• 专利附录
  • 专利说明
  • 权利要求
  • 类似技术
  • 同族专利
  • 引用文献
  • 法律状态
  • 优先权
    • 专利摘要:
    METHOD OF OPERATING A FLOATING CAPACITOR MULTILEVEL CONVERTER AND MULTILEVEL CONVERTER SYSTEM The realizations of the invention refer to multilevel converters and, more specifically, to a method system for voltage balance in multilevel converters. It is a method (160) of operating a multi-level floating capacitor converter (60) which has a direct current link (62) and a plurality of phase conductors (14, 50, 70, 72, 74) , each of which has a plurality of floating capacitors (28, 30, 54) which includes employing redundant states to balance floating capacitor voltages when charging or discharging floating capacitors (28, 30, 54). The redundant states are used when obtaining a load current from the multi-level floating capacitor converter (60). If a charge current value is less than a limit value, then a capacitor current from a phase terminal capacitor (144) is used to determine redundant states, otherwise, a charge current direction is used to determine the redundant states.
    公开号:BR102014007746B1
    申请号:R102014007746-4
    申请日:2014-03-31
    公开日:2021-02-23
    发明作者:Stefan Schroeder；Qingyun Chen
    申请人:Ge Energy Power Conversion Technology Ltd；
    IPC主号:
    专利说明:

    FIELD OF THE INVENTION
    [001] The realizations of the invention refer to multilevel converters and, more specifically, to a method system for voltage balance in multilevel converters. BACKGROUND OF THE INVENTION
    [002] Multilevel converters are generally used in high-power industrial applications, such as variable speed drive systems (VSD) or in energy conversion applications such as a solar (or photovoltaic) generation system, wind turbine generators and high voltage direct current transmission (HVDC) systems. An example of a multilevel converter is a multi-level floating capacitor (FC) converter. The multi-level floating capacitor converter includes several capacitors called floating capacitors. The floating capacitors are charged at various voltage levels and when changing switching states, the floating capacitors and a dc source are connected in different ways and produce various phase-to-neutral output voltages.
    [003] Floating capacitor voltage balancing is a challenge when operating the multi-level floating capacitor converter. Voltage imbalance in floating capacitors (ie uneven voltages across floating capacitors) can override capacitors and switching devices and cause overvoltage and undervoltage peaks during operation of the converter. The voltage imbalance of floating capacitors also causes an increase in the total harmonic distortion (THD) of the output voltage and can cause the control loop to become unstable.
    [004] One of the known solutions used to balance voltages of floating capacitors uses an additional balancing circuitry. However, the balancing circuitry adds costs, losses, volume to the multi-level floating capacitor converter and also needs to be carefully designed.
    [005] Therefore, it is desirable to provide a method and a system that addresses the problems mentioned above. DESCRIPTION OF THE INVENTION
    [006] According to an embodiment of the present invention, a method of operating a multi-level floating capacitor converter that includes a direct current link and a plurality of phase conductors, each of which has a plurality of floating capacitors, is provided. The method includes employing redundant states to balance floating capacitor voltages when charging or discharging floating capacitors. Employing redundant states includes obtaining a charge current from the multi-level floating capacitor converter and using a capacitor current from a phase terminal capacitor to determine redundant states when a charge current value is less than a threshold value. When the load current value is greater than the limit value, the load current direction is used to determine redundant states.
    [007] According to another embodiment of the present invention, a multilevel converter system comprising a converter that includes a direct current link and a plurality of phase conductors, each of which has a plurality of floating capacitors and a converter controller is provided. The converter controller is configured to employ redundant states to balance floating capacitor voltages when charging or discharging the plurality of floating capacitors. The converter controller employs redundant states when using a capacitor current from a phase terminal capacitor to determine redundant states when a load current value is less than a threshold value, and use the load current direction to determine redundant states when the load current value is higher than the limit value. BRIEF DESCRIPTION OF THE DRAWINGS
    [008] These and other aspects, characteristics, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings, in which similar characters represent similar parts through drawings.
    [009] Figure 1 is a schematic diagram of a conductor of an exemplary multi-level floating capacitor converter and the output waveform for use in accordance with an embodiment of the present technique.
    [010] Figure 2 is a schematic diagram of a conductor of another multi-level floating capacitor converter for use in accordance with an embodiment of the present technique.
    [011] Figure 3 is a schematic diagram of a five-level three-phase floating capacitor converter for use in accordance with an embodiment of the present technique.
    [012] Figure 4 is a state machine diagram for controlling the operation of a conductor of a five-level floating capacitor converter according to an embodiment of the present technique.
    [013] Figure 5 is a circuit diagram of a conductor of a multi-level floating capacitor converter with a phase terminal capacitor for use in accordance with an embodiment of the present technique.
    [014] Figure 6 is a block diagram that represents a method of operating a multi-level floating capacitor converter according to an embodiment of the present technique. DESCRIPTION OF ACCOMPLISHMENTS OF THE INVENTION
    [015] Figure 1 illustrates a schematic 10 of an example conductor or phase of an exemplary floating capacitor (FC) multilevel converter and the output waveform 12. It should be noted that scheme 10 is just an example of the multilevel converter floating capacitor, and other variations of multi-level floating capacitor converter such as one shown in Figure 2 are well within the scope of the present technique. A conductor 14 of the multi-level floating capacitor converter includes four unidirectional switching devices 16, 18, 20, and 22, two bidirectional switching devices 24 and 26 and two floating capacitors 28 and 30. In one embodiment, two capacitors 32 and 34 of DC links are controlled to each have a voltage approximately equal to Vcc / 2, where Vcc is the total DC link voltage. A Van output phase voltage of conductor 14 is measured between a center point or neutral point 36 of a DC link 38 and a phase terminal 40. As shown in output waveform 12, the output phase voltage Van has five voltage levels, two positive voltage levels (Vcc / 2 and Vcc / 4), one zero voltage level (0) and two negative voltage levels (-Vcc / 2 and - Vcc / 4). Since the Van output phase voltage has five levels, this converter is called a five-level converter. It should be noted that in this embodiment, a line-to-line output voltage (that is, a voltage between two phase terminals) will have nine voltage levels.
    [016] In schematic 10, if the voltage across floating capacitors 28 and 30 is not equal to Vcc / 4 then this can result in uneven voltage steps in the output phase voltage. Switching devices 18, 20 and 26 are also generally evaluated on the assumption that the voltage across floating capacitors 28 and 30 is Vcc / 4. Now, when the voltage across the floating capacitor 28 or 30 becomes greater than Vcc / 4, switching devices 18, 20 and 26 can also see higher voltages across them (that is, they will be surpassed). Thus, in a realization of the present technique, a control system and method is revealed to balance the voltages along the floating capacitors in a multi-level floating capacitor converter.
    [017] Figure 2 illustrates a schematic diagram of a conductor or a phase 50 of another multi-level floating capacitor converter for use in accordance with an embodiment of the present technique. In this embodiment, a conductor 50 of the multi-level floating capacitor converter includes a DC link 52, floating capacitors 54, top switching devices 56 and bottom switching devices 58. An output phase voltage measured at phase terminal 59 will have five voltage levels, two positive voltage levels (Vcc / 2 and Vcc / 4), one zero voltage level (0) and two negative voltage levels (-Vcc / 2 and - Vcc / 4) obtained with various switching combinations of top and bottom switching devices 56 and 58 respectively.
    [018] Figure 3 illustrates a schematic of a five-level three-phase floating capacitor converter 60 for use in accordance with an embodiment of the present technique. In general, converter 60 is a three-phase representation of the converter shown in Figure 1. Converter 60 includes a split DC link 62 with a neutral point 64 and capacitors 66 and 68 and three switching conductors 70, 72, and 74 with three terminals phase 76, 78, and 80 respectively (for phases a, b, and c). A three-phase load 82 is connected along the three-phase output terminals 76, 78, and 80. A converter controller 84 provides switching pulse commands to the switching devices of the switching conductors 70, 72, and 74 based on multiple inputs (not shown) such as a three-phase reference voltage, a phase current direction, a DC link voltage and floating capacitor voltages, for example. In one embodiment, the term "controller" refers to any combination of elements of software and hardware, or any system, process or functionality that performs or facilitates the processes described in this document. As described above, the phase output voltages at phase terminals 76, 78, and 80 will have five levels, namely Vcc / 2, Vcc / 4, 0, -Vcc / 4 and -Vcc / 2.
    [019] Figure 4 illustrates a state machine diagram 100 for controlling the operation of a conductor 14 (Figure 1) of the five-level floating capacitor converter according to an embodiment of the present technique. In one embodiment, the state machine 100 can be employed by converter controller 84. The state machine 100 includes two switching states 102 and 118 for the two output voltage levels Vcc / 2 and -Vcc / 2 respectively. In addition, there are two switching states each (104, 106 and 114, 116) for the two output voltage levels Vcc / 4 and -Vcc / 4. In other words, switching states 104 and 106 are redundant and either can be used to obtain the output voltage level Vcc / 4. Similarly, switching states 114 and 116 are redundant and can be used to obtain the output voltage level -Vcc / 4. For the zero output voltage level (0) there are three redundant switching states 108, 110 and 112. In each state, the floating capacitors 28 and 30 can be charged, discharged or remain unchanged depending on a current direction. If the charging and discharging of floating capacitors is not regulated properly, then this will result in unbalanced voltages across them. The voltage imbalance of floating capacitors can overwhelm capacitors and switching devices and an increase in total harmonic distortion (THD) of the output voltage and can cause the control loop to become unstable. In one embodiment, the selection of redundant switching states depends on charging the floating capacitor or discharging the floating capacitor.
    [020] In another embodiment, for a given switching state, if the charge state of the floating capacitor remains unchanged, then the other redundant switching states for that voltage level are not considered. For example, for the zero output voltage level, only state 108 needs to be used as state 108 nor does it charge or discharge the floating capacitors.
    [021] The switching states that correspond to the positive, negative and zero output voltage of the state machine 100 and related switching positions of the switching devices on conductor 114 are given below in Table 1. Table 1 also provides information on whether the floating capacitors 28, 30 (Figure 1) will be charged (+), discharged (-) or will remain unchanged (*) for a given switching state when a phase current flows out of the converter (ie, the phase current exiting the phase terminal 40). When the phase current direction is opposite (that is, the phase current flows into the converter or phase terminal 40), then the capacitor's charge or discharge state is reversed. In other words, when the phase current direction reverses, the capacitor being charged begins to discharge, the capacitor being discharged begins to charge and that capacitor that remained unchanged, remains unchanged. In an embodiment of the present technique, the phase current can be a load current of the converter if it exceeds a limit value, or the phase current can be a load current of the phase terminal capacitor if it does not exceed the limit values, as will be described subsequently. For the zero output voltage level, only a switching state 108 that neither charges nor discharges the floating capacitors is shown. TABLE 1

    [022] As can be seen from Table 1, for an outgoing phase current, 2 redundant switching states each for output voltage levels + Vcc / 4 and -Vcc / 4 can be used following the charging requirements or unloading of the floating capacitors. For the zero output voltage level, only one switching state (108) is used as this does not result in any charging or discharging of floating capacitors. However, in another embodiment, other redundant switching states to the zero voltage level can also be employed to meet the charging and discharging requirements of the floating capacitors. As discussed above, for phase input current, charging and discharging of floating capacitors merely reverses. During normal operation, that is, when the phase current is the load current, redundant switching states are used to balance the floating capacitor voltages with the help of the load current. However, when there is no load or there is a low load on the converter, using the load current to charge or discharge the floating capacitors, and thus, to balance the floating capacitor voltages, is a challenge. According to an embodiment of the present technique, a control system that uses a current in a phase terminal capacitor (for example, filter capacitor or stray capacitance at the output terminals of the converter) is used to balance floating capacitor voltages in conditions load or low load.
    [023] Figure 5 shows a circuit diagram 140 of a conductor of a multi-level floating capacitor converter with a phase capacitor 144. In one embodiment, the phase capacitor 144 can be part of an output filter 146 that it also includes a resistor 142 and an inductor 143. In another embodiment, a dedicated or additional capacitor can also be added to the phase terminal. In yet another embodiment, the phase terminal capacitor 144 may not be a separate component attached to the converter, instead it may simply be a capacitor formed due to its parasitic capacitance at the phase terminal, for example, capacitances of the attached cable or the windings of engine. In other words, phase capacitor 144 can be defined as either a filter capacitor, an additional capacitor or a parasitic capacitance formed at the phase terminals of the converter. In accordance with an embodiment of the present technique, phase 144 capacitor charging or discharging current is used to determine appropriate switching states in Table 1 to balance floating capacitor voltages during low-load or no-load conditions.
    [024] In general, a phase constant capacitor time 144 is much less than the carrier period. The time constant refers to the time taken by a capacitor to charge at 63% of a step voltage and the carrier period refers to a time period of a switching cycle (i.e., carrier period = 1 / switching frequency). In this way, the phase terminal capacitor 144 can charge or discharge with a switching cycle. In one embodiment of the present technique, this phase 144 capacitor charging and discharging current is used to determine redundant states to balance the floating capacitor voltages during low or no-charge currents.
    [025] A charge Q required for the phase 144 terminal capacitor to reach a Vdegrau level is given by Q = Vdegrau x Cs, where Vdegrau is a step voltage applied across the filter 146 in volts and Cs refers to the capacitance value of capacitor 144 in Farads. The Q load is measured in Coulombs and the Q load signal depends on the Vdegrau signal, that is, whether the Vdegrau voltage is going up or down. In addition, the Vdegrau value depends on the voltage at the phase terminal 40 of the converter. The value of Vdegrau, and whether it is going up or down, is known by a controller 148 that determines the switching states for the converter. In other words, controller 148 determines the expected voltage at the phase terminal of the multilevel converter from a voltage reference value that it receives as an input and then based on the expected voltage, determines the direction of the phase current. Finally, controller 148 uses the direction of the phase current to decide the redundant switching state that will balance the floating capacitor voltages.
    [026] A voltage deviation ΔVfc of a floating capacitor Cfc in relation to the phase terminal capacitor Cs will be given by the following equation:

    [027] In other words, the voltage deviation ΔVfc of the floating capacitor Cfc is directly proportional to the capacitance value of the phase terminal capacitor 144 and the deviation in an output phase voltage Vhase (or Vdegrau), at the same time that it is inversely proportional to the capacitance value of the floating capacitor Cfc. Thus, the higher the capacitance value of the floating capacitor, the smaller the voltage deviation ΔVfc of the floating capacitor Cfc.
    [028] As an example, assume that the Vhase voltage at the 40-phase terminal of the converter is 0 volts. In this way, phase 144 capacitor is also charged at 0 volts. Now, if the expected phase voltage Vhase is to be changed from 0 to Vcc / 4 according to the converter's voltage requirement, then at no charge or low cost, the phase terminal capacitor 144 needs to charge so that can reach the voltage level of Vcc / 4. In this way, controller 148 determines that, since phase terminal capacitor 144 needs to charge, current at phase terminal 40 can flow out of the converter. Now, from Table 1, it is known that for a phase current directed outward and for the voltage level Vcc / 4, there are two redundant switching states 104 and 106. However, from Table 1, it is also known that state 104 results in the charging of floating capacitor 28, while state 106 results in the discharge of floating capacitor 28. Thus, if a floating capacitor voltage sensor (not shown) detects that floating capacitor 28 is overloaded , then switching state 106 will be selected by controller 148, otherwise switching state 104 will be selected.
    [029] As discussed above, the present technique uses a current direction of charge during normal conditions and a current direction of the phase terminal capacitor during no-load or low-load currents to decide between redundant switching states to balance the voltages of floating capacitor. In one embodiment, a current limit value is used to differentiate between the normal condition and the no-load or low-load current condition. In one embodiment, the current limit value is decided by matching the load Q of the phase terminal capacitor 144, which is given by Vdegrau * Cs, with a load Q load, which can be generated by the limit load current Unlimited. The load Qload of the limit limit load Unlimited can be given as Qload = Unlimited * d / fc, where fc is a switching frequency and d is a duty cycle. The value of d can be an effective value or an average value or a value determined by an operator. Now, equating Qcarga and Q provides:

    [030] Thus, the current limit value depends on a phase terminal capacitance value, the voltage at the phase terminal of the converter and the switching frequency and duty cycle of the multi-level floating capacitor converter. In one embodiment, the load current is compared against this current limit value and, when the load current is less than the current limit value, redundant switching states are determined based on the current direction of the phase terminal capacitor. The deviations from the above equations assume that the return cables of all capacitors are connected to the dc link and, consequently, that the total voltage step of a phase inverter is transferred to its respective capacitor. If the return cables of each capacitor are connected to each other but are not connected to the dc link, then only a fraction of the total phase inverter voltage is applied to the respective capacitor, for example 2/3 in the case of a three-phase system . This factor must be incorporated into equations (2) and (3) in such a configuration.
    [031] Figure 6 shows a method 160 of operating a multi-level floating capacitor converter according to an embodiment of the present technique. In step 168, the method includes employing redundant switching states in order to balance floating capacitor voltages when charging or discharging floating capacitors. In one embodiment, redundant switching states for a given voltage level refer to the switching states that can be used in place of each other as they result in the same voltage level. Steps 162 to 166 refer to steps that may be involved in using redundant switching states. For example, employing switching states includes measuring a charge current from a floating capacitor multilevel converter in step 162. In step 164, a charge current value is compared against a limit value and if the charge current value is less at the limit value a capacitor current from a phase terminal capacitor is then used to determine redundant switching states. In one embodiment, the phase terminal capacitor may be a filter capacitor or, in another embodiment, the phase terminal capacitor may be a parasitic capacitance. In another embodiment, the current limit value depends on parameters that include the value of the phase terminal capacitor, the switching frequency, the duty cycle and the voltage at the phase terminal of the converter. The use of the phase terminal capacitor current comprises first determining the current direction of the phase terminal capacitor, based on an expected voltage transition at a phase terminal of the converter. In one embodiment, if the voltage at the phase terminal of the converter is expected to go from high to low, then the current direction of the phase terminal capacitor is marked as flowing into the phase terminal, otherwise the current direction of the phase terminal capacitor is marked as flowing out of the phase terminal. In step 166, method 160 includes using a load current direction to determine redundant switching states if the load current value is above the limit value.
    [032] The example mentioned above or part of the method and example steps mentioned above can be deployed by suitable computer program code in a processor-based system, such as a general purpose or special application computer. Computer program code can be stored or adapted for storage on one or more tangible, machine-readable media, such as memory chips, local or remote hard drives, optical discs (that is, CDs or DVDs), or other media, that can be accessed by the processor-based system to execute the stored code.
    [033] Although only a few features of the invention have been illustrated and described in this document, many modifications and changes can occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as they fall within the scope of the invention.
    权利要求:
    Claims (13)
    [0001]
    1. METHOD (160) OF OPERATING A FLOATING CAPACITOR MULTILEVEL CONVERTER (60) that includes a direct current link (62) and a plurality of phase conductors (14, 50, 70, 72, 74), each of which one has a plurality of floating capacitors (28, 30, 54), a phase terminal (40, 76, 78, 80) and a phase terminal capacitor (144), the method being characterized by comprising the steps of: a) employ redundant states to balance floating capacitor voltages when charging or discharging floating capacitors, and employing redundant states includes the steps of: a1) obtaining (162) a charging current from the floating capacitor multilevel converter (60); a2) use (164) a capacitor current from the phase terminal capacitor (144) to determine redundant states when a load current value is less than a limit value; and a3) use (166) a load current direction to determine redundant states when the load current value is greater than the limit value, in which the step of using (164) the capacitor current of the phase terminal capacitor (144 ) comprises determining a current direction of the phase terminal capacitor (144) based on an expected voltage transition at the phase terminal (40, 76, 78, 80) of the multi-level floating capacitor converter (60).
    [0002]
    2. METHOD (160), according to claim 1, characterized in that the limit value is determined based on parameters that include a capacitance value of the phase terminal capacitor (144), a voltage change at a phase terminal (40 , 76, 78, 80) of the multi-level floating capacitor converter (60), and the switching frequency and duty cycle of the multi-level floating capacitor converter (60).
    [0003]
    3. METHOD (160), according to claim 1, characterized in that the limit value is determined by equating a load of the phase terminal capacitor (144) with a load generated by a load current.
    [0004]
    4. METHOD (160) according to claim 1, characterized by the step of determining the current direction of the phase terminal capacitor (144) including marking the current direction of the phase terminal capacitor (144) as flowing into the phase terminal (40, 76, 78, 80) if the expected voltage transition is from high to low.
    [0005]
    METHOD (160) according to either of claims 1 or 4, characterized by the step of determining the current direction of the phase terminal capacitor (144) including marking the current direction of the phase terminal capacitor (144) as flowing out of the phase terminal (40, 76, 78, 80), in case the expected voltage transition is low to high.
    [0006]
    6. METHOD (160) according to any one of claims 1, 4 or 5, characterized by the expected voltage transition at a phase terminal (40, 76, 78, 80) of the multi-level floating capacitor converter (60) being determined based on a voltage reference value.
    [0007]
    METHOD (160) according to any one of claims 1 to 6, characterized in that the redundant switching states include at least two switching states that generate equal voltages.
    [0008]
    8. METHOD (160), according to claim 7, characterized by the determination of the redundant state to be decided based on the requirement of charging or discharging the floating capacitor.
    [0009]
    9. METHOD (160) according to any one of claims 1 to 8, characterized in that a time constant of the phase terminal capacitor (144) is less than a carrier period.
    [0010]
    10. METHOD (160), according to any one of claims 1 to 9, characterized by the phase terminal capacitor (144) charging or discharging within a switching cycle.
    [0011]
    11. MULTILEVEL CONVERTER SYSTEM, characterized by comprising: - a converter (60) that includes a direct current link (62), a plurality of phase conductors (14, 50, 70, 72, 74) each of which has plurality floating capacitors (28, 30, 54), a phase terminal (40, 76, 78, 80) and a phase terminal capacitor (144); and - a converter controller (148) configured to: employ redundant states to balance floating capacitor voltages when charging and discharging the plurality of floating capacitors (28, 30, 54), where the controller employs redundant states when: using a current capacitor of the phase terminal capacitor (144) to determine redundant states when a load current value of the converter (60) is less than a limit value; and use a load current direction to determine redundant states when the load current value is greater than the limit value, where the converter controller (148) is configured to use the capacitor current of the phase terminal capacitor (144) when determining a current direction of the phase terminal capacitor based on an expected voltage transition at a phase terminal (40, 76, 78, 80) of the converter (60).
    [0012]
    12. MULTILEVEL CONVERTER SYSTEM, according to claim 11, characterized in that the phase terminal capacitor (144) comprises a filter capacitor or a phase terminal parasitic capacitance.
    [0013]
    13. MULTILEVEL CONVERTER SYSTEM, according to either of claims 11 or 12, characterized in that the controller (148) is configured to determine the redundant state based on the charging or discharging requirements of the floating capacitor (28, 30, 54).
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    法律状态:
    2017-08-15| B03A| Publication of a patent application or of a certificate of addition of invention [chapter 3.1 patent gazette]|
    2018-11-13| B06F| Objections, documents and/or translations needed after an examination request according [chapter 6.6 patent gazette]|
    2020-02-11| B06U| Preliminary requirement: requests with searches performed by other patent offices: procedure suspended [chapter 6.21 patent gazette]|
    2020-12-22| B09A| Decision: intention to grant [chapter 9.1 patent gazette]|
    2021-02-23| B16A| Patent or certificate of addition of invention granted|Free format text: PRAZO DE VALIDADE: 20 (VINTE) ANOS CONTADOS A PARTIR DE 31/03/2014, OBSERVADAS AS CONDICOES LEGAIS. |
    优先权:
    申请号 | 申请日 | 专利标题
    US13/853,335|US9246407B2|2013-03-29|2013-03-29|Voltage balancing system and method for multilevel converters|
    US13/853,335|2013-03-29|
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