Mesh construction type battery energy storage system considering battery charge state

Abstract

The invention relates to a network construction type battery energy storage system considering the state of charge of a battery, and is suitable for the field of energy storage system network construction control. The system comprises an energy storage battery, wherein the energy storage battery is connected with a power grid through a DC / DC converter, a capacitor and an inverter in sequence; the inverter adopts matching control, the matching control generates a reference value of inverter side angle frequency based on the direct-current bus voltage, an inertia simulation coefficient and a damping coefficient are introduced to respectively simulate system inertia and damping, and a direct-current voltage tracking coefficient is introduced to enable the direct-current bus voltage to track a voltage reference value; the DC / DC converter applies SOC-frequency control, the SOC-frequency control comprises active power outer loop control and current inner loop control, the active power outer loop control determines the SOC reference slope based on the inverter output frequency, determines the battery output power reference value in combination with the energy storage battery SOC, and then determines the battery output current reference value, and the active power outer loop control determines the SOC reference slope based on the inverter output frequency and the battery output current reference value in combination with the energy storage battery SOC. And the reference input is used for current inner loop control.

Description

Technical Field

[0001] This invention relates to a grid-type battery energy storage system that takes into account the state of charge of the battery. It is applicable to the field of grid-based control of energy storage systems. Background Technology

[0002] With the rapid development of inverter-interface renewable energy sources (RES) such as wind and solar power, the inertia of power systems has significantly decreased, and frequency events have become more frequent, posing a severe challenge to grid stability. To address this issue, grid-forming (GFM) control has emerged as a promising solution. GFM control provides a certain amount of inertia by simulating the voltage and frequency support capabilities of synchronous generators, thereby improving the dynamic performance of low-inertia systems.

[0003] Among various power generation technologies, Battery Energy Storage Systems (BESS) are particularly suitable as power sources for GFM control systems due to their advantages such as fast response, bidirectional power flow, and high controllability. Compared to intermittent and unpredictable renewable energy sources such as wind and solar power, BESS can provide dispatchable and stable power output, while actively regulating active and reactive power to achieve key functions such as black start, frequency regulation, and fault recovery.

[0004] In GFM-controlled BESS applications, the battery state of charge (SOC) is not considered, making it difficult to fully utilize the energy of the energy storage system. In practical applications, it is necessary to fully consider the monitoring and management of the battery SOC to ensure continuous operation, especially during long system recovery processes. Summary of the Invention

[0005] The technical problem this invention aims to solve is to provide a grid-type battery energy storage system that considers the battery's state of charge (SOC) in light of the aforementioned issues. By treating SOC as a dynamic variable, a droop-like relationship is established between the battery's output power and the system frequency. The system autonomously adjusts its power output based on real-time frequency deviations, thereby achieving better frequency support and energy utilization, while also providing a certain degree of damping and inertia.

[0006] The technical solution adopted in this invention is: a grid-type battery energy storage system considering the state of charge of the battery, including an energy storage battery, which is connected to the power grid in sequence via a DC / DC converter, a capacitor and an inverter;

[0007] The inverter employs matching control, which generates a reference value for the inverter side angular frequency based on the DC bus voltage, and introduces inertia simulation coefficients and damping coefficients to simulate the system inertia and damping respectively, and introduces a DC voltage tracking coefficient to make the DC bus voltage track the voltage reference value.

[0008] The DC / DC converter uses SOC-frequency control, which includes active power outer loop control and current inner loop control. The active power outer loop control determines the SOC reference slope based on the inverter output frequency, and determines the battery output power reference value by combining it with the energy storage battery SOC, and then determines the battery output current reference value, which serves as the reference input for the current inner loop control.

[0009] The reference value for generating the inverter side angular frequency based on the DC bus voltage includes: Based on the squared difference between the DC voltage of the capacitor and the reference value of the capacitor voltage, combined with the inertial simulation coefficient, damping coefficient and DC voltage tracking coefficient, the reference value of the inverter side angular frequency is determined.

[0010] The mathematical model for the matching control is as follows: ; Among them, w n The rated angular frequency on the inverter side, w ref Here, s is the reference value for the inverter side angular frequency, s is the complex frequency variable, and K is the reference value for the inverter side angular frequency. T K is the DC voltage tracking coefficient. J K represents the inertial simulation coefficient. D v is the damping coefficient. dc V is the DC voltage across the capacitor. dcref This is the reference value for the capacitor voltage.

[0011] The process of determining the SOC reference slope based on the inverter output frequency and combining it with the energy storage battery SOC to determine the battery output power reference value includes: When the inverter output frequency decreases, the SOC reference slope increases, and the battery output power increases; when the inverter output frequency increases, the SOC reference slope decreases, and the battery output power decreases.

[0012] The determination of the SOC reference slope based on the inverter output frequency includes: ; in, As the SOC reference slope, This is the proportionality coefficient. f is the rated frequency, f is the inverter output frequency, and k is the frequency. s The given rate of decline of SOC at the active power setpoint.

[0013] The determination of the battery output power reference value based on the state of charge (SOC) of the energy storage battery includes: Based on the energy storage battery's SOC, SOC reference value, and SOC reference slope, the additional power is determined, and combined with the active power setpoint, the battery output power reference value is determined.

[0014] The differential operator in the SOC-frequency control is replaced with a high-gain observer.

[0015] The beneficial effects of this invention are: the DC / DC converter in this invention applies SOC-frequency control, and by using the SOC of the energy storage battery as a dynamic variable, a droop-like relationship is established between the battery output power and the system frequency.

[0016] The inverter in this invention employs matching control, and the DC / DC converter uses SOC-frequency control. This allows for automatic adjustment of the active power output of the battery energy storage system based on changes in system frequency and load power, compensating for the active power control missing in the matching control. It also increases the system's damping and inertia, thereby enabling the management of battery energy.

[0017] This invention enables BESS to autonomously adjust its power output based on real-time frequency deviation, thereby achieving better frequency support and energy utilization, while providing a certain amount of damping and inertia. This strategy can also be regarded as an implicit energy management mechanism.

[0018] Incorporating the State of Charge (SOC) into the control loop in this invention not only helps maintain system stability but also makes full use of the power output of the available energy balance battery, which is also the key to the sustained operation of the GFM system in black-start scenarios. Attached Figure Description

[0019] Figure 1 The diagram shows the BESS topology in the embodiment.

[0020] Figure 2 This is a control block diagram of the DC / DC converter and inverter in the embodiment.

[0021] Figure 3 The simulation waveform of BESS with the proposed SOC-frequency control strategy under a 0.5 Hz frequency change on the grid side is shown.

[0022] Figure 4 The simulation waveform of BESS without the proposed SOC-frequency control strategy is shown under a 0.5 Hz frequency variation on the grid side. Detailed Implementation

[0023] like Figure 1 , 2As shown, this embodiment is a grid-type battery energy storage system that considers the state of charge of the battery, including an energy storage battery, a DC / DC converter, a capacitor and an inverter, wherein the energy storage battery is connected to the power grid in sequence via the DC / DC converter, the capacitor and the inverter.

[0024] In this embodiment, the inverter adopts matching control. The matching control generates a reference value of the inverter side angular frequency based on the DC bus voltage, and introduces inertia simulation coefficient and damping coefficient to simulate the system inertia and damping respectively. A DC voltage tracking coefficient is introduced to make the DC bus voltage track the voltage reference value.

[0025] In some specific embodiments, the matching control determines the reference value of the inverter-side angular frequency based on the squared difference between the capacitor DC voltage and the capacitor voltage reference value, combined with the inertial simulation coefficient, damping coefficient, and DC voltage tracking coefficient.

[0026] The mathematical model for the applied matching control is: ; Among them, w n It is the rated angular frequency, w ref K is the reference value for the inverter's side angular frequency. T It is the DC voltage tracking coefficient, K J It is the inertial simulation coefficient, K D It is the damping coefficient, C dc This is the DC-side capacitance value, V. dc It is the DC voltage of the capacitor, v dcref This is the reference value for capacitor voltage.

[0027] From an energy perspective, the square of the capacitor voltage reflects the energy stored in the capacitor; therefore, the square difference of the DC bus voltage is used. As a deviation input, the energy deviation indirectly reflects the power balance state of the system, which is more in line with the energy regulation nature of matched control.

[0028] When the matching control shown in Equation (1) is applied, the inverter cannot regulate the active power of the system. Therefore, in this embodiment, SOC-frequency control is applied in the DC / DC converter (bidirectional buck-boost converter) in the front stage to realize active power control.

[0029] In this embodiment, SOC-frequency control includes active power outer loop control and current inner loop control. The active power outer loop control determines the SOC reference slope based on the inverter output frequency, determines the additional power based on the energy storage battery SOC, SOC reference value, and SOC reference slope, and determines the battery output power reference value by combining the active power setpoint, and then determines the battery output current reference value as the reference input for the current inner loop control.

[0030] In traditional DC voltage and current dual closed-loop circuits, the difference between the DC voltage reference value and the actual value is typically used as the input to the inner current loop via a PI controller. After modifying this to an active power outer loop, the active power setpoint P... set Divide by the rated voltage of the energy storage battery as the reference input for the current loop, i.e. ; Among them, i batref This is the battery output current reference value, P. set It is the active power setpoint, V batn This is the rated output voltage of the energy storage battery.

[0031] In this embodiment, the active power setting value P set Based on this, an additional power Psoc related to the SOC of the energy storage battery is added to create a droop-like function between SOC and frequency.

[0032] For a battery, the time derivative of its state of charge (SOC) is proportional to its instantaneous output power, therefore:

[0033] Among them, P bat P represents the battery's actual power output. ref For its reference value, E n State of Charge (SOC) indicates battery capacity. ref This is the reference value for SOC. In the proposed control loop, the battery output power is represented by P. ref Therefore

[0034] Among them, P set For a given battery output power, the corresponding SOC drop slope.

[0035]

[0036] Where, k s For SOC in P set The given descent slope under load power. To enable BESS to autonomously adjust its output power in response to emergencies, an additional slope term is added, namely...

[0037] Among them, f n Where f is the rated frequency, Δk is the inverter output frequency, K is the proportional gain, and f is the inverter output frequency. Combining formulas (5) and (6), the control gain of the battery SOC is:

[0038] Where, k SOCref This represents the reference slope of the SOC.

[0039] In this embodiment, when the inverter output frequency decreases, the SOC reference slope increases, the battery output power increases, compensating for the active power deficit and raising the frequency; when the inverter output frequency increases, the SOC reference slope decreases, the battery output power decreases, reducing the active power surplus and lowering the frequency.

[0040] This embodiment of SOC-frequency control includes a differential operator. Because the differential element amplifies noise, an equivalent differential operator is typically used in practice to mitigate its adverse effects on the system. This invention utilizes a high-gain observer. For a single-input single-output system, assuming the input is x and the output is y, let…

[0041] Where e is a very small positive constant, x1 can be seen as an estimate of the input y, while x2 is approximately the first derivative of y. The high-gain observer can be considered as a finite impulse response filter, effectively attenuating potential interference. Since all poles of the finite impulse response filter are located at the origin in the z-domain, i.e., z = 0, it is necessary to transform the continuous-time characteristic equation s... 2 All roots of + a1s + a2 = 0 are placed at s = -1. Therefore, we have

[0042] For a high-gain observer, solving its characteristic equation yields the poles.

[0043] Under the bilinear transformation, setting z = 0, we obtain

[0044] Among them, T s The sampling period is given. Therefore, based on the above analysis, the parameters of the high-gain observer can be successfully designed, thereby enabling the equivalent analog differential operator in the proposed SOC-frequency control scheme.

[0045] The following examples illustrate this:

[0046] BESS closes at time 0. Figure 1Switch S1 completes the soft-start operation with load within the initial 0.5 seconds. After the system stabilizes, pre-synchronization begins at 1.0 second, and within 0.5 seconds, the difference in voltage amplitude, phase angle, and frequency between the inverter and the grid side is limited to within a certain threshold. Then, at 1.5 seconds, switch S3 is closed to achieve grid connection. At 2.0 seconds, the grid frequency changes by -0.5 Hz, and the grid-side frequency recovers to 50 Hz at 3.0 seconds. At 4.0 seconds, the grid frequency changes by +0.5 Hz, and at 5.0 seconds, the grid-side frequency recovers again. Under these conditions, the BESS system waveform is as follows: Figure 3 As shown, and Figure 4 The comparison simulation results without this SOC-frequency control strategy are shown.

[0047] When the grid-side frequency drops by 0.5 Hz, the output power P of BESS e Increased to approximately 22 kW, such as Figure 3 As shown in (a), the excess active power is absorbed by the power grid. (From P) e and P SOC The consistency of the changing trend indicates that this additional power output almost entirely originates from SOC-frequency control, i.e., P SOC The current i in phase a of inductor. sa Increase accordingly, such as Figure 3 As shown in (b). Under steady state, Figure 3 In (c), the DC bus capacitor voltage Vdc dropped by approximately 50 V. Figure 3 In (d), the frequency f stabilizes at approximately 49.72 Hz. When the grid-side frequency changes to 50.5 Hz, Figure 3 In (a), the battery energy storage system begins to absorb active power, P e The trend of change is still related to P SOC Maintain consistency. Figure 3 (b) Display current i sa Corresponding reduction Figure 3 The DC bus voltage vdc in (c) is offset by approximately 35 V in steady state. Figure 3 The frequency f in (d) is stable at approximately 50.2 Hz.

[0048] Figure 4 The display shows that when the grid-side frequency drops by 0.5 Hz, Figure 4 (a) Active power P output by the inverter e After a relatively short dynamic process, it quickly stabilized at approximately 19 kW. Figure 4 (b) i sa It will also increase accordingly. Figure 4 The DC capacitor voltage in (c) eventually stabilizes at around 740 V. Figure 4(d) The steady-state system frequency is approximately 49.65 Hz. When the grid-side frequency increases by 0.5 Hz, Figure 4 (a) The inverter's output power is greatly reduced at this time, and the active power required by the load is mainly provided by the grid. Figure 4 (b) Current i sa Significantly reduced, Figure 4 (c) DC capacitor voltage v dc It eventually stabilized at approximately 852 V. Figure 4 In (d), a small overshoot occurs at frequency f, which eventually stabilizes at approximately 53.5 Hz.

[0049] Will Figure 3 and Figure 4 The simulation waveforms show that the model using the proposed SOC-frequency control exhibits smaller steady-state deviations during both frequency decrease and increase events. Dynamic changes in frequency and active power indicate that the proposed method also provides stronger damping and virtual inertia. Furthermore, the battery can flexibly adjust the charging and discharging process according to frequency events, achieving coordinated management of energy management and frequency support.

[0050] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to any specific implementation. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A grid-type battery energy storage system considering the state of charge of batteries, characterized in that, It includes an energy storage battery, which is connected to the power grid in sequence via a DC / DC converter, a capacitor, and an inverter; The inverter employs matching control, which generates a reference value for the inverter side angular frequency based on the DC bus voltage, and introduces inertia simulation coefficients and damping coefficients to simulate the system inertia and damping respectively, and introduces a DC voltage tracking coefficient to make the DC bus voltage track the voltage reference value. The DC / DC converter uses SOC-frequency control, which includes active power outer loop control and current inner loop control. The active power outer loop control determines the SOC reference slope based on the inverter output frequency, and determines the battery output power reference value by combining it with the energy storage battery SOC, and then determines the battery output current reference value, which serves as the reference input for the current inner loop control.

2. The grid-type battery energy storage system considering battery state of charge according to claim 1, characterized in that, The reference value for generating the inverter side angular frequency based on the DC bus voltage includes: Based on the squared difference between the DC voltage of the capacitor and the reference value of the capacitor voltage, combined with the inertial simulation coefficient, damping coefficient and DC voltage tracking coefficient, the reference value of the inverter side angular frequency is determined.

3. The grid-type battery energy storage system considering battery state of charge according to claim 2, characterized in that, The mathematical model for the matching control is as follows: ； Among them, w n The rated angular frequency on the inverter side, w ref Here, s is the reference value for the inverter side angular frequency, s is the complex frequency variable, and K is the reference value for the inverter side angular frequency. T K is the DC voltage tracking coefficient. J K represents the inertial simulation coefficient. D v is the damping coefficient. dc V is the DC voltage across the capacitor. dcref This is the reference value for the capacitor voltage.

4. The grid-type battery energy storage system considering battery state of charge according to claim 1, characterized in that, The process of determining the SOC reference slope based on the inverter output frequency and combining it with the energy storage battery SOC to determine the battery output power reference value includes: When the inverter output frequency decreases, the SOC reference slope increases, and the battery output power increases; when the inverter output frequency increases, the SOC reference slope decreases, and the battery output power decreases.

5. The grid-type battery energy storage system considering battery state of charge according to claim 4, characterized in that, The determination of the SOC reference slope based on the inverter output frequency includes: ； in, As the SOC reference slope, This is the proportionality coefficient. f is the rated frequency, f is the inverter output frequency, and k is the frequency. s The given rate of decline of SOC at the active power setpoint.

6. The grid-type battery energy storage system considering battery state of charge according to claim 1, characterized in that, The determination of the battery output power reference value based on the state of charge (SOC) of the energy storage battery includes: Based on the energy storage battery's SOC, SOC reference value, and SOC reference slope, the additional power is determined, and combined with the active power setpoint, the battery output power reference value is determined.

7. The grid-type battery energy storage system considering battery state of charge according to claim 1, characterized in that, The differential operator in the SOC-frequency control is replaced with a high-gain observer.

Images