VFD Resonance With AC Grid 1

[Guilhermebr]

Hello

I made some measurement in 3 phase 60Hz 380V, in the input with 3 VFD 500CV same bus.

Below waveforms voltage e current at max load, and resonance probability? maybe between capacitor DC Voltage bus x grid AC cables?

The is 250 metes low voltage cables, betwenn suplly transformes and input AC VFDs.

current_oscillations_around_zero

voltage_commutation_diodes



GS

 

[Guilhermebr]

Fase A have oscillatory transients, and, Phases b c high frequency transients. The waveforms have 512 samples for circle around 30 khz.

GS

 

[iop95]

What VFD model? What control motor setting (V/f, vector, DTC)?
Are you sure it's diode rectifier? May be controlled (SCR or IGBT).

 

[Guilhermebr]

Hello iop95. Weg CFW11. Yes, I am sure. It is diode rectifier.

GS

 

[Guilhermebr]

I believe in a resonance between inverter capacitor link dc and, cables AC supply voltage.

GS

 

[waross]

Vary the frequency of the load and check the waveforms.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Guilhermebr]

The output frequency vfd is fixed at 60Hz.

GS

 

[Marke]

Hello Guilhermebr

This could be Middlebrook instability.
Do you have a harmonic filter in series with the input to the VFD?
Do you have a Hi Z impedance on the input to the VFD?

I usually find Middlebrook in a rural installation with a single VFD on a dedicated transformer and many KM of dedicated 11KV or 22KV line supplying the transformer.

If you were to look at the input impedance presented by a VFD, you will find that at operating voltage, the VFD presents a negative impedance in that the slope of voltage against current is negative, not positive.
If the supply impedance is high and inductive, and there is significant capacitance across the input to the VFD (i.e. harmonic filter capacitors), then you effectively have a low frequency tuned circuit with a negative resistance device in parallel. This is an Oscilator. You will probably find that as you reduce the speed, the oscillation will reduce and stop.
In my world, we find that Middlebrook only kicks in around 45Hz.
Reduce the capacitance on the input, reduce the speed, reduce/eliminate the DC Bus compensation in the VFD will all help.
Best regards,
Mark.

Mark Empson
Advanced Motor Control Ltd

 

[davidbeach]

Mark, nice to see you back again.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[waross]

Welcome back, Mark.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[iop95]

It's input filter + cable oscillation, at about 2kHz.
Power supply seem to be week and cable oversized (very low resistance).
May try to add some resistor across filter output (VFD input).
If no input filter, cable only may do the same for such lenght.

 

[Guilhermebr]

Hello.

There is more oscillation just commutating diode, not all the time in the waveform. Maybe problems some any diode?

GS

 

[iop95]

No diodes issue.
As Mark said, may be Middlebrook instability - when output impedance of input filter (or just power supply if no filter) is (or become at some frequency value/range) greater than input impedance of rectifier/converter.

Such oscillations after diode commutation are "normal" for a non-damped line.
Red curve has lower frequency - seems that non-simmetic/equal elements exist on each phase.

 

[Guilhermebr]

The voltage is not equal in the 3 phases, see below.

The impedance not equal, I think that are the power cables from transformer to VFDs



GS