To Parellel Or Not to Parellel? 1

[Mbrooke]

Onsite power/co-gen for a remote building. What are the pros and cons of paralleling each gen vs running them independently?


For those asking the 100kw units (1B & 2B) are going to be replaced with 200kw units so as to be a full back-up to units 1A and 2A.

https://files.engineering.com/getfi...2-4926-9c81-8f9f80ca26e4&file=Local_Power.jpg

 

[cranky108]

Con, increased fault current. Pro, more reliable for a single generator trip.

 

[Mbrooke]

Agree.

What about cost? Obviously replicating gens is pricey, but how much does paralleling gear cost?

 

[mparenteau]

Rather than guess...it might be worth a call to a company like Powell to get a budget cost.

Mike

 

[waross]

Price;
Not just the purchase price.
Fuel costs will be greater.
Maintenance cost will be greater with twice the total running hours.
Diesel engines typically work best at higher loading.
Complexity. By adding paralleling gear you may require a higher level of technical ability to troubleshoot.
There are many more electricians with ATS experience available than there are electricians with experience with auto-synchronizing gear.
By not paralleling you avoid a number of possible failure modes, some of them serious.
Compare the cost to your customer of the lost time as a generator takes 8 to 10 seconds to start and come on-line with the cost of lost time and repairs due to a serious paralleling gear failure.
PS I am not a fan of paralleling if you don't need to.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Mbrooke]

@Waross: I agree. I don't like paralleling unless I have too.

FWIW the units are natural gas except BCN which is diesel for loss of gas supply. Why twice the running hours?

 

[waross]

I see that you have a back-up to a back-up to a back-up to the prime power generator.
I would have considered using a UPS normally on bypass for the last stage of back-up.
That would both provide back-up and also carry over the start and transfer times.
I have done a number of back-ups to the back-up. (I can remember about six.)
Grid, back-up and back-up to the back-up.
Considering:
The installations were NOT critical power.
Grid failures were almost weekly.
I would often go one or two years between fail to start calls for over 10 automatic standby installations.
There was very little in the way of good technical help available to the customers.
In the event of a generator "Fail-to-start" the owner would operate a manual transfer switch and operate a separate control switch to transfer the control to the second generator.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

Running two sets at sharing the load rather than one set with all the load.
Did I misunderstand something?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Mbrooke]

1A and 2A run at 100%. If 1A fails 1B starts and transfers automatically. Also done for maintenance of 1A and 2A.

But I get what your saying, with cyclic or seasonal load 1A can run the whole building during light loads and 1A and 2A can parallel during peak load.

 

[FreddyNurk]

At that size (depending on existing board ratings) you're more likely to be unable to trip due to lack of capable fault current than have issues with overall ratings and fault withstand. On that basis paralleling may be credible if the gear is rated for it.

waross has a good point, having worked with a few decent electricians I tend to forget that not all of them have the required experience to handle paralleling systems.
Other benefits to a full-sync system would include the option to feed back to the network for load testing rather than using load banks, and the ability to avoid having to go black to transition back to power (I'm assuming there's a network feed somewhere here).

The system would have to be reviewed from a protection point of view to make sure that the faulted section can be separated out rather than dropping the whole lot.
Lastly, with motor operated breakers and relatively commonly available control gear, the synchronising and load sharing is generally well handled these days, although not necessarily cheap. If you've already got generators with their own mains-fail start equipment, its likely to be a lot more expensive to retrofit than if it was a greenfields site and the units can already be supplied with the equipment.

EDMS Australia

 

[cranky108]

Maybe I don't understand the small generator paralleling issues.

Most power companies that own generation parallel all the time. In fact there can be up to 1.5 TeraWatts of capacity in parallel at any time. Not only that the generation can be mixed as to the type.
Gas, coal, hydro, solar, wind, etc.

I guess the balancing maybe an issue, and that in our case is done on a cst basis, by unit cost, and a few other factors, such as fuel availability.
Likely you don't have those issues during your run time.

Help me understand the paralleling issues for small generators.

 

[Mbrooke]

@FreddyNurk: Can you go further into not having enough fault current to open a breaker? This issue needs to be addressed.

 

[catserveng]

I get this quite a bit, and usually a lot of things need to be considered when making this decision.

I attached a paper I pretty regularly give to my customers, usually the plant or site manager or other technical staff for them to go talk to their management about the issues.

The attached started out as a small class for generator dealers technical staff, its not meant to provide all the answers, and hopefully you will find it helpful.

MikeL.

 

[waross]

Basic automatic paralleling gear may cost the same for a 100MVA generator as for a 100KVA generator.
Part of the skill set of the technicians is a familiarity with auto sync-check relays and automatic paralleling gear.
Such expertise is rare outside of the power companies.

I have to correct myself on fuel consumption.
I just took a look at Cummins fuel consumption data for the first time in over 10 years.
Fuel consumption has dropped from 13 Gallons per KWHr to 6.6 Gal per KWHr.
Fuel consumption penalty at 50% load only 1.5%
Disclaimer:
I have not cross checked these figures with other brands.
I would do that before I made a purchase decision.

Fault current.
Self excited generators were subject to voltage collapse under short circuit conditions.
Very low short circuit current was a known issue.
With the widespread use of PMGs this is less of an issue.
However the various impedances of generators is much lower than the impedances of comparably sized transformers.
Instantaneous trip settings must be much lower for generator circuits.
Many generators are unable to generate the 10 times current required by an instantaneous breaker set on "HI". (10 X)
There are CT based boost circuits to increase the excitation in the event of a fault.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

Generators with mismatched pitch operated in parallel.
It is my understanding that circulating currents are an issue only at no load.
With any appreciable load on the sets, the circulating currents cease and become complimentary harmonics.


Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Mbrooke]

Will a generator survive or sustain itself trying to trip a breaker thermally? What about an RK or low-peak fuse? Do those help?

 

[catserveng]

Winding pitch and impact on paralleled units, can based on my own experience be not a problem to a huge PITA based on the system design, loads and a number of other things.

The Cummins paper attached is an excellent resource for understanding that issue a bit better. Too bad their engines are the wrong color

MikeL.

 

[FreddyNurk]

MBrooke, waross has already summarised the issues with small sources and fault capability.

To elaborate on what waross said though, a fair few sets can give say, 8x current at the point of a fault, but can't sustain it long enough to trip a breaker that needs to coordinate with downstream breakers. In the really small sets, that current level can disappear within say, 1/2 cycle, not long enough to give confidence that the breaker trip element will see it before it disappears.

EDMS Australia