GENSET DRIVING A UPS LOAD

[basiliou]

Dear All ,

Since i work for five years on Power generation business , i have a question to make in the forum .

Fist of all i live in Greece , and the mains supply here is rated at 400/240 Volts - 50 Hz.

I have to size a diesel generating set , able to provide power in 2 parallel connected UPS rated at 40 KVA each (that means total 80 KVA). UPS brand name is Liebert.

Can somebody propose a gooud source to retrieve data for my calculations ?

Liebert refers a Harmonics distortion better that 4% (no load) .

UPS , do not have a Starting current to take into account when sizing generating sets , but they are having a lot of Harmonics.

How do i have to proceed in order to calculate the correct KVA size of my Diesel Generating Set ?

Best Regards


George

 

[busbar]

A safe approach may be to contact 2 or 3 genset manufacturers for recommendations.

 

[basiliou]

I was looking to got a litle bit deeper in that problem.
Thank you busbar anyway for your help.

Any other ideas to make my clalculations ?


Regards

George

 

[basiliou]

I recently read , that a golden rule for sizing that genset is

GenSet KVA = 2 times x UPS KVA (minimum !)

Does anybody knows why ?

 

[rbulsara]

Most gen mfr. will say 1.6 to 2 times the UPS rating. 2 will definitely do the job.

The reason includes:

1. Generator will heat more with harmonics.
2. Some generator's voltage regulators do not react well to harmonic voltage distortion.

However this assumes 100% UPS load, all harmonics. Also consider the following and you will see why there is no strating answer. In my opinion if the motor load equals the UPS loads, the effect of the UPS harmonics is pretty much negated. This assumes no VFDs.

UPS loads, usually have associated cooling and need to be suppported by the generator. The cooling (motor) loads acts as dampner for total harmonics.

Also it is important to know what order harmonics are invlolved. Different generator pitch winding eliminates or permits different harmonics (for example, 2/3 pitch will eliminate circulation of triplen harmonics). Ask gen mfrs.

Also incorporation input isolation transformer for UPS also help emliniate triplen harmonics.

Digital voltage regulators on generator are not (or much less) sensitive to harmonics. Using a isolation transfomer for voltage sensing circuits for AVR is also helpful.

 

[RonShap]

I would also suggest that you shoot for 1.5-2 times the UPS size. Try to buy the UPS that has a filter that is dropped out of the circuit under low load.

 

[alehman]

You should plan to avoid loading the genset over 80% with any type of load. If a major portion of the load is non-harmonic producing, you can reduce the 1.5 =- 2 factor. My clients have facilities operating up to 80% with 50/50 load mix without difficulty with gensets in the 500 to 2000kW range.

Smaller gensets tend to have less expensive regulators so I would be cautious and carefully consult with the suppliers' engineering folks as previously suggested. You may have difficulty getting a definitive answer.

Ron was alluding to UPS input filters which have very large capacitors. Under light load conditions, if the filter stays connected, the generator may well see leading power factor if there are not enough inductive loads to absorb the VARs. This can cause problems with genset votlage regulators as well. Some UPS mfg's have recognized this problem and include switching to disconnect the filters automatically when load is light.

 

[PWR]

I would second the opinions about this being an area of some uncertainty and you won't be able to do any calculations to "solve" the problem. I would rely most heavily on what Liebert tells you. I have seen UPS units whose load characteristics sufficiently distort the generator voltage output waveform such that the UPS sensing circuitry forces it to switch back to battery input.

 

[PAFred]

If I were you I'd make my life a lot easier and just install as many Caterpillar Rotory UPS's as I need to get the output I need and be done with it.

Then you wont have (ANY) problems with dirty power, or (Harmonics)! They are a bit pricy but they done require hardly any maintenance and certainly no batteries to deal with. Check into them you can fine info at most Cat web sites.

I've installed them an I'm pretty sceptibale at new things especially when they are so small for what they do. But after going to the school on them I was really surprised at them and I also found out after going through the electronics that they are virtually unbreakable! They are that good.

 

[rbulsara]

Just a reminder to last poster:

Caterpillar rotary UPS is still a static UPS (rectifier/inverter) and equally susceptible to harmonics as any.

The rotary part there in is spinnig DC generator in lieu of battery (lasting a few seconds). This is different than a conventional rotary UPS consisting of a motor-generator such as a piller Uniblock.

I only stated this response to the last post. I have nothing against Caterpillar 'rotary ups' but it is important to know the difference.

 

[PAFred]

Thank you for your input, but it seems that you arent familiar with the Cat line of UPS, by the way Cat dosent make them they are a an afiliate. But you might be surprised to see just waht they can and cannot do. The amount of discharge time at full load for a model 300 has (by myself) been tested at a full discharge load lasted for 13 seconds which is more than enough time for a gen set to come on line and supply the power to keep the rotor going and the output remains constant as for the load.

Please take a look at the following supplied link and scroll down slightly to the section heading "Product Benefits" in particular (Harmonics.

Sometimes it pays to take a second look. Your input is appreciated.

 

[rbulsara]

pafred:

No offence..but read again what I wrote...and I am familiar with it..and one who really makes them..Active Power.

CAT is a Static UPS with rotary "battery" (a dc generator)not a conventional rotary UPS which includes a M-G set in essence. M-G sets isolate back up generators from harmonics as motor is electrically isolated from UPS generator. I am not disputing your "battery" time..

 

[ScottyUK]

PAFred,

Have a look for the a true rotary UPS made by a company called EuroDiesel. They were sold to GE by Seghers group in the last year or so - I'm not sure of the current website.

It is a very interesting design of electrical machine with a diesel generator and clutch. No electronics in the power circuit. They are popular with the financial institutions, who are among the few people who can afford them, but they are an exceptional product.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[ruggedscot]

13 seconds support - Man you really have no fear ! what if you have a fail to start for some reason - is your process vulnerable and what kind of hit do you take if it goes belly up !

We have quite a UPS here on site - Piller made and its something worth seeing. Basically a rotary double wound that uses power to turn the machine to give out on the critical services bus. when the mains fails the battery supplies the machine to keep it turning and there is no dip in output. If it exceeds the duration of time then the diesel kicks in and turns the machine through a clutch. While this is happening the motor windings that were used to turn the machine now act as a generator and supply the non critical supported load.

These machines are so rock solid in construction and can take some abuse without affecting the critical load.....

When something depends on secure power Piller do some good units but they are pricy....

http://www.piller.com/asp/dusv.asp?sessionid=&stateid=2&orderid=44&site=1&sid=2

Rugged

 

[PAFred]

To all, the only thing that can be said is simply this (Seeing is beleaving) if you havent studied , havent experienced havent been schooled on a product then (NO ONE) can objectively provide any honast real opinnion on anything. As mentioned in other replies go to the web site "Active Power.com" download or read as much information as can be obtained on line then you may be able to offer something worth while. I'll be the first to agree 13 seconds dont seem like much at all but consider this nearly all gen sets are stand by application and nearly all are sold and installed by spec to be on line in 10 seconds or less. With that in mind consider that 13 seconds is truley enough time (if) your system is up to par. Also note that the Active Power Rotary UPS is constantly on line regardless of the gen set and constantly cleaning the voltage while at the same time regulating it. The 13 seconds is a figure based on (1) unit in an N+1 system. That time can be what ever you make it. Like everything else it boils down to the dollar and how many you want to spend. I began my original statement stating that I was also skeptible of the system until I'd been to school on them and had seen them in operation. I too am familiar with the Pillar units and with over 30 years in the generator business I can with experience state that I wouldn't put a single Pillar in any job that I had a choice or controlling factor in for a lot of reasons not just one. But for something to think about consider the maintenance involved alone with the Pillar set up. The Active Power rotary units are basically maintenance free, self protected and un-maned . I was able to come up with the only possible flaw that could bring the system down according to the engineers that designed them, and for that to happen would truly be a one in a million to happen. Again I can only empthasize have an open mind and check out what these people have come up with. You will be surprized if you take a hard look at them and exactly how they work. These people knew what they were doing. Look into it for your selve's