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06-02-2013 10:17 AM

What kinds of VFD solution do you prefer to solve this?

What Kind of Solution Do u prefer for the Following Problem for the VFD :

Dear Fellows,
I have an application of 110kw VFD, Driving a motor which is about 800 meters away from VFD Panel. I think even with an Output choke we cant run the system.
What i have decided to Use a RC filter with respect to ground in the DC bus which will be placed opposite to the internal VFD Reactor. Ie., if the VFD Reactor is in "-" the RC will be connect in "+" and Ground and Vise Versa .

I would like to have opinion and share the experience from various sector Professionals.
06-04-2013 12:19 PM
Top #2
06-04-2013 12:19 PM
In my understanding, All Voltage Source Inverter drives essentially require an Output Sine wave filter to meet cable length requirements due to VSI PWM design and PWM waveforms.
However, a latest generation CSI MV (Current Source Inverter) drive adopts a different approach and can go for longer cable lengths upto 15KM's without any output sine wave filter requirements (For Medium Voltage only).
06-04-2013 01:20 PM
Top #3
06-04-2013 01:20 PM
Hi, with respect to longer cable lengths, there are two concerns.
1) Reflected wave effect - Depends on Motor CIV (corona Inception Voltage) and cable length distance. Mere putting RC circuit may not help and can burn motor outer layer winding due to voltage spikes during operation in case of VSI drives. Some drive vendors have dedicated simulation softwares available which can help get right solution with above mentioned inputs.Simulation Software can also show the voltage spike trends at drive output and motor input to make sure that motor is safe with the solution voltage spikes are below motor CIV value all time..
2) Voltage drop in longer cable - It depends on Cable length, Cable size and application also to see impact of voltage drop and current demand by load to meet torque requirements. By right cable size selection and utilizing simulation softwares, right solution can be generated.

It needs better engineering rather than mere product selection to ensure safe and longer operations.
06-04-2013 03:21 PM
Top #4
06-04-2013 03:21 PM
I agree Kuoz, but i would have a suggestion to use

1. Sinus filter which will produce a sine wave at the VFD output ,which will eliminate the PWM waves at the output and hence there is no standing wave problems.

2. There could be about 15% voltage drop at the motor terminals (3-5% sine filter drop and about 10% due to the longer cable distance.My recommendation is to go for 480V supply transformer at the drive input to compensate this drop voltage. Drive shall accept 380-500V and required voltage at the motor terminals can be programmed in the VFD (considering these drop voltages)
06-04-2013 06:22 PM
Top #5
06-04-2013 06:22 PM
It all depends upon the quality of the sinewave. For a cable length of 15km an LC circuit is not required as there is sufficient capacitance in the cable, just some inductance will be OK.

I know something of CSI drives having worked for Hill Graham in the past. For an induction machine it is necessary to provide capitance at the output (motor side) of the drive for commutation of the thyristors. This capacitor bank has the benifit of also cleaning up the current waveform (the voltage is already good). So in essence there is a filter there anyway.
06-04-2013 07:22 PM
Top #6
06-04-2013 07:22 PM
The normal approach is to have a VFD producing a good sinusoidal waveform (like the MV Perfectr Harmony drive). This should be good enough for a cable length of 800m, for very long cables an LC filter can be connected at the output of the VFD (motor side) to eliminate transmission line effects.
06-04-2013 09:23 PM
Top #7
06-04-2013 09:23 PM
By mentioning 15KM's, I wish to share that CSI topology drive does not have any drive to motor cable length distance limitation due to excellent output current and voltage waveform, whether cable length is 10m, 100m or 800 meters. As an example, if distance is 200m, VSI would need an additional filter while CSI does not need.

True CSI has integral capacitor that helps in voltage generation due to CSI topology. But clean current and voltage wave forms are due to other technical attributes of this technology.
06-04-2013 11:25 PM
Top #8
06-04-2013 11:25 PM
Its not so clear as to say all VSI always need a filter, I know of many MV variable frequency drives with muli-level waveforms that do not need a filter on cable lengths of 800m or even to 2km.

It depends on the quality of the sinewave.

Most LV VSI drives have raw PWM waveforms so you would be correct that they would need a filter.
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