MCC fires originating from current transformers 2

[Eng2013]

I am an Electrical Engineers in a flammable material processing plant and I kindly need assistance from the forum regarding two fires we had in Motor Control Centers (MCCs).
In the last three months we have had two fires incidences in two different MCC’s, both originated from primary wound current transformers (CT). The first incident was a fire that burnt the entire motor starter equipment of a 3 phase, 50Hz, 500V, 18KW motor. The damage was so bad to do proper analysis of what caused the fire. The investigation was shelved when it reached a dead-end, with hopes that this was an isolated incident that will unlikely happen again.
3 month down we have a similar CT incident on a different MCC. The CT smoldered to ashes but never led to a fire. The affected motor was a 22KW, 500V, 50Hz, 3 phase motor starter. In this incident the circuit breaker tripped, the earth leakage unit tripped, the contactor welded closed and the overload relay tripped. It is possible that the CT heated up so much after laboring on start-up currents for so many years, and decided to blow. The CT might have disconnected one of the lines and led to a single phasing motor. Due to the high current the contactor welded before the overload tripped and the circuit breaker tripped during the fire.
In both incidents the circuit breaker, earth leakage unit, contactor, overload relay and the CT were properly sized. In the latest fire incident the pump was faultless but the motor windings overheated and burnt.
There is a speculation that there was a loose connection on the CT but there is no evidence to support it. The CT manufacturer explained that it is very unlikely for the entire CT to burn due to a loose connection. CT terminals would burn and open the circuit way before the CT burns.
The secondary side of the CT were connected to a current meter (for indication purposes), and as per design, one leg of the current meter was earthed/grounded without a protection fuse.
Routine maintenance is almost never carried out due to lack of equipment availability. On close inspection it has been noted that most of other similar motor starters have CTs that are cracked and discolored due to heat. The two motors described start and stop as many as 25 times per day.
Now:
- Is it common for primary wound CTs to burn as in the two incidents explained?
- Why should one leg of the CT secondary be earthed/grounded, and should there be a protection fuse between the CT terminal and the earthing point?
- Is it a poor design to have primary wound CTs installed on a direct on line starters that are subjected to frequent starts/stops?
- On inspection of an MCC how serious is it to find cracked and heat-discolored CTs?
- Is there a different explanation to the cause of the two fires other than the cause I have explained?
Thanking you in advance for your assistance.

 

[Eng2013]

I have attached a photos

 

[jraef]

Are you sure they were properly sized and not being pushed into saturation during starting current situations?


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[Eng2013]

Hi jraef
Thanks for the question. Today we spend the day in an investigation, and it was agreed that the CB, O/L relay and the contactor were properly sized. However, the CT was determined to be the root cause of the incident. Although the current ratio was correctly specified, the fault withstand current was exceeded. The explanation is that wound primary CTs should not be used on direct online starters of motors that frequently start and stops. You have correctly mentioned that the CT was in saturation for long.
- Is it common for primary wound CTs to burn as in the two incidents explained?
- Why should one leg of the CT secondary be earthed/grounded?
- Should there be a protection fuse between one of the secondary CT terminal and the earthing?

 

[waross]

CT heated up so much after laboring on start-up currents for so many years, and decided to blow.

It seems that the installation has worked for years.

Although the current ratio was correctly specified, the fault withstand current was exceeded.

This does not imply that wound primary CTs should not be used for DOL motor applications.
Rather it suggests that either:
1> The CT was specified with too low a fault current withstand rating.,
or
2> The fault was not cleared quick enough.
I would expect most of the heat to be generated in the CT secondary. Saturation is a good thing. When the CT is saturated any further increase in secondary current with increasing primary current is negligible.
Is it possible that the CT insulation had deteriorated after years in service and the overload incident was the trigger that caused a failure in a CT that no longer was close to the original specs for winding insulation.
Don't put fuses in CT secondary circuits or in any grounding/bonding circuits.



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

 

[Eng2013]

Thanks a lot Waross for the response and advice.
You are right - it has been noticed that the the CT gave-in due to insulation failure caused by repetitive heating. It is likely that the CT burnt and broke the line. Then the motor single phased. This theory holds as pump had no blockage whatsoever. I have attached photos of other discolored CTs found in other MCCs.
The CT manufacturer does not specify the withstand fault current on the CTs which is a big problem. He has promised to give us indicative calculations, although this is coming very late. The circuit breaker tripped, but probably only when the CT burnt and shorted to ground.
Would you agree that wound primary CTs are not desirable for direct on line starters that start and stop frequently?

 

[ScottyUK]

My opinion is to avoid wound primary CTs where possible. It can be awkward when using very low ratios, but consider using a lower secondary current to keep the turns ratio high, e.g. use an n:1 CT instead of an n:5 CT.

 

[jraef]

I've never seen a wound primary CT used in a motor starter application of that's any help. I had no idea anyone ever did that.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[Eng2013]

Hi jraef. Once again thanks for the response.
Are you saying you have never seen primary wound CTs used in DOL starters with characteristics I explained? A lot of panel manufacturers on my side of the woods use them. It means that there has been a general industrial misapplication that require correction. I will take it upon myself to write a short paper on the topic.

Warros; what is your comment on this?

 

[Eng2013]

Thanks Scott UK
Thanks for the suggestion of using CTS with low secondary winding ratio to help register low currents. I was thinking in the direction of changing the ratio by increasing the primary winding from one to two or three. Can both options meet the same objective???

 

[waross]

Hi Jeff.
Did a large pulp and paper mill mill project years ago.
The software people wanted a signal from zero to 150% of motor current for every motor in the plant.
Every motor below about 50 HP had a 100:5 do-nut CT on one motor lead.
The CTs were fairly cheap do-nut type CTs. We wound both the primary and secondary to adjust the ratios.
For example;
The 100:5 Amp CT would have a 20 turn secondary or 20:1 ratio.
If we wound 6 turns on the primary and 3 turns in the reverse direction on the secondary, the new ratio would be (20-3):6 or 17:6
17:6 would be 2.83:1 or 14.2:5
14.2 x 100%/150% = 9.44
So for a motor with a full load current of 9.44 Amps we would use 6 turns on the primary and 3 reverse turns on the secondary.
For some motors we wound the secondary in the reverse direction, for some we wound the secondary in the forward direction. It depended on the ratio we needed for that particular motor current.
Some of the smaller motors needed a lot of turns, both primary and secondary, to get the needed ratio. The windows were quite full.

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

 

[MatthewDB]

- Is it common for primary wound CTs to burn as in the two incidents explained?
I have only seen it with faults and not normal motor starts. But then I've never seen them applied to motors. In my experience I've seen smaller motors use motor overload relays and larger motors use CT's. Larger motors have a high enough current that wound primary is not needed.

- Why should one leg of the CT secondary be earthed/grounded?
It depends on the system voltage. If the system is medium or high voltage, grounding is a MUST because stray capacitance can induce hazardous voltages onto the CT secondary and flashover somewhere.
It is a good practice to ground even in low voltage applications. If there is a fault to the CT secondary, it will insure that the fault is interrupted instead of leaving LV on the secondary. If the secondary is shorted, it would be an easy mistake to presume the secondary is dead and worked resulting in a shock.

- Should there be a protection fuse between one of the secondary CT terminal and the earthing?

No, because now you have an unreliable ground if you did. The CT secondary should be designed to carry the saturation current safely for the maximum possible fault duration. Designed that way, there is no risk of "over-current" because the maximum conceivable fault current is handled without damage.

 

[Eng2013]

Hi MatthewDB
That is very helpful - highly appreciated. There are so many engineers, companies and panel manufacturers who use wound primary CTs for smaller motors - less than 45Kw. an 30Kw. almost in all cases such CTs are used for metering purposes. Operations crew always like to confirm that motors are running by reading an analogue current meter in the MC, and verifying that the load current is healthy.
In the resent weeks we did numerous experiments to prove that such CTs can burn, and the results we saw were positive.
The conclusion of our investigation had two root causes: Selection of the wrong CTs in MCC design and lack of maintenance of our MCCs. From now hence forth we will change our design to doughnut CTs and implement MCC routine maintenance as part of our asset management strategy. I will also be writing an awareness document and possibly a presentation to assist my colleagues and neighbors ...hahaha!
Thanks for the explanation on the earthing the CT primary.
Best regards.