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21 - MHO Distance Relay


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 Va, Vb, Vc, Ia, Ib and Ic. The zero sequence current phasor is calculated as follows, using the three-phase currents:

The phasors are used to compute the 6 impedances: Za, Zb, Zc, Zab, Zbc and Zca. 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.


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

Line Parameters Tab

NameUnitDescriptionDefault value

The frequency of the measured current signals. Both 50
and 60 Hz are supported within the model.



kmThe total length of the line200


ohm/kmThe positive sequence resistance of the line0.011748


ohm/kmThe zero sequence resistance of the line0.29014


H/kmThe positive sequence inductance of the line0.00077518


H/kmThe zero sequence inductance of the line0.0030241

Settings Tab

NameUnitDescriptionDefault value

Zone 1


The forward looking distance of the line that is protected
in the first zone.


Zone 2


The forward looking distance of the line that is protected
in the second zone.


Zone 1
Time Delay


The time delay setting for Zone 1. A half-cycle delay is
recommended to avoid overreach.


Zone 2
Time Delay

sThe time delay setting for Zone 2.0.3333


n/aTo enable the POTT logic.disabled

Input and output signals

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

I/O nameType (unit)Description
VabcInput (V)

A 3-dimension voltage input for three-phase voltages, in
the order of Va, Vb and Vc.

IabcInput (A)

A 3-dimension current input for three-phase currents, in
the order of Ia, Ib and Ic.

RPTInput (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.

ResetInput (binary)

The reset signal for the relay. The relay will be reset when
this signal becomes 1.

TripOutput (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.

SPTOutput (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

R_0ohmRa measured by the relay.
R_1ohmRb measured by the relay.
R_2ohmRc measured by the relay.
R_3ohmRab measured by the relay.
R_4ohmRbc measured by the relay.
R_5ohmRca measured by the relay.
X_0ohmXa measured by the relay
X_1ohmXb measured by the relay
X_2ohmXc measured by the relay
X_3ohmXab measured by the relay.
X_4ohmXbc measured by the relay.
X_5ohmXca 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

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