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Home > Archive > Electrical Engineering > January 2007 > Transformer Impedance Question
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Transformer Impedance Question
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| Steve Lockridge 2007-01-26, 9:25 am |
| We have a situation where a customer needs to put three single-phase,
50 KVA pole mounted transformers into a three-phase bank. The voltage
of each transformer is 13800-277/480Y. Two of the units have an
impedance of 2.9%. The other unit has a 1.6% impedance.
Here's the problem. One transformer design engineer is telling me that
the Wye connection will prevent any significant load and/or voltage
imbalances. Another transformer design engineer is telling me that the
Wye connection will not correct for the impedance difference.
Are there any applications or systems engineers out there who can give
me their thoughts on this? Thanks.
Steve Lockridge
Alfa Transformer
http://www.alfatransformer.com
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| Transformers don't care how they're hooked up, Y,D, whatever. The
impendence difference sets you up for imbabance.
"Steve Lockridge" <steve@websitewarehouse.com> wrote in message
news:1169824114.541389.250370@l53g2000cwa.googlegroups.com...
> We have a situation where a customer needs to put three single-phase,
> 50 KVA pole mounted transformers into a three-phase bank. The voltage
> of each transformer is 13800-277/480Y. Two of the units have an
> impedance of 2.9%. The other unit has a 1.6% impedance.
>
> Here's the problem. One transformer design engineer is telling me that
> the Wye connection will prevent any significant load and/or voltage
> imbalances. Another transformer design engineer is telling me that the
> Wye connection will not correct for the impedance difference.
>
> Are there any applications or systems engineers out there who can give
> me their thoughts on this? Thanks.
>
> Steve Lockridge
> Alfa Transformer
> http://www.alfatransformer.com
>
| |
| Salmon Egg 2007-01-26, 1:25 pm |
| On 1/26/07 7:08 AM, in article
1169824114.541389.250370@l53g2000cwa.googlegroups.com, "Steve Lockridge"
<steve@websitewarehouse.com> wrote:
> We have a situation where a customer needs to put three single-phase,
> 50 KVA pole mounted transformers into a three-phase bank. The voltage
> of each transformer is 13800-277/480Y. Two of the units have an
> impedance of 2.9%. The other unit has a 1.6% impedance.
>
> Here's the problem. One transformer design engineer is telling me that
> the Wye connection will prevent any significant load and/or voltage
> imbalances. Another transformer design engineer is telling me that the
> Wye connection will not correct for the impedance difference.
>
> Are there any applications or systems engineers out there who can give
> me their thoughts on this? Thanks.
>
> Steve Lockridge
> Alfa Transformer
> http://www.alfatransformer.com
>
There is no substitute for understanding the problem yourself. You should
understand it well enough to solve it yourself. If you do not know enough to
tell which engineer (if any) is giving good advice, you are in over your
head and should not be practicing electrical engineering at a level that
requires a PE license.
Bill
-- Fermez le Bush--about two years to go.
| |
| phil-news-nospam@ipal.net 2007-01-26, 1:25 pm |
| On 26 Jan 2007 07:08:34 -0800 Steve Lockridge <steve@websitewarehouse.com> wrote:
| We have a situation where a customer needs to put three single-phase,
| 50 KVA pole mounted transformers into a three-phase bank. The voltage
| of each transformer is 13800-277/480Y. Two of the units have an
| impedance of 2.9%. The other unit has a 1.6% impedance.
If each transformer is single phase to begin with then it is either a
277 volt secondary or a 480 volt secondary. If 277 then wired WYE they
can be 480Y/277. If 480 then the best they can do is 480 delta (unless
you want to go with 831Y/480).
| Here's the problem. One transformer design engineer is telling me that
| the Wye connection will prevent any significant load and/or voltage
| imbalances. Another transformer design engineer is telling me that the
| Wye connection will not correct for the impedance difference.
What is the primary configuration? Wye or delta?
What kind of imbalance is your concern? I'd probably worry more about
the imbalance between transformers being paralleled. To know what kind
of imbalance you would have with 3 277 volt pigs in a WYE configuration
it might be nice to know what kind of load you are expecting. The more
load, the more the 1.6% one will be overpowering the others. But still,
1.6% would be just a few volts more than 2.9% even at full rated load.
Fault conditions might get weirder.
| Are there any applications or systems engineers out there who can give
| me their thoughts on this? Thanks.
Not me. Power is an "interesting hobby" for me. When I have built data
centers I have specified necessary loads and voltages, and that it for
my professional "power experience".
| Steve Lockridge
| Alfa Transformer
| http://www.alfatransformer.com
So are you just doing sales consulting? You guys should at least have a
professional engineer educated or certified in power design on contract
to call upon.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2007-01-26-1246@ipal.net |
|------------------------------------/-------------------------------------|
| |
| Don Kelly 2007-01-28, 3:25 am |
|
"Pablo" <sertec@snet.net> wrote in message
news:qRquh.56102$wc5.7821@newssvr25.news.prodigy.net...[color=darkred]
> Transformers don't care how they're hooked up, Y,D, whatever. The
> impendence difference sets you up for imbabance.
> "Steve Lockridge" <steve@websitewarehouse.com> wrote in message
> news:1169824114.541389.250370@l53g2000cwa.googlegroups.com...
The Y connection will not correct unbalances. However, depending on the
load, the voltage unbalances would likely be small. Single phase loads will
create larger unbalances.
--
Don Kelly dhky@shawcross.ca
remove the X to answer
----------------------------
| |
| Steve Lockridge 2007-01-29, 3:25 am |
| > So are you just doing sales consulting? You guys should at least have a
> professional engineer educated or certified in power design on contract
> to call upon.
We have several engineers that we consult with. That is where the
problem began. The answers were split about 50/50 between "it is a
problem" and "it isn't a problem." Those were transformer design
engineers. The reason I posted here was to see if there were some
applications or systems engineers who could give me a definitive
answer.
Steve Lockridge
slockridge@alfatransformer.com
| |
| Ben Miller 2007-01-29, 1:25 pm |
| "Steve Lockridge" <steve@websitewarehouse.com> wrote in message
news:1169824114.541389.250370@l53g2000cwa.googlegroups.com...
> We have a situation where a customer needs to put three single-phase,
> 50 KVA pole mounted transformers into a three-phase bank. The voltage
> of each transformer is 13800-277/480Y. Two of the units have an
> impedance of 2.9%. The other unit has a 1.6% impedance.
>
> Here's the problem. One transformer design engineer is telling me that
> the Wye connection will prevent any significant load and/or voltage
> imbalances. Another transformer design engineer is telling me that the
> Wye connection will not correct for the impedance difference.
>
> Are there any applications or systems engineers out there who can give
> me their thoughts on this? Thanks.
No connection will compensate for the difference. Voltage imbalance
(voltage at each phase divided by the average) will be minimal under normal
balanced load, but as was pointed out it could be worse with single-phase
loads.
There is a bigger problem, however. Maximum fault current on the low
impedance phase could be up to 80% higher than on the other phases. There is
a danger that someone will analyze this using the higher impedance, not
realizing that they differ, which will give too low a value and result in
inadequately rated load equipment.
Recommended practice is to use three identical transformers. Why ask for
problems?
Ben Miller
--
Benjamin D. Miller, PE
B. MILLER ENGINEERING
www.bmillerengineering.com
>
> Steve Lockridge
> Alfa Transformer
> http://www.alfatransformer.com
>
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