Medium-voltage uninterruptible power supply system based on cascading super-capacitance energy storage and control method

Abstract

The invention relates to a power electronics and energy storage technology, and aims to provide a medium-voltage uninterruptible power supply system based on cascade super-capacitance energy storage and a control method. According to the system, a cascade super-capacitance energy storage module with a three-phase structure is connected between an isolation reactor and a load side switch through a filter reactor and is used for stabilizing the voltage of a load bus; according to the invention, operations of adaptive power reference calculation, voltage phase control and voltage amplitude stable control are carried out; according to the method, the over-capacitance wide voltage operation characteristic is fully considered, and the control on the over-capacitance charging and discharging process is adaptively adjusted according to the power grid fault state and the over-capacitance voltage deviation, so that the system can provide the required power support when the fault occurs, stable energy supplementation is performed after recovery, the risks of over-capacitance voltage out-of-limit and the like are favorably avoided, and the system reliability is improved. And the effectiveness of energy management and the stability of the system are improved. And during a power grid fault period, the load voltage is recovered to be close to a rated value in a short time, voltage drop and overshoot are reduced, and the response speed and the voltage supporting capability of the system are enhanced.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics and energy storage, and particularly relates to a medium-voltage uninterruptible power supply system and a control method based on cascaded supercapacitor energy storage. Background Art

[0002] In recent years, highly sensitive industrial loads typified by precision manufacturing, data centers, and mining operations have imposed stringent requirements on the power supply reliability of medium-voltage power grids. Among them, voltage sags (usually lasting from milliseconds to seconds) are the most common power quality problems, which may trigger major accidents such as equipment downtime and production line paralysis, causing immeasurable economic losses. In existing solutions, although a dual-loop power supply system can provide basic protection through standby bus switching, the action delay of the switch will result in inevitable power supply interruptions during the switching process, making it difficult to achieve millisecond-level uninterruptible power supply. Conventional uninterruptible power supplies are limited by voltage and power ratings and are only applicable to low-voltage scenarios (usually with voltages less than 1 kV), unable to meet the megawatt-level power supply requirements of large-capacity medium-voltage (usually with voltages of 6 - 35 kV) loads.

[0003] To address this technical bottleneck, researchers have proposed various medium-voltage uninterruptible power supply solutions. However, the common problem with these solutions is that at the topological architecture level, a step-up transformer must be configured, introducing additional losses (efficiency loss of about 1 - 2%), and the leakage inductance of the transformer may affect the voltage control accuracy; at the same time, for a low-voltage battery energy storage system to match the power requirements of large-capacity loads, multi-machine parallel connection and relatively complex coordinated control are required, which may lead to stability problems; in addition, the batteries are configured in series and parallel on the DC side, making it difficult to avoid the circulating current problem between the batteries and easily triggering the short-board effect of local accelerated aging, threatening the overall reliability of the system. In view of the above limitations of battery energy storage, supercapacitors (abbreviated as "supercaps" in the present invention) can effectively meet the short-term high-power support requirements in voltage sag scenarios due to their superior characteristics such as extremely long cycle life, high power density, and millisecond-level fast response. This gives it great potential for application in medium-voltage uninterruptible power supply systems, promising faster dynamic response, lower investment costs, and smaller floor space.

[0004] However, since supercaps have a wide variation range and fast dynamic characteristics during the charge and discharge process, if supercaps are to be combined with a medium-voltage uninterruptible power supply system, special and stringent requirements are imposed on the control system in terms of fast response and energy management. From the existing public literature, researchers generally have not paid enough attention to this and no targeted solutions have been seen. If an attempt is made to directly apply existing control methods to this system, the following key technical problems may be faced: 1. Existing control strategies mostly stem from the design experience of battery energy storage systems with relatively constant DC voltage, and do not fully consider the characteristics of the wide variation range of supercapacitors during charge and discharge. Currently, there is still a lack of a scheme for adaptively adjusting the charge and discharge strategy based on the real-time voltage of supercapacitors and the system operating conditions. This results in the inability to optimally discharge when power support is needed, or low efficiency and poor stability during recharge, and it is impossible to achieve effective adaptive energy management of supercapacitors.

[0005] 2. Existing control strategies usually do not take the load voltage amplitude as the direct optimization target, and lack an active suppression mechanism for fault transient shocks and system oscillations. As a result, it is difficult for the system to balance the response speed and voltage stability. Eventually, the voltage amplitude shows a significant drop or overshoot, and may be accompanied by oscillations for a long time before stabilizing, unable to meet the requirements of high-sensitivity loads for power supply quality.

[0006] Therefore, there is an urgent need to develop a new type of medium-voltage uninterruptible power supply system based on cascaded supercapacitor energy storage and its control method to overcome the limitations of existing solutions, achieve adaptive management of supercapacitor energy, and fast, precise, and stable control of the load voltage, so as to meet the stringent requirements of medium-voltage high-sensitivity loads for power supply continuity and stability. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to overcome the deficiencies in the prior art and provide a medium-voltage uninterruptible power supply system and control method based on cascaded supercapacitor energy storage. This solution aims to achieve fast and stable control of the load voltage and ensure efficient adaptive management of supercapacitor energy.

[0008] To solve the technical problem, the solution of the present invention is:

[0009] First, a control method for medium-voltage uninterruptible power supply based on cascaded supercapacitor energy storage is provided, including the following steps: S201. Adaptive power reference calculation Collect the three-phase voltages on the grid side u gx , the three-phase voltages on the load side v x and the three-phase output currents of the cascaded supercapacitor energy storage module i x , x = a , b , c (referring to the three phases of alternating current respectively); according to the voltage v x and the current i x , calculate the load voltage amplitude V m and the active power output by the cascaded supercapacitor energy storage module P; Collect the ultracapacitor voltages of each sub-module in the cascaded ultracapacitor energy storage module U sc,xj , j= 1, 2, …, N , and calculate the average ultracapacitor voltage U SCavg ; Combine the grid fault status and the average ultracapacitor voltage to calculate the reference value of the active power of the cascaded ultracapacitor energy storage module P ref ; S202, Voltage Phase Control According to the reference value of the active power P ref , generate the reference value of the voltage phase through the voltage phase control law, and adjust the output phase of the cascaded ultracapacitor energy storage module; S203, Voltage Magnitude Stability Control Generate the reference value of the modulation voltage through the dynamic damping voltage control law and the voltage magnitude control law to drive the cascaded ultracapacitor energy storage module to stabilize the load voltage magnitude.

[0010] As a preferred solution of the present invention, in the step S201, the average ultracapacitor voltage is calculated by dividing the sum of the ultracapacitor voltages of each sub-module by the total number of sub-modules U SCavg , as shown in the following formula: ; Wherein, N is the number of sub-modules in each phase, j= 1, 2, …, N ; U sc,aj , U sc,bj , U sc,cj are the ultracapacitor voltages of each sub-module in the three phases respectively.

[0011] As a preferred solution of the present invention, in the step S201, the reference value of the active power of the cascaded ultracapacitor energy storage module is calculated according to the following formula P ref : ; Wherein, P SC is the reference value of the active power for ultracapacitor voltage stabilization; P is the output active power of the cascaded ultracapacitor energy storage module; F is the grid voltage sag fault status value, and its judgment condition is: when the grid voltage amplitude fluctuation range exceeds ±10%, F = 1; otherwise, F = 0.

[0012] As a preferred embodiment of the present invention, the reference value of the active power of the supercapacitor for voltage regulation is P SC calculated as follows: ; where is the limit value of the active power of the supercapacitor charge and discharge, used to limit the supercapacitor charge and discharge power value; k SC is the supercapacitor power ratio coefficient; Δ U SC is the supercapacitor voltage deviation value, and its calculation formula is as follows: ; where U SCref is the supercapacitor voltage reference value, U SCavg is the average supercapacitor voltage.

[0013] As a preferred embodiment of the present invention, in the step S202, the voltage phase control law is shown as the following formula: ; where θ ref is the voltage phase reference value; s is the Laplace operator; ω 0 is the rated angular frequency of the load; Δ ω is the angular frequency correction amount, and its calculation formula is shown as follows: ; where s is the Laplace operator; ω c is the cut-off frequency of the voltage phase control; m is the proportional coefficient of the voltage phase control.

[0014] As a preferred embodiment of the present invention, the step S203 includes two steps of dynamic damping voltage calculation and voltage amplitude control, specifically including: S2031. Dynamic damping voltage calculation The dynamic damping voltage is used to eliminate the oscillation risk caused by transient impact disturbances; the d-axis and q-axis dynamic damping voltages v zd and v zq are calculated respectively according to the dynamic damping voltage control law, and the specific calculation formula is shown as follows: ; where s is the Laplace operator; ω zis the dynamic damping control cut-off frequency; i d and i q are the d-axis and q-axis output currents respectively; k z is the proportional coefficient of the high-pass filter; r z is the damping coefficient; S2032, voltage amplitude control The stable control of the load voltage amplitude is achieved through the voltage amplitude control law, and the specific calculation formula is as follows: ; wherein, m d and m q represent the d-axis and q-axis modulation voltage reference values respectively; s is the Laplace operator, k pv is the proportional coefficient of the load voltage control; k iv is the integral coefficient of the load voltage control; V ref is the reference value of the load voltage amplitude; V m is the amplitude of the load voltage; Then, using the voltage phase reference value θ ref , the d-axis modulation voltage reference value m d and the q-axis modulation voltage reference value m q perform coordinate inverse transformation to obtain the three-phase modulation voltage reference values m a , m b , m c ; Using m a , m b , m c perform modulation to drive the cascaded supercapacitor energy storage module to operate.

[0015] As a preferred embodiment of the present invention, according to the operation status of the cascaded supercapacitor energy storage module, the medium-voltage uninterruptible power supply system operates in the following manner: (1) When the cascaded supercapacitor energy storage module can operate normally, the medium-voltage uninterruptible power supply system operates in the voltage stabilization mode; in this mode, the load side switch S 1 and the grid side switch S2 is in the closed state, and the bypass switch S 3 is in the open state; by executing the above steps to control the operation of the cascaded ultracapacitor energy storage module, the stable load voltage is achieved; (2)When the cascaded ultracapacitor energy storage module fails and is removed, the medium-voltage uninterruptible power supply system operates in the bypass mode: in this mode, the load-side switch S 1 and the grid-side switch S 2 are in the open state, and the bypass switch S 3 is in the closed state, and the load is directly powered by the grid.

[0016] The present invention further provides a medium-voltage uninterruptible power supply system based on cascaded ultracapacitor energy storage, which applies the control method of the medium-voltage uninterruptible power supply based on cascaded ultracapacitor energy storage as described above. The system includes: an isolation reactor L i , a filtering reactor L f , a cascaded ultracapacitor energy storage module, a load-side switch S 1 , a grid-side switch S 2 , a bypass switch S 3 and a controller; the load-side switch S 1 , the isolation reactor L i , the grid-side switch S 2 are sequentially connected in series between the load and the grid, and are connected in parallel with the bypass switch S 3 ; the cascaded ultracapacitor energy storage module is connected to the isolation reactor L f through the filtering reactor L i between the isolation reactor and the load-side switch S 1 ; among them, the isolation reactor L i is used to limit the fault current during grid faults, the cascaded ultracapacitor energy storage module is used to stabilize the load bus voltage, and the filtering reactor L f is used to suppress the harmonic output of the cascaded ultracapacitor energy storage module; the controller is configured to execute the above control method; The cascaded ultracapacitor energy storage module is of a three-phase structure, and each phase consists of NIt is composed of several sub - modules with the same structure connected in series. Each sub - module includes a supercapacitor (abbreviated as super - cap), a filter capacitor, and an H - bridge converter connected in parallel. The number of sub - modules in each phase of the cascaded super - cap energy storage module N and the capacitance value of the super - cap in the sub - module are configured according to the actual requirements of the power supply power and power supply duration of the medium - voltage load.

[0017] Description of the invention principle: 1. Through the adaptive power reference adjustment based on the fault state and the super - cap voltage deviation, the system can maintain the super - cap voltage within a reasonable working range while meeting the power demand, avoiding problems such as over - limit of the super - cap voltage that may be caused by conventional control methods, and realizing optimized energy management.

[0018] 2. Through voltage phase control and combined with adaptive power reference calculation, when the power grid is normal, the super - cap voltage is controlled to the given super - cap voltage reference value U Scref . During the grid voltage sag fault, the cascaded super - cap energy storage module will automatically discharge to provide power support for the load; after the fault is cleared, the super - cap voltage will smoothly recover to U SCref . And through this voltage phase control link, the active power of the system can accurately track its reference value, thereby completing the management of the super - cap energy and ensuring that the output voltage of the system is synchronized with the grid voltage during grid - connected operation.

[0019] 3. The voltage amplitude stability control includes dynamic damping control and voltage amplitude control, both of which are implemented in the dq coordinate system θ ref defined. The dynamic damping voltage is used to eliminate the oscillation risk caused by disturbances such as transient shocks, and the stable control of the load voltage amplitude is realized through the voltage amplitude control law.

[0020] Through the voltage amplitude stability control, the present invention constructs a closed - loop control with the load voltage amplitude as the direct feedback, which can quickly and accurately correct the amplitude deviation, thus ensuring that the load voltage amplitude accurately tracks the reference. At the same time, the dynamic damping voltage is introduced to generate a compensation voltage signal opposite to the oscillation trend and superimpose it on the modulation command to actively suppress the oscillation of the system during the transient process. And due to its high - pass characteristic, it can improve the dynamic voltage stability performance of the system without affecting the steady - state voltage accuracy.

[0021] 4. The medium-voltage uninterruptible power supply system based on cascaded ultracapacitor energy storage of the present invention has two operating modes: a voltage stabilization mode and a bypass mode. Under normal circumstances, the medium-voltage uninterruptible power supply system operates in the voltage stabilization mode to provide stable power supply to the load. In the case of a need to shut down due to a fault in the cascaded ultracapacitor energy storage module, etc., the medium-voltage uninterruptible power supply system operates in the bypass operation mode. The cascaded ultracapacitor energy storage module is in a shutdown state, and the load is directly powered by the power grid.

[0022] 5. The controller of the system of the present invention includes a processor and a computer-readable storage medium, and a computer program is stored on the computer-readable storage medium; when the computer program is executed by the processor, it can implement the adaptive power reference calculation, voltage phase control, and voltage amplitude stabilization control.

[0023] Compared with the prior art, the beneficial effects of the present invention are: 1. The control method of the medium-voltage uninterruptible power supply system based on cascaded ultracapacitor energy storage of the present invention, through adaptive power reference calculation, fully considers the wide-voltage operation characteristics of ultracapacitors, and adaptively adjusts the control of the charging and discharging process of ultracapacitors according to the grid fault state and the voltage deviation of ultracapacitors, enabling the system to provide the required power support during a fault and perform smooth energy replenishment after recovery, which helps to avoid risks such as ultracapacitor voltage over-limit and improve the effectiveness of energy management and the stability of the system.

[0024] 2. The control method of the medium-voltage uninterruptible power supply system based on cascaded ultracapacitor energy storage of the present invention, through the voltage amplitude stabilization control strategy, uses the load voltage amplitude error as a direct feedback signal to achieve high-precision closed-loop control. At the same time, the introduced dynamic damping voltage calculation and compensation mechanism actively generates damping based on the high-frequency information of the output current to suppress the system oscillation during disturbance. Due to its high-pass filter characteristics, this damping link mainly acts on the transient state without affecting the steady-state accuracy. During a grid fault, through the rapid adjustment of this strategy, the load voltage can be restored to near the rated value in a short time, reducing voltage dips and overshoots.

[0025] 3. The system of the present invention adopts a cascaded converter topology. The cascaded ultracapacitor energy storage module is directly connected to the medium-voltage power grid without a step-up transformer. This design avoids the adverse effects of transformer-related losses and leakage inductance. Its sub-module architecture is convenient for expanding the power level as needed, improving the applicability of the system. At the same time, a supercapacitor is selected as the energy storage medium, and its fast response characteristics are used to enhance the response speed and voltage support ability of the system to transient events such as voltage sags. Combining with the high power density characteristics of ultracapacitors, in the scenario of short-term high-power voltage sag treatment, this system solution helps to optimize the investment cost and equipment footprint. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is the topology diagram of the medium-voltage uninterruptible power supply system based on cascaded ultracapacitor energy storage in the present invention.

[0027] Figure 2 It is the control block diagram for the method of realizing medium - voltage uninterruptible power supply in the present invention.

[0028] Figure 3 It is the waveform diagram of multiple parameters of the load and the power grid when the three - phase grid voltage drops to 10% temporarily, adopting the control method of the present invention.

[0029] Figure 4 It is the waveform diagram of multiple parameters of the cascaded ultracapacitor energy storage module when the three - phase grid voltage drops to 10% temporarily, adopting the control method of the present invention. Detailed implementation manners

[0030] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further clearly and completely described below in conjunction with the accompanying drawings and embodiments. It should be understood that the described embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. Any modification, equivalent replacement made to the present invention under the inspiration of the present invention, or any product similar to the present invention obtained by combining the present invention with other technical features are all within the protection scope of the present invention.

[0031] 1. Overview of the implementation solution of the present invention 1.1 Topology of the medium - voltage uninterruptible power supply system Figure 1 It is the topology diagram of the medium - voltage uninterruptible power supply system with cascaded ultracapacitor energy storage in the present invention. The topology of this system and the structure of the cascaded ultracapacitor energy storage module are as described above.

[0032] 1.2 Operating mode of the cascaded ultracapacitor energy storage module The cascaded ultracapacitor energy storage module can generate the required AC voltage according to the control instruction to stabilize the load bus. After each sub - module receives the control signal generated by the controller, it operates on its own supercapacitor through the internal H - bridge converter. Specifically, the control signal commands the H - bridge to perform switching actions, selectively connecting the voltage of the supercapacitor (usually positive voltage, negative voltage or zero voltage) to the loop formed by all the sub - modules in series in this phase.

[0033] Within the same phase of the cascaded ultracapacitor energy storage module, modulation techniques applicable to cascaded H - bridge converters, such as carrier - phase - shifted pulse - width modulation or carrier - stacked pulse - width modulation, etc., are used to make the voltages output by all sub - modules be serially superimposed, jointly synthesizing a multi - level stepped voltage approaching the target waveform. Finally, the multi - level stepped voltage output by the cascaded ultracapacitor energy storage system passes through the filter reactor L f After filtering, the load bus voltage is constructed. By precisely controlling the input or bypass of each sub - module, the dynamic, fast regulation and stabilization of the amplitude and phase of the load bus voltage are realized.

[0034] 1.3 According to the operation status of the cascaded ultracapacitor energy storage module, this medium-voltage uninterruptible power supply system has two operating modes: a voltage stabilization mode and a bypass mode, and the specific operation method is as described above.

[0035] Based on common understanding, the controller of the medium-voltage uninterruptible power supply system includes a processor and a computer-readable storage medium, on which a computer program is stored; when the computer program is executed by the processor, it can implement the control method for medium-voltage uninterruptible power supply described below in the present invention.

[0036] 1.4 Control method for medium-voltage uninterruptible power supply Figure 2 It is the control block diagram of the medium-voltage uninterruptible power supply system based on cascaded ultracapacitor energy storage in the present invention. According to this figure, it can be understood that the control method described in the present invention mainly includes three parts: adaptive power reference calculation, voltage phase control, and voltage amplitude stabilization control, and the specific implementation method is as described above.

[0037] When using the three-phase modulated voltage reference value for modulation, modulation techniques applicable to the cascaded H-bridge converter can be adopted, such as carrier phase-shifted pulse width modulation or carrier stacked pulse width modulation, etc. Taking carrier phase-shifted pulse width modulation as an example, for each N sub-module generates N uniformly phase-shifted triangular carrier waves. Inside each sub-module, the corresponding modulated voltage reference value of this phase (such as m a ) is compared with its triangular carrier wave, and the comparison result is used to generate the gate pulse signal for driving the H-bridge switch tubes of this sub-module. The H-bridges of each sub-module act according to the received gate signals, and selectively connect their internal supercapacitors (forward, reverse, or bypass) to the series circuit. After the output voltages of each sub-module in the same phase are connected in series and superimposed, a multi-level stepped voltage approaching a sine wave is formed, and after filtering through the filter reactor L f filtering, the load bus voltage is constructed. Through precise adjustment of the voltage amplitude stabilization control m a , m b , m c , the control of the load bus voltage amplitude and phase is finally realized.

[0038] 2. A specific embodiment and verification results In this embodiment, first, the PLECS simulation software is used to construct the circuit topology structure and control model of the medium-voltage uninterruptible power supply system with reference to the content described above, in this way to verify the specific feasibility and corresponding technical effects of the present invention.

[0039] Set the relevant operating conditions in the simulation model as follows: the power grid is a 10 kV / 50 Hz medium-voltage power grid; the load is a 10 kV / 50 Hz / 4 MW medium-voltage load; the rated power of the cascaded ultracapacitor energy storage module is 5 MW, and the number of sub-modules per phase N is 15, and the ultracapacitance value of each sub-module is 10 F, meeting the power demand of 5 MW for 25 seconds of discharge; the filter reactor L f is 8 mH; the isolation reactor L i is 63 mH.

[0040] S201, Adaptive Power Reference Calculation Collect the three-phase voltages on the grid side u gx (in this application x = a , b , c ), the three-phase voltages on the load side v x and the three-phase output currents of the cascaded ultracapacitor energy storage module i x ; according to v x and i x , calculate the load voltage amplitude V m , the active power output of the cascaded ultracapacitor energy storage module P . Collect the ultracapacitor voltages of each sub-module U sc,xj ( j= 1, 2, …, N ), and calculate the average ultracapacitor voltage of the cascaded ultracapacitor energy storage module U SCavg .

[0041] According to the grid voltage fault state and the average ultracapacitor voltage U SCavg , calculate the active power reference value P ref of the cascaded ultracapacitor energy storage module: ; where, P SC is the active power reference value for ultracapacitor voltage regulation; P is the active power output of the cascaded ultracapacitor energy storage module; F is the grid voltage sag fault state value, and its judgment condition is: when the grid voltage amplitude fluctuation range exceeds ±10%, F = 1; otherwise, F = 0.

[0042] Reference value of active power for supercapacitor voltage stabilization P SC , and the calculation formula is as follows:

[0043] Among them, is the limit value of active power for supercapacitor charge and discharge, set to 1 MW; k SC is the power ratio coefficient of the supercapacitor, set to 8500; Δ U sc is the voltage deviation value of the supercapacitor, and the calculation formula is as follows: ; Among them, U SCref is the reference voltage of the supercapacitor, set to 990 V.

[0044] Through adaptive power reference calculation, the system can autonomously adjust the reference value of active power P ref to achieve continuous stability of the load voltage and automatic management of the supercapacitor voltage.

[0045] S202. Voltage phase control Calculate the reference value of the voltage phase of the cascaded supercapacitor energy storage module through the voltage phase control law θ ref . The voltage phase control law is shown in the following formula: ; Among them, s is the Laplace operator; ω 0 is the rated angular frequency of the load, which is 100π rad / s; Δω is the angular frequency correction amount, and its calculation formula is shown as follows: ; Among them, s is the Laplace operator; ω c is the cut-off frequency of voltage phase control, set to 200π rad / s; m is the proportional coefficient of voltage phase control, set to 7.53×10 -6 .

[0046] Through voltage phase control and combined with the adaptive power reference calculation described in step S201, when the power grid is normal, the supercapacitor voltage is controlled to 990 V; during the power grid sag fault, the supercapacitor will automatically discharge to provide power support for the load; after the fault is cleared, the supercapacitor voltage will smoothly recover to 990 V.

[0047] S203. Voltage amplitude stabilization control The voltage amplitude stability control includes dynamic damping control and voltage amplitude control, both of which are implemented in the θ ref defined dq coordinate system.

[0048] S2031. Calculation of dynamic damping voltage The dynamic damping voltage is used to eliminate the oscillation risk caused by disturbances such as transient shocks. The d-axis and q-axis dynamic damping voltages are calculated respectively according to the dynamic damping voltage calculation formula v zd and v zq , and its calculation formula is: ; where s is the Laplace operator; ω z is the cut-off frequency of dynamic damping control, set to 20π rad / s; i d and i q are the d-axis and q-axis output currents respectively; k z is the proportional coefficient of the high-pass filter, set to 1.2; r z is the damping coefficient, set to 28.

[0049] S2032. Voltage amplitude control The stable control of the load voltage amplitude is achieved through the voltage amplitude control law. The voltage amplitude control law is as follows: ; where s is the Laplace operator, k pv is the proportional coefficient of load voltage control, set to 0.5; k iv is the integral coefficient of load voltage control, set to 7500; V ref is the reference value of load voltage amplitude, set to 8163.97 V; m d and m q represent the d-axis and q-axis modulation voltage reference values respectively. Using θ ref , m d and m q to perform coordinate inverse transformation to obtain the three-phase modulation voltage reference value m a , mb , m c ; By using m a , m b , m c for modulation. In this embodiment, by way of example, through the modulation method of carrier phase shift, the cascaded ultracapacitor energy storage module is driven to operate.

[0050] Figure 3 is the waveform diagram of the load, grid voltage amplitude, three-phase grid voltage, three-phase load voltage, and three-phase load current when the three-phase grid voltage drops to 10% and the control method of the present invention is adopted. In the simulation, the grid voltage is set as follows: when the simulation time is 0 - 1 s, the grid voltage is normal; when the simulation time is 1 s, a severe voltage dip fault occurs in the three-phase grid voltage, dropping to 10%; when the simulation time is 1.5 s, the grid voltage recovers.

[0051] From Figure 3 it can be seen that when the grid voltage is normal, the system operates stably, and the load voltage is stable at the rated value; during the whole process of the severe voltage dip fault of the grid voltage, the load voltage is not interrupted, and the load voltage amplitude can be stable at the rated value; when the grid voltage dip fault occurs and the grid voltage recovers, the load voltage amplitude only fluctuates slightly, and the maximum fluctuation amplitude does not exceed ±5%, and the fluctuation is reduced to within ±1% within one power frequency cycle.

[0052] Figure 4 is the waveform diagram of the load, grid voltage amplitude, output power of the cascaded ultracapacitor energy storage module, output current, and average ultracapacitor voltage when the control method of the present invention is adopted under the same simulation as Figure 3 . From Figure 4 it can be seen that when the grid voltage is normal, the ultracapacitor voltage is stable at the set value of 990 V, and the cascaded ultracapacitor energy storage module provides reactive power to stabilize the load voltage; when the grid voltage dip fault occurs, the cascaded ultracapacitor energy storage module quickly provides power support, and the ultracapacitor discharges automatically to stabilize the load voltage; when the grid voltage recovers, the cascaded ultracapacitor energy storage module quickly adjusts the power to stabilize the load voltage, and at the same time the ultracapacitor charges smoothly and recovers to 990 V.

[0053] In summary, the medium-voltage uninterruptible power supply system and control method based on cascaded ultracapacitor energy storage proposed by the present invention achieve the continuous stability of the load voltage under steady-state and transient conditions, as well as the adaptive management of the ultracapacitor voltage. During the severe grid voltage dip fault, the system can quickly stabilize the load voltage without switch switching, effectively suppress voltage fluctuations, and ensure the continuity and stability of the medium-voltage sensitive load power supply.

Claims

1. A control method for medium voltage uninterruptible power supply based on cascaded super-capacitor energy storage, characterized in that: The following steps are involved: S201, adaptive power reference calculation Collect three-phase voltage on the grid side u gx , three-phase voltage on the load side v x And the three-phase output current of the cascaded super-capacitor energy storage module i x , x=a , b , c ; According to the voltage v x and current i x , calculate the load voltage amplitude V m , Cascaded super-capacity energy storage modules output active power P ; Collect the supercapacity voltage of each submodule in the cascade supercapacitor energy storage module U sc,xj , j= 1,2,…, N , calculate the average overcapacitance voltage U SCavg ; Combine the grid fault status and the average voltage of the overcapacity to calculate the active power reference value of the cascaded overcapacity energy storage module P ref ; S202, voltage phase control According to the active power reference value P ref , generating a voltage phase reference value through a voltage phase control law to adjust the output phase of the cascaded super-capacitor energy storage module; S203, voltage amplitude stability control The modulation voltage reference value is generated by the dynamic damping voltage control law and the voltage amplitude control law to drive the cascaded ultra-capacitor energy storage module to stabilize the load voltage amplitude.

2. The method according to claim 1, characterized in that In step S201, the average over-capacity voltage is calculated by dividing the sum of the over-capacity voltages of each submodule by the total number of submodules. U SCavg , as shown in the following formula: ； in, N is the number of submodules in each phase, j= 1,2,…, N ; U sc,aj , U sc,bj , U sc,cj They are the over-capacity voltage of each submodule in the three phases.

3. The method according to claim 1, characterized in that In step S201, the active power reference value of the cascaded super-capacity energy storage module is calculated according to the following formula: P ref : ； in, P SC It is the reference value of active power for over-capacity voltage stabilization; P Output active power for cascaded super-capacity energy storage modules; F It is the grid temporary drop fault state value, and its judgment condition is: when the grid voltage amplitude fluctuation range exceeds ±10%, F =1; otherwise, F =0.

4. The method according to claim 3, characterized in that The over-capacity voltage-stabilized active power reference value P SC The calculation formula is as follows: ； in, It is the over-capacity charging and discharging active power limit, which is used to limit the over-capacity charging and discharging power value; k SC is the excess capacity power ratio coefficient; Δ U sc is the over-capacitance voltage deviation value, and its calculation formula is as follows: ； in, U SCref is the overcapacitance voltage reference value, U SCavg is the average overcapacitance voltage.

5. The method according to claim 1, characterized in that In step S202, the voltage phase control law is as follows: ； in, θ ref is the voltage phase reference value; s is the Laplace operator; ω 0 is the rated angular frequency of the load; Δ ω is the angular frequency correction, and its calculation formula is as follows: ； in, s is the Laplace operator; ω c is the voltage phase control cut-off frequency; m is the voltage phase control proportional coefficient.

6. The method according to claim 1, characterized in that The step S203 includes two steps: dynamic damping voltage calculation and voltage amplitude control, specifically including: S2031, Dynamic damping voltage calculation The dynamic damping voltage is used to eliminate the oscillation risk caused by transient impact disturbances. The d-axis and q-axis dynamic damping voltages are calculated according to the dynamic damping voltage control law. v zd and v zq , the specific calculation formula is as follows: ； in, s is the Laplace operator; ω z Control the cutoff frequency for dynamic damping; i d and i q are the d-axis and q-axis output currents respectively; k z is the high-pass filter proportional coefficient; r z is the damping coefficient; S2032, voltage amplitude control The voltage amplitude control law is used to achieve stable control of the load voltage amplitude. The specific calculation formula is as follows: ； in, m d and m q Respectively represent the d-axis and q-axis modulation voltage reference values; s is the Laplace operator, k pv is the load voltage control proportional coefficient; k iv is the load voltage control integral coefficient; V ref is the load voltage amplitude reference; V m is the load voltage amplitude; Then, using the voltage phase reference θ ref , d-axis modulation voltage reference value m d and q-axis modulation voltage reference value m q Perform inverse coordinate transformation to obtain the three-phase modulation voltage reference value m a , m b , m c ;use m a , m b , m c Modulation is performed to drive the cascaded ultra-capacity energy storage module to operate.

7. The method according to any one of claims 1 to 6, characterized in that According to the commissioning of the cascaded super-capacity energy storage module, the medium voltage uninterruptible power supply system is operated in the following manner: (1) When the cascaded super-capacity energy storage module can operate normally, the medium voltage uninterruptible power supply system operates in a voltage stabilization mode; in this mode, the load side switch S 1 and grid-side switches S 2 In the closed state, the bypass switch S 3 The device is in disconnected state; and the operation of the cascaded super-capacity energy storage module is controlled by executing the steps to achieve a stable load voltage; (2) When the cascaded super-capacity energy storage module fails and is removed, the medium voltage uninterruptible power supply system operates in bypass mode: in this mode, the load side switch S 1 and grid-side switches S 2 In the disconnected state, bypass switch S 3 In the closed state, the load is directly powered by the grid.

8. A medium voltage uninterruptible power supply system based on cascaded super-capacity energy storage, applying the control method of medium voltage uninterruptible power supply based on cascaded super-capacity energy storage as described in any one of claims 1 to 6, characterized in that: The system includes: Isolation reactor L i , filter reactor L f , cascaded supercapacitor energy storage module, load-side switch S 1 , Grid side switch S 2 , Bypass switch S 3 and a controller; the load side switch S 1 , Isolation Reactor L i , Grid side switch S 2 Connected in series between the load and the grid, and connected to the bypass switch S 3 The cascaded super-capacitor energy storage module is connected in parallel through a filter reactor. L f Connect to the isolation reactor L i With load side switch S 1 Between; Among them, the isolation reactor L i Used to limit fault current during grid faults, cascaded super-capacitor energy storage modules are used to stabilize load bus voltage, filter reactors L f Used to suppress the output harmonics of the cascaded super-capacitor energy storage module; the controller is configured to execute the control method; The cascaded super-capacity energy storage module has a three-phase structure, each phase consists of N The submodules of the same structure are connected in series, each submodule includes a supercapacitor, a filter capacitor and an H-bridge converter connected in parallel. The supercapacitor is referred to as a supercapacitor. The number of submodules in each phase of the cascade supercapacitor energy storage module is N The capacity of the submodule and the excess capacity are configured according to the actual needs of the power supply power and power supply duration of the medium voltage load.

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