I am trying to calculate fault currents of IEEE 33-bus system using Zbus matrix approach. Without considering loads I am getting good results of fault currents in each branch by applying faults on each bus. However, after putting the effects of the load in the diagonal elements of Ybus (to get the Zbus) as (PL+jQL)*/|V2|, I am not getting good results.
In this case, fault current coming from the source becomes less than the fault current at the fault point or fault branch current.
I like to know expert opinion on this problem.
Regards,
Mahamad Nabab Alam
Hi,
If your loads have jXL or jXC terms; i.e. reactive terms, stored energy in them (i.e. initial values of IL/ VL at the instance of fault) also contributes to fault currents. Also, mechanical energy stored in spinning rotors of induction motors (i.e. back emf's) also feed the fault.
So the resultant fault current will be phasor sum of those fed by generators & those due to such loads.
Regards,
Nikhil
Interesting question and thanks for posting. I think when you represent the loads as the equivalent admittance and adding to the ybus leads to an increase in the effective impedance existing between the source and the fault point which would resulting less fault currents (steady state) ......
Dear Mahamad Nabab Alam,
As per my simulation of 8-bus system in ETAP, when I connect or disconnect entire loads of the system, the fault current is not affected (in both cases fault current is same). I haven't gone through the mathematical terms as you are discussing, but I am answering based on the simulations.
Hi,
If your loads have jXL or jXC terms; i.e. reactive terms, stored energy in them (i.e. initial values of IL/ VL at the instance of fault) also contributes to fault currents. Also, mechanical energy stored in spinning rotors of induction motors (i.e. back emf's) also feed the fault.
So the resultant fault current will be phasor sum of those fed by generators & those due to such loads.
Regards,
Nikhil
Dear all, through the equation of load modeling (that I have mentioned), it is clear that loads modeled as a constant impedance so it cannot be able to store energy to supply during the fault (correct me if I'm not right).
Hi, I completely agree with Nikhil.
Every inductive element (lumped or distributed) can store the energy in electromagnetic form. The contribution to the fault current depends on whether the corresponding reactance is involved in the equations of calculating steady state fault currents.
Moreover, the contribution to the fault current can be negative (reducing the fault current) and positive (increase fault current). Positive can only happen when the corresponding inductive element keep on getting the input energy even during the fault. That’s the reason why synchronous generators or simply generators are contributing positively to the fault current as the mechanical input is available even during the fault (with the assumption generator breaker cannot trip instantaneously after the fault) . And all other elements: transmission lines, transformers, and etc., be inductive or capacitive, existing in the fault path are just passive and at most they can provide negative contribution to the fault current.
The detailed analysis concludes that all the loads are contributing to the fault current during the fault. As a result, the fault current coming from the substation is becoming less than the value of the fault current at the fault point.
Hey, the following files can be used to short-circuit calculations to help you. Thanks
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