Can you describe your turbine-generator configuration in more detail? Most large electric utility turbine-generators that are connected to a grid have under frequency protection to protect the equipment.
As metengr says, most turbines are equipped with underfrequency protection. It is not uncommon for one of the shaft critical speeds to be fairly close to the lower safe operating speed. Operation in that speed range must be prevented to protect the machine.
Do you have any underfrequency protection on your machine?
For our US machines we have a 3 step under frequency protective device to protect the machine the to protect the grid; normal frequency is 60Hz, depending on time exposure we can go as low as 58.5 Hz before we would trip the unit. I agree with ScottyUK and the danger of operating the turbine at lower speeds. I would expect an increased risk of blade failures.
Turbine buckets (blades) will have several resonances. the resonance should be tuned so that synch frequency will be between the bucket resonances. For example an L-0 might be running between its 5th and 6 resonance. The LP longer buckets will be more supspetacle to off frequency operation.
For all the units I commissioned 70s thru 80s, none had any off frequency protection. However, there was a warning that accumulated time at certain off frequency ranges might lead to failure. I recall greater than 1/2 hertz for 90 minutes and 1 1/2 hertzs for 15 minutes.
Some customers did install timers to record off frequency durations. In US, I rarely saw units operate 1/2 hertzs off (except for some large system disturbance for a few minutes). But when in the far and middle east in the 80s, the units would accumalate "possible failure operating time" and there were several units that throw L-0 and L-1s with less than a year operation. The fix was in tuning the buckets.
On the 'excitation system' view, you can use V/Hz limiters to provide either V/Hz trip, limiting, or both. V/Hz is a.k.a. over-fluxing. This limiter puts a ceiling for the maximum voltage allowed for a specific speed. Typical settings for hydro gen's are 1.10 [V/Hz], I would check the IEEE standards for steam gens. Example, 1.10 maximum terminal voltage for a 1.0 frequency (60Hz base). The limiter drops proportional with speed. Also note, the V/Hz limiter should be coordinated with max V/Hz allowed by the GSU (generator step up transformer).
Kudos to Byrdj for mentioning the blade resonance, that may be more of a concern since the turbine section of a unit should be protected. No different than operating a hydro turbine away from the rough zone.
A little extra clarification
Resonance, the tuning fork effect of a single bucket. Not to be confussed with the critical speed of the rotor.
If the frequency of rotational speed speed and/or the stimulus of passing steam from a partion section drives the bucket resonance mode frequency, high cyclic fatigue failure will occur.
The tuning of the buckets (and to some extent the diaghram partions) can be designed (or modified) to allow off frequency operation (also off pressure operation). The limits and deviations I quoted were for the popular US utility standard turbine code I work on back then. The OEM should provide the information for the unit and for the actual buckets installed.
Why would you want to operate your turbine at a reduced frequency? It is almost inconceivable that a turbine such as yours can tolerate off-synchronous operation and stay together.
Much smaller turbines, such as those used for powering auxiliary equipment (boiler feed pumps, combustion air fans, and smaller loads) are almost always designed to operate over very wide speed and load ranges.
Most generators supplying into utility grids are contractually obliged to provide some degree of off-frequency capability. The last thing the grid operator needs when the grid is heavily loaded and frequency is dropping, such as would happen if a major interconnector or a large generating site tripped, is for the remaining generating stations to trip on under-frequency. For gas turbine prime movers this can be a tough requirement because the turbine output inherently falls as the frequency drops. The grid operator will typically require that power output is maintained as frequency falls, down to a certain frequency. In the UK this is governed by the Grid Code.
The UK's requirements are (crudely):
Full output maintained down to 49.5 Hz
Linear derating from 100% output at 49.5 Hz down to 95% of output at 47.0 Hz
A CCGT plant is not capable of maintaining operation under these conditions indefinitely because the over-firing of the engine typically required will cause accelerated damage to the hot parts. Months of service life can be consumed in minutes under high levels of over-firing.
CC 6.3.3 (Page 143) of the following document gives the chapter & verse. Most utilities will have a similar document.
Sorry that my response was misleading. I am very much aware of the need to carry substantial load at slightly less than synchronous frequency under system load stress situations. The needs and practices for US systems are substantially identical to yours except for the 60 hz vs. 50 hz issue. I interpreted cstron's question as asking about carrying substantial loading at much below the designed synchronous frequency. It is in that context that I made the comments above. Your comments should be helpful in clarifying the situation for those not familiar with the perspective of utility power generation and utility power grids.
