secondary rated amps for TRANSFORMER rated OA/FA/FOA, 55/65 C, CAN FA Rated KVA BE USED IN MOTOR STA

[sarawakmiri]

Have searched all over and can find no technical answer. If the Transformer is rated as OA/FA/FOR, 55 / 65 C. WOULD YOU ASSOME THE FOA KVA RATING WOULD BE REQURED FOR 3 HOURS OR MORE AND SIZE THE SECONDARY FEEDER CONDUCTORS AT. FOA. KVA. AMPS x 1.25 %. and can the Rated KVA at FA rating be used in starting the largest motor load. Please help if you can. Not bing able to find an answer on GOOGLE or in any forums is upsetting my technical ability.

 

[RRaghunath]

The issue is voltage drop during largest motor starting, I understand.
If true, what is important to consider in voltage drop calculation is the maximum pre-load (load before the motor start command is given) and the %Z of the transformer.
The important point to remember is that the %Z may be referred to the OA rating of transformer.
Thus, while calculating the ohmic value of transformer impedance, the base MVA shall be considered accordingly.

 

[waross]

It depends:
Are you in IEC land or NEMA land?
You must determine what rating the %imp is based on.
In NEMA land it is based on the ONAN rating.
For normal loads the transformer regulation is used to calculate voltage dip.
Motor starting current, however, is highly reactive at and near locked rotor.
Using the %imp for motor starting will yield acceptable results.
While the %imp is calculated at the ONAN rating, you may use the transformer maximum rating for transformer sizing.
Size the conductors for the type of cooling, (FOA)
Calculate starting voltage dip with the %imp at the rating that the %imp is calculated at.

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

 

[dpc]

The ratings are for continuous operation. If you plan on operating at the maximum FOA rating, you should probably size the conductors and overcurrent protection accordingly. Without any additional information on the actual load or applicable codes, I would size overcurrent protection and the cabling for the highest possible kVA rating with a 25% safety factor to allow short-term overloading as you suggest.

The forced cooled kVA ratings are irrelevant for a short-term event such as motor starting. The main issue is voltage drop, not transformer overheating. The voltage drop is a function of the transformer impedance and this is a constant that is not changed by the cooling of the transformer. The impedance is generally expressed in a percentage based on the transformer kVA rating as a base. In the US, this kVA is generally the lowest self-cooled rating. In IEC land, it is generally the highest forced cooled rating. But this is a just a change in kVA base. The impedance is basically a constant regardless of how it is expressed.

Cheers,

Dave

 

[sarawakmiri]

Thank you for your answers. I understand the Transformer Impedance is Given with the Transformer and the available short circuit current is calculated using the base Rated KVA I,e OA Rated KVA. I understood the normal design load on the transformer was based on the OA Rated KVA. The max motor load that could be started, as a rule of thumb was less than or equal to 65% of the Transformer OA Rated KVA. I have searched all the technical written data on Transformers I could find and can find nothing that is written on sizing the Secondary feeders. My opinion is the max available transformer nameplate KVA should be used and considered as a continuous load,
. BUT it makes no logical sense to size the load base on this max KVA load. Can we all share info on written technical reference that addresses this question. Seems like a simple question which in reality must be standard industrial application. Thank you for your help.

 

[davidbeach]

One transformer by itself, or a transformer surrounded by others? Typically a transformer is loaded to something less than base nameplate rating most of the year. Occasional seasonal loading may push it up beyond that. The additional capacity is often reserved for when an adjacent transformer, or part of an adjacent feeder, is out and load gets switched around. Local practices will vary depending on location, relevant codes, etc.

 

[sarawakmiri]

Davidbeach, thank you that makes me in agreement with you. How would you size the secondary feeders and have you a technical reference for support. Love this forum.

 

[sarawakmiri]

Bill and Dave thank you for your answers much appreciate your technical expertise.

 

[davidbeach]

Sizing the secondary feeders - it depends. What's the governing code? In the US, the answers for NEC and NESC will be vastly different. Is top nameplate your limit or do you have LBNR (Loading Beyond Nameplate Rating) ratings? Most utilities have LBNR ratings, most non-utilities don't. What works wondrously at utility X fails horrendously at utility Y; perhaps because one is winter peaking and the other summer peaking. In other words, there's no answer that anybody here can "give" you; you need to derive the answer based on your circumstances and needs. Non-utility installations will have far less wiggle room than utility installations.

 

[cuky2000]

The minimum feeder size shall meet three basic criteria:
1) Continuous current to meet load demand.
2) Voltage drop
3) Insulation temperature withstands under short circuit.
Although uncommon, feeders can be sized under the transformer rating.

 

[7anoter4]

From IEEE 242/2001:
9.5.2.4 Development of intermediate characteristics
Cable, with the thermal inertia of its own and of its surroundings, takes from 1 h to 6 h to change from initial to final temperature as the result of a current change. Consequently, overloads substantially greater than its continuous rating may be placed on a cable for this range of times.
If functioning on forced oil or/and forced air is limited in time you may use a reduced size cable.
However, if the transformer is a part of a power station where the continuation of energy supply it is more important , I'll consider the transformer maximum power as cable required ampacity as the cost of cable is less than 1% of the delivered energy.

 

[sarawakmiri]

Thank you for your reply. When and why would you even want to specify OA/FA/FOA rated Transformer instead of ONAN ??

 

[bacon4life]

Fans and pumps allow the transformer to be physically smaller, and likely cheaper.

 

[7anoter4]

I know 2 cases in which a transformer will be overloaded. One it is in a heavy industrial complex -as an integrated iron [and steel] mill where the production chain interruption is very expensive. In this case a local substation is provided with 2 transformer-usually each one covering 70% of the load. However, if one of the transformers is out of order for a while, the healthy other must provide the full load.
The second case is in a power station system when a unit could falls and then other units work in overload.
Because this situation can last a long time it is necessary for the transformer to be able to withstand the overload in good conditions.

 

[davidbeach]

For an industrial customer, the loading may be fairly constant and it makes sense to size the transformer based on that load.

For an electric utility the loading can be highly variable; from very low, now even negative, at certain times of the year to very high at other times. The transformer is the transformer and the no-load losses are there all the time. A smaller transformer with lower no-load losses, that gets pushed, even pushed really hard, for a relaitively short amount of time each year. That can be far more economical than having a transformer sized for a load that might exist for a few hours every few years.

 

[sarawakmiri]

7anoter4, Thank you for your reply. I start to understand the application of transformers and would size such secondary feeders for continuous duty at the largest Rated KVA.