Electromechanical assembly comprising an alternator

Abstract

The invention relates to an electromechanical assembly comprising: an alternator with a wound rotor; a regulator acting on the excitation of the alternator; a rectifier at the outlet of the alternator, supplying a rectified voltage to a continuous bus; and a booster circuit connected by means of a filter to the outlet of the alternator and supplying a voltage to the continuous bus.

Claims

1. An electromechanical assembly, including: a wound-rotor alternator, a regulator acting on the excitation of the alternator, a rectifier at output phases of the alternator, delivering a rectified voltage to a DC bus, a step-up circuit linked by means of a filter to the output phases of the alternator, and delivering a voltage to the DC bus, the filter including an inductor in series at each output phase of the alternator, the inductor being connected directly to an output phase of the alternator and linking the output phase of the alternator to the step-up circuit, and two capacitors being connected directly to each output phase of the alternator and linking each output phase of the alternator to the DC bus.

2. The assembly as claimed in claim 1, the rectifier being a diode rectifier.

3. The assembly as claimed in claim 2, the diode being standard recovery time diode.

4. The assembly as claimed in claim 3, the diodes having a t.sub.rr greater than or equal to 4 s.

5. The assembly as claimed in claim 1, the capacitors having a capacitance that is chosen such that the frequency of an LC filter, L being the inductance of an inductor between 1 kHz and 5 kHz.

6. The assembly as claimed in claim 1, the step-up circuit being dimensioned for a fraction of the maximum power to be transmitted.

7. The assembly as claimed in claim 6, the step-up circuit being dimensioned for less than of the nominal power.

8. The assembly as claimed in claim 1, the rectifier being dimensioned for the maximum power to be transmitted.

9. The assembly as claimed in claim 1, the wound-rotor alternator being supplied with DC current by an exciter of the alternator, wherein the exciter is associated with an AC-to-DC converter.

10. The assembly as claimed in claim 9, the regulator acting on excitation current of the exciter.

11. The assembly as claimed in claim 9, the AC-to-DC converter being a rotary converter.

12. The assembly as claimed in claim 1, wherein, in a first mode of operation, the excitation current is regulated in such a way as to servo-control the voltage of the DC bus to a setpoint value.

13. The assembly as claimed claim 1, wherein, in a second mode of operation, the voltage of the alternator is rectified and stepped up by the step-up circuit to a setpoint voltage Udc_ref.

14. The assembly as claimed in claim 1, an excitation current of the alternator being regulated such that the level of magnetic saturation of the alternator does not exceed a coefficient of saturation of between 1.25 and 1.6.

15. The assembly as claimed in claim 1, the step-up circuit including an inverter.

16. The assembly as claimed in claim 15, the inverter being controlled by a pulse width modulation (PWM) technique.

17. A method for producing electricity using the electromechanical assembly as claimed in claim 1, wherein the alternator is driven in rotation.

18. The method as claimed in claim 17, wherein, in a first mode of operation, an excitation current is regulated in such a way as to servo-control the voltage of the DC bus to a setpoint value, the first mode of operation being selected when the speed of the alternator is between 80% and 120% of its nominal speed, and wherein, in a second mode of operation, the voltage of the alternator is rectified and stepped up by the step-up circuit to a setpoint voltage Udc_ref, the second mode of operation being selected when the speed of the alternator is lower than in the first mode.

19. The assembly as claimed in claim 1, the inductor having an inductance that is chosen such that the voltage drop across the inductor is between 4% and 12% of the nominal voltage of the alternator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1 to 3, described previously, illustrate the prior art,

(2) FIG. 4 is a schematic drawing of an example of an assembly according to the invention,

(3) FIG. 5 shows an example of the variation of the power as a function of the rotational speed,

(4) FIG. 6 illustrates an example of a regulation circuit, and

(5) FIG. 7 is a timing diagram of the voltages of the inverter.

DESCRIPTION OF EXAMPLE EMBODIMENTS

(6) The assembly 100 according to the invention includes, as illustrated in FIG. 1, a wound-rotor synchronous alternator 110, the rotor of which is supplied with DC current by an exciter 112 associated with a rotary AC-to-DC converter 119. A voltage regulator 113 connected to the field winding of the exciter makes it possible to regulate the voltage of the DC bus.

(7) A rectifier 111 formed of diodes d1 to d6, in particular of standard recovery time (t.sub.rr) diodes, rectifies the voltage of the alternator 110. This rectifier 111 is dimensioned for the maximum power to be delivered.

(8) A step-up circuit 115 is connected to the alternator and to the DC bus. This step-up circuit 115 is composed of a voltage inverter, for example an inverter with switching elements i1 to i6 that are formed by IGBTs. The inverter 115 is controlled by pulse width modulation.

(9) An RLC filter links the alternator 110 to the inverter 115. This filter includes inductors 120 in series with the output phases of the alternator, and pairs of capacitors 130, 131 linked to the DC bus. The filter includes six capacitors c1 to c6, linked in pairs to each phase. The step-up circuit 115 is dimensioned for only a fraction of the maximum power to be delivered.

(10) The alternator is driven by a combustion engine. As a variant, the driving element is a wind turbine, for example.

(11) In one variant that is not illustrated, the alternator still has a wound rotor, but the rotor is supplied with DC current by an assembly composed of rings and brushes.

(12) In a first mode of operation, the AC voltage of the alternator is rectified by the diodes d1 to d6. The excitation current of the rotor. If is continuously adjusted by the regulator 113 in order to servo-control the DC voltage Udc to a setpoint value Udc_ref, which may be constant or variable.

(13) The regulator is for example an off-the-shelf regulator, such as the one referenced D510 from Leroy Somer.

(14) Only a residual current that is not factored into the design passes through the switching elements i1 to i6 of the inverter. This mode of operation is preferably used in a zone B of the power-speed plane, where the excitation current If of the rotor that is required in order to achieve the setpoint DC voltage does not lead to saturation of the magnetic circuit of the alternator. This zone of the power-speed plane is located, as is visible in FIG. 5, where the power to be supplied is the greatest.

(15) In a second mode of operation, preferably used at the lowest speeds, corresponding to the zone A in FIG. 5, the voltage of the alternator is rectified and stepped up, by the inverter, to the setpoint voltage Udc_ref.

(16) In this mode of operation, the excitation current of the rotor is regulated such that the saturation level of the alternator does not exceed a certain defined value in order, inter alia, to minimize losses, for example in order to maintain a coefficient of saturation of less than 1.25.

(17) The filter formed of the induction coils 120 and of the capacitors c1 to c6 significantly attenuates, at the points Vau, Vav, Vaw, the harmonics of the voltages Viu, Viv, Viw of the inverter in differential mode and in common mode.

(18) The advantage of this filtering guarantees that the switching operations of the inverter do not have an impact on the reliability of the winding of the alternator. It also guarantees the absence of steep voltage edges between the 3 phases of the alternator and the ground of the system, which edges would be liable to destroy the bearings of the alternator by creating common-mode currents, and the non-conduction of the diodes d1 to d6, in particular at the instants of inverter switching operations, thereby enabling the use of standard recovery time diodes.

(19) In the preferred use zone of this second mode of operation, the element driving the alternator may deliver only a portion of its maximum power, thereby making it possible to dimension the inverter, the induction coils and the capacitors only for a fraction of the nominal power, for example of the nominal power for a generator set in which the power of the combustion engine decreases in terms of N.sup.3.

(20) For this second mode of operation, there are a plurality of known methods for regulating the voltage Udc to the setpoint value Udc_ref and for generating the commands for controlling the inverter.

(21) One of these methods will be described with reference to FIG. 6.

(22) The reference frame d,q that is used is an orthonormal reference frame, rotating at the frequency of the fundamental voltage of the alternator. The imaginary currents id, iq are obtained after having applied a three-phase/two-phase transformation, followed by a rotation of angle , to the 3 currents Iiu, Iiv, Iiw. The angle is chosen such that a modification of the value of the current iq acts only on the active power at the input of the inverter, and such that a modification of the value of the current id acts only on the reactive power.

(23) The value of the voltage of the DC bus is regulated to the setpoint value Udc_ref by a PID (proportional-integral-derivative) controller 201, the output of which forms the current setpoint iq_ref.

(24) The current setpoint id_ref is chosen, for example, so as to minimize the losses of the alternator, as described in the publication WO 2012/110979 A1.

(25) Two PI (proportional-integral) controllers 202 and 203 make it possible to servo-control the currents id and iq to the respective setpoints id_ref and iq_ref. The output of these two current regulators, in the rotating reference frame, represents the 2 orthonormal components Vd, Vq of the voltage vector that has to be applied at the input of the inverter.

(26) The condition regarding the angle cited above is met when the voltage Vd is equal to 0. The PI and I (integral) controllers 204 and 205 act as a PLL (phase locked loop). They servo-control the voltage Vd to 0, and make it possible to define the angle .

(27) The Modulation unit defines the instants of closure and of opening of the switching elements of each of the arms of the inverter, in accordance with a known pulse width modulation (PWM) method.

(28) In the example under consideration, the chosen technique is a pulse width modulation PWM at a set modulation frequency, where only 2 of the 3 arms of the inverter switch for each chopping period, as illustrated in FIG. 7. The inverter arm that does not switch is, for example, the one for which the absolute value of the current is the highest of the 3 arms, in order to minimize the losses of the inverter. The duty cycles of the 2 phases that switch depend on the values Vd_ref, Vq_ref, on the angle and on the voltage Udc, as illustrated in FIG. 7.

(29) The invention is not limited to the example that has just been described. In particular, it is possible to replace the diode bridge 111 with a thyristor bridge or with a mixed bridge.

(30) It is also possible to use a plurality of diode bridges.

(31) It is possible to replace the inverter with a step-up chopper.

(32) The expression including a(n) should be understood as a synonym for comprising at least one.