Switched capacitor type photovoltaic subcascade equalizer convenient to expand

Abstract

The invention discloses a switched capacitor type photovoltaic subcascade equalizer convenient to expand, discloses a switched capacitor type photovoltaic subcascade equalizer topology convenient to expand, and relates to the field of photovoltaic power generation. The invention provides a switched capacitor type photovoltaic subcascade equalizer convenient to expand, aiming at the problem that the generated power of a photovoltaic system is reduced due to a partial shielding phenomenon and the problems that a traditional differential power processing (DPP) equalizer based on a transformer is large in size, high in cost and poor in expansibility. And the generating capacity of the photovoltaic system under the condition of partial shielding can be effectively improved. The switched capacitor type equalizer has the advantages that the switched capacitor type equalizer is small in topological size, low in cost and simple in control method, meanwhile, one ends of all flying capacitors in the equalizer are connected together to form a star-shaped structure, equalization between any photovoltaic substrings can be achieved, and the equalization performance is remarkably improved. In addition, the design parameters of the sub-cascade equalizer of each photovoltaic panel are fixed, and the method has high expansibility.

Description

Technical Field

[0001] The invention belongs to the field of photovoltaic power generation, and in particular relates to a study of a new topology of a photovoltaic equalizer based on a switched capacitor converter. Background Art

[0002] Considering that the voltage value of a single photovoltaic module is not high, photovoltaic modules are usually connected in series to achieve the required voltage level. When a module is shaded, its corresponding bypass diode will be turned on to protect the module, which not only causes huge power loss, but also causes multiple local maximum power points to appear on the power-voltage characteristic curve of the system, bringing challenges to the maximum power point tracking (MPPT) technology of the subsequent stage.

[0003] In order to overcome the power reduction problem caused by partial shading, the academic community has proposed the concepts of full power processing (FPP) and differential power processing (DPP). Considering that the FPP balancer must process all the power of the system, the power loss will increase accordingly with the increase of the system power level, while the DPP balancer only processes a small amount of mismatched power in the system, with lower losses and costs. In recent years, it has been widely used in photovoltaic systems. Common DPP balancers can be divided into component-bus (PV-Bus), component-component (PV-PV) and component-isolated ports (PV-PVwithisolatedports).

[0004] In the component-bus structure, the balancing current flows between the PV module and the bus, such as the flyback converter and LLC topology with voltage multiplier. This topology only requires a few control switches, but the switch voltage stress is often equal to the bus voltage, and the scalability is poor. The component-isolation port structure is usually based on a bidirectional isolated DC-DC converter, which can transfer energy directly in the component without passing through the bus. However, the expansion of the component-isolation port architecture requires additional windings for each PV module, resulting in high cost and large size of the balancing system.

[0005] Taking into account scalability, volume, cost and other factors, the component-component structure will be an ideal choice for distributed photovoltaic applications, especially for photovoltaic sub-string-level balancing with a large number and low power, because it avoids the use of transformers. Common component-component structures include switched capacitors, switched inductors, buck-boost, etc. Among them, the switched capacitor type equalizer has become a hot topic in academic research due to its small size, low cost and simple control method. However, the traditional switched capacitor type equalizer can only realize energy transmission from adjacent modules to adjacent modules, and the system's balancing performance will decrease as the number of modules increases. Although some improved switched capacitor type balancing structures have been proposed in recent years to improve the balancing efficiency, they also bring problems such as too many components and high voltage stress, which poses challenges to the scalability and cost of the system. Summary of the invention

[0006] In order to solve the above problems, the present invention proposes a switch capacitor photovoltaic sub-string level balancer that is easy to expand and can be applied to photovoltaic strings of any scale. The balance of N photovoltaic panels is taken as an example. Figure 1 The architecture of the photovoltaic balancing system is given.

[0007] The photovoltaic balancing system includes N photovoltaic panels and N photovoltaic balancers, where N is an integer approximately or equal to 2; each photovoltaic panel includes M photovoltaic sub-strings, where M is an integer greater than or equal to 3, that is, the photovoltaic string contains a total of M*N photovoltaic sub-strings PV k (k=1.1~NM). Each photovoltaic balancer includes 2*M+2 switch tubes Q i.1 ~Q i.2M+2 , M flying capacitors C i.1 ~C i.M , M filter capacitors C pvi.1 ~C pvi.M and 1 capacitor C pvU.i , where i is the number of the photovoltaic panel, (i=1~N). The output side of the photovoltaic string is connected to the maximum power point tracking (MPPT) controller and the load. The MPPT controller controls the voltage of the entire photovoltaic string, and the photovoltaic balancing system realizes the voltage balancing of M*N substrings.

[0008] Each photovoltaic panel is equipped with a sub-string level balancer. The i-th photovoltaic balancer is composed of: M sub-string PV i.1 ~PV i.M Respectively with the filter capacitor C pvi.1 ~C pvi.M In parallel, they are connected in parallel with the half-bridge structure composed of two complementary switching tubes, Q i.1 ~Q i.2 , Q i.3 ~Q i.4 ...Q i.2M-1 ~Q i.2M , flying capacitor C i.1 ~C i.M One end of each photovoltaic string is connected to the midpoint of the half bridge, that is, the capacitor C i.1 One end of the switch tube Q i.1 ~Q i.2 Midpoint, capacitor C i.2 One end of the switch tube Q i.3 ~Q i.4 Midpoint...Capacitance C i.M One end of the switch tube Q i.2M-1 ~Q i.2M Midpoint, M flying capacitors C i.1 ~C i.MThe other end is connected in a star configuration to form a bus and connected to capacitor C pvU.i The corresponding half-bridge circuit Q i.2M+1 ~Q i.2M+2 midpoint, while the capacitor C pvU.i and the substring PV in the i+1th photovoltaic panel i+1.1 Connect in parallel.

[0009] The equalizer is driven by a pair of complementary signals PWM+ and PWM- with a fixed duty cycle of 50%, where PWM+ drives the even-numbered switch tube Q in the half-bridge circuit. i.2 ~Q i.2M+2 , PWM-driven odd-numbered switch tube Q i.1 ~Q i.2M+1 In one switching cycle, the equalizer has two working modes, namely mode A and mode B.

[0010] Mode A: Even switch Q i.2 ~Q i.2M+2 On, odd switch Q i.1 ~Q i.2M+1 Shut down, the PV string PV at the top of the i-th balancer i.2 ~PV i.M and capacitor C pvU.i They are connected to the corresponding star-type switched capacitor converters. During this state, the unshaded PV substrings will store the extra energy in the flying capacitors, and the flying capacitors will also transfer energy to the shaded PV substrings.

[0011] Mode B: odd number of switches Q i.1 ~Q i.2M+1 On, even switch Q i.2 ~Q i.2M+2 Shut down, the PV string PV at the lower end of the i-th balancer i.1 ~PV i.M They are connected to the corresponding star-type switched capacitor converters. During this state, the unshaded PV substrings will store the extra energy in the flying capacitors, and the flying capacitors will also transfer energy to the shaded PV substrings.

[0012] Capacitor C pvU.i PV i+1.1 The energy transfer between them always exists, regardless of the switch state. Finally, through the alternating operation of these two modes, the voltage balance of M*N photovoltaic substrings in the photovoltaic system is achieved.

[0013] Due to the star structure, all substrings in each PV balancer and C pvU.iThere is a direct energy transmission path between them. In one switching cycle, energy can be directly transmitted from an unshaded photovoltaic sub-string to a shaded photovoltaic sub-string. Compared with the disadvantage that the traditional switched capacitor type equalizer can only transmit energy adjacently, the photovoltaic equalizer designed in the present invention has better balancing performance and faster balancing speed.

[0014] The capacitor C in the i-th photovoltaic balancer pvU.i A substring PV in the adjacent i+1th photovoltaic panel i+1.1 The substrings are connected in parallel, so each substring-level balancer balances the three substrings in a photovoltaic panel and one substring in an adjacent photovoltaic panel at the same time, and the substrings of the adjacent photovoltaic panels are balanced by their corresponding photovoltaic balancers, ultimately achieving the balance of all substrings in the photovoltaic string. In other words, the capacitor C pvU.i The main function is to balance the adjacent photovoltaic panels. When the photovoltaic balancing system needs to be expanded, it only needs to stack photovoltaic panels and photovoltaic balancers. Compared with the high voltage stress problem of the device caused by the traditional photovoltaic balancer during expansion, the balancer proposed in the present invention has fixed design parameters and good scalability, and can adapt to photovoltaic systems of any scale.

[0015] In addition, C pvU.i The substrings of adjacent photovoltaic panels are connected in parallel, and the balance between them is DC coupled, so the cables that transfer energy between adjacent photovoltaic panels do not carry high-frequency AC current. This allows different photovoltaic balancers to work when the switch drive signals are not synchronized. At the same time, there is no need to have strict requirements on cable length and parasitic parameters, which greatly facilitates the installation and expansion of photovoltaic panels and photovoltaic balancers in actual photovoltaic systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 A switch capacitor photovoltaic sub-string equalizer is proposed which is easy to expand.

[0017] Figure 2 This is a typical application diagram of a photovoltaic balancing system.

[0018] Figure 3 is a diagram of two working modes of the photovoltaic balancer 1

[0019] Figure 3 (a) Mode A

[0020] Figure 3(b) Mode B DETAILED DESCRIPTION

[0021] The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0022] The present invention can be applied to a photovoltaic system with any number of photovoltaic panels by superimposing photovoltaic panels and substring-level equalizers. As a typical embodiment of the present invention, taking the balancing of 6 photovoltaic substrings of 2 photovoltaic panels as an example, Figure 2 The architecture of the photovoltaic balancing system is given, in which each photovoltaic panel is composed of three sub-strings connected in series. k Represents each PV substring, (k = 1.1 ~ 2.3). The system contains a total of 6 PV substrings PV 1.1 ~PV 2.3 , 16 switch tubes Q 1.1 ~Q 2.8 , 6 flying capacitors C 1.1 ~C 2.3 , 6 filter capacitors C pv1.1 ~C pv2.3 , 2 capacitors C pvU.i (i=1, 2), where i is the number of the photovoltaic panel. Two photovoltaic panels are connected in series, and the output side is connected to the maximum power point tracking (MPPT) controller and the load. The MPPT controller controls the voltage of the entire photovoltaic string, and the photovoltaic balancing system realizes the voltage balancing of the 6 photovoltaic sub-strings.

[0023] Each photovoltaic panel is equipped with a sub-string level balancer. The photovoltaic balancer 1 corresponding to photovoltaic panel 1 is composed of: three sub-string PV 1.1 ~PV 1.3 Respectively with the filter capacitor C pv1.1 ~C pv1.3 In parallel, they are connected in parallel with the half-bridge structure composed of two complementary switching tubes, Q 1.1 ~Q 1.2 , Q 1.3 ~Q 1.4 , Q 1.5 ~Q 1.6 , flying capacitor C 1.1 ~C 1.3 One end of each photovoltaic string is connected to the midpoint of the half bridge, that is, the capacitor C 1.1 One end of the switch tube Q 1.1 ~Q 1.2 Midpoint, capacitor C 1.2 One end of the switch tube Q 1.3 ~Q 1.4 Midpoint, capacitor C 1.3 One end of the switch tube Q 1.5 ~Q 1.6 Midpoint, three capacitors C 1.1 ~C 1.3 The other end is connected in a star configuration to form a bus and connected to capacitor C pvU.1 The corresponding half-bridge circuit Q 1.7 ~Q 1.8 Midpoint, CpvU.1 With the substring PV in PV panel 2 2.1 Connect in parallel.

[0024] The photovoltaic balancer 2 corresponding to the photovoltaic panel 2 is composed of three sub-strings PV 2.1 ~PV 2.3 Respectively with capacitor C pv2.1 ~C pv2.3 In parallel, they are connected in parallel with the half-bridge structure composed of two complementary switching tubes, Q 2.1 ~Q 2.2 , Q 2.3 ~Q 2.4 , Q 2.5 ~Q 2.6 , flying capacitor C 2.1 ~C 2.3 One end of each photovoltaic string is connected to the midpoint of the half bridge, that is, the capacitor C 2.1 One end of the switch Q 2.1 ~Q 2.2 Midpoint, capacitor C 2.2 One end of the switch tube Q 2.3 ~Q 2.4 Midpoint, capacitor C 2.3 One end of the switch tube Q 2.5 ~Q 2.6 Midpoint, three capacitors C 2.1 ~C 2.3 The other end is connected in a star configuration to form a bus and connected to capacitor C pvU.2 The corresponding half-bridge circuit Q 2.7 ~Q 2.8 midpoint.

[0025] Next, the balancing principle is explained in detail in combination with the working mode of the photovoltaic balancer 1. 2.1 Connect in parallel to C pvU.1 The two ends of the photovoltaic substring PV 2.1 It also participates in the balancing process of photovoltaic balancer 1. Assuming that the light intensity PV of the photovoltaic substring is 2.1 >PV 1.3 >PV 1.2 >PV 1.1 The photovoltaic balancer 1 contains 8 switch tubes Q 1.1 ~Q 1.8 , 3 flying capacitors C 1.1 ~C 1.3 , 3 filter capacitors C pv1.1 ~C pv1.3 , and capacitor C pvU.1 , as shown in Figure 3. 4 photovoltaic substrings PV 1.1 ~PV 2.1 Series connection, photovoltaic substring PV1.1 ~PV 1.3 There are three PV strings in the first PV panel, PV 2.1 It is the lowest photovoltaic sub-string in the second photovoltaic panel.

[0026] The equalizer is driven by a pair of complementary signals PWM+ and PWM- with a fixed duty cycle of 50%, where PWM+ drives the even-numbered switch tube Q in the half-bridge circuit. 1.2 ~Q 1.8 , PWM-driven odd-numbered switch tube Q 1.1 ~Q 1.7 In one switching cycle, the photovoltaic balancer 1 has two working modes, namely mode A and mode B.

[0027] Mode A: As shown in Figure 3(a), the even-numbered switches Q 1.2 ~Q 1.8 On, odd switch Q 1.1 ~Q 1.7 Shut down, PV string 1.2 ~PV 2.1 Connected with a star switched capacitor converter. During this state, the PV string PV 1.2 ~PV 2.1 The energy is stored in the flying capacitor, and the balancing paths are i 11A ~i 16A .

[0028] Among them, on the equilibrium path i 11A In the circuit, the capacitor C pvU.1 Through the switch Q 1.8 and Q 1.6 Flying capacitor C 1.3 Transfer energy; on the equilibrium path i 12A In the filter capacitor C pv1.3 Through the switch Q 1.6 and Q 1.4 Flying capacitor C 1.3 and C 1.2 Transfer energy; on the equilibrium path i 13A In A, the filter capacitor C pv1.2 Through the switch Q 1.4 and Q 1.2 Flying capacitor C 1.3 and C 1.1 Transfer energy; on the equilibrium path i 14A In the filter capacitor C pv1.3 and capacitor C pvU.1 Series through switch Q 1.8 and Q 1.4 Flying capacitor C 1.2 Transfer energy; on the equilibrium path i 15A In the filter capacitor Cpv1.2 and C pv1.3 Series through switch Q 1.6 and Q 1.2 Flying capacitor C 1.3 and C 1.1 Transfer energy; on the equilibrium path i 16A In the filter capacitor C pv1.2 , C pv3 and capacitor C pvU.1 Series through switch Q 1.8 and Q 1.2 Flying capacitor C 1.3 and C 1.1 Transfer energy.

[0029] Mode B: As shown in Figure 3(b), odd-numbered switches Q 1.1 ~Q 1.7 On, even switch Q 1.2 ~Q 1.8 Shutdown, star switched capacitor converter and PV substring PV 1.1 ~PV 1.3 During this state, the energy stored in the flying capacitor in mode A is transferred to the photovoltaic string PV 1.1 ~PV 1.3 In the equation, the equilibrium paths are i 11B ~i 16B .

[0030] Among them, on the equilibrium path i 11B In the flying capacitor C 1.3 Through the switch Q 1.5 and Q 1.7 To the filter capacitor C pv1.3 Transfer energy; on the equilibrium path i 12B In the flying capacitor C 1.2 and C 1.3 Series through switch Q 1.3 and Q 1.5 To the filter capacitor C pv1.2 Transfer energy; on the equilibrium path i 13B In the flying capacitor C 1.1 and C 1.2 Series through switch Q 1.1 and Q 1.3 To the filter capacitor C pv1.1 Transfer energy; on the equilibrium path i 14B In the flying capacitor C 1.2 Through the switch Q 1.3 and Q 1.7 To the filter capacitor C pv1.2 and C pv1.3 Transfer energy; on the equilibrium path i 15B In the flying capacitor C1.1 and C 1.3 Series through switch Q 1.1 and Q 1.3 To the filter capacitor C pv1.1 and C pv1.2 Transfer energy; on the equilibrium path i 16B In the flying capacitor C 1.1 Through the switch Q 1.1 and Q 1.7 To the filter capacitor C pv1.1 , C pv1.2 and C pv1.3 Transfer energy.

Claims

1. A switch capacitor photovoltaic sub-string equalizer that is easy to expand, characterized in that: The photovoltaic balancer can be applied to photovoltaic strings of any scale. Taking the balance of N photovoltaic panels as an example, N is an integer approximately or equal to 2, and the photovoltaic balancing system includes N photovoltaic panels and N photovoltaic balancers; each photovoltaic panel includes M photovoltaic sub-strings, and M is an integer approximately or equal to 3, that is, the photovoltaic string contains M*N photovoltaic sub-strings PV k (k=1.1~NM); Each photovoltaic balancer includes 2*M+2 switch tubes Q i.1 ~Q i.2M+2 , M flying capacitors C i.1 ~C i.M , M filter capacitors C pvi.1 ~C pvi.M and 1 capacitor C pvU.i , where i is the number of the photovoltaic panel, (i=1~N); the output side of the photovoltaic string is connected to the maximum power point tracking (MPPT) controller and the load, the MPPT controller controls the voltage of the entire photovoltaic string, and the photovoltaic balancing system realizes the voltage balancing of M*N substrings.

2. A switch capacitor photovoltaic sub-string equalizer that is easy to expand, characterized in that: Each photovoltaic panel is equipped with a sub-string level balancer. The i-th photovoltaic balancer is composed of: M sub-string PV i.1 ~PV i.M Respectively with the filter capacitor C pvi.1 ~C pvi.M In parallel, they are connected in parallel with the half-bridge structure composed of two complementary switching tubes, Q i.1 ~Q i.2 , Q i.3 ~Q i.4 ...Q i.2M-1 ~Q i.2M , flying capacitor C i.1 ~C i.M One end of each photovoltaic string is connected to the midpoint of the half bridge, that is, the capacitor C i.1 One end of the switch tube Q i.1 ~Q i.2 Midpoint, capacitor C i.2 One end of the switch tube Q i.3 ~Q i.4 Midpoint...Capacitance C i.M One end of the switch tube Q i.2M-1 ~Q i.2M Midpoint, M flying capacitors C i.1 ~C i.M The other end is connected in a star configuration to form a bus and connected to capacitor C pvU.i The corresponding half-bridge circuit Q i.2M+1 ~Q i.2M+2 midpoint, while the capacitor C pvU.i and the substring PV in the i+1th photovoltaic panel i+1.1 Connect in parallel.

3. A switch capacitor photovoltaic sub-string equalizer that is easy to expand, characterized in that: The equalizer is driven by a pair of complementary signals PWM+ and PWM- with a fixed duty cycle of 50%, where PWM+ drives the even-numbered switch tube Q in the half-bridge circuit. i.2 ~Q i.2M+2 , PWM-driven odd-numbered switch tube Q i.1 ~Q i.2M+1 ; In a switching cycle, the equalizer has two working modes, namely mode A and mode B; (3-1) Mode A: Even switch Q i.2 ~Q i.2M+2 On, odd switch Q i.1 ~Q i.2M+1 Shut down, the PV string PV at the top of the i-th balancer i.2 ~PV i.M and capacitor C pvU.i They are respectively connected to the corresponding star-type switched capacitor converters; during this state, the unshaded photovoltaic substring will store the extra energy generated in the flying capacitor, and the flying capacitor will also transfer energy to the shaded photovoltaic substring; (3-2) Mode B: odd number of switches Q i.1 ~Q i.2M+1 On, even switch Q i.2 ~Q i.2M+2 Shut down, the PV string PV at the lower end of the i-th balancer i.1 ~PV i.M They are respectively connected to the corresponding star-type switched capacitor converters; during this state, the unshaded photovoltaic substring will store the extra energy generated in the flying capacitor, and the flying capacitor will also transfer energy to the shaded photovoltaic substring; (3-3) Capacitor C pvU.i PV i+1.1 The energy transfer between them always exists, regardless of the switch state; finally, through the alternating operation of these two modes, the voltage balance of M*N photovoltaic substrings in the photovoltaic system is achieved.

4. A switch capacitor photovoltaic sub-string equalizer that is easy to expand, characterized in that: Due to the star structure, all substrings in each PV balancer and C pvU.i There is a direct energy transmission path between them. In one switching cycle, energy can be directly transmitted from the unshaded photovoltaic substring to the shaded photovoltaic substring. Compared with the disadvantage that the traditional switched capacitor type equalizer can only transmit energy adjacent to each other, the photovoltaic equalizer designed by the present invention has better balancing performance and faster balancing speed. The capacitor C in the i-th photovoltaic balancer pvU.i A substring PV in the adjacent i+1th photovoltaic panel i+1.1 The substrings are connected in parallel, so each substring-level balancer balances three substrings in a photovoltaic panel and one substring in an adjacent photovoltaic panel at the same time, and the substrings of the adjacent photovoltaic panels are balanced by their corresponding photovoltaic balancers, and finally all substrings in the photovoltaic string are balanced; that is, the capacitor C pvU.i The main function is to balance the adjacent photovoltaic panels. When the photovoltaic balancing system needs to be expanded, it only needs to stack photovoltaic panels and photovoltaic balancers. Compared with the high voltage stress problem of the device caused by the traditional photovoltaic balancer during expansion, the equalizer proposed in the present invention has fixed design parameters and better scalability, and can adapt to photovoltaic systems of any scale. In addition, C pvU.i The substrings of adjacent photovoltaic panels are connected in parallel, and the balance between them is DC coupled, so the cables that transfer energy between adjacent photovoltaic panels do not carry high-frequency AC current; this allows different photovoltaic balancers to work when the switch drive signals are not synchronized, and there is no need to have strict requirements on cable length and parasitic parameters; this greatly facilitates the installation and expansion of photovoltaic panels and photovoltaic balancers in actual photovoltaic systems.

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