CN115864520A - A control method and system based on a high proportion of photovoltaic energy connected to a hybrid power grid - Google Patents

Abstract

The invention belongs to the technical field of power grid regulation, and discloses a control method and system based on high-proportion photovoltaic energy access to a hybrid power grid, including: obtaining operating parameters of a synchronous generator, and establishing a phase angle model of the synchronous generator and an inverter power supply respectively Active frequency control model; obtain the operating parameters of the photovoltaic energy storage inverter in the active and reactive states of the microgrid, and establish the reference voltage model and reactive voltage control model of the photovoltaic energy storage inverter; according to the established synchronous generator The phase angle model, the active frequency control model of the inverter power supply, the reference voltage model of the photovoltaic energy storage inverter and the reactive voltage control model realize the control of the grid-connected point voltage and frequency fluctuations caused by the microgrid connecting to the AC-DC hybrid grid. Through the model calculation, the control of the voltage and frequency fluctuations of the grid-connected point caused by the micro-grid connecting to the AC-DC hybrid grid is realized, and the voltage and frequency fluctuations in the micro-grid connecting to the DC hybrid grid are reduced.

Description

A control method and system based on high proportion photovoltaic energy access to hybrid power grid

Technical Field

The present invention belongs to the technical field of power grid regulation and control, and in particular relates to a control method and system based on access of a high proportion of photovoltaic energy to a hybrid power grid.

Background Art

In order to meet the challenges brought by global climate change and environmental pollution and achieve sustainable economic development, reducing carbon emissions is an important measure and strategy. Building a clean, low-carbon, safe and efficient energy system, controlling the total amount of fossil energy, focusing on improving utilization efficiency, implementing renewable energy substitution actions, deepening power system reform, and building a new power system with new energy as the main body have become the mainstream trend and leading direction.

In response to the intermittent problems of renewable energy power generation systems, the concept of AC/DC hybrid microgrids was proposed and began to be applied in actual engineering practice. The normal operation of microgrids is inseparable from efficient and reasonable control. Large-scale distributed power sources have intermittent and volatile energy. The integration of renewable energy with small capacity into traditional centralized power grids will pose a huge threat to the existing power grid system. When intermittent and volatile photovoltaic power sources are connected to the AC/DC hybrid power grid, the voltage and frequency fluctuations at the grid connection point will be caused.

Summary of the invention

In order to solve the problems in the prior art, the present invention provides a control method and system based on the access of a high proportion of photovoltaic energy to a hybrid power grid, which can control the voltage and frequency fluctuations at the grid connection point caused by the access of a microgrid to an AC/DC hybrid power grid, and reduce the voltage and frequency fluctuations when the microgrid is connected to a DC hybrid power grid.

In order to achieve the above object, the present invention provides the following technical solutions:

A control method based on access of high proportion of photovoltaic energy to a hybrid power grid, comprising:

Obtain the operating parameters of the synchronous generator, and establish a phase angle model of the synchronous generator and an active frequency control model of the inverter power supply;

Obtain the operating parameters of the photovoltaic energy storage inverter in the active and reactive states of the microgrid, and establish the reference voltage model and reactive voltage control model of the photovoltaic energy storage inverter;

Based on the established phase angle model of the synchronous generator, the active frequency control model of the inverter power supply, the reference voltage model of the photovoltaic energy storage inverter and the reactive voltage control model, the voltage and frequency fluctuations at the grid connection point caused by the microgrid being connected to the AC/DC hybrid power grid are controlled.

其中，

为同步电机的相位角，ω和ω_g分别为积分上下限中0时刻和t时刻对应的角速度。Furthermore, the phase angle model of the synchronous generator is

in,

is the phase angle of the synchronous motor, ω and ω _g are the angular velocities corresponding to time 0 and time t in the upper and lower limits of the integral, respectively.

其中，H为转动惯量J对应的虚拟惯性时间常数，P_in为逆变电源输入功率，P_out逆变电源输出功率，ω为逆变电源角频率，ω_g为公共母线的角频率，K_d为阻尼系数。Furthermore, the inverter power supply active frequency control model of the synchronous generator is:

Among them, H is the virtual inertia time constant corresponding to the moment of inertia J, P _in is the inverter input power, P _out is the inverter output power, ω is the inverter angular frequency, ω _g is the angular frequency of the common bus, and K _d is the damping coefficient.

其中，Q_ref和D_q分别为无功功率设定输入值和下垂系数；T_a为延迟环节时间常数；E_set和E_g分别为分布式逆变电源参考端电压和无功功率控制器输出信号；k_p1和k_i1都是各自环节的比例积分系数。Furthermore, the reference voltage model of the photovoltaic energy storage inverter is

Among them, Q _ref and D _q are the reactive power setting input value and droop coefficient respectively; _Ta is the delay link time constant; E _set and E _g are the reference terminal voltage of the distributed inverter and the output signal of the reactive power controller respectively; k _p1 and k _i1 are the proportional integral coefficients of their respective links.

其中，Q₀为额定功率；k_Q为无功调节系数；Q_e为逆变器瞬时无功功率；E₀为虚拟空载电势；虚拟励磁电势E；k_U为机端电压调节系数；U为逆变器输出电压的有效值。Furthermore, the reactive voltage control model of the photovoltaic energy storage inverter is

Among them, _Q0 is the rated power; _kQ is the reactive power regulation coefficient; _Qe is the instantaneous reactive power of the inverter; _E0 is the virtual no-load potential; virtual excitation potential E; _kU is the terminal voltage regulation coefficient; U is the effective value of the inverter output voltage.

Furthermore, the E ₀ =311V.

Furthermore, when the microgrid enters the off-grid working state, the power supply in the microgrid provides voltage and frequency to the main grid, and the voltage and frequency generated by the virtual synchronous generator are used to control the operation of the photovoltaic inverter to maintain the stable operation of the photovoltaic system.

A control system based on access of high proportion of photovoltaic energy to a hybrid power grid, comprising:

A synchronous generator control module is used to obtain the operating parameters of the synchronous generator and establish a phase angle model of the synchronous generator and an inverter power supply active frequency control model;

The photovoltaic energy storage inverter control module is used to obtain the operating parameters of the photovoltaic energy storage inverter in the active and reactive states of the microgrid, and to establish a reference voltage model and reactive voltage control model for the photovoltaic energy storage inverter;

The integrated control module is used to control the voltage and frequency fluctuations at the grid connection point caused by the microgrid being connected to the AC/DC hybrid grid based on the established phase angle model of the synchronous generator, the active frequency control model of the inverter power supply, the reference voltage model of the photovoltaic energy storage inverter, and the reactive voltage control model.

Compared with the prior art, the advantages of the present invention are:

The present invention provides a control method based on the access of a high proportion of photovoltaic energy to a hybrid power grid. By acquiring the operating parameters of a synchronous generator and the operating parameters of a photovoltaic energy storage inverter in the active and reactive states of a microgrid, a phase angle model of the synchronous generator, an inverter power supply active frequency control model, a reference voltage model of the photovoltaic energy storage inverter, and a reactive voltage control model, the method analyzes the voltage and frequency under reactive power-voltage, active power-frequency, and photovoltaic storage coordinated control in the photovoltaic power grid, and through model calculation, the voltage and frequency fluctuations at the grid connection point caused by the access of the microgrid to an AC/DC hybrid power grid are controlled, thereby reducing the voltage and frequency fluctuations in the access of the microgrid to a DC hybrid power grid.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

In the attached picture:

FIG1 is a topological structure diagram of an AC/DC hybrid microgrid in a control method based on access of a high proportion of photovoltaic energy to a hybrid power grid according to the present invention;

FIG2 is a vector relationship diagram of an equivalent circuit of a synchronous generator in a control method based on access of a high proportion of photovoltaic energy to a hybrid power grid according to the present invention;

3 is a block diagram of active power-frequency control of a power system in a control method based on access of a high proportion of photovoltaic energy to a hybrid power grid according to the present invention;

FIG4 is a block diagram of reactive power-voltage control of a photovoltaic system in a control method based on access of a high proportion of photovoltaic energy to a hybrid power grid according to the present invention;

5 is a structural diagram of a photovoltaic-storage AC/DC hybrid microgrid system in which the microgrid is in an island working mode in a control method based on high-proportion photovoltaic energy access to a hybrid power grid according to the present invention;

FIG6 is a structural diagram of a photovoltaic-storage hybrid microgrid system in a control method based on access of a high proportion of photovoltaic energy to a hybrid power grid according to the present invention;

7 is a block diagram of the front-stage control structure of a photovoltaic energy storage system in a control method based on access of a high proportion of photovoltaic energy to a hybrid power grid according to the present invention;

FIG8 is a control flow chart of a main inverter including VSG control in a preferred embodiment of a control method based on high-proportion photovoltaic energy access to a hybrid power grid according to the present invention;

9 is a schematic diagram of the constant power control principle of the slave control unit in a preferred embodiment of a control method based on high-proportion photovoltaic energy access to a hybrid power grid according to the present invention;

FIG10 is a flow chart of a control method based on access of a high proportion of photovoltaic energy to a hybrid power grid according to the present invention;

DETAILED DESCRIPTION

The present invention will be described in detail below with reference to the accompanying drawings and in combination with embodiments. It should be noted that the embodiments and features in the embodiments of the present invention can be combined with each other without conflict.

The following detailed description is an exemplary description, which is intended to provide further detailed description of the present invention. Unless otherwise specified, all technical terms used in the present invention have the same meaning as those generally understood by those skilled in the art to which the present invention belongs. The terms used in the present invention are only for describing specific embodiments, and are not intended to limit the exemplary embodiments according to the present invention.

The present invention provides a control method based on the access of a high proportion of photovoltaic energy to a hybrid power grid. Through research on hybrid microgrid systems in grid-connected and off-grid (island) modes, a multi-loop control method and a synchronous generator control method are proposed to coordinate the power control of a photovoltaic-storage hybrid microgrid, thereby achieving stable operation of the system and avoiding frequent power conversion between subgrid systems.

Example 1

A control method based on access of a high proportion of photovoltaic energy to a hybrid power grid, as shown in FIG10 , includes:

Obtain the operating parameters of the synchronous generator, and establish a phase angle model of the synchronous generator and an active frequency control model of the inverter power supply;

Obtain the operating parameters of the photovoltaic energy storage inverter in the active and reactive states of the microgrid, and establish the reference voltage model and reactive voltage control model of the photovoltaic energy storage inverter;

Based on the established phase angle model of the synchronous generator, the active frequency control model of the inverter power supply, the reference voltage model of the photovoltaic energy storage inverter and the reactive voltage control model, the voltage and frequency fluctuations at the grid connection point caused by the microgrid being connected to the AC/DC hybrid power grid are controlled.

其中，

为同步电机的相位角，ω和ω_g分别为积分上下限中0时刻和t时刻对应的角速度。Specifically, the phase angle model of the synchronous generator is

in,

is the phase angle of the synchronous motor, ω and ω _g are the angular velocities corresponding to time 0 and time t in the upper and lower limits of the integral, respectively.

其中，H为转动惯量J对应的虚拟惯性时间常数，P_in为逆变电源输入功率，P_out逆变电源输出功率，ω为逆变电源角频率，ω_g为公共母线的角频率，K_d为阻尼系数。Specifically, the inverter power supply active frequency control model of the synchronous generator is:

Among them, H is the virtual inertia time constant corresponding to the moment of inertia J, P _in is the inverter input power, P _out is the inverter output power, ω is the inverter angular frequency, ω _g is the angular frequency of the common bus, and K _d is the damping coefficient.

Specifically, the AC/DC hybrid microgrid is the most widely used microgrid structure in field projects at present, as shown in Figure 1, which is divided into an AC subgrid system and a DC subgrid system, each of which is connected by its own busbar, and each subgrid can directly supply power to the AC load and the DC load. Distributed power sources, energy storage devices and power electronic components are usually connected to the subgrid. The present invention is based on the research of the AC/DC hybrid microgrid. After analyzing and studying its structural characteristics and advantages, the typical control method of the bidirectional converter has insufficient power flow and power reference value selection when off-grid, and proposes a new control method of the bidirectional converter based on comprehensive droop in off-grid mode. At the same time, the energy storage access microgrid introduces a complex upper-level power management system to coordinate the access of the photovoltaic system and the power balance of the energy storage unit, which increases the complexity of the system. In order to realize the power coordination control of the hybrid microgrid, the present invention adopts a comprehensive control method to realize the power coordination control of high-proportion photovoltaic energy and energy storage.

According to the analysis of hybrid microgrids, microgrids composed of distributed power sources usually have two completely different working modes: grid-connected and off-grid (island). The grid-connected mode is mainly connected to the grid system. The grid system will exchange electricity with the microgrid system. The microgrid can obtain power regulation from the grid, so that it can maintain stable operation. In the grid-connected operation mode, the electricity generated by new energy sources such as photovoltaics can be effectively utilized. If the microgrid is separated from the grid and operates independently, the microgrid in the off-grid (island) mode needs to rely on distributed energy such as photovoltaics and energy storage devices in the system to achieve power balance and voltage regulation. In order to ensure the continuity of power supply, the microgrid system is usually adjusted through energy storage devices to ensure that important loads can obtain continuous electricity.

For an AC/DC hybrid microgrid, if it is directly connected to the grid, the system will mainly achieve power exchange through the public connection point connected to the grid, thereby achieving internal power balance. In this state, distributed power sources such as photovoltaics will operate in maximum power mode. If there is excess power in the microgrid system, it will be directly incorporated into the large grid. If there is insufficient power in the microgrid system, the grid will supply power to the microgrid system. The energy storage system connected to the microgrid will determine the working state of the microgrid based on the connected bus voltage. If power fluctuations are found in the microgrid, power balance will be achieved by controlling charging and discharging. The AC/DC power converter as the system control needs to be in a stable working state at all times to ensure that the DC side can exchange power with the grid.

If the hybrid microgrid is disconnected from the main grid, it will enter the off-grid (island) mode, and the AC side will no longer be connected to the grid, so the microgrid can no longer achieve its own power balance through the main grid. At this time, it is usually necessary to set the distributed power supply in the constant voltage control working mode, and the energy storage system also needs to release electric energy to ensure that the microgrid system can get power regulation in time to maintain stable operation. The bidirectional AC/DC converter needs to adjust its own working mode according to the working mode of the grid to ensure that the power in the microgrid can be accurately adjusted, ensuring that important loads are stably powered while maintaining system stability, and also to maximize the utilization efficiency of the power supply.

Specifically, the photovoltaic unidirectional inverter in the photovoltaic energy storage system needs to have the working characteristics of a synchronous generator, so the vector relationship principle of the synchronous generator is analyzed first. As shown in Figure 2, the working circuit diagram of the synchronous generator after connecting to the large power grid is shown in the figure below. The vector relationship between the voltage at both ends of each device in the circuit and the circuit current is shown.

为相位角。如果定旋转坐标系直轴d和网端电压U_g两者方向相同，交轴q与d轴两者相互垂直。将图2中所示的各个矢量分解到d轴和q轴上，可以得到如下所示的直轴和交轴输出电流，具体表达式如下所示。In the figure, _Us is the electromotive force in the synchronous generator, R is equivalent to the inverter resistance, jX is equivalent to the inverter reactance, _Ug is the terminal voltage, _Ig is the current,

is the phase angle. If the direct axis d and the grid voltage _Ug of the fixed rotating coordinate system are in the same direction, the quadrature axis q and the d-axis are perpendicular to each other. Decomposing each vector shown in Figure 2 to the d-axis and q-axis, the direct axis and quadrature axis output currents shown below can be obtained, and the specific expressions are shown below.

Among them, admittance Y and U _sd , U _sq are respectively

为转子和系统两者角速度ω和ω_g差值的积分，即Phase Angle

is the integral of the difference between the angular velocities ω and ω _g of the rotor and the system, that is,

The synchronous generator uses a speed regulator to adjust the rotor angular velocity ω. The angular frequency and active power of the generator directly determine the magnitude of the angular velocity. The internal potential Us of the generator is controlled by the excitation system, and the magnitude of the potential is determined by the voltage and reactive power. Therefore, the speed regulator model of the synchronous generator is introduced into this converter control method. In addition, the excitation system model is also introduced, so that the control method of the photovoltaic energy storage system has similar characteristics to the synchronous generator.

The control method under reactive power-frequency is:

Because of the inertia of the synchronous generator rotor, there will not be a sudden change in frequency in a short period of time; therefore, according to the rotor motion equation, virtual inertia control is introduced in the inverter control method so that the converter has working characteristics similar to the rotor motion in the synchronous generator. The following is the inverter power supply active frequency control equation.

Where H is the virtual inertia time constant corresponding to the moment of inertia J, P _in is the inverter input power, P _out is the inverter output power, ω is the inverter angular frequency, ω _g is the angular frequency of the common bus, and K _d is the damping coefficient.

When the system is connected to the main grid, the frequency _ωg of the photovoltaic unidirectional inverter is clamped. At this time, the main grid system directly determines the frequency of the microgrid, so there is no need to adjust the frequency of the microgrid through a distributed inverter. However, if the photovoltaic inverter is connected to the microgrid and the penetration rate is high, the microgrid needs a distributed power supply to adjust the frequency of the system to ensure that the system can maintain normal operation when the load changes. The present invention provides an increase in active power-frequency droop control link to form a frequency modulation controller in the system, and its adjustment process is as follows.

Where _Pref is the active power, _Dp is the droop coefficient, and _ωref is the reference angular frequency. By combining the two equations, we get the "speed regulator" model shown below, that is, the transfer function of the active-frequency controller is:

The active power-frequency control block diagram is shown in Figure 3.

If the photovoltaic inverter is connected to the large power grid at this time, the system frequency is the reference frequency, and the droop control mode will no longer work, which has similar working characteristics to the rotor in the synchronous motor. If a microgrid is incorporated, the controller can achieve droop control by adjusting the rotor characteristics. In this way, even if the internal frequency fluctuates, the controller can also play a regulatory role, reduce the fluctuation of the system frequency, and ensure that the system is in a stable operating state. The damping control module _Kd (ω- _ωg ) can also keep the frequencies of the microgrid system and the photovoltaic inverter consistent.

其中，Q_ref和D_q分别为无功功率设定输入值和下垂系数；T_a为延迟环节时间常数；E_set和E_g分别为分布式逆变电源参考端电压和无功功率控制器输出信号；k_p1和k_i1都是各自环节的比例积分系数。Specifically, the reference voltage model of the photovoltaic energy storage inverter is:

Among them, Q _ref and D _q are the reactive power setting input value and droop coefficient respectively; _Ta is the delay link time constant; E _set and E _g are the reference terminal voltage of the distributed inverter and the output signal of the reactive power controller respectively; k _p1 and k _i1 are the proportional integral coefficients of their respective links.

其中，Q₀为额定功率；k_Q为无功调节系数；Q_e为逆变器瞬时无功功率；E₀为虚拟空载电势；虚拟励磁电势E；k_U为机端电压调节系数；U为逆变器输出电压的有效值。Specifically, the reactive voltage control model of the photovoltaic energy storage inverter is:

Among them, _Q0 is the rated power; _kQ is the reactive power regulation coefficient; _Qe is the instantaneous reactive power of the inverter; _E0 is the virtual no-load potential; virtual excitation potential E; _kU is the terminal voltage regulation coefficient; U is the effective value of the inverter output voltage.

Preferably, E0=311V.

Specifically, when the microgrid enters the off-grid working state, the power supply in the microgrid provides voltage and frequency to the main grid, and the voltage and frequency generated by the virtual synchronous generator are used to control the operation of the photovoltaic inverter to maintain the stable operation of the photovoltaic system.

The control method under reactive power-voltage is:

The design of the present invention provides a photovoltaic energy storage inverter control system as shown in FIG4 , which is a reactive-voltage controller model circuit, based on the analysis of the structural operation mode of the synchronous generator excitation system.

Among them, Q _ref and D _q are the reactive power setting input value and droop coefficient respectively; _Ta is the delay link time constant; E _set and E _g are the reference terminal voltage of the distributed inverter and the output signal of the reactive power controller respectively; k _p1 and k _i1 are the proportional integral coefficients of their respective links.

From this, the reference voltage of the photovoltaic energy storage inverter can be obtained as shown below.

Controlling the reactive power output of the system through the PI controller has the characteristics of fast response speed, but the reactive power in the synchronous generator takes some time to reach a steady state. If the change is too fast, it will cause a sharp fluctuation in the active power. Therefore, it is necessary to add a corresponding delay link on the basis of PI control. The reactive power also slowly transitions to the steady state value due to the existence of the delay link, reducing the impact on the system. The reactive-voltage droop characteristic is mainly determined by Dq.

By simulating the working characteristics of the synchronous generator rotor, the output power and frequency, reactive power and voltage of the inverter in the photovoltaic storage system can form characteristic changing relationships. The distributed inverter therefore has a virtual rotational inertia similar to that of the synchronous motor rotor. Under this control mode, when the distributed inverter is connected to the microgrid, the total rotational inertia of the system increases, further improving the frequency stability of the power grid system.

Specifically, the implementation process of the photovoltaic storage coordinated control method in this preferred embodiment is: because the hierarchical control method requires communication for management, if the communication equipment fails, the system may crash. The design of the present invention provides a comprehensive control method for realizing the coordinated control of the photovoltaic storage AC/DC system. The specific control process is as follows:

For a photovoltaic-storage AC-DC hybrid microgrid as shown in Figure 5, it includes two small photovoltaic-storage DC microgrid systems, units 1 and 2. The photovoltaic array in the photovoltaic power generation system can output power to the next level of the microgrid through a boost-type unidirectional DC/DC converter. The energy storage system is connected to the DC sub-microgrid through a bidirectional DC/DC converter, and the two can exchange electrical energy. The DC microgrid is connected to the DC load and can be switched as needed. The DC microgrid is also connected to the AC side through an AC/DC bidirectional converter, which can provide electrical energy to the AC load. An LC filter is connected to the AC/DC converter to reduce the output ripple. The hybrid microgrid is disconnected from the grid and therefore operates in off-grid (island) mode.

At this time, unit 1 serves as the main power source of the system, and the frequency and voltage of the microgrid are supported by this unit. Unit 2 serves as the slave power source of the system. When the system is working, the virtual synchronous generator (VSG) is used to control the bidirectional converter of the main power source, and the constant power control method is used to control the bidirectional converter of the slave power source. The DC bus voltage of unit 1 and unit 2 is DC750V, and the load demand will change when the hybrid system is working. If the microgrid is disconnected from the grid and enters the off-grid (island) working state, the energy storage system will play a key role in regulating the voltage, and can also balance the internal power of the system and maintain the stable operation of the system. For example, it can be used to adjust the DC side bus voltage and maintain the stability of the system voltage. It can also provide stable power for the system to ensure timely replenishment or consumption of power when the load power changes, and ensure that the system maintains stable operation. The photovoltaic system operates at the maximum power point according to the actual working conditions.

The energy storage system and the photovoltaic array system can be combined in the following two ways: the first is to connect the energy storage system and the photovoltaic array system to the next level of the system respectively. At this time, the two cannot be adjusted with each other, and the photovoltaic system is difficult to be adjusted, which can easily affect the operation of the system. The second connection method is to connect the two in parallel and then connect them to the DC side together. This connection method can improve the flexibility of system adjustment. The present invention adopts the connection method shown in Figure 6, which belongs to the second connection method mentioned above. _{P pv} and _PB are the power of the photovoltaic array and the energy storage system respectively, and the capacitors Cdcl and Cdc2 are connected to the output end of the bidirectional converter in the circuit. This connection method can fully control the energy storage system and flexibly select the rated voltage of the energy storage system. The energy storage system can store energy when the photovoltaic power generation system has excess power and release electric energy when the photovoltaic power generation system is short of power, thereby maintaining the stability of the entire system.

In the above hybrid microgrid, the bidirectional DC/DC converter directly controls the DC bus voltage. The photovoltaic system is adjusted to the maximum power operating point through the perturbation observation method (P&O), thereby realizing the maximum power tracking MPPT control. As shown in Figure 7, the first-level control framework of the photovoltaic storage system, the DC bus voltage Vdc, the real-time energy storage current iB, the photovoltaic input voltage Vpv and current ipv are all filtered to reduce the switching noise.

In the control block diagram shown in Figure 7, the reference voltage output by the photovoltaic array can be obtained according to the system charge state and photovoltaic system parameters. There are certain differences between the three modes of normal, charge state over-limit regulation and DC bus voltage over-limit regulation.

Specifically, the operation of the photovoltaic system is divided into three modes according to whether the microgrid participates in the photovoltaic system according to the voltage, that is, the output voltage of the microgrid, the grid-connected output voltage and the common output voltage of the two in the photovoltaic grid system;

Normal operation mode

When operating within the safe range, the MPPT VMPPT is consistent with the reference voltage _Vpvref of the photovoltaic system, and the SOC over-limit control loops PI0 and PI1 and the bus voltage deviation over-limit control loops PI2 and PI3 all enter the idle state. The photovoltaic output voltage Vpv and output current ipv obtained by the MPPT algorithm, the photovoltaic system is in the maximum power output state at this time. The voltage deviation between Vpvref and Vpv is input into the unidirectional DC/DC converter controller, and the photovoltaic output high power is transmitted to the DC bus. The energy storage system adjusts the bus voltage through the bidirectional DC/DC converter to ensure that the reference voltage Vdcref is reached.

The system can achieve power balance of the circuit through the charging and discharging method of the energy storage system. At this time, the state of charge of the energy storage system is within the normal range, and the charging and discharging current iB and the DC bus voltage fluctuation are all within the normal range. If the state of charge SOC is less than the maximum upper limit, SOC _max and the actual SOC form a positive deviation signal, and the Pl1 controller of the forward saturation limit has no signal output, so the SOC limit control loop will enter the idle state.

State of charge over-limit regulation mode

When the photovoltaic output power exceeds the load demand, the energy storage system will absorb excess power through charging, and the energy storage system's state of charge SOC will also increase. However, if the energy storage state of charge exceeds SOC _max , the system will be affected and may enter an unstable state. The photovoltaic system needs to reduce output to ensure that the system is within a safe range. If the energy storage state of charge exceeds the set SOC _max , the PI1 control loop starts to work, and the deviation between the SOC _max and the real-time SOC signal begins to become negative, which is finally added to the voltage VMPPT through the PI1 controller. At this time, the photovoltaic system will not work at the maximum power point. The photovoltaic system will reduce power output because of the PI1 control loop until the energy storage system enters an idle state and the real-time state of charge drops below SOC _max .

DC bus voltage over-limit regulation mode

The PI2 control loop enters an idle state when the system is working normally. At this time, the energy storage system will maintain the system in a stable operating state through charging and discharging. If the output power of the photovoltaic system is very large, resulting in an excessive charging current iB that exceeds the tolerance range of the energy storage system, the energy storage system will be greatly affected and the system will enter an unreliable working state. Therefore, if the output power of the photovoltaic system is very large and the load demand of the entire network is relatively small, the charging current iB of the energy storage system exceeds the allowable range, and the energy storage system cannot consume the output of the photovoltaic system in a short period of time. At this time, the DC bus voltage rises and the system cannot achieve power balance.

In the circuit shown in FIG6 , the DC bus voltage deviation is input into the PI2 control loop, and the output value is input into the photovoltaic power controller. At this time, the photovoltaic system can be prompted to reduce the output power, thereby avoiding a rapid rise in the DC bus voltage.

In this control method, a certain voltage safety deviation signal can be set, and the specific value is determined by the application scenario and practical working conditions. The specific adjustment process is as follows:

Detect the real-time voltage Vdc of the DC bus of the system. If the voltage value exceeds the allowable value, the PI2 control circuit will start to work. The voltage upper limit Vdc+△V is subtracted from the instantaneous value Vdc of the bus. The obtained deviation is input into the PI2 controller, and the output signal is sent to the photovoltaic power generation system, thereby reducing the output power of the photovoltaic system. In contrast to the above situation, if the power output of the photovoltaic system is much lower than the load demand, the DC bus voltage will drop rapidly because the energy storage system cannot provide the necessary power compensation. The PI3 control loop will start to work at this time. The deviation value obtained by subtracting the bus voltage Vdc-△V from the instantaneous value Vdc is input into the PI3 controller, and then the system will cut off part of the load to avoid the system from collapsing due to excessive load.

In the master-slave control of the next-level inverter, when the microgrid is coordinating, the master-slave control of the next-level inverter adopts the virtual synchronous generator control mode and the constant power mode respectively, so as to adjust the system voltage and frequency and follow the load changes. If the microgrid enters the off-grid (island) working state, some of the power sources are needed to provide voltage and frequency support for the main grid. When the virtual synchronous generator control mode is adopted, the photovoltaic inverter will also have certain frequency and voltage regulation capabilities during the working process, which can enable the system to maintain stable operation. The inverter that simulates the working mode of the synchronous generator is called a synchronous inverter, which mainly realizes the autonomous distribution of active and reactive power by adjusting the grid frequency and voltage.

The working mode of this inverter is analyzed in detail below. The following equation is the reactive voltage control equation under simulated synchronous control.

In the formula, _Q0 is the rated power; _kQ is the reactive power regulation coefficient; _Qe is the instantaneous reactive power of the inverter; virtual no-load potential _E0 = 311V; virtual excitation potential E; _kU is the terminal voltage regulation coefficient; U is the effective value of the inverter output voltage. Figure 8 shows the structural block diagram of the VSG control method.

The inverter adopts constant power control and can output constant power according to the command, which is usually called a current-type inverter. Constant power control consists of two parts: the power outer loop and the current inner loop. P* is the rated active power, Q* is the rated reactive power, P and Q are the actual active and reactive power respectively, as shown in Figure 9 for the corresponding control block diagram.

This control method first summarizes the virtual synchronous generator control method, and then proposes a comprehensive control method for the power coordination control problem of the photovoltaic storage hybrid microgrid in the off-grid (island) mode. The previous level and the next level adopt multi-loop power control and virtual synchronous generator control methods respectively. This control method can avoid the sudden voltage change of the DC bus on the basis of maintaining the power balance of the previous level, and at the same time avoid the occurrence of the energy storage system charge limit state. In addition, the main inverter of the next level can be controlled by the control method of the virtual synchronous generator, and a constant power method is adopted, so that the internal power of the entire system is balanced and maintained in a stable operating state.

Specifically, although the typical droop control method of the bidirectional converter can achieve autonomy well, it has certain shortcomings. Therefore, the present invention uses different bidirectional converter control methods in off-grid (isolated island) and grid-connected mode, that is, it describes the mode switching method of the bidirectional converter based on comprehensive droop control. At the same time, in order to enable the photovoltaic storage hybrid microgrid to achieve power coordination control under different working conditions in off-grid mode, a comprehensive control method is given. The first level adopts a multi-loop control method to achieve power balance and suppress power fluctuations, and the second level adopts a virtual synchronous generator control method to maintain the inertia and damping of the system.

The present invention has made a specific analysis and study on the AC/DC hybrid microgrid topology and characteristics of a high-proportion photovoltaic access energy storage system, and at the same time, modeled and analyzed energy storage and photovoltaics. The working principles of photovoltaic unidirectional converters and bidirectional energy storage device DC/DC converters are studied. The previous level system formed by the photovoltaic connection unidirectional converter and the energy storage device connection bidirectional energy storage device DC/DC converter can be equivalent to a DC voltage source. According to the power distribution principle of the bidirectional converter, the control methods of the bidirectional converter in different modes of the hybrid microgrid are analyzed and studied respectively, and the threshold method is used to realize the autonomous coordination of power control between the bidirectional converter and the energy storage device, which can avoid unnecessary action of power electronic devices and reduce the loss of electric energy. A new bidirectional converter control method is designed based on the operation of the typical droop control method: fixed DC bus voltage control is adopted in the grid-connected mode, and a comprehensive droop control method is adopted in the off-grid (island) mode. The simulation results verify the correctness of the method: that is, when connected to the grid, the power transmission size and direction are determined according to the DC bus voltage; when off-grid, the bidirectional converter works only when there is and only one microgrid power shortage, the other microgrid power is abundant, and the frequency and voltage deviation values exceed the threshold, which reduces the frequent flow of power. To the greatest extent, it is guaranteed that at least one microgrid can be in a normal or near-rated operating state, avoiding the phenomenon of frequent power conversion between sub-microgrids caused by power fluctuations between grids. The design of the present invention realizes the power coordination control of the photovoltaic storage AC/DC island microgrid through a comprehensive control method, that is, considering the energy storage charge state and the DC bus reference voltage deviation exceeding the limit, a multi-loop power control method is constructed in the previous level structure to achieve the power balance of the previous level of the system and make the system operate within a safe range. The simulation results show that it can effectively prevent the energy storage charge state from exceeding the limit and the DC bus voltage from changing suddenly and achieve the power balance of the previous level. In addition, in the control of the next level, the virtual synchronous generator control method is adopted for the main converter, and the constant power control method is adopted for the slave converter to realize the coordinated control of the entire system.

Example 2

A control system based on access of high proportion of photovoltaic energy to a hybrid power grid, comprising:

A synchronous generator control module is used to obtain the operating parameters of the synchronous generator and establish a phase angle model of the synchronous generator and an inverter power supply active frequency control model;

The photovoltaic energy storage inverter control module is used to obtain the operating parameters of the photovoltaic energy storage inverter in the active and reactive states of the microgrid, and to establish a reference voltage model and reactive voltage control model for the photovoltaic energy storage inverter;

The integrated control module is used to control the voltage and frequency fluctuations at the grid connection point caused by the microgrid being connected to the AC/DC hybrid grid based on the established phase angle model of the synchronous generator, the active frequency control model of the inverter power supply, the reference voltage model of the photovoltaic energy storage inverter, and the reactive voltage control model.

Those skilled in the art will appreciate that embodiments of the present invention may be provided as methods, systems, or computer program products. Therefore, the present invention may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present invention may take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

The present invention is described with reference to the flowchart and/or block diagram of the method, device (system), and computer program product according to the embodiment of the present invention. It should be understood that each process and/or box in the flowchart and/or block diagram, as well as the combination of the process and/or box in the flowchart and/or block diagram can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device produce a device for implementing the functions specified in one or more processes in the flowchart and/or one or more boxes in the block diagram.

These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing device to work in a specific manner, so that the instructions stored in the computer-readable memory produce a manufactured product including an instruction device that implements the functions specified in one or more processes in the flowchart and/or one or more boxes in the block diagram.

These computer program instructions may also be loaded onto a computer or other programmable data processing device so that a series of operational steps are executed on the computer or other programmable device to produce a computer-implemented process, whereby the instructions executed on the computer or other programmable device provide steps for implementing the functions specified in one or more processes in the flowchart and/or one or more boxes in the block diagram.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents, and any modifications or equivalent replacements that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims (8)

1. A control method based on high proportion of photovoltaic energy access to a hybrid power grid, characterized by comprising: Obtain the operating parameters of the synchronous generator, and establish a phase angle model of the synchronous generator and an active frequency control model of the inverter power supply; Obtain the operating parameters of the photovoltaic energy storage inverter in the active and reactive states of the microgrid, and establish the reference voltage model and reactive voltage control model of the photovoltaic energy storage inverter; Based on the established phase angle model of the synchronous generator, the active frequency control model of the inverter power supply, the reference voltage model of the photovoltaic energy storage inverter and the reactive voltage control model, the voltage and frequency fluctuations at the grid connection point caused by the microgrid being connected to the AC/DC hybrid power grid are controlled. 2. A control method based on high-proportion photovoltaic energy access to a hybrid power grid according to claim 1, characterized in that the phase angle model of the synchronous generator is

in,

is the phase angle of the synchronous motor, ω and ω _g are the angular velocities corresponding to time 0 and time t in the upper and lower limits of the integral, respectively. 3. A control method based on high-proportion photovoltaic energy access to a hybrid power grid according to claim 1, characterized in that the inverter power supply active frequency control model of the synchronous generator is:

Among them, H is the virtual inertia time constant corresponding to the moment of inertia J, P _in is the inverter input power, P _out is the inverter output power, ω is the inverter angular frequency, ω _g is the angular frequency of the common bus, and K _d is the damping coefficient. 4. A control method based on high-proportion photovoltaic energy access to a hybrid power grid according to claim 1, characterized in that the reference voltage model of the photovoltaic energy storage inverter is

Among them, Q _ref and D _q are the reactive power setting input value and droop coefficient respectively; _Ta is the delay link time constant; E _set and E _g are the reference terminal voltage of the distributed inverter and the output signal of the reactive power controller respectively; k _p1 and k _i1 are the proportional integral coefficients of their respective links. 5. A control method based on high-proportion photovoltaic energy access to a hybrid power grid according to claim 1, characterized in that the reactive voltage control model of the photovoltaic energy storage inverter is:

Among them, _Q0 is the rated power; _kQ is the reactive power regulation coefficient; _Qe is the instantaneous reactive power of the inverter; _E0 is the virtual no-load potential; virtual excitation potential E; _kU is the terminal voltage regulation coefficient; U is the effective value of the inverter output voltage. 6 . The control method based on access of high-proportion photovoltaic energy to a hybrid power grid according to claim 5 , wherein E ₀ =311V. 7. According to claim 1, a control method based on high proportion of photovoltaic energy access to hybrid power grid is characterized in that when the microgrid enters the off-grid working state, the power supply in the microgrid provides voltage and frequency to the main grid, and the voltage and frequency generated by the virtual synchronous generator are used to control the photovoltaic inverter to work, so as to maintain the stable operation of the photovoltaic system. 8. A control system based on high proportion of photovoltaic energy access to hybrid power grid, characterized by comprising: A synchronous generator control module is used to obtain the operating parameters of the synchronous generator and establish a phase angle model of the synchronous generator and an inverter power supply active frequency control model; The photovoltaic energy storage inverter control module is used to obtain the operating parameters of the photovoltaic energy storage inverter in the active and reactive states of the microgrid, and to establish a reference voltage model and reactive voltage control model for the photovoltaic energy storage inverter; The integrated control module is used to control the voltage and frequency fluctuations at the grid connection point caused by the microgrid being connected to the AC/DC hybrid grid based on the established phase angle model of the synchronous generator, the active frequency control model of the inverter power supply, the reference voltage model of the photovoltaic energy storage inverter, and the reactive voltage control model.

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