Detecting An Out-Of-Step Condition - ABB RES670 Technical Manual

1MRK 511 408-UUS A
7.2.7.2
Phasor measurement unit RES670 2.2 ANSI
Technical manual
system, or as a single machine – infinite bus equivalent system, the following
information is necessary: 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 Figure
and, therefore, the previous quantities shall be referred to 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
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 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, 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 Figure 42. 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
46 the relay is connected to the terminals of the generator
46 looks into the system and the impedances in that
Section 7
Impedance protection
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