There are several assumptions to made before we do electrical fault analysis study on a electrical system.Power systems are inherently dynamic which are subjected to constant voltage and current variations and therefore to maintain the stability of the system and also to choose appropriate switch gears and settings for the relays, we need to know the worst case scenarios. Fault Study comes in handy for that purpose.Following are some of the assumptions which take in to account while consideration of the electrical fault analysis on electrical engineering .
Assumptions made in a Electrical Fault Analysis Study and the validation of those assumptions
In fault analysis study its necessary to make assumptions , because we cant predict 100% natural scenarios on this electrical energy. Following are some of the assumptions commonly made in three phase fault studies for the ease of calculations,
- Transformers are on nominal tap position. This will let us take nominal voltages of transformers in calculations.
- All sources are balanced and equal in magnitude and phase. We neglect the slight differences in magnitude and phase of the source voltages as it is nothing when compared with the fault.
- High Voltage Power Lines are assumed fully transposed and all 3 phase have same impedance. Transposed lines have more or less equal inductance’s in all three phases.
- Loads currents are negligible compared to fault currents. Usually fault currents are about several kilo amperes, but load currents are mostly in ampere range. Therefore the effect of the load current on the final result is negligible.
- Sources represented by the Thevenin’s voltage prior to fault at the fault point.
- Large systems may be represented by infinite bus bars. When comparing with large systems with a small one, effect from the small one will not make much effect on the larger system. Therefore there is not much different taking the large system as an infinite bus bar.
- Line charging currents can be completely neglected as line charging currents are smaller compared to load current.
- Resistances are negligible compared to the reluctance. Usually in power lines, the dominant component of the impedance is the reluctance as it is several times higher than the resistance.
Analogue methods of studying the fault flow in a system
Symmetrical components method:
This Method can only be used to study asymmetric faults by decomposing the phase components into a sequence of symmetric components.(Positive, Negative and Zero sequence.)
Bus impedance or admittance method:
This can be use for the purpose of analyzing both symmetrical and asymmetrical faults with the use of bus-bar impedance’s of the system.
Assumptions like generators and loads are balance in the system are not very reasonable because generator capacities are vary with the time. And also when fault happen some loads like large motors they supply the power to the system instead of getting power from the system.
We are plan to discuss more about the fault analysis method which use in electrical analysis in our future articles.