Power Transformer Micarta Diaphagms vs Relief Valves

[Bambie]

thread238-9659

The manufacturer's manual for their 30 year old power transformer infers that the micarta diaphragm and goose neck vent protects the gas detector relay from seeing pressure transients above 5 psig, which will cause loss of calibration. It also credits the diaphragm with mitigating the severity of damage resulting from explosive pressurization from a fault.
Can a relief valve set to 5 psig with a discharge pipe routed down to the berm (to satisfy environmental concerns) also claim to provide these attributes?
Would the insurance company find a loop hole if a fault occurred?

 

[Bambie]

prc,

How else could you provide overpressure protection for the relay?

If the PRD can provide this protection, why wait for the conservator to go solid?

 

[Bambie]

Scotty and prc,

IEEE C57.12.10 has answered some of my questions:

5.1.7 The pressure relief device must relieve 5000 CFM at 10 psig (but it doesn't specify the fluid)

5.1.9 The sudden pressure relay must withstand a pressure of +/- 15 psig without damage.

PRD protecting the SPR seems to be implied.

 

[ScottyUK]

Bambie,

Just so you appreciate that the PRD really doesn't guarantee to prevent a tank rupture:

For a sense of scale, the top plate is 40mm thick and the cover bolts are M24. The Buchholz relay has been torn in half toward the centre right of the image.

 

[Bambie]

ScottyUK,

Exactly my point; the PRD cannot protect the transformer from a fault.

Since the PRD can only respond to slow rates of pressure rise that the SPR cannot detect and moderate expansion rates from decay heat following successful trips by the much quicker SPR, then (ipso-facto) it's purpose is really to protect the SPR and should be set-up accordingly.

 

[ScottyUK]

Hi Bambie,

My point is that the energy release was so violent and massive that the transformer was doomed regardless of the SPR / PRD / Buchholz because the fault can't be cleared fast enough for an alternative outcome. The fastest-acting relays would be the differential schemes, but they are still limited by the breaker operating times. When the transformer is completely destroyed, protecting one minor component isn't worth worrying about.

 

[prc]

True, PRD cannot protect transformer from fault. It is intended for protecting tank from rupture. But effectiveness depends on the point of fault with respect to PRD position on tank, intensity of fault etc, etc. Its purpose is not to protect SPR,sure. In fact SPR is used mainly in US ( it was introduced to serve the function of Buchholz relay in nitrogen sealed units),but PRD is provided universally. From the discussions that I have seen in this forum, Buchholz is many times reliable than SPR. In case of a fault inside tank, the dynamic pressure developed is many bars (see the earlier mentioned IEEE std) SPR is expected to trip unit on change of pressure rise( C57.12.10 specifies within 3 cycles) and PRD operates many cycles later.

 

[bacon4life]

There are 4 pressure cases to consider:
1. Full vacuum while filling the transformer. I found it odd that the relay did not explicitly state whether it could withstand full vacuum or that it needed an isolation/equalization valve.
2. Slow overpressure due to a maintenance mistake such as:

a. Forgetting to open the valve to the conservator​

b. Over filling the transformer with cold oil, then having the oil warm up and expand.​

c. Filling the LTC with a 1 inch oil hose and having an 1/8th inch breather hole​

d. Malfunctioning nitrogen blanket system.​

3. Moderate speed pressure rise from a moderate energy fault
4. Rapid pressure rise from a high energy fault

In cases 2 & 3, the PRD is provided to prevent tank failure. As a side effect, the PRD would also protect the SPR only for case 2. Also note that the SPR doesn’t prevent anything in case 2. A big advantage of the PRD over the rupture disk is that the transformer reseals itself. A bit of spilled oil is far better than emptying the entire conservator, then having to spend a week doing a vacuum refill.

In case 3, the most important aspect is prevention of a catastrophic fire and collateral damage by preventing tank rupture. If the damage is minimized enough so the transformer can be repaired, consider it a bonus.

In case 4, the tank may fail no matter what protection is provided.

 

[Bambie]

bacon4life,

For cases 2 and 3 what set pressure would you recommend for the PRD (knowing your gas relay detector bellows needs protection from 5 psig excursions)?

 

[bacon4life]

For case 3, even if you can salvage the transformer, you replace the gas detector relay.

 

[Bambie]

bacon4life,

What pressure would you advise prc or ScottyUK to set their PRDs at?

(Hint: you get to choose either 5 psig or 10 psig or 15 psig)

 

[bacon4life]

I am indebted to Scotty and PCR for all they have shared with me through eng-tips, so I doubt they need advice from me. Qualitrol makes PRDs at 2.9 PSI and 4 PSI. The setting needs also to account for the head above the PRD.

 

[prc]

Bacon4life, we are all learning from each other.
Qualitrol and others(Commem, Messko, Sukrut ) all make PRDs up to an operating pressure of 25 psi. Manufacturer/user selects the appropriate one.
Normally in India, customers prefer a setting of 7 psi normally. If we select a too low setting chances of mal operation is more. Thumb rule for selection is as below:
Nominal operating pressure in psi = ( maximum conservator oil head over PRD + 1-2 psi tolerance in setting+ 1 psi, pressure drop for PRD)1.3 resealing factor.

 

[ScottyUK]

Hi bacon4life,

As prc says - this is a place for all of us to learn. I learn from prc almost every time he posts something.


bambie,

My opinion - as an equipment operator / maintainer, not a designer - would be to keep the setting as low as was consistent with reliability. The design features of the transformer are very significant factors influencing the chosen set pressure:
[ul]
[li]conservator?[/li]
[li]silica breather?[/li]
[li]hermetically sealed?[/li]
[/ul]


If a SPR on one of 'my' transformers had been subjected to overpressure due to a serious fault I would likely replace it. The transformer would be going out for rebuild anyway because we don't have that capability in-house and the cost of the new relay wouldn't be a large additional cost in the overall scheme of things. If the fault was less aggressive but the PRD had still operated I'd get the SPR inspected, function tested and calibrated by the instruments group.