Charging current method, charging current device, and electronic converter with the charging current device

Abstract

In a charging current method for limiting a charging current in a DC voltage circuit, the charging current is conducted from an electric supply grid into the DC voltage circuit via diodes. A DC voltage of the DC voltage circuit is ascertained and is based on the supply voltage of the electric supply grid. A variable undervoltage threshold on the DC voltage circuit is determined and a limit of the charging current is activated when the DC voltage reaches or falls below the variable undervoltage threshold.

Claims

1. A charging current method for limiting a charging current in a DC voltage circuit, the charging current method comprising: conducting the charging current from an electric supply grid into the DC voltage circuit via diodes; ascertaining a DC voltage on the DC voltage circuit, with the DC voltage being dependent on a supply voltage of the electric supply grid; determining and updating a variable undervoltage threshold on the DC voltage circuit when the ascertained DC voltage embodied as an averaged DC voltage increases over a time period; and activating the limiting of the charging current when the DC voltage reaches or falls below the variable undervoltage threshold.

2. The charging current method of claim 1, further comprising forming a voltage difference between the DC voltage and the variable undervoltage threshold, with the voltage difference remaining unchanged irrespective of the value of the DC voltage.

3. The charging current method of claim 1, further comprising forming a voltage difference between the DC voltage and the variable undervoltage threshold, with the voltage difference varying as a function of the value of the DC voltage.

4. The charging current method of claim 1, further comprising activating an electric precharging of the DC voltage circuit and/or a reduction in energy extraction on the DC voltage circuit in order to limit the charging current.

5. The charging current method of claim 1, further comprising determining the variable undervoltage threshold as a function of a capacitance on the DC voltage circuit.

6. The charging current method of claim 1, further comprising determining the variable undervoltage threshold as a function of a grid impedance on the electric supply grid.

7. The charging current method of claim 1, further comprising: determining the variable undervoltage threshold by a modulation index of a controller for switchable semiconductors of a bridge circuit; connecting the diodes as backward diodes into the DC voltage circuit in parallel with the switchable semiconductors; and embodying the modulation index as a quotient of the supply voltage of the electric supply grid and the DC voltage of the DC voltage circuit.

8. A charging current device for limiting a charging current, the charging current device comprising: a DC voltage circuit; a plurality of diodes connectable to an electric supply grid and connected to the DC voltage circuit, with the charging current capable of being conducted via the diodes from the electric supply grid Into the DC voltage circuit; and a processor unit designed to ascertain a DC voltage on the DC voltage circuit, with the DC voltage being dependent on a supply voltage of the electric supply grid, for determining and updating a variable undervoltage threshold on the DC voltage circuit by an increase checking function when the ascertained DC voltage increases over a time period, and for activating the limiting of the charging current when the DC voltage reaches or falls below the variable undervoltage threshold.

9. The charging current device of claim 8, wherein the processor unit is designed to form a voltage difference between the DC voltage and the variable undervoltage threshold, with the voltage difference remaining unchangeable irrespective of a value of the DC voltage or being variable as a function of the value of the DC voltage.

10. The charging current device of claim 8, further comprising a precharging unit designed to activate an electric precharging of the DC voltage circuit to limit the charging current and/or a disconnect unit designed to activate a reduction in energy extraction on the DC voltage circuit to thereby limit the charging current.

11. The charging current device of claim 8, wherein the processor unit is designed to determine the variable undervoltage threshold as a function of a capacitance on the DC voltage circuit and/or as a function of a grid impedance on the electric supply grid.

12. The charging current device of claim 8, wherein the processor unit is designed to provide a modulation index of a controller for switchable power semiconductors of a bridge circuit in order to determine the variable undervoltage cutoff threshold, wherein the diodes are connected as backward diodes into the DC voltage circuit in parallel with the switchable power semiconductors, and wherein the modulation index is embodied as a quotient of the supply voltage of the electric supply grid and the DC voltage of the DC voltage circuit.

13. An electric converter, comprising: a charging current device comprising a DC voltage circuit, a DC voltage circuit, a plurality of diodes connectable to an electric supply grid and connected to the DC voltage circuit, with a charging current capable of being conducted via the diodes from the electric supply grid into the DC voltage circuit, and a processor unit designed to ascertain a DC voltage on the DC voltage circuit, with the DC voltage being dependent on a supply voltage of the electric supply grid, for determining and updating a variable undervoltage threshold on the DC voltage circuit by an increase checking function when the ascertained DC voltage increases over a time period, and for activating the limiting of the charging current when the DC voltage reaches or falls below the variable undervoltage threshold; and an electric machine electrically connectable to the DC voltage circuit and operatable on the electric supply grid via the diodes and the DC voltage circuit.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The above-described characteristics, features and advantages of this invention, as well as the manner in which these are realized, will become clearer and more readily understandable in connection with the following description of the exemplary embodiments, which are explained in more detail with reference to the figures, in which:

(2) FIG. 1 shows a schematic representation of a structure chart of the inventive charging current method,

(3) FIG. 2 shows a first schematic representation of a diagram containing a variable undervoltage threshold for the inventive charging current method according to FIG. 1,

(4) FIG. 3 shows a second schematic representation of a diagram containing an exemplary embodiment of the variable undervoltage threshold according to FIG. 2 for the inventive charging current method according to FIG. 1,

(5) FIG. 4 shows a third schematic representation of a diagram containing the variable undervoltage threshold according to FIG. 2 or FIG. 3 at the time of a voltage failure of a DC voltage on a DC voltage circuit as a function of a voltage failure of a supply voltage of a supply grid for the inventive charging current method according to FIG. 1, and

(6) FIG. 5 shows a schematic representation of an inventive electric converter with an inventive charging device for the inventive charging current method according to FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(7) FIG. 1 shows a schematic representation of a structure chart of the inventive charging current method 1.

(8) The charging current method 1 limits a charging current I.sub.L in a DC voltage circuit, the charging current I.sub.L being conducted from an electric supply grid into the DC voltage circuit via diodes.

(9) In a first step of the charging current method 1, a DC voltage U.sub.DC on the DC voltage circuit is ascertained 5, said DC voltage U.sub.DC being dependent on a supply voltage U.sub.V of the electric supply grid. To that end, both the supply voltage U.sub.V on the electric supply grid and the DC voltage U.sub.DC on the DC voltage circuit can be measured in each case, but can also be determined or calculated from other electrical variables.

(10) In a further step, a variable undervoltage threshold 7 is determined 6 on the DC voltage circuit.

(11) In order to determine 6 the variable undervoltage threshold 7, a voltage difference U is formed 9 between the DC voltage U.sub.DC and the variable undervoltage threshold 7.

(12) Further, the voltage difference U can be formed such that it remains unchanged irrespective of the value of the DC voltage U.sub.DC.

(13) The voltage difference U can also be formed such that the voltage difference U varies as a function of the value of the DC voltage U.sub.DC.

(14) The variable undervoltage threshold 7 in the exemplary embodiment according to FIG. 1 can be determined 6 both as a function of a capacitance C.sub.GK on the DC voltage circuit and as a function of a grid impedance Z.sub.N on the electric supply grid.

(15) The variable undervoltage threshold 7 is determined 6 only if the DC voltage U.sub.DC increases over a specific time period t.sub.B.

(16) The variable undervoltage threshold 7 can also be determined 6 by means of a modulation index M of a controller for switchable semiconductors. In this case the diodes are each connected into the circuit as backward diodes in parallel with the switchable semiconductors of a bridge circuit, wherein the bridge circuit can then come into use for example in an active infeed between the supply grid and the DC voltage circuit in the rectifier mode of operation. The modulation index M is in this case embodied as the quotient of the supply voltage U.sub.V of the electric supply grid and the DC voltage U.sub.DC of the DC voltage circuit.

(17) The activation of a limiting 8 of the charging current I.sub.L commences if the DC voltage U.sub.DC reaches or falls below the variable undervoltage threshold 7. The limiting 8 becomes effective when the charging current starts to flow in the DC voltage circuit following the voltage drop and the voltage recovery.

(18) In FIG. 1, an electric precharging 10 of the DC voltage circuit and a reduction in energy extraction on the DC voltage circuit are activated for the purpose of limiting 8 the charging current I.sub.L.

(19) FIG. 2 shows a first schematic representation of a diagram containing a variable undervoltage threshold 7 for the inventive charging current method 1 according to FIG. 1.

(20) The diagram has two axes, one axis being labeled with a time t and the other axis with a voltage U.

(21) A DC voltage U.sub.DC plotted on the axis of the voltage U as a ripple DC voltage 20 is followed at a spacing of a voltage difference U by the variable undervoltage threshold 7, which is embodied as a variable undervoltage threshold 22 adjusted to match the ripple DC voltage 20.

(22) The DC voltage U.sub.DC plotted as a ripple DC voltage 23 on the axis of the voltage U is followed by the static undervoltage threshold 19, which is embodied as a static undervoltage threshold 23 fitted to the ripple DC voltage 20, at a spacing of a further voltage difference U. It is apparent here that the variable undervoltage threshold 7 of the DC voltage U.sub.DC embodied as a ripple DC voltage 20 can follow flexibly at a narrower spacing, wherein the static undervoltage threshold 19 remains unchanged.

(23) A time period t.sub.B stretches between a first time point t.sub.P1 and a second time point t.sub.P2. The time period t.sub.B is chosen in this case such that it is shorter than a likely fluctuation in the supply voltage of the supply grid. A time period of 10 ms is proposed as a typical value.

(24) FIG. 3 shows a second schematic representation of a diagram containing an exemplary embodiment of the variable undervoltage threshold 7 according to FIG. 2 for the inventive charging current method 1 according to FIG. 1.

(25) Analogously to the diagram of FIG. 2, the diagram has two axes, one axis likewise being labeled with the time t and the other axis with the voltage U and a time period t.sub.B stretching between the first time point t.sub.P1 and a second time point t.sub.P2, as in FIG. 2.

(26) A DC voltage U.sub.DC plotted on the axis of the voltage U as an averaged DC voltage 21 is followed at a spacing of the voltage difference U by the variable undervoltage threshold 7, which is embodied as a variable undervoltage threshold 24 adjusted to match the averaged DC voltage 20.

(27) The DC voltage U.sub.DC plotted on the axis of the voltage U as an averaged DC voltage 25 is followed at a spacing of the further voltage difference U by the static undervoltage threshold 19, which is embodied as a static undervoltage threshold 25 fitted to the averaged DC voltage 21. Here in FIG. 3 also it is evident that the variable undervoltage threshold 7 of the DC voltage U.sub.DC embodied as an averaged DC voltage 21 can follow flexibly at a narrower spacing, the static undervoltage threshold 19 remaining unchanged.

(28) Both FIG. 2 and FIG. 3 show that the variable undervoltage threshold 7 lies significantly higher and closer to the ascertained DC voltage U.sub.DC compared to the conventional static undervoltage threshold 19. Accordingly, the voltage difference U between variable undervoltage threshold 7 and DC voltage U.sub.DC is likewise significantly reduced compared to the further voltage difference U. As a result, the current flow of the charging current starting following the voltage drop and voltage recovery, and consequently the i.sup.2t value, is also significantly reduced compared to the known solution with the static undervoltage threshold 19.

(29) FIG. 4 shows a third schematic representation of a diagram containing the variable undervoltage threshold 7 according to FIG. 3 at the second time point t.sub.P2 of a voltage failure of a DC voltage U.sub.DC on a DC voltage circuit for the inventive charging current method according to FIG. 1.

(30) The diagram of FIG. 4 is to be understood analogously to the diagram of FIG. 3. However, a voltage drop of the DC voltage U.sub.DC occurs at the second time point t.sub.P2, driven by the voltage drop of the supply voltage of the supply grid, wherein the DC voltage U.sub.DC reaches and falls below the variable undervoltage threshold 7.

(31) It becomes clear in FIG. 4 that if the variable undervoltage threshold 7 were to be updated further starting from the second time point t.sub.P2, this would decrease to the same extent as the ascertained DC voltage U.sub.DC actually decreases. A reaching and falling below of the variable undervoltage threshold 7 desired in the case of an imminent voltage drop would therefore be scarcely possible.

(32) Accordingly, the undervoltage threshold 7 must be updated when the ascertained DC voltage U.sub.DC increases over the time period t.sub.B. An increase in the ascertained DC voltage U.sub.DC for determining or updating the variable undervoltage threshold 7 over the specific time period t.sub.B is present if an increase in the DC voltage U.sub.DC, in this case embodied as an averaged DC voltage 21, occurs over the specific time period t.sub.B.

(33) FIG. 5 shows a schematic representation of an inventive electric converter with an inventive charging device for the inventive charging current method according to FIG. 1.

(34) In addition to the charging device 13, the electric converter 17, also shown in more detail here as a frequency converter having a DC voltage circuit 2also referred to as an intermediate DC link voltage circuitincludes a passive rectifier 26 with diodes 3 which are electrically connected to the DC voltage circuit 2 by means of three bridge branches of a bridge circuit. The rectifier 26 together with its diodes 3 is connected to a supply grid 4, depicted here as a three-phase grid. The supply voltage U.sub.v is applied to the three-phase supply grid 4. The supply grid 4 is also determined by its grid inductance Z.sub.N.

(35) An electric machine 18 is electrically connected to the DC voltage circuit 2, in this case via an inverter 27. A capacitance C.sub.GK, in this case an intermediate DC link voltage circuit capacitance, is also connected to the DC voltage circuit 2. The DC voltage 2 is applied to the DC voltage circuit 2. In the case of voltage recovery following a voltage failure, the charging current I.sub.L flows in the DC voltage circuit 2. It can be limited as a result of an electric precharging by means of a precharging unit 15 or as a result of a reduction in energy extraction by means of a disconnect facility 16in this case the inverter 27which isolates the electric machine 18 electrically from the DC voltage circuit 2.

(36) The inventive charging method can furthermore be performed by means of a processor unit 14, the processor unit 14 having an increase checking function 12 for ascertaining the increase or reduction in the DC voltage U.sub.DC.