METHOD, ELECTRICAL CIRCUIT ARRANGEMENTS AND INSULATION MONITORING DEVICES FOR INTERFERENCE-RESISTANT INSULATION MONITORING OF AN UNGROUNDED POWER SUPPLY SYSTEM WITH A CONVERTER

Description

[0001] The invention relates to a method and an electrical circuit arrangement for interference-resistant insulation monitoring of an ungrounded power supply system with a converter (frequency converter).

[0002] Furthermore, the invention relates to an insulation monitoring device with the proposed electrical circuit arrangement.

[0003] For the supply of electrical equipment with increased requirements for operational, fire, and contact safety, an ungrounded (IT) power supply system (French: Isolé Terre) is used. In this type of power supply system, all active parts of the supply network are isolated from earth potential.

[0004] Due to the inherent safety of the ungrounded power supply system against dangerous touch voltages, a continuous power supply to the consumers connected to the ungrounded power supply system can be maintained even if an initial insulation fault occurs.

[0005] The electrical resistance of active conductors of the power supply system to earth (insulation resistance - in case of fault also insulation fault resistance or fault resistance) is therefore constantly monitored, since a possible further fault on another active conductor (second fault) would create a fault loop and the fault current flowing in conjunction with an overcurrent protection device would result in a shutdown of the system with operational standstill.

[0006] Insulation resistance is typically monitored by an insulation monitoring device (IMD), which actively applies a measurement voltage to the power supply system and captures and evaluates a measurement signal corresponding to the insulation resistance.

[0007] However, there are applications in ungrounded power supply systems, for example in mining or power plants, where the power supply should be switched off as quickly as possible even if an initial insulation fault occurs. The insulation monitoring device should then detect an initial fault as quickly as possible and trigger a switching signal.

[0008] The requirement for the shortest possible shutdown time, unlike in a grounded power supply system, is not determined with the aim of avoiding the risk of electric shock – this risk does not exist by definition in the first fault case in a standard-compliant ungrounded power supply system – but by the requirements with regard to fire protection and plant protection.

[0009] Especially in the case of insulation monitoring in conjunction with high-quality (frequency) converters and controlled drives, short shutdown times in the range of a few tens of seconds would theoretically be possible with active insulation monitoring, since the network leakage capacitances in these drives often have sufficiently small values.

[0010] In practice, these shutdown times are not achievable in many inverter applications, because the inverter drive (inverter-controlled electric motor) generates application-related low-frequency displacement voltages, i.e., voltages between an active conductor and earth, with an amplitude that is often many times the amplitude of the measuring voltage applied by the insulation monitoring device and can therefore significantly interfere with the measured signal.

[0011] Furthermore, if the frequency of the low-frequency displacement voltage components is very close to the (fundamental) frequency (measurement frequency) of the applied and specially clocked measurement voltage, monitoring is not possible in this operating state.

[0012] Across all manufacturers, the interference caused by low-frequency displacement voltage components is addressed in the technical documentation of insulation monitoring devices by specifying a (monitoring) gap in the specified frequency range. For example, the monitorable frequency range of a state-of-the-art insulation monitoring device is specified such that reliable insulation monitoring is only possible for mains voltages "DC and 15 to 460 Hz," meaning a monitoring gap exists between DC (direct current) and 15 Hz.

[0013] Due to the low-frequency interference components in the recorded measurement signal, the evaluation of the recorded measurement signal is complex, and the shutdown times achievable according to the state of the art are significantly higher than required in some critical applications (mining, power plants).

[0014] Extreme demands on shutdown times arise particularly when, for example, an existing 3AC 50 / 60 Hz IT system with very high network leakage capacitances is expanded with a high-performance frequency converter drive. In the event of a ground fault on a motor phase of the controlled converter drive, displacement voltages can occur, leading to the destruction of equipment not designed for such loads within the existing 3AC 50 / 60 Hz IT system. In such cases, shutdown times of less than 100 ms are often required.

[0015] In addition to low-frequency disturbances, which require computationally intensive and time-consuming evaluation of the recorded measurement signal and thus impair rapid shutdown, switching-frequency disturbance components, i.e., corresponding to the switching frequency of the inverter, also occur in the range of approximately 10 kHz or above, which, due to their detrimental effect on the equipment, necessitate a rapid shutdown of the power supply.

[0016] For insulation monitoring, it is known from the prior art to select the measurement voltage component from the interference-prone, recorded measurement signal by means of analog or digital filter measures.

[0017] High-quality insulation monitoring devices typically employ sophisticated digital signal processing methods to suppress interference components whose frequency is very close to the measurement frequency. The closer the interference signal frequency is to the measurement frequency, the more computational effort and processing time are required for sufficient interference suppression.

[0018] One approach pursued in grounded power supply systems – in contrast to the ungrounded power supply systems considered here – to achieve a rapid shutdown of system components in the event of a critical fault is to use residual current-based protective devices (residual current protective devices RCDs).

[0019] In applications with high-performance frequency converter drives, however, a particularly critical fault condition—a ground fault in a motor phase—is characterized by a large DC component in the fault current. Therefore, the use of a Type A residual current device (for detecting alternating fault currents and pulsating DC fault currents) is usually not possible. Suitable Type B residual current devices (for detecting alternating fault currents, pulsating DC fault currents, and pure DC fault currents) for high-performance frequency converter drives are often unavailable.

[0020] However, the use of residual current-based protective devices in ungrounded power supply systems is fundamentally problematic, as this type of protective device only functions reliably if it is ensured that the residual current transformer of the residual current protective device is actually located in the fault loop.

[0021] The German patent application EP 2 230 522 A1 discloses a method and a device for insulation monitoring of a DC IT network that is coupled to a grounded AC network via a galvanically connected inverter. A resistance measuring device is provided to determine the insulation resistance. This device uses a switch to alternately connect a reference resistor to the conductors of the DC IT network. An additional current measuring device on the DC side can detect any leakage current flowing through the operating inverter.

[0022] EP 2 490 032 A2 discloses a method for detecting earth faults in a motor drive system with a converter. In this method, the drive system is supplied with a common-mode voltage to generate motor control signals that enable the unambiguous identification of fault currents.

[0023] EP 2 219 041 A1 describes a robust detection of an AC fault current in an electric vehicle using sideband harmonics of a pulse width modulated signal.

[0024] EP 1 229 629 A2 discloses a device and a method for detecting earth faults in a solar power generation system. In this method, a differential current between the output lines of a solar battery is detected, an AC leakage current component attributable to the capacitance of the solar battery relative to earth is removed from the differential current, and by comparing a current value after removal of the AC leakage current component with a predetermined threshold value, it is determined whether an earth fault condition has occurred.

[0025] EP 2 530 801 A1 describes a residual current protection system for the interference-resistant determination of the earth fault resistance in electrical installations affected by system noise. The measurement method is based on the injection of a measurement signal voltage, the frequency of which is adaptively adjusted depending on a measured noise spectrum.

[0026] The requirement for short measurement and shutdown times, which is necessary in specific applications, cannot be fully met according to the current state of the art.

[0027] The present invention is therefore based on the objective of providing a method and a device for interference-resistant insulation monitoring in an ungrounded power supply system with a converter drive.

[0028] The problem underlying the invention is solved by applying a measuring voltage between an active conductor of the power supply system and earth and detecting a measurement signal driven by the measuring voltage, replicating a pulse width modulation signal of the converter, evaluating the replicated pulse width modulation signal with filter characteristics that correspond to filtering the detected measurement signal in order to obtain a low-frequency interference signal, and subtracting the low-frequency interference signal from the detected measurement signal.

[0029] This approach achieves interference resistance in insulation monitoring against low-frequency interference generated by the inverter by suppressing this interference to such an extent that continuous (frequency-)disruptive monitoring of the insulation resistance is possible. This allows even symmetrical faults caused by low-frequency interference to be detected quickly and accurately.

[0030] Unlike the switching-frequency interference components described above, disruptive low-frequency components arise in the displacement voltage of an ungrounded power supply system with an inverter drive due to the pulse width modulation (PWM) used in the inverter to generate control signals. These interference components can exhibit amplitudes equal to the DC link voltage of the frequency converter.

[0031] Instead of using the slow-settling analog and / or digital filters known from the prior art to eliminate switching-frequency interference components, system knowledge about the frequency converter generating the interfering low-frequency components is used to simulate the pulse-width modulation (PWM) signal. Accessible internal system parameters of the converter can include, for example, the frequency and duty cycle of the pulse-width modulation signal or the amplitude of the DC link voltage.

[0032] The pulse width modulation signal from the frequency converter is reconstructed from the existing system knowledge and evaluated with the same filter characteristics that correspond to the filtering of the acquired measurement signal. To obtain a low-frequency interference signal, the same filter algorithms that the measurement signal acquired for insulation monitoring undergoes in an insulation monitoring device are applied to the reconstructed pulse width modulation signal.

[0033] The resulting low-frequency interference signal is then subtracted from the measured signal, which as a mixed signal contains both a useful component - due to the applied measuring voltage - and the interference component caused by the inverter drive, in order to obtain a measurement signal free of low-frequency interference for determining the insulation resistance.

[0034] If the modulation type (e.g. block or sine commutation) of the frequency converter is also known, the PWM signal of the frequency converter can be replicated without knowledge of other signal components such as mains frequency or measurement signal.

[0035] The evaluation of the simulated pulse width modulation signal using the same filter algorithms that are applied to the acquired measurement signal in an insulation monitoring device - preferably low-pass filtering - extracts the desired low-frequency interference components caused by the frequency converter.

[0036] If this low-frequency interference signal is subtracted from the measured signal in the insulation monitoring device, significant interference suppression can be achieved. In this way, in an ungrounded power supply system with an inverter, standard-compliant, fast-acting, and, above all, interference-resistant insulation monitoring in the low-frequency range is possible in all operating states of the inverter, without creating a monitoring gap in low-frequency ranges.

[0037] The invention eliminates the need to select the low-frequency interference from the measured signal using complex and / or slow filter algorithms, allowing interference suppression to work significantly faster – even in cases where the frequencies of the interference signal and the measured signal are close to each other.

[0038] In a further development, the frequency and duty cycle of the pulse width modulation signal are provided by the converter via a digital interface or can alternatively be calculated using signal processing methods.

[0039] The frequency and duty cycle of the pulse width modulation signal can be provided directly by the frequency converter, for example via a digital interface, or calculated using very fast control engineering methods from communications engineering (e.g. PLL circuit).

[0040] Furthermore, the amplitude of the pulse width modulation signal is either directly specified or measured as the amplitude of an intermediate circuit DC voltage of the converter.

[0041] Preferably, the amplitude of the pulse width modulation signal is determined by measuring a DC voltage at the intermediate circuit of the converter or by measuring a line voltage with a correction factor.

[0042] The measurement can be performed as a DC voltage measurement at the converter's intermediate circuit or by measuring the phase-to-phase voltage using a known correction factor. The following relationship often applies: Uzwk = (1.32 ... 1.4) Un, where Uzwk is the intermediate circuit DC voltage, Un is the nominal network voltage, and the correction factor is between 1.32 and 1.4.

[0043] The problem underlying the invention is further solved by an electrical circuit arrangement with a coupling device for applying a measuring voltage between an active conductor of the power supply system and earth and for detecting a measuring signal driven by the measuring voltage, with a reconstruction unit for replicating a pulse width modulation signal of the converter, with an evaluation circuit for evaluating the replicated pulse width modulation signal, wherein the evaluation circuit has filter characteristics that correspond to filtering the detected measuring signal in order to obtain a low-frequency interference signal, and with a subtraction circuit for subtracting the low-frequency interference signal from the detected measuring signal.

[0044] To carry out the method according to the invention, the electrical circuit arrangement according to the invention comprises as essential functional blocks a coupling device for applying a measuring voltage, a reconstruction unit for reproducing a pulse width modulation signal, an evaluation circuit for evaluating the reproduced pulse width modulation signal, and a subtraction circuit for subtracting the low-frequency interference signal from the detected measuring signal.

[0045] With this circuit arrangement, the pulse-width modulation signal of the inverter is simulated in the reconstruction unit and low-pass filtered in the evaluation circuit. The signal thus modeled corresponds to the low-frequency interference signal generated by the inverter and is subtracted from the acquired measurement signal in the subtraction circuit to obtain a measurement signal free of low-frequency interference components.

[0046] In a further embodiment, the electrical circuit arrangement includes a digital interface via which a frequency and a duty cycle of the pulse width modulation signal are provided by the converter.

[0047] Alternatively, the electrical circuit arrangement can include a computing unit for calculating a frequency and duty cycle of the pulse width modulation signal using signal processing methods.

[0048] Advantageously, the insulation monitoring device for monitoring an insulation resistance in an ungrounded power supply system with a converter comprises an electrical circuit arrangement according to the invention for interference-resistant insulation monitoring, which replicates the pulse width modulation signal of the converter and eliminates the resulting low-frequency interference signal from the detected measurement signal.

[0049] This extension of the insulation monitoring device, as required by the standard, allows insulation monitoring to be carried out reliably even in the presence of low-frequency interference signals.

[0050] Further advantageous design features will become apparent from the following description and the drawing, which illustrates a preferred embodiment of the invention by way of an example. It shows: Fig. 1: an embodiment of the circuit arrangement according to the invention with simulation of a pulse width modulation signal.

[0051] In Fig. 1 A functional block diagram shows a circuit arrangement according to the invention for a disturbance-resistant insulation monitoring of an ungrounded power supply system 2 with a converter 4.

[0052] This circuit arrangement emulates a pulse-width modulation signal from the inverter 4 to obtain an interference-free measurement signal in the low-frequency range, preferably between DC and approximately 15 Hz. For older insulation monitoring devices, this low-frequency range can extend to 40 Hz DC due to the larger monitoring gap; current, high-quality insulation monitoring devices reduce the monitoring gap from DC to below 1 Hz.

[0053] In this embodiment, the circuit arrangement comprises, in addition to the coupling device 8 located in the insulation monitoring device 10, a reconstruction unit 30 which replicates the pulse width modulation signal of the inverter 4. For this purpose, the reconstruction unit 30 is connected to the inverter 4 via a digital interface 32 in order to directly utilize internal system parameters of the inverter, such as the frequency and duty cycle of the pulse width modulation signal, for the replication of the pulse width modulation signal.

[0054] The circuit arrangement further includes an evaluation circuit 34, which filters the simulated pulse-width modulation signal to obtain a low-frequency interference signal that reflects the interference components generated by the inverter 4 in the event of a fault. The filter characteristic used corresponds to that which is also used in the insulation monitoring device 10 for filtering the acquired measurement signal.

[0055] In a subtraction circuit 36 ​​downstream of the evaluation circuit 34, the low-frequency interference signal obtained by filtering is subtracted from the acquired measurement signal, so that the low-frequency interference component contained in the acquired measurement signal and the low-frequency interference component obtained from the reconstructed pulse width modulation signal cancel each other out and a measurement signal that is largely free of interference in the low-frequency range is available.

Claims

1. A method for an interference-resistant insulation monitoring of an ungrounded power supply system having a converter (4), the method comprising the following steps: applying a measuring voltage between an active conductor of the power supply system and ground and detecting a measuring signal driven by the measuring voltage, replicating a pulse width modulation signal of the converter (4), assessing the replicated pulse width modulation signal using filter characteristics corresponding to a filtering of the detected measuring signal in order to obtain a low-frequency interfering signal, subtracting the low-frequency interfering signal from the detected measuring signal.

2. The method according to claim 1, characterized in that a frequency and a duty factor of the pulse width modulation signal are provided by the converter (4) via a digital interface (32).

3. The method according to claim 1, characterized in that a frequency and a duty factor of the pulse width modulation signal are calculated using signal processing methods.

4. The method according to any one of the claims 1 to 3, characterized in that an amplitude of the pulse width modulation signal is directly predetermined or metrologically determined as an amplitude of an intermediary-circuit direct voltage of the converter (4).

5. The method according to claim 4, characterized in that the amplitude of the pulse width modulation signal is metrologically detected via a direct voltage measurement at the intermediate circuit of the converter (4) or via a conductor voltage measurement having a correction factor.

6. An electrical circuit arrangement for an interference-resistant insulation monitoring of an ungrounded power supply system having a converter (4), having a coupling device (8) for applying a measuring voltage between an active conductor of the power supply system and ground and for detecting a measuring signal driven by the measuring voltage; having a reconstruction unit (30) for replicating a pulse width modulation signal of the converter (4); having an assessment circuit (34) for assessing the replicated pulse width modulation signal, said assessment circuit (34) having filter characteristics corresponding to a filtering of the detected measuring signal in order to obtain a low-frequency interfering signal; and having a subtraction circuit (36) for subtracting the low-frequency interfering signal from the detected measuring signal.

7. The electrical circuit arrangement according to claim 6, characterized by a digital interface (32) via which a frequency and a duty factor of the pulse width modulation signal are provided by the converter (4).

8. The electrical circuit arrangement according to claim 6, characterized by a computing unit for computing a frequency and a duty factor of the pulse width modulation signal using signal processing methods.

9. An insulation monitoring device (10) for monitoring an insulation resistance in an ungrounded power supply system using a converter (4), characterized by an electrical circuit arrangement for an interference-resistant insulation monitoring according to any one of the claims 6 to 8.