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# Description

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. The model works for 50 Hz or 60 Hz system.

# 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 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. The figure below shows the relay tripping logic.

## General Parameters

NameUnitDescription

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/kmThe zero sequence resistance of the line

Zero sequence inductance

H/kmThe zero sequence inductance of the line

## Protection Settings

NameUnitDescription

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

sThe time delay setting for Zone 2.

Enable permissive transfer trip

-To enable the POTT logic.

# Inputs and Outputs

## Inputs

NameDescription

Vabc

Three-phase voltages: Va, Vb, Vc

Iabc

Three-phase currents: Ia, Ib, Ic

RPTReceived 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

NameDescription
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.

SPTThe 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

NameUnitDescription
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
RXR_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.
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