proving reverse power protection relay is working through real reverse power
proving reverse power protection relay is working through real reverse power
(OP)
We have a couple of diesel generators and gas turbine on a ship. The class society guy wants us to prove the integrity of reverse power relays as part of his intermediate (2.5yr) survey. He suggested other operators choose to actually induce a real reverse power situation by taking load off one set and driving the set frequency up on the other so as to motor the unloaded generator. What is general view out there on this? I would have thought less risk of damage to just get a contactor in and make protection tests on the relay without the generator running.






RE: proving reverse power protection relay is working through real reverse power
I've heard of other people doing similar occasionally (for diesels at least), but I've never seen it myself as in all cases I've been involved with, a test set was acceptable instead. In theory the small duration for motoring shouldn't affect anything, but that's without knowing the size of your vessel or its engines. I don't know about the gas turbine though.
EDMS Australia
RE: proving reverse power protection relay is working through real reverse power
Set the frequency lower on the set that you want to test for reverse power.
Many operators unload a set before taking it off-line.
Unload the set as if you were going to take it off-line.
Then take the governor setting a little lower and it should take itself off-line with almost no system upset on the reverse power trip.
Bill
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"Why not the best?"
Jimmy Carter
RE: proving reverse power protection relay is working through real reverse power
The prevailing load aboard the vessel would have to be served by sufficient generating capacity to carry both it and the pumping load of the gas turbine.
The settings of the reverse power relay would have to be lower than the pumping load drawn by the gas-turbine-driven generator when motoring on line; if not, the test is not only pointless but doomed to failure.
Gas-turbine-driven generators may be designed to trip off line and out of service upon detection of fuel flow failure; such detection would have to be temporarily defeated for the duration of the test.
It may not be immediately obvious, but if such a live test is performed, the complete unit start-up sequence from rest, including purge, would have to be completed, as once combustion ceases at operating speed it can not be otherwise re-established. Prudent pre-planning would therefore be required.
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: proving reverse power protection relay is working through real reverse power
RE: proving reverse power protection relay is working through real reverse power
Simpler and better approach would be to reverse polarity of the CT inputs to the reverse power protection relay briefly to simulate reverse power condition.
Of course,the connections have to be restored once test is completed carefully.
Rompicherla Raghunath
RE: proving reverse power protection relay is working through real reverse power
Consider:
When you are at sea and a prime mover fails for some reason and the reverse power relay operates, what will happen?
If you are not operating on an N + 1 basis things may get interesting.
If a set fails and you must depend on the reverse power protection to take it off-line the system frequency will stay the same or drop a little.
Function testing is simulating real world conditions.
eg: a set has failed for some reason.
You can simulate this by cutting the fuel or steam to the set under test. This will not only test the reverse power protection but will also show the ability of the system to recover from a reverse power event on one set.
QUESTION:
Do you want to get a box checked and signed on a report form or do you want to be assured that the the system will provide stable continuing power in the event of a set failure while at sea?
Different types of prime mover have different negative reactions to reverse power. That is why we use reverse power protection.
Your best function test is to induce reverse power and let the reverse power protection operate. This may also be the quickest and safest.
You can induce reverse power quickly by cutting the fuel or induce reverse power smoothly by setting the speed slower.
If you do not use an N + 1 scheme an induced reverse power test may give you some preview of what to expect when a set fails at sea at 2:30 A.M.
Bill
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"Why not the best?"
Jimmy Carter
RE: proving reverse power protection relay is working through real reverse power
RE: proving reverse power protection relay is working through real reverse power
Respectfully, this has not been my experience; with the fuel rack forced to zero injection the set will simply motor from the mains at full operating speed, therefore there will be no loss of coolant flow, lube oil pressure, or any other adverse effect.
Think about it: a Diesel engine vehicle with a standard transmission descending a steep enough grade goes to zero fuel rack in the short term with no issues whatever that I'm aware of. I had a VW golf with a TDI Diesel engine, and the only problem I encountered was that on long descents in bitterly cold weather there would be zero heater output and my feet would start to get cold.
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: proving reverse power protection relay is working through real reverse power
I'd also expect what you mentioned with gas turbines to be correct, although I only have limited experience with turbines.
waross is right though, the best way to test the total system under those circumstances is to let it do what its supposed to, which is a similar issue to running up a standby set, but never connecting it to load. It gets around the risks with secondary kits in terms of having to temporarily jumper or disable things and then forgetting to reinstate them too.
EDMS Australia
RE: proving reverse power protection relay is working through real reverse power
That being said, I wouldn't expect the underload condition to prevail for more time than it takes for the reverse power protection to operate, typically measured in seconds.
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: proving reverse power protection relay is working through real reverse power
I have only done it on diesel engines between 300kW and 4000kW, not turbines. But have done it quite a few times in almost 40 years with no problems except for one unit that was pretty old and not well maintained, it suffered a surge suppressor failure (blew open) when the unit was intentionally reverse powered.
Most of the time we would just parallel it to one of the other units on board and trip the fuel rack, never had to change frequency. Only concern usually was to make sure ships load was not too high so that the other unit online would stay online.
I will say that the last time I had to do these kinds of tests, the engineer who developed the protection settings used 10% of nameplate for his settings, Which I think was a pretty common rule of thumb for a lot of years based on what I have seen. These was newer engines with lower parasitic power levels. The Diesel Inspector felt vindicated since the unit failed to trip, even though the relay was tested and "proven".
MikeL.
RE: proving reverse power protection relay is working through real reverse power
Rompicherla Raghunath
RE: proving reverse power protection relay is working through real reverse power
Diesel engines don't like light loads. The lighter the load the less the engine likes it. Comparing 5% forward power with 5% reverse power, there will not be much difference. Either case may cause similar concerns.
Diesel trucks will often be in a reverse power situation for 5, 10 15 or more minutes at a time descending some of our mountain grades. The trucks configured for the mountains will have either a Jacobs type engine brake or an exhaust brake that will increase the amount of reverse power dissipated considerably.
With properly functioning reverse power protection, the set will be tripped off-line hours before any problems will arise.
Running lightly loaded has the possibility of creating much worse problems than a few seconds or minutes of reverse power.
Bill
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"Why not the best?"
Jimmy Carter
RE: proving reverse power protection relay is working through real reverse power
The surveyor just wants to see proof of relay operation, and would be satisfied with the injection test. That would take time to organise though and he was simply saying that during meeting on ship it could be proved there and then in his experience with other operators. I didn’t have so much concern with DGs (9MW each) knowing they could take a fair percentage of reverse power without damage but GT (24MW) I felt a bit apprehensive having little experience with these. He was a bit of an old sea dog (as most are) and I felt what he might see week in week out on a small 500kw generator on a fishing boat might not play out so well for us hence my caution and preference of injection test to satisfy his need. He needs to see it done on next visit so bit of time to decide yet.
I like the idea of full system check as I guess that is what is there for in first place I.e prevent damage. I guess apprehension comes from fact fairly new ship and not had time yet to get up to speed with the plant layout and protection philosophy And lack knowledge of GTs so would be a expensive mistake to make and for him just to tick a box saying failed to work!! Then go home.
RE: proving reverse power protection relay is working through real reverse power
Bill
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"Why not the best?"
Jimmy Carter
RE: proving reverse power protection relay is working through real reverse power
RE: proving reverse power protection relay is working through real reverse power
A steam turbine operating under condenser vacuum presents such a small load that a reverse power relay may not see it at all: they are notoriously difficult to set to reliably protect the turbine, and very sensitive to any phase shift introduced by the instrument transformers.
RE: proving reverse power protection relay is working through real reverse power
Thanks for that explanation, I'm not a turbine guy, but have some smaller steam turbine generators I take care of once in a while. Instead of reverse power they have a low forward power settings instead, below the unload trip point. Never got a good explanation why that was, your description was helpful.
MikeL
RE: proving reverse power protection relay is working through real reverse power
RE: proving reverse power protection relay is working through real reverse power
The real world test proves the settings as well.
If you hesitate to do a real world test, then consider the possible consequences of an actual real world reverse power when the reverse power protection fails to operate due to improper settings.
Bill
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"Why not the best?"
Jimmy Carter
RE: proving reverse power protection relay is working through real reverse power
If the goal is simply proving the relay is working and healthy the most appropriate way forward is testing the relay with an injection test set.
If the goal is proving the system as a whole works than you want to look at doing real world tests.
Typically a maintenance cycle on electrical protections begins by testing all the individual components (ie: relays) and ends with an overall test of the system based on a real world event.
All of my experience is with utilities so if there are other industry practices for oil&gas I would be unaware.
RE: proving reverse power protection relay is working through real reverse power
Some of these inspectors can be very hard to please, and your arguments need to be pretty darn good to get them to accept something other than what they think they need to see. But most of these inspectors were also working engineers with lots of experience who in just about every case I dealt were passionate about getting it right.
Last year I dealt with 9 separate instances of units failing to trip on reverse power after an engine control or mechanical failure. In all cases the relay performed as expected, asserting the trip output when tested using a test set. In all cases the setpoint used was too high for the actual parasitic power it took to drive the unit when paralleled to the bus. In 7 of those cases we were able to go back to the engine manufacturer and get a estimated parasitic power number, in two cases we could not and in both cases, a "real world" test was done to see what the reverse power level really was and develop an appropriate trip setting. In two of the cases where we got a number from the supplier, the end user insisted on a real test due to the amount of damage sustained by the units for not tripping (the units had been driven for several hours before being discovered). Also in 6 of these cases the electronic engine controls or monitoring system were not properly integrated with the breaker controls, so instead of getting a proper signal telling the switchgear the engine died, the switchgear controls relied on the reverse power setting to open the breaker.
Every industry segment has its own quirks, utilities have different concerns than ships, even in maritime what the ship does has a big impact on how the regulatory folks deal with them and to what extent they will test things out. At from what I've seen
MikeL.
RE: proving reverse power protection relay is working through real reverse power
The difference between making sure before or making excuses after.
Bill
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"Why not the best?"
Jimmy Carter
RE: proving reverse power protection relay is working through real reverse power
More important, imho, is having a documented maintenance process that covers everything and isn't reactionary.
Should the specific relays be tested individually (ie: bench tested)? Absolutely. Should real world tests be done as well? Absolutely.
Should we understand the purpose, impact and difference between these things and why they are both needed to prove overall system health? Absolutely.
So... if the question is whether or not to do secondary injection tests on individual relays, or to just to overall high level system tests ('real world tests') the answer is yes. To both.
This person seems to be required to provide proof a relay works. The appropriate thing to do is to test the relay. I'm assuming that system level tests are required as well for other purposes. To simply fire off a real world test for any maintenance scenario is not a good way to test protections.
RE: proving reverse power protection relay is working through real reverse power
Off subject, but a document you may be interested in mike ......
http://www.globalmaritime.com/library/media/148610...
RE: proving reverse power protection relay is working through real reverse power
Great article, DP has always been a really interesting topic, where it started being pretty loose to holding really tight, especially for directional drilling systems. A few years ago ABB also had a very good paper about open vs closed bus ties on larger marine power systems, fast operating sectionalizing schemes seemed to play a big role in those ideas. I don't do much of that work anymore and I think it has changed a lot in the last few years. I know we used to keep a lot of units on line at low loads to support open bus tie schemes, but the newer engines or in some areas having to comply with emissions regulations started to make that less attractive. Some operators mixed schemes based on what the load profiles actually were, less critical loads staying on with fewer generators trying to load them up, or adding ballast load in some cases.
When I started doing a lot of offshore work, the inspectors were brutal, especially to us "smart ass american whiz kids" as some of the old DNV guys used to call us. Was able to get to a point where we could actually talk about their concerns, what the end result of the inspections needed to be, what steps in testing did they want, and what areas did they feel they got mislead before. I think what has changed is that we had more time to actually talk things thru, a lot of this was newer and we all were learning together. It seems like no one has the time or desire anymore to actually work it out together, just get it done as fast and cheap as you can and get back to work seems to be the goal.
Good luck in your venture, thanks for sharing the article, was a great read.
MikeL.
RE: proving reverse power protection relay is working through real reverse power
RE: proving reverse power protection relay is working through real reverse power
When you consider Scotty's description of the damage that may be done to turbines by reverse power and Mike's experiences with failed reverse power schemes, give the inspector what he wants.
You will be done in less time than it will take to even plan a bench test.
Bill
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"Why not the best?"
Jimmy Carter
RE: proving reverse power protection relay is working through real reverse power
RE: proving reverse power protection relay is working through real reverse power
If your control system incorporates a Historian, or if you set up a Dranetz or similar power analyser, then you'll be able to see what the real reverse power load is during the test, data which will help you immensely if your relay doesn't operate because the prime mover doesn't require quite as much motoring power as the manufacturer thinks. I imagine that a diesel engine being driven at rated speed consumes a fair bit of power so it should be reasonably easy to establish usable settings.
RE: proving reverse power protection relay is working through real reverse power
Years ago there was an engine that was commonly used in transport trucks that was noted for almost zero hold-back on down grades. This engine had so little hold-back that the time to lose enough RPMs to synchronize an up-shift was many times the time of competing engines.
A setting that works for 98% of diesel engines may not work for all engines.
Bill
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"Why not the best?"
Jimmy Carter
RE: proving reverse power protection relay is working through real reverse power
Although I take your earlier point about Diesel engines not liking to run at low or zero load for extended periods [as this almost certainly will cause fouling], reverse power tripping commonly occurs with no intentional time delay, rendering this observation, with respect, not applicable to this instance.
It is my understanding that once the equipment need to perform the type of data capture ScottyUK is suggesting [whether it be that type of equipment or even the more primitive method employing temporary raised-zero reverse-reading-capable meters[ is in place and reverse power protection has been blocked, the process of forcing fuel rack to zero, recording readings, and releasing fuel rack for normal operation could all be accomplished within at most thirty seconds; although one might exist, I do not know of any Diesel engine that will foul in that short a period.
And unless I'm way off base, if the reverse power prot on an in-service gas turbine is blocked beforehand, the sequence of forcing fuel flow to zero, capturing readings and manually tripping the unit out of service to normal shutdown should be capable of completion within a similar time frame, with no resulting damage [anybody who knows otherwise, please say so; I'm still learning].
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: proving reverse power protection relay is working through real reverse power
On a single-shaft machine the power consumed by the compressor at sync speed is so large that workable relay settings are almost guaranteed - you're looking at a load of about 30% of the power turbine's gross output: put another way, the compressor consumes the equivalent of half the set's electrical output. That's a hell of a big target to miss, even with calculated settings.
You could probably do as you suggest with a GT, but I haven't seen it done that way. You'd have to defeat a fair bit of the control logic in order to run that test.
RE: proving reverse power protection relay is working through real reverse power
Now look at all the improvements in horsepower to weight/size ratios in modern gas and diesel IC engines, a 500kW genset was a big chunk of iron, and not just the engine but the tail end as well. And internal improvements like smaller bearing/crankpin surface area per BHP than older engines, improved piston ring to liner interface, improved cam to lifter profiles, all this stuff made the engines frictional horsepower go way down. But did that really get passed on down the trail to the folks doing the electrical protection? Do the current modeling softwares like SPM and ETap take that into account? I have an older protection handbook that states "Maximum motoring power of a diesel engine is 25% of rating", that was probably a good guideline for a large slow speed engine many years ago, not sure it would be of any use today.
I still see a lot of units in the field with 10/10 reverse power protection, on a high speed highly turbocharged engine and a smaller, lighter generator end, I will bet money that engine has nowhere near a 10% of rated parasitic horsepower demand. But they still showup, and almost every time I bring it up it, it becomes an argument. Of the 9 units I mentioned last year, I was involved on three of them early in their service life. In all three cases I expressed my concern, in all three cases the settings were not changed. On top of the fact the engines have reduced frictional losses, modern engines also have more extensive protection system with very large numbers of active shutdown protections. Take an "old" style mechanical diesel engine with a standard protection package, it likely had low oil pressure, high water temperature, overspeed and emergency stop shutdowns. It was also in most cases an Energize to Shutdown protection system, so a loss of control power would allow the engine to continue to run. Take an engine driving a similar size generator today with modern electronic engine controls operating under some sort of emissions compliance. I would bet at a MINIMUM there are at least 20 active shutdown protections, and if control voltage drops while the engine is running it will operate erratically or shutdown altogether. I'm not sure if this is an issue on the smaller end of the size range or because it driven by and IC engine. Sounds like the gas and steam turbine folks are more diligent in their protection schemes.
Nice thing about a forum like this is the differing perspectives everyone has based on what they work around, not everyone may have all the "right" answers but it sure brings in a lot of great info.
Hope that helps, MikeL.
RE: proving reverse power protection relay is working through real reverse power
Also, most new sets have the rings seated well, but the odd set will not have the rings set and will pump oil under light loading.
I installed a set for a meat processing plant. They had one large machine called the cutter that ran for a couple of hours each day. The genset was flat out, blowing black smoke and leaning on the mounts when the cutter started. Then it dropped down to about 20% to 25% of rated load. When the cutter was not in use the load may be as low as 10% of rated load.
When the dealer became aware of our load profile he demanded 5 days on a load bank to ensure that the rings were properly seated.
(With a 275 kW set I sure boiled a lot of water that week.)
The damage from reverse power on a diesel may be long term and is often reversible by simply loading the set.
Consider the Duromax diesel engine. GMC announced back in 2007 that they had just built the 1,000,000th Duramax engine.
By now they must be close to 3,000,000 engines. Every time the driver takes his foot off the gas, the engine is in reverse power mode. In the mountains the engine may be in reverse power mode for miles at a time.
No-one has problems with a Duramax engine due to coasting down hills.
I don't see any problem with the majority of diesel engines in reverse power mode for a few seconds or even a few minutes until the protection trips.
There is the rare exception. That is some old worn out standby sets. These are in such bad shape that they wouldn't last more than a few months as prime power sets but the owners won't replace them until they fail completely.
The rings and cylinders are worn so badly that the pressure of combustion is the only thing sealing the rings.
Under reverse power they will pump oil so badly that there is an actual danger of throwing globs of burning lube oil out of the exhaust stack. Still the reverse power does not damage these engine any further until either they run dry of oil (seen that) or set fire to the generator shack. (haven't seen that yet but I have heard stories)
A diesel engine is not subject to the kind of immediate, expensive damage that a gas turbine is under reverse power and there is no danger to the engine by a real world test.
A suggestion. If you have instrumentation installed that will measure reverse power, disable the reverse power protection and see what it takes to drive the set. Then enable the reverse power protection and check that the set point is well below the maximum reverse power. If the set point is too close to the actual maximum reverse power the action may not be reliable.
Bill
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"Why not the best?"
Jimmy Carter