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This post was last edited by chenjinfeng on 2020-3-27 at 13:50. 1. Basic principle of bus differential protection: In simple terms, the basic principle of bus differential protection is to make judgments and take actions based on the principle of balance between inputs and outputs. Since there are only incoming and outgoing lines on the busbar, under normal operating conditions, the magnitudes of the incoming and outgoing currents are equal and their phases are the same. If the bus fails, this balance is disrupted. Some protections compare whether the currents are balanced, while others check whether the current phases are identical; some combine both approaches. Once a bus fault is detected, the protection mechanism is activated immediately, causing all circuit breakers on that bus to trip. In the case of dual-bus parallel operation, some protective devices will selectively trip the bus tie switch as well as all the incoming and outgoing circuit breakers on the faulty bus, in order to reduce the scope of the power outage. 2. What is differential protection? Why is it called differential? What are the advantages of this? Differential protection is the primary protection for transformers, and it is installed based on the principle of circulating current. It is primarily used to protect against various inter-phase short-circuit faults that occur within the windings of two-winding or three-winding transformers as well as on their leads; it can also be used to protect against single-phase winding short-circuit faults in transformers. Current transformers are installed on both sides of the wound transformer, and their secondary sides are connected according to the circulating current method; that is, if the similar terminals of the current transformers on both sides point toward the busbar side, then those similar terminals are connected together, with a current relay connected in parallel between these two connections. The current flowing through the relay coil is the difference between the secondary currents of the current transformers on both sides; in other words, the differential relay is connected to the differential circuit. Theoretically, the current in the differential circuit is zero under normal operation and in the event of external faults. In fact, due to reasons such as the impossibility of the current transformers on both sides having exactly identical characteristics, an unbalanced current Iumb still flows in the differential circuit during normal operation and external short circuits. At this time, the current flowing through the relay is Ik=I1-I2=Iumb. It is necessary to keep this unbalanced current as small as possible to ensure that the relay does not operate erroneously. When a phase-to-phase short circuit occurs inside the transformer, in the differential circuit the direction of I2 changes or it becomes zero (on the side without power supply). As a result, the current flowing through the relay is the sum of I1 and I2, that is, Ik = I1 + I2 = Iumb, which enables the relay to operate reliably. The scope of transformer differential protection includes the electrical equipment located between the current transformers that make up the transformer differential protection, as well as the wires that connect these devices. Since differential protection does not operate in response to faults outside the protected area, it does not need to coordinate with the protection of adjacent components outside that area in terms of operating values and timing; as a result, it can act instantly in the event of a fault within the protected area. 3. Why is the voltage used for protecting 220KV high-voltage lines taken from the busbar TV rather than from the line TV? In fact, both voltages are fed into the protection device, but they serve different purposes. The busbar voltage is generally used to determine whether a fault occurs in the forward direction or the reverse direction, while the line voltage is used, based on the angle between current and voltage, for purposes such as resetting circuits and determining whether there is voltage in the line. Currently, for 220KV line protection, a set of fiber-optic current differential protection and a set of high-frequency distance protection are commonly used; the use of two sets of fiber-optic current protection and two sets of high-frequency protection is less common. 4. The basic principle of transformer differential protection 1. The working principle of transformer differential protection is the same as that of line longitudinal differential protection – it involves comparing the phase and magnitude of currents on various sides of the equipment being protected. 2. Difference between transformer differential protection and line differential protection: The rated currents on the high-voltage side and low-voltage side of a transformer are not equal, and moreover, the phases of the currents on different sides of the transformer are often different. Therefore, to ensure the proper operation of the longitudinal differential protection, it is necessary to appropriately select the turns ratios of the current transformers on each side, as well as to compensate for the phase differences in the currents, so that the secondary currents on both sides are equal during normal operation and external short-circuit faults.