Is an isolation transformer needed on a VFD input side? 3

[edison123]

This week, we trialed a 125 KW VFD for testing LV motors in decoupled condition. During the trials, I noticed our LED lights started flickering even though they were switched off.

When asked, the OEM drive guru said there will be harmonics fed into the system, and we need to install an isolation transformer to prevent harmonics getting back into other loads.

Is this correct? If yes, what will be transformer connection, delta input/wye output, or delta/delta or wye/delta?

What will be the KVA rating of the isolation transformer?

If isolation transformer is required, can I go for 415 V input / 690 V output so that I can get a 690 V VFD output for testing some motors which are rated at 690 V?

Will a 690 V VFD give 415 V output (which will be the 95% of the motors we test) without any derating in current capacity?

 

[Gr8blu]

In general, drives only care about the current. This means a 690V-400A drive will happily operate at around 415V-400A. But the harmonic content on both the input and output sides of the drive will be somewhat higher than when operated at rated (690V) output.

Isolation transformers are (usually) rated 1:1 in terms of voltage, but there is no reason you can't go to 415/690 if that's what works for you. Given that the drive is on the output side of the transformer, it's probably a good idea to have the transformer secondary as a WYE connection.

 

[waross]

Just a thought;
We have had posts that pointed out that while a 690 Volt drive will produce an effective voltage of 415 Volts, the peak of the individual pulses will still be based on 690 Volts.
The windings of the 415 Volt motor will see the same stress as if they were tested at 690 Volts.
If this is a concern, You may want to look for a 415:690 Volt transformer with 415 Volt taps so that you may test 415 Volt motors without the 690 Volt stress.

 

[edison123]

Thanks, gr8. So it would be a delta/wye transformer? Any reason why the VFD input should be a wye?

Bill. Yes, I had the same idea. 415/690-415 trafo. Since the drive is 690 V rated, I guess there is a better safety factor (in terms of voltage) when it is used in 415 V?

 

[waross]

An anecdote for illustration:
A new armoured cable tripping the breaker at 120 VAC to ground.
The cable rang clear on a continuity test.
The cable rang clear on a 250 Volt Megger test.
The cable rang dead short on a 500 Volt megger test.
I happened once in a lot of years.
But, you will know better than me:
How likely is it that a motor will pass at 415 Volts but fail at 690 Volts?

Possibly worth a try if capital is short;
Wind a primary reactor around a CT core.
I am thinking a core about 5" or 6" in diameter.
It may not take a lot of inductive reactance to attenuate higher frequency harmonics.

 

[waross]

Add to that an auto-transformer for the 690 Volts.
Here we go from 480 Volts to 600 Volts with a pair of 489:120 Volt lighting transformers in open delta.
I'll leave it to you to select your common voltages for an auto-transformer boost.

 

[waross]

I am thinking a core about 5" or 6" in diameter.

How about a stator core from a discarded motor?

 

[FacEngrPE]

Most VFD'd will misbehave if connected to a delta supply.
This is solved by using a Y winding on the output of the isolation transformer, and deriving an isolated ground from that.
See the electric code for details.

If you use the autotransformer approach (boost buck) the ground reference is provided from upstream, but you do not get isolation.
If you only need to prevent the VFD from disturbing other equipment by it's harmonic injection into the power supply, install inductors at the panel input. Often the best size for input inductors is found in the O&M manual.

480 VAC x 1.414 = 678 volts
690 VAC x 1.414 = 973 volts.

Max DC Bus Link Voltage is approximately equal to AC Peak voltage
I would order a transformer with taps to adjust the input voltage as Bill recommends, unless you are intentionally stress testing the motors.

 

[edison123]

Wye secondary with grounded neutral it is.

I have an old 250 KVA trafo which I will rewind for 415 Delta and 690 V Wye with a 415 V tap.

 

[waross]

Sounds good and at the right price. grin

 

[waross]

During the trials, I noticed our LED lights started flickering even though they were switched off.

I hope that it wasn't Rf interference rather than conducted interference.
While the isolation transformer with 690 Volts available is a good idea, you may consider a test with the drive and drive cables shielded to rule out RF interference.
Alternately, you may be able to shield the led drivers.
It will be embarrassing to install the transformer and find the lights still flickering.
That said, even if the flickering is RF the transformer is still a good idea.

 

[edison123]

Update: Drive OEM informed a 690 V drive cannot be used for 415 V input because of capacitor charging capability.

Their recommendation is a 3-winding D/D-Y isolation transformer with 30 deg phase shift for 12 pulse firing for harmonics reduction.

Or go for an AC to AC drive eliminating all the DC circuit & the capacitors and with 5% current harmonics, but the cost will be much higher.