CN218733278U

CN218733278U - Control system for improving adaptability of grid-connected inverter to weak power grid

Info

Publication number: CN218733278U
Application number: CN202222147713.3U
Authority: CN
Inventors: 齐革军; 牛晨晖; 陈新宇; 蒋俊荣; 李冬; 施俊佼; 胡皓; 肖华锋; 李明明
Original assignee: Huaneng Guanyun Clean Energy Power Generation Co ltd; Southeast University; Clean Energy Branch of Huaneng International Power Jiangsu Energy Development Co Ltd Clean Energy Branch; Huaneng International Power Jiangsu Energy Development Co Ltd
Current assignee: Huaneng Guanyun Clean Energy Power Generation Co ltd; Southeast University; Clean Energy Branch of Huaneng International Power Jiangsu Energy Development Co Ltd Clean Energy Branch; Huaneng International Power Jiangsu Energy Development Co Ltd
Priority date: 2022-08-16
Filing date: 2022-08-16
Publication date: 2023-03-24
Anticipated expiration: 2032-08-16

Abstract

The utility model discloses a promote control system of grid-connected inverter weak power grids adaptability belongs to grid-connected inverter control field. The method relates to a control system which is formed by a main control loop and a voltage feedforward branch of a common coupling point of a novel current controller. Firstly, reference current respectively passes through a first proportion controller, a second proportion controller and a delay module, and the outputs of the first proportion controller and the delay module are added through a first adder to obtain a first output signal; meanwhile, the voltage of the public coupling point is sampled, and a second output signal is generated through a virtual capacitor link and a repeated prediction link. The first output signal and the second output signal are added through an adder and then divided by the gain coefficient of the inverter bridge to obtain a modulation wave. The modulation wave passes through the PWM module to generate a driving signal to drive a power device of the inverter and control the network access current.

Description

Control system for improving adaptability of grid-connected inverter to weak power grid

Technical Field

The utility model relates to a grid-connected inverter control field, concretely relates to promote control system of the weak electric wire netting adaptability of grid-connected inverter.

Background

As the proportion of distributed energy resources in the power grid increases, the influence of the distributed energy resources on the power grid becomes increasingly non-negligible. From the Point of Common Coupling (PCC), the grid can be equivalent to a voltage source in series with an inductive impedance. Moreover, the larger the grid impedance, the more severe the impact on the grid-connected inverter stability. Compared with a strong power grid, the weak power grid has the following two characteristics: (1) The impedance of the power grid cannot be ignored and changes along with the operation mode of the power grid; and (2) the power grid contains rich background harmonics.

Although there are many documents on the stability of the inverter under the weak grid, most documents are directed to the LC/LCL type inverter, and the study on the L type inverter is very little. Compared with an LC/LCL type inverter, the PCC voltage of the L type inverter is not only influenced by the background harmonic wave of a power grid, but also influenced by the inversion voltage, so that the distortion degree of the voltage of an access point of the L type inverter is far greater than that of an LC/LCL type filter under the same power, and the quality of the network access current of the L type filter under a weak power grid is poor.

Furthermore, most of the literature has not been investigated under extremely weak grids. As the grid becomes increasingly weaker, especially when the Short Circuit Ratio (SCR) is equal to 1, if the grid current and the PCC voltage are kept in phase and the grid impedance is a pure inductive reactance, the PCC voltage will drop to zero, which seriously affects the stability of the inverter. Although documents prove that the stability margin of the system can be increased by operating the inverter in a voltage source mode or additionally adding a reactive power compensation device under an extremely weak power grid, the dynamic characteristic of the system and the quality of the grid-entering current are required to be further improved when the inverter is operated in the voltage source mode, and the cost of the system is increased by adding the reactive power compensation device.

Compared with an LC/LCL type inverter, the L type inverter has the characteristics of simple structure and control, and has lower cost in the household photovoltaic low-power occasions, so that the L type inverter still has wide application. However, when the power grid is a weak power grid, especially a very weak power grid, the L-type inverter is affected by the PCC voltage, resulting in poor quality of the current entering the power grid, and the power grid impedance results in a very easy instability of the system. Therefore, if the problem that distorted PCC voltage influences the quality of current of the L-shaped inverter in a network can be solved, a method is found to enable the inverter to keep stable operation under the extremely weak power network, the problem that the application of the L-shaped inverter under the extremely weak power network is limited is solved, and the method has important practical value.

SUMMERY OF THE UTILITY MODEL

The utility model provides a not enough to prior art, the utility model provides a promote control system of grid-connected inverter weak electric wire netting adaptability.

The purpose of the utility model can be realized by the following technical scheme:

a control system for improving adaptability of a grid-connected inverter weak power grid comprises: a main control loop and a PCC voltage feedforward branch of the current controller;

the input end of the PCC voltage feedforward branch circuit is connected to a loop of the inverter, and the PCC voltage is collected;

the main control loop of the current controller comprises: the system comprises a first proportion controller, a second proportion controller, a delay module, a first subtraction controller, a first addition controller and an inverter bridge gain reciprocal module; the reference current is respectively input into the input ends of the first proportional controller and the second proportional controller, and the output ends of the first proportional controller and the second proportional controller are connected with the input end of the first subtraction controller; and the output ends of the first subtraction controller and the PCC voltage feedforward branch circuit are connected with the input end of the first addition controller.

Optionally, an output end of the first addition controller is connected to an input end of the inverter bridge gain reciprocal module, and an output end of the inverter bridge gain reciprocal module is connected to the PWM module.

Optionally, the PCC voltage feed-forward branch includes a virtual capacitor link and a repetitive prediction controller, the PCC voltage obtained by sampling is connected to an input end of the virtual capacitor link, and an output end of the virtual capacitor link is connected to an input end of the repetitive prediction controller; the output end of the repeated prediction controller is connected with the input end of the first addition controller.

Optionally, a virtual filter capacitor is configured on the inverter circuit, and the collected PCC voltage is calculated according to the virtual filter capacitor.

Optionally, a delay module is connected in series between the second proportional controller and the first addition controller. The utility model has the advantages that:

compared with the existing inverter current loop controller, the inverter current loop controller does not need to feed back the network current, can save one sensor, and can effectively increase the stability margin of the inverter under a weak current network compared with the conventional PI controller.

The virtual capacitor module effectively filters abundant background harmonic waves contained in the PCC voltage, and the influence of the PCC voltage on the quality of the network access current is reduced.

The repeated control prediction module can solve the problem of phase lag of the PCC voltage after passing through the virtual capacitor module, and can not amplify high-order harmonics in the PCC voltage like other phase compensation modules.

The utility model discloses can solve L type dc-to-ac converter effectively and advance the relatively poor problem of net current quality under the weak current net, adopt novel current controller simultaneously, under the prerequisite that does not influence system dynamic characteristic and incremental cost, can increase the stability margin of dc-to-ac converter under the extremely weak current net by a wide margin, ensure that the dc-to-ac converter can steady operation under the extremely weak current net, have important meaning.

Drawings

The present invention will be further described with reference to the accompanying drawings.

Fig. 1 is a schematic diagram of the connection of an L-type grid-connected inverter power circuit and a control circuit.

Fig. 2 is a waveform of an access point voltage of an L-shaped inverter under a weak power grid.

Fig. 3 is a schematic diagram of the deviation of the modulated wave caused by the distorted PCC voltage.

FIG. 4 is a schematic diagram of the circuit after adding the dummy capacitor.

FIG. 5 is a Bode diagram of GRLC(s).

FIG. 6 is a bode plot of lead correction and repetitive control prediction transfer functions.

Fig. 7 is a vector relationship between various physical quantities under a weak grid.

Fig. 8 shows the PCC voltage versus the magnitude of the grid inlet current for SCR = 1.

Fig. 9 is a simplified circuit diagram of a single-phase inverter system under a weak grid.

Fig. 10 is a block diagram of system control when a conventional PI controller is used.

Fig. 11 shows the inverter output impedance versus the grid impedance when using a conventional PI controller.

Fig. 12 is a control block diagram of a system when the controller of the present invention is used.

Fig. 13 shows the relationship between the inverter output impedance and the grid impedance when the controller of the present invention is used.

Fig. 14 (a) shows PCC voltage waveform and grid-in current waveform when PCC voltage is directly fed forward.

Fig. 14 (b) shows PCC voltage waveform and net-in current waveform after adding the dummy capacitor and repetitive control prediction module.

Fig. 15 (a) is a current waveform of an inverter destabilization when the conventional PI controller is employed in a weak grid.

Fig. 15 (b) shows the current waveform of the inverter that remains stable when the controller of the present invention is used in a weak grid.

Fig. 16 is a PCC voltage waveform and a grid inlet current waveform of the controller according to the present invention when SCR = 1.

Detailed Description

The technical solutions in the embodiments of the present invention will be described clearly and completely with reference to the accompanying drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by a person of ordinary skill in the art without creative efforts belong to the protection scope of the present invention.

In some specific embodiments of the present invention, an example of the design for improving the adaptability of the L-type inverter under the weak grid is given in conjunction with fig. 1. Fig. 2 shows the waveform of the PCC voltage of the L-type inverter in a weak grid. It can be seen that, under a weak power grid, the voltage distortion of the access point of the L-shaped inverter is serious, and the L-shaped inverter is rich in various harmonics.

Fig. 3 shows a schematic diagram of the modulated wave at the peaks and valleys of the PCC voltage feed-forward, compared to the carrier wave to produce a PWM wave. For better comparison, the ideal modulation wave is compared with the triangular carrier wave, and the deviation between the PWM wave generated by comparing the actual modulation wave with the carrier wave and the ideal PWM wave is also given. It can be seen that the distorted PCC voltage produces significant current deviation, which affects the grid-incoming current quality.

Considering the LC/LCL inverter, since the filter capacitor C is included, there is no serious problem of PCC voltage distortion, so a virtual filter capacitor C is constructed in the circuit, as shown in fig. 4. Because the impedance of the power grid is far larger than the corresponding inductive reactance of the L-type filter under the condition of an extremely weak power grid, the PCC voltage u _PCC Can be expressed approximately as:

in the formula u _inv The inverter voltage is represented, the frequency domain complex frequency is represented by s, the inverter filter inductance is represented by L, the virtual capacitance is represented by C, and the damping coefficient is represented by R.

From equation (1), the transfer function G from the inverter voltage to the PCC voltage can be determined _RLC (s) expression:

by using the parameters of the LC filter for reference, the virtual capacitor C is increased to be beneficial to stabilizing the voltage, and in addition, the damping factor R in the formula (2) can be effectively increased to be beneficial to G _RLC (s) suppression of harmonics in the PCC voltage. It can be found that when C =20uf, r =20 Ω, G at this time _RLC (s) has a good suppression effect on harmonics including 3 rd order and above, and the bode diagram is shown in FIG. 5.

As can be seen from FIG. 5, although G _RLC (s) has good suppression of harmonicsHowever, the phase lag of 27 ° is generated in the fundamental wave component of the PCC voltage, and the phase of the fundamental wave of the PCC voltage needs to be compensated. The conventional phase compensation method comprises methods such as advanced correction, interpolation prediction and the like, but both methods have differential characteristics and are easy to amplify harmonic components in the PCC voltage, and the repeated control prediction has an internal model link, so that the static-error-free prediction can be realized on fundamental waves and each harmonic in theory. The transfer functions of the look-ahead correction and the repetitive predictive control may be expressed as:

/>

in the above formula, G _LC (s) denotes the lead correction transfer function, G _RP (s) represents a repeated prediction transfer function, T represents a differential element constant, α T represents an integral element constant, and Q is generally a low-pass filter or a constant less than 1; n represents the number of switching cycles of one power grid cycle; k represents the number of switching cycles to be predicted; m represents a gain factor, typically less than 1. Taking prediction of 29 ° as an example, bode plots of the patterns (3) and (4) are plotted, as shown in fig. 6. It can be seen that the objective of compensating the fundamental wave by 29 ° can be achieved by using both the look-ahead correction, which amplifies the amplitudes of the harmonics, and the repetitive prediction, which does not amplify the amplitudes of the harmonics.

In weak grids, in addition to the distorted PCC voltage affecting the grid-incoming current quality, the stability of the system is also affected as the grid impedance increases. Fig. 7 shows the vector relationship of each physical quantity when the inverter operates at a unit power factor under a weak grid. It can be seen that as the grid impedance increases, u _PCC The amplitude of (c) is continuously decreasing with increasing grid impedance. The short circuit ratio SCR is usually used to measure the strength of the power grid, and the expression is:

u in formula (5) _g Representing the grid voltage, Z _g Representing the grid impedance and S the apparent power of the inverter. As can be seen from equation (5), the SCR decreases with increasing grid impedance. And according to equation (5), when SCR =1 and the grid impedance is pure inductive reactance, the apparent power of the inverter is equal to reactive power, i.e. the inverter cannot generate active power at this time, and the inductive reactance voltage is equal to and opposite to the grid voltage at this time, which results in the PCC voltage amplitude being zero. From fig. 7, a general expression for the PCC voltage can be found:

in the formula I _g Representing the grid current, X _g Representing the inductive reactance, R, of the grid _g Expressing the grid impedance. Plotted at different R according to formulas (5) and (6) _g /X _g When SCR =1, PCC voltage and grid-inlet current amplitude I _g FIG. 8 shows the relationship of (1). It can be seen that at SCR =1, if the grid impedance is pure inductive, then the inverter is operating at rated power, no active power will be emitted and the PCC voltage drops to 0. Thus, for the inverter, if SCR =1, and there is no additional reactive compensation device, the grid is an extremely weak grid.

Fig. 9 shows an equivalent circuit of a single-phase inverter under a weak grid. Wherein i _s (s) is a reference command current, Z _op (s) represents inverter output impedance, Z _g (s) is the grid impedance. If Z is _op Amplitude-frequency curve of(s) and Z _g The amplitude-frequency curve of(s) has an intersection point, and if the system is to be stable, Z should be ensured at the intersection point _op The phase of(s) is greater than-90 deg..

FIG. 10 shows a control block diagram of the system when a conventional PI controller is used, where G _i (s) denotes a current loop controller, K _pwm Representing the gain of the inverter; g _d (s) indicating system controlAnd (4) delay control. From FIG. 10, the grid inlet current i can be determined _{g_b(} s) expression:

further, the output impedance Z of the inverter can be obtained _{op_b} (s) expression:

as can be seen from equation (8), if the control delay can be completely eliminated, the inverter output impedance is infinite, and the influence of the grid impedance can be eliminated, but in practical cases, the control delay cannot be completely eliminated, so that the control delay becomes 0.1T _s The current loop controller is a conventional PI controller for example, and Z is drawn _op (s) and Z _g The relationship of(s) is shown in FIG. 11.

It can be seen that with Z _g (s) increasing, Z _g (s) and Z _op And(s) at the intersection point of the amplitude-frequency curve, the corresponding phase angle margin is closer to minus 90 degrees, and the stability margin of the system is proved to be continuously reduced. When L is _g When increasing to 51mH, when SCR =1, the phase angle margin is already less than 30 °, it can be considered that the system has not guaranteed stability. Although the gain of the PI controller is greatly reduced, the stability margin of the system can be increased, but this will undoubtedly reduce the bandwidth of the inverter and the quality of the grid-connected current.

The conventional PI controller is simple to implement, strong in robustness and most widely applied in practice, but the PI controller causes phase lag due to the existence of an integration link. As can be seen from fig. 11, this is a substantial cause of the instability of the inverter in the weak grid. In order to improve the adaptability of the inverter under a weak power grid, the problem that the phase lag of the output impedance of the inverter is caused by a PI controller is essentially solved. The utility model provides a by the novel current loop controller that first proportion controller, second proportion controller and delay module constitute, can carry effectivelyLiter Z _op (s) phase angle margin. The system block diagram of the new controller is shown in fig. 12.

K in FIG. 12 ₁ /K ₂ Are respectively:

where Ts represents the switching period. From fig. 12, the expression for the net current with the proposed controller can be found:

in the formula e ^-sTs Representing the internal delay of the proposed controller. The inverter output impedance Z when the controller is used can be obtained from the equation (11) _{op_PPD} (s) expression:

respectively drawing Z under weak current network, when using conventional PI controller and the proposed controller _op The bode diagram of(s) is shown in FIG. 13. It can be seen that although the proposed controller is used, Z _op The amplitude of(s) is reduced at low frequencies, but when the SCR is close to 1, the system phase angle stability margin with the proposed controller is close to 90 °, and when the grid impedance continues to increase substantially, there is still a large margin to ensure system stability.

FIG. 14 (a) shows the PCC voltage waveform and the net-in current waveform when the PCC voltage is fed forward directly; fig. 14 (b) shows the PCC voltage waveform and the grid inlet current waveform after adding the virtual capacitance module and the repetitive control prediction module. It can be seen that virtual capacitance fuzzy and repeated predictive control can effectively solve the problem that the PCC voltage distortion affects the quality of the network-entering current.

Fig. 15 shows the net current waveform for both the case of PI and the controller of the present invention when the SCR is reduced to 1.64. It can be seen that adopt the PI controller, the system is unstability, and adopts the utility model provides a controller, the dc-to-ac converter still can guarantee stability, this proves the utility model discloses can promote the stability margin of L type dc-to-ac converter under the weak current net.

Likewise, fig. 16 shows the network current and the PCC voltage waveform of the system when the SCR is reduced to 1, using the controller of the present invention. Can find out that PCC voltage distortion is very serious this moment, but the system still can keep steady operation, proves the utility model discloses can ensure L type dc-to-ac converter steady operation under the extremely weak electric wire netting.

In the description herein, references to the description of "one embodiment," "an example," "a specific example," etc., mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the schematic representations of the terms used above do not necessarily refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

The foregoing shows and describes the general principles, essential features, and advantages of the invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only illustrative of the principles of the present invention, and that various changes and modifications may be made without departing from the spirit and scope of the present invention, and all such changes and modifications fall within the scope of the present invention as claimed.

Claims

1. A control system for improving adaptability of a grid-connected inverter weak power grid is characterized by comprising:

a main control loop and a PCC voltage feedforward branch of the current controller;

2. The control system for improving the adaptability of the grid-connected inverter weak grid according to claim 1, wherein an output end of the first addition controller is connected with an input end of an inverter bridge gain reciprocal module, and an output end of the inverter bridge gain reciprocal module is connected with a PWM module.

3. The control system for improving the adaptability of the grid-connected inverter weak grid according to claim 1, wherein the PCC voltage feedforward branch comprises a virtual capacitor link and a repeated prediction controller, the sampled PCC voltage is connected to an input end of the virtual capacitor link, and an output end of the virtual capacitor link is connected to an input end of the repeated prediction controller; the output end of the repeated prediction controller is connected with the input end of the first addition controller.

4. The control system for improving the adaptability of the grid-connected inverter to the weak grid according to claim 1, wherein a virtual filter capacitor is constructed on the inverter loop, and the collected PCC voltage is calculated according to the virtual filter capacitor.

5. The control system for improving the grid-connected inverter weak grid adaptability according to claim 1, wherein a delay module is connected in series between the second proportional controller and the first addition controller.

6. The control system for improving the grid-connected inverter weak grid adaptability according to claim 4, wherein the PCC voltage u is _PCC The expression is as follows:

u _inv To u _PCC Transfer function G of _RLC (s) the expression is:

7. the control system for improving the grid-connected inverter weak grid adaptability according to claim 3, wherein the transfer function G of the repeated predictive control _RP Comprises the following steps:

where z is a mapping of the frequency domain s in the discrete domain, and Q is typically a low-pass filter or a constant less than 1; n represents the number of switching cycles of one power grid cycle; k represents the number of switching cycles to be predicted; m represents a gain coefficient.