In recent years, new energy generation has achieved a leapfrog development. The large-scale electrochemical energy storage technology has significant strategic significance for the implementation of energy structure adjustment in the country. As an important direction for the development and application of energy storage, energy storage power supply technology has become one of the key energy storage technologies supported and promoted in the field of new energy with its own advantages.
| Installation location | Scale | Battery route |
| Shanxi Jingyu Power Plant | 9 MW/4.500 MWh | Lithium manganese oxide |
| Shanxi Jinneng Sunshine Power Plant | 9 MW/4.500 MWh | Ternary lithium |
| Shanxi Tongda Power Plant | 9 MW/4.478 MWh | Lithium iron phosphate |
When accurately controlling any power point within the rated power, battery energy storage systems can achieve full power output within milliseconds to seconds due to their advantages such as fast response speed, strong short-term power throughput, and flexible adjustment. Due to the urgent need for battery energy storage to assist in power frequency regulation, it can be combined with conventional frequency regulation power sources to effectively improve the frequency regulation capability of the power system, or to compensate for frequency deviation issues by independently participating in grid frequency regulation services as a frequency regulation power source. At present, typical design schemes for battery energy storage systems participating in power frequency regulation are constantly being explored both domestically and internationally. Tables 1 and 2 show demonstration projects for battery energy storage participating in frequency regulation both domestically and internationally.
| Energy storage type | Installation location | System scale |
| Lead-acid battery | Kodiak, Alaska, USA | 3 MW × 0.25 h |
| Lead-acid battery | Notrees, Texas, USA | 36 MW × 0.25 h |
| Liquid flow battery | Hokkaido, Japan | 4 MW × 1.5 hours |
| Lithium ion batteries | Elkins, West Virginia, USA | 32 MW × 0.25 h |
| Flywheel energy storage | Stephen Town, New York | 20 MW × 0.25 h |
By analyzing the participation of battery energy storage systems in power frequency regulation demonstration projects, it can be seen that this technology has broad application prospects. Studying the coordinated control problem of battery energy storage systems participating in power frequency regulation can help energy storage better participate in power frequency regulation. This article first summarizes the existing typical frequency modulation strategies and introduces the scheme design of battery energy storage capacity configuration system. At the same time, its economy is analyzed, providing strong support for the direction, planning, and construction of battery energy storage systems applied in the frequency modulation field.
1. Coordinated control strategy
At present, energy storage power sources mainly participate in the primary frequency regulation of the power grid through two control methods: inertial control and droop control.
When the system changes in frequency due to changes in load, etc., it is required that the active power change of the power grid and the frequency change meet a certain characteristic relationship, that is, the power change can compensate for the frequency change. The change in active power of the power grid can be to change the output of the generator, or to regulate the power of the grid by using other devices such as energy storage systems for charging and discharging. When the system frequency decreases, the energy storage system discharges and releases electrical energy to the grid, causing the active power of the grid to increase; When the system frequency increases, the energy storage system charges, absorbs energy from the grid, and reduces the active power of the grid, which is the primary frequency regulation of the energy storage system.
When using the virtual inertial control strategy (Figure 1), the unit adjustable power coefficient (denoted as ME) is defined as the ratio of energy storage output to frequency change rate increment, and its output calculation formula is:
When using the virtual droop control strategy (Figure 2), the unit regulated power coefficient (denoted as KE) is defined as the ratio of energy storage output to frequency increment, and its output calculation is shown in the formula.
The control strategy adopted by energy storage power sources to assist in grid frequency regulation is influenced by the application scenario. A linear output control strategy for energy storage batteries is proposed based on factors such as the rated power of the energy storage power supply system and the dead zone of primary frequency regulation; A comprehensive control strategy is proposed for two types of energy storage participating in primary frequency regulation of the power grid, which can effectively reduce the maximum frequency deviation and steady-state frequency deviation; By combining droop control and inertial control, a comprehensive control strategy is proposed based on the various requirements of power grid frequency regulation to achieve precise control.
2. Capacity configuration
2.1 Rated power design
Energy storage capacity configuration refers to the capacity of energy storage to replace conventional units in adjusting active power output through prime movers and participating in grid frequency regulation through charging and discharging. As one of the fundamental issues involved in the optimization planning of grid assisted frequency regulation applications, the capacity configuration method of battery energy storage systems has received considerable attention.
Assuming the frequency modulation period and starting time are T and t0, respectively, Prated is the rated power (charging is positive), Δ PE (t) is the power demand instruction, ∆ Pmax surplus is the maximum excess power. Based on PCS considerations, the configuration calculation of the energy storage system can be obtained as follows:
In the formula, η DC/DC and η DC/AC represents the efficiency of the converter; η CH and η Dis is the charging and discharging efficiency of the system.
2.2 Rated capacity design
According to the Prated in the formula, obtain the real-time power sequence, set Erated as the rated capacity, and calculate Erated using the formula. Therefore, considering that the charging and discharging cut-off voltages of QSOC (State of Charge) are 1 and 0, respectively, their State of Charge values are:
Among them, the unit of Erased is MWh; Ed represents the cumulative discharge of energy storage.
Let QSOC, ref represent the state of charge of the initial operating value. Considering the upper and lower limits of its operating range, the QSOC and k values at time k are as follows:
Among them, PiE represents the power command at the i-th moment; Δ T is the power command time interval. At this point, if the system is running normally, QSOC and k should meet the formula:
The prediction of the rated energy storage capacity based on the participation of energy storage power sources should meet the following equation:
By collecting battery SOC and analyzing its output depth, an adaptive control strategy is proposed; Research on the reconstruction of energy storage systems for primary frequency regulation in the power grid, in order to achieve fast and accurate response of energy storage participating in power grid frequency regulation. The proposed control strategy reduces the capacity configuration of energy storage, thereby maximizing the economic efficiency of energy storage power supply configuration.
3. Economic analysis
From the perspective of the effect model of energy storage participating in power grid frequency regulation and the theory of cycle life cost, construct an economic model for evaluating participation in energy storage power grid frequency regulation.
3.1 Cost and expense analysis of large-scale energy storage systems
The cost of energy storage usually includes two parts: first, operating costs; Next is investment cost.
The cost of energy storage investment is composed of replacement investment and initial investment cost. The so-called initial investment cost is actually a fixed one-time investment in the construction of energy storage projects. And in the total cost, this cost accounts for a very large proportion, including the cost of rated capacity and the power cost generated by the rated power of energy storage. In general, PCS will have an impact on power costs. The value of energy storage equipment itself will be reflected through capacity cost, and the so-called replacement investment cost actually refers to the maintenance and updating of corresponding energy storage equipment during the operation of energy storage, which requires financial investment.
Based on the aforementioned cost analysis, the expression for the present value of cost CLCC can be obtained as follows:
3.2 Economic benefits analysis of large-scale energy storage systems
The participation efficiency of power grid frequency regulation and energy storage includes fixed benefits on the one hand; Usually, it also includes environmental, dynamic, and static benefits.
And among them, real-time electricity, backup power, and efficiency constitute fixed benefits.
The expression for fixed benefits is:
From the above expression, it can be seen that the standby power efficiency is represented by PCAPACITY, while the real-time electricity efficiency is represented by PENERGY. The power service supply, also known as the frequency modulation generated by energy storage, is represented by P1, with MW as its unit; The corresponding unit price of capacity reserve is expressed in R1, and the frequency modulation task quantity, which is the frequency modulation electricity generated by energy storage, is expressed in E1, with MWh as its unit; R2 is an adjustment that fluctuates the electricity price based on the actual situation.
A large number of programs are required to calculate the environmental, dynamic, and static benefits, so the fixed benefit of Pprofit is usually used in an equivalent way to replace the energy storage frequency regulation benefit. Therefore, based on the present value method, the cycle benefits are converted into the following expression at the initial stage of project investment:
In the formula, Ry represents annual benefits.
Propose a battery life calculation model based on equivalent life loss, which can accurately estimate the life cycle of energy storage under frequent irregular cycles; Propose a coordinated and complementary method for optical energy storage to suppress power fluctuations. This method can reduce the demand for energy storage capacity, significantly reduce the number of cycles of energy storage charging and discharging, and extend the service life of energy storage in optical energy storage power plants.
4. Key scientific issues
Due to the volatility and intermittency of new energy sources such as wind power and photovoltaics, which have an impact on grid connection, it is necessary to optimize the control strategies, capacity allocation methods, and economic evaluation methods of energy storage power sources to lay a solid foundation for their introduction into the frequency modulation market. The author believes that the key scientific issues for the future are as follows:
(1) Handling the issue of coordinated control of energy storage participating in frequency regulation. Based on primary frequency regulation, comprehensive consideration should be given to its characteristics. Firstly, the relevant parameters of battery energy storage power supply should be sorted out to ensure that primary frequency regulation will have a good effect, and thus ensure that the generator does not have too much pressure during secondary frequency regulation.
(2) Handle issues related to the configuration capacity of energy storage power sources. When dealing with this issue, sufficient research should be conducted on dynamic programming and reasonable means should be chosen.
(3) Solving various problems in the energy storage power supply model based on economic evaluation. People need to study the life assessment model of energy storage power sources in what state of electricity they are in.
5. Conclusion
This article provides a detailed analysis and exploration of the configuration capacity, control coordination, and economic evaluation model of energy storage power sources. The following conclusions have been drawn:
(1) In terms of assisting traditional frequency regulation generator sets with energy storage power, the frequency regulation characteristics are fully considered, and the proposed strategy effectively improves the power utilization of energy storage power while achieving the optimal frequency regulation effect.
(2) In terms of the configuration capacity of power sources involved in power grid frequency regulation, the first step should be based on the comprehensive satisfaction of power grid frequency regulation needs, and diversified means should be used to ensure the optimal configuration of energy storage power sources.
(3) In the economic evaluation model of power grid frequency regulation involving storage power sources, various indicators of energy storage power sources (including their own and external indicators) should be quantified, and the economic evaluation model should be constructed based on the actual demand of the power grid.