Abstract: To improve grid stability and security and alleviate resource shortages and environmental problems caused by traditional power generation systems, microgrid systems have attracted widespread attention and research in recent years due to their flexible configuration and ease of operation. Among these systems, energy storage inverters are crucial energy conversion devices and key to peak shaving and valley filling, and the core technology of energy storage inverters lies in their internal control strategies, which directly affect the system's power quality and voltage and current control effectiveness. This paper proposes control structures and methods for energy storage inverters under two operating states, based on their working principles and characteristics. When the system operates in inverter mode, a single-loop current control structure is adopted to simplify the analysis and ensure the current meets system requirements. When the system operates in energy storage mode, a dual-loop voltage and current control structure is used. The system and controller models are built in the Matlab/Simulink simulation environment. Simulation results show that the adopted control strategy has good anti-interference capability against load disturbances.
1 Introduction
To achieve sustainable energy development and improve the utilization rate of renewable energy, microgrids always aim to enable flexible integration of renewable energy sources such as solar photovoltaic and wind power. However, due to the intermittent and random nature of renewable energy output, their large-scale integration can impact power quality and grid stability. Adding a management system to an energy storage system can more effectively promote the utilization of renewable energy. To achieve effective connection between the grid and the energy storage system and complete the charging and discharging process of the energy storage system, energy storage inverters have emerged. The energy storage system managed by the energy storage inverter can act as a load, absorbing surplus electricity from the microgrid, or as a grid power source, suppressing the volatility and uncertainty of renewable energy to a certain extent and ensuring the safe, stable, and reliable operation of the grid. According to national requirements for distributed generation and microgrid construction, energy storage inverters provide active and reactive power support to the grid, stabilize grid voltage and frequency, and facilitate the charging and discharging of various energy storage devices connected to the grid.
2. Working principle and model of energy storage inverter
2.1 The principle of bidirectional energy flow in energy storage inverters
To simplify the analysis by focusing on the vector relationships within the circuit, circuit resistance is ignored. Figure 1 shows the model circuit of the energy storage inverter, which mainly consists of the grid electromotive force e, the grid-side inductance L, the energy storage inverter bridge circuit, the load resistor RL, and the DC-side electromotive force eL. The AC-side voltage and current are v and i, respectively. The DC-side voltage and current are Vdc and Idc, respectively.
When the switching losses of the bridge circuit are not considered, the power conservation relationship on both sides of the system yields the following:
iv=idc Vdc （1）
As can be seen from equation (1), we can control the AC side parameters of the energy storage inverter by controlling the DC side current parameters, and we can also change the DC flow by controlling the AC side parameters.
Assuming the AC current and inductor voltage vectors are represented by L and VL respectively, we can obtain VL = ωLI, and the current lags the voltage by 90°. If the grid electromotive force vector is represented by E, and the AC voltage of the energy storage inverter is represented by V, then according to Kirchhoff's voltage law, the voltage relationship on the AC side of the system can be expressed as E = V + VL = V + ωLI, with the magnitudes being [equation missing]. Assuming | I | remains constant, then |VL| = ωL| I | also remains constant. Assuming the grid electromotive force E is constant, the trajectory of the system voltage vector V in space is circular, and the radius |VL| is [equation missing]. To visually represent this relationship, Figure 2 shows the relationship between the endpoints of V at the following four operating points A, B, C, and D.
The specific operational status and analysis are as follows:
(1) When the voltage vector endpoints are between points A and B, the energy storage inverter operates in energy storage mode, and the grid supplies active and inductive reactive power to the system. When operating at point A, the grid only supplies inductive reactive power.
(2) When the voltage vector endpoints are between points B and C, the energy storage inverter operates in energy storage mode, and the grid supplies active and capacitive reactive power to the system. When operating at point B, the grid only supplies active power.
(3) When the voltage vector endpoints are between points C and D, the energy storage inverter operates in inverter mode, and the system supplies active and capacitive reactive power to the grid. When operating at point C, it only supplies capacitive reactive power to the grid.
(4) When the voltage vector endpoints are between points D and A, the energy storage inverter operates in inverter mode, and the system supplies active and inductive reactive power to the grid. When operating at point D, it supplies active power to the grid. Based on the above analysis, we can control the energy storage inverter to operate in any state by controlling the grid-side current. In the control of the three-phase energy storage inverter, we hope that the system can operate at points B and D, and only active power is transmitted between the grid and the energy storage inverter system. That is, in the energy storage operating state, the grid-side voltage vector and current vector are in the same direction, and in the inverter operating state, the grid-side voltage vector and current vector are in opposite directions.
2.2 Topology of Energy Storage Inverter
The large capacitor connected in parallel to the voltage-source energy storage inverter can effectively suppress DC voltage fluctuations. Therefore, this paper adopts the voltage-source topology shown in Figure 3.
2.2.1 Main circuit structure
The single-stage main circuit topology is shown in Figure 4. The energy storage medium is connected to the power grid through a first-stage DC/AC conversion stage and an isolation transformer. This is to achieve the access and maximum utilization of the energy storage medium with low port voltage and wide voltage range.
To facilitate the distributed integration of large-scale energy storage components, a multi-branch design is adopted. The single-stage design consists of multiple independent DC/AC branches, connected in parallel to the grid via isolation transformers.
When using multi-branch converters, energy storage units can be connected in groups. The converter will monitor and control the status of each group of energy storage media separately, and control each branch separately according to the management system or preset data. This effectively improves the reliability of the energy storage system, avoids circulating current and capacity imbalance caused by large-scale parallel connection of battery packs, and can effectively improve the utilization rate and lifespan of energy storage media.
The main circuit structure is shown in Figure 5. The battery pack input passes through a DC EMC filter and a switch, and is connected to the PCS DC bus. The DC voltage is converted into a high-frequency three-phase chopped voltage by a three-phase bridge converter, and then converted into sinusoidal AC voltage by an LCL filter. After passing through an AC switch, an AC EMC filter, and an AC circuit breaker, it is sent to the internal AC bus.
3. Energy Storage Inverter Control Structure and PI Control Strategy
The main function of an energy storage inverter is to facilitate the flow and conversion of energy between the three-phase power grid and the DC energy storage device. It typically operates in two modes: inversion and energy storage. In both modes, we primarily control the grid-side current and DC voltage.
3.1 Single-loop control structure in inverter mode
Based on the topology of the energy storage inverter in inverter operation and the control objective in this state, we consider the DC-side voltage to be stable, therefore only the system current needs to be controlled. Since the grid current and voltage are three-phase sinusoidal AC quantities, for ease of control, we choose to design and analyze the controller in the dq rotating coordinate system. The single-loop control structure shown in Figure 6 is typically used. To achieve the unit power control requirement of the system, the reactive power reference current on the q-axis needs to be set, and the active power reference current on the d-axis needs to be set according to requirements. The current controller generates switching signals for the energy storage inverter bridge circuit based on the deviation between the actual d- and q-axis current values and the reference current values, ultimately causing the system current to reach the set value.
3.2 Dual-loop control structure in energy storage mode
When the system operates in energy storage mode, it needs to meet current requirements while maintaining a stable DC output voltage. Therefore, we adopt a dual-loop control structure for voltage and current, as shown in Figure 7. The outer voltage loop controller enables rapid, steady-state error-free tracking of the DC voltage and generates a d-axis reference current; the inner current loop controller tracks and regulates the d- and q-axis currents, ultimately meeting the requirements of the control system.
3.3 PI Control Strategy
Due to its simple algorithm and easy-to-tune controller parameters, PI control has become a widely adopted control scheme in current energy storage inverter control systems. For energy storage inverter systems, when operating in inverter mode, the current loop controller in Figure 6 adopts a PI control strategy; when operating in energy storage mode, both the voltage and current controllers in Figure 7 adopt a PI control strategy, which is referred to as dual-loop PI control.
3.3.1 Control Equations under Energy Storage Inverter State
With the system operating in inverter mode and the grid-side control voltage, the system equations can be obtained as follows:
When the system operates in energy storage mode, the current control analysis is the same as that in inverter mode, and will not be repeated here. The control equations for Ud and Uq are as follows.
When the system operates in energy storage mode, a voltage-current dual-loop control structure as shown in Figure 7 is adopted to maintain voltage stability. The DC-side voltage is regulated by the outer loop PI controller to generate a reference value i*d for the d-axis. The proportional gain and integral gain of the controller are Kup and Kui, respectively. Assuming the q-axis reference current i*d is 0, the control equation is as follows:
Combining equations (5) and (6), the PI voltage and current dual-loop control structure diagram under energy storage state is shown in Figure 9.
3.3.2 PI Controller Parameter Design
(1) Design of current inner loop parameters
Since the controlled objects of the d-axis and q-axis are the same, their controller structures and parameters are also the same. The closed-loop block diagram of the d-axis current is shown in Figure 10. If a fast dynamic response is required, the current loop controller parameters can be obtained as follows:
(2) Design of outer loop voltage parameters
For a three-phase inverter system, when SVPWM modulation is used, its DC-side current IDC can be expressed as:
4 Control Simulation
4.1 Simulation of PI Single-Loop Control under Inverter State
The parameters of the control system in inverter mode are set as shown in Table 1. The simulation diagram of the system in inverter mode is shown in Figure 12. The simulation results are shown in Figures 13 and 14.
The system's d-axis and q-axis currents can converge to the set values quickly and without error, resulting in good control performance.
4.2 Simulation of PI dual-loop control under energy storage conditions
The system control system parameter settings in energy storage mode are shown in Table 2. The system main circuit model and PI dual-loop controller model in energy storage mode are shown in Figure 15.
The voltage waveform at the DC end of the system is shown in Figure 16.
5. Conclusion
Energy and environmental issues have spurred research into microgrids, driving the development of energy storage inverter systems and leading to widespread scholarly attention and research on energy storage inverters in recent years. This project considers the comprehensive control objectives and practical problems that energy storage inverters need to meet under both inversion and energy storage operating states, and studies the control strategies of their internal current and voltage loop controllers. Mathematical models of the energy storage inverter under both inversion and energy storage operating states are established, and the SVPWM modulation algorithm used in the system is implemented. Based on the control objectives of the energy storage inverter, the controller structure for both inversion and energy storage operating states is determined. The method and principle of PI control are analyzed, and the control equations are established and the control parameters are tuned. Simulation verification of PI control under both operating states is completed in the Matlab/Simulink simulation environment.
