Power-generating system with improved treatment of charging impacts, load-shedding and harmonics

Abstract

The invention relates to a system for generating electric power, comprising: an alternator (1) for coupling with a drive system (7), supplying an AC voltage to an output bus (10); a reversible AC/DC converter (2) in which the AC bus (6) is connected to the output bus (10) of the alternator (1); an electricity-storage element (3) connected to the DC bus (9) of the converter (2); a controller (4) arranged to react to a transient state of load-shedding or charging impact by controlling the converter (2) so as to collect energy on the output bus (10) of the alternator (2) and to store same in the storage element (3) in the case of load-shedding, and to collect energy in the storage element (3) and to inject same into the output bus (10) in the case of charging impact, the converter (2) being controlled so as to inject currents to compensate for harmonic currents into the AC bus (10) of the alternator (1).

Claims

1. An electrical energy generation system comprising: an alternator to be coupled to a driving system, delivering an alternating voltage to an output bus, an AC/DC reversible converter whose AC bus is linked to the output bus of the alternator, an electrical storage element linked to the DC bus of the converter, a controller arranged to react to transient load-shedding or charging impact conditions by controlling the converter in such a way as to take energy from the output bus of the alternator and store the energy in the storage element in load-shedding cases, and to take the energy from the storage element and inject the energy onto the output bus in charging impact cases, the converter being controlled to inject harmonic current neutralization currents on the AC bus of the alternator, the converter comprising at least one active filter function for generating harmonic current neutralization control voltages, the harmonic current neutralization control voltages being added to power transfer control voltages to obtain control signals of the converter.

2. The electrical energy generation system as claimed in claim 1, the storage element being invoked to discharge only in transient conditions.

3. The electrical energy generation system as claimed in claim 2, the storage element being invoked to discharge only in at least one of a transient active power condition and a reactive power condition.

4. The electrical energy generation system as claimed in claim 1, the controller being arranged to react to at least one of a transient reactive power condition and an active power condition on a micronetwork, upon load-shedding or charging impact, to allow control of imbalance(s) in form, in amplitude and in response time by virtue of the controlling of the converter managing the energy on the output bus of the alternator, the controller allowing dynamic control of motor torque from the storage element in the charging impact and load-shedding cases.

5. The electrical energy generation system as claimed in claim 1, the storage element being a supercapacitor.

6. The electrical energy generation system as claimed in claim 1, comprising a passive filter, the passive filter comprising an inductance connected in series between each phase of the output bus of the alternator and a corresponding phase of the AC bus of the converter.

7. The electrical energy generation system as claimed in claim 1, the converter comprising three arms each comprising a first group of p switches electrically connected in series between a terminal of the storage element and a phase of the AC bus of the converter, and a second group of p switches electrically connected in series between the same phase of the AC bus of the converter and an other terminal of the storage element, p being an integer greater than 2.

8. The electrical energy generation system as claimed in claim 7, each arm comprising p1 balancing capacitors, each balancing capacitor being connected to a first pole between n.sup.th and n+1.sup.th switches of the first group, counted from a respective phase of the AC bus and to a second pole between n.sup.th and n+1.sup.th switches of the second group, counted from the same phase of the AC bus, n being an integer.

9. The electrical energy generation system as claimed in claim 1, the converter comprising three arms each comprising an inductance linked by a first terminal to a first terminal of the storage element, a first electronic switch linked to a second terminal of the inductance and to a corresponding phase of the AC bus of the converter, a second electronic switch linked to the second terminal of the inductance and to a second terminal of the storage element, and a balancing capacitor arranged between the corresponding phase of the AC bus of the converter and the second terminal of the storage element.

10. An electrical energy generation system comprising: an alternator to be coupled to a driving system, delivering an alternating voltage to an output bus, an AC/DC reversible converter whose AC bus is linked to the output bus of the alternator, an electrical storage element linked to a DC bus of the converter, a controller arranged to react to transient load-shedding or charging impact conditions by controlling the converter in such a way as to take energy from the output bus of the alternator and store the energy in the storage element in load-shedding cases, and to take the energy from the storage element and inject the energy onto the output bus in charging impact cases, the converter being controlled to inject harmonic current neutralization currents on the AC bus of the alternator, in which control signals of the converter are generated upon implementation of a method for driving the converter comprising calculating active and reactive components of the current of the AC bus of the converter, in a Park reference frame of the same frequency, calculating reference active and reactive currents in the same Park reference frame, calculating reference voltages, from differences between the reference active and reactive currents and the active and reactive components of the current of the AC bus of the converter, calculating control voltages for power transfers of the converter from the reference voltages by inverse Park transformation in a three phase reference of the current of the AC bus of the converter.

11. The electrical energy generation system as claimed in claim 10, reference active and reactive currents being obtained from the active and reactive components of an output current of the electrical energy generation system in the same Park reference frame.

12. The electrical energy generation system as claimed in claim 11, the active component of the output current of the electrical energy generation system being filtered by a filter to eliminate from the output current of the electrical energy generation system high frequencies and a continuous component in order to avoid having the converter exchange energy in steady-state conditions from the output current of the electrical energy generation system.

13. The electrical energy generation system as claimed in claim 12, the reactive component of the output current of the system being filtered so as obtain a zero reactive current on the alternator.

14. The electrical energy generation system as claimed in claim 11, each of the active and reactive components of the output current of the electrical energy generation system being filtered by a filter to eliminate from the output current of the electrical energy generation system high frequencies and a continuous component in order to avoid having the converter exchange energy in steady-state conditions from the output current of the electrical energy generation system.

15. The electrical energy generation system as claimed in claim 10, reference active and reactive currents being calculated respectively as a function at least of speed of the driving system and of excitation current of an exciter of the alternator.

16. The electrical energy generation system as claimed in claim 10, the harmonic current neutralization control voltages being obtained at least by calculation of active and reactive components of output currents of the alternator.

17. The electrical energy generation system as claimed in claim 16, the harmonic current neutralization control voltages being obtained at least by calculation of the active and reactive components of the output currents of the alternator in a Park reference frame of a frequency n times greater than a frequency of the three-phase reference frame, n being an integer greater than 2.

18. The electrical energy generation system as claimed in claim 10, the harmonic current neutralization control voltages being obtained at least by calculation of the active and reactive components of the current of the AC bus of the converter.

19. The electrical energy generation system as claimed in claim 18, the harmonic current neutralization control voltages being obtained at least by calculation of the active and reactive components of the current of the AC bus of the converter in a Park reference frame of a frequency n times greater than a frequency of the three-phase reference frame, n being an integer greater than 2.

20. The electrical energy generation system as claimed in claim 10, a value of the reference active current being subtracted from a supercapacitor control current before being used for calculating the reference voltages in order to avoid exceeding predefined safety voltage thresholds, the supercapacitor control current being a function at least of a voltage at terminals of the storage element and of a direct nominal current of the alternator.

21. The electrical energy generation system as claimed in claim 10, the control voltages for the power transfers of the converter being calculated by inverse Park transformation in a three-phase reference of the current of the AC bus of the converter.

Description

(1) The invention will be able to be better understood on reading the following detailed description of nonlimiting examples of implementation thereof, and on studying the attached drawing, in which:

(2) FIG. 1 is a schematic representation of a system according to the invention, in the case of a three-phase generator,

(3) FIG. 2 represents a triple-boost type converter structure,

(4) FIG. 3 represents a multi-level converter structure with (p+1) levels for p cells,

(5) FIG. 4 is an example of a control scheme allowing management of the storage element during the transient phases,

(6) FIG. 5 is a view similar to FIG. 4, of a variant control scheme with a free reference for the reactive power allowing the user to choose any reactive power level to be neutralized,

(7) FIG. 6 presents a variant control scheme from a model based on the speed of rotation of the group and the excitation current of the alternator,

(8) FIG. 7 presents a harmonic current control structure in order to produce an active filtering,

(9) FIG. 8 illustrates a method for managing the voltage at the terminals of the supercapacitor,

(10) FIG. 9a illustrates an example of a method for charging the supercapacitor with a safety function, and

(11) FIG. 9b illustrates an example of thresholds for triggering the charging and the safe shutdown of the system as a function of the voltages at the terminals of the storage element V.sub.DC.

(12) FIG. 1 shows an example of an installation according to the invention, for energy production delivered to an output bus 10 linked to the three-phase network or to one or more loads R.

(13) The installation comprises a driving means 7 such as a heat engine for example, or any other driving means, wind- or hydro-powered. The driving means 7 rotationally drives the rotor of an alternator 1, also called generator, comprising an exciter powering a main inductor arranged on the rotor, the main armature being linked to the output bus 10.

(14) The alternator 1 is driven in rotation at a regulated speed, but the output bus 10 may be subject to charging impact or load-shedding cases.

(15) The installation comprises an AC/DC reversible converter 2 driven by a controller 4, and a storage element 3.

(16) The controller 4 may know, from current sensors in the example of FIG. 1, the current in each of the phases 5 of the load R and the current of each phase of the AC bus 6 of the converter 2, as well as the voltage of each of the phases of the output bus 10 of the alternator 1.

(17) In the example illustrated in FIG. 1, each phase of the AC bus of the converter 2 comprises an inductance 8, in series between the converter 2 and the corresponding phase of the output bus of the alternator 1. The dimensioning of these inductances 8 depends on the power of the installation.

(18) In normal operation, the controller 4 ensures the voltage regulation of the alternator 1 through the detection of the voltage of the output bus 10 of the alternator 1. In the case of a conventional wound exciter, the controller 4 may be provided with a power element allowing it to supply the exciting inductor with the excitation current required to ensure the desired regulation of the output voltage of the alternator 1.

(19) To ensure the charging/discharging of the storage element 3, the controller sends control commands to the reversible electronic converter 2. A continuous supervision of the charging/discharging voltage V.sub.DC, of the charging/discharging currents of the storage element 3, and of the transient state (impact, load-shedding) of the system is carried out for this.

(20) In a load-shedding case, a command to charge the storage element 3 is given to make it possible to best reduce the voltage overshoot of the alternator. There is a recharging of the storage element 3 also when the level of the storage element 3 is below certain predefined thresholds.

(21) For its part, the discharging command is given in charging impact cases, to limit the voltage drop at the terminals of the alternator 1, or when the level of the storage element 3 is above a predefined threshold.

(22) The control commands may be sent by wire or wirelessly without departing from the scope of the present invention.

(23) The controller 4 also ensures, in the example illustrated, the communication with the driving motor 7, advantageously allowing an anticipation of the speed variation due to an impact or load-shedding case. In effect, by virtue of the measurement of the output current of the system i.sub.la, i.sub.lb, i.sub.lc and of that exchanged with the storage element 3, the controller 4 may estimate the power involved and determine a set point that makes it possible to anticipate the intake of fuel to the engine in order to limit the disturbances on the output bus of the alternator in transient conditions.

(24) The controller 4 thus advantageously supervises: the control of the active and reactive power exchanges on the one hand between the three-phase network created by the alternator 1 and on the other hand the load or loads R, using the three-phase converter 2 connected to the storage element 3 with direct current, the management of the harmonic currents added by the electromotive force of the machine or by a non-linear load R connected to the alternator 1, and in the case where the storage consists of a supercapacitor, the management of the voltage 9 at the terminals of this element.

(25) FIG. 2 illustrates a system comprising a converter 2 of triple boost type.

(26) The converter 2 comprises three arms 12, each comprising an inductance L.sub.f linked by a first terminal to the positive terminal 31 of the storage element 3, a first electronic switch T.sub.a, T.sub.b or T.sub.c linked between a second terminal of the inductance L.sub.f and a corresponding phase a, b or c of the AC bus of the converter, a second electronic switch T.sub.a, T.sub.b or T.sub.c linked between the second terminal of the inductance L.sub.f and the negative terminal 32 of the storage element 3 and a balancing and filtering capacitor C.sub.f arranged between the corresponding phase of the AC bus of the converter and the negative terminal 32 of the storage element 3.

(27) The currents i.sub.a, i.sub.b, and i.sub.c of the AC bus 6 of the converter 2 are injected on the output bus 10 of the alternator 1, the latter delivering a three-phase current i.sub.pa, i.sub.pb, and i.sub.pc for the respective phases a, b and c.

(28) The three-phase output current of the system i.sub.la, i.sub.lb and i.sub.lc may also be measured, for example by sensors 5.

(29) The controller 4 ensures the control of the converter 2, may also be configured to act on the speed regulator 27 of the heat engine 7 and/or on the voltage regulator 21 of the alternator 1.

(30) FIG. 3 illustrates a variant converter with p cells, p being an integer greater than 2.

(31) The converter comprises three arms 22 each comprising a first group of p switches K1.sub.1, K1.sub.2 . . . K1.sub.p electrically connected in series between a positive terminal 31 of the storage element 3 and a phase of the AC bus of the converter, and a second group of p electronic switches K1.sub.1, K1.sub.2 . . . K1.sub.p connected in series between the same phase of the AC bus of the converter and the negative terminal 32 of the storage element 3.

(32) Each arm 22 may comprise p1 balancing capacitors C.sub.f, 2p IGBT, each balancing capacitor C.sub.f being connected by a terminal between the n.sup.th and the n+1.sup.th electronic switches of the first group counted from the respective phase of the AC bus and by the other terminal between the n.sup.th and n+1.sup.th electronic switches of the second group, counted from the same phase of the AC bus.

(33) The balancing capacitor C.sub.f connected between the n.sup.th and n+1.sup.th electronic switches of the first group counted from the respective phase of the AC bus and by the other terminal between the n.sup.th and n+1.sup.th electronic switches of the second group has a voltage of nV.sub.dc/p, where V.sub.dc represents the voltage at the terminals of the storage element 3.

(34) The basic control of the active and reactive power exchanges is presented in FIG. 4.

(35) For a fixed amplitude three-phase network, controlling the powers amounts to controlling the active and reactive currents. These currents may be controlled in a Park reference frame R.sub.dq synchronous with the simple voltage v.sub.a of the first phase of the three-phase network, but the use of any of the other phases falls within the scope of the invention. A phase-locked loop 30 may be used for the synchronization.

(36) The control structure may be arranged to generate reference active i.sub.d* and reactive i.sub.q* currents in the Park reference frame R.sub.dq, and then lock them to the active and reactive components i.sub.d and i.sub.q, obtained by Park transformation in the same reference frame R.sub.dq, of the currents i.sub.a, i.sub.b, i.sub.c actually exchanged by the converter 2 with the network, by calculating the three duty cycles of the reversible converter 2.

(37) The generation of the reference currents i.sub.d* and i.sub.q* may be created from the active i.sub.ld and reactive i.sub.lq components of the output currents of the load or loads R in the Park reference frame R.sub.dq. The active component i.sub.ld is filtered to eliminate the high frequencies and the continuous component of the active current in order to avoid having the reversible converter 2 exchange active or reactive energy in steady-state conditions. Thus, the active current i.sub.d* takes account only of the transient conditions of the load (impact or load-shedding) and thus makes it possible to invoke the storage system 3 only in transient conditions. The reactive component i.sub.q* may be treated in the same way as the active component i.sub.d*, in order to minimize the time for which the reversible converter 2 is invoked.

(38) In a variant, the reactive power is neutralized by modifying the structure of the filter of FIG. 4 which generates i.sub.q*, which makes it possible to fully neutralize the reactive power exchanged with the load and obtain a zero reactive current i.sub.qp in the alternator. If necessary, the neutralization may be arbitrary, the user having the possibility of defining the reactive current level that is to be neutralized, as illustrated in FIG. 5.

(39) Another variant of generation of the reference currents i.sub.d* and i.sub.q* consists in using other measured quantities. Reference active and reactive currents (i.sub.d*, i.sub.q*) is no longer directly measured but calculated respectively as a function at least of the speed () of the driving system and of the excitation current (i.sub.f) of an alternator exciter. That makes it possible to dispense with the measurement of the output currents of the system i.sub.la, i.sub.lb, and i.sub.lc. The information on the variation of the torque may be taken by measuring the speed variation and the information on the variation of the magnetic state of the machine may be taken by measuring the excitation current i.sub.f of the exciter, as illustrated in FIG. 6.

(40) From the measurement of the speed of the revolving group and from a mathematical model 100 of the system, it is possible to calculate a variation of the torque on the alternator and to deduce therefrom the variation of the active current in the alternator i.sub.dp. The reactive current in the alternator i.sub.qp may be calculated from the measurement of the excitation current of the exciter and from the mathematical model 110 of the system. These calculations may also be found in the publication by P. Wetzer entitled Machines synchronesexcitation. Techniques de l'ingnieur, D3545, 1997 [Synchronous machinesexcitation. Engineer techniques, D3545, 1997] cited above. Thus, it is possible to deduce the two references i.sub.d* and i.sub.q*, from the currents of the alternator calculated i.sub.dp and i.sub.qp and the currents i.sub.d and i.sub.q of the converter 2 and then the regulation function is identical to the basic control. A combination of these two variants remains within the scope of the present invention.

(41) To generate the control signals S.sub.a, S.sub.b and S.sub.c of the converter 2, the reference currents i.sub.d* and i.sub.q* are compared to the active and reactive components i.sub.d and i.sub.q of the three-phase current i.sub.a, i.sub.b, i.sub.c measured at the output of the reversible converter 2, obtained after a transformation into the Park reference frame R.sub.dq. The current error is by a regulator 33, of PID type for example, which generates voltage references .sub.d and .sub.q in the Park reference frame, which, after inverse Park transformation in the three-phase reference frame R.sub.abc, give the power transfer control voltages m.sub.a, m.sub.b and m.sub.c in the latter reference frame. Finally, a pulse width modulation (PWM) function 34 makes it possible to generate the signals S.sub.a, S.sub.b and S.sub.c to control the reversible converter 2.

(42) The system is advantageously arranged to act on the harmonic currents delivered by the machine or induced by a non-linear load, by virtue of the active filtering function made possible with the structure described in the present application.

(43) An example of control for the active filtering function is described in FIG. 7. The converter comprises at least one active filter function allowing the converter to generate neutralization voltages to avoid the harmonic currents in the converter due to the voltage harmonics on the AC side. Control voltages V*n.sub.abc are added to the control voltages due to the power transfers m.sub.a, m.sub.b, m.sub.c to obtain the control signals S.sub.a, S.sub.b, S.sub.c of the converter.

(44) The currents i.sub.pa, i.sub.pb, i.sub.pc measured at the output of the alternator are transformed into a Park reference frame R.sub.ndq of a frequency n times greater than the fundamental frequency of the three-phase current i.sub.a, i.sub.b, i.sub.c of the AC bus of the converter, the transformed currents being combined respectively with a filtering to extract the amplitude of the harmonic n in the two axes d.sub.n and q.sub.n of this Park reference frame R.sub.ndq. These extracted currents i.sub.dn and i.sub.qn are then locked onto zero references so as to act on the output voltages of the regulator and sent to the input of a regulator 41, for example of PID type or other advanced regulator structure, whose outputs are added to the preceding operation of the converter 2 corresponding to the voltages that the converter 2 must deliver in this Park reference frame R.sub.ndq. An inverse Park transformation, at the speed and in the direction of the harmonic n, makes it possible to obtain the neutralization control voltages V*n.sub.abc of the harmonic n in the three-phase reference frame R.sub.abc. The latter may be added to the voltages m.sub.a, m.sub.b and m.sub.c delivered by the preceding control, which makes it possible to act on the active and reactive power transfers.

(45) A variant consists in considering the currents of the converter instead of the currents of the alternator, that is to say i.sub.a, i.sub.b, i.sub.c instead of i.sub.pa, i.sub.pb, i.sub.pc, illustrated in FIG. 7. That allows the converter to produce purely sinusoidal currents at the fundamental frequency and avoid having the harmonic voltages of the three-phase network created by the alternator produce harmonic currents in the converter. The latter control is equivalent to a plug circuit for the voltage harmonics resulting from main generator with its load.

(46) This active filtering principle may be applied to all the harmonic currents that are to be neutralized, by reproducing the neutralization structure and by adding all the resulting voltages to obtain the control voltages of the converter.

(47) Whatever the structure of the converter and the control laws applied, it is desirable to control the state of charge of the supercapacitor, given by its voltage V.sub.DC. Since this is an active current in the Park reference frame which may modify this charge state, it is sufficient to modify the active reference current i.sub.d* by subtracting a current that makes it possible to act on the voltage V.sub.DC. A safety shutdown should be provided so as to avoid exceeding the maximum voltage for the supercapacitor while maintaining the minimum voltage making it possible to ensure the correct operation of the converter 2.

(48) FIGS. 8, 9a and 9b illustrate an example of this control of the voltage V.sub.DC of the supercapacitor.

(49) The thresholds a1%, a2% and a3% representing the output current levels of the system or discharge current levels in relation to the direct nominal current Idn of the alternator, these thresholds being able to be set within the 100% band without departing from the scope of the invention.

(50) The values a1, a2 and a3 are determined by experimentally to avoid activating the safety function and excessively disrupting the operation of the hybrid function.

(51) An example of control of the mean current of the supercapacitor consists in delivering an amplitude and a sign of Id.sub.VDC dependent on the voltage V.sub.DC according to programmed thresholds and applying a comparison algorithm by adding a hysteresis at the threshold level to avoid a beat effect. A safety output gives a true or false logic level to allow a current in the supercapacitor or prevent it if the thresholds are reached; when the safety function is active then Id=0. The voltage thresholds and output current levels of the system and discharge current levels may be modified without departing from the scope of this invention.