A control system and method for energy storage system assisting frequency modulation of thermal power

Abstract

The application discloses a control system and method for energy storage system assisting frequency modulation of thermal power, and relates to the technical fields of power systems and inverter control, and comprises a power calculation module, a master unit current support control module, an inter-phase adjustment module, a slave unit virtual impedance control module, a current amplitude compensation module, a master unit energy storage control module and a slave unit energy storage control module. The application adopts a master-slave structure combined with virtual impedance control, accurately adjusts output voltages of each energy storage submodule while ensuring current consistency, realizes power self-adaptive distribution among multiple modules, introduces a power distribution based on an SOC state and an inter-phase SOC driven current amplitude compensation mechanism, dynamically balances state of charge of each energy storage unit during system operation, improves overall service life and utilization efficiency of the energy storage system, and solves the problems that multiple module cooperation, power balance, three-phase coordination and system robustness in the prior art cannot meet the requirements of new power systems on high-performance energy storage network control.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power systems and inverter control technology, in particular to a control system and method for frequency regulation of thermal power assisted by energy storage system. BACKGROUND

[0002] With the continuous increase of new energy penetration, the output of traditional thermal power units has decreased significantly, the system inertia of the power system has gradually weakened, and the frequency regulation capability has significantly deteriorated. Especially in the scenario of high proportion of photovoltaic access, the system frequency is increasingly affected by intermittent disturbances, the steady-state frequency deviation increases, and the traditional thermal power unit is difficult to undertake the task of fast frequency response and inertia support. Therefore, it is urgent to build a new energy storage network scheme with fast regulation and frequency support capability. Among them, the high-voltage direct-hanging type network energy storage system is particularly suitable for joint operation with large thermal power units, participates in system frequency regulation and inertia support, and its representative is a high-voltage direct-hanging type three-phase cascaded H-bridge energy storage system. However, the network control of the energy storage system under the condition of high-voltage direct-hanging has high complexity, higher control accuracy and stability requirements, and coordination control faces significant challenges.

[0003] At present, the high-voltage direct-hanging type three-phase cascaded H-bridge energy storage system still has coordination control problems under the condition of network operation, and the main problems are as follows:

[0004] First, the series output of the energy storage module leads to the consistency of the current, the lack of flexible module-level power regulation capability, and the difficulty in realizing flexible power distribution among multiple modules while meeting the consistent current constraint; second, the overall frequency support capability of the system is limited, the response is lagging, and the inertia support demand after high proportion of new energy access cannot be met; third, the inconsistent state of charge (SOC) between modules may lead to the "short board effect" limiting the overall operation performance of the system; fourth, under the independent operation of the three-phase system, there are problems of frequency drift, uneven power distribution and imbalance of inter-phase energy.

[0005] In order to overcome the above problems, the existing technology proposes a network type energy storage control scheme, mainly including three types:

[0006] (1) Voltage type virtual synchronous generator control (VSG) scheme: by constructing a voltage controller similar to a synchronous generator to simulate inertia and damping response, suitable for centralized large power energy storage scenarios; limitations: difficult to coordinate multiple modules, module response speed is limited, poor adaptability; difficult to realize independent adjustment and energy balance among modules;

[0007] (2) Current-type grid-following control scheme (GFL): using phase-locked loop to extract grid phase and frequency, and using current reference to control energy storage output; limitations: dependent on high-precision sensing devices, weak anti-interference ability; sensing delay easily causes frequency drift and stability decline; lack of multi-module power distribution and SOC balancing mechanism;

[0008] (3) Master-slave grid-forming control scheme: some studies propose using master unit to adjust voltage and slave unit to follow current reference, but do not systematically introduce inter-phase synchronization and energy balance control. At the same time, master unit synchronization with grid relies on phase-locked loop, which is not robust; limitations: ignores inter-phase imbalance and three-phase power dynamic imbalance, overall system regulation granularity is coarse, and fine-grained distributed control cannot be realized.

[0009] In summary, these schemes have obvious deficiencies in multi-module cooperation, energy balance, three-phase coordination and system robustness, and are difficult to meet the comprehensive needs of new power systems for high-performance energy storage grid-forming control, especially thermal power frequency modulation. Therefore, a control system and method for thermal power frequency modulation assisted by energy storage system are needed to intelligently schedule power supply to solve the above technical problems and provide protection and defense for large-scale grid safety. SUMMARY

[0010] In view of the above technical problems in the related art, the present application proposes a control system and method for thermal power frequency modulation assisted by an energy storage system, which designs energy storage system parameters from the aspects of frequency response capability, power regulation range and SOC regulation margin to solve the above problems.

[0011] In a first aspect, the present application provides a control system for thermal power frequency modulation assisted by an energy storage system, the energy storage system being a three-phase cascaded H-bridge energy storage system, the energy storage system comprising an A-phase system, a B-phase system and a C-phase system, each phase being composed of a linear impedance and N energy storage sub-modules in series, forming an overall series three-phase structure connected to a power grid; the energy storage sub-modules are identical in structure and each comprise a filter circuit, an energy storage converter and an energy storage unit; the uppermost energy storage sub-module of each phase of the energy storage system is taken as the master unit of each phase, and the other N-1 energy storage sub-modules are taken as the slave units of each phase; comprising the following modules:

[0012] a power calculation module for sampling and calculating the active power output of each phase system and the reactive power output of each phase system;

[0013] The main unit current support control module is configured to calculate, based on the per-phase system output active power, the per-phase system output reactive power, and the inter-phase phase angle balance adjustment amount, a per-phase main unit output current frequency and a per-phase main unit output current amplitude by a virtual inertia calculation and a first PI controller calculation, and send the two to all slave units in the current phase as a current phase reference, and calculate a per-phase corrected active power and a per-phase equivalent phase angle variable and send them to other phase main units;

[0014] The inter-phase adjustment module is configured to calculate a per-phase alternating current sinusoidal signal according to a per-phase system output current phase angle, and perform phase angle compensation on the per-phase system output current phase angle to obtain a per-phase equivalent phase angle variable and send it to other phase main units, and calculate an inter-phase phase angle balance adjustment amount according to the per-phase equivalent phase angle variable and adjust a per-phase system frequency output based on a second PI controller to make the three-phase phases consistent.

[0015] The slave unit virtual impedance control module is configured to combine the current phase reference sent by the main unit with a cascaded power distribution coefficient, and adjust the output voltage of the slave unit by using virtual impedance control.

[0016] The current amplitude compensation module is configured to correct a per-phase current active power according to a per-phase system output active power and a per-phase SOC average value to obtain a per-phase corrected active power, and calculate a per-phase current amplitude compensation component according to the per-phase corrected active power and inject it into the calculation of the main unit output current amplitude.

[0017] The main unit energy storage control module is configured to generate a main unit PWM signal by a proportional-resonant controller from a per-phase alternating current sinusoidal signal and a per-phase current of the utility grid, and adjust the state of a corresponding energy storage converter of the main unit.

[0018] The slave unit energy storage control module is configured to generate a slave unit PWM signal by a voltage-current double closed loop control from a per-phase system slave unit output voltage reference, a per-phase current of the utility grid, and a per-phase system slave unit output voltage, and adjust the state of a corresponding energy storage converter of the slave unit.

[0019] Specifically, the formula of the virtual inertia calculation and the first PI controller calculation is shown in formula (2):

[0020]

[0021] wherein, is an x-phase main unit output current frequency; I x is an x-phase main unit output current amplitude; P x is an x-phase system output active power; Q x is an x-phase system output reactive power; H is a virtual inertia time constant; and D is a virtual damping coefficient. represents an x-phase system at a standard frequency fn the active power reference of the system; the x-phase system output current phase angle at the standard frequency f n the reactive power reference of the system; Δf x the inter-phase phase angle balancing adjustment amount of the x-phase; I * the system rated current amplitude; the reactive regulation proportional coefficient; the reactive regulation integral coefficient; s represents the Laplace operator; the frequency of the x-phase main unit output current; the derivative with respect to the time variable t; ΔI x the x-phase current amplitude compensation component; x ∈ {A, B, C}.

[0022] Specifically, the calculation formula of the inter-phase regulation module for calculating the AC current sinusoidal signal of each phase according to the phase angle of the system output current of each phase is shown in formula (4):

[0023]

[0024] wherein, the current sinusoidal signal conversion function is used to combine the current amplitude and phase into a sinusoidal signal, and the output of the the x-phase AC current sinusoidal signal; δ B I the B-phase system output current phase angle; δ C I the C-phase system output current phase angle.

[0025] Specifically, the phase angle compensation is performed on the phase angle of the system output current of each phase according to formula (5) to obtain the equivalent phase angle variable of each phase:

[0026]

[0027] wherein the A-phase equivalent phase angle variable after phase angle compensation; the B-phase equivalent phase angle variable after phase angle compensation; the C-phase equivalent phase angle variable after phase angle compensation; δ A I the A-phase system output current phase angle; δ B I the B-phase system output current phase angle; δ C I the C-phase system output current phase angle.

[0028] Specifically, the inter-phase phase angle balancing adjustment amount is calculated by the main unit of each phase according to the equivalent phase angle variable of each phase, and the frequency output of each phase system is adjusted based on the second PI controller to make the three-phase phase consistent, which is realized based on formula (6):

[0029]

[0030] where Δf x represents the inter-phase phase angle regulation amount of the x-phase; Ω = {A, B, C} is used to represent a set of subscripts of the three phases A, B, and C; k p and k I is a frequency regulation coefficient, k p is an inter-phase synchronization control frequency regulation coefficient; k I is an inter-phase synchronization control frequency regulation coefficient; represents a set of equivalent phase angle variables of all phases; finally, the inter-phase phase angle regulation amount Δf x of the x-phase is passed to the formula (2) of the main unit current support control module.

[0031] Specifically, the expression of the virtual impedance control regulation is shown in formula (7):

[0032]

[0033] where i x represents the x-phase system output current; V x represents the x-phase output voltage reference of the unit; s represents the Laplace operator; x represents the phase, x ∈ {A, B, C}; I x is the x-phase main unit output current amplitude; represents the x-phase virtual resistance amplitude; g(s) is a second-order transfer function; represents the virtual impedance size of the unit; represents the active power required to be provided by the x-phase, which is calculated according to formula (8):

[0034]

[0035] Specifically, the second-order transfer function g(s) is implemented based on formula (9):

[0036]

[0037] where, is the x-phase angle frequency, that is, f n is the standard frequency; represents the cascade power distribution coefficient of each unit of the x-phase.

[0038] Specifically, the corrected active power of each phase is obtained by correcting the current active power of each phase according to the system output active power of each phase and the average value of the SOC of each phase, and the corrected active power of each phase is implemented based on formula (11), which is shown as follows:

[0039]

[0040] wherein P x is the x-phase system output active power; P' x is the x-phase corrected active power; is the x-phase SOC average value, n is the number of all energy storage units in the x-phase system; x∈{A, B, C}; k soc is the SOC inter-phase equalization proportionality coefficient.

[0041] Specifically, the calculation formula of the per-phase current amplitude compensation component according to the per-phase corrected active power is shown in formula (12):

[0042]

[0043] wherein ΔI x is the x-phase current amplitude compensation component; k0 is the proportionality coefficient of current compensation control; is the average value of the three-phase corrected active power; sgn() is the sign function.

[0044] In a second aspect, the present application provides a control method for a thermal power frequency modulation assisted by an energy storage system, based on the control system for a thermal power frequency modulation assisted by an energy storage system according to any one of the first aspect, comprising the following steps:

[0045] S1, sampling and calculating the per-phase system output active power and the per-phase system output reactive power;

[0046] S2, calculating the per-phase main unit output current frequency and the per-phase main unit output current amplitude by the virtual inertia calculation and the first PI controller based on the per-phase system output active power, the per-phase system output reactive power and the inter-phase phase angle equalization adjustment amount, and sending the two to all slave units in the current phase as the current phase reference, and calculating the per-phase corrected active power and the per-phase equivalent phase angle variable and sending them to other phase main units;

[0047] S3, calculating the per-phase alternating current sinusoidal signal according to the per-phase system output current phase angle, and performing phase angle compensation on the per-phase system output current phase angle to obtain the per-phase equivalent phase angle variable and send it to other phase main units, and calculating the inter-phase phase angle equalization adjustment amount according to the per-phase equivalent phase angle variable and adjusting the per-phase system frequency output based on the second PI controller to make the three-phase phase consistent;

[0048] S4, combining the current phase reference received by the slave unit from the main unit with the cascaded power distribution coefficient, and adjusting the output voltage of the slave unit by the virtual impedance control;

[0049] S5, correct the current active power of each phase according to the average value of the active power output by each phase system and the SOC of each phase, and calculate the current amplitude compensation component of each phase according to the corrected active power of each phase, and then inject it into the calculation of the output current amplitude of the main unit;

[0050] S6, the main unit PWM signal is generated by the proportional resonant controller of the sinusoidal signal of each phase alternating current and the current of the public power grid to adjust the state of the corresponding energy storage converter of the main unit;

[0051] S7, the slave unit PWM signal is generated by the voltage-current double closed loop control of the output voltage reference of each phase system from the unit and the current of the public power grid and the output voltage of each phase system from the unit to adjust the state of the corresponding energy storage converter of the slave unit.

[0052] The control system and method for assisting the frequency regulation of the thermal power of the energy storage system can effectively assist in adjusting the climbing rate of the high proportion of new energy penetration of the thermal power, thereby improving the frequency regulation capability of the entire power system under high proportion of new energy penetration.

[0053] Compared with the prior art, the present application has the following significant progress:

[0054] 1) stronger multi-module power regulation capability

[0055] Compared with the traditional voltage type VSG and current type GFL scheme which cannot realize flexible power regulation at the module level, the present application adopts a master-slave structure combined with virtual impedance control, which can accurately regulate the output voltage of each energy storage sub-module while ensuring current consistency, and realize power adaptive distribution among multiple modules.

[0056] 2) complete SOC balancing mechanism

[0057] Unlike the prior control method which does not consider or only partially considers energy balance, the present application introduces a power distribution factor based on the SOC state, and introduces an inter-phase SOC driven current amplitude compensation mechanism in a three-phase system, which can dynamically balance the state of charge of each energy storage unit during system operation, and improve the overall life and utilization efficiency of the energy storage system.

[0058] 3) better system-level frequency support capability

[0059] The present application introduces a virtual inertia element in the main unit, so that the energy storage system has frequency dynamic response characteristics; at the same time, the slave unit improves the system disturbance suppression capability through a dynamically optimized second-order controller, and the overall frequency support performance is obviously better than the slow response scheme of the voltage type VSG and the unstable scheme of the current type GFL.

[0060] 4) outstanding inter-phase coordination capability

[0061] Traditional schemes often ignore three-phase coordinated control, which is easy to cause power imbalance and phase angle drift problems. The application realizes the three-phase power coordination and synchronous control under the current uniformity constraint by constructing the inter-phase phase angle regulating quantity and the current compensation quantity injection strategy based on the SOC difference, effectively inhibiting three-phase dynamic imbalance.

[0062] 5) clear structure, easy to implement engineering

[0063] The scheme of the application adopts a hierarchical architecture of master-slave + inter-phase control, has clear function division, low coupling degree between controllers, is suitable for distributed deployment and modular expansion of actual projects, has low dependence on sensor accuracy and external power grid, has stronger robustness and field adaptability, more reasonably and intelligently schedules power supply, and provides more excellent guarantee and defense for large-scale power grid safety. BRIEF DESCRIPTION OF DRAWINGS

[0064] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments will be briefly introduced as follows. Obviously, the drawings in the following description only constitute some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of these drawings.

[0065] Figure 1 is a control block diagram of a control system of a power storage system assisting frequency modulation of thermal power provided by the embodiment of the present application;

[0066] Figure 2 is a main unit current support control schematic diagram taken A phase as an example provided by the embodiment of the present application;

[0067] Figure 3 is a slave unit virtual impedance control schematic diagram taken A phase as an example provided by the embodiment of the present application;

[0068] Figure 4 is a schematic diagram of a control method of a power storage system assisting frequency modulation of thermal power provided by the embodiment of the present application. DETAILED DESCRIPTION

[0069] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments only constitute some embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art belong to the scope of protection of the present application.

[0070] In addition, the terms "first", "second", "third", etc. are used only for descriptive purposes and are not to be construed as indicating or implying relative importance or an ordered ranking of the indicated technical features. Thus, features defined with "first", "second" or "third" can explicitly or implicitly include one or more of the features. In the description of the present application, the meaning of "a plurality" is two or more, unless otherwise explicitly and specifically limited.

[0071] The embodiment of the present application provides a control method and system for frequency modulation of thermal power with the aid of an energy storage system.

[0072] Embodiment one.

[0073] Reference Figure 1 The embodiment provides a schematic diagram of a control system for frequency modulation of thermal power with the aid of an energy storage system. The energy storage system is a three-phase cascaded H-bridge energy storage system. The three phases include phase A, phase B and phase C. The energy storage system includes phase A system, phase B system and phase C system. Each phase is composed of linear impedance and N energy storage sub-modules in series, forming an overall series three-phase structure and being connected to a power grid.

[0074] The power grid provides a voltage amplitude V g and a phase angle θ g as a synchronous reference between three phases of a general energy storage system control.

[0075] In a conventional power system, a phase-locked loop (PLL) is generally used to track the phase and frequency of the power grid in real time to provide a synchronous reference signal for the control system. For example, by measuring the zero-crossing point of the power grid or using a complex algorithm (such as a synchronous coordinate system phase-locked loop), the PLL outputs a phase angle θ g and a voltage amplitude V g as a reference for subsequent control (such as current control and power regulation). That is, the conventional power system needs real-time information (voltage amplitude V g and phase angle θ g ) of the power grid to achieve synchronization, otherwise it cannot accurately control the phase and amplitude of the output current or the power output, which may result in grid connection failure or low power transmission efficiency. The control method of the embodiment realizes a lock-free phase-locked loop control between the three phases of the power grid, without the need for real-time tracking of the synchronous control of the grid voltage reference (voltage amplitude V g and phase angle θ g ). This technology not only simplifies system design, but also improves dynamic response, reliability and adaptability, and is particularly suitable for new energy grid connection, microgrid and other scenarios with high synchronization performance requirements and complex grid conditions.

[0076] A-phase system, B-phase system and C-phase system are connected to the public power grid, responsible for converting the direct current of the energy storage unit into alternating current and feeding back to the power grid;

[0077] The energy storage sub-modules are structurally identical, each including a filter circuit, an energy storage converter and an energy storage unit; the energy storage unit is connected to the energy storage converter and connected to the public power grid through the filter circuit; the filter circuit includes an inductor L and a capacitor C for filtering and stabilizing the output voltage; each energy storage converter includes an SOC measurement device, a voltage sampling device and a current sampling device; the SOC measurement device is used to collect the remaining capacity of the energy storage unit, i.e., the real-time SOC state of the energy storage unit; the voltage sampling device is used to sample the voltage of the energy storage sub-module, and the current sampling device is used to sample the current of the energy storage sub-module, with a sampling frequency of 10 kHz; that is, each energy storage sub-module is equipped with independent SOC measurement, voltage and current sampling functions;

[0078] The energy storage sub-module adopts an IGBT full-bridge structure supporting SVPWM control, with voltage and current double-loop control;

[0079] The energy storage unit is a storage battery or other energy storage device, used to store and release electrical energy. The energy storage unit is connected to the controller through a direct current link to provide or absorb direct current;

[0080] The energy storage converter PCS (Power Conversion System) is the core device connecting the energy storage unit and the power grid, realizing bidirectional conversion of electrical energy. Its main functions and features include: converting the direct current of the battery into alternating current (discharging) or converting the alternating current of the power grid into direct current (charging), supporting AC / DC flexible conversion; receiving control instructions through communication to adjust the active / reactive power of the power grid and balance the load and the power grid interaction;

[0081] In the system, the energy storage sub-modules are connected in series to form a network, constituting a whole directly connected to the high-voltage power grid. This structure, while achieving unified current output, also presents the following control difficulties:

[0082] 1) All modules must output the same current, otherwise inconsistent current will cause module imbalance and even overvoltage and overcurrent failure;

[0083] 2) How to coordinate the synchronization and power distribution of each module;

[0084] 3) Power regulation must take into account the SOC balancing goal to avoid the "short board effect" in series modules;

[0085] 4) Can suppress current under grid voltage fluctuation.

[0086] The embodiment proposes a hierarchical and graded collaborative control strategy based on "current control + virtual impedance control + inter-phase coordination control", each phase selects the uppermost energy storage submodule as the master unit, the master unit adopts current support control method to be responsible for current control, the remaining energy storage submodules are slave units, the slave units adopt virtual impedance control method, the inter-phase behavior control adopts inter-phase phase supplement control and inter-phase current compensation control, which are respectively used for three-phase phase balance adjustment and three-phase inter-phase SOC balance control.

[0087] The embodiment takes the uppermost energy storage submodule of each phase of the three-phase energy storage system as the master unit of each phase, and takes the other N-1 energy storage submodules as the slave unit of each phase.

[0088] For example, in the embodiment, N is 10, the energy storage submodules in the A-phase system are divided, and are numbered from top to bottom according to the series order of the energy storage submodules, which are the first energy storage submodule of the A-phase, the second energy storage submodule of the A-phase,..., and the tenth energy storage submodule of the A-phase. The first energy storage submodule of the A-phase is taken as the master unit of the A-phase system, and the second energy storage submodule to the tenth energy storage submodule of the A-phase are taken as the slave unit of the A-phase system.

[0089] The division rules of the energy storage submodules in the B-phase system and the C-phase system are consistent with those in the A-phase system, that is:

[0090] The first energy storage submodule of the B-phase is taken as the master unit of the B-phase system, and the second energy storage submodule to the tenth energy storage submodule of the B-phase are taken as the slave unit of the B-phase system.

[0091] The first energy storage submodule of the C-phase is taken as the master unit of the C-phase system, and the second energy storage submodule to the tenth energy storage submodule of the C-phase are taken as the slave unit of the C-phase system.

[0092] The output voltage of the first energy storage submodule of the A-phase is v A1 , the output voltage of the second energy storage submodule of the A-phase is v A2 , and the output voltage of the Nth energy storage submodule of the A-phase is v AN ; due to the reason that the current is shared by each energy storage submodule in the series system, the output current of the energy storage submodules in the A-phase system is consistent, which is i A ;

[0093] i A is the A-phase current of the public power grid, and the A-phase voltage of the public power grid is equal to the cumulative sum of the output voltages (v A1 , v A2 ,..., and v AN ) of the energy storage submodules of the A-phase.

[0094] The input current and input voltage of each level of energy storage submodule in the B-phase system and the C-phase system are similar to those in the A-phase, that is:

[0095] The output voltage of the first energy storage submodule in the B phase is v B1 The output voltage of the second energy storage submodule in the B phase is v B2 The output current of the Nth energy storage submodule in the B phase is v BN Due to the current sharing among the energy storage submodules in the series system, the output currents of the energy storage submodules in the B phase system are consistent, all being i B ;

[0096] i B The B-phase current of the utility grid, and the B-phase voltage of the utility grid is equal to the cumulative sum of the output voltages (v B1 , v B2 ,..., v BN ) of the energy storage submodules in the B phase.

[0097] The output voltage of the first energy storage submodule in the C phase is v C1 The output voltage of the second energy storage submodule in the C phase is v C2 The output current of the Nth energy storage submodule in the C phase is v CN Due to the current sharing among the energy storage submodules in the series system, the output currents of the energy storage submodules in the C phase system are consistent, all being i C ;

[0098] i C The C-phase current of the utility grid, and the C-phase voltage of the utility grid is equal to the cumulative sum of the output voltages (v C1 , v C2 ,..., v CN ) of the energy storage submodules in the C phase.

[0099] The energy storage submodules each include a unit controller, i.e., each phase system includes 1 master unit controller and N-1 slave unit controllers;

[0100] The master unit controller of each phase system is responsible for coordinating the operation of the entire system, and the master unit controller of each phase system communicates with the master unit controllers of other phase systems through inter-phase communication messages; the master unit controller of each phase system sends inter-phase communication messages belonging to itself to the master unit controllers of other phase systems, and receives inter-phase communication messages sent by the master unit controllers of other phase systems; the inter-phase communication messages include: the A-phase corrected active power P' A ; the B-phase corrected active power P' B ; the C-phase corrected active power P' C ; the A-phase equivalent phase angle variable the B-phase equivalent phase angle variable the C-phase equivalent phase angle variable

[0101] For example, the A-phase system master controller sends the A-phase modified active power P A Equivalent phase angle variable corresponding to the A phase to the B-phase system master controller and the C-phase system master controller, and also receives the B-phase modified active power P A Equivalent phase angle variable corresponding to the B phase and the C-phase modified active power P C Equivalent phase angle variable corresponding to the C phase

[0102] The master unit communicates with the per-phase slave unit controller for receiving the slave unit SOC information sent by the slave unit and calculating the per-phase SOC average value and sends the master controller instruction to the slave unit controller;

[0103] The per-phase SOC average value is calculated according to the following formula: n is the number of all energy storage units in the x phase; x∈{A, B, C}, used to represent three phases;

[0104] The master controller instruction includes the master unit output current frequency Master unit output current amplitude I x and the per-phase SOC average value to the slave unit controller; wherein x represents a phase, x∈{A, B, C};

[0105] The master unit controller also controls the charging and discharging process of the energy storage unit in the master unit, and adjusts the output current and voltage through signals;

[0106] Each slave unit has a slave unit controller, which is responsible for controlling the corresponding energy storage unit, and receives the master controller instruction sent by the per-phase master unit controller, adjusts the charging and discharging process according to the master controller instruction, and feeds back the slave unit SOC information to the master unit controller;

[0107] The inter-phase communication (interaction between master units) is mainly for adjusting inter-phase cooperation, including inter-phase synchronization and inter-phase SOC balancing; the intra-phase communication (interaction between master units and slave units) is mainly for intra-phase unit synchronization, inertia support and intra-phase SOC balancing;

[0108] The inter-phase communication (for example, between the A-phase master unit and the B-phase master unit) and the intra-phase communication (for example, between the A-phase master unit and the A-phase slave unit) both adopt wired communication, which is realized through serial communication technology, which is prior art and will not be described here.

[0109] Figure d AThe signal is a duty cycle control signal output by an A-phase master unit controller, and is used to adjust the switching state of an IGBT in a power conversion system (PCS). Ai represents a duty cycle control signal output by an A-phase i-th slave unit controller.

[0110] In a power electronic system, the switching state of an IGBT (Insulated Gate Bipolar Transistor) in a PSC (Power Conversion System) includes two working modes, namely ON and OFF, which are controlled by a gate voltage, and the switching state of the IGBT is controlled to achieve accurate regulation of current and voltage, thereby meeting the operation requirements of the power electronic system.

[0111] Generally, a reference value (such as a target current or voltage) is first obtained by a controller, and then compared with an actual value, and then passed through a current loop or voltage loop controller, and then converted into a normalized duty cycle signal through a modulation strategy (such as SPWM or SVPWM).

[0112] The master unit controller is a current VSG controller, and the slave unit controller is a virtual impedance controller.

[0113] A VSG controller (Virtual Synchronous Generator Controller) is a control strategy used to simulate the characteristics of a synchronous generator, and is mainly applied to new energy power generation systems (such as wind power and photovoltaic power) and energy storage systems to enhance the inertia and stability of the system.

[0114] The energy storage system has the ability to operate in a network, and the control system acts on the energy storage system.

[0115] The control system includes the following modules:

[0116] A power calculation module is used to sample and calculate the active power output by each phase system and the reactive power output by each phase system.

[0117] Specifically, the active power output by each phase system P x and the reactive power output by each phase system Q x The output voltage v x and the output current i x are calculated, and the calculation expression is shown in formula (1):

[0118]

[0119] Where s represents a Laplace operator, represents v x lagging 90°, which can be represented by v xAfter Hilbert transform, τ is low-pass filter time constant, generally set to 0.008s~0.03s; Parameter x in the formula is used to represent three-phase number, x∈{A,B,C}; Namely i x represents x-phase system output current; v x represents x-phase system output voltage;

[0120] Specifically, each phase system output active power P x includes A-phase system output reactive power P A , B-phase system output reactive power P B and C-phase system output reactive power P C ; Each phase system output reactive power Q x includes A-phase system reactive power Q A , B-phase system reactive power Q B and C-phase system reactive power Q C ;

[0121] The output voltage v x is obtained by the voltage sampling device of each phase main unit, generally the voltage signal of the capacitor C is collected; The output current i x is obtained by the current sampling device of each phase main unit, generally the current signal of the inductor L is collected;

[0122] For example, the output voltage v A and the output current i A of the A-phase system are obtained by the voltage sampling device and the current sampling device of the A-phase main unit, and the overall output reactive power and active power of the A-phase are obtained by calculation, and the expression is as follows:

[0123]

[0124] Similarly, the output current and output voltage of the B-phase system and the C-phase system are collected, and the active power and the reactive power are calculated, which is similar to the A-phase system, and will not be described here.

[0125] Figure 1 In i A , v A is the output current of the A-phase system, v A2 , …, v An represents the output voltage of the A-phase system.

[0126] The main unit current value support control module is configured to calculate, based on per-phase system output active power, per-phase system output reactive power, and inter-phase phase angle balance adjustment amount, per-phase main unit output current frequency and per-phase main unit output current amplitude through virtual inertia calculation and a first PI controller calculation, and send the two to all slave units in the current phase as current phase reference, and calculate per-phase corrected active power and per-phase equivalent phase angle variable and send them to other phase main units;

[0127] The formula of the virtual inertia calculation and the PI controller calculation is shown in formula (2):

[0128]

[0129] wherein, is the x-phase main unit output current frequency; I x is the x-phase main unit output current amplitude; P x is the x-phase system output active power; Q x is the x-phase system output reactive power; H is a virtual inertia time constant, the value of which is related to energy storage capacity and system allowed frequency change rate, and in the embodiment, the value is 0.039; D is a virtual damping coefficient, the value of which is related to energy storage capacity and allowed frequency deviation range, and in the embodiment, the value is 80; represents the x-phase system output active power reference at the standard frequency f n , which is generally determined according to the preset value of the grid demand; represents the x-phase system output reactive power reference at the standard frequency f n , which is generally determined according to the preset value of the grid demand (if the grid does not require a reactive power reference value, it is set to 0); Δf x represents the inter-phase phase angle balance adjustment amount of the x-phase; I x * is the x-phase system rated current amplitude, which can be obtained by dividing the rated grid voltage; is a frequency regulation coefficient, is a reactive power regulation proportion coefficient; is a reactive power regulation integral coefficient; s represents a Laplace operator; the standard frequency f n is 50 Hz by default; represents the x-phase main unit output current frequency derivative with respect to the time variable t, i.e., the rate of change of frequency (ROCOF); ΔI x is the x-phase current amplitude compensation component; x represents a phase, and x∈{A, B, C};

[0130] ​​The first formula in formula (2) is a virtual inertia calculation; the second formula in formula (2) is used to calculate an output current amplitude reference, and the first PI controller is used for system reactive power regulation control, and the output current amplitude reference is adjusted according to system reactive power.

[0131] The PI controller generates a control signal by performing proportional and integral operations on the error signal of the system to adjust the controlled object, so that the output of the system is as close as possible to the expected value.

[0132] In the second formula of formula (2) represents the error between the reference value and the actual value of the reactive power; is the first PI controller; is a proportional coefficient, which performs proportional operation on the error, quickly responds to the change of the error, and generates a control component proportional to the size of the error; is an integral element, which performs operation on the accumulation of the error with time in the time domain, and is used to eliminate the steady-state error of the system; through the joint action of the proportional and integral, the first PI controller adjusts the output current amplitude I x of the main unit to be close to the expected value, so as to realize accurate control of the reactive power, that is, the second formula of formula (2) is the embodiment of the calculation of the first PI controller;

[0133] Each phase main unit calculates the output current frequency f of each phase main unit and the output current amplitude I x of each phase main unit, and then sends the output current frequency f of each phase main unit and the output current amplitude I x of each phase main unit to all slave units in the same phase as the current phase reference; each phase main unit also calculates the SOC average value of each phase according to the SOC information (main unit SOC information and all slave unit SOC information) of each phase energy storage subsystem, and sends the SOC average value

[0134] Specifically, the output current frequency f of the main unit includes the output current frequency f of the A-phase main unit, the output current frequency f of the B-phase main unit, and the output current frequency f of the C-phase main unit. The output current amplitude I x of the main unit includes the output current amplitude I A of the A-phase main unit, the output current amplitude I B of the B-phase main unit, and the output current amplitude I C of the C-phase main unit.

[0135] Reference Figure 2 , Figure 2A-phase main unit current VSG controller schematic diagram; in particular, the calculation formula corresponding to the A-phase main unit output current frequency and the A-phase main unit output current amplitude is as follows:

[0136]

[0137] Wherein, Δf A represents the inter-phase balance adjustment amount of the A-phase; represents the A-phase main unit output current frequency; I A * is the A-phase system rated current amplitude; I A represents the A-phase system main unit output current amplitude; represents the A-phase system active power reference under the standard frequency f n ; represents the A-phase system reactive power reference under the standard frequency f n ; is the A-phase system reactive power regulation proportional coefficient, taking the value of 0.0006; is the A-phase system reactive power regulation integral coefficient, taking the value of 0.0005; represents the derivative with respect to the time variable t;

[0138] In particular, the calculation formula corresponding to the B-phase main unit output current frequency and the B-phase main unit output current amplitude is as follows:

[0139]

[0140] Wherein, Δf B represents the inter-phase balance adjustment amount of the B-phase; represents the B-phase main unit output current frequency; I B * is the B-phase system rated current amplitude; I B represents the B-phase system main unit output current amplitude; represents the B-phase system active power reference under the standard frequency f n ; represents the B-phase system reactive power reference under the standard frequency f n ; is the B-phase system reactive power regulation proportional coefficient, taking the value of 0.0006; is the B-phase system reactive power regulation integral coefficient, taking the value of 0.0005; represents the derivative with respect to the time variable t;

[0141] In particular, the calculation formula corresponding to the C-phase main unit output current frequency and the C-phase main unit output current amplitude is as follows:

[0142]

[0143] wherein, Δf C represents the inter-phase balance adjustment amount of phase C; represents the output current frequency of the main unit of phase C; I C * is the rated current amplitude of the C-phase system; I C represents the output current amplitude of the main unit of the C-phase system; represents the active power reference of the C-phase system under the standard frequency f n ; represents the reactive power reference of the C-phase system under the standard frequency f n ; is the reactive adjustment proportional coefficient of the C-phase system, and is 0.0006; is the reactive adjustment integral coefficient of the C-phase system, and is 0.0005; represents the derivative with respect to the time variable t; ;

[0144] the inter-phase adjustment module is configured to calculate an AC current sinusoidal signal of each phase system according to a phase angle of an output current of each phase system, and to perform phase angle compensation on the phase angle of the output current of each phase system to obtain an equivalent phase angle variable of each phase, and send the equivalent phase angle variable to other phase main units, and each phase main unit is configured to calculate an inter-phase phase angle balance adjustment amount according to the equivalent phase angle variable of each phase, and adjust a frequency output of each phase system based on a second PI controller to make the three-phase phase angles consistent;

[0145] the output phase angle of each phase is obtained by the main unit controller through the output current frequency of each phase main unit integration, as shown in equation (3):

[0146]

[0147] that is, the output phase angle of the A-phase system is δ A I , the output phase angle of the B-phase system is δ B I , and the output phase angle of the C-phase system is δ C I ;

[0148] Since each phase system operates independently, to achieve overall coordinated control of the system, the following two problems need to be solved: 1) inter-phase synchronization; and 2) inter-phase power distribution. In this scheme, the control method of each phase is the same, and thus the overall output of the system can be obtained, that is, the overall output frequency and overall output amplitude of the energy storage system are obtained:

[0149] The per-phase AC current sinusoidal signal calculated by the per-phase system master unit comprising As shown in formula (4):

[0150]

[0151] wherein, is a current sinusoidal signal conversion function for combining the current amplitude and phase into a sinusoidal signal, and the output is the x-phase AC current sinusoidal signal; that is, is the A-phase AC current sinusoidal signal; is the B-phase AC current sinusoidal signal; is the C-phase AC current sinusoidal signal; δ A I , δ B I , δ C I are the per-phase system output current phase angles calculated by the A, B, and C-phase system master unit controllers through formula (3); δ A I is the A-phase system output current phase angle; δ B I is the B-phase system output current phase angle; δ C I is the C-phase system output current phase angle; I A is the A-phase system current master unit output current amplitude, which is also the current flowing through all the units in the A-phase; I B is the B-phase system current master unit output current amplitude, which is also the current flowing through all the units in the B-phase; I C is the C-phase system current master unit output current amplitude, which is also the current flowing through all the units in the C-phase;

[0152] In order to eliminate the fixed phase difference of 2 / 3π(rad) of the three-phase current phase, the per-phase equivalent phase angle variable is obtained by compensating the per-phase system output current phase angle based on the A-phase through formula (5); formula (5) is as follows:

[0153]

[0154] wherein is the A-phase equivalent phase angle variable after phase angle compensation; is the B-phase equivalent phase angle variable after phase angle compensation; is the C-phase equivalent phase angle variable after phase angle compensation; the per-phase master unit controller transmits the equivalent phase angle variable after phase angle compensation of the per-phase system to the master unit controllers of other phases;

[0155] For example, the A-phase master controller transmits the phase angle compensated A-phase equivalent phase angle variable to the B-phase master controller and the C-phase master controller; similarly, the B-phase master controller transmits the phase angle compensated B-phase equivalent phase angle variable to the A-phase master controller and the C-phase master controller; the C-phase master controller transmits the phase angle compensated C-phase equivalent phase angle variable to the A-phase master controller and the B-phase master controller;

[0156] The second PI controller is used to adjust the system frequency output of each phase to make the three-phase phases consistent, as shown in equation (6):

[0157]

[0158] where Δf x represents the inter-phase phase angle equalization adjustment amount of the x-phase; Ω = {A, B, C} is used to represent a set of A, B, and C three-phase subscripts; k p and k I are frequency adjustment coefficients, k p is an inter-phase synchronization control frequency adjustment coefficient, which is taken as 1.3 in the embodiment, and k I is an inter-phase synchronization control frequency adjustment coefficient, which is taken as 2.5 in the embodiment; represents a set of all phase equivalent phase angle variables; finally, the inter-phase phase angle equalization adjustment amount Δf x of the x-phase is transmitted to the formula (2) of the master unit current support control module;

[0159] The formula (6) first calculates which represents the sum of the phase differences between the x-phase and the other two phases, and the sum of the phase differences reflects the overall phase difference between the x-phase and the other two phases;

[0160] Then, the sum of the phase differences is processed through the second PI controller (k P +k I / s); the proportional part k p is directly multiplied by the sum of the phase differences to generate an adjustment amount proportional to the current phase difference; the integral part k I / s integrates the sum of the phase differences to generate an adjustment amount accumulated over time, which is used to eliminate steady-state errors;

[0161] The final Δf x is used to adjust the frequency output of the x-phase system, and by adjusting the frequency output, the speed of phase change is changed, so that the three-phase phases tend to be consistent.

[0162] The steady state of the formula (6) is that is, when δ AI = δ B I + 2π / 3 = δ C I -2π / 3 meets the system three-phase balance requirements, because the phase is the frequency integral, the steady-state phase balance, the frequency must be synchronized, so it can keep the three-phase system frequency synchronization between phases through formula (6), while also maintaining the three-phase system phase balance.

[0163] From the unit virtual impedance control module, for combining the current phase reference sent by the slave unit from the master unit with the cascaded power distribution coefficient, the output voltage of the slave unit is adjusted by the virtual impedance control;

[0164] Reference Figure 3 , Figure 3 The virtual impedance control schematic diagram of the A-phase slave unit, since the output current of the energy storage sub-module in each phase system is consistent, the current phase reference (the output current frequency and the output current amplitude I x of the master unit) can be used for slave unit control, the slave unit uses a virtual impedance control method without synchronization, adjusts the output voltage according to the current reference power and system current information, and the expression is shown in formula (7):

[0165]

[0166] Where, V x represents the output voltage reference of the x-phase slave unit; s represents the Laplace operator; x represents the phase, x∈{A,B,C}; represents the active power required to be provided by the x-phase; I x is the output current amplitude of the x-phase master unit; represents the amplitude of the x-phase virtual resistance; g(s) is a second-order transfer function, which is used to optimize the dynamic response of the system and improve the stability of the system; represents the virtual impedance size of the slave unit;

[0167] Where, the active power required to be provided by the x-phase According to formula (8), we get:

[0168]

[0169] Where, H is the virtual inertia time constant, and D is the virtual damping coefficient; s represents the Laplace operator; is the output current frequency of the x-phase master unit; f n is the standard frequency, generally 50Hz;

[0170] Where, the second-order transfer function g(s) is realized based on formula (9):

[0171]

[0172] wherein x represents a phase, x∈{A,B,C}; is the phase angle frequency, i.e. f n is the standard frequency; represents the cascade power distribution coefficient of each unit in phase x, which needs to be determined according to the real-time SOC state of each energy storage unit in each phase system, so that the output power can be adjusted according to the real-time SOC state, thereby realizing the SOC balancing of each energy storage unit in the phase;

[0173] the cascade power distribution coefficient of each unit in phase x According to formula (10), we have:

[0174]

[0175] wherein, represents the residual capacity of the i th energy storage unit in phase x, i.e. SOC, which is the real-time SOC state of the energy storage unit, The value of can be directly read according to the estimation of the BMS system of the energy storage unit; n represents the number of all energy storage units in phase x, and i represents the serial number of the energy storage unit from top to bottom in each phase, the serial number of the energy storage unit of the master unit is 1, the serial number of the energy storage unit of the first slave unit is 2, and the serial number of the energy storage unit of the N-1 th slave unit is N;

[0176] The current amplitude compensation module is used for correcting the current active power of each phase according to the active power of each phase system and the average value of each phase SOC to obtain the corrected active power of each phase, and calculating the current amplitude compensation component of each phase and injecting it into the calculation of the output current amplitude of the master unit.

[0177] Since each phase system operates independently, in order to realize the overall coordinated control of the system, the following two problems need to be solved: (1) inter-phase power distribution; (2) inter-phase SOC balancing adjustment.

[0178] In order to further realize the power coordination and SOC balancing among the three-phase systems, the current amplitude compensation control strategy is introduced in this embodiment: the strategy constructs a corrected power expression according to the average value of the SOC of each phase to correct the current active power of each phase, so as to obtain the corrected active power of each phase, and calculates the current amplitude compensation component of each phase and injects it into the calculation of the output current amplitude I x of the master unit in formula (2), so as to dynamically adjust the energy flow among the three phases, realize the inter-phase SOC balancing, and realize the active power coordination and smooth energy balancing among the three phases.

[0179] The current active power of each phase is corrected according to the current active power of each phase and the average value of the SOC of each phase to obtain the corrected active power of each phase, which is realized based on formula (11) as shown below:

[0180]

[0181] wherein P x is the output active power of the x-phase system, i.e., the current active power of the x-phase system; P' x is the corrected active power of the x-phase; is the average value of the SOC of the x-phase, n is the number of all energy storage units in the x-phase system; x∈{A, B, C} is used to represent three phases; k soc is the SOC inter-phase balancing proportionality coefficient, which can generally be selected between 0.01-0.1 of the rated power, and the greater the value, the faster the SOC convergence speed, and too large will also affect stability;

[0182] The calculation of the current amplitude compensation component of each phase according to the corrected active power of each phase is shown in formula (12) as shown below:

[0183]

[0184] wherein ΔI x is the current amplitude compensation component of the x-phase; k0 is the proportionality coefficient of the current compensation control, which needs to be selected according to the power and current and system stability, and in the embodiment, k0 is between 1.4e-4-2.86e-4; is the average value of the corrected active power of three phases, i.e., sgn() is a sign function.

[0185] The SOC is defined as follows:

[0186]

[0187] wherein, represents the SOC (remaining capacity) of the i-th energy storage unit of the x-phase system, wherein represents the initial remaining capacity of the i-th energy storage unit of the x-phase system, represents the output current of the DC side of the i-th energy storage unit of the x-phase system, represents the capacity of the i-th energy storage unit of the x-phase system, which can be found in the battery module data manual; represents the maximum energy sum of the i-th energy storage unit of the x-phase system, which can be obtained by multiplying P xi represents the output active power of the i-th energy storage unit of the x-phase system;

[0188] Because power and SOC are also closely related, power disturbance changes will also cause SOC imbalance, and if power distribution is uneven, it will cause the SOC of some energy storage units to be too high or too low, thereby affecting the overall performance and life of the system; therefore, by introducing feedback regulation of SOC, the SOC of each energy storage unit can be balanced to avoid overcharging or overdischarging.

[0189] Formula is a current adjustment formula, and its control significance is that in discharge mode P x >0, is too large, then AI x is positive, the output current amplitude of the phase system increases, forcing the discharge power of the phase system to increase and accelerating the SOC drop speed; in charge mode P x <0, when is too large, then AI x is positive, the output current amplitude of the phase system decreases, forcing the charging power of the phase system to decrease and reducing the SOC rise speed.

[0190] The main unit energy storage control module is used to generate a main unit PWM signal by a proportional resonant controller to adjust the switching state of IGBT in the corresponding energy storage converter of the main unit, by inputting the sinusoidal signal of each phase alternating current and the current of each phase of the utility grid.

[0191] The sinusoidal signal of each phase alternating current and the current of each phase of the utility grid i x are input as input signals into the proportional resonant controller, which processes and calculates according to the difference between the input signals to generate a main unit PWM signal d x ; the duty cycle of this signal will be adjusted according to the change of the input signal to achieve accurate control of the energy storage converter; the generated d x signal is used to adjust the switching state of IGBT in the corresponding energy storage converter of the main unit, and IGBT as the core switching device of the energy storage converter, its switching state directly determines the output voltage and current of the converter, so by adjusting the switching state of IGBT, the flow of electrical energy between the energy storage system and the utility grid can be controlled, such as realizing the charging (absorbing electrical energy from the grid) or discharging (delivering electrical energy to the grid) of the energy storage system, while ensuring the quality and stability of the output current.

[0192] The proportional resonant (PR) controller is a controller for tracking and eliminating specific frequency harmonics of alternating current signals, especially suitable for power electronics and motor control fields. The proportional resonant controller adds a resonant link to the traditional proportional control, which realizes high-precision tracking of specific frequency signals and suppresses harmonic interference. This is prior art and will not be described here.

[0193] From the unit energy storage control module, for each phase system from the unit output voltage reference and the utility grid per phase current and per phase system from the unit output voltage by voltage and current double closed loop control generation from the unit PWM signal adjustment from the unit corresponding energy storage inverter IGBT switch state.

[0194] Double closed loop control input includes each phase system control based on two key input quantities (each phase system from the unit output voltage reference V x With the utility grid per phase current i x ) and the actual output of each phase system from the unit output voltage v Ai , output voltage reference V x Set the voltage output target that the system expects to achieve, while the utility grid per phase current i x And each phase system from the unit output voltage v Ai Then provide real-time running state information of the system; voltage and current double closed loop control is a common control strategy, which includes voltage loop and current loop, voltage loop mainly ensures the stability of output voltage, so that it is close to output voltage reference V x ; current loop is used to quickly respond to the change of current, improve the dynamic performance of the system. Through the synergistic effect of the two closed loops, the system can accurately adjust the control quantity according to the real-time feedback of voltage and current information; the output result of double closed loop control is used to adjust the from the unit PWM signal d Ai ; the duty ratio of PWM signal determines the on and off time ratio of IGBT, by adjusting the duty ratio of dAi, the switching state of IGBT can be controlled, that is, the generated PWM signal d Ai Is used to adjust the switching state of IGBT in energy storage inverter, and the switching state of IGBT directly affects the output voltage and current of energy storage inverter, so as to realize the control of energy exchange between energy storage system and utility grid, such as realizing the functions of charging and discharging of energy storage system, and maintaining the stability of output voltage and good power quality.

[0195] The voltage-current double closed-loop control is a kind of high-performance control strategy widely used in power electronic systems (such as inverters, frequency converters, motor drives, etc.), which realizes accurate and fast system regulation through nested voltage loop and current loop, including outer loop (voltage loop) and inner loop (current loop): the outer loop takes the output voltage as the controlled quantity, compares the error between the target voltage and the actual voltage through the PI controller, and outputs the current reference value; the inner loop (current loop) takes the current as the controlled quantity, receives the current reference value output by the voltage loop, and adjusts the actual current to track the current reference value through the PI controller. The voltage-current double closed-loop control works through the cooperation of the inner and outer two closed loops, the inner loop is responsible for the rapid regulation of the current, and the outer loop manages the steady-state accuracy of the voltage, and the two cooperate to improve the overall performance, so that the system outputs stable voltage and current, which is the prior art and will not be described here.

[0196] The embodiment also includes a protection warning module for setting overvoltage protection and undervoltage protection for each energy storage sub-module and setting SOC upper and lower threshold values, when the remaining power of the energy storage unit is lower than the SOC lower threshold value or higher than the SOC upper threshold value, the single energy storage sub-module immediately enters the protection state and reports the exception; when the communication between the master unit and the slave unit in the phase is abnormal or the voltage and current are out of limit, the single energy storage sub-module immediately enters the protection state and reports the exception.

[0197] Generally, after entering the protection state, two coping strategies can be adopted: one is to start the reserved redundant unit to replace the abnormal module to ensure the continuous operation of the system; the other is to dynamically redistribute the power among the remaining units through the proposed control method to realize function compensation. If the power distribution has reached the system limit and still cannot meet the operation demand, the shutdown maintenance mechanism needs to be triggered to ensure the safety of the system.

[0198] In addition, it needs to be explained that under normal control conditions, the SOC of all energy storage units is consistent and there is no over-limit problem of a certain energy storage unit.

[0199] The embodiment is based on a set of three-phase cascaded H-bridge energy storage systems with a rated capacity of 100 MW and a voltage level of 15.75 kV, the overvoltage protection of a single energy storage sub-module is set to 1800V, and the undervoltage protection is 1200V; the SOC upper and lower threshold values of the energy storage unit in the embodiment are set to 95% and 10%; when communication abnormality or voltage and current out-of-limit occurs, the module immediately enters the protection state and reports the exception;

[0200] The control strategy of the embodiment does not rely on a traditional phase-locked loop (PLL) for synchronization signal extraction. The master unit establishes a synchronous relationship with the grid or inter-phase reference through current support control, and the slave unit achieves adaptive synchronization through virtual impedance regulation, effectively avoiding phase-locked instability and control uncertainty caused by sensing errors. At the same time, since the system is essentially a current-type control framework, it has natural current limiting capability and can operate stably in weak grid or large line impedance variation scenarios, significantly reducing the risk of overcurrent, and has good distributed deployment capability and field engineering implementability.

[0201] Embodiment two.

[0202] Reference Figure 4 The embodiment provides a control method for frequency modulation of thermal power assisted by an energy storage system, based on the control system for frequency modulation of thermal power assisted by an energy storage system described in embodiment one, comprising the following steps:

[0203] S1, sampling and calculating the active power output of each phase system and the reactive power output of each phase system;

[0204] S2, based on the active power output of each phase system and the reactive power output of each phase system and the inter-phase phase angle balance adjustment amount, the frequency of the output current of each phase master unit and the amplitude of the output current of each phase master unit are calculated through virtual inertia calculation and a first PI controller, respectively, and each phase master unit sends the two as the current phase reference to all slave units in the phase, and calculates the modified active power of each phase and the equivalent phase angle variable of each phase and sends it to other phase master units;

[0205] S3, according to the phase angle of the output current of each phase system, the sinusoidal signal of the alternating current of each phase is calculated, and the phase angle of the output current of each phase system is compensated to obtain the equivalent phase angle variable of each phase and send it to other phase master units, and each phase master unit calculates the inter-phase phase angle balance adjustment amount according to the equivalent phase angle variable of each phase and adjusts the frequency output of each phase system based on a second PI controller to make the three-phase phase consistent;

[0206] S4, combining the current phase reference sent by the master unit with the cascaded power distribution coefficient, the output voltage of the slave unit is adjusted by using virtual impedance control;

[0207] S5, according to the active power output of each phase system and the average value of each phase SOC, the current active power of each phase is modified to obtain the modified active power of each phase, and after the current amplitude compensation component of each phase is calculated according to the modified active power of each phase, it is injected into the calculation of the output current amplitude of the master unit;

[0208] S6, the sinusoidal signal of the alternating current of each phase and the current of each phase of the public power grid are generated through a proportional resonant controller to generate a master unit PWM signal to adjust the state of the energy storage converter corresponding to the master unit;

[0209] S7, the output voltage reference of each phase system from the unit is adjusted by the per-phase current of the utility grid and the per-phase system from the unit output voltage through a voltage-current double closed loop control to generate a slave unit PWM signal to adjust the state of the corresponding energy storage converter of the slave unit.

[0210] As described in the background, due to the high penetration of new energy, thermal power is in long-term low-load operation, which leads to the decrease of the frequency regulation capability of the whole power system. Due to the limitation of the thermal power ramp rate, the system lacks inertia support. The control system and method of the energy storage system provided in the embodiment of the application can effectively assist in adjusting the thermal power ramp rate under high penetration of new energy, thereby improving the frequency regulation capability of the whole power system under high penetration of new energy.

[0211] Compared with the prior art, the embodiment of the application has the following significant progress:

[0212] 1) Stronger multi-module power regulation capability

[0213] Compared with the traditional voltage-type VSG and current-type GFL scheme which cannot realize flexible power regulation at the module level, the embodiment adopts a master-slave structure combined with virtual impedance control, which can accurately regulate the output voltage of each energy storage sub-module while ensuring current consistency, and realize power adaptive distribution among multiple modules.

[0214] 2) Complete SOC balancing mechanism

[0215] Unlike existing control methods that do not consider or only partially consider energy balancing, the embodiment introduces a power distribution factor based on the SOC state, and introduces a phase-to-phase SOC-driven current amplitude compensation mechanism in a three-phase system, which can dynamically balance the state of charge of each energy storage unit during system operation, and improve the overall life and utilization efficiency of the energy storage system.

[0216] 3) Better system-level frequency support capability

[0217] The embodiment introduces a virtual inertia element in the master unit, so that the energy storage system has frequency dynamic response characteristics; at the same time, the slave unit improves the system disturbance suppression capability through a dynamically optimized second-order controller, and the overall frequency support performance is significantly better than the slow response scheme of the voltage-type VSG and the unstable scheme of the current-type GFL.

[0218] 4) Outstanding phase-to-phase coordination capability

[0219] Traditional schemes often ignore three-phase coordinated control, which can easily cause power imbalance and phase angle drift. The embodiment constructs a phase-to-phase phase angle adjustment amount and a current compensation amount injection strategy based on the SOC difference, and for the first time realizes phase-to-phase power coordination and synchronous control of a three-phase system under current consistency constraints, effectively suppressing three-phase dynamic imbalance.

[0220] 5) clear structure, easy to implement engineering

[0221] The scheme of the embodiment adopts a hierarchical architecture of master-slave + inter-phase control, has clear function division, low coupling degree between controllers, is suitable for distributed deployment and modular expansion of actual projects, has low dependence on sensor precision and external power grid, has stronger robustness and field adaptability, performs more reasonable intelligent scheduling on power supply, and provides more excellent guarantee and defense for large-scale power grid safety.

[0222] The above merely describes preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A control system for energy storage system assisting frequency modulation of thermal power generation, characterized in that, The energy storage system is a three-phase cascaded H-bridge energy storage system, and the energy storage system comprises an A-phase system, a B-phase system and a C-phase system, each phase being composed of linear impedance and N energy storage sub-modules in series, forming an overall series three-phase structure and being connected to a power grid; the energy storage sub-modules are of the same structure and each comprises a filter circuit, an energy storage converter and an energy storage unit; the uppermost energy storage sub-module of each phase of the energy storage system is taken as a master unit of each phase, and the other N-1 energy storage sub-modules are taken as slave units of each phase; and the energy storage system comprises the following modules: a power calculation module for sampling and calculating active power output of each phase system and reactive power output of each phase system; a master unit current support control module for calculating, based on the active power output of each phase system, the reactive power output of each phase system and an inter-phase phase angle balance adjustment amount, an output current frequency of each phase master unit and an output current amplitude of each phase master unit through virtual inertia calculation and a first PI controller calculation, and sending, by each phase master unit, the two as a current phase reference to all slave units of the current phase and calculating a corrected active power of each phase and an equivalent phase angle variable of each phase and sending the corrected active power and the equivalent phase angle variable to master units of other phases; an inter-phase adjustment module for calculating an AC current sinusoidal signal of each phase according to a phase angle of an output current of each phase system, performing phase angle compensation on the phase angle of the output current of each phase system to obtain an equivalent phase angle variable of each phase and sending the equivalent phase angle variable to master units of other phases, and calculating, by each phase master unit, an inter-phase phase angle balance adjustment amount according to the equivalent phase angle variable of each phase and adjusting a frequency output of each phase system based on a second PI controller to make the three phases consistent in phase; a slave unit virtual impedance control module for combining, by a slave unit, a current phase reference sent by a master unit and a cascaded power distribution coefficient, and adjusting an output voltage of the slave unit through virtual impedance control; a current amplitude compensation module for correcting a current active power of each phase according to an active power output of each phase system and an SOC average value of each phase to obtain a corrected active power of each phase, calculating a current amplitude compensation component of each phase according to the corrected active power of each phase and injecting the current amplitude compensation component into calculation of an output current amplitude of each phase; a master unit energy storage control module for generating a master unit PWM signal through a proportional-resonant controller based on an AC current sinusoidal signal of each phase and a current of each phase of a power grid to adjust a state of an energy storage converter corresponding to the master unit; a slave unit energy storage control module for generating a slave unit PWM signal through a voltage-current double-loop control based on an output voltage reference of a slave unit of each phase system, a current of each phase of the power grid and an output voltage of the slave unit of each phase system to adjust a state of an energy storage converter corresponding to the slave unit.

2. The control system of claim 1, wherein, The formula of the virtual inertia calculation and the first PI controller calculation is shown in formula (2): wherein is the x-phase main unit output current frequency; I x is the x-phase main unit output current amplitude; P x is the x-phase system output active power; Q x is the x-phase system output reactive power; H is a virtual inertia time constant; D is a virtual damping coefficient; denotes the x-phase system output active power reference at the standard frequency f n denotes the x-phase system output reactive power reference at the standard frequency f n Δf x denotes the x-phase inter-phase angle equalization regulation quantity; I * is the system rated current amplitude; is the reactive regulation proportional coefficient; is the reactive regulation integral coefficient; s denotes the Laplace operator; denotes the x-phase main unit output current frequency derivative with respect to the time variable t; ΔI x is the x-phase current amplitude compensation component; x e {A, B, C}.​ 3. The control system of claim 2, wherein, The calculation formula of the AC current sinusoidal signal of each phase according to the phase angle of the output current of each phase system in the inter-phase adjustment module is shown in formula (4): wherein, is a current sine signal transformation function for combining current magnitude and phase into a sine signal, output of is the x-phase ac current sine signal; δ A I is the A-phase system output current phase angle; δ B I is the B-phase system output current phase angle; δ C I is the C-phase system output current phase angle.

4. The control system of claim 1, wherein, The equivalent phase angle variable of each phase is obtained by performing phase angle compensation on the phase angle of the output current of each phase system according to formula (5): wherein is the equivalent phase angle variable for phase A after phase angle compensation; is the equivalent phase angle variable for phase B after phase angle compensation; is the equivalent phase angle variable for phase C after phase angle compensation; delta A I for the A-phase system output current phase angle; delta B I is the phase angle of the B-phase system output current; delta C I The phase angle of the output current for the C-phase system.

5. The control system of claim 4, wherein, The inter-phase phase angle balance adjustment amount is calculated by each phase master unit according to the equivalent phase angle variable of each phase, and the frequency output of each phase system is adjusted based on the second PI controller to make the three phases consistent in phase, which is realized based on formula (6): where Δf x represents the inter-phase phase angle equalization adjustment amount of the x phase; Ω = {A, B, C}; k p is the inter-phase synchronization control frequency adjustment coefficient; k I is the inter-phase synchronization control frequency adjustment coefficient; represents the set of equivalent phase angle variables of all phases; the inter-phase phase angle equalization adjustment amount Δf x of the x phase is finally passed to the formula (2) of the main unit current support control module.

6. The control system of claim 2, wherein, The expression of the virtual impedance control adjustment is shown in formula (7): where i x represents the x-phase system output current; V x represents the x-phase cell output voltage reference; s represents the Laplace operator; x represents the phase, x e {A, B, C}; represents the x-phase virtual resistance magnitude; g(s) is a second-order transfer function; represents the virtual impedance magnitude of the cell; represents the active power that the x-phase needs to provide, calculated according to equation (8):

7. The control system of claim 6, wherein, The second-order transfer function g(s) is implemented based on formula (9): wherein is the x-phase angle frequency, i.e. denotes the cascade power distribution factor of each unit of the x-phase.

8. The control system of claim 1, wherein, The corrected active power of each phase is obtained according to the active power of each phase and the average value of each phase SOC, and the calculation formula of the corrected active power of each phase is implemented based on formula (11) and is as shown in the following formula (11): where P x is the active power output of phase x; P' x is the corrected active power of phase x; is the average value of SOC of phase x, n is the number of all energy storage units in phase x; x ∈ {A, B, C}; k soc is the SOC inter-phase balancing proportionality coefficient.

9. The control system of claim 8, wherein, The calculation formula of the current amplitude compensation component of each phase is as shown in formula (12): where ΔI x is the x-phase current amplitude compensation component; k0is the proportional coefficient of current compensation control; is the average value of the three-phase corrected active power; sgn() is the sign function.

10. A control method of frequency modulation of thermal power assisted by an energy storage system, based on the control system of frequency modulation of thermal power assisted by an energy storage system according to any one of claims 1-9, characterized in that, The method comprises the following steps: S1, sampling and calculating the active power and the reactive power of each phase; S2, the output current frequency of each phase and the output current amplitude of each phase are calculated based on the active power and the reactive power of each phase and the inter-phase phase angle balance adjustment amount through virtual inertia calculation and a first PI controller, and the two are sent to all slave units in the phase as the current phase reference, and the corrected active power of each phase and the equivalent phase angle variable of each phase are calculated and sent to other master units; S3, the AC current sine signal of each phase is calculated according to the phase angle of the output current of each phase, and the phase angle of the output current of each phase is compensated to obtain the equivalent phase angle variable of each phase, which is sent to other master units, and the inter-phase phase angle balance adjustment amount is calculated according to the equivalent phase angle variable of each phase, and the frequency output of each phase is adjusted based on a second PI controller to make the three-phase phase angles consistent; S4, the current phase reference sent by the master unit is received by the slave unit, and the output voltage of the slave unit is adjusted by adopting virtual impedance control in combination with the cascade power distribution coefficient; S5, the corrected active power of each phase is obtained according to the active power of each phase and the average value of each phase SOC, and the current amplitude compensation component of each phase is calculated and then injected into the calculation of the output current amplitude of the master unit; S6, the AC current sine signal of each phase and the current of each phase of the utility grid are generated into the master unit PWM signal through a proportional resonant controller to adjust the state of the energy storage converter corresponding to the master unit; S7, the output voltage reference of each phase of the slave unit and the current of each phase of the utility grid and the output voltage of each phase of the slave unit are generated into the slave unit PWM signal through a voltage-current double closed loop control to adjust the state of the energy storage converter corresponding to the slave unit.

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