Dear All.
I have a problem at hand which needs some answer. We are building an Ocean going vessel which is twin screw diesel propelled. Each Marine engine has a shaft generator installed coupled to its shaft. The question is why cannot we operate both of these shaft generators in parallel when both engines are running at a constant speed ? What are the potential problems if we want to them to be operated in parallel. In case it is not recommended, what could be the reasons why it is not recommended to operate them in Parallel.
If anyone has any inputs, please let me know at the earliest.
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Sincerely,
Suryanarayana,
Visakhapatnam,
India

 

The ship design firm needs to be involved in that descision.

 

First of all, who told you they couldn't be synchronized (paralleled)?

I'm presuming the two diesel generators would be synchronous AC (Alternating Current) generators (also known as alternators).

What would happen if you needed to slow down one engine while the two were paralled (synchronized)? (That's a rhetorical question, because when the two are synchronized (paralled) BOTH have to run at the same frequency--which is directly related to their speed. This means if you tried to slow one engine down (say for some kind of maneuvering) the other diesel generator would try to keep the slower one running at the same speed and the faster one. So, if you suddenly needed some kind of emergency maneuver it would be necessary to unsynchronize (un-parallel) the two machines to do so.)

When two (or more) synchronous generators are synchronized (paralleled) together they ALL will and must run at the same frequency. One machine can't run at 54.37 Hz, and another at 50.02 Hz. The laws of physics won't allow that if they are synchronized together (paralleled). Not on land. Not in the sky. Not on the water. Not under water. Nowhere on planet Earth. And you would need some kind of electrical load-sharing system (such as Isochronous Speed Control and Droop Speed Control) to even have the two diesel-generators operate in synchronism (parallel) together. And, if you want slow the ship down when both propellers are operating with their diesel-generators synchronized (paralleled) together you will need to have some kind of control system to make sure both diesels maintain the same speed--because speed and frequency are directly related at all times when two (or more) machines are synchronized.

This doesn't seem like a very good idea (operating the two diesel-generators being used for main propulsion engines) in parallel (synchronism).

 

Stadimet
Hopefully this isn't the same Cruise Ship we discussed in 2018?

Does the Cruise ship you are talking about now have very large, say greater than say 20-15 MW, 4.16 to 11kV, Diesel powered generators or is it Azipod powered?
Regards, Phil Corso

 

Stadimet
Hopefully this isn't the same Cruise Ship we discussed in 2018?

Does the Cruise ship you are talking about now have very large, say greater than say 20-15 MW, 4.16 to 11kV, Diesel powered generators or is it Azipod powered?
Regards, Phil Corso

Phil, this is not the same cruise ship that was under discussion earlier with you. This is new project. We are evaluating on how many generators , their capacities and the operating philosophies during which time, this issue has come up. I am aware that both Engines have to be at the same speed to ensure that both generators if needed to operate in parallel, be at the same frequency. We are about to decide on the configuration and am in complete agreement with WTF. Just thought of bringing up the discussion just in case I am not aware of some subtle issue to be taken care of.
Thanks a lot for the inputs.
The generator ratings in question are in the ball park of 1.7 to 1.8 MW each while the shaft generators in the 1 MW range all operating at 415V AC, 3Ph all diesel powered.
Will come back requesting you for further help.
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Regards,
Surya

 

I think WTF covered it well. Synchronized generators WILL be at the same speed....period. Since the vast majority of the torque produced by the diesel is being used to turn the prop, I would expect there to be big problems trying to operate those generators in parallel. If your intent is to have them provide power to the whole ship, they would need to be sized accordingly.

Also, the best speed for a generator is very different from the best speed for a ship's propeller. I can definitely see a use case for a shaft-powered generator for use on an ocean crossing or other scenario where the engine can run for a long time at a fixed speed, but there will definitely need to be other generators ready to take the load in case the main needs to stop (or even just change speed) on short notice. Unless you plan to also use large rectifiers/inverters that can take a wide range of input voltage & frequency and output a good clean AC signal at fixed voltage & frequency. You just made your power plant a lot more complex, though, and I don't know what might be available in that world at that scale.

 

Ok here a small variation from the original question. If it is not twin screw - but single screw and both generators are connected to the gearbox on the other side, which on the one end is connected to both Main Diesel Engines, then it should not be a problem to operate them in parallel at a range of speeds. isn't it. ? Here both engines are connected to single gear box. The same gearbox is used to connect to the shafts of the Shaft AC Synchronous generators - two of them.
Now there are other generators to take of regular loads. The shaft generators are only for redundancy.
What do you think of the parallel operation and any consequences to be thought of.? Please let me know.
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Sincerely,
Surya

--snipped--
Also, the best speed for a generator is very different from the best speed for a ship's propeller. I can definitely see a use case for a shaft-powered generator for use on an ocean crossing or other scenario where the engine can run for a long time at a fixed speed, but there will definitely need to be other generators ready to take the load in case the main needs to stop (or even just change speed) on short notice. Unless you plan to also use large rectifiers/inverters that can take a wide range of input voltage & frequency and output a good clean AC signal at fixed voltage & frequency. You just made your power plant a lot more complex, though, and I don't know what might be available in that world at that scale.

 

What is your ship's electrical system's required frequency? Frequency is directly related to speed. If your shaft changes speed, so will the output frequency. Will you put a rectifier/inverter between the shaft generator(s) and the ship's electrical system? Unless you have a variable gearbox (like a car's automatic transmission but fancier) that can maintain constant generator speed across all shaft speeds, you can only use the shaft generator at a particular shaft speed and have it still provide power to the ship. Unless, again, you have some sort of power conditioning system. Your ship's electrical system will need to be at a fixed frequency. so you need some method to make sure your generator output is at that frequency.

 

We will operate the shaft generator only in a minor speed range and will not allow the generators to operate beyond that range. The generators are connected to the switchboard each with its own panel and they are not operated in parallel with each feeding its section of the Bus. The frequency is 50 Hz. It is a Ring mains system with one side of the ring connected to one shaft generator while another connected the second side of the ring. The ring is disconnected and is split into two sections.

 

Just a question on my part. Wouldnt you have to be operated them independently in order to sync them together or is it possible to build the two generators so they would always be synchronized. The rotors would have to be built in the exact same position so the generated sine wave would always be identical as far as phase position. If not, you'd have to be able to control one independently to match the other in order to tie them together.

 

Oh I see now you said they wouldnt be operated in parallel. Never mind.

 

stadimet,

THEY say: A schematic is worth a thousand words. (In some parts of the world, a simplified schematic for a three-phase system is called a one-line, or single-line, diagram.)

And it's true.

It's really, really true.

 

Your shafts have to be running at a steady speed the whole time they're in use. You can't change that speed....ever....without redesigning the generator/gearbox/inverter/etc.

Is that your planned use case? To use the shaft generators while underway at normal cruising speed? ANY variation in shaft speed will show up as a variation in output frequency, which EVERY load on the bus will see. Are the generators designed to run at shaft speed? If not, and you have a gearbox driving them, then any variation in shaft speed will be multiplied by the gearbox and have a bigger impact on your output frequency.

Is this something like what you're intending?

 

Joseph_e2, you are right in your schematic. That is how they are going to be operated with a Bus tie at the bottom to separate them from being in parallel. Also it is correct, that the shaft speed is constant through a gearbox and within a narrow band when these generators are operating. Shaft generators are proposed to be used in normal cruising mode with just the ships load.

 

@stadimet
If the propeller-shafts are driven by diesel-engines, then it very unlikely that any other generator connected to those diesel-engines will ever operate at the 50/60-Hz that utility generators must operate at for normal electrical loads!
I think you mean that the gearing is such that when operating at a fixed speed for very long time, as suggested by Joseph_e2, there should be a Propulsion-Bus and a Utility-Bus for 50/60-Hz Generators. And the Utility-Bus should be transferable to the Propulsion-Bus... The reason is the larger Propulsion Generators are more efficient!
You should contact a company like Wartsila, or Cummins.
Regards, Phil Corso

 

I've been out of the maritime world for a long time (woah, 20 years now), so forgive my ignorance. Is there a good reason to not make the propulsion diesel-electric? I know cruise ships at least are heading in that direction. The props are driven by electric motors.

Another thought... If you're committed to diesel main engines driving the props and generators via gearboxes, you need to make sure enough of your other generators are on hot standby so they can start automatically and quickly in case of casualty. If one of your engines stops (water in the fuel, debris in the prop, a jillion other things that can go wrong at sea), you will lose that generator and the loads powered by its bus. And you'll most likely have to throttle back the other main engine very quickly to maintain stability, which means that the other bus will also lose power. I know you're aware that it's not like a shoreside facility that loses power. At sea, during rough weather especially, lives are at stake just from a blackout.

 

It's not recommended to run these two shaft generators at the same time due to several reasons. These include:

1. Synchronization Issues
2. Load Sharing Problems
3. Reactive Power Imbalance
4. Voltage and Frequency Regulation
5. Control Complexity

 

Stadimet
For very large Bulk Cargo ships, the generators are driven by extremely large Diesel-Engines. They are much larger than any used on even the the largest Cruise Ship!
Regards, Phil Corso

 

Operating two shaft generators in parallel on a twin-screw diesel-propelled vessel can be a complex task. While it might seem efficient to harness power from both engines simultaneously, there are several potential issues to consider. Firstly, ensuring precise synchronisation between the generators is crucial to prevent any phase mismatches or frequency variations, which could lead to unstable power output. Additionally, if there are slight differences in the load distribution between the engines, it could result in an imbalance in power generation, potentially overloading one generator while underutilizing the other. Moreover, if the generators aren't designed for parallel operation, they may lack the necessary control systems and protections, increasing the risk of damage or malfunction. It's imperative to consult with the manufacturer or a marine engineering expert to evaluate the compatibility and feasibility of running these generators in parallel. They can provide specific insights into the design and capabilities of the equipment, ensuring safe and efficient operation for your ocean-going vessel.