Bucholz trip 1

[seanleed]

Hi all, first thread so be gentle with me please.

We have had a new CHP plant installed on site and during the testing an issue has arisen. When the CHP plant trips its 11kV breaker (during functional testing) an unrelated breaker also trips out on Bucholz trip. The CHP plant was wound down and not generating each time so there was no instant load transfer. The breakers are in the same room, on the same bus. The transformer whos breaker trips sits beside another transformer which also has a Bucholz relay- this does not trip. It's the same one each time. The breakers do not sit beside each other, one is panel 5, the other panel 7.

Does anyone have any ideas what could be up? We have Schneider coming on site as soon as they can free someone up, but it would good to get some ideas beforehand.

Thank you, Sean

 

[RRaghunath]

It is wiring mix-up I suppose, since the plant is still under commissioning.
Buchholz relay maloperation I know when a sudden large step load is put on the transformer. Your case doesn't seem one such.
You get the indication in the annunciation system that it is Buchholz trip but how sure are you about it?? It could be some other trip wrongly tagged as Buchholz, couldn't it be!
Checking the scheme drawings and wiring will provide you some clues.

 

[seanleed]

The wiring has been checked back to the inputs on the SEPAM and according to the drawings it is correct. The existing system has been in for 10 years without issue, it's just since the new plant was fitted that it has tripped.
When Schneider come on site we will check the input is correctly assigned and see what goes from then.

 

[cranky108]

"The existing system has been in for 10 years without issue, it's just since the new plant was fitted that it has tripped."

Look at what has changed. The problem is likely there.

 

[slavag]

Seems as some mistake in wiring or protective terminal configuration.
Second option is some dip of DC and unwanted peak up of SEPAM input.

 

[seanleed]

Schneider were on site yesterday for a couple of hours investigating it. No wiring issues, no SEPAM setting issues. The Bucholz input has been going high as it has been recorded in the SEPAM as such. No gas showing in the Bucholz viewing chamber, oil is clean. No one has any idea. They've downloaded everything out of both SEPAMs and will get it all analysed on Monday. Next step is to do a functional trip test of panel 5 (CHP) and record what happens in 7 at the time. They'll switch off the trip output from the SEPAMs initially to see what happens without the physical trip happening and we'll go from there.

 

[RRaghunath]

One possibility is Electromagnetic interference (EMI).
The control cable that brings Buchholz trip from field to the relay is affected by cables nearby along its run.
Check if there are any power cables (especially HT) running along the control cable in parallel and probably for a good distance.
SEPAM must have a solution to make the Binary inputs immune to EMI.

 

[sushilksk]

For what time Buchholz signal remains high? And how buchholz is wired to Trip relay... wired to mechanical auxilliary relay or directly to numerical relay. Numerical relays have low burden for input signals and more susceptible to EMI.

 

[slavag]

Add 20ms delay for input operation

 

[seanleed]

We're thinking along similar lines just now, the Bucholz relay is wired directly back to the SEPAM so we are also thinking induced voltage, the cable supplying the transformer that trips and the multicore with the signal I/O are running in the same trench. I had asked Schneider to put a delay timer in to see if that solved the issue but they didn't want to do that yet.
As things progress I'll keep this updated.

 

[RRaghunath]

Introducing delay in such an important trip as Buchholz results in compromise on transformer protection. Not a good idea.

 

[sushilksk]

for 50Hz, 20ms delay means one extra cycle. During internal fault, fault clearance time would increase from ~3 cycles to ~4 cycles. That may be be risky, but avoids false trips due to DC transients.

Other way is to connect resistor, say 10K ohm (with suitable wattage) in parallel to binary input. With this, time delay of lower value ~2ms can be used.

 

[slavag]

Best way is connect to high speed trip relay, and contact of trip relay to BI of SEPAM...

 

[prc]

I could not understand the connection or position of the Buchholz tripped transformer.A layout sketch may help. But it is a well known fact that Buchholz may trip when a transformer is tripped on over currents. Any overcurrent will result in axial oscillations of winding stack and consequent movement of oil surges in to conservator. This oil surge in pipe can result in relay tripping. Solution - slightly desensitise the relay by raising oil velocity setting in the relay.

 

[seanleed]

The control cable for the transformer that unintentionally trips was running beside the HV cable of the transformer that we intentionally trip.
I rerouted the control cable to avoid all HV sources. We did another test yesterday.
When we tripped the CHP (intentional) transformer 7 also tripped again (unintentional)

Once we can arrange a suitable time to test again, I'm going to disconnect the Buchholz trip input and see what happens then.
If it still trips it's nothing to do with the input and everyone is stumped.
If it doesn't trip, I've done a new Logipam program to load into the SEPAM with a 20ms delay for the Buchholz input and we'll see what happens from there.

 

[waross]

Try disconnecting the leads at the Bucholz relay.

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

 

[oldfieldguy]

The talk of signal coupling between adjacent power and control cables brings to mind an issue I had a few years back with the upgrades to motor controls. The client issued drawings to upgrade the controls for a large (several thousand HP) induction motor from electro-mechanical protection and control relays to solid-state, microprocessor equivalents.

I wasn't unhappy to install a GE SR-469 in place of adoorful of electromechanicals, but the plan also included replacing an old Agastat electro-pneumatic time delay relay with a solid-state time delay relay. The problem came in when the enabling device for the Agastat, a zero-speed switch, was used as the enabling input to the solid-state replacement. On the Agastat, 120 VAC went out to the switch, came back and energized the Agastat coil. The new relay was simply looking for a closing contact (no current load) for initiation.

The controls mis-operated horribly. Wiring was checked and recheck. The problem was determined to be cross-talk on the pair of wires from the speed switch. In troubleshooting, a portable resistive element ( a 100-watt incandescent lamp) was placed in parallel with the input to the relay. The problem disappeared, the resistance acting as a ballast to keep crosstalk below the point needed to initiate the timer relay. The semi-permanent solution was to use a 100-watt Chromalox heater element to replace the lightbulb as a ballast resistor.

This was not the only problem I uncovered when upgrading and mixing older and newer technologies. Another painful lesson learned between me and GE was the pitfalls of timing between electromechanical contacts and virtual solid-state contacts. Control systems that worked fine when all electromechanical went bonkers when parts of the system were modernized with microprocessor based elements.

old field guy