Abstract : This paper introduces a novel full-line fast-acting protection system for responding to short-circuit faults on short and ultra-short lines. It utilizes optical fiber as a channel and operates using frequency shift key (FSK) control. The protection scheme is based on a direction-discriminatory out-of-range permissible transmission trip (POTI) method. To accommodate situations where one side has no power supply or weak power supply, this microprocessor-based protection device also includes forwarding frequency hopping and weak-feed trip circuits. Keywords: Longitudinal protection; Optical fiber channel; Relay 0 Introduction With the continuous growth of power load, the power grid structure has undergone significant changes. More and more short and ultra-short lines have emerged, especially in the power supply networks of industrial and mining enterprises, where power distribution lines hundreds of meters long are commonplace. The load characteristics supplied by these lines are often very important, and the stepped multi-stage protection method can no longer meet the requirements of safe, reliable, and continuous power supply. To ensure that the residual voltage of the main substation busbars does not remain excessively low during accident situations, and that the load motors can successfully and reliably self-start, it is essential to achieve full-line fast-acting protection. Line longitudinal protection is an ideal protection method to achieve full-line fast operation, but it requires a communication channel to synthesize the actions of protection devices on both sides of the line, thereby determining whether the fault point is located inside or outside the line zone. Currently widely used longitudinal protection schemes include: direction comparison for blocking; phase comparison; transmission of trip signals, including under-range direct (DUTF), under-range permissive (PUTT), over-range permissive (POTT), and unlocking; and current differential, including equalizing differential, frequency modulation, or pulse code modulation current differential. In terms of channel configuration, various methods are used, including the use of lead wires (including dedicated overhead lines or cables, or leased telephone cables), power line carrier, microwave, and optical fiber. The same approach applies. The combination of all these protection schemes and channel schemes must be rationally selected based on the specific conditions of the protected line. For medium and low voltage lines, circuit breakers operate on three phases, the power supply network is a low-current grounding system, and some substation current transformers are still configured as two phases. Therefore, to achieve full-line fast-acting longitudinal protection, it is advisable to prioritize over-range permissible trip (POTF) protection using fiber optic channels. This allows the use of simple current direction elements covering the entire line as initiators and measurers; the fiber optic channel, unaffected by electromagnetic interference, provides uninterrupted continuous monitoring; and, provided sufficient protection sensitivity is achieved, longitudinal operation of the line will not occur. Clearly, satisfactory results will be obtained in actual system operation. 1. Fiber Optic Longitudinal Current Direction Protection Device The new MDM-B1P(A) type fiber optic longitudinal current direction protection device is a complete short-line protection device developed and designed according to the above principles. It includes full-line high-speed longitudinal protection, backup protection for directional current and voltage, zero-sequence current protection to prevent multi-point grounding, three-phase single-phase automatic reclosing with no-voltage, synchronizing, and adjacent-line current verification, a quasi-synchronizing closing device, and related operating circuits for the circuit breaker. Therefore, it is suitable for power grids with voltage levels of 35kV and below, serving as the main and backup protection for short-line or ultra-short-line protection. [b][align=center]For details, please click: Practice of Fast Fiber Optic Longitudinal Current Direction Protection for Ultra-Short Lines[/align][/b]