8405-011 WOODWARD

Monitoring of the Differential Current 

The monitoring of the differential current is carried out in two stages.. 

The first monitoring level serves as a warning and can be enabled or disabled. Should the adjustable warning 

characteristic be exceeded, a text appears in the display and a relay contact is enabled. The pick-up time and the 

dropout delay of the relay output are adjustable. The warning stage of the monitoring is auto-resetting. 

The second stage of monitoring (main stage) serves to initiate tripping. In contrast to the first stage, it offers the 

possibility to monitor the overstepping of an adjustable tripping characteristic (Id < In) and additionally, a fixed 

tripping-threshold of 100%, relative to the generator rated current (Id > In). The trigger-delay for each limit value 

may be independently adjusted, thus allowing a shorter triggering time at higher differential currents. When one 

or both tripping characteristics are exceeded, a text display is initiated and two relay contacts are energized. The 

tripping characteristics possess a 2% hysterisis relative to the generator rated current. . 

The signal relay is only automatically reset if the function "automatic reset relay" in the Entry field on the screen 

is configured to "on". Otherwise, the resetting is carried out by pressing the "Clear" button on the front of the 

unit or via the discrete input terminal 18 "reset". 

The two monitoring levels can also be used to change the characteristics of the control function (stage 1: small 

value and a long time; stage 2: high value and a short time) 

Tripping Characteristic 

The following figure shows the tripping and warning characteristics (with sample values for X12, Y1,and Y2). It 

represents the tripping and warning thresholds (Y) relative to the restraint current (X) The positions of the corner 

points are determined by the coordinates P (X12/Y2) and P (X12/Y1). The selection of these positions is dependent on the generator being protected. The following gives the ranges of tripping and warning thresholds: 

IS / IN 0 to X12 The threshold Id is independent of the restraint current.. 

IS / IN X12 to 5 × IN The threshold Id is dependent on the restraint current. A change of 100% in the 

restraint current causes an increase of 10% in the tripping threshold. 

IS / IN > 5 × IN The threshold Id stays constant at 85%. 

Different characteristics can be chosen for the first and second monitoring levels, whereby the horizontal position 

(X-coordinate) is valid for both stages. The vertical position (Y-coordinate) can be chosen separately for each 

monitoring level. This results in a fixed difference in thresholds of the first and second monitioring levels for 

each restraint current Is.