ABB Relion 670 Series RES670, Technical Manual

Page: 170 / 888

Figure 41 . When a synchronous machine is out-of-step, pole-slips occur. To recognize a
pole-slip, the complex impedance Z(R,X) must traverse the lens from right to left in case
of a generator and in the opposite direction in case of a motor. Another requirement is that
the travel across the lens takes no less than a specific minimum traverse time, typically
40...60 milliseconds. The above timing is used to discriminate a fault from an out-of-step
condition. In
Figure 41 , some important points on the trajectory of Z(R, X) are designated.
Point 0: the pre-fault, normal load Z(R, X). Point 1: impedance Z under a three-phase fault
with low fault resistance: Z lies practically on, or very near, the Z-line. Transition of the
measured Z from point 0 to point 1 takes app. 20 ms, due to Fourier filters. Point 2: Z
immediately after the fault has been cleared. Transition of the measured Z from point 1 to
point 2 takes approximately 20 ms, due to Fourier filters. The complex impedance then
travels in the direction from the right to the left, and exits the lens on the opposite side.
When the complex impedance exits the lens on the side opposite to its entrance, the 1st
pole-slip has already occurred and more pole-slips can be expected if the generator is not
disconnected.
Figure 41 shows two pole-slips. Figures like Figure 41 and Figure 43 are
always possible to draw by means of the analog output data from the pole-slip function,
and are of great help with eventual investigations of the performance of the out-of-step
function.
7.2.7.3 Maximum slip frequency
A pole-slip may be detected if it has a slip frequency lower than a maximum value fsMax .
The specific value of fsMax depends on the setting (parameter) PickupAngle (which
determines the width of the lens characteristic). A parameter in this calculation routine is
the value of the minimum traverse time,
traverseTimeMin . The minimum traverse time is
the minimum time that the travel of the complex impedance Z(R, X) through the lens, from
one side to the other, must last in order to recognize that a pole-slip has occurred. The value
of the internal constant
traverseTimeMin is a function of the set PickupAngle .For values of
PickupAngle <= 110°, traverseTimeMin = 50 ms. For values PickupAngle > 110°,
traverseTimeMin = 40 ms. The expression which relates the maximum slip frequency
fsMax and the traverseTimeMin is as follows:
fsMax Hz
traverseTimeMin ms
PickupAngle
[ ]
≅
[ ]
⋅
°
[ ]
°
1000
1 000
180
. -
[[ ]






ANSIEQUATION2319 V1 EN
(Equation 7)
The maximum slip frequency fsMax for traverseTimeMin = 50 ms is:
PickupAngle = 90° → fsMax = 20 × 0.500 = 10.000 Hz
PickupAngle = 100° → fsMax = 20 × 0.444 = 8.888 Hz
PickupAngle = 110° → fsMax = 20 × 0.388 = 7.777 Hz
(default 110°)
The maximum slip frequency fsMax for traverseTimeMin = 40 ms is:
Section 7
1MRK 511 365-UUS A
Impedance protection
164 Phasor measurement unit RES670 2.1 ANSI
Technical manual