Electric ship standby battery control strategy based on improved droop control

Abstract

The invention discloses an electric ship standby battery control strategy based on improved droop control. The electric ship standby battery control strategy comprises the following steps: step 1, constructing a cooperative work model of an electric ship standby battery and a main battery; 2, improving a traditional droop control strategy through a translation droop curve, and achieving power equalization; step 3, based on the improved droop control strategy in the step 2, compensating voltage offset by using a fuzzy algorithm to improve the voltage control precision; and 4, accurately distributing the output current of each direct current bidirectional converter through output current information interaction between the main battery direct current bidirectional converter and the standby battery direct current bidirectional converter. According to the electric ship standby battery control strategy based on improved droop control, the output power of the standby battery can be rapidly and accurately adjusted according to the real-time operation state of the electric ship, and the electric ship standby battery control strategy has the practical application value for maintaining the voltage and frequency stability of a ship electric power system and improving the comprehensive utilization efficiency of energy.

Description

Technical Field

[0001] The present invention relates to the technical field of electric ship battery control, and particularly relates to a control strategy for an electric ship backup battery based on improved droop control. Background Technique

[0002] With the accelerating global energy transition, electric ships are increasingly widely used in the maritime field. Electric ships are powered by electricity, reducing greenhouse gas emissions caused by traditional fuel ships. However, the stability of the energy supply of electric ships is crucial, especially in the face of emergencies or main power failures. The role of backup batteries becomes extremely critical. During the navigation of electric ships, sudden situations such as bad weather and equipment failures may occur, resulting in the abnormal operation of the main power supply. At this time, the backup battery needs to be quickly put into use to ensure the basic navigation safety of the ship and the operation of key equipment. Therefore, exploring the control strategy of electric ship backup batteries is of great significance for ensuring the normal operation of the ship, balancing power output, and maintaining the stability of system voltage and frequency.

[0003] The traditional droop control strategy has certain limitations. The operating conditions of electric ships are complex and changeable, and the load characteristics are quite different from those of conventional distributed energy systems. The response speed of the traditional droop control strategy is slow. When the ship load suddenly changes, the backup battery cannot quickly adjust the output power, resulting in large voltage and frequency fluctuations and affecting the stability of the electric ship power system. It is difficult for the traditional droop control strategy to accurately consider the battery state of the electric ship backup battery, which may lead to overcharge and overdischarge of the backup battery, reducing the reliability and service life of the backup battery. Summary of the Invention

[0004] To address problems such as unstable system voltage and uneven power output caused by load mutations or main power failures, the present invention provides a control strategy for an electric ship backup battery based on improved droop control. By translating the droop curve to improve the traditional droop control, introducing a fuzzy algorithm to compensate for voltage deviation, and constructing a current distribution compensator to evenly divide the output current, it can meet the power requirements of electric ships in a complex and changeable environment, significantly reduce voltage and frequency fluctuations, and improve system stability.

[0005] The technical solution adopted by the present invention is as follows:

[0006] A control strategy for an electric ship backup battery based on improved droop control includes the following steps:

[0007] Step 1: Construct a collaborative working model for the electric ship backup battery and the main battery;

[0008] Step 2: Improve the droop control strategy by translating the droop curve to achieve power sharing;

[0009] Step 3: Based on the improved droop control strategy in Step 2, use the fuzzy algorithm to compensate for the voltage offset;

[0010] Step 4: Through the output current information interaction between the main battery DC bidirectional converter and the backup battery DC bidirectional converter, accurately allocate the output currents of each DC bidirectional converter.

[0011] In the said Step 1, the collaborative working model of the backup battery and the main battery of the electric ship specifically includes:

[0012] The DC bidirectional converter can be represented by the Thevenin equivalent model. According to Kirchhoff's voltage law, the following equation is listed:

[0013]

[0014] In Equation (1): U load represents the voltage across the load and is also the DC bus voltage; are the initial voltages of the main battery and the backup battery respectively; R1 and R2 are the equivalent impedances of the main battery and the backup battery respectively; R line1 , R line2 are the cable impedances from the main battery and the backup battery to the common coupling point respectively; I1 and I2 are the output currents of the main battery and the backup battery respectively.

[0015] According to Equation (1), the current ratio between the main battery DC bidirectional converter and the backup battery DC bidirectional converter can be obtained as:

[0016]

[0017] Let R line1 be equal to 0, and the droop relationship curve of I-U is expressed as:

[0018]

[0019] In Equation (3): U bus is the actual voltage value of the common DC bus; represents the i-th DC bidirectional converter, i.e., the voltage reference value of the main battery converter or the backup battery converter; I i represents the output current of the i-th DC bidirectional converter, i.e., the magnitude of the current delivered by the converter to the DC bus. R i is the droop coefficient, which determines the rate of voltage change with current.

[0020] In the said Step 2, let the initial working points A and B of the main battery DC bidirectional converter and the backup battery DC bidirectional converter be located on the droop curve 1 and the droop curve 2 respectively, and the initial voltage reference value The deviation is ΔU, and the output current deviation of the main battery DC bidirectional converter and the backup battery DC bidirectional converter is ΔI i . By the translation curve method, the operating points of the main battery DC bidirectional converter and the backup battery DC bidirectional converter are moved to point C, that is, the droop curves 1 and 2 are translated to the droop curves 1' and 2' respectively. At this time, the main battery DC bidirectional converter and the backup battery DC bidirectional converter can meet the requirements of operating at the initial voltage reference value and achieve power sharing at the same time.

[0021] In order to realize the translation of the above-mentioned main battery DC bidirectional converter and backup battery DC bidirectional converter, re-analyze the droop relationship of I-U in the droop control model when the main battery DC bidirectional converter and the backup battery DC bidirectional converter are operating in parallel;

[0022] For the first term on the right side of the I-U equation in Equation (3) add the voltage compensation amount ΔU i , and for the second term -R i I i add the current compensation amount ΔI i , and let k i be the droop coefficient corresponding to different converters, thus obtaining the improved droop control equation as:

[0023]

[0024] In Equation (4): U dcref is the output voltage reference value of the improved DC bidirectional converter; is the initial voltage reference value of the DC bidirectional converter, that is, the target output voltage of the converter under no-load condition when no compensation amount is added; ΔU i can be obtained by adding a voltage deviation compensator to the droop controller; ΔI i can be obtained by adding a current distribution accuracy compensator.

[0025] In step 3, the output current I of the main battery DC bidirectional converter and the backup battery DC bidirectional converter i is used as the input quantity of the fuzzy logic control unit FIS, and the voltage deviation compensation is used as the output of the fuzzy logic control unit FIS;

[0026] Define the fuzzy subsets as {NB, NM, NS, ZO, PS, PM, PB}, corresponding to negative large, negative medium, negative small, zero, positive small, positive medium, and positive large respectively; the input domain of the fuzzy logic control unit FIS is {0, 4, 6, 10, 14, 16, 20}, and the corresponding output domain is {0, 2.5, 4, 5, 6, 7.5, 10}.

[0027] Based on the characteristic that the more load power borne by the main battery DC bidirectional converter and the backup battery DC bidirectional converter, the more obvious the terminal voltage drop, fuzzy rules are established. According to the membership function of the fuzzy subset, the input current fuzzy level is established, and the output voltage compensation amount ΔU is obtained. i The mapping relationship of the fuzzy levels is calculated by the fuzzy inference system to dynamically adjust the droop curve to compensate for the voltage drop.

[0028] The output quantity ΔU i is superimposed on the given DC bus voltage, thereby realizing the translation of the fixed droop curve. According to Equation (4), if ΔU i > 0, the droop curve moves up along the voltage axis, increasing the output voltage reference value to compensate for the voltage drop caused by the load; if ΔU i < 0, the droop curve moves down along the voltage axis to suppress the overvoltage situation.

[0029] In step 4, to improve the power and current sharing accuracy of the main battery DC bidirectional converter and the backup battery DC bidirectional converter, it is necessary to make the DC power supply system of the electric ship meet the following conditions during operation through an improved droop control strategy:

[0030] I1k1 = I2k2 (5);

[0031] In Equation (5): k1 and k2 are the droop coefficients of the DC bidirectional converters corresponding to the main battery and the backup battery respectively, representing the rate of change of the converter output voltage with the output current.

[0032] The current sharing accuracy compensator only needs the output currents and droop coefficients of the main battery DC bidirectional converter and the backup battery DC bidirectional converter, and adjusts by multiplying them. According to Equation (5), the current sharing accuracy compensator collects the output current I of each converter i and its droop coefficient k i . When the I i k i of the two converters are not equal, the compensator generates a current compensation amount to make them equal, realizing current deviation compensation.

[0033] During operation, the output current compensation amounts ΔI1 and ΔI2 are respectively:

[0034]

[0035] In Equation (6): G pic is the PI controller of the current sharing accuracy compensator.

[0036] The control strategy of the backup battery of the electric ship based on the improved droop control of the present invention has the following technical effects:

[0037] 1) The improved droop control strategy of the present invention can quickly and accurately adjust the output power of the backup battery according to the real-time operating state of the electric ship. When the main power supply fails or the load suddenly changes, the backup battery can respond quickly to maintain the voltage and frequency stability of the ship's power system.

[0038] 2) The present invention compensates for voltage deviation through a fuzzy algorithm and constructs a current distribution compensator to achieve equal sharing control of output current and power, and can meet the power demand of the electric ship in a complex and changeable environment. Therefore, this method is of great significance for maintaining the normal operation of the ship's key equipment and reducing the voltage and frequency fluctuations of the system.

[0039] 3) The control strategy of the backup battery of the electric ship based on the improved droop control can quickly and accurately adjust the output power of the backup battery according to the real-time operating state of the electric ship, and has practical application value for maintaining the voltage and frequency stability of the ship's power system and improving the comprehensive energy utilization efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The present invention will be further described below in conjunction with the drawings and examples;

[0041] Figure 1 FIG. is a flow block diagram of the control strategy of the backup battery of the electric ship based on the improved droop control.

[0042] Figure 2 FIG. is a collaborative working model diagram of the backup battery and the main battery of the electric ship.

[0043] Figure 3 FIG. is a control diagram of the translational droop curve.

[0044] Figure 4 FIG. is a membership function diagram of the fuzzy inference system.

[0045] Figure 5 FIG. is a waveform diagram of the DC bus voltage.

[0046] Figure 6 FIG. is a waveform diagram of the output current. DETAILED DESCRIPTION OF THE INVENTION

[0047] The control strategy of the backup battery of the electric ship based on the improved droop control includes the following steps:

[0048] Step 1: Construct a collaborative working model of the backup battery and the main battery of the electric ship to solve the problems of unstable system voltage and uneven power output caused by sudden load changes or main power supply failures during the navigation of the electric ship;

[0049] Step 2: Improve the traditional droop control strategy by translating the droop curve to achieve equal power sharing, so as to reduce the interference of line impedance on power distribution and the influence on voltage regulation;

[0050] Step 3: Based on the improved droop control strategy in Step 2, use the fuzzy algorithm to compensate for the voltage offset and improve the voltage control accuracy;

[0051] Step 4: Through the output current information interaction between the main battery DC bidirectional converter and the backup battery DC bidirectional converter, accurately allocate the output currents of each DC bidirectional converter.

[0052] In the above-mentioned Step 1, the collaborative working model of the backup battery and the main battery of the electric ship specifically includes:

[0053] The DC bidirectional converter can be represented by the Thevenin equivalent model. According to Kirchhoff's voltage law, the following equation can be listed:

[0054]

[0055] In Equation (1): U load represents the voltage across the load and is also the voltage of the DC bus; are the initial voltages of the main battery and the backup battery respectively; R1 and R2 are the equivalent impedances of the main battery and the backup battery respectively; R line1 and R line2 are the cable impedances from the main battery and the backup battery to the common coupling point respectively; I1 and I2 are the output currents of the main battery and the backup battery respectively. According to Equation (1), the current ratio between the main battery DC bidirectional converter and the backup battery DC bidirectional converter can be obtained as:

[0056]

[0057] Let R line1 be equal to 0, and the droop relationship curve of I-U can be expressed as:

[0058]

[0059] In Equation (3): U bus is the actual voltage value of the common DC bus; represents the i-th DC bidirectional converter, that is, the voltage reference value of the main battery converter or the backup battery converter; I i represents the output current of the i-th DC bidirectional converter, that is, the magnitude of the current delivered by the converter to the DC bus. Droop control realizes power distribution through the linear relationship between voltage and current, and its expression is U = U * - kI, where k is the droop coefficient. Therefore, R i in Equation (3) is the droop coefficient, which determines the rate of voltage change with current.

[0060] In the above-mentioned Step 2, as Figure 3As shown in the figure, let the initial operating points A and B of the main battery DC bidirectional converter and the backup battery DC bidirectional converter be located on the droop curves 1 and 2 respectively, with a deviation of ΔU from the initial voltage reference value and the output current deviation of the main battery DC bidirectional converter and the backup battery DC bidirectional converter be ΔI i . By the curve translation method, the operating points of the main battery DC bidirectional converter and the backup battery DC bidirectional converter are moved to point C, that is, the droop curves 1 and 2 are translated to the droop curves 1' and 2' respectively. At this time, the main battery DC bidirectional converter and the backup battery DC bidirectional converter can satisfy operating at the initial voltage reference value while achieving power sharing;

[0061] To achieve the translation of the above-mentioned main battery DC bidirectional converter and backup battery DC bidirectional converter, re-examine the I-U droop relationship in the droop control model when the main battery DC bidirectional converter and the backup battery DC bidirectional converter are operating in parallel. For the first term on the right side of the I-U equation in Equation (3) add a voltage compensation amount ΔU i , and for the second term -R i I i add a current compensation amount ΔI i , and let k i be the droop coefficient corresponding to different converters, thus obtaining the improved droop control equation as:

[0062]

[0063] In Equation (4): U dcref is the output voltage reference value of the improved DC bidirectional converter; is the initial voltage reference value of the DC bidirectional converter, that is, the target output voltage of the converter in the no-load state before adding the compensation amount; ΔU i can be obtained by adding a voltage deviation compensator to the droop controller;

[0064] ΔI i can be obtained by adding a current distribution accuracy compensator; the voltage deviation compensator collects the deviation between the actual DC bus voltage and the ideal reference voltage in real time, and calculates the voltage compensation amount ΔU i through a control algorithm; after the current distribution accuracy compensator obtains the output current of each converter, it calculates the current distribution error according to the current sharing target in Equation (5), and generates ΔI i through an algorithm.

[0065] In step 3, the fuzzy inference system (FIS) has strong robustness and good control performance for nonlinear and complex objects. The voltage deviation compensator based on the fuzzy inference system can reduce the sudden drop of the DC bus voltage caused by load mutation;

[0066] Take the output currents I of the main battery DC bidirectional converter and the backup battery DC bidirectional converter i as the input quantity of the fuzzy logic control unit FIS, and the voltage deviation compensation as the output of the fuzzy logic control unit FIS.

[0067] Define the fuzzy subsets as {NB, NM, NS, ZO, PS, PM, PB}, corresponding to negative large, negative medium, negative small, zero, positive small, positive medium, and positive large respectively; the input universe of discourse of the fuzzy logic control unit FIS is {0, 4, 6, 10, 14, 16, 20}, and the corresponding output universe of discourse is {0, 2.5, 4, 5, 6, 7.5, 10}. Based on the characteristic that the greater the load power borne by the main battery DC bidirectional converter and the backup battery DC bidirectional converter, the more obvious the terminal voltage drop, establish fuzzy rules. According to Figure 4 the membership functions of the fuzzy subsets, establish the fuzzy grades of the input current, and the output voltage compensation amount ΔU i the mapping relationship of the fuzzy grades, and through the operation of the fuzzy inference system, dynamically adjust the droop curve to compensate for the voltage drop.

[0068] Superimpose the output quantity ΔU i onto the given DC bus voltage, so as to realize the translation of the fixed droop curve. According to Equation (4), if ΔU i > 0, the droop curve moves up along the voltage axis, improving the output voltage reference value to compensate for the voltage drop caused by the load; if ΔU i < 0, the droop curve moves down along the voltage axis to suppress the overvoltage situation.

[0069] In step 4,

[0070] To improve the power and current sharing accuracy of the main battery DC bidirectional converter and the backup battery DC bidirectional converter, it is necessary to make the DC power supply system of the electric ship meet the following during operation through an improved droop control strategy:

[0071] I1k1 = I2k2 (5);

[0072] In Equation (5): k1 and k2 are the droop coefficients of the DC bidirectional converters corresponding to the main battery and the backup battery respectively, indicating the rate of change of the converter output voltage with the output current.

[0073] The current distribution accuracy compensator only needs the output currents and droop coefficients of the main battery DC bidirectional converter and the backup battery DC bidirectional converter, and adjusts the product thereof. According to Equation (5), the current distribution accuracy compensator collects the output current I of each converter i and its droop coefficient k i . When the I i k i of the two converters are not equal, the compensator generates a current compensation amount to make them equal, realizing current deviation compensation.

[0074] It can achieve the purpose of compensating the output current deviation of the main battery DC bidirectional converter and the backup battery DC bidirectional converter when the electric ship DC power supply system is in a steady state.

[0075] During operation, the output current compensation amounts ΔI1 and ΔI2 are respectively:

[0076]

[0077] In Equation (6): G pic is the PI controller of the current distribution accuracy compensator.

[0078] Figure 5 is the DC bus voltage waveform diagram. As can be seen from Figure 5 , after voltage deviation compensation by introducing the fuzzy algorithm, the influence of load changes on the voltage becomes smaller. When the load power suddenly changes at 0.1 s, the DC bus voltage slightly decreases as the load increases. However, the presence of the voltage deviation compensator keeps the voltage basically maintained at 400 V.

[0079] Figure 6 is the output current waveform diagram. As can be seen from Figure 6 , the current distribution accuracy compensator improves the current distribution accuracy by using the current information between adjacent units. When the load power suddenly changes at 0.1 s, the output currents of the two batteries increase rapidly. However, the presence of the current distribution accuracy compensator makes the output currents basically evenly divided.

Claims

1. The control strategy of the backup battery for electric ships based on improved droop control is characterized in that It includes the following steps: Step 1: Construct a collaborative working model for the backup battery and the main battery of the electric ship; Step 2: Improve the droop control strategy by translating the droop curve to achieve equal power sharing; Step 3: Based on the improved droop control strategy in Step 2, use the fuzzy algorithm to compensate for the voltage offset; Step 4: Through the output current information interaction between the main battery DC-DC converter and the backup battery DC-DC converter, accurately distribute the output current of each DC-DC converter.

2. The control strategy for the backup battery of an electric ship based on improved droop control according to claim 1, wherein: In the said Step 1, the collaborative working model for the backup battery and the main battery of the electric ship specifically includes: The DC-DC converter can be represented by the Thevenin equivalent model, and the following equation is listed according to Kirchhoff's voltage law: In Equation (1): U load represents the voltage across the load and is also the voltage of the DC bus; are the initial voltages of the main battery and the backup battery respectively; R1 and R2 are the equivalent impedances of the main battery and the backup battery respectively; R line1 and R line2 are the cable impedances from the main battery and the backup battery to the common coupling point respectively; I1 and I2 are the output currents of the main battery and the backup battery respectively.

3. According to the electric ship backup battery control strategy based on improved droop control described in Claim 2, wherein According to Equation (1), the current ratio between the main battery DC-DC converter and the backup battery DC-DC converter can be obtained as:

4. The backup battery control strategy for electric ships based on improved droop control according to claim 3, characterized in that: Let R line1 be equal to 0, and the droop relationship curve with respect to I-U is expressed as: In formula (3): U bus is the actual voltage value of the common DC bus; represents the voltage reference value of the i-th DC bidirectional converter, i.e., the main battery converter or the backup battery converter; I i represents the output current of the i-th DC bidirectional converter, i.e., the magnitude of the current delivered by the converter to the DC bus; R i is the droop coefficient, which determines the rate of voltage change with current.

5. The control strategy for the backup battery of an electric ship based on improved droop control according to claim 4, characterized in that: In step 2, let the initial operating points A and B of the main battery DC bidirectional converter and the backup battery DC bidirectional converter be located on droop curve 1 and droop curve 2 respectively, with a deviation of ΔU from the initial voltage reference value and the output current deviation of the main battery DC bidirectional converter and the backup battery DC bidirectional converter be ΔI i ; by the curve translation method, move the operating points of the main battery DC bidirectional converter and the backup battery DC bidirectional converter to point C, that is, translate droop curve 1 and droop curve 2 to droop curve 1' and droop curve 2' respectively; at this time, the main battery DC bidirectional converter and the backup battery DC bidirectional converter can meet the requirement of operating at the initial voltage reference value and achieve power sharing simultaneously.

6. The control strategy for the backup battery of an electric ship based on improved droop control according to claim 5, characterized in that: In order to realize the translation of the above main battery DC-DC converter and backup battery DC-DC converter, re-analyze the droop relationship of I-U in the droop control model when the main battery DC-DC converter and the backup battery DC-DC converter are operating in parallel; For the first term on the right side of the I-U equation in Equation (3) Increase the voltage compensation amount ΔU i , the second term -R on the right side of the I-U equation in Equation (3) i I i Increase the current compensation amount ΔI i , and let k i be the droop coefficient corresponding to different converters, so as to obtain the improved droop control equation as follows: In Equation (4): U dcref is the output voltage reference value of the improved DC bidirectional converter; is the initial voltage reference value of the DC bidirectional converter, that is, the target output voltage of the converter under no-load condition when no compensation amount is added; ΔU i can be obtained by adding a voltage deviation compensator to the droop controller; ΔI i can be obtained by adding a current distribution accuracy compensator.

7. The control strategy for the backup battery of an electric ship based on improved droop control according to claim 6, characterized in that: In the step 3, the output currents I of the main battery DC bidirectional converter and the backup battery DC bidirectional converter i are used as the input quantities of the fuzzy logic control unit FIS, and the voltage deviation compensation is used as the output of the fuzzy logic control unit FIS; Define the fuzzy subsets as {NB, NM, NS, ZO, PS, PM, PB}, corresponding to negative large, negative medium, negative small, zero, positive small, positive medium, and positive large respectively; the input universe of discourse of the fuzzy logic control unit FIS is {0, 4, 6, 10, 14, 16, 20}, and the corresponding output universe of discourse is {0, 2.5, 4, 5, 6, 7.5, 10}; Based on the characteristic that the greater the load power borne by the main battery DC bidirectional converter and the standby battery DC bidirectional converter, the more obvious the voltage dip at the machine terminal, a fuzzy rule is established. According to the membership function of the fuzzy subset, a fuzzy level of the input current is established, and the output voltage compensation amount ΔU is obtained. i The mapping relationship of the fuzzy levels is calculated by the fuzzy inference system to dynamically adjust the droop curve to compensate for the voltage dip.

8. The control strategy for the backup battery of an electric ship based on improved droop control according to claim 7, wherein: The output quantity ΔU i is superimposed on the given DC bus voltage to achieve the translation of the fixed droop curve; according to Equation (4), if ΔU i > 0, the droop curve moves up along the voltage axis, increasing the output voltage reference value to compensate for the voltage drop caused by the load; if ΔU i < 0, the droop curve moves down along the voltage axis to suppress the overvoltage condition.

9. The control strategy for the backup battery of an electric ship based on improved droop control according to claim 1, characterized in that: In the said Step 4, to improve the power and current equal sharing accuracy of the main battery DC-DC converter and the backup battery DC-DC converter, it is necessary to make the DC power supply system of the electric ship satisfy the following during operation through the improved droop control strategy: I1k1 = I2k2 (5); In Equation (5): k1 and k2 are the droop coefficients of the DC-DC converters corresponding to the main battery and the backup battery respectively, indicating the rate of change of the converter output voltage with the output current; The current distribution accuracy compensator only needs the output currents and droop coefficients of the main battery DC bidirectional converter and the backup battery DC bidirectional converter, and adjusts the product thereof; according to Equation (5), the current distribution accuracy compensator collects the output current I of each converter i and its droop coefficient k i , when the I i k i of the two converters are not equal, the compensator generates a current compensation amount to make them equal, realizing current deviation compensation.

10. The control strategy for the backup battery of an electric ship based on improved droop control according to claim 1, characterized in that: During operation, the output current compensation amounts ΔI1 and ΔI2 are respectively: In Equation (6): G pic is the PI controller of the current distribution precision compensator.

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