transformer voltage drop

[jmlinder]

I am being required by a Electrical Building Inspector to submit voltage drop calculations to support a permit application. The Inspector has stated that the calculations must include the voltage drop accross the 480/208 distribution transformers. It seems to me that the voltage drop of a 3.7% impedance transformer will always exceed the NEC guideline of 3% and when added to the remaining circuit voltage drops will exceed the 5%. My first thought was to raise the secondary voltage through tap settings but this would create a high voltage condition during times of small current draw. I would appeciate any other thoughts?

 

[augie47]

Curious as to the application. There are few areas where NEC requires voltage drop to be addressed. Your inspector may, of course, be enforcing local rules.

 

[Julius Right]

The actually, voltage drop across the transformer will be only 1.5%:
DeltaV = RI cos(fi)+XI sin(fi) let's say cos(fi)=0.8 sin(fi)=0.6
Let's say X/R=8 and S[KVA]=1000 and the transformer impedance will be:
Z=VL-L^2/S*uk%/100 VL-L=480 V uk%=3.7 Z=0.008525 ohm
Z^2=X^2+R^2 If R=X/8 then Z^2=X^2*(1+1/64) X=Z/sqrt(1+1/64)= X=0.00859 ohm and R=X/8=0.001074 ohm.
Irat=S/sqrt(VL-L)=1000/SQRT(3)/480=1.2028 KA=1202.8 A.
DeltaV=0.001074*1202.8*.8+0.00859*1202.8*0.6=7.23 DeltaV% =1.507

 

[Julius Right]

Sorry! I forgot sqrt(3) then DeltaV=SQRT(3)*7.23=2.6%

 

[jmlinder]

The application is simply a 1200A 480 volt switchboard feeding a 400A 480 volt panelboard feeding a 75 KVA distribution transformer and a 225A 208/120 panelboard.

Curious as to the application. There are few areas where NEC requires voltage drop to be addressed. Your inspector may, of course, be enforcing local rules.

 

[Julius Right]

Still I think your consumers could be supplied with 2.5 % overvoltage- as usual around the world in no-load case- and to get rated voltage at full load.

 

[wirenut1980]

The actually, voltage drop across the transformer will be only 1.5%:
DeltaV = RI cos(fi)+XI sin(fi) let's say cos(fi)=0.8 sin(fi)=0.6
Let's say X/R=8 and S[KVA]=1000 and the transformer impedance will be:
Z=VL-L^2/S*uk%/100 VL-L=480 V uk%=3.7 Z=0.008525 ohm
Z^2=X^2+R^2 If R=X/8 then Z^2=X^2*(1+1/64) X=Z/sqrt(1+1/64)= X=0.00859 ohm and R=X/8=0.001074 ohm.
Irat=S/sqrt(VL-L)=1000/SQRT(3)/480=1.2028 KA=1202.8 A.
DeltaV=0.001074*1202.8*.8+0.00859*1202.8*0.6=7.23 DeltaV% =1.507

How do you know what the voltage drop will be without knowing the load? At full load the voltage drop will be 3.7%. Will the transformer be fully loaded? As others have said, voltage drop is not enforceable in the NEC, but it could be a local rule.

I would not mess with changing the taps until I knew more about the typical voltage profile there. If the voltage typically runs lower than nominal it might be ok. But a lot of locations will typically run higher than nominal voltage (due to oversize utility transformer), and during lightly loaded periods, the voltage may run high and if you add 2.5% on top of that, it could be enough to damage equipment.

 

[templdl]

That is a curious request as there are primary taps that are included on most transformers that can be connected to correct for a voltage drop that occurs befor the transformer, the transformers primary taps would compensate for that voltage drop at the secondary taps.
The only other thought that I would have would be in regard to the load on the transformer where the voltage may drop on the secondary of the transformer as the load increases which as I recall would be regulation.

 

[Julius Right]

Since Vsc=sqrt(3)*Irated*Z transformer short-circuit voltage [Volts- by definition] and
DeltaV= R*I cos(fi)+X*I sin(fi) Voltage drop across the transformer [for any I equal or different of Irated] if I=Irated then DeltaV=Vsc only if R=X and cos(fi)=0.707.Usually DeltaV<Vsc

The no-load of 480 V secondary transformer is usually 525 V [I think :happysad:].Also
As on MV system voltage drop on cable or OHL is negligible and most of
Utilities transformers are automatic on-load tap changer provided no [usually!] overvoltage is expected on transformer MV terminals.

 

[Hv&Lv]

The no-load of 480 V secondary transformer is usually 525 V [I think :happysad:].

No it isn't. The no load secondary voltage would be determined by the supply voltage. 525 is over 9%. We really can't be over 6%. That would be like a 120 volt service having 131 volts.

Also
As on MV system voltage drop on cable or OHL is negligible and most of
Utilities transformers are automatic on-load tap changer provided no [usually!] overvoltage is expected on transformer MV terminals.

What?

 

[beanland]

No so simple - in fact Complex

No so simple - in fact Complex

A transformer is an R + jX impedance. The load has power factor, W + jQ. (Watts and VARs). That load has a complex current, say A + jB. Thus the voltage drop is (R +jX)(A +jB) = (RA - XB) +j(RB + XA). The bottom line is that the voltage drop is based on the power factor of the load and complex transformer impedance. A low power factor load leads to more voltage drop. A capacitive load can lead to a voltage rise!

 

[Julius Right]

That was not good English, I am afraid. Therefore, I think it has to be formulated again as following:
"The utility transformer is provided with on-load tap changer which could be [automatically] controlled".
In addition, I agree, in zone A [for good performances] the maximum supply voltage has to be 1.05*Vrated. However, it is still tolerable up to 1.1*Vrated [zone B]

 

[weressl]

I am being required by a Electrical Building Inspector to submit voltage drop calculations to support a permit application. The Inspector has stated that the calculations must include the voltage drop accross the 480/208 distribution transformers. It seems to me that the voltage drop of a 3.7% impedance transformer will always exceed the NEC guideline of 3% and when added to the remaining circuit voltage drops will exceed the 5%. My first thought was to raise the secondary voltage through tap settings but this would create a high voltage condition during times of small current draw. I would appeciate any other thoughts?

That is just silly.

There is no 'voltage drop' per see across a transformer. It is a voltage 'changer' by design. There is an output voltage variation, between no load and fully loaded conditions and the appropriate tap CAN be selected based on the matrix of average supply voltage and expected average load conditions, but either of those are seldom known. The transformers are designed to supply voltage in the acceptable range when the supply voltage is in the acceptable range. Normally no tap change is necessary.

If there are STRICTER than ANSI acceptable voltage regulations are required by local regulations then those regulations must have some parameters spelled out. Your inspector should supply these details along with the requirements for voltage drop calculations which are only recommendations, not mandates by NEC.

 

[jim dungar]

"The utility transformer is provided with on-load tap changer which could be [automatically] controlled".

Voltage regulators may be found on distribution/substation yard transformers, but they are rarely found on units smaller than 7.5MVA.

 

[weressl]

No it isn't. The no load secondary voltage would be determined by the supply voltage. 525 is over 9%. We really can't be over 6%. That would be like a 120 volt service having 131 volts.

See acceptable voltage tolerances.

http://www.pge.com/includes/docs/pd...ergystatus/powerquality/voltage_tolerance.pdf

 

[kwired]

I am being required by a Electrical Building Inspector to submit voltage drop calculations to support a permit application. The Inspector has stated that the calculations must include the voltage drop accross the 480/208 distribution transformers. It seems to me that the voltage drop of a 3.7% impedance transformer will always exceed the NEC guideline of 3% and when added to the remaining circuit voltage drops will exceed the 5%. My first thought was to raise the secondary voltage through tap settings but this would create a high voltage condition during times of small current draw. I would appeciate any other thoughts?

What is there to calculate? If it is wound for 480 in and 208 out at rated full load then if you input 480 and load to full load you will get 208 out.

If you have any other input voltage and any other load besides full load then output voltage is going to change. Input voltage will drop as voltage on the supply conductors drops does need consideration.

What am I missing besides needing to know loading conditions and voltage drop on the supply conductors, and the fact the transformer primary likely has taps for compensation of input voltage?

 

[Hv&Lv]

See acceptable voltage tolerances.

http://www.pge.com/includes/docs/pd...ergystatus/powerquality/voltage_tolerance.pdf

I've seen that page many times. It backs up what I stated.

 

[weressl]

I've seen that page many times. It backs up what I stated.

Hmmmmm....you stated

"No it isn't. The no load secondary voltage would be determined by the supply voltage. 525 is over 9%. We really can't be over 6%. That would be like a 120 volt service having 131 volts."

Seems to me that the NEMA tolerance, which is what equipment is built to, is +/- 10%.

 

[Hv&Lv]

Hmmmmm....you stated

"No it isn't. The no load secondary voltage would be determined by the supply voltage. 525 is over 9%. We really can't be over 6%. That would be like a 120 volt service having 131 volts."

Seems to me that the NEMA tolerance, which is what equipment is built to, is +/- 10%.

I wasn't referring to NEMA tolerance. That is for electrical equipment that utilizes the voltages supplied.. I was referring to service and utilization voltage.

You have to read my response with the statement I was referring to...."The no-load of 480 V secondary transformer is usually 525 V [I think ]."

 

[Besoeker]

I am being required by a Electrical Building Inspector to submit voltage drop calculations to support a permit application. The Inspector has stated that the calculations must include the voltage drop accross the 480/208 distribution transformers. It seems to me that the voltage drop of a 3.7% impedance transformer will always exceed the NEC guideline of 3% and when added to the remaining circuit voltage drops will exceed the 5%. My first thought was to raise the secondary voltage through tap settings but this would create a high voltage condition during times of small current draw. I would appeciate any other thoughts?

The nature of the load has a bearing on transformer regulation.