A marine hybrid power system undisturbed switching method

Abstract

The application discloses a kind of marine hybrid power system undisturbed switching method, based on containing diesel engine main engine, clutch, gear box, frequency converter and asynchronous motor marine hybrid power system, frequency converter is monitored each equipment state by PLC controller;The application is through accurate control and strategy, effectively reduce the current, voltage mutation in switching process;The application is through smooth control process, gradually adjusts control mode according to the monitoring other equipment state in switching process, reduces the mechanical impact and thermal shock that equipment is additionally subjected to, prolongs the service life of equipment.

Description

Technical Field

[0001] This invention belongs to the field of marine hybrid power systems, and specifically relates to a method for seamlessly switching from electric mode to power generation mode in a marine hybrid power system using a frequency converter. Background Technology

[0002] With social development and technological advancements, the shipbuilding industry, as a vital component of global trade, faces increasing demands for more efficient, environmentally friendly, and safe ship propulsion systems. Hybrid power systems have garnered significant attention due to their advantages in improving energy efficiency, reducing emissions, and enhancing flexibility. A hybrid power system typically consists of an internal combustion engine, an electric motor, and corresponding energy storage devices. Among these, the frequency converter plays a crucial role, enabling smooth switching between electric and generator modes.

[0003] However, traditional marine hybrid power systems, especially during power source switching, present several challenges. First, due to the varying operating characteristics and response speeds of different power sources, disturbances can easily occur during switching, leading to increased hull vibration and noise. Second, the switching process can also cause grid impacts and equipment damage, highlighting the urgent need for more advanced, disturbance-free switching methods.

[0004] In traditional marine hybrid power systems, when the electric motor switches from electric mode to generator mode, it is typically shut down completely first, and then restarted in generator mode only after the diesel engine has been connected to the grid. This often involves sudden changes in current, frequency, and voltage. These sudden changes can cause voltage fluctuations in the power grid and even affect other networked devices. Furthermore, during the connection process, because the diesel engine is still idling, mechanical equipment is susceptible to thermal and mechanical shocks, accelerating wear and reducing its lifespan.

[0005] Currently, some frequency converters on the market use independent control methods. This switching control method is relatively simple, such as switching states at fixed intervals. This method results in slow system response, making it unsuitable for the complex and ever-changing ship operating environment. Furthermore, due to the abrupt control changes, there may be temporary power losses during switching, leading to a decrease in overall system efficiency. Meanwhile, some manufacturers employ methods such as using damping circuits or adding filters to the main circuit to mitigate current changes.

[0006] Some scholars have focused on optimizing the switching process from electric to generator modes by improving control strategies. They employ optimization methods such as model predictive control (MPC) and genetic algorithms to improve the dynamic performance of the system and reduce current and voltage spikes during switching. However, these methods often increase the complexity and cost of the system, making them difficult to implement in engineering. Summary of the Invention

[0007] This invention addresses the impact and instability issues caused by inverter switching in marine hybrid power systems. It provides a disturbance-free switching method for marine hybrid power systems. By monitoring the status of system sub-equipment and introducing closed-loop control for step-by-step adjustment, the current and mechanical shocks during the switching process are effectively reduced, making the marine hybrid power system more economical and sustainable.

[0008] To achieve the above objectives, the technical solution adopted by the present invention to solve its technical problem is: a disturbance-free switching method for a marine hybrid power inverter, based on a marine hybrid power system including a diesel engine, clutch, gearbox, inverter, and asynchronous motor. The inverter monitors the status of each sub-device through communication with an installed PLC controller, and obtains information such as combined / separated operation status and diesel engine speed. The steps are as follows:

[0009] Step 1: In electric mode, the frequency converter adopts a speed closed-loop control method. When it receives the instruction to switch to generator mode, it obtains the current idle speed of the diesel engine by interacting with the PLC controller, and reduces the motor speed to the same as the diesel engine speed by using the slope setting method.

[0010] Step 2: After the speed reduction is completed, the frequency converter sends a disengagement signal to the PLC controller. Then, the PLC controller controls the gearbox-side clutch to engage and disengage, and sends a disengagement success signal to the frequency converter after engagement.

[0011] Step 3: After receiving the successful connection signal, the frequency converter switches the speed closed-loop control to current control mode. At this time, the q-axis current of the asynchronous motor will gradually decrease from a positive value to 0. The frequency converter adjusts the output frequency and voltage by monitoring the real-time current and grid test status to ensure smooth current change and reduce the impact on the system.

[0012] Step 4: After the q-axis current gradually decreases to 0, the inverter sends a switching completion command to the PLC controller. At this time, the speed of the diesel engine in the hybrid system can be controlled by the handle.

[0013] Step 5: When the diesel engine's speed rises above the preset value, the frequency converter switches the asynchronous motor to generator mode, and the PLC controller operates in constant voltage and current limiting control mode, outputting a stable DC bus voltage on the DC side. At this time, the frequency converter provides the DC bus voltage and uses the q-axis current to control the DC bus voltage.

[0014] Furthermore, in the power generation mode, the inverter provides a DC bus voltage, controls the DC bus voltage using the q-axis current, constructs the d-axis current using the flux linkage amplitude, and limits the output of the d-axis current during the pre-switching process to prevent the inrush current during the switching process from causing an overcurrent fault in the inverter and affecting the normal operation of the system.

[0015] Furthermore, the inverter adopts a three-closed-loop vector control system based on an outer speed loop, an outer voltage loop, and an inner current loop. In the motoring state and the pre-switching state, the inverter uses the outer speed loop to control the motor speed. After the diesel engine is connected and the power of the main engine has not reached the generator speed threshold, the inverter uses the inner current loop to limit the inverter power output. When operating in generator mode, the inverter uses the outer voltage loop to successfully switch the operating mode and output power to the outside.

[0016] Furthermore, the inverter adopts a control system based on a reactive power control loop: when the grid-side voltage drops, the reactive power component at the grid connection point is detected and then fed to the input terminal of the inverter, and its output reactive power is adjusted in real time to provide a certain amount of reactive power to the system to help the grid voltage recover and achieve low voltage ride-through; when the system voltage drops, the output range of the reactive power control reference value is appropriately limited on the basis of stabilizing the DC-side bus voltage.

[0017] Furthermore, when compensating the power grid, a threshold can be preset. When the reactive power is lower than the threshold, the frequency converter starts the reactive power control loop to automatically compensate the grid side.

[0018] The beneficial effects of this invention are:

[0019] This invention achieves smooth switching of the inverter's operating state by gradually adjusting the control mode through a preset control strategy, effectively reducing current and voltage surges during the switching process, reducing additional mechanical and thermal shocks to the equipment, and extending the equipment's service life.

[0020] The frequency converter of this invention, through its smooth and precise control characteristics when switching from electric mode to generator mode, helps to reduce the operational risks of the system and improve the stability and reliability of the equipment.

[0021] This invention adopts a step-by-step adjustment control method. By monitoring the status of other equipment during the switching process, it reduces various impact problems caused by the switching process. The switching control method is adjusted according to the engagement / disengagement status of the diesel engine and gearbox clutch to ensure a smooth current transition during the switching. Attached Figure Description

[0022] Figure 1 This is a flowchart of the frequency converter's disturbance-free switching control strategy of the present invention;

[0023] Figure 2 This is a block diagram of the inverter power generation control algorithm of the present invention. Detailed Implementation

[0024] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0025] In marine hybrid power systems, when the system operates in electric mode during navigation, the gearbox is in a split-drive state. At this time, the frequency converter drives the asynchronous motor to power the propeller. In this state, the diesel engine, to ensure fuel efficiency and system flexibility, idles after starting. If the diesel engine is stopped and then rejoined during the switch from electric to generator mode, it will cause significant thermal and mechanical shocks, affecting the equipment's lifespan. Therefore, this invention provides a disturbance-free switching method for marine hybrid power systems, the specific flowchart of which is shown below. Figure 1 As shown.

[0026] This invention discloses a disturbance-free switching method for a marine hybrid power system. Based on a marine hybrid power system including a diesel engine, clutch, gearbox, frequency converter, and asynchronous motor, the frequency converter monitors the status of each sub-device through communication with an installed PLC controller (Programmable Logic Controller) to obtain information such as combined / separated status and diesel engine speed. The steps are as follows.

[0027] Step 1: In electric mode, the frequency converter adopts a speed closed-loop control method. When it receives the instruction to switch to generator mode, it obtains the current idle speed of the diesel engine by interacting with the PLC controller, and reduces the motor speed to the same speed as the diesel engine by using a slope setting method.

[0028] Step 2: After the speed reduction is completed, the frequency converter sends a disengagement signal to the PLC controller. Subsequently, the PLC controller controls the gearbox-side clutch to engage, meaning the asynchronous motor speed synchronizes with the diesel engine speed before disengagement. After disengagement, a disengagement success signal is sent to the frequency converter. The asynchronous motor then switches to constant current mode.

[0029] Step 3: After receiving the successful connection signal, the inverter switches the speed closed-loop control to current control mode. At this time, the q-axis current of the asynchronous motor will gradually decrease from a positive value to 0. The inverter adjusts the output frequency and voltage by monitoring the real-time current and grid test status to ensure smooth current change and reduce the impact on the system.

[0030] Step 4: After the q-axis current gradually decreases to 0, the inverter sends a switching completion command to the PLC controller. At this time, the speed of the diesel engine in the hybrid system can be controlled by the handle.

[0031] Step 5: When the diesel engine's speed rises above the preset value, the frequency converter switches the asynchronous motor's operating mode to generator mode. When the PLC controller operates in constant voltage and current limiting control mode, it outputs a stable DC bus voltage on the DC side. The generator mode frequency converter control method in this invention is as follows: Figure 2As shown, the inverter provides a DC bus voltage, controls the DC bus voltage using the q-axis current, and constructs the d-axis current using flux linkage amplitude observation. During the pre-switching process, the d-axis current is limited to prevent inrush current during switching from causing overcurrent faults in the inverter and affecting normal system operation. The output current limit value is designed rationally based on the diesel engine, load, and battery status.

[0032] The overall control block diagram has an independent reactive power control loop. When the grid-side voltage drops, the reactive power component at the grid connection point is detected and fed back to the inverter input. The inverter's output reactive power is adjusted in real time to provide a certain amount of reactive power to the system to help the grid voltage recover and achieve low-voltage ride-through. When the system voltage drops, while stabilizing the DC bus voltage, the reactive power compensation range of the grid-side inverter is limited due to its own capacity constraints. The output range of the reactive power control reference value is correspondingly limited. When compensating the grid, a threshold can be preset. When the reactive power is lower than the threshold, the inverter activates the reactive power control loop to automatically compensate the grid side.

[0033] The frequency converter of this invention adopts a three-closed-loop vector control system based on speed outer loop, voltage outer loop and current inner loop. The three closed loops are used for different control modes. In the electric state and pre-switching state, the frequency converter uses speed outer loop to control the motor speed. After the diesel engine is connected and the power of the main engine has not reached the generator speed threshold, the frequency converter uses current inner loop to limit the power output of the frequency converter. When working in generator mode, the frequency converter uses voltage outer loop to successfully switch the working mode and output power to the outside.

[0034] The inverter obtains the host speed and gearbox engagement / disengagement status through interaction with the top-level controller PLC. This reduces unnecessary sensor installation and additional communication lines. The overall control algorithm switching logic is also implemented in the top-level controller, but the specific implementation of the control algorithm is in the inverter's bottom-level controller. The bottom-level controller monitors the asynchronous motor's operating status in real time through sensors, including speed sensors and voltage sensors. Using these sensor systems, the current position and operating status of the motor can be determined.

[0035] This invention combines current hybrid marine equipment and introduces a smooth switching process of speed-current-power to study a simple, stable, and disturbance-free switching control method, aiming to provide a more reliable technical solution for the stable and efficient operation of marine hybrid power systems.

[0036] This invention is not limited to the preferred embodiments described above. Any person skilled in the art can derive other variations and improvements based on the inspiration of this invention. However, regardless of any changes made to its shape or structure, any technical solution that is the same as or similar to this application falls within the protection scope of this invention.

Claims

1. A method for seamless switching of a marine hybrid power system, based on a marine hybrid power system comprising a diesel engine, a clutch, a gearbox, a frequency converter, and an asynchronous motor, characterized in that: The frequency converter monitors the status of each device through a PLC controller, and the steps are as follows: Step 1: In electric mode, the frequency converter adopts a speed closed-loop control method. When it receives the instruction to switch to generator mode, it obtains the current idle speed of the diesel engine by interacting with the PLC controller, and reduces the motor speed to the same as the diesel engine speed by using the slope setting method. Step 2: After the speed reduction is completed, the frequency converter sends a closing signal to the PLC controller. The PLC controller controls the gearbox-side clutch to close and close, and sends a closing success signal to the frequency converter after closing. Step 3: After receiving the successful connection signal, the frequency converter switches the speed closed-loop control to current control mode. The q-axis current of the asynchronous motor gradually decreases. The frequency converter adjusts the output frequency and voltage by monitoring the real-time current and grid test status. Step 4: After the q-axis current decreases to 0, the frequency converter sends a switching completion command to the PLC controller, and the diesel engine speed is controlled by the handle. Step 5: When the speed of the diesel engine reaches above the preset value, the frequency converter switches the asynchronous motor to the generator mode, and the PLC controller operates in the constant voltage and current limiting control mode, outputting a stable DC bus voltage on the DC side. In power generation mode, the inverter provides DC bus voltage, controls DC bus voltage using q-axis current, constructs d-axis current by observing the amplitude of flux linkage, and limits the output of d-axis current during pre-switching. The inverter adopts a three-closed-loop vector control system based on speed outer loop, voltage outer loop and current inner loop. In motoring and pre-switching states, the speed outer loop is used to control the motor speed. After the diesel engine is combined and the power of the main engine has not reached the generation speed threshold, the current inner loop is used to limit the inverter power output. In generation mode, the voltage outer loop is used to switch the working mode and output power to the outside. The inverter adopts a control system based on a reactive power control loop: when the grid-side voltage drops, the reactive power component at the grid connection point is detected and fed to the inverter input terminal, and its output reactive power is adjusted in real time to provide a certain amount of reactive power to the system to help the grid voltage recover and achieve low voltage ride-through; when the system voltage drops, the output range of the reactive power control reference value is appropriately limited on the basis of stabilizing the DC bus voltage. When compensating the power grid, a threshold is preset. When the reactive power is lower than the threshold, the frequency converter starts the reactive power control loop to automatically compensate the grid side.

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