emmjea001
Electrical
- Jul 17, 2013
- 24
Dear Members,
Please can I ask you to consider a technical query. Say there are generator sets rated at 1250kVA before de-rating. After de-rating they available power on site is: 912.5kVA and 730kW at 0.8 PFC, so for both units this is 1825kVA and 1460kW.
Now currently the plant maximum load (actually only the peak MD) is 1465kVA, as measured off the utility meter, and at 0.98 PF, as the PFC units are operating. When the generators run the PFC units are dropped out, and the uncorrected PF is about 0.72.
So 1465kVA at 0.98 is 1435 KW of "real load", which worked back again at 0.72 is 1994 kVA.
Please comment on the following points - if you can below each actual point this would be helpful.
1. the de-rated actual engine power available on site is as per the de-rated value of 1460kW, and the plant at the MD full load is 1435, so actually this is quiet close to the prime rating, and although there would appear to be lots of "room" if you look at the kVA values - i.e. Generator Capacity of 1825 kVA versus actual of 1465 kVA - this is at 0.98 PF, where-as in fact the 1825 kVA is only the value at 0.8 kVA, and if you were to run the sets at 0.98 PF you would only have an output of 1460kW (the de-rated on site kW power) x 1/0.98 = 1490kVA - do I have this correct? (it always amazes me how the simple concept of PF and kW and kVA every now and again "undoes" me!)
2. From this, if I go back to the actual load, which in kW is 1435 kW, at 0.72 PF this is 1994 kVA - which is over the rating - however, so long as the engine is not overloaded, which we assume it is not as it is rated prime at 1460kW, and we have peak of 1435kW (and constant around the 1350kW range), then all we have to worry about is the current rating of the alternator - is this correct?
3. following above, if we look at the 1994 kVA value, I do not believe this is actually over the rated values for the alternator, as you should not have to de-rate the alternator for altitude, and so its full data sheet rating of 1250 kVA should apply / for each alternator, which is 2500kVA. which is comfortably above the 1994kVA - is this correct?
4. in fact, the case could be made that at an unchanging real load of 1435 kW, all the way down to a hypothetical worst case value of PF = 0.57 which would yield at load of 2490kVA, the sets should be ok, and there is no need to look at running them on a corrected PF scheme.
Your technical comment and further advice on the above will be appreciated.
Please can I ask you to consider a technical query. Say there are generator sets rated at 1250kVA before de-rating. After de-rating they available power on site is: 912.5kVA and 730kW at 0.8 PFC, so for both units this is 1825kVA and 1460kW.
Now currently the plant maximum load (actually only the peak MD) is 1465kVA, as measured off the utility meter, and at 0.98 PF, as the PFC units are operating. When the generators run the PFC units are dropped out, and the uncorrected PF is about 0.72.
So 1465kVA at 0.98 is 1435 KW of "real load", which worked back again at 0.72 is 1994 kVA.
Please comment on the following points - if you can below each actual point this would be helpful.
1. the de-rated actual engine power available on site is as per the de-rated value of 1460kW, and the plant at the MD full load is 1435, so actually this is quiet close to the prime rating, and although there would appear to be lots of "room" if you look at the kVA values - i.e. Generator Capacity of 1825 kVA versus actual of 1465 kVA - this is at 0.98 PF, where-as in fact the 1825 kVA is only the value at 0.8 kVA, and if you were to run the sets at 0.98 PF you would only have an output of 1460kW (the de-rated on site kW power) x 1/0.98 = 1490kVA - do I have this correct? (it always amazes me how the simple concept of PF and kW and kVA every now and again "undoes" me!)
2. From this, if I go back to the actual load, which in kW is 1435 kW, at 0.72 PF this is 1994 kVA - which is over the rating - however, so long as the engine is not overloaded, which we assume it is not as it is rated prime at 1460kW, and we have peak of 1435kW (and constant around the 1350kW range), then all we have to worry about is the current rating of the alternator - is this correct?
3. following above, if we look at the 1994 kVA value, I do not believe this is actually over the rated values for the alternator, as you should not have to de-rate the alternator for altitude, and so its full data sheet rating of 1250 kVA should apply / for each alternator, which is 2500kVA. which is comfortably above the 1994kVA - is this correct?
4. in fact, the case could be made that at an unchanging real load of 1435 kW, all the way down to a hypothetical worst case value of PF = 0.57 which would yield at load of 2490kVA, the sets should be ok, and there is no need to look at running them on a corrected PF scheme.
Your technical comment and further advice on the above will be appreciated.