Exploring the Dynamics of Grid-Connected to Off-Grid Transition in Power Conversion Systems

Introduction to Power Conversion Systems (PCS) and Their Importance

Power Conversion Systems (PCS) are integral components of modern energy systems, tasked with the crucial function of converting electrical power from one form to another. These systems are pivotal in integrating renewable energy sources, managing energy storage, and ensuring consistent energy quality and supply. PCS are designed to operate in both grid-connected and off-grid modes, thereby ensuring that energy delivery is maintained even during grid disturbances or failures.

The ability of PCS to switch seamlessly between on-grid and off-grid modes is not just a convenience but a critical requirement for the resilience of power systems, especially in the face of increasing occurrences of extreme weather events and cyber-attacks. The experiments conducted on PCS functions provide valuable insights into the dynamics of these transitions and highlight the technological advancements in this field.

On-grid and Off-grid Switching Control

The transition between grid-connected and off-grid modes can be categorized into two types: active and passive off-grid switching.

Active Off-grid Switching

Active off-grid switching is characterized by a seamless transition from a grid-connected to an off-grid state. This process is essential when there is a sudden grid failure. In such cases, the energy storage system within the PCS must quickly detect the failure and switch to the off-grid operation mode. The transition time must be brief to minimize any interruption in power supply.

To effectively manage this transition, a combination of frequency and amplitude detection methods is employed. These methods allow for a comprehensive judgment and rapid detection of grid faults, facilitating a smooth and impact-free switching process. Figure 1 illustrates the waveform diagram of an active mode transition from grid-connected to off-grid, showing the critical parameters such as phase voltage and phase current during the switch.

Passive Off-grid Switching

In contrast, passive off-grid switching is another method for transitioning from grid-connected to off-grid mode, involving a passive control strategy. Here, the PCS monitors the voltage at the grid connection point (Vm). If the voltage at this point falls or rises beyond a set threshold for a predefined number of consecutive sampling points, it indicates a disconnection or failure of the main grid. The PCS then automatically transitions to off-grid control mode and triggers the disconnection of the main grid switch to realize passive off-grid operation. Figure 2 depicts the waveform of this process, demonstrating the voltage behavior during the transition.

Synchronous Grid-Connected Switching Control

Reconnecting to the grid from an off-grid state requires careful synchronization to ensure that the PCS output matches the grid's voltage in amplitude, frequency, and phase. This is crucial to prevent excessive surge currents, which could pose a risk to the converter's safety. Two methods are primarily used for this:

Passive synchronization involves the use of a protection device for grid-connection closing. The energy storage converter changes from a voltage/frequency (V/f) control mode to a constant power control mode during the transition. A synchronization protection device facilitates this process, with real-time judgment by the protection device to ensure immediate closure when conditions are met. Figure 3 presents the off-grid to grid-connected switching waveform diagram.

Automatic synchronization control allows the PCS to autonomously determine the synchronization point without relying on a separate synchronization protection device. It tracks the grid side voltage and, upon receiving a synchronization command, starts phase tracking. Once the tracking is complete, a grid-connection closing command is issued, and the corresponding line switch is closed to achieve automatic synchronization. Figure 4 outlines this automatic synchronization control process.

Managing Off-grid Nonlinear Loads and Harmonic Suppression

When operating off-grid, PCS might encounter nonlinear loads, which can lead to significant voltage distortion. This distortion is undesirable as it can affect the quality of power supply and damage sensitive equipment. To mitigate this, harmonic suppression methods are employed to ensure a clean and stable output voltage waveform, even under nonlinear loading conditions. Figures 5 and 7 show the waveform improvements with harmonic suppression techniques and the load waveform of a switching reactor when off-grid and loaded, respectively.

Off-grid Black Start Control

In the event of a complete system shutdown or "blackout," the PCS must have the capability to perform a black start. This process involves progressively re-energizing parts of the power system without external power. Figure 10 illustrates the load shedding strategy employed during a black start, highlighting the DC output voltage waveform and the PCS output voltage for different loads.

Multi-machine Parallel Testing

The reliability and effectiveness of PCS are also tested in multi-machine parallel configurations. Figure 11 and Figure 12 showcase the performance of multiple PCS units running in parallel under different load conditions. These figures demonstrate the power balancing and stable operation of PCS units when faced with the introduction of impact loads, such as motor starts, and the equalization of power among units when one is cut out and then reintroduced.

In these tests, the PCS units must work collaboratively to maintain system stability, adjusting their output in real-time to handle the dynamic load changes. The waveforms for voltage and current at different stations (A, B, and C) reveal the intricacies of parallel operation and the sophisticated control mechanisms necessary to prevent overloads and ensure a steady power supply.

Conclusion

The transition from grid-connected to off-grid modes in Power Conversion Systems represents a critical area of research and development in the energy sector. The ability to switch seamlessly between these states ensures that power delivery remains reliable and efficient, regardless of grid stability. The experiments conducted on PCS functions, including active and passive off-grid switching, synchronization control, harmonic suppression, off-grid black start, and multi-machine parallel operation, demonstrate the advanced capabilities of modern PCS. These systems provide a robust foundation for the resilient and flexible power grids of the future, accommodating the growing integration of renewable energy sources and the evolving demands of energy consumers.