Off-grid energy storage system and control method for off-grid energy storage system

The present application claims priority under 35 U.S.C. § 119 to Chinese Application No. 201710131395.2 filed Mar. 7, 2017, the entire content of which is incorporated herein by reference.

The present disclosure relates to the field of the microgrid technology, and in particular to an off-grid energy storage system and a control method for the off-grid energy storage system.

FIG. 1 shows an off-grid energy storage system, which includes multiple energy storage inverters, a direct current side of each of the energy storage inverters is connected to an independent storage battery, and alternate current sides of the energy storage inverters are connected in parallel with the same alternate current bus, thereby converting multiple direct current inputs into one alternate current output.

Ensuring consistency of operating states of the storage batteries in the off-grid energy storage system, for example, ensuring balance among the states of charge (SOC) of the storage batteries and ensuring balance among voltages across the storage batteries is an important mean for ensuring safe and stable operation of the off-grid energy storage system. However, the current technical force still cannot provide technical support for maintaining the balance among the SOCs of the storage batteries or maintaining the balance among voltages across the storage batteries.

In view of this, an off-grid energy storage system and a control method for the off-grid energy storage system are provided according to the present disclosure, to balance the operating states of storage batteries in an off-grid energy storage system.

An off-grid energy storage system is provided, which includes n energy storage inverters identified by P₁, P₂, . . . ,P_nsequentially. A direct current side of each of the energy storage inverters is connected to an independent storage battery, and alternate current sides of the energy storage inverters are connected in parallel. The storage battery connected to the direct current side of P_jis identified by B_j, where j=2, 3, . . . , n. A control system for an output waveform of P₁has a voltage-current double loop control structure. A control system for an output waveform of P₁has a current loop control structure, a current loop given value of P_jis equal to a value obtained by adding a current correction value of P_jto a current loop given value of P₁, the current correction value of P_jis an output quantity of an automatic control system for causing B_jto have the same operating parameter as B₁.

Specifically, the operating parameter includes a state of charge of the storage battery.

Specifically, the operating parameter includes a voltage across the storage battery.

Specifically, the automatic control system is an open-loop control system.

Specifically, the automatic control system is a closed-loop control system.

Specifically, the automatic control system is configured to adjust the current correction value of P_jbased on a difference between the operating parameter of B_jand the operating parameter of B₁with a variable step size.

A control method for an off-grid energy storage system is provided. The off-grid energy storage system includes n energy storage inverters identified by P₁, P₂, . . . , P_nsequentially, a direct current side of each of the energy storage inverters is connected to an independent storage battery, and alternate current sides of the energy storage inverters are connected in parallel, the storage battery connected to the direct current side of P_jis identified by B_j, with j=2, 3, . . . , n. The control method includes: controlling, by P₁, an output waveform in a voltage-current double loop control mode; adjusting, by P₁, a current correction value of P₁based on a difference between an operating parameter of B_jand an operating parameter of B₁to cause B_jto have the same operating parameter as B₁; and controlling, by P_j, an output waveform in a current loop control mode, where a current loop given value of P_jis equal to a value obtained by adding a current loop given value of P₁to the adjusted current correction value of P_j.

Specifically, P_jadjusting the current correction value of P_jbased on the difference between the operating parameter of B_jand the operating parameter of B₁includes: adjusting, by P_j, the current correction value of P_jbased on a difference between a state of charge of B_jand a state of charge of B₁; or adjusting, by P_j, the current correction value of P_jbased on a difference between a voltage across B_jand a voltage across B₁.

Specifically, P_jadjusting the current correction value of P_jbased on the difference between the operating parameter of B_jand the operating parameter of B₁includes: adjusting, by P_j, the current correction value of P_jin an open-loop mode or a closed-loop mode based on the difference between the operating parameter of B_jand the operating parameter of B₁.

Specifically, P_jadjusting the current correction value of P_jbased on the difference between the operating parameter of B_jand the operating parameter of B₁includes: adjusting, by P_j, the current correction value of P_jbased on the difference between the operating parameter of B_jand the operating parameter of B₁with a variable step size.

It can be seen from the above technical solution that, according to the present disclosure, any one energy storage inverter P₁is selected to use a voltage-current double loop control mode, and the remaining energy storage inverters (i.e., P_j, j=2, 3, . . . , n) use a current loop control mode. On this basis, according to the present disclosure, the operating parameter of the storage battery B₁is taken as a reference for the operation of the storage battery B_j, and P_jdynamically adjusts the current correction value of P_jwhen the operation of the B_jis deviated, to reduce the deviation. The current loop given value of P_jis equal to a value obtained by adding the current correction value of P_jto the current loop given value of P₁. When the difference between the operating parameter of B₁and the operating parameter of B₁is zero, the automatic control system reaches a steady state. In this case, the current correction value of P_jis zero, that is, P_joperates in accordance with the current loop given value of P₁. It can be seen that, according to the present disclosure, it is ensured the consistency of operating states of the storage batteries in the off-grid energy storage system.

The technical solutions according to the embodiments of the present disclosure will be described clearly and completely as follows in conjunction with the drawings in the embodiments of the present application. It is apparent that the described embodiments are only a part of the embodiments according to the present disclosure, rather than all the embodiments. Any other embodiments obtained by those skilled in the art based on the embodiments in the present disclosure without any creative efforts fall within the protection scope of the present disclosure.

Referring to FIG. 2, an off-grid energy storage system is disclosed according to an embodiment of the present disclosure, to balance operating states of storage batteries in the off-grid energy storage system. The off-grid energy storage system includes n energy storage inverters identified by P₁, P₂, . . . , P_nsequentially (the identifiers of the energy storage inverters are merely used to distinguish the energy storage inverters, and the allocation of the identifiers is not targeted). A direct current side of each of the energy storage inverters is connected to an independent storage battery, and alternate current sides of the energy storage inverters are connected in parallel. The storage battery connected to the direct current side of P_jis identified by B_j, where j=2, 3, . . . , n. A control system for an output waveform of P₁has a voltage-current double loop control structure. A control system for an output waveform of P_jhas a current loop control structure. A current loop given value of P_jis equal to a value obtained by adding a current correction value of P_jto a current loop given value of P₁. The current correction value of P_jis an output quantity of an automatic control system for causing B_jto have the same operating parameter as

The technical solution of this embodiment is described in detail below.

The off-grid energy storage system should include at least one energy storage inverter performing V/F (voltage/frequency) control to establish a voltage and a frequency of an alternate current bus. In this embodiment, any one energy storage inverter P₁is selected to use a voltage-current double loop control mode, and P₁establishes the voltage and the frequency of the alternate current bus using a voltage loop. Alternate current sides of the remaining energy storage inverters (i.e., P_j) and the alternate current side of P₁are connected in parallel with the same alternate current bus, and the remaining energy storage inverters have the same output voltage and frequency as P₁. In a case that the P₁uses a voltage-current double loop control mode and the remaining energy storage inverters use a current loop control, the energy storage inverters operate in parallel in accordance with their own current loop given values, a current loop given value of P₁is the output of the voltage loop.

During the actual operation of the off-grid energy storage system, the operating states of the storage batteries connected to the direct current sides of the energy storage inverters may be asynchronous due to factors such as different manufacturers or different production batches of the storage batteries. If the operating states of the storage batteries are not balanced, the difference among the operating states of the storage batteries and among the operating states of the energy storage inverters may be larger and larger.

In this regard, in this embodiment, the operating parameter of B₁is taken as the reference of the operation of B_j, and an automatic control system (denoted as Z_j) in P_jdynamically adjusts the current correction value of P₁based on the deviation of the operating parameter of B_jfor synchronous operation between B_jwith B₁. The current loop given value of P_jis equal to a value obtained by adding the current loop given value of P₁to the output of Z_j(that is, the adjusted current correction value of P_j). When the difference between the operating parameter of B_jand the operating parameter of B₁is zero, the automatic control system reaches a steady state. In this case, the current correction value of P_jis zero, that is, P_joperates in accordance with the current loop given value of P₁. It can be seen that, according to this embodiment, it is ensured the consistency of operating states of the storage batteries in the off-grid energy storage system.

Specifically, Z_jmay be an open-loop control system, or may be a closed-loop control system. Specifically, in a case that Z₁is an open-loop control system, the structure as shown in FIG. 3 may be used, in which the difference between the operating parameter of B_jand the operating parameter of B₁is processed by a proportional adjustment unit 10 and an output amplitude limiting unit 20 to obtain the current correction value of P_j. In a case that Z_jis a closed-loop control system, the structure as shown in FIG. 4 may be used, in which the difference between the operating parameter of B_jand the operating parameter of B₁is processed by a PI controller 10 (or other types of controller such as PID regulator or I regulator) and an output amplitude limiting unit 20 to obtain the current correction value of P_j. It should be noted that, each of a proportional factor k_nof the proportional adjustment unit 10 and the parameter of the PI controller 10 may be a fixed parameter, or may be adjusted based on the difference between the operating parameter of B_jand the operating parameter of B₁with a variable step size. That is, the parameter becomes larger in a case of a large difference to achieve fast balance, and the parameter becomes smaller in a case of a small difference to achieve fine adjustment.

Specifically, Z_jcontrolling B_jto have the same operating parameter as B₁may mean that there is no difference between the voltage across B_jand the voltage across B₁or may mean that there is no difference between the state of charge of B_jand the state of charge of B₁.

Specifically, a sampling signal, a control signal, and the like, may be transmitted in the off-grid energy storage system in a CAN bus communication mode, RS485 communication mode, Ethernet communication mode, or power line carrier communication mode, etc., which is not limited.

In addition, referring to FIG. 5, a control method for an off-grid energy storage system is disclosed according to the present application. The off-grid energy storage system includes n energy storage inverters identified by P₁, P₂, . . . , P_nsequentially, a direct current side of each of the energy storage inverters is connected to an independent storage battery, and alternate current sides of the energy storage inverters are connected in parallel, the storage battery connected to the direct current side of P_jis identified by B_j, where j=2, 3, . . . , n. The control method includes steps S01 to S03.

In step S01, P₁controls an output waveform of P₁in a voltage-current double loop control mode.

In step S02, P_jadjusts a current correction value of P_jbased on a difference between an operating parameter of B_jand an operating parameter of B₁to cause B_jto have the same operating parameter as B₁.

In step S03, P_jcontrols an output waveform of P_jin a current loop control mode. A current loop given value of P_jis equal to a value obtained by adding a current loop given value of P₁to the adjusted current correction value of P_j.

Specifically, P_jadjusting the current correction value of P_jbased on the difference between the operating parameter of B_jand the operating parameter of B₁includes: adjusting, by P_j, the current correction value of P_jbased on a difference between a state of charge of B_jand a state of charge of B₁; or adjusting, by P_j, the current correction value of P_jbased on a difference between a voltage across B_jand a voltage across B₁.

Specifically, P_jadjusting the current correction value of P_jbased on the difference between the operating parameter of B_jand the operating parameter of B₁includes: adjusting, by P_j, the current correction value of P_jin an open-loop mode or a closed-loop mode based on the difference between the operating parameter of B_jand the operating parameter of B₁.

Specifically, P_jadjusting the current correction value of P_jbased on the difference between the operating parameter of B_jand the operating parameter of B₁includes: adjusting, by P_j, the current correction value of P_jbased on the difference between the operating parameter of B_jand the operating parameter of B₁with a variable step size.

In summary, according to the present disclosure, any one energy storage inverter P₁is selected to use a voltage-current double loop control mode, and the remaining energy storage inverters (i.e., P_j, j=2, 3, . . . , n) use a current loop control mode. On this basis, according to the present disclosure, the operating parameter of the storage battery B₁is taken as a reference for the operation of the storage battery B_j, and P_jdynamically adjusts the current correction value of P_jwhen the operation of the B_jis deviated, to reduce the deviation. The current loop given value of P_jis equal to a value obtained by adding the current correction value of P_jto the current loop given value of P₁. When the difference between the operating parameter of B_jand the operating parameter of B₁is zero, the automatic control system reaches a steady state. In this case, the current correction value of P_jis zero, that is, P_joperates in accordance with the current loop given value of P₁. It can be seen that, according to the present disclosure, it is ensured the consistency of operating states of the storage batteries in the off-grid energy storage system.

In the present specification, the embodiments are described in progressive manner. Each embodiment mainly focuses on an aspect different from other embodiments, and reference can be made to these similar parts among the embodiments. The control method for the off-grid energy storage system in the embodiment corresponds to the off-grid energy storage system disclosed in the embodiment, and is described relatively simply. For detailed description of the control method, reference may be made to the related description of the off-grid energy storage system.

The above illustration of the disclosed embodiments enables those skilled in the art to implement or practice the present disclosure. Many changes to these embodiments are apparent for those skilled in the art, and general principles defined herein can be implemented in other embodiments without departing the spirit or scope of the present disclosure. Hence, the present disclosure is not limited to the embodiments disclosed herein, but is to conform to the widest scope consistent with principles and novel features disclosed herein.