1)Loads are typically not included in the fault currents studies in this Chapter
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Question
1)Loads are typically not included in the fault currents studies in this Chapter. How do loads impact the faults currents in practical applications?
2)What about the impedances of this system, and many of the examples out of the books, tell you that we are using simplified system? What would you expect to see in the impedance of real systems that we neglect in many of the learning examples for short circuit fault calculations?
((Prefault load current is neglected. Because of this, the positive- sequence internal voltages of all machines are equal to the pre- fault voltage VF. Therefore, the prefault voltage at each bus in the positive-sequence network equals VF.)) from textbook
3)For steady state short circuit fault modeling, (besides possibly load) what part of the fault response is ignored?
4)When a fault impedance is zero, what two descriptions are often used to description the fault?
Explanation / Answer
1.
Static Loads do not contribute to fault currents. However rotating machines such as induction motors connected as a load in the network contribute to the fault current during the first 2-3 cycles if it was operating when the fault occured. This happens because as the motor was operating when the fault occured it had a rotating magnetic field in its air gap. As the fault occurs the voltage will dip and as the air gap flux cannot change instantaneously, it dies down by feeding the fault current for first few cycles. As a result the subtransient fault current increases and hence the peak fault current increases during the asymmentrical part of short circuit currents. Apart from induction motors which are very commonly connected loads, synchronous motors also contribute to fault current. However synchronous motors are not used as loads generally.
2.
We generally neglect the resistances of all components connected in the system while doing short circuit calculations unless it is specified to used them. We deal with only reactances for short circuit calculations. Practically every equipment has some resistance which opposes the fault current and hence actual fault current is lower. Secondly, In general while doing short circuit calculationa we assume a prefault voltage of 1.0 pu when the fault occurs. However this might not be the case for a big network with a lot of loads connected. The voltage at the the fault point is generally less than 1.0 pu and hence the short circuit currents are generally less. Apart from prefault voltage, we assume a bolted fault while doing short circuit calculations. A bolted fault is a fault which offers zero resistance to the fault current. A bolted short circuit fault results in a higher value of short circuit current and gives the worst value of short current which may occur. However practically every short circuit fault occurs with short circuit current opposed by a fault impedance. The value of fault impedance is not under anyone's control and it may vary according to the type of fault. Hence while doing short circuit calculations, we assume a bolted short circuit fault and calculate the maximum value of short circuit current. So you can notice that the short circuit currents calculated by us are generally on the higher side when compared to a practical scenario. Practically the fault current may be opposed by some value of fault impedance :may be small or may be high; with a prefault voltage less than 1 pu. Both these scenarios lead to a lower short circuit current when compared to our calculations.
3.
For steady short circuit fault modelling the subtransient and transient part of tbe fault current is ignored. The subtransient and transient behaviour of fault current is mainly due to the generator's inherent behaviour of offering a lower reactance to short circuit currents during first few cycles due to generator dynamics during short circuit. Apart from generators, motors connected to a system also contribute to fault current during the first few cycles. Hence before reaching a steady state value of short circuit current, a dc offset is introduced in the fault current due to the above factors which dies down in the first 5-6 cycles and steady state short circuit current is obtained. During steady state short circuit model we neglect the short circuit response during the first 5-6 cycles which are known as subtransient and transient period of fault current. Subtransient period is the starting 2-3 cycles and transient period is the next 3-4 cycles. Eventually after 5-6 cycles steady short circuit current is achieved.
4.
A zero fault impedance is often described as a bolted short circuit fault in short circuit analysis. Alternatively it is sometimes also referred as a solid fault. Both these terminologies refer to a fault with zero fault impedance.
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