Patent Application
20250119049
The invention relates to a brake adjuster for a converter with a DC-link having a resistor, a switching group with submodules and also a first connection point and a second connection point. The invention furthermore relates to a modular multilevel converter comprising such a brake adjuster. The invention also relates to a converter assembly having such a modular multilevel converter and a feed device. In addition, the invention relates to a method for operating such a brake adjuster or such a modular multilevel converter or such a converter assembly.
In various applications of converters with a DC-link, in particular with a voltage source converter, and modular multilevel converters, a brake adjuster is advantageous for converting electrical energy into heat. A typical example is the operation of a motor with a modular multilevel converter fed by a diode rectifier. If braking operation is to be provided in this configuration, a brake adjuster is commonly used. But also in regenerative configurations that can feed braking energy from the motor back into a power supply network, for example with two modular multilevel converters connected via the DC-link, i.e. connected on the DC voltage side, a brake adjuster can be advantageous if, for example, the braking operation of a motor is required with a simultaneous power failure.
The modular multilevel converter is disclosed in DE 101 03 031 A1. This converter has submodules with which a finely graduated AC voltage can be generated from a DC voltage.
Typically, such a brake adjuster for a modular multilevel converter is implemented with a central resistor, also known as a brake resistor. Alternatively, solutions with several brake resistors distributed across the submodules of the modular multilevel converter are possible. One of the possible solutions for implementing a brake adjuster with a central brake resistor is disclosed in EP 1 917 712 B1.
A pulsed resistor is known from EP 1 917 712 B1. This pulsed resistor relates to an application for a converter in the higher voltage and power range. This pulsed resistor has at least two two-pole subsystems and a resistor element, with these subsystems and the resistor element being electrically connected to each other in series. This results in a pulsed resistor with which a braking current can be finely controlled and which can be adapted to any medium voltage with simple means.
The resistor of the brake adjuster, also known as the brake adjuster resistor, is connected in series with several submodules of the modular multilevel converter to form a brake adjuster branch, also referred to simply as the brake adjuster. Apart from the SINAMICS PERFECT HARMONY GH150 and SINAMICS PERFECT HARMONY SH150 products, such a configuration is also known from the publication “Braking chopper solutions for Modular Multilevel Converters” (S. Schoening, P. K. Steimer and J. W. Kolar, Proceedings of the 2011 14th European Conference on Power Electronics and Applications, 2011, pp. 1-10.) and “Performance of a Distributed Dynamic Brake for an Induction Motor Fed by a Modular Multilevel DSCC Inverter,” (Y. Okazaki, S. Shioda and H. Akagi, in IEEE Transactions on Power Electronics, Vol. 33, No 6, pp. 4796-4806 June 2018, doi: 10.1109/TPEL.2017.2737678). In these configurations, one of the two poles of the brake resistor is directly connected to one of the two conductors of the DC-link, i.e. to a connection point of the brake adjuster.
To operate the brake adjuster, the averaged current through the brake adjuster, also known as the brake adjuster current, is set, for example, by pulse width modulation. For this purpose, the series-connected submodules, designed as a half bridge, for example, are either in the “On” state (submodule capacitor is connected to the connection points of the submodule) or the “Off” state (connection points of the submodule are short-circuited). If the submodules or at least some of the submodules are in the “Off” state, depending on the DC-link voltage, the number of submodules switched off and the resistance value of the brake resistor, a current flows through the brake adjuster branch. To control or regulate the current, individual submodules can be switched on (increasing the brake adjuster current) or switched off (reducing the brake adjuster current).
A series circuit is defined as a circuit of components through which the same current flows. The series circuit of partial impedances acts as a single two-pole circuit with an impedance that corresponds to the sum of the partial impedances.
The object of the invention is to improve a brake adjuster, particularly with regard to its usability in a converter.
This object is achieved by means of a brake adjuster having the features of claim 1. This object is further achieved by means of a modular multilevel converter having the features of claim 6. In addition, this object is achieved by means of a converter assembly having the features of claim 7. This object is also achieved by means of a method having the features of claim 8.
Advantageous embodiments of the invention are stated in the dependent claims.
Among other things, the invention is based on the realization that the design of a brake adjuster can be improved in that in a series circuit of submodules and brake resistor the submodules are arranged on both sides of the brake resistor between the brake adjuster connection points. This makes it possible to reduce the maximum voltage occurring within the brake adjuster or within a converter comprising such a brake adjuster. This also reduces the insulation requirements for the brake adjuster or the corresponding converter. The submodules of the brake adjuster are also referred to as brake adjuster submodules.
The transition between the two switching states (ON and OFF) in normal operation of the brake adjuster is carried out, for example, by sequential switching of the submodules in order to avoid high voltages within the brake adjuster or the converter. To enable simultaneous switching of all submodules, a design suited to high insulation voltages is required. Due to insulation distances, the converter dimensions are correspondingly large.
If the submodules are set to the ON state, the maintained current due to the inductances present, in particular due to the inductive behavior of the cables and the brake resistor, brings about the charging of the submodule capacitors of the brake adjuster's submodules. Depending on the parameters of the components, the number of submodules of the brake adjuster and the delay during staggered switching, this results in operating states in which the voltage provided by the series-connected brake adjuster submodules is higher than the DC-link voltage. Since the highest voltage occurring in the inverter is also taken into account for the insulation requirements, for example according to IEC 61800-5-1, such operating states increase the insulation requirements. This consideration is independent of the structure of the submodules and applies in particular to other structures such as double modules and full-bridge modules in addition to the half-bridge module structure.
In the proposed design, the brake resistor is not connected directly with one of its two poles to one of the connection points of the brake adjuster, i.e. to the DC-link. Instead, the submodules of the brake adjuster are divided into two groups of submodules connected in series. These two groups are referred to as the first and second switching groups. The brake resistor is then connected within the brake adjuster between these two groups of submodules.
The behavior of the brake adjuster with its submodules is virtually unchanged in terms of the ability to be controlled and regulated in this setup and the sum of the voltages provided by the submodules can still be greater than the DC-link voltage provided by the converter, in particular of a modular multilevel converter. This negative voltage across the brake resistor results in a current through it and therefore also in a reduction in the voltage between two points on different sides of the resistor. This means that the voltage with the greatest drop between two points of the converter is still the DC-link voltage, or the increase in the voltage with the greatest drop between two points compared to the DC-link voltage is lower than with the already known arrangement of the brake adjuster.
Compared to the previous connection, with the connection of the brake resistor proposed here, the insulation requirements are reduced due to the operating conditions described above. This results in a higher possible brake adjuster performance for existing products and their insulation coordination or, depending on the boundary conditions, makes brake adjuster operation possible in the first place. An increase in the maximum permissible DC-link voltage is also possible. For future products, the lower insulation requirements enable a reduction in costs due to smaller distances to be maintained and lower requirements being placed on the components used. If the voltages occurring during the described operation of the brake adjuster are taken into account, the requirements with regard to clearances and partial discharge extinction voltage in accordance with IEC 61800-5-1 in particular can be implemented in a simpler, more space-saving and cost-effective manner.
The modular multilevel converter can have a large number of phase modules. It has proven to be particularly advantageous for connection to three-phase grids or three-phase electrical machines if the modular multilevel converter has precisely three phase modules. To increase performance, it also makes sense if the number of phase modules is a multiple of three. The phase modules are arranged electrically in parallel with regard to their DC voltage connection points. The DC voltage connection points form the DC-link of the modular multilevel converter.
The brake resistor is particularly advantageously used in the modular multilevel converter if the latter cannot feed electrical energy back into a power supply network. This is the case, for example, with the converter assembly in which the feed device is formed by one or more diode rectifiers and therefore does not allow the direction of energy flow to be reversed. In this case, the electrical energy generated by braking a drive is converted into heat by the resistor of the brake adjuster. This enables wear-free braking of the drive, in particular a motor of the drive, in which the modular multilevel converter of the drive can be designed to be particularly compact and cost-effective.
This results in the possibility of very highly dynamic operation. With the proposed design, it is possible to switch on all submodules simultaneously to quickly reduce the current through the brake resistor without generating inadmissibly high voltages in the converter. There are also no increased insulation requirements for such operation, as the voltages arising during highly dynamic operation, especially when all submodules are switched on simultaneously, are uncritical in the proposed design.
Apart from the submodules, the switching groups have no further resistor that makes a significant contribution to the conversion of electrical energy into heat compared to the resistor arranged between the first switching group and the second switching group. The switching groups contribute only slightly to the conversion of electrical energy into heat, for example through switching losses of the semiconductors, balancing resistors of the capacitors, etc. Unlike the resistor, which is located between the first switching group and the second switching group, in this heat generation what these components have in common is that they are parasitic or have a function other than the conversion of electrical energy into heat. In this sense, the switching groups can be described as resistance-free.
The arrangement of the resistor between the first switching group and the second switching group has proven to be advantageous, as the resistor is often arranged spatially separated from the switching groups. Due to the spatial separation, the environment for the switching group, for example inside a building to protect it from environmental influences, and for the resistor, for example outdoors to better dissipate heat to the surroundings, can be optimized independently of each other.
Other than the first and second switching groups, the brake adjuster has no further switching group. In other words, the brake adjuster has precisely two switching groups, with the switching groups not contributing significantly to the conversion of electrical energy into heat.
In an advantageous embodiment of the invention, the first and second switching groups each have at least two submodules, the submodules of the respective switching group being arranged electrically in series. The number of submodules in the individual switching groups can be increased so that the brake adjuster can also be used for converters with a higher DC-link voltage. The number then results from the level of the DC-link voltage and the voltage blocking capacity of the semiconductors used in the submodules, taking into account the overvoltages that occur in the brake adjuster submodules due to operation. In addition, it may be useful to increase the number of submodules for redundancy reasons, so that the brake adjuster remains operational even if one or more submodules fail. It may also be advantageous to increase the number of submodules in order to provide a greater discharging current for the modules and thus reduce the minimum time required in the ON state.
It has proven to be advantageous to select the same number of submodules for the first switching group and the second switching group, for a maximum reduction in the highest occurring voltage. Alternatively, it is also possible to define and optimize the number of submodules per switching group according to other criteria, such as the available installation space.
In a further advantageous embodiment of the invention, the first switching group and the second switching group are each designed to be resistance-free. In this context, resistance-free means that there is no significant conversion of electrical energy into heat by the switching groups compared to the resistor arranged between the first switching group and the second switching group. Only operational losses are present in the switching groups. This means that the entire part of the brake adjuster that is relevant for the conversion into heat is located between the two switching groups. This makes it particularly easy to arrange the resistor separately from the other components of the brake adjuster. In the case of an outdoor installation being arranged outdoors, outside of a building. for example, only two supply lines are required. This means that this spatial separation for optimum heat dissipation can be easily implemented.
In a further advantageous embodiment of the invention, the brake adjuster has precisely two connection points corresponding to the first connection point and the second connection point. By its very definition as a series circuit, the brake adjuster has precisely two connection points. There are no other connection points, for example for connection to a ground potential. It has been found that such a further connection point is not necessary to reduce the requirements regarding the insulation voltage.
In a further advantageous embodiment of the invention, the resistor comprises a plurality of partial resistors. Since the resistor can convert a comparatively high amount of electrical energy into heat, it has proven advantageous to implement this resistor by means of a large number of partial resistors arranged in series and/or parallel. These can be cooled separately or at least spaced apart to ensure good cooling or heat dissipation.
In an unclaimed embodiment, the brake adjuster has a third switching group and a further resistor, with the third switching group having at least one submodule, with the further resistor and the third switching group being arranged in a further series circuit, and with the further series circuit being arranged between the second switching group and the second connection point and being arranged electrically in series with the series circuit. This structure makes it possible to further reduce the occurring voltages, which are decisive for the insulation requirements within the resistor structure. By spreading these across several resistors and switching groups, they can be arranged flexibly in the converter simultaneously and thus take into account the structural requirements, so that the converter can be manufactured particularly compactly in terms of its size. In addition, the components can be distributed in the converter with the flexibility to make them particularly easy to access. This ensures good maintainability despite the small dimensions of the converter.
It is particularly advantageous if the third switching group is electrically connected directly to the second connection point of the brake adjuster. In this case, the further resistor is then connected directly to the second switching group. If this arrangement is represented in an electrical equivalent circuit diagram, the parasitic inductances could be represented by one or more inductances at each point of the series circuit or the further series circuit. The voltages generated at the parasitic inductances by the current driven by the parasitic inductances do not then have a negative effect on the design of the brake adjuster.
The invention is described and explained in more detail below on the basis of the exemplary embodiments shown in the figures, in which:
The number of submodules 2 of the respective switching groups 21, 22 can assume any value greater than or equal to one. The number of submodules 2 of the respective switching groups 21, 22 can be the same, for example to keep the maximum occurring voltages US1 and US2 across the switching groups 21, 22 as equal as possible and the overvoltage for the insulation coordination as low as possible. Alternatively, the number of submodules 2 of the respective switching groups 21, 22 can also be different, for example in order to make advantageous use of the available installation space of the converter 5 and to make the brake adjuster 1 as compact as possible. The resistor 3 is arranged between the first switching group 21 and the second switching group 22. Since resistor 3 is located between the first and second switching groups 21, 22, the DC-link voltage Uzk must be used to consider the insulation requirements and no longer the sum of the voltage US1 across the first switching group 21 and the voltage US2 across the second switching group 22. This is due to the fact that for such critical operating points, for example when the current is rapidly reduced by the brake adjuster 1, there is a negative voltage drop across the resistor 3 in the direction of the reference arrow of UR. However, by dividing the submodules 2 between the two switching groups, this negative voltage drop does not have a negative effect on the voltages present in the brake adjuster 1 with respect to ground, for example, as the voltages US1 and US2 of the individual switching groups 21, 22 do not add up to a high voltage relevant for the insulation considerations.
This results in a reduction of the voltage relevant for the design of the insulation, without the need for further components, but solely due to the arrangement of the submodules 2 in at least two switching groups 21, 22 in accordance with the invention.
The modular multilevel converter 1 has a large number of further submodules 25 to provide voltages at the phase connection points 54. If the semiconductors of the submodules 2 and the further submodules 25 have the same voltage blocking capability, it has proven to be advantageous if the number of submodules 2 of the brake adjuster 1 corresponds to half the number of further submodules 25 of the modular multilevel converter or is slightly larger, in particular one or two larger. This allows the use of semiconductors and submodules 2, 25 and thus the optimization of the manufacturing costs for the proposed modular multilevel converter 5.
Although the diode rectifiers 62 do not allow any power reversal, i.e. a feedback of electrical energy from the modular multilevel converter 5 into the power supply network 8, the brake adjuster 1 has the advantage of being able to brake the motor 7 in a regenerative manner, i.e. without wear. For this purpose, the electrical energy generated by the motor 7 in regenerative mode is converted into heat by means of the brake adjuster 1.
In summary, the invention relates to a brake adjuster for a modular multilevel converter, having a resistor, a first switching group, and a first connection point and a second connection point. To improve the usability of the brake adjuster in a converter, it is proposed that the brake adjuster further comprises a second switching group, with the first switching group and the second switching group each comprising at least one submodule, with the first switching group being arranged between the resistor and the first connection point and the second switching group being arranged between the resistor and the second connection point such that the first switching group, the resistor and the second switching group form an electrical series circuit between the first connection point and the second connection point. The invention furthermore relates to a modular multilevel converter comprising such a brake adjuster. The invention also relates to a converter assembly with such a modular multilevel converter and a feed device, with the feed device comprising at least one diode rectifier. Furthermore, the invention relates to a method for operating such a brake adjuster, such a modular multilevel converter or such a converter assembly, with all submodules of the brake adjuster being switched on simultaneously for rapid reduction of a braking current.
| Number | Date | Country | Kind |
|---|---|---|---|
| 22151801.2 | Jan 2022 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/080492 | 11/2/2022 | WO |
