Gas Turbine Excitation System

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cyclones

With reference to GE frame9E GT Excitation system(rotaduct/rotating diodes), why DC excitation voltage is not directly applied to rotor?

I mean that DC excitation voltage coming from excitation panel is first applied to exciter stator & consequently on exciter rotor, we get AC voltage which is then again converted to DC voltage through rotating diodes which give this DC voltage to generator rotor? What if we apply DC voltage to generator rotor directly which is coming from excitation panel?
 
There are basically two types of excitation systems used with synchronous generators (alternators). One uses slip rings and carbon "brushes" to apply DC voltage and current (for a Frame 9E I think the voltage can be as high as 700 VDC and the current can be greater than 1200 A at Base Load--but it's been many years since I last worked on a Frame 9E) directly to the rotating field so those figures may be off by a little bit (high or low). The brushes wear down and must be replaced; they can be replaced on-line, but it's not a job for the squeamish. The DC voltage and current must be produced by some method, and that's usually done by tapping the generator terminal voltage with a transformer, and then converting that voltage into variable voltage DC which is then applied to the field of the generator through the brushes and slip rings. So, the power for the exciter comes from the generator terminals in this system (usually).

The second type of excitation uses a system that applies a small DC voltage and current to the stationary field of a small AC generator. The rotating portion of this generator is on one end of the generator shaft (where the slip rings and brushes would be located if the other system was in use). The rotating portion of the small rotating exciter produces an AC voltage which is applied to diodes which are mounted on a ring on the shaft and convert the AC voltage into DC, which is then applied to the rotating field of the synchronous generator. The AC voltage and current produced by the rotating exciter and converted to DC is similar to that applied to slip rings by brushes in the other type of excitation system. The power for the rotating exciter comes from the torque applied to the generator rotor by the turbine shaft. Only a small DC voltage, which is varied as required, to control the generator terminal voltage.

So, you can apply DC voltage directly to the rotating field of a synchronous generator through slip rings and brushes, but in your case it would require a new generator rotor with slip rings and a brush rigging and a new system for developing the DC voltage and current. And it would require a much more powerful DC source than the excitation system provided with a rotating exciter.

Usually, rotating exciters are less expensive than brushes and slip rings and it's believed they require less maintenance. In practice, that generally means that little or no maintenance is performed and that leads to dirt and moisture collecting on the rotating exciter components and rotating diode wheel components and that leads to shorts and grounds. Carbon brushes must be monitored and periodically changed and that is believed to require more maintenance. Also, the DC excitation power source (a high voltage transformer and a rectifier bridge and associated control system) require more maintenance and is necessarily more complicated than a rotating exciter.

Remember: The generator rotor is spinning at 3000 RPM and you can't
just plug the field leads into a DC outlet; the cord would quickly get twisted and broken. So, there must be a method of applying or supplying DC voltage to the rotating magnetic field. And that's done with either slip rings and brushes or a rotating exciter, which is a mini-generator that produces AC with a variable DC source, and that AC is rectified and the output of the rectifier is directly connected to the rotating magnetic field.

You should try googling rotating exciters and static exciters and 'brushes and slip rings'.
 
In GT Frame-6 While doing regular Ratcheting every time first AOP starts then Hydraulic ratcheting pump starts and then EOP starts. This was not happening before but now we are facing this issue. Is this normal or we are doing some mistake in operation.
 
In GT Frame-6 While doing regular Ratcheting every time first AOP starts then Hydraulic ratcheting pump starts and then EOP starts. This was not happening before but now we are facing this issue. Is this normal or we are doing some mistake in operation.
Hi Jayp,

I guess and avdvise you, to open a new thread regarding your issue at site, as this one is dedicated for other subject.

We would glad to help you, by telling us:

-Type/model of controls systems associated
- what kind of work have been performed before this occured
-Did you review the sequence logic for the sub systems and check if it is correct or not?

You can also have a look on this thread, it may give you some indications:
https://control.com/forums/threads/problem-in-ratcheting-in-gas-turbine.41987/

Regards,
James
 
syed taha ahmed,

So, there are two types of exciters (also called AVRs--Automatic Voltage Regulators): brushed and brushless. Most GE-design F-class units use an exciter (AVR) to control the voltage on the synchronous generator rotor (through brushes and slip-rings--so it's a brushed system of applying voltage to create the DC magnetic field of the synchronous generator rotor), and a high-power variable frequency "drive" (usually called an LCI (Load-Commutated Inverter), sometimes referred to as a SFC (Static Frequency Converter or Control)).

When starting F-class machines, the generator is used as a synchronous motor to spin up the turbine during starting, purging, firing and acceleration. The excitation system is used to create the magnetic field on the generator rotor (again, through the brushes and slip-rings), and the variable frequency system is used to apply a variable frequency AC voltage and -current to the generator stator windings. The variable frequency voltage and -current causes the generator--being used as a motor--to run at variable speeds during starting and acceleration.

Once the unit reaches approximately 95% speed and is self-sustaining (meaning the hot gases passing through the turbine section are sufficient to keep the unit spinning), the variable frequency voltage and -current being applied to the generator stator windings is removed. There is some other switching that goes on to re-convert the generator back to a generator. But, the DC that's being applied to the generator rotor through the brushes and slip-rings to control the generator stator voltage.

But, that's a brushed excitation system that's applying DC to the synchronous generator rotor--whether the generator is being used as a generator (normal operation) OR a motor (during gas turbine starting). The DC from the exciter ("AVR") is passed through carbon brushes which are in contact with the rotating slip rings on the generator rotor.

It surprises most people to learn that there is usually very little difference (electrically) between motors and generators--except the directions of ampere flow and the "direction" of torque flow. (There are two main types of AC electrical machines: synchronous and induction (asynchronous). Most high-voltage/current AC generators are synchronous; while the overwhelming majority of AC motors (small AND large) are induction. Synchronous machines require DC current to produce the magnetic field on the generator rotor; inductions do not (the rotor magnetic field is induced by the application of AC to the machine stator windings).

Now, brushless exciters for synchronous generators are commonly used, but I, personally, have never seen them used on GE-design F-class synchronous generators used as starting means for the gas turbines. Brushless exciters DO NOT use carbon brushes to apply DC voltage and -current to the synchronous generator rotor through slip-rings. Instead, there is a small rotating generator in place of the slip-rings, and a DC voltage and -current (from the brushless exciter (also commonly referred to as an AVR)) to the stationary windings of this rotating generator (sometimes called a rotating exciter, too). When the generator rotor shaft is spinning, the presence of DC on the stator windings induces AC in the small generator rotor windings--and that AC is applied to the rotating diodes which convert the AC into DC which is then applied to the main generator rotor windings to control the main generator terminal voltage. But, again--I have never seen a brushless exciter with a rotating diode wheel used as the source for the synchronous generator DC voltage and -current on a GE-design F-class heavy duty gas turbine. (And one reason is that for AC to be induced in the brushless exciter rotor there MUST be rotation--and when starting from zero or low speed there is insufficient speed to induce the AC to be rectified by the rotating diodes.)

I believe you will find that the 6F machine you are speaking about, and/or working on, does not have rotating diodes (it may--I haven't seen every GE-design 6F machine ever produced). I believe the two "exciters" you are referring to are actually one exciter (which is used both during starting (when the generator is being used as a motor) and during normal operation when the machine is at rated speed/frequency and producing electricity), and the LCI (or SFC, or variable frequency "drive") which is only applying variable frequency AC voltage and -current to the generator stator windings during starting (again, when the generator is being used as a motor to start the gas turbine).

Brushless exciters are inexpensive and don't have the maintenance requirements of brushed systems (brush replacement; slip ring maintenance), and while they are used on some large synchronous generators they are not as widely used as brushed exciters. Brushed exciters have some characteristics which make them very desirable for some types of grid problems--to help "ride through" some grid problems, and even to help "burn out" or identify (destructively) some other types of grid problems. Brushless exciters don't have these capabilities, and are really only used on very simple, basic systems (in general).

Hope this helps!

I ask that if you discover that the GE-design Frame 6F heavy duty gas turbine you are writing about DOES, in fact, have a brushless exciter with a rotating diode wheel that you write back to let us know. I believe that would be a one-off unit--not impossible, but pretty unusual. And, lots of people read these threads (now and in the future!) and would benefit from knowing if such a system did exist on GE-design F-class heavy duty gas turbine-generators. (And I am presuming the 6F machine you are referring to is being used to power a synchronous generator, and not some kind of large, centrifugal compressor.)
 
syed taha ahmed,

Let me explain again, ...

Actually, you need to provide pictures of the equipment you are describing. Diode set.... pn [sic] excitor compartment.... During startup the diode wheel pull down....

The SFC (Static Frequency Converter) uses diode-like devices to convert AC (Alternating Current) to DC (Direct Current) back into AC at a variable frequency (controllable, variable frequency) that will be applied to the stator windings of the generator. The speed of the rotor of a synchronous AC machine (motor or generator) is directly related to the frequency of the AC applied to the stator windings of the machine. As the frequency increases the speed of the generator rotor will increase. This is how the turbine-generator shaft is accelerated from my ear zero speed to near rated speed: by controlling the frequency of the AC being applied to the generator stator.

To produce the variable frequency AC it is common to use two sets (sometimes called ”bridges”) of diode-like devices, usually called thermistors or SCRs (Silicon-Controlled Rectifiers). One set produces DC from AC that is applied to a second set that then converts (sometimes called 'inverts') the DC into variable frequency AC. IT IS NOT customary for either of these sets of diode-like devices to spin or revolve or rotate. They are NOT generally connected to the generator rotor shaft; they are usually very solidly mounted on stationary insulators in an air-conditioned compartment. Sometimes the diode-like devices are cooled by a dedicated water cooling system. These sets of diode-like devices would be connected to the generator stator windings during startup, and would be disconnected when the turbine-generator was near rated speed and frequency. At this point, the turbine is capable of maintaining rated turbine-generator speed for synchronizing and loading the unit.

Now, in order for the synchronous machine to spin during startup there has to be a second magnetic field--this one on the generator rotor. To produce this magnetic field it is necessary to apply DC to the generator rotor windings (called the generator field). When using the generator as a motor during starting the generator exciter (often called the"AVR”) is used to convert AC using diode-like devices into DC and that DC is passed through carbon brushes to slip rings (often called the”collector”) to the generator rotor windings. This is typically called a ”static excitor.” The diode-like devices would not rotate, they would be stationary.

I would think that using a ”rotating exciter” (a brushless exciter with a rotating diode wheel on the end of the generator rotor shaft in place of the brushes and slip rings (the collector assembly)) would be difficult and expensive. BUT pretty much everything is possible these days, and if GE Belfort France is involved very little would surprise me.

The typical synchronous generator static exciter also consists of diode-like devices arranged into bridges to produce variable DC. I suppose it could be located in the PEECC of a GE-design heavy duty gas turbine, but it's usually in a second compartment, often where the SFC (often called the"LCI (Load-Commutated Inverter)) is located.

We don't know what your experience is or your job at the power plant; you may even be a student or intern trying to equate something at a real power plant to something in a textbook or reference book used in coursework. You may have worked at a smaller power plant where brushless exciters with rotating diode wheels were used on a synchronous generator driven by a gas turbine that had a different starting means (induction electric motor, or diesel engine) or the synchronous generator was driven by a steam turbine. (The power plant where the 6FA machine is located may also have a steam turbine-generator and that generator may use a brushless, rotating exciter with a rotating diode wheel. BUT, that doesn't mean the gas turbine-generator and the steam turbine-generator will or must have the same type of exciters (the steam turbine-generator will not require a SFC to accelerate it to rated speed).

It is possible--and easy--to attach pictures to responses to threads on Control.com. Without seeing pictures OR electrical drawings of the gas turbine-generator starting means and exciter I can't understand what you are describing. That may be because I would not expect to encounter a rotating, brushless exciter on a GE-design Frame 6FA heavy duty gas turbine-generator. But, at this point, we need to ”see” what you are talking about.

I am anxious to help you understand the system, and if I learn something new in the process--I am happy and lucky! There is a widespread tendency for to people to believe that every GE-design Frame 6FA heavy duty gas turbine-generator is like every other GE-design Frame 6FA heavy duty gas turbine-generator--and I may even be making that mistake at this point. But, at this point, without pictures or electrical drawings I am not able to properly understand or explain this.

So, please provide additional information to help us to help you.

Thank you.
 
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