proving reverse power protection relay is working through real reverse power 17

[jimmy2times]

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.

 

[FreddyNurk]

If you have access to the CT and VT terminals for the relay, and you can isolate them (i.e. just feed the relay, rather than everything else that also might use the same terminals), you can use a relay test set to verify the reverse power condition.

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

 

[waross]

Don't set the frequency up on your ship system.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

Although this doesn't fall strictly within my field of expertise, I believe I'm warranted in saying that "live" reverse power testing of a gas turbine may pose some issues...

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]

 

[davidbeach]

I'd find a secondary injection test to be far more controllable and repeatable.

 

[RRaghunath]

I don't think reverse power can be allowed in to Diesel engines without subjecting the reciprocating engines to damage.
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

 

[waross]

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.

Do you want to comply with this, or argue.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[JG2828]

I did some relay testing at a trash burning power plant a while back. Their standard operating procedure was to turn off the conveyor belt and stop feeding trash (fuel) in. They let it continue to generate until the boiler cooled down enough and the generator went into reverse power, which tripped it off line. But that was a steam turbine, not a gas turbine or a diesel.

 

[crshears]

I don't think reverse power can be allowed in to Diesel engines without subjecting the reciprocating engines to damage.

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]

 

[FreddyNurk]

crshears, that's been my understanding as well, although I'd expect that one of the significant differences is that in an unmanned station (or ship's machinery space...) is that the reverse power situation is likely to persist long enough to cause issues due to lack of sufficient cylinder temperatures and lube oil flow, whereas in your car the load is variable enough that the engine heat is maintained. Its a different application, but if motoring was completely unacceptable for a diesel, then the Jacobs Brake would never have been developed.

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

 

[crshears]

Hey Freddy, agreed that service tests are best; I for one wouldn't want to be called out of my rack while under way to deal with a morning watch generator trip.

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]

 

[catserveng]

I have also run into having to do a "real" test, even when a test set was available and a test plan proposed. Some Diesel Inspectors for certain agencies can be pretty set on it.

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.

 

[RRaghunath]

CR & MikeL, Thanks for enlightening me with your experiences.

Rompicherla Raghunath

 

[waross]

It is not a case of "No reverse power is okay." "Reverse power is bad."
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
--------------------
"Why not the best?"
Jimmy Carter

 

[jimmy2times]

Thanks for many responses, I didn’t realise there had been any as I used to get email notifications but must have disabled that without knowing!

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.

 

[waross]

I'll defer to Scotty on the turbine issue.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[marks1080]

This can be (and normally is) proven with a relay test set and the relay. I do not think it's a good idea to do a real world test for this, when the goal is only to prove the relay correctly operates.

 

[ScottyUK]

The thinnest parts of a GT - the blades and vanes - cool down quickly if the fuel is cut off and airflow through the engine is maintained, which will rapidly eat up hot parts life due to thermal cycling. My experience is on single-shaft machines bigger than have ever set sail (except as cargo) but a prolonged motoring event is worse than a trip and coast to a halt because of the sustained flow of air from the compressor causing rapid differential cooling within the engine. On a single-shaft machine a GT compressor usually consumes about 1/3 of the turbine's gross output so the relay should have no difficulty is seeing a motoring load of that magnitude. On a multi-shaft machine with separate power turbine and gas generator (i.e. an aero-derivative) the braking load is lower because the HP compressor is on the high speed shaft which doesn't get motored by the alternator.

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.

 

[catserveng]

Scotty,

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

 

[ScottyUK]

Glad to help Mike, you help me out far more often than I get to return the favour.