distribution transformers cxalculation

[zorgglub]

Dear All,
we have 3 padmount distribution transformers , rated for 13.200Y / 208y-120.
the line voltage in Haiti is 12.470v.
we keep on experiencing low secondary voltage , and the installer of the transformer tell us that it is because of grid power , not transformer.
my guess is that the transformation ratio is 13.200/208 = 63.5
the secondary voltage should be 12470 / 63.5 = 196v.
am I right ?
maybe we can adjust the tap on the transformers , 196v + 5% = 206v
what do you think ?
thanks for the support.

best regards

Jean Michel

 

[magoo2]

That should work fine.

 

[waross]

It is the wrong transformer. Should be a 13,200 Volt transformer.
We used to ave similar problems in Central America. We frequently received 13,800 Volt transformers for our 13,200 Volt system.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[zorgglub]

the fun part of the story is that the generators connected via an ATS are delivering...220/127 !
so we alternate between 196v to 220 and back , the transitions are rather brutal. we are losing AC units and fluo balasts at an accelerated rate.
the plan to correct this issue is to re-tap the tranformers and adjust the AVR of the gensets to have +/- 206v from both sources.

best regards

Jean Michel

 

[waross]

"the transitions are rather brutal". I hope that you are not doing a very fast transfer without synchronizing. An abrupt discontinuity in the sine wave may be more damaging than an abrupt voltage change.
The plan to set everything to 206 volts sounds good.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[zorgglub]

we are using Kutai BTS3BE800 transfer switch

the DSE331 used for switching control is not synchronizing , since main transformers and generators nominal voltage do not match (196v for tx , 220 for gen), and the "genius vendor-installer-engineer who setup the power stations" doesn't have a clue about how to make it works ...
i can post the conf. file of the DSE331 , it's prety sad to see the timers values for this kind of switch.
we are planning to redo the entire system with the DSE distributor for Latin American & Caribbean , plus redoing the gensets installation (ducting of radiator smaller than radiator surface , no flexibles on exhaust pipes , etc.).
thanks for the support guys , it cheers me up !

jean michel

 

[waross]

A very fast transfer switch operates so fast that an out of phase closure may damage motors including possible bent or broken shafts on larger motors. A very fast transfer switch incorporates a sync check relay that delays retransfer until the generator output drifts into synchronism with the grid.
With large motors it is good practice to allow several seconds (5 or 8 or more) to allow the residual back EMF of the motors to decay to a safe level.
Fluorescent lights and refrigerators withstand fast transfers quite well. I've done a couple of dozen residential installations with fast retransfer without issues. It is larger motors that experience problems with a fast retransfer.
I suspect that the 196 Volts (and possibly less at times) is the cause of the failures.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Yep, sub 200V will normally toast a lot of 208V fully loaded motors like refrigeration compressors. Sub 208V will toast motors that were re-plated from 240 to 208.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[zorgglub]

indeed , in Haiti we are running almost 14h/day on generators ,and overheating of AC and (chinesse) ballasts are commons.
about the ATS switch , i think it is the return from genset to main city power that cause brutal transitions.
according to the configutation file of the DSE331 , the auxilliary switches are not used , i wonder if setting a pause when switching from generator and main will permit to synchronize the return.

best regards

Jean Michel

 

[melspuds]

zorgglub;
If the incoming voltage is 12,470 volts and you have a 13,200 volt primary transformer the secondary will be always low. Most transformers have 2-2.5%(= 5%) below normal voltage taps. Changing the tap changer would help, but most likely the 12,470 line is already low when loaded. Your transformer 13,200 - 5% = 12,540 volts. 12,540-12,470=70 volts and most likely the 12,470 is low to say about 2.5% = 12,158 volts. Therefore when the line is loaded the secondary will be approx. 202.8 volts. As I see it, that transformer must be changed to a 12,470 volt primary one.
I might be wrong, but I bet the 13,200 volt transformer is already on 5% low tap.
The main problem with most 208 volt motors is that they are wound for 230 volts service and relabeled for for 208 volt service. Which means the motors are very sensitive to low voltage.
Your ballasts will overheat due to low voltage.
Suggest that you make the installer install the correct voltage transformer. That is the only answer to your problem.

 

[zorgglub]

Thanks Melspuds (is this your real name ?!?).
here it is not so easy to find such transformers , as they all come from Florida or US .
the budget for the project is not extensible , I thinks we can manage to replace motors from time to time , and switch fluo tubes by led models. all criticals equipments (operating rooms , ICU , emergency) can sustain power from 100 to 240v.X-ray generator will need to be recalibrated eventualy.
thanks gentlemens , you were helpfull !

best regards

Jean Michel

 

[waross]

If you are stuck with the existing transformer you may consider adding auto-transformers to the lighting panels to boost the voltage.
120V:6V transformers should boost your 206V:119V up to 216V:125V. That should make your equipment happy.
three 500 KVA transformers should be able to handle 83 Amps each at 6 Volt boost. That should be enough for most 100 amp panels at 80% loading.
I feel your pain on the transformer. I used to have a new, unusable, 13,200V:277V transformer sitting behind the power house in a third world location. It wasn't worth the cost to return it to the US for the 13,200V:480V transformer that we needed.
This was a case of a general manager who thought that he didn't need a system engineer (myself) to vet electrical orders and procedures. Every time that he tried to prove that he didn't need me, he ended up proving how much he really did need me.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[melspuds]

zorgglub,
Just keep in mind that this is a hospital, that is a life or death facility. I just do not understand how the electrical system is so screwed up. You need a very good electrical engineer and a electrical contractor, which have experience with hospitals. I know that Haiti is a poor third world country. I can not see how somebody would install a wrong primary voltage transformer and a generator with the also the wrong voltage. Was there any oversight of the installation by the hospital or anybody else?
What size are the padmount transformers? Are all three the same size? Waross has a idea with using auto-transformers, which would work, but still is not the correct thing to do. Those transformers need to be changed to 12,470 volt primaries. Also get the generator to the correct voltage if possible or replace the generator.
Good luck,
Dave

 

[waross]

I've run into other generators running at 220:127Volts or 129Volts. Usually someone who has always heard 220Volts looks at the generator voltmeter reading 208 volts and cranks the AVR all the way up trying to get the 220 Volts that he thinks he needs.
Just turn the voltage adjustment down on the AVR to 208 Volts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[zorgglub]

Dear all,
we are going the have the transformers adapted to the 12470v line and adjust the AVR to 208 v, and see how things are going.
thanks for the informations and advices !

best regards from Haiti

Jean Michel