Distributed photovoltaic power station main converter

Abstract

The utility model relates to a distributed photovoltaic power station main converter, and belongs to the field of photovoltaic power generation. The main current transformer comprises a T-type three-level current transformer and a Buck-Boost current transformer; one end of the main converter is connected with a power grid, and the other end is connected with a photovoltaic module system and energy storage equipment of the distributed photovoltaic power station; the T-type three-level converter comprises three bridge arms, two output capacitors and the like; each bridge arm is provided with four power switch tubes; the two output capacitors are connected in series, and the connection point is called as the midpoint O of the T-type three-level converter. The Buck-Boost converter comprises a direct current inductor, two power switch tubes and the like; and one end of the direct-current inductor is connected with the midpoint O so as to realize midpoint potential automatic balance. The system is low in cost, simple in control, small in current harmonic wave, low in power consumption, and capable of realizing fast and automatic balance of neutral-point potential, thereby effectively coping with the influence of illumination and network voltage fluctuation on system control, and realizing fast and stable control of a distributed photovoltaic power station.

Description

TECHNICAL FIELD

[0001] The utility model relates to a kind of current transformer, especially a kind of distributed photovoltaic power station main current transformer, belong to photovoltaic power generation technical field. BACKGROUND

[0002] More and more enterprises build distributed photovoltaic power station, wherein distributed photovoltaic power station main current transformer is crucial. On the one hand, direct current generated by photovoltaic module system and the power of energy storage device is inverted into alternating current to supply power to enterprise load, on the other hand, power grid can be rectified to charge energy storage device at night, to realize peak clipping and valley filling. Therefore, distributed photovoltaic power station main current transformer must have bidirectional power transmission function, and must have functions such as small harmonic on AC side, low power consumption, effectively coping with the influence of light fluctuation, to meet the requirements of high efficiency photovoltaic power station.

[0003] T-type three-level current transformer has been applied to charging pile, energy storage system and other advantages such as small harmonic, low voltage stress of switching device, low cost and bidirectional energy transmission.

[0004] But the topology of existing T-type three-level current transformer has inherent defect of midpoint potential imbalance, which will increase harmonic content on AC side and voltage stress of switching device, and even damage switching device and break down DC side capacitor, thereby increasing complexity of existing control method. In order to solve the inherent midpoint potential imbalance problem, existing methods include PI control method, zero sequence voltage injection method, finite control set model predictive control method (FCS-MPC), finite time control method (FTC) and other methods. But these methods have defects such as complex operation, long time consumption, poor fast response ability, and cannot effectively cope with the disturbance of light fluctuation (and power fluctuation) and power grid voltage fluctuation to system stable operation. SUMMARY

[0005] The main purpose of the utility model is to provide a three-level current transformer capable of automatically adjusting midpoint potential balance as distributed photovoltaic power station main current transformer, which has advantages such as low harmonic content of AC side current, small current transformer self-loss, automatically balanced midpoint potential and low cost.

[0006] In order to achieve the above purpose, the main current transformer of the utility model comprises a T-type three-level current transformer and a Buck-Boost DC current transformer. One end of the T-type three-level current transformer is connected with power grid, and the other end is connected with the Buck-Boost DC current transformer. The other end of the Buck-Boost DC current transformer is connected with DC bus. The DC bus is connected with photovoltaic module system of distributed photovoltaic power station. The DC bus is also connected with energy storage device.

[0007] The T-type three-level converter comprises three LCL filters, three bridge arms, two output capacitors, each of the bridge arms has four power switch tubes with anti-parallel continuous current diodes, S x1 ~S x4 , x=a, b, c; one end of the three LCL filters is connected with A phase, B phase and C phase of the power grid respectively; the two output capacitors are connected in series, wherein the output capacitor connected with the positive pole of the DC side of the T-type three-level converter is referred to as an upper capacitor, the output capacitor connected with the negative pole of the DC side of the T-type three-level converter is referred to as a lower capacitor, and the connection point of the two output capacitors is referred to as a midpoint O of the T-type three-level converter.

[0008] The Buck-Boost DC converter comprises a DC inductor L, two power switch tubes VT1 and VT2 and an output capacitor C; one end of the DC inductor L is connected with the midpoint O of the T-type three-level converter; and the DC inductor L functions to realize automatic balancing of the midpoint potential.

[0009] The utility model discloses a T-type three-level converter is adopted to realize that the alternating current side current tracks and the direct current side voltage is stable, adopts the Buck-Boost DC converter to realize the midpoint potential balance, and compared with prior art, the utility model discloses the beneficial effects are:

[0010] 1. The utility model discloses a T-type three-level converter, can effectively reduce the converter alternating current side current harmonic distortion rate, reduces the power loss of converter itself, and the voltage stress of power switch tube is reduced by half compared with conventional VSC converter, can greatly reduce the cost.

[0011] 2. The utility model discloses a T-type three-level converter, can effectively reduce the converter alternating current side current harmonic distortion rate, reduces the power loss of converter itself, and the voltage stress of power switch tube is reduced by half compared with conventional VSC converter, can greatly reduce the cost. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 It is the topological structure schematic drawing of the utility model discloses the distributed photovoltaic power station main converter.

[0013] Figure 2 It is the power supply system topological structure schematic of containing the distributed photovoltaic power station that adopts the main converter of the utility model discloses to constitute.

[0014] Figure 3 It is the control block diagram of the utility model discloses the distributed photovoltaic power station main converter.

[0015] Figure 4 The utility model discloses an experimental waveform of T type three -level inverter midpoint potential.

[0016] Wherein, 1 - main converter;11 - T type three -level inverter;12 - Buck - Boost DC converter;3 - AC power grid;4 - DC bus;5 - photovoltaic module system;6 - energy storage device Specific embodiments

[0017] The utility model will be further explained in detail in connection with the drawings.

[0018] As Figure 1 The utility model discloses a photovoltaic power station main converter topology structure, including: T type three -level inverter 11, Buck - Boost DC converter 12, T type three -level inverter 11 one end is connected with power grid, and the other end is connected with Buck - Boost DC converter 12;Buck - Boost DC converter 12 the other end is connected with DC bus 4;DC bus 4 is connected with the photovoltaic module system 5 of distributed photovoltaic power station;DC bus 4 is also connected with energy storage device 6.

[0019] As Figure 1 The utility model discloses a photovoltaic power station main converter topology structure, including: T type three -level inverter 11, Buck - Boost DC converter 12, T type three -level inverter 11 one end is connected with power grid, and the other end is connected with Buck - Boost DC converter 12;Buck - Boost DC converter 12 the other end is connected with DC bus 4;DC bus 4 is connected with the photovoltaic module system 5 of distributed photovoltaic power station;DC bus 4 is also connected with energy storage device 6. x -C x -L gx , x=a, b, c), three bridge arms, two output capacitors C1, C2;Each bridge arm has four power switch tubes with antiparallel continuous flow diode: S x1 ~S x4 , x=a, b, c;Three LCL filters are connected with the A phase, B phase and C phase of power grid respectively;Two output capacitors C1 and C2 are connected in series, wherein the output capacitor C1 connected with the positive pole of the DC side of T type three -level inverter 11 is called upper capacitor, and the output capacitor C2 connected with the negative pole of the DC side of T type three -level inverter is called lower capacitor, and the connecting point of C1 and C2 is called the midpoint O of T type three -level inverter 11.Three bridge arms are short-circuited at the other end of bidirectional switch unit S x2 ~S x3 (x=a, b, c) and connected with O point.

[0020] As Figure 1 The utility model discloses a photovoltaic power station main converter topology structure, including: T type three -level inverter 11, Buck - Boost DC converter 12, T type three -level inverter 11 one end is connected with power grid, and the other end is connected with Buck - Boost DC converter 12;Buck - Boost DC converter 12 the other end is connected with DC bus 4;DC bus 4 is connected with the photovoltaic module system 5 of distributed photovoltaic power station;DC bus 4 is also connected with energy storage device 6.

[0021] As an example, as Figure 2As shown, the photovoltaic module system 5, the energy storage device 6 and the main converter 1 are connected to the DC bus 4. During the day, the DC power generated by the photovoltaic module system 5 and the power of the energy storage device 6 are inverted into AC power to supply the enterprise load; at night, the commercial power can be rectified into DC power to charge the energy storage device 6, realizing peak load shifting.

[0022] The working process of the distributed photovoltaic power station main converter is as follows:

[0023] As shown in the figure, Figure 3 the first step, the T-type three-level converter 11 adopts PI control and carrier pulse width modulation (CB-PWM) strategy to realize harmonic minimization:

[0024] Firstly, through the phase-locked loop PLL, the phase angle θ of the three-phase grid voltage e a , e b , e c is obtained, and the phase information is provided to the Park transformation and iPark transformation of the system, to ensure the synchronization of the three-phase modulation wave of the main converter 11 and the grid voltage;

[0025] Secondly, the AC side voltage u a , u b , u c and the current i a , i b , i c of the T-type three-level converter 11 are transformed by abc / dq to obtain u d , u q and i d , i q ;

[0026] The difference between the DC output voltage reference value U dc * of the T-type three-level converter 11 and the current actual measurement value U dc is controlled by the PI controller, and the output is the reference value i d of i d * ; the difference between this reference value i d * and the current actual value i d is controlled by the PI controller, and the output is the reference value u d of u d * ; the difference between the reference value i q of i q * (which can be i q * =0) and the current actual value i q is controlled by the PI controller, and the output is the reference value u q of uq * ;

[0027] Finally, the reference value u d * and u q * , the voltage u a , u b , u c reference value u a * , u b * , u c * , carrier pulse width modulation (CB-PWM) T type three-level inverter 11 switch unit S x1 ~S x4 The driving signal of the driving T type three-level inverter 11 three bridge arm work. Note: S x1 and S x3 complementary, S x2 and S x4 complementary, S x1 and S x4 can not be turned on at the same time.

[0028] Second, the Buck-Boost DC converter 12, the midpoint potential of T type three-level inverter 11 automatic balance control:

[0029] When the midpoint potential is higher than its reference value, then make Buck-Boost DC converter 12 work in Boost state, from the midpoint O extracted current, make the midpoint potential drop; When the midpoint potential is lower than its reference value, then make Buck-Boost DC converter 12 work in Buck state, to the midpoint O injection current, make the midpoint potential rise, so as to realize the midpoint potential automatic balance.

[0030] Figure 4 For T type three-level inverter 11 given fixed modulation ratio, only with Buck-Boost DC converter 12 to realize the midpoint voltage balance experiment waveform. As can be seen from the figure, through the Buck-Boost converter 12 can realize the balance control of the upper and lower capacitor voltage, the midpoint potential oscillation is also well suppressed.

[0031] In short, the utility model discloses low in cost, control is simple, can suppress main converter ac side current harmonic distortion, has reduced power consumption, can realize T type three-level inverter midpoint potential fast automatic balance, thereby effectively cope with the influence of light and grid voltage fluctuation on system control, realizes the quick stable control of distributed photovoltaic power station.

Claims

1. A distributed photovoltaic power station master converter, characterized in that: The utility model relates to a kind of distributed photovoltaic power station, including: T-type three-level inverter, Buck-Boost DC converter;The T-type three-level inverter one end is connected with power grid, the other end is connected with the Buck-Boost DC converter;The other end of the Buck-Boost DC converter is connected with DC bus;The DC bus is connected with the photovoltaic module system of distributed photovoltaic power station;The DC bus is also connected with energy storage equipment; The T-type three-level converter comprises three LCL filters, three bridge arms, two output capacitors; each of the bridge arms has four power switch tubes: S x1 ~ S x4 , x = a, b, c; one end of the three LCL filters is connected with A phase, B phase and C phase of the power grid respectively; the two output capacitors are connected in series, wherein the output capacitor connected with the positive pole of the DC side of the T-type three-level converter is called the upper capacitor, the output capacitor connected with the negative pole of the DC side of the T-type three-level converter is called the lower capacitor, and the connection point of the two output capacitors is called the midpoint O of the T-type three-level converter.

2. The distributed photovoltaic power station master converter according to claim 1, characterized in that: The Buck-Boost DC converter includes a DC inductor L, two power switch tubes VT1, VT2 and an output capacitor C;The one end of the DC inductor L is connected with the midpoint O of the T-type three-level inverter, to realize the midpoint potential automatic balance of the T-type three-level inverter.

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