Index

# RELAYS 101

### In-Depth Create 3-Phase Fault

Whenever you attempt to simulate a fault for a relay test you must ensure that the phase current value(s) are equal to or greater than any overcurrent supervision involved in the circuit (or trip equation). A 3-Phase Fault will have all three voltages equal and balanced around the circle and all three currents are equal and balanced around the circle. This yields a fault that has no Zero Sequence and no Negative Sequence components. The voltages will be a faulted value and remain equal in magnitude, remain 120 degrees apart with the original phase sequence. The currents will be a faulted value, original phase sequence and because the current value is high then the angle of the currents will increase towards 90 degrees Lagging. Since all of the voltages and currents are equal and 120 degrees apart then it can be shown that the phase-to-phase values are equal to the phase value times the square root of 3. With both the voltages and currents shown to be equal to the phase value times the square root of three then when the impedance is calculated it can be shown simply as "(Vx√3)/(Ix√3)=V/I=Z" Since all of the voltages and currents are equal and 120 degrees apart then there will be no Zero Sequence. There will be no Neutral Current. There will be no Compensated Neutral Current (K0IN=0). Therefore when calculating a simple phase value of impedance then it could be seen as Z=V/(I+K0IN). And when Compensated Neutral Current is zero then the final calculation is Z=V/I. Thus whether you calculate Z as phase-to-ground or phase-to-phase then Z=V/I. In a 3-phase fault simply take any one phase voltage and divide it by its phase current. Or to set up a simulated test then take a fault voltage and divide it by the impedance to the fault and the calculation produces the simulated fault current. Iflt = Vflt / Zflt

Faulted VA magnitude: Angle: degrees

Fault Z Magnitude: Fault Z Angle: degrees

Step 1: Enter a desired test phase voltage and the desired impedance to the fault. Divide the voltage by the impedance. The resultant will be the magnitude of the simulated test current.

(Itest = Vtest/Ztest)

Your calculated test current: amps

Step 2: The current test angle(s) will be the Fault Z Angle subtracted from the phase voltage angle(s) (example IA Angle will be 0 degrees - Fault Z Angle)

Your calculated test current angle: degrees

VA: volts VA Angle: degrees

VB: volts VB Angle: degrees

VC: volts VC Angle: degrees

IA: amps IA Angle: degrees

IB: amps IB Angle: degrees

IC: amps IC Angle: degrees

The following diagram is a sampling of 3-phase vectors during a 3-phase fault. Phase-A is colored black, Phase-B is colored green and Phase-C is colored red. Since there is no scale shown, it is difficult to view values but the three voltages are faulted substantially, however they are still equal in magnitude to one another and the angles between the three phases are still 120 degrees apart. This is what happens during a 3-phase fault. Additionally, all three currents are equal in magnitude and 120 degrees apart. It is noteworthy that the angle of the current is extremely lagging, indicative of very heavy balanced load or a 3-phase fault. This is the system behavior that will be simulated in a test set when a 3-phase fault is set up. All voltages are at a fault level and are equal and 120 degrees apart. All currents are at a fault level, are equal, 120 degrees apart and at a lagging phase angle. This is a snapshot of the phase vectors at the same time and from the same fault as the "Symmetrical Components" diagram that is displayed on the "In-Depth Positive Sequence" page. When the phases are equal in magnitude to one another and 120 degrees apart then there is no Negative Sequence and there is no Zero Sequence. Only Positive Sequence, no collapsing phase angles (like on a phase-to-phase fault) no imbalances, no ground currents. This is the easiest fault to simulate. This is the easiest to calculate. Z=E/I

Faulted Phase vectors during a sampled 3-Phase Fault
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