Steam Turbine Generator Set

Hello rangacharya,

Over Frequency and Low Frequency are not used for Generators.

Generators are Machines whose electrical output is DC.

Alternators are AC output machines, and require both Over Frequency and Low Frequency sensors, included in feedback loops to the Prime Mover.

The Prime Mover may be Diesel Engine, Other Gas Engine, Steam, Gas, or Water Turbine, Windmill, or other originating mechanical machine, mechanically coupled to the Alternator, to produce useful Electric Power from an original fuel source.

If you need further help..... always helps....

Dear Sparkstation,

You have corrected my question thank you for the same. However my question still remains unanswered. I would like to know whether the Frequency limit is to protect turbine or protect alternator.

rangacharya

Hello rangacharya,

"I would like to know whether the Frequency limit is to protect turbine or protect alternator."

The short answer to your dual question is: Both.

The Frequency Limit Control System is to protect the Alternator, plus all connected electrically to it, as load devices. (motors, transformers, gas-discharge lamp circuits, medical equipment, and so on)

The Frequency Limit Control System is also to protect the Turbine, which if the automatic over-speed mechanical device fails, then relies on the over-speed control on the Alternator. (A cheap Insurance Method against permanent damage)

In all the Water Turbine/Alternator, Steam Turbine/Alternator, Diesel Engine/Alternator, Petrol Engine/Alternator, the Control Systems are basically similar.

Kind Regards.....

Over-frequency and under-frequency protection are used for most generator/prime-mover ( diesel engine , steam turbine, combustion turbine and even wind turbines) appications to protect the machines .

In some cases , the critical parameter may belong to the generator in others it may bebelong to the engine(prime-mover).

The method of aplying the protection may vary.

In the case of the generator or electrical parameters , the generator frequency may be measured and a frequency relay used to trip the generator off-line or even shutdown the engine.

In the case of an engine over-frequency and under-frequency which is then interchangeable with over-speed and under-speed , shaft speed probes or some other method of speed detection is used to activate an underspeed or overspeed trip.

In a large grid application , it is not normal to have large fluctuations in the generator frequency when it is on-line.

In smaller grids , where the generator size is comparable to the grid size , then generator load changes can affect the grid frequency and result in an over or under frequency condition.

For single generators (in island mode) such as in off-shore platforms or standby generator appications, an integrated under/over frequency relay sold by companies such as Siemens or Basler can achieve adequate protection.

Hello djacob,

Thank you for your well-thought out answer.

Please note the vital difference between Generators and Alternators.

Generators output DC.

Alternators output AC.

Thus Generators, by their very nature, cannot use Over Frequency or Low Frequency controls, because the output being DC, has zero Frequency.

I realize that the term "Generator" is often misused for Alternator output machines, but it is better to get these terms used correctly.

That was what I tried to explain, but evidently not sufficiently, in my Post above:

http://cr4.globalspec.com/comment/159963/Re-Steam-Turbine-Generator-Set

Kind Regards.....

i may be up the wrong tree here but assuming it is a alternator i think under and over frequency is controled by the automatic voltage regulator through its droop funtion for under frequency and overspeed funtion for over frequency both of which cut dc supply to exciter fields.

Excellent point Sparky..i wonder why in the real world, people still say Diesel Generator and not Diesel Alternator...!!

Aussie king..frequency is controlled by speed and voltage is controlled AVR..and there are separate relays which do the job...and are called OF and Uf relays..

Cheers !!

Dear Sparkstation,

You may be correct.I just wish everyone would stick to the definitions.

Maybe, also, any chance that your insistence on the terms may have stemmed from the automotive industry where "generators" and "alternators" had specific meanings for the electrical systems in the cars or trucks.

Early vehicles all had generators producing DC to charge the battery and power headlights and other loads..

They even had two versions where you may have the positive side earthed as in the case of some English-made cars. Most had the negative side earthed which is the universal practise today.

Approx 40 years ago , vehicle manufacturers have been weaning over to "alternators" producing multi-phase AC which is then rectified to produce DC to power to charge batteries , power headlights and other loads.Someone purchasing replacement spare parts for a vehicles' battery charging or electrical system during this era had to know whether the vehicle had one type or the other.

The advent of solid state thyristor electronics boosted the changeover from generators to alternators since it became easier and more cost effective to control vehicles' voltage and charging current using electronics rather than electrical coil type relays.

Unfortunately in my 30 years or so in the field I have seen everyone from OEMs such as Sienmens, ABB, GE .... to endusers such as myself use the term "generator" to refer to our machines in the range from 0.5 MW to 150 MW !!!

Even the machine O&M manuals use the term "generator".

Many thanks for the prompting , though.

It depends on where the generator is located. In general, an OEM will spec the generator as "capable" of handling +/-5% of the grid frequency. But in parts of the world where there are great distances involved between the generation source and the demand, there is usually a requirement for the generator to stay connected to the grid for "x-cycles" if there is a severe degradation in the system voltage or frequency. In reality, not all generators are created equal. Some generators tied to steam turbines are designed to trip off line if the speed drops below 2 Hz or above 1 Hz of the nominal. As for generators/alternators. Generators on cars put out DC; generators that produce power to the grid are indeed called generators, and they put out three-phase AC, with amp outputs as high as 13,000 or more.

Hello bonzoboy,

"....generators that produce power to the grid are indeed called generators, and they put out three-phase AC...."

They may be called generators at your location.

But they are called Alternators in most locations, because as I said earlier, Alternators are what they are, because their output is AC, rather than Generators, which are machines with DC output.

I do realize that in some areas, perhaps as in your own, technical terms may have become "fuzzy", but it is really best to keep things correct, don't you think?

Once a term or word has lost its meaning, it is meaningless.

Kind Regards....

Actually, I've never heard the term used outside of automobile use (alternator). I concur that the alternator produces AC, but every turbine I've seen shipped (and that's a lot of gigawatts), either 50 Hz or 60 Hz countries, it's been called a generator. Even site drawings and design manuals refer to the units as "steam turbine generators" or "gas turbine generators". I'd have no problem defining them as alternators for some special bid specification, but it seems much of the world that makes electricity refers to them as generators (well, at least the OEM's do) TTFN.....

Hello bonzoboy,

We mostly use water turbines here in New Zealand, in what is called Hydropower.

These are mostly Kaplan, Pelton wheel, Francis types, all connected mechanically to the main Alternator, which connects mechanically in turn to a small Generator, which excites the main Alternator.

That small Generator is excited in turn by a very small permanent magnet Pilot Generator, which puts out a few hundred watts, and is an extremely accurate unit, with very fine control system, used as the master Control of each Alternator.

All per complete unit are on a common shaft, which sectionally reduces in diameter as it progresses towards the Pilot Generator.

There are Steam Turbine and Gas turbine Alternators used in NZ's North Island, variously powered by coal or natural gas, and they appear to use a similar method of control, by DC Pilot Generator, then a larger Generator which outputs DC into the exciting rotor connections of the main Alternator.

Diesel-powered Alternators have also been used in New Zealand, some of these units are quite large, generally being used as emergency power at Hydro-power Stations.

I have installed these units, worked inside them, Inspected and Commissioned them.

Electric Generation Equipment, plus the controls, as installed in New Zealand has been mainly sourced from UK: England - Scotland - Belfast (Northern Ireland), Italy, Germany, France, Sweden, Switzerland, Japan.

As far as I know, we have not sourced major Electric Generation Equipment from the US, in particular your State of Florida.

The historic reason for non-US equipment, has always been the high $US versus the low $NZ exchange rate.

No doubt future such equipment may be sourced from China, but I do hope they improve their Quality Assurance Testing and Guarantees beforehand.

As you can see, there is a basic difference between "Generation Equipment" and the Standard Terms of "Alternator" and "Generator".

So, your experience of the situation is different from mine.

When I started working as one of my many hobbies over the years, on cars etc, while quite young, most had Generators (DC) to charge the batteries.

Most earlier manufactured cars, trucks and tractors I have worked on, had crank handle start, magneto ignition, and carbide headlamps.

Some even earlier tractors had manual "spin the flywheel" system to start the engine on petrol first, then when hot, run on kerosene (UK = Paraffin).

I purchased a 55 Amp 12 Volt car Alternator, in approximately 1964, made by Joseph Lucas & Son, UK, as the first ever special order for one of that rated output into their Christchurch Agent's Warehouse (Woof & Salvesen - long gone).

Since that time, I have preferred the Bosch pattern of Alternator, because it is far simpler to repair - the Ingram Alternators made in Australia use the Bosch design, as do the Japanese car Alternators.

All of these actually output DC, as I'm sure you are aware, but that's only because of the diode assemblies internally.

Please note that I am not a pedant, just suggesting correct Standard Terms are used.

After all, one day some reader might misuse the Terms in a Specification, and large problems, or a Lawsuit or two (not against you or I), may develop as a result.

So that's my explanation, in the usual brief manner to which I have become accustomed.

Thank you for your patience.

Kind Regards....

Dear All,

This is proving to be a very healthy discussion.

Automatic Voltage regulator(AVR) controls voltage and VAR reactive loading.

Droop/speed controller controls speed/frequency.

To address the original question: as I had contributed before , operating off-design , either under frequency or overfrequency had dangers for the generator ( sorry Sparkstation... habit I guess, but we must talk about cricket and driving on the left hand side of the road which you and I know about) and the prime-mover , whether turbine, diesel engine etc.

Combustion turbines have a turbine blade resonance characteristic and continuous operation below 57.00 Hz can result is a fatigue failure of turbine blading and possibly a compressor surge , if the gas turbine has an axial flow compressor.Most of the turbines which we have in service in the 30 to 100 MW range will trip off-line on underspeed at a falling speed of 3420 rpm.Which is equivalent to 57.00 HZ . This protection is for the prime-mover: a gas turbine in this application.

There is no off-line underspeed trip.

Most turbines( steam or gas) will trip on overspeed at 110% , in our case 3960 rpm.

This is an off-line condition since it may be impossible to have machines which are connected to the grid to reach to this frequency.

Even if all the transmission lines trip out, the machines droop controller will not allow overspeed.

Many overspeed trips occur if the generator breaker trips with the machine at full load AND the fuel control valve do not reposition fast enough.The combination of excess fuel and inertia carries the machine to the overspeed value and sometimes even higher.I have seen it.

On the AVR side , if you are connected to the grid and attempt to under-excite a synchronous generator, under-excitation limits on the AVR should prevent you from going below the under-excitation limit ,if the limit fails and you do under-excite the generator will start to slip poles and fall out of sync with a resultant trip on overcurent , assuming it did not trip on under-excitation.

Ther are special cases where some machines , turbine/generator combination ( as in the case of blackstart operation ) where the ubnderspeed and lower excitation limit does not apply since the machine was designed to allow operation beyond these limits.