Generator and Alternator kW and KVA Confusion

[stevesummers]

Hi All,

I have what is hopefully a simple problem - but just having a bit of brain fog and cant see the way through it.

We have a 2MWe rated gas engine, which is driving a 2481kVA @0.8PF alternator set, equivalent to 1984.8kW (its IEC world so alternators are rated on kVA)

The alternator supplies a 100kVA auxiliary load (say 90kW at 0.9PF), for the various cooling and auxiliary systems etc.

Where i am getting confused is how much power can we get out at the connection point in terms of both kW and kVA. On paper we want the system to be able to deliver 2000kW at 0.95 PF lag through to 0.95 lead.... in practice it will probably sit at around 0.98 lag, but that's beside the point.

In alternator terms, everything looks ok i.e. we are have a nominal 100kVA (90kW) load and then if we are exporting 2000kW, we have a total load of 2105kVA (2000/0.95) +100kVA = 2205kVA so below the alternator rating, and everything checks out.

But if we look at the engine rating, this is only given in kW i.e. actual real power (as expected)... but the load we are asking the alternator to deliver is an active power of 2000kW for the generation and an additional 90KW for the auxiliaries.

So this makes me think the alternator is oversized, or the engine is undersized and if we tried to actually run it this way, then the engine would fall over, and would we have to accept we can only deliver 2000kW - 90kW = 1910kW.

For the purpose of this i am ignoring any short term overload capabilities etc.

Hope that makes sense - any help / sanity appreciated.

S

 

[dpc]

The generator is basically a kVA machine. The generator capability curves define the region (lagging and leading pf) that it can operate within. At a pf of 1.0, it could likely run happily at 2481 kW. The generator kVA rating should always be higher than the prime mover's kW rating allow for operating at a lagging power to produce vars needed for voltage regulation, etc. The engine has a maximum rating of 2000 kW - plus any overload capacity, so that will generally limit the kW production, not the generator.

 

[Hoxton]

Your 'alternator' is really a 'current generator'. In actual fact an ‘alternating current generator’ (of course you can have a direct current generator).

As dpc says, it is a kVA machine. The generator is given a power factor rating as that defines the mechanical design of the generator shaft. Otherwise, in simple terms, the generator does not care about kW!
If your gas engine is rated at 2 MW (mechanical) then, if we assume that the generator is 97% efficient, the electrical kW will be 1940 kW. If you exceed this then you must reduce the power factor, so as not to exceed the line current rating and thereby exceed the rated temperature rise of the generator (and overheat it).

As usual with a mains parallel set (you talk about ‘export’ and from that I assume you are not in ‘island mode’) you will operate the set at near 1.0 pf, this minimises the line current and increase the generator efficiency (reduce the losses). There will still be some electrical losses in the generator, so you will never achieve 2 MW electrical with a 2MW mechanical engine.

In practical terms, the mechanical Kw is not monitored…

So, in reality you could run the set at 2 MW electrical and to achieve this, run the engine at slightly more than 2 MW, it will probably be ok.

The gas engine generating sets I worked with were rated at 2 MW (or whatever the particular set was sold as) at 1.0 pf. It may seem strange to rate the genset at a fixed Kw from what I said above, but what it means is that if you operate at a lower power factor (and higher generator losses) then you must reduce the kW rating of the set.
In practice, the generator controller runs the set at a constant kW which it will not allow to be exceeded. Just like cruise control on a car. This is because in mains parallel if the system frequency drops, then you do not want the controller to increase the genset power to try and restore system frequency. If the genset is small relative to grid capacity (here in UK, we have about 40 000 MW of grid capacity).

If we sold the set as being capable of island mode, often as standby to the site when the grid failed, then the set was rated at 0.8 pf and the rated kW was lower, due to the higher anticipated losses.

We never got around to having a set that changed its control set point when the grid failed!

My final point is that if, as expected, you have 90 kW of site power, or genset auxiliaries, then you will never supply this and export 2 MW with a 2 MW engine.

 

[waross]

KVA depends on the ampacity of the generator windings and on the output voltage.
A 100 KVA generator at 120/208 will safely develop 115 KVA at 240 Volts. (139/240 Volts or 240 Volts delta)
The maximum KW depends on the power of the prime mover.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[RRaghunath]

The Gas engine Generator data sheet is required here.
It is possible that 2MWe is
* the shaft output of Gas engine or
* that of generator output or
* generator output after deducting the auxiliary power consumption.

The data sheet alone can clarify this point depending on what is designed / contracted.
If 2MWe happens to be Gas engine shaft output, the generator output will be less than 2MW after deducting for generator losses.
The power available for export will be further less once auxiliary losses are deducted from the generator output.

 

[Hoxton]

Good idea, the genset data sheet will help

 

[LittleInch]

Also the engine will be rated at sea level and probably something like 20C air temp with a set gas Calorific Value.

There's usually a de-rating factor for real life operation...

and 90kW sounds like quite a lot of auxiliary load. Is that a constant 90kW?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ppedUK]

The 'e' on the end of the MW rating normally means electrical power, therefore the set should be able deliver 2MW electrical output. The engine shaft power output will be slightly higher to allow for losses.

If you want 2MW export in addition to 90kW of Aux. load, then your set is undersized unless you continually run the set overloaded. Also if you consistently want 2MWe now and in the future, your set needs be rated even higher to allow for engine wear. You won't get 2MWe out of the set when the engine is worn.

Gensets of this rating normally have the alternator rating at 0.8pf as standard (off-the-shelf). You can have (within reason) whatever pf you want on a non-standard machine.

90kW is not really a lot of auxiliary load, particularly if you are not using the waste heat off the engine in a CHP scheme. If you can't use the waste heat, then a fair portion of the aux power goes into the cooling fans on an external heat exchanger.

 

[waross]

You haven't said if the machine is prime rated or standby rated.
It makes a difference.
No overload is allowed for a standby machine.
A prime rated machine is allowed a 10% overload for one hour out of twelve, up to 25 hours per year.A 2 MW standby rated set will be prime rated at 18.2 MW.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[GroovyGuy]

Hey Bill,
Let's not loose site of the fact that diesel gen-sets can have up to three power ratings, including; [ul]
[li]Standby[/li]
[li]Prime[/li]
[li]Continuous[/li]
[/ul]
Would a 2MW standby unit not be rated 1.6MW continuous (ie 80% of standby rating)? 18.2MW (ie 91% of standby rating) seems a tad high to me.
Regards,
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[Hoxton]

Gas engines of that size are usually rated COP to ISO8528, i.e. continuous power with no overload.

 

[waross]

GG you are confusing KVA to KW, based on 80% with Prime to Standby based on 10%.
KW x 0.8 = KVA
Prime plus 10% = Standby.

Hoxton, The engine will be the same basic engine with the same power rating.
A standby set will be based on the full HP.
The prime set will be the same engine with the set de-rated 9%.
Our prime power sets (that were mostly operated as continuous sets) ran 15,000 hours before a top end overhaul.
Can your rating produce full rated power for 15,000 hours without a drop in maximum power?
I am sorry if I have misunderstood your information.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter