How to detect earth leakage in DC power systems ? - Page 1
Forum section: electronics / projects

author: station240
author_rank: Unknown
timestamp: February 08, 2016, 11:13:59 pm
content: I've been trying to design a simple circuit to detect earth leakage currents in DC cabling, but I just cannot wrap my head around the concept.Lets assume neither DC cable in earthed, measuring between a cable and earth ground should be zero (or close to), if their is a fault the voltage increases.Catch is the DC voltage is something like 400V, so high value resistors are needed to protect the circuit.It does seem like something that can be done with an Opamp and a few resistors, but I just cannot picture the layout or component values needed.Oh and a simple core balance design won't work, as the leakage could be in an input supply as well.

author: uncle_bob
author_rank: Unknown
timestamp: February 09, 2016, 02:18:32 am
content: HiIs the voltage on *both* cables 4KV? (if so .. how's that work ?)What about the "stuff" on the ends of the cables (leakage)?=====In an AC system. you have transformers that break things up. You can talk about the leakage of a cable. In a DC system, there are no breaks. Everything is hooked directly to everything else in a DC system. You can not measure a live cable, you can only measure a live system.So let's say that we are ok so far. The power source and load are both highly insulated and the cable is the only thing that can ever possibly leak. You are measuring a system and assigning all the leakage to the cable.Since the system is active and termed "power", the assumption is that the current though the load is way bigger than any possible leakage. You can check *either* cable and the data is the same. Both give you the leakage of the system. Simply put, if you are looking for 10 ua, and have 100A on the lines, even at 4KV the load resistance is massively less than the leakage resistance.So far all pretty basic.Now we need to come back to the voltage on the lines. If they both are 4KV, how do you know it's 4KV? Is something keeping it at 4KV? If so what? (it likely is a path to ground and violates our "everything well insulated" rule).So let's say that one conductor is 4KV to the other and both are indeterminate to ground. Take the lower voltage conductor (pick one) and bias it up through a very high value resistor. Pick a voltage that does not nuke anything, say 1KV. If you want about a 1 ua leakage test, you grab a resistor that is about a giga ohm (yes they make them). Put it in series with the 1KV supply output and wire it to the chosen conductor. Take a second equal resistor and run it from the conductor to the input of your op amp. You now have 500V to ground on the chosen conductor. If you have 1 ua of leakage at 500V, you have a 250 mega ohm source resistance on a 500 V supply. The voltage on the conductor will go down. The current to the op amp will drop. You can work out the leakage. If it's leakage *into* the conductor, the voltage goes the other way.That's pretty much how it's done. There are a whole bunch of qualifiers. In most systems one side of the circuit is grounded. In that case you can't measure leakage on a live power carrying conductor unless it's a noticeable fraction of the current on the cable. In that case it is a current into the cable and current out of the cable measure.Bob

author: station240
author_rank: Unknown
timestamp: February 09, 2016, 07:23:26 am
content: Quote from: uncle_bob on February 09, 2016, 02:18:32 amI clearly said 400V, not 4KV.One cable is +400V, the other is ground, but not connected to Earth.The real trick is to determine what resistors to use that won't in themselves create leakage currents.Eg give me a circuit.Maybe I could dispense with the Opamp and just run a opto isolator off the resistor network. As the LED inside would need 20mA to operate.

author: Jeroen3
author_rank: Super Contributor
timestamp: February 09, 2016, 07:35:36 am
content: Put a clamp around both conductors to measure leakage. Why should it be different from AC leakage measurement?

author: Zero999
author_rank: Super Contributor
timestamp: February 09, 2016, 08:56:31 am
content: Quote from: station240 on February 09, 2016, 07:23:26 amIf the negative conductor is not connected to earth, then there will be no earth leakage.

author: tautech
author_rank: Super Contributor
timestamp: February 09, 2016, 09:08:02 am
content: Megger test each conductor to ground, that's how sparkies do it.

author: penfold
author_rank: Frequent Contributor
timestamp: February 09, 2016, 10:27:19 am
content: You have to remember that the DC supply wires will be 400 V apart from one another, if one is measured at 0 V wrt earth, then the other will measure 400 V away from that (positive or negative depending which wire is which).You can use resistors to weakly pull the conductors to something like 200 V of earth either side. Assuming you have gig ohms of isolation to earth, then adding 1 meg from each conductor to earth will pull them pretty close to +/- 200 V to earth, and indeed any fault to earth with resistance in the region of, or less than, 1 meg ohm will have a significant effect on the two supply voltages.Measuring those kinds of voltages is a great purpose for a Norton input opamp which really shines with high common mode voltages and wide voltage differences when used correctly.But say a fault to earth develops near the center of a transformer winding, to earth, then the two cables are quite rigidly referenced to earth at +/- 200 V, your circuit will not know the difference.You may find that your solution requires both a balanced core current transformer (DC type, hall effect, fluxgate based etc) and the voltage measurement method.

author: station240
author_rank: Unknown
timestamp: February 09, 2016, 12:26:21 pm
content: Quote from: Hero999 on February 09, 2016, 08:56:31 amCorrect, unless there is a fault.I'm trying to detect that fault, before it turns into a fire.

author: max_torque
author_rank: Super Contributor
timestamp: February 09, 2016, 05:09:35 pm
content: Most commercial earth leakage detectors, such as those used on electric (or hybrid) passenger cars AC couple an oscillating signal into each conductor of the DC bus. Google turns up a lot of info on such a technique......

author: UncleBog
author_rank: Newbie
timestamp: February 09, 2016, 05:46:21 pm
content: A few years ago I developed a system that had to deliver 400 VDC underwater, I used a Bender 'Isometer', seehttp://www.bender-uk.com/products/c/insulation_monitoring__1/main_circuits__3.html. They have a technical document section which may help you to understand how they do it.

author: station240
author_rank: Unknown
timestamp: February 11, 2016, 05:22:52 am
content: Quote from: max_torque on February 09, 2016, 05:09:35 pmAfraid google isn't being very nice to me, all I've found so far is patents (which have enough detail to build a circuit) and commercial products.I also found mitsubishi electric are working on an Arc fault detector for DC circuits, patent pending.http://www.mitsubishielectric.com/news/2015/0217-a.html