CN103532214A - Photovoltaic power generation system structure integrating energy storage and grid-connected and off-grid power supply functions and control method - Google Patents

Abstract

The invention discloses a photovoltaic power generation system structure integrating energy storage and grid-connected and off-grid power supply functions and a control method, which can realize storage and conversion of renewable energy sources such as solar energy to electric energy and meanwhile have gird-connected and off-grid power supply functions. The structure comprises a photovoltaic component, a direct current-direct current booster circuit, a three-phase inverter circuit, an isolating transformer, a PCC (Programmable Computer Controller) grid-connected switch, a local load and the like. According to the control method, seamless switching between grid-connected/off-grid working modes of the system can be realized, and stepless regulation of output power of photovoltaic batteries and optimal management of accumulators can also be realized. Compared with the conventional photovoltaic grid-connected inverter, the photovoltaic power generation system has the prominent advantages of simple structure, rich function, high power supply reliability and the like, contributes to improving the utilization efficiency of renewable energy resources for power generation and has excellent application and generalization prospect.

Description

Photovoltaic power generation system structure and control method integrating energy storage and off-grid power supply functions

technical field

The invention relates to a system structure and control method in the technical field of photovoltaic power generation, which can realize the conversion and storage of solar energy into electric energy, and has two power supply modes of grid-connected and off-grid. the

technical background

With the gradual depletion of conventional energy, the increasingly prominent problem of environmental pollution and the advancement of global power market reforms, distributed power generation technologies based on renewable energy such as solar energy and wind energy have received more and more attention and attention. According to statistics, in 2011, the global newly installed photovoltaic capacity was about 27.79GW, compared with 17.5GW in 2010, an increase of 62%. However, my country's installed capacity of photovoltaic power generation systems in 2011 only accounted for about 7% of the world's total, which is incongruous compared with the world's largest output of domestic photovoltaic cell modules. On the one hand, this is of course related to the restrictions on domestic new energy grid-connected power generation policies, and on the other hand, it is also related to the relative backwardness of domestic core technologies. According to the 2012 global photovoltaic inverter (the core device of photovoltaic power generation system) ranking given by IMS Research, none of the top ten companies came from China. Therefore, it is of great significance and value to research and develop advanced photovoltaic power generation system structures and related control methods with independent intellectual property rights, and to master core technologies. the

At present, the structure of photovoltaic power generation system mainly has two types: grid-connected and off-grid, among which grid-connected power generation is the most important way. Due to the random and intermittent characteristics of solar and wind energy, the large-scale grid-connected power generation of such renewable energy will increase the difficulty of power dispatching and power flow control of the existing grid, and threaten the safe and stable operation of the existing grid. In addition, the country has special regulations on renewable energy grid-connected power generation. When the power system fails, the grid-connected power generation system will automatically shut down, which is not conducive to the fault removal and automatic recovery of the grid. Off-grid systems are often used in remote areas such as islands that cannot be covered by the grid to solve the problem of electricity consumption for residents or important equipment. In off-grid photovoltaic power generation systems, energy storage devices such as batteries are usually required to ensure uninterrupted power supply when there is insufficient sunlight. However, in this type of off-grid system, due to the lack of a truly continuous and controllable power source in the system, it is difficult to achieve optimal management of the storage battery. the

In fact, as people's requirements for power supply reliability are getting higher and higher, it is hoped that distributed power generation systems such as photovoltaics and wind power can not only operate on-grid, but also off-grid. Fast and smooth switching can be achieved. Moreover, the introduction of energy storage devices can improve the power fluctuation problem of renewable energy grid-connected power generation, and solve the negative impact of existing renewable energy grid-connected power generation on the power system. the

Based on the above reasons, the present invention proposes a practical photovoltaic power generation system structure and control method, which has the functions of energy storage and on/off-grid operation, and can solve the optimal control of the storage battery through reasonable and effective control, which is of great importance. research significance and engineering application value. the

Invention content:

The invention aims to provide a system solution with simple structure, flexible power supply mode and high reliability for the distributed photovoltaic power generation system. The structure of the photovoltaic power generation system is shown in Figure 1, consisting of photovoltaic modules (1, 2), direct boost circuits (3, 4), energy storage batteries (5), three-phase inverter circuits (6), It is composed of an isolation transformer (7), a PCC grid-connected switch (8) and a local load (9). It is characterized by:

1. The outputs of photovoltaic modules 1(1)…N(2) are respectively transformed by direct booster circuits I(3)…N(4) and imported into the DC bus, and then transformed by three-phase inverter circuit (6). The output three-phase AC voltage is connected to the three-phase power frequency isolation transformer (7). The primary side winding of the transformer is connected in delta, the secondary side is connected in star shape, and the neutral point is drawn out to form a three-phase four-wire power supply mode. Load power supply or connected to the grid through PCC grid-connected switch;

2. An independent device is formed in the dotted line box in Attachment 1, and there are 3+N external power connection ports in total, among which ports a ₁ ... a _N are the input ports of N groups of photovoltaic modules, and port b is the connection port of the energy storage battery , port c is the local load connection port, and port d is the grid-connected port;

3. According to the needs of system energy balance, the direct boost circuit I(3)...N(4) can work in either the maximum power tracking (MPPT) mode or the power limiting mode to realize stepless adjustment of power;

4. The inverter circuit (6) can work in two modes of P/Q or v/f as required, and free and smooth switching can be realized between the two modes. In the grid-connected mode, the PCC grid-connected switch is closed, and the inverter works in P/Q mode, that is, it can supply power to the local load through it, and can also manage the charge and discharge of the battery through it. In the off-grid mode, the PCC grid-connected switch is disconnected, and the inverter works in v/f mode, which is responsible for the control of the three-phase voltage on the AC side and supplies power to the local load;

5. In addition to realizing electrical isolation and forming a neutral point to supply power to three-phase asymmetrical loads, the isolation transformer can also adjust the DC bus voltage by adjusting the turn ratio of the primary and secondary side windings, change the number of battery packs connected in series, and improve the system Configuration flexibility;

6. The PCC grid-connected switch (8) can use a contactor or a solid-state fast switch, both of which can realize remote control to complete the parallel and de-parallel actions, and the solid-state fast switch is realized by a thyristor or IGBT, which has a higher Fast switching speed and special current limiting capability. the

7. Accompanying drawing 2 shows a specific circuit implementation of the system. In the figure, C ₁ , L ₁ , S ₁ , and D ₁ constitute the first group of direct boost circuits, and C _N , L _N , S _N , D _N constitute the Nth group of direct boost circuits, the output terminals of the N groups of direct boost current are connected in parallel with the bus capacitor and storage battery to form a DC bus, S _a1 , S _a2 , S _a3 , S _b1 , S _b2 , S _b3 and L _f , C _f constitute a three-phase inverter circuit, T _r is a three-phase isolation transformer;

In order to be able to supply power to three-phase asymmetrical loads, in addition to the three-phase three-leg inverter circuit plus isolation transformer structure shown in Figure 2, a three-phase four-leg topology can also be used, as shown in Figure 3 , S _a4 and S _b4 in the figure together constitute the fourth bridge arm, and the midpoint of the bridge arm is the neutral point of the three-phase output. After adopting the three-phase four-leg circuit, the bulky power frequency isolation transformer can be omitted, which not only reduces the cost, but also effectively improves the power density of the system. But there are also technical problems such as leakage current and complicated control.

In addition, in the photovoltaic power generation system proposed by the present invention, its energy source is not limited to photovoltaic cells, but may also be wind power generators, fuel cells, etc., or any combination thereof. The difference between different power generation units lies in the power control strategy of the DC boost circuit. the

The control strategy algorithm flow chart of the photovoltaic power generation system proposed by the present invention is shown in Figure 4. According to whether the external power grid is normal, there are two working modes: grid-connected and off-grid. It is characterized by:

1. The control system detects the faults of the power grid in real time. When the power grid is normal, it works in the grid-connected mode and the PCC switch is closed; otherwise, when the power grid fails, the PCC switch is quickly disconnected, and the system works in the off-grid mode;

2. In the grid-connected mode, all DC boost circuits operate in MPPT mode, and the inverter circuit operates in P/Q mode (the corresponding control block diagram is shown in Figure 5), and its power is given by the charging of the battery. Discharge management requirements are given: in the constant current charging stage, the grid-connected power of the inverter circuit is adjusted through the charging current ( _ic ) closed-loop control; in the constant voltage charging stage, the DC bus voltage (U _DC ) closed-loop control is used to determine Grid-connected power of the inverter circuit;

3. In off-grid mode, the inverter circuit works in v/f mode (its control block diagram is shown in Figure 6), providing stable voltage for local loads, and the output power is determined by the local loads. The working mode of the direct boost circuit depends on the working state of the system, that is, it can be operated with MPPT or with limited power. When the terminal voltage or charging current of the battery does not reach the upper limit, the DC boost circuit operates in MPPT; otherwise, when the battery terminal voltage or charging current reaches a certain upper limit, the DC boost circuit automatically exits the MPPT mode and enters the limited power mode . In addition, when the battery power is lower than a certain value (such as 5%), the system will stop after running for a certain period of time. the

Beneficial effect:

Compared with the existing conventional photovoltaic grid-connected power generation system, the photovoltaic power generation system proposed by the present invention realizes some problems that the conventional photovoltaic power generation system does not have through the improvement and innovation of the control strategy while maintaining a small increase in the number of hardware. Functions and Features:

1. It has two working modes: independent and grid-connected. When the grid fails, the system can still use new energy to generate power and supply power to local loads, which can effectively improve the reliability of power supply and the utilization rate of new energy;

2. The system can realize seamless switching between independent and grid-connected working modes, ensuring uninterrupted power supply of important local loads;

3. The energy storage link is introduced. In the grid-connected operation mode, it can also stabilize the power fluctuation of photovoltaics when necessary, and reduce the negative impact of distributed power generation on the grid;

4. The DC boost circuit has two working modes: MPPT and power limiting. Even in the off-grid mode, the battery can get better charge and discharge management, which helps to prolong the service life of the battery. the

Description of drawings

Accompanying drawing 1 is the overall structural block diagram of the photovoltaic power generation system that the present invention proposes;

Accompanying drawing 2 is the specific realization circuit that adopts isolation transformer and three bridge arm inverter circuit;

Accompanying drawing 3 is the specific realization circuit that adopts four-bridge arm inverter circuit;

Accompanying drawing 4 is the integrated control strategy flowchart of photovoltaic power generation system;

Accompanying drawing 5 is the specific control realization block diagram of the system under grid-connected mode;

Accompanying drawing 6 is the specific control realization block diagram of the system in off-grid mode. the

Accompanying drawing 7 is the system structural block diagram when single photovoltaic input;

Figure 8 shows the key simulation waveforms for single photovoltaic input. the

The symbols in accompanying drawing 1-accompanying drawing 6 are defined as shown in Table 1

Table 1 Description of the accompanying symbols

name illustrate name illustrate PV Photovoltaic modules U _DC DC bus feedback voltage C ₁ , C _N DC boost circuit input capacitance U _DCref DC bus voltage reference given L ₁ , L _N DC boost circuit input inductance i _C Battery charge and discharge current S ₁ , S _N Direct boost circuit power switch tube i _Cref Battery charge and discharge current given D ₁ , D _N DC boost circuit freewheeling diode i _d , i _q AC and direct axis feedback current C _B DC bus capacitor i _dref, i _qref AC and D axis current given bat Battery pack u _d , u _q AC and direct axis feedback voltage S _a1 ... S _a4 Inverter circuit bridge arm upper tube U _dref, u _qref AC and direct axis voltage given S _b1 ... S _b4 Inverter circuit bridge arm lower tube P.I. closed loop regulator L _f Inverter circuit output filter inductor S _w1 , S _wN Control circuit selection switch _f Inverter circuit output filter capacitor P&O Perturbation and Observation MPPT T _r isolation transformer driver Drive circuit

Detailed ways

The technical implementation of the present invention will be further explained below in conjunction with the accompanying drawings, but the following content is not intended to limit the protection scope of the present invention:

Accompanying drawing 1 is the structural diagram of the photovoltaic power generation system proposed by the present invention, which is composed of photovoltaic modules, N groups of direct boost circuits, storage batteries, three-phase three-leg inverter circuits, isolation transformers and PCC grid-connected switches and other components. Their specific connection relationship is as follows: the input terminals (a ₁ -a _N ) of N groups of direct booster circuits are connected to respective photovoltaic modules, and their outputs are connected together and connected in parallel with the storage battery to form a DC bus. The busbar is connected to the input of the inverter circuit, and the three-phase output of the inverter circuit is connected to the primary side of the power frequency isolation transformer, and the primary side winding adopts a delta connection mode, and the secondary side of the transformer adopts a star connection mode, and the neutral Point leads to provide single-phase and three-phase power supply capabilities for local loads. At the same time, the secondary side of the transformer is connected to the grid through the PCC switch. When the power grid is normal, the PCC switch is closed, and the system runs in parallel to the grid, which can transmit power to the grid or absorb power from the grid; when the power grid fails, the PCC switch is disconnected, and the system automatically switches to the off-grid mode. The batteries together provide electrical energy for local loads.

The part shown in the dotted line box in Figure 1 constitutes an independent device, with a total of N+3 input and output ports: a ₁ ~ a _N are the input ports of the photovoltaic module, and the energy of this port flows in one direction, and flows into the system from the photovoltaic module; b is the connection port of the energy storage battery, and the energy of this port allows two-way flow, which can charge and discharge the battery; c is the connection port of the local load, and the energy flows from the system to the load; d is the grid-connected port, and the energy of this port can also be Bi-directional flow, the system can transmit energy to the grid and absorb energy from the grid.

Accompanying drawing 2 is the specific implementation circuit of accompanying drawing 1, in the figure C ₁ , L ₁ , S ₁ , D ₁ constitute the I-th direct boost circuit, C _N , L _N , S _N , DN constitute the N-th group Direct boost circuit, S _a1 , S _a2 , S _a3 , S _b1 , S _b2 , S _b3 and L _f , C _f form a three-phase inverter circuit, and T _r is a power frequency isolation transformer. By controlling the power switches S ₁ ...S _N of the direct boost circuit, the maximum power output of the photovoltaic cell module can be realized, and any power output lower than the maximum power can also be realized. For the three-phase inverter circuit, it is divided into two modes: grid-connected and off-grid. When off-grid, it works in v/f mode for the purpose of stabilizing the output AC voltage; when grid-connected, it works in P/Q mode to optimize storage. The goal is to control the grid-connected power according to the constant current and constant voltage charging requirements. It can be seen that the photovoltaic power generation system proposed by the present invention is completely different from the conventional photovoltaic grid-connected inverter in terms of control objectives. The conventional inverter aims to stabilize the bus voltage, and its steady-state grid-connected power is equal to the power generated by photovoltaic modules.

When the three-phase inverter circuit adopts a four-leg topology, the output of the fourth bridge arm is the neutral point, and the inverter itself has the ability to supply single-phase or three-phase unbalanced power supply, so the power frequency isolation transformer can be omitted , suitable for occasions without isolation requirements. Attached Figure 3 shows a specific implementation circuit when using a three-phase four-arm inverter topology. the

Figure 4 shows the system integrated control method proposed by the present invention. When the power grid is normal, the PCC switch is closed and the system is connected to the grid. At this time, the direct boost circuit operates in MPPT mode, and the photovoltaic modules output at maximum power. In grid-connected mode, the main inverter works in P/Q mode, and the active power is determined by the charging and discharging requirements of the battery. When the battery capacity is insufficient, the battery is charged with a constant current, that is, the current closed loop is formed with the charging current, and the output of the current loop regulator is used as the given _idref of the active power of the inverter, as shown in Figure 5. The reactive power is directly given by the system comprehensive control, usually set to zero. When the battery is charged to a certain voltage, the constant current mode is switched to the constant voltage mode, that is, the selection switch S is switched from switch point 2 to switch point 1. At this time, the active power of the inverter is given by the output of the bus voltage closed-loop regulator.

When the power grid fails, the PCC switch is disconnected, and the system switches to the off-grid operation mode. Due to the loss of the support of the power grid, the AC side voltage is controlled by the main inverter at this time, and the inverter works in v/f mode (its control block diagram is shown in Figure 6), and its output power cannot be directly controlled. Local load determination. The working mode of the direct boost circuit depends on the working state of the system and the battery. In grid-connected mode, they all operate in MPPT mode. In the off-grid mode, its working mode is different according to the difference of working conditions. When the terminal voltage or charging current of the battery does not reach the upper limit, the DC boost circuit operates in MPPT; otherwise, when the battery terminal voltage or charging current reaches a certain upper limit, the DC boost circuit automatically exits the MPPT mode and enters the limited power mode . But when the battery current is too low, the system will stop after a certain time delay. the

specific embodiment

In order to preliminarily illustrate the correctness and feasibility of the present invention, a set of direct boost circuits (as shown in Figure 7) is used as an example to simulate and verify the system, and the simulation results are shown in Figure 8. the

Before 0.2 seconds, the system was working in off-grid mode, and the photovoltaic and storage batteries jointly supplied power to the local load (5kW purely resistive). At this time, the inverter works in v/f mode to provide a stable reference voltage for the AC bus. In 0.2 seconds, the inverter completes phase pre-synchronization, the PCC grid-connected switch is closed, and the system changes from off-grid operation to grid-connected operation. Near the switching point, the working state of the battery remains unchanged. When the switching process is completed and after a period of stable operation (set to 0.1 second in this embodiment), the constant current charging starts. During constant current charging, even if the output power of photovoltaics changes, the charging current of the battery can remain unchanged, and only the grid-connected power (expressed as grid-connected current) will change. In 0.8 seconds, the system switches from the grid-connected mode to the off-grid mode again, and the battery exits constant current charging, and its charging and discharging power is determined by the instantaneous power difference between the photovoltaic and the load. It can be seen from the simulation results that the photovoltaic power generation system proposed by the present invention can work well in the on-grid/off-grid mode, and the AC bus voltage does not fluctuate significantly during the dynamic process, realizing seamless switching between the two working modes . the

Claims (5)

1. A photovoltaic power generation system structure and control method that integrates energy storage and off-grid power supply functions, consisting of photovoltaic modules (1, 2), direct boost circuits (3, 4), energy storage batteries (5), three A phase inverter circuit (6), an isolation transformer (7), a PCC grid-connected switch (8), and a local load (9), are characterized in that: The outputs of photovoltaic modules 1...N are respectively transformed by the direct boost circuit I...N, and connected in parallel with the battery to form a DC bus, and then pass through the three-phase inverter circuit to output three-phase AC voltage and connect to the three-phase power frequency isolation transformer , the primary side winding of the transformer adopts delta connection, the secondary side adopts star connection and the neutral point is led out, thus forming a three-phase four-wire power supply mode, supplying power for local loads or connecting to the power grid through the grid-connected switch at the PCC; As an independent device, there are a total of 3+N external input/output ports, namely the input ports of N groups of photovoltaic modules (a ₁ ... a _N ), the battery pack connection port (b), the local load connection port (c) and parallel net port (d). 2. The isolation transformer according to claim 1, characterized in that the primary side windings are connected in delta, the secondary side is connected in a star shape, and the turn ratios of the primary and secondary sides can be reasonably designed according to the DC bus voltage requirements, which is convenient for the battery optimized configuration. 3. The PCC grid-connected switch according to claim 1, characterized in that a contactor or a solid-state fast switch can be used, wherein the solid-state fast switch is realized by a semiconductor switching device such as a thyristor or IGBT. 4. The input power of the direct boost circuit according to claim 1 is not limited to photovoltaic modules, but also renewable energy sources such as wind power generators and fuel cells, or any combination thereof. 5. A kind of system control strategy proposed for claim 1, is characterized in that: The control system detects the faults of the power grid in real time. When the power grid is normal, it works in the grid-connected mode and the PCC switch is closed; on the contrary, when the power grid fails, the PCC switch is quickly disconnected, and the system works in the off-grid mode; In the grid-connected mode, all DC boost circuits work in MPPT mode, and the inverter circuit works in P/Q mode, and its power is given by the charge and discharge management requirements of the battery. Current closed-loop control to adjust the grid-connected power of the inverter circuit; in the constant voltage charging stage, the grid-connected power of the inverter circuit is determined by the DC bus voltage closed-loop control; In the off-grid mode, the inverter circuit works in v/f mode to provide a stable voltage for the local load, the output power is determined by the local load, and the working mode of the direct boost circuit depends on the working state of the system, which can be MPPT can also be a power limit; when the terminal voltage or charging current of the battery does not reach the upper limit, the direct boost circuit operates at MPPT; otherwise, when the battery terminal voltage or charging current reaches a certain upper limit, the direct boost circuit automatically Exit MPPT mode and enter power limited mode.

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