Flexible access ship double-closed-loop direct-current integrated power system

Abstract

The invention relates to a flexible access ship double-closed-loop direct-current comprehensive power system which is composed of an inner-stage closed-loop power grid and an outer-stage closed-loop power grid, each of the inner-stage closed-loop power grid and the outer-stage closed-loop power grid is composed of two buses, each bus is segmented by a bus tie switch, and the tail ends of the two buses are connected by a bridging switch to form closed-loop direct-current power supply. A first-stage outer ring adopts a general electrical system, a second-stage inner ring adopts a flexible electrical system, flexible adjustment can be realized according to load requirements, flexible access and plug and play of multiple types of loads are realized, two-stage power grids are connected through a DCDC converter, and the rated voltage of a second-stage power grid can be dynamically adjusted. By utilizing the characteristics of controllable power flow of a direct current system, plug and play of a multi-type energy system, flexible and controllable load voltage and the like, unification of energy efficiency level, power supply reliability and load access flexibility of the whole system is realized, and the system has important significance on high energy efficiency, high performance and high flexibility of a ship integrated power system and is suitable for various different types of power systems. Manned and unmanned ships with high energy efficiency level and multi-task load access requirements are provided.

Description

Technical Field

[0001] This invention relates to the field of DC integrated power system design technology, and in particular to a ship dual-closed-loop DC integrated power system with flexible mission access and its construction method. Background Technology

[0002] Shipboard DC integrated power systems have advantages such as high overall energy efficiency, good maneuverability, and flexible equipment access. With green and intelligent technologies becoming the industry's recognized trend in shipboard development, shipboard DC integrated power systems, with their unique advantages, will inevitably become an important development direction for future shipboard environmentally friendly power systems.

[0003] Existing shipboard DC integrated power systems are mainly used in research vessels, special engineering vessels, inland waterway cruise ships, and government vessels. They generally adopt a segmented structure with a single letter, and the DC bus voltage is generally DC 750V or DC 1000V. Source-end equipment (such as generator sets, energy storage systems, etc.) and load-end equipment (such as propulsion equipment, daily power distribution equipment, and work equipment) are generally connected to the bus through converters (including ACDC, DCDC, DCAC, etc.). They have advantages such as simple structure and mature technology; however, they also have problems such as inflexible access to multiple loads and low system reliability.

[0004] As ships become increasingly intelligent and unmanned, more and more unmanned equipment or multi-voltage level operating equipment need to have flexible power system access interfaces to achieve plug-and-play functionality and require high power supply reliability. The current network topology cannot meet this requirement. Summary of the Invention

[0005] To address the challenges of flexible multi-task access and high-reliability power supply requirements for future unmanned and minimally staffed vessels, a flexible dual-closed-loop DC integrated power system for ships is proposed. This system utilizes a two-tiered closed-loop power grid architecture to meet the needs for flexible access, plug-and-play functionality, and redundant reliable power supply for multiple loads. The outer loop uses a universal power supply system, while the inner loop employs a flexible power supply system, allowing for adjustments based on load demands. This enables flexible access and plug-and-play functionality for various load types. A DC-DC converter connects the two power grids, enabling dynamic adjustment of the rated voltage of the second-level grid.

[0006] The technical solution of this invention is as follows: a flexible access ship dual-closed-loop DC integrated power system, consisting of an inner and outer closed-loop power grid. Both the inner and outer loops consist of two buses, each segmented by a bus tie switch. The ends of the two buses are connected by a jumper switch to form a closed-loop DC power supply. Both the inner and outer loops are divided into several bus segments, with the number of outer loop bus segments being greater than or equal to the number of inner loop bus segments. Several types of power sources and energy storage systems are connected to the outer loop via converters. Each segment of the outer loop bus is configured with the ship's essential loads according to system requirements. The bus tie switch and jumper switch use power electronic switches to ensure the speed and sensitivity of system protection. Each segment of the inner loop bus is connected to a corresponding segment of the outer loop bus via a DC-DC converter, enabling control of the inner loop bus voltage level and power flow regulation. Multi-tasking loads in the inner loop are connected to the inner loop busbar via standard connectors, achieving flexible access and plug-and-play functionality for multi-tasking loads.

[0007] A method for establishing a flexible access shipboard dual-closed-loop DC integrated power system specifically includes the following steps: 1) Design the system network structure and adopt a double closed-loop network structure. The outer loop adopts a general electrical system. The operating voltage of the inner loop bus is dynamically adjusted according to the load demand within a range not exceeding 95% of the outer loop bus voltage. When the inner loop is open, the two bus sections that are not directly physically connected operate independently at different voltage levels. The inner and outer loops are connected by DC-DC converters, which have both bus voltage regulation and power flow control functions. 2) Select the generator set and energy storage system. The generator set is connected to the outer ring as a DC power module, and the energy storage system is connected to the outer ring with a power supply set on the outer ring. 3) Control operation mode: The ship's dual closed-loop DC integrated power system has multiple operation modes. The switching between different operation modes is realized through the integrated energy management system (EMS). Under different operation modes, the EMS can dynamically adjust the number and output of grid-connected generators and the operation status of the energy storage system according to the real-time operation status of the system, so as to ensure that the system operates in a high-efficiency state. 4) Design power flow control schemes and strategies. Under normal conditions, the optimal control objective is to achieve the highest energy efficiency level. When some equipment is in an abnormal operating state, the control objective is to ensure the safe and stable operation of the system. Under normal system operation, the generator sets should be operated at the optimal operating point to minimize network losses and effectively improve the ship's energy efficiency. When some equipment in the system is in an abnormal state, the load level of the abnormal equipment should be effectively reduced to provide a safety guarantee for the safe operation of the faulty equipment in subsequent fault replacement or emergency situations. 5) Design selective protection schemes and strategies for the system to ensure system security and effective fault isolation; 6) Task system access and management: A task management system is set up in the inner ring to manage the access and operation of task systems with different loads; 7) Design a network reconfiguration scheme. In the event that a load loses power for reasons beyond its control, the EMS will plan a feasible network reconfiguration path based on the real-time operating status of the system and implement it automatically or after manual confirmation in real time to restore power to the unexpectedly power-out load.

[0008] Furthermore, in the dual closed-loop operation mode of step 3), all switches on the inner and outer power grids are closed, and any power source has at least two power supply paths to supply power to the load. However, the inner network can only use a single voltage level to supply power to loads of the same voltage level. At the same time, the outer loop is controlled by EMS to minimize the network loss of the system.

[0009] Furthermore, in step 3) of the operating mode, when operating in a single closed-loop mode, there are two scenarios: one with an open outer loop and a closed inner loop, and the other with a closed outer loop and an open inner loop. In the case of an open outer loop and a closed inner loop, at least one of the jumper switches on the outer loop bus is in the open state, and the outer loop power supply can only supply power to a specific load through a single path, while the inner loop load still has sufficient power supply reliability and redundancy. In the case of a closed outer loop and an open inner loop, at least one of the jumper switches on the inner loop is in the open state, and the outer loop power supply can still supply power to a specific load through multiple paths. This operating state is suitable for situations where the inner loop load has diverse voltage levels and high power supply reliability requirements.

[0010] Furthermore, in the dual open-loop operation mode of step 3), at least one of the jumper switches on both the inner and outer loop busbars is in the open state, and the bus tie switch is in the closed state. The entire operation adopts an open-loop structure. The outer loop power supply can only supply power to a specific load through a single path. The inner loop busbar can be divided into multiple sections as needed to adapt to the usage requirements of different voltage level loads. This mode is suitable for situations where some equipment in the outer loop fails, the inner loop has requirements for power supply redundancy, or the load voltage levels are diverse.

[0011] Furthermore, in the independent operation mode of step 3), all bus tie switches and jumper switches of the inner and outer rings are in the open state, all busbars operate independently, each section of the outer ring busbar needs to have power supply on the grid, and all inner ring busbars can operate at different voltage levels. This mode, as an emergency operating condition, has the highest independence, the lowest power supply flexibility and energy efficiency level, and is suitable for situations where some equipment in the inner ring fails or the load voltage levels are diverse, or some equipment in the outer ring fails; it is also suitable for special tasks where the independence of each device is required to be high.

[0012] The beneficial effects of this invention are as follows: The flexible access ship dual-closed-loop DC integrated power system of this invention is designed with two sets of closed-loop network system structures. The outer loop adopts a common voltage level, while the inner loop adopts a unified or diversified voltage level. By utilizing the characteristics of controllable DC power flow, plug-and-play for multiple types of energy systems, and flexible controllable load voltage, the system achieves a unified level of energy efficiency, power supply reliability, and load access flexibility. This is of great significance for the high energy efficiency, high performance, and high flexibility of ship integrated power systems and is suitable for various types of manned and unmanned ships with high energy efficiency and multi-task load access requirements. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the shipboard dual closed-loop DC integrated power system of the present invention. Detailed Implementation

[0014] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0015] like Figure 1 The diagram illustrates an embodiment of the shipboard dual-closed-loop DC integrated power system of the present invention. Both the outer and inner rings consist of two busbars, each segmented by a bus tie switch. The ends of the two busbars are connected by a jumper switch to form a closed-loop DC power supply. Both the inner and outer ring networks are divided into multiple busbar segments (each ring is divided into four segments in the diagram). The number of busbar segments in the outer ring is greater than or equal to the number of busbar segments in the inner ring. Various types of power sources and energy storage systems are connected to the outer ring via converters (ACDC or DCDC). Each busbar segment in the outer ring can be configured with essential ship loads (powered by DCAC) according to system requirements, such as main propellers, side propellers, and daily loads. The bus tie switch and jumper switch use power electronic switches to ensure the speed and sensitivity of system protection. Each busbar segment in the inner ring is connected to a corresponding busbar segment in the outer ring via a DCDC converter to control the voltage level and power flow of the inner ring busbar. Multi-tasking loads in the inner ring are connected to the inner ring busbar via standard connectors, enabling flexible access and plug-and-play functionality for multi-tasking loads.

[0016] The method for establishing an integrated DC power system with internal and external double closed loops includes the following steps: 1. The system network structure is designed with an inner and outer double closed-loop network structure. The outer loop uses a general DC 1000V or DC 750V power supply, which can also be adjusted to a medium-voltage power supply, such as DC 10kV or DC 4kV, according to the actual ship load requirements. The inner loop bus voltage can be dynamically adjusted according to the load requirements, within a range not exceeding the outer loop bus voltage. For example, if the outer loop uses DC 1000V, the inner loop bus voltage can be dynamically adjusted within the range of DC 100V to DC 950V (approximately 95% of the external network voltage); if the outer loop uses DC 750V, the inner loop bus voltage can be dynamically adjusted within the range of DC 100V to DC 720V. In the case of an open inner loop, two bus sections that are not directly physically connected can operate at different voltage levels. If the outer loop uses a medium-voltage power supply, the inner loop generally uses a low-voltage power supply, and the bus voltage should not exceed DC 1500V in principle. The inner and outer rings are connected by a DC-DC converter, which has both bus voltage regulation and power flow control functions (in the case of multiple buses operating in parallel).

[0017] 2. Regarding the selection of generator sets and energy storage systems, the DC integrated power system does not have the synchronization requirements of the AC system for generator sets. As long as the access voltage is within the allowable range, it can be quickly connected, providing convenience for the system's energy access. The generator sets involved in this invention can be diesel generator sets, LNG generator sets, methanol generator sets, ammonia fuel generator sets, hydrogen fuel generator sets, wind turbine generator sets, photovoltaic power generation systems, etc., and can be modularly designed as DC power modules. The usable DC-based energy storage systems include various types such as lithium battery energy storage systems, supercapacitor systems, hydrogen fuel cell systems, and flywheel energy storage systems. There are no restrictions on fuel type, new energy type, or energy storage type, offering great convenience and flexibility.

[0018] 3. Controlling Operating Modes: The dual-closed-loop DC integrated power system has multiple operating modes, and switching between different modes can be achieved through the integrated energy management system (EMS). During system operation, the EMS automatically adjusts the system's operating modes based on factors such as real-time system operating status, load access requirements, and changes in power supply and load. In different operating modes, the EMS can dynamically adjust the number and output of grid-connected generators, as well as the operating status of the energy storage system, based on the real-time operating status within the system, ensuring the system operates at high energy efficiency.

[0019] In the dual-loop operation mode, all switches on both the inner and outer power grids are closed, resulting in the highest system redundancy. However, the operation control and protection settings are also the most complex. Each power source has at least two power supply paths to the load, ensuring the highest system reliability. However, the inner network can only use a single voltage level to supply loads of the same voltage level. Simultaneously, the outer loop can be controlled by the EMS to minimize network losses, which is crucial for improving the ship's self-sufficiency. This operation mode is suitable for loads with high power supply reliability requirements and relatively uniform electrical systems for the connected task loads.

[0020] In single-loop operation mode, this mode has two scenarios: outer loop open-loop with inner loop closed-loop, and outer loop closed-loop with inner loop open-loop. In the case of outer loop open-loop and inner loop closed-loop operation, at least one jumper switch on the outer loop bus is open, and the outer loop power supply can only supply power to a specific load through a single path, while the inner loop load still has sufficient power supply reliability and redundancy. In the case of outer loop closed-loop and inner loop open-loop operation, at least one jumper switch on the inner loop is open, and the outer loop power supply can still supply power to a specific load through multiple paths. This operating state is suitable for situations where the inner loop load has diverse voltage levels and high power supply reliability requirements. The single-loop operation mode can balance the system's high energy efficiency level with the diversity of load voltage levels, and is suitable for the dynamic and flexible access of multiple loads, which is of great significance for multi-tasking systems.

[0021] In the dual open-loop operation mode, at least one of the jumper switches on both the inner and outer busbars is in the open state, while the bus tie switch is in the closed state, employing an open-loop operation structure. The outer loop power supply can only supply power to a specific load through a single path, while the inner loop busbar can be divided into multiple sections as needed to adapt to the usage requirements of different voltage level loads. This mode is suitable for situations where some equipment in the outer loop fails, the inner loop has certain requirements for power supply redundancy, or the load voltage levels are diverse; it is a less common operating condition.

[0022] In independent operation mode, all bus tie switches and jumper switches in both the inner and outer loops are open, and all busbars operate independently. Each section of the outer loop busbar requires a power supply to the grid, while all inner loop busbars can operate at different voltage levels. This mode, designed as an emergency operation, offers the highest level of independence, the lowest power supply flexibility, and the lowest energy efficiency. It is suitable for situations where some equipment in the inner loop fails or the load voltage levels are diverse, or where some equipment in the outer loop fails; it is also suitable for special missions where high independence of each piece of equipment is required.

[0023] 4. Design power flow control schemes and strategies. A typical characteristic of DC integrated power systems is the controllability of power flow (referring to switch control in ring networks, which can form various ring networks, and the current direction in the ring network can be controlled by converters in modular power supplies and energy storage systems). This feature provides great convenience for the operation of complex network structures. Power flow control strategies are mainly divided into two cases: under normal conditions, the optimization control objective is to achieve the highest energy efficiency level; under abnormal conditions of some equipment, the control objective is to ensure the safe and stable operation of the system. That is, under normal system operation, on the one hand, power flow control can be used to make the generator sets operate at the optimal operating point; on the other hand, network power flow can be adjusted to minimize network losses. The combination of the two can effectively improve the energy efficiency level of ships, which has a significant effect on the green upgrading of ships. For unmanned ships, a significant improvement in energy efficiency means an effective improvement in the ship's self-sufficiency, which is of great significance for improving the mission completion capability of unmanned ships. When some equipment in the system is in an abnormal state and its load-carrying capacity is reduced, the load level of the abnormal equipment can be effectively reduced through power flow adjustment, providing a safety guarantee for subsequent fault replacement or safe operation of faulty equipment in emergency situations. Power flow control of the system is implemented by the integrated energy management system (EMS).

[0024] 5. Design a selective protection scheme and strategy for the system. The basic selective protection strategy proposed for the dual closed-loop network structure is as follows: (1) All generator sets, energy storage systems, loads and other equipment are connected to the bus through controllable converters (including ACDC, DCDC, DCAC, etc.). Faults of the equipment itself and its lines (such as internal short circuits, grounding, etc.) are protected by the controllable converters, which have the characteristics of speed and selectivity. Internal short circuit faults of the converters are protected by high-speed fuses, and engineering practice has proven that selectivity can be achieved.

[0025] (2) All bus tie and jumper switches are power electronic switches, and their fastest operating time can reach within 30μs. Under the closed-loop operation of the system, the operation of the power electronic switch is judged in combination with the magnitude and direction of the current, so as to quickly isolate the faulty bus and realize the selectivity of bus protection.

[0026] (3) Line faults between the inner and outer loops are protected by the DC-DC converter. In the event of a line fault, a blocking signal is quickly issued to shut down the output, and a tripping command is sent to the mechanical disconnect switch to effectively disconnect the faulty line. At the same time, the DC-DC converter has a certain short-circuit current sustaining capability. In the event of failure of the lower-level protection, it can be used as a backup protection to disconnect the corresponding faulty bus to ensure system safety.

[0027] (4) When a fault occurs in the inner ring bus, the power electronic switches at both ends of the bus section are disconnected simultaneously and a blocking signal is sent to the converter supplying power to the bus section. After receiving the blocking signal, the converter immediately blocks the output and sends a trip signal to the isolation mechanical switch to ensure effective isolation of the fault.

[0028] (5) All circuits protected by semiconductor devices are equipped with mechanical isolation circuit breakers. When the semiconductor protection device is activated, the mechanical isolation circuit breaker is immediately disconnected to ensure reliable disconnection of the faulty circuit or equipment.

[0029] (6) All power supply side equipment, such as generator sets and energy storage systems, are equipped with independent controllers to provide overload protection. The action threshold and time threshold of the overload protection are set according to the system requirements and the characteristics of the equipment itself.

[0030] 6. Inner ring task load access and management: Supports flexible access and plug-and-play functionality for task devices with different electrical systems. All task loads are accessed through standard interfaces reserved in the inner ring. Accessible devices include towed sonar, underwater robots, UUVs, and AUVs. To facilitate comprehensive control and management of the task system, a task management system is set up in the inner ring, responsible for the access and operation management of different task systems.

[0031] The task load can be automatically or manually connected to the inner ring power supply network depending on the actual situation.

[0032] After a task load is connected, the task management system provides the necessary power supply according to the standard communication protocol. When multiple task loads of different voltage levels are connected, the task management system automatically adjusts the inner ring network topology, supporting the simultaneous connection and operation of devices with multiple different voltage levels. The total number of voltage levels is determined by the number of busbar segments in the ring network. Figure 1 It consists of 4 segments, supporting 4 different voltage levels of equipment. Loads of different voltage levels must not be connected to the same non-disconnectable busbar segment. The task management system has corresponding interlocking functions to ensure that it does not supply power to loads of different voltage levels that are connected later.

[0033] 7. A network reconfiguration scheme is established, featuring a dual-closed-loop network structure with multiple power supply paths and strong network reconfiguration capabilities. Theoretically, the system can provide at least two power supply paths for each load. In the event of a load losing power for reasons beyond its control, the EMS can plan feasible network reconfiguration paths based on the real-time system operating status (such as the number and capacity of power sources on the grid and the importance of the load), and automatically or after manual confirmation, restore power to the unexpectedly power-out load in real time. Network reconfiguration is implemented based on artificial intelligence algorithms.

[0034] The embodiments described above merely illustrate specific implementations of the present invention, and while the descriptions are 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 flexible access shipboard dual-closed-loop DC integrated power system, characterized in that, The system consists of two closed-loop power grids, one inner and one outer. Each inner and outer ring consists of two buses, each segmented by a bus tie switch. The ends of the two buses are connected by a jumper switch to form a closed-loop DC power supply. Both the inner and outer rings are divided into several bus segments, with the number of bus segments in the outer ring being greater than or equal to that in the inner ring. Several types of power sources and energy storage systems are connected to the outer ring via converters. Each bus segment in the outer ring is configured with the necessary ship loads according to system requirements. The bus tie switch and jumper switch use power electronic switches to ensure the speed and sensitivity of system protection. Each bus segment in the inner ring is connected to a corresponding bus segment in the outer ring via a DC-DC converter, enabling control of the voltage level and power flow regulation of the inner ring bus. Multi-tasking loads in the inner ring are connected to the inner ring busbars via standard connectors, enabling flexible access and plug-and-play functionality for multi-tasking loads.

2. The method for establishing a flexible access ship dual-closed-loop DC integrated power system as described in claim 1, characterized in that, Specifically, the steps include: 1) Designing the system network structure, adopting a double closed-loop network structure. The outer loop uses a general power system, and the inner loop bus voltage is dynamically adjusted according to load demand within a range not exceeding 95% of the outer loop bus voltage. When the inner loop is open, the two bus sections that are not directly physically connected operate independently at different voltage levels. The inner and outer loops are connected by a DC-DC converter, which combines bus voltage regulation and power flow control; 2) Selecting generator sets and energy storage systems. The generator sets are connected to the outer loop as DC power modules, and the energy storage system is connected to the outer loop with a power system set in the outer loop; 3) Controlling the operating modes. The ship's double closed-loop DC integrated power system has multiple operating modes. The switching between different operating modes is achieved through the integrated energy management system (EMS). Under different operating modes, the EMS can dynamically adjust the number and output of on-grid generator sets and the operating status of the energy storage system according to the real-time operating status of the system to ensure that the system operates in a high-efficiency state; 4) Designing power flow control schemes and strategies. Under normal conditions, the highest energy efficiency level is the optimization control objective; under abnormal operating conditions of some equipment, the safe and stable operation of the system is the control objective. Under normal system conditions, the generator set operates at its optimal operating point to minimize network losses and effectively improve the ship's energy efficiency. When some equipment in the system is in an abnormal state, the load level of the abnormal equipment is effectively reduced, providing a safety guarantee for the safe operation of the equipment in subsequent fault replacement or emergency situations. 5) Design selective protection schemes and strategies for the system to ensure system security and effective fault isolation; 6) Task system access and management: Set up a task management system in the inner ring to manage the access and operation of different load task systems; 7) Design network reconstruction scheme: In the event that a load loses power for reasons other than its own, the EMS will plan a feasible network reconstruction path based on the real-time operating status of the system and restore power to the unexpectedly power-out load in real time, either automatically or after manual confirmation.

3. The method for establishing a flexible access ship dual-closed-loop DC integrated power system according to claim 2, characterized in that, In the dual closed-loop operation mode of step 3), all switches on the inner and outer power grids are closed, and any power source has at least two power supply paths to supply power to the load. However, the inner network can only use a single voltage level to supply power to loads of the same voltage level. At the same time, the outer loop is controlled by EMS to minimize the network loss of the system.

4. The method for establishing a flexible access ship dual-closed-loop DC integrated power system according to claim 2, characterized in that, Step 3) Operating modes: In single closed-loop operation mode, there are two scenarios: one with an open outer loop and a closed inner loop, and the other with a closed outer loop and an open inner loop. In the case of an open outer loop and a closed inner loop, at least one of the jumper switches on the outer loop bus is in the open state, and the outer loop power supply can only supply power to a specific load through a single path, while the inner loop load still has sufficient power supply reliability and redundancy. In the case of a closed outer loop and an open inner loop, at least one of the jumper switches on the inner loop is in the open state, and the outer loop power supply can still supply power to a specific load through multiple paths. This operating state is suitable for situations where the inner loop load has diverse voltage levels and high power supply reliability requirements.

5. The method for establishing a flexible access ship dual-closed-loop DC integrated power system according to claim 2, characterized in that, In the dual open-loop operation mode of step 3), at least one of the jumper switches on both the inner and outer loop busbars is in the open state, and the bus tie switch is in the closed state. The entire operation adopts an open-loop structure. The outer loop power supply can only supply power to a specific load through a single path. The inner loop busbar can be divided into multiple sections as needed to adapt to the usage requirements of different voltage level loads. This mode is suitable for situations where some equipment in the outer loop fails, the inner loop has requirements for power supply redundancy, or the load voltage levels are diverse.

6. The method for establishing a flexible access ship dual-closed-loop DC integrated power system according to claim 2, characterized in that, In the independent operation mode of step 3), all bus tie switches and jumper switches of the inner and outer rings are in the open state, all busbars operate independently, each section of the outer ring busbar needs to have power supply on the grid, and all inner ring busbars can operate at different voltage levels. This mode is an emergency operating condition with the highest independence, the lowest power supply flexibility and energy efficiency level. It is suitable for situations where some equipment in the inner ring fails or the load voltage level is diverse, and some equipment in the outer ring fails; it is also suitable for special tasks where the independence of each device is required to be high.

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