Patent Description:
Storage battery facilities have been increasingly used to make use of renewable energy and cope with power failures. For example, a DC power supply and distribution system that supplies and distributes DC power is used as a power supply system used in application of solar power generation and storage batteries. In a power supply system such as a DC supply and distribution system, power converters are sometimes connected in parallel to increase device capacity. However, when power converters are connected in parallel, the magnitudes of output currents of the power converters may become unbalanced and, if the unbalance increases, the operation of the power converters may become unstable.

To cope with this, for example, Japanese Patent <CIT> (PTL <NUM>) discloses a power supply system that suppresses unbalance in output current by correcting the operation of power converters, based on a voltage control signal composed of an error signal comparing a voltage detection value of a load with a reference voltage oscillation of each power converter and an unbalance detection value calculated from output current of each power converter.

DC power supply and distribution systems are advantageous in that the number of times of power conversion is reduced and cost efficiency is improved because DC power output from a DC power source need not be converted into AC power. Furthermore, DC voltage output from the DC power supply and distribution systems has no fixed standards and has flexibility. Systems for outputting DC power suitable for each load device have been proposed.

For example, Japanese Patent <CIT> (PTL <NUM>) proposes a power supply system including a current detector to detect current consumption of a load circuit receiving a predetermined power supply voltage and having a plurality of load states, in which an optimum DC voltage depending on the states of output current of the load is supplied based on the current consumption detected by the current detector and the predetermined power supply voltage.

Additional prior art, related to the preamble of claim <NUM>, is given in <CIT>.

Document <CIT> discloses a load state detector which detects a state (voltage, current, etc.) of a DC system connected to a plurality of loads as an operating state of the loads and outputs the detection result as a load operating information to command the generator.

Further related disclosure is seen in <CIT> and <CIT>.

However, in Japanese Patent <CIT> (PTL <NUM>), it is necessary to provide each power converter with a communication interface for communicating information such as output currents of the power converters connected in parallel between a plurality of power conversion devices, leading to increase in size and weight of the power conversion devices.

In the DC/DC converter disclosed in Japanese Patent <CIT> (PTL <NUM>), it is necessary to prepare a data table indicating the relation of power supply voltages of the load, and when the kinds of connected loads are not fixed, power consumption of the load is unable to be appropriately reduced.

The present invention has been made to solve the problem above, and an object of the present invention is to provide a power supply system including a plurality of power conversion devices connected in parallel, in which power consumption of loads is reduced without performing communication between the power conversion devices.

A power supply system according to the present invention is described in independent claim <NUM> and converts power received from a main power source to supply AC power or DC power and outputs the converted power to a DC system. This power supply system includes a plurality of power conversion devices provided between the main power source and the DC system and connected in parallel with each other, a state detector to detect an operating state of at least one load connected to the DC system, and a command generator to generate a voltage command that is a command value of voltage distributed from the power supply system to the DC system. Each of the power conversion devices includes a voltage controller to generate a power command based on a voltage of the DC system and the voltage command and a converter to convert power received from the main power source based on the power command and output the converted power to the DC system. The command generator generates the voltage command such that loss of at least one load connected to the DC system is reduced, based on a detection result of the state detector. The voltage controller (<NUM>) is configured to set droop characteristics (DL) that define characteristics of the power command for the voltage of the DC system (L, LA, LB), based on the voltage command, and to generate the power command from the set droop characteristics (DL) and the voltage of the DC system (L, LA, LB).

According to the present invention, in a power supply system including a plurality of power conversion devices connected in parallel, power consumption of loads is reduced without performing communication between the power conversion devices.

Embodiments of the present invention will be described in detail below with reference to the drawings. Like or corresponding parts in the drawings are denoted by like reference signs and a description thereof is not repeated.

<FIG> is a diagram schematically showing an exemplary configuration of a power supply system <NUM> according to the present embodiment. Power supply system <NUM> is connected to a main power source <NUM> and a plurality of loads <NUM>. Although <FIG> shows an example in which main power source <NUM> is an AC system power source, main power source <NUM> may be a DC power source. The number of loads <NUM> is not necessarily more than one and may be one.

Power supply system <NUM> supplies power received from main power source <NUM> to a plurality of loads <NUM> connected to a DC system L. Power supply system <NUM> includes a plurality of power conversion devices <NUM>, a load state detector <NUM>, and a command generator <NUM>.

Each power conversion device <NUM> converts power received from main power source <NUM> into DC power based on a distribution voltage command Vref from command generator <NUM> and outputs the converted DC power to DC system L. Distribution voltage command Vref is a command value of voltage distributed from power supply system <NUM> to DC system L.

Load state detector <NUM> detects a state (voltage, current, etc.) of DC system L connected to a plurality of loads <NUM> as an operating state of loads <NUM> and outputs the detection result as load operating information to command generator <NUM>.

Command generator <NUM> calculates distribution voltage command Vref for each power conversion device <NUM> in accordance with a state of loads <NUM> detected by load state detector <NUM> and outputs the calculated distribution voltage command Vref to the corresponding power conversion device <NUM>. In the present embodiment, command generator <NUM> calculates distribution voltage command Vref every predetermined update cycle. Distribution voltage command Vref output from command generator <NUM> therefore may vary stepwise every predetermined update cycle.

Power supply system <NUM> according to the present embodiment is a power supply and distribution system that supplies and distributes DC power. In the present embodiment, main power source <NUM> is an AC system power source, and power supply system <NUM> converts AC power from main power source <NUM> into DC power with power conversion devices <NUM> and supplies the DC power to DC system L. Main power source <NUM> may be a DC system, and a DC/DC converter may be used for power conversion device <NUM>.

The kinds of loads <NUM> connected to DC system L are classified into, for example, general loads such as lighting loads and office automation devices and power loads such as air conditioning loads. When power supply system <NUM> is applied to factories, the power loads include factory power loads such as conveyors and press machines.

The power loads including at least general power loads and factory power loads significantly differ in operating characteristics from the general loads including lighting loads, and input voltage characteristics in which power efficiency in loads is highest differ depending on individual operating states. Supplying a suitable distribution voltage according to the kinds of loads facilitates improvement of power distribution efficiency. It is therefore preferable that a plurality of loads <NUM> connected to DC system L are of the same kind of loads, if possible.

<FIG> is a diagram schematically showing an exemplary configuration of power conversion device <NUM>. Power conversion device <NUM> includes an AC/DC converter <NUM>, sensors <NUM> and <NUM>, a transformer <NUM>, an output controller <NUM>, a DC voltage controller <NUM>, and a command value filter <NUM>.

Transformer <NUM> is provided between main power source <NUM> and AC/DC converter <NUM> and insulates main power source <NUM> from AC/DC converter <NUM>. In the present embodiment, assuming that main power source <NUM> is an AC system, transformer <NUM> is provided in power conversion device <NUM>. However, when a power receiving transformer provides insulation from the outside or when insulation from an AC system is not required, transformer <NUM> can be eliminated.

AC/DC converter <NUM> is provided between transformer <NUM> and DC system L, converts AC power from main power source <NUM> into DC power, and outputs the converted DC power to DC system L.

Sensor <NUM> is provided between transformer <NUM> and AC/DC converter <NUM>, detects voltage and current of AC power supplied from main power source <NUM> to AC/DC converter <NUM>, and outputs the detection result to output controller <NUM>.

Sensor <NUM> is provided on DC system L, detects voltage Vdc and current on DC system L, and outputs the detection result to output controller <NUM> and DC voltage controller <NUM>.

Command value filter <NUM> performs a filter process (low-pass filter process) on distribution voltage command Vref from command generator <NUM> and outputs distribution voltage command Vdc_ref after the filter process to DC voltage controller <NUM>. As described above, distribution voltage command Vref output from command generator <NUM> may vary stepwise every predetermined update cycle. When the variation range of distribution voltage command Vref is large, an overshoot may occur in voltage on DC system L due the operation of power conversion device <NUM>. Insertion of a low-pass filter to distribution voltage command Vref in command value filter <NUM> therefore suppresses an abrupt change of voltage on DC system L. The time constant of the low-pass filter is adjusted and set in advance in accordance with the control characteristics (specifications) of power conversion device <NUM>. When the control response of AC/DC converter <NUM> is small and there is no concern of overshoot, command value filter <NUM> can be eliminated.

DC voltage controller <NUM> generates an output power command Pdc_ref for AC/DC converter <NUM> in accordance with voltage Vdc on DC system L detected by sensor <NUM> and distribution voltage command Vdc_ref after the filter process input from command value filter <NUM>. Output power command Pdc_ref is a command value of output power of AC/DC converter <NUM>. DC voltage controller <NUM> outputs the generated output power command Pdc_ref to output controller <NUM>.

<FIG> is a diagram illustrating control characteristics in DC voltage controller <NUM>. In <FIG>, the horizontal axis is voltage Vdc (in volts) on DC system L, and the vertical axis is output power command Pdc_ref (in watts).

DC voltage controller <NUM> sets droop characteristics that define the characteristics of output power command Pdc_ref for voltage Vdc of DC system L, based on distribution voltage command Vdc_ref after the filter process, in which voltage Vdc of DC system L is an input signal and output power command Pdc_ref is an output signal.

Specifically, DC voltage controller <NUM> sets a droop characteristics line DL with a slope Kp having a dead zone having a width d in which distribution voltage command Vdc_ref after the filter process is at the center, as shown in <FIG>. Output power command Pdc_ref in the dead zone of droop characteristics line DL is set to zero. Distribution voltage command Vdc_ref after the filter process of the same value is specified in a plurality of power conversion devices <NUM>.

Here, slope Kp of droop characteristics line DL is preset based on the specifications of power conversion device <NUM>, such as the rated operating range and the protection threshold of power conversion device <NUM>, and stored in a not-shown memory. Width d of the dead zone of droop characteristics line DL is preset to a value in accordance with the amount of variation in detection values from sensors <NUM> and <NUM> and stored in a not-shown memory. DC voltage controller <NUM> sets droop characteristics line DL as shown in <FIG>, using distribution voltage command Vdc_ref after the filter process input from command value filter <NUM> and slope Kp and width d of the dead zone stored in the memory.

DC voltage controller <NUM> then refers to droop characteristics line DL set as described above and generates output power command Pdc_ref corresponding to voltage Vdc detected by sensor <NUM>. In the present embodiment, the droop characteristics in which the output signal is output power command Pdc_ref as shown in <FIG> are used. However, the droop characteristics in which the output signal is an output current command value may be used.

Returning to <FIG>, output controller <NUM> controls the operation of AC/DC converter <NUM>, based on the detection results by sensors <NUM> and <NUM> and output power command Pdc_ref from DC voltage controller <NUM>. In doing so, when output power command Pdc_ref exceeds a preset control range from a power minimum value Pmin to a power maximum value Pmax, output power command Pdc_ref is set to a value within the control range. This prevents device destruction and the like due to overcurrent.

The unbalance in output power among a plurality of power conversion devices <NUM> connected in parallel is often caused by variations in sensor detection value of each of power conversion devices <NUM> and variations in line impedance between DC system L and power conversion device <NUM>.

When DC voltage controller <NUM> is mounted to generate output power command Pdc_ref using the droop characteristics shown in <FIG>, unbalance of output power occurs to some extent, but the amount of unbalance can be suppressed, thereby preventing unstable operation and the like. In this case, the amount of unbalance steadily converges to a constant value based on variations in voltage detection value occurring among a plurality of power conversion devices <NUM> and the droop characteristics shown in <FIG>. In particular, variations in voltage detection value are determined by detection accuracy of sensors <NUM> and <NUM> and a voltage drop amount in line impedance and are generally as small as approximately <NUM> to <NUM>%. Therefore, the amount of output unbalance among a plurality of power conversion devices <NUM> can be restricted to an acceptable magnitude by setting the droop characteristics in line with the worst case of variations. That is, DC voltage controller <NUM> that generates output power command Pdc_ref using the droop characteristics shown in <FIG> can be used to level out the output power while suppressing the amount of unbalance in output power among a plurality of power conversion devices <NUM>.

In addition, since DC voltage controller <NUM> according to the present embodiment performs control of DC system L only with information from sensors <NUM> and <NUM> included in power conversion device <NUM> itself, there is no need for constructing a high-speed communication interface with another power conversion device <NUM>.

Furthermore, DC voltage controller <NUM> is configured to have a dead zone with width d in the droop characteristics in order to suppress cross current (current circulating between power conversion devices <NUM>) caused by variations (errors) in detection value by sensors <NUM> and <NUM>. Supposing that variations in detection value occur in a case with no dead zone, DC power command values having positive polarity and having negative polarity may be mixed in each power conversion device <NUM>, causing cross current. Since cross current itself is irrelevant to the work of a load, power loss caused by cross current is unnecessary. However, in DC voltage controller <NUM> according to the present embodiment, the amount of variations in detection value is assumed and a dead zone with width d corresponding to it is preset. This suppresses occurrence of cross current.

A DC power supply and distribution system like power supply system <NUM> according to the present embodiment is more flexible in the value of output voltage of DC power than an AC power supply and distribution system with a standardized output voltage. Thus, the power efficiency may be improved by changing the voltage of DC power in accordance with a state of loads. However, it is preferable that the voltage of DC power falls within an acceptable range set for each load so as not to inhibit the operation of the load. The acceptable range of voltage input to the load may vary based on an operating state of the load.

In view of these, command generator <NUM> according to the present embodiment determines and generates distribution voltage command Vref based on load operating information acquired by load state detector <NUM> such that power consumption of a plurality of loads <NUM> is minimized or the power efficiency of a plurality of loads <NUM> is optimized.

Power consumption of load <NUM> can be represented by a sum of power consumption consumed in work in a main functional part of load <NUM> and loss occurring in a power supply circuit and an input interface of load <NUM>. In particular, power consumed in the main functional part of load <NUM> varies with a load command. Power consumption of the main functional part therefore hardly changes if a load operation command is a constant value. Based on this, command generator <NUM> according to the present embodiment adjusts distribution voltage command Vref such that power loss in the power supply circuit and the input interface of load <NUM> decreases in a period in which the load operation command does not vary. This can reduce power consumption of load <NUM> and enables improvement of cost efficiency.

<FIG> is a flowchart showing an exemplary process performed when command generator <NUM> generates distribution voltage command Vref. This flowchart is repeatedly performed every time a predetermined condition is satisfied (for example, in predetermined cycles).

First, command generator <NUM> performs a process of updating operating information of a plurality of loads <NUM> (step S10). For example, command generator <NUM> acquires voltage and current of DC system L, power information at a measurement point, operation commands of loads <NUM>, and the like from load state detector <NUM> and stores the acquired information into a memory.

The load operating information recorded in a computation cycle this time is used as the previous value of load operating information in the next computation cycle.

Subsequently, command generator <NUM> determines whether a state of load <NUM> has suddenly changed (steps S20 and S30). Specifically, command generator <NUM> determines whether the amount of variation in operation of load <NUM> (command value or detection value of operation) exceeds a threshold (step S20) and determines whether the amount of variation in power consumption of load <NUM> exceeds a threshold Pth1 (step S30).

If there is no significant variation in operation and power consumption of load <NUM> (NO at step S20 and NO at step S30), command generator <NUM> performs a process of calculating an acceptable range of distribution voltage command Vref (step S40).

<FIG> is a diagram conceptually showing a method of determining an acceptable range of distribution voltage command Vref. Command generator <NUM> according to the present embodiment calculates an acceptable range of distribution voltage command Vref, based on the control characteristics (droop characteristics) of power conversion device <NUM> and load operating information.

Specifically, when a point of intersection of an output power upper limit PHlim and a distribution voltage upper limit VHlim of power conversion device <NUM> is "intersection point I1", command generator <NUM> sets an upper limit VH to distribution voltage command Vdc_ref (the center value of the dead zone) in the case where the droop characteristics line DL of DC voltage controller <NUM> passes through intersection point I1.

Similarly, when a point of intersection of an output power lower limit PLlim and a distribution voltage lower limit VLlim of power conversion device <NUM> is "intersection point I2", command generator <NUM> sets a lower limit VL to distribution voltage command Vdc_ref (the center value of the dead zone) in the case where the droop characteristics line DL of DC voltage controller <NUM> passes through intersection point I2. Command generator <NUM> then sets an acceptable range of distribution voltage command Vref to a range Ra from lower limit VL to upper limit VH. In this way, unstable operation can be prevented by setting an acceptable range of distribution voltage command Vdc_ref using the droop characteristics of DC voltage controller <NUM>, and limiting distribution voltage command Vdc_ref to fall within the set acceptable range.

Output power lower limit PLlim and output power upper limit PHlim of power conversion device <NUM> are preset based on the preset power maximum value Pmax and power minimum value Pmin or the rated output capacity of power supply system <NUM>.

When load <NUM> is a power load such as a motor, an acceptable range of distribution voltage command Vref may be calculated as follows. When load <NUM> is a power load such as a motor, the voltage required for load <NUM> may vary greatly with the operating state of load <NUM>. This is because induced voltage from a motor that is a power load varies greatly with the rotation speed of the motor.

In view of this, when load <NUM> is a power load such as a motor, command generator <NUM> may calculate distribution voltage upper limit VHlim and distribution voltage lower limit VLlim, for example, based on the operating state of the power load (for example, the rotation speed of the motor) and sets an acceptable range of distribution voltage command Vref to a range Rb from distribution voltage upper limit VHlim to distribution voltage lower limit VLlim.

When load <NUM> is a general load, the voltage required for load <NUM> less varies with the operating state of load <NUM>. In view of this, an acceptable range of distribution voltage command Vref may be predetermined fixed values according to the specifications of load <NUM>.

Returning to <FIG>, after an acceptable range of distribution voltage command Vref is calculated at step S40, command generator <NUM> performs a process of updating distribution voltage command Vref (step S50). In this step, effect verification is performed as to whether power consumption of load <NUM> has been reduced by varying distribution voltage command Vref in the previous computation cycle, and it is determined whether the updating direction of distribution voltage command Vref is set to the increasing direction or the decreasing direction, in accordance with the result of verification. Specifically, command generator <NUM> compares the value of load operating information this time with the previous value of load operating information and determines the variation direction of the power consumption of load <NUM>. Then, if the power consumption of load <NUM> is reduced or not changed, distribution voltage command Vref is varied in the same direction as the previous one. If the power consumption of load <NUM> is increased, the distribution voltage command value Vref is varied in a direction opposite to the previous one.

For example, command generator <NUM> updates distribution voltage command Vref as follows. If the power consumption of load <NUM> is reduced or not changed, distribution voltage command Vref is updated as indicated by the following Equation (<NUM>).

On the other hand, if the power consumption of load <NUM> is increased, the sign of an adjustment direction flag F is reversed as indicated by the following Equation (<NUM>) and distribution voltage command Vref is updated as indicated by the following Equation (<NUM>). <MAT> <MAT>.

In Equation (<NUM>) and Equation (<NUM>), "Vref" in the left side is distribution voltage command Vref after update, "Vref" in the right side is distribution voltage command Vref before update, and "ΔV" is a predetermined minute voltage. "F" in the right side is the adjustment direction flag, which is a parameter for determining the updating direction of distribution voltage command Vref. The initial value of adjustment direction flag F can be set to "<NUM>" or "-<NUM>".

In this way, when the power consumption of load <NUM> is reduced or not changed, distribution voltage command Vref can be varied in the same direction as the previous one, and when the power consumption of load <NUM> is increased, distribution voltage command Vref can be varied in a direction opposite to the previous one. With such a process repeated, distribution voltage command Vref can be adjusted such that the power consumption of load <NUM> is minimized (minimal).

After the process of updating distribution voltage command Vref is performed at step S50, command generator <NUM> performs a limiter process for distribution voltage command Vref (step S60). Specifically, when distribution voltage command Vref after update calculated at step S50 does not fall within the acceptable range calculated at step S40, command generator <NUM> limits distribution voltage command Vref such that distribution voltage command Vref falls within the acceptable range.

Distribution command generator <NUM> updates distribution voltage command Vref in a direction to reduce the power consumption of load <NUM> by comparing the value of load operating information this time with the previous value. Therefore, if distribution voltage command Vref is fixed to upper limit VH or lower limit VL of the acceptable range by the limiter process, the operating state information does not change to make comparison of power consumption variation of loads <NUM> difficult. It is therefore preferable to prevent distribution voltage command Vref from being fixed to upper limit VH or lower limit VL in the limiter process.

In the process at step S60, therefore, the following process is performed using upper limit VH or lower limit VL so that distribution voltage command Vref is not fixed to upper limit VH or lower limit VL.

When distribution voltage command Vref is less than lower limit VL, the sign of adjustment direction flag F is reversed as indicated by the following Equation (<NUM>) and distribution voltage command Vref is updated as indicated by the following Equation (<NUM>). <MAT> <MAT>.

On the other hand, when distribution voltage command Vref exceeds upper limit VH, the sign of adjustment direction flag F is reversed as indicated by the following Equation (<NUM>) and distribution voltage command Vref is updated as indicated by the following Equation (<NUM>). <MAT> <MAT>.

A series of processes described above is repeatedly performed at predetermined cycles, whereby the distribution voltage can be adjusted such that power consumption of load <NUM> is reduced in power supply system <NUM>, thereby contributing to improvement in power distribution efficiency.

With such processes repeated, distribution voltage command Vref is adjusted in a range included in the acceptable range such that the power consumption of load <NUM> is minimized (minimal).

In the present embodiment, distribution voltage command Vref is adjusted such that the actual power consumption of load <NUM> is minimized (minimal), and therefore, even when the characteristics of load <NUM> have changed due to the connection environment between load <NUM> and power supply system <NUM> or degradation of load <NUM> over time, the change can be followed.

The process performed when it is determined that a state of load <NUM> has suddenly changed at steps S20 and S30 will now be described. If there is a significant variation in the operation command for load <NUM> (YES at step S20) or if there is a significant variation in power consumption of load <NUM> (YES at step S30), command generator <NUM> performs a sudden change operation exception process (step S70).

If the kinds of load <NUM> include a motor such as a compressor of an air conditioner and the rotation speed of the motor as a main functional part increases abruptly, the voltage to be distributed to load <NUM> also increases abruptly, which may cause unstable operation and deterioration in control characteristics. Therefore, if it is determined that a state of load <NUM> has suddenly changed at steps S20 and S30, command generator <NUM> performs a sudden change operation exception process at step S70 for resetting a distribution voltage as an exception operation. Lower limit VL of the acceptable range set at step S40 is a lower limit voltage for a steady operation, and applying lower limit VL for a steady operation as it is to a transient response may cause deterioration of control characteristics such as overshoot. At step S70, therefore, distribution voltage command Vref is set at a voltage larger than lower limit VL.

For example, when the rotation speed (command value or detection value) of a motor serving as load <NUM> is monitored at step S20, and if there is an abrupt increase such that increase of the rotation speed of the motor exceeds a threshold (YES at step S20), command generator <NUM> performs a sudden change operation exception process at step S70. The "threshold" used for determining an abrupt increase of the rotation speed of the motor can be preset according to the specifications of load <NUM>.

For example, if there is an abrupt change such that the amount of change of power consumption of load <NUM> exceeds a preset threshold Pth1 at step S30 (YES at step S20), command generator <NUM> performs a sudden change operation exception process at step S70. The "threshold Pth1" used for determining a sudden change of power consumption of load <NUM> can be preset by the user in accordance with the kind of load <NUM>, and the like.

In the sudden change operation exception process at step S70, command generator <NUM> temporarily stops the control of optimizing distribution voltage command Vref such that power consumption of load <NUM> is minimized (step S50) and performs a process of updating distribution voltage command Vref to a fixed value preset based on the specifications of load <NUM> and the like. This can prevent unstable operation when a state of load <NUM> suddenly changes. Distribution voltage command Vref may be updated using a distribution voltage command received by load state detector <NUM> from the load <NUM> side.

As described above, in power supply system <NUM> according to the present embodiment, while output powers of a plurality of power conversion devices <NUM> connected in parallel are leveled out, power consumption of load <NUM> can be reduced by adjusting the distribution voltage in accordance with an operating state of load <NUM>.

The configuration of a power supply system 1A according to a second embodiment will now be described. Power supply system 1A differs in that power conversion device <NUM> in power supply system <NUM> described above is changed to a power conversion device 10A shown in <FIG>. The other configuration of power supply system 1A is the same as power supply system <NUM> described above.

<FIG> is a diagram schematically showing an exemplary configuration of power conversion device 10A in power supply system 1A according to the present embodiment. Power conversion device 10A differs from power conversion device <NUM> shown in <FIG> described above in that it additionally includes a DC link LK, a DC/DC converter <NUM>, a sensor <NUM>, a DC link voltage controller <NUM>, and an output controller <NUM>.

DC link LK connects AC/DC converter <NUM> and DC/DC converter <NUM>. Sensor <NUM> detects a state (voltage, current, etc.) of DC link LK and outputs the detection result to DC link voltage controller <NUM> and output controller <NUM>.

AC/DC converter <NUM> controls the voltage of DC link LK in accordance with the operation of DC link voltage controller <NUM> and output controller <NUM> to stabilize operation. DC/DC converter <NUM> performs voltage control of DC system L in accordance with the operation of DC voltage controller <NUM> and output controller <NUM>.

<FIG> is a flowchart showing an exemplary process performed when command generator <NUM> according to the present embodiment generates distribution voltage command Vref. In the flowchart shown in <FIG>, steps S90 and S91 are added instead of steps S20, S30, and S70 in the flowchart shown in <FIG>, step S80 is added before the process at step S40, and step S50 is changed to step S50a. The other steps in <FIG> (the steps denoted by the same numerals as those of the steps shown in <FIG> described above) have already been described and will not be further elaborated.

At step S90, command generator <NUM> determines whether a distribution voltage request value is received from load <NUM> or an external device (a server or an energy management system (EMS), etc.) that outputs an operation command to load <NUM>. Thus, command generator <NUM> according to the present embodiment is configured to perform communication with load <NUM> or an external device.

If a distribution voltage request value is received from load <NUM> or an external device (YES at step S90), command generator <NUM> performs an exception process of outputting the distribution voltage request value received from load <NUM> or an external device as it is as distribution voltage command Vref to command value filter <NUM> (step S91). That is, command generator <NUM> according to the present embodiment permits exceptional updating of distribution voltage command Vref only when a distribution voltage request value is received from load <NUM> or an external device.

On the other hand, if a distribution voltage request value is not received from load <NUM> or an external device (NO at step S90), command generator <NUM> calculates loss information of DC/DC converter <NUM> based on the voltage and current of DC system L acquired by load state detector <NUM> (step S80).

In the present embodiment, since there is no other device connected between load state detector <NUM> and a plurality of power conversion devices 10A and output currents of a plurality of power conversion devices 10A are leveled out, the operating states (for example, output voltage, output current) of DC/DC converters <NUM> of a plurality of power conversion devices 10A can be predicted based on the voltage and current of DC system L detected by load state detector <NUM>.

In the present embodiment, a loss map that defines the correspondence between the operating state of DC/DC converter <NUM> and loss of DC/DC converter <NUM> is recorded in advance in the memory of command generator <NUM>. Based on these, command generator <NUM> predicts the operating state of DC/DC converter <NUM> based on the voltage and current of DC system L and calculates the loss of DC/DC converter <NUM> in the predicted operating state of DC/DC converter <NUM> by referring to the loss map.

Subsequently, at step S50a, command generator <NUM> compares the value of the sum of power consumption of load <NUM> and loss of DC/DC converter <NUM> this time with the previous value and generates distribution voltage command Vref such that the amount of power received by main power source <NUM> is reduced.

By doing so, in the present embodiment, distribution voltage command Vref can be adjusted in a direction to reduce the sum of power consumption of load <NUM> and loss of DC/DC converter <NUM>. Therefore, the amount of power received by main power source <NUM> can be reduced considering the operating state and loss of power conversion device 10A, thereby reducing electricity costs.

<FIG> is a diagram schematically showing an exemplary configuration of a power supply system 1B according to the present embodiment. In power supply system 1B, four power conversion devices 10B are connected in parallel and an external control device <NUM> is added, and external control device <NUM> includes a command generator <NUM> and a device manager <NUM>. External control device <NUM> may be, for example, a server or an EMS describe above that outputs an operation command to load <NUM>.

<FIG> is a diagram schematically showing an exemplary configuration of power conversion device 10B according to the present embodiment. Power conversion device 10B differs from power conversion device 10A shown in <FIG> described above in that it additionally includes an operating state manager <NUM> that performs communication with device manager <NUM> of external control device <NUM>.

For example, in a case where the output capacity per power conversion device 10B is <NUM> kW and the rated output capacity of loads <NUM> as a whole is <NUM> kW, in the present embodiment, even when two power conversion devices 10B among four power conversion devices 10B fail, the remaining two power conversion devices 10B perform rated operation to enable continuous operation of loads <NUM> as a whole.

Power supply system 1B of the present embodiment is a parallel redundancy system in which four power conversion devices 10B are connected in parallel and may ensure operation stability even when the droop characteristics in DC voltage controller <NUM> are varied with the number of operating power conversion devices 10B. In power conversion device 10B in <FIG>, operating state manager <NUM> outputs information such as the number of operating power conversion devices 10B to DC voltage controller <NUM>, based on information specified by device manager <NUM>, and the like. DC voltage controller <NUM> can vary slope Kp and width d of the dead zone of the droop characteristics of DC voltage controller <NUM>, based on information from operating state manager <NUM>, even during operation of power conversion device 10B.

Command generator <NUM> according to the third embodiment performs a process mostly similar to the process by command generator <NUM> according to the second embodiment (the flowchart shown in <FIG> describe above). However, command generator <NUM> according to the present embodiment performs operation different from that of command generator <NUM> according to the second embodiment only at step S40 in <FIG>. In the following, the process at step S40 by command generator <NUM> according to the third embodiment will be described.

The operation of command generator <NUM> according to the present embodiment differs from the operation shown in the flowchart in <FIG> described above only in the operation at step S40. The description is the same and only a modification in the process at step S40 by command generator <NUM> according to the present embodiment will be described.

Step S40 is a process of calculating an acceptable range Ra of distribution voltage command Vref. For the acceptable range Ra according to the present embodiment, upper limit VH and lower limit VL of the acceptable range Ra are calculated based on the control characteristics and number of operating devices information of power conversion devices 10B managed by device manager <NUM>, and load operating information.

Even in the present third embodiment, like the other embodiments, upper limit VH of the acceptable range Ra is calculated based on intersection point I1 of output power upper limit PHlim and distribution voltage upper limit VHlim of power conversion device 10B, and lower limit VL of the acceptable range Ra is calculated based on intersection point I2 of output power lower limit PLlim and distribution voltage lower limit VLlim of power conversion device 10B. However, in the present third embodiment, the values of output power upper limit PHlim and output power lower limit PLlim of power conversion device 10B change with the operating state of power conversion device 10B.

The following Equations (<NUM>) and (<NUM>) show an example of calculation formulas of output power upper limit PHlim and output power lower limit PLlim. In the following Equations (<NUM>) and (<NUM>), "Pr" is the rated power of loads <NUM> as a whole connected to DC system L, "Ab" is the number of power conversion devices 10B operating at present, and "Ar" is the number (preset value) of backup power conversion devices 10B for providing redundancy. When the number of operating power conversion devices 10B changes due to failure or the like, it is necessary to promptly update the acceptable range Ra. <MAT> <MAT>.

With the setting above, the acceptable range Ra can be expanded in accordance with the number of power conversion devices 10B operating at present. The setting of distribution voltage command Vref therefore can be flexible, and the efficiency of power distribution to loads <NUM> can be increased.

<FIG> is a diagram schematically showing an exemplary configuration of a power supply system 1C according to a fourth embodiment. Power supply system 1C includes a plurality of DC systems LA and LB, a plurality of power conversion devices 10C, and a command generator 30C. A plurality of power conversion devices 10C convert the power received from main power source <NUM> into different DC powers based on the corresponding distribution voltage commands and supply the converted DC powers to DC systems LA and LB. Command generator 30C includes controllers <NUM> and <NUM> corresponding to a plurality of DC systems LA and LB.

Power conversion device 10C may use a device that converts the power received from main power source <NUM> directly into a plurality of DC powers or may convert the received power into one or more DC powers with a power converter and include a distribution device in the power converter to further converts the power into a plurality of DC powers.

A plurality of DC systems LA and LB are provided, for example, for each kind of loads <NUM>, and loads <NUM> connected to the same DC system are of the same kind. The operating characteristics differ with the kinds of loads <NUM>, and the suitable distribution voltages also differ. In a power supply system having only a single DC system, loads <NUM> of different kinds are connected to the same system, and therefore, it is difficult to adjust the distribution voltage of each individual load <NUM>. In comparison, in power supply system 1C having a plurality of DC systems LA and LB, a DC system is easily configured and routed for each kind of loads <NUM>, and a distribution voltage suitable for each individual load <NUM> is easily supplied. <FIG> shows an example in which a plurality of general loads 3A are connected to DC system LA, and a plurality of power loads 3B are connected to DC system LB.

With the function of power conversion device 10C or a distributor included in power conversion device 10C, supply powers to a plurality of DC systems LA and LB can be controlled independently of each other. Therefore, the determination method for the corresponding command generator 30C and the calculation method for distribution voltage command Vref can be set individually, depending on the kind of loads <NUM> connected to DC systems LA and LB, so that the operation of command generator 30C can be easily adjusted for the kind of loads <NUM>.

Claim 1:
A power supply system (<NUM>, 1A, 1B, 1C) to convert power received from a main power source (<NUM>) to supply AC power or DC power, and output the converted power to a DC system (L, LA, LB),
the power supply system (<NUM>, 1A, 1B, 1C) comprising:
- a plurality of power conversion devices (<NUM>, 10A, 10B, 10C) provided between the main power source (<NUM>) and the DC system (L, LA, LB) and connected in parallel with each other;
characterized by further comprising:
- a state detector (<NUM>) to detect an operating state of at least one load connected to the DC system (L, LA, LB); and
- a command generator (<NUM>, 30C) to generate a voltage command that is a command value of voltage distributed from the power supply system (<NUM>, 1A, 1B, 1C) to the DC system (L, LA, LB),
each of the power conversion devices (<NUM>, 10A, 10B, 10C) including
- a voltage controller (<NUM>) to generate a power command based on a voltage of the DC system (L, LA, LB) and the voltage command, and
- a converter (<NUM>) to convert power received from the main power source (<NUM>) based on the power command and output the converted power to the DC system (L, LA, LB),
wherein the command generator (<NUM>, 30C) is configured to generate the voltage command such that loss of the at least one load (<NUM>) connected to the DC system (L, LA, LB) is reduced, based on a detection result of the state detector (<NUM>),
wherein the voltage controller (<NUM>) is configured to set droop characteristics (DL) that define characteristics of the power command for the voltage of the DC system (L, LA, LB), based on the voltage command, and to generate the power command from the set droop characteristics (DL) and the voltage of the DC system (L, LA, LB).