How to calculate short-circuit current

In power system, short circuit fault is a common serious fault, which may lead to safety accidents such as equipment damage, system instability and even fire. Therefore, accurate calculation of short-circuit current is of great significance to the design of power system, the selection of protection devices and the safety of operation.

1. What is short-circuit current?

Short-circuit current refers to the current flowing through the fault point in the power system under short-circuit fault state. Common short circuit types include three-phase short circuit, two-phase short circuit, single-phase grounding short circuit and two-phase grounding short circuit. Among them, although the probability of three-phase short circuit is low (about 5%), it produces the largest short-circuit current, so the three-phase short-circuit current is usually used as the design basis in the selection and protection setting of electrical equipment.

---

Second, the purpose of short-circuit current calculation

1. Selection of electrical equipment: such as circuit breakers, disconnectors, buses, etc. should have sufficient short-circuit tolerance.

2. Setting the relay protection device: ensure that the protection device can cut off the fault current quickly and reliably.
　　. Evaluate system stability: analyze the influence of short circuit on system voltage and frequency stability.

4. Formulate safety measures: to prevent electrical fire and equipment damage caused by short circuit.
　　--

Third, the calculation method of short-circuit current

The calculation of short-circuit current is usually based on symmetrical component method or standard unit value method, and combined with the equivalent circuit of power system for analysis. The following are common calculation steps:

# 1. Establish the equivalent model of the system

The generators, transformers, transmission lines and loads in the power system are simplified into corresponding equivalent circuit parameters, such as reactance, resistance and voltage source.

# 2. Determine the position of short circuit point

According to the actual needs, the location of the short circuit point is selected, usually at key nodes such as buses at all levels and feeder terminals.

# 3. Calculate the short-circuit parameters of each component

The parameters of each component are reduced to a unified reference capacity and voltage level, which is convenient for calculation. For example, using the scalar value method:

$$

X^* = frac{X}{Z_B} = frac{X S_B}{V_B^2}

$$

Where: $X$ is the actual reactance, $S_B$ is the reference capacity and $V_B$ is the reference voltage.

# 4. Calculation of Equivalent Total Impedance

The equivalent total impedance $Z_{total}$ from the power supply to the short circuit point is obtained through the series-parallel relationship.

# 5. Calculate short-circuit current

The initial value of short-circuit current (secondary transient short-circuit current) is:

$$

I_k = frac{U}{sqrt{3} Z_{total}}

$$

Among them, $U$ is the rated voltage line voltage of the system, and the unit is V or kV; $I_k$ is the short-circuit current, and the unit is a or kA.

---
　　V. Matters needing attention in practical application

1. Consider multi-power supply system: When multiple power supplies are running in parallel, the short-circuit current provided by each power supply should be calculated separately and then superimposed.

2. Feedback current of asynchronous motor: When a large-capacity motor is short-circuited, it may feed back current to the system, which cannot be ignored.
　　3. Time factor: the short-circuit process includes sub-transient, transient and steady-state stages, and the current value in different stages is different, so the action time of the protection device should match it.

4. Using professional software to assist calculation: such as ETAP, PSCAD, DIgSILENT and other software can greatly improve the calculation efficiency and accuracy.

---

V. Conclusion

The calculation of short-circuit current is the basis and key content in power system analysis. By accurately calculating the short-circuit current, the safe and stable operation of power system can be effectively guaranteed, the reliability of electrical equipment can be improved, and accidents caused by short-circuit can be prevented. Therefore, electric power engineers and technicians should master the calculation principle and method of short-circuit current and apply it flexibly in combination with engineering practice.

(The full text is about 950 words)