New Marine Diesel Electric Hybrid Systems, Potential Problems? 4

[ee2sea]

While I posted this in the Marine/Ocean Engineering Forum for general comments, I would most appreciate any comments of specific electrical difficulties with these new hybrid type diesel electric systems being proposed by both Siemens and ABB. See here:

http://www.eng-tips.com/viewthread.cfm?qid=301280&page=1

The whole key to the fuel savings is primarily variable speed operation of the diesels and thus AC gens. Does that imply that the DC bus would have to be a variable DC value or could it still be fixed? With respect to transmission problems such as harmonics, would the "front-end" portion attached to these variable speed generators likely have to operate as an AFE?

I believe wind power has used some elements of this? Does this system exist at all in other hybrid vehicles? Other examples in process industries with a DC bus usually mean a DC bus derived from a fixed frequency, fixed-voltage source, ie a utility. Assume that is a totally different beast. Would the stored energy of this larger "vehicle" system require (lithium-based) batteries or would ultra-capacitors still be feasible?

The whole idea is very exciting to me but would appreciate any ideas on the electrical difficulties it may run into.

 

[Drivesrock]

Hi ee2sea.

To me these solutions are making a vessel wide DC bus of LV inverters. I would have thought there would still have to be some kind of a distribution board with protection and load isolation devices, bus-tie contactor, perhaps pre-charge circuits for DC link capacitors when connecting dc buses and inverters, the diesel engine governor controllers and manual control and monitoring equipment.
Something the ac distribution board already does in part and therefore not eliminated – space or efficiency wise. And then the active rectifier, batteries or ultra-capacitors require space and weight considerations too.

I believe the front end “portion” would indeed be an active front end and the generator could supply variable voltage and frequency that the rectifier would synchronize to. No issues here and as you mention, consolidated wind power technology uses that. This would allow for a fixed or variable DC link voltage as may be required but I think fixed voltage (or within a 20% range for some optimization) is probably more likely required for the inverters for the propulsion motors. Fixed dc voltage perhaps for the auxiliary load inverters.
Active rectifiers with their voltage boost and low harmonics benefits are not new and companies such as Converteam are already supplying these for the propulsion drives in offshore support vessels thus eliminating the need for 12 pulse transformers with subsequent gains in efficiency, weight, space and cost.

With regards to being able to run the diesels engines at their optimum efficiency this will depend on the load cycle of the vessel. Perhaps a PSV will only be in DP mode for 25% of the time when the load will be reduced and engine speed could be optimized? Steaming to/from location, when the higher loads are required of the diesel engines anyway, could represent 40% of the cycle and the rest is standby in the field or port. So I don’t see much room for (the) great fuel savings over the ac electrical solutions already employed. The biggest savings are/were in the move from a diesel mechanical propulsion system to diesel electric.

As with many things the proposed solutions will be more or less suited to one application or another. This could well be more applicable to ferries with well defined work-cycles.

Single-point failures will be of specific concern.
I would have thought that the dc supply routed around the vessel should employ twisted cables to eliminate increased inductance. And these cables may need special routing and/or installation.
Generators and their controls may need to be special and perhaps the choice of manufacturer and maintenance supplier will be restricted.
Another concern would be the (future) availability of ultra-capacitors or Lithium batteries anywhere in the world where an OSV would find itself working. These are not off-the-shelf items with immediate delivery anywhere at reasonable cost. Any initial benefits could be quickly lost due to vessel downtime or inability to use a certain feature.

Perhaps the solutions presented are more ‘concepts’ than reality?

 

[rhatcher]

Drivesrock's comments on the ABB and Siemens systems are good. I will not add to them but I will give a star.

Referring to the WSDOT link:

http://www.wsdot.wa.gov/Ferries/Business/Contracts/Contracts.aspx?category=7&fiscalyear=&awarded

The RFI is interesting.

The 'Desired Operating Criteria' for the Hyak’s propulsion system include:
- Fuel consumption to be less than the existing system
- Lower exhaust emissions: NOx, SOx, CO2, PM Lower maintenance costs.
- Upgrade to modernize Hyak’s propulsion system for the next 25 years.
- Existing 16cyl EMD 645 diesel prime movers to be reused

It is not clear to me how the first three criteria can be achieved if the fourth criteria specifies using the same engines. Perhaps the use of cleaner fuels and more efficient fuel control could allow the existing engines to run cleaner and more efficiently.

If you are going to keep the diesel engines, then why not keep the DC generators, the DC bus, and the DC motors? AC technology is not inherently more efficient than DC technology. The only advantage to switching from DC to AC would be elimination of the brushes from maintenance concerns.

The disadvantage to the AC design versus the existing DC system for this upgrade is that an AC system will require new generators and new motors at a great cost compared to keeping part or all of the existing components.

The AC hybrid design does require rectifiers and inverters but, the DC hybrid design will require chopper circuits to regulate the flow of DC power between the batteries and motors (or generators) when depleting (or charging) the batteries.

Finally, the main point of inefficiency for the existing design is the size of the four engines versus the operating points for this duty cycle, not the system voltage of DC versus AC.

If you are required to make a new AC design based on keeping all four of the existing engines, then you will be making a compromise on the new design. There will be no advantage over modification of the existing DC system (except the brushes).

Existing System
The RFI gives the vessel's duty cycle as running at 60% propulsion power for 20-50 minutes while crossing and at 10% power for 20-30 minutes while at the dock.

According to the RFI, the disadvantage for this current system is that all four generators must run all of the time. This is because of the safety requirement to have one 'extra' generator on line at all times. Clearly they are doing something wrong at the dock. This will become apparent in a moment.

Notice that propulsion system shown for the MV Kaleetan, a sister ship to the MV Hyak, is similar to the US design for WWII diesel electric submarine propulsion systems. Although it is not stated on the drawing, the rated armature voltage for the propulsion motors is twice the rated armature voltage for the generators.

As shown in the motor-generator loop diagram (see attachment) , any one (or more) generator(s) can be connected in parallel to the propulsion motors using the 'half power' contacts. I think the correct name should be 'half speed' contacts. This is because full voltage from the generator(s) connected with these contacts provides half voltage to the propulsion motor for up to 'half speed' operation. However, for the sake of clarity, I will stick to the contact names as given on the drawing.

For operation above half speed, at least two generators must be connected in series using the 'full power' contacts. For high speed operation, all four generators would be connected using the 'full power' contacts. In this case one pair of series connected generators is connected in parallel with the second pair of series connected generators.

The normal operating profile would be:

0-50% speed = 0-25% propulsion power = one generator/low power contacts.
50-70% speed = 25-50% propulsion power = two generators/high power contacts
70-100% speed = 50%-100% propulsion power = four generators/high power contacts

Note that 60% propulsion power = 78% speed.

When you add the requirement for providing additional generators on line for safety considerations:

0-50% speed = 2 generators/low power contacts (Very inefficient at the 10% power operating point)
50-100% speed = four generators/high power contacts (Essentially the same as above for the 78% speed, 60% power operating point. Note that the loss of any one generator means losing a series generator pair; power is reduced to 50% and speed is reduced to 70%)

Although the vessel could improve efficiency by operating in the proper power mode when at dock (2 gens instead of 4), greater efficiency gains could be realized with a hybrid system.

Hybrid System
Suppose that you remove one generator and replace it with a pair of battery banks. Size the battery banks so that each one has voltage equal to one generator and that the pair of banks has amp/hour capacity to provide 60% propulsion power for 20 minutes when connected in the series, 'high power', operating mode.

Install the battery banks in a pair using 'low power' and 'high power' contacts to allow the battery banks to be connected in parallel or in series in the same manner as a generator pair.

Install an additional pair of contacts for each of the remaining three generators so that they can be cross connected such that any two generators can form a series pair for 'high power' operating mode. Keep the existing 'low power' contacts for parallel, 'low power' operation.

Consider the duty cycle of 60% propulsion power for 20-50 minutes when crossing and 10% power for 20-30 minutes while docked.

For crossing, two generators would be connected in series operating mode using the 'full power' contacts and running at full engine power during the crossing time to produce 50% of propulsion power. The additional 10% of propulsion power required to reach 78% speed would be drawn from the battery banks that are connected to the DC bus using the 'full power' contacts (series connected).

At the dock, the two generators would operate in parallel and at full load using the 'low power' contacts to provide 10% propulsion power for dock operations and using the remaining power to quickly charge the battery banks. Once enough of the charge cycle is complete (a very short time), one generator would be dropped and you would run on a single generator for the remainder of the dock period. For this operating mode, the battery banks would be parallel connected to the DC bus using the 'low power' contacts.

For the concern of safety redundancy, the redundancy is in the batteries and the third generator. For failure of one generator during the crossing, the batteries allow sufficient power to operate the vessel at 60% propulsion power long enough for the bad generator to be switched out of the series generator pair and the third generator to be started and switched back in.

Redundancy at the dock is less of a concern since you will always have at least one generator and two battery banks connected to the DC bus and are operating at a very small load.

Obviously, this hybrid design using DC generators and motors requires some form of additional electronics to regulate the flow of power between the batteries, the motors, and the generators when charging or depleting the batteries. However, this is relatively simple technology that can be obtained for no greater cost or size than the equivalent rectifiers and inverters as required for the AC system.

 

[rhatcher]

Wow, sorry for the long post. I have some experience in diesel electric propulsion and, I also find the hybrid technology exciting. I guess that I got carried away.

 

[ee2sea]

Drivesrock,

I truly appreciate the very helpful reply. Yes, the protection scheme does present some, at least, regulatory concerns. Interestingly, Siemens keeps their entire DC bus inside the enclosure lineup shown in the Blue Drive Plus C architecture for Ostensjo Rederi's OSV vessel. ABB is pushing the envelope with an actual DC transmission bus. They have detailed this a bit in their new website

http://www.dc-grid.com.

I find their concerns about DC breakers a bit perplexing as I believe many DC breakers (Hawker Siddely, Secheron, etc) would do splendidly as bus-tie breakers of sorts, separating the DC system into two halves. A variable DC bus of limited range is interesting.

The 20% fuel savings may be overstated by ABB minus the most extreme of load profiles. Still, I believe that 5-10% fuel savings could be found for most vessels with any significant variance in load profile. The elimination of a full scale AC switchboard and especially 12-pulse phase shifting transformers is just icing on the cake.

I think the battery or capacitor banks would have to be large enough of multiple modules that one could have small inventory of each module on board as spares eliminating concern of availability? A small bank could easily be composed of 100+ modules such that a spare stock of 5 would not be of much concern.

Thank you, rhatcher, for the very detailed response. I am not as concerned about existing installations where perhaps the older DC systems could still be reused. I am more focused on new builds. Would doubt any new vessels would be constructed with either DC generators or motors but maybe I could be wrong?

Cheers

 

[rhatcher]

ee2sea,

The difficulty with DC breakers is with interrupting fault current. For an AC breaker, the fault is always interrupted at the zero crossing of the current flow in the AC cycle.

For a DC breaker, the fault must be interrupted at the full fault current. This is much more difficult and requires breakers that are much larger than the AC equivalent ratings.

Otherwise, I would say that in order to understand the principles of diesel-electric propulsion that it is necessary to understand the design philosophy and the progress in existing technology.

The system that I discussed, although based on 70 year old DC technology, is not very different from the existing AC design criteria. The idea is that multiple, smaller generators can more efficiently handle various load conditions, especially alternate heavy/light duty cycles, than a single large generator.

We have been at that point in technology for 70 years with various DC (parallel/series) designs followed by the modern standard of multiple small AC generators connected in parallel as needed.

In the example that I discusssed, we can see that there could be some improvement in efficiency by simply improving the existing operating practices to better suit the orignal equipment design operating points.

Even with the improved practices, there is a mismatch between the desired operating points and the designed operating points such that there is significant idle generating capacity during parts of the duty cycle.

The basic idea for a hybrid design is that the idle capacity that is present during one part of the duty cycle can be efficiently utilized to generate power that is stored (batteries, capacitors, flywheels, etc..) for use in another part of the cycle.

This philosophy, as demonstrated on the DC system, is the same that would be applied for a more modern AC system.

Take another look at the ABB and Siemens proposals and decide for yourself what is the difference in approach besides the difference in AC or DC.

 

[astronut01]

Ya dc gens are fine unless you watcj this. Triple digit arc #s are why they deep sixed $5 mil of them. Teco must be laughing all the way to the bank

http://www.king5.com/news/investiga...y-generators-that-cant-be-used-100081499.html

 

[rhatcher]

Astronut01,

In my opinion, the reason that they were forced to abandon this project is poor and/or incomplete engineering design practices, not the choice of DC power over AC power.

Specifically, they purchased the larger, multi-million dollar generators without also considering everything else that must be purchased for the necessary changes required when the generators are changed. At a minimum, this includes a new switchboard and cables as mentioned in the article.

In my opinion, it is easy to speculate that the problem may extend beyond the requirement to purchase a new switchboard and cables.

Think about it. They are proposing that buying an entirely new system (new generators, switchboards, and cables) would be more economical than installing the already purchased generators and buying a new switchboard and cables to go with it. (?)

Also, consider that the request for new proposal specifies that DC distribution of propulsion power will be "strongly considered." This makes me think that DC power is not the problem.

Perhaps the larger engines cannot fit in the existing engine room (height would be the limiting factor). Even if the new engines do fit, perhaps the amount of cutting to the hull and bulkeads required to rig the new engines into place is too costly. Maybe this is the reason for the design criteria that I questioned in my first post which states that the new design must reuse the existing engines. (?)

Another consideration is that two large engine/generator sets are a poor chioce in terms of efficiency for this load profile of 60% propulsion power for 1/2 time and 10% propulsion power for 1/2 time. This is especially true when you add the requirement that one 'extra' generator must be running at all times. This means that both large generators must run 100% of the time, even during the 50% of the load cycle time that only 10% power is required. The four smaller generators would be much more efficent for this load cycle if they were operated properly (see my post from 11 july).

In my opinion, the choice of DC power is not the problem here. The problem is the poor engineering design practice of failing to think the problem through to completion before beginning the project and before spending milions of taxpayer dollars.

This is just my opinion based on the facts that are available to me; it is up to you to decide what it is worth.