Synchronizing Energy Storage Converters with the Grid: Methods and Processes

The integration of renewable energy resources into the electrical grid has necessitated the development of advanced energy storage systems, with Power Conversion Systems (PCS) playing a central role. These systems are crucial in maintaining a stable power supply and ensuring that the transition between grid-connected and off-grid modes happens seamlessly. This article delves into the various methods and processes involved in synchronizing energy storage converters with the grid.

 

Introduction to PCS and its Important Functionality in Experiments

 

PCS is a technology integral to the functionality of energy storage systems. It serves as a bridge between the storage elements and the power grid. The main functions of a PCS include the conversion of AC to DC (and vice versa), ensuring the quality of power, and controlling the flow of energy to and from the grid. Advanced PCS units are capable of performing complex experiments to test and improve these functions, thereby enhancing the overall reliability and efficiency of energy storage systems.

 

On-grid and Off-grid Switching Control

 

The ability of a PCS to switch between on-grid and off-grid modes is essential for the reliability of power supply systems. Two types of control strategies are employed for this purpose: active and passive.

 

Active Off-grid Switching

 

Active off-grid switching occurs when there is a failure or instability detected in the grid. The PCS must quickly identify this issue and switch to off-grid operation mode to ensure an uninterrupted power supply. The switchover needs to be executed swiftly to minimize the impact on the load and the continuity of the power supply.

 

The process typically involves the detection of anomalies in the frequency and amplitude of the grid power. By using a combination of both detection methods, a PCS can comprehensively judge and quickly detect grid faults. This enables a smooth transition that is free from any significant impact on the load or power supply. Figure 1 illustrates the waveform diagram of an active mode switching from on-grid to off-grid, highlighting the importance of maintaining a phase voltage and a phase current that align with the requirements of the off-grid operation.

 

Passive Off-grid Switching

 

In contrast, passive off-grid switching is initiated when the voltage at the grid connection point falls or rises beyond a certain threshold for a set number of consecutive sampling points. This indicates that the main network is either disconnected from the microgrid or has failed. The PCS then automatically transitions to off-grid control mode and triggers the disconnection of the main network switch. This passive off-grid mode is depicted in Figure 2, showcasing the seamless transition and the importance of detecting voltage changes at the grid connection point.

 

Synchronous Grid-connected Switching Control

 

Reconnecting to the grid, or grid synchronization, is equally crucial and can be achieved through passive or automatic control.

 

Passive Synchronization Grid-connected Control

 

Passive synchronization involves the use of a protection device to manage the grid connection. Before reconnecting to the grid, the energy storage converter must synchronize its output voltage with the grid voltage in terms of amplitude, frequency, and phase. This is achieved through a phase-locked loop tracking control. Figure 3 outlines the process, emphasizing the role of the synchronization protection device and the conditions for safe reconnection to the grid.

 

Automatic Synchronization Grid-connected Control

 

Automatic synchronization control allows the PCS to autonomously determine the synchronization point without relying on an external synchronization protection device. The PCS detects the grid-side voltage and, upon receiving a synchronization command from the monitoring system, starts to track the grid phase. Once phase tracking is complete, the grid-tied closing command is issued, and the switch is closed to achieve synchronization with the grid. This process is illustrated in Figure 4.

 

Off-grid Nonlinear Load Handling and Harmonic Elimination

 

When an energy storage system with PCS operates off-grid with a large nonlinear load, the output voltage can become significantly distorted. This is particularly problematic when rectifier electrical equipment is uncontrolled. Harmonic suppression methods must be employed to ensure the quality of power is maintained and to prevent damage to sensitive equipment. Figures 4 and 5 compare the output voltage waveforms with and without harmonic suppression, demonstrating the effectiveness of these methods.

 

Off-grid Switching Load

 

The ability to handle switching loads while off-grid is another important aspect of a PCS. Figure 7 shows the load waveform of a switching reactor when off-grid and loaded. The PCS must be able to manage these dynamic load conditions without causing instabilities in the power supply.

 

Off-grid Black Start Control

 

A black start is the process of restoring an electric power station to operation without relying on external electric power. PCS plays a key role in this process, especially in a microgrid scenario. Figure 10 shows the load shedding process and how the PCS manages the transition, with the green waveform representing the DC output voltage and the purple waveform representing the PCS output voltage.

 

Multi-machine Parallel Operation Testing

 

The ability of multiple PCS units to operate in parallel is critical for scalability and redundancy in energy storage systems. Figure 11 illustrates the parallel operation of two 50kW PCS units and one 100kW PCS, all running with an adjustable RLC load. The figure demonstrates how the units share the load and maintain stable operation even when one PCS is taken offline or brought back online.

 

Figure 12 further showcases the performance of three PCS units running in parallel with a resistive load and the subsequent introduction of a motor load. The impact on current and voltage is minimized, indicating that the PCS units are effectively managing power equalization and ensuring smooth operation under varying loads.

 

Conclusion

 

The synchronization of energy storage converters with the grid involves a range of methods and processes designed to maintain stability, efficiency, and reliability in power supply systems. PCS technology is at the heart of these operations, providing critical control functions that allow for seamless transitions between on-grid and off-grid modes, handling nonlinear loads, and facilitating black start operations. As the demand for renewable energy sources grows, the role of PCS and the sophistication of synchronization techniques will become increasingly important to ensure a resilient and sustainable energy infrastructure.