11. Try to analyze why the zero-sequence reactance of a single-loop transmission line is less than the equivalent zero-sequence reactance of a double-loop transmission line from the physical concept?
Parallel to the erection of double-circuit lines are through the zero sequence current, in a return line with a phase wire cross-linked flux not only with the other two-phase zero sequence current mutual inductance flux, there is another loop three-phase zero Sequence current generated mutual inductance flux, and are formed through the earth loop. Zero-sequence current in the same direction, so mutual magnetic flux from the magnetic effect. (0), then the zero-sequence impedance of the single-loop transmission line is Z (0), Z (0) is the impedance of each line, = Zs + 2zm, zero-sequence impedance of the double-loop transmission line 'Z (0) = Z (0) + Z (0).
13. Why does the non-damper winding of synchronous generator transient potential Eq in the short-circuit remain the same? What is the practical value of this point?
Damped synchronous generator because of its Eq and excitation winding flux linkage f is proportional to, and f in the running state of the sudden moment of conservation, so Eq in the running state of sudden changes in the moment is also conserved. For generators with undamped windings, this value can be calculated from the steady state at the moment before the mutation and directly applied to the calculation after the moment of the mutation.
14. What is the physical meaning of the Park transformation (ie, why do Park transforms)?
Park transformation from the physical point of view is actually a reference system transformation. By Park transformation, the observer's view is transferred from the stationary stator to the rotating rotor. Stator windings from the stator, the stator winding is three stator winding, dd winding, qq winding, 00 windings to replace, in which dd winding, qq windings were d axis, q axis axis, with the rotor synchronization Movement, 00 winding axis in the rotor rotation axis direction (with d axis, q axis perpendicular). Because there is no relative motion between the stator equivalent windings and the rotor windings, the mutual inductance between the equivalent windings of the stator, the mutual inductance between the equivalent windings, and the mutual inductance between the stator and rotor windings do not vary with the stator- Of the change, that does not change with the α, does not change with time. In this way, Park's flux linkage equation and the Park voltage equation are combined to obtain constant coefficient differential equation to solve.
15. With the damper winding of the synchronous generator-side three-phase short-circuit, short-circuit current in the stator winding cycle component can be divided into several parts? Which parts are attenuated?What time constant attenuation?
At this time, the periodic components in the stator windings can be divided into steady-state forced component and free component, the former is not attenuated, the latter is divided into sub-transient component and transient component of the two parts to zero attenuation, sub-transient component to vertical The time constant Td of the shaft-damper winding is attenuated and the transient component decays to zero with the time constant Td 'of the field winding.
16. Why is it necessary to implement reactive power in-place balance?
power transmission voltage loss is V = '' 'P QPR QX (R jX), in the high-voltage power line, the power loss is 2VV22
In most cases, the line reactance is larger than the resistance, and the reactance of the transformer is much larger than the resistance. Therefore, the long-distance transmission of reactive power increases the voltage drop and power loss of the grid during transmission. Therefore, from improving the voltage quality of reducing the loss of the network point of view, should try to achieve the reactive power of the local balance.
17. Describe why series capacitor compensation can improve power system stability.
If the line reactance is XLeq = XL-XC <XL, the total reactance X of the system decreases, and the power limit Pm = EV of the simple power system increases accordingly. The operation of the line Stability also increased accordingly with X
18. Short-circuiting of the power system, the closer to the short-circuit point, the more positive sequence voltage (low), negative sequence voltage (high), zero sequence voltage (high).
19. Non-damper winding The synchronous generator transient potential Eq 'is directly proportional to the flux linkage of the (excited) winding, and can be maintained at the instant of a sudden change in operating conditions.
20. Power system three-phase short circuit, the impact of current (that is, the shortest possible maximum instantaneous current value) as iim, RMS (short-circuit current maximum possible Of the effective value) as Iim, the existence of iim = 2im? why?
Iim refers to the maximum possible instantaneous value of the short-circuit current, which occurs 0.01 seconds after the fault. Iim is short-circuit after the first cycle of short-circuit current instantaneous value of the root mean square value, that is famous value. Since the short-circuit current includes the period component and the non-cycle component, the instantaneous value of the non-periodic component
