3 Phase Voltage stabilizer unestable 3

[Mario JGG]

Hello,
We have an installation with the following configuration,
Connection to the grid with a Y-Y Stepdown Transformer 750 KVA, from 12470/7200 to 480/277V, 60 Hz.
The load are 3 phase air conditioners, the compressors are connected in Delta, the current of consumption is around 25 Amps, each, we have normally 02 runnings, and 01 in stand by, like there are voltage fluctuations from 410 V to 550 V, we have installed one new 3 phase Voltage Stabilizer type Servo, internal arrangement type Y with neutral.
We have the following situation, when the compressors are running at least one, with 20 Amps, the stabilizer is working fine, when the compressor stopped(minimal current in the load around 1 Amp), the stabilizer starts with erratic comportment, noise in the internal transformer, one of the arms of the servo going to full movement and the equipment is tripped for high current in one phase, it just only happens with no load.
My Hypothesis are:
- The neutral in the inlet transformer does not have a good reference with earth, and we do not have a good reference with no load, the neutral is causing the unbalance in the equipment.
- Some problems of earth.
- We are thinking install one small 3 phase load (5 Amps) to cause that the stabilizer has all-time one triphasic load.
Please any advice or experience is really appreciated
Regards,
Mario JGG

 

[FacEngrPE]

Is this something like your device?

I think the three phase unit is three single phase units having a common neutral. For this to sense voltage correctly the input neutral must a solid connection to the transformer neutral. In the US we rarely see these as our power suppliers usually keep the power supply within prescribed limits.

 

[Mario JGG]

Thanks, FacEngrPE,
I appreciate your help
Yes very similar, as do you indicating are 3 units with a common neutral, our suspect is neutral is not correctly earthed, or there are some is making it unstable because we have all connected to the neutral of the transformer, the stabilizer works fine when having 3 phasic loads, but it fails when the loads are sttopped.

 

[FacEngrPE]

I am going to make a guess here.

Loose neutral connections, in this case between the transformer and the stabilizer will in the presence of a single phase load, make the neutral point of the voltage stabilizer wander. That would explain the strange behavior of the regulating control driving one of the tap changers to it's stops.

See this thread thread238-19735, similar to your situation.

This could be at the transformer, the connecting wiring, or inside the stabilizer. The voltage drop in the neutral between the transformer and the voltage stabilizer should be measured - with a single phase load (a large load is better) in place, and the voltage regulator either out of the circuit or set to a fixed position.

Given the voltage level I would recommend engaging an electrician skilled in troubleshooting.

 

[Mario JGG]

Thanks, FacEngrPE, good information.

 

[waross]

There is a wide difference between the purpose of a ground and a neutral.
Line workers typically use the same connections and conductors for both grounding and for neutral connections.
If the primary wye point of the 12.47 kV transformer is not properly connected AS A NEUTRAL, you may have floating and wandering voltages on the secondary, regardless of secondary neutral/ground connections. If the secondary wye points are not properly connected, it gets worse.

If all else fails, try a 100 KVA transformer with a 480/277 Volt primary, the secondary voltage is unimportant.
Connect the secondary in delta, tape it up and forget it.
Connect the wye point to the wye point of both the 480 Volt winding of the step down transformer and to the neutral of the voltage regulator. The delta winding will use circulating current to balance the phase voltages in the wyes. The stabilizing effect will reflect back onto the primary side.
By the way, the delta will circulate enough current to counter whatever is causing the effect. 100 KVA is probably overkill.
A 7.5 KVA transformer is probably enough.
With a motor connected, the back EMF will counter whatever is causing the effect.

The Original Poster is not technically proficient in this area and may be making guesses and suppositions.
I say this not to criticize but to explain why I despair of determining the cause of the problem.
Despite that, and whatever the issue is, I damm well know how to fix it.

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

 

[edison123]

Issues with Y-Y trafo with floating neutrals.

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

Muthu

http://www.edison.co.in