Buchholz relay for transformer protection 3

[electricuwe]

In Europe the Buchholz relay is a very common protection device for oil-filled transformers, even for small (several 100 kVA) power ratings. It looks like that in other parts of the world the Buchholz relay is also known, but not very common. I would like to know if my assumption is correct at what type of protecion is used instead.

 

[SALMANENG]

In Pakistan, normally more than 250 KVA rating transformers are with buchholz protection. It is very common to have buchholz relay for transformer protection.

 

[redtrumpet]

I have seen the Buchholz relay on several transformers in Canada at sizes 7.5 MVA and above.

 

[gordonl]

As Redtrumpet mentioned, Buchholz relays are common in Canada. (required on conservator design transformers over 7.5MVA by CSA C88)

The Buchholz relay performs two functions:
1) Sudden pressure trip - this based on oil flow to the conservator, so if a sudden rush of oil (or gas) flows to the conservator then the transformer trips. This fuction is can be replaced by a rapid pressure rise relay on a transformer. This provides protection for major faults, (fast expanding gas) and major oil leaks.

2)Gas Accumulation - The Buchholz relay also gathers bubbles rising to the conservator. Once enough gas has accumulated an alarm is set off. This provides early warning alarms for several types of faults: core faults, interturn faults, faulty joints. These types of faults may be detected with some of the new electronic monitoring equipement such as partial discharge or online dissolved gas monitoring, but is much more expensive than a Buchholz.

The buchholz also provides a point for drawing these gasses for analysis, which can provide valuable diagnostic information.

They can only be installed on conservator type transformers, they cannot be fitted on sealed tank design transformers. I'm not sure why they are more popular in Canada, perhaps because of former ties to British electrical companies, or perhaps our large ambient temperature swings.

 

[peterb]

Gordonl, I think you've hit it with the large ambient changes. The conservator design allows room for expansion/contraction of the oil, where a sealed tank design will need to absorb this by the gas cushion. I have also seen hybrid designs, where the conservator tank is sealed by a type of gas-filled bladder - this avioids the need for a dehydrating breather on the conservator.

The Sudden Pressure relay is common on sealed tank transformers - this is a feature on many US transformers and is the equivalent to the Buchholz relay.

 

[dpc]

My experience in the U.S. has been that these are always provided on conservator type transformers. But the vast majority of transformers in the U.S. do not have conservators, but rather use inert gas (nitrogen) in a sealed tank. These use sudden pressure relays.

The only conservator units we see come from Europe.

Hope this helps.

 

[jbartos]

Suggestions:
1. ANSI/IEEE Standard 242 describes various types relays for transfomer protection; however, it does not address the Buchholz Relay explicitly.
2. Buchholz relay is convenient to use for the transformers that have their oil reservoir. Since the oil reservoirs cause oil contamination, this relay principle has become obsolete in USA.
3. In transformers having a gas cushion instead of a conservation tank, the tripping of the Buchholz relay is not appicable and it is replace by a sudden pressure relay which is built into the tank and operates on the basis of rate-of-increase of pressure.
4. There are more postings on this relay topic in this Forum, just a few clicks away.

 

[peterb]

A quick look at both the ABB & GE websites showed pictures of conservator type transformers under the category of large power transformers. Another quick look at the GE website showed a listing of Buchholz protection as an available option for power transformers (

http://www.geindustrial.com/products/applications/pt-optional-accessories.htm).

I don't think that a sealed tank design is really an option at the higher ratings, regardless of the country of origin.

Re the oil reservoirs causing oil contamination, refer to my earlier post for a mention of a couple of commonly accepted methods of protecting against this ( dehydrating breathers and sealed reservoirs).

 

[jwerthman]

Most of the US transformer manufacturers do not use conservator systems as a standard, but will provide it if specified. For smaller units (roughly 20 MVA OA and less), most use a sealed tank with some oil expansion space above the core and coils. The space above the oil is usually filled with nitrogen or dry air. The tank is not allowed to "breathe" except under severe changes in pressure that could damage the tank. As a result, the pressure in the tank varies over a wide range from low ambient temperature no load to high ambient temperature full operation.

For larger units, most US manufacturers use a nitrogen blanket system to keep a constant pressure of gas above the oil. As the oil heats and expands, nitrogen is expelled to atmosphere, and as the oil cools and contracts, new nitrogen is added from a nitrogen bottle through a pressure regulator.

Buchholz relays can only be used on conservator designs and US manufacturs will furnish them if required (but only if the conservator system is used). Sudden pressure relays can be used on conservator systems, nitrogen systems, or true sealed tank designs.
Every engineer has their own list of pros and cons about each of these systems. And most feel quite strongly one way or the other, usually based on some bad experience with one system or the other.

 

[electricuwe]

Thanks to all for the fast and detailed response.

 

[prc]

Electricuwe,let me add something on the subject eventhough bit late:
Since one engineer from your country-Buchholz- invented a gas operated relay in 1923 for protecting trfs from incipient faults ,no other instrument has given so much reliable operation for protecting trfs and evenafter several decades this is going strong all arond the world with out very little change from the original design.It actually gives three protections 1)If a minor fault occurs inside trf and persists ,then gas bubbles get collected in the relay and gives an alarm by the operation of the top float.2)If a devastating fault occurs inside trf then enormous gas is generated with the movement of windings .This creates a sudden surge of oil towards the conservator causing the upper float to cause tripping of trf and there by isolating the trf from system to prevent further damage to trf.3)If the oil level in conservator goes down and become empty ,then the relay will trip trf .

To achieve the above three functions,three instruments are used with gas cushion system.1)Gas detector relay connected to trf cover-this is basically a magnetic type liquid level gauge with a float operating in an oil filled chamber and a snap action switch to give alarm. 2)Sudden pressure relay -connected to either at tank bottom (oil sudden pressure relay) or in the gas cushion area (gas sudden pressure relay)3) Magnetic oil level gauge with low level alarm contacts.
For more details of these systems refer to IEEE std C37.91-1985 Guide for protection Relay applications to Power Trfs .
But it must be emphasised that sudden pressure relay is slightly inferior to buccholz relay when reliability of operation is concerned.Bucholtz relay is used not only in Europe but all over Asia and Japan too (They were earlier using a modified version called Pitot relay)I believe in US also Buchholtz relay is quite common as I remember supplying several trfs to US market with conservator and Buchholtz relay.
Coming to the "suggestions",I dont think oil reservoir will cause oil contamination . Many years back old BBC made a site study with various types of oil preservation arrangements (See BBC Review 1965)and found not much difference between them in results.Gas cushion has the serious disadvantage with EHV trfs ie the dissolved nitrogen will bubble out when trf oil temp changes with suden load change or ambient temp and these bubbles when stick between live winding discs cause risk for dielectric integrity.So probably the trend in US also will be for conservator tank with Buccholtz relay.Pl correct if my understanding is wrong.

 

[jbartos]

Suggestion: Check with the manufacturer anticipated trends, e.g.

http://www.geindustrial.com/products/brochures/sst_brochure.pdf

etc.

 

[electricuwe]

I'd like to add some additional information to my initial post to avoid misleading interpretation:

Also in Europe transformers with hermetic seal are known and used, but only in the lower power range and for distribution transformers.

 

[dpc]

There has been some implication that the inert gas oil preservation systems are not used in the U.S. for very large transformers. This is not actually the case.

There are many 500 MVA to 1000 MVA transformers in the U.S. without conservators, using nitrogen blanket above the oil.

Just wanted to clarify that. We are seeing more conservator-type designs due to increased competition from manufacturers outside the U.S. but there is no size limit on the inert gas type systems. Also, I think the newer conservator designs utilizing the internal bladder in the conservator to eliminate the direct interaction of oil and air have made the conservator approach more acceptable in the U.S. prc has an interesting point regarding nitrogen absorption issue in EHV transformers. I have not heard of this before.

Each approach has advantages and disadvantages. I think if one had a dominant advantage, the other style would have disappeared long ago. But that's just my opinion - I could be wrong.

 

[electricpete]

I know one large US utility that has the large Generator Stepup Unit (GSU) transformers sudden pressures and gas relays wired for alarm only.

Protection for the GSU's is provided by two independent differential schemes. There is also a distance relay (21) looking from the generator partway into the GSU.

In my years at one of those plants, we have had one false trip of a GSU sudden pressure relay. It occurred when a switchyard breaker failed, creating a very close-in fault for our generators (through-fault for the GSU's). Three out of four of the GSU suddent pressure trip devices (we have 4 GSU's) all tripped. Luckily we only got an alarm, not a trip. Apparently it was the winding movement that caused the pressure surge?

Perhaps the propensity for false trip is the basis for not using these devices? (I'm just guessing based on limited experience discussed above).

 

[jbartos]

Suggestion: Visit

http://www.tde.alstom.com/transmission/

http://www.abb.com/global/abbzh/abb...1C616&e=us&c=B3BB2C2CC01FF2A6412567E700352D8A

for more info on large and medium power transformers

 

[sparky1976]

Guys,
Good thread,
I have experience problems with bucholz relay, it not seismic proof, we had earth quake 5.7 ritcher scale, and trips big Xfmrs, we nearly had black out that day.

we are thinking to avoid such event by adding time delay to bucholz relay maybe 100 ms to 500 ms.

On our Xfmr we have Bucholz and Rapid pressure rise relay and both now trip the Xfmr instantenously,

Any comment of adding time delay to bucholz and leave the RPRR to trip Xfmr instantenously ?

Thanks

 

[dommdd]

Mid Mediterranian we have a Brithish Clonial history and we use to have this protection for Txs of 1MVA and higher. Due to our high rate of change in temeprature we chose to use Buckholts on 500KVA as well.

 

[SphincterBoy]

Electricpete: Excellent comments. I am (now) having my doubts about the sudden pressure relay (we have them on all our transformers, based on the experience you related.

If installing the conservator tank is required to utilize the Bucholz relay, then it's worth it.

Another large utility in our state advised us to configure
the SCADA system for ALARM ONLY on the sudden pressure relays. The problem you mentioned might be more widespread,
and needs to be addressed by IEEE.

Also, can't the oil in the conservator tank be changed
periodically, or/if it becomes contaminated? This would avoid any problems with contamination migrating into the main tank.

Thanks for the great post!

 

[prc]

Yamin,solution for your problem is to use special Bucholtz relays with reed type cotacts instead of normal mercury contacts. Trf industry is using such relys for many years. A company in Italy ,Cedaspe (earlier ASPE) is specialising in such relays .Pitot relays used in Japan also meet such seismic requirements.
Boy, the oil in conservator is always in contact with main tank oil and moisture will spread to main tank immediately.We we cannot achieve much by changing the oil in conservator.