Observer: "Most synchronous generators are designed to support 300% of their rated kVA for 10 seconds. From the OP, [(600 amps starting current)(230 volts single phase)]/1000 = 138 starting kVA"

Genset:"a generator with the correct insulation class (H), may also handle high overcurrent for short periods (such as that mentioned in Naams question), but the potential for alternator insulation failure is high', I followed this up more recently with the comment that things just arent made the way they used to be..."

Gentlemen, assuming both claims are valid it is beyond my comprehension that the supplier of generators (main parts engine and alternator) would try to save cost by being stingy as far as insulation class of the alternator wiring is concerned. in Nigeria i bought the generator together with his twin second hand from a german construction company which withdrew from the country. that was end of 1981, both units were most probably 2-3 years old. and now i am told that after THIRTY years of technical and manufacturing development the products are inferior?

And Naam,here is your drawing of the switching using a manual rotary changeover switch. 1-0-2.

Naam,

No, contemporary alternators are not inferior, far from it, but 300% of rated Kva for 10 seconds equates to an enormous amount of heat being generated in the alternator windings. These recommendations exist to extend the life of an alternator at the end of the day Naam... the manufacturer doesnt want you to burn your alternator out through misuse.

Having said that, competition is rife and the bottom line does lead to cost cutting in some areas which irritate me to be honest. Mecc Alte exciter coils is one such gripe... they need to spend a little more money on improving the insulation of their exciter coils in my opinion, far too many of them short out in only moderate damp conditons and can be a real pain to change sometimes. I have taken to disassembling every Mecc Alte alternator that I fit and spraying additional insualtion on the exciter coil to reduce the frequency of these specific faults.

Let me qualify the remark I made, if I may... I regularly see alternators (belt driven PTO Tractor sets mostly), which are thirty + years old. These belong to old farmers who work the things hard, constantly (and dont like to spend money...!). I see them when the belts need changing, these things are overwound and heavily insulated, I have no doubt that they will be providing power to the milking parlours of future farming generations for years to come... 'They just dont build them like they used to...' (is the term I should have used), is a reference to the fact that these generators take a lot of stick and last for years because they were overbuilt and not because they were technically superior.

Genset.

This post has been edited by genset: 2009-11-23 07:09:20

QUOTE

The neutral conductor is not switched.

You can make a working drawing from this information

Hi David96,

You went on to add, (a note at the end of the drawing you subsequently posted), that;

QUOTE

'main neutral and generator neutral are connected to the neutral bar of the switchboard directly, they are not switched'

With respect, I disagree with you on the issue of a permanent neutral connection to a secondary power source (generator) for domestic use.

It is my preference to always treat a standby generator for domestic use (and not designed to sync and run parallel with the utility supply), as a separately derived source of power. As such the star point of the alternator is connected to earth via a grounding electrode at the site of installation and the alternator neutral is bonded to the body of the alternator.

When configured in this manner, the neutrals (utility and generator) should be switched and not permanently connected, if ground fault protection devices are in use.

If the utility supply neutral and the generator supply neutral are both, separately grounded and the neutrals of both sources are permanently connected at the transfer switch; should a ground fault occur, any ground fault current would be split along the neutral into two parallel paths to earth. If the amount of sensed ground fault current does not exceed the settings of any ground fault protection devices fitted (now or in the future) to the faulty circuit, they will be rendered ineffective, quite an important point to bear in mind.

The two main ways to solve this problem are to remove the neutral/earth bond from the alternator, or to switch the neutrals at the transfer switch (both of which would return the full ground fault current through the protection device). In my opinion, switching the neutral is a must.

I understand that MEN (PME) is a consideration, but as it is still not implemented country wide throughout Thailand, I prefer to focus on the viability of the local grounding system and proceed as if MEN is not in place (even if it is).

Genset

...Not sure what happened to the formating of this post, I wrote it in Msoft Word and pasted it into the window... the spacing is a little tight but I cant amend it... sorry.

The attached document explains this problem in more detail and quite clearly...



This post has been edited by genset: 2009-11-23 05:49:41

one has to add "the manufacturer wants me to spend a multiple of my dough because he is not willing to produce with an additional minimum of my aforesaid dough the sturdiness of a product i have experienced three decades ago".

At the end of the day Naam, if you size your generator correctly, use it appropriately and service it regularly, it should give you years of trouble free service... I wouldnt like to see you slide into a fit of depression over this now...

Genset

This post has been edited by genset: 2009-11-23 05:47:19

i am not at all depressed Genset. i stated facts and i see a problem. when i see a problem i am looking for a solution. i do not consider "[over]sizing my generator correctly" an acceptable solution assuming there are other ways, assuming Observer's statement "Most synchronous generators are designed to support 300% of their rated kVA for 10 seconds" is correct and especially based on the actual experience i had years ago with a generator (actually alternator) which quite obviously took loads in a stride which seem to be beyond the capability of some nowadays alternators.

so let's look for a solution apart from oversizing. i am neither a stingy nor a poor man. i've told you what i spent on my home. it does not make a big difference to my bank account whether i spend 300k or one million Baht on my generator. but having grown up as a poor boy in a poor environment and working hard for whatever i own now i hate wasting a single penny.

Hi Genset, regarding your post 154.

One would agree with you there. I was pre supposing a MEN system. The TT system is the system most used in Thailand and they may switch the incoming neutral.

AS3000/2007 and AS3010 provide information on generator connection the majority of gensets connected to an MEN system do not switch the neutral and use a 3 pole ATS or 2 x 3 pole contactors (for small sets) or a manual 3 pole C/O switch and only one MEN connection per consumer, the MEN is at the main switchboard.

AS3000/2007 7.3.8.1.
The neutral-earth connection (MEN connection) shall be made within the installation at the switchboard to which the electricity generation system is connected.
Note: This may require disconnection of any neutral-earth connection within an electricity generation system, such as an engine driven generating set.

The incoming neutral to a MEN switchboard shall not be switched.

Neutral and earth conductors shall not operate in parallel.

7.3.8.1.2 Switching.
The changeover device for an alternative supply shall be selected to maintain the function of,and prevent damage to, the electrical installation being supplied.
Note: Examples of functions to be maintained include the maintainance of the continuity of the neutral conductor (overlapping of neutrals), the operation of RCDs or the continued operation of uninterruptible power supplies.

The changeover device shall open all active conductors of the normal supplies when the alternative supply is connected.

Note.
One has to make allowances for Thailand. In Australia we regard neutrals, earthing and equipotential bonding seriously. In Thailand they do not. One may have to use alternative methods as long as electrical safety is not compromised.

Naam:

According to the NEMA MG1 Standards, a synchronous generator should have the capability of supporting 300% overcurrent for 10 seconds. The question thus becomes how much of a voltage dip (sag) are you willing to tolerate and still be able to start all your motor loads. Is it acceptable for the lights to noticeably flicker or dim every time an AC starts, what about all your electronics? A lot depends on the generator windings, exciter, voltage regulator, and motor characteristics. If you require an exact answer the manufacturer will have to supply a motor starting capacity curve, or similar analysis, that details the ability of the generator set (as sold) to handle motor starting loads. Without such data being readily available we can only speak in generalizations. And yes, they don't build 'em like they used to.

This post has been edited by InterestedObserver: 2009-11-23 12:53:41

Hi Naam,

I appreciate your comments, have attempted to answer your questions to date as honestly as I can and will continue to do so.



Ok, the first thing we need to correctly size a generator for your needs, is an accurate load calculation based on the starting load requirement of each motor you intend to run and all your other appliances.

Genset

This post has been edited by genset: 2009-11-23 15:22:08

NEMA MG1 further states that (although each insulation class (A,B,F,H) has different characteristics (maximum operational temperature at each class)), under normal, continuous operating conditions, the insulation will have a minimum life expectancy of 30,000 hrs... thats 60yrs at the recommended standby (LRP) rating of 500hrs operation a year... so with a change of engine, you should be good to hand it down to the next generation...




Agreed, although I appreciate Naams comments, without the accurate starting load data (as part of a fully comprehensive load calculation) it is difficult to give a precise answer to this question and recommend an appropriate generator size for Naams requirement.

Genset

This post has been edited by genset: 2009-11-23 15:29:45

Hi Naam

I missed this initially (rushing to read your message before running out to work...), but I wouldnt categorize the statement underlined above, as the alternator taking the load in its stride... or as a reflection of an alternators capability to handle the load requirement of motors on start up. Under normal operating conditions (starting motors etc), the alternator should not be required to venture into this territory (we discussed alternator insulation ratings in post #75, if anybody is confused by this disussion I would ask them to read that post again, it contains information relevant to this exchange).

The ability to support 300% of rated Kva for 10 seconds is a safety factor, built in to protect the alternator windings from catastrophic failure and it should not be used as a basis for calculating a load requirement.

The 10 second rating is designed to allow the overcurrent protection device fitted on the generator, to operate effectively and trip the supply from the generator should such a situation occur.

Genset

This post has been edited by genset: 2009-11-23 16:48:52

that goes without saying but at the same time i read in the recommendations of an alternator manufacturer (you sent by e-mail) of starting currents which are FIVE to TEN times the flying amps. for an electrical layman like me that all sounds very confusing. what i will do is measure the starting currents of the two types of a/c units (12 and 24k btu) i am planning to use as well of the two huge fridges/freezers in my kitchen. unfortunately the chap whom i sent out to buy a "clamp amp" brought a model without peak hold. calling the shop the answer was "no habb". inspite of this i will still try to measure the two units. besides considering these loads i still insist on either manually or otherwise "one after the other switching (after the breaker or with a gadget that replaces the braker) first off and than on.

all my electronics (PCs, TVs, etc. as well as my house water suplly) are on inverters and batterie back-ups. there seem to be another problem to be solved, i.e. (according some experts here) separating the inverters from the net as long as the generator is running. other experts beg to disagree but if necessary that should not be a big problem to solve.

if you look at the sketch i posted even the "comfort version" is a total of only 12 kW out of which the highest "consumer" draws 2.2 kW. the 12kW contain already an aircon which is not really needed. as i don't have split personalities it is highly unlikely that i sit in front of my screens in my study and watch at the same time news in my TV-room using in both room an aircon simultaneously

needless to say is that during generator use neither pool pump, nor pond pump, nor waterfall, nor irrigation pump, nor deep well pump will be used.

Alternator protection. General.

Overvoltage
Undervoltage
Overspeed (Hz)
Underspeed (Hz)

Short circuit protection.
Overcurrent protection.

Earth fault protection. TT systems.

1. Overvoltage, undervoltage, overspeed and underspeed characteristics should be
incorporated in the AVR.

2. Short circuit and overcurrent protection by a MCB or MCCB of suitable rating.

3. Earth fault protection for generators connected to a TT system.

Earth fault protection is not mandatory in Thailand on TT systems. An earth fault may not operate a protective device.

On a TN (MEN) system an earth fault must be cleared within 0.4secs, the fault current path via the neutral conductor to the source of supply.

If you are not in a rush and in an effort to get an accurate load requirement and generator sizing, perhaps you would prefer to wait until I come back in the new year, I would be happy to help you out with this in person.

Genset

This post has been edited by genset: Yesterday, 2009-11-24 09:00:42

Observer,
i think i emphasised from the beginning that i am planning to go with my idea of switching loads one after the other. if this problem is solved the generator of my liking

http://www.kipor.com/prouducts_detail.asp?info_id=577

with a continous output of 30kW should not have any problems to deal with the "comfort" solution (12kW) i want to achieve. the unit has to deal only at start with the heavier load of the selected aircons. reason: whenever we lose a phase or even two the remaining aircons work continously, never switch off and because of that no fluctuation (except perhaps a minor one from the fridges) occurs. all my electronics are (as mentioned) on inverters (three sets of them, each with a 24v battery back-up capacity 320ah).

light flickering should also be eliminated as no aircon will switch off and then on again during generator use (see above). besides, having lived and worked 15 years in the desert, the bush and the swamps a little light flickering wouldn't do any harm