ABB Switzerland Ltd REG 316*4 1MRB520049-Uen / Rev. F
3-264
E. Detecting rotor slip and shift
Rotor shift is detected by monitoring the voltage U·cos j, i.e. the
component of the voltage in phase with the current.
If the generator is faster than the power system, the rotor
movement in the impedance and voltage diagram is from right to
left and generating is signalled. If the generator is slower than
the power system, the rotor movement is from left to right and
motoring is signalled (the power system drives the generator as
if it were a motor).
The movements in the impedance plain can be seen from Fig.
3.5.26.1. The transient behaviour is described by the transient
e.m.f’s E
A
and E
B
,
and by X
d
' , X
T
and the transient system im-
pedance Z
S
.
The detection of rotor angle is enabled when
· the minimum current exceeds 0.10 I
N
· the maximum voltage falls below 0.92 U
N
· the voltage U·cos j has an angular velocity of 0.2...8 Hz
and
· the corresponding direction is not blocked.
An alarm is given when movement of the rotor in relation to the
slip line and the rotor angle exceeds the angle set for ‘WarnAngle’.
Slipping is detected when
· a change of rotor angle is detected
· the slip line is crossed between Z
A
and Z
B
· the direction of movement has remained the same since
pick-up and has lasted at least for the time ‘t-slip’
· the direction of movement has remained the same
throughout ‘t-slip’.
When the impedance crosses the slip line between Z
B
and Z
C
it
counts as being in zone 1 and between Z
C
and Z
A
in zone 2. The
entire distance Z
A
-Z
B
becomes zone 1 when ‘EnableZone1’ is
enabled (external device detects the direction of the centre of
slipping).
After the first slip, the signals ‘Zone1’ or ‘Zone2’ and - depending
on the direction of slip - either ‘Generator’ or ‘Motor’ are issued.
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