5-244 L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUALGROUPED ELEMENTS CHAPTER 5: SETTINGS5Figure 5-130: Ground distance zones 2 and higher pickup logicGround directional supervisionA dual (zero-sequence and negative-sequence) memory-polarized directional supervision applied to the ground distanceprotection elements has been shown to give good directional integrity. However, a reverse double-line-to-ground fault canlead to bad operation of the ground element in a sound phase if the zone reach setting is increased to cover highresistance faults.Ground distance zones 2 and higher use an additional ground directional supervision to enhance directional integrity. Theelement’s directional characteristic angle is used as a maximum torque angle together with a 90° limit angle.The supervision is biased toward operation in order to avoid compromising the sensitivity of ground distance elements atlow signal levels. Otherwise, the reverse fault condition that generates concern has high polarizing levels so that a correctreverse fault decision can be reliably made.
