Thermal power units, as the primary frequency regulation resource providing AGC (Automatic Guided Vehicle) auxiliary services, suffer from several drawbacks: long response time (typically in the tens of seconds range); slow regulation rate (the standard regulation rate (MWmin) for thermal power units does not exceed 3% of rated power); and poor regulation accuracy (the allowable deviation for thermal power units is 1% of rated power).
However, employing battery energy storage systems in conjunction with thermal power units to respond to AGC commands can fully leverage the advantages of energy storage systems: short response time (<100ms), fast regulation rate (regulation time from no-load to full-load less than 20ms), and high regulation accuracy. This improves the overall regulation performance index K of the unit while avoiding the need for large-capacity energy storage systems, enabling the project to achieve better economic benefits.
The basic principles and process of combined thermal and energy storage frequency regulation:
1) Electrically, energy storage and thermal power units can operate in parallel at the grid connection end, working together to track AGC dispatch commands, thus greatly improving the overall regulation performance;
2) Without changing the original AGC control of the thermal power unit, construct the output command of the energy storage system based on the difference between the AGC command and the real-time output of the thermal power unit, and make up for the power demand gap caused by the difference by utilizing the fast and accurate power control characteristics of the energy storage system.
3) As the output of the thermal power unit responds to the AGC command and approaches it, the output of the energy storage system is withdrawn accordingly until the thermal power unit finally takes over the output of the AGC command. It can be seen that the high-power operation time of the energy storage system during a single AGC adjustment is on the order of 1 to 2 minutes.
As can be seen from the above process, the maximum output power of the BESS is the difference between the AGC command and the current output of the thermal power unit. The performance requirement emphasizes high-power, rapid, and precise regulation, while the capacity requirement is limited, making it a typical power-type BESS application. Although the capacity and power of the BESS can theoretically be optimally configured based on the grid frequency and the fluctuation characteristics of the control error signal in the region, comprehensively considering the impact of load fluctuations, the grid AGC dispatching principles, and optimizing economic benefits, most current design processes are based on the analysis and statistical data of the unit's past AGC commands, striving to completely track more than 90% of the AGC dispatching commands and, during operation, to maintain the battery SOC around 50%.
Furthermore, based on the technical requirement that the maximum power change rate of the thermal power unit is 3%P per minute, and since AGC command changes are mostly on a minute-by-minute cycle, configuring a 2C energy storage system at 3% of the thermal power unit's rated power P is more reasonable.
The basic principle is shown in the figure.
In combined thermal power and energy storage systems, BESS (Boiler Energy Storage System) grid connection methods generally fall into two categories: one utilizes the surplus capacity of the existing plant auxiliary transformer and connects it to the generator outlet via a secondary voltage booster;
the other configures an independent step-up transformer to directly connect the energy storage system to the generator outlet. Both connection methods require attention to line short-circuit capacity and harmonic variations to ensure the safe operation of the existing thermal power units, main transformers, boiler actuators, and auxiliary systems. Currently, the plant auxiliary transformer connection scheme is more common.
Regarding communication and control systems, both the RTU (Remote Control Unit) and DCS (Distributed Control System) should be modified accordingly, as shown in Figure.
The equipment's technical upgrades and basic functions include:
The RTU (Regional Unit) will add a BESS (Balanced Energy Storage System) power measurement package, which will be merged with the generator output measurement values and transmitted to the power grid dispatch center as the basis for AGC (Automatic Gain Control) assessment. A new communication channel with the BESS will be established to allocate AGC commands and, as needed, transmit the output and status information of the combined thermal power and energy storage system to the BESS for preliminary AGC regulation performance index K evaluation and benefit analysis locally.
The DCS (Distributed Control System) will establish a new communication channel with the BESS to transmit AGC commands, generator output feedback, actual generator load indicators, generator AGC activation feedback, generator primary frequency regulation action flags, generator output limits, and generator regulation rate limits.
BESS, based on AGC commands and the real-time output of the generator unit, combined with the SOC of the energy storage system battery, constructs power commands for the energy storage system to achieve rapid power control and regulation, as shown in the figure.
Image: BESS Auxiliary AGC Controller
In a combined thermal and energy storage frequency regulation system, the energy storage system mostly consists of a PCS+step-up transformer container, a battery container, a high-voltage access container, and a local monitoring container. Among them, the PCS+step-up transformer container houses the ring main unit, the step-up transformer, and the PCS. It is connected to the battery container on the DC side, and on the AC side, it is connected in parallel with the adjacent energy storage system before being connected to the plant service transformer via a central switch cabinet.
In the specific implementation of the project, the details of the design and modification may vary, but all must adhere to the principle of minimizing the impact on the original thermal power units and should not pose any safety hazards to the normal operation of the DCS and the units.
With increasingly stringent requirements for power quality, particularly the rapid increase in the capacity of renewable energy sources such as wind and solar power, the power grid has a growing demand for high-quality frequency regulation resources. However, frequent large-scale AGC (Automatic Gain Control) adjustments by thermal power units can negatively impact equipment and hinder stable operation. Furthermore, ultra-low emission retrofitting further limits the regulation rate of thermal power units, reducing the regulation performance index K. Therefore, integrated thermal power and energy storage frequency regulation systems offer direct technical benefits and substantial economic advantages.
Taking a thermal power and energy storage integrated project in Northwest China as an example, before the addition of energy storage, the AGC regulation performance index K of independent thermal power units ranged from 1.97 to 2.62. After adding energy storage, the integrated thermal power and energy storage system improved this to 4.95 to 5.91; the compensation cost also increased from less than 10,000 yuan/day to nearly 110,000 yuan/day.
However, during periods of relatively stable load, the grid's demand for frequency regulation resources has an upper limit, and the market space for this application will be rapidly squeezed. Due to the adoption of a "zero-sum" rule and the influence of policies and related interest distribution mechanisms, the project's revenue, especially that of energy storage system owners, is subject to certain uncertainties.