Generator Relay Burning Out 1

[whycliffrussell]

I have a client with a 100kW, 600VAC, Caterpillar SRCR generator set that they are having some problems with. I don't have much information on the unit only a wiring diagram that I have attached. Through a telephone conversation with my client I have gathered the following details on what is happing:


1) The generator pick-up relay contacts, K1 keep burning out.
2) The generator is very slow getting up to speed (it idles too long).
3) The client put a new fuel pump + injects on the generator – but there was little improvement to the performance.
4) The generator does not have a static exciter (self excited?)
5) Sometimes the generator will start fine, other times it will not. When it does not it ends up burning out BOTH contacts on relay K1.

This wiring diagram is a little over my head. From what I can tell it looks like the field power comes from some taps off the stator through rectifier diodes. The K1 relay appears to be gating the SCR that supplies the field, and the K1 relay is also connected to the regular (on the top right hand side of the drawing). Do you have any idea of what the control sequence for this type of operation would typically be, as I'm not too familiar with generator controls.

My client said that he was talking ot a mfg. rep. about replacing the regular (which according to him will cost $20,000!) This seems rather excessive to me.

I have contacted the mfg. and they just want to send a guy to site to look at it; however, we would like to get a preliminary idea of what is going on as this is a very remote site and it may cost thousands of dollars to get a service rep. to look at it and who knows how much to fix it (as it is a vintage unit we may just decide that the generator should be replaced).

Any ideas?

 

[rbulsara]

100kW unit? It may be cheaper/wiser to repalce the unit then try to troubleshoot from what you are describing.

Some would have to pay the cost of service.

 

[rbulsara]

And by the way the issue seems to be mechanical, slow to come to speed? Could be a variety of reasons, impediement to fuel supply, air supply (0xygen), mechanical valve assembly, etc. Looks the like the engine needs a tune up.

 

[marks1080]

Hey Wycliff! I went through a few terms of uni with you at MUN (I'm assuming this is the same guy)... if not, sorry... also sorry for posting this here, I can see no way of sending a private msg to someone.

 

[whycliffrussell]

My client has already replaced the fuel pump and injectors (on his own accord) on the unit, but it has seen little impreovement in performance. I definetly see some problems on the mechanical side; however, should this relay be burning up because of this?

 

[marks1080]

A more related post:

When the unit is running do they have any idea of shaft vibrations or if the unit may be running hotter than expected? What kind of a load is connected to the unit?

 

[whycliffrussell]

The connected load is approximately 52kW, however my client did a measurement at one point in time to see what the load was and he measured about 30A (~31kW).

They have gave me no indication of shaft vibrations or that the unit is running hotter than normal.

The problem seems to happen when the unit is starting up. It seems to idle longer than it should and sometimes "sputters" (this is when BOTH of the contacts on the K1 relay burn out). Other times this does not happen, and the unit will start and get up to running ok.

There definelty seems to be a mechanical issue here. We're just wondering why this would cause the contacts on the relay to repeatedly burn out?

The load consists of six small motors driving some pumps and compressors for a boiler, and some emergency lighting. I would not imagine that is is a load problem as the problem is with starting the generator (ie: when it is unloaded). There does not appear to be any problem one the generator is up and running and loaded.

 

[waross]

The pickup relay bypasses the regulator to give the maximum residual voltage to the field for fast voltage buildup. At low speed the frequency is down as is the reactances of inductors. As a result the current is high.
But why is the engine building up slow?
Check fuel filter and fuel lines for restrictions. If they got a shot of water in the fuel, they may have changed a waterlogged fuel filter just to have the replacement become immediately waterlogged.
Look for pinches in the fuel lines.
Check the air filter for restrictions. I have seen new filter elements installed without removing the plastic wrapping.
Check the exhaust for restrictions.
SMOKE, If the exhaust is fairly clear as the set is running slow it often points to a fuel restriction.
If the set is blowing white smoke, you may not have enough engine compression or the air intake may be restricted.
Look for damage or slippage in the mechanical throttle linkage. The linkage may not exist on all sets.



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

 

[whycliffrussell]

waross:

Am I correct in assuming:

1) The engine is building up slow.
2) Since this is happening there is little voltage on the stator therefore the contacts on K1 relay (terminal 1,7 and 9,3) stay "in" keeping the SCR fired.
3) Since the frequency is down (inductor impedance is frequency dependant) there is a high current going to the field which is causing the contacts on the relay to burn up? (Luckily the field windings or the SCR do not seem to be damaged yet...at least not that we know)

4) When does the voltage regulator pick up and open the relay contacts on K1 and start firing the SCR (PIN 8?). Once this happens i guess the regulator controls the SCR and the K1 relay is out of the picture (The K1 relay is only used for starting?)

 

[waross]

First fix the engine. Then look for the collateral damage.
I believe that the relay takes all the residual voltage available, rectifies it and bypasses the regulator. When the generator output rises enough to pull in the build up relay, the normally closed contacts open and pass control to the Automatic Voltage Regulator.
The older plants were very prone to AVR failure at low RPMs because of the combination of calling for maximum output and low inductive reactance allowing excess current.
Again, fix the engine first. Then, on an older set that has been running slow, suspect AVR damage.

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

 

[catserveng]

The K1 relay on a CAT SRCR generator is also called a "buildup relay" and does what some of the others above have mentioned. The circuit was originally designed for intermittant duty, if it is burning the contacts there are two possible problems. Longer than expected time runnng below rated speed, or a problem with the slip rings and/or rotor.

The wiring diagram should show an optional connection for a voltage regulator "stop" switch. Until you get your slow to rated speed issue corrected I would use that feature, and only let the regulator come on once the unit is at rated speed.

This is an obsolete generator, went out of production in the late 70's or early 80's. The regulators and parts are very hard to come by in the CAT System, but there is an aftermarket supplier I have used in the past with great results if you do find there is a regulator problem after fixing the prime mover issues.

http://www.power-tronics.com/

I would guess the prime mover for that unit is a D333, a 6 cylinder NA diesel. There are alot of reasons it may not be coming up to speed, but many of the suggestions posted above are great starting points.

Hope that helps

 

[LaffeyDD724]

whycliffrussell,
It seems that from your description of what is taking place, that the K1 contacts are overloaded for their rating. Having said that and not being familiar with the machine and not having access to it for trouble analysis and not knowing the maintenance history of it and not knowing when the K1 contacts began to fail under load or what their current rating is or what the normal field current is.By analyzing the schematic that you posted I would be compelled to check the R5 resistance wire and try to determine if it is OK. It may be that somewhere along the machines history that the R5 resistance wire was for what ever reason replaced with ordinary wire. Taking into account the resistance of the field circuit plus the R5 resistance, if the R5 resistance was absent from the circuit it might cause the K1 contacts to operate at a current above their rating. This phenomenon may not have manifest itself had the engine not malfunctioned on start up. Sometimes things like this happen and go undetected until further degradation takes place. Trouble shooting from afar is always less desirable than having hands on troubleshooting, but I will risk it and add my 2 cents worth in anyway.

Tom


None of us is as smart as all of us.

 

[waross]

The modern Automatic Voltage Regulators used on smaller diesel sets are almost universally equipped with an "Under Frequency Roll Off" feature. It wasn't always so. Older AVRs were prone to failure at lower frequencies. The sets were generally equipped with a switch to turn off the AVR when starting or stopping.
catserveng and I have both seen and replaced AVRs that have been destroyed by running at reduced speed.
FIX the engine FIRST and then check for electrical damage.

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