BESS Ancillary Services and Grid Control Functions — Beyond Charge and Discharge
The BESS (Battery Energy Storage System) refers to the battery-based energy storage system used to store and deliver electrical energy on demand, contributing to grid stability and flexibility. It is characterized by its capacity to provide operational flexibility and, at a more sophisticated level, ancillary services to the grid. This is what we will address here.
Overview of BESS Grid Support Capabilities
Through rapid control of active power (P) and reactive power (Q), supporting both voltage and frequency, the BESS helps damp oscillations and reduce ramp variations associated with renewable generation. It likewise improves power quality by mitigating voltage sags and fluctuations, enables peak demand reduction (peak shaving), and can provide contingency support — enhancing system reliability and resilience, including enabling islanded operation and black start capability when implemented with Grid-Forming/VSG strategies.
Why Rapid P and Q Control Matters
“Rapid control” of active power (P) and reactive power (Q) is critically relevant because frequency (P–f) and voltage (Q–V) are system variables governed by physical equilibria that can deteriorate within just a few cycles when a disturbance occurs. Blackouts typically result from imbalances in these two quantities, caused by instantaneous P/Q mismatches in the network.
A P deficit (load > generation) is normally supplied by the kinetic energy of synchronous machines (or equivalent sources), resulting in frequency decay and an elevated ROCOF (Rate of Change of Frequency, typically expressed in Hz/s).
In this same scenario, a BESS connected to the grid — by injecting or absorbing active power within tens of milliseconds — reduces this imbalance, lowers the ROCOF, and arrests the frequency deviation, buying time for the slower mechanical response of synchronous machines. This is a typical example of an ancillary service, which should form part of the new BESS regulatory framework in Brazil — already established and remunerated in other countries, including Latin American markets such as Chile and Argentina.
In cases of Q deficit, also common in power systems, an installed BESS modifies reactive power flow and the local voltage profile, mitigating voltage sags/swells and improving voltage stability margins.
P and Q Competition — Priority Logic Under Contingency
Since the VSC (Voltage Source Converter) has a defined current limit, P and Q demands can occasionally compete with one another. During contingencies and undervoltage events, it is common practice to prioritize Q (voltage support) and temporarily curtail P to remain within equipment limits and comply with ride-through requirements — i.e., dynamic stability.
Power Electronics: The VSC as the Core Interface
From an engineering standpoint, these functions are enabled by power electronic converters — bidirectional VSCs (Voltage Source Converters) — with an AC interface and a DC bus, capable of operating as both an inverter and a rectifier, absorbing and injecting power in both directions.
In a BESS, the VSC interfaces the battery with the grid (via DC/DC converter and DC bus, followed by DC/AC converter and AC bus), enabling bidirectional power flow: discharge (DC→AC, injecting active power) and charge (AC→DC, absorbing active power).
Output Filters: L and LCL
Between the VSC and the grid, L filters (a series inductor at the converter output) and LCL filters (inductor–capacitor–inductor arrangement) are used to attenuate switching harmonics (ripple) generated by PWM modulation, reduce current distortion (THD), and limit the rate of current rise (di/dt).
The L filter is simpler and more robust but requires larger inductances to achieve the same attenuation at high frequencies. The LCL filter offers greater harmonic attenuation with smaller components, but introduces a resonant frequency that must be actively damped to prevent current amplification and converter–grid interaction instabilities — a phenomenon conceptually analogous, at the extreme, to the noise-cancelling principle in modern high-end headphones.
Four-Quadrant P–Q Operation
In addition to commanding active power, the VSC controls reactive power for voltage support, capable of operating across all four quadrants of the P–Q plane within the converter’s current and voltage limits.
At the innermost control layer, the converter regulates currents (and, where applicable, voltages) in the P/Q axes, respecting semiconductor current and voltage limits. At higher control layers, the controller generates active and reactive power references to fulfill system objectives through:
Frequency regulation via active power (P) control, with responses in tens of milliseconds (FFR) and/or participation in primary control via P–f droop characteristic;
Voltage regulation via reactive power (Q) control and/or bus voltage control with Q–V droop and impedance voltage drop compensation;
Electromechanical oscillation damping of low-frequency modes via supplementary controllers that modulate P and/or Q; and
Contingency and power quality event support via ride-through strategies (LVRT/HVRT), in which the BESS injects/absorbs reactive current and limits active current during voltage sags/swells in accordance with grid code requirements.
Ancillary Services in Detail
Frequency Support (P–f Control)
The controller measures frequency (or ROCOF — Rate of Change of Frequency, in Hz/s) at the bus and adjusts BESS active power via a droop law (primary control): when frequency falls below nominal, the BESS increases P (discharge); when it rises, P is reduced or the BESS absorbs P (charge).
In modern implementations, a Fast Frequency Response (FFR) mode may apply a rapid power impulse within tens of milliseconds to arrest the ROCOF and improve the frequency nadir, ahead of the activation of slower responses.
From the converter engineering perspective, this P modulation is translated into an active current reference (Id*), bounded by Imax, DC bus voltage limits, and State of Charge (SOC). Furthermore, in weak grids, frequency estimation may be obtained via PLL (grid-following) or via internal dynamics (grid-forming/VSG), which significantly affects both performance and stability.
Voltage Support (Q–V Control)
The Q–V function (reactive power versus voltage control) aims to maintain the voltage profile and reduce voltage variations at buses of interest. The controller measures the RMS voltage (or the positive-sequence component) and adjusts reactive power injection/absorption via a Q–V droop characteristic: when voltage drops, Q is injected (capacitive reactive current); when voltage rises, Q is absorbed (inductive reactive current).
Power Oscillation Damping (POD)
At a finer level of tuning, POD (Power Oscillation Damping) is a supplementary control function designed to reduce low-frequency electromechanical oscillations (typically in the range of ~0.1 to 2 Hz) that arise from interactions between system areas, large generation blocks, and transmission lines.
Conclusion
The BESS goes far beyond a capacity charge-and-discharge system. It is a robust and sophisticated platform that enhances the natural dynamics of the grid and its contingency responses, complementing the adjustments provided by synchronous machines and stabilizing, to the greatest extent possible, the natural disturbances caused by load variations and intermittent power sources such as solar PV and wind generation.
Flavio Marqueti — Engineer and Executive Director, EVENERGY