Diesel- Generator Set

[raisinbran]

I am looking for possible causes of damage to the gears/bearings in a speed-change gearcase that couples a large diesel engine (2000 kW) to a 2700 kVA generator. This system is connected to other generators and the utility thru a set of circuit breakers. One theory is that the gen-set circuit breaker recloser allows the circuit breaker to reclose when the voltages are extremely out of phase. Aside from tripping the circuit breaker, what effect would this have on the drive train? In other words, would there be a corresponding significant torque spike associated with the out-of-phase reclose?
Thanks,
Raisinbran

 

[TheBlacksmith]

Absolutely. I have seen directly coupled (no red gears)gensets rocking violently on their mounts as a result of out of phase breaker closures. The generator wants to sync up with the mains in an extremely short time and the engine cannot accelrate or decelerate nearly that fast.

 

[waross]

Depending on the amount of displacement when the breaker closes, you can get broken couplings and drive keys and more severe mechanical damage. There is indeed a torque spike.
respectfully

 

[ScottyUK]

Add bent or broken con rods on the engine, generator winding movement, other bad things. Same sort of problems occur with a pole slip. Not good, especially if it is happening repeatedly.

Why does the generator have an auto recloser? That is very strange, at least in the world of big generators. Normally generators would have a breaker equipped with synchronising gear specifically to prevent this type of problem. Or are you saying the utility has a recloser somewhere out on their network?


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[raisinbran]

ScottyUK
My fault. I am not familiar with all of the various equipment terminology, and I used the words "recloser" and "synchronizer" inter-changeably. Synchronizer is the correct word.

To prevent the problem that I am concerned about, what are accepted values for phase angle difference and voltage difference for a generator to utility circuit breaker application? Do these values depend on the utility short circuit capacity?

Raisinbran

 

[slavag]

Hi.
And frequency difference also.
These parametrs are not depend on the utility SC.
Actully, generator mnf. must send to you these limits.
Regards.
Slava

 

[waross]

If the breaker is closed early, the set will accelerate to "catch up" to the grid . When it reaches the point of synchronism, it will be above synchronous speed. It will overshot and then decelerate. There may be more than one speed swing before the speed stabilizes. The magnitude, period and damping will depend on the characteristics of the genset assuming that the genset is small in comparison to the grid system.
If the breaker is closed late, the set will decelerate first and then accelerate.
respectfully

 

[edison123]

"One theory is that the gen-set circuit breaker recloser allows the circuit breaker to reclose when the voltages are extremely out of phase."

Use of synch-check relays will prevent this.

For synchronizing your generator to the grid, you need

1. Voltage match
2. Frequency match
3. Phase sequence match

 

[ScottyUK]

Hi Raisinbrain,

The synchronising relay controls two parameters: speed and voltage. By manipulating speed the relay effectively has control of phase too if the governor is sensitive enough. The damage you are describing indicates a problem caused by speed or phase mismatch rather than voltage mismatch.


The initial places I would check are:

Governor control - how well does the set hold a constant speed at idle (no load)? If the governor can't control the speed of the set accurately then the synchronising relay will struggle to perform its job. waross might have some ideas on this as he works on diesels more than me, but governor hardware and governor control loop tuning are where I would be looking.

Breaker timing - is the generator breaker sluggish to respond? Is the breaker closure time consistent and repeatable? You'll need a Kelman tester or similar to measure breaker timing, or any switchgear maintenance company should be able to do it for you. Note the closure time from the demand until operation - see why later on.

What are the synchronising relay settings for breaker closure? These are a trade off between the perfect condition of zero phase error and zero voltage error and the real-world capability of the set. Bigger machines tend to have better control and instrument systems and can hold tighter margins so their synchroniser settings are tight. On a small set the commissioning techs sometimes open up the window where breaker closure is permitted in order to mask some other problem which is preventing correct synchronising conditions being met.

Normally there are adjustments for (minimum of) voltage and phase window to allow breaker closure, and pulse length from the synchroniser to the governor / AVR. There is a setting to compensate for breaker closing delay. Too long a raise / lower pulse causes instability and hunting around the setpoint. An incorrect breaker timing compensation setting will guarantee a bad synch even if everything else is perfect.

Hope this gives you some ideas.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[raisinbran]

Thanks to all for your valuable input. I am sure that I will have more questions, but I now have an excellent starting point.
Regards,
Dave