Hi.

After two decades my family could finally build an electric line to the farm, giving a rest to the old diesel engine and to our pockets as each day running the irrigation pump would burn 200L of fuel. Now a whole month running the pump 24/7 will cost 1/4 of a day with diesel.

Unfortunately, one day we found a fuse had blown, without any load (see attached image as an example of the fuse, although this is a three phase line). My father (they live alone there now) replaced it but the power was not restored, as apparently other fuse was blown 4km away and another one 10km away. He replaced them all, and again they blew up within 5 minutes: first the one on the transformer, then the one 4km away, and then the one 10km away. This situation repeated itself, sometimes with days between fuse blowouts, sometimes minutes. If the pump is running the fuses will not trip.

The motor is ok, the transformer is ok and line to ground isolation is ok.

The line is 20km long from where it got to originally, with 10m tall poles, separated 80m each with 60cm between conductors and we are its sole consumers. It is in a remote rural area and has no transpositions, nor capacitors or inductors. its voltage is 13.2kV, our transformed rated at 40kVA, the motor consumes 23kVA when working and the HV fuses are 8~12A.

Distance to the substation is about 40km.

The transformer manufacturer told us there could be an issue with line capacitance blowing the fuses, as the transformer is fine, and that transpositions along the line would solve the issue.

What do you think about this?

Thank you very much

 

Hello there

Welcome to AAC
My thoughts are any two conductors separated by an insulating medium constitute a condenser or a capacitor. In case of an overhead line two conductors form the two plates of a capacitor and the air between the conductors behaves as the dielectric medium. Thus an overhead line can be assumed to have capacitance between conductors throughout the length of the line. The capacitance is uniformly distributed over the total length of the line and may be regarded as a uniform series of condensers connected between the conductors,draws a leading current, even when supplying no load. Which is why your fuses keep blowing. This leading current is in quadrature with the applied voltage and is termed as the charging current.charging current is due to the capacitive effect between the conductors of the line and is not in any way dependent on the load. Another reason why your fuses keep blowing The strength of the charging current depends upon the voltage of transmission, the capacitance of the line and the frequency of ac supply. These are my thoughts.

 

Transpositions, same concept as behind twisted pair wires to equalize the fields surrounding the wires. The PE who signed off the electrical design should be in the loop as the need for transpositions should have been calculated before being approved.

https://quickfield.com/advanced/transposition.htm

 

A faulty insulator could be the cause, this may involve checking every insulator. When i worked for a Line company that process was a pain. But will cause intermittent fuse blowing when theres no load.

 

Is it 13.2kV to the grounded neutral or is it 13.2kV between two ungrounded wires?

 

Before going any deeper I'd request to have someone equipped with MV instrumentation come out and get a line current reading at full load and idle. Might be worth measuring both at the load and at the source. Fuses blow because of excessive current - chances are this is going to be a matter of figuring out why excessive current is present. Could be as simple as poor power factor - utility might need to add a capacitor, etc. Could be something stranger like a wonky lightning arrestor or an MV cable intermittently faulting phase-to-shield. Could be worth having the cables / transformer megged if nothing stands out with the current measurements.

 

Sorry, I forgot to mention the line consists of three naked aerial conductors, one at the top of the poles and the other two at the sides of the crossbar. It does not have a neutral wire nor a ground return line, only three identical phase conductors.

The line was tested with a megaohm meter before commissioning it and it was fine. What's curious is that the fuses that blow are the ones closest to the end of the line, farthest from the substation, and then progressively going towards the substation, what i believe would rule out an insulation problem. Also, if there is a load on the line, the fuses don't blow

 

Hello there

Welcome to AAC

Well, thanks, check my joining date and yours though! haha

 

I am so happy you have a good sense of humor and at last it was nice to finally meet you so to speak
@nsaspook hit the target heads should roll because of your predicament

 

there's no living without humor

And your welcome is valid nonetheless, as you clearly are more active here than i'll ever be

nice meeting you too

 

Could be that the line is too capacitive without a load and connecting the motor/transformer brings it closer to unity? If only there were a recloser or some other form of datalogging overcurrent device involved which could shed light on whether this is an overload or short circuit issue. Forensic analysis of an MV fuse is pretty useless since they rely on expulsion for arc quenching - not like cutting open a 300/600V dual-element fuse to see which element opened.

 

Since the load is inductive, is it possible that the inductive 'kick' at motor shutdown is causing the problem?