A method of power distribution for a marine hybrid power system

Abstract

The application discloses a power distribution method of a ship hybrid power system. The method comprises the following steps: collecting a main engine rotating speed signal, a lithium battery pack state of charge signal and a shaft generator frequency converter DC bus voltage in real time; judging whether the shaft generator system is in a constant power rotating speed interval according to the main engine rotating speed signal; when the main engine rotating speed is in the constant power rotating speed interval, establishing a DC bus voltage dynamic model as a physical constraint boundary of power distribution; establishing a lithium battery volt-ampere characteristic dynamic model in real time according to the signals, and calculating a battery compensation power instruction meeting a volt-ampere constraint; superimposing the battery compensation power instruction on an inverter power reference value of the shaft generator, generating a composite power instruction, and adjusting inverter output through the shaft generator frequency converter.

Description

Technical Field

[0001] This invention relates to the field of ship electric propulsion and hybrid power control technology, and in particular to a power distribution method for a ship hybrid power system. Background Technology

[0002] With the ongoing green transformation of the global shipping industry, the International Maritime Organization (IMO) is imposing increasingly stringent restrictions on sulfur oxides, nitrogen oxides, and greenhouse gas emissions from ships. Against this backdrop, hybrid-powered ships, due to their combination of fuel economy, clean emissions, and range flexibility, have become an important direction for the development of marine propulsion systems. The hybrid power system consisting of a shaft-driven generator (PTO) and a lithium-ion battery pack is one of the mainstream marine hybrid power architectures currently available. In a DC common bus architecture, the stability of the DC bus voltage is a core indicator of system safety. Fluctuations in the DC bus voltage directly affect not only the torque output accuracy and power quality of the propulsion motor, but also the protection threshold settings of the entire power system. When the bus voltage drops below the inverter undervoltage protection threshold, the propulsion motor will lose its driving capability, posing a threat to the ship's navigation safety; when the bus voltage overshoots above the overvoltage protection threshold, it may damage power semiconductor devices and bus capacitors. Traditional control strategies typically maintain the DC bus voltage through simple voltage-power droop control or regular power distribution based on state of charge (SOC). However, in actual operation, especially when the ship faces sudden high-power load demands or rapid changes in main engine speed, these methods do not fully consider the nonlinear relationship between SOC and the internal electromotive force of the battery.

[0003] In actual operation, the terminal voltage of a lithium iron phosphate battery is not a one-to-one linear relationship with its state of charge (SOC). Especially in the region where the SOC is below 20% or above 80%, the terminal voltage is extremely sensitive to changes in SOC. When the SOC is in the low-charge range, if a high-power discharge command is still issued to the battery according to the traditional lookup table strategy, the battery terminal voltage will drop sharply below the safe operating threshold, triggering the protection action of the battery management system.

[0004] Conventional control strategies do not model the battery's polarization memory effect and ohmic internal resistance changes with temperature and SOC in real time, thus failing to accurately assess the battery's instantaneous discharge capacity before load surges occur. When fluctuations in the main unit's rotational speed cause a sudden drop in shaft generator output, the system relies on the battery to quickly fill the power gap. However, if the battery's polarization state has not yet recovered from the previous discharge process, its actual output transient current will be far lower than the estimate based on static parameters, resulting in a voltage drop exceeding expectations. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a power distribution method for a marine hybrid power system, so as to solve the problem that the battery voltage drops too much under load impact and the energy storage unit is unexpectedly disconnected from the DC grid because the real-time voltage characteristics of the lithium battery are not fully considered in the prior art.

[0006] To achieve the above objectives, the technical solution provided by the present invention is as follows.

[0007] A power distribution method for a marine hybrid power system includes the following steps: S1. Real-time acquisition of host speed signal and lithium battery pack state of charge. Signal and shaft inverter DC bus voltage ; S2. Determine whether the shaft generator system is in the constant power speed range based on the main unit speed signal; S3. When the main engine speed is within the constant power speed range, establish a dynamic model of DC bus voltage as the physical constraint boundary for power distribution. S4, according to the above Real-time signal establishment of a dynamic model of the lithium battery's current-voltage characteristics; S5. Using the host speed signal as the first feedforward and the volt-ampere characteristic model as the second feedforward, combined with the DC bus voltage... Calculate the battery compensation power command that satisfies the volt-ampere constraint. ; S6. Send the battery compensation power command The composite power command is generated by superimposing it onto the shaft-generator inverter power reference value. The inverter output is adjusted by the shaft-driven frequency converter.

[0008] Optionally, the dynamic model of the DC bus voltage established in step S3 is as follows: in This is the equivalent capacitance of the DC bus. The output power of the shaft rectifier unit. The charging and discharging power of the battery's bidirectional chopper unit, For the output power of the shaft inverter unit, For time; The dynamic model of lithium battery voltage-current characteristics established in step S4 includes: open-circuit voltage. Series ohmic internal resistance Battery current Polarization voltage Polarized capacitors With polarization resistance This forms a first-order RC equivalent circuit model: Optionally, in step S5, the battery compensation power command The calculation method is as follows: in Within each control cycle, the following must be satisfied: And it is implicitly constrained by the following formula: Optionally, in step S5, It is also subject to the voltage difference dynamic adjustment factor. l Correction: in This is the base current command calculated based on the host feedforward. It is used to achieve instantaneous power compensation by utilizing the voltage difference between the DC bus and the battery port.

[0009] Optionally, the maximum discharge current With maximum charging current Determined by the following current-voltage limiting function: in , The battery's safe operating voltage threshold, The temperature-dependent maximum continuous current provided by the manufacturer.

[0010] Optionally, a polarization state tracking step is also included: In each control cycle, based on the actual battery current Update polarization voltage: And the updated Substitute this into the current-voltage constraint calculation for the next cycle.

[0011] Optionally, it also includes an online internal resistance identification step: Under steady-state navigation conditions, the system is updated online using the recursive least squares method. .

[0012] Optionally, it also includes Correction steps: By integrating the ampere-hour integral method with voltage feedback: in For Coulomb efficiency, For rated capacity, This is the fusion coefficient, used to suppress current integral drift.

[0013] Optionally, the shaft-driven frequency converter adopts a DC-DC common bus topology, including a shaft-driven rectifier unit, a battery-driven bidirectional chopper unit, and a shaft-driven inverter unit. The battery-driven bidirectional chopper unit outputs according to the volt-ampere characteristic model. It operates in voltage source mode or current source mode, and according to... and The difference automatically switches the control mode.

[0014] Optionally, the method further includes a host speed fluctuation suppression step: performing a first-order low-pass filter on the host speed signal to obtain a filtered speed signal, and substituting the filtered speed signal into the current-voltage characteristic model for calculation to suppress instantaneous fluctuations in the host speed.

[0015] The advantages and beneficial effects of this invention are as follows: This invention directly embeds the dynamic model of lithium battery current-voltage characteristics into the feedforward control link, enabling the generation process of power distribution commands to sense changes in battery terminal voltage, polarization state, and internal resistance in real time. This allows for automatic current-voltage constraint limiting of the current command within each control cycle. When the battery's state of charge (SOC) is in a low-charge range or the polarization state has not yet recovered, the current-voltage model automatically applies a stricter upper limit to the discharge current, avoiding issuing power commands to the battery that exceed its current physical capacity. This effectively prevents the energy storage unit from accidentally disconnecting from the grid due to excessively low terminal voltage. This invention establishes a dynamic model of the DC bus voltage, using the charge-discharge balance of the bus capacitor as a necessary constraint for power distribution. The power flow among the shaft-driven rectifier unit, the battery bidirectional chopper unit, and the shaft-driven inverter unit satisfies the instantaneous power balance relationship required by this model, ensuring that the power distribution process always operates within physically feasible boundaries without requiring repeated corrections via an additional voltage outer loop. Detailed Implementation

[0016] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the specific embodiments of this application are described in detail below. It is to be understood that the specific embodiments described herein are merely illustrative of this application and not intended to limit it. All other embodiments obtained by those skilled in the art based on the embodiments in this application without inventive effort are within the scope of protection of this application.

[0017] The terms “comprising” and “having”, and any variations thereof, used in this application are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the steps or units listed, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to such process, method, product, or apparatus.

[0018] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly or implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0019] The marine hybrid power system involved in this application uses lithium iron phosphate batteries as energy storage units. Lithium iron phosphate batteries have been widely used in marine energy storage due to their advantages such as good thermal stability, long cycle life, and low raw material cost. However, lithium iron phosphate batteries also have several technical drawbacks in practical engineering applications.

[0020] The voltage plateau characteristics of lithium iron phosphate batteries lead to Estimation is difficult. The open-circuit voltage of lithium iron phosphate batteries varies extremely smoothly within the range of 30% to 80% of their state of charge (SOC), with the single-cell voltage fluctuating only between 3.20 volts and 3.30 volts. The voltage's sensitivity to SOC is extremely low, making estimation based on the open-circuit voltage method challenging. The estimation contains a large error within this range, which is precisely where marine hybrid power systems require batteries to undergo frequent high-power charging and discharging. When the ampere-hour integration method accumulates errors due to current sensor drift, the system may misjudge the actual state of charge of the battery, thereby issuing power commands that exceed the battery's current capacity.

[0021] Meanwhile, at low temperatures, the solid-phase diffusion coefficient of lithium ions in lithium iron phosphate cathode materials decreases significantly, the electrochemical reaction rate on the electrode surface decreases, and both the usable capacity and maximum discharge power of the battery decrease. If the power distribution strategy is set according to the parameters at room temperature under low-temperature conditions, the battery terminal voltage will drop rapidly during high-current discharge, triggering undervoltage protection.

[0022] Due to the low intrinsic electronic conductivity and lithium-ion diffusion coefficient of the olivine-type crystal structure of lithium iron phosphate materials, significant concentration polarization easily forms on the electrode surface during battery discharge. The accumulation of polarization voltage causes the actual terminal voltage of the battery to deviate from the off-circuit voltage, and the decay time constant of the polarization voltage is typically on the order of several seconds to tens of seconds. In marine hybrid power systems, frequent fluctuations in main engine speed necessitate multiple consecutive charge-discharge pulses for the battery. If the polarization generated by the previous discharge has not fully recovered, the discharge capacity of the subsequent pulse will be severely limited.

[0023] The power distribution method of this application applies to a marine hybrid power system including the following main equipment: a shaft generator (PTO permanent magnet through-shaft motor, rated power 630 kW, operating speed 80 to 110 r / min, output power 600 kW in constant power range), a lithium battery pack (total capacity 774.144 kWh, rated discharge power 600 kW), a shaft generator inverter (including a shaft generator rectifier unit, a battery bidirectional chopper unit, and a shaft generator inverter unit, DC bus rated voltage 1050 volts, inverter efficiency not less than 96%), and a power management controller. The system adopts a DC common bus topology, with the shaft generator rectifier unit and the battery bidirectional chopper unit connected in parallel to the DC bus, and then connected together to the shaft generator inverter unit, which drives the marine propulsion motor.

[0024] For ease of description, this application defines the following parameters: This represents the measured value of the DC bus voltage; This indicates the state of charge of the lithium battery pack, with a value ranging from zero to one percent or from zero percent to one hundred percent. This indicates the active power output of the shaft-driven rectifier unit; This represents the active power injected into the DC bus by the battery's bidirectional chopper unit; a positive value indicates discharging, and a negative value indicates charging. This indicates the active power consumed by the shaft-generated inverter unit; This indicates the current flowing through the battery; positive values ​​indicate the direction of discharge. Indicates the battery terminal voltage; Indicates the battery open-circuit voltage; Indicates the series ohmic internal resistance; Indicates polarization voltage; Indicates polarization capacitance; Indicates polarization resistance; Indicates the control cycle.

[0025] The stability of the DC bus voltage depends on the instantaneous power balance of the bus capacitor. When the total power flowing into the bus (the sum of the output power of the shaft rectifier unit and the output power of the battery bidirectional chopper unit) is inconsistent with the total power flowing out of the bus, the excess or insufficient power will cause the bus capacitor to charge and discharge, resulting in changes in the DC bus voltage.

[0026] Within any given control cycle, the output power of the shaft generator rectifier unit is determined by the main engine speed and the shaft generator excitation state, while the power consumption of the shaft generator inverter unit is determined by the propulsion load; neither can be adjusted instantaneously. Therefore, the controllable variable for maintaining a stable DC bus voltage is the charging and discharging power of the battery bidirectional chopper unit. This application correctly calculates the power command value of the battery bidirectional chopper unit while satisfying the real-time volt-ampere constraint of the battery.

[0027] Specifically, when a power gap exists between the rectified output power of the generator and the power consumed by the inverter, if the bidirectional chopper unit of the battery cannot inject sufficient compensation power in time, the bus capacitor will be forced to release its stored energy to maintain power balance, resulting in a drop in bus voltage. The rate of decrease in bus voltage is directly proportional to the magnitude of the power gap and inversely proportional to the equivalent capacitance of the bus. Conversely, when the generator output power exceeds the inverter power consumption, the excess power will charge the bus capacitor, causing the bus voltage to rise.

[0028] The current-voltage characteristics of lithium battery packs vary with Since temperature and current direction change in real time, this application uses a first-order RC equivalent circuit model to describe the dynamic voltage-current characteristics of lithium batteries. This model treats the battery as an ideal voltage source connected in series with an ohmic internal resistance, and then connected in parallel with an RC network, which can better reflect the terminal voltage response characteristics of the battery under dynamic operating conditions.

[0029] Open circuit voltage Characterizing the electromotive force of a lithium battery pack in equilibrium state, and The relationship is non-linear. For lithium iron phosphate batteries, the open-circuit voltage is... The change is gradual within the middle range of 20% to 80%, while... The change is steep in the ranges below 20% and above 80%. This nonlinear characteristic makes it possible to achieve this at low... and high In certain regions, even small changes in current can cause terminal voltage fluctuations. In the actual operation of marine hybrid power systems, the battery... They are usually limited to a range to extend cycle life, but under high-power load impact, It may cross the above boundaries rapidly, at which point the nonlinear change in open-circuit voltage places higher demands on the adaptability of power allocation strategies.

[0030] Ohmic resistance Ohmic loss, characterizing the migration of lithium ions in the electrolyte and electrode materials, is related to both temperature and state of charge (SOC). Lower temperatures result in higher ohmic resistance; at lower temperatures... In certain regions, the ohmic resistance increases due to decreased utilization of active materials and reduced ion concentration. At low temperatures, the ohmic resistance of lithium iron phosphate batteries can reach more than 2.5 times that at 25 degrees Celsius, meaning that the maximum discharge current of the battery needs to be significantly reduced under these conditions.

[0031] Polarized capacitor With polarization resistance This describes the concentration polarization and electrochemical polarization processes of lithium ions at the electrode surface. During battery discharge, the ion concentration gradient establishes a polarization voltage on the electrode surface, which increases exponentially with discharge time; after discharge stops, the polarization voltage decays exponentially. The polarization memory effect reduces the battery's transient discharge capability between consecutive discharge pulses. For lithium iron phosphate batteries, the polarization time constant is typically between five and fifteen seconds, meaning that after a high-current discharge, the battery needs tens of seconds to fully recover its polarization state.

[0032] The first-order RC equivalent circuit model is described by both the terminal voltage equation and the polarization dynamic equation. The terminal voltage equation expresses the balance between the battery terminal voltage, open-circuit voltage, ohmic voltage drop, and polarization voltage; the polarization dynamic equation describes the variation of polarization voltage with battery current and time. Using this model, the power management controller can adjust the current voltage according to the battery current in each control cycle. Based on temperature and polarization state, the maximum current that the battery can safely output under the current physical state is calculated, thus providing hard constraint boundaries for power distribution commands.

[0033] This application uses the main engine speed signal as the first feedforward and the lithium battery volt-ampere characteristic model as the second feedforward, calculating the battery compensation power command through product coupling. The first feedforward reflects the changing trend of the power demand of the ship's propulsion system. When the main engine speed drops sharply, the shaft generator output will decrease accordingly, requiring timely battery power compensation to maintain the stability of the DC bus voltage. The advantage of the main engine speed signal is that its change precedes the actual occurrence of the power shortage. A decrease in speed means a reduction in the main engine output torque, while the response of the shaft generator rectified power has mechanical and electromagnetic inertia. Using the speed signal as feedforward allows battery compensation to be initiated before the power shortage forms.

[0034] The second feedforward reflects the battery's instantaneous output capability. The volt-ampere characteristic model adjusts the current output capability according to the current voltage level in each control cycle. The maximum discharge current and maximum charging current are calculated based on temperature and polarization state, and these are used as boundary constraints for the battery reference current command. When the battery... When the battery is in a low charge range or its polarization has not yet recovered, the current-voltage model automatically applies a stricter upper limit to the discharge current to avoid issuing power commands to the battery that exceed its current physical capacity.

[0035] The battery compensation power command is calculated as the product of the battery reference current command and the DC bus voltage. The battery reference current command must simultaneously satisfy both current limiting constraints and volt-ampere constraints. The current limiting constraint ensures that the battery reference current does not exceed the current... The maximum safe current under the specified temperature conditions; the essence of the volt-ampere constraint is that the battery bidirectional chopper unit must adjust the duty cycle to make the battery port voltage follow the DC bus voltage. When the difference between the DC bus voltage and the sum of the open circuit voltage and polarization voltage exceeds the product of the ohmic internal resistance and the maximum discharge current, even if the system requires a larger discharge current, the battery reference current will be clamped at the maximum discharge current.

[0036] To further improve the control response speed, this application introduces a voltage difference dynamic adjustment factor to correct the battery reference current command. The voltage difference reflects the instantaneous voltage gradient between the DC bus and the battery port. When the DC bus voltage is higher than the battery port voltage, the voltage gradient causes current to flow from the bus to the battery, i.e., the charging direction; when the DC bus voltage is lower than the battery port voltage, the voltage gradient causes current to flow from the battery to the bus, i.e., the discharging direction.

[0037] The physical significance of dynamic voltage difference regulation lies in its use of the natural potential difference between the DC bus voltage and the battery port voltage to drive power flow. Ideally, if the battery port voltage is exactly equal to the DC bus voltage, the bidirectional chopper unit can maintain power balance without operating. When a deviation occurs, the chopper unit establishes or eliminates the deviation by adjusting the switching duty cycle, and the dynamic voltage difference regulation factor provides gain control for this regulation process.

[0038] Polarization voltage is a dynamic quantity that changes continuously with time, and its rate of change depends on the instantaneous current of the battery and the polarization time constant. In this application, the polarization voltage is updated according to the actual battery current in each control cycle, and the updated polarization voltage is substituted into the volt-ampere constraint calculation for the next cycle.

[0039] When a battery undergoes a large current discharge, the polarization voltage increases, which reduces the voltage margin left for the ohmic voltage drop in the current-voltage constraint equation, thereby automatically reducing the maximum discharge current.

[0040] The implementation of polarization state tracking also needs to consider the time-varying characteristics of polarization parameters. Polarization resistance and polarization capacitance drift with battery aging and temperature changes, but the drift amplitude of polarization parameters is smaller compared to the ohmic internal resistance. Furthermore, the accuracy requirements for polarization parameters in polarization state tracking are not as high as the accuracy requirements for ohmic internal resistance in online internal resistance identification. Therefore, in the embodiments of this application, offline calibration values ​​are used for the polarization parameters.

[0041] The ohmic internal resistance drifts with battery aging and temperature changes, and the factory-specified value is insufficient to meet the control requirements throughout the battery's lifecycle. This application updates the ohmic internal resistance parameter online using a recursive least squares method under steady-state navigation conditions to eliminate model biases caused by aging and temperature changes.

[0042] The implementation of online internal resistance identification requires certain operating conditions. The recursive least squares method requires the input signal to have continuous excitation, meaning the battery current needs to contain a sufficiently rich range of frequency components. Under steady-state ship navigation conditions, small fluctuations in main engine speed and random disturbances in propulsion load can provide the necessary excitation signals for the algorithm. Under conditions of violent ship maneuvering or berthing, the current changes too drastically. In such cases, updates can be paused, the previous estimate used, and identification resumed once the operating conditions stabilize.

[0043] The ampere-hour integration method is The basic method for estimation calculates the change in charge by integrating the battery current over time. However, zero-point drift and accumulated errors in the current sensor can lead to... A significant deviation can occur between the estimated and actual values. Over long-term voyages, the cumulative error of the ampere-hour integral can reach 5% to 10%.

[0044] This application employs a fusion of ampere-hour integration and voltage feedback. Correction: When the battery is in a quiescent or low-current condition, the terminal voltage is close to the open-circuit voltage, and the SOC derived from the open-circuit voltage has high reliability. At this time, the voltage feedback term plays a dominant role and can effectively suppress the cumulative drift of the ampere-hour integral. When the battery is in a high-current dynamic condition, the terminal voltage is greatly affected by the ohmic voltage drop and polarization voltage. The weight of the voltage feedback term is automatically reduced, and the ampere-hour integral term becomes dominant.

[0045] Under static or low-current conditions, the fusion coefficient takes a larger value, making the voltage feedback term dominant. Correction: Under high-current dynamic conditions, the fusion coefficient is set to a smaller value to avoid the impact of terminal voltage fluctuations on... Estimated interference. The switching of the fusion coefficient can be achieved by detecting the amplitude and rate of change of the battery current. When the current amplitude is lower than the set threshold and the rate of change is small, it is determined to be a static or low-current condition, and the fusion coefficient is increased; otherwise, the fusion coefficient is decreased.

[0046] The ohmic internal resistance of lithium batteries increases at low temperatures, leading to a decrease in their transient discharge capability. This application proactively limits the battery compensation power command when the battery temperature is detected to be below a set low-temperature threshold. When the temperature is below the minimum allowable operating temperature, the battery's power compensation function is completely disabled, and the shaft generator system maintains the bus voltage independently, ensuring the battery is not damaged at extreme low temperatures. When the temperature is between the minimum allowable operating temperature and the low-temperature threshold, the compensation power decreases linearly, allowing the battery to still provide limited power support under low-temperature conditions while preventing excessive voltage drop. When the temperature recovers, the compensation power returns to normal.

[0047] When ships navigate in polar regions or high-latitude seas during winter, ambient temperatures may drop below minus ten degrees Celsius. In such cases, low-temperature compensation strategies are crucial for ensuring the safe operation of batteries. By actively limiting the charging and discharging current at low temperatures, this application not only avoids the risk of the terminal voltage dropping below the protection threshold but also slows down the accelerated aging process of batteries caused by low temperatures.

[0048] When the ship is berthed and shore power is connected, the shaft generator system is in a shutdown state, and the DC bus voltage is maintained by the shore power through the rectifier unit. Under this condition, the power management controller adjusts the operating mode of the battery bidirectional chopper unit according to the SOC signal of the lithium battery pack and the upper limit of the available power of the shore power.

[0049] The main unit's rotation speed signal may contain high-frequency noise introduced by factors such as combustion fluctuations and mechanical vibrations. If the unprocessed raw rotation speed signal is used directly for feedforward control, it may cause unnecessary jitter in the battery compensation power command, increasing the switching losses of power electronic devices and the number of battery charge-discharge cycles.

[0050] This application performs first-order low-pass filtering on the host speed signal. The filtered speed signal retains the low-frequency components reflecting changes in power demand and suppresses high-frequency noise interference with power allocation commands. The selection of the filtering time constant needs to strike a balance between response speed and noise suppression: if the time constant is too small, the filtering effect is insufficient, and high-frequency noise will still enter the control loop; if the time constant is too large, the signal delay increases, and the fast response advantage of feedforward control is weakened.

[0051] In engineering practice, the filtering time constant is typically set to five to ten times the control cycle. For power management systems with a control cycle of ten milliseconds, the filtering time constant can be set to 50 to 100 milliseconds, which effectively suppresses noise components with frequencies higher than ten hertz while retaining useful signal components with frequencies lower than two hertz. The filtered speed signal is then substituted into the volt-ampere characteristic model for calculation.

[0052] Example 1 This embodiment employs a hybrid-powered marine system consisting of a 600 kW shaft generator, a 774.144 kWh lithium battery pack, and a 1050 V DC bus voltage. The power management controller is configured with the following parameters: a control cycle of 10 milliseconds; first-order RC equivalent circuit model parameters calibrated through offline battery testing; and a forgetting factor of 0.98 for the recursive least squares method.

[0053] Power distribution is performed according to the method of this application: within the constant power range where the main engine speed is stable at 90 r / min, The DC bus voltage is 65% during normal system operation and remains stable at around 1050 volts. When the main engine speed decreases from 90 r / min to 82 r / min due to changes in sailing conditions, the shaft generator rectifier output power decreases from 580 kW to 480 kW.

[0054] The current-voltage characteristic model is based on the current Based on a 65% discharge rate and a temperature of 25 degrees Celsius, the maximum discharge current is calculated to be approximately 1200 amperes (equivalent to approximately 630 kilowatts), while polarization state tracking shows a polarization voltage of 15 volts. Current constraint calculations yield a battery reference current of approximately 1150 bar. The corresponding battery compensation power is approximately 1207.5 kilowatts.

[0055] The maximum allowable discharge current is calculated to prevent the battery terminal voltage from dropping below the safe operating voltage threshold under the current bus voltage.

[0056] The battery compensation power is superimposed on the shaft generator inverter power reference value.

[0057] The composite power command enables the battery bidirectional chopper unit to quickly output approximately 110 kW of compensation power, while the DC bus voltage drop is controlled to within 25 volts.

[0058] Tests showed that, under typical navigation conditions with a main engine speed variation rate of 0.5-1.5 r / min per second, the DC bus voltage fluctuation did not exceed ±30 volts, the maximum battery discharge current did not exceed the maximum discharge current limit calculated by the volt-ampere model, and the system did not trigger any protection actions.

[0059] Example 2 The difference between this embodiment and Embodiment 1 is that the ship experienced two high-power load impacts with only a 5-second interval between them. During the first impact, the battery discharge current reached 1000A and lasted for 3 seconds.

[0060] During the first impact, the polarization dynamic equation is continuously integrated based on the actual discharge current I_BAT=1000A: The polarization voltage U_pol is increased from its initial value of 5V to 42V. After the first surge, the load returns to normal, the battery current drops to a minimum, and the polarization voltage begins to decay exponentially. The time constant is 8s. When the second impact occurs 5s later, the polarization voltage is still in a state of incomplete recovery at approximately 30V.

[0061] At this point, the current U_pol = 30V is substituted into the current-voltage constraint equation in the current-voltage characteristic model: Because the polarization voltage term occupies a large voltage margin, the voltage margin left for the ohmic voltage drop I_BAT_REF × R_0 in the equation is reduced, thereby automatically reducing the maximum discharge current limit I_DIS_MAX from 1200A without polarization memory to 900A, successfully limiting the actual discharge current within a safe range.

[0062] Testing revealed that without polarization state tracking, the system would issue a discharge command of I_DIS_MAX=1200A during the second impact, but the actual battery terminal voltage would drop from 1050V to below 950V, triggering the low-voltage protection of the battery management system. This embodiment, through polarization state tracking, accurately assessed the battery's transient discharge capability based on the polarization dynamic equation before the second impact and automatically reduced the limit, keeping the lowest terminal voltage above 990V, ensuring normal system operation.

[0063] Example 3 The difference between this embodiment and Embodiment 1 is that the battery temperature drops to -5 degrees Celsius, below the low-temperature threshold of 0 degrees Celsius but above the minimum allowable operating temperature of -10 degrees Celsius. The system calculates a low-temperature compensation coefficient of 0.5, limiting the battery compensation power to 50% of its normal value. When the host machine speed fluctuates similarly, the output power of the battery bidirectional chopper unit is limited to approximately 55 kilowatts. Testing showed that under low-temperature conditions, the lowest point of the battery terminal voltage remained above 1020 volts, and no protection action was triggered.

[0064] Example 4 The difference between this embodiment and Embodiment 1 is that after the ship berths, shore power (6600V / 500kW) is connected, and the shaft generator system is shut down. The power management controller collects a SOC of 82%, and the maximum available shore power is 500kW. The controller adjusts the battery's bidirectional chopper unit to absorb shore power energy at a first charging power of 50kW, while simultaneously switching the shaft generator rectifier unit to standby mode. When the SOC drops to the preset operating range, the battery stops charging. This embodiment, through shore power coordination, maintains the battery's SOC within a favorable range before the ship departs.

[0065] Comparative Example 1 The difference between this comparative example and Example 1 is that the dynamic model of lithium battery voltage-current characteristics is not enabled; only traditional voltage droop control is used. In Example 1, when the host speed decreases from 90 r / min to 82 r / min, the droop controller's response lags by approximately fifty milliseconds (limited by the integral time of the proportional-integral regulator). Furthermore, because voltage-current constraints are not included, the droop controller still issues a discharge command with the same droop coefficient as when the SOC drops to 60% when it reaches 30%, causing the battery terminal voltage to drop to 925 volts. The battery management system triggers low-voltage protection and disconnects the battery bidirectional chopper unit. After the battery is disconnected, the DC bus voltage further drops to 870 volts, and the shaft inverter unit shuts down due to undervoltage start-up protection.

[0066] As can be seen from Example 1 and Comparative Example 1, this application incorporates the dynamic model of lithium battery current-voltage characteristics into the feedforward control link, so that the battery compensation power command is subject to both current limiting constraint and current-voltage constraint in each control cycle, effectively avoiding the energy storage unit disconnection accident caused by the command exceeding the physical capacity of the battery.

[0067] Comparative Example 2 The difference between this comparative example and Example 1 is that the ohmic internal resistance is fixed at the factory nominal value of 0.8 milliohms and is not updated online. After the battery has run for 1,000 cycles, the actual ohmic internal resistance has risen to 1.3 milliohms. When the first impact occurs in Example 2, the volt-ampere model calculates the maximum discharge current based on an ohmic internal resistance of 0.8 milliohms, resulting in a maximum discharge current of approximately 1500 amps. However, the actual ohmic internal resistance is 1.3 milliohms, and the actual maximum discharge current is only about 1100 amps. The system issues a command based on 1500 amps, causing the battery terminal voltage to drop from 1050 volts to 940 volts, triggering the battery management system protection.

[0068] As can be seen from Example 2 and Comparative Example 2, the increase in ohmic internal resistance caused by battery aging is not negligible. This application updates the ohmic internal resistance online using the recursive least squares method, so that the current-voltage characteristic model maintains high parameter accuracy throughout the battery's entire life cycle, avoiding overestimation of discharge capacity due to model parameter mismatch.

[0069] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A power distribution method for a marine hybrid power system, characterized in that, Includes the following steps: S1. Real-time acquisition of host speed signal and lithium battery pack state of charge. Signal and shaft inverter DC bus voltage ; S2. Determine whether the shaft generator system is in the constant power speed range based on the main unit speed signal; S3. When the main engine speed is within the constant power speed range, establish a dynamic model of DC bus voltage as the physical constraint boundary for power distribution. S4, according to the above Real-time signal establishment of a dynamic model of the lithium battery's current-voltage characteristics; S5. Using the host speed signal as the first feedforward and the volt-ampere characteristic model as the second feedforward, combined with the DC bus voltage... Calculate the battery compensation power command that satisfies the volt-ampere constraint. ; S6. Send the battery compensation power command Superimposed on the shaft-generator inverter power reference value to generate a composite power command. The inverter output is adjusted by the shaft-driven frequency converter.

2. The method according to claim 1, characterized in that, The dynamic model of DC bus voltage established in step S3 is as follows: ； in This is the equivalent capacitance of the DC bus. The output power of the shaft rectifier unit, The charging and discharging power of the battery's bidirectional chopper unit, For the output power of the shaft inverter unit, For time; The dynamic model of lithium battery voltage-current characteristics established in step S4 includes: open-circuit voltage. Series ohmic internal resistance Battery current Polarization voltage Polarized capacitors With polarization resistance This forms a first-order RC equivalent circuit model: ； 。 3. The method according to claim 2, characterized in that, In step S5, the battery compensation power command The calculation method is as follows: ； in Within each control cycle, the following must be satisfied: ； And it is implicitly constrained by the following formula: 。 4. The method according to claim 3, characterized in that, In step S5 It is also subject to the voltage difference dynamic adjustment factor. λ Correction: ； ； in This is the base current command calculated based on the host feedforward. It is used to achieve instantaneous power compensation by utilizing the voltage difference between the DC bus and the battery port.

5. The method according to claim 3, characterized in that, The maximum discharge current With maximum charging current Determined by the following current-voltage limiting function: ； ； in , The battery's safe operating voltage threshold, The temperature-dependent maximum continuous current provided by the manufacturer.

6. The method according to claim 3, characterized in that, It also includes a polarization state tracking step: In each control cycle, based on the actual battery current Update polarization voltage: ； And the updated Substitute this into the current-voltage constraint calculation for the next cycle.

7. The method according to claim 3, characterized in that, It also includes the online internal resistance identification step: Under steady-state navigation conditions, the system is updated online using the recursive least squares method. .

8. The method according to claim 3, characterized in that, It also includes SOC correction steps: By integrating the ampere-hour integral method with voltage feedback: ； in For Coulomb efficiency, For rated capacity, This is the fusion coefficient, used to suppress current integral drift.

9. The method according to claim 2, characterized in that, The shaft-driven inverter adopts a DC-DC common bus topology and includes a shaft-driven rectifier unit, a battery-powered bidirectional chopper unit, and a shaft-driven inverter unit. The battery-powered bidirectional chopper unit outputs according to the volt-ampere characteristic model. It operates in voltage source mode or current source mode, and according to... and The difference automatically switches the control mode.

10. The method according to claim 3, characterized in that, It also includes a host speed fluctuation suppression step: performing a first-order low-pass filter on the host speed signal to obtain a filtered speed signal, and substituting the filtered speed signal into the current-voltage characteristic model for calculation to suppress instantaneous fluctuations in host speed.

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