METHOD FOR PROTECTING CONVERTERS AGAINST DOWNSTREAM SHORT CIRCUITS

Abstract

Method for implementing a power supply system for a downstream circuit of an electrical network using a voltage source type converter having at least one half-arm comprising at least one semiconductor switch Txy and having at least one protection circuit comprising one or more power protection components, which includes: - as a function of a maximum thermal stress I2·tmax that the converter can withstand, an estimated short-circuit current of the network flowing in said half-arm as a function of time Icc(t) and an estimated short-circuit duration tcc for said downstream circuit, - a dimensioning of said at least one power protection component of the protection circuit to withstand a portion K·Iccmax of the peak short-circuit current Iccmax such that I2·t converter remains less than I2·tmax throughout the entire duration tcc of the short circuit, - the calculation of a resistance ratio between a first resistance R1x,series resistance of said half-arm and a second resistance R2x, series resistance of said protection circuit, said protection circuit being connected in parallel with said half-arm, such that for the short-circuit current value Iccmax, the short-circuit current through said half-arm is limited to the maximum acceptable value (1-K)·Iccmax for said at least one semiconductor switch, the additional portion of short-circuit current K·Iccmax through said protection circuit, - the introduction in said half-arm of a balancing resistance such that the sum of the balancing resistance of a connecting resistance of said half-arm and a conduction resistance of said at least one semiconductor switch of said half-arm divided by the sum of the connecting resistance of said protection circuit and the conduction resistance of said at least one protection component is equal to the ratio R1x / R2x. Figure 2A,

Description

Title of the invention: METHOD FOR PROTECTING CONVERTERS AGAINST DOWNSTREAM SHORT CIRCUITS technical field

[0001] The invention relates to the field of direct current networks supplied by a power converter of the voltage source type and applies in particular to public low voltage direct current (LVDC) distribution networks. Previous technique

[0002] It is known to produce converters with short-circuit protection in motor control applications or for inverters.

[0003] Document CN104467390 (A) relates for example to a protection circuit of a converter comprising protection diodes for which the electronic switches or semiconductor switches are connected to the supply phases by inductors and for which it is indicated that the impedance of the inductors being greater than that of the protection diodes, a short-circuit current passes mainly through the protection diodes which protects the semiconductor switches until the fuses upstream of the converter are cut. Technical problem

[0004] Such a solution, suitable for relatively modest short-circuit currents compared to the maximum current allowed by the protected semiconductor switch, is not usable in an electrical network environment where the short-circuit currents on the network downstream of the converter can reach extremely high values, for example ten to one hundred times the maximum value allowed by a freewheeling diode of a semiconductor switch and are not limited by the current available in the upstream circuit of the converter. Description of the invention

[0005] In view of the foregoing, this disclosure proposes a method for powering a downstream DC-type electrical network circuit with an upstream AC-type circuit by means of a voltage source-type converter having at least one half-arm comprising at least one Txy semiconductor switch and having at least one protection circuit comprising one or more power protection components which includes: a. as a function of a maximum thermal constraint according to the product I2-tmax that the converter can withstand, an estimated short-circuit current of the network flowing in said half-arm as a function of time Icc(t) and an estimated short-circuit duration tcc for said downstream circuit, b. a sizing of said at least one power protection component of the protection circuit to support a portion K-Iccmax of the peak short-circuit current Iccmax such that I2-t converter remains less than I2-tmax throughout the entire duration tcc of the short circuit, c. the calculation of a resistance ratio between a first resistance R1x, series resistance of said half-arm and a second resistance R2x, series resistance of said protection circuit, said protection circuit being connected in parallel with said half-arm, such that for the short-circuit current value Iccmax, the short-circuit current through said half-arm is limited to the maximum acceptable value (1K)-Iccmax for said at least one semiconductor switch, the additional portion of short-circuit current K-Iccmax through said protection circuit, d. the introduction into said half-arm of a balancing resistance R$ such that the sum of the balancing resistance R$, a connecting resistance Rb of said half-arm and a conduction resistance of said at least one semiconductor switch of said half-arm divided by the sum of the connecting resistance of said protection circuit R^p and the conduction resistance of said at least one protection component Rpp is equal to the ratio Rlx / R2x.

[0006] Preferably, a measurement of the connection resistance Rp of each half-arm is carried out at the level of the converter wiring, prior to the calculation of the balancing resistance ^5.

[0007] Said at least one protection component may advantageously be chosen such that its threshold voltage is less than the threshold voltage of a freewheeling diode of said at least one switch.

[0008] According to different embodiments:

[0009] Said half-arm may include a semiconductor switch, the protection circuit may include a protection component, said balancing resistor R$ being calculated such that the freewheeling diode current of said semiconductor switch is given by the equation

[0010] iD = ———— (p~ d)+(Rp + RRp)icÀt)) \ 1 1X.C \ i—f O

[0011] and the current in the protection component is given by the equation

[0012] ip - ((VD - VP) + (Rd + Rs + RRD)icJ, t))

[0013] with: has. b. yd the freewheeling diode threshold voltage, Rsc — Rd the conduction resistance of the freewheeling diode of said switch, c. Rp = Rrd, the equivalent connection resistance of said semiconductor switch, d. R$ the balancing resistance, e. the threshold voltage of a protection component, f. Rrp is the connection resistance of the protection component, g. Rcp ~ the conduction resistance of the protection component, h. said protection component being chosen such that VP < VD and (Rp + Rkp) < (Rd + Rrd), with RP + Rrp = R2x and RD + Rrd + Rs = lx.

[0014] Said half-arm comprising several semiconductor switches in series, each of said semiconductor switches may comprise said protection circuit.

[0015] Said half-arm may include a semiconductor switch, the protection circuit may include N protection components wired in parallel, each having a connecting resistance Rrp and a conduction resistance RP, the balancing resistance R$ is then calculated so that the freewheeling diode current of said semiconductor switch is given by the equation:

[0016] iD _ (N (yp -) + (RP + RRp)iC({t))

[0017] and, the current in a protection component is given by the equation

[0018] ip = —_ ((VD - VP) + (RD + Rs + R^i^t))

[0019] with: a. yd the freewheeling diode threshold voltage, b. R$c — Rd the conduction resistance of the freewheeling diode of said in switch, c. Rb — Rrd the equivalent bonding resistance of said semiconductor switch, d. Rs the balancing resistance, e. yP the threshold voltage of a protection component, f- Rrp the connection resistance of a protection component, g. RP the conduction resistance of a protective component, h. N the number of protective components connected in parallel, i. said protective components being chosen such that VP Vp> and (RP + Rrp) £ (RD + RrpÙ j. and such that RP + Rrp = R2x and N(Rd + Rs + Rrd) = Rlx

[0020] The half-arm may comprise M semiconductor switches in series and the protection circuit connected in parallel with a half-arm may comprise N protection components wired in parallel with said arm, the balancing resistance Rs being such that the freewheeling current of said half-arm is given by the equation: [00211 VF- Va„,)

[0022] and, the current in a protection component is given by the equation

[0023] + (RD,eq + RS+RRD)ic^

[0024] with: a. V d „ y^. the equivalent threshold voltage of the freewheeling diodes of the semiconductor switches of the half-arm, b. p _ p _ VM p the equivalent resistance of the freewheeling diodes of the — KD.eq — Lii=^Di semiconductor switches of the half-arm c. Rrd the equivalent connection resistance of the half-arm linkage, d. the balancing resistance, e. the threshold voltage of a protection component, f. Rrp is the connection resistance of a protection component, g. Rp the conduction resistance of a protection component, h. N the number of protection components connected in parallel, i. with VD,eq and Rp + Rrp - Rn,eq + Rrd.

[0025] The half-arm may comprise M semiconductor switches in series, the protection device may comprise Q protection components in series, distributed over N parallel branches, a balancing resistance being such that the freewheeling current of said half-arm is given by the equation: 100261 * = Vp-VD„) y(RF + RRp)i^

[0027] and, the current in a protection component is given by the equation

[0028] ip = 1 ((vDeq - VP^q) + (R^eq + Rrd + Rs)icc(t)) ' y-lf fu i? /

[0029] with: a. y^ „ y^ the equivalent threshold voltage of the freewheeling diodes of the semiconductor switches of the half-arm, b. n _ p _ Vw p the equivalent resistance of the freewheeling diodes of the semiconductor switches of the half-arm c. Rrd the equivalent connection resistance of the half-arm linkage, d. Rs the balancing resistance, e. _ yQ TZ the equivalent threshold voltage of the protection components P,i in series of the half-arm, f- Rrp the connection resistance of a protection component, g. D yÔ D the equivalent conduction resistance of the components of serial protection, h. N the number of branches in series of protection components connected in parallel, i. and Vp eq < Vet Rp^j + Rrp + Rrd.

[0030] This disclosure further proposes a system for supplying a downstream circuit of an AC or DC type electrical network by means of an upstream circuit of the opposite type by means of a voltage source type converter comprising at least one arm having half-arms equipped with protection circuits made according to the method as proposed above for which the balancing resistance Rs of each half-arm is disposed at one of the two ends of the half-arm, or distributed at the two ends of the half-arm.

[0031] The said protection component(s) are advantageously diodes but can also be thyristors equipped with a control circuit.

[0032] The system preferably includes an input filter having an impedance IZfl = IRf+jXfl greater than Rs and limiting the fault current Icc(t).

[0033] The balancing resistance R$ can be associated with a balancing inductance Ls to realize a balancing impedance. Brief description of the drawings

[0034] Other features, details and advantages of the invention will become apparent from the following detailed description of non-limiting embodiments and from the analysis of the accompanying drawings, in which:

[0035] [Fig-1] shows a schematic view of a converter arm;

[0036] [Fig.2A], [Fig.2B], [Fig.2C] show schematic views of different assemblies according to this disclosure;

[0037] [Fig.3A], [Fig.3B], [Fig.3C] show different levels of models for the implementation equation of a converter arm;

[0038] [Fig.4] shows curves representing the influence of the number of power diodes on the current of a freewheeling diode;

[0039] [Fig.5] shows curves representing the evolution of the current as a function of different values ​​of balancing resistances;

[0040] [Fig.6] shows curves representing the evolution of the number of diodes in parallel as a function of the nominal current accepted by the diodes and the value of the filtering inductance;

[0041] [Fig.7] shows surfaces and curves related to short-circuit duration as a function of the short-circuit current to nominal current ratio. Description of the implementation methods

[0042] The drawings and the description below contain elements that may not not only to serve to better explain the present invention, but also to contribute to its definition, if necessary.

[0043] Reference is now made to [Fig. 1] representing a converter arm B, in which an upper half-arm Hu is provided with N semiconductor switches (also called valves) Txy=Tui,..., TuN of transistor type (IGBT, MOSFET, etc.), each connected in antiparallel to a diode as shown in Figures 2A to 2C for transistors 110, 120a, 120b, 130a, 130b, respectively, in antiparallel to diodes 111, 121a, 121b, 131a, 131b, in order to achieve bidirectional current flow. These diodes are often called freewheeling diodes. Integrated within the same module as the switch, these diodes are sized to conduct a current lower than the nominal current of the transistor since they are sized in proportion to the effective current they conduct.Thus, unlike power diodes which aim to minimize conduction losses, freewheeling diodes aim to minimize switching losses and therefore switching time to allow for higher frequency response of transistors with higher conduction resistance. This pursuit of speed results in a reduction in the thermal resistance of the switches.

[0044] Returning to [Fig. 1], an alternating current phase is connected at the midpoint between the half-arms Hu and Hb of the converter through an inductor f, while the distal ends of the half-arms constitute the poles of a DC bus, between which are one or more capacitors C in a series / parallel arrangement. Several arrangements corresponding to this description are possible, and the scheme can be reproduced when several phases supply the DC bus.

[0045] The method of this disclosure consists of connecting in parallel with each half-arm one or more additional power diodes, with well-defined characteristics, as well as one or more series impedances. The choice of characteristics will be detailed in the following paragraph, while an example of the arrangement of these components is shown in Figures 2A to 2C for the case of a 2L-VSC, i.e., a two-level voltage source converter. In these examples, one or more protection diodes 115, 125a, 125b, 135a to 135d are used. These protection diodes can be integrated into a module containing the switch(es).

[0046] Hereafter, icc will be used interchangeably to refer to a short-circuit current or a fault current.

[0047] According to Figures 2A to 2C, connection impedances 114 are found at the level of the protection diode of [Fig. 2A], for example, and 113 at the level of the half-arm containing the transistor 110, also at [Fig. 2A]. These connection impedances correspond to the impedances introduced by the various connectors and possible connection methods of the transistors and the protection diodes. These impedances There are numerous pedances distributed throughout the system. For simplicity, these impedances are grouped by arm in figures 2A to 2C.

[0048] These figures also represent balancing impedances made up here of balancing resistors 112, 122, 132 according to the present disclosure.

[0049] In the case of [Fig.2A], a diode 115 is connected in parallel with the single transistor 110 of the half-arm 11, in [Fig.2B], a pair of parallel diodes 125a, 125b is connected in parallel with two series transistors 120a, 120b while in the case of [Fig.2C] four diodes 135a, 135b, 135c, 135d forming a parallel / series network are connected to the half-arm 13 having two switches 130a, 130b in series.

[0050] Indeed, the protection diodes do not necessarily need to be in parallel with each switch: it is possible to insert the protection diode(s) in parallel with the half-arm, as shown in the examples in Figures 2A to 2C. For better current withstand, the diodes can be wired in parallel as in [Fig. 2B]. Furthermore, to achieve better voltage withstand, the protection diodes can be connected in series, as shown in [Fig. 2C]. The circuit in [Fig. 2A] is called "valve protection" or "switch protection," considering that if several switches were present on the half-arm, as in [Fig. 1], each switch would have a protection diode in parallel. The circuit in [Fig. 2B] is called "half-arm protection," and the circuit in [Fig. 2C] is called "half-arm protection with series connection."

[0051] In the case of a three-phase (or two-phase) configuration, three (or two) arms would be connected in parallel. Each would be supplied by a different phase. The external capacitors would then be common to the whole assembly.

[0052] Finally, diodes can be replaced by controlled devices, such as thyristors, without impacting operation: the objective then being to continue to use components with high thermal resistance, but no longer to use them in nominal mode.

[0053] The implementation of the semiconductor switch protection depends heavily on the design of the converter arm. When discrete components are used, each component—transistor, diodes, and impedances—can be individually selected and freely positioned. In this configuration, freewheeling diodes can be omitted, and only power diodes can be used.

[0054] In cases where modules are used, the freewheeling diodes are already integrated into the switches, for example, IGBTs. The general solution would then be to use discrete protection diodes to best adapt to each semiconductor switch. While the balancing impedances can only be located on the module's connection terminals.

[0055] Note that in the particular case of a two-level voltage converter, the protection diodes can come from the arm of a diode rectifier, or from the arm of another two-level voltage converter whose IGBTs are not controlled.

[0056] Finally, a last solution would be to directly adjust the doping of the diodes and / or transistors and the positions of the semiconductor layers to obtain the desired function. The whole assembly would then be integrated within a single module.

[0057] Under normal operating conditions, the operation of the converter equipped with power diodes and balancing resistors or impedances differs only when transferring power from the AC network to the DC network. Indeed, in this operating mode, and for a voltage source type converter, it is the freewheeling diodes that normally conduct the current. However, the presence of the protection diodes and balancing impedances allows the total equivalent resistance to be divided, which ultimately results in lower conduction losses.

[0058] Equation (1) reflects this fact, giving the losses P^^fj in a diode:

[0059] p (f\_(Rf*RRp)vD^(RD+Rs+Rim)vf1j • ​​ / .\2 (1) conc^ J (Rp+RKp)+N(Rd+Rs+Rrd) lol\ (Rp+R^p^NfR / jt-Rg+R^)

[0060] where: - VD is the threshold voltage of the freewheeling diode, - Rp is the conduction resistance of the freewheeling diode, - Rrd is the equivalent coupling resistance of the freewheeling diode, - Rs is the balancing resistance, - Vp is the threshold voltage of a protection diode, - Rrp is the equivalent connecting resistance of the protection diode, - Rp is the conduction resistance of a protection diode, - N designates the number of protection diodes put in parallel, - itot designates the current in the half-arm.

[0061] The assumed embodiment for writing equation (1) is that of a 2L-VSC, as shown in Figure 2A. Furthermore, it is assumed that the protection diodes all have the same characteristics (Vp, RP), while being placed in parallel with a semiconductor switch.

[0062] Note that in the case where a half-arm incorporates M switches, the total losses are obtained by multiplying equation (1) by M.

[0063] It should also be noted that the reduction of conduction losses is valid only if the protection diodes are selected so as to have VP < VD and (Rp + Rpp) (Rp + Rrp): the current then passes mainly through the diodes of

[0064]

[0065]

[0066]

[0067]

[0068]

[0069]

[0070]

[0071]

[0072]

[0073] protection. The phenomenon is accentuated in the case where there is a balancing impedance, since (Rd + Rrd) < (RD + RS + Rrd). In the configuration shown in Figure 2A, each switch can be analyzed independently of the total number of switches on the arm. This is because the fault current, denoted Èc, is primarily limited by the filter and cable impedances; the influence of the protection diode impedances on the fault current can be considered negligible. Figures 3A to 3C represent successive steps in simplifying one half-arm of a converter when putting it into equations. Figure 3A shows a detailed model including the components transistor 140, freewheeling diode 142, balancing resistor 142, connecting resistors 143, 144a, 144b, and protection diodes 145a, 145b. Figure 3B shows an intermediate model with the freewheeling diode 141, balancing resistor 142, connecting resistor 143, and an equivalent diode 151 with a threshold voltage Vp and equivalent series resistance (Rrp+Rp) / N. Figure 3C shows an equivalent model with a diode 161 corresponding to the parallel connection of the freewheeling diode and the protection diode. Thus, using this assumption and the equivalent model of Figure 3C, it is possible to establish that the current in the freewheeling diode during a fault is given by equation (2), while the current in a protection diode is given by equation (3), / «(0 here denoting the half-arm current. “ (Rp+RRf:)+N^^ (N(Vp-VD) + + RRp)icc(t) ) (2) ip ~ {R^+R^N^+Rs+R^ ((VD~Vp) + (RD + RS + RRD)^c(t) ) (3) By selecting protection diodes such that Vp VD and (7?P + rrp) - (rd + RRD^ 'c current passes mainly through the protection diodes during the fault, which limits the thermal stress on the freewheeling diodes. Note that increasing the number of protection diodes increases the current deflection as illustrated in [Fig.4] describing the influence of the number N of protection diodes on the freewheeling diode current with curve 1, N=0, curve 2, N=1, curve 3, N=2. Furthermore, by selecting VP < VD, a period of time appears during which only the protection diode conducts. This occurs as soon as the condition in equation 4 below is met. It «011 Within the scope of this application, [Fig. 5] shows, during the conduction time of the freewheeling diode, in the case of a VSL2C converter with a diode protection, the current in the freewheeling diode without balancing resistor in curve 5, the current in the protection diode without balancing resistor in curve 6, the current in the freewheeling diode with a 10 milliohm balancing resistor in curve 4 and the current in the protection diode with a 10 milliohm balancing resistor in curve 7.

[0074] Embodiments:

[0075] Half-arm protection is primarily used in cases where the converter to be protected is directly integrated into a module, without individual access to the switches. Protection is then achieved by combining this basic module (NPC3L, VSC2L, etc.) with another module containing the protection diodes. These protection diodes are obtained from a three-phase or single-phase rectifier arm.

[0076] Just as in the previous case, a two-level voltage converter whose transistors are not controlled can be used to perform this function, but these modules generally have a lower thermal constraint.

[0077] Finally, it should be noted that the use of discrete components also makes it possible to achieve this type of assembly, as does the adjustment of the semiconductor layers. The half-arm protection minimizes the total number of components.

[0078] Behavior under nominal operating conditions:

[0079] As with protection by switches, the operation of the converter differs from the state of the art only when it comes to transferring power from the AC network to the DC network. Indeed, in this operating mode and for a voltage source type converter, it is the diodes that conduct the current.

[0080] In the case of a two-level voltage architecture, the difference ends there and it is possible to reuse equation (1). On the other hand, if the converter has an architecture with more than two voltage levels, or several switches in series, the protection diode conducts only when all the IGBTs are blocked, which leads to equation (5).

[0081] p ( A ~ (Rp+^rp^di^Rrd+Rs-) • / \2 (5) (Rp+RRp)+N(R[)eiI+RRa+R^) OR : - vD = eSt 'a tensæn de scu'' equivalent of freewheeling diodes, - "VM o is the equivalent resistance of the freewheeling diodes, - Rrd is the equivalent bonded connection resistance of the freewheeling diode, - R$ is the balancing resistance, - Vp is the threshold voltage of a protection diode, - Rp is the conduction resistance of a protection diode, Rrp is the equivalent bonded connection resistance of the protection diode, N denotes the number of protection diodes connected in parallel, itot denotes the total current coming from the AC network. VDi is the equivalent threshold voltage of the freewheeling diode of the i-th in switch, - Rdj is the equivalent conduction resistance of the freewheeling diode of the i-th switch, - M denotes the number of switches in series per half-arm.

[0082] As with the previous paragraph, it should be noted that the reduction in conduction losses is valid only if the protection diodes are selected so that Vp < V^eq and Rp

[0083] Behavior in fault mode:

[0084] In fault mode, the current flowing in the half-arm, denoted k-XO, can be considered independent of the diode parameters. Under this condition, the current diverted in each of the protection diodes is given by equation (6). [00851 (6)

[0086] Analysis of this equation highlights that the current deviation in the protection diodes increases with the number of switches in the half-arm, the value of the balancing impedance, and the number of protection diodes in parallel. An optimum balancing is obtained by selecting protection diodes such that Vp < Vet Rp RD,eq.

[0087] Furthermore, the phenomenon of non-conduction of intrinsic diodes, described by Equation (4) still exists in this configuration, but appears this time only when the condition in the following equation (7) is satisfied.

[0088] It Ut) || * N(yDiarVp) (7) Rp+Rrp

[0089] Half-arm protection with a series assembly:

[0090] Embodiment method:

[0091] The half-arm protection with series assembly meets the same constraints as the half-arm protection, with the addition of voltage withstand: if the protection diodes cannot withstand the voltage alone, we propose to put several of them in series.

[0092] This can be done either directly at the semiconductor layer level or by using discrete components. Where compactness is a critical criterion, a particular solution could be to use a rectifier arm instead of two discrete diodes. The module's midpoint is then not connected to a phase.

[0093] Steady-state behavior:

[0094] In steady state, the behavior does not diverge from that described previously for half-arm protection. However, the equation describing the conduction losses evolves as follows:

[0095] p / \ / \ ■ / \2 co / kA y (Rpw+RR^N(Rfy.q+Rit[)+Rs) to! (Rpf.q+RRD)+N(Rf)ec+RRD+Rs) M (8) where - y _ y is the equivalent threshold voltage of the freewheeling diodes of the half-arm switches, - is the equivalent resistance of the freewheeling diodes of the in half-arm switches, - Rs is the balancing resistance, - y? "yp is the threshold voltage of a protection diode, - p _ V£ p is the conduction resistance of a protection diode, KP£q — Lq=fiPj - N denotes the number of protection diodes connected in parallel, - itot designates the total current coming from the alternating current network, - VDi is the threshold voltage of the freewheeling diode of the i-th switch, - Red is the conduction resistance of the freewheeling diode of the i-th in switch, - M denotes the number of switches in series on the same half-arm, - L denotes the number of protection diodes in series on the same half-arm.

[0096] As with the previous paragraph, it should be recalled that the reduction of conduction losses is valid only if the protection diodes are selected so as to have Vpeq < Vp)eq and Rp^q Rn.eq.

[0097] Behavior in fault mode:

[0098] The current deflection in the protection diodes increases with the number of switches in the half-arm, the value of the balancing impedance, and the number of protection diodes in parallel. An optimum balancing is obtained by selecting protection diodes such that Vp < VDeq and Rp < Rd^.

[0099] Note, however, that equations (6) and (7) describing the behavior of the converters are replaced respectively by equations (9) and (10): 101001 ++ W) )

[0101] He II s <w)

[0102] Component selection:

[0103] Taking current deviation into account:

[0104] The first criterion, for selecting the components and implementing our invention, relates to the estimation of the current deviation required by the protection diodes.

[0105] Once the basic converter topology has been chosen, the desired implementation mode must be selected. As a reminder, the three implementation possibilities are shown in Figures 2A, 2B, and 2C. Depending on the topology chosen and referring to equations (3), (6), or (9), the first step is to select a power diode reference that maximizes the current deflection and / or achieves a target current deflection.

[0106] In a second step, it is possible to add or modify the balancing impedances Rs to achieve a specific deflection target and / or minimize overvoltages. This current deflection can also be influenced by the number of protection diodes.

[0107] By way of example, if the protection of a two-level voltage converter as shown in [Fig.2A] is sought with an objective of having 5 times more current in the protection diode than in the freewheeling diode of the converter and knowing the parameters of the components available with Table 1 below, it is possible to obtain a first dimensioning.

[0108] [Tables] Rp vP Rd Vd Iccmax R RP — R RD 2.3 mQ 0.75 V 6.5 m<> IV 500 A 1 m<>

[0109] In this case study, condition (11) is satisfied if N > 4 or if Ry ≤ 6 mΩ. If only two protection diodes can be inserted in parallel, then it is necessary that R ≤ -3.5 mΩ to obtain the desired current deflection:

[0110] L _ (V [yVP) camix > j- (1 1) -N(VD-Vp)^RP+RRP)lmmK

[0111] Taking into account the thermal constraint

[0112] In parallel with the estimation of the current deviation, it is necessary to verify that the diodes are capable of thermally withstanding the stress represented by the fault current, using the i2t criterion. This criterion is calculated using equations (11) and (12).

[0113] pt > cx / 2.^ (12)

[0114] Where 1 corresponds to a safety factor on the dimensioning

[0115] Where / 2. { _ denotes the maximum permissible thermal stress of a protection diode and where

[0116] -, [R / 2 / 3 rL (13) ^System = LjXO 2 dt = 2 + 2 + 2 dt

[0117] The integral's decomposition stems from the very behavior of the DC short circuit. Indeed, during a low-impedance fault on the DC bus, the converter goes through three distinct stages: a. Discharging of capacitors; b. The discharge of DC line inductances and the possible reversal of the DC voltage; c. The fault is powered by the AC network through the freewheeling diodes.

[0118] From this sequence, it is possible to define five characteristic instants which are found in equation (13): a. the moment of onset of the fault and phase 1; b. ^2 the start time of phase 2 and the end time of phase 1; c. h the moment of the start of phase 3 and the end of phase 2; d. The moment the fault ends, with the latter being cut off; e. ^blrxk the instant of blocking of the semiconductor switches.

[0119] As a first approximation, it is possible to consider that f [ — thhxk- Although in reality, there is a slight difference due to the sampling frequency and the detection thresholds, which leads to fi < tblock-

[0120] Note that this criterion takes into account the maximum switching time of the protection associated with the converter. A significant switching time mechanically leads to the selection of components with high i2-t values. Note also that, in addition to the current deflection, it is necessary to consider the conduction nonlinearities of the protection diodes, described in equations (4), (7), and (10), to calculate this criterion.

[0121] Taking into account inductances

[0122] Increasing the converter's filter inductance limits the fault current from the AC network, which ultimately reduces the need for thermal oversizing of the semiconductor components. Inserting and / or oversizing a filter inductance on the DC bus is another way to limit the fault current.

[0123] Fig. 6 represents an illustration with the ratio of diode current associated with the IGBT module divided by the nominal current of the converter on the abscissa and the number of protection diodes in parallel on the ordinate for three different inductance values, curve 8 inductance of 0.1mH, curve 9 0.5mH, curve 10 2mH, assuming no balancing resistor. The diodes of the IGBT modules considered had nominal currents of 35 A, 50 A, 75 A, 100 A, 150 A, 200 A, 225 A, 300 A, 450 A, 600 A and 900 A corresponding to the points aligned vertically from left to right, which correspond respectively to 1.02, 1.43, 2.21, 2.94, 4.41, 5.88, 6.62, 8.82, 13.24, 17.65 and 26.48 In. Calculation of protections

[0124] In view of the foregoing, the present disclosure proposes to protect the semiconductor switches of a power supply system of a downstream circuit of an AC or DC type electrical network by an upstream circuit of the opposite type by means of a voltage source type converter by means of protection components and the addition of one or more balancing resistors which will redirect a portion of the short-circuit current into the protection components.

[0125] The converter is then provided with at least one half-arm 11, 12, 13 comprising at least one semiconductor switch Txy 110, 120a, 120b, 130a, 130b and provided with at least one protection circuit comprising one or more power protection components 115, 125a, 125b, 135a, 135b, 135c, 135d as in Figures 2A to 2C.

[0126] The process then comprises: - as a function of a maximum thermal constraint F-tmax that the converter can withstand, an estimated short-circuit current of the network flowing in said half-arm as a function of time Icc(t) and an estimated short-circuit duration tcc for said downstream circuit, - a sizing of said at least one power protection component 115, 125a, 125b, 135a, 135b, 135c, 135d of the protection circuit to support a part K-Iccmax of peak short-circuit current Lcmax such that Ft converter remains less than F-tmax throughout the entire duration tcc of the short circuit, - the introduction into said half-arm of a balancing resistance Rs 112, 122, 132 such that the sum of the balancing resistance Rs 112, 122, 132 of a connecting resistance 115, 125a, 125b, 135a, 135b, 135c, 135d of said half-arm and of a conduction resistance of said at least one semiconductor switch 110, 120a, 120b, 130a, 130b of said half-arm divided by the sum of the connecting resistance 114, 124a, 124b, 134a, 134b of said protection circuit Rrp and the conduction resistance of said at least one protection component Rcp 115, 125a, 125b, 135a, 135b, 135c, 135d is equal to the ratio Rlx / R2x.

[0127] The measurements can be carried out in situ and then, prior to calculating the balancing resistance Rs, a measurement of the connection resistance Rp of each half-arm is carried out at the level of the converter wiring.

[0128] Said at least one protection component is chosen such that its threshold voltage is less than the threshold voltage of a freewheeling diode of said at least one switch.

[0129] In the case where the half-arm includes a semiconductor switch, the protection circuit includes a protection component, and said balancing resistor Rs is calculated such that the freewheeling diode current of said semiconductor switch is given by the equation 101301 '0 = ^1^ <14>

[0131] and the current in the protection component is given by the equation 101321 = <15)OÎ,: - VD is the freewheeling diode threshold voltage, - Rsc ~ Rd is the conduction resistance of the freewheeling diode of said in switch, - Rp = Rrd is the equivalent connection resistance of said semiconductor switch, - Rs is the balancing resistance, - Vp is the threshold voltage of a protection component, - Rrp is the connection resistance of the protection component, - RCP = Rp is the conduction resistance of the protection component,

[0133] said protection component being chosen such that VP < VD and + Rrp) < (Rd + Rrd\ with Rp + RRp = R2x and RPJ + Rrd + Rs = #1%.

[0134] The half-arm may include several semiconductor switches in series, each of said semiconductor switches including said protection circuit.

[0135] The half-arm may include a semiconductor switch, and the protection circuit may include N protection components wired in parallel to reduce the current to which each protection circuit is subjected. This corresponds to Figure 2A but with two protection power diodes as in Figure 2B. In this case, each protection component includes a connecting resistance Rrp and a conduction resistance Rp, and the balancing resistance Rs is calculated so that the freewheeling diode current of said semiconductor switch is given by the equation:

[0136] iD = (N (yp - yD) + (Rp + RrpMï)) (16)

[0137] and, the current in a protection component is given by the equation

[0138] ( VD~ Vp) +(Rd + Rs + Rrd)^ (17) with: - VD is the freewheeling diode threshold voltage, - Rsc — Rd the conduction resistance of the freewheeling diode of said in switch, - Rb = Rrd, the equivalent connection resistance of said semiconductor switch, - Rs is the balancing resistance, - yP the threshold voltage of a protection component, - Rrp is the connection resistance of a protective component, - Rp is the conduction resistance of a protective component, - N the number of protection components connected in parallel,

[0139] said protection components being chosen such that:

[0140] + Rpp)<,(RD + R / lD) and with RP + Rrp = A2x and NtRn + Rs + R^Rlx.

[0141] The half-arm may comprise M semiconductor switches in series, and the protection circuit connected in parallel with a half-arm may comprise N protection components wired in parallel with said arm as in Figure 2B, or two transistors in series are wired in parallel with a set of two diodes in parallel. In this case, the balancing resistance Rs is such that the freewheeling current of said half-arm is given by the equation: [01421 Vp - VDy ^R^R^i» (18)

[0143] and, the current in a protection component is given by the equation

[0144] ip = 77---r-ri---77-77— ( ( VDeq - VP) + (RDeq + Rs + RRD)iJt)) (19) with: - yyD the equivalent threshold voltage of the freewheeling diodes of the semiconductor switches of the half-arm, - R, — Rd = Rd the equivalent resistance of the freewheeling diodes of the semiconductor switches of the half-arm - Rrd the equivalent connection resistance of the half-arm link, - Rs is the balancing resistance, - the threshold voltage of a protection component, - Rrp is the connection resistance of a protective component, - RP is the conduction resistance of a protective component, - N is the number of protective components connected in parallel, and with VP <VDeq et Rp + Rrp Rd^ + Rrd.

[0145] According to a general case, the half-arm comprises M semiconductor switches in series, the protection device comprises Q protection components in series, distributed over N parallel branches, and the balancing resistance Rs is such that the freewheeling current of said half-arm is given by the equation: [01461 .....) W Vp-V^ ^Rp^Ut}) <20>

[0147] and, the current in a protection component is given by the equation [°1481 'p = * [ ( ( W Vp„ ) + Rrd+RsM) ) (21) with : - v — V 'a equivalent threshold voltage of the freewheeling diodes of the semiconductor switches of the half-arm, - p _ n _ n the equivalent resistance of the freewheeling diodes of the ^0 — KD <eq — semiconductor switches of the half-arm - Rrd the equivalent connection resistance of the half-arm link, - the balancing resistance, - iz _ v the equivalent threshold voltage of the protection components VP^q ~ 'PJ in series of the half-arm, - Rrp is the connection resistance of a protection component, - o _ d the equivalent conduction resistance of the components of Kp^q - L^Rpj series protection, - N is the number of branches in series of protection components connected in parallel, and with Vp^.q < VDeq and Rp^q + Rrp Ro,eq + Rrd.

[0149] The described method makes it possible to make a power supply system for a downstream circuit of an AC or DC type electrical network by means of an upstream circuit of the opposite type by means of a voltage source type converter comprising at least one arm provided with half-arms equipped with protection circuits for which the balancing resistance Rs of each half-arm is disposed at one of the two ends of the half-arm, or distributed at the two ends of the half-arm.

[0150] The said protection component(s) are diodes or thyristors provided with a control circuit, for example.

[0151] The system may include an input filter having an impedance IZfI =1 Rf+jXfl » Rs, for example IZfl> 50 • Rs limiting the fault current Icc(t).

[0152] Furthermore, within the framework of the invention, the balancing resistance can be associated with a balancing inductance Ls to achieve a balancing impedance.

[0153] The method and system of this disclosure are primarily intended for low-voltage direct current (LVDC) public distribution networks and low-voltage direct current (LVDC) industrial networks supplied by the alternating current network. A direct application would be the supply of power to public lighting networks or charging stations directly from a dedicated feeder at a medium-voltage / low-voltage substation.

[0154] However, it is worth noting that the proposed solution can also be applied more specifically to converters used for electric motors. Indeed, during start-up, these motors can experience very high inrush currents, ranging from 4 to 7 times the nominal current. The invention would therefore make it possible to limit the oversizing of converters.

[0155] Fig. 7 illustrates the surfaces and curves related to the thermal resistance of the converter in a time plane as a function of the short-circuit current ratio to the nominal current of the converter.

[0156] Surface 11 is the damage-free operating surface of the converter, curve 12 is the protection tripping curve, surface 14 is the safety margin between the damage-free operating surface of the converter provided by the protection circuit of this disclosure and curve 12 of fuse or switching device tripping, curve 13 is the maximum thermal stress curve of the converter, surface 15 is the surface corresponding to protection given by fuse devices, zone 16 is an area that cannot be reached due to the limitation of the downstream short-circuit current while curve 17 is the maximum thermal stress of the converter without the protections of this disclosure.

[0157] Assuming the equipment in a network is correctly sized, the switching device closest to the fault should interrupt the most quickly; in this case, it is the one located on the DC side. However, if this switching device fails, the switching device on the AC network must interrupt the fault. The converter must therefore be able to withstand the load long enough to allow the switching devices to operate selectively. Figure 7 illustrates this principle: thanks to the device of this disclosure, the acceptable thermal stress of the converter is increased, which makes it possible to coordinate with the existing protection and even to provide a safety margin, represented by the area 14. In the example given, the maximum short-circuit current is 14 In, and for fault currents below 14 In, the protection will systematically have interrupted before the converter reaches its limit.

[0158] The invention is not limited to the examples described above, which are given only by way of example, but encompasses all the variations that a person skilled in the art could consider within the framework of the desired protection. In particular, the various device variants described in relation to the upper half-arm Hu of [Fig. 1] also apply to the lower half-arm Hb. Similarly, for a multi-arm converter, the protection and balancing resistances or impedances are applied to the different arms of the converter.

Claims

Demands

1. A method for implementing a system for supplying a downstream circuit of a DC-type electrical network by means of an upstream AC-type circuit using a voltage source-type converter having at least one half-arm (11, 12, 13) comprising at least one Txy-type semiconductor switch (110, 120a, 120b, 130a, 130b) and having at least one protection circuit comprising one or more power protection components (115, 125a, 125b, 135a, 135b, 135c, 135d), characterized in that it comprises: - as a function of a maximum thermal constraint I2-tmax that the converter can withstand, an estimated short-circuit current of the network flowing in said half-arm as a function of time Icc(t) and an estimated short-circuit duration tcc for said downstream circuit, - a sizing of said at least one power protection component (115, 125a, 125b, 135a, 135b, 135c, 135d) of the protection circuit to support a K-Iccmax portion of peak short-circuit current Iccmax such that I2-t converter remains less than I2-tmax throughout the entire duration tcc of the short circuit, - the calculation of a resistance ratio between a first resistance R1x, series resistance of said half-arm and a second resistance R2x, series resistance of said protection circuit, said protection circuit being connected in parallel with said half-arm, such that for the short-circuit current value Iccmax, the short-circuit current through said half-arm is limited to the maximum acceptable value (1K)-Iccmax for said at least one semiconductor switch, the additional portion of short-circuit current K-Iccmax through said protection circuit, - the introduction into said half-arm of a balancing resistance R$ (112, 122, 132) such that the sum of the balancing resistance (112, 122, 132) of a connecting resistance Rb (113, 123, 133) of said half-arm and of a conduction resistance of said at least one semiconductor switch (110, 120a, 120b, 130a, 130b) of said half-arm divided by the sum of the connecting resistance (114, 124a, 124b, 134a, 134b) of said protection circuit Rpp and of the conduction resistance of said at least one protection component RCp (115, 125a, 125b, 135a, 135b, 135c, 135d) is equal to the ratio Rlx / R2x.

2. Method according to claim 1 wherein, prior to calculating the balancing resistance Rs, a measurement of the connection resistance Rp of each half-arm is carried out at the level of the converter wiring.

3. Method according to claim 1 or 2 wherein said at least one protection component is chosen such that its threshold voltage is less than the threshold voltage of a freewheeling diode of said at least one switch.

4. A method according to claim 1, 2 or 3 wherein said half-arm comprises a semiconductor switch, the protection circuit comprises a protection component, and said balancing resistance Rs is calculated such that the freewheeling diode current of said semiconductor switch is given by the equation: ~ ((VP Vd) + + ) and the current in the protection component is given by the equation ip “ (r^rrMrd+r^ ^^D~Vp) + (rd + Rs + ) with: Vd the freewheeling diode threshold voltage, Rsc ~ Rd the conduction resistance of the freewheeling diode of said switch, Rp ~ Rrd^ the equivalent coupling resistance of said semiconductor switch, Rs the balancing resistance, Vp the threshold voltage of a protection component, Rpp the coupling resistance of the protection component, Rcp ~ RP the conduction resistance of the protection component,said protection component being chosen such that VP < VD and (RP + R^) « (Rd + RRp^ with RP + Rrp = R2x and Rd + Rrd + Rs = RLv,

5. The method according to claim 3, wherein said half-arm comprises several semiconductor switches in series, each of said inter- Semiconductor breakers include a so-called protection circuit.

6. A method according to claim 1, 2, or 3, wherein said half-arm comprises a semiconductor switch, the protection circuit comprises N protection components wired in parallel, each comprising a connecting resistance Rpp and a conduction resistance Rp, and wherein the balancing resistance Rs is calculated such that the freewheeling diode current of said semiconductor switch is given by the equation: “(RP + RPP^R[P^VD) + (Rp + RRp)i(T)) ) and, the current in a protection component is given by the equation: Rp ~ (Rp + Rrp) + N(Rd + R^D - VP) + (RD + RdK^) ) with: Rsc ~ the freewheeling diode threshold voltage, Rb ~ the freewheeling diode conduction resistance of said switch, Rd ~ the equivalent connecting resistance of said semiconductor switch, Rs the balancing resistance, Vp the threshold voltage of a component protection,Rpp is the connection resistance of a protective component, Rp is the conduction resistance of a protective component, N is the number of protective components connected in parallel, said protective components being chosen such that: VP <VD et (Rp + Rrp) < (Rd + R^ et avec RP + Rrp = R2x et N(RI} + Rs + Rw) - Rlx,

7. A method according to claim 1, 2, or 3, wherein the half-arm comprises M semiconductor switches in series and the protection circuit connected in parallel with a half-arm comprises N protection components wired in parallel with said arm, and the balancing resistance is such that the freewheeling current of said half-arm is given by the equation: and, the current in a protection component is given by the equation: IP = ARD,eq + RS + Rl^l^ with : y® _ y^ the equivalent threshold voltage of the freewheeling diodes of the semiconductor switches of the half-arm, p _ „ _ r> the equivalent resistance of the freewheeling diodes ~ Rü^q ~ L^RdJ of the semiconductor switches of the half-arm Rrd the equivalent coupling resistance of the half-arm, Rs the balancing resistance, VP the threshold voltage of a protection component, Rrp the coupling resistance of a protection component, RP the conduction resistance of a protection component, N the number of protection components connected in parallel, and with Vp < V[)eq and Rp + Rrp Rd^ + Rrd.

8. A method according to claim 1, 2, or 3, wherein the half-arm comprises M semiconductor switches in series, the protection device comprises Q protection components in series, distributed over N parallel branches, and the balancing resistance Φy is such that the freewheeling current of said half-arm is given by the equation: Φy = (Vp - Vp) and the current in a protection component is given by the equation Φy = (Rp + RpNv + Rrd + Rs) (Vp + Vp) with: Φy = Rd - Rd'a, the equivalent resistance of the freewheeling diodes of the semiconductor switches of the half-arm; Rrd = Rd - Rd'a, the equivalent coupling resistance of the half-arm; Rrd = Rd - Rs, the balancing resistance; and Vp = Vp. equivalent threshold voltage of the components of 'P,eq — 1 Pi series protection of the half-arm,Rrp is the connection resistance of a protection component, pp is the equivalent conduction resistance of the Rp+'q protection components in series, N is the number of series branches of protection components connected in parallel, and with Vp£q < Viyq and Rp,eq + Rrp Ro.eq + Rrd.,

9. A system for supplying a downstream circuit of an AC or DC type electrical network by means of an upstream circuit of the opposite type by means of a voltage source type converter comprising at least one arm having half-arms equipped with protection circuits made according to the method of any one of the preceding claims wherein the balancing resistance of each half-arm is disposed at one of the two ends of the half-arm, or distributed at both ends of the half-arm.

10. Power supply system according to claim 9 wherein said protection component(s) are diodes.

11. Power supply system according to claim 9 wherein said protection component(s) are thyristors provided with a control circuit.

12. Power supply system according to claim 9, 10 or 11 comprising an input filter having an impedance IZfl = IRf+jXfl » Rs limiting the fault current Icc(t).

13. Power supply system according to any one of claims 9 to 12 wherein the balancing resistance is associated with a balancing inductance Ls for realizing a balancing impedance.

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