Voltage Transformer Fault and Blowing Up

The failure of your VTs could be caused by any of a number of factors, none of which we can determine from afar. We cannot see the age and condition of the equipment, we cannot tell what maintenance and cleaning had been done in the past. We can't tell if there has been any modification or change to the electrical system that could have affected its operating conditions. We don't know anything about the power source, to kdetermine if there might have been surges, impulses, or sags in the voltage, or if there are harmonic prproblems. We cannot look at the VT to determine what portion of it failed (insulation, windings, terminations, etc.). In short, you need to hire a qualified electrical testing contractor immediately who can investigate your problem and recommend solutions.

The interesting think is it happened after ''5 years maintenance'', All maintenances and tests were done, we re-operated then VT belowed up (only 2 hours later). Maintenance time nothing changed, all conditions, cleaning were/are well. System is 6 years old (not too old). We dont think any power problem because, it is in common busbar, so not possible to effect only one Clad one VT. We checked everywhere and every connection again again.

If nothing about the transformer has changed then something that surrounds the transformer has. Did you add a big VFD or a large non-linear load? These types of things can cause very high harmonics which in turn cause unexpected equipment failures.

You must turn this over to a qualified professional organization; anyone who re-energizes high voltage equipment without thoroughly investigating, understanding, and removing the cause of a transformer failure has clearly demonstrated their lack of knowledge of the subtleties surrounding this type of problem.

Obviously your problem is the weakest link in your power grid is this transformer. It should be a ganged circuit breaker or relay that protects this transformer. The protection device should prevent an intermittent failure from damaging equipment. Once a proper protection system has been installed, somebody will have to investigate up and down stream of this panel for bad wiring. You may have to use thermal imaging to scan your facility.

reson why. there is a short circuit some where or some thing is overload it. the only to stop the vt from blowing up is to trace where it's overlaoding or shorting out. and

check that it not over heating

hope this can help

blessing regards

What is the country of manufacture,is there warranty,did you check prequalifications of manufacturer before purchasing?.

@RAMConsult; we didn't add any load, nothing was changed.

@hi-fi power station sound; i was thinking trying your way, i will try

@pnaban Manufacturer from İstanbul/Turkey, the company is reliable, before we didn't face any proble with this transformer, i called manufacturer they told me send us blowed transformers they will assess VTs that what happened. But i don't have time for this research results.

I look at the picture and it has got phase on it. and it was great to help

take care & peace

Did you do any checks on failed VTs? Winding resistance? Winding -ground, winding-winding? Any fuses blown?

secondary overload due to switch fault has been mentioned.

What kind of system do you have?

Unearthed systems have their appeal (you mention secondary, but not primary earthing checked), but when they grow in size,capacitance builds up to a point where resonance can cause frequent high voltages when arcing faults occur. The symptom is often CTs blowing up.

So more information please.

Hi all,

I too have encountered the same problem of PT bursting in INdia. We are setting up a new factory with 33 kV voltage level. The PT 33 kv / 110 (/ rt3) / 110 (rt3) has failed both in the 33 kV ICOG panel at the metering yard as well as in the indoor 4 panel switch board. The PT is earthed type with star / star connection in Pri and Sec. This is three single phase dual core VTs of KALPA make. It has been supplied with routine test certificate as per the governing INdian Standard Code and the test voltages as 36/70/170 kVp as per the name plate.

These PT have been working for about a month without any problems or signs of deterioration.

No lightning has been observed during the failures.

No change in the transformer tappings have also been observed during the failure.

No switching ON / OFF of any loads have also been observed during the failures.

One similarity observed during all these 3 failure occasions are that they had occurred in the evening hours i.e, about 6.30 PM to 7.00 PM IST.

One thing to note is that we have 2 Nos. 2.5 MVA trafos 33/0.433 kVoperating in parallel.

I also saw some article about the NCT loose connection can effect the PT. I am nut however clear about the logic / rationale / technicalities behind this statement.

Can any of you throw some more light on the possible reasons for the failures of PTs

shivvaram

See my post number #10.

A sparking bad connection could cause repeated resonant voltage peaks.

Is the time you mention the local load maximum?

The greater the current broken, the higher the voltage spikes.

How is the system to which PTs are connected earthed?

Is the secondary of each PT fully loaded? It may be that adding resistive loading will damp resonance enough to help with your problem.

Is it possible to monitor PT secondaries for magnitude and number of voltage spikes, particularly at the 7 pm "witching hour"? This will establish if you have a high magnitude spike happening often.

If you have a laptop computer, you can get software, which will trigger on overvoltage above normal peak and record multiple events e.g. Sigview (which has 21 day free trial) using sound-card input. You need to take care with insulation and isolation of course - my approach is to use two 3.5 kV rated 1 Megohm VRW37 type metal glaze resistors in series in the divider [e.g. 2 meg/2 kohm, if you cannot get VRW37 easy, use 10 x 220 kohm 500 volt axial lead resistors in series] to the audio input.

Hi,

i am facing similar problem in solar plant. Can you tell what was the reason and how it is rectified

Pravin

See my original reply in #1. We cannot diagnose your situation from afar. You need to hire a qualified high-voltage electrical testing firm to come and investigate the situation. There is certainly a cause, but we can't see it from here, especially with the lack of information provided.

We have no knowledge of you system unless you tell us.

One cause of trouble can be seen in the photo of the VT at the start of this thread & the reported voltages from the 3 VTs.

The system is 11 kV line to line with 3 wires. That means the balanced voltage to earth is 11/√3 kilovolts, that is 6.35 kV, as reported voltages A & B confirm - C is low. Note the text on the VT "U1: 11000/√3" means its primary is rated for 6.35 kV continuous, that is its normal voltage Un. The next line is 1.9 x Un 6h (max 12 kV) - that means it can survive 12 kV for 6 hours, when one line is faulted to earth.

The "devil" is that many remote lines made to minimum cost have just 3 wires and are only very lightly earthed, so that any earth fault is such a small current no consumer is affected.

At the most basic, at a board with several outgoing lines, the operator observes that phase A has gone from 6.35 to 0 and B & C are now 11 kV to earth. He turns off each feeder in turn to see if the zero volts disappears, re-closing if not, until he finds the faulty line - maybe when he recloses the fault has gone - if necessary he isolates faulty line & calls maintenance. If he isolates any alarm & sleeps for 6 hours, two VTs may have been damaged & fail later. A greater degree of automation may disconnect a faulty line automatically - but automation can fail.

Unless you have some monitoring & recording you may not even Know you had an event. Small solar systems may automatically reconnect after a mains fail but have no memory of that.

An elementary monitor could be an AC 60V relay on each VT, held by it own normally closed contact - a released relay would signal a volt drop of > 70% of several cycles duration. Note in the photo each VT has a da-dn winding of 33 volts, correct correction of these windings in series A-B-C [with 2 wires da-dn', leaving two unconnected terminals da-dn] makes an "open delta" circuit which delivers zero volts if all VTs have normal balanced voltage & phase but 57 volts if one high voltage phase is earthed by a fault. A contact of the detector relay could connect a battery powered mechanical dial hour-second hands clock to record fault duration.

A problem with these essentially un-earthed systems is that, as they grow, the system capacitance may become enough to resonate with VT primary inductance & cause damaging overvoltages on the VTs. Eventually, they get modified to low resistance earth with fast earth fault protection.

There might have been a one-off fault on your supply system, but if it were a persistent problem not fixed by power supplier, you could consider 11 kV VTs with 100V secondaries - bigger dimension?] in place of 6.35 kV [same 1.9 times overvolt withstand i.e. 21kV ] and Surge arrestors.