Isolating transformer to deal with inrush currents?

[RichardMichael]

I have a problem at a remote site in Africa where we are running a couple of prime power generator sets (FG Wilson 220/380, 50 Hz (1500 rpm) 450 kVA (409 kVA prime power) single set operation, no synchronization, controlled by a transfer switch. The alternator is connected star, and earth neutral TNS connection is provided inside the unit.

The system is running during normal operation at between 40 and 50% of the prime power rating, which should not present us with problems, ambient temperatures remaining constant at around 30 – 32 degrees Celsius. We are at sea level.

The connected load is around 65% inductive, comprising reefers, chiller/freezer units –the remaining 35% is made up of general single phase small power and lighting for offices and accommodation. The phases are balanced to within around 5%.

Our problem is that we also supply power to our landlord, who has a construction yard. He uses a lot of heavy duty three phase equipment such as pressing machines and industrial wood saws. There is obviously a fluctuating load on his site, something between 40 and 45 kVA.When this equipment is being used on the site, the inrush current is causing voltage dips, predominantly on L3, which drops top around 160 volts.

We have all gthe usual symptoms – lights flicker and compressors are in constant need of repair. Due to complicated inter-company politics, it is not possible to implement the obvious solution of supplying the landlord’s installation from a separate generator.

I was thinking that an isolating transformer might be a solution – but I am in need of further technical advice.

 

[Engineer54]

Surely the simplest and overall best solution would be for the landlords supply to be derived from it's own generator system!! An isolating transformer will not cure your standing problem!! Sounds as if your alternator loading is rather unbalanced, try sorting that out, before digging any deeper!!...

By the way, prime and stand-by generators should be run at a minimum of around 70% of it's rated continuous load, not 40%/50%... Generators prefer to run loaded, rather than under loaded, continue and you'll end up coking up your engines

 

[Knobhead]

An isolating transformer isn’t the way forward, it’s just adding to losses. If the generator is the back up supply then you’re going to have to look at some load shedding when it’s in use. 5% imbalance shouldn’t cause any major problems. you need to look more closely at your set up.
What imbalance is your landlord adding in to the system.

 

[GLENNSPARK]

Surely the simplest and overall best solution would be for the landlords supply to be derived from it's own generator system!! An isolating transformer will not cure your standing problem!! Sounds as if your alternator loading is rather unbalanced, try sorting that out, before digging any deeper!!...

By the way, prime and stand-by generators should be run at a minimum of around 70% of it's rated continuous load, not 40%/50%... Generators prefer to run loaded, rather than under loaded, continue and you'll end up coking up your engines

yep....diesels dont like overspeed.....and they certainly dont like to `run light` either...lol.....there grafters by nature are diesels....keep em under load at a steady pace.....

 

[GLENNSPARK]

and as for imballences between phases is concerned.....better ballancing of phases and a larger neutral?....

 

[WDMDL]

Why L3?
If phase loading is balanced to within 5% as you say, a 3ph machine should not cause a significant drop on one phase compared to the others.
How long does the higher starting current for the large machines last for? How much higher than running current is it?
Sounds like the output voltage of the generator is falling due to current limiting in its regulator circuitry.

While the generator may be 400kVA and be running at just under 50% load, the starting current for the yard's machines may be in excess of 200kVA, hence the current limiting and therefore voltage reduction.

Load shedding has been mentioned and could be the answer.
One possible load that could be disconnected is your compressors, which would then prevent the damage to them from deficient supply (subject to adequate reservoir capacity - something else to think about)
They only need to be disconnected for the duration of the starting current (plus a time delay, if the duration is so short that the motor may not have stopped before re-energisation)

I think some more detailed power analysis is required to see exactly where the problem lies before altering anything though...

Simon.