ABB RELION REX640 Technical Manual page 1012

Protection and control

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Protection functions

Out of step protection functions detect stable power swings and out of step

conditions based on the fact that the voltage/current variation during a power

swing is slow compared to the step change during a fault. Both faults and

power swings may cause the measured impedance to enter into the operating

characteristic of a distance relay element. The apparent impedance moves from the

pre-fault value to a fault value in a very short time, a few milliseconds, during a fault

condition. However, the rate of change of the impedance is much slower during a

power swing or out of step condition than during a fault depending on the slip

frequency of the out of step. The impedance measurement should not be used by

itself to distinguish between a fault condition and an out of step condition from a

phase fault. The fundamental method for discriminating between faults and power

swings is to track the rate of change of the measured impedance.

The function measures the rate of change of the impedance using two impedance

measurement elements known as blinders together with a timing device. If the

measured impedance stays between the blinders for a predetermined time, the

function declares a power swing condition and asserts an output that can be

used to block the distance protection. However, if the impedance passes the inner

blinder and exits on the other side of the mho characteristics (that is, the resistive

component of impedance has opposite sign as at the time of point of entry) an out

of step operate is issued by the function.

Figure 562: Example of out of step detection

The shaded region indicates a fault zone in a distance protection function. For curve

A, the impedance moves into the out of step zone and leaves slowly, indicating the

occurrence of a swing that quickly stabilizes. For curve B, the impedance moves

slowly into the out of step zone and exits the zone indicating that the network is

becoming unstable. For curve C, impedance rapidly moves into, and remains in, the

fault zone indicating an actual fault and not an out of step condition.

1MRS759142 F

gives an example of out of step

Technical Manual