Model Overview

The distance relay uses the three-phase voltages and the three-phase currents measured at its end of the transmission line. It uses a generic MHO phase distance element and has two protection zones. The relay characteristics are illustrated on the R-X plane in the figure below. The dotted line represents the total impedance vector of the transmission line in the forward direction. The two circles represent the protection zones, commonly known as MHO characteristics. Their centers are both located in the impedance line axis and pass through the origin. The diameter of a given circle will be the total line impedance multiplied by the protected zone percentage.

In the figure above, we have

Where R₁ and X₁ are the total positive sequence resistance and reactance of the line. For the example of the figure above, zone 1 MHO circle has an 80% reach thus the diameter of the zone 1 circle equals to 80% of the total line impedance. In the example, zone 2 MHO circle has a 120% reach.

The model uses Discrete Fourier Transform to calculate the fundamental phasor (magnitude and angle) of V_a, V_b, V_c, I_a, I_b and I_c. The zero sequence current phasor is calculated as follows, using the three-phase currents:

The phasors are used to compute the 6 impedances: Z_a, Z_b, Z_c, Z_ab, Z_bc and Z_ca. Take phase A as an example, the impedances are calculated as follows:

Where k is the residual compensation factor:

If any of the computed impedances falls in the circle zone and stays inside for the time delay setting of this zone, the trip signal changes from 0 to 1. A permissive overreach transfer tripping (POTT) logic is also implemented in this model. For the local relay, when zone 2 picks up, a permissive tripping signal (SPT) will be sent out to the other relay at the remote end of the line. If the permissive tripping option is enabled on the remote relay, when it receives the permissive tripping signal and detects the fault in its zone 2, it will trip immediately, instead of waiting for the time delay of zone 2. Similarly, if the local relay receives the permissive tripping signal (RPT) from the remote relay, and detects a fault in its zone 2, it will send out the trip command immediately. The figure below shows the relay tripping logic.

Mask and Parameters

General Parameters

Name	Unit	Description
Frequency	Hz	The nominal frequency of the measured signals. Both 50 and 60 Hz are supported in the model.
Line length	km	The total length of the line
Positive sequence resistance	ohm/km	The positive sequence resistance of the line
Positive sequence inductance	H/km	The positive sequence inductance of the line
Zero sequence resistance	ohm/km	The zero sequence resistance of the line
Zero sequence inductance	H/km	The zero sequence inductance of the line

Protection Settings

Name	Unit	Description
Enable MHO distance relay	-	Enable and disable relay operation
Zone 1	% of line length	The forward looking distance of the line that is protected in the first zone.
Zone 2	% of line length	The forward looking distance of the line that is protected in the second zone.
Zone 1 delay	s	The time delay setting for Zone 1. A half-cycle delay is recommended to avoid overreach.
Zone 2 delay	s	The time delay setting for Zone 2.
Enable permissive transfer trip	-	To enable the POTT logic.

Inputs and Outputs

Inputs

Name	Description
Vabc	Three-phase voltages: Va, Vb, Vc
Iabc	Three-phase currents: Ia, Ib, Ic
RPT	Received permissive tripping signal from the remote relay. If the relay receives a RPT and at the same time detects a fault in Zone 2, it will discard the time delay setting for Zone 2 and assert a trip signal directly.
Reset	The reset signal for the relay. The relay will be reset when this signal becomes 1.

Outputs

Name	Description
Trip	The trip command. By default, it is 0. it will become 1 if a fault condition is detected either in Zone 1 or Zone 2, and the fault condition keeps being true for the certain time delay; or if a permissive trip command is received and the fault is detected to be in Zone 2.
SPT	The permissive tripping signal sent from the local relay. It will be asserted if the fault is detected in Zone 2.

Available monitoring signals in m

Name		Unit	Description
Zone1_trip		-	The trip command from Zone 1. By default, it is 0. it will become 1 if a fault condition is detected in Zone 1, and the fault condition keeps being true for the certain time delay.
Zone2_trip		-	The trip command from Zone 2. By default, it is 0. it will become 1 if a fault condition is detected in Zone 2, and the fault condition keeps being true for the certain time delay.
RPT_trip		-	The trip command from RPT. By default, it is 0. it will become 1 if a permissive trip command is received and the fault is detected to be in Zone 2
RX	R_0	ohm	Ra measured by the relay.
	R_1	ohm	Rb measured by the relay.
	R_2	ohm	Rc measured by the relay.
	R_3	ohm	Rab measured by the relay.
	R_4	ohm	Rbc measured by the relay.
	R_5	ohm	Rca measured by the relay.
	X_0	ohm	Xa measured by the relay
	X_1	ohm	Xb measured by the relay
	X_2	ohm	Xc measured by the relay
	X_3	ohm	Xab measured by the relay.
	X_4	ohm	Xbc measured by the relay.
	X_5	ohm	Xca measured by the relay.

ARTEMiS User Documentation

21 - MHO Distance Relay

Description