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The double blinder OOS (out of step) relay is based on the conventional rate of change of impedance method in order to differentiate a stable and an unstable power swing (out-of-step) using positive sequence impedance.

As shown in the figure below, the relay characteristic consists of two parallel blinders on the impedance plane that are used to detect the rate of change of impedance during any disturbance. The idea is based on the fact that the rate of change of impedance is faster during fault compared to that during power swing.

A timer is triggered when the impedance trajectory enters the outer blinder to measures the time spent between the inner and outer blinders. If this time exceeds a predetermined threshold, known as PSB time delay, it is determined as a power swing.

Once a power swing is detected, if this trajectory enters the inner blinder, it is identified as an unstable swing, whereas, if it leaves the outer blinder without entering the inner blinder, it is identified as a stable swing, as shown in the figure below.

Model Description


The mask of the ePHASORSIM Double Blinder OOS Relay model is as shown above. In order to calculate the positive sequence impedance trajectory, the relay requires the voltage and current magnitude as well as angle in p.u.

When the Reset pin is 1, it resets the output signals to normal condition.


The out-of-step trip signal OST is inverse of the OOS signal to make it compatible with the I/O 'status' pin of Positive Sequence Components. When unstable swing is detected, the output changes as follows:

Relay Logic

Normal condition01
OOS detected10


The settings for the OOS relay can be summarized as follows:

Blinder Angle (deg)

The blinder angle of inclination in degrees (refer to relay characteristic figure above).

Inner Blinder (p.u.)

The perpendicular distance of inner blinder from ZTotal (refer to relay characteristic figure above).
Outer Blinder (p.u.)The perpendicular distance of outer blinder from ZTotal (refer to relay characteristic figure above).
PSB Delay (ms)The threshold time delay to distinguish between fault and power swing.
OOS Tripping logicThe OST can be set to '0' for Trip on-the-way-in (TOWI), i.e. when the trajectory enters the inner blinder, or to '1' for Trip on-the-way-out (TOWO), i.e. when the trajectory leaves the inner blinder.
Number of RelaysSpecifies the number of relays to be modeled. This relay model can take vector inputs, and thus, can be used as more than one relay.


See inside the subsystem Power System Protection Relays in demo PHASOR-02.

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