Areva MiCOM P441, Technical Manual

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P44x/EN HW/E33 Relay Description
Page 36/44 MiCOM P441/P442 & P444
4.7 Power swing detection
Power swings are caused by a lack of stability in the network with sudden load fluctuations.
A power swing may cause the two sources connected by the protected line to go out of step
(loose synchronism) with each other.
The power swing detection element may be used to selectively prevent when the measured
impedance point moves into the start-up characteristic from a power swing and still allows
tripping for a fault (fault evolving during a power swing). The power swing detection element
may also be used to selectively trip once an out-of-step condition has been declared.
figure 14 illustrates the characteristics of power swing.
Z3
P3038ENa
Unstable
Swing
Stable
Swing
Z4
X
R
Powerswing
Boundary
Characteristic
FIGURE 14 - POWER SWING
4.7.1 Power swing detection
The power swing detection element is used to detect a stable power swing or loss of
synchronism condition (out-of-step) as it passes through near the loop convergence (start-
up) characteristic (Z3 and Z4 if enabled). Power swing detection is based on the status of
the line to be protected:
Power swings are characterised by the simultaneous appearance of three impedance points
in the start-up zone, passing through the power swing boundary
∆
R/
∆
X .Their speed of entry
(passing through the resistance limits that define the power swing detector) is slower than
that in the case of three-phase faults, which is instantaneous.
The P44x does not differentiate a stable power swing from loss of synchronism condition.
A power swing is detected and declared if:
•
At least one single-phase impedance is within the start-up zone after having crossed
the power swing band in more than 5 ms.
•
The three impedance points have been in the power swing band for more than 5 ms.
•
At least two poles of the breaker are closed (impedance measurement possible on two
phases).