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 shake 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.
The fault current flows into the polarity mark of the CT connected to Circuit Breaker 3, so the Directional Overcurrent (67) relay sees the fault in the forward direction. If the fault current is larger than the overcurrent setting, the relay will trip.
Directional overcurrent protection schemes were replaced with line impedance relays (21) to prevent a situation like this from occurring. This relay’s primary purpose is to trip for faults on the transmission line, not for faults somewhere else on the system, as would happen here. A line impedance relay would recognize that the fault was not on the transmission line and ignore this fault unless it was programmed to also provide backup protection with a significant time delay.
The fault current flows into the non-polarity mark of the CT connected to Circuit Breaker 4, so the Directional Overcurrent (67) relay sees the fault in the reverse direction. The orange/red shaded region indicates the typical region for a fault behind a relay.
Overcurrent directional relays can be set to trip for faults in the forward direction, which will protect the equipment in front of the relay. Or they can also be set to trip for faults behind the relay in the reverse direction. Forward and reverse are typically determined by the normal flow of current into the relay, so be sure to confirm the CT connections before you make any assumptions.
Incorrectly determining forward and reverse is an easy mistake to make. If I ever have doubts about some relay settings or directional overcurrent tests, I usually ask the design engineer, “Did you mean to trip if the fault is on the transmission line, or on the buss?” (You can use whatever easy-to-define characteristic for your situation.) Once they answer that question, I will review the CT connections and build a test on the transmission line and see if it trips. I then apply the fault in the reverse direction to make sure it doesn’t trip. Always ask the engineer what they intended if there is any doubt.
You could also perform a test in either direction first and see what direction the relay is set to trip. If it doesn’t make sense to you, you can ask the engineer, “Did you mean for the relay to trip if there is a fault on the buss?”
I hope this helps clear up the definitions of forward and reverse for Directional Overcurrent (67) relays. The next post on this topic, “Testing Directional Overcurrent Relays“, will hopefully help clarify the characteristic angle.
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