Forums » Electrical Engineering

Faults through the neutral

    • 5 posts
    July 13, 2019 11:40 AM PDT

    if a transformer or a generator solidly grounded then there is a possibility of flow of current for outside faults through the neutral. Is this a safe method for generator or tranformer?

    • 103 posts
    July 13, 2019 6:26 PM PDT

    I may not be addressing your question, because I have never heard of a through fault unless some form of differential protection was being used.

    Line to ground faults are the most common & should be cleared by a properly set protective relay or OCPD without damage to the source, generator or transformer.  The protection must be set with respect to the damage curve for the source.

    In grounded systems the neutral is sized for line to neutral loads and/or for fault clearing.  Any fault current on the EGC must use the system neutral conductor to return to the source at some point.  The transition point is the main or system bonding jumper that connects the EGCs to the system neutral conductor.

    There are several grounding methods that can be used, the particular method chosen is dependent on the application.

     

     

    • 75 posts
    July 13, 2019 7:32 PM PDT
    Matt -
    Give us some context. Is this 480V, 1000KVA?
    208V, 500KVA?
    13.8 KV?

    What works at one level is not necessarily even legal at other levels.
    • 5 posts
    July 13, 2019 8:58 PM PDT

    Thank you Steve.

    @Carl: 2.5MVA, 20kV/0.415kV

     

     

    • 200 posts
    July 15, 2019 2:51 PM PDT

    Hi, Matt, thanks for added me as Friend. I am attending to a Hameed suggestion to reply here for the sake of knowledge of Z4E Members. Thanks, Hameed. Sorry for the late reply.

    For Industrial and Power applications: equipment grounding conductor is sized in relation to the Over-Current Device (OCD) and it is installed/bonded to the case of equipment, in this way a low-impedance grounding electrode system is established by using the electrodes installed (no matter electrode type). This set of electrodes becomes the Grounding Electrode Grid.

    The most usual power distribution network (related to Transformer) is:
    - Delta-side: no Neutral, power generation side, high voltage (low current), long distance distribution;
    - Wye-side: with Neutral, load/consumer side, low voltage (high current), short distance distribution;

    Since Z4E is an international site, it is important to note that not all countries or regions in the world have (or had) a mandatory grounding system, the third (central) pin of a plug, by any code or legislation for Residential or even Commercial installations, at the Wye-side, as the Industrial and Power ones. In such cases, the power utility company may (or may not) provide a grounded Neutral, sometimes as Local, Neutral grounded as near as possible of Wye-side of a Transformer; sometimes as Individual, Neutral grounded at Metering box.

    Power grids with mandatory grounding systems the "possibility of flow of current for outside faults through the neutral" as you putted is more rare than in power grids with grounded Neutral systems, and even these are more inclined to return faults to the ground at the Wye-side than to the Delta-side of the Transformers and Generators.

    Assuming there is Neutral in both Generator and Transformer (Wye-side connected to power generation side), there are the following options:

    If NEUTRAL IS SWITCHED: both Transformer and Generator are separately derived systems, they should be grounded and bonded. The better option in such cases is to establish Neutral-to-Ground bonds at both equipment and also bond Neutral, equipment grounding conductor, and equipment case all together - meaning there are two Neutral-to-Ground bonds, usually away of the Main Breaker or Switch. However, a special attention shall be given to Neutral-to-Ground bond must be avoided in some downstream devices that could cause current to flow across the metallic parts of electrical installation.

    If NEUTRAL IS NOT SWITCHED: both Transformer and Generator have the same the Neutral-to-Ground connection but ISOLATED from Ground - meaning there are only one Neutral-to-Ground bond, usually connected to an isolated Neutral bus near of or inside of the Main Breaker or Switch.

    Steve is correct: "There are several grounding methods that can be used, the particular method chosen is dependent on the application", it depends on what the specifications of the project call for, a design decision sometimes taking into account the safety x cost relation when some specific grounding method is not mandatory by a code.

    Grounding is a very common discussion here in Z4E, please check if these topics could be of any help:

    grounding requirements
    https://zone4engineer.com/forums/topic/6/grounding-requirements

    grounding system
    https://zone4engineer.com/forums/topic/44/grounding-system

    MCCB switchgear
    https://zone4engineer.com/forums/topic/31/mccb-switchgear

    MV Transformer
    https://zone4engineer.com/forums/topic/36/mv-transformer

    two point grounding system
    https://zone4engineer.com/forums/topic/403/two-point-grounding-system

    I hope this helps. Regards.