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Installing Pfc After Vsd


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#1 RichieLee

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Posted 21 April 2007 - 04:42 PM

Hi Marke and others,

I am new to this Forum.


#1. I have read many comments that PFC must not be installed after a VSD (i.e. downstream of the VSD on the motor side). But what if one install a PFC with a detuned reactor near to the motor after (downstream of) the VSD with a dV/dt filter after the VSD so that the dV/dt or THDv% is insignificant ? Will this then be ok to install the PFC after the VSD ?


#2. Will #1. above achieve some energy savings ?


#3. Will the followings achieve some energy savings for a motor with VSD :

a. Install a motor choke or dV/dt filter after the VSD ?

b. Install a line reactor or other harmonic filter before the VSD (on line side) ?


Many Thanks

#2 mariomaggi

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Posted 21 April 2007 - 07:51 PM

Welcome RichieLee,
#1 - If you need variable frequency, will be very difficult to calculate the right capacitor bank. Usually IGBTs are not enough oversized to charge also capacitors

#2 - No, as far as I know. Could be that for very long cables you can have some energy saving, I'm not sure.

#3a - small improvement, due to reduced motor losses
#3b - some energy savings, thanks to reduced active current from the main supply, and less heating in cables, therefore ohmic losses are reduced.

Regards
Mario

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#3 marke

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Posted 21 April 2007 - 08:16 PM

Hello RichieLee

Welcome to the forum.

#1. The only way that I would contemplate adding Power Factor Correction capacitors after an inverter would be if there was a sinusoidal filter added to the output of the inverter. This is a very expensive option, but the voltage wave form needs to be a sine wave with very low voltage distortion.
The output of the inverter would need to be a constant voltage and frequency as the KVAR of the capacitors varies with the square of the voltage and also with frequency.
The big question though, is why would you even consider such a thing as there will be no difference in current draw from the supply, so absolutely zero advantage!!! The Vars to the motor are supplied by the inverter, not the supply.

#2 No. If you had very long cables between the output of the inverter and the motor, you may reduce the copper loss in those cable, but the additional losses in the sinusoidal filter would be higher than any gain.

#3a Installing an output reactor after the inverter wil reduce the peak currents due to the capacitance of the cables, this will reduce the heat losses in the inverter, but the heat losses in the output reactor would be higher except in very long cable runs. So I expect an energy loss rather than a gain.

#3b A line reactor could reduce the harmonic content of the input current and this will reduce the losses in the supply. There will be losses in the line reactor which will probably offset any gain from the addition of the line reactor. The reduction in harmonics in the total supply system is of benefit to the electricity supplier, but probably not to the consumer.

Best regards,

#4 RichieLee

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Posted 22 April 2007 - 02:13 AM

Hi Mariomaggi and Marke,

Thanks for the reply.

1. Assuming everything else is not an issue, if one installs a correctly sized PFC Capacitor near to the motor downstream of and far away (say 100M) from the inverter, then the I2R losses saved by the PFC Capacitor installed near the motor (I2R along the cable between the inverter and the PFC Capacitor near the motor) is a form of energy savings in KW, and this energy savings should also be reflected upstream of the inverter.

I2R Energy saved downstream of an inverter would be translated into energy savings upstream of the inverter, right ?

2. Using choke and reactor for energy savings doesn't sound too worthwhile because of their inherent losses. What better ways can one achieve additional energy savings by reducing harmonics for motor with inverter -- And are these better done downstream or upstream of the inverter ?

Thanks

#5 marke

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Posted 22 April 2007 - 04:28 AM

Hello RichieLee

1.
QUOTE
Assuming everything else is not an issue,
You can not make this assumption as it is not a case of maybe, or possibly, it is a case of absolute madness to even consider it possible!! The output voltage from the inverter is a chopped waveform at the carier frequency of the inverter and this is typically between 3kHz and 12kHz. The pf capacitor will look like a short circuit at these frequencies, draw lots of current, blow the inverter and probably the capacitor as well. If you fitted a sinusoidal filter on the output of the iverter such that the voltage distortion was very low, then you may not cause damage, but any energy that you would save in the cable would be more than offset by the losses in the filter.

2. Current harmonics in the output of modern inverters is not too much of an issue, however, the input current is very distorted. If you use an "active front end" inverter, the distortion is much reduced.

Best regards,

#6 RichieLee

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Posted 22 April 2007 - 10:11 AM

QUOTE(marke @ Apr 22 2007, 04:28 AM) View Post

Hello RichieLee

1. You can not make this assumption as it is not a case of maybe, or possibly, it is a case of absolute madness to even consider it possible!! The output voltage from the inverter is a chopped waveform at the carier frequency of the inverter and this is typically between 3kHz and 12kHz. The pf capacitor will look like a short circuit at these frequencies, draw lots of current, blow the inverter and probably the capacitor as well. If you fitted a sinusoidal filter on the output of the iverter such that the voltage distortion was very low, then you may not cause damage, but any energy that you would save in the cable would be more than offset by the losses in the filter.

2. Current harmonics in the output of modern inverters is not too much of an issue, however, the input current is very distorted. If you use an "active front end" inverter, the distortion is much reduced.

Best regards,



#7 RichieLee

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Posted 22 April 2007 - 10:18 AM

Hi Mark,
Thanks for the reply.
Point 1 is well noted.

Point 2 : if there is already a line reactor on the input side of the inverter, in order to further reduce the current distortion (THDi), could one install a properly sized Capacitor (and how to determine the size?) in series with the existing Line Reactor to form some sort of passive harmonic filter on the input side of the inverter ?

Thanks

#8 marke

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Posted 23 April 2007 - 07:12 AM

Hello RichieLee

To gain any appreciable advantage, you would need to design a complex filter and if this is not done correctly, you can have resonance problems that can raise the voltage on the input to the inverter and cause other problems. This is another science and needs to be done by a professional with the appropriate training and experience. I do not believe that such a filter will reduce your power bill, but may reduce the harmonics and be of benefit to your electricity supplier.
Reducing the harmonics, is an expensive exercise and will not slow down the KWHr meter appreciably.

The options for reducing the harmonics on the input current are:
Line reactor (small improvement.)
Passive Input filter
12 pulse input
18 pulse input
active front end

Best regards,

#9 RichieLee

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Posted 23 April 2007 - 03:18 PM

Hi Mark,
Thanks for the reply.

#10 mariomaggi

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Posted 25 April 2007 - 08:08 AM

Hi Mark,
QUOTE
The options for reducing the harmonics on the input current are:
Line reactor (small improvement.)
Passive Input filter
12 pulse input
18 pulse input
active front end

and also

Active filters
Hydrid filters

QUOTE
I do not believe that such a filter will reduce your power bill, but may reduce the harmonics and be of benefit to your electricity supplier.


Some convenient reduction in power bill due to harmonic mitigation is possible in some cases. It depends from the plant layout, from type of loads and from service factors (intermittent, 8 hours per day, 24h/24 etc).
In any case, after harmonics mitigation there is a better reliability in general (less motor failures, less electronics failures, also less mechanical failures, etc.)

Regards
Mario

Mario Maggi - Italy - http://www.evlist.ithttps://www.axu.it


#11 marke

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Posted 25 April 2007 - 09:40 AM

Hi Mario
Yes, good points as usual.
There certainly can be benefits from the reduction in harmonics, some are difficult to quantify in monitary terms, and in many cases will not directly impact on the energy bill, but they certainly impact on the overall distribution efficiency.
I expect that there will be a change in tariffs to reflect poor harmonic levels and this may make harmonic improvements more attractive.

Best regards,

#12 kens

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Posted 25 April 2007 - 08:34 PM

There can be a reduction in power factor penalties (if charged) by reducing distortion related p.f. issues.

Ken
An expert is one who knows more and more about less and less until he knows absolutely everything about nothing

#13 marke

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Posted 25 April 2007 - 08:58 PM

Depends on how the power factor is measured and the tariff applied.
Often, I see power factor tariffs based on an infrequent measurement made with a clip on phase angle measurement. That will certainly not highlight distortion power factor.

Best regards,




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