Operating Electromagnetic Processes in a Three-Phase Bridge Inverter with Pole Commutation in Sinusoidal PWM Mode

OPERATING ELECTROMAGNETIC PROCESSES IN A THREE-PHASE BRIDGE INVERTER WITH POLE COMMUTATION IN SINUSOIDAL PWM MODE

M. A. Zhitkov, V. A. Dobruskii

(Presented by the scientific-technical seminar of the Scientific Research Institute of AEM)

The implementation of pulse-width modulation of an inverter’s output voltage according to a sinusoidal law makes it possible effectively to improve the shape of the load-current curve. In this case, the most expedient is the realization of such a mode in the simplest types of thyristor inverters—in inverters with pole or common commutation.

Fig. 1.

In the inverter with pole commutation shown in Fig. 1, this proved possible under modulation by shifting the leading edge of the pulses with the simultaneous use of power thyristors in the mode of a controlled zero valve [2].

Owing to the substantial difference in the duration of the operating and commutation electromagnetic processes and to the complete autonomy of the commutation circuit with direct recharging of the commutating capacitor, the operating electromagnetic processes in the inverter may be considered separately from the commutation processes [1].

The inverter operates as follows. Control pulses, shown respectively in Fig. 2 (b, c, d), are applied to the control electrodes of thyristors 1, 2; 3, 4; 5, 6 (Fig. 1).

During the interval \(t_1—t_2\) (Fig. 2, a), amounting to 60 electrical degrees, thyristor 14 (Fig. 1) is turned on by the trailing edge of the control pulses (Fig. 2a); it applies commutating pulses to the thyristors of the cathode group, and therefore simultaneously turns off

Fig. 2.

only thyristors 4 and 6 are switched on, while the valve of the anode group 1 remains open and passes the reactive currents of the disconnected loads of phases B and C, which close, respectively, through the circuits (Fig. 1): \(Z_B\) — diode 9 — choke 19 — thyristor 1 — \(Z_A—Z_B\) and \(Z_C\) — diode 11 — choke 19 — thyristor 1 — \(Z_A—Z_C\).

Thus, in the interval \(t_1—t_2\), thyristor 1 is open; therefore the formation of the current in the loads of phases B and C occurs according to the sinusoidal law incorporated in the control signals. In accordance with the equality known from the theory of three-phase circuits, \(i_A+i_B+i_C=0\), it may be concluded that the load current of phase A, equal to \(|i_A|=|i_B+i_C|\), will also have a sinusoidal form.

In the interval \(t_2—t_3\) (Fig. 2, a), on the trailing edge of the control pulses thyristor 13 is switched on, which supplies commutating pulses to the thyristors of the anode group; therefore only thyristors 1 and 3 are switched off, while the valve of the cathode group 6 is used as a zero valve, through which the reactive currents of the disconnected loads of phases A and B close.

Thereafter the commutation process proceeds similarly. The operation of the inverter may be considered only over \(1/6\) of the period, since the subsequent cyclic permutation makes it possible to obtain line diagrams of voltages and currents for the entire period.

Let us consider \(1/6\) part of the period, coinciding with the beginning of the formation of the positive half-wave of the voltage in phase A, i.e., with the origin of coordinates (Fig. 2). In accordance with the adopted method of controlling the inverter, the durations of the open states of the thyristors of all phases will be as shown in Fig. 2 (b, c, d), where the bold lines indicate the intervals within which the corresponding thyristors are not switched off and are used as working and zero valves.

Determination of the form of the phase and line voltages on the load, taking into account the load power factor, is carried out with the aid of

Fig. 3. \((\cos \varphi_{\mathrm{n}} = 0.866)\).

equivalent circuits, compiled for each interval corresponding to the current state of the inverter valves. In this case it is not difficult to determine the time intervals for each phase when the current is drawn from the power source, or when reactive energy is exchanged between phases or returned to the power source.

Figures 3 and 4 show diagrams of the phase voltages \(U_A, U_B\), and \(U_C\), and of the line voltage \(U_{AB}\), for the cases in which a three-phase active-inductive load is connected in a star without a neutral conductor and has a power factor \(\cos \varphi_{\mathrm{n}}\), respectively, of 0.866 and 0.5, while the commutation frequency, in accordance with Fig. 2, is \(f_{\mathrm{k}} = 24 f_{\mathrm{m}}\).

From the voltage curves presented it is seen that the phase voltages are a sequence of unipolar and bipolar pulses of variable duration with an amplitude taking values \(1/3 \cdot E\) or \(2/3 \cdot E\), while the line voltage is a sequence of unipolar pulses of variable duration with amplitude \(E\). The shape of the pulses, their duration, and even their number per half-period of the output frequency change when the load power factor changes.

From the figures presented it is seen that the phase-shift angle between the current and voltage in each phase receives an increment in the direction of current lag equal to \(\pi/6\). This is a drawback of the circuit, since it limits the operability of the inverter for a load with a low power factor. The occurrence of the additional shift angle is explained by the fact that, with this method of control, the phase voltages are formed...

Fig. 4. \((\cos \varphi_{\mathrm{н}} = 0.5)\).

voltages, while the current in a phase is determined by the line-to-line voltages, since the neutral point of the load star is not connected to the zero point of the power supply. To eliminate this drawback, it is advisable to form the line-to-line voltages.

REFERENCES

1. M. A. Zhitkov. Bridge inverter with pole commutation in sinusoidal PWM mode. Izvestiya TPI, vol. 262, 1972.

2. A. I. Zaitsev, M. A. Zhitkov. Independent inverters with pole commutation in pulse-width modulation mode. Reports of the VI Scientific and Technical Conference on Problems of Production Automation. Vol. V, Tomsk, VINITI Publishing House, 1969.