Method for configuring dc link capacitor of ship hybrid power station energy storage system

Abstract

The application discloses a DC link capacitor configuration method for a ship hybrid power station energy storage system, and the method comprises the following steps: constructing a ship hybrid power station energy storage system model containing an energy storage frequency modulation system and an AC power supply system, setting an inertia time constant and a damping coefficient of a VSG, and calculating an inertia frequency modulation required power; in an online stage, according to an actually measured AC frequency fluctuation and an energy storage output power considering a power ramping constraint, a DC link capacitor is calculated with the target of maintaining a DC voltage fluctuation in a safe range under a maximum frequency step fluctuation. The application quantitatively analyzes the frequency modulation required power / energy, the battery ramping power / energy and the link capacitor power / energy, uses the DC link capacitor energy as a fast frequency modulation resource in the energy storage inertia frequency modulation system, and makes up the difference between the energy storage system ramping power output and the VSG inertia frequency modulation power requirement. According to the power and energy conservation, the DC link capacitor size under the best frequency modulation power requirement of the ship comprehensive power system is obtained, the method is suitable for different types of energy storage systems and system frequency modulation requirements under different disturbances, and the inertia frequency modulation capability and the stable operation capability of the system under multiple working conditions are effectively improved.

Description

Technical Field

[0001] This invention relates to a technology in the field of ship power control, specifically a method for configuring DC link capacitors in a ship hybrid power station energy storage system for inertial frequency regulation. Background Technology

[0002] Existing shipboard hybrid power station energy storage systems typically treat the power supply side as a rigid DC source, neglecting the limitations of battery power ramping capability. When ramping capability is insufficient, DC link capacitors are needed to support part of the inertial frequency modulation capability. Current capacitor calculation methods are based solely on DC-side ripple current suppression, resulting in conservative calculations that may lead to DC-side voltage exceeding limits during inertial regulation, thus affecting system stability. Summary of the Invention

[0003] This invention addresses the problem in existing DC link capacitor configuration techniques that fail to consider battery ramping capabilities, potentially leading to excessive DC voltage fluctuations during inertial frequency regulation. It proposes a DC link capacitor configuration method for shipboard hybrid power station energy storage systems. This method quantifies and analyzes the frequency regulation demand power / energy, battery ramping power / energy, and link capacitor power / energy. The DC link capacitor energy is utilized as a rapid frequency regulation resource in the energy storage inertial frequency regulation system, bridging the gap between the energy storage system's ramping power output and the VSG inertial frequency regulation power demand. Based on power and energy conservation, the optimal DC link capacitor size is obtained to meet the optimal frequency regulation power demand of the ship's integrated power system. This method is applicable to different types of energy storage systems and system frequency regulation requirements under various disturbances, effectively improving the system's inertial frequency regulation capability and stable operation under multiple operating conditions.

[0004] This invention is achieved through the following technical solution:

[0005] This invention relates to a method for configuring the DC link capacitor in a ship hybrid power station energy storage system for inertial frequency regulation. The method involves constructing a ship hybrid power station energy storage system model that includes an energy storage frequency regulation system and an AC power supply system, setting the inertial time constant and damping coefficient of the VSG, and calculating the power demand for inertial frequency regulation. During the online phase, based on the measured AC frequency fluctuations and considering the energy storage output power under power ramp-up constraints, the DC link capacitor is calculated with the goal of maintaining DC voltage fluctuations within a safe range under the maximum frequency step fluctuation.

[0006] The DC link capacitor is obtained by calculating the inverter-side inertial frequency modulation power demand during system frequency step change and the energy storage output power considering power ramp-up constraints, and then configuring the DC capacitor according to the steady-state DC voltage ripple suppression effect and the DC link voltage fluctuation limit during frequency modulation transient, and comparing the maximum values ​​of the two.

[0007] This invention relates to a system for implementing the above-described method, comprising: an energy storage frequency regulation system and an AC power supply system, wherein: the DC / DC converter in the energy storage frequency regulation system is controlled by a constant DC voltage, i.e., a constant Udc voltage is used to boost the voltage of the battery before connecting it to the DC side of the DC / AC converter, ensuring the stability of the DC voltage. The DC / AC converter acquires the three-phase voltage and current signals on the AC side and employs a virtual synchronous generator (VSG) control strategy to simulate the inertia characteristics of a synchronous machine, thereby realizing the frequency regulation function of the energy storage system. The AC power supply system consists of a diesel generator and a load, connected to the energy storage frequency regulation system through a transformer to form an integrated operation mode. The diesel generator and the energy storage frequency regulation system work together to provide a stable and reliable power supply to the AC load.

[0008] Technical effect

[0009] This invention clarifies the AC-side inertial frequency modulation power requirement by constructing a small-signal model of a virtual synchronous generator (VSG) and analyzes the key influencing factors of battery power ramping capability, including the ramping limit of the inductor current reference value in the DC / DC controller and the battery SOC-VOC characteristics. Based on this, the minimum limit of the DC link capacitor value within the voltage variation range during inertial frequency modulation is calculated using the DC link capacitor voltage-energy change formula. The capacitor value is then calculated using the ripple configuration method, and the maximum value of the two is selected. Compared with existing technologies, this invention accurately quantifies the maximum AC-side inertial frequency modulation power requirement and clarifies the power ramping capability of the source-side battery and its influencing factor, SOC. Considering both the source-side battery ramping capability and the inertial frequency modulation power requirement, the DC link capacitor value is configured to ensure that the capacitor voltage drop range remains within a safe range. Comparing the DC link capacitor value calculated using the traditional steady-state filtering formula, the maximum value of the two is taken as the DC link capacitor value, simultaneously satisfying the inertial frequency modulation requirement and the steady-state filtering effect, thereby significantly improving the system's safety, reliability, and dynamic response performance. Attached Figure Description

[0010] Figure 1 This is a flowchart of the present invention;

[0011] Figure 2 This is a control principle diagram of a ship-based hybrid power station energy storage system.

[0012] Figure 3 A schematic diagram of a DC / DC converter control system with a ramp rate limiter;

[0013] Figure 4 A schematic diagram of the VSG inertial frequency modulation power demand under underdamped conditions with a frequency step.

[0014] Figure 5 A schematic diagram showing the relationship between battery open-circuit voltage and SOC;

[0015] Figure 6 This is a schematic diagram showing the relationship between the terminal voltage and the state of charge (SOC) of a lithium nickel manganese cobalt oxide (LiNMC) battery at 15℃.

[0016] Figure 7 A schematic diagram showing the VSG frequency modulation power demand and energy storage active power output during a frequency step.

[0017] Figure 8 Example 1: (a) System output power verification; (b) Schematic diagram of DC voltage fluctuation.

[0018] Figure 9 Example 2 shows the verification of (a) system output power and (b) schematic diagram of DC voltage fluctuation. Detailed Implementation

[0019] like Figure 1 As shown in this embodiment, a DC link capacitor configuration method for a shipboard hybrid power station energy storage system oriented towards inertial frequency modulation is provided, including:

[0020] Step 1, construct as follows Figure 2 The model shown is a ship hybrid power station energy storage system model that includes an energy storage frequency regulation system and an AC power supply system. The energy storage frequency regulation system includes a DC / DC converter and a DC / AC converter connected in sequence. The AC power supply system includes an AC bus connected to the energy storage frequency regulation system, as well as a diesel generator and load connected in parallel. The battery energy storage is boosted by the DC / DC converter and connected to the DC side of the grid-connected inverter. It is interconnected with the diesel generator in the AC system through AC interconnection to form a ship hybrid power station to supply power to the AC load.

[0021] The DC / AC converter described above employs a virtual synchronous generator (VSG) control strategy, specifically including:

[0022] Step a, the motion equation of the VSG rotor of the DC / AC converter in the energy storage frequency regulation system satisfies: Where: the superscript "*" indicates the per-unit value; P m P e Input mechanical and electromagnetic power to VSG; ω0, ω g δ is the rated frequency and system frequency; H is the power angle; D is the inertial constant; and D is the damping coefficient.

[0023] Step b: Obtain the small-signal model of the system by performing a Laplace transform on the frequency modulation power demand of the virtual synchronizer. Where: Δ· represents the change in each of the above quantities; the derivation of the active and reactive power output of VSG is as follows: Where: α is the impedance angle of the filter circuit; Z is the impedance. This leads to the expression for active power, and the small-signal relationship between active power and power angle is satisfied: This yields the transfer function between the change in active power output of the virtual synchronous machine and the change in system frequency. Wherein: S n The rated capacity of the inverter, This is the power synchronization coefficient.

[0024] Step c: When the system frequency experiences a step change, the change in active power output by the virtual synchronous machine is:

[0025]

[0026] Step 2: Set the inertia constant and damping coefficient of the VSG, calculate the power required for inertial frequency modulation and the energy storage output power considering power ramp-up constraints, and then obtain the DC link capacitor, specifically including:

[0027] 2.1 Calculation of inverter-side inertial frequency modulation power demand during system frequency step: Considering the optimal operating state of the system, VSG control typically operates at the optimal damping ratio, i.e., underdamped state. At this time, the poles of the change in active power output by the virtual synchronous machine are two conjugate complex roots. Performing an inverse Laplace transform on these roots yields: Where: e is the natural constant. This leads to the peak value of the inertial frequency modulation power requirement. With peak time in: B = D / 4H, C = m / 4H

[0028] 2.2 Calculation of Energy Storage Output Power Considering Power Ramp-up Constraints: When the battery SOC changes, the terminal voltage also changes, causing the battery output power ramp-up rate to change accordingly. Therefore, given the battery's inherent characteristics, the battery output power ramp-up rate is jointly determined by the battery SOC and the inductor current ramp-up rate limit. The calculated energy storage output power P considering power ramp-up constraints is then obtained. ESS (t)=k P_ramp ·t=(α1z 6 +α2z 5 +α3z 4 +α4z 3 +α5z 2 +α6z 1 +α7)·k Lref_ramp ·t, where: battery output power ramp rate k P_ramp =U Bat ·k Lref_ramp k Lref_ramp To limit the inductor current ramp rate in a DC / DC controller, U Bat This represents the battery terminal voltage. Since batteries typically have low internal resistance, the terminal voltage U is used here. Bat With open circuit voltage Uoc They are considered equal. The open-circuit voltage U of a lithium nickel manganese cobalt oxide (LiNMC) battery is considered equal. oc =α1z 6 +α2z 5 +α3z 4 +α4z 3 +α5z 2 +α6z 1 +α7, z is the state of charge (SOC), α i The coefficient is defined.

[0029] like Figure 5 As shown, this represents the open-circuit voltage (U) of the battery at a certain temperature. oc The discharge curve of a battery can be fitted as a function of its state of charge (SOC). It can be observed that the open-circuit voltage decreases exponentially during periods when the SOC is between 90% and 100%, then levels off, accelerating its decline when the SOC reaches 20%. In actual operation, when the battery temperature is 15℃, α... i The values ​​are 2.08, -4.16, 0.43, 4.86, -4.02, 1.43, and 3.57, respectively.

[0030] 2.3 Configure the DC capacitor according to the steady-state DC voltage ripple suppression effect, specifically as follows:

[0031] in:

[0032] 2.4 Configure a DC capacitor to limit DC link voltage fluctuations during frequency modulation transients, specifically as follows: in:

[0033] Energy provided by the DC link capacitor during inertial frequency modulation t p The time at which the energy storage power ramp-up curve intersects the VSG inertial frequency modulation power curve is based on the equation. Using t p The boundary endpoints (0,0) and (π / C,0) of the existence range (0,π / C) are related to Ae -Bt The function sin(Ct) / t is obtained by interpolation approximation based on its monotonicity.

[0034] 2.5 Compare the maximum values ​​obtained from steps 2.3 and 2.4 to obtain the DC link capacitance. Where: t c =-ln[k P_ramp (1-e -Bπ / C ) / AC+e -Bπ / C ] / B; B = D / 4H; C = m / 4H; U dc U is the DC-side voltage of the inverter. dc.min The maximum voltage drop limit of the inverter's DC side; ΔU dc纹波 P0 is the DC capacitor voltage ripple limit for the inverter; P1 is the steady-state output power of the DC / DC converter; D1 is the duty cycle of the DC / DC converter, which is 1-U. Bat / U dc ;f s k is the switching frequency of the DC / DC converter. P_ramp The ramp-up rate of the energy storage system's output power.

[0035] Based on specific practical experiments, a MATLAB / Simulink simulation model of a typical ship hybrid power station energy storage system was established, including batteries, DC / DC converters, DC / AC converters, diesel generators and other equivalent loads, with parameters shown in Table 1.

[0036] Table 1 System Parameters

[0037]

[0038]

[0039] Through specific practical experiments, lithium nickel manganese cobalt oxide (LiNMC) batteries were used for testing, and the battery temperature was set at 15℃. In Example 1, the inductor current ramp rate was limited to 2000 A / s, and the state of charge (SOC) of the battery storage was 20%. This example aims to verify the accuracy of the proposed DC link capacitor calculation method under the condition of limited output power ramp of the energy storage system. In Example 2, the inductor current ramp rate was also limited to 2000 A / s, but the SOC of the battery storage was increased to 90%. This example was used to verify the impact of changes in battery SOC on DC voltage drop during inertial frequency modulation, thereby demonstrating the applicability of the proposed DC link capacitor calculation method under different SOC states of energy storage.

[0040] Example 1: When the ship's hybrid power station energy storage system has been running for 1 second, a load is connected to the AC network, causing the grid connection frequency to drop sharply to 49.95Hz. At this time, because the energy storage system uses VSG (Virtual Synchronous Generator) control, it immediately provides inertial frequency modulation power support to the AC network, such as... Figure 8 As shown in (a). However, since the output power of the energy storage system is limited by the ramp rate, it cannot immediately meet the VSG's demand for inertial frequency modulation power. As a result, the DC link capacitor acts as a supporting capacitor at this time, participating in the power output. According to the requirement that the maximum allowable DC voltage drop limit of the system is 10%, the DC link capacitor calculated according to this method is 4400μF, while the configuration method based solely on steady-state capacitor voltage ripple suppression is only 3300μF. Figure 8As shown in (b), the DC voltage dropped to 673V with a drop error of 0.3%, verifying the accuracy of this method.

[0041] Example 2: When the ship's hybrid power station energy storage system has been running for 1 second, a load is connected to the AC network, causing the grid connection frequency to drop sharply to 49.95Hz. At this time, the energy storage system controlled by VSG (Virtual Synchronous Generator) immediately provides inertial frequency regulation power support. For example... Figure 9 As shown in (a), with the increase of battery energy storage SOC, the ramp rate of energy storage output power also increases, resulting in a reduction in the energy provided by the DC capacitor during the inverter's participation in inertial frequency modulation, causing the DC voltage to drop to 700V, with a reduced voltage drop amplitude. This verifies the applicability of this method under different battery energy storage SOC states.

[0042] Compared with existing technologies, this invention proposes a DC link capacitor configuration method to address the challenges of large fluctuations in AC side frequency and limited DC side energy storage power ramping during inertial frequency modulation. This method ensures effective AC side inertial frequency modulation power supply while maintaining DC voltage fluctuations within a safe and stable range, thereby optimizing the DC link capacitor configuration. The specific implementations described above can be locally adjusted by those skilled in the art without departing from the principles and spirit of this invention. The scope of protection of this invention is defined by the claims and is not limited to the specific implementations described above; all implementations within the scope of these claims are bound by this invention.

Claims

1. A method for configuring DC link capacitors in a shipboard hybrid power station energy storage system for inertial frequency modulation, characterized in that, By constructing a ship hybrid power station energy storage system model that includes an energy storage frequency regulation system and an AC power supply system, the inertial time constant and damping coefficient of the VSG are set, and the inertial frequency regulation power requirement of the system is calculated based on the frequency step disturbance. Considering the energy storage power ramping constraint, the required DC link capacitance value is calculated with the goal of keeping the DC link voltage within the safe fluctuation range under the maximum allowable frequency step condition of the system. The DC link capacitor is obtained by calculating the inverter-side inertial frequency modulation power demand during system frequency step change and the energy storage output power considering power ramp-up constraints, and then configuring the DC capacitor according to the steady-state DC voltage ripple suppression effect and the DC link voltage fluctuation limit during frequency modulation transient, and comparing the maximum values ​​of the two. The aforementioned ship hybrid power station energy storage system model includes an energy storage frequency regulation system and an AC power supply system. The energy storage frequency regulation system includes a DC / DC converter and a DC / AC converter connected in sequence. The AC power supply system includes an AC bus connected to the energy storage frequency regulation system, as well as a diesel generator and load connected in parallel. The battery energy storage is boosted by the DC / DC converter and then connected to the DC side of the grid-connected converter. It is interconnected with the diesel generator in the AC system through AC interconnection to form a ship hybrid power station to supply power to the AC load. The DC / AC converter mentioned above adopts a Virtual Synchronous Machine (VSG) control strategy, specifically including: Step a, the motion equation of the VSG rotor of the DC / AC converter in the energy storage frequency regulation system satisfies: Where: the superscript "*" indicates the per-unit value; , The VSG outputs mechanical and electromagnetic power; , For rated frequency and system frequency; H is the angle of work; H is the constant of inertia; D is the damping coefficient. Step b: Obtain the small-signal model of the system by performing a Laplace transform on the frequency modulation power demand of the virtual synchronizer. ,in: The changes in the above quantities are: the output current of the virtual synchronizer is: Where E is the port potential of the virtual synchronous machine; U is the grid connection point voltage; and δ is the power angle of the virtual synchronous machine. Furthermore, the apparent power S output by the virtual synchronous generator can be expressed as: The superscript "*" indicates the conjugate operation for complex numbers. The impedance angle of the filter circuit. ; For impedance, , , The parasitic resistance and inductance of the LC filter are given; thus, the active and reactive power outputs of the VSG are obtained as follows: , , The VSG outputs electromagnetic active and reactive power; thus, the small-signal relationship between active power and power angle is obtained: ,in: The rated capacity of the inverter, Let be the power synchronization coefficient, thus obtaining the transfer function between the change in active power output of the virtual synchronizer and the change in system frequency. ; Step c: When the system frequency experiences a step change, the change in active power output by the virtual synchronous machine is: ; The DC link capacitor is obtained by setting the inertia constant and damping coefficient of the VSG and calculating the power required for inertial frequency modulation and the energy storage output power considering power ramp-up constraints. Specifically, it includes: 2.1 Calculate the power requirement of the inverter-side inertial frequency modulation when the system frequency increases by a step; 2.2 Calculation of energy storage output power taking into account power ramping constraints: 2.3 Configure the DC capacitor according to the steady-state DC voltage ripple suppression effect, specifically as follows: ,in: , which is the DC-side voltage ripple amplitude, representing the allowable ripple size of the inverter's DC capacitor voltage; 2.4 Configure DC capacitors according to the DC link voltage fluctuation limit during frequency modulation transients. 2.5 Compare the maximum values ​​obtained from steps 2.3 and 2.4 to obtain the DC link capacitance. ,in: , is the time point at which the energy storage output power curve intersects with the VSG inertia frequency modulation power curve; , is the inertia frequency modulation amplitude coefficient, which reflects the amplitude of the system output inertial power under a given frequency disturbance; , is the damping inertia ratio coefficient, a system damping characteristic parameter determined by both the damping coefficient and the inertial constant; , which is the normalized characteristic frequency coefficient, determined by the characteristic frequency factor and the inertia constant, and is used to characterize the oscillation characteristics of the system; , is the characteristic frequency factor, a dynamic characteristic parameter jointly determined by the inertial constant, damping coefficient, and power synchronization coefficient; This refers to the DC-side voltage of the inverter. This is the maximum voltage drop limit for the DC side of the inverter; Limit the DC capacitor voltage ripple of the inverter; This refers to the steady-state output power of the DC / DC converter. The duty cycle of the DC / DC converter is... , This is the voltage at the energy storage terminal. This is the voltage of the DC link capacitor; The switching frequency of the DC / DC converter; The ramp rate of the energy storage system's output power is the limit that characterizes the rate of change of the energy storage device's output power.

2. The DC link capacitor configuration method for a shipboard hybrid power station energy storage system oriented towards inertial frequency modulation according to claim 1, characterized in that, Step 2.1 specifically involves: considering the optimal operating state of the system, VSG control typically operates in the optimal damping ratio, i.e., the underdamped state. At this point, the poles of the change in active power output by the virtual synchronous machine are two conjugate complex roots. Performing an inverse Laplace transform on these roots yields: Where: e is the natural constant, Thus, the peak power required for inertial frequency modulation is obtained. With peak time ,in: , , , .

3. The DC link capacitor configuration method for a shipboard hybrid power station energy storage system oriented towards inertial frequency modulation according to claim 1, characterized in that, Step 2.2 specifically involves the following: When the battery SOC changes, the terminal voltage also changes, causing the battery output power ramp-up rate to change accordingly. Therefore, given the battery's inherent characteristics, the battery output power ramp-up rate is jointly determined by the battery SOC and the inductor current ramp-up rate limit. The energy storage output power taking into account the power ramp-up constraint is then calculated. Among them: battery output power ramp-up rate , To limit the inductor current ramp rate in a DC / DC controller, This refers to the battery terminal voltage. Since batteries typically have low internal resistance, the terminal voltage is used here. With open circuit voltage Assuming they are equal, the open-circuit voltage of a lithium nickel manganese cobalt oxide (LiNMC) battery is... , State of charge (SOC) The coefficient is defined.

4. The DC link capacitor configuration method for a shipboard hybrid power station energy storage system oriented towards inertial frequency modulation according to claim 1, characterized in that, Step 2.4, as described above, specifically includes: ,in: Energy provided by the DC link capacitor during inertial frequency modulation , The time of the intersection of the energy storage power ramp-up curve and the VSG inertial frequency modulation power curve is based on the equation. ,use Existence range boundary endpoints and and The function is obtained by interpolation approximation based on its monotonic properties.

5. A DC link capacitor configuration system for a shipboard hybrid power station energy storage system oriented towards inertial frequency modulation, implementing the method of any one of claims 1-4, characterized in that, include: The system comprises an energy storage frequency regulation system and an AC power supply system. The DC / DC converter in the energy storage frequency regulation system uses constant DC voltage control (i.e., constant Udc) to boost the voltage of the battery before connecting it to the DC / AC converter's DC side, ensuring stable DC voltage. The DC / AC converter acquires three-phase voltage and current signals from the AC side and employs a virtual synchronous machine (VSG) control strategy to simulate the inertia characteristics of a synchronous machine, thereby realizing the frequency regulation function of the energy storage system. The AC power supply system consists of a diesel generator and a load, connected to the energy storage frequency regulation system via a transformer, forming an integrated operation mode. The diesel generator and the energy storage frequency regulation system work together to provide a stable and reliable power supply to the AC load.

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