Solidly grounded transformer

  1. Hello everybody,

    I'm a bit confused about the concept of solidly grounded tranformers. From my understanding, the wye connection of the transformer side would be followed by a resistor of a certain magnitude.

    Hopeful that my doubt would be clarified.

  2. jcsd
  3. NascentOxygen

    Staff: Mentor

    It's unclear to me what you are asking. Just guessing .... perhaps the words "star point" should get a mention in your question?
  4. @nascentoxygen I'm not too sure of the definition of solidly-ground earth.. please and thanks :)
  5. im not too sure of the definition of solidly grounded transformer.. please and thanks! :)
  6. NascentOxygen

    Staff: Mentor

    Maybe you should cite the source where you saw it mentioned?
  7. jim hardy

    jim hardy 5,460
    Science Advisor
    Gold Member
    2014 Award

    That term usually describes an electrical system whose connection to earth is through a low enough impedance that a fault between an energized conductor and ground will pass enough current to trip an overcurrent protective device.
    To achieve that one might connect the neutral of a three phase transformer supplying a system directly to earth and to the building's steel.

    In US homes the single phase transformer supplying the house with 240VAC has a centertap that is connected to earth. So each of the two incoming lines is 120VAC to earth. A fault on either will trip an upstream protective device.

    In industry it is not uncommon to connect a system to earth through an impedance high enough that a fault to ground will not trip anything and instead provide an alarm. That allows an important process to run uninterrupted while operators isolate and fix the fault. That is called a "High impedance grounded system".

    You ought to locate a copy of IEEE "Green Book", standard 142. It is a very well written treatment of the subject that is clear and easy to understand. It should be in every electrical and industrial engineers' knowledge base. Try a search on IEEE Green Book..

    FOIWATER 420
    Gold Member

    Yeah I think Jim has it right.

    It's the concept of neutral grounding resistance (NGR)

    Usually, as you said, you wouldn't want high resistance in the ground circuit. For power systems though, depending on where you are engineering, there might be a code or regulation against allowing a certain level of ground current to flow.

    For example, for mining, no more than 25 amps is allowed to flow at voltages of 4160v (2400 single phase). If you have a wye connected transformer with grounded neutral you need to input a NGR to limit the ground fault current to 25A.

    Also its required to use 'watch-dog' circuits to ensure your NGR is in the circuit

    Here is some literature on the watchdog im familiar with from a company called startco, who make protective relays as well.

    FOIWATER 420
    Gold Member

    So just to reiterate - a solidly connected wye to ground would be for a residential case, where voltage are low enough that you don't want any ground impedance
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