RELAYS 101

Plotting Z on the R-X Diagram with a Mho Circle Characteristic

Enter data into the text boxes. Click the Plot Z Button.
One topic of note on this page is the idea that each quadrant in this RX diagram also has a notation from a "Power Diagram" (the "PQ Diagram"). While the scaling would be completely different between the two diagarms, there is a commonality - "Watts and VARS".
Because of the Sine and Cosine functions the first quadrant works out to be watts out and VARs out.
Because of the Sine and Cosine functions the second quadrant works out to be watts in and VARs out.
Because of the Sine and Cosine functions the third quadrant works out to be watts in and VARs in.
And finally, because of the Sine and Cosine functions the fourth quadrant works out to be watts out and VARs in.

The important thing to remember about this is - a fault is a system event that drains the power system. A fault would be an event that is watts and VARs LEAVING the system. A fault is watts and VARs out.

If you think about quadrant two (for example), what kind of "fault" makes your voltage sag (VARs out), but brings power in to the system (watts in)?
Or (Q3), is it a "fault" if real and reactive power flow in to your system?
And (for Q4), watts out and VARs in - power leaving the system but the voltage gets boosted?
While there are certainly "System Events" that can occur in any of the 4 quadrants, only Q1 has watts and VARs leaving the system.
Protective relays have focused upon Q1 for protecting the system from faults mainly through the "Mho" Circle characteristic that can be "tuned" to the Q1.
Other system events, in other quadrants, can be sensed and acted upon.
For example one type of "Loss of Field" relay is concentrated to sense "Watts out and VARs in". (Q4).
Another example: Q3 (Watts In/VARs In) sensing is used if the protection engineer needs a device to sense "Reverse Fault", perhaps in a pilot scheme or an Out-of-Step protection relay action.