ABB RELION REX640 Technical Manual page 1016

Protection and control

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Protection functions

Function block

Figure 563: Function block

Functionality

The three-phase underexcitation protection function UEXPDIS is used to protect the

synchronous machine against the underexcitation or loss of excitation condition.

The protection is based on the offset-mho circular characteristics on the impedance

plane. The function calculates the apparent impedance from the machine terminal

voltages and currents. If the impedance vector enters the offset-mho circle, the

function gives the operating signal after a set definite time. The operating time

characteristics are according to definite time (DT).

This function contains a blocking functionality. It is possible to block the function

outputs, timer or the function itself.

Analog channel configuration

UEXPDIS has two analog group inputs which all must be properly configured.

Table 945: Analog inputs

There are a few special conditions which must be noted with the configuration.

Table 946: Special conditions

U3P connected to real measurements

See the preprocessing function blocks in this document for the possible

signal sources.

Three-phase currents

Three-phase voltages

The function can work with one voltage chan-

Phase Sel for Z Clc set to "A

nel connected (

or AB", "B or BC" or "C or CA"), if the setting

Impedance Meas mode is set to "1Phase-to-

earth" or "1Phaseto- phase".

The function requires that at least two (the

third voltage will be derived) or all three volt-

age channels are connected if

Meas mode is set to "3Phase-to-earth",

"3Phase-to-phase" or "Pos sequence".

1MRS759142 F

Technical Manual