Here the scenario;

We have 4 engines on line on our main switchboard. The switchboard is split by a bus-tie, so 2 engines are supplying Bus A and 2 engines are supplying Bus B.

In order to stop 1 engine we have to close the bus-tie and make the board common. We do this by syncing the 2 boards together through our Power Management System (PMS). The PMS then either raises or lowers the speed of engine 1 and 2 together to match that of engine 3 and 4 so the bus tie can be closed.

Then this happens; Engine 2 seems to take all of the load and engine 1 goes into reverse power. Its almost as if engine 2 responds quicker than engine 1, or engine 1 is responding a lot slower.

We are unsure why this is happening; it runs fine on its own and when paralleled in with any of the other engines. It only happens then syncing in the bus tie between the 2 switchboards.

Could this be a lack of excitation on the alternator of engine 1 or the AVR on engine 1 responding to slowly?

Any ideas guys?

Cheers
Martin

 

Martin Crummy,

I suggest you detemine what voltages sources are connected to the auto-sychronizer for each of the conditions you cited!

Regards, Phil Corso

 

Martin Crummy,

The first question to ask is: When did the problem start? Has this scheme worked properly in the past, but just recently it has started misbehaving?

I'm not very clear on the sequence of events. When does Engine 2 take all the load--before the bus-tie is closed, I presume? What happens to the frequency when Engine 2 takes all the load?

When you say the PMS "...raises the speed..." do you mean it raises the speed reference? What happens to the frequency (which is directly related to speed)? Do you know if the two machines (Engines 1 and -2, and Engines 3 and -4) are both operating in Droop Speed Control and being controlled by the PMS--or are they operating in Load-sharing Isochronous Speed Control Mode? Is it possible the PMS is switching Engine 2 into Isochronous Speed Control mode prematurely?

At some point in this process, probably after the bus-tie breaker closes, the PMS has to reduce the load on Engine 1 in order to be able to take Engine 1 off, and I'm guessing that the load reference to Engine 2 is raised to do this.

My money is on the operators doing something different than they've done in the past. Are they following a written SOP (Standard Operating Procedure) the same way they've followed it in the past?

Or, has the SOP been changed recently?

Contrary to popular belief, watts (load) is directly related to fuel; reactive power is related to excitation (which the AVR regulates). Increase the fuel, the load goes up; decrease the fuel the load goes down. Increase the excitation the reactive current changes; decrease the excitation the reactive current changes. Decrease the excitation too far and the loss of excitation relay should actuate to protect against slipping a pole and/or overheating the generator end-turns.

Many AVRs have a LEL--Lower Excitation Limit--which prevents reducing the excitation excessively to protect against slipping a pole or overheating the generator end-turns, and to protect against tripping the breaker because of loss of/low excitation (which would mean the load being carried by that generator-set would be lost).

Something's changed: The SOP. Or the way the operators are sequencing things. Or the operators have arbitrarily decided to not follow the SOP. Or, if there's no SOP then someone has decreed things be done differently and someone doesn't understand, or the decree is wrong. It's hard to imagine the PMS or the engine governors suddenly not working like they have in the past.

Please write back with more information, and any troubleshooting results.

Cheers!

 

Said Gen 1 and 2 are on bus A. PMS is sending speed adjustment signals to these generators in order to synchronize with bus B. For some unknown reason Gen 1 and gen 2 received different signals resulting gen 2 had higher fuel admission and took the load on bus A.

 

Many thanks for the responses.

I'm new to the vessel and have been told that it has always been like this.

Yeah Engine 2 takes the all the load and causes Engine 1 to reverse power before the Tie breaker gets closed.

I believe they are all running in Load-sharing Isochronous Speed Control. But will look into this.

This only happens when we stop either Engine 3 or 4, Stopping either engine 1 or 2 is no problem. So what we now do is stop engine 2 then close the bus tie and all is OK

I will get back to you with what happens to the frequency of the 2 engines the next time this happens.

Cheers
Martin

 

Martin,

Thanks for clarifying if this has been an ongoing issue for some (since commissioning???) or a more recent issue.

I presume all the engines are the same, and the governors are the same, as well.?.?.? Can you compare the parameters for speed control and gain and deadband of all the governors to determine if they are all configured/programmed the same?

I read the original post as saying that when you want to stop Engine 1--not when you want to stop one engine ("an" engine), so that's one issue I can see with my response. (You wrote "In order to stop 1 engine ...." which I interpreted as Engine 1. You seem to have meant, "In order to stop one engine ..." or, "In order to stop an engine ...")

You wrote:

"...The PMS then either raises or lowers the speed of engine 1 and 2 <b>together</b> to match that of engine 3 and 4 so the bus tie can be closed." (The emphasis is mine.)

When two generator-sets are synchronized together they are running at the same speed (frequency). So, if the PMS raises the speed reference for one machine the speed of both machines is going to increase at the same rate. What may be happening is that the PMS is raising the speed reference of Engine 2 when synchronizing. This will definitely allow some control of frequency to allow synchronization, but because both machines are operating in Isochronous Speed Control mode Engine 1 starts backing down, very quickly because it's in Isochronous Speed Control Mode (albeit Isoch Load-sharing Mode).

If Engine 1 could be switched to Droop Speed Control mode before starting the synchronization procedure then Engine 1 won't care about load, and it's frequency will follow that of Engine 2 as the PMS is changing the frequency during synchronization. Once the tie-breaker is closed Engine 1 could be switched back to Isochronous Load-sharing Mode and any engine could then be stopped.

I'm curious, though. When you want to stop an engine once the bus is common, how is that done?

I also presume that the PMS can't ever use Engine 3 & -4 for synchronizing, but that if it could the behaviour would be the same: Engine 3 or Engine 4 would take all the load and Engine 4 or Engine 3, respectively, would unload to reverse power. Again, that's because I suspect the PMS isn't really changing the speed references of BOTH engines--it's only changing the speed reference of one engine.

Hopefully there's some way to monitor the speed reference signals to Engine 1 and Engine 2 during the synchronization procedure to see what's happening.

It could also be that the PMS isn't programmed properly, and that when it's increasing the speed reference for Engine 2 it's also lowering the speed reference for Engine 1, or that there's some "bug" in the programming that doesn't recognize that synchronizing is going on.

It's really going to be necessary to monitor the speed reference signals to both engines during the synchronization process.

If the speed reference to <b><i>both</i></b> engines is changing at the same rate, then it's possible that the governors of Engines 1 and -2 are not tuned similarly (the Isoch Load-sharing parameters are not the same).

And many times when problems like this are encountered it turns out that there is more than contributing factor--it could be a combination of PMS programming/configuration and governor tuning.

Please write back and let us know how you fare in troubleshooting and resolution.

Cheers, mate!