RELAYS 101

Faulted Phase-Phase Voltage Will Have No V0 In Real Life

This routine demonstrates how the voltages and angles will behave in a phase to phase fault (B-C voltage in this example). Visit the page for In-Depth B-C Fault to learn the calculations involved. The system will behave in such a manner as to not produce zero sequence voltage during the phase to phase fault. The three phase voltages will add together to produce 0 volts of zero sequence voltage during this fault. Thus there will be an action that collapses the B to C voltage angle difference. Such that a fault at the generator will have 120 degrees difference between B & C but at the point of the fault the phase angle difference between B & C phases will be 0 degrees. At this point the two voltages will each be one-half the value of VA and the B & C phase angles will each be 180 degrees away from VA. Thus these three phases of voltage will add up to be 0 volts of zero sequence. In this example the value of VA Magnitude can be user adjusted between 34 and 200 volts. VA phase angle is set (internally) at 0 degrees. VB, VB angle, VC and VC angle cannot be changed by the user. The user can also adjust the percentage distance to the fault. Adjustment from 0% (at the bus - or generator) to 100% of the distance to the fault. The point of the fault would represent the most extreme distortion of the three-phase system (thus maintaining the fact that there is no V0 during a phase to phase fault; (side note: there is no I0 during a phase-phase fault either!) Why should you care? Because if the smart relay (like a microprocessor-based relay) detects I0 or V0 then it will shift to its internal programming to trip accurately for a phase to ground fault, eventhough you might be attempting to test for accurate action on a phase to phase fault. Also, a smart relay will BLOCK TRIPPING (if enabled) if it detects V0 with no I0. V0 without I0 simply does not happen in real life unless a voltage potential is missing (like from a blown fuse). Thus a smart relay has "Fuse Failure" or "Loss Of Potential" protection built in. When attempting to simulate a phase-to-phase fault, you must create a true-to-life simulation or the relay may not behave in the manner that you predict. Note that "FF" and "LOP" protection has been in protective relays before the inception of microprocessor relays.

Back in the olden days an incorrect simulation was allowed (even taught) in testing the old electro-mechanical distance relays. Those relays simply measured the voltage applied to the relay's terminals; didn't matter if it was a single phase or two applied phases. Didn't matter if V0 would have been created because it was not a concern in those old relays. Today's relays care about V0.

There are two simulations available. The first is a correct simulation where you will see the voltage phase angles collapse as the voltages are faulted. This produces the true-to-life situation where the phases are faulted to one another and there is NO V0 created.
The second situation is the case that occurs during incorrect relay testing, or there is a blown fuse. That case will be the voltage magnitudes are faulted but the phases remain 120 degrees apart. (Just doesn't happen!)

Try both simulations and take note of the phase voltage and angle values when you reach a specified VBC (for example when VBC=50 volts or so). Then take note of the V0 with each simulation.

Operational note: You may have to "RESET" or reload the page semi-frequently as buffers may fill and yield unexpected and incorrect results. A good browser should be able to allow phase collapse all the way to 180 degrees and back to normal before a page reload might be necessary. Also, be aware that this was written with the adjustable number fields with a stepper switch capability. Some browsers ignore the stepper switch and just give you a text field. The app works either way. If your browser just gives you a text field then simply enter the number that you wish and then click (or tab) away from your entry field. The changing of the text in the field will commence the calculations.