6.6kV Terminal Box explosion 1

[neps3]

Hi

This is an intersting piece of puzzle and I would like to welcome those interested to analyse and contribute.

Transformer size -20MVA, 15.75/6.6kV
15.75kV connection is via the PIB.
Terminal Box 6.6kV- 3 cables per phase. 400sqmm Aluminium.
6.6kV bushing is vertical and the palm is Copper.
Aluminium Cable with Aluminium lugs 400sqmm.
Connection - with two bolts 12mm dia. per lug. 70mm long x 50mm width. Two holes per lug but very close. Since the hole on the bushing palm did not align the hole was drilled on the lug. This results in one hole to overlap 50% on the other.

SOE Sequence of Events show that a 900HP Pulveriser motor was started before the explosion of the terminal box which ripped off the bleed pipe to the Conservator and caught fire.

Although the transformer did not fail, the rewinders advised that the coils have moved. The business warranted rewind of the transformer.

On receipt I did some modification.

I would welcome your analysis please.

Regards

 

[nraju]

kantor,

Can you explain more about the modification you have done? Also, what are the relays connected on the secondary side?

 

[neps3]

nraju

Standard HV transformer relays. diff,o/c,e/f,nef etc.

I like to welcome a root cause analysis please. The relays that operated are e/f, diff. I will brief on the mod later.

 

[Shortstub]

Just to start:

What relay flags were up?

Approximate installation age?

Mod age?

Was 900 Hp start successful?

Details on neutral earthing method?

SOE for e/f and diff?

 

[Shortstub]

kantor,

Some additional food for thoughts based on similar incident caused by JB being too small to dissipate termination losses:

Conductors: single-core; 3-core; 3-1/2; insulation-type; round; segmental?

Connection: Crimped; soldered; welded?

Junction Box: Dimensions.

Observation: Was JB uncomfortable to touch?

Note: In previous message, I asked about start of 900 Hp. I meant was there a noticable time interval between start and explosion?

 

[Shortstub]

Kantor,

Additional thoughts on connection component details:

Washers: Steel? Aluminum? Conventional? Belleville-type?

Nuts: Aluminum? Steel?

Active area on copper palm?

 

[DanDel]

I'm not too clear on what you are saying and what you are asking, but it sounds like you had a fault inside the secondary (6.6kV) terminal box of your transformer very soon after the start of a 900HP motor which was fed from this point.
If this is true, I'm not sure why there was a question about the secondary relays.
I understand that a differential relay would trip in this case, but what is an e/f relay?
You seem to be concerned about the cable lugs failing due to the holes that were drilled to make them fit. Give us some more (un-convoluted) information about the actual fault. What actually failed(burnt, melted, flashed-over, etc) inside the terminal box? This should be easy to see and simple to analyze, unless your terminal box was destroyed.
And anytime you want to give us more 'pieces of the puzzle', especially as far as your 'modifications', please feel free.

 

[Shortstub]

DanDel,

An e/f is an Earth-fault relay!

I may be wrong, but I believe Kantor was describing a transformer connection to the switchgear via multi-cable runs instead of bus duct!

 

[neps3]

Dandel

I mentioned about the terminal box explosion. What was found are as follows:

TB side plates ripped open.
Cable lugs melted- all phases.
One side of the TB vertical plate was melted by the arcing.
The palm was melted in the areas concerned.
The TB was on fire. Not the transformer inside.
The TB was on the transformer with cable connections.

Within a few ms of the 900HP motor start the event happended. I would not worry about the modification at this stage.

Regards

 

[Shortstub]

It is agreed that determination of connector installation details at transformer JB is impossible! What technique was used on opposite end of cable?

 

[sengn]

You should have used a differential relay with arc sensor. This way the relay had tripped the breakers extremely fast.

 

[neps3]

sengn

How many terminal boxes are fitted with infra red sensors in the world. Do you think we can justify. I have fitted sensors for generator slip rings but not terminal boxes.

Shortstub

Unfortunately the cable lug is Aluminium and the bushing palm is Copper. The guys here have used Alminox instead of a bimetal plate. Three cables per phase. The opposite end goes into the Switchboard cell termination about 150 metres away.

Regards

 

[DanDel]

Shortstub, thanks for the definition of the e/f relay. We(in the good ol' US of A) would call that a g/f, or ground-fault relay.

Without any forensic evidence at the site of the fault, you might have to assume that there was a bad connection in the terminal box which failed, melted, and plasma-arced within the short time of the motor start. I would make sure any of these connections, whether mechanical or compression, are made correctly and without any drilling.

I don't think anything would operate faster than a well-set-up differential relay.

 

[neps3]

Dandel

I am sorry for mingling words. Yes it is ground fault relay (g/f). Sometimes I forget that I am not is the US of A.

Yes your analysis is correct.

Over time the Aluminium to Copper reacted and with the bad bolt connection had become loose. During a start of motor, you know the cables move, transients, arcing etc. The arc isonisation caused a g/f to the Blue phase causing further ionisation of the TB chamber resulting in a three phase to ground fault. Lead to explosion of the TB. The lugs and palms were melted.

Modification.

New bushing & palm.
New cable (Aluminium). Only a short length in each phase.
New bolted design to evenly pressurise the connection with belville washers. But with an additional plate on the bolt head side.
Finally, each bolted connection was covered with insulation. Raychem. The purpose is to contain any arcing in the event any repeats occur. I am getting all 6.6kV terminations to be wrapped in Raychem/self amalgamating tapes. This is only for Al/Copper termination.

A star for you Dandel.

Regards

 

[neps3]

Danel and All

Yes relay operation as follows:

1.diff relay
2.o/c relay
3.g/f relay

Regards

 

[Shortstub]

Kantor,

No need to apologize! Some of us also worked outside of USA!

Regarding Al/Cu connections you might want to look into direct tension indicators as substitute for Belleville washer. Former measures "force!" Latter uses "torgue" which may not alway result in proper force!

 

[sengn]

DanDel

Well, there are arc protection relays that work within 7 msec.

 

[neps3]

sengn

Yes I have used infra red detection (arc detection) relays particularly in the slip ring of 45MW Alternators. But have you seen anyone installing arc sensors in terminal boxes. Can we justify the installation.I would be happy if you can quote me a station that has infra red detectors in teminal boxes.

Cost Optimisation is a term in this era.

Regards

 

[sengn]

kantor

I cannot quote you that but take a look at

http://www.vamp.fi,

maybe you can have some info there

regards

 

[dpc]

kantor,

With all due respect, if you already knew the answer, you should have skipped the quiz and saved everyone a lot of time. If you had a specific question, you should have asked it straight away.

I think we've all taken enough tests in our life, already.

This is the second time you've asked a question for which you already knew the answer. I'm not sure what the point it, but it doesn't seem too useful.

Dave