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This post was last edited by chenjinfeng on 2020-2-24 14:34 Optical fiber current differential protection evolved on the basis of current differential protection. The basic protection principle is also based on Kirchhoff's basic current law. It can ideally realize unitization of protection. The principle is simple and is not affected by changes in operating modes. Moreover, since the protection devices on both sides have no electrical connection, the reliability of operation is improved. Currently, current differential protection is widely used in main transformers, lines and busbars of power systems. Its advantages such as high sensitivity, simple, reliable and fast action, ability to adapt to power system oscillations and non-full-phase operation are unmatched by other forms of protection. While inheriting these advantages of current differential protection, optical fiber current differential protection uses its reliable and stable optical fiber transmission channel to ensure that the amplitude and phase of the transmitted current are correctly and reliably transmitted to the opposite side. 1 Principle Introduction Optical fiber phase split current differential protection uses the line fiber channel to transmit sampling data to the opposite side in real time and receive sampling data from the opposite side at the same time. Each side protection uses local and opposite side current data to calculate differential current by phase. According to the braking characteristic equation of the current differential protection, the protection will trip when it is judged to be a fault within the zone, and the protection will not operate when it is judged to be a fault outside the zone. The typical structure of an optical fiber current differential protection system is shown in Figure 1. When the line is operating normally or an out-of-area fault occurs, the current phases on both sides of the line are reversed. As shown in the figure, assuming that the M side is the power transmitting end and the N side is the power receiving end, then the current on the M side flows from the bus to the line, and the current on the N side flows from the line to the bus. The currents on both sides are equal in magnitude and opposite in direction. At this time, the difference current on both sides of the line is zero. ； When a line fault occurs in the area, the fault current flows from the bus to the line in the same direction. The difference current between the currents on both sides of the line is no longer zero. When it meets the operating characteristic equation of the current differential protection, the protection device issues a trip order to quickly remove the fault phase. For optical fiber phase current differential protection, the differential protection generally adopts the double slope braking characteristic as shown in Figure 2 to ensure stability when a ride-through fault occurs. In the figure, Id represents the differential current, Ir represents the braking current, and K1 and K2 represent different braking slopes respectively. Using such a braking characteristic curve can ensure higher sensitivity when the current is small, and higher reliability when the current is large. That is, when an out-of-area fault occurs at the end of the line, a transmission error will occur due to saturation of the current transformer. At this time, the braking characteristic with a higher slope is more reliable. Due to factors such as the measurement error of the current transformers on both sides of the line and the charging capacitor current generated during the operation of the ultra-high voltage line, when the differential protection uses local and opposite side current data to calculate the real-time differential current by phase, its value is not zero, that is, there is a certain unbalanced current. Optical differential protection must be set to avoid this current value, which is why the minimum difference current setting value Isl is not zero in Figure 2 shown above. How to avoid the impact of the unbalanced current on differential protection? Different types of protection devices use different setting methods. Generally, the fixed threshold method is used for setting, that is, the differential current measured by the protection device during normal operation is regarded as the pure capacitive current of the protected line, and the current value is multiplied by a coefficient (usually 2-3) as the action threshold of the differential current. When the differential element determines that there is a fault within the zone and issues a tripping command, in addition to tripping the circuit breaker on this side of the line, it also sends a joint trip signal to the opposite side of the line with the help of the optical fiber channel, causing the circuit breaker on the opposite side to trip quickly.