A text to understand the meaning of short-circuit current of the power supply
Short-circuit current is the current that flows through the power system during operation, when an abnormal connection (ie, a short circuit) occurs between phases or between phases and ground (or neutral). The value can be much greater than the rated current and depends on the electrical distance of the short circuit from the power supply.
The purpose and significance of short-circuit current calculationIn addition to effective technical measures to prevent short circuits in the power supply and distribution system. A sensitive and reliable relay protection device and a circuit breaker with sufficient current interruption capability should be provided to quickly cut off the short circuit and minimize the short circuit hazard. For this purpose, the short-circuit current calculation must be carried out in order to correctly select and calibrate the protection device. The components that limit the short-circuit current and the switching device calculate the short-circuit current in order to limit the hazard of the short circuit and narrow the scope of the fault. In the design and operation of substations and power supply systems, short-circuit current calculations must be performed based on:
1) Select electrical equipment and current-carrying conductors, and the thermal stability and dynamic stability must be verified by short-circuit current.
2) Select and adjust the relay protection device so that it can correctly cut the short circuit
3) Determine the reasonable main wiring scheme, operation mode and current limiting measures.
4) The electrical equipment protecting the power system is not damaged in the most severe short-circuit condition, and the damage caused by the short-circuit fault is minimized.
The meaning of the short-circuit current calculation can be used to select the fuse to prevent the device from burning out. Provide basis for system design, equipment selection of new station, and establishment of relay protection setting.
1. Assume that the system has an infinite capacity. After the user is shorted, the system bus voltage can be maintained. That is, the calculated impedance is much larger than the system impedance.
Specific provisions: For the calculation of short-circuit current in 3~35KV-class power grid, it can be considered that the capacity of the system of 110KV and above is infinite. Just calculate the impedance of the network components of 35KV and below.
2. When calculating the short-circuit current in the high-voltage electrical appliance, only consider the reactance of the generator, transformer, and reactor, and ignore the resistance; for overhead lines and cables, only when the resistance is greater than 1/3 of the reactance Resistance, generally only counts the reactance and ignores the resistance.
3. The short-circuit current calculation formula or calculation chart is calculated by three-phase short circuit. Because the single-phase short circuit or the two-phase short circuit short-circuit current is less than the three-phase short-circuit current. An electric appliance capable of breaking a three-phase short-circuit current must be capable of breaking a single-phase short-circuit current or a two-phase short-circuit current.
(1) Verifying the mechanical stability and thermal stability of electrical equipment;
(2) verifying the interrupting capacity of the switch;
(3) Determine the value of the relay protection and safety automatic device;
(4) Provide a basis for system design and selection of electrical main wiring;
(5) Perform failure analysis;
(6) Determining the electromagnetic interference of the transmission line to the adjacent communication line
Commonly used calculation methods are:
Impedance matrix method, and use the superposition principle to make the network state after short circuit = network state fault component state before short circuit, add a potential equal to the voltage of the node and the opposite direction before the fault, and then use the impedance matrix to find The voltage value of the fault component of each node is added, and the voltage of the node after the short circuit fault is obtained by adding the voltage before the fault of the node. In turn, the current through the branches can be determined for short-circuit faults.
In the event of a short-circuit fault in the power system, the sudden change in the power distribution of the system and the severe drop in voltage may damage the stability of the parallel operation of the power plants and cause the entire system to be unloaded. At this time, some generators may be overloaded. Cut off some users. The greater the voltage drop during a short circuit, the longer the duration, and the greater the likelihood of disrupting the overall operation of the power system. The short-circuit current limiting measures are to ensure the safe and reliable operation of the system and to mitigate the effects of short-circuit. In addition to all the reasons that may cause short-circuit in operation and maintenance, the short-circuit fault should be removed as soon as possible so that the system voltage is higher. Return to normal value in a short time.
Short-circuit current prevention measures1. Calculate the short-circuit current, correctly select and verify the electrical equipment, and the rated voltage of the electrical equipment should match the rated voltage of the line.
2. Correctly select the setting value of relay protection and the rated current of the melt, and use the quick-break protection device to quickly cut off the short-circuit current when the short-circuit occurs, reduce the duration of the short-circuit current, and reduce the loss caused by the short-circuit.
3. Install a lightning rod in the substation, install a lightning arrester around the transformer and on the line to reduce lightning damage.
4. Ensure the construction quality of overhead lines, strengthen line maintenance, and keep the line sag consistent and meet the requirements.
5, with electrical installation and maintenance of electrical equipment, focus on the focus to prevent mis-wiring, misuse, work in the vicinity of the live parts, take measures to prevent short-circuit.
6. Strengthen governance to prevent small animals from entering the power distribution room and climbing electrical equipment.
7. Remove conductive dust in time to prevent conductive dust from entering electrical equipment.
8. Mark the cable in the embedding area. Some people excavate the construction around. They should send special personnel to take care of them, and explain the cable laying position to the construction personnel to prevent the cable from being damaged and causing a short circuit.
9. The operation and maintenance personnel of the power system should carefully study the regulations, strictly abide by the rules and regulations, operate the electrical equipment correctly, and prohibit the load-carrying knives and the grounded knives. After the line is constructed and the maintenance personnel have finished working, the grounding wire should be removed immediately. It is necessary to inspect the lines and equipment frequently, find defects in time, and quickly carry out maintenance.
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