Four-quadrant asynchronous electric drives with three-phase PWM rectifiers, страница 2

The equation for voltage on the DC filter capacitor can be written as:

,                                      (2)

where: u_d - the relative voltage in DC link ; i_acd - current in supply circuit of the inverter of the electric drive; X_C - relative reactance of the condenser in DC link. After substitution in the equations (2) expressions for voltage components the equation is reduced to:

.                 (3)

Having added to the equation (3) equation for the DC current of the induction drive with vector control system and projection to X axis of the rotor flux vector Ψ_2x=const [3], we shall receive the equations for a PU voltage in DC link:

,            (4)

where: r₁, r₂ - the PU resistance of the stator and rotor of the induction machine, X₁, X₂ - the PU inductive reactance of the stator and rotor; X₀ – the PU inductive reactance of the magnetizing circuit; υ – the PU frequency of rotor speed ; μ - the PU rotor moment.

The equations (4) connect mechanical variable of the induction drive with vector control system, the voltage in DC link, the phase current vector in power network and the phase voltage. The equations (1) - (2) allow synthesizing current control systems and DC voltage control systems in the rotating reference frame for IGBT PWM rectifiers.

It is necessary to mark, that the equations (1) - (4) are correct at the following limitations:

1. The voltage in the DC link of PWM rectifiers cannot be set less then rectified voltage. It is caused of IGBT power module free-wheel diodes.

2. The upper bound of the DC link voltage is limited only by parameters of power elements.

3. The voltage in DC link should be enough for support a phase voltage of the asynchronous electric drive and PWM rectifier.

Thus, for a sinusoidal PWM

                                                                                   (5)

and for the space vector PWM

                                                                                          (6)

where: u_1x, u_1y - components of the average for period of PWM relative (PU) voltage vector on the induction machine stator.

The equations (1) - (6) have been used in designing the four-quadrant asynchronous electric drive for escalator stations of Saint-Petersburg subway.

For maintaining a required voltage in DC link is used the control system in the rotating reference frame. In that control system the voltage controller output signal is used as input for the controller of the current vector active component. For compensation of the current vector reactive component is used the feedback current signals from respective point of the power network.

In this control system there is a useful opportunity to set any DC supply voltage of the electric drive in limits 620-800V for the better use of induction machine characteristics. Moreover, there is enough free choice of a value of DC voltage, allows reducing losses in the electric drive due to set of the optimum magnetizing current for the induction machine [3].

3. MAIN RESULTS

In the induction electric drive research and development team of Scientific Research Institute of Fine Mechanics has been made and tested of 200kW four-quadrant AC induction drive with PWM inverters and vector control systems for subway escalators. This work has been made under supervision of the head of electric drive research team Goncharenko M.R. and senior researcher Baev A.P.

This 200kW four-quadrant AC electric drive includes two identical cases. But cases differ one from another by software of the control system built on the basis of TMS-320 microprocessor. It is necessary to note, that significant number of program blocks are identical. Data exchange between control systems of the PWM rectifier and the electric drive control system is realized on high-speed and noiseproof CAN- channel.

4. CONCLUSION

The tests results of four-quadrant AC drive with the vector control system shows the full correspondence schematic solutions and selected control algorithms with solved problems. The results received at the tests, confirm theoretical conclusions made in works [1], [3] and equations (1) - (6).

References

1. Goncharenko R.B., Goncharenko M.R., Rudomazina I.A. Way of increase efficiency an electric machine systems of energy conversion renewed sources. Izvestia Akademii Nauk. Energetika, no.2, pp. 36-45, 1998 (in Russian).
2. Rusong Wu, S. B. Dewan, G. R. Slemon A. PWM AC to DC converter with fixed switching frequency. IEEE Trans. Ind. Appl., vol. 26, no. 5, pp. 880-885, 1990.
3. Goncharenko M.R.  Electromagnetic processes in a power circuit high-speed asynchronous electric drive. The abstract of the Phd. Thesis. Leningrad, IFMO, 1989, p. 17.