阅读说明：本技术 用于由转换器供给的供电系统的绝缘监测的方法 (Method for insulation monitoring of a power supply system supplied by a converter ) 是由 迪特尔·黑克尔 于 2019-09-06 设计创作，主要内容包括：本发明涉及用于由转换器供给的供电系统的绝缘监测的方法。本发明涉及用于在供电系统中确定绝缘电阻和定位绝缘故障的方法,该供电系统的有源部件未接地,并且通过接地操作且配备有受控功率半导体开关的转换器对所述供电系统进行供给。在转换器的输出处产生对地共模电压,并且将该对地共模电压叠加在未接地的网络上作为有源测量电压以测量绝缘电阻。对转换器中共模测量电压的产生的直接集成允许具有成本效益的实现方式,包括与完全未接地供电系统中可能实现的功能类似全面的绝缘监测功能。此外,可以将用于确定绝缘电阻的方法扩展到用于定位绝缘故障并且从而用于定位故障系统分支的方法中。(The invention relates to a method for insulation monitoring of a power supply system supplied by a converter. The invention relates to a method for determining the insulation resistance and locating insulation faults in a power supply system whose active components are ungrounded and which is supplied by a converter operated by grounding and equipped with controlled power semiconductor switches. A common-mode voltage to ground is generated at the output of the converter and superimposed on the ungrounded network as an active measurement voltage to measure the insulation resistance. The direct integration of the generation of the common mode measurement voltage in the converter allows for a cost-effective implementation, including a comprehensive insulation monitoring function similar to that possible in a completely ungrounded power supply system. Furthermore, the method for determining the insulation resistance can be extended to methods for locating insulation faults and thus for locating faulty system branches.)

1. An apparatus for determining the insulation resistance (R) of a power supply system (20, 30, 40, 50)_f) The active components of the power supply system are ungrounded and the power supply system is supplied through a converter (10, 14, 24, 34) which is grounded and is equipped with controlled power semiconductor switches (SW10, SW20, SW30, SW40, SW50, SW60), the method comprising the steps of:

-generating a common mode voltage to ground at the output of the converter (10, 14, 24, 34),

it is characterized in that the preparation method is characterized in that,

-superimposing the common mode voltage as the insulation resistance (R) for determining the ungrounded power supply system (20, 30, 40, 50)_f) Measured voltage (U)_g)，

-detecting the measurement voltage (U) in the ungrounded power supply system (20, 30, 40, 50)_g)，

-detecting the voltage (U) due to said measurement_g) While passing through said insulation resistance (R)_f) A measurement current (I) flowing in the ungrounded power supply system (20, 30, 40, 50)_g)，

-by evaluating the measurement current (I)_g) Determining the insulation resistance (R)_f)。

2. The method of claim 1,

the common mode voltage is generated by generating a pulse pattern (sa, sb, sc) for controlling the power semiconductor switches (SW10, SW20, SW30, SW40, SW50, SW 60).

3. The method of claim 2,

the pulse mode (sa, sb, sc) is by using a bias voltage (U)_m) A reference voltage (U) per phase (L1, L2, L3) of the converter (10, 14, 24, 34) is carried out_L1，U_L2，U_L3) Is generated by the voltage shift (12).

4. The method of claim 3,

the bias voltage (U)_m) Is arranged such that it has a square-wave waveform with a fundamental frequency below the network frequency of the supply system (20, 30, 40, 50), resulting in the measurement voltage (U)_g) At said bias voltage (U)_m) Of the fundamental frequency.

5. Method according to any of claims 1 to 4, characterized by the following application:

the converter (10, 14, 24, 34) is configured as an inverter (14) or as a rectifier (24) or as a frequency converter (34).

6. Method for locating an insulation fault in a branch power supply system (40, 50) whose active components are ungrounded and which is supplied by a converter (10, 14, 24, 34) which is grounded and which is equipped with controlled power semiconductor switches (SW10, SW20, SW30, SW40, SW50, SW60), comprising the determination of the insulation resistance (R) according to any one of claims 1 to 5_f) The method of (1), further comprising the steps of:

-detecting, by means of a residual current measuring device (6), a measured voltage (U) in a system branch (41, 42, 51, 52) to be monitored in an ungrounded power supply system (40, 50)_g) Induced residual current (I)_x)，

-evaluating the residual current (I)_x) To detect the presence of insulation faults (R)_x) The system branch (41, 51).

7. The method of claim 6,

a partial insulation resistance and/or a partial network leakage capacitance of the system branch (41, 42, 51, 52) to be monitored is determined.

Technical Field

The invention relates to a method for determining the insulation resistance of a power supply system whose active components are ungrounded and which is supplied by a converter operated by grounding and equipped with controlled power semiconductor switches. A common mode voltage to ground is generated at the output of the converter.

The invention also relates to a method for locating an insulation fault in a branched ungrounded power supply system, comprising the features of the method for determining the insulation resistance.

Background

With the increasing automation and digitization of industrial processes, it is necessary to convert the energy provided by the power supply for current transmission in terms of network configuration, network voltage and network frequency, or to adapt it to the requirements of the loads connected to the power supply system.

According to the standards (DIN VDE 0100-. Such power supply systems are also referred to as ungrounded power supply systems or IT systems (french), and, therefore, have an ungrounded network configuration.

For example, in addition to the use of inverters for supplying the power supply system (alternating current) with energy (direct current) provided by photovoltaic power stations, and rectifiers in direct current network architectures, the demand for variable speed electric drives supplied by the power supply system via frequency converters is also growing. As used hereinafter, the term "converter" includes inverters, rectifiers, and frequency converters.

As power electronics technology advances towards more and more powerful semiconductor circuits, power semiconductor switches are increasingly used to electronically convert currents.

In the most common converter configurations, the circuit for implementing the converter basically has (depending on the number of phases in the ac power supply system) at least one bridge circuit consisting of a plurality of parallel paths, each having at least two power semiconductor switches, whether the converter is used as an inverter, rectifier or converter. The power semiconductor switches are controlled by a control circuit by means of a control pulse pattern such that a desired output signal in the form of an alternating voltage of a desired frequency or in the form of a direct voltage is produced at the output of the converter.

The converter is used for various network configurations. When the converter is used in an ungrounded power supply system, the following options exist: according to the requirements of the standard IEC61557-8, different network parameters related to the safe operation of the converter are determined when monitoring an ungrounded power supply system using an Insulation Monitoring Device (IMD). In particular, the mandatory continuous monitoring of the insulation resistance in an ungrounded power supply system can take place reliably and largely independently of the size of the network leakage capacitance of the ungrounded power supply system in the two-digit megaohm range up to the kiloohm range.

Symmetric and asymmetric insulation faults can be detected using insulation monitoring devices that comply with the product standard IEC 61557-8. The additional risk of fire caused by a symmetrical fault is detected by standard insulation monitoring devices, allowing timely action to be taken. By observing how the insulation resistance value develops over time, conclusions can be drawn about humidity, contamination, ageing and degradation. Maintenance measures can be planned in time, thereby avoiding system shutdown. Thus, continuous insulation monitoring in an ungrounded power supply system provides an information advantage.

Disclosure of Invention

It is therefore an object of the present invention to achieve a technical monitoring option for a power supply system which is comparable in terms of insulation resistance to options available in a completely ungrounded power supply system, the active components of which are isolated to ground or grounded by a sufficiently high impedance (ungrounded power supply system) and which is supplied by a converter which does not operate in the ungrounded situation. In particular, the aim is to reliably detect a symmetric insulation fault and to minimize the influence of the magnitude of the network leakage capacitance on the determination of the insulation resistance.

With reference to a particular embodiment, this object is achieved by a method comprising the following steps according to the invention: the common-mode voltage is superimposed as a measurement voltage for determining the insulation resistance of the ungrounded power supply system, the measurement voltage is detected in the ungrounded power supply system, a measurement current flowing in the ungrounded power supply system through the insulation resistance as a result of the measurement voltage is detected, and the insulation resistance is determined by evaluating the measurement current.

The basic idea of the invention is based first on the realization of an active measurement of the insulation resistance of an ungrounded power supply system supplied by a converter operated with ground and on the generation of a measurement voltage required for the active measurement in the converter.

Thus, there is no need for a common-mode measurement voltage source (e.g. in the form of an Insulation Monitoring Device (IMD)) which is normally used for measuring insulation resistance in an ungrounded power supply system, since the common-mode voltage to ground generated in the converter is superimposed on the ungrounded network at the converter output, i.e. connected between each active conductor of the power supply system and ground. This measurement voltage is detected at the output of the converter at the interface with the power supply system to be monitored for evaluation. By using the common-mode voltage as the active measurement voltage, a symmetrical impedance to ground can be determined, in particular in the case of symmetrical fault states. Since the common-mode voltage can be set largely independently of the supply voltage, the determination of the insulation resistance is possible in the following cases: largely independent of network leakage capacitance, largely unaffected by interfering components and at the same time complying with regulations and/or standards.

The superimposed measurement voltage results in a measurement current which is closed in a measurement circuit across a leakage impedance inherently present in the power supply system and across the grounded converter. When the frequency of the measuring current is sufficiently low, the magnitude of the measuring current is mainly determined by the insulation resistance, i.e. by the ohmic part of the leakage impedance, because the conductance of the leakage capacitance, i.e. the conductance of the capacitive part of the leakage impedance, is negligible compared to the ohmic part at low frequencies. The measurement current, which is proportional to the insulation resistance, is detected at the central location, preferably at the output of the converter, and the insulation resistance can be determined in combination with the known measurement voltage.

Thus, the direct integration of the generation of the common-mode measurement voltage in the converter allows a cost-effective implementation, including a comprehensive insulation monitoring function similar to those possible in a completely ungrounded power supply system (a power supply system not supplied by a converter operated by ground).

In particular, possible application areas of electrical installations connected to ground-operated converters are expanding in the following respects: a small top-mounted photovoltaic facility operated at ground; a variable driver; a wind power plant operating at ground; an industrial direct current power supply system; direct current power supplies operating at ground, for example in telecommunications, computer centres or in the medical field; for use in a ground operated converter for blast furnace applications or in a ground operated converter for electric to gas applications.

In a further advantageous embodiment, the common-mode voltage is generated by generating a pulse pattern for controlling the power semiconductor switches.

In order to generate the output signal of the converter as required in terms of signal waveform and signal frequency, the power semiconductor switches of the converter are controlled using a suitable pulse pattern. For multiphase converters, a symmetrical pulse mode is typically used, which produces a negligible common mode voltage at the output of the converter. However, by varying the pulse mode control, for example by controlling the asymmetry, a significant common mode voltage to ground can be generated at the output of the converter which can be used according to the invention for measurement and monitoring tasks in the power supply system.

Advantageously, the pulse pattern is generated by using a voltage shift of the bias voltage to the reference voltage of each phase.

In order to obtain the required direct voltage signal at the output of the converter, the pulse pattern for controlling the power semiconductor is modified in a suitable manner. This is achieved by using a voltage shift of the bias voltage to the reference voltage of each phase.

In a further embodiment of the method, the bias voltage is set such that it has a square-wave waveform with a fundamental frequency below the network frequency of the supply system, resulting in the measuring frequency of the measuring voltage being at the level of the fundamental frequency of the bias voltage.

The magnitude, frequency and waveform of the bias voltage are the main settable parameters of the bias voltage, which map the parameters of the desired common mode voltage. In particular, the waveform and frequency of the bias voltage are reflected in the waveform and frequency of the common mode voltage, thereby forming a measurement voltage suitable for determining the insulation resistance and which may advantageously be adapted to the magnitude of a system parameter of the power supply system to be monitored, such as the leakage capacitance.

Therefore, in case of large network leakage capacitances, for example in widely branched ungrounded power supply systems, it is advantageous to choose the frequency of the bias voltage and thus the common mode voltage, i.e. the final constant frequency of the measurement voltage, which is significantly lower than the network frequency of the ungrounded power supply system. In addition, when a square waveform is used, known algorithms for signal processing in the insulation monitoring device of the ungrounded power supply system may be employed.

In addition to taking into account the adaptive adjustment of the settable parameters of the bias voltage by the existing network leakage capacitance, these parameters can also be optimized taking into account the influence of possible interfering signals.

The method may advantageously be used in electrical installations wherein the converter is configured as an inverter or rectifier or frequency converter.

The method according to the invention, in particular for controlling the generation of the pulse pattern of the power semiconductor switches, is therefore independent of the function in which the converter is used. Thus, the method is applicable, for example, when a direct current supply system connected to a converter serving as a rectifier is to be monitored, or in the case of operating an alternating current supply system downstream of a converter serving as an inverter or frequency converter.

Possible applications of the method according to the invention are therefore, for example: a photovoltaic facility operating with ground at an input side of the photovoltaic inverter; a dc power supply supplied from an industrially used rectifier having a ground network arrangement on an ac side; the AC input side is provided with a grounding network configuration frequency converter driver; a DC power supply supplied from a rectifier having a ground network configuration on the AC side of the computer center; a rectifier-based Uninterruptible Power Supply (UPS) having a ground network configuration on an ac side; an inverter drive of an electric vehicle has a galvanic connection between a drive side and a vehicle body.

The object of monitoring in a branch power supply system is further achieved on the basis of the method for determining an insulation resistance according to the invention by a method for locating an insulation fault, which method for locating an insulation fault further comprises the following steps: the residual current caused by the measured voltage in the system branch to be monitored in the ungrounded power supply system is detected by means of a residual current measuring device, and the residual current is evaluated to detect a system branch exhibiting an insulation fault.

The method for determining the insulation resistance according to the invention can be enhanced by the following steps to obtain a method for locating an insulation fault and thus for locating a faulty system branch: the residual current caused by the measured voltage in the system branch to be monitored in the ungrounded power supply system is detected by measuring the residual current.

For this purpose, a residual current measuring device is installed in each system branch to be monitored in a manner known in the art, which residual current measuring device detects a residual current driven by the common-mode voltage as a measurement voltage in the system branch affected by the insulation fault.

In its function as a measurement voltage (test voltage), the common-mode voltage generated in the converter replaces the test current generator normally used for insulation fault locating devices, which would require a complicated installation.

Advantageously, the partial network leakage capacitance of the system branch to be monitored is determined.

Based on the measured voltage detected in the system branch to be monitored and the residual current caused by the measured voltage, a (partial) network leakage capacitance of the system branch to be monitored can be determined.

Drawings

Other advantageous features will be apparent from the following description and the accompanying drawings, which illustrate by way of example preferred embodiments of the invention.

Figure 1 is a diagram of the principle of monitoring for a notch "symmetrical fault" of a passive residual current measurement technique,

figure 2 is a graphical representation of the principle of the effect of the capacitive leakage current on the measurement sensitivity of the residual current measurement,

figure 3 shows a simulation of a converter with voltage displacement for generating a common mode voltage (inverter with STC modulation),

figure 4a shows the simulation result (signal waveform) of the converter simulation of figure 3 without voltage displacement,

fig. 4b shows the simulation results (signal waveforms) of the converter simulation of fig. 3 with voltage displacement, fig. 5a shows insulation monitoring according to the invention in an ungrounded dc supply system,

figure 5b shows insulation monitoring according to the invention in an ungrounded ac supply system,

fig. 6a shows the localization of an insulation fault according to the invention in a branched ungrounded dc supply system, an

Fig. 6b shows the localization of an insulation fault according to the invention in a branched ungrounded ac supply system.

Detailed Description

Fig. 1 is a diagram of the principle of monitoring for gap "symmetry faults" that can occur when using passive residual current measurement techniques. Shown is a power supply system 2 comprising active conductors (phase conductors, phases) L1, L2, and L3 and being grounded by a power supply 4. The residual current measuring device 6 detects the vector sum of the currents flowing in the conductors L1, L2, and L3. The residual current measuring device 6 may be part of a residual current protection device (RCD) or a Residual Current Monitor (RCM).

Due to the generally symmetrical structure in the electrical system, a symmetrical fault situation can be assumed in general in the case of insulation degradation, i.e. an insulation fault which affects all three conductors L1, L2 and L3 in the same way. In FIG. 1, the resistance R is measured by the insulation resistance_iso-in case of failure also by R_f(insulation fault or insulation fault resistance) — to account for such an insulation fault, wherein the insulation resistance R_isoActing between the conductors L1, L2, L3 (including all connected devices) and ground, and modeling the complex-valued leakage impedance of the entire power supply system 2 as a concentration element and a leakage capacitance C_e。

Due to symmetrical phasing, by means of the insulation resistance R_isoResidual current I flowing in conductors L1, L2, and L3_f1、I_f2And I_f3The vector sum of (a) and thus the resulting residual current Δ I is close to zero.

Despite the residual current I_f1、I_f2And I_f3Is significant, but when using passive residual current measurement techniques, this residual current cannot be detected and leads to monitoring gaps associated with the occurrence of symmetrical faults.

Fig. 2 is a diagram of the principle of the influence of the capacitive leakage current on the measurement sensitivity of the residual current measurement.

One disadvantage that occurs when using residual current-based monitoring systems in comparison with the use of Insulation Monitoring Devices (IMD) installed according to the standard is that the measuring behavior of the residual current-based monitoring systems is significantly dependent on the network leakage capacitances present and from the network leakage capacitancesA determined leakage current. If significant capacitive leakage current I_ablAlready flowing, then with a smaller but still harmful residual current I_fThe vector addition of (a) results in only a small increase in residual current, which cannot be reliably detected in the case of an insufficient sensitivity of the residual current measuring device. The numerical example in fig. 2 shows the residual current I when 30mA is present_fExcept for the inherent leakage current I_ablThe residual current Δ I increases only slightly by 1.5mA, apart from (irrespective of the operating current).

FIG. 3 shows a voltage source with a circuit for generating a measurement voltage U_gVoltage displacement 12 of the common mode voltage of the converter 10.

In this simulation model, the converter 10 is modeled as an inverter 14, whose power semiconductor switches SW10 to SW60 are controlled by the control circuit 16 by means of pulse patterns sa, sb, sc. In this model, it is shown that pulse patterns sa, sb, sc for controlling the power semiconductor switches SW10 to SW60 are generated using the STC modulation method (sinusoidal triangular comparison in center aligned mode). The control pulse patterns sa, sb, sc may also be modified for generating an optimized measurement voltage U using other known modulation methods (STC-THI, CSVM, DSVM type A, DSVM type B, DSVM type C) as described below_g。

In the STC process, the control pulse patterns sa, sb, sc are essentially implemented in a known manner by using a standardized sinusoidal reference voltage U_L1、U_L2、U_L3Modulating high frequency triangular carrier signal C_rIs modulated so as to produce an output voltage U which is as sinusoidal as possible_ab、U_ac、U_bc。

Without voltage shift 12, i.e. when the carrier signal C is controlled symmetrically_rAt the output of the inverter, no significant voltage U appears as a measurement voltage_gIs measured (compare the measured voltage U in fig. 4 a)_gSignal waveform of (d).

However, if the control pulse pattern generation in the control circuit 16 is modified by the voltage shift 12 such that the bias voltage U is_mA respective sinusoidal reference voltage superimposed on each phase L1, L2, L3U_L1、U_L2、U_L3Above, a measurement voltage U then appears at the output of the inverter 14_gThe common mode voltage to ground (compare the measured voltage U in fig. 4 b) is measurable and available_gSignal waveform of (d).

Fig. 4a shows the signal waveform as a result of simulation from the converter simulation of fig. 3 without voltage displacement 12.

To illustrate the basic effect of inverter operation, three sinusoidal reference voltages U are shown_L1、U_L2And U_L3Which are offset from each other by 120 DEG and have a frequency of 50Hz, and which each modulate a high-frequency (2kHz) triangular carrier signal C_r. The resulting output voltage U is due to the superposition of the center taps (tapping) of the bridge branches of the inverter 14_ab、U_acAnd U_bcDo have substantially square-wave pulses, but they contain a sinusoidal reference voltage U as an undesired fundamental oscillation_L1、U_L2And U_L3Of (c) is detected.

Common mode voltage that occurs (measurement voltage U)_g) A total of a few hundred millivolts and are not available, or are available only to a certain extent, as measurement voltages U for insulation monitoring or for locating insulation faults in electrical supply networks_g。

Fig. 4b shows the signal waveform as a result of simulation from the converter simulation of fig. 3 in the presence of the voltage shift 12.

Voltage shift 12 at each sinusoidal reference voltage U_L1、U_L2And U_L3A bias voltage U with square wave waveform, amplitude of +/-40 mV and fundamental frequency of 10Hz is superposed on the base_m。

As a result, a common-mode voltage to ground with an amplitude of ± 10V and a fundamental frequency of also 10Hz appears as the measurement voltage Ug (note the compressed time scale compared to fig. 4 a). Whereas the measuring signal frequency at 10Hz is significantly lower than the network frequency at 50Hz, the resulting measuring voltage U is generated_gCan be used to determine the insulation resistance even in the presence of large network leakage capacitance (fig. 2), because of the capacitive leakage current I_ablHas been greatly reduced and does not distort the measurement results.

Fig. 5a and 5b show the application of insulation monitoring according to the invention to an ungrounded direct current supply system (fig. 5a) and an ungrounded alternating current supply system (fig. 5 b).

Fig. 5a shows a converter 10 configured as a rectifier 24 and operated with ground by the power supply 4. An ungrounded dc supply system comprising active conductors L + and L-is connected to the output side of the rectifier 24. Leakage capacitance C of ungrounded power supply system 20_eAnd insulation resistance R_fShown as a concentrating element to ground PE (guard conductor PE).

The monitor 26 measures, on the one hand, the measurement voltage U provided according to the invention by the rectifier 24_gOn the other hand by a measurement voltage U_gInduced measurement current ((common mode) residual current) I_gThe value of (a), the measurement current I_gDetected by the residual current measuring device 6 as a sum current on the active conductors L + and L-. Insulation resistance R_fCalculated from these two variables.

By modifying the control pulse pattern sa, sb, sc in the controlled rectifier 24 (fig. 3), the measurement voltage U can be made_gAdapted to ungrounded power supply system 20 such that leakage capacitance C is_eAnd the accompanying leakage current I_ab1Without any substantial deteriorating effect on the measurement results (unlike in the case of residual current-based detection of the measurement signal without adaptation according to fig. 2).

Therefore, the method for determining the insulation resistance R according to the invention_fThe method of (a) provides the same functionality with the same reliability as a standard Insulation Monitoring Device (IMD), with the advantage that no external common mode measurement voltage source needs to be installed with care.

In addition, the insulation resistance R is excluded from the monitoring device 26_fBesides, the leakage capacitance C can also be determined_eThe size of (2).

Fig. 5b shows an application of the frequency converter 34 in which the converter 10 is configured to operate via the power supply 4 to ground. The ac power supply system 30 is connected to the output of a frequency converter 34. Here, the insulation resistance R_fAnd (if applicable) leakage capacitance C_eIs also determined in a monitor 26, the monitor 26 evaluating the measurement power generated by the frequency converterPress U_gAnd from said measurement voltage U_gInduced measuring current I_g。

Fig. 6a and 6b show the localization of insulation faults in a branched ungrounded direct current supply system (fig. 6a) operated by the ground rectifier 24 and in a branched ungrounded alternating current supply system (fig. 6b) fed by the grounded frequency converter 34 according to the invention.

The branched ungrounded dc supply system 40 of fig. 6a has two system branches 41, 42, which are each connected to a load 43, 44 and which can each be separated from the main system using a separating device 49.

Like the main system, the system branches 41, 42 and the system branches 51, 52 of fig. 6b have, like any power supply system, an inherently present (part of) insulation resistance and (part of) network leakage capacitance, which are not shown.

As shown, an insulation fault R in the system branch 41 due to an insulation fault on the load 43_xIn the case of (2), a closed current path is formed which passes through the insulation fault R_xAnd ground rectifier 24 closed and residual current I therein_xIs flowing.

An evaluation device 46 is provided in each of the system branches 41, 42. Each evaluation device 46 has a function for a (potential) residual current I_xResidual current measuring device 6 for branch-selective detection of, residual current I_xBy means of a measuring voltage U generated in a rectifier 24_g(common mode voltage) is driven as a common mode current. Each evaluation device 46 further comprises a microprocessor 47 and a device for transmitting the detected residual current I_xThe interface 48.

The enhanced monitor 45 is also intended to receive the residual current I sent by the evaluation means 46_xAnd (receive) interface 48.

Based on the residual current measuring device 6 installed in the main system and system branches 41, 42, the residual current I can be tracked_xThe resulting current path and thus the faulty system branch 41 can be detected.

In the event of a failure of the power supply system, the enhanced monitor 45 can be operated in accordance with the enhanced monitor 45 measured measurement voltage U_gAnd calculating (part of) the insulation resistance and/or (part of) the network leakage capacitance of the respective monitored system branch 41, 42 from the (common mode) measurement current flowing in the respective monitored system branch 41, 42 and sent to the enhanced monitor 45 by the evaluation means 46.

The evaluation results can be stored locally, displayed locally or transmitted to a superordinate control center in order to initiate corresponding safety measures.

Alternatively, the (partial) insulation resistance values of the system branches 41, 42 to be monitored and, optionally, the (partial) network leakage capacitances of the system branches 41, 42 may be determined directly in the respective evaluation device 46. In this case, the enhanced monitor 45 will be about measuring the voltage U_gTo the evaluation devices 46 so as to allow "on-site" branch selective determinations to be made at the respective evaluation devices 46.

The system branches 41, 42 evaluated as critical can be disconnected by means of the respective microprocessor 47 and the separating device 49.

In terms of structure and function, the illustration in fig. 6b substantially corresponds to the illustration in fig. 6a, except for the following: instead of the branched ungrounded direct current supply system 40, a branched ungrounded alternating current supply system 50 comprising two system branches 51, 52 is connected to the output of the frequency converter 34 operated at ground. Polyphase loads 53, 54 are connected to the system branches 51, 52, an insulation fault R occurring in one load 51_x。

Each system branch 51, 52 has an evaluation device 56 comprising a microprocessor 47, a residual current measuring device 6 and a three-phase separating device 59 controlled by the microprocessor 47.