1. Output voltage during commutation?

Taking the three-phase half-wave controllable rectifier with large inductance load as an example, the influence of leakage reactance on the rectifier circuit is analyzed. ?

During commutation (commutation), the leakage reactance prevents the current from changing, so the current can't change suddenly, so there is a changing process. ?

Ω t1triggers the V2 tube, so that the current changes from phase A to phase B. The phase current A cannot drop from Id to zero instantly, and the phase current B cannot suddenly rise from zero to Id. It takes some time to complete current commutation until ωT2, as shown in Figure 2-23(c). This process is called commutation process. The time corresponding to commutation process is calculated by phase angle, which is called commutation overlap angle and expressed by γ. During the overlapping angle γ, A and B thyristors are turned on at the same time, which is equivalent to a short circuit between the two phases. The difference between the two-phase potentials ub-ua is called short-circuit voltage, which generates an imaginary short-circuit current ik in the two-phase leakage reactance circuit, as shown by the dotted line in Figure 2-23(a) (in fact, thyristors are unidirectional, which is equivalent to adding an ik to the original current), and the A-phase current ia=Id- ik, which gradually decreases with the increase of ik; And ib= ik gradually increases. When Id increases, that is, ia decreases to zero, V 1 turns off, and the current of V2 tube reaches the stable current Id, thus completing the commutation process.

In the commutation process, the ud waveform is neither ua nor ub, but the average value of the commutation two-phase voltage. Compared with the case that the transformer leakage reactance is not considered, that is, γ=0, the rectified output voltage waveform reduces the shadow area, thus reducing the average output voltage Ud. This reduced area is caused by the commutation of the load current Id, so the average value of this area, that is, the voltage drop caused by Id, is called commutation voltage drop, and its value is the average value of the three shaded areas in the figure within a period. For other rectifier circuits with M commutations in a period, its value is the average of shadow areas of M blocks in a period. According to the formula (2-2 1), during commutation, the output voLTage ud = ub -LT(dik/dt)= ub -LT(dib/dt), and the output voltage without considering the influence of leakage reactance is ub, so the voltage caused by LT drops to ub -ud=? LT(dib/dt？ ), so the area of the shadow is

Second, the commutation overlap angle γ.

In order to facilitate the calculation, move the coordinate origin to the natural commutation points of phase A and phase B, and set

According to the working principle of the circuit, when the current in the inductor LT changes from 0 to Id, it just happens that ωt changes from α to α+γ. According to these conditions, it can be obtained by mathematical operation.

The above formula is a general formula, which can be obtained by substituting m=3 for a three-phase half-wave circuit.

For the three-phase bridge circuit, it is equivalent to a six-phase half-wave rectifier circuit with a phase voltage of m=6, and the result after substitution is the same as that of the three-phase half-wave circuit.

For single-phase double half-wave circuit, it is equivalent to two-phase half-wave circuit, as long as? Substitution can get M=2?

For single-phase fully-controlled bridge, the leakage reactance XT of transformer plays a role in two commutations in one cycle, and its current range is Id-Id. Although m=2 at this time, the commutation angle equation is

As long as Id, XT, U2φ and control angle α are known, the overlap angle γ can be calculated. When α is constant, Id XT increases and γ increases, which is because of the overlap angle caused by the leakage inductance of transformer when commutation. The greater the Id XT, the greater the energy stored by the transformer. When Id XT is constant, the smaller α is, the greater γ is, and the largest γ is when α is 0.

The leakage reactance of transformer, like the series reactance of AC incoming line, can limit the short-circuit current and mitigate the current change, and can also limit the current change rate and voltage change rate on thyristor. However, due to the existence of leakage reactance, it is equivalent to a short circuit between two phases during commutation, which leads to a gap in the phase voltage waveform of the power supply. When observing the phase voltage waveform with oscilloscope, there will be burrs at the commutation point, which will seriously distort the voltage waveform of the power grid and affect the normal operation of itself and other electrical equipment.