CN115441487B - SOC equalization method and terminal of common direct current bus energy storage system - Google Patents

Abstract

The present invention provides a SOC balancing method and terminal for a common DC bus energy storage system, including the steps of: initializing the upper limit and lower limit of the voltage sampling value of the DC bus in the energy storage system, and acquiring the working state of the energy storage system and the DC bus voltage sampling value of each subsystem in real time; determining the first charging/discharging subsystem according to the voltage sampling value, and collecting the SOC of the electric cabinet in the first charging/discharging subsystem in real time, if the SOC of the electric cabinet is charged/discharged to a first charging value/first discharging value, then stopping the operation of the first charging/discharging subsystem; setting the overvoltage threshold/undervoltage threshold of the DC/DC in the first charging/discharging subsystem to the first overvoltage threshold/first undervoltage threshold; repeating the above steps for the remaining subsystems until the overvoltage threshold/undervoltage threshold of the DC/DC in each subsystem is set to the first overvoltage threshold/first undervoltage threshold. The present invention can effectively solve the problem that the energy storage system cannot be charged and discharged at the same time, resulting in the SOC balancing of each subsystem becoming worse and worse.

Description

A SOC balancing method and terminal for a common DC bus energy storage system

Technical Field

The present invention relates to the technical field of energy storage systems, and in particular to a SOC balancing method and a terminal for a common DC bus energy storage system.

Background Art

The common DC bus energy storage system is a new type of energy storage architecture. A common DC bus energy storage system is usually composed of multiple energy storage subsystems connected to the power grid. Each subsystem includes a cabinet, DC/DC (bidirectional DC converter), AC/DC (i.e. energy storage converter PCS, also called inverter) and EMS system, and each EMS system is independent of each other and does not communicate with each other. The subsystems are connected in parallel by sharing a DC bus, and the DC bus of the entire energy storage system is established and maintained by the DC/DC of each subsystem.

Ideally, during the charging and discharging process of the common DC bus energy storage system, each subsystem should be in the same charging and discharging state with equal power. But in reality, due to the long distribution distance of each subsystem, the length of the DC bus is several kilometers or even dozens of kilometers, which has the effect of voltage drop; and the sampling voltage of the DC bus by the DC/DC of each subsystem is not consistent, which will result in errors. Due to these two reasons, the sampling results of the DC/DC of each subsystem for the same DC bus voltage are inconsistent, and there is a certain energy storage subsystem that is charged and discharged first, resulting in low charging and discharging efficiency of the entire common DC bus energy storage system. In the long run, the balance between the sub-energy storage systems of the common DC bus energy storage system will become worse and worse.

Summary of the invention

The technical problem to be solved by the present invention is to provide a SOC balancing method and terminal for a common DC bus energy storage system, so as to solve the problem that the energy storage system cannot be charged and discharged at the same time, resulting in the SOC balance of each subsystem becoming worse and worse.

In order to solve the above technical problems, the technical solution adopted by the present invention is:

A SOC balancing method for a common DC bus energy storage system comprises the following steps:

S1. Initialize the upper and lower limits of the voltage sampling value of the DC bus in the energy storage system, and obtain the working status of the energy storage system and the DC bus voltage sampling values of each subsystem in real time;

S2. Determine the first charging/discharging electronic system according to the voltage sampling value, and collect the SOC of the electric cabinet in the first charging/discharging electronic system in real time. If the SOC of the electric cabinet is charged/discharged to a first charging value/first discharging value, stop the operation of the first charging/discharging electronic system;

S3, setting the overvoltage threshold/undervoltage threshold of the DC/DC in the first charging/discharging electronic system to a first overvoltage threshold/first undervoltage threshold, wherein the first overvoltage threshold is higher than the upper limit, and the first undervoltage threshold is lower than the lower limit;

S4, repeating steps S2-S3 for the remaining subsystems until the overvoltage threshold/undervoltage threshold of the DC/DC in each subsystem is set to the first overvoltage threshold/first undervoltage threshold;

S5. The upper limit value and the lower limit value are changed from initial values to the first overvoltage threshold value and the first undervoltage threshold value, and each subsystem is charged and discharged simultaneously.

In order to solve the above technical problems, another technical solution adopted by the present invention is:

A SOC balancing terminal for a common DC bus energy storage system comprises a memory, a processor and a computer program stored in the memory and executable on the processor. When executing the computer program, the processor implements the steps in the SOC balancing method for a common DC bus energy storage system as described above.

The beneficial effects of the present invention are as follows: the present invention provides a SOC balancing method and terminal for a common DC bus energy storage system. When there is an imbalance among the subsystems in the common DC bus energy storage system (for example, the DC bus sampling error and the SOC imbalance between each other caused by the first charging/discharging, the overvoltage threshold/undervoltage threshold of the subsystem that is first charged/discharged to the first charging value/first discharging value is adjusted in sequence to the first overvoltage threshold/first undervoltage threshold, so that the initial upper/lower limit value of the DC bus is less than/greater than the overvoltage threshold/undervoltage threshold of the subsystem DC/DC that is first charged/discharged, and is within the normal range. According to the DC/DC droop strategy, the subsystem that is first charged/discharged will It is in a state of neither charging nor discharging, thereby stopping power output, while the energy storage system needs to continue charging/discharging, and the charging/discharging power is continued to be borne by the remaining subsystems, that is, the remaining subsystems continue to maintain the output upper/lower limit values of the DC bus, until the overvoltage threshold/undervoltage threshold of the DC/DC in each subsystem reaches the first overvoltage threshold/first undervoltage threshold, the upper/lower limit value of the DC bus is changed from the initial value to the first overvoltage threshold/first undervoltage threshold, and the subsequent DC bus voltage can be evenly maintained by each subsystem, that is, the same charging and discharging is achieved, thereby avoiding the increasing imbalance of SOC between the subsystems, and greatly improving the utilization efficiency of the common DC bus energy storage system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a system architecture diagram of a common DC bus energy storage system according to an embodiment of the present invention;

FIG2 is an overall flow chart of a SOC balancing method for a common DC bus energy storage system according to an embodiment of the present invention;

FIG3 is a schematic diagram of the working principle of a DC/DC converter that maintains the DC bus through droop control;

FIG4 is a specific flow chart of a SOC balancing method for a common DC bus energy storage system according to an embodiment of the present invention;

FIG5 is a schematic structural diagram of a SOC balancing terminal of a common DC bus energy storage system according to an embodiment of the present invention.

Description of labels:

10. A SOC balancing terminal for a common DC bus energy storage system; 20. Memory; 30. Processor.

DETAILED DESCRIPTION

In order to explain the technical content, achieved objectives and effects of the present invention in detail, the following is an explanation in combination with the implementation modes and the accompanying drawings.

Referring to FIGS. 1 to 4 , a SOC balancing method for a common DC bus energy storage system includes the following steps:

S1. Initialize the upper and lower limits of the voltage sampling value of the DC bus in the energy storage system, and obtain the working status of the energy storage system and the DC bus voltage sampling values of each subsystem in real time;

S2. Determine the first charging/discharging electronic system according to the voltage sampling value, and collect the SOC of the electric cabinet in the first charging/discharging electronic system in real time. If the SOC of the electric cabinet is charged/discharged to a first charging value/first discharging value, stop the operation of the first charging/discharging electronic system;

S3, setting the overvoltage threshold/undervoltage threshold of the DC/DC in the first charging/discharging electronic system to a first overvoltage threshold/first undervoltage threshold, wherein the first overvoltage threshold is higher than the upper limit, and the first undervoltage threshold is lower than the lower limit;

S4, repeating steps S2-S3 for the remaining subsystems until the overvoltage threshold/undervoltage threshold of the DC/DC in each subsystem is set to the first overvoltage threshold/first undervoltage threshold;

S5. The upper limit value and the lower limit value are changed from initial values to the first overvoltage threshold value and the first undervoltage threshold value, and each subsystem is charged and discharged simultaneously.

As can be seen from the above description, the beneficial effect of the present invention is that: in the case where there is an imbalance among the subsystems in the common DC bus energy storage system (for example, the DC bus sampling error and the first charging/discharging causing the SOC imbalance between each other), by sequentially adjusting the overvoltage threshold/undervoltage threshold of the subsystem that is first charged/discharged to the first charging value/first discharging value to the first overvoltage threshold/first undervoltage threshold, the initial upper/lower limit value of the DC bus is less than/greater than the overvoltage threshold/undervoltage threshold of the first charging/discharging subsystem DC/DC, which is within the normal range. According to the DC/DC droop strategy, the subsystem that is first charged/discharged will be in a state of neither charging nor discharging. , thereby stopping power output, while the energy storage system needs to continue charging/discharging, and the charging/discharging power is continued to be borne by the remaining subsystems, that is, the remaining subsystems continue to maintain the output upper/lower limit values of the DC bus, until the overvoltage threshold/undervoltage threshold of the DC/DC in each subsystem reaches the first overvoltage threshold/first undervoltage threshold, the upper/lower limit value of the DC bus is changed from the initial value to the first overvoltage threshold/first undervoltage threshold, and the subsequent DC bus voltage can be evenly maintained by each subsystem, that is, the same charging and discharging is achieved, thereby avoiding the increasing imbalance of SOC between the subsystems, and greatly improving the utilization efficiency of the common DC bus energy storage system.

Further, the initial values of the upper limit value and the lower limit value are 800V and 750V respectively;

The first charging value and the first discharging value are 95% and 20% of the SOC of the electric cabinet, respectively;

The first overvoltage threshold and the first undervoltage threshold are 805V and 745V respectively.

From the above description, it can be seen that the first charging value is 95% of the SOC of the electric cabinet, that is, 5% of the SOC is reserved to prevent overcharging of the electric cabinet. At the same time, the first discharging value is 20% of the SOC of the electric cabinet. This is because due to the small current in the DC bus, energy is flowing, and the electric cabinet will have a small current charging and discharging situation. Therefore, a part of the power should be reserved to absorb the small current to avoid overcharging and over-discharging of the electric cabinet. All subsystems that have not reached this value subsequently can achieve the same charging and discharging until the highest and lowest SOC values. At the same time, the first overvoltage threshold is slightly higher than the initial upper limit value of the DC bus, and the first undervoltage threshold is slightly lower than the initial lower limit value of the DC bus to avoid switching back and forth between charging-not charging and discharging-not discharging.

Furthermore, the step S2 further includes:

If the charge/discharge voltage of the single cell of the electric cabinet reaches the first charge voltage/first discharge voltage, the operation of the first charge/discharge electronic system is stopped.

Furthermore, the first charging voltage and the first discharging voltage are 3.5V and 3.1V respectively.

From the above description, it can be seen that the timing of first stopping the charging/discharging electronic system can be further improved by judging whether the voltage of the single battery cell in each subsystem electrical cabinet reaches the first charging voltage of 3.5V/the first discharging voltage of 3.1V, so as to ensure the timely setting of the first overvoltage threshold/the first undervoltage threshold.

Furthermore, the step S5 is specifically as follows:

When the energy storage system is in a charging state, the upper limit value is changed from an initial value to the first overvoltage threshold value. Subsequently, when the energy storage system is changed from a charging state to a discharging state, when each subsystem is discharged simultaneously until the SOC of the electric cabinet in each subsystem reaches a second charging value, the first overvoltage threshold value is reset to the initial value of the upper limit value.

When the energy storage system is in a discharging state, the lower limit value is changed from an initial value to the first undervoltage threshold value. Subsequently, when the energy storage system is changed from a discharging state to a charging state, when each subsystem is charged simultaneously until the SOC of the electric cabinet in each subsystem reaches a second discharging value, the first undervoltage threshold value is reset to the initial value of the lower limit value.

Furthermore, the second charging value and the second discharging value are 90% and 30% of the SOC of the power cabinet, respectively.

From the above description, it can be seen that when the overvoltage threshold or undervoltage threshold of the DC/DC of the subsystem is set to the first overvoltage threshold or the first undervoltage threshold, the sampling voltage of the DC bus will adapt to the working state of the energy storage system and the DC/DC setting value of each subsystem, that is, the upper limit value = overvoltage threshold, the lower limit value = undervoltage threshold, but at this time, the SOC of the cabinets of each subsystem has reached 95% in the charging state and is about to be fully charged. In order to form a closed-loop strategy, the SOC of each cabinet is discharged to 90% and then the overvoltage threshold of the DC/DC of each subsystem is set as the initial upper limit value of the DC bus. Then the upper limit value of the DC bus will also be restored to the initial value of 800V, that is, the system is restored. The state during charging can avoid the DC bus sampling voltage getting higher and higher after the subsequent outgoing line executes the waiting strategy multiple times until it exceeds the extreme overvoltage threshold and causes the entire system to shut down; similarly, the SOC of the cabinets of each subsystem has reached 20% in the discharge state. In order to form a strategy closed loop, the SOC of each cabinet is charged to 25% and then the undervoltage threshold of the DC/DC of each subsystem is set as the initial lower limit of the DC bus. The lower limit of the DC bus will also be restored to the initial value of 750V, that is, the state of the system just discharged is restored, avoiding the DC bus sampling voltage getting lower and lower until it is lower than the extreme undervoltage threshold after the subsequent outgoing line executes the waiting strategy multiple times, causing the entire system to shut down.

Furthermore, the step S5 further includes:

When the energy storage system is in a charging state, the upper limit value is changed from an initial value to the first overvoltage threshold value. Subsequently, when the energy storage system is changed from a charging state to a discharging state, when each subsystem is discharged at the same time until the single cells of the electric cabinets in each subsystem are all charged to a second charging voltage, the first overvoltage threshold value is reset to the initial value of the upper limit value.

When the energy storage system is in a charging state, the lower limit value is changed from an initial value to the first undervoltage threshold value. Subsequently, when the energy storage system is changed from a discharging state to a charging state, when each subsystem is charged simultaneously until the single cells of the electrical cabinet in each subsystem are charged to a second discharge voltage, the first undervoltage threshold value is reset to the initial value of the lower limit value.

Furthermore, the second charging voltage and the second discharging voltage are 3.45V and 3.25V respectively.

From the above description, it can be seen that, similarly, in the charging state, when the charging voltage of the single battery cells of each subsystem cabinet reaches the first charging voltage of 3.5V, in order to form a closed-loop strategy, the single battery cells of each cabinet are discharged to the second charging voltage of 3.45V, and then the overvoltage threshold of each subsystem DC/DC is set to the initial upper limit value of the DC bus. Then, the upper limit value of the DC bus will also be restored to the initial value of 800V, that is, the state when the system is just charged is restored, avoiding the DC bus sampling voltage getting higher and higher after the subsequent outgoing line executes the waiting strategy multiple times until it exceeds the limit overvoltage threshold, causing the entire The system shuts down; and in the discharge state, when the single cells of each subsystem cabinet are discharged to a voltage reaching the first discharge voltage of 3.1V, in order to form a closed strategy loop, the single cells of each cabinet are charged to a voltage of the second discharge voltage of 3.25V, and then the undervoltage threshold of each subsystem DC/DC is set to the initial lower limit of the DC bus. The lower limit of the DC bus will also be restored to the initial value of 750V, that is, the state of the system just discharged is restored, avoiding the DC bus sampling voltage getting lower and lower until it falls below the limit undervoltage threshold after the subsequent outgoing line executes the waiting strategy multiple times, causing the entire system to shut down.

Furthermore, the step S1 also includes:

Obtain the SOC, short-term unbalanced power, subsystem periodic load or sampling error of each subsystem;

The steps S1 and S2 also include:

According to the SOC difference of each subsystem, the short-term unbalanced power, the subsystem periodic load or the sampling error, it is judged whether the imbalance degree between the subsystems reaches the upper limit condition. If so, it is judged that there is a sampling error in each subsystem, and step S2 is executed. Otherwise, the DC/DC of each subsystem uses the initial values of the upper limit value and the lower limit value as the overvoltage threshold and undervoltage threshold to perform charging/discharging.

From the above description, it can be seen that there are bound to be differences between subsystems. These differences are not only the differences in the sampling voltage of the DC/DC of each subsystem to the common DC bus, but may also be due to differences in the SOC of each subsystem itself and differences caused by short-term unbalanced electricity and subsystem periodic load. The SOC difference, short-term unbalanced electricity, subsystem periodic load or sampling error are used to comprehensively confirm the final imbalance degree between the subsystems, thereby improving the accuracy of judging the imbalance between the subsystems.

Please refer to Figure 5, a SOC balancing terminal of a common DC bus energy storage system includes a memory, a processor, and a computer program stored in the memory and executable on the processor. When executing the computer program, the processor implements the steps in the SOC balancing method of a common DC bus energy storage system as described above.

From the above description, it can be seen that the beneficial effects of the present invention are: based on the same technical concept, in conjunction with the above-mentioned SOC balancing method of a common DC bus energy storage system, a SOC balancing terminal of a common DC bus energy storage system is provided. When there is an imbalance in the subsystems in the common DC bus energy storage system (for example, the DC bus sampling error and the first charging/discharging causing the SOC imbalance between each other), the overvoltage threshold/undervoltage threshold of the subsystem that is first charged/discharged to the first charging value/first discharging value is adjusted in sequence to the first overvoltage threshold/first undervoltage threshold, so that the initial upper/lower limit value of the DC bus is less than/greater than the overvoltage threshold/undervoltage threshold of the subsystem DC/DC that is first charged/discharged, and is within the normal range. According to the DC/ In the DC droop strategy, the subsystem that is charged/discharged first will be in a state of neither charging nor discharging, thereby stopping power output, while the energy storage system needs to continue charging/discharging, and the charging/discharging power will continue to be borne by the remaining subsystems, that is, the remaining subsystems continue to maintain the output upper/lower limit values of the DC bus, until the overvoltage threshold/undervoltage threshold of the DC/DC in each subsystem reaches the first overvoltage threshold/first undervoltage threshold, the upper/lower limit value of the DC bus is changed from the initial value to the first overvoltage threshold/first undervoltage threshold, and the subsequent DC bus voltage can be evenly maintained by each subsystem, that is, the same charging and discharging is achieved, thereby avoiding the increasing imbalance of SOC between the subsystems and greatly improving the utilization efficiency of the common DC bus energy storage system.

The present invention provides a SOC balancing method and terminal for a common DC bus energy storage system, which is suitable for a situation where there is an error in the sampling voltage of the DC/DC to the DC bus in each subsystem of the common DC bus energy storage system, resulting in SOC imbalance between the subsystems and the inability to charge and discharge at the same time. The method and terminal are described in detail in conjunction with the embodiments below.

Please refer to Figures 1 to 3, the first embodiment of the present invention is:

A SOC balancing method for a common DC bus energy storage system, in this embodiment, as shown in FIG1, taking a common DC bus energy storage system composed of four energy storage subsystems A, B, C, and D as an example, each subsystem is composed of 1 electric cabinet, 1 DC/DC, 1 AC/DC (i.e., energy storage converter PCS, also called inverter) and 1 EMS system, and each EMS system is independent of each other and does not communicate with each other. Each subsystem is connected in parallel by sharing a DC bus, and the DC bus of the entire energy storage system is sampled by the DC/DC of each subsystem, and the voltage of the DC bus is established and maintained by the DC/DC of each subsystem. Subsystems A, B, C, and D can independently transmit power to the DC bus, and then output it to their respective loads via AC/DC to the power grid, and the energy scheduling of the entire process is controlled by the EMS system.

Before that, the working principle of DC/DC is explained as follows:

The side of the energy storage subsystem connected to the battery has a certain voltage operating range, matching the battery voltage, and the voltage sampling is located at the connection point between the battery and the DC/DC. Within this voltage range, the DC/DC can charge or discharge the battery at a certain power, and the power and current can be adjusted for the battery to work in constant power or constant current mode.

The side connected to the DC-BUS also has voltage sampling. The control goal is to adjust the DC bus within an allowable voltage range, which can be understood as voltage stabilization, but the voltage stabilization accuracy may not be very high, for example, 740V~800V, which is determined by the load's tolerance and the allowable fluctuation of the DC bus. In this range, the droop method is used for control, which is an automatic feedback function. When the sampling voltage of the DC bus is lower than 750V, the battery replenishes energy to the bus. When it is higher than 750V, energy is extracted from the bus to the battery. This power has a proportional coefficient. The greater the deviation from the bus voltage stabilization point, the higher the power. When the DC/DC collects the bus voltage to, for example, below 745V, it starts full-power discharge to reach the maximum value of regulation, and vice versa during charging.

Take discharge as an example: when a single module (battery + DC/DC) forms a DC bus, the load and DC/DC are relatively close, and the load fluctuation is directly reflected at the DC/DC bus voltage sampling point. The adjustment is relatively rapid, and the DC bus can be controlled to a relatively small fluctuation. When multiple modules form a DC bus, the source and load are distributed in different positions, and each load has uncertainty in operation. When a load closer to a module is discharged, the bus voltage sampling of this module is lower than that of other farther modules, and the discharge power is greater than that of other farther modules.

When the voltage sampling has errors, considering the extreme conditions, the sampling of the modules closer to each other is also low, and their discharge power is relatively the largest. The original plan is to use the dynamic adjustment of the voltage lower limit threshold to stop the full power discharge of the module with too low SOC, but at this time it is necessary to determine whether the module stops working completely or enters the droop control range.

The reason for the SOC imbalance between subsystems in the common DC bus energy storage system is that the DC/DC of a certain module continuously collects a bus voltage below the discharge threshold. Since the position and power of the load, as well as the voltage sampling errors of each DC/DC, including module hardware differences, are all random, a self-correcting negative feedback mechanism needs to be introduced. The goal is to keep all modules working simultaneously and the degree of imbalance within a certain range, which is not affected by the above random factors.

In this embodiment, as shown in FIG3 , the DC/DC establishes a DC bus voltage U _x . The DC/DC has two parameters, namely, an undervoltage threshold V _Uvr and an overvoltage threshold V _Our . These two parameters determine the upper and lower limits of the DC bus voltage, which is equivalent to V _Uvr being the lower limit of the DC bus and V _Our being the upper limit of the DC bus. In addition, the DC/DC also includes an undervoltage limit value V _Ulimit and an overvoltage limit value U _Qlimit , wherein the undervoltage limit value V _Ulimit is less than the undervoltage threshold V _Uvr , and the overvoltage limit value U _Qlimit is greater than the overvoltage threshold V _Our . When the voltage of the DC bus acquired by the DC/DC of a certain subsystem is less than the undervoltage limit value V _Ulimit of its own DC/DC or greater than the overvoltage limit value U _Qlimit of its own DC/DC, the DC/DC will shut down for protection, that is, the subsystem will stop charging or discharging, and may even cause the entire energy storage system to stop charging or discharging.

When the DC/DC is maintaining the DC bus voltage U _x , after the DC/DC establishes the bus voltage U _x , it maintains the DC bus voltage U _x through droop control. The so-called droop control can be understood as the DC/DC sampling the DC bus voltage U _x . If the DC bus voltage U _x is lower than 750V, the DC/DC will automatically draw power from the cabinet, discharge the cabinet, and maintain the DC bus voltage U x at 750V; if the DC bus voltage is higher than 800V, the DC/DC will automatically transmit power to the cabinet, charge the cabinet, and keep the DC bus voltage lower than 800V. When there is no energy input or output in the DC bus, the DC bus voltage will be maintained at V _Uvr , that is, 750V. Since there is no energy input to the DC bus, after the DC/DC establishes a DC bus voltage of 750V, the DC bus voltage will decrease. At this time, the DC/DC uses droop control to discharge the cabinet and maintain the DC bus voltage at 750V. The cabinet stops discharging, and then the DC bus voltage decreases again, and the cabinet discharges again, and the cycle repeats. Since this process is dynamic, the external appearance is that the cabinet continuously discharges the DC to maintain the DC bus. The other energy storage subsystems maintain the DC bus voltage in the same way as energy storage subsystem A.

The charging/discharging process of the common DC bus energy storage system is as follows:

Discharge process: Taking energy storage subsystem A as an example, PCS outputs 50kw of power to the grid. At this time, the DC bus voltage will be pulled down to below 750V. After the DC/DC detects that the DC bus voltage is lower than V _Uvr , it will draw 50kw of power from the switch cabinet (the switch cabinet discharge power must be consistent with the PCS discharge power to achieve power balance. If the switch cabinet discharge power is small, the DC bus voltage is lower than 750V, and if the switch cabinet discharge power is large, the DC bus voltage is greater than 800V). The switch cabinet discharges and maintains the DC bus voltage at 750V.

Charging process: Taking energy storage subsystem A as an example, PCS draws 50kw of power from the grid and transmits it to the DC bus. At this time, the DC bus voltage will increase and be higher than 800V. After the DC/DC detects that the DC bus voltage is higher than V _Our , it inputs 50kw of power into the switch cabinet (the charging power of the switch cabinet must be consistent with the charging power of the PCS to achieve power balance. If the charging power of the switch cabinet is small, the DC bus voltage will be higher than 800V. If the charging power of the switch cabinet is large, the DC bus voltage will be less than 750V). The switch cabinet is charged and the DC bus voltage is maintained at 800V.

The charging and discharging process of other energy storage subsystems is consistent with that of energy storage subsystem A. Therefore, if only the PCS of subsystem A is discharged or charged, the DC/DC of the four energy storage subsystems will be charged or discharged, because the DC buses of the four energy storage subsystems are connected in parallel, the changes in the DC bus voltage are consistent, and the power will be evenly distributed to the cabinets of the four energy storage subsystems. When multiple PCSs are charged or discharged, the power is still evenly distributed to the cabinets of the four energy storage subsystems. Compared with the charging and discharging of a single PCS, the power is just evenly distributed.

Under normal circumstances, when the common DC bus energy storage system of the architecture of Figure 1 is charged and discharged, the four energy storage subsystems are charged and discharged at the same time. However, due to the long distribution distance of the four energy storage subsystems, the length of the DC bus is several kilometers or tens of kilometers, which has the influence of voltage drop; and the sampling voltage of the DC bus by the four energy storage subsystems DC/DC is not consistent, and there will be errors. Due to these two reasons, the sampling results of the four DC/DCs for the same DC bus voltage are inconsistent. Assuming that the lower limit value V _Uvr of the DC bus is 750V and the upper limit value V _Uvr is 800V, it may happen that the energy storage subsystem A samples the DC bus voltage U _xA of 749V, the subsystem B samples the DC bus voltage U _xB of 750V, the subsystem C samples the DC bus voltage U _xC of 751V, and the subsystem D samples the DC bus voltage U _xD of 752V. This situation will cause the DC bus voltage of energy storage subsystem A to always be lower than V _Uvr . When the shared DC bus energy storage system is discharged, the cabinet of subsystem A will be discharged first, and the other three subsystems will not work. The cabinet of subsystem A will be emptied, and the entire shared DC bus energy storage system will stop running. Similarly, when charging, a certain subsystem will be charged first, and the shared DC bus energy storage system will stop charging after the cabinet is fully charged. The charging and discharging of a certain energy storage subsystem first causes the charging and discharging efficiency of the entire shared DC bus energy storage system to be low. In the long run, the balance between the sub-energy storage systems of the shared DC bus energy storage system will become worse and worse.

In addition, the difference in short-term unbalanced electricity of the common DC bus energy storage system and the difference in subsystem periodic load will also cause sampling differences between subsystems. Short-term unbalanced electricity can filter the power imbalance caused by transient load fluctuations, and can also reflect the actual situation such as voltage sampling error, hardware inconsistency between subsystems, position difference and imbalance caused by lead impedance. The imbalance can be a function of SOC, charging and discharging power, energy storage subsystem capacity and time. For example: the power integral Wn of subsystem x in a judgment cycle (such as 2 minutes); and the subsystem periodic load is the integral of the subsystem discharge rate in the judgment cycle, which is used to represent the cumulative heat generation Qn of the subsystem. This parameter allows batteries of different specifications to operate in parallel and participate in charging and discharging according to their own load capacity, so that it is not limited to subsystems with the same power.

In view of the problem that the energy storage system cannot be charged and discharged simultaneously due to the SOC imbalance between the subsystems, this embodiment provides a SOC balancing method for a common DC bus energy storage system, as shown in FIG2 , including the steps of:

S1. Initialize the upper and lower limits of the voltage sampling value of the DC bus in the energy storage system, and obtain the working status of the energy storage system and the DC bus voltage sampling values of each subsystem in real time;

S2. Determine the first charging/discharging electronic system according to the voltage sampling value, and collect the SOC of the electric cabinet in the first charging/discharging electronic system in real time. If the SOC of the electric cabinet is charged/discharged to the first charging value/first discharging value, stop the operation of the first charging/discharging electronic system;

S3, setting the overvoltage threshold/undervoltage threshold of the DC/DC in the first charging/discharging electronic system to a first overvoltage threshold/first undervoltage threshold, the first overvoltage threshold being higher than an upper limit, and the first undervoltage threshold being lower than a lower limit;

S4, repeating steps S2-S3 for the remaining subsystems until the overvoltage threshold/undervoltage threshold of the DC/DC in each subsystem is set to the first overvoltage threshold/first undervoltage threshold;

S5. The upper limit value and the lower limit value are changed from the initial values to the first overvoltage threshold value and the first undervoltage threshold value, and each subsystem is charged and discharged at the same time.

That is, in this embodiment, when there is an imbalance among the subsystems in the common DC bus energy storage system (for example, the DC bus sampling error and the first charge/discharge result in the SOC imbalance between each other), by sequentially adjusting the overvoltage threshold/undervoltage threshold of the subsystem that is first charged/discharged to the first charge value/first discharge value to the first overvoltage threshold/first undervoltage threshold, the initial upper/lower limit value of the DC bus is less than/greater than the overvoltage threshold/undervoltage threshold of the first charged/discharged electronic system DC/DC, which is within the normal range. According to the DC/DC droop strategy, the subsystem that is first charged/discharged will be in a state of neither charging nor discharging, thereby stopping the power. rate output, and the energy storage system needs to continue charging/discharging, and the charging/discharging power is continued to be borne by the remaining subsystems, that is, the remaining subsystems continue to maintain the output upper/lower limit values of the DC bus, until the overvoltage threshold/undervoltage threshold of the DC/DC in each subsystem reaches the first overvoltage threshold/first undervoltage threshold, the upper/lower limit value of the DC bus is changed from the initial value to the first overvoltage threshold/first undervoltage threshold, and the subsequent DC bus voltage can be evenly maintained by each subsystem, that is, the same charging and discharging is achieved, thereby avoiding the increasing imbalance of SOC between the subsystems, and greatly improving the utilization efficiency of the common DC bus energy storage system.

As mentioned above, the imbalance of the DC/DC of each subsystem to the DC bus may be caused by various reasons. Therefore, in this embodiment, step S1 also includes:

Obtain the SOC, short-term unbalanced power, subsystem periodic load or sampling error of each subsystem;

The steps between step S1 and step S2 also include:

Whether the degree of imbalance between the subsystems reaches the upper limit condition is determined according to the SOC difference, short-term unbalanced power, subsystem periodic load or sampling error of each subsystem. If so, step S2 is executed. Otherwise, the DC/DC of each subsystem uses the initial values of the upper limit and lower limit as the overvoltage threshold and undervoltage threshold to perform charging/discharging.

That is, as shown in FIG4 , the degree of imbalance among the final subsystems is comprehensively confirmed by combining SOC difference, short-term unbalanced power, subsystem cycle load or sampling error, for example, whether the short-term unbalanced power (Wnmax-Wnmin)/Wnavg is greater than 20%, whether the subsystem cycle load Qn is greater than 1, the SOC difference is greater than 20%, and the sampling error is greater than 0.5%, thereby improving the accuracy of judging the imbalance among the subsystems.

Please refer to FIG. 4 , the second embodiment of the present invention is:

A SOC balancing method for a common DC bus energy storage system, based on the above-mentioned embodiment 1, in this embodiment, the initial values of the upper limit value and the lower limit value are 800V and 750V respectively; the first charging value and the first discharging value are 95% and 20% of the SOC of the electric cabinet respectively; the first overvoltage threshold and the first undervoltage threshold are 805V and 745V respectively. In other embodiments, the first overvoltage threshold and the first undervoltage threshold can be set as needed, for example, the upper limit value can be increased by 0.2% as the first overvoltage threshold, and the lower limit value can be decreased by 0.2% as the first undervoltage threshold.

That is, in this embodiment, the first charging value is set to 95% of the SOC of the power cabinet, that is, 5% of the SOC is reserved to prevent overcharging of the power cabinet. At the same time, the first discharging value is 20% of the SOC of the power cabinet. This is because due to the small current in the DC bus, energy is flowing, and the power cabinet will be charged and discharged with a small current. Therefore, a part of the power should be reserved to absorb the small current to avoid overcharging and over-discharging of the power cabinet, and all subsystems that have not subsequently reached this value can achieve the same charging and discharging until the highest and lowest SOC values. At the same time, the first overvoltage threshold is slightly higher than the initial upper limit value of the DC bus, and the first undervoltage threshold is slightly lower than the initial lower limit value of the DC bus, so as to avoid switching back and forth between charging-not charging and discharging-not discharging.

Wherein, step S2 also includes:

If the charge/discharge voltage of the single cell of the electric cabinet reaches the first charge voltage/first discharge voltage, the first charge/discharge electronic system is stopped, wherein the first charge voltage and the first discharge voltage are 3.5V and 3.1V respectively.

That is, in this embodiment, the timing of first stopping the charging/discharging electronic system can be further improved by judging whether the voltage of the single battery cell in each subsystem electrical cabinet reaches the first charging voltage of 3.5V/the first discharging voltage of 3.1V, so as to ensure the timely setting of the first overvoltage threshold/the first undervoltage threshold.

Meanwhile, in this embodiment, step S5 is specifically as follows:

When the energy storage system is in a charging state, the upper limit value changes from the initial value to the first overvoltage threshold value. Subsequently, when the energy storage system changes from a charging state to a discharging state, when each subsystem is discharged simultaneously until the SOC of the electric cabinets in each subsystem reaches a second charging value, the first overvoltage threshold value is reset to the initial value of the upper limit value.

When the energy storage system is in a discharging state, the lower limit value changes from the initial value to the first undervoltage threshold. Subsequently, when the energy storage system changes from a discharging state to a charging state, when each subsystem is charged simultaneously until the SOC of the power cabinet in each subsystem reaches the second discharging value, the first undervoltage threshold is reset to the initial value of the lower limit.

The second charging value and the second discharging value are respectively 90% and 30% of the SOC of the electric cabinet.

Similarly, step S5 may also be:

When the energy storage system is in a charging state, the upper limit value changes from the initial value to the first overvoltage threshold value. Subsequently, when the energy storage system changes from a charging state to a discharging state, when each subsystem is discharged simultaneously until the single cells of the electric cabinets in each subsystem are all charged to the second charging voltage, the first overvoltage threshold value is reset to the initial value of the upper limit value.

When the energy storage system is in a charging state, the lower limit value changes from the initial value to the first undervoltage threshold. Subsequently, when the energy storage system changes from a discharging state to a charging state, when each subsystem is charged simultaneously until the single cells of the electrical cabinets in each subsystem are charged to the second discharge voltage, the first undervoltage threshold is reset to the initial value of the lower limit.

The second charging voltage and the second discharging voltage are 3.45V and 3.25V respectively.

That is, in this embodiment, in the charging state, when the charging voltage of the single cells of each subsystem cabinet reaches the first charging voltage of 3.5V, in order to form a closed-loop strategy, the single cells of each cabinet are discharged to the second charging voltage of 3.45V, and then the overvoltage threshold of each subsystem DC/DC is set to the initial upper limit value of the DC bus. Then, the upper limit value of the DC bus will also be restored to the initial value of 800V, that is, the state of the system just charged is restored, so as to avoid the DC bus sampling voltage getting higher and higher until it exceeds the limit overvoltage threshold after the subsequent outgoing line executes the waiting strategy multiple times, causing the entire system Shut down; in the discharge state, when the single cells of each subsystem cabinet are discharged to the first discharge voltage of 3.1V, in order to form a strategy closed loop, the single cells of each cabinet are charged to the second discharge voltage of 3.25V, and then the undervoltage threshold of each subsystem DC/DC is set to the initial lower limit of the DC bus. The lower limit of the DC bus will also be restored to the initial value of 750V, that is, the state of the system just discharged is restored, avoiding the DC bus sampling voltage getting lower and lower until it is lower than the limit undervoltage threshold after the subsequent outgoing line executes the waiting strategy multiple times, causing the entire system to shut down.

Specifically, in this embodiment, during the discharge process of the energy storage system, due to the problem of line sampling error, only subsystem A in the energy storage system of Figure 1 is discharging, and the other three subsystems B, C, and D are neither charged nor discharged. Subsystem A is always discharging until the SOC of subsystem A is ≤20% or the voltage of the single cell is ≤3.1V. Subsystem A sets its DC/DC undervoltage threshold V _Uvr to the first undervoltage threshold 745V. At this time, the voltage of the DC bus is maintained at the initial value of 750V by subsystems B, C, and D, which means that the DC bus voltage U _x is greater than the V _Uvr (745V) of subsystem A, that is, it belongs to the normal range. According to the DC/DC droop strategy, the electric cabinet of subsystem A will be in a state of neither charging nor discharging, so subsystem A stops power output, while the common DC bus energy storage system continues to discharge to the outside, and then the discharge power is borne by subsystems B, C, and D. There are two situations at this time:

Case 1: The power discharged by the energy storage system is evenly divided among subsystems B, C and D until the SOC of the three subsystems is ≤20% or the voltage of the single cell is ≤3.1V, at which point the discharge stops.

Case 2: In subsystems B, C and D, subsystem B is also discharged due to the DC/DC sampling error, and subsystems C and D do not operate. Similarly, subsystem B implements the same strategy as subsystem A. When subsystem B is discharged to SOC≤20% or the single cell voltage≤3.1V, the DC/DC undervoltage threshold V _Uvr of subsystem B is also set to the first undervoltage threshold 745V. In this way, subsystem B is also in a state of neither charging nor discharging due to the DC/DC droop strategy. The remaining subsystems C and D share the discharge power of the energy storage system or still implement the corresponding separate discharge strategy like subsystem A or B, until each subsystem sets V _Uvr to the first undervoltage threshold 745V. At this time, the voltage of the DC bus needs to be equal to that of each subsystem, that is, it will change from 750V to 745V. By implementing this strategy, it is possible to avoid the problem of simultaneous charging and discharging of subsystems due to DC/DC sampling errors or voltage drops, as well as premature discharge of a single subsystem causing the common DC bus energy storage system to stop discharging. When the SOC of the four subsystems is recharged from ≤20% or the voltage of the single cell is recharged from ≤3.1V to SOC≥30% or the voltage of the single cell is ≥3.25V, the V _Uvr of each subsystem is reset to the initial undervoltage threshold of 750V, that is, the initial state when the system is just started to discharge is restored, forming a closed loop of the strategy. If the V _Uvr is not restored to the initial value of 750V, the DC bus voltage will drop lower and lower after multiple executions of the waiting strategy, until it exceeds the limit undervoltage value V _Ulimit , causing the entire energy storage system to shut down.

Similarly, the energy storage system executes the corresponding strategy during the charging process as in the above-mentioned discharging process. When the overvoltage thresholds of the DC/DC subsystems are all set to the first overvoltage threshold, the sampling voltage of the DC bus will adapt to the working state of the energy storage system and the DC/DC setting values of each subsystem, that is, the upper limit value = overvoltage threshold. In order to form a closed strategy loop, the SOC of each cabinet is discharged to ≤90% or the voltage of the single cell is ≤3.45V, and then the first overvoltage threshold V _Our of the DC/DC of each subsystem is set from 805V back to the initial upper limit value of 800V of the DC bus. The upper limit value of the DC bus will also be restored to the initial value of 800V, that is, the state when the system is just started for charging is restored, that is, the problem of the DC bus sampling voltage becoming higher and higher after the subsequent outgoing line executes the waiting strategy multiple times until it exceeds the limit overvoltage threshold and causes the entire system to shut down is avoided.

It is worth noting that the 20%, 30%, 90% and 95% SOC set in each subsystem and the voltage values of the single cell 3.1V, 3.25V, 3.45 and 3.5V, etc., can be equivalent to the waiting value of the entire strategy, which can be set according to the actual situation. That is, a subsystem discharges first, and after reaching the waiting value, the waiting strategy is executed, and the overvoltage threshold or undervoltage threshold is set. During the waiting strategy period, no discharge or charging is performed. When all subsystems have reached this waiting value, they can continue to discharge or charge to the protection value, that is, ultimately achieve the same charging and discharging among the subsystems, which can be understood as asynchronous charging and discharging.

Please refer to FIG. 5 , the third embodiment of the present invention is:

A SOC balancing terminal 10 for a common DC bus energy storage system includes a memory 20, a processor 30, and a computer program stored in the memory 20 and executable by the processor 30. When executing the computer program, the processor 30 implements the steps in a SOC balancing method for a common DC bus energy storage system in the above-mentioned embodiment 1 or embodiment 2.

In summary, the SOC balancing method of a common DC bus energy storage system provided by the present invention has the following beneficial effects:

1. Solved the problem that the subsystems in the common DC bus energy storage system cannot be charged and discharged at the same time due to DC/DC sampling errors and voltage drops caused by the DC bus being too long;

2. It avoids the increasing imbalance of SOC between subsystems and greatly improves the utilization efficiency of the common DC bus energy storage system.

The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent transformations made using the contents of the present invention's specification and drawings, or directly or indirectly applied in related technical fields, are also included in the patent protection scope of the present invention.

Claims (9)

1. A SOC balancing method for a common DC bus energy storage system, characterized by comprising the steps of: S1. Initialize the upper and lower limits of the voltage sampling value of the DC bus in the energy storage system, and obtain the working status of the energy storage system and the DC bus voltage sampling values of each subsystem in real time; S2. Determine the first charging/discharging electronic system according to the voltage sampling value, and collect the SOC of the electric cabinet in the first charging/discharging electronic system in real time. If the SOC of the electric cabinet is charged/discharged to a first charging value/first discharging value, stop the operation of the first charging/discharging electronic system; S3, setting the overvoltage threshold/undervoltage threshold of the DC/DC in the first charging/discharging electronic system to a first overvoltage threshold/first undervoltage threshold, wherein the first overvoltage threshold is higher than the upper limit, and the first undervoltage threshold is lower than the lower limit; S4, repeating steps S2-S3 for the remaining subsystems until the overvoltage threshold/undervoltage threshold of the DC/DC in each subsystem is set to the first overvoltage threshold/first undervoltage threshold; S5, the upper limit value and the lower limit value are changed from the initial values to the first overvoltage threshold value and the first undervoltage threshold value, and each subsystem is charged and discharged at the same time; The step S5 is specifically as follows: When the energy storage system is in a charging state, the upper limit value is changed from the initial value to the first overvoltage threshold value, and subsequently when the energy storage system is changed from a charging state to a discharging state, when each subsystem is discharged simultaneously until the SOC of the electric cabinet in each subsystem reaches a second charging value, the upper limit value is reset to the initial value of the upper limit value; When the energy storage system is in a discharging state, the lower limit value is changed from an initial value to the first undervoltage threshold. Subsequently, when the energy storage system is changed from a discharging state to a charging state, when each subsystem is charged simultaneously until the SOC of the electric cabinet in each subsystem reaches a second discharging value, the lower limit value is reset to the initial value of the lower limit value. 2. A SOC balancing method for a common DC bus energy storage system according to claim 1, characterized in that the initial values of the upper limit value and the lower limit value are 800V and 750V respectively; The first charging value and the first discharging value are 95% and 20% of the SOC of the electric cabinet, respectively; The first overvoltage threshold and the first undervoltage threshold are 805V and 745V respectively. 3. The SOC balancing method of a common DC bus energy storage system according to claim 1, characterized in that step S2 further comprises: If the charge/discharge voltage of the single battery cell of the electric cabinet reaches the first charge voltage/first discharge voltage, the operation of the first charge/discharge electronic system is stopped. 4. The SOC balancing method of a common DC bus energy storage system according to claim 3, characterized in that the first charging voltage and the first discharging voltage are 3.5V and 3.1V respectively. 5. The SOC balancing method of a common DC bus energy storage system according to claim 1, characterized in that the second charging value and the second discharging value are 90% and 30% of the SOC of the electric cabinet, respectively. 6. The SOC balancing method of a common DC bus energy storage system according to claim 1, characterized in that step S5 further comprises: When the energy storage system is in a charging state, the upper limit value is changed from the initial value to the first overvoltage threshold value. Subsequently, when the energy storage system is changed from a charging state to a discharging state, when each subsystem is discharged at the same time until the single cells of the electric cabinets in each subsystem are all charged to the second charging voltage, the upper limit value is reset to the initial value of the upper limit value; When the energy storage system is in a discharging state, the lower limit value is changed from an initial value to the first undervoltage threshold. Subsequently, when the energy storage system is changed from a discharging state to a charging state, and when each subsystem is charged simultaneously until the single cells of the electrical cabinet in each subsystem are charged to a second discharging voltage, the lower limit value is reset to the initial value of the lower limit value. 7. A SOC balancing method for a common DC bus energy storage system according to claim 6, characterized in that the second charging voltage and the second discharging voltage are 3.45V and 3.25V respectively. 8. The SOC balancing method of a common DC bus energy storage system according to claim 1, characterized in that the step S1 further comprises: Obtain the SOC, short-term unbalanced power, subsystem periodic load or sampling error of each subsystem; The steps S1 and S2 also include: According to the SOC difference of each subsystem, the short-term unbalanced power, the subsystem periodic load or the sampling error, it is judged whether the imbalance degree between the subsystems reaches the upper limit condition. If so, it is judged that there is a sampling error in each subsystem, and step S2 is executed. Otherwise, the DC/DC of each subsystem uses the initial values of the upper limit value and the lower limit value as the overvoltage threshold and the undervoltage threshold to perform charging/discharging. 9. A SOC balancing terminal for a common DC bus energy storage system, characterized in that it comprises a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the steps in a SOC balancing method for a common DC bus energy storage system as described in any one of claims 1 to 8 when executing the computer program.