i assume you are talking about 3-phase circuit. so be careful as there are 3 inductors and 3 capacitors.. but typical equations for cut-off frequency is considered as line-to-netural. your other phases will contribute to this single line-to-neutral so you have to take that into account.

good luck building for that power range. hope you get it right as you will have only ONE try at it.

Mr.Cool
 

Thank you Mr. Cool for your reply.. The question has changed a bit now.. I used some equations to get the output 3 phase AC voltage of the inverter. The Line to neutral voltage is 11 kV, making it a 19 kV line to line voltage.

I calculated both the 3rd and 5th harmonics. The fifth harmonics, being the dominant one resulted to be 3120 Volts. Generally the harmonics are referred in amps (Current harmonics). but in my case, I do not know the load and hence can you kindly help me in finding the filter capacitor and inductor values for this value of Voltage harmonics.

Any equations, material regaring this issue are very highly appreciated.

thank you

Chaitanya

Added after 2 hours 37 minutes:

And forgot to mention...The cut off frequency would be 60 Hz as I am trying to eliminate every signal other than the fundamental AC.

Chaitanya
 

You know what, man, you need to predefine the requirements for the ripple of output current/voltage so that you can get start to calculate the specific values.

Besides, since i assume u r doing the simulation, u could try some values of L&C in simulink 2 see the results, even w/o any cal... How that sound? Good luck.


 

Thank you yzou_ua . Unfortunately Im not doing any kind of simulation. This filter calculation is a very small part of my project. hence, Im not simulating it.

I have one question, as the frequency of the harmonics is multiple times the frequency of the fundamental signal; Can I use a low pass filter with a cutoff frequency of 60 HZ so that it allows only the fundamental signal and suppresses all other voltage signals. The doubt I have in my mind is that if I find out the value of R and C of a low pass filter by the equation

F. cutoff = 1/2*pi*R*C

Are the values of R and C I get by the above equation can be used to the voltage level I am using. i.e., for 19kV

Regards,
Chaitanya
 

Obviously, ur idea is correct to use cutoff freq...
In practice, u'd better to use larger value than ur cal. results of L&C and see how is the outputs. The basic principle is the larger L&C, the better outputs, but more cost.


 

Unfortunately Im not doing any kind of simulation.
Click to expand...
I think, it's unfortunately indeed. O.K., you don't necessarily need a simulation. Some old fashioned
hand calculation of AC networks should do the same.

Can I use a low pass filter with a cutoff frequency of 60 HZ so that it allows only the fundamental signal?
Click to expand...
I fear, it gonna be difficult with a varying load.

I understand now, that you're apparently using an inverter operating at the mains fundamental frequency.
Hopefully, 3rd harmonics can be suppressed by inverter design, so 5th/7th would be the strongest harmonics.

Apart from having a cut-off frequency and an attenuation for certain harmonics, a low-pass filter also creates
a voltage drop for the fundamental. This basically sets a limitation to the series inductance value and thwarts
the idea of "the larger L&C, the better outputs", which is valid for DC filters.

As another issue, the filter also acts as a reactant load to the inverter, increases it's required current rating
and creates additional losses.
 

Thx for explaining this "thwart"...

 

right.. so if your fundamental is 60Hz and your first major harmonic is the 5th.. then you would set your cut-off frequency somewhere aboge 60Hz but below 5*60 = 300Hz to achieve the level of attenuation that you want.

if it were an ideal world,, you'd chose 60Hz.. but because of practical limiations that FvM points out.. you must make trade-offs.

also.. don't pick a cut-off frequency near a resonance point of your system. and make sure your L-C resonant frequency is NOT near any of your expected harmonic frequencies or you'll regret it. that's an often missed point of filter design.

it's always about trade-offs....

Mr.Cool
 

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