Power supply system comprising hybrid energy storage and diesel generator two-stage emergency power supply

Abstract

The invention discloses a power supply system containing a hybrid energy storage and diesel generator two-stage emergency power supply, which relates to the technical field of power supply systems and comprises an alternating current bus, a system power supply, an energy storage device, a diesel generator, an important load and an unimportant load. The system power supply comprises a frequently-used power supply and a standby power supply which are respectively connected to the AC bus through switches; the energy storage device consists of a hybrid energy storage system, a three-phase inverter, a filtering unit and an energy storage switch, and is connected to the alternating current bus; the diesel generator is connected to the bus through the grid-connected switch; the loads are divided into important loads and unimportant loads, and the system has four switchable states of common power supply-energy storage parallel connection, standby power supply-energy storage parallel connection, energy storage power supply and diesel generator-energy storage parallel connection so as to ensure stable power supply.

Description

Technical Field

[0001] The present invention relates to the technical field of power supply systems, and in particular to a power supply system comprising a two-level emergency power supply of a hybrid energy storage and a diesel generator. Background Art

[0002] With the widespread use of power systems and power-consuming equipment, power supply continuity and stability are of great significance in many important scenarios (such as data centers, hospitals, industrial control, etc.). In particular, in the event of a power failure in the power grid, being able to switch to emergency power in a timely manner to ensure the continuous operation of important loads is a key requirement of modern power supply systems. For this reason, traditional power supply systems usually use a single backup power source (such as a diesel generator) or energy storage device to meet emergency power supply needs.

[0003] The prior art has the following defects:

[0004] Single emergency power supply is susceptible to load fluctuations: Traditional emergency power supply systems mostly use diesel generators as the main backup power supply, but diesel generators start and adjust power relatively slowly and cannot respond quickly to sudden load changes, easily causing power shortages or overloads.

[0005] The instantaneous power supply capacity of energy storage devices is limited: Although a single energy storage device (such as a lithium battery) can provide power for a load in a short period of time, frequent high-power charging and discharging will accelerate battery aging and shorten its service life. Although supercapacitors have fast response capabilities, their energy storage capacity is small and it is difficult to provide long-term power support.

[0006] Large current shock during the switching process: When switching between the normal power supply and the backup power supply, due to the difference in voltage amplitude and phase angle, a large shock current is likely to occur, causing instantaneous shock to the power grid and load, which may affect the safety and stability of the equipment.

[0007] Lack of flexible control strategies: The existing emergency power supply system lacks efficient switching control between grid-connected or isolated grid operation modes, which can easily lead to discontinuous or unstable power supply of the system and cannot flexibly respond to different power supply conditions.

[0008] Therefore, in order to overcome the above-mentioned defects in the prior art, it is necessary to design a dual emergency power supply system containing a hybrid energy storage device and a diesel generator, which can effectively achieve rapid response, seamless switching, reduce current shock, extend battery life, and ensure continuous power supply to important loads during grid failure or power switching. Summary of the invention

[0009] The object of the present invention is to provide a dual emergency power supply system comprising a hybrid energy storage device and a diesel generator to ensure continuous and stable power supply to important loads when the normal power supply fails.

[0010] The technical solution of the present invention is: to provide a power supply system containing a two-level emergency power supply of a hybrid energy storage and a diesel generator, the power supply system comprising: an AC bus, a system power supply, an energy storage device, a diesel generator, important loads and unimportant loads;

[0011] The system power supply includes: normal power supply, backup power supply, normal power switch and backup power switch; the normal power supply and backup power supply are both grid power supplies, and both are connected to the AC bus through the normal power switch and the backup power switch respectively;

[0012] The energy storage device includes: a hybrid energy storage system, a three-phase inverter, a filter unit and an energy storage switch; the hybrid energy storage system is connected to the AC busbar through the three-phase inverter, the filter unit and the energy storage switch in sequence;

[0013] The diesel generator includes: a diesel generator set and a diesel generator grid-connected switch; the diesel generator set is connected to the AC busbar through the diesel generator grid-connected switch;

[0014] The power supply system also includes: an important load switch and an unimportant load switch; the important load is connected to the AC bus through the important load switch, and the unimportant load is connected to the AC bus through the unimportant load switch;

[0015] The power supply system has four switchable working states: normal power supply-energy storage parallel working state, backup power supply-energy storage parallel working state, energy storage power supply working state, diesel generator-energy storage parallel working state; the energy storage power supply working state is a transitional state. When the fault conditions of each power supply meet the corresponding conditions, the other three states will switch to each other. During the state switching process, there will be a certain period of time in the energy storage power supply working state, and the energy storage device provides short-term power support.

[0016] The hybrid energy storage system includes a lithium battery and a supercapacitor. The supercapacitor bears the high-frequency power part of the load, and the lithium battery bears the low-frequency power part of the load. The energy distribution strategy established by the two is:

[0017] In order to control the low-frequency part of the load power provided by the lithium battery, the DC side current is given a value i dc_ref Perform low-pass filtering to obtain the given value of the lithium battery output current As shown below:

[0018]

[0019] Among them, f L is the cutoff frequency of the low-pass filter, and s is the Laplace operator;

[0020] Supercapacitor output current setpoint is the given value of DC side current idc_ref Subtract the lithium battery output current given value As shown below:

[0021]

[0022] In any of the above technical solutions, further, the power supply system has a control principle, including:

[0023] Use grid power supply alone: ​​It is prohibited to connect the normal power supply and the backup power supply in parallel;

[0024] Prioritize grid power supply: Diesel generator is used as emergency power supply when both normal power supply and backup power supply fail. If one of the normal power supply and backup power supply is normal, the diesel generator will not be turned on.

[0025] Prioritize the use of common power supply: give priority to the use of common power supply among the two power grids;

[0026] The energy storage power supply system works all the time: The energy storage power supply system is always in working condition, which plays a role in improving power quality and providing emergency power supply during power switching intervals;

[0027] Important load priority principle: When the diesel generator is providing emergency power, unimportant loads need to be disconnected to ensure the normal power supply of the diesel generator to important loads.

[0028] In any of the above technical solutions, further, the logic and process of switching the common power supply-energy storage parallel working state to other working states include:

[0029] When the common power supply and energy storage system are working normally, the system remains in this state by default; if the common power supply fails and the backup power supply is normal, the common power switch is disconnected and the backup power switch is closed, and the system enters the backup power supply-energy storage parallel working state; if the common power supply fails and the backup power supply fails, the common power switch is disconnected, the diesel generator is started, and the unimportant load switches are disconnected. The system first enters the energy storage power supply working state for a longer period of time. When the diesel generator voltage is started and synchronized with the energy storage output voltage, the diesel generator switch is closed, and the system enters the diesel generator-energy storage parallel working state.

[0030] In any of the above technical solutions, further, the logic and process of switching the backup power supply-energy storage parallel working state to other working states include:

[0031] If the normal power supply has returned to normal, disconnect the backup power switch and then close the normal power switch, and the system will return to the normal power supply-energy storage parallel working state; if the normal power supply has not returned to normal and the backup power supply has also detected a fault, disconnect the backup power switch, start the diesel generator, disconnect the unimportant load switches, and wait until the diesel generator is started and synchronized with the energy storage output voltage, the system will enter the diesel generator-energy storage parallel working state.

[0032] In any of the above technical solutions, further, the logic and process of switching the diesel generator-energy storage parallel working state to other working states include:

[0033] If the normal power supply returns to normal, the diesel generator switch is disconnected, the normal power switch is closed, and the normal power supply-energy storage parallel working state is entered; if the normal power supply does not return to normal, but the backup power supply returns to normal, the diesel generator switch is disconnected, the backup power switch is closed, and the backup power supply-energy storage parallel working state is entered.

[0034] In any of the above technical solutions, further, the three-phase inverter in the energy storage device has multiple control modes to adapt to different working conditions, and the control modes of the three-phase inverter include:

[0035] PQ control mode: When the hybrid energy storage device is connected in parallel with the grid or diesel generator, the three-phase inverter adopts the PQ control strategy by default. In the PQ control mode, the inverter controls the output current according to the reference value of active power P and the reference value of reactive power Q, where the reactive power set value Q is set to 0 to ensure that the energy storage device only provides active power to the system; when the energy storage device needs to be charged, the reference value of P is set to the charging power of the energy storage device, and the inverter is controlled according to the charging power;

[0036] Dynamic load balancing mode: When the energy storage device is connected in parallel with the diesel generator and does not need to be charged, the energy storage device is mainly used to smooth the load power fluctuation. At this time, the given value of active power P is calculated through the proportional link according to the difference between the AC side voltage and the rated voltage and the difference between the AC side frequency and the rated frequency. In this way, when the voltage or frequency of the power grid or generator deviates from the rated value, the energy storage device can supplement power in time to balance the system load;

[0037] VF control mode: When the energy storage device operates independently, the inverter adopts the VF control mode to maintain the voltage and frequency stability of the system. In the VF mode, the AC side frequency feedback controls the active power output of the inverter, and the AC side voltage feedback controls the reactive power output, ensuring the constancy of the system frequency and voltage in the isolated grid state.

[0038] The beneficial effects of the present invention are:

[0039] The present invention enhances the power supply reliability of the system: through the two-level emergency power supply design, seamless switching is achieved when the power grid fails, ensuring stable power supply for important loads;

[0040] Improve dynamic response capability: The supercapacitor in the hybrid energy storage device bears the high-frequency power demand, and the lithium battery bears the low-frequency part, so that the system can quickly respond to sudden load changes and ensure the stability of voltage and frequency;

[0041] Extend battery life: through reasonable power distribution, reduce the instantaneous power output fluctuation of lithium batteries, avoid frequent charging and discharging, and thus extend the battery life;

[0042] Reduce current shock during switching: When switching between different power sources, the output voltage amplitude and phase angle of the energy storage device are automatically adjusted synchronously, effectively reducing current shock and protecting equipment safety;

[0043] Improve system flexibility and energy utilization: By adopting multiple inverter control modes, the system can operate in multiple states such as grid-connected and isolated grid, making full use of electric energy resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The advantages of the above and additional aspects of the present invention will become apparent and easily understood in the description of the embodiments in conjunction with the following drawings, in which:

[0045] Figure 1 It is a topological structure diagram of a power supply system of a power supply system including a hybrid energy storage and a diesel generator two-level emergency power supply according to an embodiment of the present invention;

[0046] Figure 2 It is a circuit topology diagram of a hybrid energy storage system of a power supply system including a hybrid energy storage and a diesel generator two-level emergency power supply according to an embodiment of the present invention;

[0047] Figure 3 It is a switching logic diagram of four working states of a power supply system of a power supply system including a hybrid energy storage and a diesel generator two-level emergency power supply according to an embodiment of the present invention;

[0048] Figure 4 The initial working state of the power supply system including a hybrid energy storage and a diesel generator two-stage emergency power supply according to one embodiment of the present invention is a common power supply-energy storage parallel state switching control flow chart;

[0049] Figure 5 The initial working state of the power supply system containing a hybrid energy storage and a diesel generator two-stage emergency power supply according to one embodiment of the present invention is a control flow chart of the backup power supply-energy storage parallel state switching;

[0050] Figure 6The initial state of the power supply system containing a hybrid energy storage and a diesel generator two-stage emergency power supply according to an embodiment of the present invention is a control flow chart of switching between a diesel generator and an energy storage parallel state;

[0051] Figure 7 It is a hybrid energy storage and inverter control block diagram of a power supply system including a hybrid energy storage and a diesel generator two-level emergency power supply according to an embodiment of the present invention;

[0052] Figure 8 It is a waveform diagram of a sudden change in load when an energy storage device alone supplies power to a power supply system containing a hybrid energy storage and a diesel generator two-level emergency power supply according to an embodiment of the present invention;

[0053] Fig. 9 It is a waveform diagram of a common power supply of a power supply system including a hybrid energy storage and a diesel generator two-level emergency power supply - energy storage in parallel switching to energy storage power supply alone according to an embodiment of the present invention;

[0054] Fig.10 It is a waveform diagram of a power supply system including a hybrid energy storage and a diesel generator two-stage emergency power supply, wherein the common power supply-energy storage is directly switched to the backup power supply-energy storage in parallel according to an embodiment of the present invention;

[0055] Fig.11 It is a waveform diagram of a power supply system including a hybrid energy storage and a diesel generator two-stage emergency power supply, in which a common power supply-energy storage is connected in parallel and switched to a backup power supply-energy storage in parallel after adjusting the amplitude and phase angle according to an embodiment of the present invention;

[0056] Fig.12 It is a power distribution between a diesel generator and an energy storage device of a power supply system including a hybrid energy storage and a diesel generator two-level emergency power supply according to an embodiment of the present invention;

[0057] Fig.13 It is a load increase and decrease waveform in the diesel generator-energy storage parallel state of a power supply system containing a hybrid energy storage and a diesel generator two-level emergency power supply according to an embodiment of the present invention. DETAILED DESCRIPTION

[0058] In order to more clearly understand the above-mentioned purpose, features and advantages of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0059] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited to the specific embodiments disclosed below.

[0060] like Figure 1 As shown, this embodiment provides a power supply system including a hybrid energy storage and a diesel generator two-level emergency power supply, and the power supply system includes: an AC bus, a system power supply, an energy storage device, a diesel generator, important loads and unimportant loads.

[0061] The system power supply includes: normal power supply, backup power supply, normal power switch and backup power switch.

[0062] The energy storage device includes: a hybrid energy storage system, a three-phase inverter, a filter unit and an energy storage switch.

[0063] The diesel generator includes: a diesel generator set and a diesel generator grid-connected switch.

[0064] The power supply system also includes: important load switches and unimportant load switches.

[0065] The normal power supply and the backup power supply are both grid power supplies, and the normal power supply and the backup power supply are connected to the AC bus through the normal power switch and the backup power switch respectively.

[0066] like Figure 2 As shown, the hybrid energy storage system is composed of a lithium battery and a supercapacitor, and the hybrid energy storage system is connected to the AC busbar through a three-phase inverter, a filter unit and an energy storage switch in sequence; considering the advantages and disadvantages of the two energy storage methods of lithium batteries and supercapacitors, the supercapacitor bears the high-frequency power part of the load, and the lithium battery bears the low-frequency power part of the load. The reason is that the supercapacitor has the characteristics of fast charging and discharging, and is suitable for bearing the high-frequency power part of the load. When the load suddenly changes (for example, the load suddenly increases or decreases), the supercapacitor can respond quickly and output or absorb the required current instantly, which can effectively buffer the impact of load changes on the system and avoid the lithium battery from frequently outputting large currents; lithium batteries are more suitable for long-term stable output, so the low-frequency power part of the load is borne by the lithium battery to provide stable power support. In this embodiment, the given value of the DC side current is processed by a low-pass filter to obtain the output current of the lithium battery, which ensures that the lithium battery mainly bears a stable low-frequency load, thereby reducing the impact of frequent changes on battery life.

[0067] The diesel generator set is connected to the AC bus through the diesel generator grid-connected switch. When the grid power fails, the diesel generator serves as the last emergency power source.

[0068] Important loads are connected to the AC bus through important load switches, and unimportant loads are connected to the AC bus through unimportant load switches; considering the limited capacity of diesel generators and hybrid energy storage devices, in order to avoid overloading of diesel generators and energy storage devices, they must ensure normal power supply to the loads. According to the importance of the loads, the loads are divided into important loads and unimportant loads, which are connected to the bus through important load switches and unimportant load switches respectively. When the normal power supply or backup power supply is in use, the unimportant load switch is closed to ensure that both important loads and unimportant loads can be powered normally. When the diesel generator is in use, the unimportant load switch is disconnected to ensure that the diesel generator is not overloaded.

[0069] In order to ensure the effective operation of the power supply system, the control principles of the dual power supply system including hybrid energy storage and diesel generator emergency power supply are established as follows:

[0070] Use grid power supply alone: ​​It is prohibited to connect the normal power supply and the backup power supply in parallel;

[0071] Prioritize grid power supply: Diesel generator is used as emergency power supply when both normal power supply and backup power supply fail. If one of the normal power supply and backup power supply is normal, the diesel generator will not be turned on.

[0072] Prioritize the use of common power supply: give priority to the use of common power supply among the two power grids;

[0073] The energy storage power supply system works all the time: The energy storage power supply system is always in working condition, which plays a role in improving power quality and providing emergency power supply during power switching intervals;

[0074] Important load priority principle: When the diesel generator is providing emergency power, unimportant loads need to be disconnected to ensure the normal power supply of the diesel generator to important loads.

[0075] like Figure 3-6 As shown, under the above control principle, the power supply system has four switchable working states: common power supply-energy storage parallel working state, backup power supply-energy storage parallel working state, energy storage power supply working state, diesel generator-energy storage parallel working state; among which the energy storage power supply working state is a transitional state, and the other three states will be in the energy storage power supply working state for a certain period of time during the switching process. The logic and process of switching between the three states are as follows:

[0076] Normal power supply-energy storage parallel working state: When the normal power supply and energy storage system are working normally, the system remains in this state by default; if the normal power supply fails and the backup power supply is normal, the normal power switch is disconnected and the backup power switch is closed, and the system enters the backup power supply-energy storage parallel working state; if the normal power supply fails and the backup power supply fails, the normal power switch is disconnected, the diesel generator is started, and the unimportant load switches are disconnected. The system first enters the energy storage power supply working state for a longer period of time. When the diesel generator voltage is started and synchronized with the energy storage output voltage, the diesel generator switch is closed, and the system enters the diesel generator-energy storage parallel working state.

[0077] Backup power supply-energy storage parallel working state: If the normal power supply has returned to normal, disconnect the backup power switch and then close the normal power switch, and the system returns to the normal power supply-energy storage parallel working state; if the normal power supply has not returned to normal and the backup power supply also detects a fault, disconnect the backup power switch, start the diesel generator, disconnect the unimportant load switch, and wait until the diesel generator is started and synchronized with the energy storage output voltage, the system enters the diesel generator-energy storage parallel working state.

[0078] Diesel generator-energy storage parallel working state: If the normal power supply returns to normal, the diesel generator switch is disconnected, the normal power switch is closed, and the normal power supply-energy storage parallel working state is entered; if the normal power supply does not return to normal, but the backup power supply returns to normal, the diesel generator switch is disconnected, the backup power switch is closed, and the backup power supply-energy storage parallel working state is entered.

[0079] like Figure 7 As shown in the figure, since the supercapacitor bears the high-frequency power part of the load and the lithium battery bears the low-frequency power part of the load, the two need to establish an energy distribution strategy:

[0080] DC side current given value i dc_ref The DC side voltage given value u dc_ref and the actual value of the DC side voltage u dc The difference is obtained through the PI link (proportional-integral control). In order to control the low-frequency part of the load power provided by the lithium battery, the DC side current is given a value i dc_ref Perform low-pass filtering to obtain the given value of the lithium battery output current As shown below:

[0081]

[0082] Among them, f L is the cutoff frequency of the low-pass filter, and s is the Laplace operator.

[0083] Supercapacitor output current setpoint is the given value of DC side current i dc_refSubtract the given value of lithium battery output current As shown below:

[0084]

[0085] By distributing the output current of the lithium battery and the supercapacitor, the sudden change of the lithium battery output power can be avoided, which is beneficial to prolonging the life of the lithium battery and can make the energy storage device have better dynamic response performance.

[0086] The three-phase inverter in the energy storage device has multiple control modes to adapt to different working conditions, ensuring that the energy storage system can maintain system stability when connected to the grid, paralleled with diesel generators, and isolated. The control modes of the three-phase inverter include:

[0087] PQ control mode: When the hybrid energy storage device is connected in parallel with the grid or diesel generator, the three-phase inverter adopts the PQ control strategy by default. In the PQ control mode, the inverter controls the output current according to the reference value of active power P and the reference value of reactive power Q, where the reactive power set value Q is set to 0 to ensure that the energy storage device only provides active power to the system, thereby achieving efficient use of energy. When the energy storage device needs to be charged, the reference value of P is set to the charging power of the energy storage device, and the inverter is controlled according to this charging power.

[0088] Dynamic load balancing mode: When the energy storage device is connected in parallel with the diesel generator and does not need to be charged, the energy storage device is mainly used to smooth load power fluctuations. At this time, the given value of active power P is calculated through a proportional link based on the difference between the AC side voltage and the rated voltage and the difference between the AC side frequency and the rated frequency. In this way, when the voltage or frequency of the power grid or generator deviates from the rated value, the energy storage device can supplement power in time to balance the system load.

[0089] VF control mode: When the energy storage device operates independently, the inverter adopts the VF control mode to maintain the voltage and frequency stability of the system. In the VF mode, the AC side frequency feedback controls the active power output of the inverter, and the AC side voltage feedback controls the reactive power output, thereby ensuring the constancy of the system frequency and voltage in the isolated grid state.

[0090] In order to verify the performance of energy distribution of the hybrid energy storage device, a simulation model was built on the MATLAB / Simulink platform for experimental verification. Figure 8 The following are the waveforms of the energy storage device supplying power to the load when it changes suddenly. (a) is the DC bus voltage waveform, (b) is the DC bus current waveform, (c) is the lithium battery output current waveform, (d) is the supercapacitor output current waveform, (e) is the DC total power waveform, (f) is the lithium battery output power waveform, and (g) is the supercapacitor output power waveform.

[0091] In the time period of 0 to 1s, the load power is 0, and the output currents of the lithium battery and supercapacitor are both 0. At 1s, the load current increases from 0 to 150A. Since the supercapacitor bears the high-frequency current part of the load and the lithium battery bears the low-frequency current part of the load, the supercapacitor output current increases rapidly to 150A and immediately drops back to 0, while the lithium battery output current gradually increases from 0 to 150A.

[0092] At 3s, the load current jumps from 150A to 300A, the supercapacitor output current increases rapidly from 0 to 150A and immediately drops back to 0, while the lithium battery output current gradually increases from 150A to 300A. At 5s, the load current jumps from 300A to 150A, the supercapacitor output current drops rapidly from 0 to -150A, starts to absorb power, and immediately rises back to 0, while the lithium battery output current gradually decreases from 300A to 150A.

[0093] It can be seen that when the load power changes, the supercapacitor takes advantage of its fast charging and discharging, and can instantly output or absorb part of the load power, which can avoid drastic changes in the output power of the lithium battery when the load power changes, thereby effectively extending the service life of the lithium battery. The supercapacitor has a smaller capacity, and after the load change ends, the lithium battery takes advantage of its larger energy storage capacity and bears the steady-state power of the load. The simulation proves the effectiveness of the power distribution control strategy of the hybrid energy storage system, which can give full play to the advantages of the two energy storage methods.

[0094] like Fig. 9 The figure shows the waveform when the common power supply and the energy storage device are operated in parallel and the common power supply fails and switches to the hybrid energy storage for power supply alone. (a) is the common power supply voltage waveform, (b) is the load side voltage waveform, (c) is the load side current waveform, (d) is the energy storage device output voltage waveform, (e) is the energy storage device output current waveform, (f) is the energy storage device DC side current waveform, (g) is the energy storage device DC side voltage waveform, (h) is the lithium battery output current waveform, and (i) is the supercapacitor output current waveform.

[0095] Before the common power supply fails, the system is in the common power supply-energy storage parallel working state, the load active power is 4.5kW, and the reactive power is 1kVar. The hybrid energy storage device works in the grid-connected mode, and the three-phase AC voltage output by the energy storage device is consistent with the amplitude and phase of the common power supply voltage, and the energy storage device has only a small charging current. The output current of the common power supply is equal to the sum of the load current and the charging current of the energy storage device. The CA line voltage of the common power supply has a voltage drop fault at t=0.4s, as shown in Figure (a). 4.2ms after the fault occurs, the system detects the common power supply fault and stops the common power supply thyristor drive signal. 10ms after the fault occurs, it is detected that the common power supply thyristor is completely disconnected, and the system switches to the energy storage device alone.

[0096] After the common power supply is disconnected, the output current of the energy storage device increases to be equal to the load current, and the load is completely powered by the energy storage device. After the common power supply fails, the DC side current of the energy storage device increases from nearly 0A to 9A. The lithium battery bears the low-frequency part of the output power, and its output current harmonics are small. The supercapacitor bears the high-frequency part of the output power. At the moment when the power of the energy storage device suddenly increases, the output current of the supercapacitor has a spike. After entering the steady state, the supercapacitor outputs harmonic current. During the switching process, the DC voltage of the energy storage device basically does not change.

[0097] When the dual power supply system with two-level emergency power supplies, energy storage and diesel generator, is working normally, the system works in the common power supply-energy storage parallel working state, and the energy storage device works in the grid-connected mode. After the common power supply fails, the common power switch is disconnected. In order to shorten the power-off time of the load, the energy storage device switches to the isolated grid mode after the common power switch is disconnected, and the energy storage device supplies power to the load alone. Due to the limited capacity of the energy storage device, if the backup power supply is normal, the energy storage device needs to be connected in parallel with the backup power supply as soon as possible. If the backup power supply is directly connected in parallel with the energy storage device at this time, a large impact current will appear due to the difference in the voltage amplitude and phase angle of the backup power supply and the energy storage device. In order to reduce the impact when the backup power supply is put into use, the energy storage device adjusts the phase and amplitude of the output voltage to be consistent with the backup power supply, and then closes the backup power switch, the energy storage device switches to the grid-connected mode, and the system enters the backup power supply-energy storage parallel working state.

[0098] The voltage of commonly used power supplies is shown in the following formula:

[0099]

[0100] The voltage of the backup power supply is shown as follows:

[0101]

[0102] After the common power supply fails and is disconnected, the energy storage device enters a separate power supply state. In order to reduce the impact on the load after the common power supply is disconnected, the amplitude and phase angle of the output voltage of the energy storage device are consistent with the common power supply, so the following formula holds:

[0103]

[0104] At this time, there is a certain amplitude and phase difference between the output voltage of the energy storage device and the backup power supply voltage. If the energy storage device is directly connected in parallel with the backup power supply, a large impact current will appear. Fig.10 As shown. Among them, (a) is the waveform of the common power supply voltage, (b) is the waveform of the backup power supply voltage, (c) is the waveform of the load side voltage, (d) is the waveform of the load side current, (e) is the waveform of the energy storage device output voltage, (f) is the waveform of the energy storage device output current, (g) is the waveform of the DC side voltage of the energy storage device, (h) is the waveform of the energy storage device output voltage per unit value, and (i) is the waveform of the energy storage device output voltage phase angle.

[0105] The voltage amplitude of the backup power supply is 1.1 times that of the common power supply, and the phase angle of the backup power supply voltage lags behind the common power supply voltage by 15°. The active power of the load is 4.5kW, and the reactive power is 1kVar. The common power supply experienced a voltage drop failure at t=0.1s, and the common power switch was disconnected. Then the output current of the energy storage device increased to the load current, and the energy storage device supplied power to the load alone. In order to reduce the impact of the common power failure disconnection, the amplitude and phase angle of the output voltage of the energy storage device are consistent with those of the common power supply. At t=0.3s, the energy storage device is directly connected in parallel with the backup power supply. Due to the moment when the energy storage device is connected in parallel with the backup power supply, there is a certain amplitude difference and phase angle difference between the output voltage of the energy storage device and the voltage of the backup power supply, and the load voltage and current have a large impact, and the peak value of the impact current of the energy storage device reaches 210A.

[0106] After the energy storage device is connected in parallel with the backup power supply, it switches back to the grid-connected operation mode, and the output voltage amplitude and phase angle suddenly change to be consistent with the backup power supply at t = 0.305s. It can be seen that when the energy storage device supplies power to the load alone, if the output voltage amplitude and phase angle are not adjusted and it is directly connected in parallel with the backup power supply, it will bring a greater impact to the power supply system.

[0107] In order to eliminate the impact when the energy storage device is connected in parallel with the backup power supply, the amplitude and phase angle of the output voltage of the energy storage device need to be adjusted to be consistent with the backup power supply when the backup power supply is connected in parallel. The time when the energy storage device supplies power alone is Tcn, then the change rate of the output voltage amplitude and phase angle during the energy storage device's power supply phase is as follows:

[0108]

[0109] like Fig.11 The following is a waveform diagram of the energy storage device connected in parallel with the backup power supply after adjusting the amplitude and phase angle. (a) is the load voltage waveform, (b) is the load side current waveform, (c) is the energy storage device output voltage waveform, (d) is the energy storage device output current waveform, (e) is the energy storage device DC side voltage waveform, (f) is the energy storage device output voltage per unit value waveform, and (g) is the energy storage device output voltage phase angle waveform.

[0110] The voltage amplitude of the backup power supply is also 1.1 times that of the common power supply, and the voltage phase angle of the backup power supply is also 15° behind that of the common power supply. The common power supply also has a voltage drop fault at 0.1s. After the fault occurs, the common power supply is disconnected, the energy storage device is powered alone, and the backup power supply is connected in parallel with the energy storage device at 0.3s. During the period from 0.1s to 0.3s, the amplitude and phase angle of the output voltage of the energy storage device gradually change to be consistent with the backup power supply. The peak value of the impact current when the energy storage device is connected in parallel with the backup power supply is reduced to 20A, and the load current and voltage do not undergo a significant mutation at the time of parallel connection, reducing the impact on the load. And during the period when the energy storage device is powered alone, the change rate of the amplitude and phase angle of its output voltage is small, and the impact on the load is small. It can be seen that after the common power supply is disconnected, adjusting the amplitude and phase angle of the output voltage of the energy storage device to be consistent with the backup power supply can significantly reduce the impact when the energy storage device is connected in parallel with the backup power supply.

[0111] like Fig.12 The figure shows the power distribution diagram when the diesel generator and the energy storage device are in parallel operation. In the time period (0-t1), the system is powered by the common power supply, the energy storage device is in parallel with the common power supply, and no power is output. At t1, the common power supply fails, and the backup power supply fails, the common power switch is disconnected, and in the time period (t1-t2), the load is only powered by the energy storage device, and the diesel generator starts at no load. At t2, the diesel generator is started, and the energy storage device is in parallel with the diesel generator. In the time period (t2-t3), the load power is gradually switched from the energy storage device to the diesel generator. In the time period (t3-t4), the load power is completely borne by the diesel generator, and the energy storage device is always in parallel with the diesel generator. At t4, the load suddenly increases, and accordingly, the output power of the diesel generator increases. Since the loading process of the diesel generator is longer than that of the energy storage device, when the power of the diesel generator is not fully increased to the load power, the energy storage device supplements the output of the difference between the output power of the diesel generator and the load power, and is in a discharging state. At time t5, the load suddenly decreases and the output power of the diesel generator drops. Since it takes time for the diesel generator to reduce its load, when the power of the diesel generator has not been completely reduced to the load power, the energy storage device absorbs the excess power output of the diesel generator and is in a charging state.

[0112] When the common power supply and energy storage device are in parallel working state, if the common power supply fails, and if the backup power supply also fails, the system switches to the control strategy of the diesel generator and energy storage device in parallel. The switching strategy is the same as the aforementioned strategy of switching to the backup power supply and energy storage device in parallel.

[0113] like Fig.13 As shown, (a) is the diesel generator speed per unit waveform, (b) is the diesel generator excitation voltage per unit waveform, (c) is the diesel generator output power waveform, (d) is the energy storage device output current waveform, (e) is the energy storage device output power waveform, and (f) is the energy storage device DC side voltage waveform. At the beginning, the load power is 0, and the load side voltage amplitude and frequency are both at the rated value. At 2s, the active power of the load increases to 415kW, and the reactive power increases to 250kVar. It can be seen that the speed of the diesel generator drops from the rated speed to 0.93pu, and the corresponding load side voltage frequency drops to 0.93pu, and the load side voltage amplitude drops to 0.9pu. At this time, the voltage amplitude and frequency on the load side are lower than the rated value, and the energy storage device begins to output power. At the same time, the diesel generator begins to increase the active power and reactive power output. As the load side voltage amplitude and frequency gradually increase to the rated value, the output power of the energy storage device gradually decreases. At 7s, the voltage amplitude and frequency on the load side return to the rated value, and the output power of the energy storage device is zero. At 8s, the active power of the load drops to 207kW, and the reactive power drops to 125kVar. At this time, the speed of the diesel generator rises from the rated speed to 1.03pu, the corresponding load-side voltage frequency rises to 1.03pu, and the load-side voltage amplitude rises to 1.05pu. At this time, the load-side voltage amplitude and frequency are higher than the rated value, and the energy storage device begins to absorb power. At the same time, the diesel generator begins to reduce the active power and reactive power output. In the process of the load-side voltage amplitude and frequency gradually decreasing to the rated value, the absorbed power of the energy storage device gradually decreases. At 11s, the load-side voltage amplitude and frequency return to the rated value, and the output power of the energy storage device here returns to zero. During the load fluctuation, the DC side voltage value of the hybrid energy storage device remains basically unchanged.

[0114] In summary, the present invention proposes a power supply system containing a two-level emergency power supply of hybrid energy storage and a diesel generator, wherein the power supply system comprises: an AC bus, a system power supply, an energy storage device, a diesel generator, important loads and unimportant loads.

[0115] The system power supply includes: a normal power supply, a backup power supply, a normal power switch and a backup power switch; the normal power supply and the backup power supply are both grid power supplies, and both are connected to the AC bus through the normal power switch and the backup power switch respectively.

[0116] The energy storage device includes: a hybrid energy storage system, a three-phase inverter, a filter unit and an energy storage switch; the hybrid energy storage system is connected to the AC bus through the three-phase inverter, the filter unit and the energy storage switch in sequence.

[0117] The diesel generator includes: a diesel generator set and a diesel generator grid-connected switch; the diesel generator set is connected to the AC bus through the diesel generator grid-connected switch.

[0118] The power supply system also includes: an important load switch and an unimportant load switch; the important load is connected to the AC bus through the important load switch, and the unimportant load is connected to the AC bus through the unimportant load switch.

[0119] The power supply system has four switchable working states: normal power supply-energy storage parallel working state, backup power supply-energy storage parallel working state, energy storage power supply working state, diesel generator-energy storage parallel working state; among them, the energy storage power supply working state is a transitional state. When the fault conditions of each power supply meet the corresponding adjustment, the other three states will switch with each other. During the state switching process, there will be a certain period of time in the energy storage power supply working state, and the energy storage device provides short-term power support.

[0120] The hybrid energy storage system includes a lithium battery and a supercapacitor. The supercapacitor bears the high-frequency power part of the load, and the lithium battery bears the low-frequency power part of the load. The energy distribution strategy established by the two is:

[0121] In order to control the low-frequency part of the load power provided by the lithium battery, the DC side current is given a value i dc_ref Perform low-pass filtering to obtain the given value of the lithium battery output current As shown below:

[0122]

[0123] Among them, f L is the cutoff frequency of the low-pass filter, and s is the Laplace operator.

[0124] Supercapacitor output current setpoint is the given value of DC side current i dc_ref Subtract the lithium battery output current given value As shown below:

[0125]

[0126] In the present invention, the terms "install", "connect", "connect", "fix" and the like should be understood in a broad sense. For example, "connect" can be a fixed connection, a detachable connection, or an integral connection; "connect" can be a direct connection or an indirect connection through an intermediate medium. For those skilled in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0127] The shapes of various components in the drawings are schematic, and certain differences from their actual shapes are not excluded. The drawings are only used to illustrate the principles of the present invention and are not intended to limit the present invention.

[0128] Although the present invention is disclosed in detail with reference to the accompanying drawings, it should be understood that these descriptions are merely exemplary and are not intended to limit the application of the present invention. The scope of protection of the present invention is defined by the appended claims and may include various modifications, alterations and equivalents made to the invention without departing from the scope and spirit of the present invention.

Claims

1. A power supply system containing a hybrid energy storage and a diesel generator two-level emergency power supply, characterized in that: The power supply system includes: an AC bus, a system power supply, an energy storage device, a diesel generator, important loads and unimportant loads; The system power supply includes: normal power supply, backup power supply, normal power switch and backup power switch; the normal power supply and backup power supply are both grid power supplies, and both are connected to the AC bus through the normal power switch and the backup power switch respectively; The energy storage device includes: a hybrid energy storage system, a three-phase inverter, a filter unit and an energy storage switch; the hybrid energy storage system is connected to the AC busbar through the three-phase inverter, the filter unit and the energy storage switch in sequence; The diesel generator includes: a diesel generator set and a diesel generator grid-connected switch; the diesel generator set is connected to the AC busbar through the diesel generator grid-connected switch; The power supply system also includes: an important load switch and an unimportant load switch; the important load is connected to the AC bus through the important load switch, and the unimportant load is connected to the AC bus through the unimportant load switch; The power supply system has four switchable working states: normal power supply-energy storage parallel working state, backup power supply-energy storage parallel working state, energy storage power supply working state, diesel generator-energy storage parallel working state; the energy storage power supply working state is a transitional state. When the fault conditions of each power supply meet the corresponding conditions, the other three states will switch to each other. During the state switching process, there will be a certain period of time in the energy storage power supply working state, and the energy storage device provides short-term power support. The hybrid energy storage system includes a lithium battery and a supercapacitor. The supercapacitor bears the high-frequency power part of the load, and the lithium battery bears the low-frequency power part of the load. The energy distribution strategy established by the two is: In order to control the low-frequency part of the load power provided by the lithium battery, the DC side current is given a value i dc_ref Perform low-pass filtering to obtain the given value of the lithium battery output current As shown below: Among them, f L is the cutoff frequency of the low-pass filter, and s is the Laplace operator; Supercapacitor output current setpoint is the given value of DC side current i dc_ref Subtract the given value of lithium battery output current As shown below:

2. The power supply system comprising a hybrid energy storage and a diesel generator two-stage emergency power supply as claimed in claim 1, characterized in that: The power supply system has a control principle, including: Use grid power supply alone: ​​It is prohibited to connect the normal power supply and the backup power supply in parallel; Prioritize grid power supply: Diesel generator is used as emergency power supply when both normal power supply and backup power supply fail. If one of the normal power supply and backup power supply is normal, the diesel generator will not be turned on. Prioritize the use of common power supply: give priority to the use of common power supply among the two power grids; The energy storage power supply system works all the time: The energy storage power supply system is always in working condition, which plays a role in improving power quality and providing emergency power supply during power switching intervals; Important load priority principle: When the diesel generator is providing emergency power, unimportant loads need to be disconnected to ensure the normal power supply of the diesel generator to important loads.

3. The power supply system comprising a hybrid energy storage and a diesel generator two-stage emergency power supply as claimed in claim 1, characterized in that: The logic and process of switching the common power supply-energy storage parallel working state to other working states include: When the common power supply and energy storage system are working normally, the system remains in this state by default; if the common power supply fails and the backup power supply is normal, the common power switch is disconnected and the backup power switch is closed, and the system enters the backup power supply-energy storage parallel working state; if the common power supply fails and the backup power supply fails, the common power switch is disconnected, the diesel generator is started, and the unimportant load switches are disconnected. The system first enters the energy storage power supply working state for a longer period of time. When the diesel generator voltage is started and synchronized with the energy storage output voltage, the diesel generator switch is closed, and the system enters the diesel generator-energy storage parallel working state.

4. The power supply system comprising a hybrid energy storage and a diesel generator two-stage emergency power supply as claimed in claim 1, characterized in that: The logic and process of switching the backup power supply-energy storage parallel working state to other working states include: If the normal power supply has returned to normal, disconnect the backup power switch and then close the normal power switch, and the system will return to the normal power supply-energy storage parallel working state; if the normal power supply has not returned to normal and the backup power supply has also detected a fault, disconnect the backup power switch, start the diesel generator, disconnect the unimportant load switches, and wait until the diesel generator is started and synchronized with the energy storage output voltage, the system will enter the diesel generator-energy storage parallel working state.

5. The power supply system comprising a hybrid energy storage and a diesel generator two-stage emergency power supply as claimed in claim 1, characterized in that: The logic and process of switching the diesel generator-energy storage parallel working state to other working states include: If the normal power supply returns to normal, the diesel generator switch is disconnected, the normal power switch is closed, and the normal power supply-energy storage parallel working state is entered; if the normal power supply does not return to normal, but the backup power supply returns to normal, the diesel generator switch is disconnected, the backup power switch is closed, and the backup power supply-energy storage parallel working state is entered.

6. The power supply system comprising a hybrid energy storage and a diesel generator two-stage emergency power supply as claimed in claim 1, characterized in that: The three-phase inverter in the energy storage device has multiple control modes to adapt to different working conditions. The control modes of the three-phase inverter include: PQ control mode: When the hybrid energy storage device is connected in parallel with the grid or diesel generator, the three-phase inverter adopts the PQ control strategy by default. In the PQ control mode, the inverter controls the output current according to the reference value of active power P and the reference value of reactive power Q, where the reactive power set value Q is set to 0 to ensure that the energy storage device only provides active power to the system; when the energy storage device needs to be charged, the reference value of P is set to the charging power of the energy storage device, and the inverter is controlled according to the charging power; Dynamic load balancing mode: When the energy storage device is connected in parallel with the diesel generator and does not need to be charged, the energy storage device is mainly used to smooth the load power fluctuation. At this time, the given value of active power P is calculated through the proportional link according to the difference between the AC side voltage and the rated voltage and the difference between the AC side frequency and the rated frequency. In this way, when the voltage or frequency of the power grid or generator deviates from the rated value, the energy storage device can supplement power in time to balance the system load; VF control mode: When the energy storage device operates independently, the inverter adopts the VF control mode to maintain the voltage and frequency stability of the system. In the VF mode, the AC side frequency feedback controls the active power output of the inverter, and the AC side voltage feedback controls the reactive power output, ensuring the constancy of the system frequency and voltage in the isolated grid state.

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