Patent Publication Number: US-8541904-B2

Title: Apparatus and system for power conversion

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of, and claims priority to, U.S. non-provisional application Ser. No. 12/840,423, filed Jul. 21, 2010, now U.S. Pat. No. 8,310,083, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Embodiments of the invention relate generally to power electronics systems and, more particularly, to control of power conversion in a power electronics system. 
     In power electronics systems, a power conversion process may include converting power from a source into a load power and supplying the load power to a load. In one example, a DC-link or voltage bus may supply the load power to a load coupled thereto such as connecting DC/DC converters to DC/AC inverters or other DC energy sources/sinks. 
     In an exemplary system, a hybrid electric vehicle may employ one or more common DC-link(s) coupled to available energy sources such as, for example, batteries, capacitors, flywheels, combustion engines, fuel cells, gas turbines, or the like. The DC-link voltage of the one or more common DC-link(s) should be kept within a defined operation band. This operation band could change depending on actual load on the traction inverter(s) due to efficiency optimization reasons. In other cases, it may be preferred to have the DC-link voltage at a constant value. 
     One method to hold the desired DC-link voltage is to exactly balance the power commands directed into the DC-link and coming out of the DC-link so that the energy in the DC-link remains constant. This strategy involves taking into account the dynamics of all involved converters as well as any communication delay/sample rate limitations that may be manifest in the supervisory controller. However, measurement errors (e.g., noise, offset), unknown dynamics (e.g., such as those due to nonlinearities in power conversion), and time delays of communication as well as sample and hold delay by the supervisory controller can limit an exact balancing of power into and out of the DC-link. 
     Typically, the DC-link is equipped with one or more capacitive devices (e.g., a battery or ultracapacitor) that provides substantial capacitance to filter the voltage ripple resulting from current ripple and to buffer energy in case of high frequency mismatch or imbalance of power flow into and out of the DC-link. The substantial capacitance helps buffer energy for preventing a drop/rise of DC-link voltage between subsequent or new power commands/demands of power conversion that may be due to an imbalance between power into and out of the DC-link. For example, a large capacitor may be used where voltage on the DC-link changes slowly compared to the power imbalance. Using a large capacitor, the supervisory controller is able to balance the voltage on the DC-link. However, these large-capacitance DC-link capacitors tend to be physically large components having a significant size and weight, which may be disadvantageous, especially for mobile applications such as in a hybrid electric vehicle application. In addition, these capacitors add extra costs to the systems/applications using them. 
     Therefore, it would therefore be desirable to provide an apparatus and system for controlling power conversion that reduces a capacitance, size, and weight of a capacitor used to buffer energy on a power conversion DC-link. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one aspect of the invention, an apparatus includes a DC-link, a load coupled to the DC-link, and an energy conversion system. The energy conversion system includes a first energy storage device, a first voltage converter coupled to the first energy storage device and to the DC-link, a first bus voltage controller coupled to the DC-link and to the first voltage converter, and a supervisory controller coupled to the first voltage converter and to the first bus voltage controller. The first voltage converter is configured to convert a first DC voltage from the first energy storage device into a second DC voltage based on a command signal and based on a first adjustment signal and supply the second DC voltage to the DC-link. The first bus voltage controller is configured to iterate calculation of the first adjustment signal based on the command signal and based on a measured voltage of the DC-link and communicate each iterated calculation of the first adjustment signal to the first voltage converter. The supervisory controller is configured to iterate calculation of the command signal based on the load and based on a desired DC-link voltage for the load and communicate each iterated calculation of the command signal to the first voltage converter and to the first bus voltage controller. A frequency of the first bus voltage controller to communicate each iterated calculation of the first adjustment signal is higher than a frequency of the supervisory controller to communicate each iterated calculation of the command signal. 
     In accordance with another aspect of the invention, an apparatus includes a voltage bus, a load coupled to the voltage bus, and a voltage converter coupled to the voltage bus and to an energy storage device. The voltage converter is configured to convert energy from the energy storage device into a voltage bus voltage. The apparatus also includes a supervisory controller coupled to the voltage bus and to the voltage converter. The supervisory controller programmed to iteratively determine a desired voltage bus voltage based on the load and to iteratively calculate a control signal based on the desired voltage bus voltage, the control signal configured to cause the voltage converter to convert the energy from the energy storage device into the voltage bus voltage. The apparatus further includes a voltage bus controller coupled to the voltage bus, to the voltage converter, and to the supervisory controller. The voltage bus controller is programmed to receive the voltage feedback from the voltage bus, receive the control signal from the supervisory controller, and calculate the desired voltage bus voltage based on the control signal The voltage bus controller is also programmed to determine if the voltage feedback is within the threshold of the desired voltage bus voltage and if the voltage feedback is outside of the threshold of the desired voltage bus voltage, calculate a regulatory setpoint signal based on the voltage feedback and based on the desired voltage bus voltage, the regulatory setpoint signal configured to cause the voltage converter to adjust the conversion of the energy from the energy storage device such that the voltage on the voltage bus voltage is within the threshold. A bandwidth of the voltage bus controller to receive the voltage feedback and calculate the regulatory setpoint signal is higher than a bandwidth of the supervisory controller to iteratively receive the voltage feedback and calculate the control signal. 
     In accordance with yet another aspect of the invention, a system includes a DC-link, a voltage inverter coupled to the DC-link and configured to convert a DC voltage from the DC-link into a first AC voltage, an electromechanical device coupled to the voltage inverter and configured to convert the first AC voltage into a mechanical output, and an energy conversion system. The energy conversion system includes a plurality of energy storage devices configured to store DC energy, a plurality of voltage converters, a power management controller coupled to the plurality of voltage converters, and a first DC-link voltage controller coupled to a first voltage converter of the plurality of voltage converters and to the power management controller. Each voltage converter is coupled to a respective energy storage device and configured to convert a stored voltage from the respective energy storage device into a DC supply voltage based on a setpoint signal and supply the DC supply voltage to the DC-link. The power management controller is coupled to the plurality of voltage converters and configured to iteratively calculate the setpoint signal based on a target voltage for the DC-link and to iteratively supply the setpoint signal to the plurality of voltage converters. The first DC-link voltage controller is configured to iteratively determine the target voltage based on the setpoint signal, to iteratively calculate a first adjustment signal based on a difference between the target voltage and the DC supply voltage, and to iteratively supply the first adjustment signal to the first voltage converter, wherein the first voltage converter is further configured to convert the stored voltage based on the first adjustment signal. A frequency of the power management controller to iteratively calculate the setpoint signal and supply the setpoint signal to the plurality of voltage converters is lower than a frequency of the first DC-link voltage controller to calculate and supply the first adjustment signal to the first voltage converter. 
     Various other features and advantages will be made apparent from the following detailed description and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate embodiments presently contemplated for carrying out the invention. 
       In the drawings: 
         FIG. 1  is a schematic block diagram of a power electronics system according to an embodiment of the invention. 
         FIG. 2  is a schematic block diagram of a power electronics system according to another embodiment of the invention. 
         FIG. 3  is a schematic block diagram of a power electronics system according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic block diagram of the infrastructure of a power electronics system  10  according to an embodiment of the invention. Power electronics system  10  includes a supervisory controller  12  coupled to a plurality of DC-DC voltage converters  14 ,  16 ,  18 . DC-DC voltage converters  14 - 18  are coupled to a plurality of energy storage devices  20 ,  22 ,  24 . In one embodiment, each energy storage device  20 - 24  may be a power battery, a flywheel system, a fuel cell, an ultracapacitor, or the like. While three pairs of DC-DC voltage converters/energy storage devices are shown, embodiments of the invention are not limited as such, and more or less than the number of DC-DC voltage converters and energy storage devices shown are contemplated. 
     DC-DC voltage converters  14 - 18  are also coupled to a DC-link or voltage bus  26 , which supplies voltage or energy from DC-DC voltage converters  14 - 18  to a DC-AC inverter  28  or other load. DC-AC inverter  28 , which inverts DC voltage or energy on DC-link  26  into AC voltage or energy is coupled to an electromechanical device or motor  30  to electrically drive motor  30 , which mechanically drives a wheel  32  of a hybrid electric or a purely electric vehicle in one embodiment. Hybrid electric vehicles may combine an internal combustion engine (not shown) in addition to electric motor  30  and energy storage device  20 - 24  to propel the vehicle. Such a combination may increase overall fuel efficiency by enabling the combustion engine and the electric motor  30  to each operate in respective ranges of increased efficiency. Electric motors, for example, may be efficient at accelerating from a standing start, while combustion engines may be efficient during sustained periods of constant engine operation, such as in highway driving. Having an electric motor to boost initial acceleration allows combustion engines in hybrid vehicles to be smaller and more fuel efficient. 
     Purely electric vehicles use stored electrical energy to power an electric motor such as electromechanical device  30 , which propels the vehicle and may also operate auxiliary drives. Purely electric vehicles may use one or more sources of stored electrical energy such as energy storage device  20 - 24 . For example, a first source of stored electrical energy may be used to provide longer-lasting energy while a second source of stored electrical energy may be used to provide higher-power energy for, for example, acceleration. 
     In another embodiment, power electronics system  10  may be a non-vehicle system, and motor  30  may be coupled to mechanically drive a shaft to perform work. 
     According to one embodiment, supervisory controller  12  determines a target or desired voltage for DC-link  26  based on the power demands of the load such as DC-AC inverter  28  and motor  30 . For example, based on a given speed and load of motor  30 , supervisory controller  12  calculates an efficiency optimization for the system. Based on the calculations, supervisory controller  12  calculates a command or power setpoint control signal and transmits the command signal to DC-DC voltage converters  14 - 18 . The command signal is calculated to cause DC-DC voltage converters  14 - 18  to convert energy stored in energy storage devices  20 - 24  into a voltage that substantially matches the target voltage. Supervisory controller  12  is configured to iteratively update the calculation of the command signal and supply the updated command signal calculation to DC-DC voltage converters  14 - 18  to address the speed and load demands of motor  30 . 
     A voltage measurement device  34  coupled to DC-link  26  provides a voltage measurement feedback signal. In one embodiment, the voltage measurement feedback signal represents an average voltage on DC-link  26 . In one embodiment, voltage measurement device  34  provides the voltage measurement feedback signal to supervisory controller  12 . Using the voltage measurement feedback signal, supervisory controller  12  determines if the voltage on DC-link  26  matches the target voltage or is within a given threshold of the target voltage. For example, supervisory controller  12  may find the difference between the voltage on DC-link  26  and the target voltage. If the voltage on DC-link  26  does not substantially match the target voltage or if the difference between the voltage on DC-link  26  and the target voltage is greater than a threshold, supervisory controller  12  re-calculates the control signal to cause DC-DC voltage converters  14 - 18  to adjust the voltage such that the voltage on DC-link  26  substantially matches or is within a threshold of the target voltage. 
     Power electronics system  10  also includes a DC-link voltage controller  36  coupled to supervisory controller  12 . DC-link voltage controller  36  receives the command signal from supervisory controller  12  and determines the target or desired DC-link voltage therefrom. DC-link voltage controller  36  is also coupled to voltage measurement device  34  for receiving the voltage measurement feedback signal from voltage measurement device  34 . Using the voltage measurement feedback signal, DC-link voltage controller  36  determines if the voltage on DC-link  26  matches or is within a given threshold of the target voltage. For example, DC-link voltage controller  36  may find the difference between the voltage on DC-link  26  and the target voltage. If the voltage on DC-link  26  does not substantially match the target voltage or if the difference between the voltage on DC-link  26  and the target voltage is greater than a threshold, DC-link voltage controller  36  calculates an adjustment or regulatory setpoint signal. 
     DC-DC voltage converter  14  is coupled to DC-link voltage controller  36  and is configured or programmed to convert energy from energy storage device  20  based on the command signal from supervisory controller  12  as well as based on the adjustment signal from DC-link voltage controller  36 . Accordingly, DC-link voltage controller  36  sends the adjustment signal to DC-DC voltage converter  14  to cause DC-DC voltage converter  14  to adjust the voltage to be converted such that the voltage on DC-link  26  substantially matches or is within the threshold of the target voltage. DC-link voltage controller  36  is configured to iteratively update the calculation of the adjustment signal and supply the updated adjustment signal to DC-DC voltage converter  14  to address any imbalance between power into and out of DC-link  26 . 
     According to an embodiment of the invention, DC-link voltage controller  36  has a higher bandwidth than supervisory controller  12 . That is, the frequency of DC-link voltage controller  36  to iteratively update the calculation of the adjustment signal and to supply the updated adjustment signal to DC-DC voltage converter  14  is faster or higher than the frequency of supervisory controller  12  to iteratively update the calculation of the command signal and supply the updated command signal calculation to DC-DC voltage converters  14 - 18 . The higher bandwidth control of DC-link voltage controller  36  thus reduces time delays of communication in the feedback loop with DC-link  26 . A capacitive device  38  such as a battery, capacitor, or ultracapacitor may be coupled to DC-link  26  to reduce a drop or rise of DC-link voltage between subsequent or new power commands/demands or to filter voltage ripple or buffer energy on DC-link  26 . The high bandwidth of the feedback loop of DC-DC voltage converter  14 , voltage measurement device  34 , and DC-link voltage controller  36  allows for a reduced capacitance requirement for capacitive device  38  and thus a reduced capacitor size and weight. 
     DC-DC voltage converter  14  similarly has a higher bandwidth than supervisory controller  12 . In one embodiment, the bandwidth of DC-DC voltage converter  14  substantially matches the bandwidth of DC-link voltage controller  36 . It is also preferred that energy storage device  20  be able to read quickly and to be cycled with microcycles. 
       FIG. 1  further illustrates a plurality of voltage measurement devices  40 ,  42 ,  44  coupled to supervisory controller  12  and configured to measure the voltage or state-of-charge of energy storage devices  20 - 24 . The measured voltages may indicate the availability of power and energy of energy storage devices  20 - 24 . According to an embodiment of the invention, supervisory controller  12  may be configured or programmed to receive and monitor the measured voltages over time. Supervisory controller  12  may use the measured voltages to calculate the command signal to DC-DC voltage converters  14 - 18  such that the operational lifetime and/or short-term operation of energy storage devices  20 - 24  are optimized. 
       FIG. 2  is a schematic block diagram of power electronics system  10  according to another embodiment of the invention. As shown in  FIG. 2 , power electronics system  10  includes a second DC-link voltage controller  46  coupled to second DC-DC voltage converter  16  and a third DC-link voltage controller  48  coupled to third DC-DC voltage converter  18 . While the embodiment in  FIG. 2  shows a DC-link voltage controller coupled to each DC-DC voltage converter, embodiments of the invention may include less voltage controllers than voltage converters. For example, an embodiment of the invention may include two DC-link voltage controllers and three or more pairs of DC-DC voltage converters and energy storage devices. 
     Similar to DC-link voltage controller  36 , DC-link voltage controllers  46 ,  48  are coupled to supervisory controller  12  and receive the command signal from supervisory controller  12  and determine the target or desired DC-link voltage therefrom. DC-link voltage controllers  46 ,  48  are also coupled to voltage measurement device  34  for receiving the voltage measurement feedback signal. Using the voltage measurement feedback signal, DC-link voltage controllers  46 ,  48  determine if the voltage on DC-link  26  matches or is within a given threshold of the target voltage. For example, DC-link voltage controllers  46 ,  48  may find the difference between the voltage on DC-link  26  and the target voltage. If the voltage on DC-link  26  does not substantially match the target voltage or if the difference between the voltage on DC-link  26  and the target voltage is greater than a threshold, DC-link voltage controllers  46 ,  48  calculate respective adjustment or regulatory setpoint signals. 
     DC-DC voltage converters  16 ,  18  are respectively coupled to DC-link voltage controllers  46 ,  48  and are configured or programmed to convert energy from energy storage devices  22 ,  24  based on the command signal from supervisory controller  12  as well as on the respective adjustment signals from DC-link voltage controllers  46 ,  48 . Accordingly, DC-link voltage controllers  46 ,  48  send the respective adjustment signals to DC-DC voltage converters  16 ,  18  to cause DC-DC voltage converters  16 ,  18  to adjust their respective converted voltages such that the voltage on DC-link  26  substantially matches or is within a threshold of the target voltage. DC-link voltage controllers  46 ,  48  are configured to iteratively update the calculation of the respective adjustment signals and supply the respective updated adjustment signals to DC-DC voltage converters  16 ,  18  to address any imbalance between power into and out of DC-link  26 . 
     DC-link voltage controllers  46 ,  48  have a higher bandwidth than supervisory controller  12 . That is, the frequency of DC-link voltage controllers  46 ,  48  to iteratively update the calculation of the respective adjustment signals and to supply the updated adjustment signals to DC-DC voltage converters  16 ,  18  is faster or higher than the frequency of supervisory controller  12  to iteratively update the calculation of the command signal and supply the updated command signal calculation to DC-DC voltage converters  14 - 18 . DC-DC voltage converters  16 ,  18  similarly have a higher bandwidth than supervisory controller  12 . Furthermore, it is also preferred that energy storage devices  22 ,  24  be able to read quickly and to be cycled with microcycles. 
     DC-link voltage controllers  36 ,  46 - 48  may be configured or programmed to calculate or modify their respective adjustment signals based on independent threshold levels. For example, DC-link voltage controller  36  may be configured to adjust the voltage on DC-link  26  for differences between the DC-link/target voltages above a first threshold. DC-link voltage controller  46  may be configured to adjust the voltage on DC-link  26  for differences between the DC-link/target voltages below the first threshold but above a second threshold, while DC-link voltage controller  48  may be configured to adjust the voltage on DC-link  26  for differences between the DC-link/target voltages below the second threshold. DC-link voltage controllers  36 ,  46 - 48  may thus be designed to provide optimum voltage level adjustment of DC-link  26  for their respective ranges or threshold levels. 
       FIG. 3  is a schematic block diagram of power electronics system  10  according to another embodiment of the invention. As shown in  FIG. 3 , DC-DC voltage converter  14  is coupled to one or more of DC-DC voltage converters  16 ,  18  via one or more communication lines  50 . DC-DC voltage converters  16 ,  18  are configured or programmed to convert energy from respective energy storage devices  22 ,  24  based on the command signal from supervisory controller  12  as well as on an adjustment or regulation setpoint signal from DC-DC voltage converter  14 . 
     According to one embodiment, DC-DC voltage converter  14  is configured or programmed to receive the adjustment signal from DC-link voltage controller  36  and to determine a total desired output voltage of DC-DC voltage converter  14  from the command signal and from the adjustment signal. DC-DC voltage converter  14  then determines its output capacity or ability to provide the total desired output voltage. If DC-DC voltage converter  14  determines that it can provide the total desired output voltage, then DC-DC voltage converter  14  supplies the total desired output voltage. However, if DC-DC voltage converter  14  determines that it cannot provide the total desired output voltage, DC-DC voltage converter  14  determines a difference between the output voltage it can provide and the total desired output voltage. 
     DC-DC voltage converter  14  then calculates and transmits a regulation setpoint signal to either or both of DC-DC voltage converters  16 ,  18 . The regulation setpoint signal calculated by DC-DC voltage converter  14  is configured to cause either or both of DC-DC voltage converters  16 ,  18  to convert and supply voltage to make up at least the difference between the output voltage that DC-DC voltage converter  14  can provide and the total desired output voltage. DC-DC voltage converters  16 ,  18  may also be high bandwidth converters and may be positioned on the same hardware or control board as DC-DC voltage converter  14 . In one embodiment, DC-DC voltage converters  16 ,  18  are controlled with similar or identical regulation setpoint signals. 
     In another embodiment, DC-DC voltage converter  14  may be configured to calculate the regulation setpoint signal based on one or more threshold levels such that DC-DC voltage converters  16 ,  18  respectively provide increased voltages according to respective first and second adjustment ranges. For example, according to the second adjustment range, DC-DC voltage converter  18  may be caused to increase or decrease its voltage output by smaller or lower amounts than DC-DC voltage converter  16  according to the first adjustment range. In this manner, DC-DC voltage converter  16  may be controlled to address large voltage differences on DC-link  26  while DC-DC voltage converter  18  may be controlled to address small voltage differences on DC-link  26 . 
     According to another embodiment, DC-DC voltage converter  14  may be programmed to handle large or small voltage adjustments, while calculating the regulation setpoint signal to cause DC-DC voltage converter  16 , for example, to handle small or large adjustments. 
     Embodiments of the invention thus allow high bandwidth or frequency control of DC-link voltage variations between a target DC-link voltage and the actual DC-link voltage. In this manner, the size and weight of a DC-link capacitor may be reduced, thus providing higher operational efficiency as well as providing reduced size, weight, and cost constraints. 
     A technical contribution for the disclosed apparatus is that it provides for a controller implemented technique for controlling power conversion in a power electronics system. 
     According to one embodiment of the invention, an apparatus includes a DC-link, a load coupled to the DC-link, and an energy conversion system. The energy conversion system includes a first energy storage device, a first voltage converter coupled to the first energy storage device and to the DC-link, a first bus voltage controller coupled to the DC-link and to the first voltage converter, and a supervisory controller coupled to the first voltage converter and to the first bus voltage controller. The first voltage converter is configured to convert a first DC voltage from the first energy storage device into a second DC voltage based on a command signal and based on a first adjustment signal and supply the second DC voltage to the DC-link. The first bus voltage controller is configured to iterate calculation of the first adjustment signal based on the command signal and based on a measured voltage of the DC-link and communicate each iterated calculation of the first adjustment signal to the first voltage converter. The supervisory controller is configured to iterate calculation of the command signal based on the load and based on a desired DC-link voltage for the load and communicate each iterated calculation of the command signal to the first voltage converter and to the first bus voltage controller. A frequency of the first bus voltage controller to communicate each iterated calculation of the first adjustment signal is higher than a frequency of the supervisory controller to communicate each iterated calculation of the command signal. 
     In accordance with another embodiment of the invention, an apparatus includes a voltage bus, a load coupled to the voltage bus, and a voltage converter coupled to the voltage bus and to an energy storage device. The voltage converter is configured to convert energy from the energy storage device into a voltage bus voltage. The apparatus also includes a supervisory controller coupled to the voltage bus and to the voltage converter. The supervisory controller programmed to iteratively determine a desired voltage bus voltage based on the load and to iteratively calculate a control signal based on the desired voltage bus voltage, the control signal configured to cause the voltage converter to convert the energy from the energy storage device into the voltage bus voltage. The apparatus further includes a voltage bus controller coupled to the voltage bus, to the voltage converter, and to the supervisory controller. The voltage bus controller is programmed to receive the voltage feedback from the voltage bus, receive the control signal from the supervisory controller, and calculate the desired voltage bus voltage based on the control signal The voltage bus controller is also programmed to determine if the voltage feedback is within the threshold of the desired voltage bus voltage and if the voltage feedback is outside of the threshold of the desired voltage bus voltage, calculate a regulatory setpoint signal based on the voltage feedback and based on the desired voltage bus voltage, the regulatory setpoint signal configured to cause the voltage converter to adjust the conversion of the energy from the energy storage device such that the voltage on the voltage bus voltage is within the threshold. A bandwidth of the voltage bus controller to receive the voltage feedback and calculate the regulatory setpoint signal is higher than a bandwidth of the supervisory controller to iteratively receive the voltage feedback and calculate the control signal. 
     In accordance with yet another embodiment of the invention, a system includes a DC-link, a voltage inverter coupled to the DC-link and configured to convert a DC voltage from the DC-link into a first AC voltage, an electromechanical device coupled to the voltage inverter and configured to convert the first AC voltage into a mechanical output, and an energy conversion system. The energy conversion system includes a plurality of energy storage devices configured to store DC energy, a plurality of voltage converters, a power management controller coupled to the plurality of voltage converters, and a first DC-link voltage controller coupled to a first voltage converter of the plurality of voltage converters and to the power management controller. Each voltage converter is coupled to a respective energy storage device and configured to convert a stored voltage from the respective energy storage device into a DC supply voltage based on a setpoint signal and supply the DC supply voltage to the DC-link. The power management controller is coupled to the plurality of voltage converters and configured to iteratively calculate the setpoint signal based on a target voltage for the DC-link and to iteratively supply the setpoint signal to the plurality of voltage converters. The first DC-link voltage controller is configured to iteratively determine the target voltage based on the setpoint signal, to iteratively calculate a first adjustment signal based on a difference between the target voltage and the DC supply voltage, and to iteratively supply the first adjustment signal to the first voltage converter, wherein the first voltage converter is further configured to convert the stored voltage based on the first adjustment signal. A frequency of the power management controller to iteratively calculate the setpoint signal and supply the setpoint signal to the plurality of voltage converters is lower than a frequency of the first DC-link voltage controller to calculate and supply the first adjustment signal to the first voltage converter. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.