Research on Control Strategy of Hybrid Energy Storage System Participating in Primary Frequency Regulation of Power Grid

With the ongoing development of China’s power system, there is a gradual increase in the proportion of new energy power generation. However, the randomness and volatility associated with new energy power generation can lead to increased frequency fluctuations in the power grid, posing a significant challenge to power system frequency regulation. In this paper, we investigate the control strategy of a hybrid energy storage system (HESS) that participates in the primary frequency modulation of the system. We analyze the advantages and disadvantages of various types of new energy storage from both technical and economic perspectives and perform an applicability analysis system to select lithium batteries and supercapacitor energy storage for forming a HESS. We propose a virtual droop control strategy to regulate the output of the HESS in the primary frequency regulation of the system. Finally, we build a simulation model that includes the HESS and power grid on the MATLAB/Simulink simulation platform and conduct a simulation analysis. The results show that the proposed strategy is superior and effective in controlling the HESS’s output when participating in the primary frequency modulation of the system.


Introduction
With the in-depth advancement of the clean transformation strategy of the energy system, the proportion of clean energy in China has gradually increased [1].The new energy has the characteristics of randomness and strong volatility.When a large amount of unstable electric energy flows into the grid, it will lead to aggravated power fluctuations on the power side and cause serious damage to the system.Therefore, it is urgent to introduce a new frequency regulation method to alleviate the frequency regulation pressure of traditional frequency regulation units [2] [3].
Energy Storage System (ESS), as an effective means to solve clean energy grid connection, has attracted much attention in the field of primary frequency regulation due to its advantages of precise tracking, fast response, high control precision, and two-way adjustment capability [4].The HESS can effectively suppress power fluctuations and improve energy supply stability, thereby reducing transmission loss, improving power quality and regulating power transmission, and solving voltage and frequency distortion problems.Deng et al. [5] analyzed the operation characteristics of energy storage such as supercapacitors, superconducting magnets, batteries, and flywheels, and combined these four energy storage devices with the improved system model to form a general model for frequency modulation simulation.Through the simulation, analyze and summarize the advantages and applicable application scenarios of each energy storage.Delile et al. [6] used the frequency change rate and 2 frequency deviation as control variables.When the frequency change rate exceeds its predetermined value, the battery energy storage system (BESS) will output at rated power.In the frequency recovery phase, it is controlled by the droop control strategy.In Ma's study [7], a virtual dip control strategy was used for ESS virtual droop control.The efficacy of the control strategy was reflected by comparing the results of frequency modulation with and without ESS.A strategy and a global regulation that comprehensively considers grid frequency regulation requirements and the SSE's responsibility status were proposed [8].Based on the adaptive optimization of the drop coefficient and the balancing control strategy in different charging states, implement adaptive adaptation and balance of the ESS state.
This paper conducts an in-depth study on the control strategy of the HESS participating in the primary frequency modulation of the system in a microgrid with wind storage, proposes a virtual droop control strategy suitable for HESS, and constructs a system response model with HESS primary frequency modulation.For the hybrid, the frequency regulation control strategy of the ESS provides a reference.

Hybrid energy storage type selection analysis
The addition of an energy storage device to a system significantly improves the frequency modulation effect in terms of response speed and frequency deviation.Various types of energy storage devices are available for use as frequency modulation units, each with its respective advantages and disadvantages, as evidenced by existing research and practical applications.HESS often yields better frequency modulation results than single energy storage and has a longer life.When selecting HESS, it is essential to combine the strengths of single energy storage while avoiding their weaknesses, considering both technological and economic aspects.
To determine the best HESS combination for participation in system frequency regulation, the paper summarizes the technical characteristics of multiple types of energy storage.And the HESS model is combined with the system frequency modulation model to provide a model for subsequent example simulations.To optimize the frequency modulation effect and prolong the service life of each energy storage device, various conditions are analyzed, and the optimal combination is selected based on the characteristics of the devices.These characteristics include response time, full-capacity discharge time, power limit, conversion efficiency, and technology maturity, as discussed in Li's research [9].In this study, several new energy storage technologies are analyzed for their technical characteristics, as shown in the Table .1In summary, supercapacitors and lithium batteries are optimal energy storage devices for different types of energy storage, respectively, and are often employed in system frequency regulation.Theoretically, these two technologies have complementary characteristics in terms of both technology and economy.By conducting research and optimizing the relevant control strategies involved in frequency modulation, the advantages of supercapacitors and lithium batteries can be maximized.As a result, this paper proposes a HESS, consisting of both supercapacitors and lithium-ion batteries, to participate in frequency modulation.

Energy storage battery model
The energy storage battery is generally connected to the power grid through the inverter after being connected in series and parallel, and the active and reactive power output is adjusted by controlling the inverter.Its structure is shown in Figure 1.In practical applications, to ensure consistent output characteristics and prolong the service life of an ESS, battery packs of the same system should be comprised of single batteries with the same model and performance, and each battery should operate under the same conditions.Fig. 2 illustrates the simplified equivalent circuit model of an energy storage battery group.

Fig.2. Simplified equivalent circuit model of energy storage battery pack
At a certain operating state of the battery, the battery potential is denoted as E , and the resistance R is composed of two components: the ohmic resistance 0 R and the polarization resistance 1 R : In the formula, 0 R is the constant equivalent ohmic resistance of the battery; 1 R is the equivalent polarization resistance of the battery at SOC 1 S = ; k is a coefficient.

Super capacitor model
A supercapacitor can be modeled as a series connection of a capacitor C and a resistor R. To analyze its output characteristics, we establish an RC equivalent circuit model of the supercapacitor, as shown in I represent the voltage and current at both ends of the supercapacitor, respectively.

Fig.3. Simplified Equivalent Circuit Diagram of Supercapacitors
The state of charge of the available supercapacitor is: In the formula, o V is the initial voltage; max V and min V are the maximum charging voltage and discharge cut-off voltage of the supercapacitor, respectively.

Hybrid energy storage model
In the model of ESS equipment, the expression for the change in the state of charge of ESS is: The ESS transfer function and power transfer function are respectively: In the formula, E T is the time constant of ESS.

Virtual droop control strategy for primary frequency modulation of hybrid energy storage participating system
To adjust the frequency, the ESS is treated as a conventional frequency modulation unit, simulating the droop characteristics of the generator set.This characteristic describes the relationship between the power stored or emitted by the ESS and the deviation in the frequency of the system.The droop characteristic formula is as follows: 0 1 ( ) In the formula, P is the output active power of the inverter, 0 P is the reference active power, 1 k is the droop coefficient, d f is 50 Hz and N P is the rated frequency.The ESS adopts the virtual droop control method, and the dynamic model of the load frequency of the ESS when the system performs a frequency adjustment is shown in Fig. 5: It is evident from Formula (7) that the frequency deviation value in the steady state of the system is directly influenced by several factors, including the damping coefficient of the system, the load disturbance, and the unit regulation power of both the traditional unit and the ESS.Notably, the ESS plays a critical role in this stage, and its unit adjustment power is a key determining factor.A higher unit adjustment power of the ESS results in a smaller steady-state frequency deviation.

Simulation Results and Analysis
Apply a step load fluctuation of 0.05 p.u. to the system at t=1 and simulate the frequency deviation curve of the primary frequency modulation of the system without the control strategy and virtual droop control HESS.Fig. 7 shows the results.Fig. 7. System frequency deviation curves of different control strategies under step disturbance Fig. 7 illustrates the effectiveness of the HESS using the virtual droop control strategy in regulating frequency under step disturbances.The system can restore the steady-state frequency value within the specified limit, with a maximum frequency deviation that is smaller than without ESS, and the fluctuation of deviation of primary frequency modulation of the system is also smaller.Moreover, the final steady-state frequency deviation is smaller than that of the control strategy without energy storage.The frequency deviation value of the HESS changes smoothly during the frequency regulation process, and the frequency drop speed is prompt, resulting in a favorable outcome.Furthermore, the HESS with virtual droop control can reach the max frequency deviation value earlier, and the time required to reach the stable value of frequency deviation is earlier than that of the no-control strategy.The virtual droop control strategy's frequency deterioration rate is also minimal, which indicates a robust ability to maintain the grid frequency.In summary, the HESS based on the virtual droop control strategy demonstrates superior performance, with a greater capacity to regulate frequency changes caused by step disturbances and a stronger ability to maintain grid frequency.

Conclusions
This paper conducts a comprehensive analysis of various energy storage technologies and their economics and selects battery and supercapacitors to form a HESS.Based on virtual droop control, proposes a strategy for this hybrid system to participate in primary frequency regulation.The main conclusions of this study are as follows: (1) The paper analyzes and selects a combination of HESS types, namely supercapacitors and lithium batteries, to participate in frequency modulation.Supercapacitors serve as power-type s, providing high power and fast response, with the ability to undergo multiple charge and discharge cycles, and albeit with a small capacity, they cannot produce power for extended periods.On the other hand, lithium batteries serve as energy-type energy storage devices, with a large capacity and long output time, but with low instantaneous power and a limited number of charge and discharge cycles.By complementing each other, these two energy storage types can form a HESS.
(2) The droop control strategy fully embodies the respective advantages of supercapacitors and batteries and gives full play to the role of hybrid energy storage in grid frequency regulation.Under step disturbance, the droop controls HESS compared to without control measures, and the system has a smaller maximum frequency deviation, a quicker frequency drop rate, and reaches the maximum frequency deviation sooner, which contributes to a more stable system operation and faster frequency recovery.
In summary, using the droop control HESS can effectively improve the frequency modulation effect of the system, and future research can consider HESS composed of multiple energy storage groups to participate in system frequency regulation.But the paper only conducts related research on hybrid energy storage control strategies and does not study the capacity optimization configuration of hybrid energy storage, which needs to be further explored.

Fig. 1 .
Fig.1.Structural diagram of battery ESS When BESS is utilized to mitigate power fluctuations in new energy power generation, the active power output of the ESS and the change in the state of charge are the primary technical parameters to consider.The equivalent model of an ESS consists of a battery-equivalent circuit and a state-of-charge model.Commonly used equivalent circuit models include the Thevenin model, GNL, PNGV, dynamic model, and RC model, among others [10].In practical applications, to ensure consistent output characteristics and prolong the service life of an ESS, battery packs of the same system should be comprised of single batteries with the same model and performance, and each battery should operate under the same conditions.Fig.2illustrates the simplified equivalent circuit model of an energy storage battery group.

Figure 3 .
Figure 3. Here, SC V and SC I represent the voltage and current at both ends of the supercapacitor, respectively.

Fig. 5 .
Fig.5.Load frequency response model of virtual droop control of ESS In the figure, G ( ) G s represents the transfer function of the traditional unit, G K represents the unit regulation power of the traditional unit, E ( ) P s Δ represents the frequency modulation output of the ESS, E ( ) G s represents the transfer function of the ESS, E K represents the unit regulation power of the ESS, and the rest of the parameters are the same as The meaning of the two chapters is the same.According to Fig.5, calculate and simplify the relationship among G ( ) P s Δ , E ( ) P s Δ , L ( ) P s Δ and ( ) f s Δ to obtain the relationship between ( ) f s Δ and each parameter as follows:

Table 1 .
. Technical and economical comparison of energy storage equipment