Control strategy and research on energy storage unit participation in power system frequency regulation based on VSG technology

When large-scale photovoltaic power plants are integrated into the grid, it will cause the grid-connected capacity to gradually expand, causing greater harm to the safe and stable operation of the power system, which will affect the FM characteristics of the power system. This requires the PV power plant to actively participate in power system frequency control. Through the PV virtual synchronous generator frequency control technology, coupled with the virtual synchronous PV power plant modeling, the PV new energy units can have the same frequency control characteristics as synchronous generator sets. In the virtual synchronous technology, due to the existence of damping windings, electromagnetic damping will be generated, and it is also due to virtual damping that the virtual synchronous technology has the ability to damp power oscillations. It is also because of the excellent virtual inertia and damping characteristics of the virtual synchronous generator in the grid that this FM technology helps to regulate the frequency fluctuations of the new energy grid, meet the requirements of the power system to maintain stable operation, and then realize the large-scale PV grid connection.


Preface
In recent years, with the overexploitation of fossil energy and the increasing greenhouse effect, the development and utilization of renewable energy have become particularly important, so photovoltaic power generation will be a new type of clean energy.With photovoltaics as the mainstream energy in China's energy structure and as an important development direction for renewable energy, the resource potential of photovoltaic power generation is very obvious.Therefore, in recent years, the scale of photovoltaic power plants that are grid-connected has become increasingly powerful.Since the 1980s, developed countries have been conducting photovoltaic grid-connected power generation research.After more than 20 years of development since the 1990s, the global photovoltaic power generation industry has begun to rise rapidly.Solar energy resources are abundant and widely distributed, and they have the highest development potential for renewable energy.In the next 10 years, the solar photovoltaic industry will experience a period of rapid development [1] .
As a country with abundant solar energy resources, China's PV industry is developing rapidly and experiencing explosive growth.For this reason, this paper verifies the feasibility of VSG technology for photovoltaic power generation systems by adding energy storage units and FM.

Strategy and research of grid-connected energy storage units
2.1 Energy storage units are involved in power compensation.The penetration of solar PV as a renewable energy source in the grid is increasing, and at the same time, the impact on system stability must not be ignored [2] .Because PV arrays usually work at the maximum power point, their output is the maximum power and cannot be connected to the grid for frequency fluctuations to provide more stable power support.Therefore, in the PV array integrated into the power system at the same time parallel connected to an energy storage unit, when the active power consumption of the load is less, the PV generation system side can provide sufficient output power for the load; the same energy storage unit in the system can be the PV generation system side excess output power storage; when the active power consumption of the load is too large, the PV generation system cannot provide sufficient output power for the load at this time.When the active power of the load is consumed too much and the PV power system cannot provide sufficient output power for the load, the storage unit side will release the previously stored output power to meet the load output requirements, avoiding power and frequency collapse on the load output side, thus improving the utilization rate of power and energy and improving the operating characteristics of the energy storage device.

Operating characteristics of PV energy storage systems controlled by VSG technology
Virtual synchronous generator (VSG) technology is a technology in which a grid-connected inverter can simulate the operating principle of a synchronous motor and its internal structure is shown in Figure 1 below.The grid-connected inverter is an important conversion device for distributed power sources to feed power to the grid and has many advantages over the traditional synchronous generator (SG), such as flexibility, controllability, and fast response time [3] .The work is to simulate the frequency regulation characteristics and active and reactive voltage regulation of synchronous motors by VSG technology and make the grid-connected inverter also comparable to the traditional synchronous generator in terms of operating characteristics, mechanism, and external characteristics.In the literature [4], it has been mentioned that frequency and voltage sag for the control of active and reactive power of virtual synchronous generators.It is also because of the integrated advantages of VSG technology that it has made it so far in the study of participation in modern power system applications that it is favored by many researchers and scholars.However, the grid-connected devices of a distributed photovoltaic power supply are mainly power electronic devices with poor overload capacity and inverters that inevitably introduce harmonics and affect the power quality of the grid voltage and current [5] .

Figure 1.
Internal working principle of the VSG virtual synchronous generator.

Photovoltaic array technology
Photovoltaic array technology is mainly a matrix-type module formed by connecting several photovoltaic modules in series or parallel.In the case of photovoltaic modules, they are themselves several solar panels.Through this semiconductor device, the photovoltaic effect converts solar energy into electricity.The so-called photovoltaic effect is a material that absorbs light energy to produce an electric potential effect, which therefore also forms the basic components of the photovoltaic array when grid-connected power generation and through the inverter are converted from direct current to alternating current.When the light source shines on the PN junction on the surface of the PV array and the energy of the incident photons is greater than the forbidden band width Eg of the semiconductor device, then the N region will produce a small amount of carriers for diffusion, and in the middle of the P and N regions there is a space charge region, known as the depletion region.At this point, the depletion region forms a positive electric field, and the direction of the electric field is from the N region to the P region.The electron-hole pairs formed by the aforementioned diffused carriers due to the built-in electric field are gradually pulled toward the P and N regions, resulting in the formation of a photogenerated electric potential at the PN junction.The voltage generated by the PV array during power generation is proportional to the number of PV modules connected in series, while at the same time, the amount of current flow during PV array operation is proportional to the number of PV modules connected in parallel [6][7] .

Energy storage unit batteries compared to photovoltaic array characteristics
This paper uses PV and storage inverter power in a complementary way to achieve the VSG technology strategy research purpose, but also in the grid-connected mode to simulate the SG external characteristics.The role of the storage unit battery is to maintain the stability of the bus voltage and provide and achieve energy balance for the power system.The structure of the photovoltaic energy storage grid-connected system is shown in Figure 2 below.
Photovoltaic power generation systems and energy storage unit batteries are independent and connected to the grid.The difference between a photovoltaic array and an energy storage unit is that the battery is connected to the grid through a bi-directional DC-DC converter, while the photovoltaic array is connected to the grid through a boost converter.The battery should play a storage, transmission, or compensation role when participating in power system frequency and power regulation, so bidirectional DC conversion is indispensable.When the front-stage boost converter of the PV energy storage system realizes MPPT, the back-stage is added with a DC-AC inverter to realize grid-connected control after the virtual synchronizer algorithm.The energy storage unit provides power support according to the demand of the PV power system in steady state and provides rotational inertia J and damping factor D in dynamic processes when the system has PV power fluctuations and load changes.The system frequency and voltage are maintained by the VSG inertial damping power to improve the penetration of the PV power.

Model building in MATLAB/Simulink simulation
The simulation model is built on the MATLAB/Simulink simulation platform, and its system simulation circuit diagram is shown in Figure 3.The system simulation circuit diagram is divided into a main circuit and a control circuit; the main circuit's front DC/DC inverter is driven by MPPT, and the rear DC/AC is driven by VSG control; the control circuit is divided into a VSG control system and an energy storage system.The parameters are set as shown in Table 1.The VSG control circuit module simulation circuit diagram is shown in Figure 4: The simulation circuit diagram of the energy storage control system is shown in Figure 5: Set the simulation duration to 8 s, The light intensity of 0~3 s is 1000 W/m 2 , reduce the light intensity to 600 W/m 2 in 3 s~6 s, observe the active power waveform, reactive power waveform, frequency, voltage, and current waveform of the system output on the grid side, and the simulation results are shown in Figure 6~7:  Compare the power output from the grid-connected side of Figure 6 with the power output from the storage unit side of Figure 7, The initial active load power mentioned in Table 1 is 1.2×10 8 W. From 0 to 3 seconds, when the light intensity is 1000K/m 2 , it can be observed that the active power output from the grid-connected side reaches stability at 1.199×10 8 W, which decreases by 1×10 5 W, The active power output from the energy storage unit side releases 1×10 5 W when it reaches stability, just to make up for the lack of power output from the grid-connected side of the grid and maintain the grid output power balance; In the process of 3~6 s, the set light intensity is reduced to 600 K/m 2 and the active power output from the grid side rises to 1.1995×10 8 W, which is 5×10 4 W compared with the light intensity not reduced, while the active power output from the energy storage unit is absorbed in the grid by 5×10 4 W. Therefore, it is known that the difference between the partial active power output from the grid side and the absorbed active power from the energy storage unit side is equal to that absorbed by the storage unit side.When the light intensity is restored to 1000K/m 2 , the active power output on the grid side is again the same as the previous experimental data, and so on in turn.And when the reactive power output from the grid side and the reactive power output from the storage unit side are observed, it can be concluded that the reactive power is always maintained near the initial value set and in a more stable state the rest of the time, except for a brief sudden change in the initial moment when the storage unit is connected to the grid.Therefore, it is concluded that the power emitted from the storage unit side is equal to the power emitted from the grid-connected side of the PV power system.So far, when analyzing Figure 8, it can be observed that, except for a short change at the initial access moment, the stability is basically maintained at 50 Hz, and the stability response time is rapid, probably within 0.1 s.The fluctuation range is also maintained within the allowable range of power system frequency fluctuations, and the fluctuation range and the stability response speed can be achieved by the virtual inertia J and damping factor D in VSG control.The rate of frequency change of the grid is related to the magnitude of the system inertia, and the total system inertia is inversely proportional to the rate of change of frequency [8] .And as far as the damping coefficient is concerned, it is mentioned in the literature [9] that the damping control coefficient directly affects the time-domain characteristics of the system.Comparing the grid-side three-phase voltage and current waveforms in Figure 9 and the inverter-side voltage and current waveforms in Figure 10, the grid-side three-phase voltage waveforms and the inverter-side voltage and current waveforms are both relatively smooth sine waves, and both voltage amplitudes are basically stable at 300 V, but the inverter-side current amplitude changes more obviously.As mentioned in the literature [10], when the inertia constant J is certain, the larger the damping factor D of the system, the smaller the overshoot will be and the better the stability of the system.Therefore, it can be concluded that the system is well controlled without affecting the three-phase current amplitude on the grid side.

Summary
This chapter focuses on the study of PV power generation systems and energy storage units as independent grid-connected processes, the VSG virtual synchronous motor control technology for energy storage units, and proposes a strategy for regulating the frequency of PV energy storage based on VSG control technology.The model is built in the MATLAB/Simulink simulation platform to analyze its dynamic response characteristic curve, in which the energy storage unit of the PV power generation system supplies inertial support and damping for the PV power generation system in this dynamic response process, which effectively reduces the frequency fluctuation response caused by grid connection and can simulate the primary frequency regulation characteristics of the synchronous generator, verifying that in this PV power generation system's The feasibility of the VSG control technology to control the energy storage unit to participate in frequency regulation under the structure of this PV power system is verified.

Outlook
This paper is based on the strategy and study of frequency control of the VSG-based PV energy storage grid-connected power system, but an in-depth study can still be done on the virtual inertia and damping under the energy storage PV grid-connected system.In the literature [11], precisely on the basis of the adaptive control strategy of dynamically increasing virtual inertia, virtual damping, and sag coefficient, the VSG active control structure is optimized, and a VSG active control method based on compensating the frequency difference is proposed.In the actual virtual synchronous motor control PV energy storage unit grid-connected, it should be established in the study of the relationship between energy storage unit and PV system capacity to find the relationship between energy storage energy, power, virtual inertia, and damping parameters to get the lowest control cost; VSG control technology experimental parameters should also meet the stability and dynamic performance balance of the power system while also considering the relationship between active and reactive power.

Figure 2 .
Figure 2. Structure of a grid-connected PV energy storage system based on VSG control technology.

Figure 3 .
Figure 3. Simulation circuit of a PV energy storage control system based on VSG control technology.

Table 1 .
Parameter setting of PV energy storage control systems based on VSG control.adjusts the integration link(Kul) 0.1

Figure 5 .
Figure 5. Simulation circuit diagram of an energy storage control system.

Figure 6 .
Figure 6.Power output from the grid side.

Figure 7 .
Figure 7. Power output of the energy storage unit.

Figure 9 .
Figure 9. Three-phase voltage and current waveforms on the grid side.