1. What are the composition of the power system?
By the power generation, transmission, substation, distribution and electricity and other sectors of the energy production and consumption system. Its function is to use the natural primary energy through the power generation device into electrical energy, and then by transmission, substation and distribution of electricity supply to the user.
2. What is the main role of power transformers?
The main role is to transform the voltage, in order to facilitate the transmission of power. In the same section of the line, the transmission of the same power, the voltage step-up transformer step-up voltage, the transmission line current decreases,it reduces line losses, improve transmission economy, to achieve long-distance power transmission purposes, and buck To meet the needs of users at all levels of voltage.
3. What is the cause of the short circuit fault?
(1) Lightning and other forms of over-voltage and natural aging of insulation materials, or suffer mechanical damage, resulting in the insulation of the current-carrying conductor is damaged;
(2) unpredictable natural damage, such as overhead lines due to high winds or ice caused by the collapse of electric ice, etc., or because birds and beasts bare conductor, etc.;
(3) natural pollution increased to reduce the insulation capacity; (4) operating personnel in violation of safe operating procedures and misuse, such as lines or equipment repair is not removed after the ground wire to add voltage and so on.
4. What are the consequences of a short-circuit fault?
(1) short circuit will produce a short-circuit current from the power to the short-circuit fault point, up to several times the normal load current to several times. Short-circuit currents create a large amount of electromotive force between the conductors of an electrical device, resulting in distortion, distortion, or damage to the conductor. Short-circuit current through the conductor heat generated by its sharp rise in temperature, will lead to damage to equipment due to heat.
(2) short circuit will cause the system voltage suddenly dropped significantly. In the system, the asynchronous motor will decelerate or stop due to the torque decrease.
(3) A short circuit will cause a sudden change in the power distribution in the system. Resulting in parallel operation of the power plant out of synchronization, destruction of system stability
(4) asymmetric short-circuit current to produce unbalanced alternating magnetic field. On the surrounding communication network, signal system, thyristor trigger system and automatic control system interference.
5. What is the purpose of short-circuit calculation?
(1) for the selection and calibration of various electrical equipment to provide the basis for mechanical stability and thermal stability;
(2) to provide the necessary data for the design and selection of electrical main wiring of power plants and substations;
(3) for the rational allocation of power systems in a variety of relay protection for automatic device and its parameters can be set correctly
Rely on the basis.
7. What is the procedure for performing the asymmetric fault analysis?
Step1: Write the boundary condition equation according to the type of short circuit;
Step2: The boundary condition equation of the phase component with the corresponding sequence component instead of, and simplified;
Step3: Compose the complex sequential network according to the simplified boundary condition equation;
Step4: Solve the sequence current of voltage and current according to the complex sequence network;
Step5: The voltage and current components of the order for synthesized voltage and current phase components can be obtained.
8. When the power system three-phase short-circuit, the short-circuit current non-periodic components are the same? why?
They are not the same. Because the non-periodic components of the DC component is to keep the stator flux winding of each coil generated. And short-circuit before and after the winding component of the instantaneous value of the difference between the three are not equal, so the non-periodic components of the initial value of three-phase is not equal.
9. With the short-circuit current calculation of the curve, how to consider the load ?
The load is negligible. Because the influence of the load has been taken into account when calculating the curve. When the calculation curve is made, the engine is rated to run at full load before the fault. 50% of the load is connected to the high-voltage busbar of the power plant. The rest of the load power is transmitted to the short-circuit point through the transmission line. Therefore, when the short-circuit current curve is calculated, it can not be repeated Consider the effect of the load.
10. From the physical concept why the three-phase power transmission line phase distance is greater in the positive sequence reactance?
The positive sequence reactance of the three-phase transmission line is composed of self-inductance and mutual inductance. The mutual inductance of one-phase current and the other two-phase currents are dephased. When the distance between phases increases, the mutual inductance demagnetization is weakened. , The positive sequence reactance increases.
