Frequency conversion for Steam Turbine Generator 1

[sakaran51]

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?

 

[davidbeach]

You're not going to make it. You'll be sitting there right at 110% of the design V/Hz ratio of the generator. That's about the maximum level, leaving no margin for error or off nominal system conditions. You need to be running closer to 375V than your 415V.

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?

 

[sakaran51]

Or will it be still further down,namely,2185kVA @ 312 volts only,because the excitor output also gets reduced in the same proportion?
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?

 

[davidbeach]

At that size are you sure it's worth the hassle? Might a new generator not be more cost effective than spending a bunch of money to modify a used machine and hope it works?

 

[sakaran51]

I fully agree with you. But,having brought the set from abroad,the client is insisting to use the machine and wants to be doubly sure that the modification is not worth.And we have to convince him with justifications and all the operational implications.Hence this query.
BTW,will the generator output be 2625kVA@375V or 2185kVA @312V? Thanks.

 

[ScottyUK]

What kind of exciter - static with sliprings, or shaft-mounted rectifier? Do you have a PMG?

I agree with David - a lot of effort for little gain. Can't you find an alternator in Europe?

 

[sakaran51]

Yes, ScottyUK,
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.

 

[waross]

It may be worth a search of this site. I believe that this issue has been discussed before. There may be some additional points to consider. I am wondering about the voltage rating of 450 Volts. Is this an old machine originally used for 440 Volt service? If so the V/Hz margin may have been greater than the 110% design limit that David mentions. If the unit is in service now, can you do some testing to determine the knee of the saturation curve? The next alternative may be to calculate the flux density to determine whether there is more than 10% margin in the allowable flux density.
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

 

[sakaran51]

According to name plate it is a marine generator.The turbine gear box has to be necessarily changed to reduce the speed from 1800 RPM to 1500 RPM to meet the system frequency requirement of 50HZ.As David has pointed out,the generator output voltage will come down to 375 Volts.But whether the output power of 2625kVA,as mentioned, is really achievable? Because of the reduced speed, the air flow thro' the generator also gets reduced,affecting the generator cooling system.Even though the power is reduced,as the current and losses remain same,the reduced generator cooling may be additional limiting factor in achieving the maximum generator output.It may not be linear.How much the power will get reduced?

 

[Strong]

Sakaran51,
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

 

[ScottyUK]

It should be possible to buy a new excitation system if necessary, but it's quite possible that the 60 Hz design will work with a few adjustments. The limiters will almost certainly be wrongly set.

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.

 

[Parchie]

@strong,
I guess you missed something there. 1800 rpm @ 60 Hz will be 1500rpm @ 50Hz.

 

[catserveng]

I may have missed it somewhere in the thread, but have you checked with the manufacturer of the generator? In that size range many manufacturers, such as Kato, CAT and Leroy Somer have dual or multiple ratings. In many cases the nameplate reflects what the machine was sold to provide, not what it is actually designed for. I know on many CAT generators when you actually dig into the technical data there are multiple rating for many of the larger frame machines. This may be the case with yours, it may not, but it may be worth contacting the manufacturer and seeing what they have to say.

MikeL.

 

[sakaran51]

Walt
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.

 

[sakaran51]

@MikeL
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.

 

[ScottyUK]

sakaran51,

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?

 

[sakaran51]

@ScottyUK
Yes.They(NSDK-NISHISHIBA) are very much in the business of Generator,Control and Marine electrical systems.Awaiting response from them for our query.

 

[waross]

A few comments on cooling at reduced speeds;
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

 

[waross]

Is it possible to run the turbine at 5000 RPM? Will the turbine produce enough usable power at 5000 RPM to satisfy the reduced demands of the generator?

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

 

[Strong]

"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."
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