No Neutral at 3 phase step down autotransformer?

[somersetsparks]

Hi all,
Been a few years since i've posted but here goes! At the factory I work in we have an old Spanish line from the mid-eighties that is fed from uk 400v 3 phase via a 3ph step down autotransformer to give 380v output.
The spanish panels Do not use a neutral, just 3 phases and further 380v -> 220v ( 110v - 0 - 110v) transformers to provide single phase power to some circuits.

Recently we have had a replacement 30kW Mentor MP DC drive fitted in one of the panels (replacing an old Simoreg analogue DC drive due to failure, ever since fitting this drive we have had issues with it tripping on a hardware fault and occasionally taking out the 100A line fuse (supplying the said DC drive only) on L2 (I am told this also happened before with the old drive). I am aware that the fuses are overrated anyway, and to have an instantaneous blow means a lot of current possibly a motor flashover !?,

Anyhow, today I was told that this drive trip occurs several times a day and is reset by the line manager, I have checked the supply voltage before and after the autotransformer and found that L2-L3 (Outgoing side only) sometimes dropped to 60v under full load for a couple of seconds at a time! I have tested the autotransformer windings and found no issues, however I have found that no supply neutral is connected, leaving it as a floating Y-Y autotransformer, we have 2 more of these transformers on different parts of the factory and both have the incoming neutral connected to the star point, could this be my issue? (this line was not originally ours it was purchased second hand and installed by contractors)

apologies for the very long post, any help much appreciated!

 

[marconi]

I looked up 30kW Mentor MP drives and found this information:

4.3.1 Supply types Drives rated for supply voltages of up to 575 V (rated up to 210 A) and 600 V (350 A and above), are suitable for use with any supply type i.e. TN-S, TN-C-S, TT, IT with grounding at any potential i.e neutral, centre or corner (“Grounded delta”).

See: http://www.dcdrive-direct.com/media/18931/Mentor_MP_User_Guide.pdf

It does seem that the 380Volt 3 phase 3 wire supply to the drive can be derived from a 3 phase 4 wire system which has an earthed neutral (ie: TN-S and TN-C-S and TT). As you mention, the current drive has supply from an autotransformer with a floating star point ie virtual neutral( - at some potentials with respect to earth and supply neutral) albeit not required to be connected to the drive. The other two drives do have their star points connected to neutral. So based on this information it seems odd the auto-transformer you mention does not have a 4 wire supply and the star point connected to neutral which would fix its potential to the supply neutral.

Hope this helps in some way.

 

[marconi]

Some further thoughts. The magnetic core of a transformer does not have a linear B/H curve - think hysteresis curve. This means(see * below) that a zero sequence current (with higher order odd harmonics - 3rd, 5th etc - which add up) 'need' to flow in order to produce a sinusoidal excitation field and thence sinusoidal secondary voltage. In a three phase star wired system this requires a neutral current to/from the star point. So the the absence of the neutral connection to the star point may be causing distortion of the voltage waveforms at the output of the A/T and hence troublesome currents large enough to rupture fuses.

*(Whenever a sinsusoidal emf is applied across a non-linear component the current which flows has the fundamental and harmonics of it, their ratios being dependent on the nature of the non-linearity).

 

[marconi]

I have just turned this up which explains more :

http://electrical-engineering-portal.com/transformer-connection-star-star

 

[marconi]

somersetsparks - If after all the appropriate continuity, insulation, connection checks of the drive and the auto-transformer you find nothing untoward then I recommend you connect the star point to the neutral of the supply's source of electricity.

 

[somersetsparks]

Hi marconi, thanks for your reply, it's been a while since I've thought about it, but I don't think it would cause to many problems if I connected the supply Neutral to the auto transformer, as shown in part of the link you have posted, we would then have somewhere for the zero sequence current to go, I'm back in on Monday and will probably try it Monday night ready for Tuesday! Thanks for all your replies, it's been a real headscratcher of a fault, mentor/emersson have been helpful over the phone but it seems to be a supply fault, anyway, fingers crossed, I'll let u know how it goes!

 

[somersetsparks]

Hi all, happy new year and apologies for the late reply, we have cracked the issue, it was sort of related to lack of neutral at the transformer although the voltage drops were also a symptom really.

First of all, I connected a neutral to the supply side of the auto transformer (although obviously the supply Neutral and the load neutral are the same, it is important to note that none of the loads in the panel use a neutral and all loads are connected three phase or accross to phases). After connecting the neutral my issue nearly resolved itself, the voltage stayed much more stable, and we no longer blew any fuses although the drive still tripped with the same fault on occasion although probably 80% less.

Fast forward four months and we have been living with the problem in its much reduced state, I didn't call it a success but production were pretty happy as downtime was reduced.
HOWEVER... We developed an unrelated armature/commutator breakdown after a period of excessive load on the motor due to increased hours, speed and general abuse of a 15kw dc motor.

Whilst diagnosing that fault and being in contact with Control techniques, we came accross another issue, the line reactor for this drive was undersized at 40uH instead of the recommended 220uH, this came about as the old drive had an internal line reactor so only required a small external one. When the drive was replaced with a mentor MP, the old undersized reactor was left in place.

After having replaced the reactor we haven't had a trip since!

Thanks for your help Marco night and I'm glad to have found a full solution