The Intricacies of Off-Grid Switching Loads and Black Start Control

Introduction to PCS Important Function Experiments

Power Conversion Systems (PCS) play a crucial role in the new energy industry, especially in scenarios where the integration of renewable energy sources is becoming more prevalent. They are pivotal in managing the transition between on-grid and off-grid operation, ensuring that energy supply remains constant and reliable, even during grid disturbances. This article will delve into the nuances of on-grid and off-grid switching control, the handling of nonlinear loads, harmonic elimination, and black start control.

On-Grid and Off-Grid Switching Control

Active Off-Grid Switching

Active off-grid switching is a critical feature of modern PCS that provides seamless transition from grid-connected to off-grid modes. In the event of a grid failure, the energy storage system within the PCS must quickly identify the fault and switch to off-grid operation. The switching time is a vital factor, as it should be short enough to minimize any interruption to the power supply and the load within the power system.

To achieve a smooth and impact-free transition, a combination of frequency detection and amplitude detection methods are employed. These methods allow for comprehensive and swift detection of power grid faults. The transition process is often illustrated by waveforms showing the phase voltage and current before and after switching, as depicted in Figure 1.

Passive Off-Grid Switching

Conversely, passive off-grid switching entails a control strategy that allows for a seamless transition without active intervention. The PCS, while in a grid-connected state, constantly monitors the grid connection point voltage (Vm). If the voltage at this point drops or rises beyond a predetermined threshold for N consecutive sampling points, it is inferred that either the main network has disconnected from the microgrid or a failure has occurred. Upon detection, the PCS automatically switches to off-grid control mode and actuates the opening contact to isolate the main network switch, thus achieving passive off-grid switching. The waveform of this process is represented in Figure 2.

Synchronous Grid-Connected Switching Control

Passive Synchronization

Passive synchronization for grid-connected switching employs a protection device to manage the connection. The energy storage converter must transition from off-grid to grid-connected operation, switching from a voltage/frequency control mode to a constant power rate control mode. Prior to reconnection, the converter must synchronize its output voltage with the grid voltage in terms of amplitude, frequency, and phase through a phase-locked loop tracking control. If synchronization is not achieved, a large voltage difference upon reconnection could result in excessive surge currents, endangering the converter's safety. This switching process is showcased in Figure 3.

Automatic Synchronization

Automatic synchronization grid-connected control eliminates the need for a separate synchronization protection device. Instead, the PCS autonomously determines the synchronization point. When a synchronization command is received from the monitoring system, the PCS initiates grid phase tracking. Once phase alignment is achieved, the grid-connected closing command is issued, and the corresponding circuit switch is closed, thus completing automatic synchronization with the grid, as illustrated in Figure 4.

Off-Grid with Nonlinear Load and Harmonic Elimination Processing

When the PCS operates off-grid with a substantial nonlinear load and serves as the main power source for a microgrid, a simple V/f control can result in significant distortion of the output voltage, as shown in Figure 5. Without controlling rectifier electrical equipment, the output voltage and current waveform from the PCS can become heavily distorted, necessitating the use of harmonic suppression methods to maintain power quality, demonstrated in Figure 6.

Off-Grid Switching Load

Switching loads, such as reactors, can present challenges when operating off-grid. Figure 7 shows the load waveform of a switching reactor under off-grid conditions, highlighting the need for careful management of such loads to ensure stable operation.

Off-Grid Black Start Control

Black start control is a process that allows a power system to recover from a total or partial shutdown without relying on the external electric power transmission network. In off-grid scenarios, PCS plays a critical role in black start operations. Figure 10 illustrates the load shedding process, with a 7.5kW motor and a 20kW resistor being disconnected to facilitate the recovery of the power system.

Multi-Machine Parallel Test

Testing the parallel operation of multiple PCS units is essential to assess their collective response to load changes and their ability to share loads efficiently. Figure 11 demonstrates a scenario where two 50kWPCS units and one 100kWPCS unit operate in parallel with a 36kW adjustable RLC load. The power distribution and the operation conditions are monitored to ensure stable performance when units are disconnected and reconnected.

Figure 11 shows the current and voltage waveforms for station A, B, and C as one PCS is cut out and then reintegrated into the parallel operation, highlighting the adaptive load balancing capabilities of the system.

Figure 12 depicts a situation where three PCS units are running in parallel with a resistive load, and a motor load is introduced. The impact current is minimal, and voltage fluctuation is low, demonstrating that the PCS units can achieve power equilibrium and smooth operation when faced with impact loads.

Conclusion

The ability to switch seamlessly between on-grid and off-grid operation is fundamental to the reliability and efficiency of modern power systems, particularly those incorporating renewable energy sources. PCS technology has evolved to handle these transitions with minimal impact on the power supply and load. Black start capabilities and the management of nonlinear loads are also integral to the stability of off-grid operations. Through careful design and testing, such as multi-machine parallel tests, PCS can provide robust solutions for the new energy industry, ensuring that power remains stable and reliable, regardless of the challenges presented by the ever-evolving energy landscape.