Control system for a power generation system

Abstract

The invention lies in the field of current stabilisation in a power generation system comprising a plurality of elementary power groups connected in parallel. It relates to a control system for regulating the elementary power groups. According to the invention, the control system comprises a global current control system (510) and a plurality of local current control systems each associated with an elementary power source of the power generation system. The global current control system (510) comprises: .square-solid. a divider (511) arranged to deliver a fixed current set point I.sub.n_fix, .square-solid. correction unit (512) arranged to deliver a variable current set point I.sub.n_var and to take either a steady state or a transitory state, the variable current set point I.sub.n_var being determined as a function of a correction signal S.sub.corr in the transitory state, .square-solid. an adder (513) arranged to deliver a global current set point I.sub.n_glob as the sum of the fixed current set point I.sub.n_fix and the variable current set point I.sub.n_var, and .square-solid. a scenario management unit (514) arranged to detect when the state of at least one elementary power source (220.sub.1-220.sub.N) switches from an OFF-state to an ON-state, or vice versa, to determine the correction signal S.sub.corr and to trigger the transitory state of the correction unit for a predetermined transitory period τ.sub.trans when a change of state is detected.

Claims

1. A control system for regulating a plurality of elementary power groups in a power generation system, each elementary power group comprising an elementary power source arranged to take either an ON-state, wherein it generates an elementary current I.sub.n, or an OFF-state, wherein it is not able to provide a current, the elementary power sources being connected in parallel to deliver a total current I.sub.tot as a sum of the elementary currents I.sub.n, the control system comprising a global current control system and a plurality of local current control systems each associated with an elementary power source, the global current control system being arranged to generate a global current set point I.sub.n_glob, each local current control system being arranged to work either in a global mode, wherein it regulates the elementary current I.sub.n of the associated elementary power source as a function of said global current set point I.sub.n_glob, or in a local mode, wherein it regulates said elementary current I.sub.n as a function of a predetermined local current set point I.sub.n_loc, the global current control system comprising: a divider arranged to deliver a fixed current set point I.sub.n_fix, the fixed current set point I.sub.n_fix being equal to a total current set point I.sub.tot_ref divided by a number N.sub.ON of elementary power sources in the ON state, a correction unit arranged to deliver a variable current set point I.sub.n_var, and to take either a steady state, wherein the variable current set point I.sub.n_var is determined as a function of a difference between the total current I.sub.tot and a total current set point I.sub.tot_ref so as to minimise said difference, or a transitory state, wherein the variable current set point I.sub.n_var is determined as a function of a correction signal S.sub.corr, an adder arranged to deliver the global current set point I.sub.n_glob, the global current set point being equal to the sum of the fixed current set point I.sub.n_fix and the variable current set point I.sub.n_var, and a scenario management unit arranged to detect a change of scenario from a former scenario to a new scenario, a change of scenario occurring when the state of at least one elementary power source switches from the OFF-state to the ON-state, or vice versa, the scenario management unit being further arranged to determine the correction signal S.sub.corr, to deliver it to the correction unit and to trigger the transitory state of the correction unit for a predetermined transitory period τ.sub.trans when a change of scenario is detected, the correction signal S.sub.corr being determined so that the total current I.sub.tot remains constant in spite of the change of scenario.

2. The control system of claim 1 wherein, in the transitory state of the correction unit, the variable current set point I.sub.n_var is determined as being equal to the correction signal S.sub.corr.

3. The control system of claim 2, wherein the scenario management unit is arranged to determine the correction signal S.sub.corr as follows: $S_{\mathrm{corr}}=\underset{n=1}{\overset{N_{\mathrm{ON}}}{.Math.}}\left(\frac{I_{\mathrm{tot\_ref}}}{N_{\mathrm{ON}}}-I_n\right)/\left(N_{\mathrm{ON}}-N_{\mathrm{ON\_loc}}\right)$ with N.sub.ON_loc the number of elementary power sources in the ON-state in the new scenario associated with a local current control system working in a local mode.

4. The control system of claim 2, wherein the scenario management unit is arranged to determine the correction signal S.sub.corr as a function of the differences, for each elementary power group in the ON-state associated with a local current control system working in a local mode, between its elementary current I.sub.n and the fixed current set point I.sub.n_fix in the new scenario.

5. The control system of claim 4, wherein the correction signal S.sub.corr is determined as follows: $S_{\mathrm{corr}}=\underset{n=1}{\overset{N_{\mathrm{ON\_loc}}}{.Math.}}\left(\frac{I_{\mathrm{tot\_ref}}}{N_{\mathrm{ON}}}-I_n\right)/\left(N_{\mathrm{ON}}-N_{\mathrm{ON\_loc}}\right)$ with N.sub.ON_loc the number of elementary power sources in the ON-state in the new scenario associated with a local current control system working in a local mode.

6. The control system of claim 1, wherein the predetermined transitory period τ.sub.trans ranges between 5 milliseconds and 1 second.

7. The control system of claim 1 wherein, in the transitory state of the correction unit, the variable current set point I.sub.n_var is determined as being equal to the sum of the correction signal S.sub.corr and the difference between the total current I.sub.tot and the total current set point I.sub.tot_ref.

8. The control system of claim 7 wherein, in the transitory state of the correction unit, the correction signal S.sub.corr is determined as being equal to the difference between the fixed current set point I.sub.n_fix(t) for the new scenario and the fixed current set point I.sub.n_fix(t−1) for the former scenario.

9. The control system of claim 7 wherein, in the steady state of the correction unit, the correction signal S.sub.corr is determined as being equal to zero.

10. The control system of claim 1, wherein the global current control system is arranged to generate the global current set point I.sub.n_glob at a predetermined sampling frequency.

11. The control system of claim 7, wherein the transitory period τ.sub.trans is equal to a sampling period corresponding to the sampling frequency.

12. The control system of claim 10, wherein the transitory period τ.sub.trans is equal to a sampling period corresponding to the sampling frequency.

13. A power system comprising: a power generation system; and a control system comprising an elementary power source arranged to take either an ON-state, wherein it generates an elementary current I.sub.n, or an OFF-state, wherein it is not able to provide a current, the elementary power sources being connected in parallel to deliver a total current I.sub.tot as a sum of the elementary currents I.sub.n, the control system further comprising a global current control system and a plurality of local current control systems each associated with an elementary power source, the global current control system being arranged to generate a global current set point I.sub.n_glob, each local current control system being arranged to work either in a global mode, wherein it regulates the elementary current I.sub.n of the associated elementary power source as a function of said global current set point I.sub.n_glob, or in a local mode, wherein it regulates said elementary current I.sub.n as a function of a predetermined local current set point I.sub.n_loc, the global current control system comprising: a divider arranged to deliver a fixed current set point I.sub.n_fix, the fixed current set point I.sub.n_fix being equal to a total current set point I.sub.tot_ref divided by a number N.sub.ON of elementary power sources in the ON state, a correction unit arranged to deliver a variable current set point I.sub.n_var and to take either a steady state, wherein the variable current set point I.sub.n_var is determined as a function of a difference between the total current I.sub.tot and a total current set point I.sub.tot_ref so as to minimise said difference, or a transitory state, wherein the variable current set point I.sub.n_var, is determined as a function of a correction signal S.sub.corr, an adder arranged to deliver the global current set point I.sub.n_glob, the global current set point being equal to the sum of the fixed current set point I.sub.n_fix and the variable current set point I.sub.n_var, and a scenario management unit arranged to detect a change of scenario from a former scenario to a new scenario, a change of scenario occurring when the state of at least one elementary power source switches from the OFF-state to the ON-state, or vice versa, the scenario management unit being further arranged to determine the correction signal S.sub.corr, to deliver it to the correction unit and to trigger the transitory state of the correction unit for a predetermined transitory period τ.sub.trans when a change of scenario is detected, the correction signal S.sub.corr being determined so that the total current I.sub.tot remains constant in spite of the change of scenario; wherein the power generation system comprises a plurality of elementary power groups each associated with one of the local current control systems of the control system.

14. The power system of claim 13, wherein the power generation system is dedicated to deliver the total current I.sub.tot to a smelter or an aluminium smelter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be best understood in view of the foregoing description of exemplary embodiments, which are in no way limitative, and in view of the accompanying drawings, on which:

(2) FIG. 1 represents an embodiment of a power system comprising a power generation system and a control system;

(3) FIG. 2 represents an exemplary embodiment of a global current control system of the control system of FIG. 1;

(4) FIG. 3 is a timing diagram illustrating how the power system of FIG. 1 functions;

(5) FIG. 4 represents a power system according to a first embodiment of the invention comprising the power generation system of FIG. 1 and a control system according to the invention;

(6) FIG. 5 represents an exemplary embodiment of a global current control system of the control system of FIG. 4;

(7) FIG. 6 is a timing diagram illustrating how the power system of FIG. 4 functions.

DETAILED DESCRIPTION OF DETAILED EMBODIMENTS

(8) FIG. 1 represents an embodiment of a power system 100 comprising a power station or power generation system 200 and a control system 300. The power generation system 200 enables generating a direct current with high amperage, for instance of several thousands of kilo-amperes. It comprises a number N of elementary power groups 2101, 2102, . . . , 210N, with N an integer greater than or equal to 2. In the present description, the elementary power groups are collectively referred to by reference numeral 210 and are individually referred to by reference numeral 210n, with n an integer varying between 1 and N. The same numbering applies to other components of the power system 100, each component being linked to one of the elementary power groups 210. Each elementary power group 210n comprises an elementary power source 220n able to take either an ON-state, wherein it generates an elementary current I.sub.n or an OFF-state, wherein it is not able to provide a current. Each elementary power group 210n further comprises means for measuring its elementary current I.sub.n, not represented, for instance an ammeter. The elementary power groups 210 are electrically connected to a power line 230 so that the current I.sub.tot on the power line 230 is equal to the sum of the elementary currents I.sub.n.

(9) The control system 300 comprises a global current control system 310 and N local current control systems 3201-320N, each local current control system 320n being associated with an elementary power group 210n and more particularly with an elementary power source 220n. The global current control system 310 is arranged to generate a global current set point I.sub.n_glob that may be used locally by each of the elementary power groups 210 as a reference set point. In a first embodiment, illustrated on FIG. 1, the global current set point I.sub.n_glob is common to all elementary power groups 210. In a second embodiment, a global current set point I.sub.n_glob may be determined for each of the elementary power groups 210. Each local current control system 320n is arranged to work either in a global mode, wherein it regulates the elementary current I.sub.n of the associated elementary power source as a function of said global current set point I.sub.n_glob, or in a local mode, wherein it regulates the elementary current I.sub.n as a function of a predetermined local current set point I.sub.n_loc. Predetermined local current set points are typically determined by an operator. The local current control systems 320 may be proportional-integral (PI) controllers or proportional-integral-derivative (PID) controllers or any other controllers based on these controllers. Each local current control system 320n receives the elementary current I.sub.n delivered by its associated elementary power group 210n and at least one of the global current set point I.sub.n_glob and the local current set point I.sub.n_loc. It may control the elementary power group 210n so that its elementary current I.sub.n is equal to the current set point I.sub.n_glob or I.sub.n_loc.

(10) FIG. 2 represents an exemplary embodiment of the global current control system 310. This global current control system 310 comprises a divider 311, a correction unit 312 and an adder 313. It receives the total current I.sub.tot on the power line 230 and a total current set point I.sub.tot_ref, for instance set up by an operator. The global current control system 310 also receives information relating to the number N.sub.ON of elementary power sources in the ON-state.

(11) Alternatively, it may comprise means for determining this number N.sub.ON. The total current set point I.sub.tot_ref and the number N.sub.ON of elementary power sources in the ON-state are input to the divider 311 so that it delivers a fixed current set point I.sub.n_fix equal to the total current set point I.sub.tot_ref divided by the number N.sub.ON of elementary power sources in the ON-state:
I.sub.n_fix=I.sub.tot_ref/N.sub.ON

(12) The correction unit 312 receives the total current I.sub.tot and the total current set point I.sub.tot_ref, compares them and delivers a variable current set point I.sub.n_var as a function of this comparison in order to minimise the deviation of the total current I.sub.tot with respect to the total current set point I.sub.tot_ref. The correction unit 312 may be a proportional-integral (PI) controller or a proportional-integral-derivative (PID) controller.

(13) The adder 313 receives the fixed current set point I.sub.n_fix and the variable current set point I.sub.n_var and delivers the global current set point I.sub.n_glob as the sum of these currents:
I.sub.n_glob=I.sub.n_fix+I.sub.n_var
FIG. 3 is a timing diagram illustrating how the power system 100 as described above functions. Time t is represented in abscissa and the intensity of each current is represented in ordinate. Note that the intensity scale is not the same for all currents but only aims at showing time evolution of each current with respect to the others. For this figure, it is considered a power generation system 200 with three elementary power groups 210 (N=3), dedicated to deliver a total current of 300 kA (I.sub.tot_ref=300 kA). It is assumed that at time to, the elementary power sources 220 of all elementary power groups 210 are in the ON-state and each generate a same elementary current I.sub.n (I.sub.n=I.sub.1=I.sub.2=I.sub.3=100 kA). For some reasons, it is desired to stop the elementary power group 2101 for a certain period. Then, at time t.sub.1, elementary current I.sub.1 starts to decrease until it reaches zero at time t.sub.2. This decrease is for example managed by setting a predetermined decreasing local current set point I.sub.1_loc. The decrease of I.sub.1 leads the total current I.sub.tot to show a decrease tendency. However, this decrease of I.sub.tot is evaluated by the correction unit 312, which leads the variable current set point I.sub.n_var to increase accordingly. The global current set point I.sub.n_glob increases in the same way between t.sub.1 and t.sub.2. The variable current set point I.sub.n_var rises from 0 to 50 kA and the global current set point I.sub.n_glob rises from 100 kA to 150 kA. As can be seen on FIG. 3, the gradual decrease of the elementary current I.sub.1 is compensated for by the control system 300 without any disturbance. However, when at time t.sub.3 the elementary power source 2201 switches from the ON-state to the OFF-state, the number N.sub.ON of elementary power sources in the ON-state suddenly changes from 3 to 2. This leads to a sudden change of the fixed current set point I.sub.n_fix from 100 kA to 150 kA. The global current set point I.sub.n_glob also suddenly changes from 150 kA to 200 kA. The local current control systems 3202 and 3203 are not able to compensate for this change of current set point and make the elementary power sources 2202 and 2203 to generate a current peak 31 at time t.sub.3 lasting several seconds. This current peak 31 will gradually decrease until time t.sub.4 at which the correction unit 312 fully compensates for the difference between the total current I.sub.tot and the total current set point I.sub.tot_ref. The current peak 31 is of course passed on to the total current I.sub.tot, which so shows a current peak 32. A similar effect is observed when an elementary power source 220n switches from the OFF-state to the ON-state, whereas at least one other elementary power source is delivering a no-null elementary current I.sub.n. In particular, in the example of FIG. 3, when the elementary current source 2201 is switched in the ON-state at time t.sub.5, the number N.sub.ON of elementary elements in the ON-state now suddenly changes from 2 to 3. This leads to a sudden change of the global current set point I.sub.n_glob that the local current control systems 3201, 3202, 3203 are not able to compensate for instantaneously. It results a current peak 33 for the elementary currents I.sub.2 and I.sub.3 and so a current peak 34 for the total current I.sub.tot. The current peaks 33 and 34 gradually decrease until time t.sub.6 at which the correction unit 312 fully compensates for the difference between the total current I.sub.tot and the total current set point I.sub.tot_ref. At time t.sub.7, the elementary current I.sub.1 starts to increase until it reaches the same current as elementary currents I.sub.2 and I.sub.3 at time t.sub.8 (I.sub.1=I.sub.2=I.sub.3=100 kA). The increase of the elementary current I.sub.1 leads the variable current set point I.sub.n_var to decrease accordingly from 50 KA to 0. As a result, the elementary currents I.sub.2 and I.sub.3 also decrease from 150 kA to 100 kA. FIG. 4 represents a power system 400 according to a first embodiment of the invention. It comprises the power generation system 200 as disclosed above with reference to FIG. 1 and a control system 500. The control system 500 also comprises a global current control system 510 and N local current control systems 520, each local current control system 520n being associated with an elementary power group 210n. With respect to the power system 100 of FIG. 1, the power system 400 according to an embodiment of the invention distinguishes in that the global current control system 510 is provided with the elementary currents I.sub.n of all elementary power sources 220. The global current control system 510 is also arranged to generate a global current set point I.sub.n_glob that may be used locally by each of the elementary power groups 320 as a reference set point. The global current set point I.sub.n_glob may be common to all elementary power groups 320 or may be individual for each elementary power group 320n.

(14) FIG. 5 represents an exemplary embodiment of the global current control system 510. Similarly to the global current control system 310, it comprises a divider 511, a correction unit 512 and an adder 513. In addition, it comprises a scenario management unit 514 receiving each elementary current I.sub.n, the total current set point I.sub.tot_ref and the number N.sub.ON of elementary power sources 220 in the ON-state. This number N.sub.ON could alternatively be determined by the global current control system 510 itself. The scenario management unit 514 may further receive the total current I.sub.tot, for instance in view to be used as redundant information for reasons of reliability. The divider 511 is arranged to deliver the fixed current set point I.sub.n_fix equal to the total current set point I.sub.tot_ref divided by the number N.sub.ON of elementary power sources in the ON-state:
I.sub.n_fix=I.sub.tot_ref/N.sub.ON

(15) The correction unit 512 may be based on an integral controller, a derivative controller, a proportional-integral (PI) controller, a proportional-derivative (PD) controller or a proportional-integral-derivative (PID) controller. It is arranged to deliver the variable current set point I.sub.n_var, the determination of which depends whether the correction unit 512 is in a steady state or a transitory state. By extension, the global current control system 510 is also said to be in the steady state or in the transitory state. In the steady state, the variable current set point I.sub.n_var is determined as a function of a difference between the total current I.sub.tot and the total current set point I.sub.tot_ref so as to minimise this difference. In the transitory state, the variable current set point I.sub.n_var is determined as a function of a correction signal S.sub.corr provided by the scenario management unit 514. It may be equal to this correction signal S.sub.corr.

(16) The adder 513 receives the fixed current set point I.sub.n_fix and the variable current set point I.sub.n_var and delivers the global current set point I.sub.n_glob as the sum of these currents:
I.sub.n_glob=I.sub.n_fix+I.sub.n_var

(17) The scenario management unit 514 is arranged to detect a change of state among the elementary power sources 220, i.e. a change from the ON-state to the OFF-state or vice-versa. For the sake of simplicity, each value of the number N.sub.ON of elementary power sources 220 in the ON-state is referred to as a scenario. A change of state of at least one elementary power source 220n then leads to a change of the value N.sub.ON and of scenario, unless there is simultaneously the switch of some elementary power sources 220 from the ON-state to the OFF-state and the switch of the same number of elementary power sources 220 from the OFF-state to the ON-state. The scenario management unit 514 is further arranged to trigger the transitory state of the correction unit 512 for a predetermined transitory period τ.sub.trans when a change of scenario is detected and to determine the correction signal S.sub.corr to be delivered to the correction unit 512 and used during the transitory period. The transitory period τ.sub.trans is triggered by a signal S.sub.trig. It may range for example between 5 ms and 1 s. The correction signal S.sub.corr is determined so that the total current I.sub.tot remains constant in spite of the change of scenario.

(18) In a first embodiment, the correction signal S.sub.corr is determined as a function of the total current set point I.sub.tot_ref, the number N.sub.ON of elementary power sources 220 in the ON-state in the new scenario, the elementary currents I.sub.n and the number N.sub.ON_loc of elementary power sources 220 in the ON-state in the new scenario associated with a local current control system 320n working in the local mode. More precisely, it may be determined as follows:

(19) $S_{\mathrm{corr}}=\underset{n=1}{\overset{N_{\mathrm{ON}}}{.Math.}}\left(\frac{I_{\mathrm{tot\_ref}}}{N_{\mathrm{ON}}}-I_n\right)/\left(N_{\mathrm{ON}}-N_{\mathrm{ON\_loc}}\right)$

(20) This formula may be equally expressed as follows:

(21) $S_{\mathrm{corr}}=\underset{n=1}{\overset{N_{\mathrm{ON}}}{.Math.}}\left(I_{\mathrm{n\_fix}}-I_n\right)/N_{\mathrm{ON\_glob}}$

(22) In a second embodiment, the correction signal S.sub.corr does not take into account all elementary currents I.sub.n but only those of elementary power sources 220 in the ON-state in the new scenario associated with a local current control system 320n working in the local mode. The above formulas become:

(23) $S_{\mathrm{corr}}=\underset{n=1}{\overset{N_{\mathrm{ON\_loc}}}{.Math.}}\left(\frac{I_{\mathrm{tot\_ref}}}{N_{\mathrm{ON}}}-I_n\right)/\left(N_{\mathrm{ON}}-N_{\mathrm{ON\_loc}}\right)$$\mathrm{and}$$S_{\mathrm{corr}}=\underset{n=1}{\overset{N_{\mathrm{ON\_loc}}}{.Math.}}\left(I_{\mathrm{n\_fix}}-I_n\right)/N_{\mathrm{ON\_glob}}$

(24) FIG. 6 is a timing diagram similar to that of FIG. 3 illustrating how the power system 400 according to a first embodiment of the invention functions. The same assumptions as for the timing diagram of FIG. 3 are considered. At time to, the correction unit works in the steady state. From time t.sub.0 to time t.sub.2, the power system 400 shows a same working. In particular, the variable set point I.sub.n_var increases from 0 to 50 kA. However, when the elementary power source 2201 switches to the OFF-state at time t.sub.3, the scenario management unit 514 detects the change and triggers the transitory state by means of the signal S.sub.trig. It also recalculates the correction signal S.sub.corr. Since there is no elementary power source in the ON-state associated with a local current control system working in the local mode (N.sub.ON_loc=0), the correction signal S.sub.corr is determined as zero (S.sub.corr=0). The variable current set point I.sub.n_var so suddenly decreases from 50 kA to 0 kA. Simultaneously, at time t.sub.3, the fixed current set point I.sub.n_fix suddenly increases from 100 kA to 150 kA. As a result, the global current set point I.sub.n_glob remains constant at time t.sub.3 (I.sub.n_glob=150 kA). The total current I.sub.tot so undergoes no disturbance. In a similar way, when the elementary power source 2201 switches at time t.sub.5 to the ON-state with its associated local current control system working in the local mode, the fixed current set point I.sub.n_fix suddenly decreases from 150 kA to 100 kA. The scenario management unit 514 detects the change and triggers the transitory state by means of the signal S.sub.trig. It also recalculates the correction signal S.sub.corr as follows:

(25) $S_{\mathrm{corr}}=\underset{n=1}{\overset{N_{\mathrm{ON\_loc}}}{.Math.}}\left(I_{\mathrm{n\_fix}}-I_n\right)/N_{\mathrm{ON\_glob}}=\underset{n=1}{\overset{1}{.Math.}}\left(100-0\right)/2=50\mathrm{kA}$

(26) The variable current set point I.sub.n_var so suddenly increases from 0 kA to 50 kA. Simultaneously, at time t.sub.5, the fixed current set point I.sub.n_fix suddenly decreases from 150 kA to 100 kA. As a result, the global current set point I.sub.n_glob remains constant at time t.sub.5 (I.sub.n_glob=150 kA). The total current I.sub.tot so undergoes no disturbance.

(27) According to a second embodiment of the invention, the power system comprises the power generation system 200 as disclosed above and a control system, not represented, mainly distinguishing from the control system 500 disclosed with reference to FIG. 4 and FIG. 5 in that the scenario management unit does not take into account the elementary currents I.sub.n for determining the correction signal S.sub.corr. The correction unit is also arranged to deliver a variable current set point I.sub.n_var, the determination of which depends whether it is in a steady state or a transitory state. In the steady state, the variable current set point I.sub.n_var is determined as a function of the difference between the total current set point I.sub.tot_ref and the total current I.sub.tot.

(28) However, contrary to the first embodiment of the invention wherein the regulation of the total current I.sub.tot is temporarily interrupted, the regulation is maintained even in the transitory period τ.sub.trans. To this end, in the transitory state, the variable current set point I.sub.n_var is determined not only as a function of the correction signal S.sub.corr, but also as a function of the difference between the total current set point I.sub.tot_ref and the total current I.sub.tot. As indicated above, the elementary currents I.sub.n are not considered when determining the correction signal S.sub.corr. The correction signal S.sub.corr is determined as a function of the fixed current set point in the former scenario I.sub.n_fix(t−1) and the fixed current set point in the new scenario I.sub.n_fix(t). For example, it is determined as follows:
S.sub.corr(t)=I.sub.n_fix(t)−I.sub.n_fix(t−1)

(29) The variable current set point I.sub.n_var may be determined as follows:
I.sub.n_var(t)=f(I.sub.tot_ref(t))−+S.sub.corr(t)

(30) with f a function determined so that the difference between the total current I.sub.tot and the total current set point I.sub.tot_ref is minimised.

(31) In an exemplary implementation, the correction unit may deliver the variable current set point I.sub.n_var as being the sum of the correction signal S.sub.corr and a function of the difference between the total current set point I.sub.tot_ref and the total current I.sub.tot as well in the steady state and in the transitory state. In such a case, the correction signal S.sub.corr is then determined as being zero in the steady state.

(32) This second embodiment of the invention is well suited with a power system using digital signals. In particular, the total current I.sub.tot and the elementary currents I.sub.n may be digitized at a predetermined sampling frequency, this frequency also determining the frequency of data processing. Preferably, the transitory period τ.sub.trans is set so as to be equal to a sampling period corresponding to the sampling frequency. Then, the variable current set point I.sub.n_var is corrected by a non-zero correction signal S.sub.corr for a single sampling period.