Overloading of Generators

[jan63]

I am working in a power station.I have a simple but important question.The generators here are rated at 175MW with normal current of 7.7KA.The manufacture has allowed 15% overloading and mentioned current at that to be 8.86KA with 80 degree rise above ambient.The question is this that if temperature are normal can we load the generators above 8.86KA if all other parameters are normal.$ generators are class B insulated installed in 1977 and 6 are class F installed in 1982

 

[dpc]

Not sure what you mean by "temperatures are normal", but the main concern in the overloading range will be winding temperature and bearing temperature. But you probably are not directly measuring rotor winding temperature, so anything beyond the manufacturer's recommendation is probably not advised unless you can be certain of the temperatures in all parts of the generator.

The insulation system is specified in terms of temperature rise above a standard ambient, but the actual temperature is the primary issue.

If it was my generator, I wouldn't do it unless things were desparate.

 

[jan63]

thanku.By normal temp. i meant below 80 degrees rise.And i have been advocating the point in my plant that we have only six temp detectors for stator and no for rotor so should not take risk.

 

[ScottyUK]

Agree with DPC, the rotor is vulnerable and it is difficult to obtain temperature readings from a rotor without a lot of effort. You will need to increase excitation slightly as you push the MW higher, and the field losses will increase. If you can keep the power factor near unity - ideally just slightly lagging to keep away from the stability limit - then you have the best chance of getting the higher MW without damaging the machine. Is the prime mover rated for 15% overload? That's a generous design if so.


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

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

 

[jan63]

Yes the prime mover is also designed for 15% overlad and so is the transformer.But after 32 years of operation i think 15% overload will have its risk.

 

[edison123]

The windings are probably at the end of life cycle and any higher temperatures would only accelerate the ageing process. Also, what about the shaft's ability to carry the additional load ?

 

[jan63]

Actually generator and all associated equipement whether shaft,exciter,transformer and turbine are designed for 15% overload

 

[wolf39]

jan63:

The generator can be run at maximum permissible load continuously until temperatures at the embedded stator RTDs are stable. The temperature rise of the rotor (field) winding can then be determined easily by measuring the excitation voltage at the collector (slip) rings by use of insulated brushes or by temporary wire contacts. The excitation current is to be measured at the shunt of the excitation system. The resulting resistance has to be compared with the field winding resistance accurately measured at the time of commissioning (this resistance is usually calculated to 20 degrees centigrade). Then standard IEEE 115 is applied to calculate the actual field winding temperature at the load conditions selected.

Seems that your generator has been designed to the old ASA C.50 standard which differentiated between rated load (100% or 175 MW) and continuous overload (115% or 201 MW). For 100% load a temperature rise of 60 Kelvin was specified those years. For an overload condition of 115% a temperature rise of 80 Kelvin above ambient 40 degrees centigrade was allowed (all data valid for class B insulation only).

It's up to you to increase the generator load above the 115% in case the temperature rise of both, stator and rotor winding, is less than 80 Kelvin. The standards would allow it. But as edison123 pointed out already, this is not advisable and I certainly wouldn't do it. It's the age of the stator windings and the increased expansion of all generator parts (windings, etc.) at the new overload which is very risky. What you can do, however, is to install larger heat exchangers. The ambient temperature gain can be added to the temperature rise as long as the winding temperatures don't increase above the present generator temperature levels. But be careful: Whatever you do, you have to take into account all other power train components.

Is this a hydro generator?

Regards

Wolf

http://WWW.HYDROPOWER-CONSULT.COM

 

[jan63]

it is a hydro generator

 

[edison123]

What are your present load and winding temperatures ? I would keep the maximum stator winding under 100 deg C to prolong the winding life.

 

[jan63]

presently load is 188 MW with staor currents around 9.5 KA.This is because units are being operated below rated terminal voltages.Temperatures are 99 to 100 degrees C

 

[edison123]

You have already crossed the 15% current limit 8.86 KA and gone to 9.5 KA. I would count my luck at this point and stop.

 

[wolf39]

jan63:

With the data available I assume that the hydro generators in question have a 100% rating of 184 MVA at p.f 0.95 and 13.8 kV.

At 188 MW and 9500 A the terminal voltage is 11,43 kV for p.f. 1.0 and 12,03 kV for p.f. 0.95. These voltages are far beyond the standard plus/minus 5% variation. The current of 9500 A is only about 2% higher than the 8860/0,95 = 9326 A permitted by the manufacturer under the assumption that the generators are allowed to run at 95% rated voltage. Is there a reason why the generators are operating at such low voltage? If you increase the voltage you will end up with lower temperatures for the stator windings.

You may run the generators and all other power train components more efficiently if you increase the generator voltage as the total I2R losses possibly decrease more than the iron and field winding losses go up.

Look for the power capability diagram supplied by the OEM and compare whether you are in line with their recommendations. Since the generators are out of warranty, however, you may run them as you want them to be run.

What is the rated speed?

Regards

Wolf

http://WWW.HYDROPOWER-CONSULT.COM

 

[jan63]

i will give you data of our generators.
Units 1 to 4 are 175MW,.85 power factor,13.8KV generator voltage.
Unit no.5 to 10 are 175 MW,.95 power factor,13.8Kv
speed of all these units is 136.3 rpm.
Units 11-14 are 432 MW,.9 power factor,18Kv and speed is 90.9 rpm.No overloading allowed in these four units.

The reason for the low terminal voltages is that our whole system is extremely overloaded.We have a demand of around 17000 Mw and our total capacity is 13000MW.
We have not crossed the 15% overloading.Units 1 to around 9.6KA while units 5-10 around 8.8KA

 

[ScottyUK]

Is there any reason why you can't use the GSU transformer's tapchanger to allow you to bring the machine terminal voltage back up to nominal while maintaining whatever HV side voltage you are currently running at? You will then get more power out without pushing the current rating even further.


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

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

 

[jan63]

i agree with you.I gave the suggestion to change tap of unit transformer from 537.5/13.2KV to 525/13.2KV which will improve the ter.voltage wothout much effecting the bus bar voltages.But unfortunatly my written request not so far accepted

 

[ScottyUK]

I think the best approach is to return the terminal voltage to the correct value and adjust the tapchanger to optimise the reactive loading on the machine, then see how much over-current you actually need to achieve your required output. Did the people who did not accept the proposal give any reason why?


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

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

 

[jan63]

I think they do not realise the effect on stator currents and are more interested in bus bar voltages.Still i am trying.

 

[edison123]

May be you can tell them a la Die Hard FBI - Boyz, either lose the attitude or lose the machines. You're operating well below the machine design parameters.

Scotty's suggestion will get rid of low generator voltage and high current in one shot. What is there not to realize ?

 

[jan63]

I know that.But unfortunatly people here only act when some thing bad happens and then they start blame game.Due to these low voltages and to avoid further verload of gen.we sme time have to decrease MW thus loosing valuable energy of around one million units(KWH) per day.