STORAGE SYSTEM CONFIGURED FOR USE WITH AN ENERGY MANAGEMENT SYSTEM

A storage system configured for use with an energy management system is provided herein and comprises a battery, a battery management unit coupled to the battery and a power converter comprising plurality of microinverters operably coupled to the battery and the battery management unit, each microinverter of the plurality of microinverters configured to calculate an estimate of state-of-charge of the battery and periodically communicate a calculated estimate of state-of-charge to the other microinverters, such that each microinverter of the plurality of microinverters calculates an average state-of-charge of the battery and communicates the calculated average state-of-charge to the battery management unit for controlling charging/discharging of the battery.

BACKGROUND

1. Field of the Disclosure

Embodiments of the present disclosure generally relate to power systems and, more particularly, to methods and apparatus for distributed state-of-charge (SoC) estimation of a battery.

2. Description of the Related Art

Conventional AC storage systems comprise one or more batteries (e.g., lithium-ion batteries) and provide a required energy storage (kWh) and a required AC power (KW). Conventional AC storage systems are designed with a battery management unit (BMU) which is responsible for protecting the one or more batteries from faults, balancing the battery cells, and calculating battery telemetry such as the SoC. SoC is an important parameter of the lithium-ion battery in terms of indicating a proper battery state, providing an available energy, for calculating SoH (state-of-health), etc. A high accuracy of the SoC is required (e.g., <2%). Thus, SoC estimation algorithms such as coulomb counting and Kalman filtering rely on having a single voltage and current measurement of a battery pack of the AC storage systems to accurately estimate the SoC during operation.

The single voltage/current measurement requirement can add significant cost and complexity to the BMU, as measuring the large DC currents in the battery packs requires expensive sensors or current shunts, high current terminals, large PCB traces, and expensive analog front-ends to process the signal. Additionally, the chargers/inverters, which typically connect to the one or more batteries, often have individual current measurements. While the current measurements can be transmitted to the BMU using, for example, a communications system and aggregated digitally, if the currents have high frequency content (e.g., such as in a single-phase inverter system), the speed and synchronization requirements for the communications system make using such systems impractical for transmitting the current measurements to the BMU. Accordingly, there is a need to develop a distributed SoC estimation technique which does not rely on high frequency communications.

Therefore, the inventors have found improved methods and apparatus for distributed state-of-charge (SoC) estimation of a battery, without relying on high frequency communications.

SUMMARY

In accordance with some aspects of the present disclosure, a storage system configured for use with an energy management system comprises a battery, a battery management unit coupled to the battery and a power converter comprising plurality of microinverters operably coupled to the battery and the battery management unit, each microinverter of the plurality of microinverters configured to calculate an estimate of state-of-charge of the battery and periodically communicate a calculated estimate of state-of-charge to the other microinverters, such that each microinverter of the plurality of microinverters calculates an average state-of-charge of the battery and communicates the calculated average state-of-charge to the battery management unit for controlling charging/discharging of the battery.

In accordance with some aspects of the present disclosure, a method for managing a storage system configured for use with an energy management system comprises calculating at each microinverter of a plurality of microinverters an estimate a state-of-charge of a battery, broadcasting from each of the plurality of microinverters a calculated estimate state-of-charge of the battery and when the each microinverter of the plurality of microinverters receives the calculated estimate state-of-charge from other microinverters of the plurality of microinverters, calculating at the each microinverter of the plurality of microinverters an average state-of-charge based on the calculated estimate state-of-charge for controlling charging/discharging of the battery.

In accordance with some aspects of the present disclosure, a non-transitory computer readable storage medium has instructions stored thereon that when executed b a processor performs a method for managing a storage system configured for use with an energy management system. The method comprises calculating at each microinverter of a plurality of microinverters an estimate a state-of-charge of a battery, broadcasting from each of the plurality of microinverters a calculated estimate state-of-charge of the battery and when the each microinverter of the plurality of microinverters receives the calculated estimate state-of-charge from other microinverters of the plurality of microinverters, calculating at the each microinverter of the plurality of microinverters an average state-of-charge based on the calculated estimate state-of-charge for controlling charging/discharging of the battery.

DETAILED DESCRIPTION

In accordance with the present disclosure, methods and apparatus for calculating a state-of-charge (SoC) of a battery of a storage system are disclosed herein. For example, a storage system can be configured for use with an energy management system comprising a battery, a battery management unit coupled to the battery, and a power converter. The power converter can comprise a plurality of microinverters operably coupled to the battery and the battery management unit. Each microinverter of the plurality of microinverters can be configured to calculate an estimate of state-of-charge of the battery and periodically communicate a calculated estimate of state-of-charge to the other microinverters. Each microinverter of the plurality of microinverters can calculate an average state-of-charge of the battery and communicate the calculated average state-of-charge to the battery management unit to control charging/discharging of the battery. The methods and apparatus described herein provide improved distributed SoC estimation techniques that do not rely on high frequency communications.

FIG.1is a block diagram of a system100(energy management system) for power conversion using one or more embodiments of the present disclosure. This diagram only portrays one variation of the myriad of possible system configurations and devices that may utilize the present disclosure.

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 converters102-1,102-2, . . .102-N,102-N+1, and102-N+M collectively referred to as power converters102(which also may be called power conditioners); a plurality of DC power sources104-1,104-2, . . .104-N, collectively referred to as power sources104(e.g., converters); 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 BMUs190-1,190-2, . . .190-M (battery management units) collectively referred to as BMUs190; a system controller106; a bus108; a load center110; and an IID140(island interconnect device) (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 devices are rechargeable batteries (e.g., multi-C-rate collection of AC batteries) which may be referred to as batteries120, although in other embodiments the energy storage/delivery devices may 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 sources may 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 plurality of energy storage devices/delivery devices120-1,120-2. . .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 the inverter controllers114) for controlling operation of the power converters102-1,102-2. . .102-N+M.

In some embodiments, such as the embodiment described below, the DC 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 system 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 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.

A storage system configured for use with an energy management system, such as the Enphase® Energy System, is described herein. For example,FIG.2is a block diagram of an AC battery system200(e.g., a storage system) in accordance with one or more embodiments of the present disclosure.

The AC battery system200comprises a BMU190coupled to a battery (e.g., the battery120) and one or more inverters (e.g., the power converters102). In at least some embodiments, the battery120can comprise a plurality of cells (not shown) and the power converters102can comprise four embedded converters (e.g., four embedded microinverters). In at least some embodiments, the battery120can be the IQ Battery 3 (or the IQ Battery 10) and the microinverters can be the IQ8X-BAT microinverters, both available from Enphase®. 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 power converter102. The gate terminals of the switches228and230are coupled to the BMU190.

A second terminal242of the battery120is coupled to a second terminal246of the power converter102via a current measurement module226which measures the current flowing between the battery120and the power converter102.

The BMU190is coupled to the current measurement module226for 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 first terminal244and the second terminal246for 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 power converter102through the body diode of the switch230. When the switch228is inactive while the switch230is active, battery charge is enabled to allow current flow from the power converter102to 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(e.g., non-transitory computer readable storage medium), each coupled to a CPU202(central processing unit, e.g., processor). 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 disclosure. 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 disclosure.

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 disclosure. 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 OS208(operating system), if necessary, of the inverter 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 CPU202to perform, for example, one or more methods for discharge protection, as described in greater detail below. 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 module216. The memory206additionally stores a database218for storing data related to the operation of the BMU190and/or the present disclosure, 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 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 gauge250for 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 power converter102never 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.

The inventors have found a new algorithm for determining the SoC on a battery when the battery is being charged/discharged by multiple converters (e.g., multiple power inverters), where each of the multiple converters measures a corresponding voltage/current, but no single aggregate voltage/current measurement is used in an AC battery system200.

For example, the inventive concepts described herein combine conventional SoC estimation (e.g., calculated by the BMU190with a consensus algorithm (e.g., programming a microinverter). For example, each microinverter of the power converter102(charger/discharger) is programmed to assume it is operating on a same percentage of a battery (e.g., the battery120) and that the other converters (e.g., microinverters) are operating at the same power. Each microinverter then generates its own estimate of what the SoC estimation is using a standard technique, e.g., such as at least one of coulomb counting or Kalman Filtering. Each microinverter is configured to periodically broadcast (e.g., via wired and/or wireless communication) a calculated SoC of the battery120. Thus, when a converters receives a SoC estimation from another converters, the microinverter averages the received SoC with its own SoC estimation. For example, an average of the SoC estimations can be calculated using Equation (1):

where SoCmineis a state-of-charge calculated by one of the microinverters of the power converter102and SoCotheris a state-of-charge calculated by one of the other microinverters of the power converter102, and 2 is the number of calculated SoC. Thus, if the power converter102comprises 4 microinverters, an average of the SoC can be calculated using Equation (1) with the numerator of Equation (1) being SoCmine+SoCother+SoCother+SoCotherand the denominator of Equation (1) being four (4).

In some embodiments, the algorithm can also use the consensus algorithm to average other calculated parameters that are used as part of the SoC estimation. For example, Kalman Filter based estimation calculates the estimated open-circuit voltage as part of the SoC estimation. In such embodiments, the consensus algorithm can be calculated using to Equation (2):

where P can be any internal parameter calculated as part of the SoC estimation algorithm.

Continuing with reference toFIG.2, the inverter controller114comprises support circuits254and a memory256, each coupled to a CPU252(central processing unit). 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 disclosure. 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 disclosure.

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 disclosure. 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 OS258(operating system), 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 BMU190communicates with the system controller106to perform balancing of the batteries120(e.g., multi-C-rate collection of AC batteries) based on a time remaining before each of the batteries are depleted of charge, to perform droop control (semi-passive) which allows the batteries to run out of charge at substantially the same time, and perform control of the batteries to charge batteries having less time remaining before depletion using batteries having more time remaining before depletion, as described in greater detail below.

FIG.3is a flowchart of a method300for managing a storage system configured for use with an energy management system, in accordance with at least one embodiment of the present disclosure. The method300can be performed using power converters102-1. . .102-N to convert DC power from the DC power sources104to grid-compliant AC power, using the power converters102-N+1. . .102-N+M to convert DC power from the batteries120to grid-compliant AC power (e.g., AC-DC power converter), and/or using DC-DC power converters. For illustrative purposes, the method300is described configured for use with an AC rechargeable battery and the power converters configured for use with the AC rechargeable battery.

For example, at302, the method300comprises calculating at each of a plurality of microinverters an estimate of a state-of-charge of an AC rechargeable battery. For example, each of the plurality of microinverters of the power converter102can calculate an estimate of a SoC of the battery120. In at least some embodiments, the plurality of microinverters can use coulomb counting and/or Kalman Filtering to calculate estimate of a SoC of the battery120. In at least some embodiments, the SoC estimation algorithm at302is performed on the converters at a frequency of about 200 Hz (e.g., 200 times a second).

Next, at304, the method300comprises broadcasting from each of the plurality of microinverters the calculated state-of-charge of the AC rechargeable battery. For example, each of the plurality of microinverters can broadcast the calculated SoC of the battery120via wired or wireless communication. In at least some embodiments, each of the plurality of microinverters can broadcast the calculated SoC of the battery120to each other periodically at a predetermined time frame. For example, in at least some embodiments, each of the plurality of microinverters can broadcast the calculated SoC of the battery120at 1 Hz (e.g., once a second).

Next, at306, the method300comprises when the each microinverter of the plurality of microinverters receives the calculated estimate state-of-charge from other microinverters of the plurality of microinverters, calculating at the each microinverter of the plurality of microinverters an average state-of-charge based on the calculated estimate state-of-charge for controlling charging/discharging of the battery. For example, depending on how many microinverters the power converter102comprises, each of the microinverters of the plurality of microinverters uses Equations (1) and (2) as described above to calculate an average of the received estimate of the state-of-charge. In at least some embodiments, such as when there is four microinverters, one of the microinverters can be designated as a master microinverter that is configured to receive estimates of the state-of-charge from the other three microinverters (slave microinverters), calculate an average of the received estimates of the state-of-charge, and transmit the calculated average to the BMU190, which, in turn, can control charging/discharging the battery120based on the average of the estimates of the state-of-charge and an estimate state-of-charge calculated by the BMU190. In at least some embodiments, at306averaging the received estimate of the state-of-charge with the state-of-charge of the microinverter of the plurality of microinverter can be performed@(N−1) Hz, where N is the number of converters. Thus, when there are a total of four (4) converters,306would run three (3) times a second (e.g., every time a message is received from another converter).