Three-phase unbalanced output control method and system thereof

Abstract

The present invention discloses a three-phase unbalanced output control method and system thereof, wherein the method comprises: using the sum of the three-phase output power P ac , the sum of the three-phase grid-connected power P export And the three-phase grid-connected port power limit value P exprotLimit By adjusting the output power, the sum of the three-phase grid-connected power P export 3×P exprotLimit , and then limit the grid-connected power of two phases to P exprotLimit , achieving three-phase unbalanced output; the system includes several photovoltaic modules, each with a DC / DC converter and a DC / AC converter connected to the output side. The DC / AC converter is connected to the three-phase and neutral terminals of a three-phase mains supply via four relays. This invention simultaneously meets the requirements of being friendly to traditional power grid systems, achieving self-generation and self-consumption, and improving photovoltaic energy utilization.

Description

TECHNICAL FIELD

[0001] The present application relates to an output control method, in particular to a three-phase unbalanced output control method and system thereof. BACKGROUND

[0002] With the development of society and the progress of science and technology, people's demand for energy is also growing, and in the face of the scarcity of traditional energy, the development of new energy technology is supported by national policy, and as a clean and inexhaustible energy source, solar energy, photovoltaic power generation technology has developed rapidly, and the installed capacity of photovoltaic power generation system has also increased year by year. However, photovoltaic power generation is greatly affected by the environment, and its power generation power changes with the change of light conditions, which is an unstable energy source. When a large number of photovoltaic systems are connected to the traditional power grid system, it will impact the stability of the power grid system; Therefore, some countries and regions of the power company have proposed the requirement of self-generation and self-use for photovoltaic grid-connected power generation system users, and do not allow to have grid power.

[0003] The existing three-phase photovoltaic grid-connected power generation system is a three-phase balanced output system, which can realize zero grid-connected power control of the sum of three-phase grid-connected power. However, when the user load is three-phase unbalanced, it cannot realize zero grid-connected power control of each phase grid-connected power, so there will be some phase grid power and some phase needs to take power from the power grid. Therefore, the current three-phase grid-connected power generation system cannot simultaneously meet the requirements of friendliness to the traditional power grid system, self-generation and self-use, and improvement of photovoltaic energy utilization rate. SUMMARY

[0004] The purpose of the present application is to provide a three-phase unbalanced output control method and system. The present application can simultaneously meet the requirements of friendliness to the traditional power grid system, self-generation and self-use, and improvement of photovoltaic energy utilization rate.

[0005] The technical scheme of the present application is a three-phase unbalanced output control method, comprising the following steps:

[0006] S1, according to the collected three-phase power voltage U R , U S , U T and three-phase output current I R , I S , I T , respectively, the output power P R , P S and P T of the three-phase photovoltaic grid-connected inverter are obtained;

[0007] According to the collected three-phase power voltage U R , U S , U T and three-phase grid-connected current I export_R, I export_S , I export_T , and obtain the three-phase grid-connected port power P export_R 、P export_S 、P export_T ;

[0008] S2, according to the three-phase output power P R 、P S and P T , and obtain the sum of the three-phase output power Pac;

[0009] According to the three-phase grid-connected port power P export_R 、P export_S and P export_T , get the sum of the three-phase grid-connected power P export ;

[0010] S3, according to the user set three-phase grid-connected port power limit of each phase P exprotLimit , and the sum of the three-phase output power P ac , the sum of the three-phase grid-connected power P export , the output power limit P of the three-phase photovoltaic grid-connected inverter is obtained acLimit ;

[0011] S4. Compare P acLimit and P ac The size of the output power is adjusted accordingly to obtain the target voltage value of the photovoltaic module output, and the sum of the three-phase grid-connected power P export 3×P exprotLimit ;

[0012] S5, limit the grid-connected port power of phase R and phase S, and adjust the three-phase output current I peak_ref[R] , I peak_ref[S] and I peak_ref[T] , to achieve three-phase unbalanced output.

[0013] In the above-mentioned three-phase unbalanced output control method, the bus voltage control loop reference value Bus Ac_ref and bus voltage value U bus The difference is used as the error signal and outputs the AC current peak reference value I through the PI controller. peak ;

[0014] Set the AC grid-connected port power limit P exprotLimit The grid-connected power P of the R phase export_R The difference between the current output and I peak The smaller value is used as the peak reference value I of the R phase current peak_ref[R] ;

[0015] Set the AC grid-connected port power limit PexprotLimit The grid-connected power P of the S phase export_s The difference between the current output value and I peak The smaller value is used as the peak reference value of the S phase current I peak_ref[S] ;

[0016] Take I peak As the current peak reference value I of phase T peak_ref[T] .

[0017] In the aforementioned three-phase unbalanced output control method, the calculation formula for the three-phase output power and the three-phase grid-connected port power is: Where T represents the mains cycle time, u(t) represents the instantaneous value of the mains voltage, and i(t) represents the instantaneous value of the current.

[0018] In the above-mentioned three-phase unbalanced output control method, the sum of the three-phase output power P ac =P R +P S +P T ; The sum of the three-phase grid-connected power P export =P export_R +P export_S +P export_T .

[0019] In the above-mentioned three-phase unbalanced output control method, the output power limit P acLimit =P ac +K×(3×P exprotLimit -P export ), where K is an internal control parameter.

[0020] In the above-mentioned three-phase unbalanced output control method, step S4 is specifically as follows: comparing P acLimit and P ac The size of P acLimit ﹤P ac , then the power limit flag is set, if P acLimit ﹥(P ac + hysteresis threshold), the power limit flag is cleared; if the power limit flag is set, the power limit control loop is executed, and the voltage output value of the power limit control loop is used as the voltage target value of the PV module output; if the power limit flag is not set, the maximum power tracking algorithm is used to obtain the voltage target value of the PV module output.

[0021] In the above-mentioned three-phase unbalanced output control method, the power limit control loop is based on P acLimit As the reference value, P ac The PI controller outputs the target voltage value U of the photovoltaic module. pvref .

[0022] A system for implementing the three-phase unbalanced output control method comprises a plurality of photovoltaic assemblies, the output side of each photovoltaic assembly is connected with a DC / DC converter, the output end of each DC / DC converter is connected with the input end of a DC / AC converter through a DC bus, the output end of the DC / AC converter is connected with the three-phase line end of a three-phase power supply through a three-phase relay, and the midpoint of the DC bus is connected with the zero line end of the three-phase power supply through a zero line relay.

[0023] In the foregoing system, an output current sensor for measuring the output current of the three-phase photovoltaic grid-connected inverter is connected in series between the three-phase relay and the DC / AC converter; a grid-connected port current sensor for measuring the grid-connected port current is connected in series between the three-phase relay and the three-phase power supply, and a user-side three-phase load and a voltage sensor for measuring the three-phase power supply voltage are connected between the R line and the N line, between the S line and the N line, and between the T line and the N line, respectively.

[0024] Compared with the prior art, the present application has the following advantages:

[0025] The present application realizes the unbalanced output of three-phase power by designing a control algorithm of three-phase four-wire output mode, thereby reducing the impact of the photovoltaic system on the traditional power grid system, and realizing the optimal output power control of the photovoltaic assembly and improving the utilization rate of photovoltaic energy, so as to meet the requirements of user self-use and reduce the use of power grid system and the electricity expense.

[0026] The present application also designs a three-phase photovoltaic grid-connected inverter which can be compatible with three-phase three-wire output and three-phase four-wire output, and the user can independently select three-phase output power balance control or three-phase output power unbalance control and control the three-phase four-wire output mode to realize the unbalanced output function of three-phase four-wire. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 is a schematic diagram of the power limiting control loop of the sum of three-phase grid-connected port power in the present application;

[0028] Figure 2 is a schematic diagram of the limiting control loop of each phase grid-connected port power in the present application;

[0029] Figure 3 is a schematic diagram of the grid-connected inverter circuit structure of the three-phase unbalanced system. DETAILED DESCRIPTION

[0030] The present application will be further described below in conjunction with the embodiments, but it is not used as the basis for limiting the present application.

[0031] Embodiment:

[0032] A three-phase unbalanced output control method comprises the following steps:

[0033] S1, select the three-phase four-wire output of the three-phase photovoltaic grid-connected inverter, according to the collected three-phase mains voltage U R 、U S 、U T And the three-phase output current I R , I S , I T , and the three-phase output power P of the three-phase photovoltaic grid-connected inverter is obtained respectively. R 、P S and P T ;

[0034] According to the collected three-phase mains voltage U R 、U S 、U T And the three-phase grid-connected current I export_R , I export_S , I export_T , and obtain the three-phase grid-connected port power P export_R 、P export_S 、P export_T ;

[0035] The calculation formulas for the three-phase output power and the three-phase grid-connected port power are: Where T represents the mains cycle time, u(t) represents the instantaneous value of the mains voltage, and i(t) represents the instantaneous value of the current.

[0036] S2, according to the three-phase output power P R 、P S and P T , the sum of the three-phase output power of the three-phase photovoltaic grid-connected inverter P is obtained ac , P ac =P R +P S +P T ;

[0037] According to the three-phase grid-connected port power P export_R 、P export_S and P export_T , get the sum of the three-phase grid-connected power P export , P export =P export_R +P export_S +P export_T ;

[0038] S3, according to the user set three-phase grid-connected port power limit of each phase P exprotLimit , and the sum of the three-phase output power P ac , the sum of the three-phase grid-connected power Pexport , get the output power limit P acLimit of the three-phase photovoltaic grid-connected inverter acLimit ; P ac = P exprotLimit + K x (3 x P export - P acLimit ), wherein K is an internal control parameter, and K is 0.1 in the embodiment.

[0039] S4, compare the sizes of P ac and P acLimit , if P ac < P acLimit , the limit power flag bit is set to 1; if P ac + hysteresis threshold), the limit power flag bit is cleared, and the limit power flag bit is assigned to 0; the hysteresis threshold is an internal control parameter, which avoids frequent setting and clearing of the limit power flag bit, and in the embodiment, the hysteresis threshold is 200w; then it is judged whether the limit power flag bit is set, if the limit power flag bit is set, the power limiting control loop is executed, and the voltage output value of the power limiting control loop is taken as the voltage target value of the photovoltaic module output; if the limit power flag bit is not set, the voltage corresponding to the maximum power obtained by the maximum power tracking algorithm is taken as the voltage target value of the photovoltaic module output. Thus, the sum P export of the three-phase grid-connected port powers is 3 x P exprotLimit .

[0040] Wherein, the power limiting control loop takes P acLimit as a reference value, takes P ac as a feedback value, adjusts the output power of the photovoltaic module, and outputs the voltage target value U pvref of the photovoltaic module through the PI controller, as shown in Figure 1 .

[0041] S5, limit the grid-connected port powers of the R phase and the S phase, control the grid-connected port powers of the R phase and the S phase to be P exprotLimit , adjust the peak reference value of the three-phase output current, as shown in Figure 2 , take the difference between the bus voltage control loop reference value Bus Ac_ref and the bus voltage value U bus as an error signal, and output the AC current peak reference value I peak through the PI controller;

[0042] Take the difference between the AC grid-connected port power limit P exprotLimit and the grid-connected port power P export_R of the R phase as an error signal, control through the PI controller, and take the smaller value between the current output value and I peak as the peak reference value I peak_ref[R] of the R phase current;

[0043] Set the AC grid-connected port power limit P exprotLimit The grid-connected power P of the S phase export_s The difference between the current output and I peak The smaller value is used as the peak reference value of the S phase current I peak_ref[S] ;

[0044] Take I peak As the current peak reference value I of phase T peak_ref[T] .

[0045] When the three phases are unbalanced, I peak_ref[R] , I peak_ref[S] , I peak_ref[T] For different current values, thus achieving three-phase unbalanced output, when the grid port power limit value P exprotLimit If it is 0, the power output of each phase grid-connected port is zero.

[0046] A system for implementing the above-mentioned three-phase unbalanced output control method, such as Figure 3 As shown, it includes several photovoltaic modules, and the output side of each photovoltaic module is connected to a DC / DC converter. The output end of each DC / DC converter is connected to the input end of the DC / AC converter via a DC bus. The output end of the DC / AC converter is connected to the input end of the DC / AC converter via a three-phase relay S R 、S S 、S T Connect to the R, S and T terminals of the three-phase mains electricity, and the DC bus midpoint is connected to the zero line relay S N Connect to the N line of the three-phase mains, the three-phase relay S R 、S S 、S T There is a corresponding series connection between the DC / AC converter for measuring the output current I of the three-phase photovoltaic grid-connected inverter R , I S and I T Output current sensor; three-phase relay S R 、S S 、S T The three-phase mains R, S and T terminals are connected in series with the three-phase mains current I export_R , I export_S , I export_T The grid-connected port current sensor has user-side three-phase loads Z connected between the R line and the N line, between the S line and the N line, and between the T line and the N line. R , Z S , Z T And used to measure the three-phase mains voltage U R 、US , U T Voltage sensor of the application.

[0047] When selecting three-phase photovoltaic grid-connected inverter three-phase three-wire output, the relay S N Balanced control of three-phase output power is realized.

[0048] When selecting three-phase photovoltaic grid-connected inverter three-phase four-wire output, the relay S N Unbalanced control of three-phase output power is realized.

[0049] Three-phase output power can be calculated by three-phase photovoltaic grid-connected inverter three-phase output current and three-phase grid voltage collected by DSP controller, or by three-phase power meter. Then the above-mentioned three-phase power unbalanced output control method is used to realize zero output or unbalanced output of grid-connected port power.

Claims

1. A three-phase unbalanced output control method, characterized by: The following steps are involved: S1, according to the collected three-phase mains voltage U R 、U S 、U T And the three-phase output current I R , I S , I T , and the output power P of the three-phase photovoltaic grid-connected inverter is obtained respectively. R 、P S and P T ; According to the collected three-phase mains voltage U R 、U S 、U T And the three-phase grid-connected current I export_R , I export_S , I export_T , and obtain the three-phase grid-connected port power P export_R 、P export_S 、P export_T ; S2, according to the three-phase output power P R 、P S and P T , and obtain the sum of the three-phase output power Pac; According to the three-phase grid-connected port power P export_R 、P export_S and P export_T , get the sum of the three-phase grid-connected power P export ; S3, according to the user set three-phase grid-connected port power limit of each phase P exprotLimit , and the sum of the three-phase output power P ac , the sum of the three-phase grid-connected power P export , the output power limit P of the three-phase photovoltaic grid-connected inverter is obtained acLimit ; S4. Compare P acLimit and P ac The size of the output power is adjusted accordingly to obtain the target voltage value of the photovoltaic module output, and the sum of the three-phase grid-connected power P export 3×P exprotLimit ; S5, limit the grid-connected port power of phase R and phase S, and adjust the three-phase output current I peak_ref[R] , I peak_ref[S] and I peak_ref[T] , to achieve three-phase unbalanced output.

2. The three-phase unbalanced output control method according to claim 1, characterized in that: S5 is specifically: the bus voltage control loop reference value Bus Ac_ref and bus voltage value U bus The difference is used as the error signal and outputs the AC current peak reference value I through the PI controller. peak ; Set the AC grid-connected port power limit P exprotLimit The grid-connected power P of the R phase export_R The difference between the current output and I peak The smaller value is used as the peak reference value I of the R phase current peak_ref[R] ; Set the AC grid-connected port power limit P exprotLimit The grid-connected power P of the S phase export_s The difference between the current output value and I peak The smaller value is used as the peak reference value of the S phase current I peak_ref[S] ; Take I peak As the current peak reference value I of phase T peak_ref[T] .

3. The three-phase unbalanced output control method according to claim 1, characterized in that: The calculation formula for the three-phase output power and the three-phase grid-connected port power is: Where T represents the mains cycle time, u(t) represents the instantaneous value of the mains voltage, and i(t) represents the instantaneous value of the current.

4. The three-phase unbalanced output control method according to claim 1, characterized in that: The sum of the three-phase output power P ac =P R +P S +P T ; The sum of the three-phase grid-connected power P export =P export_R +P export_S +P export_T .

5. The three-phase unbalanced output control method according to claim 1, characterized in that: The output power limit P acLimit =P ac +K×(3×P exprotLimit -P export ), where K is an internal control parameter.

6. The three-phase unbalanced output control method according to claim 1, characterized in that: Step S4 is specifically as follows: compare P acLimit and P ac The size of P acLimit ﹤P ac , then the power limit flag is set, if P acLimit ﹥(P ac + hysteresis threshold), the power limit flag is cleared; If the power limit flag is set, the power limit control loop is executed, and the voltage output value of the power limit control loop is used as the voltage target value of the photovoltaic module output; if the power limit flag is not set, the maximum power tracking algorithm is used to obtain the voltage target value of the photovoltaic module output.

7. The three-phase unbalanced output control method according to claim 6, characterized in that: The power limit control loop is based on P acLimit As the reference value, P ac The PI controller outputs the target voltage value U of the photovoltaic module. pvref .

8. A system for implementing the three-phase unbalanced output control method according to any one of claims 1 to 7, characterized in that: It includes several photovoltaic modules, and the output side of each photovoltaic module is connected to a DC / DC converter. The output end of each DC / DC converter is connected to the input end of the DC / AC converter via a DC bus. The output end of the DC / AC converter is connected to the three-phase line end of the three-phase mains via a three-phase relay, and the midpoint of the DC bus is connected to the neutral line end of the three-phase mains via a neutral line relay.

9. A system according to claim 8, characterized in that: An output current sensor for measuring the output current of the three-phase photovoltaic grid-connected inverter is connected in series between the three-phase relay and the DC / AC converter; a grid-connected port current sensor for measuring the three-phase grid-connected port current is connected in series between the three-phase relay and the three-phase mains power. The user-side three-phase load and the voltage sensor for measuring the three-phase mains voltage are respectively connected between the R line and the N line, between the S line and the N line, and between the T line and the N line.

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