Sizing a diesel generator based on metering data

[rockman7892]

I'm in the process of determining size of 480V diesel genset for an industrial site to replace existing temporary units. I'm evaluating size based on both metering data captured via a portable power meter for site at service entrance as well through load calculation in order to evaluate both for gen sizing. The facility is a gas pumping facility which consists of general lighting, heat trace, and motor loads, to switch between utility and gen with loads connected to a common MCC. The motor loading cosists of several small 1.5 and 10hp motors as well as (2) 50hp motors as largest motors.

I have understanding of the load calculation process in accounting for all motor and non-motor loads in kW and then accounting for "starting load" of largest motor to arrive at generator size. For starting load of largest motors i've used 2.5-3x motor kW or what default values software provided. I've used Kohler's gen sizing program (similar to CAT's) to check sizing calcs.

At one point a portable power meter was setup at service entrance of facility for a short period of time to try to capture actual facility loading while operations mimic'd normal full load conditions at the time. I'm trying to determine how to best use that measured data in conjunction with load calculations to come up with generator size.

Obviously as expected the actual metering data is less than that derived by calculation. In looking at the metering data the maximum sustained current (not a spike that would represent motor starting or transient) is approximately 618A on a single phase corresponding to approximately 513kVA assuming balanced load. Unfortunately I cant seem to be able to identify the pf in the meter at this time of measurement (since it was taken of current trend) so will have to make an assumption to determine kW. Assuming a .8pf gives a measured kw value of 410kW (note: metering was not performed long enough for meter to capture max avg power based on 10min window)

In comparison when preforming gen calc using hand calcs or more specifically generator software I arrive at a running kW=675.2 and kVA=775.5. This includes a max starting kw=107 from starting of 50hp motor when all other load is running. The kw rating from calculation is approx. 275kW larger than measured value and likely has a bunch of conservativeness built into it so looking for good engineering judgement on how to evaluate both methods to arrive at optimal gen set size.

Does anyone have sound approach for using measured power for deterring gen set sizes backed up by calculations like I have in this case?

 

[waross]

Greatest average current (15 minute average) plus 2 times the largest motor current.
If you size the set as a prime power set rather than a standby set, you will get an additional 10% capacity for one hour out of ten hours. (Or out of twelve hours)
Or, review the monthly demand value for the last year or the last two years of power bills to determine peak usage. The peak monthly demand on power bills is generally a 15 minute rolling average so that motor starting surges are not included.
I would list the monthly demands and use judgment to exclude any outliers.
I would then add twice the rated KVA (Motor rated voltage multiplied by rated current to convert to KVA) of the largest motor, to get the recommended gen-set size.
Generator ends are rated in KVA, not kW.
I always worked in KVA or current and had good results and satisfied customers.
Insider tip:
The Cat software uses a default fairly conservative motor starting allowable voltage drop.
Increase the allowable voltage drop from the default setting until the next smaller size set is recommended.
Given the reduction in cost for the smaller set, most customers will be satisfied with the occasional greater voltage dip.
You will only see the maximum voltage dip in the event that the facility is fully loaded with all other loads when the largest motor is started.
In most plants that seldom happens.

When the motor is a significant portion of the load on the gen-set, 2.5 is often to little allowance for motor starting.
The motor will start, but magnetic contactors may drop out on undervoltage and the frequency may drop enough to be noticeable.
Gen-sets in that size range will have a UFRO function on the AVR. That is if a load pulls the frequency down more that 3 Hz, the Under-Frequency_Roll-Off feature of the Automatic Voltage Regulator will reduce the voltage to hold the Volts per Hertz ratio steady.

 

[rockman7892]

Greatest average current (15 minute average) plus 2 times the largest motor current.
If you size the set as a prime power set rather than a standby set, you will get an additional 10% capacity for one hour out of ten hours. (Or out of twelve hours)
Or, review the monthly demand value for the last year or the last two years of power bills to determine peak usage. The peak monthly demand on power bills is generally a 15 minute rolling average so that motor starting surges are not included.
I would list the monthly demands and use judgment to exclude any outliers.
I would then add twice the rated KVA (Motor rated voltage multiplied by rated current to convert to KVA) of the largest motor, to get the recommended gen-set size.
Generator ends are rated in KVA, not kW.
I always worked in KVA or current and had good results and satisfied customers.
Insider tip:
The Cat software uses a default fairly conservative motor starting allowable voltage drop.
Increase the allowable voltage drop from the default setting until the next smaller size set is recommended.
Given the reduction in cost for the smaller set, most customers will be satisfied with the occasional greater voltage dip.
You will only see the maximum voltage dip in the event that the facility is fully loaded with all other loads when the largest motor is started.
In most plants that seldom happens.

When the motor is a significant portion of the load on the gen-set, 2.5 is often to little allowance for motor starting.
The motor will start, but magnetic contactors may drop out on undervoltage and the frequency may drop enough to be noticeable.
Gen-sets in that size range will have a UFRO function on the AVR. That is if a load pulls the frequency down more that 3 Hz, the Under-Frequency_Roll-Off feature of the Automatic Voltage Regulator will reduce the voltage to hold the Volts per Hertz ratio steady.

Thanks Waross. So when doing load cals are they done in kW or KVA? I’ve seen most done in kW and my understanding is that gen sets were rated in kW?

Also if performing load calcs method are we typically required to adhere to NEC 700 article related to calculating gen sizes in industrial applicants or are industry approaches more accepted?

 

[waross]

I’ve seen most done in kW and my understanding is that gen sets were rated in kW?

Generator ends are rated in KVA. Rated Volts times Rated Amps /1000
Generator prime movers are rated in kW, Power.
Typically, almost universally, sets in your size range are rated at 0.8 PF.
kW = KVA x 0.8
Generator sets in your size range are typically rated in both kW and KVA.
eg: kW = 400, KVA = 500, PF 0.8
The actual PF is determined by the load.
The 0.8 PF rating is the Rated kW/KVA ratio.
If you exceed the kW capability of the Prime Mover, the set will slow down.
Note: Capability versus rating. Some sets are somewhat overpowered.
UFRO will lower the voltage so as to lessen the load and give the set a chance to recover speed.
In the field, the current rating is often more important than the KVA rating.
Example.
A set is rated at 250 KVA, 200 kW, 0.8 PF, 240 Volts, Thus rated current is 601 Amp. The windings will overheat if a continuous load of more than 601 Amps is supplied.
BUT, the set is to be used for 208/120 Volts. The rated current is still 601 Amps per phase.
The KVA rating at 208/120 Volts is 120 Volts times 601 Amps times 3 equals 216.4 KVA.
Same set, different KVA rating, same current rating.
(And the same Prime Mover, so the same kW. So, actual rating = 216.4 KVA, 200 kW, for an unstated PF of 0.92)
I don't worry about the new PF or include it in calculations BUT when the sizing is close I take comfort in knowing that this set will do a little better starting large motors the a set working at full frated voltage and full rated KVA.
It's safer to work in current when sizing the set. When using KVA to size the set make sure that you are using the correct KVA rating for the intended voltage.
Motor starting is quite reactive.
The prime mover is generally capable of supplying the real power at a 3:1 motor current ration.
The generator windings will often be momentarily overloaded by the motor starting current for the short time that the motor is starting.
And be aware, there may occasionally be special cases that are exceptions to the above rules of thumb.
For example, the larger the gen-set is in relation to the motor, the more forgiving is motor starting.
And good to know.
A prime power set may have additional accessories such as an oil cooler, a larger lube oil sump, or other additions.
But, the same basic set, before the addition of extras, is used for both Prime Power and for Standby Power.
The Prime Power set is derated 10% from the Standby Rating.
Comparing a 500 KVA Standby set with a 450 KVA Prime Power set from the same series, both will have the same engine and both will have the same generator end.
The expected lifetime run time of the prime power set may be orders of magnitude greater than the expected run time of the standby set.
At the end of a year of Prime Power use a set may have almost 9000 hours of use.
In the same year a Standby set may have less than 100 hours of use.
Hence the derating of the Prime Power set.
But, after one or two years of service, it may be difficult to draw the full allowed 10 percent over capacity.

 

[waross]

Thanks Waross. So when doing load cals are they done in kW or KVA? I’ve seen most done in kW and my understanding is that gen sets were rated in kW?

The people in the business side of our little utility tended to talk in kW, and so in conversation, I often used kW.
When it came time to do load calcs I worked in KVA or Amps.
And beware.
I was not always able to check orders before they were submitted.
I have been caught a couple of times when a set was ordered at a given output at 120/208 Volts and the set delivered was rated at 120/240 center tapped delta.
When the load is mostly fixed, (lighting in a theater) and the set is only 87% of what was needed, you have to get creative.
When a set has been shipped by ocean freighter to Central America, you often have to live with what you got.
In one case I was able to negotiate a discount for the customer on the basis of the set not being as ordered, but returning a set was out of the question.
There is probably still a 277 Volt transformer sitting outside the power house after about 20 years.
The transformer ordered was for 480 Volts. The transformer shipped was 277 Volts for use on a 277/480 volt system.
It would have cost more to return it than it cost, so it sat out back, brand new and unused.
Forgive my anecdotes.
I hope that they help you to avoid a couple of the pits that I fell into when I was learning.