Analyzing Parallel Operation of Multiple Power Conversion Systems in a Microgrid Environment

ntroduction

The rapid evolution of microgrids as localized sources of electricity, particularly in the context of the new energy industry, has necessitated the development of sophisticated Power Conversion Systems (PCS). These systems are fundamental in ensuring the reliability, efficiency, and safety of the electrical supply within microgrid environments. The PCS is responsible for the crucial function of converting electrical energy from one form to another and managing the interchange of power between the microgrid and the main power grid. This article delves into the experimental analysis of PCS functionality, focusing on on-grid and off-grid switching control, synchronization with the grid, and the management of nonlinear loads and harmonics. The parallel operation of multiple PCS units and their collective response to various operational scenarios in a microgrid environment will also be scrutinized.

On-grid and Off-grid Switching Control

The ability to switch between on-grid (grid-connected) and off-grid (isolated) modes is a vital function of Power Conversion Systems. This capability ensures that the microgrid can operate independently when necessary, such as during a main grid failure, and can otherwise remain synchronized with the main grid for normal operations.

Active Off-grid Switching

Active off-grid switching involves a seamless transition from being connected to the main grid to operating independently. This process requires the PCS to quickly recognize a grid failure and switch to off-grid mode without causing significant disruption to the load. Fast and accurate detection of grid anomalies is achieved through a combination of frequency and amplitude detection methods. For a successful transition, the switching time must be minimal to reduce the impact on power quality and to prevent disruptions to the connected loads. The transition process is illustrated in Figure 1, which depicts the waveforms of the phase voltage and current during the active mode switching event.

Passive Off-grid Switching

In contrast to active switching, passive off-grid switching is initiated by the detection of sustained voltage fluctuations at the grid connection point. If the voltage falls or rises beyond a predetermined threshold for several consecutive sampling points, it is an indication of either a disconnection from or a failure of the main grid. The PCS then automatically transitions to off-grid mode and sends a signal to isolate the main grid switch, thereby effectuating passive off-grid operation. Figure 2 presents a waveform diagram that shows the transition from grid-connected to off-grid passive mode.

Synchronous Grid-connected Switching Control

The switch back to grid-connected mode from an off-grid state requires precise synchronization to ensure that the voltage, frequency, and phase of the microgrid align with those of the main grid. This is essential to prevent large current surges that could damage the PCS and other connected equipment. There are two main methods for achieving synchronization:

Passive Synchronization: This method uses protection devices to facilitate the grid connection. The PCS transitions from a voltage/frequency control mode to a constant power control mode during the switch. Phase-locked loop tracking control is employed to align the output voltage of the PCS with the grid voltage before reconnection. Synchronization using a protection device is depicted in Figure 3, which illustrates the off-grid to grid-connected switching waveform.

Automatic Synchronization: This method does not rely on an external synchronization protection device. Instead, the PCS autonomously determines the synchronization point by monitoring the grid side voltage. Upon receiving a synchronization command from the control system, the PCS initiates phase tracking. Once synchronization is achieved, the PCS issues a command to close the grid connection switch, thereby completing the automatic synchronization process. Figure 4 details the automatic synchronization control process.

Off-grid Operation with Nonlinear Loads and Harmonic Mitigation

In off-grid mode, the PCS often has to manage large nonlinear loads, which can lead to significant voltage distortion. The V/f control mode is commonly used when the PCS serves as the primary power source for the microgrid. However, without proper control measures, this can result in distorted output voltages and currents, especially when rectifier-based electrical equipment is used without harmonic control. Figures 4 and 5 compare the output voltage waveforms of a PCS operating with a nonlinear load without and with harmonic suppression methods, respectively.

Off-grid Switching Loads and Black Start Control

The PCS must also handle the dynamic loads that occur when switching inductive elements such as reactors. The waveform of the load during these transitions is shown in Figure 7. Additionally, the PCS plays a critical role during black start conditions, where the microgrid must be restarted without support from the main grid. Figure 10 illustrates the load shedding process necessary for a successful black start, showcasing the waveforms of the DC output voltage and the PCS output voltage.

Multi-machine Parallel Operation

The coordination of multiple PCS units operating in parallel is imperative for the stability and reliability of the microgrid. Figure 11 exhibits the parallel operation of two 50kW PCS units and one 100kW PCS unit under varying load conditions. The figure illustrates how power sharing and voltage stabilization are managed when one PCS is disconnected and then reconnected. Furthermore, Figure 12 demonstrates the stability of the parallel system when resistive and motor loads are introduced, highlighting the minimal impact on current and voltage fluctuation.

Conclusion

The parallel operation of multiple Power Conversion Systems within a microgrid environment presents various challenges that require comprehensive control strategies for both on-grid and off-grid operations. Smooth and efficient transitions between these modes are paramount for maintaining power supply integrity and minimizing disruptions. Effective management of nonlinear loads, harmonic mitigation, and the synchronization of PCS units are crucial tasks that require advanced technologies and control algorithms. The experimental analysis of these systems, as discussed in this article, provides valuable insights into the complexities of microgrid management and the critical role of PCS in the evolving landscape of the new energy industry.