Dry-Type Transformer Secondary Cable Sizing

[milwaukeebob]

Hello,
I’m currently supporting a project involving the replacement of an existing generator excitation system. The upgrade involves the installation of a new excitation xfmr, static excitation equipment (controls and power section), associated power and control cabling, etc. The specific question I have is related to sizing of the cabling between the secondary of the excitation xfmr and the AC input bus of the new excitation equipment.

The 3 phase excitation xfmr has a continuous rating of 2446 kVA, 4160V (primary) /540V (secondary). The xfmr (and the associated excitation equipment) has an occasional short-time overload rating of 3334kVA (for 10 seconds maximum). There is NO secondary protection OCD between the xfmr and the excitation equipment. The primary side OCD is a vacuum circuit breaker rated 1200A with an SEL 50/51 protective relay.

I am very clear on cable derating factors as it relates to ambient temperature, raceway design, etc. What there appears to some disagreement on in my group is the starting point for the calculations. Is it the xfmr nominal rating of 2446kVA or the short-time overload rating of 3334kVA? I have suggested the determination of secondary cable sizing starts at the 2446kVA and then in accordance with NEC we multiply by calculated current by 1.25 (in this case 3273 amperes/phase). After that, we consider cable ampacities and all the ambient temperatures, raceway design deratings, etc to determine number and size of cables per phase.

I’ve done some research but haven’t found anything definitive. I’ve been in plant operation for the past 13 years of my career and haven’t had much of an opportunity to do engineering design calculations like this in that time. I have however seen the long term results of under-sizing conductors and want to make sure we do it right.
Thanks in advance for your guidance.

 

[7anoter4]

In my opinion, you have to state the cable cross-section according to NEC, but checking for 3334 kVA for 10 sec. The time of 10 sec is too long to be considered adiabatic process then so considered will be a conservative way-anyway.

 

[waross]

From the Canadian code:
"the conductor supplied by the secondary of the transformer has an ampacity not less than the
ampacity of the primary conductor multiplied by the ratio of the primary to the secondary voltage;"
I would also consider the voltage at the excitation equipment terminals under overload conditions. You may then decide to up-size the secondary conductors if there is considerable voltage drop under overload conditions that may be lessened by larger conductors. It will depend on the length of the conductors and the internal voltage drop of the transformer. In some cases, you may consider up-sizing the primary conductors also.
To put it in perspective, 136% of full load for 10 seconds may be less of an issue than the 600% we often see with motor starting.

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

 

[milwaukeebob]

Thanks guys. I've done a little more research since yesterday and confirmed a starting point of 3334kVA. I agree 10 seconds is too long to not consider the heat generation and exchange. Waross, we have considered the possibility of voltage drop during overload conditions in the xfmr design and the length of secondary conductors is very short <20feet of circuit length. Thanks again for your feedback.

 

[7anoter4]

NEC Table 450.3(A) MAXIMUM Rating or Setting of Overcurrent Protection for Transformers Over 600 Volts (as a Percentage of Transformer-Rated Current):
Primary Protection over 600 Volts [circuit breaker]: 300% [MAXIMUM]
Irated=2615 A [at 540 V].
Imax=3564.6 A [for 10 sec] = 1.363*Irat.
I think for [protection] 50 –instantaneous-300% and for 51 inverse time relay –max.300% for t=0, 3600 for 15-20 sec and 2615*1.25=3269 A for 30 min [and infinite time also].
Conductor rated for 3270 A.NEC Art. 240.4 Protection of Conductors. (F) Transformer Secondary Conductors. Conditions: transformer single phase 2 conductors or delta-delta 3 conductors.
Supervised locations only.
“shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer, provided this protection is in accordance with 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio. “The the setting of primary 51 protection:3270/4.16*0.54=425 A.
No need to take into consideration 3564.6 A for 10 sec. If it exists a pause of 10-15 minutes between 2 overloads no significant overheating will be noted[less than 1 degree C].
Calculated according IEEE Std 242-2001 9.5.2.4 Development of intermediate characteristics
Percent overload capability IE/IN% =sqrt{[(TE-To)/(TN-To)-(Io/IN)^2*EXP(-h*K)]/[1-EXP(-h*K)]*[(230+TN)/(230+TE)]} where:
his emergency operating time in hours.
IE is emergency operating current rating,
IN is normal current rating,
IO is operating current prior to emergency,
TE is conductor emergency operating temperature[90.3]oC,
TN is conductor normal operating temperature[90]oC,
TO is ambient temperature[40]oC,
K is a constant, dependent on cable size and installation type (see Table 9-5),[2.5]
230 is zero-resistance temperature value (234 for copper, 228 for aluminum)
If the cable run in conduit in air [K=2.5] the temperature [for XLPE insulation] will be 90.3 oC [40 degrees ambient].So 0.3 degrees it is negligible.