Frequency conversion for Steam Turbine Generator
Frequency conversion for Steam Turbine Generator
(OP)
Hi,
A Steam Turbine Generator has been shifted from one country to another country.The generator name plate details are as follows:
Capacity:3150 kVA/2520kW; PF 0.8; Gen.volts 450V; RPM 1800; HZ 60;Poles 4;PH 3;
Brushless excitation with exciter mounted on the same shaft extension.
This steam turbine generator has to operate in the country where the system voltage and frequency are 415V and 50HZ respectively.
The shaft speed can be reduced to 1500 RPM by changing the gear box thro' which turbine is coupled to alternator and thereby achieve 50Hz.But at the same time the exciter voltage,Generator output voltage,output power also get reduced.Also the reduced generator cooling because of reduced speed.
In order to operate the STG set in the current scenario and at the same time meeting the system requirements of 415Volts and 50Hz,may be at reduced output power,what are all the modifications to be carried out? Any suggestions from generator design experts?
A Steam Turbine Generator has been shifted from one country to another country.The generator name plate details are as follows:
Capacity:3150 kVA/2520kW; PF 0.8; Gen.volts 450V; RPM 1800; HZ 60;Poles 4;PH 3;
Brushless excitation with exciter mounted on the same shaft extension.
This steam turbine generator has to operate in the country where the system voltage and frequency are 415V and 50HZ respectively.
The shaft speed can be reduced to 1500 RPM by changing the gear box thro' which turbine is coupled to alternator and thereby achieve 50Hz.But at the same time the exciter voltage,Generator output voltage,output power also get reduced.Also the reduced generator cooling because of reduced speed.
In order to operate the STG set in the current scenario and at the same time meeting the system requirements of 415Volts and 50Hz,may be at reduced output power,what are all the modifications to be carried out? Any suggestions from generator design experts?





RE: Frequency conversion for Steam Turbine Generator
While the generator has a nameplate capacity in kVA, the real capacity limit is in Amps. You don't get more Amps as the voltage goes down, so you'll be at a capacity of 2625kVA. Is that enough?
RE: Frequency conversion for Steam Turbine Generator
Otherwise do we need to strip down the entire alternator winding and rewind it to get the required voltage of 415V output at reduced power?
RE: Frequency conversion for Steam Turbine Generator
RE: Frequency conversion for Steam Turbine Generator
BTW,will the generator output be 2625kVA@375V or 2185kVA @312V? Thanks.
RE: Frequency conversion for Steam Turbine Generator
I agree with David - a lot of effort for little gain. Can't you find an alternator in Europe?
RE: Frequency conversion for Steam Turbine Generator
Lot of effort for little gain.But before we suggest to client to go for new alternator we would like to explore all avenues.
It is an alternator with brushless exciter mounted on the same shaft at non drive end.No PMG and no static excitation with slip rings.
RE: Frequency conversion for Steam Turbine Generator
Given the reduced capacity, is it possible to generate enough torque from the turbine without changing gear ratios?
Another consideration is adding a transformer or having a Generator Step-up Transformer supplied to take the voltage from a safe level up to a usable level. Still adds cost and further reduces capacity. I would consider presenting the options to the customer including a cost per KVA figure as well as the total costs of the various options.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Frequency conversion for Steam Turbine Generator
RE: Frequency conversion for Steam Turbine Generator
Rotor vibrations should also be evaluated when changing from 1800 to 1000 rpm speed. Either the generator OEM or start-up vibration data from the original installation can be checked for possible critical speed (balance resonance) near the new operating speed. This would minimize having a very expensive surprise at startup!
Walt
RE: Frequency conversion for Steam Turbine Generator
It should be possible to replace the shaft-mounted fans with a different blade design to give equivalent cooling flow at the lower speed.
I will be surprised if this is an economically viable conversion, but this is an interesting thread.
RE: Frequency conversion for Steam Turbine Generator
I guess you missed something there. 1800 rpm @ 60 Hz will be 1500rpm @ 50Hz.
RE: Frequency conversion for Steam Turbine Generator
MikeL.
RE: Frequency conversion for Steam Turbine Generator
The speed will be reduced from 1800 RPM to 1500 RPM by means of a gearbox only.The turbine speed will remain at its original 6000 RPM and hence rotor vibrations may not occur.
ScottyUK
We would like to retain the same shaft mounted brushless exciter.However,since AVR cum excitation panel is not available,we have to buy a new excitation panel to suit the existing excitation system.
As you suggest,we should check whether the shaft mounted the generator fan can be replaced,so that generator cooling is not affected.BTW,is there any way of knowing beforehand, how much the power will be limited because of the reduced cooling,in addition, due to reduced output voltage? Ultimately,it is the permissible winding temperature which will decide the capacity of the Generator.
RE: Frequency conversion for Steam Turbine Generator
As I had already mentioned it is a Marine alternator with brushless excitation system.Make Nishishiba Electric Co.Japan.Mfr.Year 1976!Generator is having Air cooler with fresh water as coolant.As you suggest we will try to get some inputs from the manufacturer,hopefully.
RE: Frequency conversion for Steam Turbine Generator
Sorry if I wasn't clear - I meant the excitation panel in my previous post. I am fairly sure that the existing shaft-mounted excitation components will be usable, although the machine's capability curve may well be a slightly different shape and I suspect the machine's ability to produce Vars will be disproportionally reduced.
To answer the majority of the many questions we have posted above you'll need the OEM's assistance unless you can get access to the machine design data, something which is almost unheard of. Is the OEM still in business?
RE: Frequency conversion for Steam Turbine Generator
Yes.They(NSDK-NISHISHIBA) are very much in the business of Generator,Control and Marine electrical systems.Awaiting response from them for our query.
RE: Frequency conversion for Steam Turbine Generator
Yes I know there are exceptions to my comments but consider:
The air cooling system is not a finely engineered and tuned air movement system. Many designs use straight, paddle type vanes to urge the air to travel past the end turns.
Many designs use straight, symmetrical fans blades that will work the same in either direction of rotation. In many motor designs the fan blades are cast as part of the squirrel cage bars and end rings. The point is; the fans in many motors are designed more for cheap and efficient fabrication than for efficient air movement.
Given the inefficiencies of the air movement system on a typical motor or generator, I suspect and believe that an increase in speed from 1500 RPM to 1800 RPM may result in greater back pressure and less efficiency of the air movement system than in a greatly increased air flow.
Given the non linear character of heat transfer equations, if the temperature does tend to rise slightly at slower speeds, the cooling effect may increase greatly.
During the years that generator specifying, ordering, installing, trouble shooting, and repairing I installed dozens of sets and did trouble shooting on many more.
As part of the specifying and ordering process, I spent a cumulative total of many hours studying manufacturers spec sheets.
I found that one basic physical generator end may be used for a number of ratings. It may be single phase, three phase, prime power or standby power. At times I would compare generator end and engine numbers from between a prime rated set and a standby rated set to determine that the same basic set was used for both ratings or applications.
Although prime ratings and standby ratings of the same basic machine would often be on different pages, The 50 Hz and 60 Hz ratings of a given rating would be on the same chart on the same page.
Of the many hundreds of spec sheets I have seen, I have never seen a generator de-rated beyond the simple 5/6 ratio for use at 50 Hz, other than occasional slight rounding.
Some sizes such as 60 KVA, 60 Hz/50 KVA 50 Hz. would have even numbers where the ration was easy to see.
Many people notice different things depending on how chance has wired our minds. I tend to notice things such as ratios that are off.
Apart from this forum, I can't remember seeing in a text book a discussion of cooling issues when converting between 60 Hz and 50 Hz.
I admit that cooling issue at reduced speeds are real, but a reduction from 1800 RPM to 1500 RPM is not the same scale as a reduction from 1800 RPM to 18 RPM as may be encountered with VFD applications.
If a standby set is producing maximum current and exporting maximum KVARS (maximum field strength and maximum heat contribution from the field windings) for an extended period of time on a day when the ambient temperature is at the design maximum, the cooling may be a minor issue.
That said, I believe that it is well to be aware of and evaluate possible cooling issues at reduced speed, in the overwhelming number of applications it may be an insignificant factor.
Thank you for your consideration gentlemen.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Frequency conversion for Steam Turbine Generator
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Frequency conversion for Steam Turbine Generator
That is fine for the turbine with same speed, but my original comment was directed to the generator rotor that would be operating at a reduced speed. You cannot assume that the generator rotor was designed to operate at 1800 rpm and be below the first critical speed, and therefore operation at 1500 rpm would be OK. It is suggested that you verify vibration characteristics for this apparently expensive conversion project.
Walt
RE: Frequency conversion for Steam Turbine Generator
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Frequency conversion for Steam Turbine Generator
Will try to get some info.from OEM.Thanks.
RE: Frequency conversion for Steam Turbine Generator
RE: Frequency conversion for Steam Turbine Generator
There is one possible solution that hasn't been discussed, a bit off the wall. How about leave the generator as it is and fit a static frequency converter (VFD). A 3MW drive isn't cheap but it will allow your client to deliver an extra 400kW so may pay for itself in time (;provided there is space) there are issues such as harmonics to sort but a drive manufacturer will assist here. the benefit is mechanically everything stays the same and here is a saving on the gearbox modification.
Regarding striping the winding and rewinding for 415V, that may be a problem as there will be a low number of turns and configuring with more turns may not be practical. This is something I've know to be done before and a quick look inside will establish what can be done. We use a design house to reverse engineer the generator and establish it's performance etc. But that needs a strip down and a tape measure to get all the relevant dimensions.
I agree the OEM may be helpful, especially of you dangle the carrot of a new AVR panel.
Richard
RE: Frequency conversion for Steam Turbine Generator
We did look into the option of Static Frequency Converter option.But that was ruled out because of high prohibitive cost.Presently,it is decided to settle for a lower generator power output.And the speed will be reduced by adjusting the governor without changing the gearbox.
RE: Frequency conversion for Steam Turbine Generator
Incidentally: if the governor is electromechanical you will have to change out the flyball spring for one with less compressive resistance so as to achieve correct proportional response; if you don't, control characteristics will be very adversely affected.
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: Frequency conversion for Steam Turbine Generator
RE: Frequency conversion for Steam Turbine Generator
Sakaran51's OPs stated the plan was to use a different [taller] ratio gearbox so the turbine would still operate at its original speed of 6000 rpm.
waross then asked whether running the turbine @ 5000 rpm is a possibility; maybe sakaran51 has had some success in getting the necessary vibrational amplitude versus speed data from the OEM . . .
A possible side effect of operating the turbine at a reduced output, regardless of what speed it operates at, is whether the throttling losses will increase since the new loading could well and almost certainly will be off-design as compared to the original specs. Depending on what the steam-heads have to say and or what operational experience shows, there could also be a requirement to swap out the turbbine nozzle blocks for ones of a more appropriate flow profile. Yet another expense, maybe...
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: Frequency conversion for Steam Turbine Generator
I've seen people try this more than once, only to have to rip it out and replace the VFD with a proper frequency converter.