Short Circuit On MV Side Causing Trouble On LV Side

Can you post a drawing we can read?

yes sure,

sorry for that, i thought it might be visible

It is fundamental that there needs to be sufficient protection to ensure that a fault, such as a short circuit, causes a protective device to operate and disconnect the fault.

Please describe the circuit protection arrangements that are in place.

It is fundamental that after the first fault, at which point the first controller got burnt, that the fault is removed rather than persisting with changing and burning out controllers.

Please explain why no steps were taken after each fault 1, 2, 3, 4, 5 and 6 occurred to remove the cause of the fault and use the controllers as circuit protective devices instead.

controllers burnt spontaneously, all at the time of short circuit

this controller is generator starting and load sharing controller, for safety, there is Ring main unit, ring main unit tripped but in the mean time generator controller i.e. deep sea also burnt. only bus sensing input of deep sea controller is burnt.

for protection;

Ring Main unit for each transformer (IDMT relay)

Ring main unit of whole system.(IDMT relay)

do ask if i need to give any further explanation

also this drawing is for reference, i have 20 generators in same configuration at the place where this problem came. this drawing is for simplicity.

Please explain why the circuit protective equipment failed to protect the controllers.

The isolation is required.

As I know, Deep sea is just a controller with an electronic board. If it is damaged with Short circuit condition, you may guess the control power of it is connected to Main power line somewhere.

The insulation is broken with Huge current from connecting point.

And if you need to use main power line for Control power, some isolating devices (fuse, mcb, mccb) are available for fault clearance.

Hope it help you.

First of all, you need to narrow the Short Circuit sources down,.. and you can go this way:

- Separate the Generator from the transformer, and turn the first on, this way you'll judge whether the trip comes from the generator only or not,.. if the deep sea controller doesn't trip, then go the next step.

- Connect the Generator and the primary side of your transformer as they basically were, and simultaneously separate the secondary side of the transformer from the Ring Main Unit "RMU", and turn the generator on again, and watch its performance carefully, if the controller trips, then the transformer is more likely to be responsible for the trip....if not, then go next .

- Connect the RMU to the secondary side of the transformer as it was formerly, and turn the generator back on again, if the controller still trips, then the RMU is more likely to be the cause of the fault,..

-------------------------

Apply the idea "Confine and test"...

I hope this helps!

i think i didn't explain my question well, MV side is open distribution lines over head on electrical poles, when ever there is storm or rain, cables overhead gets short with each other. this short circuit on field on MV side is affecting LV side.

Ofcourse RMU is tripping but still DSE panels getting damaged.

i appreciate your support....and i definitely need help.

In my opinion if a circuit breaker burns it could be two causes:

the short-circuit current is too high -above the breaker capacity

or/and the trip time is too long.

If, for instance, the rated circuit breaker capacity is 10 kA for 1 sec and the protection will trip in 3 seconds the capacity will decrease by sqrt(3/1) that means to 5.78 kA. If the presumable short-circuit will be 8 kA at this time the breaker will be damaged.

In my opinion, the last generator tripping current could be more than the equal sharing all 4 parallel short-circuit current.

I did not understand what does mean "bus sensing input of deep sea controller is burnt".

If you may be more specific...

Sorry, as I understood now the generator circuit breaker does not open and stay in operation. Only the voltage sensing input of the DSE automatic control is damaged. The bus sensing amplifier output is not damaged, I see.

Did you check the voltage sensing amplifier? It is really a such high overvoltage on the generators common busbar?

The current interruption could produce an overvoltage mainly if the supplied transformers are no-loaded.

The generator load rejection could also produces an overvoltage by overspeed of generators.

In my opinion, you have to check if diesel controller act rapidly enough in order to reduce the speed. Also the excitation field shut-down.

You could also provide an overvoltage discharge at bus sensing output by means of a zener diode for instance.

See also:

http://www.pterra.com/project/load-rejection-overvoltage-issues-on-distributed-generation-projects/

Hello,

One thing that strikes my mind as the possible reason is an Voltage Surge. If you could introduce a SPD before your deep sea controller board, i think you should not face this trouble again.

Ramesh.A