Sizing a diesel generator based on metering data 2

[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.

 

[rockman7892]

waross - Thanks for the responses, they are very helpful.

I'm curious how often NEC style load calculations are done to determine generator size vs standard industry approach. From comparing NEC style calcs to industry approach the biggest difference is the 125% factor for all continuous loads which tends to produce overly conservative results as opposed to just using loads without factors to determine base loading. So for instance just taking all motor and non-motor loads at their values to determine base loading vs adding 125% of all continuous loads and largest motor per NEC?

 

[LittleInch]

ONe issue is that as well as the max load, you also need to work out minimum load so that you don't fall too far below 30-35% of the rated generator power for long periods of time or suddenly start those large motors when in that minimum power state when the engine might struggle to load up fas t enough before you get under voltage or drop in frequency which browns everything out.

Biggest Issue I've ever face on site is that the calculations, for me, seems to be conservative and intended for sizing transformers, where it doesn't matter so much if you oversize a bit, but it does for Generators, especially diesel ones. Too big and it will struggle a lot of the time, too small and it trips on overload.

But getting a good spread of normal loads in reality gives you a very good start.

 

[waross]

ONe issue is that as well as the max load, you also need to work out minimum load

Life is hard for a diesel generator.
On initial startup, the block loading may be well over the generators rating.
From a cold start to an overload in under 8 seconds.
One installation included one very large motor.
The gen-set struggled to start that motor.
Most of the time, when the large motor was not running, the load was 10% of the generator rating.
I have found "Wet Stacking" to be more of an issue with new sets than with old sets.
I have had a new set pump all of the lube oil out the exhaust in under an hour and shut down on low oil pressure.
I have had to load-bank more than one new set to seat the piston rings so as to avoid "Wet Stacking".
I have seldom had control over the loading of a generator set.
You do what you have to do with what you have.
Life is hard for a diesel generator!

 

[waross]

I'm curious how often NEC style load calculations are done to determine generator size vs standard industry approach. From comparing NEC style calcs to industry approach the biggest difference is the 125% factor

Are you sure that the NEC calls for 125% of everything?
Does the NEC now cover generator sizing specifically?
I worked to the Canadian Code and I haven't upgraded my codes since I retired.
My latest code covers feeder sizing.
125% of the largest motor.
For a generator, 300% for the largest motor overshadows 125% code requirement.

 

[TugboatEng]

If 2x the largest motor current significantly exceeds the greatest average current you can also consider utilizing a VFD to start that motor.

 

[edison123]

The capital and running costs of a larger DG purely from starting KVA point can be better spent on well-designed VFD's (with bypass contactor, if needed) which offer stress-free multiple starts of the motors in addition to speed control benefits, if needed.

 

[rockman7892]

Are you sure that the NEC calls for 125% of everything?
Does the NEC now cover generator sizing specifically?
I worked to the Canadian Code and I haven't upgraded my codes since I retired.
My latest code covers feeder sizing.
125% of the largest motor.
For a generator, 300% for the largest motor overshadows 125% code requirement.

The NEC for "optional standby" generators requires generator capacity to be based on NEC article 220 (demand calculations) or "other approved methods". In general article 220 requires 125% of any continuous non-motor loads.

However in seeing "other approved methods" to me means that if the industry practice we have been discussing above is recognized than we don't necessarily need to follow NEC 220 exactly.

 

[rockman7892]

If 2x the largest motor current significantly exceeds the greatest average current you can also consider utilizing a VFD to start that motor.

Load measurements show maximum average current of aprox 618A equating to 514kVA. Largest motor is 50hp so adding 2X its FLA of (65A) adds 108KVA to the measured base kVA resulting in aprox 622kVA.

When adding in consideration for largest motor starting does that simply get added to base load kVA to come up with overall gen kVA as I did above or is that motor starting kVA used for evaluation against a separate specified generator parameter (published starting kvA etc...)?

Also in previous expereince with using measured load to consider load expansion etc.. I added 25% conservative factor to the measured value (NEC requires that in come cases). However given the fact that motor starting adder adds significant kVA to calculation I don't know that it would be warranted for such a generator calculation?

 

[waross]

The NEC for "optional standby" generators requires generator capacity to be based on NEC article 220 (demand calculations) or "other approved methods". In general article 220 requires 125% of any continuous non-motor loads.

Based on that, would a home with a code mandated 100 Amp service require a 30 KVA standby generator?
I have a client running two homes, with a 100 Amp services on a 20 KVA standby generator. Both homes occupied by families.
(Two generations living on one ranch, but no A/C.)
Why a 20 KVA? That's what was available. I would have gone with a 15 KVA had one been available at the right price.
Without A/C, many homes with 100 Amp services, an electric range, an electric clothes drier, a well pump and a sewage pump will run on a 35 Amp breaker (Utility supplied to limit demand). The breaker does have a high enough instantaneous trip to allow motor starting.

My impressions;
Without A/C, NEC sizing may be oversizing.
With A/C. NEC sizing may be undersizing.
The NEC tends to over-rate non-motor loads in regards to generator capacity.
The NEC tends to under-rate motor loads in regards to generator capacity.

From the Canadian Electrical Code:

(10) For loads other than those calculated in accordance with Rules 8-200 and 8-202, feeder and service load
calculations shall be permitted to be based on demonstrated loads, provided that such calculations are
performed by a qualified person, as determined by the regulatory authority having jurisdiction.

 

[waross]

Load measurements show maximum average current of aprox 618A equating to 514kVA. Largest motor is 50hp so adding 2X its FLA of (65A) adds 108KVA to the measured base kVA resulting in approx 622kVA.

Motor starting is at a low power factor.
Short time low power factor has two effects on a generator:
The first effect is the real power or kW of motor starting and is much less than the 6 times normal current.
Three times normal current is generally adequate capacity to supply the real current demands of motor starting.
Real current impacts the Prime Mover.

The second effect of the total motor starting current at a low power factor is the six times rated motor current on the generator end.
This current causes heat buildup in the generator windings.
The duration, or the I²T is only a few seconds.
Compared to the effect on a motor, a motor sees 6 times rated current.
The generator sees 6 times the Motor Rated current, which is less than 6 times the rated current of the generator.
While there may always be exceptions for special cases, generators typically have little problem starting motors with the 300% capacity addition for the largest motor.
If the Prime Mover can hold the speed up, the generator end can easily withstand the short time overload current.

 

[waross]

When adding in consideration for largest motor starting does that simply get added to base load kVA to come up with overall gen kVA as I did above or is that motor starting kVA used for evaluation against a separate specified generator parameter (published starting kvA etc...)?

If your determination of the base load includes the largest motor, then add 200% more of the largest motor current.
If your determination of the base load does not include the largest motor, then add 200% more of the largest motor current.
That is, calculate the largest motor at 300% and calculate all other motors at 100%.

If you are using the NEC based 125% for other the base load, do two calculations;
1> Per NEC with all loads including motors at 125%.
2.> With all loads EXCEPT THE LARGEST MOTOR at 100% and the largest motor at 300%.
IF THEN:
If the first calculation is the greatest then use the first calculation and you will be code compliant.
If the second calculation is the greatest, then use the second calculation and you will still be code compliant.

Special case:
If the first calculation is the greatest, consult the AHJ for permission to use the second, smaller, calculation (Base at 100%, motor at 300%)under the provision of "other approved methods"
To that end it may be well to use the Cat sizing software with the allowable voltage drop re-set from default to a value that results in sizing at the 100% + 300% value.
Include the predicted motor starting voltage dip in your discussion with the AHJ.
You may also add the perspective of the cost of a set at 100% + 300% with the cost of a 125% rated set as per NEC.
You may want to run this past the customer. If the customer is willing to pay the additional cost of the larger set then go with the larger set.

Historically, I often saw prime power sets arbitrarily rated at an additional 25% capacity on top of the +10% included in the prime set rating.
Given the few number of hours in the typical lifetime of a standby set, I have never seen a 25% addition to a standby set rating.
But almost all of my installs were my own sizing and not subject to the NEC.
I had 100% satisfied customers.

 

[edison123]

My personal experience with DG KVA and DOL start vs VFD start.

Our 10 ton hoist has two winding two speed motor rated 415 V, 12/4 HP, 16/14 A, 1440/465 RPM.

I use a 50 KVA DG when other shop load is low or a 125 KVA DG when shop load is high.

With DOL start on high speed, the inrush current at start of the hoist lifting is 70 to 80 A with a voltage dip from both the 50 KVA & 125 KVA DG.

With DOL start on low speed, the inrush current would still be 60 to 70 A with a voltage dip and the hoist would not even lift above 5 ton. The load would slip above 5 ton with the slow speed winding.

Today, we retrofitted and commissioned a 20 HP, 31 A Fuji VFD for the same hoist motor connected to the high speed winding only. High speed was set at 50 Hz and slow speed was set at 15 Hz in the VFD.

We tested the VFD driven hoist with a 9 ton motor with the 50 KVA DG. The hoist lifted the 9 ton motor smoothly in slow speed with a steady current of only 7 Amps and at high speed with a steady current of only 16 A. Zero inrush motor current and no voltage dip whatsoever from the 50 KVA DG set at both the lifting speeds.

This is my second VFD retrofit for our 10 ton cranes and our staff are extremely happy with both the VFD retrofits, which cost me only about 10k USD (with a VFD bypass/DOL start option in case of VFD buggering off, brake resistor, new cabling, new cable suspensions system, pendant control, RF wireless control, the whole works).

With many hoist operations every day, now I don't have to worry anymore about stressing the crane motors and the DG's with frequent starts and stops.

 

[waross]

Thanks for sharing, Muthu.
As I understand VFD operation and motor Volts per Hertz response, with a VFD, the torque characteristic is the same as full load characteristic at any reasonable speed up to rated speed. That is, you can expect the same torque at low speed as at high speed.

 

[edison123]

Yes, Bill. With a constant V/F, the torque remains almost constant through the speed range.

 

[waross]

Yes, Bill. With a constant V/F, the torque remains almost constant through the speed range.

It is my understanding that if the base line of the speed/torque graph is relabelled as slip frequency or slip speed, that is with zero being at synchronous speed (zero slip)
then the graph may be used at any frequency/speed by assigning the applied frequency/speed to what was originally the 100% end of the graph.
Stated another way.
Example; A motor is rated at 1760 RPM, indicating 40 RPM or 1.33 Hz slip or 1.33 Hz rotor frequency.
If a load demanding full rated torque is applied at any frequency below rated frequency, the speed will drop by 40 RPM.
Eg: Applied frequency = 10 Hz. Synchronous speed at 10 Hz will be 300 RPM.
When the load demands 100% of rated torque, the speed will drop to 260 RPM. (300 RPM synchronous speed minus 40 RPM slip equals 260 RPM)
Labelling the graph in percent does not work as the base changes as the frequency changes.
I know that you know this Muthu. This is intended to help those who are new to VFDs.