GE Generator Hydrogen Pressure Low During Low Load

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Thread Starter

niazbibiyana

In our unit, GE generator (Model-324), 331000 MVA, Hydrogen cooled Generator, normally when runs at base load Hydrozen pressure is maintained around 4.25 bar. But at preselected load less than 200MW, the pressure gets reduced to 4.10 Bar. Power factor always .8 is maintained. But when again at base load, Hydrogen pressure again increased to 4.25 bar. I read all manual and try to find out appropriate cause. Can Anyone kindly explain about this phenomenon? I am so much confused about it.

Thanx in advance

Another thing, in Hydrogen control module, the auto hydrogen filling line is kept shut off due to some technical problem in Hydrogen station. We always fill the hydrogen through Bypass line.
 
niazbibiyana,

This is a very odd "problem" because you say the pressure is "maintained" at 4.24 barg at Base Load, and it is maintained at 4.10 barg when at Pre-Selected Load. The implication is that some regulating valve or control is performing this pressure control in response to changes in load.

The you write:

>Another thing, in Hydrogen control module, the auto hydrogen
>filling line is kept shut off due to some technical problem
>in Hydrogen station. We always fill the hydrogen through
>Bypass line.

So, are you really saying that the unit instruction manuals tell the operators to maintain 4.25 barg at Base Load and 4.10 barg at Part Load, and that the operators are manually controlling the pressure to achieve this due to some issue with the hydrogen supply system? Because, personally, I have never heard of such a requirement or recommendation. About the only thing I can think of that the generator requires more cooling at Base Load and so the pressure must be increased slightly, but the percentage change is so small as to be almost negligible.

In the past I've had great difficulty trying to explain why when the generator casing pressure is exactly what it should be at zero speed and then the unit is started and loaded to Base Load why the generator casing pressure increases above nominal. (I'm speaking of machines which are cycled on and off daily, or in peaking service.) I close the manual isolation valve to prevent hydrogen from being added to the machine when the machine is at rest and the casing pressure is at rated, and then have the operations (and maintenance) staff monitor casing pressure and generator temperature as the unit is started and loaded to Base Load. (Scavenging remains in normal service, which means that casing pressure will drop, but by a very small amount, due to the loss of air/hydrogen through the scavenging system.)

The generator casing (hydrogen) pressure increases--and I attribute this to the generator temperature increase. The Ideal Gas Law (PV = nRT) states that when the volume of a gas remains constant and the temperature is increased--the pressure increases. And because the generator temperature increases as current flows in the stator windings and the field windings but the casing volume remains the same, that the casing (hydrogen) pressure increases.

Then, as a further demonstration, when the generator casing temperature stabilizes at Base Load (after a few hours of operation), the casing pressure is reduced to rated (by venting some hydrogen) and with the supply isolation valve still closed (and scavenging in operation), and then the staff are instructed to monitor casing pressure and generator temperature as the unit is unloaded and coasts down to cooldown (turning gear; slow roll; ratchet). After a couple of hours, the generator casing pressure decreases to below rated/nominal (more than could be attributed to scavenging), and, again, I attribute this pressure decrease to the generator temperature decrease. A few operations and maintenance personnel understand this; most think it's a flaw with the system design or configuration.

Presuming automatic pressure control (which usually only increases casing (hydrogen) pressure but can't decrease it) is not in service, and operations or maintenance personnel "top off" casing (hydrogen) pressure when the unit is operating at Base Load and then casing pressure decreases when the load is reduced to 210 MW without any manual intervention or action--the only thing I can attribute the change to is that the generator temperature is also probably decreasing because currents have decreased and per the Ideal Gas Law the pressure will therefore decrease.

It's like leaving a futbol outside in direct sunlight all day and all night. If one adjusts the pressure in the ball when the ambient is high and the ball has been in direct sunlight for several hours and then checks the pressure at the coolest time of the day/night, it will be slightly less than it was when the ambient was high; and vice versa, if someone adjusts the pressure when the ambient is at it's coolest and then checks the pressure when the ball has been in direct sunlight for several hours and the ambient is at its highest the pressure will be higher. This is the Ideal Gas Law at work.

Having said all of the above, I'm still not clear about whether the pressure is changing "all by itself" after being set to nominal and load changes, or, if, since the hydrogen supply system automatic pressure regulator/control is out of service, if someone is changing pressure because of some instruction manual requirement or some plant operating procedure. 0.15 barg is a pretty small change (out of 4.25 barg--which seems to be the rated generator casing (hydrogen) pressure.?.?.?) to be concerned about (in my personal opinion).

Finally, you mentioned that at all times the power factor is maintained at 0.8 (lagging, I presume). If that's the case (and if so, why?) then there is a LOT of current flowing in the generator rotor windings--I'd estimate near maximum rated when the unit is at Base Load. I've seen plant operations personnel operate generators at nameplate power factor (0.80- or 0.85 lagging, typically) just because that's what the generator nameplate says. That's the capability of the machine, NOT how it has to be operated at all times. It's perfectly acceptable to operate the machine at 0.85, or 0.90, or 0.93 or o.99 (lagging) at any time it's producing power. In fact, the generator runs cooler (requires less cooling) and the real power output of the machine (watts; KW; MW) can be a higher (at Base Load) when the power factor is closer to unity (1.0). And, the machine (the turbine-generator) is also a little more efficient (fuel-wise versus real power) when the power factor is closer to unity--at any load. So, if you would tell us what the generator nameplate power factor rating is and you know why the unit is being operated at 0.80 power factor at most loads (it may be an operational requirement from the utility/grid the unit is synchronized to--but that would be a somewhat unusual requirement) it would be very interesting to know. (I don't really want to side-track this thread with this questions, but I am curious to understand how and why people operate their turbine-generators the way they do--especially when I encounter statements about power factor like this.)
 
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niazbibiyana

Dear CSA sir,

Now I am clear about your given opinion based on ideal gas law. Yes, from GE , they told us to maintain around 4.20 bar hydrogen pressure during low load. When machine continuous (around 24 hrs) runs on base load, I observe per 1-2 hours hydrogen pressure drops around .2 bar due to scavenging of hydrogen.

I am very very satisfied to your answer.

Your another curiosity about PF of our machine maintained, Actually, our power plant is situated on a remote zone. Another power distribution substation is very far way from our power plant. To maintain a desired voltage on long transmission line ( around 200KM), from power grid company, their requirement voltage to be maintained in the transmission line is near about 400KV . So, if we maintain power factor .8, the desired voltage can be maintained. that is why, we always maintain this power factor. Also, Auto grid control system is not available in Bangladesh grid. so, grid system is not proper stabilized.

Have a nice day. Thank you very much for your continuous comments.
 
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niazbibiyana

>Thanks for the information! It's very much appreciated.

Thank you very very much dear CSA sir...
 
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shradesh sharma

Dear Sir,

In our 300MW unit the rated hydrogen pressure for generator is 310KPa, but due to the leakages in system hydrogen pressure drops at 0.8KPa/hr rate. For this we have to fill the hydrogen cylinder.

I need to know what are the losses in generators occurs due to drop in hydrogen pressure? Although purity always maintain above 99%.
 
shradesh sharma,

It's called the Ideal Gas Law (PV=nRT). And the Ideal Gas Law basically says that if one increases the temperature of a volume of gas the pressure of the gas will also increase.

All other things being equal in a hydrogen-cooled generator at Base Load the generator is producing a lot of heat because of the current flowing in the both the stator and rotor windings. This heat causes the hydrogen pressure to increase. When the unit is unloaded, the amount of current flowing in the windings decreases, which means less heat is being produced which means the pressure decreases.

The generator casing hydrogen pressure regulator usually only increases casing pressure if it's low and never decreases pressure if it's high. So, if you fill the generator to rated hydrogen pressure at part load and then raise the load the pressure in the generator casing is going to increase above rated (because the hydrogen pressure regulator only raises pressure if the casing pressure is less than the regulator setting, and it doesn't lower pressure if it's higher than the regulator setting).

Generally, more pressure means more cooling is possible; generally. That's only true up to a point. Every hydrogen-cooled generator has a capability curve, which is really a set of curves on a graph, each line representing the limit of operation for a given hydrogen pressure and/or cold gas temperature (sometimes there is a capability curve for pressure and another for cold gas temperature). Consult the capability curve(s) for your generator for specific details about limits of operation for temperatures/pressures.

A lot of hydrogen-cooled generators that are cycled (started and stopped frequently, as much as two times per day) experience changes in generator casing pressure which are entirely related to changes in current flow and heat produced because of the current flow. Also, usually when a hydrogen-cooled generator is shut down the cooling water flow through the generator hydrogen coolers (hydrogen-to-water heat exchangers) is not reduced, so the temperature of the hydrogen gets pretty cool (cold), and this can make the generator casing pressure decrease, which causes the casing pressure regulator to increase the casing pressure which then causes the casing pressure to increase even more when the generator is started and loaded. It can be a very cyclic thing which can make some operators and supervisors very, very "nervous." But, it's just the Ideal Gas Law at work!

Hopefully there aren't large leaks in the generator casing.... That would mean there was hydrogen escaping through the leaks which means a fire/explosion hazard exists at the leaks!

Hope this helps!
 
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