EP1687646A1

EP1687646A1 - Method for determining ohmic insulation resistance

Info

Publication number: EP1687646A1
Application number: EP04818814A
Authority: EP
Inventors: Dieter Hackl; Burkhard Macht
Original assignee: Dipl-Ing Walther Bender & Co KG GmbH
Current assignee: Dipl-Ing Walther Bender & Co KG GmbH
Priority date: 2003-11-24
Filing date: 2004-11-23
Publication date: 2006-08-09
Also published as: WO2005050229A1; DE10355086A1; DE10355086B4

Abstract

The insulation relates to a method for determining ohmic insulation resistance (RF) of an earthed a.c. system. In order to determine said ohmic insulation resistance (FF) at any specific infeed point (22) of the a.c. system to all phase conductors, a common-mode voltage signal (20) which is adjusted to an at least substantially square-wave form is infed against the earth and the differential current of all phase conductors is detected (28) at any particular measuring point behind the infeed point. The differential current measuring signal part, which is produced by the infed common-mode voltage signal, is determined from the differential current by filtering and the common-mode voltage signal at the phase conductors is detected at the measuring point against the earth (24/30). The common-mode voltage signal thus detected is used to determine the common-mode voltage signal part based on the infed common-mode voltage signal by means of filtering and the ohmic part of the insulation resistance (RF) between the phase conductors and the earth is determined by the formation of quotients between the momentary common-mode voltage signal part at the measuring point and the momentary differential current measuring signal part at the measuring point.

Description

Attorney's file 2733/1054 PCT

METHOD FOR DETERMINING THE OHM'S INSULATION RESISTANCE

The invention relates to a method for determining the ohmic insulation resistance of a grounded, single-phase or multi-phase, ready-to-operate, with or without DC superimposition by connected consumers, such as converters, working AC network against earth with a signal feed into the AC network and a signal evaluation of the resulting network signals.

In a known method according to US Pat. No. 4,638,242 there is an unregulated supply of sinusoidal measurement signals (preferably two). The ohmic insulation resistance is determined using the phase relationship between the measurement signal voltage and the measurement signal current and is determined from the measurement signal current by in-phase rectification. This is a very complex method that can lead to large errors. On the one hand, when using real transformers in practice, the influence of parameters such as winding resistances, iron losses, eddy current losses, leakage inductances, winding capacities, etc. cannot be neglected. Depending on the derivation from earth, both the amplitude and the phase of the input measuring signals change. voltage to earth, and this differently for different signal frequencies. In the case of fluctuating and, in addition, quite large network leakage capacitances in the network, the determination of low ohmic derivatives is only possible with large errors with this method, since the measurement signal voltage actually present in the network can differ significantly from the signal voltage fed in.

In grounded supply or control networks, the various areas of electrical safety are mainly covered by the use of residual current devices. A distinction is made between disconnecting residual current protective devices (RCD) and non-disconnecting residual current monitoring devices (RCM). Personal protection, system protection, fire protection, securing the power supply and preventive maintenance can be mentioned as functional areas to be covered. The following problem cases are not technically or inadequately solved with the use of residual current-producing devices for the tasks described above:

1. In three-phase networks, certain combinations of ohmic and capacitive derivatives (e.g. also symmetrical ohmic derivatives) can lead to the residual current that can be detected by the residual current measuring device becoming almost zero, although an impermissibly high fault current flows through the ohmic derivative. Depending on the fault, there may be a problem with personal protection, fire protection or system protection.

2. The capacitive leakage current which is present in the "good state" of the network to be monitored is already above the permissible limit value for the residual current (ohmic leakage). As a solution, the residual current measuring device uses Limit a value is set which corresponds to the arithmetic sum of the existing leakage current and residual current limit. Since capacitive and ohmic derivatives do not add up arithmetically, but geometrically, the resulting error is sometimes very large. This solution is inadequate and only acceptable as long as there is no better solution.

3. Consumers behind the rectifier and / or inverter circuits are increasingly being used in the supply networks. Examples of this are switching power supplies and frequency converters. Leakage leads to earth behind the rectifier are detected by the residual current measuring devices, depending on the case, but mostly with very large errors.

The previous practice with residual current measuring devices has been to detect a leakage or fault current driven by the mains voltage and to trigger a defined action (switch-off, message) if a limit value is exceeded. EP 0 990 292 B1 describes a method for the selective detection of leakage and fault currents. In favorable network constellations, a partial solution to the problem described under 2 is possible. However, this does not solve the problems described under 1 and 3.

For this reason, a better method to determine the ohmic insulation resistance to earth is being sought, using a signal feed. From this, for example, other parameters can be derived from the mains voltage amplitude, such as the maximum expected fault current or the maximum expected active power. The present invention is therefore based on the object of designing a method of the type mentioned in the preamble which works with a signal feed so that the ohmic insulation resistance can be determined reliably and precisely.

To achieve the object, a method of the type mentioned in the preamble of claim 1 is distinguished according to the invention by the features listed in the characterizing part of this claim, namely by:

that at any feed point de ^ς s AC network into all phase conductors of the same at least largely regulated at the feed point to the rectangular shape common-mode voltage signal is fed to earth,

that the residual current of all network conductors is recorded at any measuring point behind the infeed point,

that the residual current measurement signal component generated by the common-mode voltage signal fed in is determined directly or indirectly from the detected differential current by filtering,

that the common-mode voltage signal to earth present at the network conductors is detected at the measuring point,

that the common-mode voltage signal component based on the common-mode voltage signal fed in is determined by filtering from the common-mode voltage signal thus detected, and that if the residual current measurement signal component remains essentially unchanged after the transient state of the discharge capacitance between the network conductors and earth has been transposed to the injected common mode voltage signal, the ohmic component of the insulation resistance between the network conductors and earth by forming the quotient between the instantaneous common mode voltage signal component is determined at the measuring point and the instantaneous residual current measuring signal component at the measuring point.

Such a method enables a very reliable and precise determination of the ohmic insulation resistance, because the detected residual current measurement signal component in the steady state depends only on this, namely because of the then existing constant of the common mode voltage signal component. Based on the differential current measurement, this method works in a directionally selective manner and only detects insulation faults behind the measuring point. This enables targeted monitoring of certain network sections. The process also works perfectly if there is a direct current superimposition by connected consumers, such as frequency converters.

The method is fundamentally not only limited to earthed AC networks, but can also be used in earthed DC networks or in earthed DC networks with an AC component.

The development of claim 2 is preferred because a transformer feed is particularly simple. In principle, however, other solutions could also be used. According to claims 3 and 4, a versatile supply of the common mode voltage signal is possible at various points in the AC network.

The further refinements of claims 5 to 7 enable effective interference suppression by signal averaging in a suitable time window. As a result, measurement errors can be avoided.

According to claim 8, digital filtering techniques can be used that work accurately and are not susceptible to interference.

According to claims 9 and 10, it is easily possible to further determine the effective power based on the insulation fault and the maximum fault current.

In a further embodiment of claims 11 and 12, complex and branched AC networks can also be monitored in a targeted manner with regard to their different areas. Theoretically, it would also be possible to use several supply points with different common-mode voltage signals, for example in different network branches. Then, however, a distinction would have to be made between the various common-mode voltage signals at the measuring points.

According to claims 13 and 14, simple resistance star points can be used to detect the common mode voltage signals at the feed-in and measuring points. Alternatively, it can be used in the event of a network phase failure or in single-phase networks may be advantageous to use the common mode voltage between the N conductor and ground.

While the measure of claim 16 is necessary to ensure proper signal filtering and evaluation, the features of claim 17 have proven to be particularly useful for suppressing offset currents and offset voltages.

The developments of claims 18 and 19 have proven themselves in practical operation.

The invention is explained in more detail below using exemplary embodiments illustrated in the drawings. Show it:

1 shows, in a simplified block diagram, an insulation monitoring device which is used to carry out the method and which has alternative supply options, directly into all network conductors or indirectly via the star earthing of the AC network to be monitored,

2 shows an overall view which is more detailed with respect to the AC network to be monitored and simplified with respect to the insulation monitoring device from FIG. 1, with a feed directly into all network conductors of the AC network to be monitored, Fig. 3 is a schematic overall view with a central feed directly into all network conductors and with decentralized measuring points in different branches of the AC network to be monitored and

Fig. 4 is a schematic overall view with a central feed via the star point earthing and with decentralized measuring points in different branches of the AC network to be monitored.

According to FIGS. 1 to 4, a three-phase network 10, which is supplied with voltage via a network feed 12, is connected to the network conductors L1 (phase), L2 (phase), L3 (phase), N (neutral conductor) via a star earthing 11 to earth (PE). The three-phase network 10 has only one network branch as shown in FIGS. 1 and 2 and two network branches as shown in FIGS. 3 and 4. In the present case, a load 14 is connected to each network branch.

In principle, the three-phase network 10 has a mixed capacitive and ohmic network derivative between the network phases L1, I_2, L3 and earth PE. The network derivation is symbolized in a simplified manner as follows: in FIG. 1 by C _A and R - in FIG. 2 by C _A ι, C _A2 , C _A3 for the individual network phases L1, L2, L3 and by R _F for the network phase L1 - in FIGS. 3 and 4 by R _F1 and R _F2 for one network phase each of the two network branches.

An insulation monitoring device, generally designated 38 in FIGS. 1 and 2, operates in such a way that a reliable and precise determination of the ohmic insulation resistance R _{F is} possible even during operation of the grounded AC network 10. A feed point 16 for a rectangular common mode voltage signal to earth PE to be fed in by the insulation monitoring device 38 can be located at any point in the AC network 10, as in its neutral earthing 11 (FIGS. 1 and 4) - i.e. directly at the beginning of the network - or behind its network feed 12 ( 1, 2, 3). The feed point 16 can also be located directly on the grid feed 12.

1, the common-mode voltage signal can be fed in, for example, in a transformer. In the case of a feed in the star earthing 11, only one feed winding 22 is required, while in all other feed cases four feed windings 22 are required for all network conductors including neutral conductors, i.e. for L1, L2, L3 and N.

A generator 20 generates a common-mode voltage signal against earth PE to be fed in via the feed winding or windings 22 and, by means of suitable control processes, ensures that the common-mode voltage signal against earth PE present at the feed point 16 on the AC network 10 is essentially rectangular. This is understood to mean a signal that is largely constant, at least between its flank areas. For this purpose, the generator 20 is supplied with an input signal via a voltage or line conductor tap 24, such as a resistance star point, which corresponds to the common-mode voltage signal to ground PE present at the supply point 16 at the line conductors L1, L2, L3 and N. If this input signal deviates from the rectangular shape, the generator 20 adjusts the feed accordingly. The differential current Δl of all network conductors L1, L2, L3 and N is detected by a residual current meter 28 at a measuring part 18 of the AC network 10 located behind the feed point 16. In an evaluator 34 using digital filter techniques for the detected differential current, all components not belonging to the common-mode voltage signal fed in are filtered out of the latter. The evaluator 34 thus ensures that the differential current measurement signal component generated by the fed common-mode voltage signal is determined in a direct or indirect manner by filtering.

At the measuring point 18, an input signal is tapped from the network conductors via a voltage or line conductor tap 30, such as a resistance star point, which corresponds to the common mode voltage signal to earth PE actually present at the measuring point 18 and is supplied to an evaluator 32. 1, in the case of directly adjacent or coincident feed-in and evaluation points 16, 18, a common voltage or mains conductor tap 24/30, such as a resistance star point, can be used. However, in contrast to the embodiment from FIG. 1, it is also possible to provide, for example in the case of feed and evaluation points 16, 18 which are further apart, completely separate device parts which work with separate voltage or mains conductor taps 24/30 and which do not require any connecting control lines.

In the evaluator 32 for the detected common mode voltage signal at the measuring point 18, which in the present case also works with digital filter techniques, all components not belonging to the fed common mode voltage signal are filtered out from this signal. The evaluator 32 thus ensures that, in a direct or indirect way, that on the injected common mode voltage signal to earth PE based common mode voltage signal component against earth PE is determined by filtering.

In a resistance evaluation element 36 connected to the evaluators 32, 34, the ohmic insulation resistance R _{F is} determined from its input signals. For this purpose, a time window is preferably determined in which the residual current measurement signal portion remains essentially unchanged after the transient state of the charge transfer capacitance C _A between the mains conductors and earth has been reached and the common-mode voltage signal fed in. This time window can be determined directly by evaluating the course of the output signal from the evaluator 34, that is to say the residual current measurement signal component. Instead, the time window can also be determined indirectly in a simple manner in that, on the basis of the time profile of the output signal, the edge regions for the common-mode voltage signal are left out by predetermined fixed time differences.

The output signals of the evaluator 34, alternatively also of the evaluator 32, are averaged within the time window in order to eliminate disturbance variables. The size of the possibly averaged output signal of the evaluator 32 is then divided by the size of the averaged output signal of the evaluator 34 in order to obtain the ohmic insulation resistance R _F as a result.

With the help of the ohmic insulation resistance R _F determined in this way, the quantities of the active power and the maximum fault current based on the insulation fault and the mains voltage components can be determined. The frequency of the square-wave common mode voltage signal that is fed in, which deviates from the mains frequency, may not correspond to harmonics of the mains frequency due to the necessary filtering. Furthermore, in view of effective interference suppression, it should be as small as possible, so that longer time windows arise during which the residual current measurement signal component is essentially constant. On the other hand, the frequency cannot be chosen too low, particularly in the case of a transformer feed. In practice, a frequency of 175 Hz has proven itself for a mains frequency of 50 Hz for the common-mode voltage signal to be fed in.

The size of the rectangular common mode voltage signal to be fed in relative to earth PE can largely be chosen as desired. In principle, it has proven to be expedient to feed in a positive and negative common-mode voltage signal that alternates with earth. When evaluating the quotient, its peak-to-peak amplitude can then be used as the instantaneous common-mode voltage signal component at the measuring point and its peak-to-peak amplitude as the instantaneous residual current measuring signal component at the measuring point. The peak-to-peak amplitudes taken into account when forming the quotient can be averaged, as described above, in order to suppress interference signals. In practice, a voltage of + and - 10 V against earth has proven itself.

It is again pointed out that the described method is fundamentally not only limited to earthed AC networks, but can also be used in earthed DC networks or in earthed DC networks with an AC component.

Claims

Attorney's file 2733/1054 PCT patent claims

Method for determining the ohmic insulation resistance of a grounded, single-phase or multi-phase, ready-to-operate, without or with direct current superimposition by connected consumers, such as converters, working alternating current network against earth with a signal feed deviating from the mains frequency into the alternating current network and a signal evaluation of the resulting signals characterized in that a common-mode voltage signal to earth, which is at least largely regulated to a rectangular shape at the feed-in point, is fed into all the line conductors at any feed-in point of the AC network, that at any measuring point behind the feed-in point the residual current of all the line conductors is detected, that from the detected residual current the residual current measurement signal component generated by the common-mode voltage signal fed in is determined directly or indirectly by filtering, that at the measuring point the common mode voltage signal to earth present there is detected, that the common mode voltage signal component based on the fed common mode voltage signal component is determined by filtering from the thus acquired common mode voltage signal and that when the residual current measurement signal component after reaching the transient state of the transfer process of the discharge capacitances between the line conductors and earth to the injected common mode voltage signal remains essentially unchanged, the ohmic part of the insulation resistance between the line conductors and earth due to the formation of a quotient between the current common mode voltage signal part at the measuring point and the current residual current Measuring signal portion is determined at the measuring point.

2. The method according to claim 1, characterized in that the common mode voltage signal is fed in transformer.

3. The method according to claim 1 or 2, characterized in that the supply of the common mode voltage signal is carried out via the mains supply or via the grounding point.

4. The method according to claim 1 or 2, characterized in that the feeding of the common mode voltage signal is carried out behind the mains feed.

5. The method according to any one of claims 1 to 4, characterized in that a time window is determined in which the residual current measurement signal portion after reaching the transient state of the recharging process of the discharge capacitances between the network conductors and earth remains essentially unchanged that the mean value in this time window of the residual current measurement signal component is formed and that the residual current measurement signal component thus averaged is used for determining the ohmic component of the insulation resistance.

6. The method according to claim 5, characterized in that the time window is determined from the common-mode voltage signal component generated by filtering by the edges and near-edge signal areas are omitted.

7. The method according to claim 5, characterized in that the time window is determined from the differential current measurement signal portion generated by filtering by the edges and near-edge signal areas are left out.

8. The method according to any one of claims 1 to 7, characterized in that digital filtering techniques are used for the filtering.

9. The method according to any one of claims 1 to 8, characterized in that the real power established by an ohmic insulation fault fixed is set by detecting the mains voltage components in a defined frequency range and determining the active power from the determined ohmic component of the insulation resistance.

10. The method according to any one of claims 1 to 9, characterized in that the maximum fault current caused by an ohmic insulation fault is determined by detecting the mains voltage components in a defined frequency range and determining the maximum fault current from the ohmic component of the insulation resistance determined.

11. The method according to any one of claims 1 to 10, characterized in that a central feed point for the regulated common mode voltage signal and several decentralized measuring points for the residual currents occurring there and common mode voltage signals are used.

12. The method according to claim 11, characterized in that for the selective monitoring of an AC network with several network branches in each network branch, a measuring point for the residual currents occurring there and common mode voltage signals is used.

13. The method according to any one of claims 1 to 12, characterized in that the injected common mode voltage signal is detected at the feed point via a resistance star point and is used for controlling its largely rectangular shape.

14. The method according to any one of claims 1 to 13, characterized in that the common-mode voltage signal to earth present at the line conductors is detected at the measuring point via a resistance star point and is used for determining the common-mode voltage signal component based on the fed-in common-mode voltage signal ,

15. The method according to any one of claims 1 to 12, characterized in that in the case of single-phase networks or in the event of failure of at least one conductor voltage at the feed point and at the measuring point, the respective common mode voltage signal between the neutral conductor and earth is detected.

16. The method according to any one of claims 1 to 15, characterized in that a common mode voltage signal deviating from the harmonics of the mains frequency is used.

17. The method according to any one of claims 1 to 16, characterized in that an alternating positive and negative common mode voltage signal is fed to earth and that in the evaluating quotient formation as a current common mode voltage signal component at the measuring point, its peak-to-peak amplitude and as a momentary Residual current measurement signal portion at the measuring point whose peak-to-peak amplitude are used.

18. The method according to any one of claims 1 to 17, characterized in that a common mode voltage signal of 175 Hz is fed in at a mains frequency of 50 Hz.

9. The method according to any one of claims 1 to 18, characterized in that at a mains voltage of 230 V to 400 V, a common mode voltage signal with about + and - 10 V is fed to earth.