CN106208782A - Cascaded H-bridges photovoltaic inverter leakage current suppressing method based on Model Predictive Control - Google Patents

Abstract

The invention discloses a method for suppressing the leakage current of cascaded H-bridge photovoltaic inverters based on model predictive control. The method includes: writing all 256 switch states of four H-bridge cascaded inverters and calculating the corresponding photovoltaic The value of the sum V _c of the battery parasitic capacitor voltage; select the switch state and form a switch state combination; calculate the output voltage V _k of the inverter AC side; online detection of the grid voltage V _g (k) and grid current i (k ), and calculate the value i(k+1) of the grid current at the moment k+1; phase-lock the grid voltage V _g (k) obtained by online detection to obtain the phase θ, and calculate the current at the moment k+1 from θ The given value i ^* (k+1) is substituted into the evaluation function h together with the current calculation value i(k+1) at time k+1; select the switch state combination corresponding to the smallest evaluation function h, and pass it as the output The drive circuit drives the switch tube. This method can not only effectively suppress the leakage current of the system, but also is easy to expand, and is suitable for single-phase non-isolated cascaded H-bridge photovoltaic grid-connected inverters with an even number of H-bridges.

Description

Leakage Current Suppression Method of Cascaded H-bridge Photovoltaic Inverter Based on Model Predictive Control

technical field

The invention relates to a method for suppressing the leakage current of a cascaded H-bridge photovoltaic inverter based on model predictive control; the method is applicable to the field of non-isolated photovoltaic grid connection.

Background technique

Compared with the traditional inverter, the cascaded H-bridge multilevel inverter has the advantages of small grid current harmonics, low switching frequency, small filter size and easy modularization, so it has attracted more and more attention from scholars. .

In addition, the DC side of each module of the cascaded H-bridge multi-level inverter can be independently powered by a photovoltaic panel, making it possible to make independent MPPT control, so the cascaded H-bridge multi-level topology is especially suitable for photovoltaic grid-connected inverters. Transformer.

Due to the modular structure of cascaded H-bridge inverters, a certain number of cascaded inverters can be cascaded to achieve the voltage required for grid connection, so the transformer that plays the role of step-up and isolation can be omitted, further reducing costs and increasing power density.

However, due to the lack of transformer isolation, there is a direct electrical connection between the photovoltaic panel and the grid, which will generate leakage current on the parasitic capacitance between the photovoltaic panel and the ground, which will affect the efficiency of the system, reduce system reliability, and threaten personal safety. Safety and electromagnetic interference, etc., so it is very necessary to suppress the leakage current.

At present, the traditional leakage current suppression methods can be mainly divided into the following three types: 1) using improved topologies, such as H5, H6 and other topologies; 2) using passive filters, such as common mode inductors, EMI filters, etc.; 3) ) to find a suitable modulation strategy.

However, different from the single-module inverter topology, the component of the leakage current in the cascaded H-bridge topology is not only related to the output of this module, but also related to the output of other modules in the cascade. Therefore, the method of suppressing the leakage current of a single H-bridge cannot be directly applied to the suppression of the leakage current of the cascaded H-bridge topology. There is a slight mismatch between suppression methods.

For this reason, scholars have made a lot of efforts and attempts in the leakage current suppression of cascaded H-bridge inverters, such as the 2016 IEEE document "Single Phase Cascaded H5 Inverter with Leakage Current Elimination for Transformerless Photovoltaic System" ("Non-isolated cascaded Analysis of Common Mode Current Characteristics of H5 Photovoltaic Inverter"—Proceedings of the 2016 IEEE Energy Society Plenary Session) proposed a modulation strategy for cascaded H5 topology to suppress leakage current, although the leakage current was suppressed to a certain extent, but As the number of modules increases, the modulation strategy will be very complicated, which is not conducive to system expansion and modular design. In addition, compared with the H4 topology, the proposed cascaded H5 topology will increase the cost and loss.

2014 IEEE document "Analysis and Suppression of Leakage Current in Cascaded-Multilevel–Inverter-Based PV Systems," Y.Zhou and H.Li, "IEEE Trans. Power Electron.", 2014, 29(10), 5265–5277 (" Leakage Current Analysis and Suppression of Cascaded Multilevel Photovoltaic Inverters", "IEEE Journal-Journal of Power Electronics", 2014, Vol. 29, No. 10, pp. 5265-5277) proposed to add common mode on the DC side and the AC side respectively The filter suppresses the leakage current, but its switching frequency is set at 10kHz, which is not in line with the original intention of using a cascode topology to reduce the switching frequency. 2013 IEEE document "A Modulation Strategy for Single-phase HB-CMI to Reduce Leakage Ground Current in Transformer-less PV Applications" ("Cascaded Multi-level Photovoltaic Inverter Leakage Current Analysis and Suppression" - 2013 IEEE Energy The modulation strategy proposed by the Proceedings of the Plenary Session of the Chinese Academy of Sciences) makes the parasitic capacitor voltage change according to the step wave of the power frequency, but the modulation strategy is relatively complicated and not easy to expand the system. In addition, the Chinese invention patent application publication CN 105450059 A published on December 22, 2015 "Modulation Method for Suppressing Leakage Current of Two H-bridge Cascaded Inverters" improved the traditional carrier layer modulation and proposed a The new modulation strategy can suppress the leakage current of a single-phase non-isolated cascaded H-bridge photovoltaic inverter containing two H-bridges, but this method is only used in the case of two H-bridges and cannot be extended to multiple H-bridges. There are certain limitations, and it is not convenient for engineering practical application.

To sum up, for single-phase non-isolated cascaded H-bridge photovoltaic inverters, the existing leakage current suppression methods mainly have the following problems:

(1) The existing technology mainly focuses on the leakage current suppression of the single-phase non-isolated cascaded H-bridge photovoltaic inverter containing two H-bridges, which largely limits the single-phase non-isolated cascaded H-bridge photovoltaic inverter. The extended application of more modules of the inverter fails to give full play to the advantages of the cascaded H-bridge photovoltaic inverter;

(2) The leakage current suppression method of the existing single-phase non-isolated cascaded H-bridge photovoltaic inverter with multiple H-bridges mainly changes the existing topology or implements it with a new topology. If common-mode filters are added on the DC side and AC side respectively, and cascaded H5 or H6 topologies are used, this will undoubtedly increase the cost and loss of the system and reduce the power density of the inverter.

Contents of the invention

The technical problem to be solved by the present invention is to overcome the limitations of the above-mentioned various schemes. Aiming at the problem that the non-isolated photovoltaic inverter generates leakage current on the parasitic capacitance between the photovoltaic panel and the earth due to the lack of transformer isolation, a new method is proposed. A leakage current suppression method based on model predictive control has the advantages of convenient expansion, simple calculation and low cost.

In order to solve the technical problems of the present invention, the main steps of the adopted technical solution are as follows:

1. A cascaded H-bridge photovoltaic grid-connected inverter leakage current suppression method based on model predictive control, including selecting a switch state combination and detecting the voltage and current of the grid. The main steps are as follows:

Step 1. Assume that the DC voltages of the four modules of the four H-bridge cascaded inverters are the same, and record it as V _dc , and use the following formula to calculate the value of the sum V _c of the corresponding photovoltaic cell parasitic capacitance voltage,

${\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; c</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}}\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; 4</span>}}{<span\; class="notranslate">\; \&Sigma;</span>}}<span\; class="notranslate">\; \&lsqb;</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&rsqb;</span>{\mathrm{<span\; class="notranslate">\; S</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; a</span>}}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; c</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}}\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; 4</span>}}{<span\; class="notranslate">\; \&Sigma;</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; S</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; b</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; g</span>}}$

Among them, V _g is the voltage of the power grid, S _1a is the switching function of the upper tube of the left bridge arm of the first H bridge, S _1b is the switching function of the upper tube of the right bridge arm of the first H bridge, and S _2a is the switching function of the second H bridge The switching function of the upper tube of the left bridge arm of the bridge, S _2b is the switching function of the upper tube of the right bridge arm of the second H bridge, S _3a is the switching function of the upper tube of the left bridge arm of the third H bridge, and S _3b is the switching function of the third H bridge The switching function of the upper tube of the right bridge arm of the H bridge, S _4a is the switching function of the upper tube of the left bridge arm of the fourth H bridge, and S _4b is the switching function of the upper tube of the right bridge arm of the fourth H bridge, and satisfies:

Arrange and combine the switching functions to obtain the value of the switching state combination S _1a /S _1b /S _2a /S _2b /S _3a /S _3b /S _4a /S _4b , and obtain four H-bridge cascaded inverters S _1a /S All 256 switch states of _1b /S _2a /S _2b /S _3a /S _3b /S _4a /S _4b ;

Step 2, according to all 256 switch states of the four H-bridge cascaded inverters written in step 1, maintain the sum of the parasitic capacitor voltage V _c of the photovoltaic cell as the sinusoidal quantity of the power frequency and Requirements, select the switch state that meets the requirements, and form the following switch state combination:

10101010-10100010-10110010-10011110-11110110-01100001-01001101-01011101-01010101, this switch combination contains a total of 9 switch states, each switch state corresponds to an output level;

Step 3, according to the 9 switch states selected in step 2, calculate the value of the output level V _k of the AC side of the inverter, where, Its value is [-4,-3,-2,-1,0,1,2,3,4]V _dc , there are 9 levels in total;

Step 4: Online detection of the single-phase non-isolated cascaded H-bridge inverter’s voltage V _g (k) and grid current i(k) of the grid at time k, and the output voltage of the AC side of the inverter calculated in Step 3 V _k is substituted into the discrete model function of the grid-connected current to predict the value i(k+1) of the grid current at time k+1, where the discrete model function satisfies the following formula:

$\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; RT</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}{\mathrm{<span\; class="notranslate">\; L</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\frac{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}{\mathrm{<span\; class="notranslate">\; L</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; k</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; g</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; )</span>$

In the formula, T _s is the sampling period of the grid voltage, L and R are the inductance value and the corresponding resistance value of the single-phase non-isolated cascaded H-bridge photovoltaic inverter, and V _k is the AC side of the inverter at time k The output voltage;

Step 5, phase-lock the grid voltage V _g (k) obtained from the online detection in step 4 to obtain the phase θ, and calculate a sinusoidal quantity with the same frequency and phase as the grid voltage from θ as the current given value at the k+1th moment i ^* (k+1), and substitute the calculated value i(k+1) of the grid current obtained in step 4 into the evaluation function h at time k+1, and select an output level that minimizes the evaluation function h; where , evaluation function h=|i ^* (k+1)-i(k+1)|;

Step 6, according to the output level selected in step 5, select the corresponding switch state from the combination of switch states selected in step 2, and drive the switch tube through the drive circuit as an output quantity.

The advantage of the present invention with respect to prior art is:

1. Strong applicability, not only can effectively suppress system leakage current, but also easy to expand, suitable for photovoltaic inverters with four H-bridge modules cascaded.

2. It does not need to be realized by changing the existing topology or adopting a new topology, and the leakage current of the single-phase non-isolated cascaded H-bridge photovoltaic inverter of four H-bridges can be realized through the switch control strategy proposed by the present invention Inhibition can not only reduce the cost and switching loss of the system, but also improve the power density of the inverter.

Description of drawings

Figure 1 is a single-phase cascaded H-bridge photovoltaic inverter topology with four H-bridges.

Figure 2 is the equivalent circuit of a single-phase cascaded H-bridge photovoltaic inverter containing four H-bridges.

Fig. 3 is a control block diagram of a method for suppressing leakage current based on model predictive control.

detailed description

In order to make the object, technical solution and advantages of the present invention more clear, the present invention will be further clearly and completely described below in conjunction with the accompanying drawings and embodiments.

The method for suppressing leakage current of cascaded H-bridge photovoltaic inverters based on model predictive control disclosed by the present invention is applicable to suppressing leakage current of four H-bridge cascaded photovoltaic inverters.

Figure 1 is the topological structure of the single-phase non-isolated cascaded H-bridge photovoltaic grid-connected inverter described in this patent, which includes four H-bridges, and each H-bridge DC side is independently powered by one or more photovoltaic panels, each The AC sides of the H-bridges are connected in series with each other and connected to the power grid through differential mode inductors L1 and L2. _The biggest feature of this cascaded H _- bridge topology is that the outputs of each H-bridge’s AC side can be superimposed to form a multi-level. Among them, the parameters C _pvk1 and C _pvk2 (k=1, 2, 3, 4) are the parasitic capacitance of the photovoltaic panel to the ground, the capacitance is related to the area of the photovoltaic panel and external factors such as weather, and L ₁ and L ₂ are the grid side filter inductance; R ₁ and R ₂ are the parasitic resistance of the grid side filter inductance; V _g is the voltage of the public coupling point on the AC side.

FIG. 2 is an equivalent circuit of a single-phase four-module cascaded H-bridge shown in FIG. 1 , where C _pvk =C _pvk1 //C _pvk2 . Take the positive half cycle of the grid current as an example for analysis, and note that the grid side filter inductance L ₁ =L ₂ , R ₁ =R ₂ .

Fig. 3 is a control block diagram based on model predictive control proposed by the patent of the present invention. The control steps include: on-line detection of the single-phase non-isolated cascaded H-bridge inverter's voltage V _g (k) and grid current i (k) of the grid at time k ), and substituted into the discrete model function of the grid-connected current to predict the value i(k+1) of the grid current at time k+1; phase-lock the grid voltage obtained by online detection to obtain the phase θ, and calculate a phase θ from θ to The sinusoidal quantity of the grid voltage with the same frequency and phase and the amplitude of A is taken as the current given value i ^{* (k+1) at the k+1th moment; the current given value i * (} k+1) at the k+1th moment Substitute into the evaluation function together with the calculated value i(k+1) of the grid current at time k+1, and select an output level that minimizes the evaluation function; according to the selected output level, further select the corresponding switch state, And as an output, drive the switching tube of the cascaded H-bridge photovoltaic inverter through the driving circuit.

For the single-phase cascaded H-bridge photovoltaic inverter with four H-bridges shown in Figure 1, the basic steps of the method for suppressing the leakage current of the cascaded H-bridge photovoltaic inverter based on model predictive control disclosed by the present invention are as follows:

See Figure 1, Figure 2, Figure 3.

Step 1. Assuming that the DC voltages of the four modules of the four H-bridge cascaded inverters are the same, and denoted as V _dc , calculate all 256 switching states (S _1a /S _1b / S _2a /S _2b /S _3a /S _3b /S _4a /S _4b ) and the corresponding PV cell parasitic capacitance voltage sum V _c value,

Among them, V _g is the voltage of the power grid, S _1a is the switching function of the upper tube of the left bridge arm of the first H bridge, S _1b is the switching function of the upper tube of the right bridge arm of the first H bridge, and S _2a is the switching function of the second H bridge The switching function of the upper tube of the left bridge arm of the bridge, S _2b is the switching function of the upper tube of the right bridge arm of the second H bridge, S _3a is the switching function of the upper tube of the left bridge arm of the third H bridge, and S _3b is the switching function of the third H bridge The switching function of the upper tube of the right bridge arm of the H bridge, S _4a is the switching function of the upper tube of the left bridge arm of the fourth H bridge, and S _4b is the switching function of the upper tube of the right bridge arm of the fourth H bridge, and satisfies:

The value of each switch function S _1a , S _1b , S _2a , S _2b , S _3a , S _3b , S _4a , S _4b is 0 or 1, and the values of the switch functions are arranged and combined as the switch state S _1a /S _1b /S _2a /S _2b /S _3a /S _3b /S _4a /S _4b values, a total of ²⁸ kinds, the value of these ²⁸ kinds of switch states is all 256 kinds of switches of four H-bridge cascaded inverters state(S _1a /S _1b /S _2a /S _2b /S _3a /S _3b /S _4a /S _4b );

According to Figure 2, it can be obtained from Kirchhoff’s voltage law that the parasitic capacitor voltage V _ck satisfies the following formula:

$\left\{\begin{array}{c}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}\mathrm{<span\; class="notranslate">\; a</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; L</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; R</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; g</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}\\ <span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; b</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; L</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; R</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}\\ <span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}\mathrm{<span\; class="notranslate">\; b</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; a</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}\\ <span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; b</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}\mathrm{<span\; class="notranslate">\; a</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}\\ <span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}\mathrm{<span\; class="notranslate">\; b</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; a</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}\mathrm{<span\; class="notranslate">\; 4</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}\end{array}\right.$

After sorting out, the parasitic capacitor voltage V _ck is calculated to satisfy the following formula:

${\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; ck</span>}}<span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; DMi</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; CMk</span>}}<span\; class="notranslate">\; -</span>\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; 4</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; DMi</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; g</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}}$

Among them, V _CMk and V _DMk represent the common-mode voltage and differential-mode voltage of the kth (k=1, 2, 3, 4) module respectively;

According to the defined switching functions S _1a , S _1b , S _2a , S _2b , S _3a , S _3b , S _4a , S _4b , calculate the output terminals A ₁ , B ₁ , A ₂ , B of each bridge arm of the four modules ₂ , the values of the voltages V _1a , V _1b , V _2a , V _2b , V _3a , V _3b , V _4a and V _4b of A ₃ , B ₃ , A ₄ and B ₄ ,

V _1a =S _1a V _dc ,

V _1b =S _1b V _dc ,

V _2a = S _2a V _dc ,

V _2b = S _2b V _dc ,

V _3a =S _3a V _dc ,

V _3b = S _3b V _dc ,

V _4a =S _4a V _dc ,

V _4b = S _4b V _dc ;

Finally, according to the system parasitic capacitance voltage V _ck obtained above, the system leakage current i _leak and the output terminals A ₁ , B ₁ , A ₂ , B ₂ , A ₃ , B ₃ , A ₄ and B of each bridge arm of the four modules ₄ The value of the voltage V _1a , V _1b , V _2a , V _2b , V _3a , V _3b , V _4a and V _4b , calculate the sum of the voltage V _c of the parasitic capacitance of the four module H bridge cascaded inverters value,

$\begin{array}{c}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; c</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}}\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; 4</span>}}{<span\; class="notranslate">\; \&Sigma;</span>}}<span\; class="notranslate">\; \&lsqb;</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&rsqb;</span>{\mathrm{<span\; class="notranslate">\; S</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; a</span>}}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; c</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}}\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; 4</span>}}{<span\; class="notranslate">\; \&Sigma;</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; S</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; b</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; g</span>}}\\ <span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; mV</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; g</span>}}\end{array}$

Among them, m satisfies the following formula:

$\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; 4</span>}}{<span\; class="notranslate">\; \&Sigma;</span>}}\frac{<span\; class="notranslate">\; \&lsqb;</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&rsqb;</span>}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; S</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; a</span>}}<span\; class="notranslate">\; -</span>\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; 4</span>}}{<span\; class="notranslate">\; \&Sigma;</span>}}\frac{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; S</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; b</span>}}$

From the 256 switch states (S _1a /S _1b /S _2a /S _2b /S _3a /S _3b /S _4a /S _4b ) calculated, the sum V _c of the 256 parasitic capacitance voltages of photovoltaic cells is obtained;

Step 2, according to all 256 switch states of the four H-bridge cascaded inverters written in step 1, maintain the sum of the parasitic capacitor voltage V _c of the photovoltaic cell as the sinusoidal quantity of the power frequency and According to the requirements, select the switch state that meets the requirements. According to the above requirements, finally select 9 switch states that meet the requirements, and form the following switch state combinations:

10101010-10100010-10110010-10011110-11110110-01100001-01001101-01011101-01010101, each switch state corresponds to an output level;

Table 1 shows the sum V _c and m value of the nine switching states of the selected four H-bridge cascaded inverters and the corresponding parasitic capacitance voltages:

Table 1 The switch state and m value of the four modules

The output voltage S _1a /S _1b /S _2a /S _2b /S _3a /S _3b /S _4a /S _4b m -4V _dc 0101 0101 2 -3V _dc 0101 1101 2 -2V _dc 0100 1101 2 -V _dc 0110 0001 2 0 1111 0110 2 V _dc 1001 1110 2 2V _dc 1011 0010 2 3V _dc 1010 0010 2 4V _dc 1010 1010 2

It can be seen from Table 1 that the values of m corresponding to the selected nine switching states are all 2, which makes the sum of the voltages of the parasitic capacitors V _c keep the power frequency sinusoidal constant, and satisfy Therefore, the selected 9 switch states can form a switch state combination;

Step 3, according to the 9 switch states selected in step 2, calculate the output voltage V _k of the AC side of the inverter, where, Its value is [-4,-3,-2,-1,0,1,2,3,4]V _dc , there are 9 levels in total;

Step 4: Online detection of the single-phase non-isolated cascaded H-bridge inverter’s voltage V _g (k) and grid current i(k) of the grid at time k, and the output voltage of the AC side of the inverter calculated in Step 3 V _k is substituted into the discrete model function of the grid-connected current to predict the value i(k+1) of the grid current at time k+1, where the discrete model function satisfies the following formula:

$\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\frac{{\mathrm{<span\; class="notranslate">\; RT</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}{\mathrm{<span\; class="notranslate">\; L</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>\frac{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}{\mathrm{<span\; class="notranslate">\; L</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; k</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; g</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; )</span>$

In the formula, T _s is the sampling period of the grid voltage, L and R are the inductance value and the corresponding resistance value of the single-phase non-isolated cascaded H-bridge photovoltaic inverter, and R=R ₁ +R ₂ , L =L ₁ +L ₂ , the value of sampling time k increases continuously with the operation of the controller, k=1,2,3,...;

Step 5, phase-lock the grid voltage obtained by online detection to obtain the phase θ, and calculate a sinusoidal quantity with the same frequency and phase as the grid voltage and amplitude A as the current given value i ^* ( k+1), and the calculated value i(k+1) of the grid current obtained in step 4 at time k+1 is substituted into the evaluation function h, and a set of switch state combinations with the smallest evaluation function h is selected. Wherein, the evaluation function h=|i ^* (k+1)-i(k+1)|. Because only when the evaluation function is minimized, the tracking effect of grid current can be guaranteed to be the best, and the basic goal of grid-connected control can be met;

Step 6, according to the output level selected in step 5, select the corresponding switch state from the combination of switch states selected in step 2, and drive the switch tube through the drive circuit as an output.

Different from the existing modulation technology, the leakage current suppression method of the cascaded H-bridge photovoltaic inverter based on the model predictive control proposed by the present invention does not require the carrier to participate in the modulation, but replaces the carrier modulation with the method of selecting the switch state through the model predictive control . According to the above steps, the sum of the parasitic capacitor voltages of the photovoltaic cells can be maintained as a power frequency sinusoidal value, thereby realizing the suppression of the leakage current of the cascaded H-bridge photovoltaic inverter.

The leakage current method of cascaded H-bridge photovoltaic inverters based on model predictive control disclosed in the present invention is applied to a cascaded photovoltaic grid-connected system of four H-bridges. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without making creative efforts shall all belong to the protection scope of this patent.

Claims (1)

1. a cascaded H-bridges photovoltaic inverter leakage current suppressing method based on Model Predictive Control, including selecting on off state Combine and detect voltage and the electric current of electrical network, it is characterised in that key step is as follows:

Step 1, if four module DC voltages of four H bridge cascaded inverters are identical, and is designated as V_dc, and calculate correspondence with following formula Photovoltaic cell parasitic capacitor voltage sum V_cValue,

$V_c=\frac{V_{dc}}{2}\underset{i=1}{\overset{4}{\mathrm{\&Sigma;}}}\&lsqb;4-2\left(i-1\right)\&rsqb;S_{ia}-\frac{V_{dc}}{2}\underset{i=1}{\overset{4}{\mathrm{\&Sigma;}}}\left(4-2i\right)S_{ib}-2V_g$

Wherein, V_gFor the voltage of electrical network, S_1aIt is the switch function of pipe, S on first left brachium pontis of H bridge_1bIt is first right brachium pontis of H bridge The switch function of upper pipe, S_2aIt is the switch function of pipe, S on second left brachium pontis of H bridge_2bIt is the opening of pipe on second right brachium pontis of H bridge Close function, S_3aIt is the switch function of pipe, S on the 3rd the left brachium pontis of H bridge_3bIt is the switch function of pipe on the 3rd the right brachium pontis of H bridge, S_4aIt is the switch function of pipe, S on the 4th the left brachium pontis of H bridge_4bIt is the switch function of pipe on the 4th the right brachium pontis of H bridge, and meets:

Switch function is carried out permutation and combination and obtains Switch State Combination in Power Systems S_1a/S_1b/S_2a/S_2b/S_3a/S_3b/S_4a/S_4bValue, obtain Four H bridge cascaded inverter S_1a/S_1b/S_2a/S_2b/S_3a/S_3b/S_4a/S_4bAll 256 kinds of on off states；

Step 2, according to all 256 kinds of on off states of four H bridge cascaded inverters that step 1 is write out, according to maintaining photovoltaic electric Pond parasitic capacitor voltage sum V_cFor power frequency sinusoidal quantity andRequirement, select Meet the on off state required, form following Switch State Combination in Power Systems:

10101010-10100010-10110010-10011110-11110110-01100001-01001101-01011101- 01010101, this Switch State Combination in Power Systems comprises 9 kinds of on off states, every kind of corresponding output level of on off state altogether；

Step 3, the 9 kinds of on off states selected according to step 2, calculate the output level V of inverter ac side_kValue, wherein,Its value is [-4 ,-3 ,-2 ,-1,0,1,2,3,4] V_dc, have 9 kinds of level；

Step 4, on-line checking single-phase non-isolated cascaded H-bridges inverter is at the voltage V of k moment electrical network_g(k) and power network current i (k), And by the output voltage V of calculated for step 3 inverter ac side_kSubstitute into the discrete model function of grid-connected current in the lump, in advance Measuring power network current value i (k+1) in the k+1 moment, wherein, discrete model function meets following formula:

$i(k+1)=(1-\frac{{\mathrm{RT}}_s}{L})i\left(k\right)+\frac{T_s}{L}(V_k-V_g(k\left)\right)$

In formula, T_sBeing the sampling period of line voltage, L and R is respectively the electricity of described single-phase non-isolated cascaded H-bridges photovoltaic DC-to-AC converter Inductance value and the resistance value of correspondence, V_kIt it is k moment inverter ac side output voltage；

Step 5, the line voltage V that step 4 on-line checking is obtained_gK () carries out phase-locked, obtain phase theta, by θ calculate one with The sinusoidal quantity of line voltage same frequency homophase is as given value of current value i in kth+1 moment^*(k+1) electrical network, and with step 4 obtained The electric current value of calculation i (k+1) in the k+1 moment substitutes into valuation functions h together, the output that valuation functions h that selects to send as an envoy to is minimum Level；Wherein, valuation functions h=| i^*(k+1)-i(k+1)|；

Step 6, the output level selected according to step 5, from the Switch State Combination in Power Systems that step 2 has been selected, select corresponding opening Off status, and drive switching tube as output by drive circuit.