DE10355086B4 - Method for determining the ohmic insulation resistance of a grounded AC network - Google Patents

Abstract

A method for determining the ohmic insulation resistance of a grounded, single- or multi-phase, operational, with or without DC superposition by connected loads, such as converters, working AC network to ground with a different from the mains frequency signal input to the AC mains and a signal evaluation of resulting signals thereby characterized in that at one of the feed point of the AC mains in all network conductors of the same at the feed point at least largely regulated to rectangular shape common mode voltage signal is fed to earth that is detected at any measuring point behind the feed point of the differential current of all network conductors that from the detected differential current directly or indirectly, the differential current measuring signal component generated by the injected common mode voltage signal is determined by filtering, that at the measuring point that is there at the network conductors nde common mode voltage signal to earth is detected, that is determined from the thus detected common mode voltage signal based on the injected common mode voltage signal common mode voltage signal component by filtering and that when the difference current Meßsignalanteil after reaching the transient condition of the reloading of Ableitkapazitäten between the Mains conductors and earth to the injected common mode voltage signal remains substantially unchanged, the ohmic portion of the insulation resistance between the network conductors and ground is determined by quotient between the current common mode voltage signal component at the measuring point and the instantaneous difference current Meßsignalanteil at the measuring point.

Description

The invention relates to a method for determining the ohmic insulation resistance of a grounded, single-phase or multi-phase, operational, with or without DC superposition by connected loads, such as converters, working AC network to ground with a signal input to the AC network and a signal evaluation of resulting network signals.

In a known method according to the US 4 638 242 A an unregulated mains supply of sinusoidal measuring signals (preferably two) takes place. The ohmic insulation resistance is determined by utilizing the phase relationship between Meßsignalspannung and Meßsignalstrom and determined by in-phase rectification of the Meßsignalstrom. This is a very complex method that can lead to large errors. On the one hand, the influence of parameters such as winding resistances, iron losses, eddy current losses, stray inductances, winding capacitances etc. can not be neglected when using real transformers in practice. Depending on the derivative with respect to earth, both the amplitude and the phase of the injected measuring signal voltage to ground change, and this differently for different signal frequencies. In the case of fluctuating and additionally quite large network discharge capacitances in the network, the determination of low ohmic derivatives in this method is only possible with large errors, since the measuring signal voltage actually present in the network can deviate significantly from the signal voltage fed in.

1. 1. In Drehstromnetzen können bestimmte Kombinationen von ohmschen und kapazitiven Ableitungen (z.B. auch symmetrische ohmsche Ableitungen) dazu führen, daß der von der differenzstrommessenden Einrichtung erfaßbare Differenzstrom nahezu Null wird, obwohl ein unzulässig hoher Fehlerstrom über die ohmsche Ableitung fließt. Je nach Fehlerfall liegt hier ein Problem im Personenschutz, Brandschutz oder im Anlagenschutz vor.
2. 2. Der im „Gut-Zustand“ des zu überwachenden Netzes vorhandene kapazitive Ableitstrom liegt bereits über dem zulässigen Grenzwert für den Fehlerstrom (ohmsche Ableitung). Als Lösung wird an der differenzstrommessenden Einrichtung als Grenzwert ein Wert eingestellt, der der arithmetischen Summe aus vorhandenem Ableitstrom und Fehlerstromgrenzwert entspricht. Da sich kapazitive Ableitungen und ohmsche Ableitungen jedoch nicht arithmetisch, sondern geometrisch addieren, ist der daraus entstehende Fehler teilweise sehr groß. Diese Lösung ist unzureichend und nur akzeptabel, solange keine bessere Lösung existiert.
3. 3. Immer häufiger kommen in den Versorgungsnetzen Verbraucher zum Einsatz, die hinter Gleichrichter- und/oder Wechselrichter-Schaltungen liegen. Beispiele hierfür sind Schaltnetzteile und Frequenzumrichter. Hinter dem Gleichrichter entstehende Ableitungen gegen Erde werden durch die differenzstrommessenden Einrichtungen, zwar fallabhängig, jedoch überwiegend mit sehr großen Fehlern erfaßt.

In grounded supply or control networks, the various areas of electrical safety are predominantly covered by the use of differential current measuring devices. A distinction is made here between disconnecting residual current protective devices (RCD) and non-disconnecting residual current monitoring devices (RCM). The functional areas to be covered include personal protection, system protection, fire safety, power supply and preventive maintenance. With the use of differential current measuring devices for the tasks described above, the following problem cases are not technically or inadequately solved:

1. 1. In three-phase networks, certain combinations of ohmic and capacitive derivatives (for example, symmetrical ohmic derivatives) may cause the differential current detectable by the differential-current measuring device to become almost zero, even though an impermissibly high fault current flows via the ohmic derivative. Depending on the error, there is a problem in personal protection, fire protection or in system protection.
2. 2. The capacitive leakage current present in the "good state" of the network to be monitored is already above the permissible limit value for the residual current (ohmic dissipation). As a solution, a value is set at the differential current measuring device as a limit value, which corresponds to the arithmetic sum of the existing leakage current and residual current limit value. However, since capacitive derivatives and ohmic derivatives do not add arithmetically, but geometrically, the resulting error is sometimes very large. This solution is inadequate and only acceptable unless a better solution exists.
3. 3. Consumers are increasingly being used in the supply networks behind rectifier and / or inverter circuits. Examples include switching power supplies and frequency converters. Discharges from earth occurring downstream of the rectifier are detected by the differential current measuring devices, although they are dependent on the case, but predominantly with very large errors.

The previous practice with differential current measuring devices is to detect a leakage current driven by the mains voltage and to trigger a defined action (shutdown, message) when a limit value is exceeded. In the EP 0 990 292 B1 A method for the selective detection of leakage and fault currents is described. In favorable network constellations hereby a partial solution of the problem described under 2 is possible. However, the problems described under 1 and 3 are not solved by this.

1. 1. Gemäß der DE 25 45 325 C3 - Siemens AG - eine Schaltungsanordnung zur Messung des Isolationswiderstandes erdfreier Starkstromleitungen.
2. 2. Gemäß der DE 38 82 833 T2 - Merlin Gerin - eine Anordnung zur Kontrolle und zur Messung der Isolierung eines elektrischen Netzes.
3. 3. Gemäß der US 4 200 835 A - Fuji Electric Co - ein Verfahren und eine Einrichtung zum Messen des Isolationswiderstandes eines im Betrieb befindlichen geerdeten Übertragungssystems.

The prior art also includes:

1. 1. According to the DE 25 45 325 C3 - Siemens AG - a circuit arrangement for measuring the insulation resistance of ungrounded power lines.
2. 2. According to the DE 38 82 833 T2 - Merlin Gerin - an arrangement to control and measure the isolation of an electrical network.
3. 3. According to the US 4 200 835 A Fuji Electric Co - a method and apparatus for measuring the insulation resistance of a grounded transmission system in service.

Therefore, it is searched for a better method working with a signal feed to determine the ohmic insulation resistance to ground. From this, other parameters can be derived, for example via the mains voltage amplitude, such as the maximum expected Fault current or the maximum expected active power.

The present invention is therefore the object of a working with a signal feed method of the type mentioned in the preamble form so that the ohmic insulation resistance can be determined safely and accurately.

To achieve the object, a method referred to in the preamble of claim 1 according to the invention is characterized by the features listed in the characterizing part of this claim, namely thereby,
that at an arbitrary feed point of the AC mains in all network conductors of the same at the feed point at least largely regulated to rectangular shape common mode voltage signal is fed to ground,
that the differential current of all network conductors is detected at any measuring point behind the feed point,
in that the difference current measurement signal component generated by the injected common-mode voltage signal is determined by filtering directly or indirectly from the detected differential current,
that the common-mode voltage signal against earth which is present there at the measuring conductors is detected at the measuring point,
in that the common-mode voltage signal component based on the injected common-mode voltage signal is determined by filtering from the common-mode voltage signal thus detected, and
that when the difference in current Meßsignalanteil after reaching the Einschwingzustandes from the recharging of the Ableitkapazitäten between the power conductors and ground to the injected common mode voltage signal remains essentially unchanged, the ohmic portion of the insulation resistance between the power conductors and ground by quotient between the current common mode voltage signal component is determined at the measuring point and the instantaneous differential current Meßsignalanteil at the measuring point.

Such a method allows a very reliable and accurate determination of the ohmic insulation resistance, because the detected difference current measuring signal component in the steady state depends only on this, and because of the then existing constancy of the common mode voltage signal component. Due to differential current measurement, this method works in a directionally selective manner and detects only insulation faults behind the measuring point. As a result, targeted monitoring of certain network sections is possible. The method also works flawlessly when DC superimposition occurs through connected loads such as frequency converters.

The method is basically not limited to grounded AC grids, but can also be used in grounded DC grids or in grounded DC grids with an AC component.

The development of claim 2 is preferred because a transformer feed is particularly simple. In principle, however, other solutions could be used.

According to claims 3 and 4, a versatile feeding of the common mode voltage signal at different points of the AC network is possible.

The further embodiments 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, which work exactly and susceptible to interference.

According to claims 9 and 10, it is possible in a simple manner, further to determine the substantiated by the insulation fault active power and the maximum fault current.

In a further embodiment of claims 11 and 12 and complicated and branched AC grids can be selectively monitored with respect to their different areas. Theoretically, it would also be possible to use several feed-in points with different common-mode voltage signals, for example in different network branches. But then it would be necessary to distinguish between the different 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 supply and measuring points. Alternatively, it may be advantageous according to claim 15 in case of failure of a mains phase or in single-phase networks to use the common mode voltage between the N conductor and ground.

While the measure of claim 16 is required to ensure proper signal filtering and evaluation, the features of claim 17 have been found to be particularly useful in suppressing offset currents and offset voltages.

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

• 1 in einem vereinfachten Blockschaltbild eine zum Durchführen des Verfahrens dienende Isolationsüberwachungseinrichtung mit alternativen Einspeisungsmöglichkeiten direkt in alle Netzleiter oder indirekt über die Sternpunkterdung des zu überwachenden Wechselstromnetzes,
• 2 eine bezüglich des zu überwachenden Wechselstromnetzes detailliertere und bezüglich der Isolationsüberwachungseinrichtung aus 1 vereinfachte Gesamtansicht mit einer Einspeisung direkt in alle Netzleiter des zu überwachenden Wechselstromnetzes,
• 3 eine schematische Gesamtansicht mit einer zentralen Einspeisung direkt in alle Netzleiter und mit dezentralen Meßstellen in verschiedenen Zweigen des zu überwachenden Wechselstromnetzes und
• 4 eine schematische Gesamtansicht mit einer zentralen Einspeisung über die Sternpunkterdung und mit dezentralen Meßstellen in verschiedenen Zweigen des zu überwachenden Wechselstromnetzes.

The invention will be explained in more detail in the drawings illustrated embodiments. Show it:

• 1 in a simplified block diagram serving for carrying out the method insulation monitoring device with alternative feed options directly into all network conductors or indirectly via the neutral earthing of the AC network to be monitored,
• 2 a more detailed with respect to the AC network to be monitored and with respect to the insulation monitoring device 1 simplified overall view with a feed directly into all network conductors of the AC network to be monitored,
• 3 a schematic overall view with a central feed directly into all network conductors and decentralized measuring points in different branches of the AC network to be monitored and
• 4 a schematic overall view with a central feed via the neutral earthing and with decentralized measuring points in different branches of the AC network to be monitored.

According to the 1 to 4 is a via a network feed 12 voltage supplied three-phase network 10 with the line conductors L1 (phase), L2 (phase), L3 (phase), N (neutral) via a neutral earthing 11 connected to earth (PE). The three-phase network 10 has according to the 1 and 2 for example, only one network branch and according to the 3 and 4 for example, two network branches. Each branch of the network has a load in the present case 14 connected.

The three-phase network 10 has in principle a mixed capacitive and ohmic power dissipation between the network phases L1, L2, L3 and earth PE. The derivation of the network is illustrated graphically by way of example as follows: In 1 through C _A and R _F - in 2 by C _A1 , C _A2 , C _A3 for the individual network phases L1, L2, L3 and by R _F for the network phase L1 - in the 3 and 4 by R _F1 and R _F2 for one net phase of the two network branches.

One in the 1 and 2 generally designated 38 insulation monitoring device operates in such a way that even in the operation of the grounded AC mains 10 a safe and accurate determination of the ohmic insulation resistance R _{F is} possible.

A feed-in point 16 for one of the insulation monitor 38 rectangular common-mode voltage signal to be injected against ground PE can be applied anywhere in the AC network 10 lie as in its star point grounding 11 ( 1 and 4 ) - ie directly at the beginning of the network - or behind its network feed 12 ( 1 . 2 . 3 ). The feed-in point 16 can also connect directly to the grid feed 12 are located.

According to 1 For example, the supply of the common-mode voltage signal can be carried out by transformation. Here, in the case of a feed in the neutral point grounding 11 only one feed winding 22 while in all other feeds there are four feed windings each 22 for all power conductors including neutral, ie for L1, L2, L3 and N.

A generator 20 generates a common mode voltage signal to ground PE to be fed via the feed winding or windings 22 and ensures by suitable control processes that the voltage at the feed point 16 on the AC mains 10 pending common mode voltage signal to ground PE is substantially rectangular. This is understood to mean a signal which is largely constant at least between its flank regions. For this purpose, the generator 20 via a voltage or network conductor tap 24 , such as a resistance star point, an input signal supplied to that at the feed point 16 corresponding to the network conductors L1, L2, L3 and N supplied common mode voltage signal to earth PE. If this input signal deviates from the rectangular form, it is done by the generator 20 a corresponding readjustment of the feed.

At one behind the feed point 16 lying measuring point 18 of the AC mains 10, the differential current .DELTA.I of all network conductors L1, L2, L3 and N by a differential current meter 28 detected. In an evaluator working with digital filtering techniques 34 for the detected differential current all components not belonging to the injected common-mode voltage signal are filtered out of this. The evaluator 34 thus ensures that the difference current measurement signal component generated by the injected common mode voltage signal is determined by filtering directly or indirectly.

At the measuring point 18 is connected to the power conductors via a voltage or mains conductor tap 30 , as a resistance star point, an input signal tapped, the at the measuring point 18 corresponds to the network conductors actually pending common mode voltage signal to earth PE and an evaluator 32 is supplied. According to 1 may occur at directly adjacent or coincidental feed-in and evaluation points 16 . 18 a common voltage or network conductor tap 24 / 30 , like resistance star point, are used. However, it is different from the embodiment 1 also possible, for example, at more distant feed-in and evaluation points 16 . 18 on these completely separate, with separate voltage or Netzleiterabgriffen 24 / 30 to provide working device parts that do not require connecting control lines.

In the present case also working with digital filtering techniques evaluator 32 for the detected common mode voltage signal at the measuring point 18 are filtered out of this signal all not to the supplied common mode voltage signal belonging components. The evaluator 32 thus ensures that the common-mode voltage signal component based on the injected common-mode voltage signal against ground PE is determined by filtering directly or indirectly.

In one to the evaluator 32 . 34 connected resistance evaluation element 36 is determined from the input signals of the ohmic insulation resistance R _F. For this purpose, a time window is preferably determined in each case in which the difference current Meßsignalanteil after reaching the transient condition of the transhipment of the Ableitkapazitäten C _A between the network conductors and ground to the injected common mode voltage signal remains substantially unchanged. The determination of this time window can be made by evaluating the course of the output signal from the evaluator 34 , So the difference current Meßsignalanteils, directly done. 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 from the evaluator 32 for the common mode voltage signal, the edge regions are omitted by predetermined fixed time differences.

Within the time window are the output signals of the evaluator 34 , alternatively also the evaluator 32 , averaged to eliminate disturbances. The size of the possibly averaged output signal of the evaluator 32 is then determined by the magnitude of the averaged output of the evaluator 34 as a result, to obtain the ohmic insulation resistance R _F.

With the help of the thus determined ohmic insulation resistance R _F , the quantities of the active power and the maximum fault current, which are justified by the insulation fault and the mains voltage components, can be determined.

The frequency of the supplied rectangular common-mode voltage signal, which deviates from the mains frequency, must not coincide with harmonics of the mains frequency, taking into account the required filtering. Furthermore, it should be as small as possible in view of an effective interference suppression, so that longer time windows arise, during which the differential current Meßsignalanteil is substantially constant. On the other hand, the frequency can not be too small, in particular in the case of a transformer feed. In practice, a frequency of 175 Hz has proven itself at 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 relation to ground PE can be chosen largely arbitrarily. In principle, it has proven expedient to feed in alternately positive and negative common-mode voltage signals relative to ground. In the evaluating quotient formation can then be used as the current common-mode voltage signal component at the measuring point whose peak-peak amplitude and the instantaneous difference current Meßsignalanteil at the measuring point whose peak-peak amplitude. The peak-to-peak amplitudes considered in the quotient formation may be averaged as described above to suppress spurious signals. In practice, a voltage of + and - 10 V has proven to earth.

It is again referred to the fact that the described method is not limited in principle only to grounded AC grids, but can also be used in grounded DC grids or in grounded DC grids with AC content.

Claims (19)

A method for determining the ohmic insulation resistance of a grounded, single- or multi-phase, operational, with or without DC superposition by connected loads, such as converters, working AC network to ground with a different from the mains frequency signal input to the AC mains and a signal evaluation of resulting signals thereby characterized in that at one of the feed point of the AC mains in all network conductors of the same at the feed point at least largely regulated to rectangular shape common mode voltage signal is fed to earth that is detected at any measuring point behind the feed point of the differential current of all network conductors that from the detected differential current directly or indirectly the difference current measurement signal component generated by the injected common mode voltage signal is determined by filtering, that at the measuring point there pending at the line conductors common mode voltage signal is detected against earth, that is determined from the thus detected common mode voltage signal based on the injected common mode voltage signal common mode voltage signal component by filtering and that when the difference current Meßsignalanteil after reaching the transient condition of the transient of Ableitkapazitäten between the power conductors and ground to the injected common mode voltage signal remains essentially unchanged, the ohmic portion of the insulation resistance between the power conductors and ground by quotient between the current common mode voltage signal component at the measuring point and the instantaneous differential current Measuring signal component is determined at the measuring point. Method according to Claim 1 , characterized in that the common mode voltage signal is fed transformer. Method according to Claim 1 or 2 , characterized in that the feeding of the common mode voltage signal is carried out via the mains supply or via the neutral earthing. Method according to Claim 1 or 2 , Characterized in that the feed of the common mode voltage signal is performed behind the grid. Method according to one of Claims 1 to 4 , characterized in that a time window is determined, in which the difference current Meßsignalanteil after reaching the transient condition of the Umladevorgang the Ableitkapazitäten between the power conductors and earth remains essentially unchanged, that in this time window, the average value of the Differenzstrom-Meßsignalanteils is formed and that the so averaged differential current measuring signal component is used for the determination of the resistive component of the insulation resistance. Method according to Claim 5 , characterized in that the time window is determined from the common-mode voltage signal component produced by filtering, by leaving out its edges and signal regions near the edge. Method according to Claim 5 , characterized in that the time window is determined from the generated by filtering differential current Meßsignalanteil by its flanks and near-edge signal areas are recessed. Method according to one of Claims 1 to 7 , characterized in that for the filtering digital filtering techniques are used. Method according to one of Claims 1 to 8th , characterized in that the substantiated by an ohmic insulation fault active power is detected by the mains voltage components detected in a defined frequency range and determined from the determined ohmic portion of the insulation resistance, the active power. Method according to one of Claims 1 to 9 , characterized in that the established by an ohmic insulation fault maximum fault current is detected by the mains voltage components detected in a defined frequency range and determined therefrom on the determined ohmic portion of the insulation resistance of the maximum fault current. Method according to one of Claims 1 to 10 , characterized in that a central feed point for the regulated common mode voltage signal and a plurality of decentralized measuring points for the differential currents occurring there and common mode voltage signals are used. Method according to Claim 11 , characterized in that for selective monitoring of an AC network with multiple network branches in each network branch a measuring point for the differential currents occurring there and common mode voltage signals is used. Method according to one of Claims 1 to 12 , characterized in that at the feed point, the injected common mode voltage signal is detected via a resistor star point and used for the regulation of its extensive rectangular shape. Method according to one of Claims 1 to 13 , characterized in that at the measuring point which there prevailing at the power conductors common mode voltage signal to earth detected via a resistor start point and used for determining the based on the injected common mode voltage signal common mode voltage signal component. Method according to one of Claims 1 to 12 , characterized in that in single-phase networks or in case 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. Method according to one of Claims 1 to 15 , Characterized in that a deviating from the harmonics of the mains frequency common-mode voltage signal is used. Method according to one of Claims 1 to 16 , characterized in that an alternatingly positive and negative common mode voltage signal is fed to earth and that in the evaluating quotient formation as the current common-mode voltage signal component at the measuring point whose peak-peak amplitude and current momentary difference signal Meßsignalanteil at the measuring point whose peak peaks Amplitude can be used. Method according to one of Claims 1 to 17 , characterized in that at a mains frequency of 50 Hz, a common mode voltage signal of 175 Hz is fed. Method according to 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.

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