Crash synchronisation

[DavH]

We have four 6.6KV 2.2MW standby generators with ability to sync together, and with the mains supply. The alternators on two of the machines are showing signs of distortion in the stator windings, one of which has failed completely.
A report by the manufacturer has stated the distortion has been caused by crash synchronisation.
The gensets are tested individually in parallel with the mains and have the ability to load and unload (starting at about 500KW and ramping up to 1270KW). The load is an export onto the utility supply.

I accept that crash synchronisation may be occurring as breakers close genset onto mains, but is it possible that the process of the generators loading and unloading is also causing some form of opposing torque to be created within the alternator?

Has anyone experienced catastrophic failures of this type and what was the solution?

 

[waross]

I have seen the coupling keys in two bearing alternators sheared off when the breaker was closed out of sync.
respectfully

 

[ScottyUK]

Synchronising faults can cause forces far in excess of a three-phase short circuit. Most machines are not braced for such high fault levels so the damage you are seeing is not unexpected.
Solution:

Check your synchroniser, calibrate or repair it and set it up properly. Fit an independent synch-check blocking relay as a backup to the synchroniser. Check your VT wiring for phasing errors.


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

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

 

[slavag]

Hi.
It's possible fault. Check with generator mnf. what is a requested "window" of dU, df and dphi and set your singronazing eq. and additional synchrocheck relay, as Scotty wrote, according to this parameters.
Please find attached data:
thread238-189772
Regards.
Slava

 

[Hoxton]

Have you considered that the mains may fail and return (i.e. due to an auto recloser)? This would give a faulty synchronisation, but with the genset synch. system working correctly.

You need a mains failure detection relay to disconnect your sets if the mains supply fails. There can be one per set or one common one, depending on how your site is configured.

 

[DavH]

Thanks for the replies.

The set up of the system is such that the genset control is fitted with a synchroniser, and a secondary sync check within the MV VCB control.

The system has been fully commissioned and tested but we are unsure if the stator distortion was present before the machines arrived on site.

What protection should be present in the system to protect against the instantaneous currents produced by crash sync. I realise the VCB protection will not operate given the short duration of the faults.
Should surge protection be fitted?
I am aware of diodes within the alternator, should these have failed?
I am really looking for evidence that this crash sync has occured and that no other factors need to be considered.

 

[RRaghunath]

I think Vector surge protection should help isolate the grid quickly if there is a possibility of grid being connected to the generator bus without ensuring synchronous conditions or if there are severe fault conditions in the grid that can cause generator pole slip.
If we are talking of synchronising the generator, checking the settings of synchroniser / synchrocheck relay should be the way.
The settings need to be sensitive when we are connecting a small generator to large grid in order to minimise the impact on the generator.

 

[slavag]

Hi.
What I can add to Raghun's post.
DavH, please pay attention again to Hoxton's post.
Are you have AR on the main?
If yes, you must add loss of main protection:
Voltage shift or Vector surge, ROCOF or df/dt this is a fast protection, also possible PPS or U1<. May be something like to underpower or reverse power protection on main for prevent reverse flow to grid, it's depend on your application.
Are you connect to main directly or via step-up xfr.?
In any case simple overcurrent 50. protection was operated in case of non-synchro connection.
Regards.
Slava

 

[ScottyUK]

What protection should be present in the system to protect against the instantaneous currents produced by crash sync.

Most of the accepted protection methods are to prevent this happening, not to detect it after it has occurred. Unless you specifically request a machine that is braced to withstand an out-of-step synchronisation you won't get one. Machines are normally braced to withstand a three phase short on the machine terminals; an out of step sync causes forces higher than this and will probably damage the bracing and cause winding movement. Protection which detects this condition then becomes a means of damage limitation. Check your spec for bracing and fault withstand ratings.

Have you considered how the crash synch might have occurred? Did some brave / foolish person operate the manual close button on the VCB rather than allowing the synchroniser to do its job? If the synchroniser and backup relay are both ok are you certain that the phasing of the VTs is correct? A Y/D connection in either of the generator or bus VTs will cause a 30[&deg;] synchronisation error. If you are certain, how did you prove it (some methods are more effective than others)?

Rotating rectifier failures are unlikely but possible. Most times we see a rectifier fail an electrical disturbance precipitates a latent defect caused by poor installation practice. Hockey puck rectifiers are very susceptible to damage from careless installation but will often test ok and appear to behave ok until the field current is boosted by the AVR when they fail.

Surge protection won't help you during an out-of-step synch because the main problem is huge mechanical stress on the machine rather than electrical stress on insulation.


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

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

 

[slavag]

Hi.
Some intresting point.
Raghun and Scotty wrote about Pole slip, Out of step or Out of Sinchronization condition.
Our practic are use this protection for generators from 100MW.
DavH's photos are seems as out of step condition result.
But according to "theory", we don't need protect this size of genrator against out of step conditions.
What is your practic?
Regards.
Slava

 

[DavH]

Our gensets are standby operation. Common mains failure relay (5second delay). Return to utility supply is a manually instigated (keyswitch) automatic sequence providing "no break" return using the synchronising method. We parallel to main directly without step up tx (6.6KV to 6.6KV).

During routine testing we have experienced consistent ROCOF trips, usually during generator loading/unloading as opposed to as the VCB closes. Is this when the damage is occuring? I want to avoid any damage to the remaining two gensets.

 

[slavag]

Hi DavH.
Could you please give more information.
What do you mean consistent ROCOF trips, what is a ROCOF setting, it's df/dt< only or df/dt< and f<. or maybe it's df/dt>?
It isn't normal situation.
Are you provide real test: loss of main?
May be problem: not equl. share of P/Q in case of two generators synch together w/o grid.
Regards.
Slava

 

[Rodmcm]

Have you had your breakers checked. You could be getting one pole closing later/earlier than the other two.

 

[slavag]

Hi DavH.
It's also seems as adsorbes of P/Q, that mean you need function 55 used.
Regards.
slava

 

[slavag]

Hi DavH,
I don't see your answers, but in all case I wouls like ask next Q.
What is a speed of alternator?
Regards.
Slava

 

[DavH]

not equl. share of P/Q in case of two generators synch together w/o grid

Have you had your breakers checked. You could be getting one pole closing later/earlier than the other two

Interesting points in so far as we have a distribution panel suppling three open rings. Two utility incomers in parallel through a bus coupler. We have noticed unbalanced load through the two incomers. Instead of load being fed through the closest incomer (closest physically) it is fed from the furthesr incomer (I hope I have describer that clearly enough).
Basic principles should be path of least resistance so this suggests possibility of one incoming VCB being higher resistance than the other. These incomers are one of the points of synchronisation when we return to utility supply after mains fail.
How can we check the VCB's to ensure they are not the problem?

Slava,

Not sure of the ROCOF setting right now, will find out.
What is function 55?
Alternator speed should be 1600rpm.

 

[slavag]

Hi DavH.
For the main contact resistance test used micro-ohmmetr.
For the timing test used CB tester ( for example Programma, today it's GE or Doble product, or Weis tester, I'm sure many other also).

function 55 by definition is out-of-synchronism function.
It's not real pole slip or out-of-step function (fast protection). Actually recommended for the synch. motors, but I think is realy good function for the small generator. Possible say is directional overpower function ( also P and also Q).
I don't found so much information about this function ( I see it in manual of Areva P243 and CEE GMSH 7002).
May be simple explanation is: Absorbe of P/Q by generator.
But isn't reverse power ( by definition is underpower).
On this moment I only learn this issue and understand only one, no rules and no data about this.
Two reason for the out-of-step condition:
1. Grid problem.
2, Some stress of generator.
For this size of generator: grid more, more strong and we can cancelled this reason.
Stress of generator, yes, and generator with high speed more sensetive for this condition.
Regards.
Slava