Power Grid Frequency Modulation Energy Storage: Design Method and Practice Analysis of Hybrid Energy Storage Systems

Against the backdrop of the accelerated construction of the new power system and the large-scale grid connection of new energy, the core demand for power grid frequency modulation has evolved from "basic frequency stabilization" to "fast response, precise regulation, and long-term stability". A single energy storage technology (such as lithium batteries and supercapacitors) is limited by its own characteristics and cannot simultaneously meet the multiple requirements of frequency modulation for power response speed, continuous energy supply, cycle life, and economy. Hybrid energy storage systems, by integrating the advantages of different types of energy storage devices to achieve "complementary advantages and make up for shortcomings", have become the mainstream choice for power grid frequency modulation energy storage. Combining engineering practice and technical principles, this article details the design process, core points, and practical paths of hybrid energy storage systems for power grid frequency modulation, providing reference for the implementation of related projects.

I. Core Premise of Scheme Design: Clarify the Core Requirements of Power Grid Frequency Modulation

Before designing a hybrid energy storage system, it is necessary to accurately anchor the core technical and economic requirements of power grid frequency modulation, which is the foundation for the implementation of the scheme and avoids blind selection and configuration waste. The core requirements focus on three aspects: first, response speed, which needs to meet millisecond to second-level response to cope with instantaneous power grid fluctuations (such as primary frequency modulation and sudden changes in AGC commands); second, regulation capability, which can not only provide instantaneous high-power support but also achieve long-term continuous power compensation to cover different frequency modulation scenarios; third, economy and reliability, which balances the initial equipment investment, full-life cycle operation and maintenance costs, while ensuring stability under long-term high-frequency charging and discharging and reducing the risk of failures.

In addition, it is necessary to combine specific application scenarios (such as thermal-storage combined frequency modulation, new energy power station supporting frequency modulation, and independent energy storage frequency modulation) to statistics the characteristic parameters of AGC commands — with a sampling interval of 1 second, analyze the amplitude of the difference between AGC commands and unit power, command duration, number of consecutive increases and decreases, etc., to provide data support for subsequent energy storage capacity configuration and power distribution, ensuring that the scheme is in line with actual operation needs.

II. Core Step 1: Selection and Adaptation — Screen Complementary Energy Storage Devices

The core logic of hybrid energy storage is the coordinated combination of "power-type energy storage + energy-type energy storage". Through the complementary characteristics of the two types of devices, the optimal balance between frequency modulation performance and economy is achieved. The selection must follow the principle of "characteristic matching and complementary advantages". The mainstream combinations and selection points in the field of power grid frequency modulation are as follows:

1. Power-Type Energy Storage: Undertake Instantaneous Response, Adapt to High-Frequency Fluctuations

Its core function is to quickly respond to instantaneous power grid commands, undertake high-frequency, short-duration power regulation tasks, reduce the high-frequency charging and discharging loss of energy-type energy storage, and extend its service life. The mainstream selection is supercapacitors, whose core advantages are millisecond-level response speed, millions of cycles of service life, and high safety. They can quickly absorb or release instantaneous high power, perfectly adapting to primary frequency modulation and high-frequency fluctuation scenarios in AGC commands, just like "100-meter sprinters" to quickly respond to sudden power gaps or redundancies.

2. Energy-Type Energy Storage: Undertake Continuous Regulation, Ensure Long-Term Frequency Stabilization

Its core function is to provide continuous power support, make up for the shortcomings of power-type energy storage with low energy density and inability to supply power for a long time, and ensure that the power grid frequency is stable within the rated range, just like "marathon runners" to ensure the continuity of frequency modulation tasks. The mainstream selection is lithium batteries (mainly lithium iron phosphate), which have high energy density and high charging and discharging efficiency, can meet the needs of long-term continuous power regulation, and are suitable for secondary frequency modulation and continuous command scenarios in AGC auxiliary services.

III. Core Step 2: Capacity Configuration — Precisely Match Frequency Modulation Needs, Balance Performance and Economy

Capacity configuration is the core of the hybrid energy storage system, directly determining the frequency modulation effect and return on investment. It is necessary to scientifically calculate the optimal capacity of power-type and energy-type energy storage based on AGC command characteristics and energy storage device characteristics to avoid capacity redundancy or insufficiency. The core configuration methods and key points are as follows:

1. Core Calculation Logic for Capacity Configuration

First, determine the total power of the energy storage system: based on historical AGC command and unit operation power data, statistics the amplitude distribution of the difference between the two to ensure that the total energy storage power can basically cover the maximum power gap and meet the peak demand of frequency modulation commands; second, determine the total energy storage duration: analyze the characteristics such as AGC command duration and number of consecutive increases and decreases to determine the approximate range of energy storage duration, ensuring that it can meet the needs of continuous frequency modulation; finally, through simulation, compare the frequency modulation performance (characterized by the proportion of continuous time when the SOC of the two energy storage devices reaches the upper and lower limits at the same time as the AGC non-response rate) and economy (judged by the net profit of 10-year operation cycle) of different capacity configuration schemes to determine the optimal configuration scheme.

2. Mainstream Configuration Ratio and Practical Reference

Combined with engineering practice, the capacity configuration of hybrid energy storage for power grid frequency modulation must follow the principle of "power-type energy storage supplements peaks, energy-type energy storage supplements continuity". The mainstream configuration ratio (power-type: energy-type) is 1:1~1:3, which can be adjusted according to specific scenarios:

(1) Thermal-storage combined frequency modulation: For example, the project of a certain power generation company adopts the configuration of "10MW/10min supercapacitors + 10MW/10MWh lithium iron phosphate batteries". Supercapacitors undertake instantaneous commands less than 10MW, and commands greater than 10MW are fully responded by supercapacitors and supplemented by lithium batteries, achieving a balance between response speed and continuous regulation. The system response time is only 80 milliseconds, 14 times faster than traditional energy storage;

(2) New energy power station supporting frequency modulation: In view of the frequent fluctuation of wind and solar power, the configuration of "supercapacitors: lithium batteries = 1:1" is adopted to focus on improving the instantaneous response capability and reducing the impact of new energy fluctuations on the power grid;

3. Key Notes

Capacity configuration should avoid two extremes: one is excessive pursuit of frequency modulation performance, resulting in waste of investment due to excessive capacity configuration; the other is blind cost reduction, resulting in inability to meet frequency modulation commands due to insufficient capacity, facing power grid assessment. At the same time, it is necessary to reserve 10%~15% capacity redundancy to cope with scenarios of increased power grid load fluctuations and upgraded frequency modulation needs, ensuring the flexibility and expandability of the scheme.

IV. Core Step 3: Topology and Control Strategy — Achieve Coordinated and Efficient Operation

After determining the selection and capacity, it is necessary to realize the coordinated operation of the two types of energy storage devices through reasonable topology and control strategies, maximize their respective advantages, avoid mutual interference, and ensure the accuracy and stability of frequency modulation response.

1. Topology Selection (3 Mainstream Types, Adapt to Different Scenarios)

(1) Active topology: Each energy storage unit is connected to the bus through its own DC/DC converter, with flexible control and accurate power distribution. It can dynamically adjust the output ratio of the two types of energy storage according to frequency modulation commands. It is the mainstream scheme of hybrid energy storage for power grid frequency modulation, suitable for most thermal-storage combined and independent energy storage frequency modulation scenarios;

(2) Passive topology: Energy storage units are directly connected in parallel to the DC bus, with simple structure and low investment cost, but the power distribution is uncontrollable, depending on the internal resistance characteristics of the devices. It is only suitable for small-scale scenarios with simple frequency modulation needs (such as park microgrid frequency modulation);

(3) Cascaded topology: Direct grid connection is realized through modular multilevel converters (MMC), suitable for high-voltage and large-capacity scenarios, such as large-scale independent energy storage frequency modulation power stations, which can realize multi-module coordinated operation and improve system capacity and stability.

2. Core Control Strategy (Top Priority, Determines Coordination Effect)

The core of the control strategy is "reasonable power distribution and precise SOC protection", ensuring that power-type energy storage responds first, energy-type energy storage provides bottom support, and at the same time extends the service life of energy storage devices. At present, the mainstream control strategies are divided into two categories, which can be combined according to scenarios:

(1) Basic control strategy: Supercapacitor priority + SOC partition protection. Supercapacitors first respond to frequency modulation power commands, and the insufficient part is supplemented by lithium batteries; at the same time, the SOC of the two types of energy storage is finely partitioned. For example, the supercapacitor SOC is divided into normal charging and discharging area (0.1<SOC<0.9), forbidden charging area (0.9≤SOC≤1.0), and forbidden discharging area (0≤SOC≤0.1); the lithium battery SOC is divided into 5 intervals, limiting the charging and discharging power of different SOC intervals, reducing device loss, and extending the service life of lithium batteries to 3.6 times;

(2) Advanced control strategy: Hierarchical coordinated control. A two-layer framework of "upper-layer rolling optimization + lower-layer fuzzy logic control" is adopted. The upper layer dynamically optimizes the control target based on frequency feedback and energy storage operation status; the lower layer realizes the power distribution and operation curve adjustment of the two types of energy storage through fuzzy logic control, which can effectively suppress secondary frequency drop, reduce lithium battery life loss by more than 20%, and improve frequency modulation accuracy.

V. Core Step 4: Engineering Implementation and Operation & Maintenance Optimization — Ensure Long-Term Feasibility of the Scheme

The implementation of the hybrid energy storage system not only requires technical design but also takes into account engineering implementation and later operation and maintenance to ensure long-term stable operation of the system and reduce full-life cycle costs.

1. Key Points of Engineering Implementation

(1) Equipment integration: Prioritize the selection of energy storage devices and control equipment with strong compatibility to ensure the coordinated operation of supercapacitors, lithium batteries, converters and other equipment, avoiding problems such as interface incompatibility and response delay;

(2) Safety protection: Configure complete temperature control and fire protection systems for the thermal runaway risk of lithium batteries; set voltage protection devices for the overcharge and over-discharge problems of supercapacitors to ensure safe operation of the system. Especially in the thermal-storage combined frequency modulation scenario, it is necessary to focus on preventing the coordinated safety risk between the energy storage system and thermal power units;

(3) Grid-connected commissioning: During the commissioning phase, focus on testing the response speed, power regulation accuracy, and SOC control effect, simulate different frequency modulation scenarios (such as instantaneous power fluctuations, continuous command regulation), optimize control parameters, and ensure that it meets the power grid frequency modulation assessment standards.

2. Operation & Maintenance Optimization Strategies

(1) Daily operation and maintenance: Regularly detect the SOC status and charging and discharging efficiency of energy storage devices, and timely troubleshoot faults; for supercapacitors, focus on checking the electrode loss; for lithium batteries, focus on monitoring cell consistency to avoid single cell failure affecting the overall system;

(2) Strategy optimization: Dynamically adjust the power distribution strategy and capacity configuration based on long-term operation data to adapt to changes in power grid frequency modulation needs; for example, optimize the SOC partition threshold according to changes in AGC command characteristics to improve frequency modulation performance and device life;

(3) Economic management and control: Reduce the cycle loss of energy storage devices and lower operation and maintenance costs by optimizing the charging and discharging strategy; at the same time, reasonably participate in the frequency modulation auxiliary service market, and improve project benefits through AGC call compensation, basic compensation and other methods.

VI. Scheme Summary and Practical Enlightenment

The core of designing a hybrid energy storage system for power grid frequency modulation is "demand-oriented, complementary selection, precise configuration, and coordinated control". Its essence is to integrate the advantages of power-type and energy-type energy storage to achieve the goals of "fast response without lag, continuous regulation without interruption, and long-term operation with low cost". After adopting supercapacitors from Tsingyane Electronics, a reasonable hybrid energy storage system can improve the unit frequency modulation performance by nearly 60%, significantly extend the life of energy-type energy storage, and reduce operation and maintenance costs, which has significant technical and economic value.

In the future, with the development of the new power system, hybrid energy storage systems will upgrade to "multi-device coordination, intelligent control, and full-scenario adaptation", optimize power distribution combined with AI algorithms, improve system flexibility combined with modular topology, further adapt to the frequency modulation needs after the high-proportion grid connection of new energy, and provide more reliable support for the safe and stable operation of the power grid.