CHAPTER 10: THEORY OF OPERATION DISTANCE ELEMENTSL90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 10-2110The 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 anoptional supplement of the ground distance protection for pilot-aided schemes. The Ground Directional Overcurrentsection describes the elements.The relay provides for an adaptive distance reach control to cope with the overreaching and sub-synchronous oscillationswhen applied to, or in a near vicinity of series compensated lines. The Application on Series Compensated Lines sectionhas information.The distance elements use phase angle comparators to shape their characteristics as described in the DistanceCharacteristics section. The voltage and current phasors are estimated using optimized techniques as explained in thenext section.10.3.2 Phasor estimationThe relay samples its input AC signals at 64 samples per power system cycle. A fast and accurate frequency trackingmechanism ensures accurate filtering and phasor estimation during off-nominal frequency conditions.The phasor estimation process for both currents and voltages is based on the commonly used Fourier algorithm. Due to adifferent nature of signal distortions in the current and voltage signals digital pre-filtering algorithms have been, however,designed and optimized separately for the current and voltage channels.The current signals are pre-filtered using an improved digital MIMIC filter. The filter removes effectively the DCcomponent(s) guaranteeing transient overshoot below 2% regardless of the initial magnitude and time constant of the DCcomponent(s). The filter has significantly better frequency response for higher frequencies as compared with a classicalMIMIC filter. This was possible without introducing any significant phase delay thanks to the high sampling rate used bythe relay.The voltage signals are pre-filtered using a special digital filter designed to cope with CVT transients. The patented filtercombines filtering and memory actions enabling the relay to cope with CVT noise under high Source Impedance Ratios(SIRs). The filter controls underestimation of the fault voltage magnitude to less than 1% of the nominal and preventscertain phase angle anomalies that can be encountered under heavy CVT noise and high SIRs.10.3.3 Distance characteristics10.3.3.1 DefinitionsThe relay shapes its distance characteristics using phase angle comparators and voltage and current phasors estimatedas described in the previous section.The following definitions pertain to all of the distance functions:• IA , I B , IC — Phase A, B, and C current phasors• IG — Ground current from a parallel line• VA , VB , VC — Phase A to ground, phase B to ground, and phase C to ground voltage phasors• ( )_1 — Positive-sequence phasor of ( ) derived from the phase quantities• ( )_2 — Negative-sequence phasor of ( ) derived from the phase quantities• ( )_0 — Zero-sequence phasor of ( ) derived from the phase quantities• ( )M — Memorized value of ( )• Z — Reach impedance (REACH ∠ RCA)• ZREV — Reverse reach impedance for non-directional applications (REV REACH ∠ REV REACH RCA + 180°)• ZD — Directional characteristic impedance (1 ∠ DIR RCA)• ZR — Right blinder characteristic impedance: ZR = RGT BLD × sin (RGT BLD RCA) × 1∠ (RGT BLD RCA – 90°)
