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Steam Turbine Voltage Issue

01/23/2015 2:59 PM

My company is planning to buy a used seimens steam turbine of 8.5 MW, 6300 volts 50 hertz. The local voltage in our country is 11000 volts and all the electrical system is based on 11000 volts. What would be the way to go from 6300v - 11000v. Do we need a step up transformer from 6300v-11000v. Please recommend the best possible solution.

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#1

Re: Steam turbine voltage issue

01/23/2015 4:33 PM

The luckiest thing would for it to be now wired in delta, but convertible to star. Otherwise, of course you need a transformer.

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#14
In reply to #1

Re: Steam turbine voltage issue

01/24/2015 3:05 AM

Dear Mr. Tornado,

Referring to your posting above, I am of the view that:

1. Generally the Generators are wound for star connection" and delta connction needed means we have to specify in the Enquiry Document.

2. Converting 6300 Volts to 11000 Volts may pose some risk, since the Insulation Value is designed for 6300 Volts.

Pl. let me know your views.

DHAYANANDHAN.S

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#16
In reply to #14

Re: Steam turbine voltage issue

01/24/2015 6:38 AM

My guess would be - if the terminations allow change from delta to star the insulation would be OK for the higher volts. But the manufacturer (or instruction manual!) could confirm.

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#19
In reply to #16

Re: Steam turbine voltage issue

01/25/2015 5:30 AM

Dear Mr.Codemaster,

I am of the view that normally termination will not allow to connect STAR to DELTA and vise versa.

You are fully aware that in Star connection the Line Voltage = Sq.Root3 x Phase Voltage and Line Current = Phase Current.

Therefore Insulation aspect is to be taken care for Higher Voltage.

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#21
In reply to #19

Re: Steam turbine voltage issue

01/25/2015 9:56 AM

I was just following on from #1, and thinking that if all 6 terminals are brought out, so that it can connected star or delta, I would expect the insulation to be good for the worse case ie star. I don't know offhand whether alternators are normally, or can be, designed that way.

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#17
In reply to #14

Re: Steam turbine voltage issue

01/24/2015 6:44 AM

I agree that a dual-voltage connection is unlikely, which is why saying "luckiest"; a transformer will probably be needed.

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#2

Re: Steam turbine voltage issue

01/23/2015 5:15 PM

People that don't already know how to do this, should not try. If your engineers are saying that they need to also buy a step up transformer then buy a step up transformer. If your engineers say that you need to buy a synchroscope to measure synchronization then buy a synchroscope. Your question is such a fundamental question of power production that you should not argue with anyone about what needs to be done.

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#3

Re: Steam turbine voltage issue

01/23/2015 5:42 PM

Please hire a qualified professional to design a proper system for you. This is not for anonymous forums.

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#4

Re: Steam Turbine Voltage Issue

01/24/2015 12:18 AM

Holy smoke(s)! I could have answered this question many years ago when I was in the third or fourth grade! This sort of question certainly explains why the entire planet seems to be suffering from a lack of progress while sitting in the doldrums. How do these people get jobs, especially jobs in areas in which mass casualties can be the result of poor decisions! If you "eff" this up, we're looking at well over a thousand amperes at 6300 Volts! In other words, a goof-up here can cause a lot of damage and, even death(s)!

Yeah, buy transformers, buy switchgear, and, most of all, get an education! Buying grades -- or trading "favors" for them -- is NOT the same as actually learning things!

HOLY CRIMINY!

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#5
In reply to #4

Re: Steam Turbine Voltage Issue

01/24/2015 12:29 AM

Actually, a bit under 1000 amps.

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#6
In reply to #5

Re: Steam Turbine Voltage Issue

01/24/2015 12:31 AM

Actually, about 1350 Amps at 6300 Volts.

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#7
In reply to #6

Re: Steam Turbine Voltage Issue

01/24/2015 12:35 AM

You forgot to divide by sqrt 3 for 3-phase. Even when you then multiply by 1.25 for power factor, you still get less than 1000A.

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#8
In reply to #7

Re: Steam Turbine Voltage Issue

01/24/2015 12:39 AM

Where is "three-phase" mentioned? I will agree that that is probably the case, but the OP makes no mention thereof. Nor did he mention power factor, which we might assume to be 1.0 at the source...

...Or 1.25...

...Or...

...In any case, there are more than enough electrons running about at high potentials to make a very bad day for someone, and this person should NOT be in charge of this "Charlie Foxtrot".

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#9
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Re: Steam Turbine Voltage Issue

01/24/2015 12:58 AM

Power factor can never be 1.25; it is always <= 1. And no, you cannot assume it to be 1.0 at the source. (The 1.25 is the reciprocal of power factor, for which 0.80 is a common preliminary assumption.) I doubt there is any such thing as a 1-phase generator of this size.

But thanks for playing.

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#10
In reply to #9

Re: Steam Turbine Voltage Issue

01/24/2015 1:04 AM

Yeah, I've had a few dollops of ethanol tonight; I messed up the inversion of the 1.25 factor, and a single-phase generator might not be practical...

...But neither was it specified.

Nevertheless, being fried by 800 Amperes at 6300 Volts or by 1300+, the result will be very similar, and the OP needs to get some help.

As for your "(T)hanks for playing" remark, I will allow those down the road to discern your intent; you are, apparently, from a younger, ruder lot than I.

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#11
In reply to #10

Re: Steam Turbine Voltage Issue

01/24/2015 1:19 AM

Thank you for your false votes. As for age, I'm 66, and I know plenty enough not to tolerate nonsense gladly. Why don't you get a name change? But don't change it to Voltamps, please.

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#12
In reply to #11

Re: Steam Turbine Voltage Issue

01/24/2015 1:26 AM

Likewise...

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#13
In reply to #12

Re: Steam Turbine Voltage Issue

01/24/2015 1:30 AM

In the present context, that makes no sense at all.

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#15
In reply to #4

Re: Steam Turbine Voltage Issue

01/24/2015 3:26 AM

Seriously there is no need to be rude. Had I known such fundamental issues I would not have used this forum. Firstly I'm a chemical engineer and secondly I'm a partner in the company so I thought why not get a second opinion from this forum, just maybe if My engineer is missing something or there is another way to go !

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#18
In reply to #15

Re: Steam Turbine Voltage Issue

01/24/2015 9:07 AM

"...no need to be rude." You come to this anonymous public forum to get an answer on a fundamental question about connecting an 8 million watt generator to an existing power grid. This is probably outside of your comprehension of electricity fundamentals but which do you think can provide more power at a rate of 100 times per second, your 8 million watt generator or this grid?

Your electrical engineer on this project is concerned with many more aspects than you are aware of. Let them do their job.

Since you claim a chemistry background, what if your electrical engineer came to you with a chemistry problem. They found out that an equal amount of lye and hydrochloric acid was mixed in the local water supply. An anonymous chemistry forum told them the only way your company could avoid liability costs for spoiling the water supply would be for you to build a purification plant next to this generator. Would you now have any faith in any chemistry problem this electrical engineer found. Would you trust their ability to wisely spend the corporate budget on an electrical problem.

Your question to us is more fundamental than this fictitious chemistry problem.

You might dismiss my reply because you think I got the power frequency wrong early in this reply. I probably did get it wrong. As others have pointed out this is probably three phase power. 300 times a second would be a correct answer for a novice technician. A good electrical engineer would know better.

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#20

Re: Steam Turbine Voltage Issue

01/25/2015 9:06 AM

Reflect on the equation

HP = Torque x RPM = Volts x Ampere x Power Factor and #Poles = (120 * f)/RPM

Using equation above where rated power of turbine is fixed, if your local supply is at 60 Hz, RPM will be more than 1/5 to producing 6300V at 50Hz

See also that Power is directly proportional to Voltage, Amperage & RPM. Increasing the voltage, you will have to increase RPM. See if rated RPM of steam turbine could tolerate increase of RPM by factor (4.7/11) which is close to one half of nominal speed. ---If steam turbine specs speed is okay with that, then you do not need transformer.

Brief summary,

RPM increase when

  • operating at 60Hz by 1/5
  • operating at higher voltage (11000V), Amperage should be re-adjusted by equation above
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#22
In reply to #20

Re: Steam Turbine Voltage Issue

01/25/2015 10:02 AM

Apart from that being mostly wrong, it doesn't help with OP's question.

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#23

Re: Steam Turbine Voltage Issue

01/26/2015 2:29 AM

Like tornado says a transformer is needed, and normally an 8.5 MW alternator are made to order thus no chance for it to do multiple connection to suit voltage. We used to do 12 wire reconnectable machine only up to 100kVA, 220v, 400v, single or 3phase to suit alternator stockist, even that these few years have stop because the low margins selling them is so unprofitable. Thus if you need any special voltage you order and pay premiun. And totally no chance at all for medium voltage.

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#24

Re: Steam Turbine Voltage Issue

01/26/2015 9:26 AM

Most power generation stations in the USA utilize a lower generation voltage at the TG delivery point and then use a "step-up" transformer to raise the voltage up to meet the distribution system voltage requirement(s).

It is not reasonable nor prudent nor cost effective to attempt rewiring the generator.

It is also much wiser and significantly more reliable to have a step up transformer with acceptable impedance between the generator output and the distribution system for fault protection reasons.

For safety and budgetary reasons the TG must remain stable at all times during generation.

The transformer serves as a filter between the distribution system that significantly reduce and/or prevent disturbances from reaching the generator that can/will cause machine mechanical & electrical operation instability resulting in tripping and/or machine damage.

This is a very simple statement that in no way includes all parameters and considerations involved in making the decision to use a step-up transformer between the generator and distribution system.

I would suggest you talk with the Siemens Application Engineering group and get their recommendations on this subject.

If Siemens does not satisfy your concerns, contact ABB, GE, Toshiba, or any of the other TG manufacturers for their opinion.

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#25

Re: Steam Turbine Voltage Issue

01/26/2015 10:11 AM

<...need a step up transformer from 6300v-11000v...>

Yes, obviously. If it were not provided then when the generator were hooked up to the grid, the grid would overwhelm the generator; instead of electricity coming out of it, smoke, sparks and molten parts would come out instead.

<...Please recommend the best possible solution...>

Consult a competent, qualified Electrical Engineer locally.

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#26

Re: Steam Turbine Voltage Issue

01/27/2015 4:43 AM

I have gone through the posts. I feel it is still the simple solution to go to the manufacturer for solution. In case of difficulty in approaching the original manufacturer, a reputed rewinding shop also can attend to such issues, in my experience.

There could be two possibilities - one is to rewire the windings in to Star as some suggested, provided the generator name plate / manual confirms that the present winding connection is Delta.

The second is redesign the excitation system to work the machine as 11kV rated one. This will require verifying the original machine design for available margins, in terms of flux density. The insulation is unlikely to be an issue (manufacturer can confirm).

The second alternative will mean uprating the kVA rating of the machine and thus provides additional capacity to the user (as said, if the original design permits).

In either case, step-up transformer is avoided.

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