New Standby Generator
New Standby Generator
(OP)
We are planning to replace one of our standby generators (4.5MW, diesel engine driven) and a consultant has proposed that we replace the single generator with two smaller manchines (2.5MW) which could be supplied as two portable units; then we can easily send them back to a manufacturers service center for maintenance. The single 4.5MW standby generator supplies a distribution bus with a variety of loads. Therefore, two smaller machines would have to be paralleled and share the load.
I'm not crazy about this idea because I don't believe two machines can be as reliable as one and I think the configuration and controls will be overly complicated. Does anyone have experience with a parallel standby diesel generator design?
I'm not crazy about this idea because I don't believe two machines can be as reliable as one and I think the configuration and controls will be overly complicated. Does anyone have experience with a parallel standby diesel generator design?






RE: New Standby Generator
RE: New Standby Generator
RE: New Standby Generator
RE: New Standby Generator
I also believe that there are enough options in terms of control systems that allow for multiple sets to operate in parallel without any problems. Woodward is the system I'm most familiar with, but I believe almost any manufacturer would allow this capability.
Note that I have no professional affiliation with any of the above mentioned companies.
RE: New Standby Generator
I would go for two units then have a third delivered in a year. Cycle thru them all on a regular basis. At Major time you can calmly schedule a ship away rebuild with no down time. If one breaks you can calmly schedule a ship away repair if required. If more power is desired in the operation you can increase one generator in the count. All the parts stay the same so spares are minimal.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: New Standby Generator
I like the idea of N+1, that is 3 x 2.5 MW sets. The reason is dependability, not repairability.
These sets typically run 15,000 hours between top end overhauls and 30,000 hours between complete top and bottom overhauls. With the newer onboard computer generated maintainance schedules the time may be extended for a lightly loaded set. It is a lot easier to transport workers than it is to transport megawatt gensets. It is quite possible that the majority of the hours on the set will be scheduled excercise.
In ten or fifteen or twenty years you may be replacing the set because of age and never will have done a 15,000 hour overhaul.
There are some good reasons to use multiple smaller sets rather than one large one, but the ability to drag the set to the service station for an oil change is not one of them.
respectfully
RE: New Standby Generator
The study included a lot of subjective analysis, especially about the cost of nuclear qualified systems. Some larger designs were already qualified but the smaller 2.5 MW units were not. Cost of qualifying testing wiped out any capital cost savings.
If this is for a non-nuclear system, I believe the multiple units will provide the same reliability at lower installed cost. It may be worth investigating.
RE: New Standby Generator
This way you must divide your installation in two, which is good if you can do it.
I propose 1x4.5 MVA + 2x 2.25 MVA if you want to have an installation that withstand any kind of problem:
If you lose the big one, the other two can enter in synchro. Probability of loss of sunchro when a generator is broken is << 1.
One of the smaller two can take the load when it is 80% of 2.25 MVA, therefore your running cost will be reduced.
RE: New Standby Generator
rcwilson - I'm interested in the Candu reliability study. Do you know if it has been published?
My experience has been that one larger unit is generally more reliable and less complex than multiple parallel units.
RE: New Standby Generator
RE: New Standby Generator
Regarding the loss of synchro, i have seen a few examples:
1. the fuel pump in one gen may be responding awkwardly, so it cannot keep same frequency no matter what the excitation, so the faulting generator disconnects automatically from the bus and you have an alarm.
2. the control may diverge, one generator increases kVAR and one absrobs more kVAR until the threshold is reached by one generator which automatically disconnects.
etc.
RE: New Standby Generator
So going for 2.5MVA sets, rather than a 4.5MVA does have logic to it.
All applications are different, however.
RE: New Standby Generator
My real question is how do we asses the relative reliability of a single generator vs. say a system with 3 parallel units, one being redundant?
It is easy mathematically to compare the relative probability of a load-impacting failure for N systems with R redundant units, if we know mean time between failure of a unit and the mean time to repair. Some reliability data for packaged gensets is available (IEEE 399, etc). But that doesn't account for the controls.
For me the harder part seems to be determining the probability of a control or switchgear failure. I have experiences with failures in two major manufacturer's computerized paralleling systems (in one case, dead bus with all gensets still running). I've not seen any reliability data on such systems.
RE: New Standby Generator
Taking the example of a manufacturer that uses a Microsoft Windows Compact Edition to run a SCADA application, the reliability is that of two items in series: WinCE + SCADA.
Whereas WinCE is harder to crash than the rest of the MS Windows operating system, it is not secure against tampering and/or malformed call procedures. For tampering, the OS might be functional, but an application developed under another environment might overwrite some critical files when installed. For malformed call procedures, it may simply be because the application has written data over the program area due to the fact that the memory is too small on the device.
The worst part is about the SCADA. The tabbed controls, buttons, text boxes, etc. are drag and drop items from an Indegrated Design/Development Environment, and as soon as the program has made the assembly, he has an application. I know a lot of tests performed on financial applications developed for banks; in contrast, i have seen SCADA programmers being called on site to investigate a malfunction, resulting in a mere "bummer" and then repairing the flaw they have identified.
Therefore, until a well established standard for testing control software is available and enforced, the reliability of the control system will continue to be zero = a best effort approach.
RE: New Standby Generator
Any-one with more knowledge on Factories Mutual, please comment. Do I even have the name right?
respectfully
RE: New Standby Generator
A recent horror story involved no fewer than 8 trips by my firm, with the vendor's software guys on site almost continuously for many weeks debugging and uploading new versions of this and that software (including MS Windows). We saw a number of "blue screen of death" incidents during the process. It had a windows front-end with a PLC controller. Our client ran out of money to pay us before we or they were comfortable with the system. Every new software update requires that the testing start over from the beginning. Admittedly, it was a customized version of the vendor's product and was delivered on very short schedule. I felt bad because of the wear on the system as well. Some of the medium-voltage breakers had several hundred operations in the process.
This type of experience is not limited to one system or vendor however.
RE: New Standby Generator
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respectfully
RE: New Standby Generator
RE: New Standby Generator