Case Study – Sometimes Extra Protection do help in really bad times

Dear Sirs / Friends

Everyday we come to new about breakdowns in various plants due to various issues in Electrical system. And our wish is that one problem must not be repeated in other plants. And sometime few problems help us to provide the best solution to other plants where such problem may arrive. Today's case is one such only.

A serious case that had happened 14 months back, but luckily, it was Fan and not mill, so there were no damage or losses. But it strikes everyone in that plant, what if that was mill instead of Fan.

On one fine day, the plant, which was running on full load just escaped a major breakdown, which could have stopped the plant for more than a fortnight. That was pure luck by chance. In that plant Coal mill (a particular machine for coal grinding) was running, now mechanical/process personnel stop the coal mill after the fine coal bin is full. Sometimes they also stop the mill if load reduction is required or if there is any unusual activity recorded in DCS.

Now there was a problem in Battery bank in particular section. 1 outgoing MCB of the 4 of battery bank tripped. Hooter rang and engineer (within 5-10 minutes) was at battery bank / charger trying to sort out the problem. Now that DC supply was Auxiliary supply for HT Panel Board (6.6 kV) Incomer-2 side. Now the Auxiliary supply was off so all the feeders of that section were in “As it is position”, the feeder ON were in ON position and the feeder OFF were in OFF position.

As the auxiliary supply was off, so Numerical relays were off too, so the breaker which were on now were un-protected. And there was no tripping taken for DC supply failure neither to Motors nor to any outgoing feeder. Such kind of tripping and logics are decided by plant process requirement, what equipment needed to be tripped during failure of DC system.

Coming back to case, now outgoing MCB was tripped and there was no DC supply to operate breakers on Incomer-2 side. Now CCR (Central control room) was unaware of this fault. The mill was just tripped 3-4 minutes before failure of DC due to vibration (as feed of coal was not coming proper) and there is interlock that If coal mill trip after 10-20 minutes coal mill fan will be stopped. When command came from CCR to stop the fan it didn’t stopped, they tried 2-3 times, but fan didn’t stop. They called instrument people to check the issue and electrical people to stop the fan motor manually. There was a little panic situation in CCR that what happened.

The coal mill fan breaker was not tripping as there was no DC supply. But what could have happened if DC supply was not there at the time of tripping of coal mill motor. As the feed was not proper, the bed of coal was not proper formed and hence vibration was coming. Now if the motor is not tripped due to heavy vibration, the motor will keep on running, the vibration may increase and may partially or fully damage the equipment.

As the running equipment was fan, chances of damage were less. Technician manually switched off the breaker. Now electrical people knew what happened and they have to avoid such case in future.

They came to us for support and we suggested that you must have AC tripping coil for at-least all the HT motor feeder, which can be seen now in HT VCB, but on request of user only and that too during Project time.

However, we can provide extra tripping coils and wire it to the control panel along with hard-wire logic.

Now we checked the breaker and there was space for installing 2nd tripping coil, following things were done

·      We installed the 2nd tripping coil

·      Wired the coil to secondary plug & socket and then to panel

·      Installed Trip circuit supervision relay for AC Tripping coil

·      Made wiring logic so that whenever there is command from CCR both trip coil operates and when there is command from relay only DC coil operate.

·      Used both TCS Relay contact in series for closing of breaker

Now you can check both name plate of breaker, where earlier there were used to be only one tripping coil, now you can see mention of “Additional trip coil supply”

Below is the pic of a breaker with both Tripping coil.

I used to think that mostly nowadays HT breaker must be having Two trip coils, but have seen in many plants that they have single Tripping coil only. And after 3-4 years of plant running, many maintenance people are of opinion that there should be two tripping coils in VCB. However, such decision must be taken during projects only.

However, For supply and installation of 2nd Tripping coil in your plant VCB contact us at [email protected]

Regards

Avinash Singh