Converter provided with a circuit for managing alternating power in an alternating part

Abstract

The invention relates to a multi-level modular converter provided with a control circuit comprising a computer to calculate an internal control setpoint of the converter and an energy management circuit allowing a power setpoint to be determined that is to be transmitted to the alternating electrical power supply network, the control circuit being configured to regulate the voltage at the point of connection of the converter to the direct electrical power supply network and to regulate the voltage at the terminals of each capacitor modelled as a function of the internal control setpoint and of the power setpoint to be transmitted to the alternating electrical power supply network.

Claims

1. A multilevel modular voltage converter for converting alternating voltage into direct voltage and inversely, comprising: a direct part intended to be connected to a direct electric power supply network; an alternating part intended to be connected to an alternating electric power network; a plurality of legs, each leg comprising an upper arm and a lower arm, each arm comprising a plurality of sub-modules controllable individually by a control member specific to each sub-module and each sub-module comprising a capacitor connectable in series in the arm when the control member of the sub-module is in a controlled state, each arm modelled by a modelled voltage source connected to a duty cycle dependent on a number of capacitors placed in series in the arm, the modelled voltage source connected in parallel to a modelled capacitor corresponding to a total capacitance of the arm; and a control circuit of the converter comprising a computer of an internal command setpoint of the converter by application of a function having an adjustable input parameter, wherein the control circuit further comprises an energy management circuit configured to deliver an operating power setpoint as a function of the voltage at the terminals of each modelled capacitor, the operating power setpoint being utilised to determine a power setpoint to be transmitted to the alternating electric power supply network, the control circuit being configured to regulate the voltage at the point of connection of the converter to the direct electric power supply network and the voltage at the terminals of each modelled capacitor as a function of the internal command setpoint and of the power setpoint to be transmitted to the alternating electric power supply network.

2. The converter according to claim 1, wherein the computer is configured to calculate the internal command setpoint by application of a derived function and a filtering function.

3. The converter according to claim 1, wherein the adjustable input parameter is an adjustable virtual inertia coefficient k.sub.VC.

4. The converter according to claim 1, wherein the internal command setpoint is an internal power setpoint P*.sub.W.

5. The converter according to claim 4, wherein the computer is configured to calculate the internal power setpoint P*.sub.W, of the converter according to the function: $P_W^{*}=\frac{1}{2}{C}_{\mathrm{eq}}{k}_{\mathrm{VC}}\times \left(v_{\mathrm{dc}}^2\times \frac{s}{1+\tau s}\right)$ where C.sub.eq=6C.sub.tot and C.sub.tot is the total capacitance in an arm of the modelled capacitor, v.sub.dc is the voltage at the point of connection of the converter to the direct electric power supply network and r is a time constant.

6. The converter according to claim 4, wherein the internal power setpoint P*.sub.W is utilised to determine a power setpoint P*.sub.dc to be transmitted to the direct electric power supply network.

7. The converter according to claim 1, wherein the internal command setpoint is an internal current setpoint I*.sub.W.

8. The converter according to claim 7, wherein the computer is configured to calculate the internal current setpoint I*.sub.W, according to the function: $I_W^{*}=C_{\mathrm{eq}}{k}_{\mathrm{VC}}\times \left(v_{\mathrm{dc}}\times \frac{s}{1+\tau s}\right)$ where C.sub.eq=6C.sub.tot and C.sub.tot is the total capacitance in an arm of the modelled capacitor, v.sub.dc is the voltage at the point of connection of the converter to the direct electric power supply network and r is a time constant.

9. The converter according to claim 7, wherein the internal current setpoint I*.sub.W is utilised to determine a current setpoint I*.sub.dc to be transmitted to the direct electric power supply network.

10. The converter according to claim 1, wherein the energy management circuit receives at input the result of comparison between a voltage setpoint at the terminals of each modelled capacitor, squared, and an average of the square of the voltages at the terminals of the modelled capacitors.

11. The converter according to claim 1, wherein the control circuit is configured to make a change in variable to control intermediate variables of current i.sub.diff and i.sub.gd and voltage v.sub.diff and v.sub.gd, where i.sub.diff and v.sub.diff are related to the direct electric power supply network and i.sub.gd and V.sub.gd are related to the alternating electric power supply network.

12. The converter according to claim 11, wherein the control circuit comprises a regulator of the current i.sub.gd having at input a setpoint i*.sub.gd corresponding to the current i.sub.gd.

13. The converter according to claim 11, wherein the control circuit comprises a regulator of the current i.sub.diff having at input a setpoint i*.sub.diff corresponding to the current i.sub.diff.

14. The converter according to claim 1, wherein the control circuit comprises a regulator of the voltage at the point of connection of the converter to the direct electric power supply network, the regulator configured to determine a power setpoint for regulation of the direct voltage of said converter as a function of a voltage setpoint at the point of connection of the converter to the direct electric power supply network and of a voltage value at the point of connection of the converter to the direct electric power supply network collected on said direct electric power supply network.

15. A control process of a multilevel modular voltage converter, the converter converting alternating voltage into direct voltage and inversely, and comprising a direct part intended to be connected to a direct electric power supply network and an alternating part intended to be connected to an alternating electric power network, the converter comprising a plurality of legs, each leg comprising an upper arm and a lower arm, each arm comprising a plurality of sub-modules controllable individually by a control member of the sub-module and comprising a capacitor connected in series in the arm in a controlled state of the control member of the sub-module, each arm capable of being modelled by a modelled voltage source connected to a duty cycle dependent on a number of capacitors placed in series in the arm, the modelled voltage source being connected in parallel to a modelled capacitor corresponding to a total capacitance of the arm, the process comprising: calculating an internal command setpoint of the converter by application of a function having an adjustable input parameter, determining an operating power setpoint as a function of the voltage at the terminals of each modelled capacitor; determining a power setpoint to be transmitted to the alternating electric power supply network from the operating power setpoint; and regulating the voltage at the point of connection of the converter to the direct electric power supply network and of the voltage at the terminals of each modelled capacitor as a function of said internal command setpoint and of said power setpoint to be transmitted to the alternating electric power supply network.

16. A control process of a converter according to claim 15, wherein the adjustable input parameter is an adjustable virtual inertia coefficient k.sub.VC.

17. A control circuit for controlling a multi-level modular converter for converting alternating voltage into direct voltage and inversely, the converter comprising: a direct part intended to be connected to a direct electric power supply network; an alternating part intended to be connected to an alternating electric power network; a plurality of legs, each leg comprising an upper arm and a lower arm, each arm comprising a plurality of sub-modules controllable individually by a control member specific to each sub-module and each sub-module comprising a capacitor connectable in series in the arm when the control member of the sub-module is in a controlled state, each arm modelled by a modelled voltage source connected to a duty cycle dependent on a number of capacitors placed in series in the arm, the modelled voltage source connected in parallel to a modelled capacitor corresponding to a total capacitance of the arm, wherein the control circuit comprises a computer of an internal command setpoint of the converter by application of a function having an adjustable input parameter, the control circuit further comprising an energy management circuit configured to deliver an operating power setpoint as a function of the voltage at the terminals of each modelled capacitor, the operating power setpoint being utilised to determine a power setpoint to be transmitted to the alternating electric power supply network, the control circuit being configured to regulate the voltage at the point of connection of the converter to the direct electric power supply network and the voltage at the terminals of each modelled capacitor as a function of the internal command setpoint and of the power setpoint to be transmitted to the alternating electric power supply network.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The invention will be more clearly understood from the following description of embodiments of the invention given by way of non-limiting examples in reference to the appended drawings, in which:

(2) FIG. 1, already described, illustrates a three-phase multi-level modular converter according to the prior art;

(3) FIG. 2, already described, illustrates a sub-module of a multi-level modular converter according to the prior art;

(4) FIG. 3, already described, illustrates a circuit equivalent to an arm of an MMC converter according to the prior art;

(5) FIG. 4, already described, shows an equivalent configuration of a multi level modular converter according to the prior art;

(6) FIG. 5 illustrates an equivalent and schematic representation of a multi level modular converter according to the invention;

(7) FIG. 6 illustrates a first embodiment of a multi-level modular converter provided with a control circuit according to the invention;

(8) FIG. 7 illustrates a computer of the converter of FIG. 6;

(9) FIG. 8 illustrates the evolution of the power of direct and alternating electric supply networks in response to disruption, for a converter of the prior art;

(10) FIG. 9 illustrates the evolution of the power of direct and alternating electric supply networks in response to a disruption, for a converter according to the invention;

(11) FIG. 10 illustrates the evolution of the internal energy in response to said disruption, for a converter of the prior art;

(12) FIG. 11 illustrates the evolution of the internal energy in response to said disruption, for a converter according to the invention;

(13) FIG. 12 illustrates a second embodiment of a multi-level modular converter provided with a control circuit according to the invention; and

(14) FIG. 13 illustrates a computer of the converter of FIG. 12.

DETAILED DESCRIPTION

(15) The invention relates to a multi-level modular converter provided with a control circuit, a circuit of equivalent behaviour of which is illustrated in FIG. 5. In a non-limiting way this figure illustrates an MMC converter 10 of direct power into alternating power. In this example, it is evident that this converter 10 comprises an alternating part 10A, connected to an alternating electric power network 110, in the left part of the diagram. The right part of the diagram shows that the converter 10 comprises a direct part 10C connected to a direct electric power supply network 120.

(16) It can be seen that a virtual capacitor C.sub.VI having adjustable capacitance (loosely put and for reasons of simplicity, the same notation will be used to designate the capacitor and its capacitance) is connected in parallel to the direct electric power supply network 120. Virtual means that this capacitor is not physically implanted in the converter 10, which comprises capacitors of sub-modules only. On the contrary, the control circuit according to the invention achieves converter operation similar to that of a converter equipped with this virtual capacitor: regulating a virtual inertia coefficient k.sub.VC, which does not appear in FIG. 5, and which is an adjustable parameter, improves the stability of the direct electric power supply network 120 and the behaviour of the converter is similar to that of a converter wherein a virtual capacitor C.sub.VI of adjustable capacitance is placed in parallel with the direct electric power supply network 120.

(17) The diagram of FIG. 5 also illustrates transfers of powers between the converter 10 and the direct and alternating electric supply networks 120 and 110. In this way, P.sub.l is the power coming from other stations of the direct electric power supply network and symbolizes sudden disruption in power on the direct network, P.sub.dc is the power extracted from the direct electric power supply network 120, P.sub.ac is the power transmitted to the alternating electric power supply network 110, P.sub.C is the power absorbed by the capacitance C.sub.dc of the direct electric power supply network 120 and P.sub.W can be considered as the power absorbed by the virtual capacitor C.sub.VI. Also, v.sub.dc is the voltage at the point of connection of the converter to the direct electric power supply network. i.sub.g is the current of the alternating electric power network and i.sub.dc is the current of the direct electric power supply network.

(18) In the converter MMC 10 according to the invention, and in contrast to a converter MMC of the prior art, a power surplus of the direct electric power supply network 120, noted P.sub.W, is absorbed by the virtual capacitor C.sub.VI and allows the converter to store internal energy W.sub.Σ in the capacitors of the sub-modules.

(19) The example of FIG. 6 illustrates a first embodiment of a multi-level modular converter 10 provided with a control circuit 20 according to the invention. In this example, the converter is controlled in terms of power. By linking in closed loop, the converter MMC 10 is configured to regulate the voltage v.sub.dc at the point of connection of the converter to the direct electric power supply network 120 and the voltage v.sub.cΣ at the terminals of each modelled capacitor.

(20) The control circuit 20 comprises a computer 22 configured to calculate an internal power setpoint P*.sub.W for the capacitors of the sub-modules of the arms. This internal power setpoint P*.sub.W is calculated from an adjustable virtual inertia coefficient k.sub.VC, at input of the computer 22, and from a nominal value of the voltage V.sub.dc at the point of connection of the converter to the direct electric power supply network 120, squared.

(21) An example of a computer 22 of a power setpoint P*.sub.W is shown in FIG. 7. This figure shows that said internal power setpoint P*.sub.W is determined according to the formula:

(22) $P_W^{*}=\frac{1}{2}{C}_{\mathrm{eq}}{k}_{\mathrm{VC}}\times \left(v_{\mathrm{dc}}^2\times \frac{s}{1+\tau s}\right)$
where C.sub.eq=6C.sub.tot and C.sub.tot is the total capacitance in an arm of the modelled capacitor, v.sub.dc is the voltage at the point of connection of the converter to the direct electric power supply network and τ is a time constant. The s au numerator represents the derived function and the filtering function consists of:

(23) $\frac{1}{1+\tau s}.$

(24) In particular, the control circuit 20 according to the invention dispenses with an intermediate step for determining a setpoint of internal energy executed in the prior art.

(25) Said internal power setpoint P*.sub.W is utilised to determine a power setpoint P*.sub.dc to be transmitted to the direct electric power supply network. It is understood that the computer 22 contributes to regulation of the internal power, and therefore of the internal energy of the converter 10 by occurring on the direct part 10C of said converter. An advantage is que in case of disruption on the alternating electric power network 110 or in the alternating part 10A of the converter, the computer 22 always regulates the voltage v.sub.dc at the point of connection of the converter to the direct electric power supply network and the voltage v.sub.cΣ at the terminals of each modelled capacitor by providing the power setpoint to be transmitted to the direct electric power supply network P*.sub.dc in the direct part of the converter.

(26) Also, the control circuit 20 of the converter 10 also comprises a power management circuit 24 configured to deliver an operating power setpoint P*.sub.f. The power management circuit 24 receives at input a comparison between a voltage setpoint v*.sub.cΣ at the terminals of each modelled capacitor, squared, and an average of the square of the voltages at the terminals of the modelled capacitors, also squared. Without departing from the scope of the invention, the average can be calculated in different ways. In the non-limiting example illustrated in FIG. 6, the average is calculated as being the sum of the squares of the voltages of the modelled capacitors in each arm, divided by six (the converter comprising six arms).

(27) The voltage setpoint at the terminals of each modelled capacitor v*.sub.cΣ is expressed as:

(28) 0$v_{c\Sigma }^{2*}=\frac{2W_{\Sigma }^{*}}{6C_{\mathrm{tot}}}$

(29) Said voltage setpoint v*.sub.cΣ at the terminals of each modelled capacitor is therefore obtained from a setpoint of internal energy W*.sub.Σ of the converter, fixed arbitrarily.

(30) Said operating power setpoint P*.sub.f is utilised to determine a power setpoint P*.sub.ac to be transmitted to the alternating electric power supply network 110. It is understood that the circuit 24 allows management of the internal energy of the converter 10 by occurring on the alternating part 10A of said converter. An advantage is that even in the presence of disruption on the direct electric power supply network 120 or in the direct part 10C of the converter 10, the power management circuit 24 effectively regulates the voltage v.sub.dc at the point of connection of the converter to the direct electric power supply network 120 and the voltage v.sub.cΣ at the terminals of each modelled capacitor by providing the power setpoint to be transmitted to the alternating electric power supply network P*.sub.ac in the alternating part of the converter 10.

(31) FIG. 6 also shows that the control circuit 20 comprises a voltage regulator 26 at the point of connection of the converter to the direct electric power supply network 120, having at input the result of comparison between a voltage setpoint v*.sub.dc at the point of connection of the converter 10 to the direct electric power supply network 120, squared, and a value v.sub.dc collected on the direct electric power supply network, also squared. The voltage regulator 26 at the point of connection of the converter to the direct electric power supply network 120 delivers a power setpoint P*.sub.m for regulation of the direct voltage of said converter 10. Said power setpoint P*.sub.m for regulation of the direct voltage of said converter is then compared to the operating power setpoint P*.sub.f to determine the power setpoint P*.sub.ac to be transmitted to the alternating electric power supply network 110.

(32) Similarly, the internal power setpoint P*.sub.W is compared to the power setpoint P*.sub.m for regulation of the direct voltage of said converter to determine the power setpoint P*.sub.dc to be transmitted to the direct electric power supply network.

(33) Also, the control circuit 20 comprises a regulator 28 of the current alternating i.sub.gd having at input a setpoint i*.sub.gd, and a regulator 30 of the current i.sub.diff having at input a setpoint i*.sub.diff.

(34) According to FIG. 3, it is known that it is possible to model the sub-modules of an arm by a modelled voltage source connected in parallel to a modelled capacitor such that the sources of modelled voltages at their terminals a voltage v.sub.mxi (with x indicating whether the arm is upper or lower and i indicating the legs). The current regulators 28 and 30 deliver voltage setpoints v*.sub.diff and v*.sub.v used following a change in variable, by a modulation member 32 and two equilibrium members 34a and 34b by means of a control algorithm (“BCA: Balancing Control Algorithm”), for regulating voltages v.sub.mxi at the terminals of the sources modelled voltages. This controls the sub-modules of the arms, or not. The voltage is therefore controlled at the terminals of the modelled capacitors v.sub.cΣxi as well as the voltage at the point of connection of the converter to the direct electric power supply network V.sub.dc.

(35) Having the virtual inertia coefficient k.sub.VC vary at input of the computer can therefore directly influence the voltage of the direct electric power supply network v.sub.dc and the inertia of this direct electric power supply network.

(36) The diagram of FIG. 6 illustrates control of active powers for control of the converter. In a non-limiting way, control of the reactive powers can be provided, in parallel with control of active powers, independently of the effect of “virtual capacitor”.

(37) FIGS. 8 to 11 illustrate the results of simulation of the behaviour of a multi-level modular converter 10 provided with a control circuit 20 according to the invention and in particular simulation by control of power. In this simulation, a test system has been created wherein the direct part of the converter is connected to an ideal source of direct power, simulating a direct electric power supply network 120, while the alternating part of the converter is connected to a source of alternating power, simulating an alternating electric power network 110. A power echelon is imposed on the simulated direct network, simulating disruption on said direct electric power supply network.

(38) FIG. 8 shows the evolution of the power P.sub.ac of the alternating electric power network in dotted lines and, in solid lines, shows the evolution of the power P.sub.dc of the direct electric power supply network in response to the imposed disruption, for a converter of the prior art. This evolution of the power P.sub.dc of the direct electric power supply network reflects the effect of “virtual capacitance”, the converter having a behaviour equivalent to that of a virtual capacitor arranged in parallel with the direct electric power supply network. FIG. 9 illustrates the same magnitudes for a converter according to the invention.

(39) FIGS. 8 and 9 disclose that in the presence of disruption on the direct electric power supply network, the evolution of the power P.sub.dc of the direct electric power supply network is identical for the converter of the prior art and for the converter according to the invention. The converter according to the invention therefore produces a “virtual capacitance” effect and is understood as a virtual capacitor arranged in parallel to the direct electric power supply network.

(40) FIG. 10 illustrates the evolution of the internal energy stored in the capacitors of the sub-modules of a converter of the prior art, in response to imposed disruption.

(41) FIG. 11 illustrates the evolution of the internal energy stored in the capacitors of the sub-modules of a converter according to the invention, in response to imposed disruption.

(42) It is evident, because of the converter according to the invention, that the energy is best regulated and that it does not increase suddenly and abruptly, as in the prior art. In particular, because of the invention, the internal energy of the converter tends more rapidly towards its nominal value. The internal energy of the converter is therefore best controlled because of the control circuit according to the invention, and especially because of the energy management circuit. In fact, the latter occurs in the alternating part of the converter and effectively controls the internal energy of the converter despite disruption on the direct electric power supply network.

(43) FIG. 12 illustrates a second embodiment of a converter 10′ according to the invention, provided with a control circuit 20′ according to the invention. In this example, the converter is controlled in terms of current. As in the example of FIG. 6, the control circuit comprises a power management circuit 24′ configured to deliver an operating power setpoint P*.sub.f. It also comprises a regulator 28′ of the alternating current i.sub.gd, a modulation member 32′ and two equilibrium members 34a′ and 34b′.

(44) In this embodiment, the control circuit 20′ comprises a computer 22′ configured to calculate an internal current setpoint I*.sub.W for the capacitors of the sub modules of the arms.

(45) Such a computer is illustrated in FIG. 13. As is evident from this figure, the internal current setpoint I*.sub.W is calculated from an adjustable virtual inertia coefficient k.sub.VC, at input of the computer 22′, and a nominal value of the voltage v.sub.dc at the point of connection of the converter to the direct electric power supply network 120. This computer 22′ also executes a derived function and a filter of the first order.

(46) The control circuit 20′ further comprises a regulator 26′ of the voltage at the point of connection of the converter to the direct electric power supply network 120, receiving at input the result of comparison between a voltage setpoint v*.sub.dc at the point of connection of the converter 10 to the direct electric power supply network 120 and a value v.sub.dc collected on the direct electric power supply network. The regulator 26′ delivers a power setpoint P*.sub.m for regulating the direct voltage of said converter 10.

(47) The control circuit 20′ additionally comprises a divider circuit 36 for dividing said power P*.sub.m by a nominal value of the voltage v.sub.dc at the point of connection of the converter to the direct electric power supply network 120, so as to determine a current operating setpoint I*.sub.m. Said current operating setpoint I*.sub.m is then compared to the internal current setpoint I*.sub.W to determine a current setpoint I*.sub.dc to be transmitted to the direct electric power supply network.