WO2011157470A1 - Circuit arrangement for determining a voltage fluctuation of conductor potentials in an unearthed electrical network - Google Patents

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), 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 voltage divider (30), in particular a symmetrical voltage divider, which is connected between supply voltage potentials (T+, T-) of the DC voltage intermediate circuit (6) and has a centre tap (M). A measuring device (31) for measuring a measurement voltage (U_M) at the centre tap (M) of the voltage divider (30) is also provided, wherein the measurement voltage (U_M) represents the voltage fluctuation of the supply voltage potentials (T+, T-) of the DC voltage intermediate circuit (6) with respect to a 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 voltage intermediate circuit and at least one self-commutated converter and a measuring arrangement for measuring the DC link voltage against ground potential known in which an offline and an online measurement are provided. It will be during the Offline measurement, during which all circuit breakers of the power converter are closed, the potentials Up and Um and the intermediate circuit voltage are measured and used to determine the insulation resistance. 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-use converter arrangements is known, wherein the converter arrangement a

Voltage link with 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 with switching elements for electrical

Connection of the phase terminals with the positive branch or the negative branch of the voltage intermediate circuit comprises. It is envisaged that a

 Operating state of the inverter during which the inverter is in operation and the electrical device, which is also in a normal mode, feeds, 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, according to the measured voltage or

Voltages and the operating state of the inverter Insulation defects at

Voltage intermediate circuit and / or determined at the phase terminals and / or 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, wherein the network is a DC voltage intermediate circuit, an n-phase network with an n-phase electrical load, with n> 1, and at least one connected to the DC link Inverter for controlling the electrical load includes. According to the invention, a voltage divider, in particular a symmetrical voltage divider, is provided which switches between supply voltage potentials of the DC intermediate circuit

connected. The voltage divider in this case has a center tap, on which by means of a measuring device one, a measuring voltage characterizing size is measured, the measuring voltage the

Voltage fluctuation of the supply voltage potentials of the

DC intermediate circuit represents a 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 the center tap of a

Voltage divider, which is connected between the two supply voltage rails of the DC intermediate circuit, its potential with respect to the conductor potentials, so the potentials of the supply voltage rails does not change, so that the potential of the center tap against the reference potential fluctuates to the same extent as the line potentials. Thus, at the

Mittelabgriff be measured by a single measurement directly a voltage which directly a voltage fluctuation of the

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.

According to one embodiment of the invention, the measuring range of the measuring device is adapted to a maximum amplitude of the voltage fluctuation, whereby the measuring accuracy is increased. Further features and advantages of embodiments of the invention will become apparent from the following description with reference to the accompanying figures.

Brief description of the drawings

Show it: a schematic block diagram of an ungrounded network with a DC voltage intermediate circuit, a thereto

 connected inverter, a 3-phase electric machine and a circuit arrangement according to the invention, a graphical representation of the time course of the

 Measuring voltage in normal operation without insulation fault, a graphical representation of the frequency spectrum of

 Measuring voltage of FIG. 2, 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. 4, a graphical representation of the time course of the

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

 Measuring voltage acc. Fig. 6.

EMBODIMENTS OF THE INVENTION In the figures, identical or functionally identical components are each identified by the same reference numeral.

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 connected thereto pulse inverter 3. The pulse inverter 3 includes

Switching elements 4a ^ lf in the form of circuit breakers, which with individual phases U, V, W of the electric machine 2 are connected 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 11 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 only two or more than three phases.

 Preferably, however, the number of phases is equal to three or at least divisible.

For example, for service purposes, it is necessary, the high-voltage battery 1 in the idle state of the DC voltage intermediate circuit 6 - often as

Traction network or high-voltage circuit - to disconnect. For this purpose, 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

Furthermore, in the DC voltage intermediate circuit 6 parallel to the

DC link capacitor 10, a voltage divider 30 is provided, which is preferably designed symmetrically. At a center tap M a measuring voltage U _{M is} measured with the aid of a measuring device 31 relative to the reference potential, which directly a voltage fluctuation of

Supply voltage potentials T + and T- represents the DC intermediate circuit 6 against the reference potential. The measuring range is the

Voltage measuring device 31 advantageously to a maximum amplitude of

Voltage fluctuation adjusted. The voltage divider 30 can, as shown, be formed from ohmic resistors 32 and 33 or else with the aid of capacitances and / or inductances. Decisive for the usability is only the voltage dividing function. Of course, the

 Voltage dividers 30 can also be formed from more than two components.

As an alternative to the direct measurement of the measuring voltage U _M , another variable derived from the measuring voltage U _M , which thus characterizes the measuring voltage U _M , can also be measured.

bei vorgegebenen elektrischen Frequenzen bzw. Winkelgeschwindigkeiten, kann dann ein Isolationsfehler detektiert werden. Dabei sind jedoch die vorgegebenen elektrischen Frequenzen bzw. Winkelgeschwindigkeiten keine Fixwerte, sondern abhängig von einer elektrischen Winkelgeschwindigkeit co_el der elektrischen Maschine 2, welche proportional zur elektrischen Frequenz der elektrischen Maschine 2 ist. Daher wird eine die elektrische Frequenz der elektrischen Maschine 2 charakterisierende Größe, wie z.B. die elektrische Winkelgeschwindigkeit co_el , bestimmt. Diese Bestimmung kann auf Basis messtechnischer Ergebnisse erfolgen. Häufig wird die elektrische Frequenz der elektrischen Maschine 2 aber auch vorgegeben, so dass diese vorbekannt ist. 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 controller 8, but alternatively can also be implemented as a separate unit, the measurement voltage U _{M of} a frequency transformation, preferably a fast Fourier transform (FFT) is subjected to calculate the frequency spectrum of the measurement voltage U _M in this way. By evaluation of the magnitude spectral amplitudes

at predetermined electrical frequencies or angular velocities, then an insulation fault can be detected. However, the predetermined electrical frequencies or angular velocities are not fixed values, but dependent on an electrical angular velocity co _{el of} the electric machine 2, which is proportional to the electrical frequency of the electric machine 2. Therefore, a variable characterizing the electrical frequency of the electric machine 2, such as the electrical angular velocity co _el , is 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 a deterioration of the insulation resistance, is noticeable in that the spectral amplitude U _M (jK ^■ co _el ) changes in absolute terms 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 _ε1 , ie at K = 3, or at the (1-fold) electrical

Angular velocity ω _ε1 , 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 2 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 measuring voltage U _M runs in the form of an alternating voltage about a zero line, which corresponds to the reference potential, so for example vehicle mass. This process is due to the fact that during the PWM operation

Supply voltage potentials T + and T- of the supply voltage rails 5 and 7 alternating voltage components are superimposed.

A fast Fourier transformation of the measurement voltage shown in FIG. 2 results in a spectral distribution (frequency spectrum) shown schematically in FIG. It can be seen that at the (1-fold) electrical angular velocity co _el no signal component is present and in the 3-fold electrical

Angular velocity 3 * co _el a signal component with a spectral amplitude of A _{0 is} present. If a single-phase unbalanced insulation fault occurs in the region of the 3-phase network 1, ie a deterioration of the insulation resistance on one of the three phases U, V or W, the result is a changed time profile of the voltage U _M (compare FIG ) and also an altered spectral distribution (see Figure 5). In particular occurs in the (1-fold) electrical

Angular velocity co _el now a signal component with a spectral amplitude of, which did not occur in the error-free case, or at least in one

Background noise went down. If one thus compares the spectral amplitude A ₁ 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 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. 6 and 7 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. 7 that such an insulation fault is manifested by the fact that the spectral amplitude has increased from a value A ₀ to a value A ₂ at 3 times the electrical angular velocity 3 * o _el . The increase in value is again a measure of the

Deterioration of the insulation resistance. By comparing the

Spectral amplitude of the 3-fold electrical angular velocity 3 * ω _{β /} with the corresponding spectral amplitude serving as the reference value in the error-free case, in this case A ₀ , thus a symmetrical insulation error can be reliably detected.

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 2> * (D _el , but in the form of a sinking of the

Amplitude value to a lower value than in normal operation, as lower than A ₀ . In this case, the amount of waste 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

Bandpass filtering with corresponding center frequencies at co _el and 3 * a> _el

(n * 6) _{e /} ) are used and the required amplitude values are then calculated from the filtered measuring voltages.

Claims

claims A circuit arrangement for determining a voltage fluctuation of conductor potentials in an ungrounded electrical network, the network comprising  a DC voltage intermediate circuit (6),  - An n-phase network (1) with an n-phase electrical load (2), with n> 1, and  at least one inverter (3) connected to the DC voltage intermediate circuit (6) for controlling the electrical load (2),  marked by  - A voltage divider (30), in particular a symmetrical voltage divider, which between  Supply voltage potentials (Τ +, T-) of the  DC intermediate circuit (6) is connected and has a center tap (M), and a measuring device (31) for measuring one, a measuring voltage (U _M ) at the center tap (M) of the voltage divider (30) Characterizing magnitude during operation of the electrical load (2), wherein the measuring voltage (U _M ) the  Voltage fluctuation of the supply voltage potentials (T +, T-) of the DC intermediate circuit (6) represents a reference potential. 2. Circuit arrangement according to claim 1, wherein the measuring range of the voltage measuring device (31) is adapted to a maximum amplitude of the voltage fluctuation.