Enhancing Frequency Stability in Modern Distributed Power Systems

Power electronics (PE) play a crucial role in modern distribution systems by enabling the integration of distributed energy resources (DERs) and improving power quality and reliability with enhanced control and flexibility. Power electronics-interfaced distributed plants are replacing massive synchronous generators as a way to make electricity as more renewable energy-based plants.

Because of this, the system's equivalent inertia and regulating power, usually provided by these synchronous generators, are likely to go down, affecting the frequency stability of the system. The unpredictability and volatility of these DERs greatly impact the frequency stability of the power grid's operation. Both primary frequency regulation and an active inertia response are being affected.

New concepts are coming on board to help balance the grid. These include wind turbines, solar generators, and energy storage systems.

Additionally, various control methods, including virtual inertia control, overspeed, and variable pitch control, are proposed to enhance the frequency stability of the system.

What is the importance of primary frequency regulation?

Primary frequency regulation (PFR) is a power system's first and fastest automatic response to frequency deviations. Its purpose is to quickly arrest and stabilize frequency changes caused by imbalances between generation and load. PFR involves adjusting the generator's power output following the fundamental frequency variations from the given value.

It is another function provided by generators connected to transmission networks. PFR responds to frequency fluctuations to keep the power system stable. The PFR modifies the active power output of generators and other controllable units in the system, including battery energy storage systems (BESSs), when frequency follows a fluctuation in loading or generation.

Various methods for primary frequency regulation are available, among which battery energy storage systems, virtual synchronized generators, droop control, and hierarchical control strategies are studied in detail in this article.

Battery Energy Storage Systems

Battery energy storage systems are a type of technology that is very efficient and responds quickly. Energy storage technology's rapid development and advantages support the frequency control of the power system. They have faster response, high accuracy, and flexibility for frequency regulation. They work well for balancing services like grid inertial response, frequency nadir, rate of change of frequency reduction, and PFR.

Key Components of BESS

A two-level voltage source converter (VSC), built with three half-bridge legs to interface the battery bank with a low-voltage AC grid, as shown in Fig. 1, is the foundation of the majority of high-power commercial BESSs, such as those ranging in capacity from 100 kW to 5 MW.

In addition, an auxiliary transformer is utilized to step up the voltage for the medium voltage grid when necessary, and an LCL filter is employed to contain the current harmonic injection within limits regulated by standards. Other protective components include electromagnetic compatibility filters and AC and DC contractors.

Fig. 1: Illustration of key components of BESS Source: IEEE Open Journal of the Industrial Electronics Society

Challenges

Lithium-ion BESSs perform effectively for PFR, but the significant initial investment needed to set them up is a problem for system managers and private investors.

The increasing penetration of renewable energy sources connected through power electronic converters is leading to reduced system inertia. Power electronic converter-interfaced sources have faster dynamics compared to traditional synchronous generators, requiring new control methods. Achieving fast response times can be challenging for some generating units.

The parts of a BESS that are most vulnerable to wear include the electrolytic capacitors, IGBT modules, and the electrochemical storage. IGBT modules and the DC-link electrolytic capacitors are the primary parts of the voltage source converter, which is prone to wear-related failure. The degradation of IGBTs and electrolytic capacitors is mainly caused by thermal stress.

Furthermore, the lifespan of lithium-ion batteries is restricted and is largely impacted by cycling. The degradation of electrochemical storage is mainly driven by its cycling pattern. It typically ranges from 3000 to 10000 cycles, depending upon the type of lithium and the cycling environment.

Virtual Synchronous Generator

To enhance the efficiency of renewable energy production systems and accomplish acceptable grid integration, the virtual synchronous generator (VSG) has been the subject of extensive research.

VSG control technology can employ the electromechanical equation of the synchronous generator in the power electronic converter's control loop to incorporate the inertia, damping, primary frequency, and voltage regulation characteristics of the synchronous generator into the converter control. The converter-interfaced renewable power generation system can imitate the conventional synchronous generator and show advantageous grid support.

Droop Control

Distributed generation units are commonly integrated through the use of power inverters as the interface. They frequently run in parallel and have high power, low cost, improved system stability, and dependability. The control design of parallel-operated inverters has become a challenge in these applications, as it aims to provide precise load sharing among many sources.

A common technique for operating inverters in parallel is droop control. Droop control systems are commonly employed for precise load sharing without communication. During the load transients, the output impedance and frequency might be adjusted to achieve accurate equal power sharing without any variations in the output voltage's amplitude or frequency.

Hierarchical Control Strategies

Hierarchical control that guarantees reliable, effective control for DERs. Depending on functionality, three levels—primary, secondary, and tertiary—are assigned to the hierarchical-based control structure. Coordinating multiple DERs through hierarchical control can optimize frequency regulation efforts.

Primary Control

Responsible for fast, local response to frequency deviations. It includes droop control and virtual inertia mechanisms.

Secondary Control

Restores frequency and voltage to nominal values after primary control action. It operates on a slower timescale.

Tertiary Control

Manages power flow between the microgrid and the main grid, optimizing economic operation.

To conclude, the future plans include investigating machine learning and artificial intelligence (AI) algorithms for detecting and mitigating frequency stability difficulties and developing advanced control systems that can handle the fluctuation and uncertainty of renewable sources.

Summarizing the Key Points

● Power electronics are essential for integrating distributed energy resources into modern power systems, enhancing reliability and power quality while addressing frequency stability challenges.

● Primary frequency regulation is crucial for quickly stabilizing frequency deviations caused by generation-load imbalances, ensuring the overall stability of the power grid.

● Battery energy storage systems provide efficient grid support, but their high initial costs and component wear, particularly in lithium-ion batteries, pose significant challenges.

● Virtual synchronous generators can mimic traditional synchronous generator characteristics, improving grid support and enhancing the efficiency of renewable energy production systems.

● Advanced control strategies, including droop control and hierarchical control, are necessary for effective load sharing and coordination among multiple distributed energy resources in the grid.

Reference

Stecca, Marco, Thiago Batista Soeiro, Laura Ramirez Elizondo, Pavol Bauer, and Peter Palensky. “Lifetime Estimation of Grid-Connected Battery Storage and Power Electronics Inverter Providing Primary Frequency Regulation.” IEEE Open Journal of the Industrial Electronics Society 2 (January 1, 2021): 240–51. https://doi.org/10.1109/ojies.2021.3064635.

Zhang, Jianwei, Huidong Wang, Guangchen Liu, Guizhen Tian, Marco Rivera, and Patrick Wheeler. “Virtual Synchronous Generator Control Strategy of Grid-Forming Matrix Converter for Renewable Power Generation.” IEEE Access 12 (January 1, 2024): 106672–84. https://doi.org/10.1109/access.2024.3436552.

Zhong, Qing-Chang, and Yu Zeng. “Universal Droop Control of Inverters With Different Types of Output Impedance.” IEEE Access 4 (January 1, 2016): 702–12. https://doi.org/10.1109/access.2016.2526616.