Research on Black Start Control technology of Energy Storage Power Station Based on VSG All Vanadium Flow Battery

To reduce the losses caused by large-scale power outages in the power system, a stable control technology for the black start process of a 100 megawatt all vanadium flow battery energy storage power station is proposed. Firstly, a model is constructed for the liquid flow battery energy storage power station, and in order to improve the system capacity, four unit level power stations are processed in parallel. Secondly, based on the energy storage of all vanadium flow batteries, the traditional voltage and frequency stability control technology has been improved to address the characteristics of low inertia and damping in power electronic interfaces. The improved VSG strategy has been applied to the overall control of a hundred megawatt level flow battery energy storage power station, solving the voltage and frequency instability problems caused by load shocks and fluctuations. Finally, stable control of multiple parallel machines during the self starting process of the liquid flow battery energy storage power station was achieved, avoiding the occurrence of multiple machine circulation and uneven power distribution problems during the self starting process.


Introduction
Nowadays, the power system is developing towards intelligence, stability, and environmental protection.With the continuous increase of power grid scale, the replacement of complex structures, and the influence and limitations of external natural conditions, the potential risk of large-scale power outages in the power system still exists.The existence of large-scale energy storage power stations can overcome the instability, randomness, and intermittency of new energy generation, and make up for the important means of increasing frequency regulation and peak shaving pressure in high proportion new energy power systems.Therefore, many scholars at home and abroad have conducted research on the construction of energy storage power stations and the control strategies of the overall energy storage system.
Reference [1] provides a detailed discussion on the multi-agent black start control scheme for microgrid systems based on parallel recovery strategies, but there is still a problem of large surge currents occurring during parallel connections.Reference [2] uses auxiliary diesel generators as the reference source for wind farms and proposes a frequency control strategy for wind turbines during the black start process.Reference [3] innovatively utilizes energy storage wind farms as the self starting power source during the black start process of the power system.A research group related to Shanghai Jiao Tong University proposed a dual adaptive inertial control measurement [4], while scholars from Chongqing University proposed a two-stage frequency adjustment strategy based on adaptive control [5].In addition, some scholars have proposed a fractional order virtual inertia strategy [6], which can improve performance and suppress oscillations.Reference [7] has improved the reactive power sharing algorithm by combining virtual impedance and virtual capacitance, while also improving the reactive power allocation problem [8].Based on fuzzy auxiliary controller VSG control, it is suitable for grid connected and islanded microgrids.For specific island operation situations, reference [9] proposes a coefficient graph method based on robust virtual inertia control.
Based on the above analysis, this article constructs a stable control technology for the black start process of a 100 megawatt level liquid flow battery energy storage power station.Through the design of a 100 megawatt level energy storage power station and the improvement of control strategies at the energy storage unit and station levels, the consistency of output power and disturbance resistance ability of each energy storage unit in the energy storage power station are improved, ensuring the voltage and frequency stability of the power supply during the black start process.Being able to once again clarify the black start issue has a great promoting effect on the application and development of 100 megawatt all vanadium flow batteries.

Working Principle of all Vanadium Flow Battery
This section conducts unit modeling of a 100 megawatt all vanadium flow battery system, studies the energy conversion mechanism and characteristics of flow batteries, and uses PSCAD software to build mathematical models for each part, laying the foundation for the reliability and stability verification of subsequent black start energy storage system models.
The all vanadium flow battery achieves the reciprocating conversion of chemical energy to electrical energy through the valence state change of vanadium ions.The positive and negative electrolytes are stored in two storage devices respectively.During operation, the external pump presses the electrolyte from the storage device into the battery stack.During the charging and discharging process, the battery is conductive internally through the directional movement of cations in the electrolyte, thereby achieving the storage and release of electrical energy.The working principle is shown in figure 1.  (2) Compared with other forms of energy storage, all vanadium flow battery energy storage technology has advantages such as good safety, long cycle life, good charging and discharging characteristics, environmental protection, independent design of power and capacity.In recent years, the market size has been huge and there has been a continuous growth trend.In large-scale fixed energy storage scenarios with output power ranging from several kilowatts to hundreds of megawatts and energy storage capacity exceeding several hours, all vanadium flow battery energy storage has obvious advantages and is one of the preferred technologies for large-scale and efficient energy storage.The parameters of the liquid flow battery module in the research content of this article are shown in table 1 In the process of studying the black start energy storage system of a 100 megawatt all vanadium flow battery, in order to analyze the applicability of the system during the black start process, a longdistance no-load line system was established.The long-distance no-load line system model consists of an energy storage system, control module, transformer, circuit breaker BRK, etc., as shown in figure 2. Four liquid flow electric energy storage systems are used as black start power sources.In order to better meet the specific needs of the engineering project, energy storage batteries with capacities of 24MW (energy storage system 1-3) and 28MW (energy storage system 4) are established.The control module adjusts power electronic devices that do not have synchronous motor characteristics, making the system equipped with virtual synchronization technology.Transformers in the line are subjected to different types of voltage regulation, The circuit breaker BRK connects different lines and loads to establish long-distance no-load lines.

Island Control Mode of Energy Storage System
Virtual Synchronous Generator (VSG) is a special type of generator that simulates the behavior of traditional synchronous generators through power electronic devices.One of the advantages of VSG is that it can better control the frequency response of the power system.
VSG exhibits a certain amount of inertia during load disturbance.If there is no inertia when adding a load, the image shows an instantaneous change, and the value change belongs to a straight line.
When it has inertia, the rising speed slows down, and inertia has the ability to maintain the original state of the system.The larger the inertia, the stronger the ability to maintain the original state.
The control principle of the 100 megawatt all vanadium flow battery energy storage system operating in zero rise and islanding mode after the collapse of the power grid is shown in figure 3.
In the formula, 0 E is the no-load potential; (6) For the main control links inside the energy storage system, a separate analysis is conducted, as shown in figure 4, where the presence of the active loop belongs to the droop of virtual synchronization P-f, and the phase angle is given ， phase angle  It reflects the reference of threephase voltage, θ It is a rotation angle, not a fixed value.Pref represents the reference point of point 0. The change in active power P causes a change in virtual speed, and the change in virtual speed causes a virtual rotation angle  changes,  The variation given to the reference value will affect the generation of the reference frequency f, and the correlation between the variation f and P, as well as the correlation between sag, active power, and virtual characteristics.
The presence of a reactive power loop, as shown in figure 5, affects the dynamic characteristics of , and the presence of a reactive power loop is the droop of the associated V-Q.If the value between the reference Qref and the actual output reactive power Q is 0, then according to the rated voltage value Un peak, Uin (three-phase forward reference voltage) can be given to simulate the droop characteristics of Q and V.
As shown in figure 6, this figure is the reference value of the three-phase reference voltage.If it is a constant voltage and frequency, then the rotation angle of the frequency And a rotation angle of 50HZ Keeping it fixed and unchanged, the reference value of voltage also remains unchanged.However, in the design of this model, the inertia and droop characteristics of virtual synchronization characteristics are all integrated into the model.Therefore, in the analysis, inertia is introduced into the control of frequency and voltage, and the droop characteristics are correlated with the control of voltage and frequency.Therefore, the final VSG characteristics mainly affect the generation of three-phase reference values, This also proves the superiority of the overall control of black start energy storage in the dual loop control part of the converter in this article.Voltage is not only related to reactive power, but also to rotation angle θ It is also related.The voltage generates a reference value for the three-phase reference voltage Uin.In fact, the three-phase reference voltage is related to the reference value and also to the rotation angle.If it is related to the rotation angle, it affects the dq transformation.During the dq transformation process, reactive power disturbance will be generated, which will not produce bias characteristics, but will be impacted during active power disturbance (the impact is caused by θ Changes lead to changes in f) Due to the dual loop control, the relationship between dq transformation and rotation angle is significant.The structural diagram of the voltage regulator is shown in figure 7. The control and drop of VSG are aimed at introducing the inertia characteristics of the synchronous machine into the reference voltage of V-f control.The inertia characteristics and virtual speed are introduced into the frequency f of the reference voltage to control the rotation angle of the reference voltage.The reactive power output of the converter is introduced into the inertia characteristics to control the voltage value of the reference voltage, and the active power is introduced into the concern characteristics to control the frequency f of the reference voltage.
The final inner loop needs to introduce real-time output current of the converter.The reference voltage on the dq axis is the component of the three-phase reference voltage corresponding to the converter on the dq axis.That is to say, if the voltage value on the dq axis of the converter tracks the reference voltage in real time, the target will be achieved.If it is different according to the d-axis, △  will be generated.After entering the PI controller for adjustment, △  will generate the reference current Iref on the d-axis, which is given as the inner loop reference value, Enable the inverter to respond to this reference current in real time, and the reference current output by the inverter responds to the actual value.Then, the outer loop can also track the target, achieving tracking control.
Converter in real-time response tracking value process.If the set id is different from the reference value Iref, a deviation of △  (deviation amount) will be generated.After the deviation of △ id passes through PI, a modulation wave on the d-axis will be generated, and the same applies to the qaxis, which will generate a modulation wave on the q-axis.
The generated modulation wave can be transformed into a three-phase modulation wave through Park inverse transformation.The modulation wave and triangular wave of the three-phase are modulated by SPWM, and the modulation wave is generated as a sine wave.The carrier wave is a triangular wave.The modulation wave of the triangular wave and the sine wave intersect to control conduction and turn off, and then adjust the continuous duty cycle in real time to achieve the control target for response.The above is the dual loop theory.

Analysis of Overvoltage Caused by Long Power Lines and Transformer Closing in Energy Storage Belt
In order to verify the effectiveness and feasibility of the proposed scheme, simulation was conducted in the PSCAD environment.During the process of setting the model, an initial load of 0.1MW was added.When simulating to 2S, an active load of 0.1MW was added.At 2S, a reactive load of 0.1Mvar was also added, and it was cut off at 3S.The relationship between rated active and reactive power and actual active and reactive power is shown in figures 8 and 9.The active and reactive droop control with virtual synchronization and characteristics is simulated as a constant impedance load, and the actual output power of the constant impedance load is related to the terminal voltage.When the reactive power load is disturbed, the excessive drop in terminal voltage causes the reactive power value to not reach the set value, resulting in a decrease in the voltage applied to the load end.The constant impedance load Z is constant, P=U2/R, the impedance is constant, and the power changes with the terminal voltage.The lower the terminal voltage, the lower the power.The higher the terminal voltage, the higher the power.
Due to the existence of reactive power output, the active and reactive power disturbances during the simulation process cannot reach the active value, resulting in the terminal voltage not reaching the rated value and therefore unable to respond.Reactive power increases and voltage decreases, while reactive power decreases and voltage rebounds, completing the Q-U response curve.The rated current and actual current of the d-axis and q-axis during the simulation process are shown in figures 10 and 11.   Figure 13.Voltage variation curve.
Figures 8, 9, 12 and 13 show the active power, reactive power, voltage, and frequency changes output by the microgrid to supply important loads in the system.The system is connected to both active and reactive loads, and the results show that the microgrid can provide power to the loads in the system in a short period of time.It can also use energy storage modules to achieve smooth voltage transition, ensure frequency stability, and meet the requirements of black start.
Based on the analysis of the overall control technology of the energy storage system in the previous section, this section analyzes the control problem of a single energy storage module, and verifies the applicability and feasibility of the control method.

Power Frequency Stability Analysis and Multi Machine Power Allocation Strategy During Sudden Load Increase
During the research process, in order to increase the overall capacity of the energy storage system in practical engineering applications and ensure that the 100 megawatt all vanadium flow battery energy storage system has enough energy to be transmitted to the power lines during use, the system was divided into four independent unit level modules.The active and reactive power change curves of the four energy storage modules are shown in figures 14 and 15, respectively.When the external power grid fails, the current and voltage changes during the black start process are shown in figures 16 and 17.In a single energy storage system, the frequency is controlled within the rated range.For an overall black start energy storage system, the frequency exhibits significant fluctuations when load fluctuations occur.We can attribute this to the higher load requirements of the system.When more circuit breakers are opened, the black start system also requires a certain buffer period.When the energy storage system detects an increase in load, the black start energy storage system can recover the frequency within a controllable time.
After analyzing a single independent energy storage module, this section explores the voltage response and power distribution of four unit level energy storage power sources, tests the numerical changes in voltage and frequency of a 100 megawatt all vanadium flow battery energy storage system, and analyzes the changes in the active and reactive power change curves during the black start process of a single energy storage module, as shown in figure 18.The reason for the large circulating current in the parallel operation of the generator set is that the power grid has insufficient capacity to withstand it.When the load borne by the power grid is large, it will jointly inject current into the power grid, which will squeeze the current on the power grid, causing a decrease in the voltage of the power grid, leading to a decrease in the quality of the entire power grid, and thus causing the problem of large level C circulating current.
Malfunctions in the control system of the generator set, such as improper regulation of the generator excitation control system, may lead to unstable or oscillating output voltage of the generator set, resulting in large parallel circulation problems.
The power distribution and voltage and current changes of the energy storage module achieve equal distribution of power according to the ratio of capacity.While achieving equal power distribution, it also to some extent reduces the occurrence of circulation phenomena.Due to the different capacities of energy storage units, during the black start process, when the load increases, as shown in the figure, the power in the black start energy storage system reaches an average distribution, making the system run smoothly.At the same time, demonstrate the effectiveness of the methods used in energy storage systems and their applicability to black start processes.

Conclusion
This article takes the all vanadium flow battery as the research object.Through the design and construction of a hundred megawatt level all vanadium flow battery energy storage system, the energy conversion mechanism of the battery is clarified.A model of all vanadium flow battery and energy storage inverter is established to achieve multi machine parallel self start control of the energy storage power station.The overall research has completed the power average distribution of the entire system and stable control of voltage and frequency.The following conclusions can be drawn: 1) This article establishes a model for the 100 megawatt all vanadium flow battery energy storage power generation and black start simulation system, and completes the dynamic control and joint operation of the multi machine energy storage system.2) Due to the fact that most traditional VSGs do not directly intervene in the output voltage, their ability to carry unbalanced loads is poor.This article proposes a dual closed-loop VSG control strategy based on a 100 megawatt all vanadium flow battery energy storage system with virtual synchronization technology, which ensures stable and reliable operation of the built black start system.
3) This article analyzes the process of active and reactive load integration into the system and verifies the output results, including parameters such as three-phase voltage output from energy storage, three-phase current output from energy storage, energy output power, system frequency, and power allocation value during load integration.This provides a reference for the application of liquid flow batteries in energy storage systems and promotes the development of energy storage systems in the process of black start implementation.

Figure 1 .
Figure 1.Working principle of all vanadium flow battery.Positive electrode reaction:

Figure 2 .
Figure 2. Model of long distance empty load line system.

Figure 3 .
Figure 3. Traditional VSG control technology for energy storage system.

vK
is the voltage regulation coefficient; .ref UU are the predicted and actual values of VSG extreme electrical RMS, respectively.In order to improve the output characteristics of VSG, virtual impedances v L and v R are often introduced to correct the modulation signal mabc U of VSG, as shown in equation (6); If virtual impedance is not introduced, take mabc

Figure 4 .
Figure 4. Active power loop control structure diagram.

Figure 5 .
Figure 5. Reactive power loop control structure diagram.

Figure 6 .
Figure 6.Three phase reference voltage reference value.

Figure 8 .
Figure 8.Rated active power and actual active power.

Figure 9 .
Figure 9.Rated reactive power and actual reactive power.

Figure 10 .
Figure 10.Rated current and actual current of daxis.

Figure 11 .
Figure 11.Rated current and actual current of qaxis.

Figure 14 .
Figure 14.Active power variation curves of four energy storage modules.

Figure 15 .
Figure 15.Reactive power variation curves of four energy storage modules.

Figure 16 .
Figure 16.Current variation curve of energy storage module.

Figure 17 .
Figure 17.Voltage variation curve of energy storage module.

Figure 18 .
Figure 18.Active and reactive power variation curves of energy storage modules.