Harmonic Filter operating characteristics 3

[pitt123]

We have an harmonic filer in our facility that is no longer in service and has not been for a while. All the origonal personel involved with this filter are long gone and there is nobody or no information left behind to explain why the filter was origonally installed.

I am curious about the origonal intent of this filter and what its specific purpose was. Is it possible to tell from just the electrical paramaters of this filter design what the purpose of the filter is? Is it possilbe to tell what frequencies or harmonics it was designed to pass or trap?

The filer is connected on a 4.16kV 3 phase system. The filter on each of the three phases has a 472 uH/phase inductor in parallel with a 3.2 ohm/phase resistor with this parallel combination then being in series with a 92uF/phase capacitor.

Is it possible from this information to come up with the intended filter design paramaters? Is there an equation that can be used?

 

[davidbeach]

Yes.

 

[pitt123]

Thanks David

Would you be so kind as to point me in the right direction for calculating this design operation based off of these paramaters?

 

[MAGTiger]

are the caps wye connected or delta?

Neil

 

[pitt123]

MAGTiger

The caps are wye connected

 

[sibeen]

Surely it's just a series LC filter with a resonant frequency of 764 Hz.

f = 1/2*pi*(sqrt L*C)

 

[pitt123]

Thanks Sibeen

Any idea why this resonant frequency of 746Hz would be of particular importance in an industrial facility.

Most of our non-linear loads consist of 6-pulse drives however we do have several power factor caps at our large motors. Would this frequency be an attempt to block or trap the 13th harmonic created from these 6 pulse drives?

 

[MAGTiger]

Yes, but I don't know why they would target the 13th. It is a small contributor. Is it possible that in the past there was a large 12 pulse system? If so then they may have been enhancing the 12 pulse performance. We have done that targeting the 11th.
Other than that, maybe the 13th is causing some specific interference problem. And this filter is trying to address that.

Neil

 

[pitt123]

Thank You MAGTiger

This filter is located on a downstream bus in close proximity to our main plant incoming bus. In this located will it filter these frequencies for the entire plant or only for equipment downstream of the bus the filter is located on?

Does the filter simply trap these third harmonic frequncies not allowing them be seen by other equipment.

Will the caps in this filter provide power factor correction for the rest of the plant.

I also assume that the point of the resistors in this filter is to not allow the filter to self resonate when the AC source is removed thus dumping all resonating energy across these resistors?

 

[MAGTiger]

The filter watns to trap (not the 3rd as you indicate ) but the 13th. And does so for all current on the bus and anywhere until another impedance (L or C) changes the tuning. This is why a more conventional trap filter tuned to the 5th harmonic and located at the front end of a VFD has 2 inductors (reactors). One (tuning reactor) in the cap branch to set the tuning frequency to just below the 5th so drive side harmonics are kept between the cap and the drive. The other reactor is on the input line to the filer and sees rated line current. The purpose of this line reactor is primarily to detune the caps (by adding it's inductance to the tuning reactor) for the source side so the source isn't supplying 5th harmoncis to the trap and thus overloading it.

In your case, it sounds as though the next up stream L is the transformer. so all harmonics from the transformer to all loads are seen by the trap. Perhaps the trap is tuned to interact with down stream reactors and transformers and the net is a different tuning frequency. Find the largest 2 or 3 VFD loads and find out how much L is in fron of them then use that in conjunction with the trap reactor to calculate the new tuning frequency.

Neil

 

[Rodmcm]

Is it possible that it is trapping external signals rather then internal. I am thinking ripple control here but that should be a block not a sink source!

 

[MAGTiger]

It isn't going to block anything. It will cause a very low impedance at the resonant frequency. Effectively a hole at 746hz. So any 746 hz on the bus goes in the hole. But keep in mind that any other L on the bus or at the front of the bus (transformer) changes the frequecy of the Z hole for THAT path.

Neil

 

[jghrist]

The resistor in parallel with the inductor makes this a second order damped filter. The damping allows a single filter to be effective over a range of frequencies. It was probably designed to trap both 12th and 13th harmonics.