Tripping of Transformer on Rapid Pressure Rise Relay 1

[kashali]

We have 08 ABB hermetically sealed transformers installed in 1996. Transformers T1 & T2 are of 8000 KVA and the remaining T3 to T8 transformers are of 1200 KVA rating. 

These transformers remain in continuous service since installation and we regularly perform inspection & testing of these transformers in our annual shutdown.

On September 03, 2014 (10:30 am), Transformer T1 tripped on Rapid Pressure Rise Relay (RPRR). 

Following are the observations taken just after tripping:
- RED indication on Seal in Relay of RPRR.
- Load Current at the time of tripping was 550 A (approximately).
- No abnormal current logged by Current Protection Relay PS-451.
- Following gauge readings were taken just after tripping of Transformer:
- Ambient Temp : 38 degree C
- Vacuum Gauge Reading : 0 PSIG (Not always zero, changes with temperature and load)
- Oil Level Reading : OK
- Temp Gauge Reading : 45 degree C

Following Tests were conducted on T2 after tripping:

- Insulation Resistance Test - OK
- TTR _ OK
- DC Winding Resistance Test - OK
- DGA/ Oil Test conducted in February, 2014 and in September, 2014 (after tripping).
No change in DGA results.

We do not find any cause of tripping. Transformer is energized at no load on September 09, 2014 and no further tripping observed.

Please, advice any suspected cause of tripping of transformer on Rapid Pressure Rise Relay.

 

[cranky108]

Have you calabrated your RPRR relay within the last three years?

NERC is concerned about thes devices misoperating and has a maintenance schedule recomendation.

 

[dpc]

These devices are fairly prone to misoperation. It's a highly sensitive mechanical devices. Agree with Cranky - time for PM on the relay if it hasn't been fully tested recently.

 

[pwrengrds]

If you have relay protection on it, have the relay take a snapshot when it rolls the lockout, then you can look at the event data and determine if a fault really happened. It makes it much easier to trouble shoot these kind of things.

 

[kashali]

The RPRR relay (Qualitrol Model 910)was tested in January, 2014 and then after tripping in September 2014 using Qualitrol Test Kit-013 and found OK.

 

[kashali]

What are the possible causes of false actuation of RPRR (Rapid Pressure Rise Relay)? Can anybody answer.

 

[collies99]

The seal-in real control board has a security circuit if wired to both "a" and "b" contacts and operates more securely than using the "a" contact alone. This then would require the transitions of both contacts before issuing a trip output.

Check if both contacts from the RPRR are use versus a lone "a" contact which can cause tripping by simple bridging of the circuit where the "a" contact is connected.

 

[kashali]

On Qualitrol Seal-in Relay (Model: 909-200-01), One contact (63X1) is used for alarm circuit and other contact (63X2) for the trip circuit.

Although , I have noticed that both contacts of Seal-in Relay energized simultaneously on actuation of RPRR.

 

[collies99]

@ kashali:
63x1 and 63x2 are the seal unit output contacts. The RPRR input contacts (3-wires) wiring to the seal-in unit is better connected for security when employing all 3 wires versus just 2 out of the 3.

 

[Jebb]

I've had this issue before when using "solid state" relays. Unlike a old school relay with a coil that may require a several mA for it to pick up, the "solid state" relays may only need a few uA before it picks up. Solid state relays really look at the voltage, so if the solid state relay has a very low equivalent impedance it only take a few uA of current flow to make the voltage rise enough for the relay to pick up. Any high impedance bridging of the circuit (moisture, corrosion, etc, on terminals) may cause enough current to flow. We solved the issue by putting a 1.5M ohm 1W resistor across the input at the relay. This eliminated the false trips. You'll have to do your own calculations to determine the resistor size you'll need.

Time to do some thevenin equivalent circuits!!!!

 

[cranky108]

You said the relay was tested, but you did not say it was calabrated. There is a difference.

Some utilities no longer use RPRR relays, and others guard there actions with 50 relays because of false trips. Some false trips were because of fast movements of the core or earthquacks, which has led to there removal.

 

[prc]

Thank you cranky and jebb for your interesting inputs.My understanding of SPRR or RPRR relays is that it is used in nitrogen sealed or hermetically sealed transformers as a substitue for gas opertaed relay like Buchholz used with conservator system of oil preservation.I have earlier also heard about many s mal operations of these relays esp from US.But in some markets where Bucholz is widely and reliably used, these relays are pushed by relay manufacturers as an additioinal protection for transformers and gullible users are specifying these relays in addition to Buchholz.

Cranky ,can you explain a little more on how 50 relays are guarded against these mal operations? But in case of sealed construction what other option is available instead of RPRR?

 

[kashali]

Qualitrol RPRR Model 910 cannot be calibrated in field.

Only you can perform "GO - NO GO Test" using Qualitrol Field Test Kit K-0013.

Yes, our transformers are hermetically sealed type with nitrogen gas. And equipped with RPRR and Sudden Pressure Rise Relays.

At the time of tripping no abnormality in electrical or physical parameters of transformers was observed.
Such as, Voltage, current, oil temperature, tank pressure.

 

[prc]

I thought RPRR (Rapid pressure Rise) & SPRR (sudden pressure rise ) relays are one and the same.Or are they different?

 

[waross]

Where there any severe weather conditions that may have caused a differential trip? That is, the transformer pressure was stable but atmospheric pressure dropped suddenly?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cranky108]

We don't use a 50 relay to guard our 63 relays, however I have heard that other companies do because they want to ensure they only operate in a high current condition.
A few companies have disabled there 63 relays because of earth movement concerns, or core movement during external faults, which trigger the sudden pressure waves.

I personally don't have any experence with any of these. I have seen where a technication taking an oil sample has tripped a sudden pressure relay, and the way he was made fun of, I doubt he will make that mastake again.

All of this should be consitered when designing a protection system, or reviewing an older one. And I have heard the case that if you have a differential, why do you need a 63 relay. It's all theory in my head and I have no you must do this case. I perfer to express it as 'it depends', on what your priorties are. Are you trying to reduce outages, or reduce equipment damage, or reduce maintenance.

 

[dpc]

prc,

Our experience with Buchholz relays is that they are equally susceptible to misoperation as the sudden pressure relays. I don't feel one has any substantial reliability edge over the other. The sudden pressure relay has been used for decades. If properly installed, tested and maintained, they are a valuable part of good transformer protection. Both types of devices are sensitive electro-mechanical devices, so if any protective device is going to cause problems, these are probably the most likely. But I don't see a need for a sudden pressure relay in a conservator unit that has a Buchholz relay.

Cranky,

Regarding the relationship between differential protection and sudden pressure relays - the sudden pressure relays are much more sensitive, especially for turn-to-turn faults.

 

[cranky108]

dpc, I have seen a differential relay pickup first on a fault in the PA winding. So they are sort of complementery in my view.
I also like the differential because it can cover more than the transformer. But you are correct that a differential has problems with a turn to turn fault.

 

[marks1080]

It's possible an outside signal was coupled onto the input conductor to the relay... I've seen that happen before in many different scenarios, including a transformer gas trip.

Check to see what other signals are in the cable that the pressure relay uses to get its signal to the relay. If you have AC mixed with DC that may very well be the problem. Also check the grounding of the cable sheild. Make sure the the bonding conductor connecting the cable sheild to ground is as short as possible. If you ground the cable sheild using a cable gland this won't (shouldn't) be an issue. If the bonding conductor to ground is too long the cable shield will actually transfer a (potentially) very large voltage to the signal conductors if the cable is exposed to high frequency noise - like the stuff you get when operating an air insulated disconnect switch.

Mark

 

[marks1080]

sorry.... *shield