Detecting An Out-Of-Step Condition - ABB REG650 Technical Manual

Section 7
Impedance protection
7.4.7.2
168
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 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,
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
Figure 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 milliseconds. (To require that the impedance Z(R, X)
travels through each of the two halves of the lens, for example, in 25 milliseconds,
results in a tendency to miss the 1st pole-slip, that one immediately after the fault has
been cleared.) The above timing is used to discriminate a fault from an out-of-step
condition. In Figure 76, 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, low-resistance fault. 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
Figure 80 looks into the system and the impedances in that
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Technical manual