21LB Load Blinder
Overview of Function
Load blinders are used with directional overcurrent protection elements to block tripping during sustained
heavy reverse load current flow in distribution networks. The reverse current flow can be caused by the gener-
ation balance moving from the primary generating centers to distributed MV generation sources.
Distributed generation can cause power flow in reverse direction to normal system operation. Load impedance
blinders are used to block overcurrent operation for reverse load current flow.
The load blinder is an impedance characteristic element with a minimum impedance level and operate angle
limits and is conveniently illustrated using a R – X plane diagram.
Structure of the Function
The load blinder function has group dependent settings. These settings can be different in each settings group
– group numbers (Gn) 1 to 4.
The function monitors the primary system impedances using the 3 phase CT inputs e.g. CT1/2/3 and 3 phase
VT inputs e.g. VT1/2/3. The function is parameterized to block operation of the overcurrent protection during
reverse load current flow, but not to block the protection for fault currents in the operate direction.
The function has both single phase and 3 phase modes of operation.
Operation of 3 phase mode can be selected to inhibit operation of the overcurrent function elements (50 and
51).
The single-phase mode is phase segregated and blocks the individual phases of the time delayed overcurrent
function elements (51).
Logic of the Function
A 3-phase overcurrent blocking signal can only be issued when:
•
Positive phase sequence voltage PPS (V
1
) is above setting, and
•
Negative phase sequence current NPS (I
2
) is below setting, and
•
Measured impedance (Z) is above setting,
•
Measured impedance (Z) is within angular limits.
[lo_7SR5_21logicdiagram3phase, 1, en_US]
Figure 5-1
Logic Diagram: 21LB Load Blinder 3-Phase
A single phase overcurrent blocking signal can only be issued when:
5.1
5.1.1
5.1.2
5.1.3
Protection and Automation Functions
5.1 21LB Load Blinder
144
Reyrolle 7SR5, Transformer Protection Device, Device Manual
C53000-G7040-C015-1, Edition 11.2019
