21 - MHO Distance Relay

Summary

This block models a generic MHO phase distance element with 2 protection zones. A permissive overreach transfer trip logic is also implemented. The relay model does not include other advanced functionalities such as polarizing quantities, memory actions,
loss of potential, reactance functions, load encroachment, etc.

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 Figure 3 - 1. 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 centres 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 Figure 3 - 1, we have:

Where R₁ and X₁ are the total positive sequence resistance and reactance of the line.

For the example of Figure 3 - 1, 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.

Distance Relay Characteristic

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.

Parameters

Figure 3 - 2 shows the parameter panel for the model. The following parameters can be modified:

Line Parameters Tab

Name	Unit	Description	Default value
Frequency	Hz	The frequency of the measured current signals. Both 50 and 60 Hz are supported within the model.	60
Line Length	km	The total length of the line	200
Positive Sequence Resistance	ohm/km	The positive sequence resistance of the line	0.011748
Zero Sequence Resistance	ohm/km	The zero sequence resistance of the line	0.29014
Positive Sequence Inductance	H/km	The positive sequence inductance of the line	0.00077518
Zero Sequence Inductance	H/km	The zero sequence inductance of the line	0.0030241

Settings Tab

Name	Unit	Description	Default value
Zone 1 reach	%	The forward looking distance of the line that is protected in the first zone.	80
Zone 2 reach	%	The forward looking distance of the line that is protected in the second zone.	120
Zone 1 Time Delay	s	The time delay setting for Zone 1. A half-cycle delay is recommended to avoid overreach.	0.0083
Zone 2 Time Delay	s	The time delay setting for Zone 2.	0.3333
Enable Permissive Tripping	n/a	To enable the POTT logic.	disabled

Input and output signals

The module has 4 inputs and 2 outputs.
21 Relay I/O

I/O name	Type (unit)	Description
Vabc	Input (V)	A 3-dimension voltage input for three-phase voltages, in the order of Va, Vb and Vc.
Iabc	Input (A)	A 3-dimension current input for three-phase currents, in the order of Ia, Ib and Ic.
RPT	Input (binary)	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	Input (binary)	The reset signal for the relay. The relay will be reset when this signal becomes 1.
Trip	Output (binary)	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	Output (binary)	The permissive tripping signal sent from the local relay. It will be asserted if the fault is detected in Zone 2.

Available monitoring signals

Other than the input and output signals, the following signals are available in the sensor list:
21 Relay monitoring signals

Name	Unit	Description
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.

These values can be used to plot the impedance trajectory on the R-X plane. For example, in ScopeView, ‘versus (R_0,X_0)’ can be used to plot Za measured by the relay on the R-X plane. The following figure shows the circles representing zone 1 and zone 2 as well
as the Za trajectory during a phase A-to-ground fault.

Impedance trajectory on R-X plane