A stability analysis method for building low-voltage DC power supply system considering photovoltaic fluctuations

Abstract

The present invention discloses a stability analysis method for a building low-voltage direct current power supply system taking into account photovoltaic fluctuations, which belongs to the technical field of building direct current power supply. The method comprises: dividing the low-voltage direct current power supply system into an active subsystem and a passive subsystem; the low-voltage direct current power supply system comprises a photovoltaic module, an energy storage module and a constant power load module; establishing an overall negative feedback model based on the active subsystem and the passive subsystem, and defining an equivalent open-loop transfer function; obtaining the current operating state parameters of each module in the low-voltage direct current power supply system, and determining the current characteristic curve of the equivalent open-loop transfer function; determining whether the current characteristic curve of the equivalent open-loop transfer function meets the stability condition, and if so, it indicates that the low-voltage direct current power supply system is stable, otherwise, the low-voltage direct current power supply system is unstable. Compared with a multi-source multi-load system, the present invention has the advantage of simplified structure, and is convenient for stability analysis of a direct current power supply system caused by photovoltaic fluctuations.

Description

Technical Field

[0001] The invention belongs to the technical field of building DC power supply, and particularly relates to a method for analyzing the stability of a building low-voltage DC power supply system considering photovoltaic fluctuations. Background Art

[0002] At present, the building mainly adopts the "photovoltaic + AC" power distribution mode, and generally does not have an energy storage part. If the building adopts a photovoltaic flexible DC power distribution system, it has the following characteristics:

[0003] (1) For a building, continuous power supply is required, and its power consumption is large. After adding "photovoltaic + DC" and equipping with energy storage, it can better achieve peak shaving and valley filling. At the same time, when the mains power fails, "photovoltaic + energy storage" can also ensure the safe operation of the system under certain conditions; (2) The traditional AC power supply system is relatively complex, while the DC power supply only needs to consider the bus voltage and power, without considering the phase, reactive power, etc., which simplifies the design of the control system; (3) The DC system directly supplies power to the load without a conversion link, reducing losses; (4) For users, their requirements for power quality are relatively high. DC power supply can reduce the power loss of the system compared with AC power supply, and can also eliminate the harmonic pollution therein, improving the power quality. Therefore, in order to meet the power supply and distribution requirements under the new situation, it is necessary to develop the building low-voltage DC (Low Voltage DC, LVDC) power supply technology.

[0004] With the substantial increase in the proportion of photovoltaic power generation in the LVDC system, the power system with a high proportion of new energy penetration shows low-damping and low-inertia characteristics. The interaction between the system and the power electronic devices with multi-time scale control characteristics leads to multi-time scale small disturbance problems of low frequency and high frequency in the LVDC system; In order to achieve efficient photovoltaic power generation, the photovoltaic control strategy usually adopts a multi-loop control system based on maximum power point tracking (MPPT); In practical applications, the photovoltaic module is greatly affected by temperature and light, and the maximum power shows a multi-peak characteristic, threatening the stability of the LVDC system.

[0005] At present, the stability problem of the LVDC system mainly focuses on the small disturbance stability analysis. Common methods include the state space method and the impedance analysis method; When analyzing the small disturbance stability problem of the photovoltaic access system at present, the droop control is usually adopted for the photovoltaic module, and the volatility of the photovoltaic is not considered, but the small disturbance stability problem caused by environmental changes cannot be ignored in actual engineering. Summary of the Invention

[0006] The purpose of the invention is to provide a method for analyzing the stability of a building low-voltage DC power supply system considering photovoltaic fluctuations, which can simplify the system structure and facilitate the stability analysis of the DC power supply system caused by photovoltaic fluctuations.

[0007] The present invention provides the following technical solutions:

[0008] A method for analyzing the stability of a building low-voltage DC power supply system considering PV fluctuations, comprising:

[0009] Dividing the low-voltage DC power supply system into an active subsystem and a passive subsystem; the low-voltage DC power supply system includes a PV module, an energy storage module, and a constant power load module;

[0010] Establishing an overall negative feedback model based on the active subsystem and the passive subsystem, and defining the equivalent open-loop transfer function of the overall negative feedback model;

[0011] Obtaining the current operating state parameters of each module in the low-voltage DC power supply system, and determining the current characteristic curve of the equivalent open-loop transfer function according to the obtained current operating state parameters;

[0012] Determining whether the current characteristic curve of the equivalent open-loop transfer function satisfies the stability condition. If it satisfies, it indicates that the low-voltage DC power supply system is stable; otherwise, the low-voltage DC power supply system is unstable.

[0013] Optionally, in the step of dividing the low-voltage DC power supply system into an active subsystem and a passive subsystem, the active subsystem includes all converters of the DC voltage control unit and the current control unit; the passive subsystem includes a transmission network and a bus.

[0014] Optionally, in the step of establishing an overall negative feedback model based on the active subsystem and the passive subsystem, and defining the equivalent open-loop transfer function of the overall negative feedback model,

[0015] When establishing the overall negative feedback model, the DC voltage control unit is represented by a Norton equivalent model of a voltage source in series with an impedance, the current control unit is represented by a Norton equivalent model of a current source in parallel with an admittance, and the passive subsystem is represented by its own voltage-current relationship.

[0016] Optionally, in the step of establishing an overall negative feedback model based on the active subsystem and the passive subsystem, and defining the equivalent open-loop transfer function of the overall negative feedback model,

[0017] The overall negative feedback model is expressed as:

[0018] [ Delta u o 1 Delta u o 2 ⋮ Delta u on ] = ( Z net + Z con ) [ Delta i 1 Delta i 2 ⋮ Delta i n ]

[0019] Wherein, is the perturbation of the Norton equivalent voltage source of the th converter, is the perturbation of the port current of the th converter, is the number of converters in the active subsystem, is the port impedance matrix of the active subsystem converter, is the impedance matrix of the passive subsystem;

[0020] The equivalent open-loop transfer function of the overall negative feedback model is:

[0021]

[0022] where, is the output admittance at the port of the active subsystem converter.

[0023] Optionally, in the step of obtaining the current operating state parameters of each module in the low-voltage DC power supply system and determining the current characteristic curve of the equivalent open-loop transfer function according to the obtained current operating state parameters,

[0024] the current characteristic curve of the equivalent open-loop transfer function is the eigenvalue locus of the impedance matrix of the equivalent open-loop transfer function in the frequency domain under the current light intensity;

[0025] Before determining the current characteristic curve of the equivalent open-loop transfer function, determine the output admittance at the port of the active subsystem converter and the impedance matrix of the passive subsystem under the current light intensity, and determine the impedance matrix of the equivalent open-loop transfer function according to the output admittance at the port of the active subsystem converter and the impedance matrix of the passive subsystem; the output admittance at the port of the active subsystem converter is the sum of the equivalent output admittances of the photovoltaic module interface converter, the energy storage module interface converter, and the constant power load module interface converter;

[0026] When determining the equivalent output admittance of the photovoltaic module interface converter, the photovoltaic cell of the photovoltaic module is connected to the DC bus through a Boost converter, and a dual-loop control based on MPPT is adopted, with the outer loop being a power control loop based on the perturbation observation method and the inner loop being a voltage loop;

[0027] When determining the equivalent output admittance of the energy storage module interface converter, the energy storage module is controlled by a droop control strategy and undergoes double closed-loop PI regulation of voltage and current;

[0028] When determining the equivalent output admittance of the constant power load module interface converter, the constant power load module adopts PI current control.

[0029] Optionally, the equivalent output admittance of the photovoltaic module interface converter is:

[0030]

[0031] where, is the frequency domain, is the equivalent capacitance on the photovoltaic cell side, is the equivalent capacitance on the output side of the photovoltaic module converter, is the current flowing through the energy storage inductor of the photovoltaic module converter, is the equivalent transfer function of the PI link of the photovoltaic module, is the output voltage on the output side of the photovoltaic module, is the short-circuit current of the photovoltaic module, is the open-circuit voltage of the photovoltaic module, is the coefficient of the short-circuit current of the photovoltaic module, is the coefficient of the open-circuit voltage of the photovoltaic module, and are the proportional coefficient and integral coefficient of the PI link of the voltage loop respectively, is the output voltage of the photovoltaic cell, is the energy storage inductor of the photovoltaic module converter, is the line resistance of the photovoltaic module converter, is the reference value of the maximum output current, is the reference value of the short-circuit current, is the reference value of the maximum output voltage, is the reference value of the open-circuit voltage, 、 and are all parameters for simplifying the formula.

[0032] Optionally, the equivalent output admittance of the energy storage module interface converter is:

[0033]

[0034] Among them, is the output impedance of the energy storage module, is the equivalent transfer function of the current loop PI link of the energy storage module, is the droop function of the energy storage module, is the equivalent transfer function of the voltage loop PI link of the energy storage module, is the frequency domain, is the duty cycle of the energy storage module converter, is the capacitance on the output side of the energy storage unit, is the output voltage on the output side of the energy storage module, is the output current of the energy storage module, is the inductor of the energy storage module, is the resistance of the energy storage module, 、 、 and are all parameters for simplifying the formula.

[0035] Optionally, the equivalent output admittance of the constant power load module interface converter is:

[0036]

[0037] where is the duty cycle of the constant power load module converter, is a pure resistive load, is the bus-side voltage, is the equivalent transfer function of the PI link of the constant power load module, is the frequency domain, is the filter capacitor, is the filter inductor, is the converter line resistance of the constant power load module, and are both parameters for simplifying the formula.

[0038] Optionally, the impedance matrix of the passive subsystem is:

[0039] { Z net = [ Z 11 Z 12 Z 21 Z 22 ] ( a + b ) × ( a + b ) Z 11 = [ 0 1 / Z s 2 ⋯ 1 / Z sa 1 / Z s 1 0 ⋯ 1 / Z sa ⋮ ⋮ ⋱ ⋮ 1 / Z s 1 1 / Z s 2 ⋯ 0 ] a × b Z 12 = [ 1 ⋯ 1 ⋮ ⋱ ⋮ 1 ⋅⋅⋅ 1 ] a × b Z 21 = [ g 1 ⋯ g a ⋮ ⋮ ⋮ g 1 ⋯ g a ] b × a Z 22 = [ P − r o 1 P … P P P − r o 2 ⋯ P ⋮ ⋮ ⋱ ⋮ P P ⋯ P − r ob ] b × b g k = 1 a ( 1 − 1 Z sk ∑ t = 1 t ≠ k a r st ) P =− 1 a ∑ i = 1 a r si

[0040] where , , and are respectively 's first block matrix, second block matrix, third block matrix and fourth block matrix, is the number of passive subsystems, is the number of load terminals, is the output impedance on the side of the th passive subsystem, is the output impedance on the side of the th passive subsystem, is the line impedance between the th passive subsystem and the DC bus, is the line impedance between the th passive subsystem and the DC bus, is the line impedance between the th load terminal and the DC bus, , and are all parameters for simplifying the formula, takes a value greater than 1 and less than or equal to .

[0041] Optionally, determine whether the current characteristic curve of the equivalent open-loop transfer function meets the stability condition. If it meets, it indicates that the low-voltage DC power supply system is stable; otherwise, the low-voltage DC power supply system is unstable.

[0042] The stability condition is that the proximity of the current characteristic curve of the equivalent open-loop transfer function to the characteristic curve corresponding to the previous light intensity is lower than the set stability margin.

[0043] When the low-voltage DC power supply system is unstable, active damping compensation is added to the control strategy of the energy storage module to improve the system stability.

[0044] The active damping compensation is to add a compensation function in series to the droop control loop of the energy storage module when the photovoltaic output decreases to improve the system stability.

[0045] The compensation function is:

[0046]

[0047] where is the frequency domain.

[0048] Compared with the prior art, the beneficial effects of the present invention are:

[0049] (1) In the building low-voltage DC power supply system of the present invention, it is divided into an active subsystem and a passive subsystem, which can simplify the system structure compared with the multi-source multi-load system; at the same time, considering the photovoltaic fluctuation, the impedance method is used to model the photovoltaic module in detail, which is convenient for stability analysis.

[0050] (2) The present invention uses the equivalent open-loop transfer function to analyze the small-signal stability problem of the system, providing a new idea for the determination and design of the small-signal stability of the system; in addition, the present invention adds active damping compensation to the control strategy of the energy storage module, which can enhance the system stability margin, increase the light intensity range for maintaining the stable operation of the system, and improve the photovoltaic utilization rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 is a flowchart of the method for analyzing the stability of a building low-voltage DC power supply system considering photovoltaic fluctuations according to the present invention.

[0052] Figure 2 is a model diagram of the overall negative feedback model of the present invention.

[0053] Figure 3 is a control strategy diagram of the photovoltaic module of the present invention.

[0054] Figure 4 is a control strategy diagram of the energy storage module of the present invention.

[0055] Figure 5 is the topological structure diagram of the passive subsystem of the present invention.

[0056] Figure 6 is the topological structure diagram of the typical LVDC system of the present invention.

[0057] Figure 7 is the result of the influence of the light intensity on the stability of the LVDC system of the present invention.

[0058] Figure 8 is the result of the waveform comparison of the influence of adding damping compensation on the stability of the LVDC system of the present invention.

[0059] Figure 9 is Figure 8 the schematic diagram of the partial enlarged structure of part A in Specific embodiments

[0060] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present invention.

[0061] Embodiment 1

[0062] As Figure 1 shown, a method for analyzing the stability of a building low-voltage DC power supply system considering photovoltaic fluctuations includes the following steps:

[0063] S1: Divide the low-voltage DC power supply system into an active subsystem and a passive subsystem; the low-voltage DC power supply system includes a photovoltaic module, an energy storage module, and a constant power load module.

[0064] Specifically, from the perspective of the overall system, the low-voltage DC power supply system can be regarded as an equivalent source - transmission network - load system in actual operation; the low-voltage DC power supply system includes a DC voltage control unit, a transmission line network, and a current control unit; for simplicity of analysis, all converters in the DC voltage control unit and the current control unit in the system can be defined as the active subsystem, and the transmission line (transmission network and bus) is equivalently defined as the passive subsystem.

[0065] S2: Establish an overall negative feedback model based on the active subsystem and the passive subsystem, and define the equivalent open-loop transfer function of the overall negative feedback model.

[0066] Specifically, according to circuit theory, in a two-port linear circuit with a source, the DC voltage control unit can be represented by a Norton equivalent model of a voltage source in series with an impedance, and the current control unit can be represented by a Thevenin equivalent model of a current source in parallel with a admittance. If it is only used for system modeling and does not affect the system stability analysis, the Thevenin equivalent model of the current control unit can be represented by a Norton equivalent model. Therefore, asFigure 2 As shown, when establishing the overall negative feedback model, the DC voltage control unit is represented by the Norton equivalent model of a voltage source in series with an impedance, the current control unit is represented by the Norton equivalent model of a current source in parallel with an admittance, and the passive subsystem is represented by its own voltage-current relationship.

[0067] Specifically, the Norton equivalent models on both sides of the passive subsystem connected to the system are expressed as:

[0068]

[0069] Among them, is the perturbation amount of the Norton equivalent voltage source of the th converter, is the perturbation amount of the port current of the th converter, is the number of converters in the active subsystem, is the th converter port impedance, is the perturbation amount of the port voltage of the th converter.

[0070] The passive subsystem can also be described as:

[0071] [ Delta u 1 Delta u 2 ⋮ Delta u n ] = Z net [ Delta i 1 Delta i 2 ⋮ Delta i n ]

[0072] Among them, is the impedance matrix of the passive subsystem;

[0073] From the above two formulas, the overall negative feedback model is expressed as:

[0074] [ Delta u o 1 Delta u o 2 ⋮ Delta u on ] = ( Z net + Z con ) [ Delta i 1 Delta i 2 ⋮ Delta i n ]

[0075] Among them, is the perturbation amount of the Norton equivalent voltage source of the th converter, is the perturbation amount of the port current of the th converter, is the number of converters in the active subsystem, is the impedance matrix of the active subsystem converter ports, is the impedance matrix of the passive subsystem;

[0076] ;

[0077] The equivalent open-loop transfer function of the overall negative feedback model is:

[0078]

[0079] wherein, is the output admittance of the active subsystem converter port.

[0080] S3: Obtain the current operating state parameters of each module in the low-voltage DC power supply system, and determine the current characteristic curve of the equivalent open-loop transfer function according to the obtained current operating state parameters.

[0081] The current characteristic curve of the equivalent open-loop transfer function is the locus of the characteristic roots of the impedance matrix of the equivalent open-loop transfer function in the frequency domain under the current light intensity, that is, the Nyquist locus.

[0082] Before determining the current characteristic curve of the equivalent open-loop transfer function, determine the output admittance of the active subsystem converter port and the impedance matrix of the passive subsystem at the current light intensity, and determine the impedance matrix of the equivalent open-loop transfer function according to and ; after obtaining the impedance matrix of the equivalent open-loop transfer function, the characteristic roots corresponding to the impedance matrix of the equivalent open-loop transfer function in the frequency domain can be obtained according to the existing technology, and the locus of the characteristic roots can be generated; the output admittance of the active subsystem converter port is the equivalent output admittance of the photovoltaic module interface converter , the equivalent output admittance of the energy storage module interface converter and the equivalent output admittance of the constant power load module interface converter sum.

[0083] When determining the equivalent output admittance of the photovoltaic module interface converter, the photovoltaic cells of the photovoltaic module are connected to the DC bus through a Boost converter, and a double-loop control based on MPPT is adopted. The outer loop is a power control loop based on the perturbation observation method, and the inner loop is a voltage loop.

[0084] Specifically, as Figure 3 shown, the photovoltaic cells are connected to the DC bus through a Boost converter, and a double-loop control based on MPPT is adopted. The outer loop is a power control loop based on the perturbation observation method (Perturbation & Observation, P&O), and the inner loop is a voltage loop to control the photovoltaic cell port voltage Upv to follow the P&O output voltage Upvm. Since the response speed of the outer loop is much slower than that of the inner loop, the characteristics of the outer loop can be ignored during analysis.

[0085] The equivalent output admittance of the photovoltaic module interface converter is:

[0086]

[0087] wherein, is the frequency domain, is the equivalent capacitance on the photovoltaic cell side, is the equivalent capacitance at the output side of the photovoltaic module converter, is the current flowing through the energy storage inductor of the photovoltaic module converter, is the equivalent transfer function of the PI link of the photovoltaic module, is the voltage at the output side of the photovoltaic module, is the short - circuit current of the photovoltaic module, is the open - circuit voltage of the photovoltaic module, is the coefficient of the short - circuit current of the photovoltaic module, is the coefficient of the open - circuit voltage of the photovoltaic module, and are the proportional coefficient and integral coefficient of the PI link of the voltage loop respectively, is the output voltage of the photovoltaic cell, is the energy storage inductor of the photovoltaic module converter, is the line resistance of the photovoltaic module converter, is the reference value of the maximum output current, is the reference value of the short - circuit current, is the reference value of the maximum output voltage, is the reference value of the open - circuit voltage, 、 and are used to simplify the formula and have no practical meaning.

[0088] Such as Figure 4 shown, when determining the equivalent output admittance of the energy storage module interface converter, the energy storage module is controlled by a droop control strategy and undergoes double - closed - loop PI regulation of voltage and current.

[0089] The equivalent output admittance of the energy storage module interface converter is:

[0090]

[0091] Wherein, is the output impedance of the energy storage module, is the equivalent transfer function of the current - loop PI link of the energy storage module, is the droop function of the energy storage module, is the equivalent transfer function of the voltage - loop PI link of the energy storage module, is the frequency domain, is the duty cycle of the energy storage module converter, is the capacitance at the output side of the energy storage unit, is the voltage at the output side of the energy storage module, is the current at the output side of the energy storage module, is the output current of the energy storage module, is the inductor of the energy storage module, is the resistor of the energy storage module, , , and are used to simplify the formula and have no practical meaning.

[0092] When determining the equivalent output admittance of the constant power load module interface converter, the constant power load module adopts PI current control.

[0093] The equivalent output admittance of the constant power load module interface converter is:

[0094]

[0095] Wherein, is the duty cycle of the constant power load module converter, is a pure resistive load, is the bus-side voltage, is the equivalent transfer function of the PI link of the constant power load module, is the frequency domain, is the filter capacitor, is the filter inductor, is the converter line resistance of the constant power load module, and are used to simplify the formula and have no practical meaning.

[0096] As Figure 5 shown, the impedance matrix of the passive subsystem is:

[0097] { Z net = [ Z 11 Z 12 Z 21 Z 22 ] ( a + b ) × ( a + b ) Z 11 = [ 0 1 / Z s 2 ⋯ 1 / Z sa 1 / Z s 1 0 ⋯ 1 / Z sa ⋮ ⋮ ⋱ ⋮ 1 / Z s 1 1 / Z s 2 ⋯ 0 ] a × b Z 12 = [ 1 ⋯ 1 ⋮ ⋱ ⋮ 1 ⋅⋅⋅ 1 ] a × b Z 21 = [ g 1 ⋯ g a ⋮ ⋮ ⋮ g 1 ⋯ g a ] b × a Z 22 = [ P − r o 1 P … P P P − r o 2 ⋯ P ⋮ ⋮ ⋱ ⋮ P P ⋯ P − r ob ] b × b g k = 1 a ( 1 − 1 Z sk ∑ t = 1 t ≠ k a r st ) P =− 1 a ∑ i = 1 a r si

[0098] Wherein, , , and are respectively the first block matrix, the second block matrix, the third block matrix and the fourth block matrix of is the number of passive subsystems, is the number of load terminals, is the output impedance on the side of the th passive subsystem, is the output impedance on the side of the th passive subsystem, is the line impedance between the th passive subsystem and the DC bus, is the The line impedance between a passive subsystem and the DC bus is the line impedance between the th load terminal and the DC bus, which is used to simplify the formula and has no practical meaning, and the value of is greater than 1 and less than or equal to

[0099] When obtaining the current operating state parameters of each module in the low-voltage DC power supply system, the current operating state parameters are mainly used to solve the output admittance of the active subsystem converter port and the impedance matrix of the passive subsystem . The current operating state parameters can be obtained by using existing technologies. All variables in the above formula are values at the steady-state operating point of the system; specifically, for the current operating state parameters of each module, reference can be made to the formula for solving the equivalent output admittance of the interface converter of each module. For example, when solving the equivalent output admittance of the photovoltaic module interface converter, it is necessary to obtain the frequency domain, the equivalent capacitance on the photovoltaic cell side, the equivalent capacitance on the outlet side of the photovoltaic module converter, the current flowing through the energy storage inductor of the photovoltaic module converter, the equivalent transfer function of the PI link of the photovoltaic module, the outlet side voltage of the photovoltaic module, the short-circuit current of the photovoltaic module, the open-circuit voltage of the photovoltaic module, the output voltage of the photovoltaic cell, the energy storage inductor of the photovoltaic module converter, the line resistance of the photovoltaic module converter, etc.; when solving the equivalent output admittance of the energy storage module interface converter, it is necessary to obtain the output impedance of the energy storage module, the equivalent transfer function of the current loop PI link of the energy storage module, the droop function of the energy storage module, the equivalent transfer function of the voltage loop PI link of the energy storage module, the frequency domain, the duty cycle of the energy storage module converter, the capacitance on the outlet side of the energy storage unit, the outlet side voltage of the energy storage module, the output current of the energy storage module, the inductor of the energy storage module, the resistor of the energy storage module, etc.; similarly, the specific parameters to be obtained can be known according to the formula for the equivalent output admittance of the constant power load module interface converter and the formula for the impedance matrix of the passive subsystem.

[0100] S4: Determine whether the current characteristic curve of the equivalent open-loop transfer function meets the stability condition. If it meets, it indicates that the low-voltage DC power supply system is stable; otherwise, the low-voltage DC power supply system is unstable.

[0101] The stability condition is that the proximity of the current characteristic curve of the equivalent open-loop transfer function to the characteristic curve corresponding to the previous light intensity is lower than the set stability margin.

[0102] In some other embodiments, the influence law of light intensity change on the stability of the low-voltage DC system can also be summarized.

[0103] Furthermore, when the low-voltage DC power supply system is unstable, active damping compensation is added to the control strategy of the energy storage module to improve the system stability.

[0104] The active damping compensation is to connect a compensation function in series in the droop control loop of the energy storage module when the photovoltaic output decreases to improve the system stability;

[0105] The compensation function is:

[0106]

[0107] where is the frequency domain.

[0108] Embodiment 2

[0109] Provide a specific example to analyze the influence of light intensity on the stability of the LVDC system.

[0110] As Figure 6 shown, take the DC bus voltage of 400V and the 8-converter system (2 sets of energy storage modules, 2 sets of photovoltaic modules, 2 sets of constant power load modules and 2 sets of constant voltage loads) as an example.

[0111] According to Figure 1 the flow of the stability analysis method of the building low-voltage DC power supply system shown, the influence result of changing light on the stability of the LVDC system can be obtained, as Figure 7 shown. When the system is under the condition of light intensity of 1000W / m2, the generalized Nyquist locus does not enclose the point (-1, j0), and the system is stable; when the light intensity suddenly changes to 1200W / m2, the distance between the generalized Nyquist locus and the point (-1, j0) becomes larger, and the system stability margin is improved; when the light intensity becomes 800W / m2, the generalized Nyquist locus encloses the point (-1, j0), and the system becomes unstable. It also verifies that as the light intensity weakens, the equivalent output impedance of the photovoltaic module decreases, and the stability margin of the system gradually decreases, revealing the influence of light intensity change on the system stability.

[0112] Embodiment 3

[0113] Provide a specific example to analyze the influence of adding active damping compensation on the stability of the LVDC system.

[0114] As Figure 6 shown, take the DC bus voltage of 400V and the 8-converter system (2 sets of energy storage modules, 2 sets of photovoltaic modules, 2 sets of constant power load modules and 2 sets of constant voltage loads) as an example.

[0115] According to Figure 1The stability analysis method process of the shown low-voltage DC power supply system for buildings can obtain the results after compensation, such as Figure 8 and Figure 9 as shown. When no compensation is added, the generalized Nyquist locus encloses the point (-1, j0), and the system is unstable. After adding compensation, the generalized Nyquist locus moves away from the point (-1, j0), and the system returns to stability, indicating that it also verifies that adding an active compensation link based on low-pass filtering in the DC voltage control unit can enhance the system stability margin, increase the illumination range for maintaining stable system operation, and improve the utilization rate of photovoltaic power.

[0116] The embodiments in this specification are described in a progressive manner. Each embodiment focuses on the differences from other embodiments. For the same or similar parts among the embodiments, reference can be made to each other.

[0117] Those skilled in the art should understand that the embodiments of the present application can be provided as methods, systems, or computer program products. Those skilled in the art can clearly understand that the technologies in the embodiments of the present invention can be implemented by means of software plus a necessary general hardware platform. Based on such an understanding, the technical solutions in the embodiments of the present invention, in essence, or the part that contributes to the prior art can be embodied in the form of a software product. This computer software product can be stored in a storage medium, such as ROM / RAM, magnetic disk, optical disc, etc., and includes several instructions to enable a computer device (which can be a personal computer, server, or network device, etc.) to execute the methods described in each embodiment or some parts of the embodiments of the present invention.

[0118] The above is only the preferred embodiment of the present invention. The protection scope of the present invention is not limited to the above embodiments. All technical solutions falling within the concept of the present invention belong to the protection scope of the present invention. It should be pointed out that for those of ordinary skill in the art, several improvements and refinements made without departing from the principle of the present invention should be regarded as the protection scope of the present invention.

Claims

1. A stability analysis method for a building low-voltage DC power supply system taking into account photovoltaic fluctuations, characterized in that: include: The low-voltage DC power supply system is divided into an active subsystem and a passive subsystem; the low-voltage DC power supply system includes a photovoltaic module, an energy storage module and a constant power load module; Establish an overall negative feedback model based on active subsystems and passive subsystems, and define the equivalent open-loop transfer function of the overall negative feedback model; Acquire current operating state parameters of each module in the low-voltage DC power supply system, and determine a current characteristic curve of an equivalent open-loop transfer function according to the acquired current operating state parameters; Determine whether the current characteristic curve of the equivalent open-loop transfer function meets the stability condition. If so, it indicates that the low-voltage DC power supply system is stable. Otherwise, the low-voltage DC power supply system is unstable. The low voltage DC power supply system is divided into an active subsystem and a passive subsystem, wherein the active subsystem includes all converters of the DC voltage control unit and the current control unit; and the passive subsystem includes the transmission network and the busbar; The overall negative feedback model based on the active subsystem and the passive subsystem is established, and the equivalent open-loop transfer function of the overall negative feedback model is defined. When establishing the overall negative feedback model, the DC voltage control unit is represented by the Norton equivalent model of voltage source series impedance, the current control unit is represented by the Norton equivalent model of current source parallel admittance, and the passive subsystem is represented by its own voltage-current relationship; The overall negative feedback model based on the active subsystem and the passive subsystem is established, and the equivalent open-loop transfer function of the overall negative feedback model is defined. The overall negative feedback model is expressed as: ； in, For the The disturbance of the Norton equivalent voltage source of the converter is, For the The disturbance of the current at each converter port is is the number of converters in the active subsystem, is the active subsystem converter port impedance matrix, is the impedance matrix of the passive subsystem; The equivalent open-loop transfer function of the overall negative feedback model is for: ； in, Output admittance of the converter port of the active subsystem; The current operating state parameters of each module in the low-voltage DC power supply system are obtained, and according to the obtained current operating state parameters, a current characteristic curve of the equivalent open-loop transfer function is determined. The current characteristic curve of the equivalent open-loop transfer function is a characteristic root trajectory of the impedance matrix of the equivalent open-loop transfer function in the frequency domain under the current light intensity; Before determining the current characteristic curve of the equivalent open-loop transfer function, determine the output admittance of the active subsystem converter port and the impedance matrix of the passive subsystem under the current light intensity, and determine the impedance matrix of the equivalent open-loop transfer function according to the output admittance of the active subsystem converter port and the impedance matrix of the passive subsystem; the output admittance of the active subsystem converter port is the sum of the equivalent output admittances of the photovoltaic module interface converter, the energy storage module interface converter and the constant power load module interface converter; When determining the equivalent output admittance of the photovoltaic module interface converter, the photovoltaic cell of the photovoltaic module is connected to the DC bus via a Boost converter, and a dual-loop control based on MPPT is adopted, wherein the outer loop is a power control loop based on a disturbance observation method, and the inner loop is a voltage loop; When determining the equivalent output admittance of the energy storage module interface converter, the energy storage module is controlled by adopting a droop control strategy and undergoing voltage and current double closed-loop PI regulation; When determining the equivalent output admittance of the constant power load module interface converter, the constant power load module adopts PI current control; The determination of whether the current characteristic curve of the equivalent open-loop transfer function satisfies the stability condition, if so, it indicates that the low-voltage DC power supply system is stable, otherwise, the low-voltage DC power supply system is unstable. The stability condition is that the degree of closeness between the current characteristic curve of the equivalent open-loop transfer function and the characteristic curve corresponding to the previous light intensity is lower than the set stability margin; When the low-voltage DC power supply system is unstable, active damping compensation is added to the control strategy of the energy storage module to improve system stability; The active damping compensation is to connect a compensation function in series in the droop control loop of the energy storage module when the photovoltaic output decreases. To improve system stability; The compensation function for: ； in, For the frequency domain.

2. A method for analyzing the stability of a building low-voltage DC power supply system taking into account photovoltaic fluctuations according to claim 1, characterized in that: The equivalent output admittance of the photovoltaic module interface converter for: ； in, is the frequency domain, is the equivalent capacitance on the photovoltaic cell side, is the equivalent capacitance at the outlet side of the photovoltaic module converter, is the current flowing through the energy storage inductor of the photovoltaic module converter, is the equivalent transfer function of the PI link of the photovoltaic module, is the outlet voltage of the photovoltaic module, is the short-circuit current of the photovoltaic module, is the open circuit voltage of the photovoltaic module, is the coefficient of the short-circuit current of the photovoltaic module, is the coefficient of the open circuit voltage of the photovoltaic module, and They are the proportional coefficient and integral coefficient of the voltage loop PI link, is the photovoltaic cell output voltage, It is the energy storage inductor of the photovoltaic module converter. is the PV module converter line resistance, is the maximum output current reference value, is the short-circuit current reference value, is the maximum output voltage reference value, is the open circuit voltage reference value, , and These are parameters used to simplify the formula.

3. A method for analyzing the stability of a building low-voltage DC power supply system taking into account photovoltaic fluctuations according to claim 1, characterized in that: The equivalent output admittance of the energy storage module interface converter for: ； in, is the output impedance of the energy storage module, is the equivalent transfer function of the current loop PI link of the energy storage module, is the droop function of the energy storage module, is the equivalent transfer function of the voltage loop PI link of the energy storage module, is the frequency domain, is the duty cycle of the energy storage module converter, is the capacitance at the outlet of the energy storage unit, is the voltage at the outlet of the energy storage module, is the output current of the energy storage module, is the energy storage module inductance, is the resistance of the energy storage module, , , and These are parameters used to simplify the formula.

4. A method for analyzing the stability of a building low-voltage DC power supply system taking into account photovoltaic fluctuations according to claim 1, characterized in that: The equivalent output admittance of the constant power load module interface converter for: ； in, is the duty cycle of the constant power load module converter, For a pure resistive load, is the bus side voltage, is the equivalent transfer function of the PI link of the constant power load module, is the frequency domain, is the filter capacitor, is the filter inductor, is the converter line resistance of the constant power load module, and These are parameters used to simplify the formula.

5. The method for analyzing the stability of a building low-voltage DC power supply system taking into account photovoltaic fluctuations according to claim 1, characterized in that: The impedance matrix of the passive subsystem for: ； in, , , and They are The first block matrix, the second block matrix, the third block matrix and the fourth block matrix, is the number of passive subsystems, is the number of load terminals, For the The output impedance of the passive subsystem side is For the The output impedance of the passive subsystem side is For the The line impedance between the passive subsystem and the DC bus, For the The line impedance between the passive subsystem and the DC bus, For the The line impedance between the load terminal and the DC bus is , and are parameters used to simplify the formula. The value is greater than 1 and less than or equal to .

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