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In three phase symmetrical fault motors(Induction and Synchronous) become generators(I guess because of the inertia). When do I take in consideration of the motors? Do I even need take them in consideration assuming there are big motors that have impact on the short circuit current or because it has short time impact I do not need to take them in the calculation?

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  • \$\begingroup\$ Depends on how accurately you want to calculate. If you are assuming the power to be supplied by an an ideal deep source like a grid, considering your motors as generators wouldnt make much of a difference as the short circuit current would primarily depend on impedance of the short circuit to earth. \$\endgroup\$ – Nouman Qaiser Jan 12 at 17:36
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If you have induction motor load you can include it in your short circuit calculation for the sub-transient timeframe. If you don’t have nameplate data you can estimate the sub-transient impedance of induction loads by assuming their sub-transient current is six per unit. You can ignore them for transient and synchronous timeframes.

If you have synchronous motors present you would include them just like you would include synchronous generators.

You might find this interesting regarding Utility Reclosing and motor loads.

When you are setting instantaneous protection elements, or calculating maximum fault current, this is when you would consider induction motor loads contribution. If you are concerned with slower protection elements then you can ignore them as their contribution is gone in a couple of cycles.

Russ

p.s. This would be a good place for you to begin studying this topic.

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Induction motor contribution can be a significant portion of the initial symmetrical fault current (Ik'' as defined in IEC 60909.) This affects the required fault rating for equipment such as switchboards, which must have a fault withstand rating exceeding the credible Ik'' that the switchboard would be exposed to.

Motor contribution is particularly important when feeding a lot of motors off an LV switchboard, as the motor contribution can be on the order of 30-50% of the prospective fault current available from the feeder transformer. This makes the difference between ordering a 25kA rated switchboard or a 40kA rated switchboard, with associated cost implications.

At medium voltage (say 11kV), the proportion of a) motor contribution current to b) the fault current available from the source, can be around 10-20% for industrial plants with a high density of motor loads. The impact is less than LV, but still significant for fault calculations and the fault withstand rating of equipment.

Fault current at HV (say 132kV) does not tend to be sensitive to motor contributions, as motor contributions at MV and LV are limited by the transformer impedances between the motors and the HV network.

In the absence of further information, it is reasonable to assume that an induction motor will have a motor contribution of approximately 6x full load current (i.e. the same as its starting current) for a fault near the motor, for a duration of 200ms. (Refer IEC 60909.)

From memory, the contribution from induction motors stops after 200ms due to loss of magnetic field, not due to loss of mechanical inertia. The motor is still spinning after 200ms, however with no magnetic field the motor is unable to regenerate any power.

For deep theory regarding the transient behaviour of induction motors, the classic textbook regarding the behaviour of motors and generators is Mulutkula S. Sarma's Electric Machines - Steady State and Dynamic Performance.

For a practical applied view, including defined criteria for when the contribution of induction motors can be disregarded, read IEC 60909-0 Short circuit currents in three-phase a.c. systems - Part 0 - calculation of currents, particularly section 3.8 Asynchronous motors.

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