DIESEL GENERATOR SIZING 1

[SphincterBoy]

I need to spec. a standby diesel generator, to
supply power to an office building, with computer equipment,
elevators, and HVAC units.

The office building is currently supplied by a
three-phase delta connected (secondary) single phase
transformer bank. The seconday voltage configuration
is 120/240V delta.

Each single phase transformer's secondary is 240 volt,
with 120 middle tap.

The phase A & C transformers are rated 50 KVA.
The middle phase B transformer is rated 100 KVA.

I am leaning towards sizing the diesel generator at 300 KVA,
with both an automatic start (on loss of power), and automatic transfer switch (to keep the generator from
backfeeding into the utility distribution system).

Any comments or suggestions?

 

[BJC]

How many stories is the building? That will determine if you have to power the elevator or just supply enough power to get it to the ground floor ( You get the people out that are on it when the power goes out).
For a real emergency system forget the computer and HVAC equipment. Emergency systems are just to get people out of the building. You need to power emergency lighting and legally require systems. Legally required systems are things like smoke control fan and fire pumps. Check artical 700 in the NEC and start from there. It will lead you to NFPA 101 and eventually to building codes like the UBC or BOCA.
You can use one generator but you will need two transfer switches and seperate the Emergency from the non emergency systems. In general if an emergency system doesn't work someone could die. If a non emergency system doesn't work you might fry a computer system or have a lot of downtime but no one is hurt but it could cost a lot of money.

 

[SphincterBoy]

Thanks for the help BJC!

There are three (3) stories in this building.

From what you have told me, there needs to be
an engineering study done, and internal re-wiring
done to the building, in addition to the
cost of the D-G, in order to comply with all
the codes.

Out of curiosity, why are two transfer switches
needed?

 

[mpparent]

Typically, you set up your emergency loads into critical/non-critical loads, with corresponding transfer switches. Loads like EM lighting, elevators, etc., fall into Priority 1 loads, which have to come on-line first.....then other loads can switched onto the emergency system provided there is no overloading on the gen set. That's why you have load-shedding schemes in more complicated systems, because if the gen can't handle the load, it can be shed and not risk the priority 1 loads. Hope this helps....

 

[BJC]

You have to seperate the emergency functions from the standby loads. The non essential loads have to be shed if the generator should have trouble powering all the loads.
The problem is that not all of this is an electrical problem, there are fire marshalls, buiding inspectors and insurance companies all in the mix. You gotta take off your engineer hat and put on your lawyer hat sometimes to sort them all out. Most of the time your local building inspectors and fire marshalls will help.
I would recommend you get the Orange Book ( IEEE 446)

 

[dpc]

As BJC notes, the determination on the need for emergency power is generally not made by the electrical engineer.

Not all building require an "emergency" generator, per the NEC definitions. If you don't need emergency power, then make sure you call it "standby" power.

Emergency power is life-safety related. Standby power is not. Generally emergency power circuits must be run in separate conduit systems.

I've had projects where the architects and others would use the "emergency" power term loosely and we had a heck of a time convincing the local inspector that we did not have to comply with NEC emergency power requirements.

Hope this helps. I don't know if you under NEC or not.

As for generator sizing, your 300 kVA should be ok - but are there any large motors to be started?

dpc

 

[meihm]

What you appear to have described is one 240 volt 3 phase, 3 wire delta distribution system with three separate 120/240 volt 1 phase 3 wire distribution systems(or possibly one 120/240 volt, 3 phase 4 wire delta sytem with 2 additional 120/240 volt 1 phase 3 wire distribution systems).

You need to very carefully review your proposed generator with your likely vendors. I don't believe you will find a generator with 120/240 volt delta output with sufficient 120 volt capability to serve your needs, much less a generator wound to provide three 120 bringouts.

In addition to dividing the loads into Emergency and Equipment (non Life safety) loads you also need to consider staring loads and their sequence. I strongly suggest that you discuss your need with someone familar with the starting issues.

 

[jbartos]

Suggestion: Reference NFPA 70-2002 National Electrical Code Article 700 Emergency Systems, Article 701 Legally Required Standby Systems, etc.
1. Check the need for an emergency power supply, building size, occupancy, etc.
2. Investigate Uninterruptible Power Supply (UPS) System since the Diesel Generator alone does not provide UPS capability.
3. If the standby source is the right solution, then investigate a load growth to size it adequately. Some load growths are difficult to predict. The intended 50% of spare capacity may or may not suffice depending on the outcome of load growth analysis.