Automatic Generation Control (AGC)

AGC functions as a secondary frequency control loop that operates on top of primary governor response. While primary frequency response arrests frequency deviations within seconds through proportional droop control, AGC integrates the residual Area Control Error (ACE) over time to restore frequency to exactly 60 Hz (or 50 Hz) and net interchange to schedule. The controller sends regulation signals to selected generating units every 2-6 seconds, adjusting their base points to drive ACE to zero. This layered architecture ensures both immediate stability and precise steady-state correction.

The ACE is the fundamental input signal to any AGC system, representing the instantaneous generation-load imbalance. Under Tie-Line Bias Control, the standard operating mode in interconnected systems, ACE is calculated as:

• ACE = (P_actual - P_scheduled) - 10B × (F_actual - F_scheduled)

Where B is the Frequency Bias Coefficient in MW/0.1 Hz. This formulation ensures that a balancing authority contributes appropriately to interconnection frequency support. A positive ACE indicates over-generation, requiring a decrease in output, while a negative ACE signals under-generation.

Balancing authorities in North America must comply with mandatory reliability metrics enforced by NERC:

• CPS1: Statistically measures ACE variability against interconnection frequency error over a rolling 12-month period. A score below 100% requires corrective action.
• CPS2: Requires ACE averaged over 10-minute periods to remain within a threshold called L10 for at least 90% of clock-hours each month.
• BAAL: Imposes real-time limits on ACE magnitude to prevent any single balancing authority from excessively contributing to frequency deviation.
• DCS: Mandates recovery of ACE to pre-disturbance values within 15 minutes following a reportable contingency event.

Once the AGC system computes the total required regulation correction, it distributes the signal among participating units using Participation Factors. Each unit's factor determines its proportional share of the regulation burden. The system must respect several constraints:

• Ramp Rate Limiters: Protect thermal equipment by capping the rate of output change in MW/min.
• Deadband: A narrow ACE range where no control pulses are issued, preventing excessive wear from minor fluctuations.
• Economic Dispatch: Periodically re-optimizes unit base points to minimize variable production cost while maintaining regulation capability.

The regulation signal is transmitted via ICCP (IEC 60870-6) to remote generating plants.

Modern AGC systems support Dynamic Scheduling, where a generator's telemetered output is electronically transferred from its physical host balancing authority to a remote balancing authority's ACE equation in real-time. This is implemented using Pseudo-Ties — telemetered readings that the receiving AGC treats as actual tie-line flows. This mechanism enables:

• Virtual power plants aggregating geographically dispersed resources.
• Joint ownership of generating units across balancing authority boundaries.
• Renewable energy credits tracking across market regions.

Pseudo-ties require high-speed, redundant telemetry to maintain control integrity.

AGC systems support multiple control modes depending on system topology:

• Tie-Line Bias: Standard mode for interconnected operation, using both frequency and interchange components in ACE.
• Flat Frequency Control: Used by isolated or islanded systems where ACE is calculated solely from frequency deviation. No tie-line component exists.
• Flat Tie-Line Control: Maintains constant interchange regardless of frequency — generally prohibited in large interconnections as it undermines frequency support.

During a microgrid islanding event, the AGC must seamlessly transition from tie-line bias to flat frequency control to maintain stability within the separated electrical island.