Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Patent
1996-11-12
1999-08-17
Wong, Peter S.
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
363132, 318432, 318807, H02M 7515
Patent
active
059402865
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for controlling the power to be transferred via a mains inverter, the mains inverter being connected to a DC intermediate circuit on the one hand and to an n-phase AC mains via reactors on the other hand, the frequency and amplitude of the voltage of which AC mains the mains inverter cannot affect independently; the mains inverter comprising n branches with appropriate switches and being arranged to transfer electric power in both directions between the n-phase AC mains and the DC intermediate circuit.
In accordance with what has been presented above, the present invention relates to a method for controlling a mains inverter, which is connected between an AC mains and a DC intermediate circuit. Typically, this DC intermediate circuit supplies the inverter, which supplies one or more electric motors. In an arrangement of this type, the power is supplied most typically from the electrical network to the motor, but for instance when the motor is decelerated, situations may occur in which it is necessary to transfer electric energy also from the load to the electric network via the DC intermediate circuit.
The situation described theoretically above can be illustrated by the diagram shown in FIG. 1 of the accompanying drawing as well as by the pointer drawing shown in FIG. 2 of the drawing. It is conceivable that the AC electromotive force existing in the electric network is produced in the generators supplying the electric network when the magnetic flux located on the generator shaft rotates as surrounded by the winding located in the generator. The voltage created in the winding of the generator can be expressed by formula (1). ##EQU1## Correspondingly, the flux generated by the voltage is obtained by integrating the voltage with respect to time according to formula (2). length abs (Vv) of the flux vector is also constant. The voltage, a variable characteristic of the AC system, thus also unequivocally represents the flux generated by it when the angular frequency of the vectors is defined. As shown in FIG. 2, the flux Vv is in a phase shift of 90.degree. with respect to the voltage Uv.
In connecting another electromotive source to the electromotive force of the AC mains described above, the voltage of which source is Uvvs via reactor L, as shown in FIG. 1, the effective power transferring between these two systems can be described by means of equation (3). precondition for a power transfer between two electromotive forces, this angle difference being also shown in FIG. 2.
As is generally known, power P can be expressed as a product of torque and angular speed. In the case described above, angular speed .omega. is the value of the angular speed calculated on the basis of the frequency of the AC electromotive power of the AC mains. The torque can be determined according to the known formula (4).
Thus, a connection between equation (4) utilized in the theory of AC motors and power equation (3) utilized in the theory of electricity transmission is determined. Since the effective power transferring between the systems is in balance in the stationary state, equation (4) can also be expressed by means of the flux Vvvs of another electromotive force in the form of formula (5).
On the basis of what has been presented above, it is apparent that if one desires to transfer power P between two electromotive systems, it is only necessary to define current vector I unequivocally on the basis of the calculations and calculate the position and magnitude of the flux vector by integrating the voltage vector according to formula (2) in either of the electromotive systems. The method according to the invention for controlling the DC level of the intermediate circuit of the mains inverter, this method being based on the above-mentioned principle, is characterized in that the power desired to be transferred via a mains inverter is controlled by defining reference values for the torque and flux of the mains inverter and comparing them with the actual values of the corresponding variables i
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Harmoinen Martti
Manninen Vesa
Pohjalainen Pasi
Tiitinen Pekka
ABB Industry OY
Jardieu Derek J.
Wong Peter S.
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