Buchholz / Sudden Pressure Relay 6

[PeterS84]

Hi All,

Can you tell me how do you connect Buchholz/SPR to trip CB - do you connect it directly to the CB or do you connect it via IED's binary inputs (or both - in parallel)?
By the way - is there a difference between Buchholz and SPR?

Regards,
Peter

 

[PHovnanian]

prc, the selection of SPR vs Buchholz depends on the tank and ventilation configuration. Like Scotty said, SPR is used where you have a sealed tank. Buchholz is used with a conservator tank. The presence of the conservator tank reduces the overpressure caused by an internal fault by giving the oil someplace to go. This desensitizes the SPR, in some cases making it useless. The Buchholz detects the fault event by monitoring oil movement into the conservator.

One can work out the hydrodynamics and figure out which system would be quicker. But the selection is dictated by the tank ventilation system. There might be a high energy fault condition where adding an SPR to a conservator/Buchholz configuration would react to HE faults faster. But in most cases, the fault conditions are covered by the Buchholz. In sealed tanks, its SPR only as there is no place to put a Buchholz.

 

[dpc]

I'm coming late to the party just to say thanks for the interesting and educational discussion on this topic. I ran into a new transformer with a conservator that had both a Buchholz as well as a SPR, and I did not understand why this was done except that the transformer was built by a US manufacturer who does very few conservator units.

The explanation by PHovnanian makes sense to me, so now I won't worry too much about the SPR in this unit.

FWIW, we specify that SPR or Buchholz relays be provided with a separate latching relay that must be reset at the transformer. The latching relay contact is used to trip an electromechanical lockout relay directly.

Cheers,

Dave

David Castor

http://www.cvoes.com

 

[bacon4life]

Phovnanian,

Although I am an EE not a mechanical engineer I tried an online flow calculator for a 5 cm diameter 2 meter long conservator pipe. At 1400 kPA (200 PSI) the flow is only 100 liters/second. With tank volumes ranging from 20,000 to 80,000 liters, I don't see how it would make much difference on the overall tank pressure. At 1400 kPA, an SPR would react in approximately 1 cycle. Is the advantage is the low energy fault detection where arcing might take multiple seconds?


We have also been installing pipes for directing the output of the pressure relief devices. Now I see that might not be such a good idea after seeing it cut the flow from 6000 l/s to 1200 l/s just by adding 3 meters of pipe.

 

[ScottyUK]

Re. PRD shields and flow directors: if you have ever seen a PRD operate in anger you'll find it makes a reasonably effective atomising nozzle, like a giant diesel injector. I'm fairly sure that the fine spray generated by the PRD contributed to the flash fire immediately after a transformer explosion which killed three friends in 2001.

Having seen that I'd fit two PRDs with flow directors rather than a single one without.


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

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

 

[prc]

PHvonanian, my views are same as yours. Iam talking about only transformers with conservator where SPR makers are pushing it as an additional accessory.

When we provide SPR in open breathing transformmer, its response will be similar to PRD ie if the internal fault is nearer to SPR or PRD,it will pick up.So I consider Buchholz more reliable than SPR.Of course there are cases where Buchholz did not pick up with serious internal fault as the tank opened out releasing pressure and gas

Scotty,normally two PRDs are provided when oil content in transforer is more than 35-40 KL. It is provided on opposite sides/corners to get maximum coverage.PRD shall be provided directly on tank or at end of short,direct pipe with out bends otherwise massive pressure wave will rupture tank before actuating PRD.But providing directing pipe on the outlet of PRD is not going to affect the PRD operation.

 

[PHovnanian]

bacon4life, that's correct. An SPR will react quickly for a fault capable of producing on the order of 100 liters/second of gas. But if I recall correctly, Buchholz relays are sensitive to oil flow velocities on the order of cm/sec which will result from much lower rates of gas generation. At the low end of the scale, a Buchholz will trip for a slow oil leak (eventually).

Its probably true that an SPR will beat a Buchholz for a high energy fault, so if you've got one, you might as well hook it up.

 

[ScottyUK]

Hi prc,

There's a good photo somewhere on the Qualitrol website which shows how different the behaviour of the oil is when it vents directly as compared to through an oil director. The discharge from the director was turbulent liquid, where the direct venting was a fairly fine mist.

I'll have a look for it later - I'm off to beat the traffic I hope!


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

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

 

[prc]

Scotty,I believe oil director will not affect the flower like flow from PRD.Director pipe normally starts from the collection box covering the PRD outlet.

bacon4life,the pressure buildup is not uniform or slow.Recently I came across a failure in a 400Kv winding at site. An arc started at line end creeped over pressboard to neutral end.Since the transformer reactance was not in circuit a very high current -14 kA RMS as per recorder-flowed.At 400kV,it was very high energy release,The pressure wave with gas blasted the outer winding and came out hitting tank,causing tank side wall distortion/weld rupture and PRD operation,but not Buchholz.Speed of response of SPR depends on how near to it is the actual fault point.

 

[Mubeenkhankashi]

@Peter: Buchholz Relay has two connections coming out from two different Mercury switches:
1. Buchholz Relay Alarm connection to ring the alarm in case of early detection of an incipient fault. This alarm connection comprises of two terminals which are connected to two terminals of a control cable. The two terminals of the control cable go into a control panel inside the Control room and gets connected to a Supervision Relay for Buchholz alarm. Once there is accumulation of gas inside the Buchholz relay (Due to a fault), the float of the mercury bulb switch goes down and so the mercury level inside the bulb rises in one end due to the slant. This mercury reaches a level where it can make contact for both the terminals of the Buchholz Alarm and so the circuit to ring the alarm is completed and the 36V DC (Generally in Asia) juice flows into the Supervision Alarm Relay which actuates the Master Supervision Relay for Alarm, which in turn supplies direct DC Volt or makes a contact to supply AC voltage to the Buzzer depending upon whether the Buzzer is DC operated or AC Operated.

2. Buchholz Relay Trip connection works exactly the same manner except that instead of connecting to the Buzzer the 36 V DC juice is fed into the Tripping coil of the circuit breaker, which operates the circuit breaker once the contact in the lower mercury bulb is made which activates the Supervision Buchholz Trip Relay which in turn actuates the Master Supervision Trip Relay which makes the DC juice to flow into the Tripping coil of the CB.

I have made a simplified diagram of the cicuit. Hope you can grasp it well from it. It is an attachment file.

 

[Mubeenkhankashi]

@Peter: Yes the above applies to the Marshal's Box as well... Marshal's Box has Two Indicators
1. OTI (Oil Temperature Indicator)
2. WTI (Winding Temperature Indicator)

Once the Temperature of the transformer oil increase beyond a certain pre-specified upper limit, contacts are made which completes the contact circuit for the same Supervision Alarm and Trip Relays in which Buchholz Unit is also installed and it operates in the same manner as Supervision Buchhol Relay operates because all these units are mounted in parallel on the same Relay i.e Supervision Alarm or Trip Relay which actuates a Master Alarm or Trip Relay...

 

[Mubeenkhankashi]

@Peter: Buchholz Relay is always used to Isolate the transformer from the incoming supply.

 

[Mubeenkhankashi]

@Slavag: It depends upon the situation you are in... Generally Buchholz relay is used to operate the circuit breaker of the incoming line rather than "all CB's". If you want to trip all the circuit breakers that is fine, You need additional circuitry but that way the risk factor increases. Electrical Equipments can malfunction anytime even if they are from the best company. If the CB on the incoming side shows a sluggish response than the CB's on the load side, the load-side CB's will operate on full load resulting in a severe flash-over inside the Arc-Quenchers resulting in damage in the arc-quenchers or the CB contacts. So its better to operate one CB i.e on the Incoming line.

 

[Mubeenkhankashi]

@prc: Why is it necessary for the Temperature Indicators to operate LV only??? Can you please explain??

 

[Marmite]

Buchholz relays need to isolate faulty transformers from all points of infeed. I'm not going to comment on the twaddle about "flashover of arc quenchers".
Winding temperature trip operates the LV breaker because once you have shed the load on the winding you remove the cause of the overtemperature.
One of the design priniples of protection is that you only trip what is necessary to isolate the fault.
Regards
Marmite

 

[Mubeenkhankashi]

@Marmite: How about the rise in winding temperature being due to the internal fault i.e Inter-turn fault??? If you shed the load, the cause of the fault persists.

 

[slavag]

Hello.

As I saied before, our practic disconnect transformer from all sides in case of any "mechanical protection" ( Buchholtz, PRD, thermal, etc).
In additioanl, practic of lot of companies desconnect LV side by HV sided circuit breaker contact, its a wiring interlock/trip, but not via some LOR.

Buchholtz relay is protection (or special electrical protection)aginst inter-turn faults,a temperature monitor is very slow.

 

[Mubeenkhankashi]

Thanks Slavag... I must tell you that i belong to a very remote region of the World and we have a very classical system of Electric substations based of very archaic system of measurement, control and protection. So, not much exposure to latest equipments being used. I like the idea of tripping the CB's of outgoing feeders by the electromechanical mechanism of the tripping system of the incoming line CB. I can imagine what that could be like.

 

[prc]

Mubeenkhankashi,mercury switches are being phased out due to environmental concerns of mercury pollution.

Frankly high temperature trip is rarely or never used in practice.Internal faults will be cleared by fast acting relays rather than slow acting tempearture indicators.Remember time constant of oil temperature is 2-4 hours and that of winding is 4-10 minutes. Overload trip will be taken care by overcurrent relays rather than sluggish oil or winding temperature indicators.

 

[slavag]

Hi Mubeenkhankashi.

Archaic system is not equivalent to bad system .
I was involved to few projects in few regions of the world: different logic of protection and control schemes, its some historical, expirience,etc issue.