Finding The Direction In Directional Overcurrent Relays

Transcript

  Finding the Direction in Directional Overcurrent Relays   A reader recently asked a question about the forward and reverse directions described in the  Directional Overcurrent Relay  section of The Relay Testing Handbook   series. I used electro-mechanical directional relays as an example, which may have been a mistake. Let’s take another look at the Directional Overcurrent (67) element from a system perspective. We will start with a simple transmission line with the source on the left and a load on the right. The current flows into the polarity mark of the CT on Breaker 3, and into the Directional Overcurrent (67) Relay using the same direction. Any current flowing into the polarity mark is considered to be the forward direction. The phasor diagram for this situation might look like the following. Every load is a combination of resistance and inductance, so the normal operating range for this line is the green shaded region when the current flows into Circuit Breaker 3.    Let’s look at what the Directional Overcurrent (67) relay connected to Circuit Breaker 4 sees under the same conditions. This relay is designed to protect the same transmission line from the other direction. The current enters the non-polarity mark of the CT, and the relay determines that current is leaving the transmission line; or the reverse direction. The phasor diagram of a meter test on the Directional Overcurrent (67) relay connected to Circuit Breaker 4 would look like the following. The current is flowing in the reverse direction  and the orange/red shaded area displays the normal region when the current flows into a load  behind the relay. If we reversed the source and load, you could swap the phasor diagrams above for each relay. Let’s sh ake things up by closing Circuit Breaker 8 and applying a Phase A-to-Ground fault 50% down the line. This is a fault, so:    The faulted voltage should drop in proportion to the severity of the fault    The fault current should be significantly larger than the normal load current.    The fault current should lag the voltage by 40-89.9 degrees depending on the line characteristics, voltage, and severity of the fault.    The non-faulted phases should stay relatively the same. Both fault currents flow into the transmission line, so the directional overcurrent relays connected to Circuit Breakers 3 and 4 will see the current in the forward direction because the current flows into both CT polarity marks.   If we pretend that the fault is exactly 50% down the line, both sources are identical, and the impedance between the sources and the fault are also identical, we can use the same phasor diagram for both relays. Obviously this won’t be true in the real world and the current magnitudes would be different. The typical region for a fault in the forward direction occurs in the green shaded area for both relays.  Now let’s look at a fault that is not on the transmission line.