Direct-current link capacitor configuration method for energy storage system of ship hybrid power station

Abstract

A ship hybrid power station energy storage system direct current link capacitance configuration method comprises the steps that a ship hybrid power station energy storage system model comprising an energy storage frequency modulation system and an alternating current power supply system is constructed, an inertia time constant and a damping coefficient of a VSG are set, and inertia frequency modulation required power is calculated; in the online stage, according to actually measured alternating current frequency fluctuation and energy storage output power of power climbing constraint, direct current link capacitance is calculated and obtained by taking the purpose of maintaining direct current voltage fluctuation within a safe range under maximum frequency step fluctuation. According to the method, the frequency modulation required power / energy, the storage battery climbing power / energy and the link capacitance power / energy are quantitatively analyzed, the direct current link capacitance energy is used as a rapid frequency modulation resource in the energy storage inertia frequency modulation system, and the difference between the energy storage system climbing power output and the VSG inertia frequency modulation power requirement is made up. According to the power and energy conservation, the direct-current link capacitance meeting the optimal frequency modulation power requirement of the ship integrated 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 inertial frequency modulation capacity and stable operation capacity of the system under multiple working conditions are effectively improved.

Description

Technical Field

[0001] The present invention relates to a technology in the field of ship power control, and in particular to a DC link capacitor configuration method for a ship hybrid power station energy storage system oriented to inertial frequency modulation. Background Art

[0002] The existing inertial control technology of ship hybrid power station energy storage system usually treats the power supply side as a rigid DC source, ignoring the limitation of battery power climbing ability. When the climbing ability is insufficient, the DC link capacitor is required to support part of the inertial frequency regulation capability. The current capacitor calculation method is only based on DC side ripple current suppression, and the calculation result is conservative. During the inertial control process, it may cause the DC side voltage to exceed the limit, thus affecting the system stability. Summary of the invention

[0003] In view of the problem that the DC link capacitor configuration technology in the prior art does not consider the battery climbing ability and may cause the DC voltage fluctuation to exceed the limit during the inertial frequency modulation process, the present invention proposes a DC link capacitor configuration method for the ship hybrid power station energy storage system, quantitatively analyzes the frequency modulation demand power / energy, battery climbing power / energy, link capacitor power / energy, and uses the DC link capacitor energy as a fast frequency modulation resource in the energy storage inertial frequency modulation system to make up for the difference between the energy storage system climbing power output and the VSG inertial frequency modulation power demand. According to the conservation of power and energy, the DC link capacitor size that meets the optimal frequency modulation power demand of the ship's integrated power system is obtained, which is suitable for different types of energy storage systems and system frequency modulation requirements under different disturbances, and effectively improves the system's inertial frequency modulation capability and stable operation capability under multiple working conditions.

[0004] The present invention is achieved through the following technical solutions:

[0005] The present invention relates to a DC link capacitor configuration method for an inertial frequency modulation-oriented ship hybrid power station energy storage system. The method comprises the following steps: constructing a ship hybrid power station energy storage system model including an energy storage frequency modulation system and an AC power supply system, setting the inertial time constant and damping coefficient of the VSG, and calculating the inertial frequency modulation required power; in the online stage, the DC link capacitor is calculated based on the energy storage output power with measured AC frequency fluctuations and considering power ramp constraints, with the goal of maintaining the DC voltage fluctuation within a safe range under maximum frequency step fluctuations.

[0006] The DC link capacitor is obtained by calculating the inertial frequency modulation power demand of the inverter side during the system frequency step and the energy storage output power taking into account the power climbing constraint, configuring the DC capacitor according to the steady-state DC voltage ripple suppression effect and the DC link voltage fluctuation limit during the frequency modulation transient process, and comparing the maximum values ​​of the two.

[0007] The present invention relates to a system for implementing the above method, comprising: an energy storage frequency modulation system and an AC power supply system, wherein: the DC / DC converter in the energy storage frequency modulation system is connected to the DC side of the DC / AC converter after boosting the battery through constant DC voltage control, that is, constant Udc, to ensure the stability of the DC voltage. The DC / AC converter collects the three-phase voltage and current signals on the AC side and adopts a virtual synchronous generator (VSG) control strategy to simulate the inertia characteristics of the synchronous machine, thereby realizing the frequency modulation function of the energy storage system. The AC power supply system is composed of a diesel generator and a load, and is connected to the energy storage frequency modulation system through a transformer to form an integrated operation mode. The diesel generator and the energy storage frequency modulation system work together to provide a stable and reliable power supply for the AC load. Technical Effects

[0008] The present invention clarifies the AC side inertia frequency modulation power demand by constructing a virtual synchronous generator (VSG) small signal model, and analyzes the key influencing factors of the battery power climbing ability, including the climbing limit of the inductor current reference value in the DC / DC controller and the battery SOC-VOC characteristics. On this basis, the DC link capacitor voltage-energy change formula is used to calculate the minimum DC link capacitor capacitance within the voltage change range during inertia frequency modulation, and the capacitor capacitance is calculated in combination with the ripple configuration method, and the maximum value of the two is finally selected. Compared with the prior art, the present invention accurately quantifies the AC side maximum inertia frequency modulation power demand, clarifies the power climbing ability of the source side battery and its influencing factor SOC, considers the source side battery climbing ability and inertia frequency modulation power demand, configures the DC link capacitor capacitance, ensures that the capacitor voltage drop range remains within a safe range, compares the DC link capacitor capacitance calculated by the traditional steady-state filtering formula, and takes the maximum value of the two as the DC link capacitor capacitance, while meeting the inertia frequency modulation demand and steady-state filtering effect, thereby significantly improving the system's safety, reliability and dynamic response performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 It is a flow chart of the present invention;

[0010] Figure 2 This is the control schematic diagram of the energy storage system of the ship hybrid power station;

[0011] Figure 3 It is a control schematic diagram of a DC / DC converter including a ramp rate limiter;

[0012] Figure 4 Schematic diagram of VSG inertial frequency modulation power demand when frequency step occurs in underdamped state;

[0013] Figure 5 Schematic diagram of the relationship between battery open circuit voltage and SOC;

[0014] Figure 6 Schematic diagram of the relationship between the terminal voltage and SOC of a lithium nickel manganese cobalt oxide (LiNMC) battery at 15°C;

[0015] Figure 7 Schematic diagram of VSG frequency modulation power demand and energy storage active power output during frequency step;

[0016] Figure 8 Schematic diagram of (a) system output power verification and (b) DC voltage fluctuation for example 1;

[0017] Fig. 9 Schematic diagram of (a) system output power verification and (b) DC voltage fluctuation for Example 2. DETAILED DESCRIPTION

[0018] like Figure 1 As shown, this embodiment relates to a DC link capacitor configuration method for a ship hybrid power station energy storage system for inertial frequency modulation, including:

[0019] Step 1: Build Figure 2 The shown model shows a ship hybrid power station energy storage system including an energy storage and frequency modulation system and an AC power supply system, wherein: the energy storage and frequency modulation 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 and frequency modulation system and a diesel generator and a load connected in parallel; the battery energy storage is connected to the DC side of the grid-connected inverter after being boosted by a DC / DC converter, and is interconnected with the diesel generator in the AC system through AC to form a ship hybrid power station to supply power to the AC load.

[0020] The DC / AC converter adopts a virtual synchronous generator (VSG) control strategy, which specifically includes:

[0021] Step a: The DC / AC converter VSG rotor motion equation in the energy storage frequency modulation system satisfies: Among them: the superscript "*" indicates the per unit value; P m , P e Input mechanical power and electromagnetic power to VSG; ω 0 ,ω g are the rated frequency and system frequency; δ is the power angle; H is the inertia constant; and D is the damping coefficient.

[0022] Step b: Laplace transform the frequency modulation power demand of the virtual synchronous machine to obtain the system small signal model Where: Δ· is the change of the above quantities; the derivation of the active power and reactive power output of VSG is: Where: α is the impedance angle of the filter circuit; Z is the impedance, Then we get the expression of active power and the small signal relationship between active power and power angle satisfies: Thus, the transfer function between the change in the active power output of the virtual synchronous machine and the change in the system frequency is obtained: Where: S n is the rated capacity of the inverter, is the power synchronization factor.

[0023] Step c: When the system frequency jumps, the change in active power output by the virtual synchronous machine is:

[0024] Step 2: Set the inertia constant and damping coefficient of the VSG and calculate the inertia frequency modulation required power and the energy storage output power taking into account the power ramp constraint, and then obtain the DC link capacitance, which includes:

[0025] 2.1 Calculate the inverter side inertia frequency modulation power demand when the system frequency steps: Considering the optimal working state of the system, VSG control usually works in the optimal damping ratio, that is, underdamped state. At this time, the poles of the change in the output active power of the virtual synchronous machine are two conjugate complex roots, and the Laplace inverse transform is obtained: Among them: e is a natural constant, Then we can get the peak value of the inertial frequency modulation power requirement. Peak time in: B=D / 4H,C=m / 4H,

[0026] 2.2 Calculation of energy storage output power taking into account power ramp constraints: When the battery SOC changes, the terminal voltage also changes, causing the battery output power ramp rate to change. Therefore, when the battery body characteristics are given, the battery output power ramp rate is determined by the battery SOC and the inductor current ramp rate limit. The energy storage output power P taking into account the power ramp constraint is calculated. ESS (t) = k P_ramp ·t=(α 1 z 6 +α 2 z 5 +α 3 z 4 +α 4 z 3 +α 5 z 2 +α 6 z 1 +α 7 )·k Lref_ramp t, where: battery output power ramp rate k P_ramp =U Bat ·k Lref_ramp , k Lref_rampis the inductor current ramp rate limit in the DC / DC controller, U Bat is the battery terminal voltage. Since batteries usually have a small internal resistance, the terminal voltage U Bat With open circuit voltage U oc The open circuit voltage U of lithium nickel manganese cobalt oxide (LiNMC) battery oc =α 1 z 6 +α 2 z 5 +α 3 z 4 +α 4 z 3 +α 5 z 2 +α 6 z 1 +α 7 , z is the state of charge (SOC), α i is the defined coefficient.

[0027] like Figure 5 As shown, the open circuit voltage (U oc ) can be fitted as a function of SOC, the typical discharge curve of the battery. It can be observed that the battery open circuit voltage decreases exponentially in the period when the SOC is between 90% and 100%, and then tends to be flat, and then accelerates when the SOC reaches 20%. In actual operation, when the battery temperature is 15°C, α i They are 2.08, -4.16, 0.43, 4.86, -4.02, 1.43 and 3.57 respectively.

[0028] 2.3 Configure DC capacitors according to the steady-state DC voltage ripple suppression effect, specifically: in:

[0029] 2.4 Configure DC capacitors according to the DC link voltage fluctuation limit during the frequency modulation transient process, specifically: in: Energy provided by DC link capacitor during inertial frequency modulation t p is the time when the energy storage power ramp curve intersects with the VSG inertia frequency modulation power curve, based on the equation Using t p The boundary endpoints (0,0) and (π / C,0) of the range (0,π / C) are related to Ae -Bt The monotonic characteristics of the function sin(Ct) / t are approximated by interpolation.

[0030] 2.5 Compare the maximum values ​​of the results obtained in 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 is the DC side voltage of the inverter; U dc.min ΔU is the maximum voltage drop limit of the inverter DC side; dc纹波 P is the voltage ripple limit of the inverter DC capacitor; 0 is the steady-state output power of the DC / DC converter; D 1 is the duty cycle of the DC / DC converter, which is 1-U Bat / U dc ;f s is the switching frequency of the DC / DC converter; k P_ramp is the output power ramp rate of the energy storage system.

[0031] After specific practical experiments, a MATLAB / Simulink simulation model of a typical ship hybrid power station energy storage system is established, including batteries, DC / DC converters, DC / AC converters, diesel generators and other equivalent loads. The parameters are shown in Table 1.

[0032] Table 1 System parameters

[0033] After specific practical experiments, the battery energy storage was tested using lithium nickel manganese cobalt oxide (LiNMC) batteries, and the battery temperature was set to 15°C. In Example 1, the energy storage inductor current climbing rate is limited to 2000A / s, and the SOC of the battery energy storage is 20%. This example aims to verify the accuracy of the proposed DC link capacitance calculation method when the output power climbing of the energy storage system is limited. In Example 2, the energy storage inductor current climbing rate limit is also 2000A / s, and the SOC of the battery energy storage is increased to 90%. This example is used to verify the impact of the change in the battery energy storage SOC on the DC voltage drop during the inertial frequency modulation process, thereby proving the applicability of the DC link capacitance calculation method proposed in this method under different SOC states of energy storage.

[0034] Example 1: When the ship hybrid power station energy storage system runs to 1 second, the AC network is connected to the load, causing the grid frequency to drop sharply to 49.95Hz. At this time, since the energy storage system adopts VSG (virtual synchronous generator) control, it immediately provides inertial frequency modulation power support to the AC network, such as Figure 8(a) is shown. However, since the output power of the energy storage system is limited by the climbing speed, it cannot immediately meet the VSG's demand for inertial frequency modulation power, causing the DC link capacitor to act as a supporting capacitor at this time and participate in power output. According to the requirement that the maximum drop limit of the system DC voltage is 10%, according to this method, the calculated DC link capacitor is 4400μF, while the configuration method based only on steady-state capacitor voltage ripple suppression is only 3300μF. Figure 8 As shown in (b), the DC voltage drops to 673 V, and the drop error is 0.3%, which verifies the accuracy of this method.

[0035] Example 2: When the ship hybrid power station energy storage system runs to 1 second, the AC network is connected to a load, causing the grid frequency to drop sharply to 49.95Hz. At this time, the energy storage system controlled by VSG (virtual synchronous generator) immediately provides inertial frequency modulation power support. Fig. 9 As shown in (a), as the battery energy storage SOC increases, the energy storage output power climbing rate also increases, resulting in a decrease in the energy provided by the DC capacitor during the inverter's participation in inertial frequency modulation, and the DC voltage drops to 700V, with a reduced voltage drop. This verifies the applicability of this method under different battery energy storage SOC states.

[0036] Compared with the prior art, the present invention proposes a DC link capacitor value configuration method for the situation where the AC side frequency fluctuates greatly and the DC side energy storage power climbing is limited during the inertial frequency modulation process. This method can ensure that the AC side inertial frequency modulation power is effectively supplied while the DC voltage fluctuation is always kept within a safe and stable range, thereby optimizing the DC link capacitor configuration. The above specific implementation can be partially adjusted in different ways by those skilled in the art without departing from the principles and purpose of the present invention. The scope of protection of the present invention shall be based on the claims and shall not be limited by the above specific implementation. Each implementation scheme within its scope shall be subject to the constraints of the present invention.

Claims

1. A method for configuring the DC link capacitor of a ship hybrid power station energy storage system for inertial frequency modulation, characterized in that: By constructing a ship hybrid power station energy storage system model including an energy storage frequency modulation system and an AC power supply system, the inertia time constant and damping coefficient of the VSG are set, and the inertia frequency modulation required power is calculated. In the online stage, the DC link capacitance is calculated based on the measured AC frequency fluctuation and the energy storage output power considering the power ramp constraint, with the goal of maintaining the DC voltage fluctuation within a safe range under the maximum frequency step fluctuation.

2. The method for configuring the DC link capacitor of the ship hybrid power station energy storage system for inertial frequency modulation according to claim 1 is characterized in that: The DC link capacitor is obtained by calculating the inertial frequency modulation power demand of the inverter side during the system frequency step and the energy storage output power taking into account the power climbing constraint, configuring the DC capacitor according to the steady-state DC voltage ripple suppression effect and the DC link voltage fluctuation limit during the frequency modulation transient process, and comparing the maximum values ​​of the two.

3. The method for configuring the DC link capacitor of the ship hybrid power station energy storage system for inertial frequency modulation according to claim 1 is characterized in that: The ship hybrid power station energy storage system model includes an energy storage frequency modulation system and an AC power supply system, wherein: the energy storage frequency modulation 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 modulation system and a diesel generator and a load in parallel, and the battery energy storage is connected to the DC side of the grid-connected inverter after being boosted by the DC / DC converter, and is interconnected with the diesel generator in the AC system through AC, together forming a ship hybrid power station to supply power to the AC load.

4. The method for configuring the DC link capacitor of the ship hybrid power station energy storage system for inertial frequency modulation according to claim 3 is characterized in that: The DC / AC converter adopts a virtual synchronous generator (VSG) control strategy, which specifically includes: Step a: The DC / AC converter VSG rotor motion equation in the energy storage frequency modulation system satisfies: Among them: superscript "*" indicates per unit value; P m , P e Input mechanical power and electromagnetic power to VSG; ω0, ω g is the rated frequency and system frequency; δ is the power angle; H is the inertia constant; D is the damping coefficient; Step b: Laplace transform the frequency modulation power demand of the virtual synchronous machine to obtain the system small signal model Where: Δ· is the change of the above quantities; the derivation of the active power and reactive power output of VSG is: Where: α is the impedance angle of the filter circuit; Z is the impedance, Then we get the expression of active power and the small signal relationship between active power and power angle satisfies: Thus, the transfer function between the change in the active power output of the virtual synchronous machine and the change in the system frequency is obtained: Where: S n is the rated capacity of the inverter, is the power synchronization coefficient; Step c: When the system frequency jumps, the change in the active power output by the virtual synchronous machine is:

5. The method for configuring the DC link capacitor of the ship hybrid power station energy storage system for inertial frequency modulation according to claim 1 is characterized in that: The DC link capacitor is obtained by setting the inertia constant and damping coefficient of the VSG and calculating the inertia frequency modulation required power and the energy storage output power taking into account the power ramp constraint, specifically including: 2.1 Calculate the inertia frequency modulation power demand on the inverter side when the system frequency steps; 2.2 Calculate the energy storage output power taking into account the power ramp constraint: 2.3 Configure DC capacitors according to the steady-state DC voltage ripple suppression effect, specifically: in: 2.4 Configure DC capacitors according to the DC link voltage fluctuation limit during frequency modulation transient process. 2.5 Compare the maximum values ​​of the results obtained in 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 is the DC side voltage of the inverter; U dc.min ΔU is the maximum voltage drop limit of the inverter DC side; dc纹波 is the inverter DC capacitor voltage ripple limit; P0 is the DC / DC converter steady-state output power; D1 is the DC / DC converter duty cycle, which is 1-U Bat / U dc ;f s is the switching frequency of the DC / DC converter; k P_ramp is the output power ramp rate of the energy storage system.

6. The method for configuring the DC link capacitor of the ship hybrid power station energy storage system for inertial frequency modulation according to claim 5 is characterized in that: The step 2.1 is specifically as follows: considering the optimal working state of the system, VSG control usually works in the optimal damping ratio, that is, the underdamped state. At this time, the poles of the change in the active power output of the virtual synchronous machine are two conjugate complex roots, and the Laplace inverse transform is performed to obtain: Among them: e is a natural constant, Then we can get the peak value of the inertial frequency modulation power requirement. Peak time in: B=D / 4H,C=m / 4H, 7. The method for configuring the DC link capacitor of the ship hybrid power station energy storage system for inertial frequency modulation according to claim 5 is characterized in that: The step 2.2 is specifically as follows: when the battery SOC changes, the terminal voltage also changes, causing the battery output power climbing rate to change accordingly. Therefore, when the battery body characteristics are given, the battery output power climbing rate is jointly determined by the battery SOC and the inductor current climbing rate limit, and the energy storage output power P considering the power climbing constraint is calculated. 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 is the inductor current ramp rate limit in the DC / DC controller, U Bat is the battery terminal voltage. Since the battery usually has a small internal resistance, the terminal voltage U Bat With open circuit voltage U oc Considered equal, the open circuit voltage U oc =α1z 6 +α2z 5 +α3z 4 +α4z 3 +α5z 2 +α6z 1 +α7, z is the state of charge (SOC), α i is the defined coefficient.

8. The method for configuring the DC link capacitor of the ship hybrid power station energy storage system for inertial frequency modulation according to claim 5 is characterized in that: The step 2.4 is specifically as follows: in: Energy provided by DC link capacitor during inertial frequency modulation t p is the time when the energy storage power ramp curve intersects with the VSG inertia frequency modulation power curve, based on the equation Using t p The boundary endpoints (0,0) and (π / C,0) of the range (0,π / C) are related to Ae -Bt The monotonic characteristics of the function sin(Ct) / t are approximated by interpolation.

9. A DC link capacitor configuration system for a ship hybrid power station energy storage system for inertial frequency modulation that implements the method described in any one of claims 1 to 8, characterized in that: include: Energy storage frequency modulation system and AC power supply system, wherein: the DC / DC converter in the energy storage frequency modulation system is controlled by constant DC voltage, that is, constant Udc is used to boost the battery and then connect it to the DC side of the DC / AC converter to ensure the stability of the DC voltage. The DC / AC converter collects the three-phase voltage and current signals on the AC side and adopts a virtual synchronous generator (VSG) control strategy to simulate the inertia characteristics of the synchronous machine, thereby realizing the frequency modulation function of the energy storage system. The AC power supply system consists of a diesel generator and a load, which is connected to the energy storage frequency modulation system through a transformer to form an integrated operation mode. The diesel generator and the energy storage frequency modulation system work together to provide a stable and reliable power supply for the AC load.

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