Response of Battery Energy Storage Systems (BESS) in Terms of Synthetic Inertia and Comparison with Conventional Resources

Synthetic inertia refers to the ability of certain power system assets, particularly inverter-based resources like Battery Energy Storage Systems (BESS), to mimic the inertial response typically provided by conventional rotating machines (e.g., synchronous generators). Synthetic inertia helps stabilize grid frequency following disturbances, such as the loss of generation or a sudden increase in demand.

In this context, let’s explore the response of BESS in terms of synthetic inertia and compare it with conventional resources like synchronous generators, which naturally provide inertia through their rotating masses.

1. Synthetic Inertia: Overview and Role in Power Systems

Inertia in Conventional Systems:

In traditional power systems, inertia is provided by the rotating mass of large synchronous generators (such as steam, gas, or hydro turbines). When there’s a sudden disturbance (like the loss of a generator), the kinetic energy stored in the rotating mass is released, slowing down the rate of frequency change and giving time for other resources to respond. This inherent inertia helps resist rapid frequency deviations, a key aspect of frequency stability.

Synthetic Inertia:

Inverter-based resources (like BESS, wind turbines, and solar PV) don’t inherently possess rotating masses and, therefore, don’t provide natural inertia. However, synthetic inertia can be emulated using advanced control algorithms in inverters. These control strategies allow inverter-based systems to respond to frequency changes by injecting or absorbing active power in response to rapid frequency changes, mimicking the effect of inertia.

2. BESS Response in Terms of Synthetic Inertia

Battery Energy Storage Systems (BESS) can provide synthetic inertia through fast-response inverter technology. Here’s how it works:

How BESS Provides Synthetic Inertia:

- Power Injection in Response to Frequency Deviations: BESS inverters are programmed to detect changes in system frequency. If the frequency drops suddenly (indicating a loss of generation), the BESS can rapidly inject active power into the grid, slowing down the rate of frequency decline. Conversely, if the frequency rises, BESS can absorb power, helping to stabilize the grid.

- Fast Response Time: Unlike conventional generators, which have mechanical limitations, BESS can respond almost instantaneously (within milliseconds) to frequency changes. This makes BESS a valuable asset in providing fast frequency response (FFR) and synthetic inertia.

- Control Strategy for Synthetic Inertia: The control algorithms in BESS can be designed to emulate virtual inertia by using the rate of change of frequency (RoCoF) as a trigger for power injection or absorption. The control system adjusts the BESS output based on the measured RoCoF to provide an effect similar to the inertia of rotating machines.

- Energy Availability: The ability of a BESS to provide synthetic inertia is dependent on its state of charge (SoC). A BESS needs to have sufficient energy stored to inject power when the frequency drops. Similarly, it must have available capacity to absorb power when the frequency rises.

Key Characteristics of BESS Response:

- Instantaneous Response: BESS systems can respond to frequency deviations within milliseconds, much faster than conventional synchronous generators.

- Power Flexibility: BESS can inject or absorb power based on grid needs, making it more versatile compared to conventional resources that only release kinetic energy when frequency drops.

- No Physical Inertia: While BESS provides synthetic inertia, it doesn’t have a rotating mass. The inertia is emulated through control algorithms and fast inverter dynamics.

3. Comparison with Conventional Resources of Synthetic Inertia

Here’s a comparison between BESS and conventional resources (such as synchronous generators and wind turbines) that provide inertia or synthetic inertia:

Key Comparisons:

1. Response Time:

- BESS responds much faster than synchronous generators (in milliseconds), providing rapid support to grid frequency. In comparison, synchronous generators have slower response times due to the mechanical nature of their inertial response (typically within a few seconds).

2. Power Injection and Flexibility:

- BESS can provide both power injection and absorption based on grid needs, offering greater flexibility. Synchronous generators can only release kinetic energy when frequency drops, and they cannot absorb power to counteract high frequencies.

3. Duration of Response:

- The duration of the response for a BESS is limited by its state of charge (SoC) and battery capacity. Synchronous generators can sustain their inertial response as long as the mechanical system continues rotating.

4. RoCoF Mitigation:

- BESS can more effectively mitigate high rates of change of frequency (RoCoF) due to its instantaneous response. Synchronous generators naturally resist RoCoF through their physical inertia but cannot react as quickly to abrupt changes.

5. Cost and Infrastructure:

- The cost of implementing synthetic inertia through BESS is typically higher due to the cost of storage systems and inverters. Conventional synchronous generators provide inertia naturally as part of their operation, so no additional cost is incurred.

4. Advantages and Limitations of BESS for Synthetic Inertia

Advantages:

- Fast Response: BESS can respond to grid events much faster than any conventional resource.

- Bidirectional Power Support: BESS can inject and absorb power, offering more dynamic control than rotating machines.

- Grid Stability: By providing fast synthetic inertia, BESS improves grid stability, especially in systems with high renewable penetration.

- Scalability: BESS systems are modular and can be scaled to provide synthetic inertia at various levels.

Limitations:

- State of Charge (SoC): The availability of synthetic inertia from BESS is limited by the battery’s state of charge. If the battery is depleted, it cannot provide inertia.

- Finite Energy: Unlike rotating mass in synchronous generators, BESS has finite energy and may only provide synthetic inertia for a limited period.

- Cost: BESS installation and maintenance costs are higher compared to conventional inertia resources.

Conclusion

BESS offers a highly responsive and flexible source of synthetic inertia, which is becoming increasingly important in grids with high renewable energy penetration. It provides several advantages over conventional synchronous generators, including faster response times and the ability to both inject and absorb power. However, BESS has limitations in terms of its energy availability (state of charge) and higher cost.

As grid systems transition to more inverter-based and renewable generation, the use of BESS for providing synthetic inertia will play a critical role in maintaining frequency stability and ensuring reliable grid operations.