AU2009312845B2

AU2009312845B2 - Device for feeding a consumer network with the electric power of a supply network

Info

Publication number: AU2009312845B2
Application number: AU2009312845A
Authority: AU
Inventors: Uwe Krebs; Jurgen Moser
Original assignee: Siemens AG; Siemens Corp
Current assignee: Siemens AG
Priority date: 2008-11-10
Filing date: 2009-11-02
Publication date: 2014-12-11
Anticipated expiration: 2029-11-02
Also published as: EP2345130B1; AU2009312845A1; EP2345130A1; WO2010052188A1; JP2012508552A; CN102204056B; HK1159332A1; CN102204056A; DE102008056581A1; JP5357262B2

Abstract

The invention relates to a device (1) for feeding a consumer network (13) with electrical power from a supply network (12), wherein the device (1) comprises a frequency converter (2), one side of which can be connected to the supply network (12) and the other side of which can be connected to the consumer network (13), said converter also being able to provide high short circuit currents for the consumer network over a sufficiently long time frame so that a short circuit in the consumer network can be localized and separated in a targeted manner from the remainder of the consumer network, which is fault-free. A mechanical switching unit (3) is disposed parallel to the frequency converter (2) and is adapted to bypass the frequency converter (2), and control means (9) are connected to said switching unit (3), said control means being adapted to actuate the switching unit (2) when a short circuit current flowing across the frequency converter (2) is detected.

Description

-1 Apparatus for feeding a consumer network with the electrical power from a power supply system Aspects of the present disclosure relate to an apparatus for feeding a consumer network with electrical power from a power supply system, wherein the apparatus has a frequency converter which can be connected on the one hand to the power supply system and on the other hand to the consumer network. Aspects of the present disclosure also relate to a method for feeding a consumer network with electrical power from a power supply system. An apparatus such as this and a method such as this are already known from DE 10 2005 004 628 Al. The apparatus described there is intended for supplying power to marine vessels in harbor. Marine vessels in harbor are generally supplied with power by running diesel engines. However, the exhaust gases from the diesel engines lead to considerable environmental pollution in the harbor towns. The apparatus of the type in question allows power to be supplied from the land. A static frequency converter, by means of which the various systems can be coupled, is provided because the voltages, system frequencies, star-point treatments and the like generally differ between the land-based power supply system and the marine-vessel distribution system. Static converters have power semiconductors which allow currents and voltages to be converted. The already known apparatus has the disadvantage that high short-circuit currents cannot be fed into the marine vessel distribution system over a sufficiently long time period. The power semiconductor valves in the frequency converter could otherwise be damaged or even destroyed. However, the short-circuit current must be fed in for a sufficiently long time to allow a short circuit in a 9293284 -2 marine vessel to be traced, to allow the faulty system part of the marine-vessel distribution system to be isolated from the rest of the marine-vessel distribution system. To ameliorate at least one of these disadvantages, aspects of the present disclosure provide an apparatus of the type mentioned initially and a method, by means of which even high short-circuit currents can be provided for the consumer network over a sufficiently long time period, thus allowing a short-circuit in the consumer network to be traced and to be deliberately isolated from the rest of the sound consumer network. Aspects of the present disclosure achieve this object by a mechanical switching unit, which is arranged in parallel with the frequency converter and is designed to bridge the frequency converter, and by control means, which are connected to the switching unit and are designed to operate the switching unit when it is found that a short-circuit current is flowing via the frequency converter. Other aspects of the present disclosure achieve this object by a method for feeding a consumer network with the electrical power from a power supply system, wherein the power supply system is connected to the consumer network via a frequency converter and a mechanical switching unit which is designed to bridge the frequency converter, in which the consumer network is monitored for a short-circuit criterion, the frequency of the consumer network is changed to the frequency of the power supply system when a short-circuit criterion is present, with the frequency converter also continuing to feed the consumer network with a rated current which is normal during normal operation without a short circuit, and with the mechanical switching unit then being closed in order to bridge the frequency converter. 9293284 -3 A mechanical switching unit is provided connected in parallel to the frequency converter and allows the frequency converter to be bridged in the event of a short circuit. The term mechanical switching unit covers all switches which have contacts which can be disconnected from one another. The contacts are arranged at distance from one another in an electrically insulating environment when in a disconnected position, thus preventing current from flowing via the contacts. Control means are provided in order to identify or to find a short circuit in the consumer network, and are designed not only to identify the short circuit on the basis of a predetermined short-circuit criterion but also to trip the mechanical switching unit. After operation of the switching unit, the short-circuit current flows essentially via the switching unit, which has an adequate current carrying capability for the short-circuit current. The mechanical switching unit is, for example, a circuit breaker, a switch disconnector or the like. Semiconductor switches, which are also referred to as electronic switches, are however precluded. The frequency converter is preferably a static frequency converter. Static converters do not have any moving parts or components, but power semiconductor valves. The frequency converter may in principle have any desired topology. For example, the static frequency converter has a rectifier and an inverter, which are connected to one another on the DC voltage side. Rectifiers and inverters are very well known to a person skilled in the art in the field of power transmission and distribution, as a result of which there is no need to describe their design in detail at this point. In general, bridge circuits comprising so-called power semiconductor valves are used, with the power semiconductor valves being connected to a closed-loop control system and being movable by this closed-loop control system specifically from a blocking 9293284 -4 position, in which a current flow via the power semiconductor valve is interrupted, to an on position, in which current can flow via the power semiconductor valve. The expedient control of the power semiconductor valves by the open-loop control unit or better, closed-loop control system, allows the desired conversion. By way of example, two-stage or three-stage converters, or else so-called multi-level converters may be used as rectifiers and inverters, which are also both jointly referred to as converters. The frequency converter is expediently designed for high voltages, and in particular for voltages between 1 kV and 52 kV. However, frequency converters which are designed for voltages above 52 kV or below 1 kV can also be used. The control means are expediently connected to the frequency converter and are designed to influence the frequency conversion of the frequency converter. According to this expedient further development, for example, care is also taken after the occurrence of the short-circuit in the consumer network to ensure that the frequency converter also continues to supply the consumer network only with the normal rated current which was fed in before the short circuit, and whose amplitude is therefore essentially unchanged. This prevents damage to the power semiconductors and the frequency converters. However, the short circuit cannot be traced in this way. The switching unit is therefore operated. However, it is operated only when the frequency converter is supplying the consumer network such that undesirable feedback resulting from the mechanical switching unit being switched on is avoided. The control means are therefore either part of the closed-loop control system for the frequency converter, or are connected to it. For example, if the power supply system frequency is 50Hz, but the consumer network in contrast uses a frequency of 60Hz during normal operation, the control means 9293284 -5 prevent the consumer network from being connected directly to the 50Hz power supply system, while the frequency converter produces a 60Hz output voltage Uout. This would result in undesirable disadvantages. According to one preferred refinement of the apparatus according to the present disclosure, input measurement sensors are provided which are designed to detect electrical input measurement variables on the power supply system side of the frequency converter, and output measurement sensors are provided, which are designed to detect electrical output measurement variables on the consumer network side of the frequency converter, wherein the control means are connected to the input measurement sensors and to the output measurement sensors, and are designed to set the output frequency of the frequency converter to its input frequency and to delay the operation of the switching unit until a selected output measurement variable and the corresponding input measurement variable are at the same frequency and phase angle. According to this refinement, consideration is given to the fact that the voltages in the input-side power supply system and those in the consumer network on the output side may be at different frequencies and phases. In order to prevent uncontrolled equalizing currents from flowing when the frequency converter is bridged, the control means delay the operation of the switching unit until in-phase measurement variables at the same frequency are present on both sides of the frequency converter. The input measurement variables and output measurement variables are expediently respectively the input voltage and the output voltage, that is to say the voltage which is respectively present in the consumer network and the voltage which is respectively present in the power supply system. Operation takes place, with the frequency converter therefore being bridged by the control means, only when the 9293284 -6 voltages in the power supply system and in the consumer network are in phase and are at the same frequency. Expediently, input connecting means are provided for connection of the apparatus to the power supply system, and possibly also output connecting means for connection of the apparatus to the consumer network. By way of example, a conventional transformer may be used as an input connecting means or output connecting means. By way of example, the short-circuit criterion is a simple threshold criterion, in other words the amplitude of the current is monitored on the consumer network or on the input power supply system side. If the amplitude of the current flowing via the frequency converter exceeds a predetermined threshold value for a predetermined time period, the presence of a short-circuit current is deduced. The amplitudes of short-circuit currents are many times greater than those of the normal rated current. By way of example, a short-circuit criterion would be satisfied if the measured measurement current exceeds a variable threshold value, which is expediently above the rated current. A further example of a short-circuit criterion, in addition to the pure load current, that is to say in addition to the current fed into the consumer network from the frequency converter, comprises additionally monitoring the output voltage. The quotient of the measured voltage and the measured current represents an impedance. According to the example of a short-circuit criterion, an impedance threshold value is defined for this impedance, whose undershooting is assessed as a short-circuit event or criterion. According to one expedient refinement of the method according to the present disclosure, at least one electrical input variable is detected on the power supply system side of the frequency converter, and at least one electrical output 9293284 -7 variable is detected on the consumer network side of the frequency converter, and the phases and/or frequency of each input variable are/is compared with the phases of the corresponding output variable. Once the short-circuit criterion is satisfied, the switching unit is closed only when it is found that the input variable and the output variable are in phase and/or are at the same frequency, as has already been stated further above. Expediently, after bridging the frequency converter, the elapsing of a transmission time period and/ or the absence of the short circuit criterion is waited for, and only then is the switching unit operated in order to cancel the bridging of the frequency converter, with the frequency of the consumer network then being changed to the frequency before the short circuit. This allows simple recreation of the normal mode of operation of the apparatus, for example after the short circuit has been traced and disconnected. Once the bridging of the frequency converter has been cancelled, the operating point before the short circuit is approached gradually again in the consumer network by closed-loop control of the frequency converter, for example by using a ramp function. By way of example, this may be the gradual production of a 60Hz frequency in the consumer network, whose frequency briefly corresponded to the frequency of the power supply system after the bridging of the frequency converter, for example 50Hz. The output frequency of the frequency converter is changed after identification of the short circuit by means, for example, of a step function or ramp function. The monitoring means expediently check whether energy is being produced in the consumer network, with the monitoring means preventing the operation of the switching unit if energy production is found in the consumer network. This is used to check whether the consumer network is a stand-alone network. 9293284 -8 Stand-alone networks are distinguished by not having their own energy generator. If the consumer network is a stand-alone network, all of the short-circuit power must therefore be applied by the frequency converter. However, if the consumer network has an energy generator, for example if a marine vessel generator in the consumer network has not yet been switched off, the short-circuit current can be provided by the generator without this loading the frequency converter. In this case, there is no need to bridge the frequency converter. According to this refinement of the present disclosure, a determination is accordingly made as to whether the consumer network is a standalone network, and bridging of the frequency converter by the control means is prevented if this is not the case. In addition to three-phase applications, aspects of the present disclosure extend expressly to arrangements with any desired number of phases. The mechanical switching unit advantageously has contacts which can move relative to one another. Mechanical switches such as these are very well known in the field of high, medium and low voltage, and the details of their design and structure therefore do not need to be described herein. They are preferably used in the field of electrical power transmission and distribution. By way of example, the contacts comprise a fixed contact, which is arranged in a fixed position, and a moving contact, which is guided such that it can move with respect to the fixed contact. The moving contact is, for example, designed such that it can pivot. However, in contrast to this, longitudinally movable moving contacts can also be used, which carry out a linear movement during switching. The contacts of linear switches such as these are, for example, arranged in a vacuum chamber of a vacuum interrupter tube, thus resulting in a so-called vacuum switch. In contrast to 9293284 -9 this, the mechanical switching unit is a gas switch, in which blowing techniques are used to quench the arc. The mechanical switching unit advantageously has a drive unit for producing a drive movement, and a switching mechanism for introducing the drive movement into the contacts. By way of example, the drive unit is a spring energy storage drive or a magnetic drive. Expediently, after operation of the mechanical switching unit as well, the frequency converter feeds the consumer network with the rated current which is normal during normal operation without a short circuit. According to one preferred refinement of the method, after bridging the frequency converter, the absence of the short circuit criterion is waited for. In the absence of the short circuit criterion, the mechanical switching unit is operated in order to cancel the bridging of the frequency converter, and the frequency of the consumer network is changed back to the normal frequency for normal operation without a short circuit. By way of example, this is done suddenly, or preferably with the aid of a ramp function. Monitoring means advantageously check whether energy is being produced in the consumer network, with the monitoring means preventing the operation of the switching unit if energy production is found in the consumer network. The short-circuit current can then be provided by the generator in the consumer network. Further expedient assignments and advantages of the invention are the subject matter of the following description of exemplary embodiments of the invention with reference to the figure of the drawing, in which: 9293284 -10 The figure schematically illustrates one exemplary embodiment of the apparatus according to aspects of the present disclosure. The apparatus 1 comprises a frequency converter 2 and a mechanical switching unit 3, which is designed to bridge the frequency converter 2 and is arranged in a three-phase parallel branch 4 in parallel with the frequency converter 2. For clarity reasons, the figure illustrates only one phase. The three short lateral strokes on the illustrated phase are, however, intended to indicate that the illustrated exemplary embodiment of the apparatus is a three-phase apparatus. In principle, however, the apparatus may have any desired number of phases. In the illustrated exemplary embodiment, the mechanical switching unit 3 is a three-pole switch disconnector. Each pole of the switch disconnector has a vacuum interrupter tube with two contacts whose ends are opposite one another. One of the contacts is guided such that it can move in the longitudinal direction, such that it can be moved to a contact position, in which the contacts rest on one another, by introduction of a drive movement from a disconnected position, in which the contacts are disconnected from one another in the vacuum chamber. The drive movement is produced by a magnetic drive, which is not illustrated in the figure, and is introduced into the contacts via a switching mechanism, which is likewise not illustrated in the figure. The magnetic drive is connected to the open-loop control unit 9 via a signal line. The frequency converter 2 has a converter which is operated as a rectifier 6, and a converter which is operated as an inverter 7, with the rectifier 6 and the inverter 7 being connected to one another via a DC voltage link 8. An open-loop 9293284 -11 control unit 9, which comprises the control means as described initially, is used for closed-loop control and protection of the converters 6, 7 and for tripping the mechanical switching unit 3. The apparatus 1 is connected to a power supply system 12, which carries a 50 Hz AC voltage, via an input transformer 10 as an input connecting means and via an output transformer 11 as an output connecting means. In the case illustrated in the figure, the consumer network 13 is a distribution system in a relatively large marine vessel which is in harbor. In this case, the marine vessel distribution system 13 is connected to the land-based power supply system 12 via the frequency converter 2. The frequency converter is therefore required to convert the 50 Hz voltage from the power supply system 12 to the 60 Hz voltage for the marine vessel distribution system 13. For closed-loop control of the frequency conversion and of the power transmission, as well as the operation of mechanical switching unit 3, the open-loop control unit 9 is connected to input current sensors 14 on the power supply system side and to output current measurement sensors 15 arranged in the marine vessel distribution system 13. Furthermore, the open loop control unit 9 is connected to input voltage measurement sensors 16 on the power supply system side and to output voltage measurement sensors 17 arranged in the marine vessel distribution system 13. If all the generators in the marine vessel have been switched off, the marine vessel distribution system is a stand-alone network. In this case, the supply current has to be provided solely by the frequency converter 2. In order to prevent the frequency converter being switched off too quickly for fault identification in the event of a short circuit, the open-loop control unit 9 uses the output current 9293284 -12 measurement sensors 15 to monitor the amplitude of the voltage Uma in the consumer network 13. If the open-loop control unit finds that a short-circuit criterion is satisfied, for example if the measured voltage is less than a previously defined voltage threshold value implemented in the open-loop control unit 9, or an impermissible undershooting of the impedance in the consumer network 13 is found, it ensures that the frequency converter 2 also continues to feed in only the rated current into the consumer network 13. Since no short-circuit current is flowing, the short circuit cannot be traced. Furthermore, the open-loop control unit uses a step function or a ramp function to ensure that the output voltage Uma of the frequency converter 2 is matched to its input voltage Umi In other words, the 60 Hz frequency of the consumer network 13 in the marine vessel is changed to a so Hz frequency. Once the frequencies of the output voltage Uma and input voltage U±, are the same, and these measurement variables U±i, Umu are in phase, the switch disconnector 3 or, in other words, the mechanical switching unit, is closed. A high short-circuit current which exceeds the rated current can now flow via the parallel branch 4. However, before the mechanical switching unit 3 is tripped, the open-loop control unit 9 compares the phases on the power supply system side of the voltage Ui, which are detected by means of the input voltage system sensors 16, with the voltages Uma on the consumer side, which are detected by means of the output voltage measurement sensors 17. After a short-circuit time period has elapsed, during which the protective devices, which are not illustrated in the figure, in the consumer network 13 in the marine vessel use the short-circuit current to trace and disconnect the short circuit, the open-loop control unit 9 identifies the absence of the short-circuit criterion, by evaluation of the signals from the connected sensors 15 and 17. The open-loop control 9293284 -13 unit 9 switches the mechanical switching unit 3 off, that is to say moves its contacts to their disconnected position. Power is now once again supplied exclusively via the frequency converter 2. Finally, after the switching unit 3 has been switched off, the closed-loop control for the inverter 7 ensures that the original 60 Hz AC voltage is gradually restored in the consumer network 13, with the aid of a so called ramp function. 9293284

Claims

1. An apparatus for feeding a consumer network with electrical power from a power supply system, wherein the apparatus comprises: a frequency converter connected between the power supply system and the consumer network, a mechanical switching unit connected in parallel with the frequency converter and configured to bridge the frequency converter, and control means connected to the mechanical switching unit and the frequency converter and configured to: monitor the consumer network for a short-circuit criterion; and upon detection of the short-circuit criterion, ensure that the frequency converter continues to feed rated current into the consumer network; change the frequency of the consumer network to the frequency of the power supply system; and close the mechanical switching unit to bridge the frequency converter.

2. The apparatus as claimed in claim 1 further comprising: an input measurement sensor configured to detect at least one electrical input measurement variable on a power supply system side of the frequency converter, and 9293284 -15 an output measurement sensor configured to detect at least one electrical output measurement variable on a consumer network side of the frequency converter.

3. The apparatus as claimed in claim 2, wherein the control means are connected to the input measurement sensors and to the output measurement sensors and are configured to: set the output frequency of the frequency converter to the input frequency of the frequency converter, and to delay the closing of the mechanical switching unit until a selected electrical input measurement variable and the corresponding electrical output measurement variable are at the same frequency and phase angle.

4. The apparatus as claimed in one of the preceding claims, further comprising: input connecting means for connection of the apparatus to the power supply system and/or output connecting means for connection of the apparatus to the consumer network.

5. The apparatus as claimed in one of the preceding claims, wherein the frequency converter comprises converters which are connected to one another via a DC voltage link, with each converter having controllable power semiconductor valves.

6. The apparatus as claimed in one of the preceding claims, wherein the mechanical switching unit comprises contacts which can move relative to one another. 9293284 -16

7. The apparatus as claimed in claim 6, wherein the mechanical switching unit comprises a drive unit for producing a drive movement, and a switching mechanism for introducing the drive movement into the contacts.

8. The apparatus as claimed in any of the preceding claims, wherein even after closing of the mechanical switching unit, the frequency converter feeds the consumer network with the rated current which is normal during normal operation.

9. The apparatus as claimed in one of the preceding claims, wherein the control means is configured to: detect absence of the short-circuit criterion after bridging the frequency converter, in the absence of the short-circuit criterion, open the mechanical switching unit to cancel the bridging of the frequency converter, and change the frequency of the consumer network back to the normal frequency for normal operation without a short circuit.

10. The apparatus as claimed in one of the preceding claims, further comprising monitoring means configured to check whether energy is being produced in the consumer network, and prevent operation of the mechanical switching unit if energy is being produced in the consumer network. 9293284 -17

11. A method for feeding a consumer network with electrical power from a power supply system, wherein the power supply system is connected to the consumer network via a frequency converter and a mechanical switching unit, which is designed to bridge the frequency converter, the method comprising: monitoring the consumer network (13) for a short-circuit criterion, changing the frequency of the consumer network to the frequency of the power supply system when the short-circuit criterion is present, feeding, via the frequency converter, the consumer network with a rated current which is normal during normal operation without a short circuit, and closing the mechanical switching unit in order to bridge the frequency converter.

12. The method as claimed in claim 11, further comprising: detecting at least one electrical input variable on a power supply system side of the frequency converter, and at least one electrical output variable on a consumer network side of the frequency converter, comparing the phases of the input variable with the phase of the output variable, and once the short-circuit criterion is satisfied, closing the mechanical switching unit when it is found that the input variable and the output variable are at the same frequency and are in phase. 9293284 -18

13. The method as claimed in claim 11 or 12, wherein even after closing of the mechanical switching unit, the frequency converter feeds the consumer network with the rated current which is normal during normal operation.

14. The method as claimed in one of the preceding claims 11 to 13, further comprising: after bridging the frequency converter, detecting absence of the short-circuit criterion, in the absence of the short-circuit criterion, opening the mechanical switching unit to cancel the bridging of the frequency converter, and changing the frequency of the consumer network back to the normal frequency for normal operation without a short circuit.

15. The method as claimed in one of claims 11 to 14, further comprising: checking whether energy is being produced in the consumer network, and preventing operation of the mechanical switching unit if energy is being produced in the consumer network. SIEMENS AKTIENGESELLSCHAFT Patent Attorneys for the Applicant SPRUSON & FERGUSON 9293284