Addressing Nonlinear Loads and Harmonic Mitigation in Off-Grid Power Conversion Systems

Introduction

The ever-evolving energy industry is increasingly adopting off-grid power conversion systems (PCS) to ensure reliable and sustainable energy supply, especially in remote areas or where grid stability is a concern. Power conversion systems play a crucial role in managing the transition between on-grid and off-grid states, maintaining power quality, and ensuring the seamless operation of connected loads. In this article, we delve into the critical functions of PCS, the intricacies of switching control between on-grid and off-grid modes, and the strategies employed to handle nonlinear loads and harmonic mitigation.

On-grid and Off-grid Switching Control

Switching control is at the heart of PCS functionality, and it can be categorized into two primary modes: active and passive.

Active Off-Grid Switching

Active off-grid switching is a critical feature that allows seamless transition from grid-connected to off-grid operation. In the event of a grid failure, the energy storage system within the PCS is designed to quickly detect and switch to off-grid mode. The transition time must be minimized to reduce the interruption to the power supply and the impact on connected loads. Active off-grid switching employs a combination of frequency and amplitude detection methods to judge and detect grid faults promptly. This ensures a smooth and impact-free switching process, as depicted in Figure 1, which illustrates the waveform of active mode switching.

Passive Off-grid Switching

In contrast, passive off-grid switching involves a control strategy that waits for a predefined condition to trigger the transition. The PCS monitors the voltage at the grid connection point (Vm), and if the voltage at N consecutive sampling points falls or rises beyond a certain threshold, it is considered indicative of a grid disconnection or failure. The PCS then automatically transitions to off-grid control mode and sends a signal to disconnect the main grid switch, completing the passive off-grid process. Figure 2 displays the waveform diagram of this passive switching mode.

Synchronous Grid-Connected Switching Control

When reconnecting to the utility grid from an off-grid state, synchronization is paramount to avoid damaging surges and ensure safety. Two forms of grid-connected switching control are employed: passive and automatic synchronization.

Passive Synchronization

Passive synchronization involves the use of a protection device that enables grid connection. The energy storage converter transitions from a voltage/frequency (V/f) control mode to a constant power control mode. Prior to grid connection, phase-locked loop tracking control is used to align the converter's output voltage with the grid voltage in amplitude, frequency, and phase. A synchronization protection device assists with grid connection by providing the PCS with the necessary voltage and frequency data. Upon meeting the closing conditions, the PCS enters a standby state after completing the synchronization, as shown in Figure 3.

Automatic Synchronization

Automatic synchronization does not rely on a separate synchronization protection device. Instead, the PCS autonomously determines the synchronization point. Upon receiving a synchronization command, the PCS begins phase tracking on the grid side and issues a grid-connected closing command to complete automatic synchronization, as outlined in the control process in Figure 4.

Off-grid Nonlinear Load and Harmonic Elimination

When PCS systems encounter nonlinear loads, which are common in microgrids, significant challenges arise. Nonlinear loads can lead to serious distortions in voltage waveform, as demonstrated in Figure 5. Harmonic suppression methods are then employed to mitigate these distortions and maintain power quality. Figure 5 compares the output voltage waveform of an off-grid nonlinear load PCS with and without harmonic suppression.

Off-grid Switching Load

The performance of PCS under varying loads is critical. Figure 7 illustrates the load waveform of a switching reactor when the system is off-grid and under load, highlighting the importance of managing such load transitions effectively.

Off-grid Black Start Control

The ability to perform a black start—restarting the PCS after a total power outage—is vital for maintaining resilience. Figure 10 showcases the load shedding strategy necessary for a successful black start, presenting the DC output voltage waveform alongside the PCS output voltage for a combination motor and resistor load.

Multi-machine Parallel Operation

The capability to run multiple PCS units in parallel is essential for scalability and redundancy. Figure 11 details the waveform of three PCS units (two 50kW and one 100kW) operating in parallel with a 36kW adjustable RLC load. The power distribution across the units is shown, and the figure demonstrates how the PCSs can adjust power sharing when one unit is disconnected and then reconnected. This ensures stable operation across the microgrid.

Figure 12 further illustrates the robustness of parallel PCS operation, this time with a resistive load and the start-up of a motor load. The minimal impact current and low voltage fluctuation displayed in the figure underscore the PCS's ability to handle dynamic loads smoothly.

Conclusion

Off-grid power conversion systems are the linchpin of modern microgrids and remote energy solutions. The seamless switching between on-grid and off-grid modes, the effective handling of nonlinear loads, and the mitigation of harmonics are all critical for maintaining power quality and ensuring reliability. Through innovative control strategies and advanced synchronization techniques, PCS technology continues to evolve, providing robust solutions for the complex energy demands of today's world. The experiments and waveform analyses presented here highlight the sophistication and capability of current PCS technology to meet these challenges head-on, paving the way for a more resilient and sustainable energy future.