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Diesel Generator Fault Levels

MG over 5 years ago

Hello all,

I am looking to specify a temporary mobile diesel generator and as such looking into the disconnection times and the maximum fault current provided by the genset.

I have contacted a provider and got the specification of the alternator in their genset, images below of the fault current graph and the alternator per unit values.

I am looking into understanding what values to use. Am I correct in getting the sustained fault level and for an earth fault to apply the multiplication factor of 2.5 for sustained? I will make sure my disconnection times are less than the sustained maximum duration in order to not trip the generator breaker and loose all of the supply.

Alternator is 50Hz 415V

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0 Former Community Member over 5 years ago

The alternator fault levels are at infinite driving torque, but your average engine cannot provide this. The instantaneous fault level is controlled by the rotating mass (alternator and flywheel) but does not last very long (a few cycles at best). The front end CPDs of full generator rating will probably never open the circuit, even with a PPP fault (which is extremely unlikely). What will happen is the engine will stall as even at full torque it cannot support the virtually infinite torque required

So, will the set have enough "whoomph" (technical term!) to be able to provide sufficient current for long enough to operate a protective device? If so in this example, will it operate an 800 A device? Or will it only be able to supply sufficient energy for long enough to operate say only a 100 A device? Or will it stall and shut down within your required disconnection time leaving the protective devices intact?

Our conventional mains supply has a vast rotating inertia to provide enough "whoomph" to the extent it will not stall and carry on providing energy till something "gives"........ The conventional basis for tabulated disconnection times assumes this.

Regards

BOD
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0 Former Community Member over 5 years ago

The alternator fault levels are at infinite driving torque, but your average engine cannot provide this. The instantaneous fault level is controlled by the rotating mass (alternator and flywheel) but does not last very long (a few cycles at best). The front end CPDs of full generator rating will probably never open the circuit, even with a PPP fault (which is extremely unlikely). What will happen is the engine will stall as even at full torque it cannot support the virtually infinite torque required

So, will the set have enough "whoomph" (technical term!) to be able to provide sufficient current for long enough to operate a protective device? If so in this example, will it operate an 800 A device? Or will it only be able to supply sufficient energy for long enough to operate say only a 100 A device? Or will it stall and shut down within your required disconnection time leaving the protective devices intact?

Our conventional mains supply has a vast rotating inertia to provide enough "whoomph" to the extent it will not stall and carry on providing energy till something "gives"........ The conventional basis for tabulated disconnection times assumes this.

Regards

BOD
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Diesel Generator Fault Levels

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