D30 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL 9-1D30 Line Distance Protection SystemChapter 9: Theory of operationTheory of operationThis chapter outlines advanced information on operation.9.1 Distance elements9.1.1 OverviewThe distance elements use memory voltage for polarization. Additional supervising functions — different for ground andphase distance zones — complement a classical mho characteristic to enhance directional integrity and reach accuracy.Note the following:• To avoid overreaching during resistive faults under heavy pre-fault load conditions, the ground distance elementsutilize a load-adaptive (zero-sequence polarized) reactance characteristic to supervise the base mho characteristic• Both negative and zero-sequence currents are compared with the memory voltage to enhance directional integrity ofthe ground distance elements• It is well-known that ground distance elements, as per the principle of distance relaying, can have limited accuracyduring double-line-to-ground faults. In order to prevent maloperation in such cases, the ground elements are blockedby an extra “fault-type comparator” that utilizes the phase angle between the negative- and zero-sequence currents.• The phase distance elements use reactance and memory polarized directional characteristics to supervise the mhocharacteristic• Both ground and phase distance elements have the current supervision functions built-inThe quadrilateral distance characteristic uses the reactance, directional, and current supervising functions as described.Right and left blinders adjustable as to both the resistive and angular positions complete the characteristic.See the Distance Characteristics section for more information regarding the distance characteristics. The DistanceElements Analysis section includes an example of analysis of the steady-state operation of the distance elements.The relay provides five zones of distance protection. All zones are identical in terms of settings. However, zone 1 has extraadaptive mechanisms built-in to enhance the transient reach accuracy even when the voltage signals are supplied frompoor quality voltage sources such as capacitive voltage transformers (CVTs). Ground zones 2 through 5, in turn, have anextra zero-sequence directional supervision implemented for their time-delayed operation after the memory expires.Consequently, zone 1 is recommended as an underreaching element, and zones 2 through 5 are recommended asoverreaching elements and for time-delayed tripping.The relay uses offset ground directional overcurrent functions as an optional supplement of the ground distanceprotection for pilot-aided schemes. The Ground Directional Overcurrent section describes the elements.
