A method for analyzing stability of a multi-machine parallel system of energy storage converters

Abstract

A kind of energy storage converter multi-machine parallel system stability analysis method, comprising: modeling and control of single PCS;Application of capacitor current feedback type active damping control strategy;Analysis of stability of single PCS under PI control;Mathematical modeling of multiple PCS parallel system;Establishment of parallel system norton equivalent circuit;Using logarithmic frequency characteristic curve, verify that active damping control can have better inhibitory effect on resonance peak of parallel system.Capacitor current feedback type active damping method can not only compensate resonance peak of parallel system, improve the stability of system, but also effectively reduce grid-connected current distortion rate, improve power quality.The stability analysis method for PCS multi-machine parallel system provided by the present application can be used for judging the operating state of parallel system, ensure the stable operation of system, and provide theoretical basis for parameter design of PCS controller.

Description

Technical Field

[0001] This invention relates to a stability analysis method for a multi-machine parallel system of energy storage converters, belonging to the field of power electronics application technology. Background Technology

[0002] Against the backdrop of the global energy crisis and the "dual carbon" problem, renewable energy power generation technology has received more attention, with large-scale energy storage technology providing crucial support for its development. As the scale of renewable energy power generation expands, the size of energy storage systems is also continuously increasing, and the number of parallel-connected energy storage converters (PCS), which serve as the interface between the energy storage system and the grid, is gradually increasing as well.

[0003] A typical PCS topology consists of an inverter and an LCL filter. As the number of PCS connected in parallel increases, various stability issues arise. Under grid-connected conditions, due to line impedance, coupling effects and interactions occur between PCS and between the PCS and the grid, making the system's resonant characteristics more complex. Therefore, to ensure the safe and stable operation of the entire system when multiple PCS are connected in parallel, system stability analysis is required, and corresponding control schemes must be provided. Summary of the Invention

[0004] The purpose of this invention is to address the resonance problem in multiple grid-connected PCS parallel systems by employing a capacitor current feedback active damping method. This control method compensates for system resonance spikes, improves system stability, reduces grid current distortion, and improves power quality. The stability analysis method for multi-PCS parallel systems proposed in this invention is used to determine the operating status of the parallel system, ensuring stable operation, and providing a theoretical basis for the parameter design of the PCS controller.

[0005] This invention is achieved through the following technical solution: a stability analysis method for a multi-machine parallel system of energy storage converters, comprising the following steps:

[0006] 1) Based on the topology of a single grid-connected PCS and the current feedforward decoupling PI control strategy adopted in the dq coordinate system, and under the condition of using capacitor current feedback active damping control, a current loop control block diagram of a single PCS is established. By using Mason's formula to simplify the block diagram, the Norton equivalent circuit model of a single PCS can be established.

[0007] 2) Based on the equivalent model of a single grid-connected PCS, an equivalent model of a parallel system of n PCS is established. Through the superposition principle and circuit simplification, the relationship matrix between the grid-side current of each PCS, the output voltage of the PCS, and the grid voltage is derived. By plotting the logarithmic frequency response curve of the transfer function of the parallel system, the resonance characteristics of the system are analyzed.

[0008] 3) After introducing the active damping strategy of capacitor current feedback, a single PCS can be represented by the Norton equivalent circuit in the form of admittance. Based on this, each parallel PCS system is equivalent to a structural model in which the current source and the output impedance are connected in parallel and connected to the power grid through the power grid equivalent impedance. Thus, the Norton equivalent circuit of the n parallel PCS system can be obtained.

[0009] 4) Plot the logarithmic frequency response curve of the transfer function of the n PCS parallel system to verify the impact of the active damping control strategy on the resonance peak of the parallel system and the system stability.

[0010] Preferably: the single PCS current loop control block diagram of the capacitor current feedback type active damping control in step 1) includes sampling and filtering the capacitor current i. c Multiply it by a feedback coefficient H i The current is applied to the modulated wave signal to achieve active damping of the system. The specific implementation of this block diagram is as follows: PCS grid-side current setpoint i 2_ref The difference between this value and its actual value i2 is calculated, and after passing through the PI controller, the capacitor current i is subtracted. c The feedback is then multiplied by the gain K of PCS. PWM The output voltage u of the PCS is obtained. i Based on the relationship between voltage and current in the LCL filter topology, a complete current loop control block diagram is obtained.

[0011] As a preferred embodiment, step 2) is as follows: Each PCS is equivalent to a controlled voltage source and connected to the power grid through the equivalent impedance of the LCL filter and the equivalent impedance of the line. Assuming that all parameters of each PCS in the parallel system are the same, including their hardware and software parameters, the relationship matrix between the grid-side current of each PCS and its own PCS output voltage, the output voltage of other PCS, and the grid voltage is derived through the superposition principle and circuit simplification. These are represented as G1, G2, and H1, respectively. The logarithmic frequency response curves of G1, G2, and H1 are plotted, thereby analyzing the resonance characteristics of the system without introducing an active damping strategy.

[0012] As a preferred embodiment, the Norton equivalent circuit of the n PCS parallel system in step 3) is as follows:

[0013] The control block diagram described above can be transformed into a simplified control block diagram, wherein:

[0014]

[0015] The loop gain of the above closed-loop system:

[0016]

[0017] The current on the grid side is:

[0018]

[0019] According to equation (3), a single PCS can be represented by a Norton equivalent circuit in admittance form, where I*(s) is the equivalent current source, and T eq (s) is the equivalent admittance, and

[0020]

[0021] Therefore, each parallel PCS system can be represented as a structural model where a current source and an output impedance are connected in parallel, and the system is connected to the grid through the grid's equivalent impedance. Assuming all PCS systems are identical, their equivalent admittances are the same. According to the superposition theorem, taking the first PCS as an example, the following grid-side current can be obtained:

[0022]

[0023] As a preferred option: Step 4) plots the logarithmic frequency response curve of the transfer function of the n parallel PCS system to analyze the system stability, specifically as follows: Equation (5) has three independent terms. For the first PCS, these three terms represent the grid-side current I1(s) and the excitation sources I1*(s), I2*(s) ~ I when these excitation sources act alone. n *(s), U g The transfer function between (s) is similar for other PCS. Based on this, the grid-side current I1(s) and the current reference value I are plotted. 1ref (s), I 2ref (s) and voltage source U g The logarithmic frequency response curve of the transfer function between (s) is used to analyze the stability of the system.

[0024] This invention employs a capacitor current feedback type active damping method to compensate for system resonance spikes, improve system stability, reduce grid current distortion, and improve power quality. The stability analysis method for PCS multi-machine parallel systems proposed in this invention is used to determine the operating status of the parallel system, ensuring stable operation, and providing a theoretical basis for the parameter design of the PCS controller. Attached Figure Description

[0025] Figure 1 This is a system architecture diagram of a single grid-connected PCS according to the present invention;

[0026] Figure 2 This is a block diagram of the PI control of a single grid-connected PCS according to the present invention;

[0027] Figure 3 It is a block diagram of PCS current loop control with active damping control strategy with capacitor current feedback.

[0028] Figure 4 yes Figure 3 Simplified equivalent block diagram;

[0029] Figure 5 It is the Norton equivalent circuit of a single PCS;

[0030] Figure 6 It is an equivalent model of a system of n PCS in parallel;

[0031] Figure 7 It is the Norton equivalent circuit of n PCS in parallel system;

[0032] Figure 8 The waveform of the grid-connected current of a single PCS is given when no active damping control strategy is adopted;

[0033] Figure 9 The grid-connected voltage and current waveforms of a single PCS are shown when an active damping control strategy is adopted.

[0034] Figure 10 The grid-connected voltage and current waveforms of a four-PCS parallel system are presented when an active damping control strategy is adopted. Detailed Implementation

[0035] The invention will now be described in detail with reference to the accompanying drawings: Figure 1 As shown, a stability analysis method for a multi-unit parallel energy storage converter system includes the following steps:

[0036] 1) Based on the topology of a single grid-connected PCS and the current feedforward decoupling PI control strategy adopted in the dq coordinate system, and under the condition of using capacitor current feedback active damping control, a current loop control block diagram of a single PCS is established. By using Mason's formula to simplify the block diagram, the Norton equivalent circuit model of a single PCS can be established.

[0037] 2) Based on the equivalent model of a single grid-connected PCS, an equivalent model of a parallel system of n PCS is established. Through the superposition principle and circuit simplification, the relationship matrix between the grid-side current of each PCS, the output voltage of the PCS, and the grid voltage is derived. By plotting the logarithmic frequency response curve of the transfer function of the parallel system, the resonance characteristics of the system are analyzed.

[0038] 3) After introducing the active damping strategy of capacitor current feedback, a single PCS can be represented by the Norton equivalent circuit in the form of admittance. Based on this, each parallel PCS system is equivalent to a structural model in which the current source and the output impedance are connected in parallel and connected to the power grid through the power grid equivalent impedance. Thus, the Norton equivalent circuit of the n parallel PCS system can be obtained.

[0039] 4) Plot the logarithmic frequency response curve of the transfer function of the n PCS parallel system to verify the impact of the active damping control strategy on the resonance peak of the parallel system and the system stability.

[0040] The block diagram of the single PCS current loop control in step 1) of the capacitor current feedback type active damping control includes sampling and filtering the capacitor current i. c Multiply it by a feedback coefficient H i The current is applied to the modulated wave signal to achieve active damping of the system. The specific implementation of this block diagram is as follows: PCS grid-side current setpoint i 2_ref The difference between this value and its actual value i2 is calculated, and after passing through the PI controller, the capacitor current i is subtracted. c The feedback is then multiplied by the gain K of PCS. PWM The output voltage u of the PCS is obtained. i Based on the relationship between voltage and current in the LCL filter topology, a complete current loop control block diagram is obtained.

[0041] Step 2) is as follows: Each PCS is equivalent to a controlled voltage source, and connected to the power grid through the equivalent impedance of the LCL filter and the equivalent impedance of the line. The specific equivalent model is as follows: Figure 6 As shown, assuming all parameters of each PCS in the parallel system are identical, including their hardware and software parameters, the relationship matrix between the grid-side current of each PCS and its own PCS output voltage, the output voltages of other PCS, and the grid voltage is derived through the superposition principle and circuit simplification. These are represented as G1, G2, and H1, respectively. The logarithmic frequency response curves of G1, G2, and H1 are plotted, thereby analyzing the resonant characteristics of the system without introducing an active damping strategy.

[0042] The Norton equivalent circuit of the n PCS parallel system in step 3) is as follows:

[0043] right Figure 3 The control block diagram shown can be transformed into a simplified control block diagram, such as... Figure 4 As shown, where:

[0044]

[0045] The loop gain of the above closed-loop system:

[0046]

[0047] The current on the grid side is:

[0048]

[0049] According to equation (3), a single PCS can be represented by a Norton equivalent circuit in admittance form, such as Figure 5 As shown. Where I*(s) is the equivalent current source, T eq (s) is the equivalent admittance, and

[0050]

[0051] Therefore, each parallel PCS system can be represented as a structural model where a current source and an output impedance are connected in parallel, and the system is connected to the power grid through the equivalent impedance of the power grid. The specific equivalent topology is as follows: Figure 7 As shown. Assuming all PCS are identical, therefore the equivalent admittance is the same. According to the superposition theorem, taking the first PCS as an example, the following grid-side current can be obtained:

[0052]

[0053] Step 4) plots the logarithmic frequency response curve of the transfer function of the n parallel PCS system to analyze the system stability, specifically as follows: Equation (5) has three independent terms. For the first PCS, these three terms represent the grid-side current I1(s) and the excitation sources I1*(s), I2*(s) ~ I when these excitation sources act alone. n *(s), U g The transfer function between (s) is similar for other PCS. Based on this, the grid-side current I1(s) and the current reference value I are plotted. 1ref (s), I 2ref (s) and voltage source U g The logarithmic frequency response curve of the transfer function between (s) is used to analyze the stability of the system. Specific Implementation

[0055] In this invention, the system architecture diagram of a single grid-connected PCS is as follows: Figure 1 As shown, this PCS consists of a voltage source inverter and an LCL filter. L1, L2, and C are the filter inductor and filter capacitor, respectively, and i1 is the inductor current on the inverter side. c i1 is the filter capacitor current, i2 is the grid-connected current, and u is the current through the filter capacitor. i It is the inverter output voltage, u a u b and u c It is the voltage of a three-phase power grid.

[0056] The grid-connected PCS adopts a PI control strategy based on current feedforward decoupling, and the control structure diagram is as follows: Figure 2 As shown. In Figure 2 in,i 2abc It is the three-phase grid-connected current, u abc This is the three-phase grid voltage, with inductance L = L1 + L2. The sampled grid-side voltage u... abc The grid voltage phase is obtained by locking the grid voltage using a three-phase phase-locked loop (PLL). This is achieved by controlling the grid-side current i. 2abc By transforming from the abc coordinate system to the dq coordinate system, the grid-side current i can be obtained. 2abc Instantaneous active current component i d and reactive current component i q Active current reference i d * Reactive current reference i q * and actual value i d i q The error between them is adjusted by the PI controller, and then the reference voltage u is output through decoupled feedback. rd u rq .

[0057] Figure 3 This is the control block diagram of the grid-connected PCS current loop when capacitor current feedback control is introduced. Figure 3 In the middle, i2_ ref It is the reference value for the grid-connected current i2, G PI (s) is a PI controller, and its transfer function expression is k p +k i / s, K PWM It is the gain of PCS, H i This is the capacitor current feedback coefficient. (Regarding...) Figure 3 The control block diagram shown can be transformed into a simplified control block diagram, such as... Figure 4 As shown, where:

[0058]

[0059] The loop gain of the above closed-loop system:

[0060]

[0061] The current on the grid side is:

[0062]

[0063] According to equation (8), a single PCS can be represented by a Norton equivalent circuit in admittance form, such as Figure 5 As shown. Where I*(s) is the equivalent current source, Teq (s) is the equivalent admittance, and

[0064]

[0065] Each PCS is equivalent to a controlled voltage source, connected to the power grid through the equivalent impedance of the LCL filter and the equivalent impedance of the line. Its specific equivalent model is as follows: Figure 6 As shown. Z 1i Z 2i (i=1···n) is the impedance of the filter inductor, Z 3i It is the impedance of the filter capacitor, Z g It is the power grid impedance, u ii It is the output voltage of the PCS, u g It is the grid voltage, i i It is the grid-side current. According to... Figure 6 The equivalent circuit shown represents the grid-side current i for each PCS. i With PCS output voltage u ii and grid voltage u g The relationship between them can be represented by a matrix:

[0066]

[0067] Assuming all parameters of each PCS in a parallel system are identical, including their hardware and software parameters, then

[0068]

[0069] Based on the above assumptions, all diagonal elements of the matrix are identical, denoted by G1, and all off-diagonal elements are identical, denoted by G2. Through the superposition principle and circuit simplification, the relationship matrices between the grid-side current of each PCS and its own PCS output voltage, the output voltages of other PCS, and the grid voltage are derived, denoted as G1, G2, and H1 respectively. Their specific expressions are as follows:

[0070]

[0071] The resonant characteristics of the system are analyzed by plotting the logarithmic frequency response curves of the relation matrices G1, G2, and H1.

[0072] After introducing a capacitor current feedback type active damping strategy, a single PCS can be represented by a Norton equivalent circuit in admittance form, such as Figure 5 As shown. Based on this, each parallel PCS system is equivalent to a current source connected in parallel with the output impedance, and connected to the power grid through the equivalent impedance of the power grid. Therefore, the Norton equivalent circuit of n parallel PCS systems can be obtained, with the equivalent topology as shown. Figure 7 As shown.

[0073] exist Figure 7 middle, I i *(s)(i=1···n) represents the equivalent current source of the i-th PCS, T eq (s) represents the equivalent admittance. Assuming all PCS are identical, the equivalent admittance is also identical. According to the superposition theorem, taking the first PCS as an example, the following grid-side current can be obtained.

[0074]

[0075] Clearly, equation (13) has three independent terms. For the first PCS, these three terms represent the grid-side current I1(s) and the excitation sources I1*(s), I2*(s) ~ I when these excitation sources act individually. n *(s), U g The transfer function between (s) is similar for other PCS.

[0076] Equation (14) gives the relationship between the equivalent current source and the current reference value. Then, the grid-side current I1(s) and the current reference value I can be obtained through equation (13). 1ref (s), I 2ref Relationship expression between (s).

[0077]

[0078] Based on this, the grid-side current I1(s) and the current reference value I can be plotted. 1ref (s), I 2ref (s) and voltage source U g The logarithmic frequency response curve of the transfer function between (s) is used to analyze the stability of the system.

[0079] In the embodiment, the DC voltage V dc The voltage is 780V; the filter inductor L1 on the PCS side is 0.25mH; the filter inductor L2 on the grid side is 0.08mH; the filter capacitor C is 220uF; the grid-connected inductor L... g It is 0.03mH; the capacitor current feedback coefficient H i The value is 3, and the switching frequency is 10kHz.

[0080] To demonstrate the correctness of the theoretical analysis proposed in this invention, the system was simulated in MATLAB / Simulink. Figure 8 The waveform of the grid-connected current of a single PCS is given when no active damping control strategy is adopted. Figure 9 The grid-connected voltage and current waveforms of a single PCS are presented when an active damping control strategy is adopted. Figure 10The grid-connected voltage and current waveforms of a four-PCS parallel system are presented when an active damping control strategy is employed. Therefore, the active damping control strategy applied in this invention can effectively suppress system resonance spikes and improve system stability. Simultaneously, the stability analysis method proposed in this invention can effectively identify the system's operating state, providing theoretical guidance for improving system stability.

[0081] The above method is only a relatively reasonable implementation of the invention. Any equivalent modifications or changes made by those skilled in the art based on the content disclosed in the invention should be included within the scope of protection of the claims.

Claims

1. A stability analysis method for a multi-unit parallel system of energy storage converters, characterized in that: It includes the following steps: 1) Based on the topology of a single grid-connected PCS, and the adopted... dq A PI control strategy with current feedforward decoupling in the coordinate system is proposed. At the same time, when using capacitor current feedback active damping control, a control block diagram of the current loop of a single PCS is established. The block diagram is simplified by using Mason's formula, and a Norton equivalent circuit model of a single PCS is established. 2) Based on the equivalent model of a single grid-connected PCS, establish n An equivalent model of a parallel PCS system is derived by using the superposition principle and circuit simplification to derive the relationship matrix between the grid-side current of each PCS, the output voltage of the PCS, and the grid voltage. The resonant characteristics of the system are analyzed by plotting the logarithmic frequency response curve of the transfer function of the parallel system. 3) After introducing the capacitor current feedback type active damping strategy, a single PCS is represented by the Norton equivalent circuit in admittance form. Based on this, each parallel PCS system is equivalent to a structural model where a current source is connected in parallel with the output impedance and connected to the power grid through the power grid equivalent impedance, thus obtaining... n The Norton equivalent circuit for a parallel PCS system is as follows: The control block diagram is transformed into a simplified control block diagram, where: (1) Loop gain of the closed-loop system: (2) The current on the grid side is: (3) According to equation (3), a single PCS can be represented by a Norton equivalent circuit in admittance form, where I *( s () is an equivalent current source. T eq ( s ) is equivalent admittance, and (4) Therefore, each parallel PCS system is equivalent to a structural model where a current source and an output impedance are connected in parallel, and the system is connected to the grid through the grid's equivalent impedance. Assuming all PCS systems are identical, their equivalent admittances are the same. According to the superposition theorem, taking the first PCS as an example, the following grid-side current is obtained: (5) 4) Drawing n The logarithmic frequency response curves of the transfer function of a parallel PCS system are used to verify the impact of the active damping control strategy on the resonance spikes of the parallel system and the system stability.

2. The stability analysis method for a multi-machine parallel system of energy storage converters according to claim 1, characterized in that: The block diagram of the single PCS current loop control in step 1) of the capacitor current feedback type active damping control includes sampling and filtering the capacitor current. i c Multiply it by a feedback coefficient H i The current is applied to the modulated wave signal to achieve active damping of the system. The specific implementation of this block diagram is as follows: PCS grid-side current setpoint. i 2_ ref Its actual value i 2. Perform the difference operation; after the difference is processed by the PI controller, subtract the capacitor current. i c The feedback, multiplied by the gain of PCS K PWM The output voltage of the PCS is obtained. u i Based on the relationship between voltage and current in the LCL filter topology, a complete current loop control block diagram is obtained.

3. The stability analysis method for a multi-unit parallel system of energy storage converters according to claim 1, characterized in that: Step 2) is as follows: Each PCS is equivalent to a controlled voltage source and connected to the power grid through the equivalent impedance of the LCL filter and the equivalent impedance of the line. Assuming all parameters of each PCS in the parallel system are identical, including their hardware and software parameters, the relationship matrix between the grid-side current of each PCS and its own output voltage, the output voltages of other PCS, and the grid voltage is derived through the superposition principle and circuit simplification. These relationships are then expressed as follows: G 1, G 2 and H 1. Draw G 1, G 2 and H The logarithmic frequency response curve of 1 is obtained, thus enabling the analysis of the system's resonant characteristics without introducing an active damping strategy.

4. The stability analysis method for a multi-machine parallel system of energy storage converters according to claim 1, characterized in that: Step 4) drawing n The logarithmic frequency response curve of the transfer function of the parallel PCS system is used to analyze the system stability, as follows: Equation (5) has three independent terms. For the first PCS, these three terms represent the grid-side current when these excitation sources act individually. I 1( s and incentives I 1*( s ), I 2*( s )~ I n *( s ), U g ( s The transfer function between the two phases is the same for all other phases. Based on this, the grid-side current is plotted. I 1( s ) and current reference value I 1ref ( s ), I 2ref ( s ) and voltage source U g ( s The logarithmic frequency response curve of the transfer function between the two systems is obtained, and the stability of the system is then analyzed based on the characteristic curve.

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