Improved droop control method and system based on energy storage converter and medium

Abstract

The invention discloses an improved droop control method and system based on an energy storage converter and a medium, and relates to the technical field of power electronic micro-grid control. The system comprises an energy storage device port, a power conversion module, an LCL output filter, a power grid port, at least one load port and a system control module, the LCL output filter is connected with a power grid port through a common coupling point; the energy storage converter converts alternating current of the power grid port into direct current and transmits electric energy to the energy storage battery port and the load port; the system control module detects the difference value between the module output power and the reference power through the improved droop control module. The method has good active and reactive power tracking performance, and can provide inertia and damping links for the system, thereby improving the stability of the system. Meanwhile, the control method can realize smooth switching of the energy storage converter between an off-grid operation mode and a grid-connected operation mode.

Description

Technical Field

[0001] The present invention relates to the field of power electronic system microgrid control technology, specifically to a control method for an energy storage converter, in particular to a control method, system and medium for improving the transient and steady-state performance of a droop control grid-connected inverter in a microgrid. Background Art

[0002] The grid-connected system is the main form of photovoltaic power generation in urban areas. In the rapid evolution of grid-connected inverter systems, energy storage inverters are the key technology to achieve efficient flow and conversion of electric energy. The importance of grid-connected control strategies for energy storage inverters has gradually increased, which is directly related to the stability and reliability of the power grid and the optimal configuration of energy. Since the droop control method uses the external characteristics of traditional synchronous generators to control distributed power generation units, it can provide the necessary voltage and frequency regulation for the inverter to operate as a voltage source and dynamically adjust the load power. It has been deeply studied in the control method of grid-connected inverters.

[0003] The traditional droop control output control loop usually consists of a capacitor voltage loop and an inductor current loop, such as using proportional-integral control in a rotating reference coordinate system, and using a proportional resonant controller in a synchronous reference coordinate system or single-phase inverter to achieve zero steady-state error tracking of the controlled signal. However, the dual-loop structure based on internal model control limits the system bandwidth to the droop control outer loop, resulting in slow dynamic response of the system when the load changes, and prone to voltage overshoot or oscillation; and the traditional droop control has problems such as large voltage control deviation, poor anti-interference ability, and low current sharing control accuracy, and many problems in power distribution accuracy and load responsiveness. With the rapid development of renewable energy, energy storage converters are increasingly widely used in microgrids. Although the traditional droop control method can achieve power synchronization, it has certain limitations in dynamic response and stability, especially under nonlinear loads and weak grid conditions, which easily lead to frequency oscillation and power overshoot. Summary of the invention

[0004] Purpose of the invention: In view of the transient and steady state of the current microgrid technology, the purpose of this invention is to propose an improved droop control method, which can improve the steady-state and dynamic performance of the energy storage converter by adding a damping link to the control signal. This method can effectively reduce the oscillation and overshoot of active and reactive power and enhance the stability of the system.

[0005] The invention relates to an improved droop control method based on an energy storage converter, comprising: detecting a system output voltage and a current through an improved droop control module, calculating the system active power and reactive power, and realizing dynamic power tracking through a control closed loop based on a power calculation deviation; improving the stability of the system and enhancing the dynamic tracking performance of the system according to a damping link of the improved droop control module; decoupling the voltage amplitude and the phase angle through the improved droop control module, realizing closed-loop real-time tracking of a reference instruction through a PI controller, and superimposing an output modulation wave signal through a voltage amplitude feedforward module to realize sinusoidal pulse width modulation.

[0006] The present invention also relates to an improved droop control system based on an energy storage converter, the system comprising an energy storage device port, a power conversion module, an LCL output filter, a grid port, at least one load port and a system control module; the LCL output filter is connected to the grid port via a common coupling point; the energy storage converter converts the alternating current of the grid port into direct current, and transmits the electric energy to the energy storage battery port and the load port; the system control module runs the above control method, and the system control module detects the difference between the output power of the system detection module and the reference power through the improved droop control module.

[0007] Depending on the power flow direction of the energy storage device, the working modes of the energy storage converter include: charging mode and discharging mode.

[0008] Depending on whether there is a grid port involved, the working modes of the energy storage inverter include: island mode, grid-connected mode, and off-grid switching mode.

[0009] The structure of the power conversion module adopts I-type NPC, mid-point clamped three-level circuit topology, supports DC-AC and AC-DC power bidirectional conversion, and realizes energy transfer between the grid port and the energy storage device.

[0010] The improved droop control module includes the following steps: detecting the output current of the energy storage inverter system; the output current includes active power P and reactive power Q; selecting a droop control coefficient; distinguishing output characteristics according to an equivalent model of the energy storage inverter connected to the grid; and establishing a small signal state space model for the microgrid system during operation, including an inverter control and interface circuit small signal dynamic sub-model and a voltage and current dual-loop control inverter small signal dynamic sub-model.

[0011] The inverter droop control is expressed by the following formula:

[0012]

[0013] Where: ω is the voltage angular velocity in droop control, E is the voltage amplitude, m and n are the active power droop coefficient and reactive power droop coefficient respectively; ω ref and Vref are the frequency reference value and the voltage reference value respectively; P and Q are the active power detection value and the reactive power detection value respectively; P ref and Q ref They are active power reference value and reactive power reference value respectively;

[0014] The selection formulas for active power droop coefficient m and reactive power droop coefficient n are as follows:

[0015]

[0016] Where: P n is the system rated active power, P max The maximum power allowed for the inverter to output when the frequency drops, Q n is the system rated reactive power, Q max The maximum reactive power that the inverter is allowed to output when the frequency decreases, ω n is the rated angular velocity, ω min is the minimum angular velocity corresponding to the maximum output power of the inverter, U n is the system rated voltage, U min To allow the inverter to output the minimum voltage.

[0017] According to the active power P and reactive power Q output by the grid-connected inverter in the dq coordinate system, including: calculating the active power P and reactive power Q output by the grid-connected inverter according to the following formula:

[0018]

[0019] Where: u d and u q is the coordinate value of the grid-connected inverter output voltage in the dq coordinate system; i gd and i gq is the coordinate value of the grid current in the dq coordinate system;

[0020] The calculation formulas for the active power P and reactive power Q output by the inverter are as follows:

[0021]

[0022] Where: E is the common bus voltage; V is the inverter terminal potential; assuming that the phase angle of E is 0, Ф is the phase angle difference between V and E; Z is the terminal impedance of the inverter, and θ is the impedance angle;

[0023] The apparent power S output by the inverter is expressed in polar coordinates as follows:

[0024]

[0025] If the impedance at the inverter end is approximately resistive, the system resistance value R>> reactance value X, the reactance value X is negligible, Z=R, θ=0°, and the voltage phase angle difference is small, sinФ=Ф, the inverter output active power is mainly affected by (VE), and the reactive power is determined by Ф;

[0026]

[0027] If the impedance at the inverter end is approximately inductive, the reactance value X>>system resistance value R, the system resistance value R is negligible, Z=X, θ=90°, the inverter output active power is determined by V, and the reactive power is affected by (VE);

[0028]

[0029] The apparent power S output by the inverter is affected by the potential V at the inverter end, the phase difference Ф between V and E, and the impedance angle θ. When the impedance Z at the inverter end is approximately pure inductive and θ=90°, Ф remains unchanged. As V increases, the inverter output powers P and Q both increase, with Q changing more significantly than P. When V remains unchanged, as Ф increases, P output by the inverter increases and Q decreases, with P changing more significantly than Q.

[0030] Based on the same inventive concept, a computer-readable storage medium of the present invention includes a memory and a processor, wherein: the memory is used to store a computer program that can be run on the processor; the processor is used to execute the above-mentioned improved droop control method based on the energy storage inverter when running the computer program.

[0031] Compared with the prior art, the present invention has the following significant technical effects:

[0032] (1) The improved droop control method and system based on the energy storage converter proposed in the present invention has good active and reactive power dynamic tracking performance, and can provide inertia and damping links for the system, thereby improving the stability of the system.

[0033] (2) The improved droop control method based on the energy storage converter proposed in the present invention can achieve smooth switching of the energy storage converter between off-grid and grid-connected operation modes.

[0034] (3) The present invention adopts modular technology. The modularization of the power converter reduces the design cost and shortens the design cycle. The introduction of the LCL compensation network module can effectively improve the attenuation characteristics of the filter in the high frequency band while maintaining a good gain characteristic in the low frequency band. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a structural schematic diagram of a system of an improved droop control grid-connected inverter provided by an embodiment of the present invention.

[0036] Figure 2 It is a circuit block diagram of an LCL filter in the intermediate frequency domain of the present invention.

[0037] Figure 3 It is a schematic diagram of judging the output power characteristics of an inverter under an inductive power grid provided by an embodiment of the present invention.

[0038] Figure 4 It is a topology diagram of a three-phase grid-connected inverter based on LCL filtering provided in an embodiment of the present invention.

[0039] Figure 5 It is a schematic diagram of simulation of transient and steady-state performance of output power during conventional droop control grid-connected loading provided by an embodiment of the present invention.

[0040] Figure 6 It is a schematic diagram of simulation of transient and steady-state performance of output power during grid-connected loading of improved droop control provided by an embodiment of the present invention.

[0041] Figure 7 It is an experimental waveform diagram of system output voltage and current in the off-grid switching mode of the improved droop control provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0042] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. By referring to the exemplary embodiments, the purposes and functions of the present invention and the methods for achieving these purposes and functions will be explained. However, the present invention is not limited to the exemplary embodiments disclosed below; it can be implemented in different forms. The essence of the description is only to help those skilled in the relevant art to comprehensively understand the specific details of the present invention.

[0043] The present invention relates to an improved droop control system based on an energy storage converter. Figure 1 1 is a schematic diagram of the structure of a system of an improved droop control grid-connected inverter provided by an embodiment of the present invention. Figure 1 As shown, the improved droop control system based on energy storage converter includes:

[0044] The system output voltage and current are detected by improving the droop control module to calculate the system active power and reactive power, and the dynamic tracking of power is realized through the control closed loop based on the power calculation deviation; the stability of the system is improved and the dynamic tracking performance of the system is enhanced according to the damping link of the improved droop control module; the voltage amplitude and phase angle are decoupled by improving the droop control module, and the closed-loop real-time tracking of the reference instruction is realized through the PI controller, and the output modulation wave signal is superimposed through the voltage amplitude feedforward module to finally realize sinusoidal pulse width modulation; the system includes a controller, a grid-connected inverter power main circuit, an LCL output filter, a grid port, an energy storage battery port and a load port. The LCL output filter is connected to the grid port through a common coupling point (PCC); the energy storage converter converts the AC power of the grid port into DC power, and transmits the electric energy to the energy storage battery port and the load port. The controller detects the difference between the output power of the module and the reference power through the improved droop control module.

[0045] Depending on the power flow direction of the energy storage device, the working modes of the energy storage converter include: charging mode and discharging mode. Depending on whether there is a grid port involved, the working modes of the energy storage converter include: island mode, grid-connected mode and off-grid switching mode.

[0046] The power conversion module structure adopts I-type NPC, midpoint clamped three-level circuit topology, supports DC-AC and AC-DC power bidirectional conversion, and realizes energy transfer between the grid port and the energy storage device. The energy storage battery port is connected to the DC side of the circuit through the filter capacitor, and the AC side is connected to the grid port through the LCL filter.

[0047] The grid port compensation network adopts an LCL compensation network, where the resonant frequency of the LCL compensation network is:

[0048]

[0049] Where: f res is the resonant frequency, L f is the inductance value of the machine side in the LCL compensation network, L g is the grid-side inductance value in the LCL compensation network, C f is the capacitance value. At the same time, in order to meet the requirements of bandwidth and attenuation characteristics, the resonant frequency f res The following requirements must also be met, where the grid frequency f AC =50Hz, and where f sw is the system switching frequency.

[0050] The LCL filter circuit block diagram of the system in the frequency domain is as follows Figure 2 As shown. Eliminate the intermediate variable i s ,i g ,ic , V c , and ignore the interference variable V g After that, V s to g The transfer function is shown below, and the system is a third-order system:

[0051]

[0052] Where: V s ,i s is the voltage and current at the input side of the LCL filter, V g ,i g is the voltage and current at the output side of the LCL filter, V c ,i c are the voltage and current on the capacitor side of the LCL filter.

[0053] The improved droop control module includes the following steps: detecting the output current of the energy storage inverter system; the output current includes active power P and reactive power Q; selecting a droop control coefficient; distinguishing output characteristics according to an equivalent model of the energy storage inverter connected to the grid; and establishing a small signal state space model for the microgrid system during operation, including an inverter control and interface circuit small signal dynamic sub-model and a voltage and current dual-loop control inverter small signal dynamic sub-model.

[0054] Inverter droop control can often be expressed by the following formula:

[0055]

[0056] Where: ω is the voltage angular velocity in droop control, E is the voltage amplitude, m and n are the active power droop coefficient and reactive power droop coefficient respectively; ω ref and V ref are the frequency reference value and the voltage reference value respectively; P and Q are the active power detection value and the reactive power detection value respectively; P ref and Q ref They are active power reference value and reactive power reference value respectively.

[0057] The selection of active power droop coefficient m and reactive power droop coefficient n can refer to the following formula:

[0058]

[0059] Where: P n is the system rated active power, P max The inverter is allowed to output maximum power when the frequency decreases; Q n is the system rated reactive power, Q max The inverter is allowed to output the maximum reactive power when the frequency decreases; ω n is the rated angular velocity, ωmin The minimum angular velocity corresponding to the maximum output power of the inverter; U n is the system rated voltage, U min To allow the inverter to output the minimum voltage. According to the national standard GB / T 15945, it is stipulated that the frequency deviation is allowed to be 1% and the voltage deviation is 5% when connected to the grid.

[0060] According to the active power P and reactive power Q output by the grid-connected inverter in the dq coordinate system, including: calculating the active power P and reactive power Q output by the grid-connected inverter according to the following formula:

[0061]

[0062] Where: u d and u q is the coordinate value of the grid-connected inverter output voltage in the dq coordinate system; i gd and i gq is the coordinate value of the grid current in the dq coordinate system.

[0063] The equivalent circuit of the inverter connected to the public AC bus of the microgrid is as follows. The active power P and reactive power Q output by the inverter are calculated according to the following formula:

[0064]

[0065] Where: E is the common bus voltage; V is the inverter terminal potential; assuming that the phase angle of E is 0, Ф is the phase angle difference between V and E; Z is the terminal impedance of the inverter, and θ is the impedance angle.

[0066] The apparent power S output by the inverter is expressed in polar coordinates as follows:

[0067]

[0068] If the impedance at the inverter end is approximately resistive, R is the system resistance value, X is the reactance value, Z is the terminal impedance of the inverter, R>>X, the reactance value X is negligible, Z=R, θ=0°, and the voltage phase angle difference is small, sinФ=Ф, then as shown in the formula, the active power output of the inverter is mainly affected by (VE), and the reactive power is determined by Ф.

[0069]

[0070] If the impedance at the inverter end is approximately inductive reactance value X>> system resistance value R, and the system resistance value R is negligible, Z=X, θ=90°. Then as shown in the formula, the inverter output active power is determined by V, and the reactive power is affected by (VE).

[0071]

[0072] Figure 3 This is a schematic diagram of determining the output power characteristics of the inverter under the inductive power grid provided by an embodiment of the present invention. Combining the above formula, it can be seen that the apparent power S output by the inverter is affected by the inverter terminal potential V, the phase angle difference Φ between V and E, and the impedance angle θ. When the inverter terminal impedance Z is approximately pure inductive, θ = 90°, the inverter output power characteristics are as follows: Figure 3 As shown in the figure, keeping Ф unchanged, as V increases, the inverter output power P and Q both increase, and Q changes more significantly than P. When V remains unchanged, as Ф increases, the inverter output P increases and Q decreases, and P changes more significantly than Q. Therefore, it can be concluded that in the case of weak grid inductive, V is positively correlated with P and Q, and its correlation with Q is stronger than that with P; Ф is positively correlated with P and negatively correlated with Q, and its correlation with P is stronger than that with Q. In order to realize the energy storage converter in the scenario of bidirectional energy flow between the energy storage port and the grid port, the system has a slow dynamic response when the load changes, and is prone to voltage overshoot or oscillation; the traditional droop control has large voltage control deviation, poor anti-interference ability, and low current sharing control accuracy; and there are problems in power distribution accuracy and load responsiveness. The present invention proposes an improved droop control method and system based on an energy storage converter. The energy storage converter can be used as an interface between the energy storage port and the grid port. Combined with the corresponding control method, it has the advantages of improving the system frequency and voltage stability. At the same time, it optimizes the power distribution accuracy and ensures efficient grid-connected operation and fast off-grid mode switching under various working conditions. The energy storage converter system of the present invention comprises: an energy storage device port, a power conversion module, a grid-side LCL circuit compensation network module, a grid port, at least one load port and a system control module. The energy storage device port is a voltage source with a rated voltage of 650-850V, and can also be replaced by a battery pack and a super capacitor, etc. The power source can be a grid or a new energy source such as photovoltaics and wind turbines; the energy storage device comprises a battery or a super capacitor, etc., which can realize the storage and feeding of electric energy; the power conversion module can adopt circuit topologies such as I-type NPC and T-type NPC, and support bidirectional power conversion of DC-AC and AC-DC; the compensation network module is composed of inductors and capacitors in series and parallel, etc., and can adopt LCL compensation topology, etc.; each power conversion unit is connected to each power port, energy storage device port and load port through a compensation network module; the load comprises a charging pile, an electric car, an electronic device, etc.; the power source or the energy storage device is connected to the grid port through its own power conversion module, and the load is connected to the power conversion module through its own LCL circuit compensation network module.

[0073] Figure 4 1 is a schematic diagram of a three-phase grid-connected inverter topology based on LCL filtering provided in an embodiment of the present invention. Figure 4As shown in the figure, the bidirectional AC / DC converter has the function of bidirectional energy flow, which can realize the energy exchange between the DC side and the three-phase grid. It mainly includes three parts: grid side, rectifier bridge and DC side. sa 、u sb and u sc As the AC side grid input, the midpoint is N; i sa 、i sb and i sc Input current from the AC grid side; the converter has three bridge arms, each phase consists of four power switching devices S with freewheeling diodes i1 ~S i4 (i=a,b,c), and two clamping diodes D i1 ~D i4 (i=a, b, c); two output bus capacitors C of the same size are connected in series on the DC side d1 and C d2 , define the midpoint of the two busbar capacitors as the zero point N,i dc1 and i dc2 is the capacitor charging current, i o is the midpoint current, p and n are the positive and negative poles respectively, and the current reference positive direction mentioned above is specified as Figure 4 .

[0074] The operating mode of the I-type three-level three-phase full-bridge converter is analyzed. Taking phase A among the three phases as an example, when S a1 and S a2 is turned on, and S a3 and S a4 Turn off, the voltage u between the bridge arm midpoint A and point N AN Output is V dc / 2, define this state as P; when S a2 and S a3 is turned on, and S a1 and S a4 Turn off, ignoring the diode voltage drop, the voltage u between the bridge arm midpoint A and point N AN The output is 0, and this state is defined as O; when S a3 and S a4 On, S a1 and S a2 Turn off, the voltage u between the bridge arm midpoint A and point N AN Output is -V dc / 2, define this state as N. It can be seen that by making corresponding logical combinations of the four power switch devices on each phase bridge arm, three level states of each phase bridge arm output can be realized, which are V dc / 2, 0, -V dc / 2.

[0075] Assume that the conduction voltage drop of power devices and diodes is ignored, such as Figure 3 The current shown in the figure refers to the positive direction, which is defined as the rectified direction of energy flowing from the AC side to the DC side. The current flow paths of the midpoint potential states P (positive), O (zero), and N (negative) of each phase bridge arm are analyzed.

[0076] P status: S a1 and S a2 is turned on, and S a3 and S a4 Turn off, clamp diode D a1 and D a2 When the current of phase A is positive, the current flows from the bridge arm midpoint A through S a1 and S a2 The freewheeling diode flows to point P; when the current of phase A is negative, the current flows from point P through S a1 and S a2 Flows to the bridge arm midpoint A. In both cases, the bridge arm midpoint A has a potential of +V relative to point N. dc / 2, the state of the rectifier bridge arm midpoint A at this time is represented by the logical switch state "1".

[0077] Depending on the power flow direction of the energy storage device, the working modes of the energy storage converter include: charging mode and discharging mode. Depending on whether there is a grid port involved, the working modes of the energy storage converter include: island mode, grid-connected mode and off-grid switching mode.

[0078] The present invention relates to a droop control power loop control system, which is mainly used to accurately control the active power, reactive power, voltage amplitude and frequency in the power system. The system realizes accurate regulation of the system output power and voltage through the droop control principle, thereby improving the stability and response speed of the power system. The system includes the following key control links: active power control, reactive power control, voltage amplitude control, frequency control, current control, and various feedback controls regulated by PI controllers.

[0079] Active power control: The system first receives the active power (P_ref) and reactive power (Q_ref) given by the host computer. Then, by real-time detection of voltage (V) and current (I) signals, active power (P) and reactive power (Q) are calculated as feedback. The calculated active power difference (ΔP) is processed by the droop coefficient m to obtain the voltage amplitude signal U1. After the voltage amplitude signal U1 is subtracted from the given voltage amplitude Un, the first step of control is entered.

[0080] Voltage amplitude control: The voltage amplitude control obtains the voltage amplitude error by subtracting the d-axis component Udref of the voltage amplitude in the dq coordinate system, and further sends it to the PI controller for the second step of adjustment. The function of the PI controller is to accurately control the voltage amplitude input to the system. Through the adjustment of the PI controller, the response speed and control accuracy of the voltage amplitude are improved, and the voltage amplitude set value is output for subsequent voltage loop decoupling control.

[0081] Reactive power control: After the reactive power difference (ΔQ) is calculated, the voltage frequency signal f1 is converted through the droop coefficient n and the difference is made between it and the given voltage frequency fn, and then the first step of control is entered.

[0082] Frequency control: Calculate the difference between the voltage frequency signal f1 and the target frequency signal f*, and send the difference to the PI controller for the second step of control. The frequency is precisely controlled by the PI controller to improve the response speed and accuracy of the system voltage frequency. After multiplying the voltage frequency value by 2π, the voltage angular velocity ω* is obtained, which is converted into the voltage phase angle θ* and decoupled from the output voltage amplitude given value, thereby providing the subsequent output voltage controller with the command value in the dq coordinate system.

[0083] Output voltage control: In the dq coordinate system, the voltage controller generates the d-axis component given value vcdref and the q-axis component given value vcqref as the output voltage command value. After these command values ​​pass through the PI controller, they generate the input signal of the current loop. The control of the current loop is achieved through closed-loop control, and the current command values ​​idref and iqref generated by the output current controller in the dq coordinate system continue to be closed-loop regulated.

[0084] Current control and inductance decoupling: The output of the current controller adjusts the current error and finally generates the current loop control output. In order to eliminate the mixed interference of the inductance effect in the dq coordinate system, the inductance value wL is used as a decoupling compensation signal to remove the influence of the inductance on the system.

[0085] The final current loop output value is superimposed on the modulation wave signal through voltage feedforward to generate a sinusoidal pulse width modulation (SPWM) signal, thereby achieving precise voltage and power regulation.

[0086] Figure 5 It is a schematic diagram of simulation of transient and steady-state performance of output power during conventional droop control grid-connected loading provided by an embodiment of the present invention. Figure 6 It is a schematic diagram of simulation of transient and steady-state performance of output power during grid-connected loading of improved droop control provided by an embodiment of the present invention. Figure 5 and Figure 6 The following are the waveform comparisons of active power P and reactive power Q before and after the droop control improvement. Figure 5As shown in the figure, when droop control is added at t=1s, there is an initial oscillation, and the instantaneous oscillation amplitude is about 3000W; when a ramp function is given at t=1.5s, the loading process is simulated, and the loading is completed at t=2.5s, P=20kW, the tracking is good, the overshoot is about 0.2%, and the reactive power oscillation is within 2kVar, while the reactive power oscillation of the traditional droop control method is within 6kVar. Compared with the waveform before optimization, the damping link of the control signal is added, and it can be seen that its steady-state performance has been significantly improved, the initial power oscillation is greatly reduced, and the overshoot is reduced, and the accuracy of the system control power is increased, which verifies the effectiveness of the proposed method.

[0087] In order to verify the effectiveness of the proposed method in off-grid switching, an off-grid switching experiment was carried out. Figure 7 : is an experimental waveform diagram of the system output voltage and current in the off-grid switching mode of the improved droop control provided by the embodiment of the present invention, such as Figure 7 As shown in the figure, the voltage difference is observed from the off-grid state, and the grid is switched on when the voltage difference is 0. Then the PCS power is adjusted to observe the response speed, and then the grid is switched on again. The experimental results are shown in the figure: the instantaneous current peak of the grid connection is 10.4A, which meets the requirements of the national standard, and the power can be accurately and quickly tracked by loading after the grid connection. The experimental results verify that the improved experimental algorithm is still applicable in the off-grid switching situation.

[0088] Based on the same inventive concept, a device of the present invention includes a memory and a processor, wherein:

[0089] A memory for storing computer programs that can be run on the processor;

[0090] The processor is used to execute the steps of the above-mentioned improved droop control method and system control method based on the energy storage converter when running the computer program, and can achieve the technical effect consistent with the above-mentioned method.

[0091] The present invention discloses an improved droop control method and system control method based on an energy storage converter. In order to improve the efficiency of bidirectional energy flow between the energy storage port and the grid port and cope with the dynamic response problem when the load changes, the traditional droop control method has some defects, such as large voltage control deviation, poor anti-interference ability, and low current sharing control accuracy. In addition, insufficient power distribution accuracy and poor load responsiveness are also common challenges. In view of the problem that the power oscillation is large during loading and the active power cannot be accurately tracked due to the lack of a damping link when the energy storage inverter system is running, an improved droop control method is proposed. The following conclusions are obtained through simulation and experimental verification based on a 60KW PCS platform: the use of the proposed improved droop control method can significantly reduce the initial oscillation of the system and improve the system stability; the experimental results verify that the system has strong system stability when the control method is adopted, and the system can achieve smooth off-grid switching. By optimizing the control method, the system can effectively improve the frequency and voltage stability of the system, while optimizing the power distribution accuracy, ensuring efficient grid-connected operation under different working conditions, and can quickly switch to off-grid mode.

[0092] The energy storage converter system of the present invention includes an energy storage device port, a power conversion module, a grid-side LCL circuit compensation network module, a grid port, at least one load port and a system control module. The energy storage device port is a voltage source with a rated voltage of 650-850V, or can be replaced by a battery pack, a super capacitor and other equipment, and the power supply can come from the grid or new energy such as photovoltaic, wind turbine and the like. The energy storage device adopts a battery or a super capacitor, etc., which can realize the storage and feeding of electric energy. The power conversion module supports bidirectional power conversion of DC-AC and AC-DC, and the circuit topology structure can adopt the form of I-type NPC, T-type NPC and the like. The compensation network module is composed of inductors and capacitors in series and parallel, and adopts LCL compensation topology and the like. Each power conversion unit is connected to each power supply port, energy storage device port and load port through a compensation network module, and the load may include a charging pile, an electric car, an electronic device and the like. The power supply or energy storage device is connected to the grid port through its respective power conversion module, and the load is connected to the power conversion module through the LCL circuit compensation network module.

Claims

1. An improved droop control method based on energy storage converter, characterized in that: include: By improving the droop control module, the system output voltage and current are detected, the system active power and reactive power are calculated, and the dynamic tracking of power is achieved through a closed-loop control based on the power calculation deviation; By improving the damping link of the droop control module, the stability of the system is improved and the dynamic tracking performance of the system is enhanced; By improving the droop control module to decouple the voltage amplitude and phase angle through the PI controller, the closed-loop real-time tracking of the reference command is realized, and then the voltage amplitude feedforward module is used to superimpose the output modulation wave signal to realize sinusoidal pulse width modulation.

2. An improved droop control system based on energy storage converter, characterized in that: The system comprises Energy storage device port, power conversion module, LCL output filter, grid port, load port and system control module; The LCL output filter is connected to the grid port through a common coupling point; the energy storage inverter converts the AC power of the grid port into DC power, and transmits the electric energy to the energy storage battery port and the load port; the system control module runs the improved droop control method based on the energy storage inverter described in claim 1 above, and the system control module detects the difference between the output power of the system detection module and the reference power through the improved droop control module.

3. The improved droop control system based on energy storage converter according to claim 2 is characterized in that: Depending on the power flow direction of the energy storage device, the working modes of the energy storage converter include: charging mode and discharging mode.

4. The improved droop control system based on energy storage converter according to claim 2 is characterized in that: Depending on whether there is a grid port involved, the working modes of the energy storage inverter include: island mode, grid-connected mode, and off-grid switching mode.

5. The improved droop control system based on energy storage converter according to claim 2 is characterized in that: The structure of the power conversion module adopts I-type NPC, mid-point clamped three-level circuit topology, supports DC-AC and AC-DC power bidirectional conversion, and realizes energy transfer between the grid port and the energy storage device.

6. The improved droop control system based on energy storage converter according to claim 2 is characterized in that: The improved droop control module comprises the following steps: Detect the output current of the energy storage converter system; the output current includes active power P and reactive power Q; select the droop control coefficient; distinguish the output characteristics according to the equivalent model of the energy storage converter grid connection; establish a small signal state space model for the microgrid system during operation, including the inverter control and interface circuit small signal dynamic sub-model and the voltage and current dual-loop control inverter small signal dynamic sub-model.

7. The improved droop control system based on energy storage converter according to claim 6 is characterized in that: The inverter droop control is expressed by the following formula: Where: ω is the voltage angular velocity in droop control, E is the voltage amplitude, m and n are the active power droop coefficient and reactive power droop coefficient respectively; ω ref and V ref are the frequency reference value and the voltage reference value respectively; P and Q are the active power detection value and the reactive power detection value respectively; P ref and Q ref They are active power reference value and reactive power reference value respectively; The selection formulas for active power droop coefficient m and reactive power droop coefficient n are as follows: Where: P n is the system rated active power, P max The maximum power allowed for the inverter to output when the frequency drops, Q n is the system rated reactive power, Q max The maximum reactive power that the inverter is allowed to output when the frequency decreases, ω n is the rated angular velocity, ω min is the minimum angular velocity corresponding to the maximum output power of the inverter, U n is the system rated voltage, U min To allow the inverter to output the minimum voltage.

8. The improved droop control system based on energy storage converter according to claim 6, characterized in that: According to the grid-connected inverter output active power P and reactive power Q in the dq coordinate system, it includes: The active power P and reactive power Q output by the grid-connected inverter are calculated according to the following formula: Where: u d and u q is the coordinate value of the grid-connected inverter output voltage in the dq coordinate system; i gd and i gq is the coordinate value of the grid current in the dq coordinate system.

9. The improved droop control system based on energy storage converter according to claim 6, characterized in that: The calculation formulas for the active power P and reactive power Q output by the inverter are as follows: Where: E is the common bus voltage; V is the inverter terminal potential; assuming that the phase angle of E is 0, Ф is the phase angle difference between V and E; Z is the terminal impedance of the inverter, and θ is the impedance angle; The apparent power S output by the inverter is expressed in polar coordinates as follows: If the impedance at the inverter end is approximately resistive, the system resistance value R>> reactance value X, the reactance value X is negligible, Z=R, θ=0°, and the voltage phase angle difference is small, sinФ=Ф, the inverter output active power is mainly affected by (VE), and the reactive power is determined by Ф; If the impedance at the inverter end is approximately inductive, the reactance value X>>system resistance value R, the system resistance value R is negligible, Z=X, θ=90°, the inverter output active power is determined by V, and the reactive power is affected by (VE); The apparent power S output by the inverter is affected by the potential V at the inverter end, the phase difference Ф between V and E, and the impedance angle θ. When the impedance Z at the inverter end is approximately pure inductive and θ=90°, Ф remains unchanged. As V increases, the inverter output powers P and Q both increase, with Q changing more significantly than P. When V remains unchanged, as Ф increases, P output by the inverter increases and Q decreases, with P changing more significantly than Q.

10. A computer-readable storage medium, characterized in that: comprising a memory and a processor, wherein: A memory for storing one or more computer programs capable of running on the processor; A processor is used to execute the improved droop control method based on energy storage converter as claimed in claim 1 when running the one or more computer programs.

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