I came across a question from colleague ...i want to load Gas turbine that supplies power to that plant only (Not connected to Grid). As soon as excitation is build up- generator is loaded without closing any breaker. (Directly supplying power). He says this quite common!!!

Firstly, is it possible to have such configuration? Is there any problem operating the machine in this way (generator or turbine)?

How the GE turbine would work in this situation? Which FSR would be take control? As i know, FSRACC (Accelaration FSR) would be controlling the governor till FSNL and FSRN (Speed Control FSR) comes in to picture after FSNL (Full Speed No Load. But whats the way forward?

Can CSA or other guys help me to understand this?

Thanks & Regards,

 

Controls_guy,

Without closing <b>any</b> breaker? I don't think so.

Having said that, there are black start GE-design heavy duty gas turbines (turbines which can be started without AC from an auxiliary transformer (usually powered by a grid connection) which excite the generator at approximately 50% of rated speed and have two or three circuits directly connected to a small transformer that is connected to the generator terminals before the generator breaker. These circuits do things like power a single cooling water pump and one or two cooling water fans. Of course, the frequency is directly proportional to rated speed (60% of rated speed = 60% of rated frequency; 90% of rated speed = 90% of rated frequency; and so on) so these devices don't operate at rated speed until the unit is at rated speed but they do provide some cooling water circulation as the unit is accelerated and some cooling air movement in the cooling water module.

Now, if the generator breaker were capable of dead bus closure then at rated speed the generator breaker could be closed on to a dead bus and energize the bus (which may include an auxiliary transformer to power the station "hotel" load (lights; motors; etc.). But, most GE dead bus closure schemes don't allow breaker closure much below 95% speed, and if they did the frequency would still be proportional to speed (if the speed is less than rated, the frequency will be less than rated).

But, without closing <b>any</b> breaker? Not on any GE-design heavy duty gas turbine I've ever seen.

And, if there were loads connected to the generator before the generator breaker and the generator is exciter at 95% speed then that would mean the turbine would experience some speed decrease due to the load and if the speed were to drop below approximately 94% speed (14HS drop-out) then the unit would automatically go into a normal fired shutdown.

I would have to guess that if there were such a machine that it would have to go on speed control before the generator were excited. And, no one really wants frequency to be less than rated, anyway (motors draw more current at low frequency and things just don't run correctly). So, why?

Now, once a GE-design heavy duty gas turbine reaches rated speed and goes on speed control (FSRN) the operator can lower the speed down 98% of rated (or whatever the low-speed stop is set for; sometimes it's 97% of rated), but why would anyone want to run at less than rated frequency?

Best not to give a lot of credence to everything you hear or are told--without a lot of details, which in cases like this are usually very, very slim or non-existent.

It just doesn't stand to reason why anyone would want to operate at less than rated frequency, especially with a small load.

Hope this helps!

 

Kind of strange question. How are you going to "load the generator" if the breaker is open? Why would you want to run a generator at 95% speed. Your question is "can I have power output", how are going to do this when you are not connected to anything?

Can you explain a bit more exactly what you are trying to do?

 

Thank you for quick response and detailed answer,

Further to the continuation of the question, I might have given more importance to speed of turbine rather non-existence of breaker it self, which is the exact case.

In other words, if say we have reached the FSNL turbine would be on speed control. Now without any breaker to close (we have considered no-breaker) can I start my auxiliaries(or any load) and loading the GT. Since FSRN is already controlling the machine can we proceed for further loading? That is the exact case that I wanted to understand. Can proceed like this?

Is there any constraint from Generator side or Turbine side?

I have also never seen this type of configuration. But technically want to understand the situation.

As you explained, I agree that if we start loading at 95%, and at higher rate then there is possibility of deceleration of turbine speed(Dropping of 14HS).

But if we proceed for slow loading or start loading at FSNL(When speed control is taking charge of FSR) then it is possible to load the machine?

I will definitely share some more details on this once I get.

Thanks & Regards,

 

Controls_guy,

Sorry, but your explanation is not clear.

Most non-black start GE-design heavy duty gas turbines get their auxiliary power (480 VAC for the MCCs, auxiliaries, lighting, air conditioning, etc.) from one--sometimes two--auxiliary power transformers. These transformers are powered by the grid--not by the turbine.

Most non-black start GE-design heavy duty gas turbines do not have the capability to power their own auxiliaries--or any kind of load--through some kind of transformer connected to the generator terminals upstream of the generator breaker. Ttypically generator terminal voltage is 11KV or 13.8 KV or similar; auxiliary loads are usually 480 VAC or less so a transformer of some sort would be necessary.

And, if the auxiliaries or loads were not connected to the generator terminals through some kind of breaker or even a switch of some sort then the frequency would be directly proportional to speed.

There must be a breaker or switch somewhere to connect--and disconnect in the event of some kind of fault (short; ground; etc.)--any load to be powered by the generator. Or else the frequency of the load will be proportional to speed.

Many times some generator rotors have enough residual magnetism to produce some generator terminal voltage before the exciter is turned on and/or the field is flashed. So, if loads (including auxiliaries) were directly connected to the generator they could be damaged by high current when the voltage and frequency were low.

Even the black start machines I described earlier have breakers with shut trip mechanisms so the breakers can be tripped by external factors (high vibration, etc.) in addition to excessive current (thermal trip). There must be some method of disconnecting the load (even if it's auxiliaries--cooling water pumps, compartment ventilation fans, etc.) from the generator in the event of some kinds of faults.

I have seen one plant that used grid AC through auxiliary transformers for auxiliaries during starting then switched to a transformer that was directly connected to the generator terminals--through a transfer switch--to power the auxiliaries (there was a split-second when the lights and motors were unpowered as the transfer switch opened the grid contacts and then closed the generator contacts to the transformer).

But, it's still not clear how you can power auxiliaries with the generator without some kind of switch or breaker and some kind of transformer.

Lastly, if the unit is <b>NOT</b> connected to a grid in parallel with other generators and their prime movers the Speedtronic would have to be in Isochronous speed control to maintain frequency as the unit was loaded and unloaded or the frequency would definitely not remain at rated without manual operator intervention.

 

> Kind of strange question. How are you going to "load the generator" if the breaker is open?

The generator output power through 11kV is taken directly, and feed directly to load. since this is isolated load and need not required to be connected to load. I am trying to understand its possiblities, requirements, and problem.

Regards,

 

"The generator output power through 11kV
is taken directly, and feed directly to
load. since this is isolated load and
need not required to be connected to
load. "

Are you saying that the generator is connected directly to load without any breaker or protection?? I find that a bit difficult to believe of not a bit dangerous!!