How to set zones for long transmission line that are valid for all faults

I am trying to implement three zonal protection in matlab and i have set zones that works good for LG fault but when i create LL fault in zone 1 the relay is responding as if fault is in zone 2.

This is the matlab simulink model

and this is single line diagram of it.[The pi section blocks are transmission lines]. Relay R1 will generate trip signal instantly when fault is in 80% of line 1 impedance[Zone1] and when fault occurs in remaining line it generates trip signal with some delay[Zone2]. I have set zones by creating LG fault at end of the zone means for Relay R1 zone 1 i have created fault at 160km of line 1.[because calculating 80% of impedance was difficult as the pi section blocks are having positive, negative and zero sequence impedance]

Digital Relay block in model consist of an algorithm which estimates Voltage and Current values and from this value impedances[Z] is calculated

ZR [R phase impedance], ZY [Y phase impedance], ZB [B phase impedance].

Yes, Red dots are impedance values calculated at different times[ The algorithm i used to estimate values uses samples so, whenever new current and voltage sample is produced impedance is calculated]

So i have created LL fault(between R and Y) at 160km of line 1 becauses set zones should work for all faults. But R phase impedances is entering into zone1 whereas Y phase is in zone 2

[William Rose]

@A Lumbini,

I would need to see some code and have more details in order to understand your problem. I am unfamiliar with three phase transmission line analysis but I am familiar with imedance anaysis of two-port sysems.

One plot title includes "ZR". The other plot title incldes "ZY". The plot axes have no titles. I assume the plots show impedances, with real and imaginary parts on x,y axes? Does each red dot rpresent an impedance measured at a different time? Does the plot with "ZR" in the title show impedance between the Z and R phases of the 3-phase system? And the plot with "ZY" in the title shows impedance between phases Z and Y? Since you are simulating a line-to-line (LL) fault between R and Y, then why not show the plot for impedance between R and Y?

You say "the relay is responding as if fault is in zone 2". Expalin what you mean by this. Specifically, how do you know "how the relay is responding"? How does a zone 2 relay response look different from a zone 1 relay response?

Each plot has two black closed loops, with one loop inside the other. Do impedances (red dots) inside the inner loop indicate a zone 1 fault, and impedances between the inner and outer loops indicate a zone 2 fault? Or something else?

[William Rose]

@A Lumbini, I'm not a simulink person so will not be able to assist you.

[A Lumbini]

Ok sir,

Thanks for your time

Answer 1

[MULI]

Hi @A Lumbini,

I understand that the digital relay is misclassifying zones during a Line-to-Line (LL) fault because it evaluates individual phase impedances.

You can follow the below steps to resolve this issue:

• For LL faults (e.g., R-Y), calculate the loop impedance instead of individual phase impedances:

• The loop impedance directly represents the fault's location and avoids inconsistencies caused by mutual coupling or independent phase evaluation.
• Modify your digital relay algorithm to compute , and for LL faults.
• Compare these loop impedances with the set zone boundaries.

You can refer to the following link which provides the distance protection relay model and offers further insights in modelling: https://www.mathworks.com/matlabcentral/fileexchange/112610-distance-protection-relay

[A Lumbini]

Thank you sir,

I will try this

[A Lumbini]

@MULI, It's working, thank you so much sir

So for LG fault at R ------ ZR falls into zone

LL fault b/w R &Y ----- ZRY falls into zone

LLG fault b/w R &Y -----ZR,ZY and ZRY falls into zone

LLL fault ------- ZRY, ZYB, ZBR falls into zone

LLLG fault ------- All Impedances falls into zone

Is it right sir?

So here the effect of decaying DC, Fault impedance etc gets eliminated.

But, can you please explain how this actually happened

[MULI]

Hi @A Lumbini,

You may find some helpful insights regarding distance protection in IEEE research papers. These resources provide a detailed explanation of fault behavior and its impact on impedance zones.

[A Lumbini]

@MULI Ok sir , thank you