Load Blinders; Load Blinder Implementation; Figure 64: Load Blinder And Angle - GE MiCOM P40 Agile Technical Manual

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LOAD BLINDERS

Load blinding is a mechanism, where protection elements are prevented from tripping under heavy load, but
healthy conditions. In the past this mechanism was mainly used for transmission systems and was rarely needed
at distribution voltage levels. In the last few years, however, distribution networks have become more subject to
periods of sustained heavy loads. This is due to a number of reasons, one of which is the increase of distributed
generation. For this reason, it has become very desirable to equip overcurrent protection, normally targeted at
distribution networks, with load blinding functionality.
Load blinders work by measuring, not only the system current levels, but also the system voltage levels and
making tripping decisions based on analysis of both of these measurements. This is known as Impedance
measurement.
When the measured current is higher than normal, this can be caused by one of two things; either a fault or a
heavy load. If the cause is a fault, the system voltage level will reduce significantly. However, if the cause is a
heavy, but healthy load, the voltage will not decrease significantly. Therefore, by measuring the both the system
voltage and currents, the protection can make a decision not to trip under heavy load conditions.
The principle of a load blinder is to configure a blinder envelope, which surrounds the expected worst case load
limits, and to block tripping for any impedance measured within this blinder region. Only fault impedance outside
the load area is allowed to cause a trip. It is possible to set the impedance and angle setting independently for the
forward and reverse regions in the Z plane.
V00645

Figure 64: Load blinder and angle

16.1

LOAD BLINDER IMPLEMENTATION

The Load blinder function is implemented in the OVERCURRENT column of the relevant settings group, under the
sub-heading LOAD BLINDER.
The settings allow you to set the impedance and angle limits for both reverse and forward directions, the
undervoltage and negative sequence current thresholds for blocking the function, and the operation mode.
There are two modes of operation; single phase and three phase;
The single phase mode uses the normal impedance (Z) of each phase. When single phase mode is selected, the
overcurrent blocking is phase segregated and is dependant on the individual overcurrent settings per phase. In
single phase mode, only the undervoltage threshold (Blinder V< Block) can block the function.
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jX
Operate area
Blind area
Operate area
Chapter 6 - Current Protection Functions
Z
Blind area
β R
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