Energy coordination and neutral point balance control method applied to bidirectional energy storage converter

Abstract

This invention discloses an energy coordination and neutral point balance control method applied to a bidirectional energy storage converter. Specifically, the difference between the voltage difference of the voltage divider capacitors and the standard deviation value (0) of the bipolar DC microgrid is subtracted, and this difference is used to generate a voltage equalization control signal after passing through a voltage regulator. This voltage equalization control signal is then subtracted from the neutral point current, and the result is used to generate an improved voltage equalization control signal through a current regulator. This improved voltage equalization control signal is added to the bus voltage control signal and transmitted to a PWM pulse signal generator to generate pulses that control the DC / DC converter. This invention employs a three-level bidirectional DC-DC energy storage converter method to suppress the neutral point voltage offset problem and proposes an improved current loop voltage equalization control loop. Compared with traditional voltage equalization control loops, the proposed voltage equalization strategy has a better effect on voltage imbalance problems.

Description

Technical Field

[0001] This invention belongs to the field of microgrid energy storage converter control technology, specifically relating to an energy coordination and neutral point balance control method applied to bidirectional energy storage converters. Background Technology

[0002] With the continuous development of new energy sources, distributed generation (DG) technologies, primarily based on renewable energy sources such as wind power and photovoltaic power, have attracted widespread attention due to their flexible power generation methods and environmental compatibility. To respond to the national call and leverage the advantages of distributed generation technology to address the intermittent, random, and decentralized nature of renewable energy, the concept of microgrids has been proposed. Compared to AC microgrids, DC microgrids can more efficiently and reliably accommodate renewable energy generation systems such as wind and solar power, energy storage units, electric vehicles, and other DC loads. Furthermore, DC microgrids do not suffer from issues such as power source synchronization and harmonics, and are easier to control. Therefore, DC microgrids have gained increasing attention in recent years. Bipolar DC microgrids, composed of a dual-bus structure, not only possess the general advantages of DC microgrids but also allow for the selection of different buses based on the load's voltage requirements. This facilitates the connection of electrical equipment and distributed power sources at different voltage levels, further enhancing the flexibility and reliability of DC power supply systems. However, in DC microgrids, unstable output from distributed generation units and load fluctuations can cause fluctuations in bus power, which can seriously threaten the stability of the microgrid system. There is no reactive power in a DC microgrid. It can achieve system power balance by ensuring the stability of the DC bus voltage. That is, the DC bus voltage is the only indicator to measure the stability of the system. Therefore, how to stabilize the DC bus voltage is the key to the research of DC microgrids.

[0003] Based on the presence or absence of a neutral line, DC microgrids can be classified into unipolar and bipolar DC microgrids. Compared to unipolar DC microgrids, bipolar DC microgrids offer more voltage level interfaces, allowing for flexible voltage level changes, and also have higher utilization of AC-DC converters. Furthermore, when one pole fails, the other pole can continue operating, resulting in higher system reliability and safety. However, imbalances in power supply, load, and line parameters between the positive and negative poles can generate unbalanced current in the neutral line, increasing line losses and causing the positive and negative bus voltages to deviate from their rated values. When the load at a node is severely unbalanced, the imbalance may exceed the limit, even triggering the neutral line's unbalanced voltage protection. To flexibly adjust the voltage imbalance of a bipolar DC microgrid while keeping the bus voltage within a reasonable range, certain distributed collaborative control strategies must be adopted to ensure the normal operation of the DC load. Currently, there are three main strategies for suppressing unbalanced voltage in bipolar DC microgrids: ① using AC-DC converters capable of suppressing unbalanced voltage; ② installing voltage balancers at the output of the AC-DC converter; ③ using load switching switches to adjust the power supply polarity of the DC load. This invention proposes a control method for suppressing unbalanced DC bus voltage in a bipolar DC microgrid using an energy storage three-level converter, thereby achieving bidirectional power flow of the grid while balancing the positive and negative voltages of the DC bus. Summary of the Invention

[0004] The purpose of this invention is to provide an energy coordination and neutral point balance control method for bidirectional energy storage converters, which uses a grid-connected current feedback active damping method to suppress resonance and improve robustness.

[0005] The technical solution adopted in this invention is an energy coordination and neutral point balance control method applied to a bidirectional energy storage converter, which is implemented according to the following steps:

[0006] Step 1: Based on the working principle of the three-level DC / DC energy storage converter, design the bus voltage controller and the voltage equalization controller. The bus voltage control adopts voltage and current dual closed-loop control.

[0007] Step 2: Add an improved current sharing control to the voltage equalization loop control of the three-level DC / DC energy storage converter.

[0008] The invention is further characterized in that,

[0009] Step 1 specifically involves:

[0010] Step 1.1, based on the rated DC bus voltage U N With DC bus voltage feedback value U ref The DC bus voltage error value ΔU is obtained as shown in equation (1);

[0011] U N -Uref =ΔU (1);

[0012] Step 1.2: The voltage error value ΔU is used to obtain the current inner loop setpoint I through the PI regulator. set I set The current error value ΔI is obtained by subtracting the current feedback value from the inductor current on the energy storage side, as shown in equation (2).

[0013] ΔI=I set -I ref (2);

[0014] In equation (2), I ref This is the feedback current value of the inductor current on the energy storage side of the DC / DC converter;

[0015] Step 1.3: The current error value ΔI is used by the current regulator to generate the bus voltage control signal P;

[0016] Step 1.4, the difference between the voltage of the voltage divider capacitor and the standard deviation value 0 of the bipolar DC microgrid is subtracted to obtain the bipolar voltage error adjustment amount ΔU1, as shown in equation (4);

[0017] 0-(U NO -U PO )=ΔU1 (4);

[0018] Among them, U NO U is the voltage across the voltage divider capacitor C1 of the converter. PO This refers to the voltage across the voltage divider capacitor C2 of the converter.

[0019] Step 1.5: The bipolar voltage error adjustment amount ΔU1 is used to generate a voltage equalization control signal P1 through the voltage regulator in the voltage divider control; after P1 is added to the bus voltage control signal P, the PWM modulation signal is obtained through the PWM regulator to control the converter switching transistor.

[0020] In step 1.5, the voltage regulator uses a PI regulator, whose equivalent transfer function G u (s), as shown in equation (5);

[0021]

[0022] Where Ku1 is the proportional coefficient of the voltage equalization control loop; Ku2 is the integral coefficient of the voltage equalization control loop.

[0023] Step 2 specifically involves:

[0024] Step 2.1: Extract the neutral point current I0 of the bipolar DC microgrid, and subtract the voltage equalization control signal P1 from the neutral point current I0 of the DC microgrid to obtain the current difference ΔI2 in the voltage equalization control loop of the bipolar microgrid, as shown in Equation (6).

[0025] ΔI1=P1-I0 (6);

[0026] Step 2.2: The current difference ΔI1 is used by the current regulator of the improved voltage equalization control loop to generate an improved voltage equalization control signal P2;

[0027] Step 2.3: The improved voltage equalization control signal P2 is added to the bus voltage control signal P and then passed through the PWM regulator to obtain the PWM modulation signal, which is used to control the converter switching transistors.

[0028] Step 2.4: The pulse signal generated by PWM controls the IGBT switching transistors of the DC / DC converter, forming a closed-loop control of the dual-level energy storage DC / DC converter.

[0029] In step 2.2, the current regulator uses a PI regulator, whose equivalent transfer function G ii (s), as shown in equation (7);

[0030] G ii (s)=G u (s)G i1 (s) (7);

[0031] in, K i1 K is the proportional coefficient of the voltage equalization control current loop; i2 Integral coefficient of voltage equalization control current.

[0032] The beneficial effects of this invention are:

[0033] 1) Develop a reasonable control strategy to achieve stable bipolar DC bus voltage and symmetrical positive and negative bus voltages;

[0034] 2) Focus on the power coordination and control among distributed energy storage modules to enable the rational allocation of power among the energy storage modules;

[0035] 3) To achieve safe and efficient utilization of distributed energy storage systems and stable operation of bipolar DC microgrids;

[0036] 4) Compared with the traditional voltage equalization control method, the improved voltage equalization loop control strategy has better effects in improving response time and suppressing neutral point voltage imbalance. Attached Figure Description

[0037] Figure 1 This is a diagram of a bipolar DC microgrid architecture for the energy coordination and neutral point balance control method of the present invention applied to a bidirectional energy storage converter;

[0038] Figure 2This is a topology diagram of a three-level bidirectional DC / DC converter with non-common ground for input and output, which is the energy coordination and neutral point balance control method of the present invention applied to bidirectional energy storage converters.

[0039] Figure 3 This is a control block diagram of a traditional bipolar energy storage converter;

[0040] Figure 4 This is an improved bipolar energy storage converter control block diagram of the energy coordination and neutral point balance control method applied to bipolar energy storage converters according to the present invention;

[0041] Figure 5 This is a simulation waveform of the energy storage current of the energy coordination and neutral point balance control method of the present invention applied to a bidirectional energy storage converter;

[0042] Figure 6 This is a bus voltage simulation waveform diagram of the energy coordination and neutral point balance control method of the present invention applied to a bidirectional energy storage converter;

[0043] Figure 7 This is a comparison waveform of unbalanced voltages in the energy coordination and neutral point balance control method of the present invention applied to a bidirectional energy storage converter.

[0044] Figure 8 This is a simulation waveform of the voltage imbalance of the energy coordination and neutral point balance control method of the present invention applied to a bidirectional energy storage converter. Detailed Implementation

[0045] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0046] This invention applies to the energy coordination and neutral point balance control method of a bidirectional energy storage converter, and is implemented according to the following steps:

[0047] Step 1: Based on the working principle of the three-level DC / DC energy storage converter, design the bus voltage controller and the voltage equalization controller. The bus voltage control uses traditional voltage and current dual closed-loop control; specifically:

[0048] Step 1.1, based on the rated DC bus voltage U N With DC bus voltage feedback value U ref The DC bus voltage error value ΔU is obtained as shown in equation (1);

[0049] U N -U ref =ΔU (1);

[0050] Step 1.2: The voltage error value ΔU is used to obtain the current inner loop setpoint I through the PI regulator. set I setThe current error value ΔI is obtained by subtracting the current feedback value from the inductor current on the energy storage side, as shown in equation (2).

[0051] ΔI=I set -I ref (2);

[0052] In equation (2), I ref This is the feedback current value of the inductor current on the energy storage side of the DC / DC converter;

[0053] Step 1.3: The current error value ΔI is used by the current regulator to generate the bus voltage control signal P;

[0054] In the bus voltage control method, both the voltage controller and the current controller use PI regulators, and their equivalent transfer function G i (s), as shown in equation (3);

[0055] G i (s)=G1(s)G2(s) (3);

[0056] in K1 is the voltage loop proportional coefficient; K2 is the voltage loop integral coefficient; K3 is the current loop proportional coefficient; K4 is the current loop integral coefficient; G1(s) represents the voltage regulator; G2(s) represents the current regulator. (The signal is multiplied by the voltage / current regulator).

[0057] Step 1.4, perform traditional voltage equalization ring design, use single voltage ring control by the voltage difference of the voltage divider capacitor, and subtract the voltage difference of the voltage divider capacitor from the standard deviation value 0 of the bipolar DC microgrid to obtain the bipolar voltage error adjustment amount ΔU1, as shown in equation (4).

[0058] 0-(U NO -U PO )=ΔU1 (4);

[0059] Among them, U NO U is the voltage across the voltage divider capacitor C1 of the converter. PO This is the voltage across the voltage divider capacitor C2 of the converter.

[0060] Step 1.5: The bipolar voltage error adjustment amount ΔU1 is used to generate a voltage equalization control signal P1 through the voltage regulator in the voltage divider control; after P1 is added to the bus voltage control signal P, the PWM modulation signal is obtained through the PWM regulator to control the converter switching transistors.

[0061] The voltage regulator uses a PI regulator, whose equivalent transfer function G u (s), as shown in equation (5);

[0062]

[0063] Where Ku1 is the proportional coefficient of the voltage equalization control loop; Ku2 is the integral coefficient of the voltage equalization control loop.

[0064] Step 2: Add an improved current sharing control to the voltage equalization loop control of the three-level DC / DC energy storage converter;

[0065] Step 2.1, improve the voltage equalization control method, extract the neutral point current I0 of the bipolar DC microgrid, and subtract the voltage equalization control signal P1 from the neutral point current I0 of the DC microgrid to obtain the current difference ΔI2 in the voltage equalization control loop of the bipolar microgrid, as shown in Equation (6).

[0066] ΔI1=P1-I0 (6);

[0067] Step 2.2: The current difference ΔI1 is used by the current regulator of the improved voltage equalization control loop to generate an improved voltage equalization control signal P2;

[0068] This current regulator uses a PI regulator, whose equivalent transfer function G ii (s), as shown in equation (7);

[0069] G ii (s)=G u (s)G i1 (s) (7);

[0070] in K i1 K is the proportional coefficient of the voltage equalization control current loop; i2 Integral coefficient of voltage equalization control current;

[0071] Step 2.3: The improved voltage equalization control signal P2 is added to the bus voltage control signal P and then passed through the PWM regulator to obtain the PWM modulation signal, which is used to control the converter switching transistors.

[0072] The basic principle of pulse width modulation (PWM) is to control the on / off state of the circuit's switching devices, resulting in a series of pulses of equal amplitude at the output. These pulses replace the sine wave or the desired waveform. In other words, multiple pulses are generated within half a cycle of the output waveform, ensuring that the equivalent voltage of each pulse is a sine wave, resulting in a smooth output with fewer low-order harmonics. By modulating the width of each pulse according to certain rules, the magnitude of the inverter circuit's output voltage can be changed.

[0073] Step 2.4: The pulse signal generated by PWM controls the IGBT switching transistors of the DC / DC converter, forming a closed-loop control of the dual-level energy storage DC / DC converter.

[0074] For a simplified bipolar DC microgrid model, the implementation method and simulation verification of the control method of the present invention are described in detail.

[0075] The proposed method is applied to bipolar DC microgrids, whose basic architecture is as follows: Figure 1 As shown, the specific converter topology used in this control method is... Figure 2 As shown, it is a topology diagram of a three-level bidirectional DC / DC converter with non-common ground input and output. Figure 3 This is a control block diagram for a traditional bipolar energy storage converter. The bus voltage control loop consists of a voltage and current double closed loop, its purpose being to stabilize the bus voltage and allow for bidirectional power flow. Secondly, a traditional voltage equalization control loop is added to address the neutral point offset problem, stabilizing the neutral point voltage and reducing voltage offset. During load switching, the single voltage outer loop control effect can cause significant voltage fluctuations, resulting in a substantial impact on the power grid during load changes. Therefore, this invention proposes an improved voltage equalization control loop based on a current loop to address this problem, such as... Figure 4 As shown. Using a neutral current as the inner voltage loop can reduce voltage imbalance while minimizing the impact of load changes on the bus voltage. Simulation models of a bipolar DC microgrid with different control methods were built on the MATLAB / Simulink platform. The models use two identical photovoltaic units connected in parallel, with a load power of 15kW. The DC bus voltage is set to 375V, and both control strategies use Uset = 375V, with a minimum allowable bus voltage Umin = 370V. During sudden load changes, the energy storage system completes bidirectional power flow: the system operates independently with a 15kW load, only the photovoltaic units work, and the output power is 20kW, while the energy storage unit absorbs 5kW of power; if a 10kW load suddenly increases by 0.5s, both the photovoltaic units and the energy storage device simultaneously supply power to the load. Figure 5 It can be seen that the photovoltaic unit reaches its rated power output after 0.05s, at which point the energy storage is in a charging state. After 0.5s, a sudden load change occurs, at which point the energy storage is in a discharging state, achieving the goal of energy coordination. During a sudden load change, the energy storage system completes bidirectional power flow. The system operates independently with a 15kW load, with only the photovoltaic unit working and outputting 20kW, while the energy storage unit absorbs 5kW of power. If a 10kW load suddenly increases within 0.5s, both the photovoltaic unit and the energy storage device simultaneously supply power to the load. A comparison is made between the traditional voltage equalization control method proposed in conventional literature and the voltage equalization control method based on the current loop improvement of this invention, under the same simulation parameters. For example... Figure 6 The simulation waveforms of the two bus voltages show that the improved control strategy can make the bus voltage stabilize more quickly. Figure 7Comparing the waveforms of the unbalanced voltage, it is clear that, under the same parameters, the improved control strategy reduces the neutral point voltage suppression from 3% to 0.05% of the bus voltage. To ensure the stable operation of the bipolar DC microgrid, according to ANSI C84, it is recommended to limit the voltage imbalance to within 3%. Figure 8 It can be seen that the voltage imbalance simulation of the present invention is basically below 0.05%, which meets the requirements.

Claims

1. A method for energy coordination and neutral point balance control applied to a bidirectional energy storage converter, characterized in that, The specific steps are as follows: Step 1: Based on the working principle of the three-level DC / DC energy storage converter, design the bus voltage control and the voltage equalization loop control of the three-level DC / DC energy storage converter. The bus voltage control uses a traditional voltage and current dual closed-loop control. Specifically: Step 1.1, based on the rated DC bus voltage. U N DC bus voltage feedback value U ref The DC bus voltage error value is obtained. ΔU As shown in equation (1); （1）； Step 1.2, Voltage Error Value ΔU The inner current loop setpoint is obtained through a PI controller. , The current error value is obtained by subtracting the inductor current feedback value from the energy storage side. ΔI As shown in equation (2); （2）； In equation (2), I ref This is the feedback current value of the inductor current on the energy storage side of the DC / DC converter; Step 1.3, Current error value ΔI The current regulator generates a bus voltage control signal. P ; Step 1.4: Subtract the voltage difference between the voltage divider capacitors from the standard deviation value 0 of the bipolar DC microgrid to obtain the bipolar voltage error adjustment amount Δ. U 1, as shown in equation (4); （4）； in, U NO Voltage divider capacitor for converter C 1. Voltage, U PO Voltage divider capacitor for converter C 2. Voltage; Step 1.5, adjust the bipolar voltage error Δ U 1. The voltage equalization control signal is generated by the voltage regulator in the voltage divider control. P 1; P 1. After being added to the bus voltage control signal P, the PWM modulation signal is obtained through the PWM regulator to control the converter switching transistors; Step 2: Add improved current sharing control to the voltage equalization loop control of the three-level DC / DC energy storage converter; specifically: Step 2.1: Extract the neutral point current of the bipolar DC microgrid. I 0, Voltage equalization control signal P 1. Neutral point current of DC microgrid I The difference between 0 and 1 is used to obtain the current difference in the voltage equalization control loop of the bipolar microgrid. ΔI 2, as shown in equation (6); （6）； Step 2.2, Current Difference ΔI 1. The current regulator with the improved voltage equalization control loop generates an improved voltage equalization control signal. P 2; Step 2.3, Improved pressure equalization control signal P 2. Bus voltage control signal P After being added together, the signals are passed through a PWM regulator to obtain a PWM modulation signal, which is then used to control the switching transistors of the converter. Step 2.4: The pulse signal generated by PWM controls the IGBT switching transistors of the DC / DC converter, forming a closed-loop control of the dual-level energy storage DC / DC converter.

2. The energy coordination and neutral point balance control method applied to a bidirectional energy storage converter according to claim 1, characterized in that, In step 1.5, the voltage regulator uses a PI regulator, whose equivalent transfer function is... G u (s) As shown in equation (5); （5）； in Ku 1 represents the proportional coefficient of the voltage equalization control loop; Ku 2. Integral coefficient of the voltage equalization control loop.

3. The energy coordination and neutral point balance control method applied to a bidirectional energy storage converter according to claim 1, characterized in that, In step 2.2, the current regulator is a PI regulator, whose equivalent transfer function is... G ii ( s As shown in equation (7); （7）； in, , K i1 This is the proportional coefficient of the voltage equalization control current loop; K i2 Integral coefficient of voltage equalization control current.

Images