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Discuss So whats my actual generator rating? in the Commercial Electrical Advice area at ElectriciansForums.net

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I posted this up on yahoo answers but I figure you guys are probably more qualified to answer my question

I have a generator in the building I am working on. It appears quite dated, but its stats are as follows: it’s a 415 Volt 3 Phase Diesel Generator.

On the plate of the generator:
415V
3 Phase
640KVA
520KW
Power Factor 0.8
902 Amps

So my question is:
We are testing the generator to its full load, now my load is at power factor 1. Would I be correct in assuming that because my load is power factor 1 that the full load of my generator is 640kW not 520kW.
Or is it that my actual power loading is rated at 520kW, but that load is allowed to be inductive or capacitive to 0.8.

Because it says 900 amps on the plate I am therefore under the assumption that regardless of my load characteristics, as long as I do not exceed 900 amps there should be no problem. Or is it a case that my load can only be 520kW and the highest I can allow the phase to go out is up to 640kVA.

Hope that makes sense, ill edit if not. Thanks for reading.
 
E

Engineer54

You can only use the KW rating if your load is purely resistive, which it will be under most testing conditions using a load bank or banks, It is also usual to test at 110% for manufactures recommended time period, but if the gen-set is an old one, it maybe better to stick to it's rated full load of 900 Amps.

Also important to check, is the signal to start to coming on line times at it's present normal load. This can be tweaked by the testing company, if outside of original specifications. It should in fact be capable of taking it's full rating load on start up, once it's up to it's designated speed. (I'm assuming this is a 4 stroke Diesel engine and not a 2 stroke jobby)

Having just read your post again, I do hope your not going to load test this generator yourself, not unless you know your generators and can identify the sign's of impending doom very quickly!! lol!!! There are a multitude of tests and checks to carry out before you start running full load tests for any period of time, as well as those after the testing is completed.
 
  • Thread Starter Thread Starter
  • #3
I am a commissioning engineer, conducting the test with engineers from the generators company.
However what has happened is someone, somewhere along the line has ordered a loadbank but the loadbank is purely resistive. (Power factor 1). (Were trying to load test the generator at 100%, but I'm not convinced that 520kW at pf 1 is 100% because its rated at 640kVA)

When I have tested transformers and UPS units at full load I have been able to run them at their full KVA rating, regardless of the phase angle applied. But I asked the engineer of the generator firm, and we came to a disagreement because lets face it, I wasn’t getting any confidence that the man had any clue what he was talking about.

So what I’m getting at is:

520kW at pf 0.8 = 640kVA or 902 amps, give or take.
However 520kW at pf 1 = 520kVA, (about 720A I think).

But if my generator is rated to 640kVA or 902 Amps, would I then not be able to increase my load to 640kW at 1 also 902 amps give or take.

Should I be going by the kVA & AMPS? Or kW?

(and why, it’d help me if you could explain to, thanks guys)
 
E

Engineer54

If your 520KW Generators Alternator states that it can supply 900A, then it doesn't really matter what that 900A is made up from, ...it should be able to supply the stated 900A. The KW and KVA ratings are just forms of measurement. So as far as i can see you should be basing your alternators rating as ''520kW at pf 0.8 =/or 640kVA or 900ish amps.... So use the 900A as your load test 100% figure...

As i stated before, during commissioning and proving tests associated with generators i generally always want to see and prove the manufacturers 110%/time rating too. In your case it depends on just how old your Gen-Set is, and how hard it was working to get to, and maintain it's 100% load.

I'd be interested to see the blank test report sheets you will be using for this load testing. Obviously without any company details and logo's etc. Any chance you can PM a copy of them to me?? Are they yours, or the generator testing company test report sheets you'll be following?? It'll be interesting to see to what depths they go too, and what sort of durations they are applying etc....
 
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E

Engineer54

Hopefully its attached.
So what did you think on the performance of the generator testing company??

I take it they didn't bring a load bank capable of 900+ amps ...lol!!. Did they bring supplementary calibrated metering equipment, or just relied on the Gen-sets integral metering facilities?? Was you seeing any oil leakages from gaskets, engine connections and the like, during the load testing regime ??

Is the load test sheet the only testing documentation carried out on your generator and associated panels etc??
 
  • Thread Starter Thread Starter
  • #8
The company I found very professional, however the one engineer who I questioned I found quite rude and aggressive when I confronted him about the accuracy of the test he was performing.

My argument being at 650KVA 0.8 p.f 520kW the current required is 900A, 100% load.

On the sheet he has put 520KVA 1 p.f 520kW the current being 730A there about. Not 100%.

The way I saw Transformers, UPS and Generators were that they don’t discriminate what type of load is applied as long as it does not exceed the kVA rating, which at the correct voltage will not exceed the current rating.

For example if the load power factor was 0.65, the kVA will still be 650, the kW is reduced to 420kW because for obvious reasons the inductors or capacitors are causing losses, but the current is still 900A. I have tried this on transformers and UPS’s during load testing and nothing has happened, the UPS flagged up an overload warning every time the kVA dipped over its rating but no matter how low I simulated a extremely inefficient load by lowering the power factor number, there was no change. The kVA stays the same, the Amps stay the same but the kW (actual) does change for obvious reasons.

So I didn’t want to carry on the argument with him because, 1 I wasn’t getting anywhere, 2 the client accepted it anyway and 3 I’d really hate to be wrong lol.


As you can see from the test sheet they took it up to 790Amps apparently that’s 10% above its full load, despite my objections. There was capacity to take it up to 900kVA at p.f 1. Obviously as it’s only resistive we couldn’t adjust the power factor.

I used a clamp meter over the cores to ensure the current readings on the gen-set meters were accurate, and it was. There were no leaks or problems during the whole test; it was run for about 5 hours.

There are other sheets, for the panels and other components etc., but as I was only commissioning on that job temporarily, I don’t have access to those documents anymore, I’m off to a new datacentre in North London and the HV’s ready to go on another in West London.
 
E

Engineer54

You are quite right, a generator will not care what type of power/current it's supplying, so long as it's not being overloaded. The 900A stated on the alternators data plate, is still 900A in anybodies language..lol!!

I agree with you, that the information stated on that official test report sheet isn't exactly now very accurate, (based on the information you have provided), and that this Gen-Set hasn't been tested at 100% or 110% at all.

If no oil leaks/seepage during the 5 hour testing was noticeable, then it's a pretty good sign that the engine is in pretty good shape, especially as you say this is an older installation...
 
Since its a 240/415v gen, 520kw/(415 * sqrt of 3)=723 amps so imho
they tested it at (792/723) = 110% of its rated 520KW output.
520kw = 640 KVA at 80% pf.
The KVA ratings arent meaningful as a true real-life output. kw is the true rated output,
on a continuous basis for prime power gens. most prime power gens are capable of providing 110% of rated output for a limited period of time defined in their definition of prime power.
 
E

Engineer54

Since its a 240/415v gen, 520kw/(415 * sqrt of 3)=723 amps so imho
they tested it at (792/723) = 110% of its rated 520KW output.
520kw = 640 KVA at 80% pf.
The KVA ratings arent meaningful as a true real-life output. kw is the true rated output,
on a continuous basis for prime power gens. most prime power gens are capable of providing 110% of rated output for a limited period of time defined in their definition of prime power.
Haha, ...So explain away the 900A rating on the alternators data plate then??
 
The alternator maybe rated 900A but does that mean the generator is?
And at what ambient temperature and elevation? Prime power or standby?
Why do you think the generator is sold at rated 640kva/520kw?
 
E

Engineer54

The alternator maybe rated 900A but does that mean the generator is?
And at what ambient temperature and elevation? Prime power or standby?
Why do you think the generator is sold at rated 640kva/520kw?
You don't know much about generators at all, ...do you??

As engines are sized accordingly to the alternator, Yes it does mean the generator is rated at the full load of the alternator, in this case around 900A!!

So what do you consider is the difference between a stand-by and a prime power generator??

All generators and distribution transformers are manufactured/sold to a KVA rating, in the case of generators generally with a stated PF % figure, which is where they derive the KW figure, which will rise or fall according to the PF %. In this case it has been stated as 0.80, which relates to the 520KW ...640KVA at 0,80 PF.... If an alternator is rated to deliver a stated current output, that's basically what the generator can deliver, it doesn't care what that current consists of, be it all resistive, all inductive or as in the majority of cases a mixture of both types of load....
 
Sir, I must respectfully disagree. You arent taken into account altitude or ambient temperature. The alternator rating is a maximum not an absolute. IMHO, altho you take every opportunity to dis me, I will not now dis you, Sir!
 

Rockingit

-
Mentor
Arms
Esteemed
As it's probably impractical for you two gents to duel at dawn using alternator data plates instead of pistols, how about we try the time proven rock and roll method instead? You know, the one where we just keep plugging stuff in until it starts coughing and chugging. Ambient conditions normally a boggy field.
 
K

Knobhead

I can’t understand this argument.
640KVA 415V @ unity PF = 890A
520KVA 415V @ 0.8 PF = 868A
As near as damn it 900A
 
  • Thread Starter Thread Starter
  • #17
The altitude and the ambient tempurature must surely be irrelevant how can it affect the generator rating that much, beside lets pretend we have ideal conditions.

@Rockingit - I wanted to turn the load bank up and take over the whole testing process but they wouldnt let me lol.
 
640 kva @ 80% pf = 520 kw, actual usable power.
but I like the duel at dawn method with alternator plates. or the rock and roll method :)
 
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E

Engineer54

640 kva @ 80% pf = 520 kw, actual usable power.
but I like the duel at dawn method with alternator plates. or the rock and roll method :)
No-ones trying to dis you...

But i can assure you, that if an alternator has a a current value stated on it's data plate, then that alternator should be expected to be able to provide that current.. You can play with KW and KVA designations all day, but they will both be based on the capability on what the alternators current output has been designed to deliver. As stated previously, the generator will not care a jot what that current is made up from, it will still provide that stated current, ...be it useful current or otherwise.

As for the ambient temperature and altitude, were talking about an in-situ generator installation in a UK City, so the altitude can be ruled out. As for ambient temperature, ...well that is clearly fine going by the test report readings... So let's take it that in this case, both are well within the manufacturers design thresholds, and that no de-rating factors need to be applied.
 

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