Is it possible for un-proper sync cause differential trip on Generator 4

[newelecteng]

Hi all,

When we try to synch 3.3kv/3.1mw generator to busbar throw CB. Some thing get wrong and the CB close without proper sync position then it trip with generator differential protection. After this trip we carry out full scale check on this unit and we don’t find any problem related 87G.

So I wonder Is it possible for un-proper sync cause differential trip on Generator

 

[Fischstabchen]

Check to see if you CTs saturated.

 

[waross]

My thought also, the CTs have probably saturated. If the current is high enough that your CTs are saturating, any trip is good news.
I know of instances where an out of sync closure sheared the key in the coupling keyway on a 600 KW diesel set, more than once.
This was paralleling with a second 600 KW set.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[newelecteng]

Thanks dears for your prompt response

This happen when we try to parallel this unit with other unit the busbar.

Let assume worst case 180dgree out of phase what will happen in this wrong CB closeing is it like 3phase fault or will be some circulating current in what direction

and I few assume CT saturated the 87G should be stable.

 

[FacEngrPE]

A 180 degree out of phase closure is like a bolted 3 phase fault, with voltage equal to twice the buss voltage. I have not seen calculations, suspicion is that the peak current is 2x of bolted fault current that would be calculated by the normal approach.
The mechanical loading of a 180 degree out of phase closure has been known to create serious damage to generators, transformers, etc.

This paper on the electrical engineering portal has more information

https://electrical-engineering-portal.com/generator-synchronizing-check-protective-function

 

[Fischstabchen]

FacEngrEE,

The Mohave subsynchronous event threw parts from the system out of the building into their retention pound. It is no joke when power from the grid starts driving the generator. When stuff becomes asynchronous, it oscillates torques between motoring and generating quickly until something breaks. The OP said this was an unsynched close in but if it really was way out of phase, the differential tripping would be on the bottom of the list of problems.

 

[newelecteng]

Yes Fischstabchen you are right.
This cause coupling failure between the generator and the turbine gearbox.

 

[waross]

180 degrees is the worst case electrically. 90 degrees is the worst case mechanically.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Fischstabchen]

You are going to see full torque one way and then full torque the other way as it oscillate between motoring and generating. Some contingencies put the angular difference between the generator and the system past 90 degrees, briefly. The generators are built for it. Large ones at least. If a swing shots past 180 degrees you are unstable. Slipping poles is worst.

 

[Hoxton]

Generator are not built or designed for faulty synchronizing

 

[RRaghunath]

I doubt a properly set and tested 7G can maloperate on out-of-sync closing of generator circuit breaker.
Suggest check the relay settings.
The relay is expected to be stable during CT saturation, especially if the protection is high impedance type and the stabilising resistor is calculated correctly.
In case of biased differential, it depends on the bias setting in the relay and the internal algorithm to detect CT saturation / blocking.

 

[newelecteng]

Hey RRaghunath

We have Basler Differential relay model BE1-87G but I think the problem is secondary path of gen neutral CTs is much larger that path of Cts near CB that they feed the 87G. Can this cause unequal saturation !!

 

[Fischstabchen]

Hoxton,

Read what I wrote. The previous poster posted that the greatest mechanical stress is 90 degrees and I said contingencies will swing a generator past 90 degrees sometimes. You are not asynchronous until you go past 180 degrees.

 

[RRaghunath]

newelecteng,
if settings are right, relay is supposed to stay stable during unequal saturation of CTs as well.
BE1-87G is a biased relay and not as simple as high impedance differential protection to set as well as to test. Check the settings as well as the relay test results.

 

[newelecteng]

Dears,

how we can compensate for different length in differential secondary circuit (Gen neutral CTs is 150Meter and CB CTs just 2Meter) see attached Pic

 

[Hoxton123]

Fischstabchen

I repeat what I say, Generators are not [need not] be built or designed for [to withstand] faulty synchronizing

According to international standards they should be designed for a three-phase fault across the generator terminals.

This is less effect, potentially, than faulty synchronising

 

[RRaghunath]

newelecteng,
Quote "how we can compensate for different length in differential secondary circuit (Gen neutral CTs is 150Meter and CB CTs just 2Meter) see attached Pic" Unquote
All that is part of pickup threshold and bias setting calculations, meaning the lead mismatch errors, errors due to CT parameter mismatch etc. can be compensated by choosing appropriate settings.

 

[Jacob Henry]

Yes. This cause coupling failure between the generator and the turbine gearbox.

https://mwi.solutions/technologies/wms-wcs-and-oms-software/

 

[electricpete]

and I few assume CT saturated the 87G should be stable.

I think you're saying you're suprised that the generator differential (87G) would trip due to CT saturation. You shouldn't be. If a line CT doesn't go into saturation at exactly the same time as the corresponding neutral CT, that creates a mismatch on the CT outputs which the differential interprets as a fault within its zone.

I think the problem is secondary path of gen neutral CTs is much larger that path of Cts near CB that they feed the 87G. Can this cause unequal saturation !!

The length plays a role to the extent it contributes to the burden that helps push the CT into saturation. The first priority it to avoid saturation on through-currents in order to avoid differential trip. But if you don't meet that and find yourself in undesirable situation where a CT reaches saturation then I think there might be some benefit to trying to balance the burdens on both line and neutral CT's (assuming they're identical). I'm recalling back in the distant past some resistors were added to our generator differential circuits in order to solve a problem with generator differential trip during fault outside the zone. I don't think it is a standard / preferred design practice but it did resolve the issue.

=====================================
(2B)+(2B)' ?

 

[jwatty]

I've seen an SA-1 generator relay misop due to a wide sync angle on a local breaker (not the generator breaker). When that breaker closed, the local angle jerked significantly enough to cause a largish inrush into the generator. There was a recommendation from ABB to add a desensitizing resistor on them at one point in time and that was never done. Your relay may be susceptible to similar transient issues.