Control Performance Standard 2 (CPS2)

The L10 is a bounding threshold calculated for each balancing authority based on its historical Frequency Bias Coefficient and the interconnection's overall frequency response characteristic. It defines a permissible ACE envelope. The standard requires that the average ACE for each 10-minute period must not exceed this L10 limit for more than 10% of the periods in a calendar month. This ensures that sustained imbalances are bounded and do not accumulate over time.

CPS2 mandates a strict 90% minimum compliance ratio. For every clock-hour, the balancing authority's ACE is averaged over six consecutive 10-minute periods. Each period is a pass/fail test against the L10 threshold. At the end of the month, the ratio of compliant periods to total periods must be at least 0.90. Falling below this threshold constitutes a violation and requires a corrective action plan to be filed with the regional reliability coordinator.

While CPS1 measures the statistical correlation between a balancing authority's ACE and the interconnection's frequency error over a rolling 12-month average, CPS2 acts as a hard boundary on sustained ACE magnitude. CPS1 can be satisfied even with large, short-lived ACE swings if they help frequency. CPS2 prevents these swings from persisting. Together, they ensure that ACE is both frequency-supportive (CPS1) and bounded in duration (CPS2).

The fundamental measurement unit for CPS2 is the 10-minute clock-hour period. The balancing authority's ACE is sampled every few seconds by the Automatic Generation Control (AGC) system, and a simple arithmetic average is calculated for each non-overlapping 10-minute block. This averaging window is designed to filter out the high-frequency noise of instantaneous ACE fluctuations and focus on the underlying, sustained imbalance trend that poses a greater risk to interconnection reliability.

A CPS2 violation occurs when the monthly compliance ratio drops below 90%. Upon violation, the balancing authority must perform a root cause analysis and submit a mitigation plan to its Regional Reliability Coordinator. Common causes include:

• Inadequate Regulation Reserve procurement
• Poor Renewable Generation Forecasting leading to large net load ramps
• Failure of a Dynamic Schedule or pseudo-tie resource Persistent violations can lead to financial penalties and increased regulatory oversight.

The Balancing Authority ACE Limit (BAAL) is a real-time operational overlay that works in conjunction with CPS2. While CPS2 is a retrospective monthly metric, BAAL provides a real-time, dynamic limit that, if exceeded, requires the operator to take immediate corrective action. BAAL is designed to prevent a balancing authority's ACE from contributing to a frequency excursion beyond a predefined limit, acting as a safety net between the long-term statistical measure of CPS1 and the bounding constraint of CPS2.

The raw input signal for CPS2 evaluation. ACE is the instantaneous algebraic sum of:

• Net Interchange Deviation: Actual tie-line flow minus scheduled flow.
• Frequency Bias Obligation: 10 × B × (Actual Frequency - Scheduled Frequency), where B is the Frequency Bias Coefficient in MW/0.1 Hz.

CPS2 requires that the 10-minute rolling average of ACE remains within the L10 envelope for at least 90% of clock-hour periods.

The specific MW boundary at the heart of CPS2 compliance. L10 is calculated as:

L10 = 1.65 × ε10 × √(10 × B × Bnet)

• ε10: A constant derived from the interconnection's historical frequency error.
• B: The balancing authority's frequency bias coefficient.
• Bnet: The interconnection's total frequency bias.

This statistical envelope ensures that ACE excursions are bounded to a level that limits unscheduled power flows.

The closed-loop control system that executes the corrective actions needed to meet CPS2. AGC processes the ACE signal every 2-6 seconds and dispatches regulation commands to generating units.

• Filtering: AGC often applies deadbands to avoid chasing noise.
• Allocation: Participation factors distribute the regulation requirement among units.
• Constraint Enforcement: Ramp rate limiters protect equipment from thermal stress.

CPS2 compliance is unattainable without a properly tuned AGC system.