Off-grid type optical firewood storage power supply system and method for avoiding frequent switching of operation modesTechnical Field
The invention belongs to the field of micro-grid operation control, and particularly relates to an off-grid type optical firewood storage power supply system and method capable of avoiding frequent switching of operation modes.
Background
In recent years, along with the continuous prominence of energy and environmental problems, the utilization of distributed power sources such as photovoltaic power, wind power and the like becomes an important means for energy conservation and emission reduction. Because the photovoltaic power generation has serious volatility and randomness, if the photovoltaic power generation is directly connected to a large power grid, larger impact is caused to the power grid, and fluctuation of the frequency and the voltage of the power grid is caused. The micro-grid is used as a power supply system integrating a distributed power supply, an energy storage system and a load, so that impact of the distributed power supply on the power grid can be effectively relieved, and the grid connection rate of the distributed power supply is improved. The micro-grid mainly comprises a grid-connected micro-grid and an off-grid micro-grid, the grid-connected micro-grid needs to be connected with a power grid, the voltage and the frequency of a micro-grid system are maintained by the power grid, and an energy storage system in the system is mainly used for smoothing fluctuation of a distributed power supply. Off-grid micro-grids do not need to rely on large grids, and typically utilize energy storage systems or diesel generators in the system to maintain the voltage and frequency of the micro-grid. Compared with a grid-connected micro-grid, the off-grid micro-grid has more practicability in remote areas, special power supply requirement places, high-speed service stations and the like.
At present, a lot of researches on an off-grid type optical storage Chai Wei power grid are carried out, but most of the researches can be summarized into an operation mode of the off-grid type optical storage Chai Wei power grid, namely when the electric quantity of a battery is normal, an energy storage system maintains the voltage and power balance of a micro power grid, a diesel generator is not put into operation, when the electric quantity of the battery is low, the diesel generator is started and put into operation, and the diesel generator is used as a main power supply to maintain the voltage and frequency of the system, and at the moment, the battery is used as a load for charging. In the running mode, the main power supply of the system is alternately replaced in the energy storage system and the diesel generator, so that the bidirectional DC/AC between the AC and DC buses in the micro-grid and the PCS of the energy storage system need to be switched to different control strategies. Since the bidirectional DC/AC and PCS belong to the power electronic type power converter, switching between different control strategies can cause fluctuation of voltage and frequency of the micro-grid, and influence the quality of electric energy. On the other hand, when the diesel generator is used as a main power supply, the power balance in the micro-grid is borne by the diesel generator, when the load suddenly increases, the output power of the diesel generator needs to be increased, but a speed regulating system has hysteresis, so that the output power cannot be rapidly increased, the system frequency is reduced, and on the contrary, the system frequency is increased because the output power cannot be timely reduced by the diesel generator. This up-and-down frequency fluctuation caused by the diesel generator is more pronounced as the load increases.
The energy storage system has bidirectional flow of power, has very fast power response, and can well solve the problem of serious hysteresis of the diesel generator. However, the energy storage system has limited installation capacity and limited time for supporting the voltage of the micro-grid, and the traditional method is to switch the main power supply to a diesel generator or a power grid when the electric quantity of the battery is low, so that the voltage of the micro-grid can be maintained by using the diesel generator and the power grid on one hand, and the battery can be charged on the other hand. As mentioned above, the mains supply may cause voltage and frequency fluctuations of the micro-grid during switching, generating a large number of harmonics. Therefore, the invention starts from the two aspects of ensuring the electric quantity of the energy storage battery of the micro-grid and avoiding the switching of the main power supply, and solves the problem of complex mode switching of the micro-source during the operation of the traditional micro-grid.
Disclosure of Invention
Aiming at the problem that the operation mode of the micro-grid needs to be frequently replaced when a diesel generator in the existing off-grid micro-grid is connected and disconnected, the invention provides an off-grid optical diesel storage power supply system and method for avoiding frequent switching of the operation mode, which reduces fluctuation of system frequency and voltage caused by frequent switching of the operation mode, starts from two aspects of ensuring the electric quantity of an energy storage battery of the micro-grid and avoiding switching of a main power supply, and solves the problem of complicated mode switching of the micro-source when the traditional micro-grid operates.
The invention is realized by adopting the following technical scheme:
an off-grid type light firewood storage power supply system capable of avoiding frequent switching of operation modes comprises a photovoltaic system, an energy storage system, a diesel generator, a load and a control system;
the photovoltaic system and the energy storage system are connected to a 750V direct current bus, the diesel generator is connected to a three-phase 380V alternating current bus, and the alternating current bus and the direct current bus are connected through a bidirectional DC/AC;
the load is divided into an alternating current load and a direct current load, the alternating current load is connected to the alternating current bus, and the direct current load is connected to the direct current bus;
the control system is used for monitoring the power of the photovoltaic system, the energy storage system, the diesel generator and the load, and when the electric quantity of the energy storage system is insufficient, the diesel generator is controlled to start and operate at specific power.
The invention is further improved in that the power converter in the photovoltaic system adopts MPPT control and power limiting control, the power converter PCS in the energy storage system adopts two-way constant voltage control, and the two-way DC/AC adopts alternating current side constant voltage constant frequency control.
The invention is further improved in that the diesel generator is connected with the micro-grid through a diesel generator power converter.
The invention is further improved in that the diesel generator power converter is structurally a back-to-back AC/AC converter, the VSG, close to the diesel generator, in the back-to-back AC/AC converter is controlled by constant voltage, the voltage class is 650V, and the VSG, close to the micro-grid, in the back-to-back AC/AC converter is controlled by PQ.
The invention is further improved in that when the energy storage battery SOCL≤SOC(t)≤SOCH is adopted, the photovoltaic system is hereinafter referred to as photovoltaic, the photovoltaic system operates in an MPPT mode, and the diesel generator is not started;
When the energy storage battery is SOCmin≤SOC(t)≤SOCL, the photovoltaic is operated in an MPPT mode, output prediction of the photovoltaic for two hours in the future is obtained, the output prediction is compared with a load prediction value, if the photovoltaic can meet the load within two hours, the diesel generator is not started, otherwise, the diesel generator is started, and the diesel generator is connected in a grid mode in a soft connection mode;
When the energy storage battery SOCH≤SOC(t)≤SOCmax is in a power limiting mode, the diesel generator is not started;
The SOC (t) is a state of charge of the energy storage battery, the SOCL is a lower limit of the state of charge corresponding to the time when the energy storage battery can be discharged with high power, the SOCH is an upper limit of the state of charge corresponding to the time when the energy storage battery can be charged with high power, the SOCmin is a lower limit of the state of charge of the energy storage battery, and the SOCmax is an upper limit of the state of charge of the energy storage battery.
The invention is further improved in that SOCL is 30% and SOCH is 80%.
The invention further improves that when the diesel generator is in soft connection and grid connection, an initial reference value of active power and reactive power of the diesel generator is set before connection, the initial reference value of active power is set to 10% of rated power of the diesel generator, and the reference value of reactive power can be set to 0, when SOC is smaller than or equal to SOCH after connection, the diesel generator is in grid connection operation, during grid connection operation, when the battery is charged with power Pb_c(t)≤0.5Pbmax, the reference value of active power of the diesel generator is increased by delta P, namely Pref(t+1)＝Pref (t) +delta P, and the reference value of reactive power is directly taken as the magnitude of alternating current reactive load, namely Qref(t+1)＝Qload (t), when the battery is charged with power Pb_c(t)＞0.5Pbmax, the active reference value at the previous moment is kept unchanged, and the reference value of reactive power is still taken as the magnitude of alternating current reactive load, namely Pref(t+1)＝Pref(t)、Qref(t+1)＝Qload (t), and when SOC is larger than SOCH, the diesel generator is withdrawn in a soft withdrawal mode.
The invention is further improved in that when the diesel generator is in soft exit, the active power reference value of the power converter of the diesel generator is gradually reduced to 0 from the current value, and finally the diesel generator is closed.
Compared with the prior art, the invention has at least the following beneficial technical effects:
According to the off-grid light diesel storage power supply system capable of avoiding frequent switching of the operation modes, the diesel generator is regarded as a power source by adopting the back-to-back PQ controlled power converter, so that the problem that the bidirectional DC/AC between the AC and DC buses frequently switches the operation modes in the process of connecting and disconnecting the diesel generator is avoided, and the micro-grid control method is simpler.
Furthermore, the off-grid type optical diesel storage power supply system provided by the invention is characterized in that the energy storage battery is used as a main power supply of the system in the whole operation process, and the power response speed of the energy storage battery is faster than that of the diesel generator, so that the electric energy quality of the whole system can be ensured.
Furthermore, the PQ-controlled diesel generator power converter is more convenient for controlling the output power of the diesel generator, can realize soft access and soft exit of the diesel generator, and solves the problem of voltage fluctuation possibly occurring during load access and exit of the traditional diesel generator.
According to the scheduling method of the off-grid type optical firewood storage power supply system for avoiding frequent switching of the operation modes, which is provided by the invention, the charge and discharge capacity of the energy storage battery is fully considered, and the SOC buffer intervals, namely [ SOCmin,SOCL ] and [ SOCH,SOCmax ], are reserved, so that the safe and stable operation of the energy storage battery can be ensured, and the service life of the energy storage battery can be ensured.
Furthermore, the diesel generator scheduling method provided by the invention allows the load to change within the range of [ -50% Pbmax,50％Pbmax ] during the grid-connected operation of the diesel generator, and when the load changes within the range, the time delay of the communication system cannot cause the fluctuation of the system voltage, namely the scheduling method ensures that the diesel generator PQ control operation has similar effect as the VF control operation.
In conclusion, the invention can not only avoid the difficult problem of seamless switching of the micro-grid operation mode, but also ensure that the system has better electric energy quality, and has wide application prospect in a small power supply system.
Drawings
FIG. 1 is a block diagram of an off-grid optical diesel storage power supply system for avoiding frequent switching of operation modes;
FIG. 2 is a block diagram of a diesel generator power converter;
FIG. 3 is a diesel generator side VSG control block diagram (VF control) in a diesel generator power converter;
FIG. 4 is a micro grid side VSG control block diagram (PQ control) in a diesel generator power converter;
FIG. 5 is a flowchart of an off-grid optical diesel storage and power supply system scheduling method for avoiding frequent switching of operation modes;
FIG. 6 is a flow chart of a method for scheduling diesel generators in an off-grid optical diesel storage power supply system that avoids frequent switching of modes of operation;
fig. 7 is a simulation diagram of an off-grid optical firewood storage power supply system constructed in an embodiment of the invention, wherein the off-grid optical firewood storage power supply system is capable of avoiding frequent switching of operation modes;
FIG. 8 is a graph of simulation results of voltages of an AC/DC bus in an embodiment of the present invention, wherein the graph is a light intensity curve, a diesel generator active reference value curve, an AC bus three-phase voltage curve and a DC bus voltage curve in order from top to bottom;
Fig. 9 is a graph of power simulation results in an embodiment of the present invention, in which a photovoltaic power curve, a diesel generator active reference value curve, a power curve transmitted from a dc bus to an ac bus (a negative value indicates power transmitted from an ac side to a dc side), and a battery state of charge curve are sequentially performed from top to bottom.
Detailed Description
The technical scheme of the invention is further described in detail through the drawings and the embodiments.
Referring to fig. 1, the off-grid type optical firewood storage power supply system capable of avoiding frequent switching of operation modes provided by the invention comprises a photovoltaic system, an energy storage system, a diesel generator, a load and a control system, wherein the photovoltaic system and the energy storage system are connected to a 750V direct current bus, the diesel generator is connected to a three-phase 380V alternating current bus, and the alternating current bus and the direct current bus are connected through a bidirectional DC/AC. The control system is the core of the off-grid power supply system and is responsible for collecting the electric quantity of each part and issuing control instructions. In the operation mode, MPPT, PCS and bidirectional DC/AC in the off-grid light firewood storage power supply system for avoiding the frequent switching operation mode adopt a control mode before and after the connection and the withdrawal of the diesel generator, thereby avoiding the problems of complex operation mode design and seamless switching among different operation modes of each power converter in the traditional off-grid micro-grid.
In particular, the structural design of the photovoltaic power converter, the energy storage PCS and the bi-directional DC/AC is consistent with the structure in a traditional micro-grid. The photovoltaic power converter adopts a boost circuit, an MPPT algorithm adopts a disturbance observation method, the photovoltaic has a limited power operation mode, the energy storage PCS adopts a buck-boost circuit with power capable of flowing bidirectionally, the control method adopts constant voltage control, namely, the control method of inductance current in the voltage inner loop of an outer loop direct current bus is adopted, and the reference voltage of the direct current bus is 750V. The bidirectional DC/AC adopts a three-phase bridge circuit and uses a four-quadrant based alternating current side constant voltage control (VF control) method. Thus, the voltage of the DC bus is maintained by the energy storage PCS, and the voltage and frequency of the AC bus is maintained by the bi-directional DC/AC. Normally, the diesel generator is not connected to the system, and is only put into the diesel generator when the electric quantity of the energy storage battery is low and the photovoltaic output is small. The diesel generator is controlled to be a power source through back-to-back AC/AC after being put into operation, only power is provided, the voltage of an alternating current bus is not influenced, the voltage of the alternating current bus before and after the diesel generator is put into operation is controlled by bidirectional DC/AC, namely the bidirectional DC/AC does not need to change a control method and switch an operation mode.
Further, the structure topology of the diesel generator power converter AC/AC is shown in FIG. 2. The Vabc terminal is connected with the diesel generator, and the Uabc terminal is connected with the micro-grid alternating current bus. The VSC near the diesel generator end uses VF control in order to provide a stable dc input to VSC 2. The VSC2 adopts PQ control, aims to control the diesel generator into a power source, avoids switching of a bidirectional DC/AC running mode before and after the diesel generator is connected and withdrawn, and simultaneously can realize soft connection and soft withdrawal of the diesel generator by adjusting the sizes of Pref and Qref, thereby avoiding influencing the electric energy quality due to sudden connection or withdrawal of the diesel generator. The control schematic diagram of the diesel generator power converter is shown in fig. 3 and fig. 4.
Specifically, the off-grid type optical firewood storage power supply system capable of avoiding frequent switching of operation modes adopts a scheduling method shown in fig. 5, and the method is specifically implemented as follows:
When the energy storage battery SOCL≤SOC(t)≤SOCH is operated in MPPT mode, the diesel generator is not started. At the moment, the battery has normal electric quantity, can be charged or discharged, and can maintain the power balance of the system no matter the photovoltaic power or the load power is changed. In this operating scenario, the power allocation in the system is determined by the control method of each module, and the control system only plays a supervisory role.
When the energy storage battery is SOCmin≤SOC(t)≤SOCL, the photovoltaic is operated in an MPPT mode, output prediction of the photovoltaic for two hours in the future is obtained, the output prediction is compared with a load prediction value, if the photovoltaic can meet the load within two hours, the diesel generator is not started, and otherwise, the diesel generator is started. The diesel generator adopts a soft access mode for grid connection, and the soft access method is as follows:
Before the connection, an initial reference value of active power and reactive power of the diesel generator needs to be set. The active power initial reference value is small and can be set to 10% of the rated power of the diesel generator, and the reactive power reference value can be set to 0. After the power is connected, when the SOC is less than or equal to SOCH, the diesel generator is in grid-connected operation, and during the grid-connected operation, the dispatching method of the diesel engine is shown in fig. 6, when the battery is charged with power Pb_c(t)≤0.5Pbmax, the active power reference value of the diesel generator is increased by delta P, namely Pref(t+1)＝Pref (t) +delta P, and the reactive power reference value is directly taken as the magnitude of an alternating current reactive load, namely Qref(t+1)＝Qload (t). When the battery is charged with power Pb_c(t)＞0.5Pbmax, the active reference value at the previous moment is kept unchanged, and the reactive reference value is still taken as the magnitude of the alternating current reactive load, namely Pref(t+1)＝Pref(t)、Qref(t+1)＝Qload (t). When SOC > SOCH, the diesel generator is ready to exit. And the soft exit mode is adopted during exit, namely the active power reference value is gradually reduced to 0 from the current value, so that voltage and current fluctuation and harmonic increase caused by power sudden decrease to the micro-grid are avoided.
When the energy storage battery SOCH≤SOC(t)≤SOCmax, the photovoltaic enters a power limiting mode, and the diesel generator is not started.
For the purpose of making the objects, technical solutions and advantages of the embodiments of the present invention more apparent, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, and it is apparent that the described embodiments are some embodiments of the present invention, but not all embodiments of the present invention. The components of the embodiments of the present invention generally described and illustrated in the figures herein may be arranged and designed in a wide variety of different configurations. Thus, the following detailed description of the embodiments of the invention, as presented in the figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of selected embodiments of the invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
Finally, the Matlab/Simulink is utilized to build the off-grid power supply system provided by the invention, a simulation diagram is shown in fig. 7, and simulation results are shown in fig. 8 and fig. 9. The illumination intensity is set to be changed in a step-type manner in the simulation process, the active power reference value of the diesel generator is increased to 5000W from zero, and the active power reference value is reduced to 0 from 5000W after being maintained for a period of time, so that the soft connection and the soft disconnection of the diesel generator are simulated. Under such a setting, it can be seen from fig. 8 that the voltage of the ac/dc bus is kept constant, wherein the voltage of the dc bus is 750V, and the voltage of the ac bus is 380V50Hz, and from fig. 9 that the power transmitted from the dc bus to the ac load is gradually reduced in the process of gradually increasing the power of the diesel generator, and finally the power is supplied to the energy storage battery from the ac side, thereby achieving the purpose of charging the battery by the diesel generator controlled by the PQ.
The above is only for illustrating the technical idea of the present invention, and the protection scope of the present invention is not limited by this, and any modification made on the basis of the technical scheme according to the technical idea of the present invention falls within the protection scope of the claims of the present invention.