Synchronize Diesel & Gas Generating sets 3

[PerkinsEngine]

Dear Engineers,

Has any gentlemen experienced a synchronization scheme for two generating sets of about 1000KW ratings, one is powered by a diesel engine and the other with a gas engine that utilizes the methane gas issued from some recycling facilities.

I guess that I can get a synchronizer/loadsharer that can work or be configured as gas or diesel, but I am wondering about the performance of both engines if they can work together or not.

And what is the best scheme to be applied, shall I use isochronous load sharing or droop as long as there could be some stipulations on the system due to some restriction on the power delivered by the gas one-since this depends on the methane fuel issuing from the plant.

Knowing that the two alternators are of same pitch, the engines are from diffrent manufacturer and the system will work at low voltage.

Any input to this post will be highly appreciated.

 

[davidbeach]

Sounds like what you should do is have one controller on the gas engine that will run it to the maximum available fuel, presuming that does not exceed the available load, and then run the diesel in load following mode to pick up the remainder of the load. That assumes that you are islanded and that the load will never be less than what the gas engine can supply all by itself. If you are paralleled to the utility, what you need to do will depend on your ability to export power. If the load can be less than the gas engine alone, you need a strategy to back off the gas engine to keep the diesel running until the load is small enough that the diesel can be shut down. The diesel would then be restarted at a total load level such that the gas engine can absorb any load that can be added while the diesel is starting.

 

[waross]

The fact that one unit is diesel and one is gas should not make any difference whatsoever.
If you are islanded and if you can accept a small frequency droop you can use a wide droop on the gas set and a narrow droop on the diesel. The frequency settings would be such that at about 75% or 80% load on the gas set, a start command would be issued to the diesel set. When the diesel comes on line, it starts to share the increased load. When the load limiting point is reached on the gas set, the diesel set will pick up any increase in load.
Your total frequency droop will be more than the best that you can achieve with a single set with droop control.
The challenging aspect of this scheme is the actual physical settings on the mechanical governors.
If you have a tight budget, a forgiving environment and are a "Hands on" type of guy, you may consider this. If you have an adequate budget and the sets already have electronic governors, then re-read davidbeach's post.

 

[choi1167]

Please don't consider this as an expertly advice. This is just my thoughts and hope it make sense.

Definitely both gensets can work together. But you'll have to consider a lot of things. What's the application? Parallel to the utility or isolated? What is the characteristic of the load and how it varies(load vs time)? For economic considerations, the methane gas could be free or cost a lot less than the diesel. Also, diesel gensets cost/kwhr is best when operated near full load.

Paralleling with the utility is less problematic in terms of operation and control. You can simply base load the gensets while the utility takes the balance of the load. On isolated system, isochronous load sharing is very challenging since the response of the gas engine is very much slower than the diesel and critical to the systems stability. An option could be to run the gas genset at base load while the diesel takes the balance (variable loads). The base load on the gasgen also depends on the gas supply.

In whatever set up you might end up, it needs prior load management and systems study.

 

[PerkinsEngine]

Thanks for the valuable inputs.

I have to clarify the system is islanded and the engines are equipped with electronic governors.

However, where is the challange when applying the isochronous loadsharing. I think that the droop may be more stable, but what is the advantage for isochronous load sharing except fixing the frequency.

Another point that I am worried about is the performance during step loads, I am afraid if the slow responce of gas engine will lead to system instability .

Thanks in advance for your comments & advices

 

[davidbeach]

Go back and re-read my first post. If you are in a position where the gas engine can always run at a fixed load, varied only in regards to fuel availability, run it at a fixed output and let the diesel absorb all of the load variability. The gas engine set will just be a negative load as far as the diesel is concerned. No attempt at load sharing, let the gas engine just ignore the load and have the diesel follow the net load.

 

[waross]

My experience is mainly with hydraulic and mechanical governors. However, I suspect that the setting will be much easier with electronic governors. I am assuming that the gas engine is configured so that it can not supply more power than the generator rating. If it is not, I suggest that it be so configured.
These figures are for the sake of illustration. You may want to run a little faster or slower. You may be able to reduce the settings.
Set the Gas governor for a 3 Hz. Droop. No-load frequency 63 Hz. Full-load frequency 60 Hz.
Set the frequency of the diesel set to corespond to the point at which you want it to start picking up excess load.
For instance, a setting of 60.5 Hz would correspond to a loading of (63-60.5)/3=83%. With these settings, the diesel will start picking up the load when the gas set is 83% loaded. The amount of load on the diesel when the gas set is fully loaded will depend on the droop setting of the governor on the diesel set. With 3 Hz. droop, The diesel will be (60.5-60/3)=17% loaded when the gas set is 100% loaded.
Change the diesel governor droop to 1 Hz. and when the gas set is 100% loaded the diesel set will be (60.5-60)/1=50% loaded.
This will give you performance similar to davidbeaches suggestion. I expect that his configuration will give tighter frequency control than mine. By all means use it if your operating parameters allow.
With my configuration, you don't have to worry about a minimum base load. It will give predictable performance from zero load to full load. Check your documentation for the minimum recommended droop settings.
Step loads. This will probably be similar to an installation I saw with similar generators but one with a hydraulic governor and one with an electronic governor. With a step load the electronic governor would respond quicker and hog the load until the other governor could catch up, sometimes to the point of tripping the generator breaker. The problem was more pronounced first thing in the morning when the units were first started and the oil was cold. After the hydraulic oil had warmed up, they worked and played together much better.
In your situation I would expect that the slower response of the gas set will result in step loads being mainly carried by the diesel set until the gas set can catch up. The frequency drop will be mostly dependent on the droop setting of the diesel set. The frequency recovery will be mostly dependant on the combined recovery of both units.

The following suggestion is something that I would try very carefully. Comments are invited.
You may be able to set the gas set on autogenous at about 60.5 Hz. and set the diesel set at either droop or autogenous at 60 Hz. Then the gas set will take all the load up to 100% of it's maximum fuel setting. As the load increases further, the gas set will start to lose RPMs. at 60 Hz. the diesel set will start to contribute, and pickup any additional load.
If the gas set is fairly heavily loaded and is step loaded, you may find that the diesel set is taking some of the load until the autogenous feature has time to act. This may happen even if the total load including the step load is less than the maximum rated load of the gas set.

Another suggestion. If you have a surplus of bio gas have you considered dual fueling the diesel set?
yours

 

[byrdj]

Haven't read all reply's in detail, just wanted to add that for stablity, an isoschyronis governor setting should have a time constait and would not be istantanous. It would work like a dispatcher, for example, load would increase, frequency droop and governor pick up some load, dispatcher sees frequency has drooped, and pulses increase to bring frequency back over a couple minutes.

 

[davidbeach]

Tell us about your loads and your generator capacity. If you your minimum load is more than the gas engine's capacity to supply and the maximum load is less than the combined total, you can run the diesel in isochronous load following mode, and base load the gas. This should give reasonably good frequency control, set by the diesel, and quick response to load fluctuations. If your minimum load is low enough that you can't set a base load on the gas set below the minimum load and still carry the maximum load with that output of the gas in addition to the diesel, you will could do something like what waross suggests, or you could use a more sophisticated control system that adjusts the baseload point of the gas engine set (very slow response load follower) and let the diesel respond to the faster changes. You don't want to try to have each generator carry a fixed proportion of the load.

 

[rcw retired EE]

Our experience with landfill gas is that the variation in BTU content makes control a little more difficult. There is not a fixed relationship between throttle opening (governor output) signal and engine output like there is on a normal diesel or gas engine. This makes some of the droop controls more complicated.

I like David's idea of baseloading the gas engine and "filling in" with the diesel. The diesel can't tell if the load increased or the gas engine output dropped. Its response is the same. I think you will have to get them parralleled and see what tweaking needs to be done.

Good Luck!