An off-grid solar-storage-diesel power supply system and method for avoiding frequent switching of operation modes

Abstract

The present invention discloses an off-grid photovoltaic-storage-diesel power supply system and method for avoiding frequent switching of operating modes. By improving the grid-connected mode and dispatching method of diesel generators in traditional off-grid power supply systems, it is achieved that when the diesel generators are connected or exited, all parts of the off-grid power supply system always maintain the same operating mode, avoiding the switching of bidirectional DC / AC between different operating modes, solving the problem of seamless switching of modes in microgrids, and also ensuring that the power supply system has good power quality.

Description

Technical Field

[0001] The present invention belongs to the field of microgrid operation control, and particularly relates to an off-grid photovoltaic energy storage diesel power supply system and method for avoiding frequent switching of operation modes. Background Art

[0002] In recent years, with the continuous prominence of energy and environmental problems, the utilization of distributed power sources such as photovoltaic power generation and wind power generation has become an important means of energy conservation and emission reduction. Due to the serious volatility and randomness of photovoltaic power generation, if directly connected to the large power grid, it will cause a large impact on the power grid and cause fluctuations in the grid frequency and voltage. As a power supply system integrating distributed power sources, energy storage systems, and loads, microgrids can effectively alleviate the impact of distributed power sources on the power grid and improve the grid connection rate of distributed power sources. Microgrids mainly include grid-connected microgrids and off-grid microgrids. Grid-connected microgrids need to be connected to the power grid and use the power grid to maintain the voltage and frequency of the microgrid system. The energy storage system in the system is mainly used to smooth the fluctuations of distributed power sources. Off-grid microgrids do not need to rely on the large power grid and usually use the energy storage system or diesel generator in the system to maintain the voltage and frequency of the microgrid. Compared with grid-connected microgrids, off-grid microgrids are more practical in remote areas, places with special power supply requirements, highway service stations, etc.

[0003] At present, there are many studies on off-grid photovoltaic energy storage diesel microgrids, but the operation modes of off-grid photovoltaic energy storage diesel microgrids in most studies can be attributed to one type, that is, when the battery power is normal, the energy storage system maintains the voltage and power balance of the microgrid, and the diesel generator does not operate; when the battery power is low, the diesel generator is started and put into operation, and the diesel generator is used as the main power source to maintain the voltage and frequency of the system. At this time, the battery is charged as a load. In this operation mode, the main power source of the system alternates between the energy storage system and the diesel generator, resulting in the need to switch different control strategies for the bidirectional DC / AC between the AC and DC buses in the microgrid and the PCS of the energy storage system. Since the bidirectional DC / AC and PCS belong to power electronic power converters, the switching between different control strategies will cause fluctuations in the voltage and frequency of the microgrid and affect the power quality. On the other hand, when the diesel generator is used as the main power source, the power balance in the microgrid is borne by the diesel generator. When the load suddenly increases, it is necessary to increase the output power of the diesel generator, but its speed regulation system has hysteresis, resulting in the inability to quickly increase the output power, causing the system frequency to drop; on the contrary, due to the inability of the diesel generator to timely reduce the output power, the system frequency will rise. This frequency fluctuation caused by the diesel generator becomes more obvious as the load increases.

[0004] The energy storage system has bidirectional power flow and very fast power response, which can well solve the problem of serious hysteresis of diesel generators. However, the installed capacity of the energy storage system is limited, and the time for supporting the microgrid voltage is limited. The traditional approach is to switch the main power supply to a diesel generator or the grid when the battery power is low. On the one hand, the diesel generator and the grid can be used to maintain the microgrid voltage, and on the other hand, the battery can be charged. As mentioned above, the switching of the main power supply during the switching process will cause voltage and frequency fluctuations of the microgrid and generate a large number of harmonics. Therefore, the present invention starts from two aspects: ensuring the power of the energy storage battery of the microgrid and avoiding the switching of the main power supply, and solves the complex mode switching problem of the micro-source during the operation of the traditional microgrid. Summary of the Invention

[0005] Aiming at the problem that the access and withdrawal of diesel generators in the existing off-grid microgrid require frequent replacement of the microgrid operation mode, the present invention provides an off-grid photovoltaic energy storage diesel power supply system and method that avoid frequent switching of the operation mode, reduce the fluctuations of the system frequency and voltage caused by frequent switching of the operation mode, and start from two aspects: ensuring the power of the energy storage battery of the microgrid and avoiding the switching of the main power supply, and solve the complex mode switching problem of the micro-source during the operation of the traditional microgrid.

[0006] The present invention is implemented by adopting the following technical solutions:

[0007] An off-grid photovoltaic energy storage diesel power supply system that avoids frequent switching of the operation mode includes a photovoltaic system, an energy storage system, a diesel generator, a load, and a control system;

[0008] Among them, the photovoltaic system and the energy storage system are connected to a 750V DC bus, the diesel generator is connected to a three-phase 380V AC bus, and a bidirectional DC / AC is connected between the AC and DC buses;

[0009] The load is divided into an AC load and a DC load. The AC load is connected to the AC bus, and the DC load is connected to the DC bus;

[0010] The control system is used to monitor the power of the photovoltaic system, the energy storage system, the diesel generator, and the load. When the power of the energy storage system is insufficient, it controls the diesel generator to start and operate at a specific power.

[0011] A further improvement of the present invention is that the power converter in the photovoltaic system adopts MPPT control and power limit control, the power converter PCS in the energy storage system adopts bidirectional constant voltage control, and the bidirectional DC / AC adopts constant voltage and constant frequency control on the AC side.

[0012] A further improvement of the present invention is that the diesel generator is connected to the microgrid through a diesel generator power converter.

[0013] A further improvement of the present invention lies in that the power converter of the diesel generator is a back-to-back AC / AC converter in structure. The VSG near the diesel generator side in the back-to-back AC / AC converter adopts constant voltage control, and the voltage level is 650V; the VSG near the microgrid side in the back-to-back AC / AC converter adopts PQ control.

[0014] A further improvement of the present invention lies in that when the SOC of the energy storage battery L ≤SOC(t)≤SOC H the photovoltaic system hereinafter referred to as: PV, operates in the MPPT mode, and the diesel generator does not start;

[0015] When the SOC of the energy storage battery min ≤SOC(t)≤SOC L the PV operates in the MPPT mode, obtains the output prediction of the PV for the next two hours, and compares it with the load prediction value. If the PV can meet the load within two hours, the diesel generator does not start, otherwise the diesel generator starts, and the diesel generator is connected to the grid in a soft connection manner;

[0016] When the SOC of the energy storage battery H ≤SOC(t)≤SOC max the PV enters the power limit mode, and the diesel generator does not start;

[0017] wherein, SOC(t) is the state of charge of the energy storage battery, SOC L is the lower limit of the state of charge corresponding to the maximum power discharge of the energy storage battery, SOC H is the upper limit of the state of charge corresponding to the maximum power charging of the energy storage battery, SOC min is the lower limit of the state of charge of the energy storage battery, SOC max is the upper limit of the state of charge of the energy storage battery.

[0018] A further improvement of the present invention lies in that SOC L is taken as 30%, and SOC H is taken as 80%.

[0019] A further improvement of the present invention lies in that when the diesel generator is connected to the grid in a soft connection manner, before connection, the initial reference values of the active and reactive powers of the diesel generator are set. The initial reference value of the active power is set to 10% of the rated power of the diesel generator, and the reference value of the reactive power can be set to 0; after connection, when SOC≤SOC H the diesel generator operates in parallel with the grid. During the parallel operation, when the battery charging power P b_c (t)≤0.5P bmax the reference value of the active power of the diesel generator increases by ΔP, that is, P ref (t + 1)=P ref(t) + ΔP, and the reactive power reference value is directly taken as the magnitude of the AC reactive load, i.e., Q ref (t + 1) = Q load (t); when the battery charging power P b_c (t) > 0.5P bmax , the active power reference value at the previous moment is kept unchanged, and the reactive power reference value is still taken as the magnitude of the AC reactive load, i.e., P ref (t + 1) = P ref (t), Q ref (t + 1) = Q load (t); when SOC > SOC H , the diesel generator exits in a soft - exit manner.

[0020] A further improvement of the present invention is that when the diesel generator exits softly, first, the active power reference value of the power converter of the diesel generator is gradually reduced from the current value to 0, and finally the diesel generator is shut down.

[0021] Compared with the prior art, the present invention has at least the following beneficial technical effects:

[0022] An off - grid photovoltaic - energy - storage - diesel power supply system for avoiding frequent switching of operation modes provided by the present invention regards the diesel generator as a power source by using a back - to - back PQ - controlled power converter, thus avoiding the problem of frequent switching of the operation mode between the AC and DC buses during the connection and disconnection of the diesel generator, and making the micro - grid control method simpler.

[0023] Furthermore, in the off - grid photovoltaic - energy - storage - diesel power supply system proposed by the present invention, the energy - storage battery serves as the main power source of the system throughout the operation process. Since the power response speed of the energy - storage battery is faster than that of the diesel generator, it can better ensure the power quality of the entire system.

[0024] Furthermore, the PQ - controlled power converter of the diesel generator is more convenient for controlling the output power of the diesel generator, and can realize the soft connection and soft disconnection of the diesel generator, solving the problem of voltage fluctuation that may occur during the on - load connection and disconnection of the traditional diesel generator.

[0025] A scheduling method for an off - grid photovoltaic - energy - storage - diesel power supply system for avoiding frequent switching of operation modes proposed by the present invention fully considers the charge - discharge capacity of the energy - storage battery and reserves a SOC buffer interval, i.e., [SOC min , SOC L and [SOC H , SOC max , which can not only ensure the safe and stable operation of the energy - storage battery, but also guarantee the service life of the energy - storage battery.

[0026] Further, the diesel generator scheduling method proposed by the present invention allows the load to vary within the range of [-50%P bmax , 50%P bmax during the grid-connected operation of the diesel generator. When the load varies within this range, the time delay of the communication system will not cause voltage fluctuations in the system, that is, this scheduling method ensures that the diesel generator has a similar effect to VF control when operating in PQ control.

[0027] In summary, the present invention can not only avoid the problem of seamless switching of the microgrid operation mode, but also ensure that the system has better power quality, and has broad application prospects in small power supply systems. Description of the Drawings

[0028] Figure 1 FIG. is a structural diagram of an off-grid photovoltaic energy storage diesel power supply system for avoiding frequent switching of operation modes;

[0029] Figure 2 FIG. is a structural diagram of a diesel generator power converter;

[0030] Figure 3 FIG. is a VSG control block diagram (VF control) on the diesel generator side in the diesel generator power converter;

[0031] Figure 4 FIG. is a VSG control block diagram (PQ control) on the microgrid side in the diesel generator power converter;

[0032] Figure 5 FIG. is a flow chart of a scheduling method for an off-grid photovoltaic energy storage diesel power supply system for avoiding frequent switching of operation modes;

[0033] Figure 6 FIG. is a flow chart of a diesel generator scheduling method in an off-grid photovoltaic energy storage diesel power supply system for avoiding frequent switching of operation modes;

[0034] Figure 7 FIG. is a simulation diagram of an off-grid photovoltaic energy storage diesel power supply system for avoiding frequent switching of operation modes built in an embodiment of the present invention;

[0035] Figure 8 FIG. is a simulation result diagram of the AC and DC bus voltages in an embodiment of the present invention. From top to bottom, they are the light intensity curve, the diesel generator active power reference value curve, the three-phase AC bus voltage curve, and the DC bus voltage curve;

[0036] Figure 9 FIG. is a simulation result diagram of the power in an embodiment of the present invention. From top to bottom, they are the photovoltaic power curve, the diesel generator active power reference value curve, the power curve transmitted from the DC bus to the AC bus (negative value indicates power transmitted from the AC side to the DC side), and the state of charge curve of the battery. Detailed Embodiments

[0037] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0038] Please refer to the attached Figure 1 , an off-grid photovoltaic energy storage diesel power supply system provided by the present invention to avoid frequent switching of operating modes, which includes five parts: a photovoltaic system, an energy storage system, a diesel generator, a load, and a control system. Among them, the photovoltaic system and the energy storage system are connected to a 750V DC bus, the diesel generator is connected to a three-phase 380V AC bus, and a bidirectional DC / AC is connected between the AC and DC buses. The control system is the core of this off-grid power supply system, responsible for collecting electrical quantities of each part and issuing control commands. In terms of structure, in an off-grid photovoltaic energy storage diesel power supply system that avoids frequent switching of operating modes, the diesel generator is connected to the AC bus through a back-to-back AC / AC power converter, which is different from the traditional off-grid microgrid where the diesel generator is directly connected to the AC bus; in terms of operating mode, the MPPT, PCS, and bidirectional DC / AC in an off-grid photovoltaic energy storage diesel power supply system that avoids frequent switching of operating modes adopt the same control method before and after the connection and disconnection of the diesel generator, avoiding the complex operating mode design of each power converter and the seamless switching problem between different operating modes in the traditional off-grid microgrid.

[0039] Specifically, the structural designs of the photovoltaic power converter, the energy storage PCS, and the bidirectional DC / AC are the same as those in the traditional microgrid. Among them, the photovoltaic power converter adopts a boost boost circuit, the MPPT algorithm adopts the perturbation observation method, and the photovoltaic has a limited power operation mode; the energy storage PCS adopts a buck-boost step-up and step-down circuit with bidirectional power flow, and the control method adopts constant voltage control, that is, the control method of the outer loop DC bus voltage and the inner loop inductor current is adopted, and the DC bus reference voltage is 750V. The bidirectional DC / AC adopts a three-phase bridge circuit and uses a four-quadrant-based constant voltage control (VF control) method on the AC side. Therefore, the voltage of the DC bus is maintained by the energy storage PCS, and the voltage and frequency of the AC bus are maintained by the bidirectional DC / AC. Usually, the diesel generator is not connected to the system, and the diesel generator is only put into operation when the energy storage battery has a low power level and the photovoltaic output is small. After the diesel generator is put into operation, it is controlled as a power source through a back-to-back AC / AC, only providing power without affecting the AC bus voltage. The voltage of the AC bus before and after the diesel generator is put into operation is controlled by the bidirectional DC / AC, that is, the bidirectional DC / AC does not need to change the control method and switch the operating mode.

[0040] Furthermore, the structural topology of the diesel generator power converter AC / AC is shown in the attached Figure 2. Among them, the Vabc terminal is connected to the diesel generator, and the Uabc terminal is connected to the AC bus of the microgrid. The VSC near the diesel generator uses VF control, aiming to provide a stable DC input for VSC2. VSC2 uses PQ control, aiming to control the diesel generator as a power source, avoiding the switching of the bidirectional DC / AC operation mode before and after the connection and disconnection of the diesel generator. At the same time, the soft connection and soft disconnection of the diesel generator can be realized by adjusting the magnitudes of Pref and Qref, avoiding the impact on power quality due to the sudden connection or disconnection of the diesel generator. The control schematic diagram of the diesel generator power converter is shown in Attachment Figure 3 and Attachment Figure 4 .

[0041] Specifically, for an off-grid photovoltaic energy storage diesel power supply system that avoids frequent switching of operation modes provided by the present invention, the adopted dispatching method is shown in Figure 5 , and the specific implementation is as follows::

[0042] When the SOC of the energy storage battery L ≤SOC(t)≤SOC H , the photovoltaic operates in the MPPT mode, and the diesel generator does not start. At this time, the battery power is normal, and it can be charged and discharged. Whether the power of the photovoltaic or the load changes, the battery can maintain the power balance of the system. In this operating scenario, the power distribution in the system is determined by the control methods of each module, and the control system only plays a supervisory role.

[0043] When the SOC of the energy storage battery min ≤SOC(t)≤SOC L , the photovoltaic operates in the MPPT mode, obtains the power output prediction of the photovoltaic in the next two hours, and compares it with the load prediction value. If the photovoltaic can meet the load within two hours, the diesel generator does not start; otherwise, the diesel generator starts. The diesel generator is connected to the grid in a soft connection manner, and the soft connection method is as follows:

[0044] Before connection, it is necessary to set the initial reference values of the active and reactive powers of the diesel generator. The initial reference value of the active power is small, usually set to 10% of the rated power of the diesel generator, and the reference value of the reactive power can be set to 0. After connection, when SOC≤SOC H , the diesel generator operates in parallel with the grid. During the parallel operation, the dispatching method of the diesel engine is shown in Figure 6 , when the battery charging power P b_c (t)≤0.5P bmax , the reference value of the active power of the diesel generator increases by ΔP, that is, P ref (t + 1) = P ref (t) + ΔP, and the reference value of the reactive power is directly taken as the magnitude of the AC reactive load, that is, Q ref (t + 1) = Qload (t). When the battery charging power P b_c (t) > 0.5P bmax , keep the active power reference value of the previous moment unchanged, and the reactive power reference value is still taken as the magnitude of the AC reactive load, that is, P ref (t + 1) = P ref (t), Q ref (t + 1) = Q load (t). When SOC > SOC H , the diesel generator is ready to withdraw. When withdrawing, a soft withdrawal method is also adopted, that is, the active power reference value gradually decreases from the current value to 0, avoiding sudden power reduction from bringing voltage, current fluctuations and harmonic increase to the microgrid.

[0045] When the SOC of the energy storage battery H ≤SOC(t)≤SOC max , the photovoltaic enters the power limit mode and the diesel generator does not start.

[0046] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Usually, the components of the present invention described and shown in the accompanying drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed present invention, but merely represents the selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

[0047] Finally, the off-grid power supply system proposed by the present invention was built using Matlab / Simulink. See the attached Figure 7 for the simulation diagram, and see the attached Figure 8 and the attached Figure 9 for the simulation results. During the simulation process, the light intensity was set to change in a stepwise manner, and the active power reference value of the diesel generator was increased from zero to 5000W, maintained for a period of time and then decreased from 5000W to 0 to simulate the soft access and soft withdrawal of the diesel generator. Under such settings, it can be seen from the attached Figure 8 that the AC and DC bus voltages are maintained constant, where the DC voltage is 750V and the AC bus voltage is 380V 50Hz; it can be seen from the attached Figure 9 that during the process of the gradual increase in the power of the diesel generator, the power transmitted from the DC bus to the AC load gradually decreases, and finally it becomes the power supply from the AC side to the energy storage battery, that is, the purpose of charging the battery by the diesel generator controlled by PQ is achieved.

[0048] The above content is only for explaining the technical idea of the present invention, and the protection scope of the present invention cannot be limited thereby. Any modification made on the basis of the technical solution according to the technical idea proposed by the present invention falls within the protection scope of the claims of the present invention.

Claims

1. A scheduling method for an off-grid solar-storage-diesel power supply system that avoids frequent switching of operating modes, characterized in that: The off-grid solar-storage-diesel power supply system includes a photovoltaic system, an energy storage system, a diesel generator, a load and a control system; The photovoltaic system and energy storage system are connected to a 750V DC bus, the diesel generator is connected to a three-phase 380V AC bus, and the AC and DC buses are connected via a bidirectional DC / AC. The load is divided into AC load and DC load. The AC load is connected to the AC bus, and the DC load is connected to the DC bus. The control system is used to monitor the power of the photovoltaic system, energy storage system, diesel generator and load. When the energy storage system is insufficient, the diesel generator is controlled to start and run at a specific power. The scheduling method includes: When the energy storage battery SOC L ≤SOC(t)≤SOC H When the photovoltaic system is referred to as: photovoltaic, it operates in MPPT mode and the diesel generator does not start; When the energy storage battery SOC min ≤SOC(t)≤SOC L When the PV is running in MPPT mode, the output forecast of the PV in the next two hours is obtained and compared with the load forecast value. If the PV can meet the load within two hours, the diesel generator will not be started. Otherwise, the diesel generator will be started and connected to the grid in a soft access manner. When the energy storage battery SOC H ≤SOC(t)≤SOC max When the PV enters the power limiting mode, the diesel generator does not start; Among them, SOC(t) is the state of charge of the energy storage battery, SOC L SOC is the lower limit of the charge state when the energy storage battery can be discharged at high power. H SOC is the upper limit of the state of charge when the energy storage battery can be charged at high power. min SOC is the lower limit of the state of charge of the energy storage battery. max The upper limit of the state of charge of the energy storage battery; When the diesel generator is softly connected to the grid, the initial reference values ​​of the active and reactive power of the diesel generator are set before connection. The initial reference value of active power is set to 10% of the rated power of the diesel generator, and the reference value of reactive power can be set to 0; after connection, when SOC ≤ SOC H When the diesel generator is connected to the grid, during the grid-connected operation, when the battery charging power P b_c (t)≤0.5P bmax When the active power reference value of the diesel generator increases by ΔP, that is, P ref (t+1)=P ref (t)+ΔP, and the reactive power reference value is directly taken as the size of the AC reactive load, that is, Q ref (t+1)=Q load (t); When the battery charging power P b_c (t)>0.5P bmax When the active reference value of the previous moment is kept unchanged, the reactive reference value is still taken as the size of the AC reactive load, that is, P ref (t+1)=P ref (t), Q ref (t+1)=Q load (t); when SOC>SOC H When the diesel generator is shut down, it will exit in a soft way.

2. According to claim 1, a scheduling method for an off-grid solar-storage-diesel power supply system that avoids frequent switching of operating modes is characterized in that: SOC L Take 30%, SOC H Take 80%.

3. According to claim 1, a scheduling method for an off-grid solar-storage-diesel power supply system that avoids frequent switching of operating modes is characterized in that: When the diesel generator is soft-exited, the active power reference value of the diesel generator power converter is first gradually reduced from the current value to 0, and finally the diesel generator is shut down.

4. The scheduling method of an off-grid solar-storage-diesel power supply system that avoids frequent switching of operation modes according to claim 1 is characterized in that: The power converter in the photovoltaic system adopts MPPT control and power limit control, the power converter PCS in the energy storage system adopts bidirectional constant voltage control, and the bidirectional DC / AC adopts constant voltage and frequency control on the AC side.

5. The scheduling method of an off-grid solar-storage-diesel power supply system that avoids frequent switching of operation modes according to claim 1 is characterized in that: The diesel generator and the microgrid are connected via a diesel generator power converter.

6. A scheduling method for an off-grid solar-storage-diesel power supply system that avoids frequent switching of operating modes according to claim 5, characterized in that: The diesel generator power converter is structurally a back-to-back AC / AC converter. The VSG close to the diesel generator side in the back-to-back AC / AC converter adopts constant voltage control, and the voltage level is 650V; the VSG close to the microgrid side in the back-to-back AC / AC converter adopts PQ control.

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