Load sensing

[Kolo]

Hi there,

The problem:
600V feed into a 600/240V transformer, Transformer runs continuously and the no load running cost is approx. $1200/yr. Want to use load sensing to turn transformer on.
Currently the transformer is manually activated using push button switch which controls a contactor.

Idea:
I would like to use a smaller rated trasformer to sense a load and when the current draw is larger than 2 amps, switch to larger transformer.

Please verify if this idea will work, and what sort of protection is required to protect the smaller transformer. If this idea is not a good one, please feel free to give some circuit suggestions.

Thank you

 

[jbartos]

Suggestion: Essentially, the idea is realizable. Depending on the load, the primary side of transformers could be controlled by contactors, preferably if switching is expected to be very frequent. If not, possibly electrically operated circuit breakers/switchgear could suffice.
Transformer loads would have to be scrutinized, e.g. if there is a bigger load coming to be loaded to the smaller transformer, it could potentially trip protective devices of the smaller transformer. Therefore, the would be larger load for the smaller transformer, would have to be delayed until the transformer transfer from smaller transformer to the bigger transformer is implemented.
The protection of the smaller transformer could be a regular protection of the smaller transformer, assuming that the call for the larger load is transferred to the bigger transformer.

 

[busbar]

What is the kVA rating or the transformer? What portion of you total electric bill is $1200? How was the $1200 determined?

 

[Kolo]

busbar

The cost was determined by measuring the no load current of the transformer and determining the kwhr/yr.

jbartos

exactly how would a person delay the load for the smaller transformer? Could you be more specific as to the type of protection equipment.

Thank you.

 

[Buhsman]

You could also purchase and inexpensive PLC and 2 CT's. Use the PLC to control your circuit. When the current exceeds 2A have it switch to the larger Xformer. When the current drops below 2A switch back to the smaller one. Just a simple idea.

 

[busbar]

It may be more economical to install a dedicated transformer for serving the 2-amp load and not use a sensing/switching arrangement, except for that existing with the larger transformer, provided that all is installed complying with applicable electrical codes.

Two amps at 600V is about 2kVA or <1kVA at 240V. Separate overcurrent protection would be needed for the proposed small transformer.

Aside — Was power factor considered in calculating kWh from no-load current?

 

[cbarn24050]

Hi, a good point busbar. An off load transformer has a poor power factor so the actual power consuption will be much lower than kolo thinks.

 

[Kolo]

The power factor was not taken into consideration for no load cost analysis.

Since the transformer running load in well balanced the power factor is high and therefore I believe the is neglegible for the running cost analysis.

During no load the only transformer losses would be core losses, since core losses are constant (primary voltage constant) then would the power factor need to be considered and if so, how would a person determine the no load Power factor.

Thanks for any assistance

 

[jbartos]

Suggestion to Kolo (Electrical) Jun 6, 2003 marked ///\\jbartos
exactly how would a person delay the load for the smaller transformer? Could you be more specific as to the type of protection equipment.
///A derivative circuit sensing the rise of the current can trip the smaller transformer SCR transfer switch and turn on the larger transformer. The fast SCR switching in less than 4ms might be used.\\\

 

[busbar]

A very rough estimate is (or maybe, was) about a 1.4-kilowatt demand, but if kW was determined by V&#8729;A without power factor—kilovoltampere demand is a more accurate term. With most power-frequency transformers, iron or no-load losses have a large reactive component {VARs} and little real component {Watts.} That is to your advantage as far as recurring costs are concerned.

The local utility rate schedule will have the most effect on true costs—whether reactive- or apparent-power/energy is used for billing.

Measurement would be best conducted with a handheld device like a Fluke 39 or 41, or a power-rated oscilloscope like a Tektronix 720P-series unit.

Just to get a general feel what your dealing with, what is the kVA rating of the subject transformer?

[An open-delta transformer set of a pair of 1ø units may be an economical fix.]

 

[nbucska]

Kolo:

The power factor of an partially trafo is not high !
About your suggested switching system:

1.) You have to make sure all of the loads can tolerate
the switch-over glitch.
2.) you may consider separate the low and hi-current
nets
3.) I don't think the sawing would justify the cost.


<nbucska@pcperipherals.com>

 

[jbartos]

Suggestion: Majority of loads tolerate 4ms glitch.

 

[moturcu]

Kolo;
No load loss in the transformer is approximately core losses which are related with the flux level in the transformer core. Flux level is proportinal with primary voltage level.

If I were you I put a solid state voltage controller on primary side and reduce voltage level when there is no or light load. This will reduce core losses on the one hand and increase harmonic related copper losses on the other hand. I think a detailed study is required for cost and performance justification. But certainly you will get much higher bandwidth than the system with a contactor.


Moturcu.

 

[jbartos]

Suggestion to the previous posting: To reduce voltage on no load transformer often poses undervoltages on the secondary side unless there is an on load tap changer. The on load tap changers are feasible on large kVA transformers only.

 

[moturcu]

Jbartos,
I agree with you. However, if the application does not care or tolerate to the reduced secondary voltage level, the idea is still applicable. By &quot; does not care&quot;, I mean no load operation of the transformer.