Principle of comprehensive protection differential protection

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This post was last edited by chenjinfeng on 2020-3-27 at 14:04. Principle of differential protection: 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 inflows and outflows. 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 methods. 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 operation, some protective devices will selectively trip the busbar switch as well as all the incoming and outgoing circuit breakers on the faulty busbar, 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 towards 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 IK flowing through the relay is given by 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 a 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 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. Therefore, 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 to determine the presence or absence of voltage in the line, and it is utilized during reclosing operations as a criterion for that purpose. Currently, the most common protection schemes for 220KV lines involve one set of fiber-optic current differential protection and one set of high-frequency distance protection; there are also cases where two sets of fiber-optic current protection and two sets of high-frequency protection are used, though this 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 – both involve 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 in the event of external short-circuit faults. For example, as shown in Figure 8-5 for the two-winding transformer at file:///C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\ksohtml\wps1A1.tmp.png, 1. Full-line quick-acting protection: On high-voltage transmission lines, it is required that the relay protection can disconnect the circuit without delay in the event of a fault occurring at any point along the line. 2. Single-side measurement protection cannot achieve rapid response across the entire line. Single-side measurement protection refers to a type of protection that measures only electrical quantities such as bus voltage and line current on one side of the line. Single-side measurement protection shares a common drawback: it is unable to quickly isolate all faults on the circuit, with the maximum isolation time being around 0.5 seconds. 3. How is the principle of bilateral measurement used to achieve full-line rapid protection? To achieve full-line rapid protection, the protection criteria are based on the electrical parameters on both sides of the line or on the behavior of the protection devices, with bilateral measurement being carried out. When measuring on both sides, corresponding protection channels are required for information exchange. The basic principles of two-sided measurement line protection mainly include the following three: (1) current differential measurement based on Kirchhoff’s current law ； (2) Phase differential measurement to compare the current phase relationship on both sides of the line ； (3) Compare the fault direction determination results of the protection systems on both sides to determine the location of the fault point. The protection measurement current is the vector sum of the currents on both sides of the line, also known as the differential current. Consider the line as a generalized node; the total current flowing into this node is zero. Under normal operating conditions or in the event of external faults, as well as internal faults within the line, ignoring the capacitive current of the line, the differential current is zero when a fault occurs at the beginning of the line ； When a fault occurs at the end of the line, the differential current is equal to the short-circuit current at the fault point, which allows for a clear distinction and enables rapid protection across the entire line. The current differential principle is used in line longitudinal differential protection, line optical fiber phase-difference differential protection, as well as in the protection of components such as transformers, generators, and busbars. What is differential protection? Differential protection operates on the basis of Kirchhoff’s current law. The types of differential protection commonly mentioned include bus differential protection, transformer differential protection, generator differential protection, and line differential protection. The basic principle behind differential protection is the same: the current transformers on each side or in each component are connected in a differential configuration. Under normal operating conditions as well as in the event of faults outside the protection range, the differential current is zero; whereas in the case of faults within the protection range, the differential current equals the fault current. The operating current setting for the differential relay is determined based on the maximum level of imbalance current that may occur due to external faults. Differential protection can be further divided into transverse differential protection and longitudinal differential protection ； Transverse difference: In parallel two-circuit lines, since the impedances are equal, the currents and phases are also equal. When one of the circuits fails, the magnitude of the fault current flowing through the two circuits becomes unequal; protection is implemented by taking advantage of this characteristic of two-circuit lines. Longitudinal difference: It involves comparing the electrical quantities on both sides of the circuit. What is full-bus differential protection? What is incomplete-bus differential protection? 1. Full-bus differential protection involves connecting the current transformers of all the components connected to the bus in the differential circuit in the same phase and with the same polarity. The characteristics and turns ratios of these current transformers must be identical; if the turns ratios cannot be made equal, compensating converters can be used to achieve ΣI=0. The operating current of the differential relay is calculated and set according to the following conditions, with the maximum value being taken: 1) to avoid the unbalanced current generated by external short circuits; 2) to avoid the maximum load current in the branch with the highest load among the busbar connection components, in order to prevent erroneous operation in case of a break in the current secondary circuit. 2. For the incomplete bus differential protection, it is sufficient to connect the current transformers attached to the components connected to the bus to the differential circuit; the current transformers attached to the components without power supply need not be connected to the differential circuit. Therefore, if a fault occurs in a power-free component, it will operate. The power-free components that are not connected to the differential circuit in a current transformer are reactors or transformers.