Virtual infinite capacitor for suppressing direct current voltage ripples of single-phase photovoltaic inverter grid-connected system

Abstract

The invention discloses a virtual infinite capacitor for suppressing direct current voltage ripples of a single-phase photovoltaic inverter grid-connected system. The virtual infinite capacitor is a bidirectional DC-DC circuit composed of a capacitor C, switch tubes Sa and Sb, freewheeling diodes Da and Db, an inductor L and a capacitor Cm. The virtual infinite capacitor is bridged on a direct-current bus of the single-phase photovoltaic inverter grid-connected system, fluctuation of charges Q in the virtual infinite capacitor is controlled through a high-frequency switch of a switching tube in the bidirectional DC-DC circuit, Q belongs to [Qmax, Qmin], charge fluctuation can be transmitted to a rear-end capacitor Cm through the DC-DC circuit, and a front-end voltage is kept at Vref. Through the effect of the virtual infinite capacitance circuit, the purpose of reducing inverter direct current bus frequency doubling ripple and grid-connected current third harmonic components is achieved, the virtual infinite capacitance uses a small capacitance circuit to replace a large direct current filtering electrolytic capacitor, the size of a direct current bus filtering capacitor is reduced, and the size of the direct current bus filtering capacitor is reduced. The problems that an inverter is large in size and low in power density are solved.

Description

Technical Field

[0001] The present invention relates to the field of power electronics technology, and in particular to the field of some direct current voltage ripple eliminators and single-phase photovoltaic grid-connected inverters. Background Art

[0002] With the rise of distributed power generation systems, power electronics technology has been widely used in distributed power generation technology, and power electronic conversion devices have also been continuously developing towards high power density, high frequency, large capacity, and integration. Capacitors play a vital role in reducing DC ripple and AC filtering, and are indispensable components in various converters. However, the emergence of large capacitors will increase the size of the converter, which is not conducive to the integration of the converter and reduces its power density, which runs counter to the development direction of today's converters. Secondly, although electrolytic capacitors have high energy density, their limited lifespan limits their application. Therefore, while ensuring the same effect of capacitors in the converter, improving the power density of the converter is one of the problems that need to be solved in today's power electronics technology research.

[0003] The single-phase grid-connected inverter is a converter used in small distributed power generation systems for grid-connected power generation. Its DC bus voltage has a large amount of double power frequency ripple. This secondary ripple causes the inverter AC output to contain a large amount of third harmonics, which seriously affects the inverter power quality. The method to reduce voltage ripple is often to use large-capacity capacitors for energy buffering, but compared with large-capacity capacitors, its buffering energy is much smaller than the average storage energy, so most of the energy storage of large capacitors is in an idle state. In order to reduce the DC bus voltage ripple and the DC bus capacitance value, researchers have proposed many improvement schemes in recent years, and proposed some hardware structures to replace traditional capacitors, but while suppressing the voltage ripple, the capacitance of the bus capacitor has not been reduced, or the bus capacitor capacitance has been reduced, the ripple suppression effect is not obvious, or a more complex control circuit is required, which increases the circuit cost. Summary of the invention

[0004] In view of the above problems, in order to suppress the DC bus voltage ripple of the inverter, reduce the inverter volume and improve the inverter power density, the present invention proposes a virtual infinite capacitor (VIC) for suppressing DC voltage ripple in a single-phase photovoltaic inverter grid-connected system.

[0005] The technical solution of the present invention to solve the above problem is: the DC side of the single-phase inverter is connected to the photovoltaic panel PV, and the DC bus is connected in series with the anti-backflow diode VD. The ordinary capacitor connected across the DC bus is replaced with a virtual infinite capacitor, and the topological structure of the virtual infinite capacitor is a non-isolated bidirectional DC-DC converter. The front end of the bidirectional DC-DC converter is connected to the DC bus of the inverter, and the front and rear ends share a negative electrode. The capacitor C is connected across the front end of the bidirectional DC-DC converter, and the two switch tubes S a , S b Connected in series with the front-end interface to form a loop, the freewheeling diode D a , D b Respectively with the switch tube S a , S b The back end of the bidirectional DC-DC converter is connected in parallel with the switch tube S a , S b Lead out the positive pole in the middle, and connect inductor L and capacitor C in series m The positive and negative electrodes at the rear end are connected to form a complete virtual infinite capacitor topology. 1 ,S 2 ,S 3 ,S 4 Divide into two branches and connect them in parallel on the DC bus, where S 1 ,S 3 To be connected in series as a branch, S 2 ,S 4 The AC output is connected in series to the S composed of switch tubes. 1 ,S 2 ,S 3 ,S 4 In the inverter bridge, the grid power supply is connected through the LCL filter, where the LCL filter is a T-type structure with two inductors L 1 ,L 2 Series connection, inductor C f It is connected in parallel with the grid power supply between the two inductors.

[0006] The technical effect of the present invention is that a virtual infinite capacitor circuit is connected across the DC bus to replace the original ordinary capacitor, and the DC voltage ripple is transferred to the back end of the virtual infinite capacitor by high-frequency switching of the virtual infinite capacitor circuit, thereby achieving the purpose of reducing the double frequency ripple of the inverter DC bus and the third harmonic component of the grid-connected current. At the same time, a small capacitor circuit is used to replace a large DC filter electrolytic capacitor, which can reduce the volume of the DC bus filter capacitor and solve the problem of large inverter volume and low power density. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The present invention will be further described below in conjunction with the accompanying drawings.

[0008] Figure 1A schematic diagram of the circuit connection of a virtual infinite capacitor for suppressing DC voltage ripple in a single-phase photovoltaic inverter grid-connected system provided by the present invention.

[0009] Figure 2 This is the equivalent circuit diagram of a virtual infinite capacitor used to suppress DC voltage ripple in a single-phase photovoltaic inverter grid-connected system.

[0010] Figure 3 This is a working characteristic diagram of the virtual infinite capacitor of the present invention.

[0011] Figure 4 This is a waveform diagram of the inductor current in the working mode of the virtual infinite capacitor of the present invention.

[0012] Figure 5 It is an equivalent circuit diagram of the virtual infinite capacitor working mode 1 of the present invention.

[0013] Figure 6 It is an equivalent circuit diagram of the virtual infinite capacitor working mode 2 of the present invention.

[0014] Figure 7 It is an equivalent circuit diagram of the virtual infinite capacitor working mode 3 of the present invention.

[0015] Figure 8 It is an equivalent circuit diagram of the virtual infinite capacitor working mode 4 of the present invention. DETAILED DESCRIPTION

[0016] In order to make the objectives, technical solutions and advantages of the present invention more clear, the present invention will be further described in detail below with reference to the accompanying drawings.

[0017] like Figure 1 As described above, the circuit of the virtual infinite capacitor for suppressing DC voltage ripple in the single-phase photovoltaic inverter grid-connected system consists of two parts. The single-phase photovoltaic inverter grid-connected system consists of a photovoltaic panel, a reverse current prevention diode VD, and four switch tubes S 1 , S 2 , S 3 , S 4 , LCL filter L 1 , L 2 , C f And grid power supply U g The virtual infinite capacitor consists of capacitor C, switch tube S a , S b , freewheeling diode D a , D b , inductor L and capacitor C m The virtual infinite capacitor single-phase photovoltaic inverter grid-connected system that suppresses DC voltage ripple can be equivalent to the following: Figure 2The system photovoltaic panel is equivalent to a relatively stable current source. Due to the instantaneous power of the inverter, the system bus voltage has a double frequency fluctuation. Figure 2 As shown, the virtual infinite capacitor is equivalent to a capacitor, and the inverter H bridge is simplified to a current-controlled current source, that is, the current i o Control the grid-connected current i through signal u g In order to ensure better power quality of the grid-connected power, the current i o Need to remain stable, that is, the voltage of the virtual infinite capacitor remains stable V ref .

[0018] Virtual Infinite Capacitor (VIC) is a nonlinear capacitor circuit. For ordinary capacitors, the capacitor voltage V is proportional to the charge. There is a linear relationship. If the capacitor charge Q∈[Q max ,Q min ], and the capacitor voltage is stable at V ref , that is, dV / dQ = 0. According to the definition of capacitance, it is considered that the capacitance here is infinite. The VQ relationship corresponding to the virtual infinite capacitor is as follows: Figure 3 As shown. The topology of VIC is a bidirectional DC-DC circuit, which is divided into three working stages. min ], it is the first stage. This stage is the charging mode, the VIC circuit works in the Buck state, the circuit works in the continuous conduction mode with gradually increasing duty cycle D, so that the voltage of capacitor C slowly reaches V ref , C m The voltage reaches V m,min =DV ref . V m,min is the minimum operating voltage of VIC. m V m,min When the charge Q = Q min At this time, the capacitance of the first stage of VIC is dV / dQ=C+D 2 C m . When Q∈[Q max ,Q min ](i.e. V m ∈[V m,min ,V m,max ]) is the second stage of VIC. In this region, the charge Q will change continuously, while V is constant at V ref , at this stage, dV / dQ is infinite, and VIC is like an infinite capacitor, so it can play a role in filtering or voltage regulation. The VIC circuit is in the state of alternating between Buck and Boost. max The third stage should be avoided as much as possible. In this stage, both switches are disconnected, only capacitor C is connected to the circuit, and the circuit is in an overcharged state.

[0019] VIC voltage regulation mainly works in the second area (working area), so it is necessary to control the fluctuation of charge Q through the switch tube in the bidirectional DC-DC circuit so that Q∈[Q max ,Q min ], where the switching frequency of the switch tube needs to be much higher than the frequency of charge fluctuations, and the charge fluctuations can be transmitted to the back-end capacitor C through the DC-DC circuit m , the front-end voltage V is maintained at V ref After entering the VIC working area, the downstream voltage V m Used to measure the charge value Q, that is, V m The interval [V m,min ,V m,max ] corresponds to the expected interval of Q [Q min ,Q max ], in order to keep the charge Q within the desired range, the bidirectional DC-DC circuit works in the Buck and Boost alternating working state, which has four working modes. The four working modes can be combined Figure 4 as well as Figures 5 to 8 To explain.

[0020] Mode 1[t 1 ~t 2 ](like Figure 5 As shown): at t = t 1 When the inductor current i L =0, switch tube S a On, i L Linear growth, when t = t 2 When the inductor current i L =i max , the front-end capacitor C gives the inductor L and the back-end capacitor C m Charging, the current flowing through the inductor in the circuit is:

[0021]

[0022] ( Figure 5 The red arrow in the figure is the actual direction of the inductor current, the same below).

[0023] Mode 2[t 2 ~t 3 ](like Figure 6 As shown): at t = t 2 When the switch tube S a Turn off, diode D b Freewheeling, inductor current i L from i max The inductor L releases energy to the back-end capacitor C. m Charging, the current flowing through the inductor in the circuit is:

[0024]

[0025] Mode 3[t 3 ~t 4 ](like Figure 7 As shown): at t = t 3 When the inductor current decreases to 0, the back-end capacitor C m The stored energy reaches the maximum value, and the switch tube S b Turn on, capacitor C m The discharge starts to charge the inductor L, the inductor starts to store energy, and the inductor current starts to increase in the reverse direction. The expression of the current flowing through the inductor in the circuit is the same as equation (2).

[0026] Mode 3[t 4 ~t 5 ](like Figure 8 As shown): at t = t 4 When the inductor current reaches the maximum reverse value, the energy storage value reaches the maximum At this time S b Shutdown, D a Start to freewheel, capacitor C m The inductor current i L From the maximum reverse current value to 0, the current flowing through the inductor in the circuit is the same as formula (1). 5 When the inductor current i L =0, the next cycle begins, and the circuit begins to repeat the above four modes.

Claims

1. A virtual infinite capacitor for suppressing DC voltage ripple in a single-phase photovoltaic inverter grid-connected system is characterized by: The virtual infinite capacitor includes a bidirectional DC-DC circuit and its application circuit. The bidirectional DC-DC circuit consists of a capacitor C, a switch tube S a , S b , freewheeling diode D a , D b , inductor L and capacitor C m The application circuit is a single-phase photovoltaic inverter grid-connected circuit, which consists of a single-phase photovoltaic inverter grid-connected system consisting of a photovoltaic panel, a reverse current protection diode VD, four switch tubes S1, S2, S 3、 S4, LCL filter L1, L2, C f And grid power supply U g The front end of the virtual infinity circuit is connected across the DC bus of the single-phase photovoltaic inverter grid-connected circuit, which can reduce the DC voltage ripple of the single-phase photovoltaic inverter grid-connected system.

2. The virtual infinite capacitor as claimed in claim 1 realizes the rear end voltage V by high frequency switching of the switch tube in the bidirectional DC-DC circuit. m The control of the charge Q, thus controlling the fluctuation of the charge Q, makes Q∈[Q max ,Q min ], the charge fluctuation can be transferred to the back-end capacitor C through the DC-DC circuit m , and the front-end voltage is kept at V ref , which solves the problem of DC voltage double frequency ripple in single-phase photovoltaic inverter grid-connected system, affecting the quality of grid-connected current and power.

3. A virtual infinite capacitor for suppressing DC voltage ripple in a single-phase photovoltaic inverter grid-connected system as claimed in claim 1, characterized in that The topology of the virtual infinite capacitor is a non-isolated bidirectional DC-DC converter. The front end of the bidirectional DC-DC converter is connected to the DC bus of the inverter, and the front and rear ends share a negative electrode. The capacitor C is connected across the front end of the bidirectional DC-DC converter, and the two switch tubes S a , S b Connected in series with the front-end interface to form a loop, the freewheeling diode D a , D b Respectively with the switch tube S a , S b The back end of the bidirectional DC-DC converter is connected in parallel with the switch tube S a , S b Lead out the positive pole in the middle, and connect inductor L and capacitor C in series m By using a bidirectional DC-DC circuit with a small capacitor to replace a large DC filter electrolytic capacitor, the size of the DC bus filter capacitor can be reduced, solving the problem of large inverter size and low power density.

Images