WO2023089248A1

WO2023089248A1 - Method for overvoltage protection in a system comprising a battery, an inverter and an electrical machine

Info

Publication number: WO2023089248A1
Application number: PCT/FR2022/000104
Authority: WO
Inventors: Gang Yang; Louis Marie SAUBLET
Original assignee: Nidec Psa Emotors
Priority date: 2021-11-17
Filing date: 2022-11-14
Publication date: 2023-05-25
Also published as: FR3129261B1; EP4434153A1; FR3129261A1

Abstract

The invention relates to a method for overvoltage protection in a system comprising a battery linked to an inverter by a high-voltage bus and an electrical machine controlled by the inverter. While the system is in a regeneration mode in which the electrical machine operates as a generator to recharge the battery, the method comprises: determining the voltage on the high-voltage bus; comparing the determined voltage with a voltage threshold (Uth), and switching the inverter to a protection mode suitable for lowering the voltage (S6) when the determined voltage exceeds said voltage threshold. In the invention, the voltage threshold is variable depending on a current state of charge of the battery. The invention also relates to a corresponding system suitable for carrying out this method.

Description

Method for protecting against an overvoltage in a system comprising a battery, an inverter and an electric machine

The present invention claims the priority of French application 2112169 filed on November 17, 2021, the content of which (text, drawings and claims) is incorporated herein by reference.

The invention relates to the field of the operational safety of an electrical system comprising an inverter and an electrical machine, and relates more particularly to protection against overvoltages.

The present invention thus relates to the protection of such an electrical system, by an appropriate method allowing suitable control of the inverter used to control the electrical machine, namely a rotating electrical machine, in the event of an overvoltage. The invention finds a preferential application in traction systems for electric or hybrid motor vehicles.

The inverter is a device that generates alternating current from direct current from an electrical source such as a battery.

Inverters comprise a power stage comprising, for example, power modules, and more generally the power electronics of the inverter. The power stage includes a set of electronic switches.

Several electronic switch technologies can be used in an inverter used in an electric traction system of a vehicle, including insulated gate bipolar transistors, also called “IGBTs”, from the English “Insulated Gate Bipolar Transistor”, and insulated gate field effect transistors also called "MOSFET", acronym for "Metal Oxide Semiconductor Field Effect Transistor", which can be translated as "field effect transistor with metal-oxide-semiconductor structure", and in especially Silicon Carbide (SiC) power transistors.

Through a set of appropriately controlled switchings (usually pulse-width modulation), the inverter modulates the source to obtain alternating current of the desired frequency to drive an electrical machine. In a mode of operation called regeneration, the electrical machine controlled by the inverter is used as a generator and allows the battery to be recharged.

However, during the regeneration phases, disconnection of one or more electrical loads (i.e. electrical consumers) and/or the battery can cause an overvoltage in the system. This phenomenon of disconnection of a load is generally designated by those skilled in the art by the English expression "load dump". We can imperfectly translate the expression “load dump” by “chute de charges” or “loss of load”, but this expression is also sometimes translated into French by its consequence, namely by the term “surtension”.

An overvoltage caused by a load dump phenomenon can have harmful consequences on the components of the inverter, in particular on the electrical capacities it contains (capacitors), and/or on other components of the system. In particular, a disconnection of the battery, for whatever reason, while it is being recharged by the electrical machine, corresponds to the disconnection of the main electrical load of the system, and can cause a significant overvoltage in the inverter.

In order to protect the components of the system, it is known, in the event of overvoltage, to apply to the inverter a safety mode called "ASC" according to the English acronym of "Active Short Circuit" meaning "active short circuit". , associated with an active discharge. The application of this mode requires the control of electronic switches (for example IGBTs).

In this UPS mode, certain electronic switches of the power stage of the inverter, namely those located on the same side of the H-bridges formed in the inverter, are closed so that they allow the passage of current. The poles of the electric machine (typically three in number for a three-phase motor) are then in a short-circuit situation. The ASC mode can be coupled with a means called active discharge, allowing current dissipation and voltage drop in the system. The application of ASC mode in particular avoids the passage of current in the capacitors of the inverter, and causes a drop in voltage in the system, which protects the electronic components of the system, and in particular the capacitors of the inverter.

In practice, the voltage in the system is observed. The electrical system comprising a high voltage part and a low voltage part (generally 12V for a motor vehicle), the voltage observed is obviously in the high part voltage, for example at the input of the inverter, at the level of a high voltage direct current bus (“HVDC” bus) connecting the battery to the inverter.

When the voltage exceeds a predefined threshold, the occurrence of a load dump situation is very likely and the ASC mode is applied to the inverter, with the prior application of a so-called FW mode for "free-wheeling", which means "freewheel", for reasons detailed below with reference to the appended figures.

The predefined voltage threshold does not correspond to the voltage limit admissible by the components, it is necessarily lower for the following reasons. First of all, the predefined voltage threshold must take into account the existence of ripples in the voltage of the current flowing in the high voltage part of the system. The predefined voltage threshold must also take into account the tolerance in the voltage measurement. In addition, controlling the inverter, in particular its transition to UPS mode with active discharge, requires some time. This delay, even if it is only from a few microseconds to a few tens of microseconds, must be taken into consideration because the rise in voltage linked to a load dump phenomenon is extremely rapid. Thus, the predefined threshold must be chosen so that, whatever the initial situation, in the event of the occurrence of a load dump phenomenon, the voltage at the time of the effective passage to ASC mode remains lower than the maximum voltage admissible by the components. of the system exposed to this voltage.

This solution, although having the merit of offering effective protection of the components, nevertheless has drawbacks. On the one hand, the system can estimate, due to the voltage ripples, that a load dump phenomenon is occurring when this is not the case, leading to an unnecessary transition to safety mode. On the other hand, the applicant has found that such a solution imposes constraints in the design and in the use of the system, in particular in terms of selection of components and/or limitation of the battery regeneration power. These constraints are explained in more detail below with reference to the appended FIG.

The solution known in the prior art can thus be optimized with a view to solving all or part of the problems set out above.

Thus, the invention relates to a method of protection against an overvoltage in a system comprising a battery, an inverter and an electrical machine controlled by the inverter. The battery is linked to the inverter by a high voltage bus, and the system operates, for the application of the process, in regeneration mode in which the electrical machine operates as a generator to recharge the battery.

The process includes: determining the voltage on the high voltage bus;

- comparing the determined voltage to a voltage threshold, and

- switching the inverter to a protection mode suitable for lowering the voltage when the determined voltage exceeds said voltage threshold. According to the invention, the voltage threshold is variable as a function of a current state of charge of the battery.

The use of a variable voltage threshold makes it possible to optimize the protection against overvoltages of such a system. In particular, it makes it possible to adapt the protection method to allow greater voltage ripples than with a known protection method, or to use components that are less reactive or quick to switch to a protection mode.

The protection mode can be an ASC mode with active discharge

The state of charge can be represented by the current battery voltage (Vbat), determined by the average of the voltage on the high voltage bus, over a predefined sliding period.

According to one embodiment, several voltage threshold levels are predefined, and the voltage threshold is chosen between the different voltage threshold levels.

The predefined voltage threshold levels can for example be two in number, namely a high threshold and a low threshold, the voltage of the high threshold being greater than the voltage of the low threshold. The low threshold is then applied when the current state of charge of the battery is less than or equal to a state of charge corresponding to a predefined limit voltage, and the high threshold is applied when the current state of charge of the battery is greater to the state of charge corresponding to the predefined limit.

According to another embodiment, the voltage threshold varies continuously over at least one state-of-charge range of the battery, according to a function of the value of the voltage of the battery in said state-of-charge range.

A maximum voltage ripple value on the high voltage bus can be determined and the function used to vary the voltage threshold can then have said maximum voltage ripple value as a parameter.

The power of the battery charging current can be determined and taken into account to determine the voltage threshold.

The method may further comprise a step of switching the inverter to FW mode immediately prior to the step of switching to ASC mode.

The invention also relates to a system comprising a battery, an inverter and an electric machine controlled by the inverter, the battery being linked to the inverter by a high-voltage bus, and which comprises a computer configured to apply a method of protection against overvoltage as previously described when the system operates in regeneration mode in which the electric machine operates as a generator to recharge the battery. Of course, the system can include:

- Means for determining the voltage on the high voltage bus;

- a voltage comparator, and

- a means of measuring the state of charge of the battery. The system may comprise a means of short-circuiting the high voltage bus when the protection mode is an ASC mode with active discharge.

Other features and advantages of the invention will appear further in the description below.

In the accompanying drawings, given by way of non-limiting examples:

- Figure 1 illustrates in the form of a graph a method of protecting a system comprising a battery, an inverter and an electric machine operating as a generator according to an embodiment in accordance with the art prior to the present invention;

- Figure 2 shows in the form of a block diagram a method according to one embodiment of the invention;

- Figure 3 illustrates in the form of a graph similar to that of Figure 1 the interest of applying a method according to the embodiment of Figure 2;

- Figure 4 illustrates in the form of a graph similar to that of Figures 1 and 3 a first aspect of the invention;

- Figure 5 illustrates in the form of a graph similar to that of Figure 4 a second aspect of the invention;

- Figure 6 illustrates in the form of a block diagram a method according to another embodiment of the intention.

FIG. 1 is a graph which presents a method for protecting a system comprising a battery, an inverter and an electric machine operating as a generator according to an embodiment in accordance with the art prior to the present invention.

The abscissa represents the time, each unit represented on the graph corresponding to 10 microseconds. The ordinate shows the voltage in a high voltage bus of the system connecting the battery to the inverter.

Curves A, B, C, D, E and F correspond respectively to the evolution of the voltage in the high voltage bus of the system when a load dump phenomenon occurs, linked to a sudden disconnection of electrical loads, in particular when suddenly disconnecting the battery. Each curve corresponds to the evolution of the voltage according to a different state of charge of the battery at the moment when the load dump occurs. Each state of charge (or level of charge, sometimes referred to by the English acronym "SOC" for "State Of Charge") results in a different voltage.

In the example represented here, which will be taken up later to illustrate the invention, the battery considered is a battery whose voltage at full charge is 460 V.

Curves A to F correspond respectively to the following conditions:

Curve A: 10% battery state of charge, voltage 400 V;

Curve B: battery state of charge of 30%, voltage of 414 V;

Curve C: 50% battery state of charge, voltage 426 V;

Curve D: 70% battery state of charge, voltage 446 V

Curve E: 80% battery state of charge, 450 V voltage

Curve F: 95% battery state of charge, voltage of 459 V .

In the example shown, the maximum admissible voltage Umax for the components subjected to the voltage present on the high voltage bus is 510 V. These components include in particular the capacitors contained in the inverter. In other words, it is necessary to set up a protection strategy making it possible to detect the occurrence of a load dump phenomenon and in this case to prevent the voltage from exceeding 510 V in these components.

The protection strategy known in the state of the art consists in determining the voltage in the high voltage bus and comparing it with a voltage threshold Uth, which is characteristic of a very probable occurrence of a load dump phenomenon.

This voltage threshold Uth, which is necessarily higher than the voltage of the battery when it is fully charged, is here set at 477 V.

Thus, when the voltage in the high voltage bus exceeds the measured 477V, action is taken to protect system components.

This measure consists in operating the inverter according to the ASC mode explained above, associated with an active discharge making it possible to drop the voltage in the system. However, a direct switch to ASC mode, in which the inverter power stage electronics located on the same side of the H-bridges formed in the inverter are closed, could cause a so-called cross-conduction situation ( more often referred to by the English term "cross conduction") if one of the H-bridges on the opposite side is still closed (switch closed), which could result in a degradation of the inverter.

It is therefore preferable to briefly put the inverter in a mode called "freewheeling" or FW mode before putting it in ASC mode.

In this mode, called "freewheeling", which means "freewheeling", the electronic switches are all in the open state, and therefore do not allow the passage of a current.

Switching to ASC mode from its nominal operating mode, in response to the detection of a load dump phenomenon, therefore takes some time, linked to the reactivity of the electronic components used to control the inverter on the one hand, and to the prior switch to FW mode on the other hand.

The choice of the voltage threshold Uth must therefore take into account the time required for the inverter to switch to UPS mode. In other words, once the voltage threshold Uth is exceeded, there must be enough time for the system to switch the inverter to UPS mode before the voltage in the high voltage bus and in the inverter reaches the maximum voltage. permissible Umax, regardless of the situation at the time the load dump occurs.

Several difficulties then arise. On the one hand, there is some uncertainty in the measured value. This uncertainty can typically be of the order of 2.5%. Thus, for a measurement of 477 V, corresponding to the voltage threshold in the example shown, the actual value of the voltage at the measurement point is between 465 V (minimum actual voltage Uthmin) and 489 V (maximum actual voltage Uthmax) .

This implies, for the determination of the voltage threshold Uth, to take into account the most unfavorable real voltage value, namely the highest real value that can correspond to measured Uth, according to the accuracy of the measurement.

In addition, depending on the state of charge of the battery, the power of the regeneration current (recharge current) applied to it is different.

Indeed, the higher the state of charge, the more the power of the charging current must be reduced compared to the maximum regeneration power of the system (we often speak of "derating" according to the corresponding English term) in order to preserve the integrity of the battery, to protect it against rapid degradation of its electrochemical components.

In the known protection strategies, the power of the maximum regeneration current of the system is taken as a reference. This is the applied power for low battery states of charge. It is therefore the evolution of the voltage represented by curve A which is taken into account in the known protection strategies, because it constitutes the most unfavorable case, that is to say the rise in the most fast.

Thus, in the example shown, it can be seen that in the most unfavorable case, if the actual voltage corresponds to llthmax, i.e. 489V, and the power of the regeneration current corresponds to the maximum regeneration power of the system, the switch to mode ASC must be performed in a time T of 30 microseconds at most.

It is of course possible to carry out the reasoning in the other direction in order to determine the voltage threshold llth according to the reactivity of the components of the system (i.e. by knowing the time T). Nevertheless, the voltage threshold llth cannot be too low, for two reasons: on the one hand, the minimum real voltage llthmin must of course be greater than the maximum voltage of the battery, but it must also be greater than the voltage maximum observable on the high voltage bus in the absence of load dump. However, the voltage experiences fluctuations, ripples, which are linked to the presence of consumers other than the battery on the high-voltage network: in an electric motor vehicle, this may be various equipment such as air conditioning, d a second inverter, etc.

The strategies known in the state of the art are thus very restrictive technically, and are not optimized.

The present invention was developed by the applicant on the basis of the observation that, depending on the state of charge of the battery which conditions the reduction applied to the power of the regeneration current of the battery, the increase in the voltage generated by the occurrence of a load dump phenomenon will be all the less abrupt as the power of the regeneration current is low, which amounts to saying that this increase will be all the less abrupt as it occurs when the state of charge battery is high.

Thus, in the example represented in FIG. 1: for curve A corresponding to a state of charge of the battery of 10%, the increase in voltage in the event of a load dump is 0.68 V/ps; - for curve B corresponding to a state of charge of the battery of 20%, the increase in voltage in the event of load dump is 0.66 V/ps;

- for curve C corresponding to a battery state of charge of 50%, voltage increase in the event of load dump is 0.64 V/ps for curve D corresponding to a battery state of charge of 70 % increase in voltage in the event of load dump is 0.51 V/ps for curve E corresponding to a state of charge of the battery of 80% increase in voltage in the event of load dump is 0.38 V/ps ps for curve F corresponding to a state of charge of the battery of 95% the increase in voltage in the event of load dump is 0.36 V/ps.

It is thus proposed in the invention to take advantage of this to optimize the protection strategy against overvoltages. More specifically, it is proposed to define a variable voltage threshold llth, depending on the state of charge of the battery (or its voltage, in that it is representative of the state of charge).

By variable is meant a non-unique threshold, varying in stages or continuously, or according to functions combining continuous variation and variation in stages.

According to a first exemplary embodiment of the invention represented in FIG. 2, the method uses two voltage thresholds.

This process is applied to a system comprising a battery capable of supplying an inverter via a high voltage DC bus and an electrical machine controlled by the inverter. The system is adapted to operate in regeneration mode, in which the battery is recharged by the electric machine operating as a generator.

In a first step S1, the current voltage of the battery Vbat is determined. Insofar as the voltage experiences ripples, and a load dump phenomenon can cause a sudden overvoltage on the high voltage bus, the current voltage of the battery can in practice be determined by carrying out a temporal average of the voltage in the DC high voltage part of the system (typically on the high voltage bus). A sliding time average can advantageously be used, over a period making it possible to cover several voltage ripples.

The current voltage of the battery Vbat, representative of its current state of charge, is then compared with a predefined limit voltage Vd (step S2). If the current voltage of the battery Vbat is less than or equal to the predefined limit voltage Vd, then a voltage threshold level called low threshold Uth1 is selected (step S3). If the voltage current of the battery Vbat is greater than the predefined limit voltage Vd, then a voltage threshold level called high threshold Uth2 is selected (step S4).

It is then determined whether an overvoltage occurs (step S5). This determination consists in comparing the current voltage in the high voltage direct current part of the system (typically on the high voltage bus) with the voltage threshold selected in step S3 or S4, namely the low threshold Uth1 or the high threshold Uth2 . This determination is made throughout the operation of the system, in particular when the electrical machine is operating as a generator to recharge the battery, by continuously measuring (i.e. as frequently as possible) the voltage in the DC high voltage part of the system.

An overvoltage, characteristic of the occurrence of a load dump phenomenon, is detected if the measured current voltage exceeds the selected voltage threshold.

As long as no overvoltage is detected, the method continues with the determination of the current voltage of the battery Vbat (that is to say by a return to step S1).

If an overvoltage is detected, i.e. the voltage in the high voltage part of the system exceeds the voltage threshold selected in step S3 or S4, then the ASC mode is applied to the inverter (step S6 ). In practice, the step S6 of applying the ASC mode includes a sub-step of briefly applying the FW mode, and the implementation of an active discharge in order to lower the voltage in the system.

Any other suitable protection mode can be applied as an alternative to the ASC mode with active discharge in step S6. For example, the ASC mode can be coupled with a means called reactive discharge, which uses resistors resulting in a slower discharge than the active discharge.

Figure 3, Figure 4 and Figure 5 illustrate the interest of such a method of protection against overvoltages.

FIG. 3 represents the voltage increases observed for example on the high voltage bus in the event of the occurrence of a load dump phenomenon at the same instant tj for the state of charge conditions of the battery A to F as defined previously with reference to figure 1 .

As indicated by the aforementioned numerical values of these increases and as clearly seen in Figure 3, up to a certain state of charge of the battery, of the order of 70% (curves A to D), the slopes of voltage increase decrease gradually but remain quite similar. On the other hand, for a state of charge of the battery of 80% (curve E), the slopes (gradients) of increase in voltage are markedly lower. Also, it appears relevant to set the predefined limit voltage Vd used in step S2 so that it corresponds to a state of charge of the battery between 70% and 80%, i.e. approximately 448V in the example of the mode of achievement represented.

This limit voltage can therefore be adapted according to the behavior of the system, i.e. the function of reducing the power of the regeneration current which is applied by the system, and which takes account of the characteristics of the battery.

The low threshold Uth1 and the high threshold Uth2 are then set, i.e. Uth1 = 477 V and Uth2=486V in the example shown.

With an uncertainty in the voltage measurement of 2.5%, this means that when a voltage corresponding to the low threshold Uth1 is measured, the actual voltage is between Uthlmin = 465 V and Uthlmax = 489 V. Similarly, when 'a voltage corresponding to the high threshold Uth2 is measured, the actual voltage is between Uth2 _m in = 474 V and Uth2max = 498 V.

In Figure 4, a graph similar to that of Figure 1 has been shown for curves A to D, i.e. for the states of charge of the battery for which the low threshold Uth1 is applied. The low threshold Uth1 corresponding to the voltage threshold Uth used in the embodiment in accordance with the prior art represented in FIG. 1, the conclusion developed in FIG. 1 is valid for the scenario represented in FIG. 4, namely that the system must allow switching to ASC mode in a maximum time T of 30 microseconds.

In Figure 5, a graph similar to that of Figure 1 has been shown for the curves E and F, that is to say for the states of charge of the battery for which the high threshold Uth2 is applied (here 80 % for curve E and 95% for curve F).

According to the reasoning developed with reference to Figure 1, the most unfavorable case must be taken into account, namely the case where the actual voltage in the high voltage part of the system corresponds to Uth2 _m ax i.e. 498 V, while the voltage Uth2 or 489V is measured (which may be the case with a measurement uncertainty of 2.5%). Nevertheless, despite the use of a higher voltage threshold than for the states of charge illustrated in FIG. 4, insofar as the voltage increase is slower when a load dump phenomenon occurs when the battery is in a high state of charge, it can be seen that in the most unfavorable case shown in FIG. 5, a time T' of 30 microseconds is available to take the necessary protective measures (i.e. typically switch the inverter to UPS mode).

Thus, the method according to the invention illustrated in FIGS. 4 and 5 has made it possible to raise the voltage threshold for which it is considered that an overvoltage occurs, while maintaining a substantially constant time T for the most unfavorable conditions associated with the two voltage thresholds used.

One of the advantages is that larger voltage ripples can be allowed in the system, without the risk of causing the inverter to go into an overvoltage protection mode by mistake, typically without risking the inverter going into UPS mode. with active discharge.

In the examples shown, while a maximum ripple of plus or minus 5 V was acceptable in the embodiment of Figure 1 according to the prior art (difference between the maximum battery voltage or 460 V and llthmin or 465 V ), a maximum ripple of 14 V is admissible in the embodiment in accordance with the invention of FIGS. 4 and 5 (i.e. the smallest of the differences between the maximum voltage of the battery, i.e. 460 V and Uth2min, i.e. 474 V of a part, and between the predefined limit voltage Vd or 448 V and Uthlmin or 465 V).

According to the principle explained above with reference to Figures 2 to 5, the high threshold and the low threshold can be chosen so as to increase the time available to apply to the inverter a mode of protection against overvoltages, while maintaining a correct allowable voltage ripple level, for example equivalent to that of the prior art (of the order of plus or minus 5 V, for example). This makes it possible, for example, to select electronic components that are less reactive, but more reliable and/or less expensive.

Of course, according to the principle described above, more than two voltage threshold levels can be defined, and used for as many distinct state-of-charge ranges.

When finite number of voltage threshold levels are used in the process (for example two thresholds, three thresholds, four thresholds, etc.), they can be integrated in a material way into the system, and actuated by switches controlled by the control device. system controller, usually a microcontroller.

It is also possible to envisage within the framework of the present invention a particularly optimized method employing a continuously variable voltage threshold, for all or part of the states of charge of the battery.

An example of such a process is shown in Figure 6.

Just as in the method of FIG. 2, the current voltage of the battery Vbat is determined in a first step S1.

In a second step S2′, the voltage threshold Uth is determined by a function of the current voltage of the battery Vbat. The function used can be very simple, for example llth = Vbat + x, with constant x (for example x=5 V, or 10 V, or 15 V, or 20 V, or 25 V). x can thus be adapted, within certain limits, to the admissible ripples of the system voltage. The function can be more elaborate, for example of the type Uth=Vbat+y(Vbat), y being a function of Vbat.

Insofar as the voltage threshold Uth can potentially take an infinity of values, it cannot be applied by a purely hardware system (based on resistors), but it will preferably be applied by a digital/analog converter.

In a step S3′, the digital/analog converter (or DAC according to the English acronym of Digital-to-Analog Converter) is updated in step S2′ to apply the determined voltage threshold Uth.

In a step S5', it is then determined whether an overvoltage occurs. This determination consists in comparing the current voltage in the high voltage direct current part of the system (typically on the high voltage bus) with the voltage threshold determined in step S3′ and applied by the DAC. An overvoltage, characteristic of the occurrence of a load dump phenomenon, is detected if the measured current voltage exceeds the voltage threshold Uth applied by the DAC.

If an overvoltage is detected, i.e. the voltage in the high voltage part of the system exceeds the voltage threshold Uth, then the ASC mode is applied to the inverter (step S6). In practice, the step S6 of applying the ASC mode includes a sub-step of briefly applying the FW mode, and the implementation of an active discharge in order to lower the voltage in the system. Any other suitable protection mode can be applied as an alternative to the ASC mode with active discharge in step S6. For example, the ASC mode can be coupled to a so-called reactive discharge means.

The examples presented above are of course purely illustrative. All numerical values given are representative of an example of an automobile electric traction system. Depending on the application considered, these values may be very different from those presented. Furthermore, those skilled in the art can adapt the teaching described above within the scope of the present invention. In particular, elements other than the current voltage alone can be envisaged to know the state of charge of the battery. Voltage can be measured or determined at various points in the system. The value of the regeneration (recharge) current of the battery can be used directly as a parameter, in addition to or as an alternative to the state of charge of the battery, to vary the voltage threshold used.

The use of a variable voltage threshold (whether it be a finite plurality of thresholds or a continuously variable threshold) proposed in the invention thus makes it possible to optimize the protection strategies against overvoltages due to a load dump phenomenon.

The optimization proposed in the invention is based on taking advantage of the fact that the increase in voltage linked to a load dump phenomenon takes place according to a significantly different dynamic depending on the current state of charge of the battery of the system, because this state of charge involves the use of more or less significant regeneration currents (charging by the electrical machine) depending on the state of charge of the battery. The optimization obtained can allow larger voltage ripples to be accepted in the system without the risk of wrongly triggering a system protection measure. It can alternatively make it possible to select less reactive (fast) components for the system in order to switch to a system protection mode, but which are more reliable and/or less expensive. Thus, in general, the design of a system using a method in accordance with the present invention is subject to fewer constraints than those of a system using a protection method known in the state of the art.

Claims

1 . Method for protecting against an overvoltage in a system comprising a battery, an inverter and an electrical machine controlled by the inverter, the battery being linked to the inverter by a high voltage bus, the system operating in regeneration mode in which the machine electricity operates as a generator to recharge the battery, the method comprising:

- determining the voltage on the high voltage bus;

- comparing the determined voltage to a voltage threshold (Uth), and

- switching the inverter to a protection mode suitable for lowering the voltage (S6), when the determined voltage exceeds said voltage threshold, the method being characterized in that said voltage threshold is variable according to a current state of charge of the battery.

2. Method according to claim 1, in which the inverter protection mode is an active short-circuit mode, called ASC mode, with active discharge.

3. Method according to claim 1 or claim 2, in which the state of charge is represented by the current voltage of the battery (Vbat), determined by the average of the voltage on the high voltage bus, over a predefined sliding period .

4. Method according to one of the preceding claims, in which several voltage threshold levels (Uth1, Uth2) are predefined, and in which the voltage threshold is chosen between the different voltage threshold levels.

5. Method according to claim 4, in which the predefined voltage threshold levels are two in number, namely a high threshold (Uth2) and a low threshold (Uth1), the voltage of the high threshold (Uth2) being greater than the low threshold voltage (Uth1).

6. Method according to claim 5, in which the low threshold (Uth 1 ) is applied when the current state of charge of the battery is less than or equal to a state of charge corresponding to a predefined limit voltage (Vd), and the high threshold (Uth2) is applied when the current state of charge of the battery is greater than the state of charge corresponding to the predefined limit (Vd).

7. Method according to one of the preceding claims, in which the voltage threshold (11th) varies continuously over at least one state-of-charge range of the battery, according to a function of the value of the voltage of the battery in said state of charge range.

8. Method according to claim 7, in which a maximum value of voltage ripple on the high voltage bus is determined and in which the function used to vary the voltage threshold (11th) has as parameter said maximum value of ripple Of voltage.

9. Method according to one of the preceding claims, in which the power of the battery charging current is determined and taken into account to determine the voltage threshold (llth).

10. Method according to one of the preceding claims, further comprising a step of switching the inverter to freewheel mode, called FW mode, immediately prior to the step of switching to ASC mode.

11. System comprising a battery, an inverter and an electrical machine controlled by the inverter, the battery being linked to the inverter by a high voltage bus, characterized in that it comprises a computer configured to apply a method according to the one of claims 1 to 10 when the system operates in regeneration mode in which the electric machine operates as a generator to recharge the battery.