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

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.

BACKGROUND

The present invention relates to the technical field of transport installations of multi-terminal high-voltage direct current (HVDC) in which stations integrate multi-level modular converters (MMC).

FIG. 1schematically illustrates a set12of sub-modules of a multi-level modular converter10according to the prior art. For a three-phase input/output current (comprising three phases φa, φband φc), this converter10comprises three conversion legs which are referenced by the indices a, b and c on the different components ofFIG. 1.

Each conversion leg comprises an upper arm and a lower arm (indicated by the indices “u” for upper and “l” for lower), each of which connects a terminal DC+ or DC− of the direct electric power supply network (DC) to a terminal of the alternating electric power network (AC). In particular, each of the legs is connected to one of the three phase lines φa, φband φcof the alternating electric power network.FIG. 1illustrates a set of 12 sub-modules, wherein a current ixipasses through each arm with (x indicating whether the arm is upper or lower and the index i indicating the legs). Also, each arm comprises a plurality of sub-modules SMxijwhich can be controlled according to a preferred sequence (with x indicating whether the arm is upper or lower, i indicating the phase line to which the arm is connected, and j being the number of the sub-module among the sub-modules in series in the arms). Here, only three sub-modules have been illustrated by arms. In practice, each lower or upper arm can comprise a number N of sub-modules, ranging from a few tens to a few hundreds.

Each sub-module SMxijcomprises a power storage system such as at least a capacitor and a control member for selectively connecting this capacitor in series between the terminals of the sub-module or to bypass them. The sub-modules are controlled according to a selected sequence to have the number of power storage elements vary progressively which are connected in series in an arm of the converter10so as to supply several levels of voltage. Also, inFIG. 1, Vdcdesignates the voltage at the point of connection of the converter to the direct electric power supply network, idcdesignates the current of the direct electric power supply network, while currents iga, ighand igcpass through the three phase lines φa, φband φc. Also, each arm has an inductance Larmand each phase line comprises an inductance Lfand a resistance Rf.

FIG. 2illustrates a sub-module SMxijbelonging to the converter10ofFIG. 1. This sub-module SMxijhas voltage vSMat its terminals. In this sub-module, each control member comprises a first electronic switching element T1such as an insulated gate bipolar transistor (IGBT) connected in series to an electric power storage element, here a capacitor CSM. This first switching element T1and this capacitor CSMare mounted parallel to a second electronic switching element T2, also an insulated gate bipolar transistor (IGBT). This second electronic switching element T2is coupled between the input and output terminals of the sub-module SMxij. The first and second switching elements T1and T2are both connected to an antiparallel diode shown inFIG. 2.

When operating, the sub-module can be controlled in two control states.

In a first state, a so-called “on” or controlled state, the first switching element T1and the second switching element T2are configured so as to connect the power storage element CSMin series with the other sub-modules. In a second state, a so-called “off” or non-controlled state, the first switching element T1and the second switching element T2are configured so as to short-circuit the power storage element CSM.

It is known that each arm, having a voltage vmat its terminals, can be modelled by a modelled voltage source, having a voltage vmat its terminals, the duty cycle of which depends on the number of controlled sub-modules, and by a modelled capacitor Ctotconnected to the voltage source. This modelling is illustrated inFIG. 3, showing an arm, with a current i passing through it and the resulting modelling. Ctotis the equivalent capacitance in an arm such that the inverse of this equivalent capacitance of the arm Ctotis equal to the sum of the inverses of the capacities of the sub-modules controlled in this arm, according to:

Accordingly, the voltage vcΣat the terminals of the modelled capacitor Ctotis equal to the sum of the voltages vcjat the terminals of the capacitors of the sub-modules in the arm (with j ranging from 1 to N and indicating the number of the capacitor and therefore of the sub-module). Also, a current impasses through each modelled capacitor Ctot. In the present application Ctotloosely designates both the modelled capacitor and the value of its capacitance. By controlling the control sequence of the sub-modules, to have the number of power storage elements connected in series vary progressively, the energy of the modelled capacitor Ctotand therefore the voltage at the terminals of each modelled voltage source can be lowered or raised.

The prior art therefore discloses an equivalent configuration of the set of the sub-modules of the converter MMC10illustrated inFIG. 4. In this figure, the converter is a converter similar to that described in reference toFIG. 1, and wherein each arm has been replaced by its modelling. Also, each phase line of the alternating electric power network is connected to a current igiand a voltage vgi(the index i indicating the number of the legs).

Here, each of the modelled sources of voltage comprises at its terminals a voltage vmxi, and a current imxipasses through each modelled capacitor Ctot, and comprises at its terminals a voltage VcΣxi(with x indicating whether the arm is upper or lower and i indicating the number of the legs). It can also be seen that it is possible to break down the converter MMC into an imaginary alternating part and an imaginary direct part (at input or output, according to whether the converter is configured to convert alternating energy into direct energy or the inverse), where the evolution of the total energy stored in the capacitors of the sub-modules is equal to the difference between the power entering the converter and the exiting power.

Converters of “Voltage Source Converter” type (familiar to the skilled person under the acronym “VSC”) are known, having a station capacitor connected in parallel of the direct electric power supply network. The disadvantage of such a capacitor in parallel is that it does not allow the converter to be disconnected from the voltage of the direct electric power supply network. Also, this type of converter needs to make use of many filters to obtain suitable converted signals.

Also, the inertia of the direct electric power supply network depends on its capacitance such that a large capacitance increases the inertia of the direct electric power supply network. Therefore, a large capacitance of the network and therefore considerable inertia allows it to best resist any disruptions. Inversely, a low network capacitance, and therefore low inertia, more easily and more precisely regulates the voltage at the point of connection of the converter to the direct electric power supply network.

In contrast to converters of Voltage Source Converter type, MMC converters do not include a station capacitor connected in parallel and which can influence the stability of the direct electric power supply network. Multi-level modular converters therefore have the advantage of offering disconnection between the total voltage of the capacitors of the sub-modules and the voltage of the direct electric power supply network. Yet, a simple variation in power can result in a substantial variation in voltage of the direct electric power supply network.

MMC converters are known the control of which is not based on energy (Non Energy-Based Control). In these converters, when any deviation in voltage appears between the voltage of the capacitors of the arms and the voltage of the direct electric power supply network, the power of the incoming direct electric power supply network varies automatically to correct said deviation in voltage. This control is executed without additional regulator since energy exchanges with the capacitors of the arms follow variations in voltage on the direct electric power supply network.

However, all variables of this type of converter are not controlled, which shows up via a lack of robustness of the converter.

Converters having their control based on energy are also known. Especially known is the document titled “Control of DC bus voltage with a Modular Multilevel Converter” (Samimi et al., PowerTech conference, 2015), which presents a multi-level modular converter comprising a control system of power transfers in the region of the alternating part, power transfers in the region of the direct part and of the internal energy of the converter. This type of converter utilises control based on energy (“Energy-Based Control”) control of the variables in current of direct and alternating electric supply networks controls the powers of these two respective networks. A difference between the powers of direct and alternating electric supply networks causes a decrease or increase in the energy stored in the capacitors of the sub-modules. But this type of converter impairs disconnection between voltages at the terminals of the capacitors of the sub-modules and voltage of the direct electric power supply network. Also, it does not adapt effectively and in real time to fluctuations in voltages on the direct electric power supply network.

These known converters are not sufficiently robust, in particular with respect to contribution to the stability of the direct electric power supply network. These existing solutions do not fully exploit the capacities of MMC converters in terms of control of the internal energy of the converter jointly with control of the stability of the network DC.

Converters such as described in document FR1557501 are also known. The behaviour of this type of multi-level modular converter is equivalent to that of a virtual capacitor placed in parallel with the direct electric power supply network.

Regulating the internal energy of this converter makes it possible to have the capacitance of the virtual capacitor vary virtually. The advantage is to be able to act on the direct electric power supply network, and contribute to its stability, while maintaining disconnection between the total voltage of the capacitors of the sub-modules and the voltage of said network.

The disadvantage of the solution of document FR1557501 is that this type of converter involves many calculation steps using a large number of intermediate variables. Also, regulation of the internal energy proves long and complex to realize and costly in terms of resources. Also, in the presence of disruption on the direct electric power supply network, it becomes particularly difficult, or even impossible, to control the internal energy of such a converter according to the prior art.

SUMMARY

An aim of the present invention is to propose a multi-level modular converter (MMC) provided with a control circuit of the converter which allows easy regulation of the internal energy of the converter. Another aim is to provide a more robust converter for effectively regulating the internal energy of the converter despite the presence of disruption on the direct electric power supply network.

To achieve this, the invention relates to a multi-level modular voltage converter for converting alternating voltage into direct voltage and inversely, comprising a so-called direct part intended to be connected to a direct electric power supply network and a so-called 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 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 which can be modelled by a modelled voltage source connected to a duty cycle dependent on a number of capacitors placed in series in the arm, each modelled voltage source being connected in parallel to a modelled capacitor corresponding to total capacitance of the arms.

The converter further comprises 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.

According to a general characteristic of the converter, the control circuit of the converter 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.

The adjustable input parameter of the computer can be set any time during regulation operations of the internal energy and done easily by the user. The internal command setpoint can be connected to different types of magnitudes. In a non-limiting way the internal command setpoint can be an internal power setpoint or even a current setpoint. The internal command setpoint calculated by the computer depends on the input parameter. Also, it is possible for the user to act directly on the internal command setpoint of the converter and accordingly 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.

The user can further adjust the input parameter as a function of disruptions on the direct electric power supply network to stabilize it.

In a non-limiting way the multi-level modular converter, the control circuit of which is provided with such a computer, behaves the same as that of a virtual capacitor arranged in parallel with the direct electric power supply network. Regulating the adjustable input parameter of the computer has the capacitance of the virtual capacitor vary virtually. The advantage is to be able to act on the direct electric power supply network while maintaining disconnection between the total voltage of the capacitors of the sub-modules and the voltage of the direct electric power supply network.

In contrast to a capacitor placed really in parallel with the direct electric power supply network, the virtual capacitor has no cost and cannot be degraded. In particular, the adjustable virtual capacitor according to the invention can take on very high capacitance values, not materially possible for a real capacitor.

The sub-modules are preferably controlled by means of two insulated gate bipolar transistors (IGBT) for placing the capacitor of said sub-module in the associated arm in series or not according to whether the sub-circuit is to be controlled in the controlled “on” state or in the non-controlled “off” state.

Each arm can be modelled by a modelled source of voltage connected in parallel to a modelled capacitor of capacitance Ctot. The sum of the voltages of the capacitors of the sub-modules of an arm is noted as vcΣ, such that the voltage at the terminals of the associated modelled capacitor in parallel with the modelled voltage source is vcΣ.

The duty cycle α, connected to the modelled voltage source, is preferably calculated as per the expression:

α=nN
where n is the number of sub-modules connected to the “on” state in the associated arm and N is the number of sub-modules in the arm.

Also, because of the invention, the energy management circuit provides a power setpoint to be transmitted to the alternating electric power supply network P*acand therefore links the voltage at the terminals of each modelled capacitor, from this setpoint. Also, this circuit contributes to regulating the internal energy of the converter by occurring on the alternating part of said converter. An advantage of the energy management circuit is to dispense with disruption on the direct electric power supply network or in the direct part of the converter. In fact, the energy management circuit allows regulation of power in the alternating part of the converter, independently of disruptions in the direct part. The robustness of the converter is therefore improved.

Regulating both 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 can further act on the stability of the direct electric power supply network. This contains any disruptions in power appearing suddenly on the direct electric power supply network and which could cause considerable variations in voltage on said network.

By way of advantage, the computer is configured to calculate the internal command setpoint by application of a derived function and a filtering function. An advantage is that application of such a filtering function consumes few calculating resources. Also, filtering dispenses with measuring noises which can damage the converter when being controlled.

The filtering function is preferably a filter of the first order, allowing measuring noises to be filtered out all the more effectively.

Advantageously, the adjustable input parameter is an adjustable virtual inertia coefficient kVC. Also, modifying this parameter kVCvirtually amounts to modifying the capacitance of the virtual capacitor and therefore contributing to the stability of the direct electric power supply network. An advantage is to propose an additional degree of liberty in the control of the internal energy of the converter MMC. The capacitance of the virtual capacitor can especially take on very high values, without additional material restrictions.

According to a first variant, the internal command setpoint is an internal power setpoint P*W. In this configuration the converter is controlled in terms of power. An advantage is that the computer directly provides a power setpoint, which dispenses especially with an intermediate calculation step of a setpoint of internal energy of the converter, as is the case in the documents of the prior art. Determining this internal power setpoint is therefore easy, as is regulating the internal energy.

In a particularly advantageous way, the computer is configured to calculate the internal power setpoint P*Wof the converter according to the function:

PW*=12⁢Ceq⁢kVC×(vdc2×s1+τ⁢⁢s)
where Ceq=6Ctotand Ctotis the total capacitance in an arm of the modelled capacitor, vdcis 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 at the numerator represents the derived function and the filtering function consists of:

It is understood that the capacitance CVCof the virtual capacitor is expressed as:
CVC=6CtotkVC

The internal power setpoint P*Wis preferably utilised to determine a power setpoint P*dcto be transmitted to the direct electric power supply network. Via determination of this power, noted P*dc, it is understood that the computer contributes to regulation of the internal power, and therefore of the internal energy of the converter by occurring on the direct part of said converter. An advantage is that in case of disruptions on the alternating electric power network or in the alternating part of the converter, the computer always regulates 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 by supplying the internal power setpoint in the direct part of the converter. As a consequence, the effect of virtual capacitance described earlier stabilizing the direct supply network is retained. The robustness of the converter is therefore improved.

According to a second variant, the internal command setpoint is an internal current setpoint I*W. In this configuration the converter is controlled in terms of current.

By way of advantage, the computer is configured to calculate the internal current setpoint I*Waccording to the function:

IW*=Ceq⁢kVC×(vd⁢⁢c×s1+τ⁢⁢s)
where Ceq=6Ctotand Ctotis the total capacitance in an arm of the modelled capacitor, vdcis the voltage at the point of connection of the converter to the direct electric power supply network and τ is a time constant.

Preferably, the internal current setpoint I*Wis utilised to determine a current setpoint I*dcto be transmitted to the direct electric power supply network. Via determination of this current setpoint I*dc, it is understood that the computer contributes to regulation of the current, and therefore of the internal energy of the converter by occurring on the direct part of said converter.

As a consequence, the effect of virtual capacitance described earlier, for stabilizing the direct supply network, is retained, despite any disruptions on the alternating electric power network or in the alternating part of the converter. The robustness of the converter is therefore improved.

In a particular embodiment, 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. The energy management circuit therefore links the voltage at the terminals of each modelled capacitor, squared, from a setpoint value of this voltage. In particular, the voltage setpoint at the terminals of each modelled capacitor v*cΣis expressed as:

vc⁢⁢Σ2*=2⁢WΣ*6⁢Ctot
where W*Σis a setpoint of internal energy selected arbitrarily.

The control circuit is preferably configured to make a change in variable to control intermediate variables in current idiffand igdand in voltage vdiffand vgd, where idiffand vdiffare connected to the direct electric power supply network and igdand vgdare connected to the alternating electric power supply network.

In a non-limiting way, in the case of a converter of direct energy into alternating energy, these variables express the variation in internal energy of the converter in the form of:

This expression reflects especially the breakdown of the converter MMC into an imaginary direct part at input, connected to the direct network and associated with the term Σi=132idiffivdiffwhich corresponds to the power of the direct part and an imaginary alternating part at output, connected to the alternating network and associated with the term igdvgdwhich corresponds to the power of the alternating part.

Advantageously, the control circuit comprises a regulator of the current igdhaving at input a setpoint i*gdcorresponding to the current igd. The regulator links the current igdby having it tend towards its setpoint i*gd. Regulating the variable igdamounts to regulating the transfers of alternating power at input or at output according to the configuration of the converter.

By way of advantage, the control circuit comprises a regulator of the current idiffhaving at input a setpoint i*diffcorresponding to the current idiff. The regulator links the current idiffby having it tend towards its setpoint i*diff. Regulating the variable idiffamounts to regulating transfers of direct power at input or at output according to the configuration of the converter.

In a non-limiting way, the variables igdand idiffcan be controlled independently. It is understood that regulating idiffand igdregulates transfers of respectively incoming and outgoing powers, and accordingly controls the internal energy of the converter stored in the capacitors of the sub-modules.

Preferably, the control circuit comprises a voltage regulator at the point of connection of the converter to the direct electric power supply network configured to determine a power setpoint for the 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. An advantage of this regulator is that it can link the voltage at the point of connection of the converter to the direct electric power supply network vdcby having its value tend towards the voltage setpoint at the point of connection of the converter to the direct electric power supply network v*dc.

The invention also relates to a control process of a multi-level modular voltage converter, the converter converting alternating voltage into direct voltage and inversely, and comprising a so-called direct part intended to be connected to a direct electric power supply network and a so-called 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 which can be modelled by a modelled voltage source connected to a duty cycle dependent on a number of capacitors placed in series in the arm, each modelled voltage source being connected in parallel to a modelled capacitor corresponding to a total capacitance of the arm, the process further comprising calculation of an internal power setpoint of the converter by application of a function having an adjustable input parameter, the process comprising:a step for determining an operating power setpoint as a function of the voltage at the terminals of each modelled capacitor;a step for determining a power setpoint to be transmitted to the alternating electric power supply network from the operating power setpoint; anda step for 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 power setpoint and of said power setpoint to be transmitted to the alternating electric power supply network.

Advantageously, the adjustable input parameter is an adjustable virtual inertia coefficient kVC.

The invention also relates to a control circuit for a multi-level modular converter such as defined hereinabove and comprising 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.

DETAILED DESCRIPTION

The invention relates to a multi-level modular converter provided with a control circuit, a circuit of equivalent behaviour of which is illustrated inFIG. 5. In a non-limiting way this figure illustrates an MMC converter10of direct power into alternating power. In this example, it is evident that this converter10comprises an alternating part10A, connected to an alternating electric power network110, in the left part of the diagram. The right part of the diagram shows that the converter10comprises a direct part10C connected to a direct electric power supply network120.

It can be seen that a virtual capacitor CVIhaving 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 network120. Virtual means that this capacitor is not physically implanted in the converter10, 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 kVC, which does not appear inFIG. 5, and which is an adjustable parameter, improves the stability of the direct electric power supply network120and the behaviour of the converter is similar to that of a converter wherein a virtual capacitor CVIof adjustable capacitance is placed in parallel with the direct electric power supply network120.

The diagram ofFIG. 5also illustrates transfers of powers between the converter10and the direct and alternating electric supply networks120and110. In this way, Plis the power coming from other stations of the direct electric power supply network and symbolizes sudden disruption in power on the direct network, Pdcis the power extracted from the direct electric power supply network120, Pacis the power transmitted to the alternating electric power supply network110, PCis the power absorbed by the capacitance Cdcof the direct electric power supply network120and PWcan be considered as the power absorbed by the virtual capacitor CVI. Also, vdcis the voltage at the point of connection of the converter to the direct electric power supply network. igis the current of the alternating electric power network and idcis the current of the direct electric power supply network.

In the converter MMC10according to the invention, and in contrast to a converter MMC of the prior art, a power surplus of the direct electric power supply network120, noted PW, is absorbed by the virtual capacitor CVIand allows the converter to store internal energy WΣin the capacitors of the sub-modules.

The example ofFIG. 6illustrates a first embodiment of a multi-level modular converter10provided with a control circuit20according to the invention. In this example, the converter is controlled in terms of power. By linking in closed loop, the converter MMC10is configured to regulate the voltage vdcat the point of connection of the converter to the direct electric power supply network120and the voltage vcΣat the terminals of each modelled capacitor.

The control circuit20comprises a computer22configured to calculate an internal power setpoint P*Wfor the capacitors of the sub-modules of the arms. This internal power setpoint P*Wis calculated from an adjustable virtual inertia coefficient kVC, at input of the computer22, and from a nominal value of the voltage Vdcat the point of connection of the converter to the direct electric power supply network120, squared.

An example of a computer22of a power setpoint P*Wis shown inFIG. 7. This figure shows that said internal power setpoint P*Wis determined according to the formula:

PW*=12⁢Ceq⁢kVC×(vdc2×s1+τ⁢⁢s)
where Ceq=6Ctotand Ctotis the total capacitance in an arm of the modelled capacitor, vdcis 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:

In particular, the control circuit20according to the invention dispenses with an intermediate step for determining a setpoint of internal energy executed in the prior art.

Said internal power setpoint P*Wis utilised to determine a power setpoint P*dcto be transmitted to the direct electric power supply network. It is understood that the computer22contributes to regulation of the internal power, and therefore of the internal energy of the converter10by occurring on the direct part10C of said converter. An advantage is que in case of disruption on the alternating electric power network110or in the alternating part10A of the converter, the computer22always regulates the voltage vdcat the point of connection of the converter to the direct electric power supply network and the voltage vcΣat the terminals of each modelled capacitor by providing the power setpoint to be transmitted to the direct electric power supply network P*dcin the direct part of the converter.

Also, the control circuit20of the converter10also comprises a power management circuit24configured to deliver an operating power setpoint P*f. The power management circuit24receives at input a comparison between a voltage setpoint v*cΣat 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 inFIG. 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).

The voltage setpoint at the terminals of each modelled capacitor v*cΣis expressed as:

Said voltage setpoint v*cΣat the terminals of each modelled capacitor is therefore obtained from a setpoint of internal energy W*Σof the converter, fixed arbitrarily.

Said operating power setpoint P*fis utilised to determine a power setpoint P*acto be transmitted to the alternating electric power supply network110. It is understood that the circuit24allows management of the internal energy of the converter10by occurring on the alternating part10A of said converter. An advantage is that even in the presence of disruption on the direct electric power supply network120or in the direct part10C of the converter10, the power management circuit24effectively regulates the voltage vdcat the point of connection of the converter to the direct electric power supply network120and the voltage vcΣat the terminals of each modelled capacitor by providing the power setpoint to be transmitted to the alternating electric power supply network P*acin the alternating part of the converter10.

FIG. 6also shows that the control circuit20comprises a voltage regulator26at the point of connection of the converter to the direct electric power supply network120, having at input the result of comparison between a voltage setpoint v*dcat the point of connection of the converter10to the direct electric power supply network120, squared, and a value vdccollected on the direct electric power supply network, also squared. The voltage regulator26at the point of connection of the converter to the direct electric power supply network120delivers a power setpoint P*mfor regulation of the direct voltage of said converter10. Said power setpoint P*mfor regulation of the direct voltage of said converter is then compared to the operating power setpoint P*fto determine the power setpoint P*acto be transmitted to the alternating electric power supply network110.

Similarly, the internal power setpoint P*Wis compared to the power setpoint P*mfor regulation of the direct voltage of said converter to determine the power setpoint P*dcto be transmitted to the direct electric power supply network.

Also, the control circuit20comprises a regulator28of the current alternating igdhaving at input a setpoint i*gd, and a regulator30of the current idiffhaving at input a setpoint i*diff.

According toFIG. 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 vmxi(with x indicating whether the arm is upper or lower and i indicating the legs). The current regulators28and30deliver voltage setpoints v*diffand v*vused following a change in variable, by a modulation member32and two equilibrium members34aand34bby means of a control algorithm (“BCA: Balancing Control Algorithm”), for regulating voltages vmxiat 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 vcΣxias well as the voltage at the point of connection of the converter to the direct electric power supply network Vdc.

Having the virtual inertia coefficient kVCvary at input of the computer can therefore directly influence the voltage of the direct electric power supply network vdcand the inertia of this direct electric power supply network.

The diagram ofFIG. 6illustrates 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”.

FIGS. 8 to 11illustrate the results of simulation of the behaviour of a multi-level modular converter10provided with a control circuit20according 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 network120, while the alternating part of the converter is connected to a source of alternating power, simulating an alternating electric power network110. A power echelon is imposed on the simulated direct network, simulating disruption on said direct electric power supply network.

FIG. 8shows the evolution of the power Pacof the alternating electric power network in dotted lines and, in solid lines, shows the evolution of the power Pdcof the direct electric power supply network in response to the imposed disruption, for a converter of the prior art. This evolution of the power Pdcof 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. 9illustrates the same magnitudes for a converter according to the invention.

FIGS. 8 and 9disclose that in the presence of disruption on the direct electric power supply network, the evolution of the power Pdcof 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.

FIG. 10illustrates 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.

FIG. 11illustrates 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.

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.

FIG. 12illustrates a second embodiment of a converter10′ according to the invention, provided with a control circuit20′ according to the invention. In this example, the converter is controlled in terms of current. As in the example ofFIG. 6, the control circuit comprises a power management circuit24′ configured to deliver an operating power setpoint P*f. It also comprises a regulator28′ of the alternating current igd, a modulation member32′ and two equilibrium members34a′ and34b′.

In this embodiment, the control circuit20′ comprises a computer22′ configured to calculate an internal current setpoint I*Wfor the capacitors of the sub modules of the arms.

Such a computer is illustrated inFIG. 13. As is evident from this figure, the internal current setpoint I*Wis calculated from an adjustable virtual inertia coefficient kVC, at input of the computer22′, and a nominal value of the voltage vdcat the point of connection of the converter to the direct electric power supply network120. This computer22′ also executes a derived function and a filter of the first order.

The control circuit20′ further comprises a regulator26′ of the voltage at the point of connection of the converter to the direct electric power supply network120, receiving at input the result of comparison between a voltage setpoint v*dcat the point of connection of the converter10to the direct electric power supply network120and a value vdccollected on the direct electric power supply network. The regulator26′ delivers a power setpoint P*mfor regulating the direct voltage of said converter10.

The control circuit20′ additionally comprises a divider circuit36for dividing said power P*mby a nominal value of the voltage vdcat the point of connection of the converter to the direct electric power supply network120, so as to determine a current operating setpoint I*m. Said current operating setpoint I*mis then compared to the internal current setpoint I*Wto determine a current setpoint I*dcto be transmitted to the direct electric power supply network.