106 9424200994 Rev NInternal Constant Purpose ValueGround current (IG) Minimum Ground (IG) current threshold for CurrentPolarization test0.50 A for5A CTsand 0.1 Afor 1A CTsNegative-SequenceVoltageMinimum V2 voltage threshold for Negative-Sequence test 1.2% ofV nominalZero-Sequence Voltage Minimum V0 voltage threshold for Zero-Sequence test 0.75% ofV nominalExternal Zero-SequenceVoltage (VX Input)Minimum external 3V0 voltage threshold for Zero-Sequencetest10% ofV nominalNegative-Sequence Ratio Minimum ratio between I1 and I2 for Negative-Sequencetest9%Zero-Sequence Ratio Minimum ratio between I1 and 3I0 for Zero-Sequence testis 9%9%If the minimum levels are not met for a particular directional test, then the test is not run and thedirectional bits are cleared for that test. For instance, if 3I1 is less than 0.50 A, the positive-sequence testis skipped and the positive-sequence directional bits are cleared.The Sequence Ratio refers to the minimum ratio required between the positive-sequence current andeither the negative or the zero-sequence current. A negative-sequence directional test would be allowed ifthe negative current were greater than 9% of the positive-sequence current. The same concept appliesfor the zero-sequence directional test.The directional tests are also supervised by the loss of potential function 60FL. If the 60FL bit is true, thenvoltage sensing was lost or is unreliable. Under this condition positive, negative, and zero-sequencedirectional tests are disabled and their bits are cleared. Current polarization is not affected by the 60FLsince it does not rely on voltage sensing.The direction bits are updated once per half-cycle. Under sudden current reversal conditions, dependingon the change in magnitude of the forward current to reverse current, the DFT (Discrete FourierTransform) could require one cycle to determine polarity of the fault. Beyond this, the 50-x element addsan additional half-cycle delay when operating in direction mode for security, for an overall response timeof a 50-x element to sudden current reversal of approximately two cycles.Theory of Using Sequence Impedances for Fault DirectionWhen using real world impedances in the Z ABC domain, it is apparent that faulted phase voltageapproaches zero as one gets closer to the fault and that the same phase's voltage becomes larger thecloser one gets to the source. However, in the sequence domain (zero-, positive-, negative-sequence),the above concept holds for positive-sequence voltage and current flow, but for negative- and zero-sequence current flow, the opposite condition occurs. Negative- and zero-sequence voltage is highest atthe fault location, and lowest at the source. This affects how the BE1-11g uses the angle of maximumtorque to prevent tripping for unusual load flow.For directional decisions, a BE1-11g is measuring the sequence impedance (Z 012 =V012 / I012 ) andcomparing the angle that it calculates to the angle of maximum torque with a window of ±90 degrees asforward (or reverse, depending on the BE1-11g setup). Suppose a radial single source condition existsrelative to the BE1-11g location. The source impedance is Z Source and the fault is downstream on a line ofimpedance Z Line . Given a source voltage of VSource and a fault current of I Relay the local substation voltagewill be shown in Equation 9. Note this equation is true independent of the fault type or the faulted phase.Directional Overcurrent (67) Protection BE1-11g
