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Disconnection times on a system if a stand by generator is used

Anthony Smith

On a fully designed power distribution system, clearly the fault levels, earth loop impedances and ultimately fuse disconnection times are determined by the system parameters Transformer %Z etc one or two in parallel for min and max fault levels levels. What needs to be considered when bringing on a stand-by generator, clearly all Fault levels, Earth loop impedances and consequently fuse disconnection times would change because of the Generator parameters. How would you design in these new parameters, bearing in mind that disconnection times on a large system may no longer be achievable 

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    I agree increasing cable sizes is not the way forward, but still interested to know how you would design your system, regardless of size on a stand-by generator parameters, as these parameters are clearly different to transformers. And I’m not sure how the transient and sub transient reactance would be plug into a power system were you are trying to establish fault levels, Ze, etc

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    I agree increasing cable sizes is not the way forward, but still interested to know how you would design your system, regardless of size on a stand-by generator parameters, as these parameters are clearly different to transformers. And I’m not sure how the transient and sub transient reactance would be plug into a power system were you are trying to establish fault levels, Ze, etc

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  • 0 in reply to Anthony Smith

    Ze is the one thing that may be lower - the local genset wll be TNS. Do not confuse it with electrode resistance - that may be quite high - several tens of ohms. You may not relay on the street earth when running on genset, as the main may be cut-off.
    Typically set the earth fault trips at 1% to 10% of the full load current for that branch if 30mA RCD function is not needed. Going the other way, final ccts will have to be 30mA RCD, now to reliably discriminate with the next layer up you need at least a factor of 3, so  be 100mA 0.1sec, and then 0.3A 1/3 second. It is not common to have more than 3-4 layers between genset and load.

    Mike.

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