WO2011157469A1

WO2011157469A1 - Circuit arrangement for determining a voltage fluctuation of conductor potentials in an unearthed electrical network

Info

Publication number: WO2011157469A1
Application number: PCT/EP2011/056662
Authority: WO
Inventors: Andreas Trautmann; Vicente Garcia Alvarez; Dragan Mikulec
Original assignee: Robert Bosch Gmbh
Priority date: 2010-06-15
Filing date: 2011-04-27
Publication date: 2011-12-22
Also published as: DE102010030133A1

Abstract

The invention relates to a circuit arrangement for determining a voltage fluctuation of conductor potentials in an unearthed electrical network, wherein the network comprises a DC voltage intermediate circuit (6), an n-phase network (1) with an n-phase electrical load (2), where n ≥ 3, and at least one inverter (3) which is connected to the DC voltage intermediate circuit (6) and is intended to control the electrical load (2). The invention provides a neutral point (P1) at which the phases (U, V, W) of the n-phase network (1) are combined via impedances (Z₁, Z₂, Z₃). A measuring device (19) for measuring a measurement voltage (U_M) at the neutral point (P1) with respect to a reference potential is also provided, wherein the measurement voltage (U_M) represents the voltage fluctuation of supply voltage potentials (T+, T-) of the DC voltage intermediate circuit (6) with respect to the reference potential.

Description

Description title

Circuit arrangement for determining a voltage fluctuation of

Conductor potentials in an ungrounded electrical network

The invention relates to a circuit arrangement for determining a

Voltage fluctuation of conductor potentials in an ungrounded electrical network.

State of the art

For the drive in hybrid or electric vehicles electrical machines in the form of induction machines are usually used, which in conjunction with inverters - often referred to as inverter - are operated. The electrical energy for the operation of the electric machine is in this case from a disconnected from the electrical system of the vehicle, ungrounded power supply, e.g. in the form of a powerful high-voltage battery. The ungrounded electrical network created in this way - often referred to as the IT network (Isole Terre) - reduces the risk of e.g. of service personnel, since with a single error, such as an insulation fault, no closed circuit is built. In addition, the operation must not be adjusted when a single fault occurs, so that an insulation fault can be reported without it already has a system failure. For this, however, it is necessary that the insulation resistance of the electrical network is continuously or at least periodically monitored during operation of the vehicle, which is possible, for example, based on a voltage fluctuation of the conductor potentials of the IT network.

From DE 10 2006 031 663 B3 is a method for measuring the

Insulation resistance in an IT network with a DC intermediate circuit and at least one self-commutated converter and a measuring arrangement for measuring the DC link voltage against ground potential known in the an offline and an online measurement are provided. During the offline measurement, during which all power switches of the power converter are closed, the potentials Up and Um as well as the intermediate circuit voltage are measured and from this the insulation resistance is determined. During the online measurement, the potentials Up and Um are measured and the time course of the measurements is evaluated. For this purpose, in particular the two potentials are summed, the sum Fourier-transformed and evaluated the change of the frequency spectrum in its time course. From EP 1 909 369 A2 a method for insulation monitoring for in-service converter arrangements is known, wherein the converter arrangement comprises a voltage intermediate circuit having at least one positive branch and one negative branch, at least one electrical device having at least two phase terminals, and at least one inverter Circuit elements for electrically connecting the phase terminals with the positive branch or the negative branch of the voltage intermediate circuit comprises. It is provided that an operating state of the inverter during the inverter is in operation and the electrical device, which also in a

Normal operation is, fed, is determined by detecting parameters of a converter control. In addition, at least one of the voltages of the positive branch or the negative branch is measured. Finally, in accordance with the measured voltage or voltages and the operating state of the inverter insulation defects at the voltage intermediate circuit and / or on the

Phase connections and / or determined on the electrical device.

Disclosure of the invention

The present invention provides a circuit arrangement for determining a voltage fluctuation of conductor potentials in an ungrounded electrical network, the network comprising a DC intermediate circuit, an n-phase network with an n-phase electrical load, n> 3, and at least one inverter connected to the DC link to

Control of the electrical load includes. According to the invention is a

(Artificial) star point provided at which the phases of the n-phase network are brought together via impedances, also is a measuring device for measuring a, a measuring voltage at the neutral point with respect to a Characterized reference potential during the operation of the electrical load consumer, wherein the measuring voltage represents the voltage fluctuation of supply voltage potentials of the DC link to the reference potential.

During operation of the electrical load and thus during operation of the inverter, the DC potentials of the

Supply voltage rails of the DC intermediate circuit

Superimposed on AC components, which leads to a voltage fluctuation of the supply voltage potentials of the DC intermediate circuit against a reference potential, which e.g. is formed by a vehicle body, lead. The invention is based on the idea that a neutral point, at which the phases of the at least three-phase network are combined, changes during operation of the electrical load and thus of the inverter depending on the respective switching state of the inverter. Related to one

DC link voltage U _ZK , the potential of the neutral point changes abruptly between the values 1/3 ^* U _Z K and 2/3 ^* U _ZK . If one refers to the potential change to half the DC link voltage, which in a symmetrical DC voltage intermediate circuit at least due to the existing

Insulation resistance drops in each case to the reference potential, the potential change of the neutral point is ± 1/6 ^* U _ZK . Furthermore, the reference potential during operation of the inverter oscillates against half

DC link voltage. The amplitude of this oscillation depends on the

Resistance conditions between the insulation resistances in the

DC link and the insulation resistors in the n-phase network from. The difference between the reference potential and the potential of the

Star point, which corresponds to the measuring voltage, thus provides a measure of a voltage fluctuation of the supply voltage potentials of

DC voltage intermediate circuit against the reference potential. Thus, at the star point by means of a single measurement directly a voltage can be measured, which directly a voltage fluctuation of

Supply voltage potentials of the DC intermediate circuit against the reference potential represents. Potential errors due to further measurements or subsequent calculations are thus reliably avoided. Alternatively to the direct measurement of the measuring voltage, another of the

Measuring voltage derived size, which thus characterized the measuring voltage, are measured. The described relationships apply regardless of the operating mode (block timing, pulse width modulation) of the inverter.

According to one embodiment of the invention, a calculation unit is additionally provided, which determines an auxiliary voltage by forming the difference between a star point voltage, which results at the star point in relation to the half of the intermediate circuit voltage, and the measuring voltage. These too

Auxiliary voltage represents voltage fluctuations of

Supply voltage potentials of the DC intermediate circuit against the reference potential and can thus also, for example. for the determination of

Insulation resistance of the IT network can be used.

According to one embodiment of the invention, the measuring range is the

Voltage measuring device adapted to a maximum amplitude of the voltage fluctuation, whereby the measurement accuracy is increased

Brief description of the drawings

Show it:

1 is a schematic block diagram of an ungrounded network with a DC intermediate circuit, a connected thereto inverter, a 3-phase electric machine and a circuit arrangement according to the invention,

2 is a graphical representation of the time course of the potential at a neutral point of the 3-phase network with respect to half the DC link voltage,

3 is a graphical representation of the time course of

Reference potential during operation of the inverter based on half the DC link voltage,

4 is a graphic representation of the time course of

Measuring voltage in normal operation without insulation fault (unfiltered and filtered), a graphical representation of the frequency spectrum of

Measuring voltage of FIG. 4, a graphical representation of the time course of the

Measuring voltage when a single-phase unbalanced insulation fault occurs in the 3-phase network, a graphic representation of the frequency spectrum of the

Measuring voltage of FIG. 6, a graphical representation of the time course of the

Measuring voltage when a symmetrical insulation fault occurs in the 3-phase network (unfiltered and filtered) and a graphic representation of the frequency spectrum of the

Measuring voltage acc. Fig. 8.

Embodiments of the invention

In the figures, identical or functionally identical components are each marked with the same reference character. 1 shows a schematic representation of a 3-phase network 1 with a three-phase electric machine 2, which may be designed, for example, as a synchronous, asynchronous or reluctance machine with a pulse inverter connected thereto 3. The pulse inverter 3 includes

Switching elements 4a-4f in the form of circuit breakers, which are connected to individual phases U, V, W of the electric machine 2 and the phases U, V, W either against a voltage applied to a positive supply voltage rail 5 of a DC intermediate circuit 6 positive

Supply voltage potential T + or one at a negative

Supply voltage rail 7 of the DC intermediate circuit 6 adjacent negative supply voltage potential T- switch. The connected to the positive supply voltage rail 5 switching elements 4a-4c are also called "high-side switch" and the negative Supply voltage rail 7 connected switch 4d-4f referred to as "low-side switch" and can be embodied for example as Insulated Gate Bipolar Transistor (IGBT) or as Metal Oxide Semiconductor Field-Effect Transistor (MOSFET) The pulse inverter 3 further comprises a plurality of freewheeling diodes 8a 8f, which are each arranged parallel to one of the switching elements 4a-4f.

The pulse inverter 3 determines the power and mode of operation of the electric machine 2 and is controlled by a controller 9, e.g. in the form of a microcontroller, driven accordingly. The electric machine 2 can be operated either in motor or generator mode.

The pulse inverter 3 also includes a so-called

DC link capacitor 10, which essentially serves to stabilize a voltage of a high-voltage energy storage in the form of a high-voltage battery 1 1 in the DC voltage intermediate circuit 6. An electrical system 12 of the vehicle with a low-voltage energy storage in the form of a low-voltage battery 13 is connected via a DC-DC converter 14 parallel to the DC-link capacitor 6.

The electric machine 2 is designed in the illustrated embodiment, three-phase, but may also have more than three phases. Preferably, however, the number of phases is divisible by three or at least by three. For example, for service purposes, it is necessary, the high-voltage battery 1 1 in the idle state of the DC voltage intermediate circuit 6 - often as

Traction network or high-voltage circuit - to disconnect. There are two

Main contactors 15 and 16 and a Vorladeschütz 17 are provided. The

Vorladeschütz allows a current limited charge of the

DC link capacitor via a pre-charge resistor 18th

The phases U, V, W in the 3-phase network 1 are combined via impedances Zi, Z ₂ and Z ₃ to an (artificial) star point P1. With the help of a

Measuring device 19 can be measured at the star point P1, a measurement voltage U _M relative to the reference potential, which directly a

Voltage fluctuation of the supply voltage potentials T + and T- of the DC intermediate circuit 6 is represented against the reference potential. The measuring range of the voltage measuring device 19 is advantageous to a maximum amplitude of the voltage fluctuation adjusted. The impedances Zi, Z ₂ and Z ₃ can, as shown, be formed from ohmic resistors or else with the aid of capacitances and / or inductances. Of course, in the individual connecting lines to the star point P1 also several

Impedances can be provided. Alternatively to the direct measurement of

Measuring voltage U _M can also be another derived from the measuring voltage U _M size, which thus characterized the measuring voltage U _M , are measured.

During operation of the electrical load 2 and thus of the inverter 3, the potential at the neutral point P1 changes depending on the respective one

Switching state of the pulse inverter 3. In Figure 2, the time course of a neutral point voltage U _P i, ie a potential difference of the neutral point P1 based on half the DC link voltage example shown for block timing of the pulse inverter. The potential of the star point P1 fluctuates abruptly by half the intermediate circuit voltage with an amplitude of ± 1/6 * U _Z K-

During operation of the pulse inverter 3, the reference potential oscillates against half the DC link voltage, as shown in Figure 3 by way of example for block timing. The amplitude of this oscillation depends on the

Resistance conditions between the insulation resistances in the

DC voltage intermediate circuit 6 and the insulation resistors in the n-phase network 1 from. The difference between the reference potential and the potential of the star point P1 corresponds to the measuring voltage U _M , which is determined by means of the

Voltage measuring device 19 is measured. This represents one

Voltage fluctuation of supply voltage potentials T +, T- of

DC intermediate circuit 6 against the reference potential (see Figure 4).

The curves of the neutral point voltage (FIG. 2) and of the reference potential (FIG. 3) are shown by way of example for block timing. Similar courses, however, also result in pulse width modulation of the pulse-controlled inverter 3.

In the following, an example of a use of the measured measurement voltage U _M in monitoring the insulation resistance in the ungrounded electrical network is explained in more detail.

By an evaluation unit 23, which is integrated in the illustrated embodiment of Figure 1 in the control unit 8, but alternatively can also be implemented as a separate unit, the measuring voltage U _M a Frequency transformation, preferably a fast Fourier transform (FFT) subjected, in order to calculate the frequency spectrum of the measuring voltage U _M in this way. By evaluation of the magnitude spectral amplitudes | U _M 0 ^ω ) I ^e i predetermined electrical frequencies or angular velocities, then an insulation fault can be detected according to the invention. However, the predetermined electrical frequencies or angular velocities are not fixed values, but dependent on an electrical angular velocity ω _β ι the electric machine 2, which is proportional to the electrical frequency of the electric machine 2.

Therefore, the electric frequency of the electric machine 2 becomes one

characterizing size, such as the electrical angular velocity ω _β ι determined. This determination can be made on the basis of metrological results. Frequently, however, the electrical frequency of the electric machine 2 is also given, so that it is already known.

An insulation fault, that is to say a deterioration of the insulation resistance, is noticeable in that the amplitude of the spectral amplitude U _M CK · ω _β 1 changes at certain frequencies. Depending on whether it is a symmetrical or a single-ended insulation fault, the change in the spectral amplitude at 3 times electrical

Angular velocity ω _β ι, ie at K = 3, or at the (1-fold) electrical

Angular velocity ω _β ι, ie at K = 1. However, this relationship will be explained in detail below. The change in the amount of

Spectral amplitude is in each case a measure of the deterioration of the

Insulation resistance.

Figure 4 shows the time course of the measuring voltage U _M in normal operation of the electric machine 2 and thus of the pulse inverter 3 without insulation fault. The unfiltered curve is shown on the left and the curve on the right after an optional low-pass filtering of the measuring voltage U _M. These

Representation form is also selected for the figure 8.

A fast Fourier transformation of the sum voltage shown in FIG. 4 results in a spectral distribution shown schematically in FIG

(Frequency spectrum). It can be seen that in the (1-fold) electrical Angular velocity ω _β ι no signal component is present and at the 3-fold electrical angular velocity 3 · ω _β ι a signal component with a

Spectral amplitude of A _{0 is} present. Now occurs in the area of the 3-phase network 1 a single-phase unbalanced

Insulation failure, that is to say a deterioration of the insulation resistance on one of the three phases U, V or W, results in a changed temporal course of the measuring voltage U _M (see FIG. 6) and also an altered spectral distribution (see FIG. In particular occurs in the (1-fold) electrical

Angular velocity ω _β ι now a signal component with a spectral amplitude of Ai, which did not occur in error-free case or at least in one

Background noise went down. If one thus compares the spectral amplitude A-i with the corresponding spectral amplitude serving as the reference value in the error-free case, in this case a spectral amplitude of 0, then in the case of

Deviation an unbalanced isolation error can be reliably detected. The magnitude amplitude change, that is in this case the so

Amplitude value A-i itself, is a measure of the deterioration of the

Insulation resistance. In this case, as with the subsequent detection of insulation faults, it is of course also possible to specify a minimum value for the deviation which must be exceeded before an insulation fault is detected.

FIGS. 8 and 9 show the time profile of the measuring voltage U _M or the resulting spectral distribution when a symmetrical insulation fault occurs in the 3-phase network 1. This affects the

Deterioration of the insulation resistance on all three phases in an analogous manner. It can be seen from FIG. 8 that such an insulation fault is manifested by the fact that the spectral amplitude has been reduced from a value A ₀ to a value A ₃ at 3 times the electrical angular velocity 3 · ω _β ₁ . The amount of waste is again a measure of the

Deterioration of the insulation resistance. By comparing the

Spectral amplitude of 3 times the electrical angular velocity 3 · ω _β ι with the corresponding spectral amplitude serving as a reference value in the error-free case, in this case A ₀ , thus a symmetrical insulation error can be detected reliably. A similar effect is also evident when symmetric

Insulation error in the DC intermediate circuit 6. Here too, there is a change in the spectral distribution in the range of 3-fold electrical

Angular velocity 3 · ω _β ι, but in the form of an increase in the

Amplitude value to a higher value than in normal operation, as higher than A ₀ . In this case, the increase in magnitude is a measure of the deterioration of the insulation resistance.

For the applicability of the invention, it is only crucial that

Spectral amplitudes at the 1-fold and 3-fold, or in the case of an n-phase network of n-fold electrical frequency or angular velocity to determine. In that sense, instead of a frequency transformation, too

Bandpassfilterungen with corresponding center frequencies at ω _β ι and 3 · ω _β ι (n · oüei) are used and the required amplitude values are then calculated from the filtered sum voltage.

As an alternative to the measurement voltage U _M itself, an auxiliary voltage U _H can also be calculated with the aid of a calculation unit not shown separately, which then determines the voltage fluctuation of the supply voltage potentials T +, T- of the DC voltage intermediate circuit 6 against the reference potential

represents. Where:

The calculation unit can be integrated in the control unit 9 or, alternatively, be implemented as an independent unit.

If the insulation resistance in the ungrounded electrical network is to be monitored on the basis of the auxiliary voltage U _H , an almost analogous evaluation method described above can be used for this purpose. It should only be noted that when the auxiliary voltage U _{H is} evaluated, a symmetrical insulation fault in the 3-phase network 1 is shown by a reduction in the spectral amplitude at three times the angular velocity and a symmetrical insulation fault in the DC intermediate circuit 6 by an increase in the spectral amplitude at three times the angular velocity. Insofar, therefore, the effects are just the opposite as in the immediate evaluation of the measurement voltage U _M -

Claims

Claims 1. Circuit arrangement for determining a voltage fluctuation of conductor potentials in an ungrounded electrical network, wherein the network comprises

a DC voltage intermediate circuit (6),

- An n-phase network (1) with an n-phase electrical

Consumers (2), with n> 3, and

- At least one of the DC voltage intermediate circuit (6) connected to the inverter (3) for controlling the electrical load (2)

marked by

a star point (P1) at which the phases (U, V, W) of the n-

Phase network (1) via impedances (Z- _\ , Z ₂ , Z ₃ ) are merged, and

- A measuring device (19) for measuring a, a

Measuring voltage (U _M ) at the neutral point (P1) with respect to a reference potential characterizing size, wherein the

Measuring voltage (U _M ) the voltage fluctuation of

Supply voltage potentials (Τ +, T-) of the DC intermediate circuit (6) represents the reference potential.

2. Circuit arrangement according to claim 1, wherein the

Circuit arrangement additionally comprises a calculation unit, which by subtraction between a neutral point voltage, which results at the neutral point (P1) against half a DC link voltage (U _Z K), and the measuring voltage (U _M ) a

Auxiliary voltage (U _H ) determines, wherein the auxiliary voltage (U _H ) represents the voltage fluctuation of the supply voltage potentials (Τ +, T-) of the DC intermediate circuit (6) against a reference potential.

3. Circuit arrangement according to claim 1 or 2, wherein the

Measuring range of the measuring device (19) is adapted to a maximum amplitude of the measuring voltage (UM).