Energy storage system-assisted renewable energy grid connection solutions are mainly divided into two types: AC parallel connection and DC coupling.
The AC parallel connection solution, as shown in the figure, refers to renewable energy power electronic grid-connected equipment, such as photovoltaic inverters or wind turbine converters, being connected to an energy storage converter (PCS) via the AC power grid. Under the unified coordination and control of the EMS, it performs functions such as peak shaving and valley filling, improving forecast accuracy, and smoothing.
The main advantages of AC parallel connection schemes include simple and clear electrical connections between devices, functional decoupling, and easy standardization of equipment development and manufacturing processes; the main disadvantages are higher costs for lines and connected equipment, faster control response speed required for PCS, and lower efficiency for multiple energy conversions.
The coupling scheme can effectively utilize the DC link inherent in most renewable energy grid-connected power generation systems, directly adding battery energy storage devices to reduce multiple power conversions of renewable energy power. This improves the system's grid connection and energy storage efficiency; it also directly utilizes existing renewable energy grid-connected equipment and grid connection channels, eliminating the need for AC equipment expansion and reducing hardware investment costs. However, there is coupling between the control system and the existing renewable energy grid-connected equipment, the tightness of which depends on the grid connection control method of the original renewable energy system.
Taking a full-power wind turbine grid-connected converter as an example, as shown in the figure, it is generally an AC-DC-AC "back-to-back" structure. The grid-side converter operates in DC-side voltage regulation mode, while the turbine-side converter operates in wind turbine power control mode or torque control mode. The two are separated by the DC side and are independently controlled, with the large capacitor bank on the DC side acting as a buffer and decoupling mechanism. Therefore, by connecting a certain capacity of BESS to the DC side to form an integrated wind and energy storage system, the grid-connected power of the wind turbine can be well controlled and the energy can be transferred over time, without having a significant impact on the wind turbine system, especially the control of the wind turbine converter.
Its basic control principle is as follows: The local controller sets the working mode, such as peak shaving and valley filling, improving prediction accuracy, or smoothing, and integrates grid dispatch information to generate the total grid-connected power target command ∑P* for the wind-storage system at a certain moment; it monitors the wind turbine power generation PNE and the energy storage system status in real time, and comprehensively calculates and generates the energy storage system charging and discharging control command P*BESS :
BESS controls the wind turbine through a DC/DC converter, tracking P*BESS commands to achieve energy storage and release between the DC side of the wind turbine converter and the battery; the grid-side converter operates in rectifier mode, stabilizing the DC side voltage Vde to achieve the total grid-connected power output ∑P of the wind turbine:
When the SOC of the energy storage system is in a critical overcharge state, the local controller also needs to limit the output command P*NE of the wind turbine by scheduling the wind turbine master controller, so as to realize the power-limited operation of the wind turbine.
The simplified diagram of the control system is shown in the figure. Vdc and Ugrid correspond to the effective values of the DC-side voltage of the wind turbine converter and the phase voltage of the grid, respectively; IBEss, Idc, INE, and Igrid correspond to the charging and discharging current of the energy storage system, the DC current of the wind turbine grid-side converter (flowing from the current-side bus capacitor bank to the grid-side converter IGBT bridge arm), the DC current of the wind turbine machine-side converter (flowing from the machine-side converter IGBT bridge arm to the DC-side bus capacitor bank), and the grid-connected current of the wind turbine grid-side converter (i.e., the total grid-connected current of the wind-storage system).
For doubly-fed induction generator (DFIG) wind turbines, the power generation system (PNE) consists of both rotor-side and stator-side output power, which the local controller must consider comprehensively when calculating the power command for the energy storage system.
It can be seen that in the wind-storage system, both the grid-side converter and the turbine-side converter maintain the original DC-side voltage regulation control mode and turbine-side power control mode, without requiring any changes to the control algorithm. However, timely and accurate acquisition of the wind-storage system status information is crucial for the local controller to achieve unified control of the energy storage system and the main wind turbine control system.
The control scheme for a parallel photovoltaic-storage DC system is shown in the figure. This control scheme does not affect the operation of the photovoltaic inverter, which always operates in the energy storage system's peak-load mode or short-term power-limiting mode. The energy storage system, in conjunction with the local controller, performs rapid power regulation to maintain the grid-connected power of the photovoltaic-storage system within the allowable control error bandwidth.
Assisting the grid connection of new energy sources is a very important application area of BESS (Balanced Energy Storage System). From a control time scale perspective, this can be divided into hourly peak-filling and minute-level improvement of forecast accuracy and smoothing of fluctuations. The former fully utilizes the existing grid's capacity to accommodate new energy sources and reduces...
Assisting renewable energy grid connection is a crucial application area for Baseline Energy Saving System (BESS). From a timescale perspective, this can be divided into hourly peak shaving and valley filling, and minute-level improvements in forecast accuracy and smoothing of fluctuations. The former is significant for fully utilizing the existing grid's renewable energy capacity, reducing conventional unit reserves, or avoiding prolonged renewable energy curtailment. The latter, in conjunction with renewable energy generation forecasting technologies, improves the planning and dispatchability of renewable energy grid connection, enhances grid friendliness, and reduces the occupation of grid fast frequency regulation resources.
In practical projects, peak shaving and valley filling applications require BESS systems capable of storing or releasing several hours' worth of electricity, necessitating large-capacity battery units. Under current business models, simply applying this function is often uneconomical, or carries a significant risk of seasonally decreasing economic benefits. However, with the continuous improvement in renewable energy generation forecast accuracy and BESS power control algorithms, it is entirely possible to integrate minute-level BESS-assisted renewable energy grid connection functions into peak shaving and valley filling projects. This allows a project to undertake comprehensive applications under unified management by an EMS or local controller, either time-sharing or simultaneously, thereby improving the project's overall economic efficiency. Furthermore, considering the power and capacity configuration requirements for improving forecast accuracy and smoothing functions, most of these applications involve low-power, high-frequency power-type energy storage. Therefore, the addition of these functions has a relatively limited impact on the configuration of existing peak-shaving and valley-filling projects, making it technically feasible.