Method and system for an AC battery

A method and system for AC battery operation. In one embodiment, the method comprises determining, at a battery management unit (BMU) coupled to an AC battery comprising a power converter and a battery that is rechargeable, a bias control voltage that indicates a state of a charge process of the AC battery; and coupling, by a bias control module of the BMU, the bias control voltage to the power converting for communicating the state of the charge process to and from the BMU and the power converter.

BACKGROUND OF THE INVENTION

Field of the Invention

Embodiments of the present disclosure relate generally to power conversion and, more particularly, to a battery management unit (BMU) to power conditioner interface for use in distributed energy generators.

Description of the Related Art

Battery systems are used in a large variety of applications for providing a source of power as well as energy storage. In addition to DC battery systems which provide DC power, there are also AC battery systems which provide AC power. Such AC battery systems may be used for storing and providing energy in distributed energy generators, for example in microgrids.

Conventional AC battery systems provide AC power via one or more DC-AC inverters coupled to the DC output from one or more rechargeable batteries. Battery devices such as lithium-based products can require dedicated electronics—i.e., a battery management unit (BMU)—to accomplish proper performance and safety. However, BMUs typically connect through a controller area network (CAN), which may not be available on some distributed energy generator an inverter employed in distributed energy generators, such as photovoltaic (PV) inverters.

Therefore, there is a need in the art for a simplified battery management unit (BMU) to power conditioner interface which does not require a controller area network (CAN) bus or a special power conditioner to be used in the battery.

SUMMARY OF THE INVENTION

Embodiments of the present invention generally relate to a method and system for AC battery operation substantially as shown in and/or described in connection with at least one of the figures, as set forth more completely in the claims.

DETAILED DESCRIPTION

FIG. 1is a block diagram of a system100for power conversion using one or more embodiments of the present invention. This diagram only portrays one variation of the myriad of possible system configurations and devices that may utilize the present invention.

The system100is a microgrid that can operate in both an islanded state and in a grid-connected state (i.e., when connected to another power grid (such as one or more other microgrids and/or a commercial power grid). The system100comprises a plurality of power converters (which also may be called power conditioners)102-1,102-2, . . .102-N,102-N+1, and102-N+M collectively referred to as power converters102; a plurality of power sources104-1,104-2, . . .104-N, collectively referred to as power sources104; a plurality of energy storage devices/delivery devices120-1,120-2, . . .120-M collectively referred to as energy storage/delivery devices120; a system controller106; a plurality of battery management units (BMUs)190-1,190-2, . . .190-M collectively referred to as BMUs190; a system controller106; a bus108; a load center110; and an island interconnect device (IID)140(which may also be referred to as a microgrid interconnect device (MID)). In some embodiments, such as the embodiments described herein, the energy storage/delivery devices120are rechargeable batteries which may be referred to as batteries120, although in other embodiments the energy storage/delivery devices120may be any other suitable device for storing energy and providing the stored energy. Generally, each of the batteries120comprises a plurality cells that are coupled in series, e.g., eight cells coupled in series to form a battery120.

Each power converter102-1,102-2. . .102-N is coupled to a DC power source104-1,104-2. . .104-N, respectively, in a one-to-one correspondence, although in some other embodiments multiple DC power sources104may be coupled to one or more of the power converters102. The power converters102-N+1,102-N+2 . . .102-N+M are respectively coupled to batteries120-1,120-2. . .120-M via BMUs190-1,190-2. . .190-M to form AC batteries180-1,180-2. . .180-M, respectively. Each of the power converters102-1,102-2. . .102-N+M comprises a corresponding controller114-1,114-2. . .114-N+M (collectively referred to as controllers114) for controlling operation of the corresponding power converter102-1,102-2. . .102-N+M.

In some embodiments, such as the embodiment described below, the power sources104are DC power sources and the power converters102are bidirectional inverters such that the power converters102-1. . .102-N convert DC power from the DC power sources104to grid-compliant AC power that is coupled to the bus108, and the power converters102-N+1 . . .102-N+M convert (during energy storage device discharge) DC power from the batteries120to grid-compliant AC power that is coupled to the bus108and also convert (during energy storage device charging) AC power from the bus108to DC output that is stored in the batteries120for subsequent use. The DC power sources104may be any suitable DC source, such as an output from a previous power conversion stage, a battery, a renewable energy source (e.g., a solar panel or photovoltaic (PV) module, a wind turbine, a hydroelectric system, or similar renewable energy source), or the like, for providing DC power. In other embodiments the power converters102may be other types of converters (such as DC-DC converters), and the bus108is a DC power bus.

The power converters102are coupled to the system controller106via the bus108(which also may be referred to as an AC line or a grid). The system controller106generally comprises a CPU coupled to each of support circuits and a memory that comprises a system control module for controlling some operational aspects of the system100and/or monitoring the system100(e.g., issuing certain command and control instructions to one or more of the power converters102, collecting data related to the performance of the power converters102, and the like). The controller106is capable of communicating with the power converters102by wireless and/or wired communication (e.g., power line communication) for providing certain operative control and/or monitoring of the power converters102.

In some embodiments, the system controller106may be a gateway that receives data (e.g., performance data) from the power converters102and communicates (e.g., via the Internet) the data and/or other information to a remote device or system, such as a master controller (not shown). Additionally or alternatively, the gateway may receive information from a remote device or system (not shown) and may communicate the information to the power converters102and/or use the information to generate control commands that are issued to the power converters102.

The power converters102are coupled to the load center110via the bus108, and the load center110is coupled to the power grid via the IID140. When coupled to the power grid (e.g., a commercial grid or a larger microgrid) via the IID140, the system100may be referred to as grid-connected; when disconnected from the power grid via the IID140, the system100may be referred to as islanded. The IID140determines when to disconnect from/connect to the power grid (e.g., the IID140may detect a grid fluctuation, disturbance, outage or the like) and performs the disconnection/connection. Once disconnected from the power grid, the system100can continue to generate power as an intentional island, without imposing safety risks on any line workers that may be working on the grid, using the droop control techniques described herein. The IID140comprises a disconnect component (e.g., a disconnect relay) for physically disconnecting/connecting the system100from/to the power grid. In some embodiments, the IID140may additionally comprise an autoformer for coupling the balanced power system100to a split-phase load that may have a misbalance in it with some neutral current. In certain embodiments, the system controller106comprises the IID140or a portion of the IID140.

The power converters102convert the DC power from the DC power sources104and discharging batteries120to grid-compliant AC power and couple the generated output power to the load center110via the bus108. The power is then distributed to one or more loads (for example to one or more appliances) and/or to the power grid (when connected to the power grid); additionally or alternatively, the generated energy may be stored for later use, for example using batteries, heated water, hydro pumping, H2O-to-hydrogen conversion, or the like. Generally the system100is coupled to the commercial power grid, although in some embodiments the system100is completely separate from the commercial grid and operates as an independent microgrid.

In some embodiments, the AC power generated by the power converters102is single-phase AC power. In other embodiments, the power converters102generate three-phase AC power.

FIG. 2is a block diagram of an AC battery system200in accordance with one or more embodiments of the present invention. The AC battery system200comprises a BMU190coupled to a battery120and an inverter102. A pair of metal-oxide-semiconductor field-effect transistors (MOSFETs) switches—switches228and230—are coupled in series between a first terminal240of the battery120and a first terminal of the inverter144such the body diode cathode terminal of the switch228is coupled to the first terminal240of the battery120and the body diode cathode terminal of the switch230is coupled to the first terminal244of the inverter102. The gate terminals of the switches228and230are coupled to the BMU190.

A second terminal242of the battery120is coupled to a second terminal246of the inverter102via a current measurement module226which measures the current flowing between the battery120and the inverter102.

The BMU190is coupled to the current measurement device226for receiving information on the measured current, and also receives an input224from the battery120indicating the battery cell voltage and temperature. The BMU190is coupled to the gate terminals of each of the switches228and230for driving the switch228to control battery discharge and driving the switch230to control battery charge as described herein. The BMU190is also coupled across the inverter terminals244and246for providing an inverter bias control voltage (which may also be referred to as a bias control voltage) to the inverter102as described further below.

The configuration of the body diodes of the switches228and230allows current to be blocked in one direction but not the other depending on state of each of the switches228and230. When the switch228is active (i.e., on) while the switch230is inactive (i.e., off), battery discharge is enabled to allow current to flow from the battery120to the inverter102through the body diode of the switch230. When the switch228is inactive while the switch230is active, battery charge is enabled to allow current flow from the inverter102to the battery120through the body diode of the switch228. When both switches228and230are active, the system is in a normal mode where the battery120can be charged or discharged.

The BMU190comprises support circuits204and a memory206, each coupled to a central processing unit (CPU)202. The CPU202may comprise one or more processors, microprocessors, microcontrollers and combinations thereof configured to execute non-transient software instructions to perform various tasks in accordance with embodiments of the present invention. The CPU202may additionally or alternatively include one or more application specific integrated circuits (ASICs). In some embodiments, the CPU202may be a microcontroller comprising internal memory for storing controller firmware that, when executed, provides the controller functionality described herein. The BMU190may be implemented using a general purpose computer that, when executing particular software, becomes a specific purpose computer for performing various embodiments of the present invention.

The support circuits204are well known circuits used to promote functionality of the CPU202. Such circuits include, but are not limited to, a cache, power supplies, clock circuits, buses, input/output (I/O) circuits, and the like. The BMU190may be implemented using a general purpose computer that, when executing particular software, becomes a specific purpose computer for performing various embodiments of the present invention. In one or more embodiments, the CPU202may be a microcontroller comprising internal memory for storing controller firmware that, when executed, provides the controller functionality described herein.

The memory206may comprise random access memory, read only memory, removable disk memory, flash memory, and various combinations of these types of memory. The memory206is sometimes referred to as main memory and may, in part, be used as cache memory or buffer memory. The memory206generally stores the operating system (OS)208, if necessary, of the controller114that can be supported by the CPU capabilities. In some embodiments, the OS208may be one of a number of commercially available operating systems such as, but not limited to, LINUX, Real-Time Operating System (RTOS), and the like.

The memory206stores non-transient processor-executable instructions and/or data that may be executed by and/or used by the CPU202. These processor-executable instructions may comprise firmware, software, and the like, or some combination thereof. The memory206stores various forms of application software, such as an acquisition system module210, a switch control module212, a control system module214, and an inverter bias control voltage module216(which may also be referred to as bias control module216). The memory206additionally stores a database218for storing data related to the operation of the BMU190and/or the present invention, such as one or more thresholds, equations, formulas, curves, and/or algorithms for the control techniques described herein. In various embodiments, one or more of the acquisition system module210, the switch control module212, the control system module214, the inverter bias control voltage module216, and the database218, or portions thereof, are implemented in software, firmware, hardware, or a combination thereof.

The acquisition system module210obtains the cell voltage and temperature information from the battery120via the input224, obtains the current measurements provided by the current measurement module226, and provides the cell voltage, cell temperature, and measured current information to the control system module214for use as described herein.

The switch control module212drives the switches228and230as determined by the control system module214.

The control system module214provides various battery management functions, including protection functions (e.g., overcurrent (OC) protection, overtemperature (OT) protection, and hardware fault protection), metrology functions (e.g., averaging measured battery cell voltage and battery current over, for example, 100 ms to reject 50 and 60 Hz ripple), state of charge (SOC) analysis (e.g., coulomb gauge for determining current flow and utilizing the current flow in estimating the battery SOC; synchronizing estimated SOC values to battery voltages (such as setting SOC to an upper bound, such as 100%, at maximum battery voltage; setting SOC to a lower bound, such as 0%, at a minimum battery voltage); turning off SOC if the inverter102never drives the battery120to these limits; and the like), balancing (e.g., autonomously balancing the charge across all cells of a battery to be equal, which may be done at the end of charge, at the end of discharge, or in some embodiments both at the end of charge and the end of discharge). By establishing upper and lower estimated SOC bounds based on battery end of charge and end of discharge, respectively, and tracking the current flow and cell voltage (i.e., battery voltage) between these events, the BMU190determines the estimated SOC.

In order to prevent inverter idle current from completely discharging the battery120(and thereby destroying the battery120) if the battery120remains unpowered for a long period of time, the BMU190detects when the battery SOC falls below a certain threshold (e.g., 20%). When the battery SOC falls below the threshold, the BMU190disconnects the inverter102from the battery120and operates in a micropower mode (e.g., <0.1 mA). The BMU190periodically attempts to reconnect by powering the inverter102until current flows in. In some embodiments, a semi-random increasing timer can be used for determining when the BMU190attempts to reconnect, for example every 2 minutes, then 5 minutes then 20 minutes, then hourly, then 18 hours, etc. The timer for determining when the BMU190attempts the reconnect is reset each time the battery120is disconnected.

The inverter bias control module216converts the battery voltage to a stabilized voltage which provides an indication of the state of the system and powers up the inverter102; the inverter bias control module216thus communicates the state of the charge process to and from the BMU190and the inverter102. When the BMU190does not allow charging or discharging, the inverter bias control module216regulates the battery output voltage to a predefined value, for example 12V. When no current is flowing between the battery120and inverter102, the inverter102is driven by the inverter bias control voltage which provides information to the inverter102indicating the system state. In certain embodiments, a buck-boost converter is used having a power level below 1 W. Exemplary values of the inverter bias control voltage and their corresponding state information are shown in Table 1 below. The bias voltages shown in Table 1 are selected so that the first six shown are below the normal operating voltage of the battery120and the last three shown are above the normal operating voltage of the battery120. In some embodiments, such as the embodiment corresponding to the bias voltages shown in Table 1, the battery operating voltage is 20 to 32V. For embodiments where the battery120has a lower or higher operating voltage range than this, the decision points from Table 1 should be scaled appropriately.

The inverter controller114comprises support circuits254and a memory256, each coupled to a central processing unit (CPU)252. The CPU252may comprise one or more processors, microprocessors, microcontrollers and combinations thereof configured to execute non-transient software instructions to perform various tasks in accordance with embodiments of the present invention. The CPU252may additionally or alternatively include one or more application specific integrated circuits (ASICs). In some embodiments, the CPU252may be a microcontroller comprising internal memory for storing controller firmware that, when executed, provides the controller functionality herein. The inverter controller114may be implemented using a general purpose computer that, when executing particular software, becomes a specific purpose computer for performing various embodiments of the present invention.

The support circuits254are well known circuits used to promote functionality of the CPU252. Such circuits include, but are not limited to, a cache, power supplies, clock circuits, buses, input/output (I/O) circuits, and the like. The inverter controller114may be implemented using a general purpose computer that, when executing particular software, becomes a specific purpose computer for performing various embodiments of the present invention. In one or more embodiments, the CPU252may be a microcontroller comprising internal memory for storing controller firmware that, when executed, provides the controller functionality described herein.

The memory256may comprise random access memory, read only memory, removable disk memory, flash memory, and various combinations of these types of memory. The memory256is sometimes referred to as main memory and may, in part, be used as cache memory or buffer memory. The memory256generally stores the operating system (OS)258, if necessary, of the inverter controller114that can be supported by the CPU capabilities. In some embodiments, the OS258may be one of a number of commercially available operating systems such as, but not limited to, LINUX, Real-Time Operating System (RTOS), and the like.

The memory256stores non-transient processor-executable instructions and/or data that may be executed by and/or used by the CPU252. These processor-executable instructions may comprise firmware, software, and the like, or some combination thereof. The memory256stores various forms of application software, such as a power conversion control module270for controlling the bidirectional power conversion, and a battery management control module272.

The battery management control module272enables state of charge (SOC) estimation by the inverter102. Upon startup, the SOC is unknown by the inverter102and must be determined (e.g., based on tracking current from an end of charge state to an end of discharge state). When the inverter102determines that the battery120refuses to take any current, the battery120is considered fully charged. The inverter102sets the required output current to be generated at zero, measures the battery voltage, and sets the estimated SOC to an upper bound, such as 100%. When the inverter102determines that no current is coming from the battery120during a discharge state, the battery120is considered fully discharged. The inverter measures the battery voltage, which may drop, for example, to 14V, and the inverter102sets the estimated SOC to a lower bound, such as 0%. Additionally, an internal coulomb counter250determines a coulomb count and couples the count to the controller114for use in tracking the current for estimating the SOC. Because a coulomb counter accumulates current measurement values over time, a small error or offset in the current measurement also accumulates, which may become significant over the integration time interval. As such, offset current accuracy is particularly important to the accuracy of the SoC estimator.

By establishing these upper and lower bounds of the estimated SOC and tracking the current flow between these events, the inverter102“learns” the total capacity as well as the present state within the cycle. The inverter102may use the estimated SOC for determining when to charge and discharge the battery120. Generally, the inverter102does not drive the battery120below a threshold percentage of discharge, where the threshold may be, for example, between 5 and 20%.

In some embodiments, the battery management control module272also provides equivalent series resistance (ESR) estimation. By estimating the ESR of the battery, which is temperature dependent and charge level dependent, information on the battery state (e.g., battery health) can be determined. As a result of the natural disturbance on the voltage and current across the battery120during power conversion, a ripple voltage exists on the DC battery voltage in addition to a ripple on the DC current (e.g., for a 120 Hz grid, a 120 Hz current ripple would be present). By correlating the ripple voltage versus the ripple current (essentially dV/dI), the ESR of the battery120can be estimated.

In such embodiments, the inverter102performs a correlation between the ripple on the battery current (Ibat) and the battery voltage (Vbat) and estimates the ESR at twice the line frequency. The estimated ESR may be used to estimate the battery's state of health (SoH) and/or degradation. Additionally, the ESR may be used to estimate actual battery voltage, versus the observed battery voltage. When the battery voltage reaches the high or low end of operation, or the high or low end of SOC, the inverter102reduces the current when the estimated ESR increases (i.e., the current is tapered).

Further detail on the functionality provided by the BMU modules and the inverter modules is described below with respect toFIG. 3

FIG. 3is a state diagram300illustrating states of the AC battery system200shown inFIG. 2in accordance with one or more embodiments of the present invention. In one embodiment, the state diagram300depicts the states associated with the charge process as implemented by the BMU190and the inverter controller114.

The system200may start in a ready to charge state302, where the battery voltage is at or less than a particular ready to charge state threshold, for example at or below 12 V. At ready to charge state switch230is active (i.e. on) to allow charging current. Since the system200may be in the ready to charge state302following a fault or an end of discharge (as described in more detail further below), the inverter102is unaware of where in the cycle the system is (i.e., whether to charge or discharge) as well as the battery SOC. In order to “unlock” the system, the inverter102attempts to charge the battery120by providing current to the battery120. If a period of time passes and the SOC measured by the BMU190falls below a minimum threshold (e.g. 5%), the state transitions from the ready to charge state302to an under voltage lockout state310via transition331. After a period of time (as determined, e.g., by a timer) in the under voltage lockout state310, the state transitions via330to a low power survival state311.

In the low power survival state311, if the inverter102attempts to charge the battery120, a transition via333to a normal state304can occur. After a timeout period (as determined, e.g., by a timer), the survivability state311transitions via332to the under voltage lockout state310.

When, during the ready to charge state302, charge current is detected, the state transitions via transition322from the ready to charge state302to a normal state304. In the normal state, both switches228and230between the battery120and the inverter102are active (i.e., on) and the battery120can charge and discharge. When the battery120is discharged to the point where a minimum cell voltage occurs (i.e., when the minimum voltage is reached on any cell within the battery120), the switch228is deactivated to disable current flow from the battery120to the inverter102and the state transitions from the normal state304to an end of discharge state306via transition324. Generally, the inverter102never allows the battery120to discharge to the point that the inverter102cannot restart. The inverter may allow for the possibility that the battery is fully discharged when the inverter102turns back on and the SOC has degraded.

In the end of discharge state306, the inverter102sets the required output current to be generated at zero in order to stop attempts to discharge the battery120, measures the battery voltage, and sets its estimated SOC to a lower bound (e.g., 0%). Additionally, the BMU190sets its estimated SOC to a lower bound (e.g., 0%). The state transitions from the end of discharge state306to a balancing state308via transition326.

During end of discharge state306, if the inverter102attempts to charge the battery120, the end of discharge state306may transition to the normal state304via335transition.

As a result of difference in cell capacities with the battery120, balancing is performed in order to balance the voltage of the cell or cells that have reached minimum voltage to the same level as the remaining cells (i.e., the voltage across all battery cells is balanced to be the same as the lowest voltage cell). In some embodiments, such as the embodiment described herein, balancing the voltage across all of the battery cells is done following both end of discharge and end of charge, although in some alternative embodiments voltage balancing of the battery cells may be done only following end of discharge or only following end of charge.

Once the voltage is balanced across the battery cells, the state transitions from the balancing state308back to the ready to charge state302via transition328.

When, during the normal state304, the battery120is charged to the point where a maximum cell voltage occurs (i.e., when the maximum voltage is reached on any cell within the battery120), the switch230is deactivated to disable the battery charging capability and the state transitions from the normal state304to an end of charge state306via transition336. In the end of charge state312, the inverter102sets the required output current to be generated at zero in order to stop attempting to charge the battery120, measures the voltage, and sets its estimated SOC to an upper bound (e.g., 100%). Additionally, the BMU190sets its estimated SOC to an upper bound (e.g., 100%). By resetting the upper and lower bounds of the estimated SOCs at both the BMU190and the inverter102at each end of charge and end of discharge state, the BMU's estimated SOC and the inverter's estimated SOC remain synchronized with one another.

During the end of charge state312, if the inverter102attempts to discharge the battery120, the end of charge state312may transition to the normal state304via337transition.

The state transitions from the end of charge state312to a balancing state314when the inverter stops attempting to charge the battery120(i.e., at transition338). Once the battery cells are balanced, the state transitions from the balancing state314to a ready to discharge state316via transition340. The BMU190activates the switch228to enable battery discharge and the state transitions from the fully charged state316to the normal state304via transition334.

The system200may enter a fault state318from any of the previously described states. The state then transitions from the fault state318to the ready to charge state302when a fault cleared event344occurs.