Technical field

The present invention relates to a control method of a multi-channel MPPT inverter, and more particularly, to a control method for improving conversion efficiency of a multi-channel MPPT inverter.

Background technology

A photovoltaic power generation system is usually composed of three parts: a photovoltaic panel array, a direct-current boost converter and a grid-connected power converter. The direct-current boost converter and the grid-connected power converter are interconnected and isolated by an intermediate direct-current bus, and are usually as a whole, called a photovoltaic inverter. The main working principle is that the direct-current boost converter boosts low-voltage direct current output by the photovoltaic panel array to high-voltage stable direct current for converting into alternating current by the back stage grid-connected power converter and then feeding into the power grid. With the increase in grid-connected power of photovoltaic power generation systems and the dropping of the maximum power point (MPP) of photovoltaic panels (PVs), single-channel direct-current boost circuits are difficult to take account of application scenarios of both high power and high boost ratio, thus use of a multi-channel MPPT input to improve grid-connected power generation of photovoltaic inverters is a research hotspot of various manufacturers.

Without loss of generality, using the more applied dual-channel MPPT input photovoltaic inverter as an example, the existing multi-channel MPPT control logic is described. FIG. 1 shows a structural diagram of a dual-channel PV input photovoltaic inverter comprising photovoltaic panels PV1, PV2, input capacitors Ci, C ₂ , input boost circuit boosti, boost ₂ , a direct-current bus capacitor Cd ₀ , an H-bridge inverter and a controller DSC. Wherein outputs of the boost circuit boosti and boost ₂ are connected to a common direct-current bus, and power is fed into the power grid through the bus capacitor Q _o and the H-bridge inverter. PWMi and PWM ₂ are drive signals of the boost circuit boosti and boost ₂ , respectively. The controller DSC generally implements dual-channel MPPT control using a digital signal processor (DSP) by acquisition of information of PV input voltages v _PV1 and v _PV2 in two channels, PV input currents i _PV1 and i _PV2 in the two channels and a direct-current bus voltage v _bus , a brief control flow chart is shown in FIG. 2. In FIG. 2, v _PV1 , v _PV2 and v _bus are reference signals of the PV voltages in the two channels and direct-current bus voltage, respectively, Δν _ρν is the difference between the PV voltages in the two channels, Δν _ρν =ν _ρνι — v _PV2 ; V _TH is a judging threshold of the difference between the PV voltages in the two channels. The controller DSC obtains PV input power signals P _PV1 and P _PV2 in the two channels by sampling the PV input voltages v _PV1 and v _PV2 in the two channels and the PV input currents i _PV1 and i _PV2 in the two channels, and obtains the PV voltage reference signals v _PV1 and v _P ^* _V through their respective MPPT module operation. At the same time, the controller DSC calculates the difference Δν _ρν between the PV voltages in the two channels, and substitutes it into a boost start and stop control logic to compare with the preset threshold V _TH . There are three cases:

( 1 ) Δν _ρν ≥ V _TH , the controller DSC turns off a boosti controller, blocks the drive signal PWMi of the boosti circuit, turns off the boosti circuit, enables a boost ₂ controller and obtains the drive signal PWM ₂ of the boost ₂ circuit, and the direct-current bus voltage reference signal v _b ^* _us = v _P ^* _Vi ;

( 2 ) v _PV ≤ V _TH , the controller DSC turns off the boost ₂ controller, blocks the drive signal PWM ₂ of the boost ₂ circuit, turns off the boost ₂ circuit, enables the boost ₂ controller and obtains the drive signal PWMi of the boosti circuit, and the direct-current bus voltage reference signal v _b ^* _us = v _P ^* _V2 ;

( 3 ) V _TH ≥ v _PV ≥—V _TH , the controller DSC enables both the boosti and boost ₂ controllers and obtains the drive signals PWMi and PWM ₂ of the boosti and boost ₂ circuits, and the direct-current bus voltage reference signal v _bus uses the maximum of the two PV voltage reference signals, i.e., v _b ^* _us = max(v _PV1 , v _PV2 ).

In the practical application, comprehensively considering thermal balance of a direct-current boost circuit of the inverter, conversion efficiency of the whole inverter and the service life of components, for the multi-channel MPPT input inverter, photovoltaic panels in all channels are usually configured almost uniformly, thus a PV curve of each input of the inverter is approximately the same. It can be seen from FIG. 2, using the existing multi-channel MPPT control method, each boost circuit is in a working state at a steady operating point, the conversion efficiency of the inverter is low, and the grid-connected power generation is small. Summary

An object of the present invention is to solve shortcomings and problems existing in the prior art, and propose a control method which can improve conversion efficiency of a multi-channel MPPT input inverter. By constructing a new boost start and stop control logic and virtual local maximum power point (VLMPP), a voltage difference between input PV voltages in multiple channels and the VLMPP is detected in real time and processed according to a certain logical relationship, and then turn-off and turn-on of a PV input boost circuit in each channel is controlled, to implement high efficient operation of the invertor.

The technical scheme adopted by the present invention is as follows.

A control method for improving conversion efficiency of a multi-channel MPPT inverter comprising following steps:

in step S 1 : collecting an input voltage v _PVm of a photovoltaic panel in each channel, an input current i _PVm of the photovoltaic panel in each channel and a direct-current bus voltage ^v bus, obtaining an input power P _PVm of the photovoltaic panel in each channel, and using input voltages of at least two channels to obtain a voltage difference Δν _ρν , wherein m = 1, 2, M, M is a number of input channels of the photovoltaic inverter MPPT;

in step S2, comparing the voltage difference Δν _ρν with a preset on-off control judging threshold to obtain a start and stop state of a boost circuit in each channel, a voltage reference signal v _PVm in each channel and a direct-current bus voltage reference signal v _b ^* _us , wherein the boost start and stop state is determined as follows,

in step S21, when |Δΐ7 _Ρ ^ | > V _THb , turning off a boost circuit in a channel corresponding to a maximum input voltage v _{PV max} , activating boost circuits in the remaining channels, wherein V _THb is a boost on-off control judging threshold 1 , and maximizing the input power Ppvm by an MPPT module in each channel to obtain the voltage reference signal v _PVm in each channel, the direct-current bus voltage reference signal v _bus using a maximum voltage reference signal v _P ^* _{V rnax} ;

in step S22, when V _THb ≥ \Δν _ρν \≥ V _THs , activating all boost circuits, whereinl^, _; is a boost on-off control judging threshold 2, and V _THs < V _THb ; and maximizing the input power Ppvm by the MPPT module in each channel to obtain the voltage reference signal v _P ^* _Vm in each channel, the direct-current bus voltage reference signal v _bus using the maximum voltage reference signal v _{PV max} ; and

in step S23, when V _THs ≥ \Δν _ρν \≥ 0, when the step is performed for a first time, obtaining a voltage V _VLMPP at a virtual local maximum power point (VLMPP), turning off all the boost circuits, maximizing a total input power P _PV _ _sum of the inverter by an MPPT module based on the direct-current bus voltage v _bus to obtain the direct-current bus voltage reference signal v _bus monitoring a voltage difference between the direct-current bus voltage ^v _b us ^an d VvLMPP , ^an d activating all the boost circuits when the voltage difference exceeds

Preferably, in step SI, using the maximum input voltage v _{PV max} , a PV input voltage ^v pv_smax slightly less than v _{PV max} and a minimum PV input voltage v _{PV min} to obtain a voltage difference v _{PV max} between v _{PV max} and v _{PV min} and a voltage difference Δν _{ρν Μ3} between v _{PV max} and v _{PV Smax} ,

in step S2, comparing the voltage differences Δν _ρν _ _max and Δν _ρν _ _MS with the preset on-off control judging threshold to obtain the start and stop state of the boost circuit in each channel, wherein the boost start and stop state is determined as follows,

in step S21, |Δΐ7 _Ρ ^ | > V _THb , turning off the boost circuit in the channel corresponding to the maximum input voltage v _PV _ _max , and activating the boost circuits in remaining channels; in step S22, V _THb ≥ \Δν _ρν \≥ V _THs , activating all the boost circuits;

in step S23, V _THs ≥ \Δν _ρν \≥ 0,when the step is performed for the first time, obtaining the voltage V _VLMPP at the VLMPP point, turning off all the boost circuits, monitoring the voltage difference between v _bus and V _VLMPP , and activating all the boost circuits when the voltage difference exceeds V _THb .

Preferably, step S23 further comprises:

in step S231, when V _THb ≥ \v _bus — V _VLMPP \≥ 0, turning off all the boost circuits, and maximizing the total input power P _PV _ _sum of the inverter by the MPPT module based on v _bus to obtain the direct-current bus voltage reference signal v _bus ;

in step S232, when V _THb ≥ \v _bus — V _VLMPP \≥ 0, activating all the boost circuits, and maximizing the input power P _PVm by the MPPT module in each channel to obtain the voltage reference signal in each channel, the direct-current bus voltage reference signal v _bus using the maximum voltage reference signal v _{PV max} .

Preferably, in step S23, the voltage V _VLMPP at the VLMPP point is obtained by following formula:

SUM{Vp _V1 ,Vp _V2 ,...,V _PVM )

VLMPP - _M

Preferably, in step SI, at least using a voltage difference between the maximum input voltage and the minimum input voltage of all input channels.

Preferably, in step S 1 , using the maximum input voltage and the minimum input voltage to obtain the voltage difference of all input channels.

In a particular embodiment, following steps are specifically comprised:

in step SI, a controller DSC collecting a PV input voltage v _PVm in each channel, a PV input current i _PVm in each channel and a direct-current bus voltage v _bus> calculating and obtaining an input power P _PVm in each channel and a total input power P _PV _ _sum of an inverter, calculating and obtaining a maximum PV input voltage v _{PV max} , a PV input voltage ^v pv_smax slightly less than v _{PV max} and a minimum PV input voltage v _{PV min} , and calculating and obtaining a voltage difference v _{PV max} between v _{PV max} and v _{PV min} and a voltage difference Δν _ρν _ _MS between v _PV _ _max and v _{PV Smax} , wherein m = 1, 2, M, M is the number of input channels of the photovoltaic inverter MPPT, Δν _ρν _ _max = ^v PV_max ^{~ v} PV_min- _> ^an d Δν _{Ρν Μ5} = V _{PV max} — V _{PV Smax} ,

in step S2, comparing the voltage differences Δν _ρν _ _max and Δν _ρν _ _MS to obtain a start and stop state of a boost circuit in each channel, a PV voltage reference signal v _P ^* _Vm in each channel and a direct-current bus voltage reference signal v _b ^* _us , specifically comprising:

in step S21, when \Δν _ρν _ _MS \≥ V _THb ,the controller DSC turning off a boost controller in a channel with the maximum PV input voltage v _PV _ _max , blocking a drive signal in the channel, turning off the boost circuit in the channel, enabling boost controllers in remaining channels and obtaining drive signals PWM _m in the channels, and at a same time the controller DSC aximizing an input power P _PVm by an MPPT module in each channel to obtain a PV voltage reference signal v _P ^* _Vm in each channel, the direct-current bus voltage reference signal using the maximum PV input voltage reference signal v _P ^* _V _ _max , i.e., v _bus = v _{PV max} , and wherein V _THb the formula is a boost on-off control judging threshold 1;

in step S22, when V _THb ≥ \Δν _ρν _ _MS \≥ V _THs ,the controller DSC enabling the boost controller in each channel and obtaining the drive signal PWM _M of the boost circuit in each channel, and at the same time the controller DSC substituting the input power P _PVm in each channel into the MPPT module in each channel to obtain the PV voltage reference signal ^v pvm ^{m eacn} channel, the direct-current bus voltage reference signal v _b ^* _us using the maximum value v _P ^* _V _ _max of PV voltage reference signals, i.e., v _bus = v _{PV max} , and wherein V _THs in the formula is a boost on-off control judging threshold 2, wherein V _THs < V _THb ; in step S23: when V _THs ≥ \Δν _ρν _ _max \≥ 0 ,the controller DSC constructing VLMPP point voltage information, turning off all the boost circuits, blocking the drive signal PWM _m of the boost circuit in each channel, maximizing a total input power P _PV _ _sum of the inverter by an MPPT module based on v _bus to obtain the direct-current bus voltage reference signal v _b ^* _us .

In step S23, the controller DSC monitoring a voltage difference between v _bus and V _VLMP p in real time, when the voltage difference between v _bus and V _VLMPP exceeds V _THb> activates the boost controller in each channel mandatorily and starting all the boost circuits; at the same time the controller DSC maximizes the input power P _PVm by the MPPT module in each channel to obtain the PV voltage reference signal v _PVm in each channel, the DC bus voltage reference signal v _bus using a maximum value v _{PV max} of PV voltage reference signals, i.e., v _us = Vp _{V max} ;

wherein if M = 2, that is, in a two-channel MPPT inverter, the PV input voltage ^v pv_smax slightly less than v _{PV max} and the minimum PV input voltage v _{PV min} are the same value, Av _{PV max} = Av _{PV MS} .

Compared to the prior art, the present invention has the following advantages using the scheme described above: a new boost start and stop control logic and a virtual local maximum power point (VLMPP) are constructed, a voltage difference between input PV voltages in multiple channels and the VLMPP is detected in real time and processed according to a certain logical relationship, and then turn-off and turn-on of a PV input boost circuit in each channel is controlled, thereby reducing the power loss in a steady state of the inverter, improving the conversion efficiency of the inverter, and implementing the economic and efficient operation of the inverter.

Brief description of the drawings

FIG. 1 is a structural diagram of a control system of a dual-channel MPPT photovoltaic inverter;

FIG. 2 is a control flow chart of a dual-channel MPPT photovoltaic inverter in prior art; and

FIG. 3 is a control flow chart of a dual-channel MPPT photovoltaic inverter according to the present invention.

Detailed description

Preferred embodiments of the present invention will be described in detail in conjunction with the accompanying drawings so that advantages and features of the present invention will be more readily understood by those skilled in the art.

Without loss of generality, using the more applied dual-channel MPPT input photovoltaic inverter as an example, a multi-channel MPPT control logic and control method according to the present invent are described.

A hardware circuit used by the present invention as shown in FIG. 1 comprises photovoltaic panels PVl, PV2, input capacitors Ci, C ₂ , input boost circuit boosti, boost ₂ , a direct-current bus capacitor Cd ₀ , an H-bridge inverter and a controller DSC. Outputs of the boost circuit boosti and boost ₂ are connected to a common direct-current bus, and power is fed into the power grid through the bus capacitor Cd ₀ and the H-bridge inverter. PWMi and PWM ₂ are drive signals of the boost circuit boosti and boost ₂ , respectively. A digital signal processor (DSP) is adopted to implements the controller DSC by corresponding hardware signal processing and acquisition of information of PV input voltages v _PV1 and v _PV2 in two channels, PV input currents i _PV1 and i _PV2 in the two channels and a direct-current bus voltage v _bus .

Compared with the existing two-channel MPPT control method, the control method according to the present invention redesigns a start and stop control logic of the dual-channel PV input boost circuit, and constructs a virtual local maximum power point (VLMMP) for judgment in restart of the double-channel boost circuit. Using the control method according to the present invention, the power loss in a steady state of the inverter can be reduced, the conversion efficiency of the inverter is improved, and the economic operation of the inverter is implemented.

The control method according to the present invention as shown in FIG. 3 comprises the steps.

In step SI, a controller DSC collecting and obtains information of PV input voltages v _PV1 and v _PV2 in two channels, PV input current i _PV1 and i _PV2 in the two channels and a direct-current bus voltage v _bus ;

a PVl input power P _PV1 , a PV2 input power P _PV2 and a total input power P _{PV sum} of the inverter are calculated and obtained, while a voltage difference a Av _PV between the two PV input voltages is obtained, and the expressions are as below:

Δν _ρν = ^V PV1 ^{~ V} PV2 IV In step S2, voltage differences v _{PV max} and Δν _ρν _ _MS are compared to obtain a start and stop state of a boost circuit in each channel, a PV voltage reference signal v _PVm in each channel and a direct-current bus voltage reference signal v _b ^* _us , which specifically comprises following steps:

in step S21 , when Δν _ρν ≥ V _THb ,the controller DSC turns off a boosti controller, blocks a drive signal PWMi of the boosti circuit, turns off the boosti circuit, enables a boost ₂ controller and obtains a drive signal PWM ₂ of the boost ₂ circuit, and at the same time the controller DSC maximizes the PV1 input power P _PV1 and the PV2 input power P _PV2 by an MPPT modules to obtain PV voltage reference signals v _PV1 and v _PV2 in the two channels, a direct-current bus voltage reference signal v _b ^* _us is given by the PV1 input voltage reference signal, i.e., v _b ^* _us = v _P ^* _V1 ;

when vpy≤—V _THb , the controller DSC turns off the boost ₂ controller, blocks the drive signal PWM ₂ of the boost ₂ circuit, turns off the boost ₂ circuit, enables the boosti controller and obtaining the drive signal PWMi of the boosti circuit, and at the same time the controller DSC maximizes the PV1 input power P _PV1 and the PV2 input power P _PV2 by the MPPT modules to obtain PV voltage reference signals v _PV1 and v _PV2 in the two channels, the direct-current bus voltage reference signal v _bus is given by the PV2 input voltage reference signal, i.e., v _b ^* _us = v _P ^* _V2 ,

in step S22, when V _THb ≥ \Δν _ρν _ _MS \≥ V _THs , the controller DSC enables both the boosti and boost ₂ controllers and obtains the drive signals PWMi and PWM ₂ of the boosti and boost ₂ circuits, and at the same time the controller DSC maximizing the PV1 input power P _PV1 and the PV2 input power P _PV2 by the MPPT modules to obtain the PV voltage reference signals v _PV1 and v _PV2 in the two channels, the direct-current bus voltage reference signal v _b ^* _us is a maximum value of PV voltage reference signals in the two channels, i.e., ^v lus ^{= max} { ^v pvi, ^v PV2) > ^{' an} d

in step S23, when V _THs ≥ \Δν _ρν \≥ 0, when the controller DSC enters this mode for the first time, the controller DSC constructs a VLMPP voltage based on formula V and the collected information of the direct-current bus voltage v _bus , which comprises the follow two step :

in step S231 , when V _THb ≥ \v _bus — V _VLMPP \ > 0, the controller DSC turns off both the boosti and boost ₂ controllers, blocks the drive signals PWMi and PWM ₂ of the boosti and boost ₂ circuits and maximizes the total input power P _{PV sum} of the inverter by the MPPT module based on v _bus to obtain the direct-current bus voltage reference signal v _bus , at which point the PV voltage reference signals v _PV1 and v _PV2 in the two channels will not work; and

in step S232, when \v _bus — V _VLMPP \≥ V _THb , the controller DSC enables both the boosti and boost ₂ controllers mandatorily, obtains the drive signals PWMi and PWM ₂ of the boosti and boost ₂ circuits and maximizes the PV1 input power P _PV1 and the PV2 input power P _PV2 by the MPPT module to obtain the PV voltage reference signals v _PV1 and v _PV2 in the two channels, the direct-current bus voltage reference signal v _bus is a maximum value of PV voltage reference signals in the two channels, i.e., v _b ^* _us = max(v _PV1 v _PV2 ) .

_ vpyi+v _PV2

^V VLMPP — ₂ ^v

The invention mainly performs logic control on the multi-channel MPPT. In specific implementation, the expected result can only be achieved in conjunction with a boost voltage, current double closed-loop controller, the existing single-channel MPPT controller, etc. At the same time, in order to reduce power sampling and calculation errors, the controller DSC will calculate power using voltage and current sampling averages within 0.2s; an operational cycle of the MPPT module is Is to reduce the phenomenon of misjudgment.

The embodiment described above is merely illustrative of the technical concept and features of the present invention and is a preferred embodiment so as to enable those skilled in the art to understand the content of the present invention and practice it accordingly, and is not intended to limit the protection scope of the present invention. Any equivalent alteration or modification made in accordance with the spirit of the present invention should be included in the protection scope of the present invention.