Derating control system and method for photovoltaic grid-connected inverter
Technical Field
The invention relates to the field of photovoltaic power generation, in particular to a derating control system and method for a photovoltaic grid-connected inverter.
Background
Fig. 1 shows a conventional grid-connected inverter, in which a Boost circuit performs MPPT tracking control on PV and transmits PV power to a bus capacitor. The grid-connected inverter feeds the power transmitted by the Boost circuit back to the power grid by stabilizing the bus voltage, so that the energy balance of the system bus is maintained, and grid-connected feeding is automatically realized. When a grid-connected inverter needs to reduce grid-connected feed power (the reason is that the inverter is over-temperature and grid-connected power needs to be reduced in power grid feed scheduling), in order to realize the rated power processing of a photovoltaic grid-connected system, as in the prior art, two main implementation modes are adopted:
firstly, as shown in fig. 2, an inversion control loop is adjusted to limit power of a system, wherein Boost continues to input power to a bus capacitor, and a grid-connected inverter suddenly reduces load, so that the input power is larger than output power, energy is accumulated in a direct current bus, overvoltage protection of the direct current bus capacitor is caused, and if the system is not protected timely, system fault damage is caused. When the grid-connected inverter needs to reduce grid-connected feed power (the reason is that the inverter is over-temperature, and grid feed scheduling needs to reduce the grid-connected power), the Boost circuit still continuously transmits power to the bus capacitor, so that the overvoltage of the bus capacitor is caused, and the system fault is caused.
Secondly, as shown in fig. 3, the power of the system is limited by adjusting the Boost control loop, and if only the Boost determines that the bus voltage is too high, the input power is limited, so that the power balance of the system can be maintained, but in this case, the bus voltage is always in a high-voltage state.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides a maximum power point tracking method of a photovoltaic system, which not only reduces the system loss, but also improves the reliability of the system.
In order to achieve the purpose, the technical scheme of the invention is as follows: a derating control system of a photovoltaic grid-connected inverter comprises a DC/DC module and a DC/AC module which are connected with a photovoltaic polar plate, and further comprises a DC/AC control unit, wherein the DC/AC control unit comprises a first PID controller, a first amplitude limiter, a first power limiter and a second PID controller which are sequentially connected;
the reference value of the first PID controller is connected with a bus voltage set value Ubusref, the feedback value of the first PID controller is connected with a bus voltage value Ubus, a first current instruction value Iref1 output by the first PID controller is subjected to amplitude limiting by the first amplitude limiter to obtain a second current instruction value Iref2, the second current instruction value Iref2 is subjected to amplitude limiting by the first power limiter to obtain a third current instruction value Iref3, the third current instruction value Iref3 is used as the reference value of the second PID controller, and the current value Iiinv output by the DC/AC module is used as the feedback value of the second PID controller;
the first amplitude limiter carries out first fixed amplitude limiting on a first current instruction value Iref 1;
and the first power limiter limits the grid-connected power according to the DC/AC module, and further dynamically limits the second current instruction value Iref 2.
In an embodiment of the present invention, the first amplitude limiter performs a first fixed amplitude limitation on the first current instruction value Iref1, where the first fixed amplitude is Iset1, iset1>0,
In an embodiment of the present invention, the first power limiter limits the second current instruction value Iref2 according to the grid-connected power of the DC/AC module, and further performs dynamic amplitude limiting on the second current instruction value Iref2, specifically, the second current instruction value Iref2 is limited according to the dynamic amplitude valuePerforming dynamic clipping, P limit Limiting the power value for the grid connection of the DC/AC module, U gird The effective value of the grid-connected voltage of the DC/AC module is obtained;
in an embodiment of the present invention, the present invention further includes a DC/DC control unit, where the DC/DC control unit includes an MPPT controller, a second limiter, a second power limiter, and a third PID controller, which are connected in sequence;
the MPPT controller carries out maximum power tracking according to an output voltage value Upv and an output current value Ipv of the photovoltaic polar plate to output a fourth current instruction Iref4, the fourth current instruction Iref4 obtains a fifth current instruction value Iref5 through amplitude limiting of the second amplitude limiter, the fifth current instruction value Iref5 obtains a sixth current instruction value Iref6 through the second power limiter, the sixth current instruction value Iref6 is used as a reference value of a third PID controller, and the output current value Ipv of the photovoltaic polar plate is used as a feedback value of the third PID controller;
the second amplitude limiter carries out fixed amplitude limiting on the fourth current instruction Iref 4;
and the second power limiter limits according to the direct current bus voltage value Ubus, and further performs dynamic amplitude limiting on the fifth current instruction value Iref 5.
In an embodiment of the present invention, the second amplitude limiter performs a second fixed amplitude limitation on the fourth current instruction value Iref4, where the second fixed amplitude is Iset2, iset2>0,
In an embodiment of the present invention, the second power limiter limits according to a dc bus voltage value Ubus, and further dynamically limits a fifth current command value Iref5, specifically, dynamically limits a current of the fifth current command value Iref5
U1 is the highest direct current bus voltage point of the full power operation of the system; u2 is the highest voltage point of the allowed work of the direct current bus voltage; m (k) = m (k-1) + Δ m, where m (k) is the fifth current command value Iref5 adjustment value in the current cycle, and m (k-1) is the fifth current command value Iref5 adjustment value in the previous cycle.
The invention also provides a derating control method of the photovoltaic grid-connected inverter, which comprises the following steps,
s1, collecting an output voltage value Upv and an output current value Ipv of a photovoltaic polar plate, a bus voltage Ubus and a grid-connected voltage effective value U in real time gird Grid-connected current value Iinv;
s2, when the photovoltaic grid-connected inverter receives the derating instruction, setting a grid-connected limit power value P of the DC/AC module limit ;
S3, obtaining a first current instruction value Iref1 by taking a difference value between a bus voltage set value Ubusref and a bus voltage Ubus through a PID algorithm, and obtaining a second current instruction value Iref2 after fixed amplitude limitation is carried out through a first amplitude limiter;
the second current instruction value Iref2 is limited according to the grid-connected power of the DC/AC module, dynamic amplitude limiting is further carried out to obtain a third direct current instruction value Iref3, the difference value of the third direct current instruction value Iref3 and the grid-connected current value Iiinv of the DC/AC module is obtained through a PID algorithm to obtain a PWM signal, and a PWM generator generates a PWM driving signal to drive a switching tube of the DC/AC module to work.
In an embodiment of the invention, the first limiter performs the first dc command value Iref1 according to the first current command value Iref1The line first fixed amplitude clipping, specifically,the first fixed amplitude is Iset1, iset1>0;
The first power limiter limits the grid-connected power according to the DC/AC module, and further dynamically limits the second current direct current instruction value Iref2, specifically, the second current direct current instruction value Iref2 is limited according to the dynamic amplitudePerforming clipping, P limit Limiting the power value for the grid connection of the DC/AC module, U gird The effective value of the grid-connected voltage of the DC/AC module is obtained;
in an embodiment of the present invention, the method further includes the following steps:
the MPPT controller carries out maximum power tracking according to the output voltage value Upv and the output current value Ipv of the photovoltaic polar plate to output a fourth current instruction Iref4, and the fourth current instruction Iref4 carries out fixed amplitude limiting through a second amplitude limiter to obtain a fifth current instruction value Iref5;
and the fifth current instruction value Iref5 is limited by the second power limiter according to the direct current bus voltage value Ubus, dynamic amplitude limitation is further performed on the fifth current instruction value Iref5 to obtain a sixth current instruction value Iref6, a difference value between the sixth current instruction value Iref6 and the output current value Ipv of the photovoltaic polar plate is obtained through a PID algorithm to obtain a PWM signal, and a PWM generator generates a PWM driving signal to drive a switching tube of the DC/DC module to work.
In an embodiment of the present invention, the second limiter performs a second fixed amplitude limiting on a fourth current instruction value Iref4, where the second fixed amplitude is Iset2, iset2>0,
The second power limiter limits the output power of the DC/DC module to dynamically limit the fifth current instruction value Iref5, specifically to limit the output power of the DC/DC module
U1 is the highest DC bus voltage point of the system in full power operation; u2 is the maximum voltage point of the allowed work of the direct current bus voltage; m (k) = m (k-1) + Δ m, where m (k) is the fifth current command value Iref5 adjustment value in the current cycle, and m (k-1) is the fifth current command value Iref5 adjustment value in the previous cycle.
Compared with the prior art, the invention has the following beneficial effects:
1. the problem of system protection or damage caused by sudden load reduction of the photovoltaic grid-connected inverter and bus voltage increase is avoided;
2. the derating method of the photovoltaic grid-connected inverter disclosed by the invention has the advantages that the voltage stress of a system switch tube is reduced, the system loss is reduced, and the reliability of the system is improved;
3. according to the invention, by introducing an m (k) mode, on the basis of the original power limitation, the Boost power is gradually reduced, so that the input power and the inversion output power of the Boost are balanced again, and the voltage drop of the Bus voltage is maintained at a normal working voltage point.
4. In the invention, a Boost power reducing mode m (k) = m (k-1) + Δ m is adopted, and in the mode, Δ m is a small amount, so that the change of power is a small amount change, the input power smoothly changes, the output power is smooth, the output power smoothly changes before and after the system reduces the power, the output power is stable, and no step change phenomenon exists.
Drawings
Fig. 1 is a schematic diagram of a conventional photovoltaic grid-connected inverter.
Fig. 2 is a derating schematic diagram of a photovoltaic grid-connected inverter in the prior art.
Fig. 3 is a derating schematic diagram of a photovoltaic grid-connected inverter in the prior art.
Fig. 4 is one of derating schematic diagrams of a grid-connected inverter of the present invention.
Fig. 5 shows a second derating schematic diagram of the grid-connected inverter according to the present invention.
Detailed Description
The invention is further explained below with reference to the drawings and the embodiments.
Example 1:
a derating control system of a photovoltaic grid-connected inverter comprises a DC/DC module and a DC/AC module which are connected with a photovoltaic polar plate, and comprises a DC/AC control unit, wherein the DC/AC control unit comprises a first PID controller, a first amplitude limiter, a first power limiter and a second PID controller which are connected in sequence,
the reference value of the first PID controller is connected with a bus voltage set value Ubusref, the feedback value of the first PID controller is connected with the bus voltage Ubus, a first current instruction value Iref1 output by the first PID controller is subjected to amplitude limiting by a first amplitude limiter to obtain a second current instruction value Iref2, the second current instruction value Iref2 is subjected to amplitude limiting by a first power limiter to obtain a third current instruction value Iref3, the third current instruction value Iref3 is used as the reference value of the second PID controller, and the current value Iiinv output by the DC/AC module is used as the feedback value of the second PID controller,
the first amplitude limiter carries out first fixed amplitude limiting on a first current instruction value Iref 1;
and the first power limiter limits the grid-connected power according to the DC/AC module, and further dynamically limits the second current instruction value Iref 2.
The DC/DC control unit comprises an MPPT controller, a second amplitude limiter, a second power limiter and a third PID controller which are connected in sequence,
the MPPT controller carries out maximum power tracking according to an output voltage value Upv and an output current value Ipv of the photovoltaic polar plate to output a fourth current instruction Iref4, the fourth current instruction Iref4 obtains a fifth current instruction value Iref5 through amplitude limiting of a second amplitude limiter, the fifth current instruction value Iref5 obtains a sixth current instruction value Iref6 through a second power limiter, the sixth current instruction value Iref6 is used as a reference value of a third PID controller, the output current value Ipv of the photovoltaic polar plate is used as a feedback value of the third PID controller,
the second amplitude limiter carries out fixed amplitude limiting on a fourth current instruction Iref 4;
and the second power limiter limits according to the direct current bus voltage value Ubus, and further dynamically limits the amplitude of the fifth current instruction value Iref 5.
The first amplitude limiter carries out first fixed amplitude limiting on a first current instruction value Iref1, and the first fixed amplitude is Iset1 and Iset1>0,
The first power limiter limits the grid-connected power according to the DC/AC module, and further dynamically limits the second current instruction value Iref2, specifically, the second current instruction value Iref2 is limited according to the dynamic amplitudePerforming dynamic clipping, P limit Limiting the power value for the grid connection of the DC/AC module, U gird Is the effective value of the grid-connected voltage of the DC/AC module,
the second amplitude limiter performs second fixed amplitude limiting on a fourth current instruction value Iref4, wherein the second fixed amplitude is Iset2 and Iset2>0,
The second power limiter limits according to the direct current bus voltage value Ubus, and then dynamically limits the fifth current instruction value Iref5, specifically, the second power limiter limits the voltage according to the direct current bus voltage value Ubus, and specifically, the second power limiter limits the fifth current instruction value Iref5 dynamically
U1 is the highest DC bus voltage point of the system in full power operation; u2 is the maximum voltage point of the allowed work of the direct current bus voltage; m (k) = m (k-1) + Δ m, where m (k) is the fifth current command value Iref5 adjustment value in the current cycle, and m (k-1) is the fifth current command value Iref5 adjustment value in the previous cycle.
A derating control method for a photovoltaic grid-connected inverter comprises the following steps,
s1, collecting an output voltage value Upv and an output current value Ipv of a photovoltaic polar plate, a bus voltage Ubus and a grid-connected voltage effective value U in real time gird The grid-connected current value Iinv;
s2, when the photovoltaic grid-connected inverter receives the derating instruction, setting a grid-connected limit power value P of the DC/AC module limit ;
S3, obtaining a first current instruction value Iref1 by taking the difference value between the bus voltage set value Ubusref and the bus voltage Ubus through a PID algorithm, obtaining a second current instruction value Iref2 after the fixed amplitude limitation is carried out by a first amplitude limiter,
the second current instruction value Iref2 is limited according to the grid-connected power of the DC/AC module to carry out dynamic amplitude limiting to obtain a third direct current instruction value Iref3, the difference value of the third direct current instruction value Iref3 and the grid-connected current value Iiinv of the DC/AC module is obtained through a PID algorithm to obtain a PWM signal, and a PWM generator generates a PWM driving signal to drive a switching tube of the DC/AC module to work.
Step S4, the MPPT controller carries out maximum power tracking according to the output voltage value Upv and the output current value Ipv of the photovoltaic polar plate to output a fourth current instruction Iref4, and the fourth current instruction Iref4 carries out fixed amplitude limiting through a second amplitude limiter to obtain a fifth current instruction value Iref5;
and the fifth current instruction value Iref5 is limited by the second power limiter according to the direct current bus voltage value Ubus, dynamic amplitude limitation is further performed on the fifth current instruction value Iref5 to obtain a sixth current instruction value Iref6, a difference value between the sixth current instruction value Iref6 and the output current value Ipv of the photovoltaic polar plate is obtained through a PID algorithm to obtain a PWM signal, and a PWM generator generates a PWM driving signal to drive a switching tube of the DC/DC module to work.
In step S2, the first limiter performs a first fixed amplitude limiting on the first current dc command value Iref1, specifically,the first fixed amplitude is Iset1, iset1>0;
The first power limiter limits the grid-connected power of the DC/AC module, and then dynamically limits the second current direct current instruction value Iref2, specifically, the second current direct current instruction value Iref2 is limited according to the dynamic amplitudePerforming clipping, P limit Limiting the power value for the grid connection of the DC/AC module, U gird Is the effective value of the grid-connected voltage of the DC/AC module,
in step S3, the second limiter performs a second fixed amplitude limitation on the fourth current instruction value Iref4, where the second fixed amplitude is Iset2, iset2>0,
The second power limiter limits the output power of the DC/DC module, and further dynamically limits the fifth current instruction value Iref5, specifically, the second power limiter limits the output power of the DC/DC module, and further dynamically limits the amplitude of the fifth current instruction value Iref5
U1 is the highest direct current bus voltage point of the full power operation of the system; u2 is the maximum voltage point of the allowed work of the direct current bus voltage; m (k) = m (k-1) + Δ m, where m (k) is the fifth current command value Iref5 adjustment value in the current cycle, and m (k-1) is the fifth current command value Iref5 adjustment value in the previous cycle.
The above is merely one preferred embodiment of the present invention. However, the present invention is not limited to the above embodiments, and any equivalent changes and modifications made according to the embodiments of the present invention, which do not bring out the functional effects beyond the scope of the present invention, belong to the protection scope of the present invention.