Detecting An Out-Of-Step Condition - ABB Relion Technical Manual

Section 7
Impedance protection
7.4.7.2
182
Zgen(Rgen, Xgen), Ztr(Rtr, Xtr), Zline(Rline, Xline), Zeq(Req, Xeq), and the setting
PickupAngle, for example 120 degrees. All impedances must be referred to the voltage
level where the out-of-step protection relay is placed; in the case shown in
relay is connected to the terminals of the generator and, therefore, the previous quantities
shall be referred to the generator nominal voltage and current level. The impedances from
the position of the out-of-step protection in the direction of the normal load flow can be
taken as forward.
The out-of-step relay, as in
direction are forward impedances:
• ForwardX = Xtr + Xline + Xeq (All values referred to generator voltage)
• ForwardR = Rtr + Rline + Req (All values referred to generator voltage)
The impedances that can be measured in the reverse direction are:
• ReverseX = Xd' (Generator transient reactance suitable for this protection)
• ReverseR = Rg (Relatively very small, can often be neglected)
Resistances are much smaller than reactances, but can in general not be neglected. The
ratio (ForwardX + ReverseX) / (ForwardR + ReverseR) determines the inclination of the
Z-line, connecting the point SE (Sending End) and RE (Receiving End), and is typically
approximately 85 degrees. While the length of the Z-line depends on the values of
ForwardX, ReverseX, ForwardR, and ReverseR, the width of the lens is a function of the
setting PickupAngle.The lens is broader for smaller values of the PickupAngle, and
becomes a circle for PickupAngle = 90 degrees.
When the complex impedance Z(R, X) enters the lens, pole slipping is imminent, and a
pickup signal is issued. The angle recommended to form the lens is 110 or 120 degrees,
because it is this rotor (power) angle where problems with dynamic stability usually begin.
Rotor (power) angle 120 degrees is sometimes called "the angle of no return" because if
this angle is reached under generator power swings, the generator is most likely to lose
step.

Detecting an out-of-step condition

An out-of-step condition is characterized by periodic changes of the rotor angle,
synchronizing power, rotational speed, currents and voltages. When displayed in the
complex impedance plane, these changes are characterized by a cyclic change in the
complex load impedance Z(R, X) as measured at the terminals of the generator, or at the
terminals of a power line connecting two power sub-systems. This was shown in
76. 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
Figure 80 looks into the system and the impedances in that
1MRK 502 048-UUS A
Figure 80 the
Figure
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