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Grounding the Transformer Neutral

  1. Mar 15, 2014 #1
    I observed the following configuration in a factory:

    Power supply from a generator (400 V line-to-line) is stepped up to 11 KV (line-to-line), which is then fed to another transformer that produces a stepped-down, 2-phase output. The step-down transformer has 3 input terminals. Step-up transformer has 3 input terminals (connected to generator) and 4 output terminals (1 for neutral), which I think should have been the other way round since the supply coming from the generator has a neutral terminal as well (earthed in this case), whereas the primary of step-down transformer has no neutral terminal.

    Now the main question: both the neutral terminal of step-up's secondary, as well as transformer's metal body were earthed! If the other end of the circuit also had a neutral, that could have helped in carrying any unbalanced current back to transformer's neutral through the earth. But in the current scenario, do you think such a configuration could serve any purpose? I think the step-up's neutral should have been left untouched (by not attaching any conductor with it) and then there would have been no need to earth the transformer's body either.
     
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  3. Mar 15, 2014 #2

    Baluncore

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    Transformer A has three phase input, with neutral probably referenced to earth somewhere nearby. It has three phase Y output with an earthed centre neutral. The three phase then goes to transformer B primary.

    A transformers body must always be earthed as protection from an insulation failure, (which would otherwise make the transformer frame live). The earth must be able to carry sufficient current to break the circuit when needed.

    Unless the 11 kV three phase coupling between the two transformers is earthed somewhere, it will be floating. That may result in a breakdown of the insulation.

    It is only by tying everything to earth that the system and protection can function reliably.
     
  4. Mar 15, 2014 #3
    Earth is not a good conductor compared to aluminum cable. If the xfmr neutral and the generator neutral miles away are both earth connected, very little current flows in the path even when loads are severely unbalanced. Most xfmrs have a delta winding which keeps the 3 phases balanced in terms of voltage even if phase currents are unbalanced. Or if the core is a 3-legged E type, this functions like a delta winding keeping balance when loads are unbalanced.

    In general, the power company delivers power over 3 wires, and voltages stay balanced despite unbalanced load currents, without using a 4th neutral wire. As mentioned above, the earth connection is for safety, and in the event of a short, such as a line touching ground, earth will conduct some fault current. But earth is not used to carry load current.

    Claude
     
  5. Mar 15, 2014 #4

    jim hardy

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    To add to Baluncore's great facts,,
    That's for safety.
    Earthing at the transformer is required by electrical code, so that fault currents returning from its load through the earth can get back into the transformer winding. Kirchoff's Current Law....



    That's for safety.
    Metal body MUST be earthed so it can't electrocute somebody leaning against it.


    Doesn't need one, it is not floating because of the earthed stepup transformer's secondary neutral which satisfies Baluncore's requirement.

    IEEE standard 142, called "Green Book" is an excellent tutorial on the why's of grounding and i really recommend it. It is easy to understand and leads your mind naturally into "safe thinking".

    Also search on phrase "Separately derived system" as defined in national electrical code.

    Are the stepup and stepdown transformers about the same size?
    Is stepdown secondary perhaps "open delta" ?, three phase derived from two windings but three terminals ?
    That is often done when the second transformer is considerably smaller than the one feeding it.
    It prevents third harmonic current from overheating the little one.

    Grounding is a fascinating and much misunderstood subject. Become the local expert !

    old jim
     
  6. Mar 16, 2014 #5
    neutral terminal of step-up's secondary was earthed
    The "load" consists of current flowing through step-down's primary winding, which is not even connected to the earth. So how can fault current travel through the earth and back to step-up's neutral terminal?

    as well as transformer's metal body were earthed!
    But if transformer's neutral isn't connected to the earth, there shouldn't be any need to earth the metal body either. Anybody touching one of the 11 KV lines (whether directly or via the metal body in case of short-circuiting) shouldn't be electrocuted if there is no electrical connection b/w 11 KV line and the ground.

    whereas the primary of step-down transformer has no neutral terminal.
    ...Doesn't need one, it is not floating..
    the primary of step down wouldn't be floating anyway because I presume it's a delta and not Y. How is the step-up's neutral "not floating"..it touches the ground, but it's not further connected to anything else...? (By "floating" I understand "not varying" even when load is unbalanced)

    Is stepdown secondary perhaps "open delta" ?
    The harmonics should be eliminated even with a delta connection, so why have an open-delta?
     
  7. Mar 16, 2014 #6

    jim hardy

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    The "load" that the step-up transformer feeds is the primary of the step-down transformer.
    That loop, consisting of stepup secondary and stepdown primary, must be earthed. That is to protect its insulation. And to meet electrical code for "separately derived system".
    It can be earthed through a " high impedance" but not an infinite one. The guideline for high impedance is it should be less than the distributed capacitance of the system.

    A fault in the stepup-stepdown loop will return current there, to the source winding.
    A fault way over on the stepdown's load side, as you said, will not return current there.


    There IS a connection to earth through the stray capacitance and the (admittedly high) insulation resistance. The current that can flow through it is presumed sufficient to electrocute any poor chap who completes that Kirchoff current loop.
    That's the main reason to ground it, protect your personnel. A falt will announce itself so people know it needs to be fixed.
    You ALWAYS earth the equipment case (unless it's plastic and completely encloses all conductive parts like on your 'double insulated' electric drill.Look inside that drill and you'll find nylon gears that insulate the chuck from armature" )

    Secondly, one earths a system to protect its insulation. Stray capacitance will be resonant with inductance of machinery at some frequency, look up "ferroresonance". It can pierce insulation.




    That IEEE green book i suggested would really straighten out your concept of earth grounding.




    "Floating" in the industry means not connected to earth, which would mean its potential to earth is completely unknown and may wll exceed your insulation's capability..
    A neutral that IS varying due to unbalance is called "unstable".

    In a balanced system third harmonics can't exist phase to phase. Write KVL around a loop through the neutral and you'll see they cancel.

    IF:
    you had your wye stepup transformer secondary winding feeding a wye stepdown's primary, with their neutrals connected, each phase to neutral voltage would include third harmonic voltage .
    The neutral would carry third harmonic current equal to the sum of the third harmonic currents in the three individual phases.
    ANDIF:
    The stepdown's secondary were delta, it'd be a short circuit for third harmonic current.
    If that stepdown transformer is physically big enough that the third harmonic current circulating in its delta secondary is insignificant to it, no problem.
    BUT if Mr Stepdown is physically puny compared to his source Mr Stepup, that third harmonic current might well melt him.

    Take a look at your metering and protection schematic, usually you see the PT's wired open delta for exactly that reason.
     
  8. Mar 16, 2014 #7

    Baluncore

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    Through step-down frame, the 11kV 3PH and the neutral on the step-up secondary.

    There is no IF, There will always be some leakage. Without earth connections to bleed off the charge, insulation will break down and one of the phases will become grounded, which puts 22 kV on the other phases that only have 11 kV insulation.

    The distribution grid is referenced to earth, so the primary of the step-up transformer is also referenced to earth through the distribution grid's neutral.

    What is the phase relationship of the step-down output phases?
     
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