Why Zone-1 of a Distance Relay is Set to Only 80-85%?
In electrical power systems, Zone 1 of a distance relay is set to cover 80-85% of a transmission line. This conservative setting prevents the relay from mistakenly acting (overreaching) for faults that occur on the next section of the line, ensuring that only the faulty section is isolated.
Zone-1 is the primary instantaneous protection zone of a distance relay.
Its purpose:
Trip faults inside the protected line
Trip without intentional delay
Avoid tripping for faults outside the line
If Zone-1 were set to 100% of the line impedance, the relay could mistakenly trip for faults beyond the protected line. This is called overreach.
Main Reasons for Keeping Zone-1 at 80-85%
CT and VT/CVT Errors
Distance relay calculates impedance from:
Z=V/I
But CTs and VTs are never perfectly accurate.
During heavy faults:
CT may saturate
VT/CVT may produce transient errors
Measured voltage/current become inaccurate
So the relay may calculate a wrong impedance.
Suppose:
Line-1 impedance = 40 Ω
Relay should protect only this 40 Ω
If relay is set exactly to:
Zset=40 Ω
then ideally:
Fault inside 40 Ω trip
Fault beyond 40 0 → no trip
But in real life measurements are not perfect.
Now Imagine a Fault Beyond the Line
Suppose fault occurs on Line-2.
Actual fault location:
Bus-A ----400---- Bus-Β----5---- Fault
Total impedance from relay:
Zactual=45 Ω
So relay SHOULD NOT trip instantly because:
Fault is outside protected line.
Now Comes the Important Part Relay calculates impedance using:
Z=V/I
During fault:
CT may saturate
CVT may distort voltage Measurement errors happen So instead of seeing 45 0, relay may incorrectly calculate Z=38 Ω Now relay thinks: “Fault is inside my protected line!”
So it trips instantly.
But actual fault was outside Line-1.
THIS WRONG OPERATION = Overreach
Meaning:
Relay “reached beyond” its intended protection zone.
Why 80-85% Prevents This?
Instead of setting relay to full 40 Ω, we intentionally reduce Zone-1 (80-85%): Z1=0.8×40=32 Ω
Now same external fault occurs.
Actual fault= 45 0 away
Even if measurement error happens Relay