Diesel Generator in parallel with steam generator

[NickParker]

We have three diesel generators (3.5 MW) that need to operate in parallel with a 1.5 MW steam turbine generator (STG) on a tanker. Since the STG manufacturer already has the unit ready off the shelf, and management has decided to use it (long lead item). I understand that since the STG has already been manufactured, its electrical characteristics, such as reactance and short-circuit data, are fixed and cannot be modified to suit our specific needs.
I would like to assess its performance when operating in parallel with the diesel generators.
Are there any additional concerns we should address? I would appreciate your input.

 

[waross]

Your biggest issue may be governor response time, Nick.
Specifically the time to respond to a step change in load.
I had a large mill on diesels.
The sets were similar but one set had an electronic governor and the other set had a hydraulic governor.
We had a large motor that needed both sets online to start.
First thing in the morning the electronic governor advanced the fuel rack faster and picked up too much of the load.
The electronic controlled set would then trip on overload, followed by the hydraulic controlled set tripping on overload.
The whole mill would then stop.
We found that if we ran the sets for 30 minutes or more, the oil would warm up and the hydraulic governor would operate fast enough to avoid tripping the electronic controlled set.
Many generators do not output a true sine wave.
Others will be more familiar with the specifics but there may be slight circulating currents between two or more sets if the winding patterns are different.
I understand that this is more evident at no load or light load and may be swamped out by heavier loads.
Running a co-generator in parallel with the grid presents a much bigger capacity difference than you anticipate.
I will be surprised if you encounter any problems that cannot be addressed by tweaking control and protection settings.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[TugboatEng]

Onboard ship, I have found that the STGs are typically driving generators through big reduction ratios (5:1 to 7:1). This gives them substantial inertia for maintaining frequency. That would me me want to use the STG in a mode that puts it in charge of maintaining frequency (isochronous). However, wear on the throttle valve may make this the less economical option. For steam turbine propulsion engines it's standard practice to turn off the governor once at sea to reduce wear on the throttle valves.

 

[waross]

I would run everything in 3% droop.
We ran a small diesel plant serving about 5000 subscribers with one to three generators in parallel depending on the load.
All in droop.
They shared the load well.
This may be implemented with the governors as supplied.
At 3% droop, the frequency will vary from 61.8 Hz at no load to 60 Hz at full load.
No-one ever noticed.
Note: Motors would rather have over-frequency than under frequency is why the range is chosen as 60 Hz to 61.8 Hz, rather than splitting the difference and running 59.1 Hz to 60.9 Hz.
If you want to favour one set to carry the base load, then consider a load control panel.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[LittleInch]

That's quite a large variance in capacity between these different units.

What's the thinking here for operating / load requirements?

Steam unit is base load / minimum load and the 9MW of diesel used for occasional duty or what?

A load control panel would seem to be a must.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[NickParker]

Steam unit is base load / minimum load and the 9MW of diesel used for occasional duty or what?

Steam is generated from the boiler using the dual fuel (mgo & burning Voc (volatile organic compounds) when the cargo is loaded into the tanker. Depending on the heat value of the VOC gases generated from the cargo loading operation, excess steam shall be used to drive a steam turbine generator(STG).

The other 3 main diesel generators provides power to the ship while sailing, loading and offloading.

 

[waross]

With the units in droop, set the governor slightly higher on the STG.
The operators may trim the governor speed settings to adjust the load sharing.
This is not as exact or precise as a load control panel, but it will often serve and is simpler and cheaper.
It sounds as if there will be manual interaction in any event.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[NickParker]

A load control panel would seem to be a must.

Can you elaborate it. Why is is a must?

 

[TugboatEng]

You'll definitely want a central panel with your speed and AVR controls, volt meters, ammeters, synchroscope, and circuit breaker controls. This is the minimum for load control.

Droop and mode can be set locally.

However, there are automatic load sharing systems that combine all of this in a module and are often less expensive to construct than manual type systems.

Don't forget your reverse power relays.

 

[NickParker]

You'll definitely want a central panel with your speed and AVR controls,

Are you referring to having a separate control panel for each generator, along with a master control panel for load sharing?