Standby Generator Sizing SKVA factors

[NLocus]

Hi Everybody

I would be grateful if someone could enlighten on the following point:

The factor to determine SKVA for (I) DOL (ii) Star Delta (iii) VSD (iv) Soft Starter

Secondly, by how much percent overload can a standby generator sustained for a few seconds

Thirdly, I noticed that standby generator in their technical datasheet, it mentioned the motor start up capability say for 100 KVA Generator, it state 250 KVA. What does this mean.

Finally, can someone share a real application sizing for a building or any other application.

Thanks in advance

 

[waross]

First; Read this thread.
Home Generator Question
thread237-420296
Second; Beware of the internet. There is some misinformation out there from supposedly dependable sources.
Third; some dependable rules of thumb;
One generator to run one motor.
Generator Amps 250% of motor FLC.
The voltage and starting dips will be unsuitable for any other loads but will actually aid the motor in starting.
Almost all Automatic Voltage Regulators for small sets incorporate Under Frequency Roll Off.
When a sudden overload such as a large motor is applied to the gen set, the prime mover slows down and the frequency drops.
The UFRO feature of the AVR reduces the voltage almost proportionally.
The reduced voltage reduces the current. The reduced current reduces the motor torque, but;
The reduced frequency reduces the slip frequency seen by the rotor and has the effect of moving the torque characteristic higher up the torque curve partly compensating for the lost torque.
This all acts to reduce the load on the set and aids recovery to normal frequency and voltage.
One generator running one or motors and other loads.
The generator current should be a minimum of the sum of all load currents and motor currents except the largest motor, plus 300% of the FLC of the largest motor.
Note: If you cannot reconcile the currents for 120 Volt loads and 208 Volt or 240 Volt loads you may not belong on this website.
Prime power sets
Use 110% of generator rated current for these calculations.
Other factors
The generator loads must be considered in regards to non-linearity and the set size may have to be increased to handle these loads.
The ratio between base load demands and largest motor demands may have an effect on generator sizing.
Staggered starting or special orders of starting may also have an effect on generator sizing.
Reduced current start methods; This gets complicated.

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

 

[catserveng]

My best answer based on what you provided so far, "it depends".

First, take a look at this for a basic understanding, Bill (along with others) has done an excellent job in the referenced thread but this article has been helpful for me in explaining the issue for a number of years,

http://power.cummins.com/sites/default/files/literature/technicalpapers/PT-7007-SizingGensets-en.pdf

From your original post,

The factor to determine SKVA for (I) DOL (ii) Star Delta (iii) VSD (iv) Soft Starter Again, it depends on several things, on type of motor, what kind of load is being driven by the motor, and on the VSD and Softstarter, how they were setup. Here is a very good resource I have used,

http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1008&context=elecengtheses

And this from ABB you may also find helpful,

http://www04.abb.com/global/seitp/s...140019cc05/$FILE/Technical+note+TM008+low.pdf

Secondly, by how much percent overload can a standby generator sustained for a few seconds. Again it depends. Two primary factors, the prime mover and the generator. On the prime mover (engine) is it turbocharged or natural aspirated, does its power rating include by design any overload capability, what kind of regulatory constraints (like exhaust emissions) are present. On the generator end, differing designs may have different SkVA rating for a design nominal kVA or ekW rating. What kind of excitation system, SE, PMG or aux winding like AREP? Pretty much now days when you buy a standby set at a given rating, that is what you get. Two major factors have driven that in my experience, cost and emissions regulations. The cost thing is easy, it is a tough market with tight margins, so none of the manufacturers want to give anything away for free. And regulatory agencies pretty much demand that the nameplate rating is what the engine produces, no extra.

Thirdly, I noticed that standby generator in their technical datasheet, it mentioned the motor start up capability say for 100 KVA Generator, it state 250 KVA. What does this mean. It means the generator set you are looking at has a rated starting kVA based on a given set of conditions.

Finally, can someone share a real application sizing for a building or any other application. Start here,

http://ecmweb.com/basics/sizing-gen-sets-large-motor-starting

A google search of the web on this issue gets you a lot of info. Generator set sizing can be easy or tough, lots of factors to consider to do it right, lot of systems out there that have mis-sized systems because someone wanted to do it easy.

Manufacturer provided sizing systems are a good start, but you need to be aware of a couple of things. The manufacturers are in the business of selling, more kW means more money. The other issue is risk. Current sizing programs take a lot of factors into consideration, but can't cover them all, so most programs add a buffer to try and cover their rear ends.

Hope that helps, MikeL.

 

[waross]

Another factor that concerns single phase sets.
When I was active sizing and installing generator sets, single phase sets were not generally made in sizes larger than 15 KVA for 1800 RPM sets. (25 KVA for 2500 RPM sets.)
For the larger sizes, a three phase set would be re-connected and relabelled for single phase use.
Often neither the generator end nor the engine would be changed.
So for example, take a 30 KVA three phase set that will be re-connected for single phase use.
Three phase specs:
KVA = 30 KVA
PF = 0.8
kW = 24 kW
Engine power = 24 kW

Now this set is reconnected for single phase use. It will lose 1/3 of the KVA rating.
Single phase specs:
KVA = 20 KVA
PF = 1 or unity
kW = 20 kW
Engine power = 24 kW.
As you can see the set is now overpowered. The extra power helps keep the frequency up during motor starting.
If the set excitation is supplied by a Permanent Magnet Generator (PMG) the generator the voltage will hold up better.
That is, with less frequency drop there will be less UFRO action dropping the voltage.
Less frequency dip combined with less voltage dip will be beneficial to the non motor loads.
I don't take this into account when sizing a set, but it does provide a nice little capacity cushion and better performance.

Naturally Aspirated versus Turbo Aspirated.
When a lightly loaded NA engine is hit with an overload such as motor starting, it is able to put out full power as fast as the governor can react.
When a lightly loaded TA motor is hit with an overload such as motor starting, it cannot develop full power until the turbo spools up.
The TA motor is more fuel efficient.
The NA motor handles block loading with less frequency and voltage dip.

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