Current Transformer Questions

Discussion in 'The Projects Forum' started by GiR, May 31, 2006.

  1. GiR

    Thread Starter New Member

    May 31, 2006
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    I'm working on a project which in its final form will need to be connected directly to a power line between 7kv-100kv. Already know I need to use a current transformer, but after looking transformer portion of AAC's, my application is a bit different and was wondering if anyone could steer me in the right direction. Not looking for circuit designs, just knowledge.

    I intend to use the current transformer to power a device which requires only 3Vdc and about 20-40ma when active, and 0ma when inactive. Device will be inactive 99.9% of the time which give me concern about it building up a crazy voltage across the secondary when it is inactive. Luckily, the other components which I intend to drive are not ICs and only 1 concerns me regarding the voltage builing up, the one component which is 1.5-3Vdc.

    When the current is generated by the CT, I know there will be no guarantee what the voltage will be but obviously I will have a ton of current at my disposal.

    Since I have no ground reference nor any other sources, I am rather limited in what I can do because I haven't ever dealt with a situation like this.

    Old school AC/DC transforming comes into mind with some diodes, and a cap/zener to limit the voltage, but am I going to be dumping around 5 amps when not in use? I really can't have that. Don't have the room to disipate that much heat. At most, I'd like to disipate no more than 100mW.

    I need to come up with a real cheap, out of the box solution for this if possible. No component that would cost more than 10-20 cents in bulk.

    Ideas? Suggestions?

    Thanks in advance.
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    The concept seems pretty far out on the fringe. I don't think you have a snowball's chance of getting what you want.
     
  3. GiR

    Thread Starter New Member

    May 31, 2006
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    Need ideas, not walls.

    Forget price restrictions at the moment. Lets start with the current transformer and assume it puts out 5A. Am I limited at only the 5A? It explained that the CTs were used in conjunction with ameters which have a very low input impedance maybe allowing those 5A to flow. But then I am stuck if I use a large load and the current is constant the voltage will skyrocket.

    Lets get into CTs a bit in depth. PTs are easy with ur turns ratios and known voltages, but the CT is a bit odd. There has got to be a way to make some type of current sink to only draw a small portion of that current and just block the rest, and then use that stepped down current to get my 3Vdc, or rather, since my only load is the 3Vdc device, I should be able to source or sink 20mA through it and call it a day, but I need to understand more fully how the CT operates.
     
  4. n9352527

    AAC Fanatic!

    Oct 14, 2005
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    What is the possible range of primary current?
     
  5. Erin G.

    Senior Member

    Mar 3, 2005
    167
    1
    Yes, you are limited to 5 amps. All metering CTs (that I'm aware of) are 0 to 5 amps. They are a single coil (donut shaped) configuration with two output leads. A current carrying conductor runs through the middle of the donut, and the output leads are attached to a metering device, such as an analog amp-meter, or even a DAQ station. The CT works solely on induction principal: When a load is drawn through the conductor, a proportional amount of electro-magnetic flux builds up around that conductor and induces current into the CT. The leads on the CT transfer the current signal to the metering device, which usually tells you how many amps are flowing through the conductor being metered.

    Your second paragraph (above) suggests that you need a power supply (3VDC) for some kind of load (20mA). This is something that the CT can not provide. If you need a regulated 3VDC power suply, then you should buy a power supply. They don't cost much and most run on 120VAC. They rectify AC, filter and regulate a pretty clean DC output. Just get one that can go up to the amp (or mili-amp) rating you need.

    The thing to remember about any electrical load is that the amps are not "produced": They are drawn by the load. If you have a transformer or power supply rated at 5 amps, then 5 amps is the most that can be drawn without creating an overload condition. If your actual load is lower than 5 amps, then it's lower: nothing bad will happen, and there will be nothing to "block". Transformers generally, and power supplies particularly, resist changes to their output voltage, which you seemed to be concerned about.

    More specific information about what you're trying to do would help us give you more specific ideas about how to do it. It sounds as though you're trying to power one device using inductance from the conductor of another, and your first post mentioned voltages in the 7- to 100KV range. If this is the case, my suggestion would be get a CT rated for the KV range, feed it's output to a suitable device (such as a DAQ station) and configure the output of that device to obtain your final desired result.

    At any rate, I hope this helps.
     
  6. GiR

    Thread Starter New Member

    May 31, 2006
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    Primary current range would probably be betwen 50amps upwards of 200-300 amps, but as I mentioned, only need 20-40mA.

    Yeah, that did help. My biggest concern was that I had to deal with all the amperage, obviously I don't. Played with PTs before, and in the end the CT is just a PT with a 1 turn primary. The DAQ idea would be great, but in use, the device must be entirely stand alone.

    The device will be connected to typical power grid distribution lines when complete, and it needs to draw 20-40 mA at most and that is only when the device is active. The load at the moment is just a 1.5-3Vdc piezo buzzer so the voltage doesn't need to be perfect. It could have as much as a 10-20% ripple without concern.

    Basically, from what I am gathering, I could do a typical AC/DC conversion using the PT with a rectifier/zener/cap combo, get my 'regulated' voltage and drive the buzzer without concern since there will be no need to draw any more current than my 20-40mA.

    Would there be any possible issues doing this?? Would the zener begin to disipate a ton of power if the voltage on the secondary was large or is this not a concern with CTs?
     
  7. Papabravo

    Expert

    Feb 24, 2006
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    Just because you think something is possible or practical does not make it so. If you wish to waste your time on such pursuits, who am I to dissuade you. You asked for advice, I gave you the benefit of a graduate education and 46 years of experience, and you are perfectly free to ignore all it.
     
  8. GiR

    Thread Starter New Member

    May 31, 2006
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    Likewise, just because you think its on the fringe or impractical or impossible, does not make it so either.

    If you want to continue to say it can't be done, be my guest, you will just keep bumping this thread.

    Its the path to the goal which is the best part, even if you never reach that goal because once you're there, its over.
     
  9. n9352527

    AAC Fanatic!

    Oct 14, 2005
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    I would suggest common 1A CT, 5A CT is a bit of a waste. If you could get one with taps, it'd be better. You need to establish the minimum primary current, at which you adjust the tap so that the secondary is just above 20mA (your requirement). Then you need to somehow sink excess secondary current when the primary current increases.

    If you have 1A CT @300A with no taps, then at 50A it would give just above ~160mA, then you need to sink approx. 140mA at minimum and 980mA at maximum. Note that at 3V and 980mA the dissipation would be approx. < 3W. This is achievable.

    This largely depends on what CT specs you can obtained.

    Careful not to open load the CT in operation or provide not enough sink, it would result in excess voltage across the terminals, which could be lethal.

    Well said ;) I certainly regard people that push the envelope, question things and are curious highly.
     
  10. GiR

    Thread Starter New Member

    May 31, 2006
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    Another concern came to mind while continuing this. The voltage I will be generating on the secondary compared to the primary could cause a huge voltage difference between the two and arc, couldn't it?

    Since neither shares a reference and the secondary would basically be floating that would cause an issue wouldnt it if the CT was't rated to handle that level of isolation.

    Also, from what I have been finding, to use a kv rated CT is going to be much more expensive than I want it to be, so here is my idea. Could I use a lower rated CT and just run a shunt between the primary and secondary so I have a reference V on the secondary so that the highest voltage difference between the primary and secondary will only be the voltage I am generating on the secondary?
     
  11. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    the isolation problem depends on the position of the CT. If it will be hanging on the power lines, then it is no problem becasue there is no reference point for the kV voltage.
    If it should be somewhere near other grounded things, it has to be well insualted, either in the primary/secondary part or in the seconadary/case part. There will by no arcing between prim/sec as long as the sec is not grounded/connected to anything else.

    Where will it be positioned?
     
  12. GiR

    Thread Starter New Member

    May 31, 2006
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    Directly upon the line. Never near any ground.

    Another thought has to do with using a core which will saturate due to the mag field. I know in typicaly power transformers you want to avoid this due to loosing the inductive characteristics of the coils and burn out the primary. Since the primary is just the straight line, is there any forseeable issue with using a small core other than that the AC signal I will get from it will get really crappy? Just want to rectify it and get 20 mA and is doens't need to be a pretty DC voltage.
     
  13. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    You could start with a transformer (pistol) soldering iron. In the secondary, there could be current like 20AmpsAC. You would have to dismantle the iron and insert the core on the secondary "wire". Using a thicker wire than for soldering could increase the current if it is too low (too different form the HV line current). Then you can see how your power supply behaves...
     
  14. bitman

    New Member

    Jan 12, 2006
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    This may be far fetched but i'll suggest it anyway.
    Perhaps you could use a simple coil in the vicinity of the powerline to gather the current , a simple series resistor network with a bridge rectifier and zener. Perhaps you could even use a parallel MOV to divert most of the voltage from before the resitor network if you wanted.
    ??? Its just an Idea

    Regards Bruce
     
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