Siemens Argus 7SR21 Technical Manual page 637

7SR210 & 7SR220 Applications Guide
5.9 Load Blinder (21)
NB: It is important to observe the convention regarding Forward and Reverse directions of current.
Load Blinders are used with directional overcurrent elements to block forward tripping during periods of high load
currents that can occur in distribution networks. The blinder is operated during user defined load conditions and is
used in conjunction with the relay protection elements.
One of the major factors contributing to high load conditions is an increase of distributed generation capacity
which can feed power back into the network contrary to the 'normal' direction of power flow. This can result in
current levels above the relay setting and can cause tripping of overcurrent elements.
One typical example is on paralleled incoming transformer circuits, forward directional overcurrent elements are
parametised such that they detect faults on the transformer LV connections for currents flowing in the direction
contrary to the 'normal' load flow. These forward elements have a relatively low current setting, typically <45% of
transformer rating, as current normally only flows in this direction during fault conditions. When embedded
generation is connected into the downstream network, these relays limit the level of current that can be fed back
into the grid.
Load Blinders measure the system impedance and are set to differentiate between a fault having a relatively low
impedance and a fault having a relatively high impedance. Additional measurands are also considered to increase
the security of operation.
Figure 5.9-1 Paralleled Transformers and Feeders
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