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6.6kV Terminal Box explosion

6.6kV Terminal Box explosion

6.6kV Terminal Box explosion

(OP)
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

RE: 6.6kV Terminal Box explosion

kantor,

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

RE: 6.6kV Terminal Box explosion

(OP)
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.

RE: 6.6kV Terminal Box explosion

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?  

RE: 6.6kV Terminal Box explosion

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?

RE: 6.6kV Terminal Box explosion

Kantor,

Additional thoughts on connection component details:

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

Nuts:  Aluminum?  Steel?

Active area on copper palm?

RE: 6.6kV Terminal Box explosion

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.

RE: 6.6kV Terminal Box explosion

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!

RE: 6.6kV Terminal Box explosion

(OP)
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

RE: 6.6kV Terminal Box explosion

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

RE: 6.6kV Terminal Box explosion

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

RE: 6.6kV Terminal Box explosion

(OP)
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

RE: 6.6kV Terminal Box explosion

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.

RE: 6.6kV Terminal Box explosion

(OP)
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

RE: 6.6kV Terminal Box explosion

(OP)
Danel and All

Yes relay operation as follows:

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

Regards

RE: 6.6kV Terminal Box explosion

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!

RE: 6.6kV Terminal Box explosion

DanDel

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

RE: 6.6kV Terminal Box explosion

(OP)
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

RE: 6.6kV Terminal Box explosion

kantor

I cannot quote you that but take a look at www.vamp.fi, maybe you can have some info there

regards

RE: 6.6kV Terminal Box explosion

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

RE: 6.6kV Terminal Box explosion

Agreed, dpc...

RE: 6.6kV Terminal Box explosion

(OP)
dpc

I have read most of the questions and answers. I find that some go around in circles. Some are very explicit and technical/engineering, but some?.

Anyway, I thought this is a useful problem of a root cause analysis which I wanted others to know and have an approach to engineering solutions.

I find that some non technical ask fundamental questions and some take their time to answer. Very basics. Things like someone working in a power station saw smoke coming from the Alternator and what the issue is?

Anyway thanks for the talk.

Regards

RE: 6.6kV Terminal Box explosion

Hopefully, you will address the terminations at the switchgear as well as the JB!

RE: 6.6kV Terminal Box explosion

(OP)
Shortstub

The 3 x 3 -400sqmm cable is 160 metres long. The Cable were spliced around the 15 metre mark from the transformer TB with new straight through joints.

No work was done at the switchgear end. Inspection at the switchgear end proved no issues. The cable cores were checked for IR/PI and was very good in the order of GOhms.

Thanks

RE: 6.6kV Terminal Box explosion

Kantor, do you do infra red monitoring/imaging on your system as part of a condition monitoring program?
Loose connections can be spotted and repairs effected before they become a problem by comparing the temperatures of adjacent connections in the same box.  We have an extensive infra red monitoring program implemented at our mine in Namibia and have purchased our own camera for the job, enabling us to repair connections and follow up on their post repair thermal condition soon after the repair.  It has proven itself worth the investment since thermal imaging contractors are pricey and the long wait for post repair follow-ups.

RE: 6.6kV Terminal Box explosion

(OP)
Adar

Thank you for the info. Yes we have an $80,000 Thermal imaging camera where we do online monitoring of thermal images. These extend to 300MVA 275kV transformer connections.

For the above the terminal box is on top of the transformer and the problem was different.

During our thermal imaging program we have picked up hot joints and have rectified. The work is on a routine cycle.

Thanks

RE: 6.6kV Terminal Box explosion

(OP)
sengn

Thanks for the info. Yes it can be fitted and good. But justifying the installation with detection system in transf and motor terminal boxes is quite difficult in the modern era. I will stick to elekies doing a good job in the termination with the engineer providing a good procedure.

RE: 6.6kV Terminal Box explosion

kantor

do you know the costs??

RE: 6.6kV Terminal Box explosion

(OP)
sengn

We have installed in a switchboard and the cost ran to tens of thous.

We have 84 HV 6.6kV motors, 6x300MVA,6x20MVA and 100s of other transformers. I am now modifying the 6.6kV motor terminals with self amalgamating tape and Raychem "COLD BOOTS". This means the terminal is completed covered and any arcing is contained. It only costs $200 for a set of three. Why would I want to go for back up detection systems when if I have not eliminated the root cause. Thats my philosophy. "Keep the house clean", "No bugs then".

Even otherwise, I strongly would recommend to contact ABB or GE on this issue.

I hope I am making the message clear.

RE: 6.6kV Terminal Box explosion

Suggestion: Generally, the nature of the malfunction as described and somewhat discussed above would indicate an abrupt failure in insulation and or isolation impedance between line conductors and potentially the ground. There, cannot be this kind of failure with the good insulation and the good isolation between the line conductors. It might have been a three phase grounded fault at very high fault level, perhaps at the higher fault level than the installation was engineered and designed.

RE: 6.6kV Terminal Box explosion

But the root-cause was a poor terminal connection not ph-ph or ph-ground insulation.  Arcing resulted by the additional current caused when the 900Hp motor was started.  Arc products then caused the ground!  Once initiated, arcing product dispersion also enveloped the other phases resulting in the 3-ph fault.  The e/f relay failed to quickly clear the fault.  Another point is that an arcing ground-fault will produce an effective or rms value far below the theoretical value.  Of course, this will affect the response of the e/f relay.  (Kantor, can you determine actual timing?)

Arc-detection as a cure seems to me to be another case of "catching the mice, and letting the elephants go free!"  But, if one has the money anything is possible.

RE: 6.6kV Terminal Box explosion

Kantor,

One last thought about the connection hardare.  Unless the bolt is steel, and the reinforcing plates as well, the problem will not go away!

RE: 6.6kV Terminal Box explosion

(OP)
Jbartos

The palm and the connection were exposed inside the outdoor closed terminal box. The design then must have taken the clearances into consideration and I checked the clearances and it complies with the requirements. But the root cause is not insulation issues, but the bad joint as shortstub points out to us.

shortstub

yes the hardware is as you have stated.

Regards

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