Probable Cause Of VT Failure At Site

I would say the serial number or, second best, the original purchase order number for the original equipment, so that a replacement can be quoted, purchased, and delivered. The sooner you can do that, the better. So you need to be getting on the phone rather than mucking about with CR4.

But by replacing the same again same failure report may come as this phenomenon is very frequent.

Why dont' you try to find out some root cause for that.

Who knows it may be some design issue & not a fault of the VT vendor!

Any idea?

without more information, it will be difficult to determine that.

Going on the assumption, that these specifications were already given to determined the requirement when it was originally purchased.

Is there something that you missed to tell us?

Did you verify this is the environment and its stable?

1PH,PT,CKT CONN,RATIO:33KV/RT3/110/RT3/110/RT3,CORE1:100VA CL-0.5,CORE2:50VA,3P BIL:36KV/70KV/170KVP,OVF:1.2 CONT,1.9 FOR 8 hrs,

and

What information needs to be communicated to local vendor?

All available information

CR4 readers are not in a position to determine the root cause of anything. That task lies entirely within the domain of the original poster's activities.

CR4 is there to provide signposts.

You haven't described anything at all about the failure. Was there arcing, sparking, burnt wiring or insulation, etc.? Or just an open winding, with no output to whatever attached instrumentation?

Insufficient information for any diagnosis.

These VTs are epoxy cast draw out type. It has primary protection with HT fuse & secondary protection with MCB s. These are 3 nos 1-phase VTs mounted on a trolley & manually draw out type.

It can be mounted in incomer side or outgoing side in switch gear panel & sometimes at the bus side also.

Failure reports are coming from site i.e VT is not measuring, or some flash over seen on the terminals of VTs on the epoxy body etc.

Now as it is epoxy VTs looking from outside it will be difficult to understand or identify the root cause & one has to break the casting to find the core , winding etc.

Now my query is whether anybody in this forum can throw some light on it so that any probable cause can be found out.

Regards

The original poster and the equipment supplier can. One might start with a simple telephone discussion. After all, the original poster can see the installation, unlike all other CR4 readers.

There are too many possible causes for us to diagnose, with no access to the equipment or the installation where they are located. You need to do an investigation, or hire someone qualified to do so.

You are suspecting the VTs, but if you have had the failure multiple times and have installed a replacement (which failed), then that is not likely a cause. The problem could then come from two directions:

1. The VTs are improperly specified for the application in the first place, and are being stressed beyond their design capabilities in some way. If proper design procedures were followed in the first place, this would not be an issue, but it is possible.

2. There are other conditions in the switchgear which are stressing the VTs. This is probably the more likely. You mention flash-over on the terminals of the VTs. Since they are on draw-out trolleys, I would suspect that the sliding terminals used to connect/disconnect the VTs from the circuit are possible culprits. You will need a bus/equipment outage to inspect them properly to see that the contacts are mating cleanly and solidly. I have seen problems in switchgear with draw-out VTs from just this cause. The fact that there are times when they are "not measuring" would reinforce this.

You have to be a detective to solve this. We cannot do that for you. You have to gather hard evidence in the field as to what REALLY failed. Is the cause from outside the VT or inside. The VT itself may have failed, but is everything outside normal? Based on experience, it is more often the outside conditions that cause the failure, not the VT just randomly failing. Post again when you have done some real investigation, or hire someone to do it for you.

A good source of qualified high voltage testing companies is the InterNational Electrical Testing Association (NETA). Check out their web site http://www.netaworld.org/. There you can search for member companies who might be able to help you.

I agree with you Mr Peter & elaborating as below for further scrutiny,

1. Regarding the VA burden of VT we have checked that the load connected across it is well within the limit & even much below the capacity. If the load connected is much less than the capacity ( here it is 100VA/Ph) the accuracy class may hamper but it will not lead to any VT failure. Also the primary & secondary neutral ( 3 nos 1-ph VT ) are earthed through earth bus ( Y/Y-OD). Open delta winding also duly connected with a damping resistance across it (value= 80 ohm,150 watt,backed up by calculation). Hence kindly judge whether from this any failure may happen or not !

2. The VT is basically GUN type with spring loaded contact which is manually draw out type. The spring loaded contacts are connected to the vertical copper riser for its primary connection while it is inserted fully. Now the switching ON & OFF of VT is purely done manually & no such primary isolator is there for disconnecting the VT. Here switching surge may arise if anybody wants to withdraw VT from charged condition which may lead VT failure which I guess ( may be wrong). Is there any other source of possibility of failure ! In case all the three phases do not touch the vertical riser simultaneously any unbalance may lead to VT failure or not ?

3. System earthing can contribute any scope of failure or not !

4. VT internal design a) Poor quality of epoxy (insulation), b)Wrong gauge of copper winding,c)The clearance between the neutral,earth terminal & phase terminal may be not sufficient which may lead to direct short ckt of VT winding.

Any other probable cause if any?

My point is as a designer if all is ok I shall catch hold of the VT vendor & shall challenge his design.

Your Views please.

I agree with RAM. More often than not it is not the equipment itself that is at fault, but how it has been installed or other factors in its working environment. Are the contact fingers making properly, both primary and secondary, on the VT? You can only ascertain that by a physical inspection IN PLACE, DE-ENERGIZED. Are there any other things in the compartment where the VT sits that are impinging on the clearances that are required for the operating voltage? Are there environmental issues such as temperature, moisture, dust or other contamination, etc., which could lead to tracking or flash-over? These are some of the questions you need to answer, not keep pointing at the VT manufacturer.

You have not mentioned the local environmental condition. Please check whether anti condensation heaters inside the circuit breaker panel are functional. Due to condensation effect water droplets may accumulate over outside surface of epoxy insulation which may cause electrical flash over. Also please check proper earthing of the carriage when put in service condition.

Manindra.

There are lots of good suggestions already posted. Speaking from a Quality perspective, where Root Cause is ALWAYS traced to a process and a Corrective Action is ALWAYS made to a process, I would have to ask a lot of questions. The first question is "what symptom(s) of failure do I see?" Followed by "why did that happen?" Followed by "why did That happen?" Repeat until you can no longer find a question.

I would think from what I have read that whoever designed the system did not actually capture all the requirements and/or incorporate them into the design, so a change to the design process may be required.

A robust design will be able to withstand large variation in its inputs and still perform. What are the inputs? Here is a short list of inputs to consider in your Root Cause Investigation:

What are the Mains voltage and current? What kind of transients are likely?Environmental temperatures? Environmental contaminants such as dust or gasses or moisture? Vibration? What about the operators? Are they trained or certified? How? Are their work or operating instructions complete and accurate? Can the operator tell if a problem is occurring now, or is likely to occur soon? What about the original installation? How is the site chosen and prepared? How was the installation accomplished? How were the installers trained and certified? What kind(s) of maintenance is required? How often? By whom? Did purchasing have the correct requirements before the purchase order was made? How was that verified? Was the correct purchase order made? How was that verified? Did the supplier understand the order correctly? How was that verified? Did the supplier supply the correct item(s)? How was that verified?

Until you know all the inputs, including design inputs, you won't know if you have uncovered the Root Cause. And nothing beats going out to the site and looking at it with your own eyes and talking to the people who work with that equipment every day. I can personally attest to finding a very conscientious worker who cleaned his area every day, but added bleach to his cleaning solution because that was obviously "better." Bleach was an unexpected input because we all "knew" it was not on any list of materials, but I could smell it when I visited the factory floor. We changed the process for controlling what materials are allowed onto the factory floor.

Also, during the design, were Failure Modes and Effects considered and planned for? Did you develop a Fault Tree and/or a Failure Modes and Effects Analysis (FMEA)? Either of these would be helpful in tracing the cause of failure.

Yes, that's the lengthy process I agreed. I can categorize in three parts to find out a probable cause though may not be accurate as all information /data may not be available.

1. Customer end, 2. Manufacturer end, 3. VT vendor end

These equipment are indoor type installed in well protected substations. IP-4X is the ingress protection of the equipment. Equipment design parameter is 50 deg ambient. Transient or harmonic presence in the main supply customer can not provide. Customer was given operation manual for their maintenance. Hope they abide by .

Routine test of the equipment being done at manufacturer's premises. HV test also done. But VT charging inside the equipment only done at customer's premises during erection testing & commissioning time.

Last is VT vendor, the internal design is with them only. They are supplying correct material as per specification but we are ignorant of the VT internal design.

Any VT protection scheme can be adopted or not ! Like surge arrester or anything beside VT fuse, MCB!

What the other countries are doing on this aspect ? How robust the design is?

Need your feedback.

Has failure been caused by momentary paralleling?

Perhaps you need to look at all the conditions, and go through the whole interconnection to make sure every thing is snug and tight...

Humidity? Salt air???

Burn evidence, hot bushings, hot sleeves, melted solder?????

The VT has OVF of 1.9 for 8-hours. This suggests that the 33kV system might be ungrounded type.

Broken delta secondary is also provided, again suggesting that this probably is meant for ground fault detection in 33kV system (again is the practice followed in ungrounded systems).

If true, the probable cause of VT failure is arcing ground in the system due to uncleared ground faults (first fault is only wired for alarm and if ignored can lead to arcing grounds and equipment failures).

In my opinion, if there is a voltage drop-or interruption-this could be a sign of intermittent short-circuit on the low voltage circuitry. Check the MCB if it is well rated.

NEC 2014 art.408.52 Protection of Instrument Circuits:

overcurrent devices has to be rated 15 amperes or less.