I have an application for a diesel 3

[sunilji]

I have an application for a diesel generator which calls for electric heating as a means of heat treatment on welded joints. The job in question is a large pressure vessel which requires about 75 kW in local heating. The heater elements are ni-chrome wires wound in-situ with suitable glass wool insulation and the resistance ( hence kW )may vary from joint to joint. Each heater element with a rating varying from 1 kW to 6 kW is controlled by a 'simmerstat' ( % ON time control ) and a contactor. Since the heaters are all single phase ( 240V ) and are randomly on & off, it appears to me a nightmare as to how to connect the load so as to achieve phase balancing. Further the application requires non-stop heating for 72 hours so that a continuous power source in the form of a diesel genset is envisaged.
Finally my question ! :
What would be the safe rating for a dg set for a 100% resistive load with a totally unbalanced load ?

 

[busbar]

Consider starting from worst case [75kW?] and split 3 ways—that is the total load with all elements on—id est at startup, everything is cold so maximum load is seen by the genset for a while. The maximum phase current for a 1.0PF load should be the limiting factor, with load balance a secondary concern.

 

[sunilji]

What about neutral current ? Would a single phase generator be better suited for the purpose ?

 

[bru]

Be aware also that any component of unbalance will contribute to rotor heating and may be an argument for upsizing the genset to accommodate this. If the genset is closely matched to the total available maximum load the rotor and field may sustain damage from heating while the KW rating is still below generator output.

 

[harshal]

Dear Sunilji,
As a practical experince , you can use a 3 phase generator with 75% derating.
But please take care of starting currents
rgds

 

[Graeme38]

Can't you step the heater elements across the three phases to balance the load?

 

[dydt]

hi..
you said that the heaters are single phase..then you should be able to rearrange them as balance as possible and form a delta configuration..

dydt

 

[busbar]

Maybe I’m missing something. Say you have a 75kW 0.8PF 480V 3ø generator serving various 480V 1ø cycling resistive loads.

75000W/480V/√3 ≈ 90 amperes

{The labeled stator current will be higher, based on kVA, but in this case the load is purely resistive.}

So then, the 1ø heaters, in whatever combination, load one phase to 90 amps, and, say, only 50 amps on the other two phases. Is this risking thermal or any other damage to the stator?

 

[sunilji]

busbar, that is exactly my worry, to what extent can a 3 phase DG set carry an unbalanced, fluctuating load.

Graeme38, yes, the distribution will be designed to form equal kW load on each phase as far as possible. But the nature of individual load control ( simmerstat )is such that it may be on or off for intervals of upto 5-10 minutes

dydt, No. it is not possible to connect a heater in a delta configuration as the individual heaters are wound in situ over the welded joints and only 2 lead wires are brought out. To make a delta connection, at least three heaters will have to linked and they may not be close by.

 

[jbartos]

Suggestion: Load is varying up to 75kW at 240V, single phase, connected to a three-phase generator. The load is resistive and its fluctuation should not pose any problem; especially as suggested by harshal posting, to have it maxed by 75%, i.e. (1/.75) x 75kW = 100kW for the generator rating. Normally, it is prudent to run the generator at some load less than 100%, e.g. 80%. It is not most economical; however, it leaves some room for unexpected changes in loads, e.g. lower leakage resistances, inter-turn shorts, etc.

 

[GOG]

In my opinion it's just a matter of good reading the description of the problematical case.
The best way to handle this is to go for single phase power generation by either a single phase generator (probably hard to get) or a three phase generator with a power transformer (3 phase primary, 1 phase secondary).
I realize this is not the cheapest option, but the advantage is that you don't have to select an oversized generator in order to withstand the expected unbalanced loads.
Hope this helps.

 

[peterb]

Not too sure how a 3-phase to 1-phase transformer would work(there are winding connections that can achieve this, but you still end up with unbalance on the 3-phase side).

As we are dealing with resistance heating elements, how about connecting the generator to a 3-phase rectifier bridge and then connecting the load to the DC output? Can this work with the existing controls? All AC load would be balanced for this configuration.

 

[GOG]

I have to confess that I was too quick with my answer. However the resulted reply of peterb was worth it.
Great thinking, and in my opinion this is the ultimate solution!!! My compliments for it.

Note:
Don't forget to calculate the power loss (heat dissipation)of the rectifiers.