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

Section 7
Impedance protection
7.3.7.2
156
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
this case shown in Figure 72
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, trouble 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 real trouble with dynamic stability usually
begins. Rotor (power) angle 120 degrees is sometimes called "the angle of no return"
because if this angle is reached under generator 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 68. 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 not 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, for example 25 milliseconds,
this is the generator nominal voltage level. The
Figure 72 looks into the system and the impedances in that
1MRK 502 043-UUS B
Technical Manual