Distributed photovoltaic power station main converter and control method thereof

Abstract

The invention relates to a distributed photovoltaic power station main converter and a control method thereof, 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; 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. And one end of a direct-current inductor of the Buck-Boost converter is connected with the midpoint O. The T-type three-level converter adopts PI control and a carrier pulse width modulation strategy to realize alternating current side current harmonic wave minimization of the T-type three-level converter; the Buck-Boost converter adopts a PI control method to realize neutral-point potential automatic balance control. The method is small in current harmonic wave, low in cost and simple to control, and can realize quick and automatic balance of the neutral-point potential, thereby effectively coping with the influence of illumination and power grid voltage fluctuation on system control, and realizing quick and stable control of the distributed photovoltaic power station.

Description

Technical Field

[0001] The invention relates to a converter, in particular to a main converter of a distributed photovoltaic power station and a control method thereof, belonging to the technical field of photovoltaic power generation. Background Art

[0002] In order to achieve the "dual carbon" goal, more and more companies have built distributed photovoltaic power stations, among which the main inverter of distributed photovoltaic power stations is crucial. On the one hand, the DC power generated by the photovoltaic component system and the power of the energy storage device are inverted into AC power to supply power to the enterprise load. On the other hand, the city power can be rectified at night to charge the energy storage device to achieve peak load reduction. To this end, the main inverter of the distributed photovoltaic power station must have a two-way power transmission function, and at the same time must have functions such as small AC side harmonics, low power consumption, and effective response to the impact of light fluctuations to meet the requirements of high-efficiency photovoltaic power stations.

[0003] T-type three-level converters have been used in charging piles, energy storage systems, etc. due to their advantages such as small harmonics, low voltage stress of switching devices, low cost, and the ability to achieve bidirectional energy transmission.

[0004] However, the topology of the existing T-type three-level converter has the inherent defect of unbalanced midpoint potential, which will increase the harmonic content on the AC side and the voltage stress of the switching device, and even damage the switching device and breakdown the DC side capacitor, thereby increasing the complexity of the existing control method. In order to solve the inherent midpoint potential imbalance problem, the 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), etc. However, these methods have defects such as complex calculation and long time consumption, so the rapid response ability is poor, and thus they cannot effectively deal with the interference of light fluctuation (and thus power fluctuation) and grid voltage fluctuation on the stable operation of the system. Summary of the invention

[0005] The main purpose of the present invention is to: in view of the shortcomings of the above-mentioned prior art, the present invention provides a distributed photovoltaic power station main converter and a control method thereof that can automatically adjust the midpoint potential balance, which has the advantages of low harmonic content of AC side current, small loss of the converter itself, automatic balance of midpoint potential, and low cost.

[0006] In order to achieve the above purpose, the main converter of the present invention includes: a T-type three-level converter and a Buck-Boost DC converter; one end of the T-type three-level converter is connected to the power grid, and the other end is connected to the Buck-Boost DC converter; the other end of the Buck-Boost DC converter is connected to a DC bus; the DC bus is connected to a photovoltaic module system of a distributed photovoltaic power station; and the DC bus is also connected to an energy storage device.

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

[0008] The Buck-Boost DC converter includes 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 to the midpoint O of the T-type three-level converter; its function is to achieve automatic balance of midpoint potential.

[0009] The present invention provides a control method for a main converter of a distributed photovoltaic power station, comprising the following steps:

[0010] Step 1: The T-type three-level converter adopts PI control and carrier pulse width modulation strategy to minimize the harmonics of its AC side current:

[0011] 11) First, the three-phase grid voltage e is obtained through the phase-locked loop PLL a 、e b 、e c The phase angle θ is obtained, and the phase information is provided to the Park transformation and iPark transformation of the system to ensure that the three-phase modulation wave of the main converter is synchronized with the grid voltage;

[0012] 12) The points after the three-phase AC passes through the three LCL filters of the T-type three-level converter are respectively denoted as points a, b, and c, and the voltages at points a, b, and c are respectively denoted as u a 、u b 、u c ; The current flowing through points a, b, and c is denoted as i a 、i b 、i c ;change i a 、i b 、i c After abc / dq transformation, we get i d and i q ;

[0013] 13) The DC side voltage reference value U of the T-type three-level converter is dc * Its current actual measured value U dcThe difference is passed through the PI controller, and its output is i d The reference value i d * ; Set this reference value i d * With the current actual value i d The difference is passed through the PI controller, and its output is u d The reference value u d * ;change i q The reference value i q * With the current actual value i q The difference is passed through the PI controller, and its output is u q The reference value u q * ;

[0014] 14) The reference value u obtained in step 13) is d * and u q * , after dq / abc transformation, we get u a 、u b 、u c The reference value u a * 、u b * 、u c * , the power switch tube S of the T-type three-level converter is obtained through the carrier pulse width modulation (CBPWM) module x1 ~S x4 A driving signal is used to drive the three bridge arms of the T-type three-level converter to work;

[0015] Step 2: The Buck-Boost DC converter adopts the following control method to realize the automatic balance control of the midpoint potential of the T-type three-level converter:

[0016] 21) Let the reference value U2 of the voltage U2 across the lower capacitor of the T-type three-level converter be * =U dc / 2, where U dc is the DC side voltage of the T-type three-level converter; U2 * The difference between the actual value U2 and the current value U1 passes through the PI controller, and the output is the DC inductor L current I2 of the Buck-Boost DC converter. L Reference value I L * ;

[0017] 22) Will I L * With its actual value IL The difference is passed through a PI controller, and its output is compared with the sawtooth wave. After passing through a PWM module, the driving signals of the power switches VT1 and VT2 of the Buck-Boost DC converter are obtained: when the midpoint potential is higher than its reference value, current is drawn from the midpoint O to reduce the midpoint potential, and the converter works in the Boost state at this time; when the midpoint potential is lower than its reference value, current is injected into the midpoint O to increase the midpoint potential, and the converter works in the Buck state at this time, thereby achieving automatic balance of the midpoint potential.

[0018] The present invention adopts a T-type three-level converter to realize AC side current tracking and DC side voltage stabilization, and adopts a Buck-Boost DC converter to realize midpoint potential balance. Compared with the prior art, the present invention has the following beneficial effects:

[0019] 1. The present invention adopts a T-type level converter, which can effectively reduce the harmonic distortion rate of the converter AC side current and reduce the power loss of the converter itself. At the same time, the voltage stress of the power switch tube is reduced by half compared with the conventional VSC converter, which can greatly reduce the cost.

[0020] 2. Overcome the influence of midpoint potential fluctuation and oscillation on the control performance of the T-type three-level converter. The present invention adopts Buck-Boost DC converter to achieve fast automatic balancing of the midpoint potential of the T-type three-level converter, greatly simplifying the control algorithm of the T-type three-level converter and improving the main converter's ability to effectively deal with interference to the stable operation of the system caused by light fluctuation (and thus power fluctuation) and grid voltage fluctuation. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of the topological structure of the main converter of the distributed photovoltaic power station of the present invention.

[0022] Figure 2 The topological structure of a power supply system including a distributed photovoltaic power station formed by the main converter of the present invention is schematically shown.

[0023] Figure 3 This is a control block diagram of the main converter of the distributed photovoltaic power station described in the present invention.

[0024] Figure 4 It is the experimental waveform of the midpoint potential of the T-type three-level converter of the present invention.

[0025] Among them, 1-main converter; 11-T-type three-level converter; 12-Buck-Boost DC converter; 3-AC grid; 4-DC bus; 5-PV module system; 6-energy storage device DETAILED DESCRIPTION

[0026] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0027] like Figure 1 As shown, the topological structure of the main converter 1 of the distributed photovoltaic power station of the present invention includes: a T-type three-level converter 11 and a Buck-Boost DC converter 12; one end of the T-type three-level converter 11 is connected to the power grid, and the other end is connected to the Buck-Boost DC converter 12; the other end of the Buck-Boost DC converter 12 is connected to the DC bus 4; the DC bus 4 is connected to the photovoltaic module system 5 of the distributed photovoltaic power station; and the DC bus 4 is also connected to the energy storage device 6.

[0028] like Figure 1 As shown, the T-type three-level converter 11 includes three LCL filters (L x -C x -L gx , x=a,b,c), three bridge arms, two output capacitors C1 and C2; each bridge arm has four power switch tubes with anti-parallel freewheeling diodes: S x1 ~S x4 , x = a, b, c; one end of the three LCL filters is connected to the A phase, B phase, and C phase of the power grid respectively; two output capacitors C1 and C2 are connected in series, wherein the output capacitor C1 connected to the positive pole of the DC side of the T-type three-level converter 11 is called the upper capacitor, and the output capacitor C2 connected to the negative pole of the DC side of the T-type three-level converter is called the lower capacitor, and the connection point of C1 and C2 is called the midpoint O of the T-type three-level converter 11. Bidirectional switch units S of the three bridge arms x2 -S x3 The other end of (x=a,b,c) is short-circuited and connected to point O.

[0029] like Figure 1 As shown, the Buck-Boost DC converter 12 includes a DC inductor L, two power switch tubes VT1, VT2 and an output capacitor C; one end of the DC inductor L is connected to the midpoint O of the T-type three-level converter 11; the function of the Buck-Boost DC converter 12 is to extract current from the midpoint O of the T-type three-level converter 11 or inject current into the midpoint O of the T-type three-level converter 11 to achieve automatic balance of the midpoint potential.

[0030] As an example, Figure 2 As shown, the AC side of the main converter 1 is connected to the grid through a circuit breaker, and its DC side is connected to the DC bus 4. The photovoltaic module system 5 and the energy storage device 6 are both connected to the DC bus 4. During the day when there is sunshine, 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 power to the enterprise load; at night, the city power can be rectified into DC power to charge the energy storage device 6, realizing peak load shaving and valley filling.

[0031] The present invention provides a control method for a main converter of a distributed photovoltaic power station, comprising the following steps:

[0032] Step 1: The T-type three-level converter 11 adopts PI control and carrier pulse width modulation (CB-PWM) strategy to achieve harmonic minimization. The specific method is as follows:

[0033] like Figure 3 As shown, 11) first obtain the three-phase grid voltage e through the phase-locked loop PLL a 、e b 、e c The phase angle θ is calculated and the phase information is provided to the Park transformation (abc / dq transformation) and iPark transformation (dq / abc transformation) of the system to ensure the synchronization of the three-phase modulation wave of the main converter 11 and the grid voltage.

[0034] 12) The voltages at points a, b, and c in the T-type three-level converter 11 are respectively denoted as u a 、u b 、u c , the current flowing through it is recorded as i a 、i b 、i c ; will u a 、u b 、u c After abc / dq transformation, we get u d and u q ;change i a 、i b 、i c After abc / dq transformation, we get i d and i q , where i d Related to active power, i q is related to reactive power, so i q The reference value can be set to 0 to achieve unity power factor operation.

[0035] 13) The DC output voltage reference value U of the T-type three-level converter 11 is dc * Its current actual measured value U dc The difference is passed through the PI controller, and its output is i d The reference value i d * ; Set this reference value i d * With the current actual value i d The difference is passed through the PI controller, and its output is u d The reference value u d * ;change i q The reference value iq * (You can make i q * =0) and the current actual value i q The difference is passed through the PI controller, and its output is u q The reference value u q * ;

[0036] 14) The reference value u obtained above d * and u q * , the voltage u is obtained by inverse transformation of abc / dq a 、u b 、u c The reference value u a * 、u b * 、u c * , the power switch tube S of the T-type three-level converter 11 is obtained by carrier pulse width modulation (CB-PWM) x1 ~S x4 The driving signal s a 、s b 、s c , driving the three bridge arms of the T-type three-level converter 11 to work. x1 With S x3 Complementary, S x2 With S x4 Complementary, S x1 With S x4 Cannot be turned on at the same time.

[0037] 2. The Buck-Boost DC converter 12 adopts the following control method to realize the automatic balance control of the midpoint potential of the T-type three-level converter 11:

[0038] 21) Let the reference value U2 of the voltage U2 across the lower capacitor C2 of the T-type three-level converter 11 be * =U dc / 2, where U dc is the DC side voltage of the T-type three-level converter 11; U2 * The difference between the actual value U2 and the current value U1 passes through the PI controller, and the output is the DC inductor L current I2 of the Buck-Boost DC converter 12. L Reference value I L * ;

[0039] 22) Will I L * Its current actual value I LThe difference is passed through a PI controller, and its output is compared with the sawtooth wave. After passing through a PWM module, the drive signals of the power switches VT1 and VT2 of the Buck-Boost DC converter 12 are obtained: when the midpoint potential is higher than its reference value, current is drawn from the midpoint O to reduce the midpoint potential, and the converter works in the Boost state at this time; when the midpoint potential is lower than its reference value, current is injected into the midpoint O to increase the midpoint potential, and the converter works in the Buck state at this time, thereby achieving automatic balance of the midpoint potential.

[0040] Figure 4 The experimental waveform of the midpoint voltage balance is given a fixed modulation ratio for the T-type three-level converter 11, and only the Buck-Boost DC converter 12 is used to achieve the midpoint voltage balance. It can be seen from the figure that the Buck-Boost converter 12 can quickly achieve the balance control of the upper and lower capacitor voltages, and the midpoint potential oscillation is also well suppressed.

[0041] In summary, the present invention can suppress the harmonic distortion of the AC side current of the main converter, reduce power consumption, have low cost, and simple control. It can realize rapid and automatic balancing of the midpoint potential of the T-type three-level converter, thereby effectively coping with the influence of light fluctuations and grid voltage fluctuations on system control, and realizing rapid and stable control of distributed photovoltaic power stations.

Claims

1. A main converter for a distributed photovoltaic power station, characterized in that: include: T-type three-level converter, Buck-Boost DC converter; one end of the T-type three-level converter is connected to the power grid, and the other end is connected to the Buck-Boost DC converter; the other end of the Buck-Boost DC converter is connected to a DC bus; the DC bus is connected to a photovoltaic module system of a distributed photovoltaic power station; the DC bus is also connected to an energy storage device; The T-type three-level converter includes three LCL filters, three bridge arms, and 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 are respectively connected to the A phase, B phase and C phase of the power grid; the two output capacitors are connected in series, wherein the output capacitor connected to the positive pole of the DC side of the T-type three-level converter is called an upper capacitor, the output capacitor connected to the negative pole of the DC side of the T-type three-level converter is called a lower capacitor, and the connection point of the two output capacitors is called the midpoint O of the T-type three-level converter.

2. A distributed photovoltaic power station main converter according to claim 1, characterized in that: The Buck-Boost DC converter includes 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 to the midpoint O of the T-type three-level converter to achieve automatic balance of the midpoint potential of the T-type three-level converter.

3. A control method for a main converter of a distributed photovoltaic power station according to claim 2, characterized in that: The steps include: Step 1: The T-type three-level converter adopts PI control and carrier pulse width modulation strategy to minimize the current harmonics on its AC side: 11) First, the three-phase grid voltage e is obtained through the phase-locked loop PLL a 、e b 、e c The phase angle θ 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 with the grid voltage; 12) The points after the three-phase AC power passes through the three LCL filters of the T-type three-level converter are respectively denoted as points a, b, and c, and the voltages at points a, b, and c are respectively denoted as u a 、u b 、u c ; The current flowing through points a, b, and c is denoted as i a 、i b 、i c ;change i a 、i b 、i c After abc / dq transformation, we get i d and i q ; 13) The DC side voltage reference value U of the T-type three-level converter is dc * Its current actual measured value U dc The difference is passed through the PI controller, and its output is i d The reference value i d * ; Set this reference value i d * With the current actual value i d The difference is passed through the PI controller, and its output is u d The reference value u d * ;change i q The reference value i q * With the current actual value i q The difference is passed through the PI controller, and its output is u q The reference value u q * ; 14) The reference value u obtained in step 13) is d * and u q * , after dq / abc transformation, we get u a 、u b 、u c The reference value u a * 、u b * 、u c * , the power switch tube S of the T-type three-level converter is obtained through the carrier pulse width modulation module x1 ~S x4 A driving signal is used to drive the three bridge arms of the T-type three-level converter to work; Step 2: The Buck-Boost DC converter adopts the following control method to realize the automatic balance control of the midpoint potential of the T-type three-level converter: 21) Let the reference value U2 of the voltage U2 across the lower capacitor of the T-type three-level converter be * =U dc / 2, where U dc is the DC side voltage of the T-type three-level converter; U2 * The difference between the actual value U2 and the current value U1 passes through the PI controller, and the output is the DC inductor L current I2 of the Buck-Boost DC converter. L Reference value I L * ; 22) Will I L * With its actual value I L The difference is passed through a PI controller, and its output is compared with the sawtooth wave. After passing through a PWM module, the driving signals of the power switches VT1 and VT2 of the Buck-Boost DC converter are obtained: when the midpoint potential is higher than its reference value, current is drawn from the midpoint O to reduce the midpoint potential, and the converter works in the Boost state at this time; when the midpoint potential is lower than its reference value, current is injected into the midpoint O to increase the midpoint potential, and the converter works in the Buck state at this time, thereby achieving automatic balance of the midpoint potential.

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