Complex power droop control method for cascaded H-bridge inverter network energy storage system

Abstract

A complex power droop control method for a cascaded H-bridge inverter network energy storage system is characterized in that a plurality of H-bridge inverter modules are cascaded to improve the overall output voltage level and the power capacity, each module only collects local power information, frequency synchronization and power balance are realized without communication, and active power and reactive power are jointly provided for a public power grid. Different from an existing cascade decentralized self-synchronization control method, the method adopts omega / P-Qamp; the V / Q unified complex power droop control solves the problem that the system is unstable due to the change of the voltage of the power grid, and also can carry out the active support of the frequency of the power grid. And finally, a three-inverter cascade grid-connected experiment platform is built, the experiment result shows that the method has better power grid robustness and support performance, and a more practical primary control strategy is provided for a future cascade energy storage system.

Description

Technical Field

[0001] The invention relates to the technical field of power electronic grid-connected control, in particular to a complex power droop control method for a cascaded H-bridge inverter grid-connected energy storage system. Background Art

[0002] The cascaded H-bridge inverter system has the advantages of high efficiency, modularity, and low economic cost. It boosts the voltage of low-voltage power generation units such as photovoltaics and energy storage and directly connects them to the grid by cascading H-bridge inverter modules. The centralized multi-level method is the most commonly used control strategy, but in complex medium and high voltage electromagnetic environments, the high-bandwidth centralized communication method will weaken the reliability and scalability of the system. Moreover, with the increase in voltage levels and the number of modules, the centralized communication method is difficult to adapt to the development of cascade systems. Therefore, how to achieve frequency synchronization and power balancing of cascaded inverter modules without communication has become an urgent problem to be solved.

[0003] However, in the existing decentralized synchronization control for cascade inverter systems, there is a problem of system instability when the grid voltage amplitude is greater than the voltage amplitude of the common coupling grid connection point. Although there are some improved control strategies, they all belong to the category of reactive power synchronization and cannot provide active frequency support for the grid. Therefore, there is currently no self-synchronization control strategy that is both robust to grid voltage changes and has frequency support capabilities and is suitable for cascade H-bridge inverter grid-connected systems.

[0004] Compared with the prior art, the differences are as follows:

[0005] The main difference is that in the cascaded H-bridge inverter grid-connected system, the existing inverter module self-synchronization control method will make it difficult to synchronize the system frequency and enter an unstable state when the grid voltage amplitude drops. This patent provides a unified droop control strategy for the cascaded H-bridge inverter grid-connected system. This method adopts ω / PQ&V / Q unified complex power droop control, introduces reactive power droop into frequency control, enhances the stability margin of the system, overcomes the problem of system instability caused by grid voltage changes, and can actively support the grid frequency. It has better grid robustness and support performance, and provides a more practical primary control strategy for future cascaded photovoltaic and energy storage systems.

[0006] 1. Technical comparison with patent CN107910903A "A distributed power balancing control method for series inverters in grid-connected mode"

[0007] 1. In patent CN107910903A, the proposed decentralized control method is based on power droop control, see patent claim formula (24), the inverter module uses local active power information to achieve frequency self-synchronization, the disadvantage is that when the grid voltage amplitude is abnormal, the system frequency is difficult to synchronize and will be in an unstable state. In contrast, the present invention introduces reactive power droop into frequency control by adopting ω / PQ&V / Q unified droop control, enhances the system stability margin, and overcomes the problem of system instability caused by grid voltage changes.

[0008] In patent CN107910903A, the proposed control method can achieve frequency synchronization between various cascade modules through power droop control. Its disadvantage is that when the grid frequency drops, the active power output decreases and cannot provide active frequency support for the grid. In contrast, the ω / PQ&V / Q unified droop control adopted by the present invention outputs more active power when the grid frequency drops, and has the ability to actively support the grid frequency.

[0009] 2. Technical comparison with patent CN111555343B "A general distributed control method and system for cascade inverters"

[0010] 1. In patent CN111555343B, the proposed decentralized control method is based on power factor angle droop control, see patent claim formula (26), the inverter module uses local power factor angle information to achieve frequency self-synchronization, and its disadvantage is that when the grid voltage amplitude is abnormal, the system frequency is difficult to synchronize and will be in an unstable state. In contrast, the present invention introduces reactive power droop into frequency control by adopting ω / PQ&V / Q unified droop control, thereby enhancing the stability margin of the system and overcoming the problem of system instability caused by grid voltage changes.

[0011] 2. The decentralized control method proposed in patent CN111555343B is based on power factor angle droop control, which has the disadvantage that the active power output decreases when the grid frequency drops. It cannot provide active frequency support for the grid. In contrast, the ω / PQ&V / Q unified droop control adopted by the present invention outputs more active power when the grid frequency drops, and has the ability to actively support the grid frequency. Summary of the invention

[0012] In view of the above problems, the present invention proposes a complex power droop control method for a cascaded H-bridge inverter grid-connected energy storage system. The present invention proposes a unified complex power droop control of ω / PQ&V / Q, which enables each module to achieve frequency self-synchronization and power self-balancing only by relying on local information. Compared with the existing decentralized synchronization control strategy of the cascaded inverter grid-connected system, the proposed method solves the problem of system instability caused by grid voltage changes while actively supporting the grid frequency. In addition, an experimental platform for cascaded grid-connected three inverters was built. The experimental results show that the method of the present invention has better grid robustness and support performance, and provides a more practical primary control strategy for future cascaded photovoltaic and energy storage systems.

[0013] To achieve the above object, the technical solution adopted by the present invention is:

[0014] A complex power droop control method for a cascaded H-bridge inverter grid-connected energy storage system comprises the following steps:

[0015] Step 1: Build a cascaded H-bridge inverter to form a grid-connected energy storage system. The H-bridge inverter modules are connected in cascade to increase the system output voltage level and power capacity.

[0016] Step 2: Each inverter module only locally collects the voltage and current signals at the output port, and calculates the active power and reactive power output by the module;

[0017] Step 3: The complex power droop control method is used to introduce both active droop and reactive droop into frequency control, and the steady-state analysis and stability analysis of the proposed control strategy are performed to verify that frequency self-synchronization and power self-balancing can be achieved without communication between modules.

[0018] As a further improvement of the present invention, in step 1, the output active and reactive power of the i-th inverter module of the cascaded H-bridge inverter grid-connected energy storage system is expressed as follows:

[0019]

[0020] Where V i and θ i Represents the output voltage amplitude and phase angle of the i-th inverter module, V g and θ g are the voltage amplitude and phase angle of the grid, |Z| and θ z is the modulus and impedance angle of the grid impedance, where the common coupling grid point voltage The output voltage of all modules is added

[0021]

[0022] In the formula, N represents the number of all modules connected in series;

[0023] In medium and high voltage applications, the grid impedance is approximately z ≈π / 2, so the output power characteristic of the i-th inverter module is

[0024]

[0025] As a further improvement of the present invention, in step 1, the cascaded H-bridge inverter grid-connected system adopts ω / PQ&V / Q unified complex power droop control, which is expressed as follows

[0026] ω i =ω rated -m P (P i -P * )-m Q (Q i -Q * )

[0027] V i =V * -nQ i

[0028] In the formula, ω i and V i are the angular frequency and voltage amplitude reference of the i-th inverter module, ω rated is the rated value of the grid angular frequency and voltage amplitude, P * and Q * Indicates the rated active power and reactive power of each module, m P and m Q is the ω / PQ frequency droop control coefficient, and n is the V / Q voltage droop control coefficient.

[0029] As a further improvement of the present invention, in step 3, the cascaded H-bridge inverter grid-connected system performs a steady-state analysis, and there is a function definition of steady-state power:

[0030]

[0031] In the formula, “-” represents the steady-state value of the physical quantity.

[0032] F can be rewritten as a function of the steady-state power factor angle:

[0033]

[0034] Assuming that the function F is monotonic with respect to the power factor angle, then for any two modules we have

[0035]

[0036] Therefore, the power factor angles of each module are equal in steady state.

[0037]

[0038] At the same time, all series modules share the same grid-connected current, and all modules have the same phase angle

[0039]

[0040] The voltage amplitude of each module is the same

[0041]

[0042] The voltage at the common coupling grid point in steady state is

[0043]

[0044] The steady-state output power of each module can be expressed as

[0045]

[0046] As a further improvement of the present invention, in step 3, the cascaded H-bridge inverter grid-connected system performs stability analysis to obtain the system's unique equilibrium point and stability conditions as follows:

[0047]

[0048] In the formula, k is the power angle difference between the module voltage and the grid voltage in steady state, I =(V p -V g ) / |Z| is the expression related to current The approximate value of

[0049] Among them, m Q / m P The design should be balanced by considering the stability margin and active power support capability. After the stability condition is met, the frequency synchronization and power balance of each module can be guaranteed. It is worth noting that if the rated voltage amplitude of the module is V * The value is reasonable to meet V p =V g , then the control parameter design can be simplified to m Q / m P >0.

[0050] Beneficial effects:

[0051] The present invention discloses a complex power droop control method for a cascaded H-bridge inverter grid-connected energy storage system. The proposed ω / PQ&V / Q unified complex power droop control enables each module to achieve frequency self-synchronization and power self-balancing only by relying on local information. Compared with the existing decentralized synchronization control strategy of cascaded inverter grid-connected systems, this method solves the problem of system instability caused by grid voltage changes while actively supporting grid frequency. In addition, three inverter cascaded grid-connected experimental platforms were built. The experimental results show that the method of the present invention has better grid robustness and support performance, and provides a more practical primary control strategy for future cascaded photovoltaic and energy storage systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 The physical structure diagram of the cascaded H-bridge inverter grid-connected system is shown;

[0053] Figure 2 shows a control schematic diagram of the i-th H-bridge inverter module;

[0054] Figure 3 The waveform diagram of the grid voltage drop of 3% is shown;

[0055] Figure 4 The experimental waveform of the power grid frequency dropping by 0.1Hz is shown. DETAILED DESCRIPTION

[0056] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. The following embodiments are used to illustrate the present invention but are not used to limit the scope of the present invention.

[0057] The ω / PQ&V / Q unified complex power droop control proposed by the method in this embodiment realizes frequency self-synchronization and power self-balancing for each module relying only on local information. Compared with the existing decentralized synchronization control strategy of cascade inverter grid-connected system, this method solves the problem of system instability caused by grid voltage changes while actively supporting grid frequency. Three inverter cascade grid-connected experimental platforms were built, and the experimental results show that the method of the present invention has better grid robustness and support performance, and provides a more practical primary control strategy for future cascade photovoltaic and energy storage systems.

[0058] The cascaded H-bridge inverter grid-connected system is composed of a number of cascaded H-bridge inverter modules, such as Figure 1 shown.

[0059] The cascaded H-bridge inverter grid-connected system adopts a ω / PQ&V / Q unified complex power droop control decentralized autonomous control method, such as Figure 2As shown, there is no need for any communication between inverter modules, and only the local controller is relied on to achieve automatic output power balancing and autonomous frequency synchronization. The ω / PQ&V / Q control strategy is described as follows

[0060] ω i =ω rated -m P (P i -P * )-m Q (Q i -Q * )

[0061] V i =V * -nQ i

[0062] In the formula, ω i and V i are the angular frequency and voltage amplitude reference of the i-th inverter module respectively. rated is the rated value of the grid angular frequency and voltage amplitude. * and Q * Indicates the rated active power and reactive power of each module. P and m Q is the ω / PQ frequency droop control coefficient. n is the V / Q voltage droop control coefficient.

[0063] Figure 3 and Figure 4 The experimental results of a complex power droop control method for a cascaded H-bridge inverter grid-connected energy storage system are given.

[0064] The robustness of the proposed method to the grid voltage amplitude and the active frequency support performance were tested respectively.

[0065] The experimental parameters are as follows:

[0066]

[0067] The grid voltage drops 3% experimental results, such as Figure 3 As shown in the figure, the grid voltage drops by 3% at 0.5s. Before and after the grid drop, the cascade inverter system remains stable, and the voltage amplitude and phase angle of each module are the same, achieving self-synchronization of the frequency and self-balancing of the power of each module. After the grid voltage drops, the common coupling grid-connected voltage is greater than the grid voltage. The system remains stable when the system outputs positive reactive power, indicating that the proposed method solves the problem of system instability when the grid voltage amplitude is greater than the common coupling grid-connected point voltage amplitude, and the system has better robustness to the grid voltage.

[0068] The experimental results of the power grid frequency dropping by 0.1Hz are as follows: Figure 4As shown in the figure, the grid frequency drops from 50 Hz to 49.9 Hz at 0.3 s, the system output active power increases, and the power of each module is evenly shared in this process, which verifies that the proposed method has active frequency support performance.

[0069] The above description is only a preferred embodiment of the present invention and does not constitute any other form of limitation to the present invention. Any modification or equivalent change made based on the technical essence of the present invention still falls within the scope of protection required by the present invention.

Claims

1. A complex power droop control method for a cascaded H-bridge inverter grid-connected energy storage system, characterized in that: The following steps are involved: Step 1: Build a cascaded H-bridge inverter to form a grid-connected energy storage system. The H-bridge inverter modules are connected in cascade to increase the system output voltage level and power capacity. Step 2: Each inverter module only locally collects the voltage and current signals at the output port, and calculates the active power and reactive power output by the module; Step 3: The complex power droop control method is used to introduce both active droop and reactive droop into frequency control, and the steady-state analysis and stability analysis of the proposed control strategy are performed to verify that frequency self-synchronization and power self-balancing can be achieved without communication between modules.

2. The complex power droop control method of a cascaded H-bridge inverter grid-connected energy storage system according to claim 1, characterized in that: In the step 1, the output active and reactive power of the i-th inverter module of the cascaded H-bridge inverter grid-connected energy storage system is expressed as follows: Where V i and θ i Represents the output voltage amplitude and phase angle of the i-th inverter module, V g and θ g are the voltage amplitude and phase angle of the grid, |Z| and θ z is the modulus and impedance angle of the grid impedance, where the common coupling grid point voltage The output voltage of all modules is added In the formula, N represents the number of all modules connected in series; In medium and high voltage applications, the grid impedance is approximately z ≈π / 2, so the output power characteristic of the i-th inverter module is 3. The complex power droop control method of a cascaded H-bridge inverter grid-connected energy storage system according to claim 1, characterized in that: In step 1, the cascaded H-bridge inverter grid-connected system adopts ω / PQ&V / Q unified complex power droop control, which is expressed as follows ω i =ω rated -m P (P i -P * )-m Q (Q i -Q * ) V i =V * -nQ i In the formula, ω i and V i are the angular frequency and voltage amplitude reference of the i-th inverter module, ω rated is the rated value of the grid angular frequency and voltage amplitude, P * and Q * Indicates the rated active power and reactive power of each module, m P and m Q is the ω / PQ frequency droop control coefficient, and n is the V / Q voltage droop control coefficient.

4. The complex power droop control method of a cascaded H-bridge inverter grid-connected energy storage system according to claim 1, characterized in that: In step 3, the cascaded H-bridge inverter grid-connected system performs steady-state analysis, and there is a function definition of steady-state power: In the formula, "" represents the steady-state value of the physical quantity. F can be rewritten as a function of the steady-state power factor angle: Assuming that the function F is monotonic with respect to the power factor angle, then for any two modules we have Therefore, the power factor angles of each module are equal in steady state. At the same time, all series modules share the same grid-connected current, and all modules have the same phase angle The voltage amplitude of each module is the same The voltage at the common coupling grid point in steady state is The steady-state output power of each module can be expressed as 5. The complex power droop control method of the cascaded H-bridge inverter grid-connected energy storage system according to claim 1, characterized in that: In step 3, the cascaded H-bridge inverter grid-connected system is subjected to stability analysis to obtain the unique equilibrium point and stability condition of the system as follows: In the formula, k is the power angle difference between the module voltage and the grid voltage in steady state, I =(V p -V g ) / |Z is the expression related to current The approximate value of Among them, m Q / m P The design should be balanced by considering the stability margin and active power support capability. After the stability condition is met, the frequency synchronization and power balance of each module can be guaranteed. It is worth noting that if the rated voltage amplitude of the module is V * The value is reasonable to meet V p =V g , then the control parameter design can be simplified to m Q / m P >0.

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