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ABSTRACT
This paper describes a new phase selector that operates reliably during many unfavorable conditions, such as high fault resistance, high load flow, power swing, Current Transformer (CT) saturation, converter-based generation, weak infeed. The phase selector is based on the angular relationship between the sequence current components and sequence voltage components. The use of voltages and currents is dynamically selected. Some complimentary methods are added for phase selection during power swings. Matlab Simulink® simulations are performed to check the operation of the mentioned phase selector and to demonstrate its reliability. Oscillographic records from real faults are also analyzed.
Introduction
Phase-selectors are core units in the line protection, as the identification of the type of fault is required for single pole tripping given by non-phase segregated units (such as neutral, negative sequence or positive-sequence differential or directional overcurrent), for the release of the corresponding fault loop in distance protection or in one-ended fault location, for signaling, etc.
Phase selectors must be reliable and fast but there are many factors that can make them fail in the fault type identification, such as fault resistance, load flow, system non-homogeneity, presence of power swings, Current Transformer (CT) saturation, weak infeed, dominant zero-sequence current contribution, converter-based generation, etc.
This paper describes the most common phase selection methods used. It focuses on the current sequence component phase selector, used in ZIV relays, explaining all the conditions that have been considered. It then explains the problems encountered by this phase selector and the improvements that were added, like complementing it with use of voltage sequence components. Results from Matlab Simulink® simulations and from oscilos coming from real faults are shown.
TYPES OF PHASE SELECTORS
There are several types of phase selectors, however, the most used ones are based on: impedance measurement, superimposed currents and sequence currents. This paper will describe the three mentioned methods and will focus on the sequence components one.
Impedance phase selectors
Phase selectors based on the impedance measurement have been implemented for many years by using impedance starters. These impedance starters are non-directional zones that surround all the tripping zones. The six units of the starter zone (AG, BG, CG, AB, BC, CA) are continuously measured. The faulted phase is selected based on the units that activate. For certain faults, the units related to the healthy phases can also operate. This makes impedance phase selection quite challenging. Some examples are included below [1].
- Phase distance elements can operate for a close-in reverse phase-to-phase-to-ground fault (AB unit can operate with BCG fault)
- Phase distance elements may operate for a phase-to-phase-to-ground fault with heavy loading (CA unit can operate with BCG fault)
- Phase distance elements can operate for a close-in phase-to-ground fault (CA unit can operate with AG fault)
- Ground distance elements can operate for a close-in reverse phase-to-ground fault (CG unit can operate with AG fault)
- Phase distance elements can operate for a close-in reverse phase-to-phase fault (BC unit can operate with CA fault)
During a phase-phase-ground fault the phase-phase unit will operate but it will also operate the leading phase ground unit [2].
The impedance phase selectors must use appropriate resistive and reactive reach settings, directionality, etc to avoid wrong operation.
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Authors:
Roberto Cimadevilla & Alberto Castañon
Presented at PACworld Conference 2022
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