Hello everybody. My city has a medium voltage (MV) grid of 20 kV. It happens many times that when a MV fault occurs, some MV-LV cabines burn their fuses in MV side. Usualy are the same cabines that burn fuses. No one knows why this happens here.
Can anyone explain this?

 

Welcome to AAC!

when a MV fault occurs

What type of fault? Voltage spike? Current surge? .....?

 

Hello everybody. My city has a medium voltage (MV) grid of 20 kV. It happens many times that when a MV fault occurs, some MV-LV cabines burn their fuses in MV side. Usualy are the same cabines that burn fuses. No one knows why this happens here.
Can anyone explain this?

I don't understand the meaning of "cabine". My explanation following presumes that it is the same as transformer, the device that reduces the 20kV to the lower voltage that supplies the users. In most installations the low voltage sides of the transformers are all tied in parallel, and thus if the primary side is faulted to neutral then the transformer will flow power from the low voltage side back to the high voltage side and fail the high voltage fuse. And while 20kV may be called Medium voltage, I call it HIGH voltage. That makes more sense than the other explanations.

 

Welcome to AAC!

What type of fault? Voltage spike? Current surge? .....?

Generaly is one phase fault to earth.

 

Generaly is one phase fault to earth.

A ground fault nearby would certainly be able to cause power to flow backward from the low voltage side and out the MV side of a step-down device (Cabine?). And it would happen where there was a better connection on the lower voltage side coupling several devices to supply that current. So it may be depending on the strength of the LV connection to other, which could provide more available current.
What is provided to protect the MV wires when there is such a phase to ground fault? Or are the MV wires not protected?

Another question: is the phase of the fuse that burns the same phase as the one with the fault?

 

I don't understand the meaning of "cabine". My explanation following presumes that it is the same as transformer, the device that reduces the 20kV to the lower voltage that supplies the users. In most installations the low voltage sides of the transformers are all tied in parallel, and thus if the primary side is faulted to neutral then the transformer will flow power from the low voltage side back to the high voltage side and fail the high voltage fuse. And while 20kV may be called Medium voltage, I call it HIGH voltage. That makes more sense than the other explanations.

Cabine stand for (secondary substation). There are 20-25 cabines per feeder (MV lines). I am not sure about your theory. The MV shield cables are grounded in the ground system of this cabine and other cabines. So, the fault current would flow through sane phases capacitances to ground, to fault cable shield, then to fault point. This should be the closed circuit. By the way, im not sure about this neither.

 

A ground fault nearby would certainly be able to cause power to flow backward from the low voltage side and out the MV side of a step-down device (Cabine?). And it would happen where there was a better connection on the lower voltage side coupling several devices to supply that current. So it may be depending on the strength of the LV connection to other, which could provide more available current.
What is provided to protect the MV wires when there is such a phase to ground fault? Or are the MV wires not protected?

Another question: is the phase of the fuse that burns the same phase as the one with the fault?

Maybe part of the fault current flows the way You said. Because normally, the fault current should pass through cable shield. Sorry for my english

 

It's not unusual for a power company to overload the feeds. It is easier to replace the fuses than to put in higher power xformers and higher capacity lines.

 

No problem with your english, it was my understanding of the word "Cabine". I had presumed that it was a pole-mounted transformer, standard in power distribution where I am.
Now I have more questions: is the ground fault between the MV distribution line or is it in the line between the distribution line and the individual Cabine (substation)? One possible explanation is that when there is a fault and an arc occurs, that the arc is causing another short circuit fault. I have seen an arc of one phase short circuit all 3 phases and burn all 3 fuses. That is common where circuits are close to each other..

 

No problem with your english, it was my understanding of the word "Cabine". I had presumed that it was a pole-mounted transformer, standard in power distribution where I am.
Now I have more questions: is the ground fault between the MV distribution line or is it in the line between the distribution line and the individual Cabine (substation)? One possible explanation is that when there is a fault and an arc occurs, that the arc is causing another short circuit fault. I have seen an arc of one phase short circuit all 3 phases and burn all 3 fuses. That is common where circuits are close to each other..

It happens even when the fault is many kilometers from the cabine, where there are many other cabines in between that does not burn fuses.

 

It happens even when the fault is many kilometers from the cabine, where there are many other cabines in between that does not burn fuses.

I will see if I can present this problem to an associate who knows a lot more about power distribution systems than I know. I have no idea if he will respond or not.

 

It happens even when the fault is many kilometers from the cabine, where there are many other cabines in between that does not burn fuses.

It may still be that the fuse that is burned is overloaded because of power being fed back through the substation. Do the substations have any way of monitoring their power flow? Any means to record the power flow direction?