Abstract:
A system that manages a supplemental energy source connected to a power grid uses a two stage control strategy to manage power transfers in and out of the power grid as well as in and out of an energy storage system, such as a battery bank. One stage uses a non-linear transfer function to control an output frequency of a DC-to-AC inverter to limit undesired effects of power transients that occur on the grid. A second stage uses control strategy for transferring energy between the energy storage system and an internal DC link based on a relationship between a voltage on a DC link connecting the first and second stages and a DC link reference voltage, the voltage on the DC link, and a voltage at the energy storage system. The control strategy includes rapid charging, over-charging protection, and grid transient stabilization.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to electrical power equipment and more specifically to an energy storage system inverter apparatus used in micro grid applications. 
       BACKGROUND 
       [0002]    The frequency of a grid, particularly a large or “infinite” grid is relatively stable. All synchronous generators on the grid will synchronize to the grid frequency. Load on the grid is inversely proportional to grid frequency, so that as the load increases relative to the combined generator output, grid frequency decreases. 
         [0003]    Droop frequency control is used in such synchronous generators to control power output. When a reference frequency is above a grid frequency, power is added to the prime move, e.g., more fuel is provided to an engine driving the generator, so that torque is increased and more power is output. Droop control systems use a linear function around the desired power level to manage the frequency of the output as a function of power. 
         [0004]    Statcom systems are used in the utility power grid to provide reactive power compensation and transient voltage support. Statcom systems are generally capacitor banks and have very short duration operational times, in the order of seconds. U.S. Pat. No. 7,508,173 teaches use of an inverter to provide reactive power in a power distribution network. 
         [0005]    In a microgrid application, a wide variety of power generation sources may contribute to a particular grid, including solar panels, wind turbines, diesel or gas generator sets, fuel cells, and even the utility grid. In this environment, power generation levels, frequency, and voltage may vary in a wide range due to the volatile nature of some of the power sources and sluggish response times of generator sets. Statcom systems may not have the capacity for grid support over the longer durations that may be occur in micro grid applications. In these cases, energy storage systems, using batteries, ultra capacitors, etc., may be used to provide supplemental power. However, energy storage systems (ESS) present additional requirements for long-term charge maintenance and ESS component protection. 
       SUMMARY OF THE DISCLOSURE 
       [0006]    In an aspect, a system for managing power transfer with a power grid includes a direct current (DC) link, an energy storage system (ESS) that stores DC power, an energy storage controller coupled between the DC link and the ESS that manages power flow between the ESS and the DC link, and a load controller coupled between the DC link and the power grid. The load controller includes a DC-to-AC inverter. The load controller is operable to adjust an output frequency of the DC-to-AC inverter using a non-linear power-frequency curve responsive to both a state of a frequency of the power grid and a power transfer state of the load controller. 
         [0007]    In another aspect, a method of managing power transfer to a power grid using a bidirectional DC-to-AC converter coupled between a power grid and an energy storage system (ESS) includes sensing a frequency of power on the grid, determining a power flow at the bidirectional DC-to-AC inverter, selecting a non-linear power-frequency curve, setting an output frequency of the bidirectional DC-to-AC inverter using the non-linear power-frequency curve and a current power flow through the bidirectional DC-to-AC inverter. 
         [0008]    In yet another aspect, a system for managing power transfer between an energy storage system component and a power grid includes a direct current (DC) bus operatively coupled to the power grid, a DC energy storage system (ESS), a DC-to-DC converter coupled between the DC bus and the energy storage system, the DC-to-DC converter adapted for bi-directional current flow, and a controller coupled to the bidirectional DC-to-DC converter. The controller operates the DC-to-DC converter to control current magnitude and direction based on a relationship between a voltage on the DC link and a DC link reference voltage, the voltage on the DC link, and a voltage at the ESS to provide a plurality of charging and discharging modes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a block diagram illustrating a system for managing power transfer with a grid; 
           [0010]      FIG. 2  is a block diagram illustrating a bidirectional DC-to-DC converter; 
           [0011]      FIG. 3  is a block diagram illustrating a DC-to-AC bi-directional inverter; 
           [0012]      FIG. 4  is a block diagram of an exemplary controller suitable for use with a DC-to-AC inverter; 
           [0013]      FIG. 5  is a control diagram for a droop frequency control scheme; 
           [0014]      FIG. 6  is a depiction of an exemplary non-linear power frequency curve for use by the droop frequency control scheme of  FIG. 5 ; 
           [0015]      FIG. 7  is a control diagram for managing current flow with an energy storage system (ESS); 
           [0016]      FIG. 8  is a three dimensional (3D) chart of current limits for charging the ESS; 
           [0017]      FIG. 9  is a 3D chart of current limits for discharging the ESS; 
           [0018]      FIG. 10  is an exemplary table showing charge mode current flow limits for the 3D chart of  FIG. 8 ; and 
           [0019]      FIG. 11  is an exemplary table showing discharge mode current flow limits for the 3D chart of  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  is a block diagram illustrating a system  10  for managing power transfer with a grid  12 . The system  10  includes a load manager  14 , an energy storage system manager  16 , and an energy storage system (ESS)  18 . The grid  12  may be a microgrid and may include both loads and power sources (not depicted). The power sources may include solar panels, wind turbines, diesel or gas generator sets, fuel cells, and/or the utility grid. In an embodiment, the primary power source is a bank of fossil fuel generators. As the demand for power on the grid  12  increases, additional generators may be brought on line. In an embodiment, a nominal or target operating range of the grid may be  440  VAC ±5% and 60 Hz±0.5 Hz. In other embodiments, other operating voltages, frequencies and nominal tolerances may be used. As will be discussed below, even though these represent target ranges operation outside these target ranges may occur for a number of reasons and may require steps to help bring operation back into the target range and thus limit damage to both the generating equipment and the loads. 
         [0021]    The ESS  18  may be any combination of known energy storage devices, but particularly may include a battery bank, a capacitor bank, or a combination of the two. In an embodiment, the ESS  18  may be exclusively batteries. 
         [0022]    The load manager  14  may be coupled to the grid  12  via a three-phase bus  20 . The load manager  14  may include a DC-to-AC bi-directional inverter  22  and a load controller  24  with a sensor connection  26  for the three-phase grid  12 . The load manager  14  may be connected to the ESS manager  16  by a DC bus  28  and a ground connection  30 . In an embodiment, the ground connection  30  may be common to the ESS  18  and the system  10 , but may be floating with respect to the grid  12 . A sensor connection  32  couples the DC bus to the controller  24 . 
         [0023]    A capacitor  34  may be used to stabilize the voltage on the DC bus  28 . Other filter arrangements may be used as well. 
         [0024]    The ESS manager  16  may include a bidirectional converter (BDC)  36  and an ESS controller  38 . The ESS controller  38  may have a sensor connection  40  to the DC bus  28  and a sensor connection  46  that reads a voltage on an ESS bus  42 . An ESS ground  44  may be coupled to the DC bus ground  30 . 
         [0025]      FIG. 2  is a block diagram illustrating the bidirectional converter  36 . The BDC  36  uses pairs of switching transistors  50  to drive three inductors  52  in a conventional manner. Each of the switching transistors  50  may be an insulated gate bipolar transistor (IGBT), often used in high voltage and high current power applications. 
         [0026]    The ESS  18  may include a filter circuit  54  and one or more batteries  56 . In an embodiment, a capacitor bank (not depicted) may be used instead of, or alongside, the battery bank. 
         [0027]      FIG. 3  is a block diagram illustrating a DC-to-AC bi-directional inverter  22 . The inverter  22  may include a filter  66  and a transistor bank  68 . Like the BDC  36  of  FIG. 2 , the transistors  69  of the transistor bank  68  may be IGBTs. 
         [0028]    The inverter circuits of  FIGS. 2 and 3  use inductive elements to allow the inherent nature of inductors and fast switching IGBT transistors to adjust the voltage on an output side of the inverter above or below that of the input. This feature allows power transfer in either direction from the DC bus  28  without particular regard to the voltage either on the grid  20  or the ESS bus  42 . 
         [0029]      FIG. 4  is a block diagram of an exemplary controller  70  suitable for use with either a DC-to-DC converter or DC-to-AC inverter. The controller  70  may include a processor  72 , a communication port  74 , and a communication bus  76  that may be used to communicate information from controller  70  externally, for example with an external diagnostics or control computer (not depicted). In another embodiment, the communication bus  76  may be used to communicate information between instances of a load controller  24  and an ESS controller  38  so that grid and ESS status may be shared between the respective controllers. 
         [0030]    The controller  70  may also include a memory  78  coupled to the processor  72  via a bus  80 . The memory  78  may store computer executable modules, operational data, settings, etc. Information stored in the memory  78  may include an operating system  82  and a control strategy  84 . The control strategy  84  may define functions specific to a particular controller application such as the load controller  24  or the ESS controller  38 . However, in an embodiment, the controller  70  may include strategy modules capable of addressing both the load controller and the ESS controller functions so that a particular controller  70  may be used in either application. In yet another embodiment, both the load controller  24  and the ESS controller  38  may be embodied in a single electronic device. Operational data and settings stored in memory  78  may include power frequency curves  86 , an ESS charge table  88  and an ESS discharge table  90 . 
         [0031]    The controller  70  may also include inputs capable of receiving and processing voltage and current information, for example at a voltage sensor input  92  and a current sensor input  94 . The voltage sensor input  92  and current sensor input  94  may have signal conditioning circuitry, such as analog to digital converters and/or a voltage measuring circuit. The controller  70  may also include one or more inverter control outputs  96  capable of driving inverter switching transistors. In some embodiments, the controller  70  or portions thereof may be embodied in a field programmable gate array (FPGA), a single chip computer, or some combination of those elements and the above-described architecture. 
       INDUSTRIAL APPLICABILITY 
       [0032]    The system  10  may operate to support the broad functions of supporting the grid  12  and keeping the ESS  18  charged and in condition for operation. Supporting the grid  12  may include providing power to the grid  12  when the generators are off-line or unable to supply the power demanded by the grid load. Supporting the grid  12  may also include providing reactive power as needed. Additionally, the system  10  may provide a short-term high current sinking capability to offset a transient power spike, for example, as may occur when a high current load is disconnected from the grid  12 . 
         [0033]    The other function of keeping the ESS  18  charged and in condition for operation may include providing rapid charging when the ESS  18  is depleted, providing a trickle charge to maintain a nominal power level, and providing a trickle drain when the ESS is in an over charged state. The system  10  may also protect the ESS  18  and other internal components from damage caused by a power surge when the grid frequency increases, particularly when grid frequency increases well above a nominal operating state, as may occur when standalone generators or renewable energy sources are primary drivers on the grid  12 . 
         [0034]    Use of the system  10  provides a comprehensive ability manage a wide range of conditions associated with micro grid operation, particularly when renewable power sources are part of the generation process including undervoltage and overvoltage correction, reactive power compensation, and up to 100% power replacement when other grid sources are unavailable. 
         [0035]    In operation, the two major functions of the system  10 , ESS management and grid support, may be reduced to two conceptual transfer functions modeled by specialized transfer functions. Using directly-measured inputs, the respective transfer functions control the DC-to-AC inverter  22  and the DC-to-DC converter  36  in accordance with its respective transfer function. 
         [0036]      FIG. 5  is a control diagram  100  for a droop frequency control scheme suitable for use in managing power interactions between the system  10  and the grid  12 . The control diagram illustrates a summation node  102  with inputs for a reference power value  104  and a measured power at a connection point  106  to the grid  12 , such as at bus  20 . The output of the summation node  102  is a difference value that drives a proportional controller  108 . The proportional controller  108  outputs a droop frequency  110 , which may be positive or negative, as a function of the power difference presented at its input. A second summation node  112  adds the droop frequency  110  to a nominal frequency reference  114  resulting in an output frequency  116  that is used to set the output frequency of the AC-to-DC inverter  22 . The proportional controller  108  may implement a non-linear transfer function illustrated in  FIG. 6 . 
         [0037]      FIG. 6 , illustrates a chart  130  of exemplary non-linear power-frequency curve  132 . As can be seen, and as is discussed below, even though a particular region may be linear, the overall curve  132  is non-linear. More than one power-frequency curve may be available and may be selected for use based on factors including, but not limited to, grid power sources, expected loads, power quality expectations, etc. 
         [0038]    The chart  130  shows a discharge region  134 , a charging region  136 , and an power rejection region  138 . A nominal frequency range  140  indicates output, that is, grid, frequencies that fall within an desired operating range. For example, in a 60 Hz environment, a micro grid may operate with a target frequency specification of 60 Hz±0.5 Hz. When the system  10  is operating in the discharge region  134 , that is, providing power to the grid  12 , the system  10  is substantially responsible for the frequency at the grid  12 . To stabilize that frequency, a portion  142  of the power-frequency curve  132  may be used that has a flat, constant frequency within the desired nominal frequency range  140  for all levels of power. When operated using this portion  142  of the power-frequency curve, the system will supply a full range of power with a constant frequency. 
         [0039]    When the system  10  is charging the ESS  18 , power is flowing into the load manager  14 . The frequency of the DC-to-AC bi-directional inverter  22  is held within the nominal range  140  using a portion  144  of the power-frequency curve  132 . The portion  144  may have a low positive slope and may be curved as shown, or may be a straight line. The low slew rate of the portion  144  improves the stability of a power control loop of the system  10 . That is, small changes in power result in small changes in frequency. 
         [0040]    In a condition occurs, such as a sudden and substantial decrease in power consumption, a generator providing power to the grid  12  may not be able to adjust its power output quickly enough and as a result it output may increase in both voltage and frequency. In some cases, the frequency may well exceed the nominal range and may exceed 63 Hz. Applying the known characteristics of phasor addition, power will flow from a higher frequency to a lower frequency source. In such a case, the load manager  14  may be suddenly overwhelmed with a power surge coming from the grid. 
         [0041]    Using the curve portion  146  in the power rejection region  138  allows the system  10 , and more specifically, the load manager  14  to rapidly increase its own frequency to reject the power injection from the generator set or sets. 
         [0042]    Note that the power-frequency curve  132  may be characterized as a single curve or may be a composite of three or more separate curves. In some embodiments, the curve  132  may be continuous or may have discontinuities, such as at the transition between charging and discharging. In addition, the curve  132  may be described via an algorithm, that is, an equation or may be embodied in a table or other transform function. 
         [0043]      FIG. 7  is a control diagram  150  for a control scheme for managing current flow with an energy storage system (ESS). Simply put, the control scheme attempts to maintain the voltage at the DC link  28  at a reference voltage. The control diagram  150  may include a summation node  152  with an inverted DC link target voltage  154  and a voltage  156  measured at the DC link  28 . The output of the summation node  152  is a difference value between the voltages. A proportional controller  158  may be used to convert and/or scale the voltage difference to a requested current value  160  that is input to a current limit block  162 . In an embodiment, the transfer function of the proportional controller  158  is a simple linear function. The requested current value  160  has both magnitude and sign. 
         [0044]    The current limit block or function  162  uses the requested current value sign and magnitude, an ESS voltage value  164 , for example measured at the output  42  of the ESS  18 , and a measured voltage  156  at the DC link  28 . As illustrated in the current limit block  162 , a transfer function of current request value  160  is converted to an current command  170  having limited maximum positive and negative values. The selection of the values is discussed further below. 
         [0045]    The current command  170  may be provided to a pulse-width modulated (PWM) block  172  that may use a known technique for generating control signals to the individual IGBT transistors  50  of the DC-to-DC converter (BDC)  36 . A feedback loop  168  from the current limit block  162  to the proportional controller  158  may be available for use in filtering the proportional controller response. 
         [0046]      FIG. 8  is an exemplary three dimensional (3D) chart  180  of current limits for charging the ESS  18 , that is when the DC link voltage  156  is above the DC link target voltage  154  and the ESS manager  16  operates, in general, to lower the DC link voltage  156 . As shown in the control diagram  150 , output of the current limit block  162  is a function of both the current request  160  and a maximum allowable current  188  taken from the chart  180 . To determine the maximum allowable current  188 , the BDC voltage  164  and the DC link voltage  156  may define a point on the x-y-plane of the chart  180  associated with a maximum current value  182  associated with the 3D sheet  184 . This value may be used by the PWM logic  172  if the current request value  160  is greater than the value from the chart  180 . If the current request value  160  is less than the value from the chart, the current request value  160  may be supplied to the PWM logic  172 . 
         [0047]      FIG. 9  is a similar 3D chart  190  for discharging the ESS  18 , for example, when the DC link voltage  156  is below the target DC link voltage  154  and the ESS manager  16  operates, in general, to increase the DC link voltage  156 . In similar fashion as described above, the chart  190  places maximum current  192  constraints on the BDC  36  based on BDC voltage  164  and the DC link voltage  156 . If the current request value  160  is below the maximum, the requested value will be used to control the PWM logic  172 . 
         [0048]    Referring to  FIG. 10 , a table  200  further illustrates exemplary maximum current limits for charge mode current flow at the ESS  18  via the BDC  36 . In this exemplary embodiment, the ESS  18  may have a nominal, fully charged, no load, voltage of 436 V. In other embodiments, another ESS may have a different storage element construction or may have a different number of cells that would affect nominal voltage. In the table  200 , the DC Link voltage  156  is shown across the top, and ESS voltage  164  is shown on the left. 
         [0049]    Both the table  200  and the table  210  following in  FIG. 12  depict maximum current values that may occur in different conditions. However, as described above, any actual current at the BDC  36  will be a function of the difference between the DC link voltage  156  and the DC link reference voltage  154  and the maximum current specified in appropriate table. Some areas of each table may not be reached for a given reference voltage  154 . In an embodiment, the reference voltage  154  may be 660 V. 
         [0050]    The table  200  is appropriate when the DC link voltage  156  is above the reference voltage  154 . In the charging mode, several charging current maximums may be imposed. For example, if the ESS voltage  166  is at or below an exemplary nominal value of 432 V and the DC link voltage is at 700 V, a maximum charging current of either  144  amps or  108  amps may be imposed in region  202 . As discussed above, the maximum is only used when the current request is above the maximum value. If, on the other hand, the DC link voltage  156  is above its reference voltage  154  (that is, in charging mode), but the BDC voltage  164  is high compared to its nominal value at 470 V, in region  206 , the maximum current may be set to −9 amps or a similar low value to provide a trickle discharge of the ESS  18 . The proportional controller  158  may set the current request to a negative value based on the feedback current limit signal  168  or for another condition. 
         [0051]    Region  204  may be entered when the DC link voltage  156  is substantially above the reference voltage  154  and the ESS voltage  164  is near a nominal value or below. This mode of operation may be encountered during a transient, such as described above where a power surge on the grid causes the load controller  14  to sink power from the grid  12 , even as the load controller  14  raises its frequency to reject a portion of the incoming power. By sinking up to 900 amps, the ESS  18  may bleed enough power from the grid  12  to reduce a high voltage situation caused by the load dump transient. 
         [0052]      FIG. 11 , is a table  210  illustrates exemplary maximum discharge mode current flow at the ESS  18  via the BDC  36 . As above, the nominal voltage at the ESS  18  may be 436 V and the reference DC link voltage  156  may be 660 V, although other voltage levels may be accommodated. Table  210  is active whenever the DC link voltage  156  is below the DC link voltage reference  154 . The region  212  may set the current limit at −900 amps when the DC link voltage  156  is low and the ESS voltage  164  is at or above is rated value. A cap of −900 powers the DC link  28  so the load controller  14  can supply power to the grid  12 . When the ESS voltage  164  is below the nominal, the maximum current limit may be set to zero even if the DC link voltage is still low in order to protect the batteries in the ESS  18 . If the DC link voltage  156  is below the reference  154  but the ESS voltage  164  is low, the current limit block  162  may set the maximum current to  9 , or a similar low value to provide a trickle charge of the ESS  18 . With the current limit set at the positive value, the proportional controller  158  may set the current request value  160  to a positive number even though the discharge mode is active based on the current limit feedback value  168 . 
         [0053]    As can be seen, the ESS manager  16  using the bidirectional DC-to-DC converter  36  and a simple control function  150  operates seamlessly between charging and discharging modes based solely on measured voltage values and does not depend on state transitions or other explicit evaluations based on power levels or current measurements to both reduce complexity and increase response times. 
         [0054]    The specific voltages, currents, and current limits used for illustration above are exemplary of an instance of an embodiment. Any of these values may vary in a 10% range or even a 20% range or more. The exact set points for a particular installation may vary from those described based on the type of power sources available on the grid, the mix of power sources, and energy efficiency considerations. For example, a micro-grid operated primarily on wind turbines may have a slightly lower slope on the power rejection curve  146  because frequency variations may be more common. In another example, the maximum current for grid support may be more or less based on an expected maximum load. In an embodiment, current values in tables  200  and  210  may be adjusted during operation to reflect changes in load, changes in generation equipment, or to reflect wear and reduced capacity of the ESS  18  over time. Similarly, the DC link reference voltage may be varied in real time based on grid conditions, environmental conditions, such as ambient temperature, etc.