FGI Technology Sharing: A Semi-physical Simulation Modeling and Analysis Method for High-Pressure Level Energy Storage
Energy storage systems have multiple key functions, such as smoothing the output of new energy power generation, peak shaving and valley filling, frequency and voltage regulation, etc. They are an important means to solve the problem of absorbing wind and solar power. With the continuous increase in the installed capacity of new energy, the capacity requirements for energy storage systems are becoming increasingly strict. Battery energy storage power stations have built multiple demonstration projects of the 100-megawatt level worldwide and are gradually developing towards the gigawatt level. It can be seen from this that large-capacity battery energy storage systems have become a research hotspot and have great practical significance.
The traditional three-phase three-level power electronic converter is used as the power conversion system (PCS). The battery stack system enhances the voltage level and capacity through a large number of series and parallel connections of individual cells. On the one hand, the large-scale parallel connection of battery clusters brings about circulating current problems; On the other hand, the power of this type of topology is generally limited to below 2MW, and it is mostly expanded through multi-level parallel connection. This method has problems such as complex control systems and stability, and the response speed of the system also has a relatively large delay.
The use of multi-level converters can separate a vast number of battery cells into different power sub-modules for segmented control. Cascaded H-bridge conversion has been widely used in high-voltage cascaded reactive power compensation devices, but it still accounts for a minority as a power conversion system in energy storage systems. The cascaded H-bridge achieves the increase of voltage level and the expansion of capacity through the cascading of power sub-modules. Its topology fundamentally avoids the direct parallel connection of a large number of battery cells, and the single-cluster control method fundamentally realizes the absence of circulating current.
At present, for various battery cells on the market, the minimum capacity of a 35KV-level energy storage system generally needs to be greater than 15MW. If the system is directly built for verification, the time and cost required will also be relatively large. Therefore, how to achieve a more efficient and concise verification of the control algorithm? Semi-physical simulation is a relatively excellent method, which adds a hardware link in the closed-loop control test. Compared with all-digital simulation, it is closer to reality, can better reflect the performance of the controller, and the results are more reliable and realistic. Compared with the real test environment, it is also more controllable and safe.
The RT-LAB simulation software developed by OPAL-RT Company of Canada can convert offline models into online models. It has high compatibility and real-time performance, and can handle complex simulation and control problems quickly and flexibly, whether in the aspects of testing, executing and controlling models, or in semi-physical real-time simulation.
This paper conducts research on the high-voltage level energy storage system. Through theoretical modeling and parameter design, a 35kV/30MW simulation model is built in MATLAB/Simulink for verification. Meanwhile, with the help of the OP5700 simulator of OPAL-RT Company, a semi-physical simulation system is built to verify the algorithm of the converter controller.
2.Control strategy of cascaded H-bridge energy storage converter
(1) The main circuit topology of the cascaded H-bridge converter
Figure 1 shows the main circuit topology of the cascaded H-bridge energy storage converter. The three phases are connected in a star configuration, with N modules in each phase. Each module has the same parameters and is connected in series. An energy storage battery is connected to the DC side of each H-bridge module. In the figure, usa, usb and usc represent the three-phase grid voltages, L is the grid connection filter reactance, R is the filter reactance and the equivalent resistance of the line, isa, isb and isc are the three-phase output currents respectively, and uia, uib and uic are the three-phase output voltages respectively.
Figure 1 Cascaded H-bridge energy storage converter
When there are N modules in each phase, the output voltage of a single-phase energy storage system is:
From Kirchhoff's voltage law, the voltage-current relationship on the grid side of the cascaded H-bridge energy storage converter can be obtained as shown in the following equation:
(2) Modulation strategy of cascaded H-bridge energy storage system
Space vector modulation and carrier phase shift modulation are the two most widely used modulation strategies in cascaded multilevel converters. Among them, the carrier phase-shift modulation technology has a simple control method, superior performance and reliable operation. It can output a higher switching frequency under the premise of a lower system switching frequency and is more suitable for the control of modular systems. This paper adopts the unipolar frequency doubling carrier phase-shift modulation strategy. The principle of the unipolar frequency doubling carrier phase-shift modulation strategy is explained in combination with an H-bridge sub-module shown in Figure 2. Its modulation principle is shown in Figure 3. Compared with the carrier wave, if the modulated wave is larger than the carrier wave, S1 is on and S2 is off; otherwise, S2 is on and S1 is off. Then compare the same modulated wave with the reverse carrier wave. If the modulated wave is larger than the carrier wave, S4 is on and S3 is off; otherwise, S3 is on and S4 is off.
(3) Grid connection control strategy for cascaded H-bridges
The cascaded H-bridge energy storage converter has multiple functions such as peak shaving and valley filling, frequency and voltage regulation, and smoothing fluctuations. The realization of these functions requires the control of active and reactive power of the energy storage converter. The common methods for grid-connected power control of cascade H-bridge energy storage converters include the current phase-separated independent control strategy and the power decoupling control strategy. Among them, the power decoupling control strategy can achieve rapid and precise control of the output power of the cascade H-bridge energy storage converter. Therefore, this paper selects the power decoupling control strategy to realize the grid-connected of the cascade H-bridge energy storage converter.
3.Sum Up
This paper studies the mathematical model, grid connection control and semi-physical simulation verification of the high-voltage cascaded H-bridge energy storage system, and draws the following conclusions:
(1) The cascaded H-bridge energy storage system can achieve single-unit large-capacity grid-connected operation without transformers, and the quality of the grid-connected current waveform is high.
(2) The verification of the controller algorithm can be achieved through the RTLAB semi-physical simulation platform, shortening the development cycle and reducing the development risks.
(3) The application of the semi-physical simulation platform provides the possibility for the subsequent connection of the cascaded H-bridge energy storage system to the large power grid for semi-physical simulation at the station.
