Detecting An Out-Of-Step Condition - ABB REL650 Technical Manual

1MRK 506 382-UEN A
6.4.7.2
Line distance protection REL650 2.2 IEC
Technical manual
the generator nominal voltage and nominal current. 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 Figure
that 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 in general can 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 StartAngle .The lens is broader for
smaller values of the StartAngle , and becomes a circle for StartAngle = 90 degrees.
When the complex impedance Z(R, X) enters the lens, pole slipping is imminent,
and a start 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,
that leads to a wild flow of the synchronizing power; so there are also periodic
changes of 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 location of the instrument transformers of a power line connecting two power
sub-systems. This was shown in
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 61, 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
Impedance protection
65 looks into the system and the impedances in
Figure 61. When a synchronous machine is out-of-
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