WTO harmonization and HS codes

List of contents of this article:

• 1,WTO harmonization and HS codes Why can asymmetric faults be calculated by the symmetric component method in the power system?
• 2, What is an asymmetric failure
• 3, Power system, there are several simple asymmetrical faults of transmission lines

Why can asymmetric faults be calculated by the symmetric component method in the power system?

1. The asymmetric voltage and current flow rate generated after an asymmetric failure in the power system can be decomposed into three sequential networks by applying the symmetrical component method, analyzed according to the sequential voltage and current symmetry in each sequence network, and then synthesized into the actual ABC quantity, so that the asymmetric failure The calculation is greatly simplified.

2. The damage to the normal operation of the power system is mainly related to asymmetric failure or asymmetric operation of circuit breakers. Since only a few points in the whole power system have unequal three-phase impedance, the method of directly solving complex three-phase asymmetric circuits is generally not used, but a simpler symmetric component method is used for analysis.

3. The method of analyzing asymmetric short circuit is the symmetrical component method. Asymmetric short circuit. Simply put, single-phase grounded short circuit, two-phase grounded short circuit, two-phase short circuit, etc. in the short circuit type are asymmetric short circuits.Three-phase short circuit belongs to symmetrical short circuit. However, for three-phase short-circuits that occur at different points, short circuits are also asymmetric short circuits.

What is asymmetric fault

1. Causes of power grid asymmetry, ultra-high-voltage overhead transmission lines The road does not change. The structure of the transformer is asymmetrical. The existence of AC and DC transformers, the system load is unbalanced, and there are nonlinear components in the system.

2. Power system short-circuit faults can be divided into three types: three relative faults, asymmetric faults and single-phase ground faults. Asymmetric fault refers to the different size and polarity of the three-phase current, and there may be two or more combinations; single-phase ground fault refers to the grounding phenomenon in one phase, and measures need to be taken to ensure personal safety.

3. Asymmetric short-circuit failure, according to different short-circuit causes, the calculation method is as follows: single-phase grounded short-circuit Single-phase grounded short-circuit current is less than the three-phase short-circuit current at the same fault point, and vice versa, the single-phase short-circuit current is greater than the three-phase short-circuit current.

4. Symmetric fault: three-phase balance fault Asymmetric fault: single-line ground fault, double-line ground fault, line-to-line fault The cause of symmetry failure is a short circuit fault between the three phases at the same time, at which time the fault current and phase difference of the three phases are equal;The magnitude of the current and the phase angle between the three phases of asymmetric faults are not equal.

5. Borrow the existing answer to the first question: asymmetric failure will produce negative sequence current and voltage. Asymmetric grounding faults or disconnection faults will produce zero-sequence voltage, and whether to generate zero-sequence current depends on the grounding operation mode of the system.

There are several types of simple asymmetrical faults of power system and transmission lines

Asymmetric faults of power systems are mainly divided into two categories: transverse asymmetrical faults and longitudinal asymmetrical faults. Horizontal asymmetry failure Two-phase short circuit This refers to the short circuit between the two phases in the power system. In this case, the two-phase current of the short circuit will increase, while the other phase current will decrease or be zero.

Zero-sequence impedance = positive-sequence impedance: the non-fault phase voltage is the pre-fault voltage, and the short-circuit current is equal to the three-phase short-circuit current. Zero-sequence impedance is infinite: the non-fault phase voltage is 5 times the voltage before the failure, and the short-circuit current is the three-phase short-circuit current √3/2.

Asymmetric operation of the power grid, including short-circuit faults and disconnection faults. Short-circuit faults include single-phase grounding, interphase short-circuit and interphase short-circuit grounding faults. In the neutral point grounding system, grounding faults and disconnection faults will generate zero-sequence current, and of course there is also negative-sequence current. Interphase short circuit does not produce negative sequence components.