Non-contact powering of LED indicator from a high voltage cable

Question

We manufacture fireproof power plugs for high-voltage transformers used in mines. The way it works is we take the incoming single-phase cable and epoxy it into the plug and attach it to the transformer. However, we want a way to show that the cable is live.

The idea is to insert an LED into the plug that will light up when the cable is live. Do you guys know of a way we can power it remotely without connecting to a source? Perhaps by the electromagnetic field produced around the cable?

The problem we have is guys remove/steal the earthing protection in the mines and then blow up the transformer when they remove a live cable. If the cable had an LED to show it was live the guys would not remove the cable and risk blowing themselves up. This is purely to improve the safety of the plugs and save lives. We install them in African mines where I am from.

Any advice or guidance would be greatly appreciated. I did a mechatronics course while doing my honours in Mechanical Engineering so hopefully that helps.

Answer 1

You don't provide details of voltage or frequency, but given that you say "high-voltage" let's assume we're talking in the 11 kV range at 50 Hz or 60 Hz.

You also don't say if the plugs are single pole (one L, one N), dual pole (L+N), or three pole (L+N+E). However, let's assume inside the plug there is at least one place where you have an insulated 11 kV 50/60 Hz wire, and another where you have access to N or E.

If all the above is true, then you can probably pick up enough current to light a LED through use of a capacitive coupling, via a metal sleeve round the HV conductor. In fact you may already have grading rings in the connector which would be used as the pickups.

100 uA should be more than enough to power a LED flashing circuit (flashing to reduce average current draw, and improve visibility), with lots of suitable circuits online including some down to nearer 10 uA for very long term battery use.

To extract 100 uA for 11 kV 50 Hz, an impedance of ~100 MΩ will serve, and at 50 Hz, this equates to a capacitor of around 30 pF. Looking at MV shielded cable specs, this is roughly the capacitance per conductor to the shield for a 0.1 m length. This suggests a 10 cm sleeve over the L conductor should allow you to extract the required power without a direct connection.

(Inductive / current transformer coupling is another option, but has the disadvantage of no power if no current. You'd want to look into the transformer magnetization current to decide if that is an option.)