Calculation and Analysis of Short-Circuit Current in Substation Design

The current flowing when an abnormal connection (short circuit) occurs between phases or between phases and the ground (or neutral line) during operation of the power system is called short-circuit current.

Short-circuit current calculation is one of the three major calculations of the power system and is the basis for selecting electrical wiring methods, electrical equipment, relay protection setting and verification. The design of this substation requires the following contents: 110 kV and 10 kV distribution devices within the substation wall, main transformers, 10 kV capacitors, self-compensation devices, AC and DC systems in the station, control protection, lightning protection and grounding, etc. The design requirements in this article are: the capacity of the main transformer is calculated by the required coefficient method; voltage level, 110/10 kV; 110 kV incoming line, 2 cable incoming lines; 10 kV incoming line, 20 cable outgoing lines (two of which are spare) as required. This article mainly analyzes the specific short-circuit current calculation method in the substation design process. 1

Overview

The short-circuit faults that may occur in the power system mainly include three-phase short circuit, two-phase short circuit and single-phase short circuit. In general, the three-phase short-circuit current is greater than the two-phase and single-phase short-circuit current. In a power grid with direct grounding of the neutral point, the short-circuit fault of one phase to the ground is the most common, accounting for about 90% of all short-circuit faults. In a power network with indirect grounding of the neutral point, the short-circuit fault is mainly various phase-to-phase short-circuits. When a short circuit occurs, due to the reduction of the impedance of the power supply circuit and the transient process of the sudden short circuit, the current in the short-circuit circuit increases greatly, which may exceed the rated current of the circuit by many times. When a short circuit occurs in the power system, the sudden change of the system power distribution and the serious drop in voltage may destroy the stability of the parallel operation of various power plants. The greater the voltage drop during the short circuit and the longer the duration, the greater the possibility of destroying the stable operation of the entire power system.

When calculating the short-circuit current, since the substation design belongs to an unknown system, we regard this power supply capacity as an infinite power system. In such a system, when a short circuit occurs somewhere, the power supply voltage remains unchanged, that is, the periodic component of the short-circuit current does not decay during the entire short circuit process.

2 Principles of short-circuit current calculation
1) When calculating short-circuit current for checking the breaking current, dynamic stability and thermal stability of electrical appliances and conductors, the calculation should be made according to the planned content of this project. Generally, the three-phase short-circuit current under the maximum operating mode should be used as the basis. For example, the substation is generally calculated based on the capacity of 2 or 3 main transformers, and the 5-10-year long-term development plan of the power grid should be appropriately considered.
2) When calculating short-circuit current, the calculation should be made according to the normal wiring method where the maximum short-circuit current may occur. The short-circuit point should be selected at the point where the short-circuit current is the largest.
3) The dynamic stability, thermal stability of conductors and electrical appliances, as well as the breaking current of electrical appliances, are generally checked according to the three-phase short-circuit current.
4) When calculating the short-circuit current of a high-voltage power grid of 10 kV and above, the resistance of the original components is generally ignored. If the total resistance ∑R in the short-circuit circuit is greater than 1/3 of the total reactance ∑X, the effective resistance of the line and other components should still be included.
5) When calculating the short-circuit current of a low-voltage power grid below 1 000 kV, it is generally not allowed to ignore the resistance value of the electrical equipment in the short-circuit loop, such as the resistance of the distribution transformer, the resistance of the low-voltage line, the busbar and cable that are not too long, the primary winding of the current transformer, the overcurrent coil of the automatic switch, and the contact resistance of the contacts of the automatic switch and the disconnector, etc., because these resistances have an impact on the low-voltage short-circuit current.
6) When calculating the short-circuit current of a certain voltage level, the average voltage is applied.
7) When calculating the current of the high-voltage system circuit, the per-unit value method and the short-circuit power method are generally used for calculation. When calculating the short-circuit current of a low-voltage distribution network below 1 000 V, the nominal value method is generally used, that is, voltage V, current kA, and resistance mΩ.

3 Three-phase short-circuit current calculation

3.1 Short circuit at K1 point on the high voltage side


of the transformer 3.2 Short circuit at K2 point on the low voltage side of the transformer

According to the above calculation, when a three-phase short circuit occurs, the maximum short circuit current on the high voltage side of the transformer is 14.673 kA, and the minimum short circuit current is 9.0827 kA; the maximum short circuit current on the low voltage side of the transformer is 27.399 kA, and the minimum short circuit current is 14.159 kA.

4 Conclusion
With the rapid development of the national economy and regional economy, the demand for electricity has grown rapidly. Substation upgrade and substation expansion have become important planning contents of power supply companies. Since substation transformation will change the network structure, the change of network structure will inevitably affect the short circuit current of each node. Accurate short circuit current calculation can reasonably determine the scheme to limit the short circuit current according to the actual situation of the substation power system, or limit the emergence of a certain operation mode, and obtain a reliable and economical main connection scheme.
In short, when evaluating and comparing various main connection schemes to select the best scheme, short circuit current calculation is a very important basic work. Therefore, when selecting electrical equipment in power design, short-circuit current must be calculated. The calculation of short-circuit current is an indispensable part of electrical design and one of the most important calculations in power design. This paper lays the foundation for the selection of electrical equipment and relay protection setting in substation design through the calculation of short-circuit current.