Flat Tie-Line Control

The standard and mandated AGC operating mode within large interconnections, directly contrasting with Flat Tie-Line Control. In this mode, the Area Control Error (ACE) is calculated using both the deviation in net interchange and a frequency deviation multiplied by a Frequency Bias Coefficient. This ensures that a balancing authority automatically assists the interconnection during frequency disturbances by temporarily altering its net schedule. Ignoring the frequency component, as Flat Tie-Line Control does, is considered a violation of good utility practice because it forces other areas to compensate for the imbalance.

The instantaneous algebraic difference between a balancing authority's net actual and scheduled interchange, combined with a frequency bias obligation. The equation is: ACE = (NIa - NIs) - 10B (Fa - Fs). In Flat Tie-Line Control, the frequency bias term 10B(Fa - Fs) is effectively set to zero, meaning the control system only strives to make NIa = NIs. This mathematically represents the abandonment of interconnection frequency support responsibility.

The operational inverse of Flat Tie-Line Control. In Flat Frequency Control, used by electrically isolated or islanded grids, the ACE equation ignores the tie-line component entirely and responds solely to frequency deviation: ACE = -10B (Fa - Fs). This is appropriate for a single balancing authority with no neighbors. Flat Tie-Line Control represents the opposite failure mode—ignoring frequency while rigidly adhering to a schedule in a synchronous interconnection.

A critical AGC setting, expressed in MW/0.1 Hz, that quantifies a balancing authority's expected governor response to frequency changes. It is the multiplier in the ACE equation that Flat Tie-Line Control effectively zeroes out. Setting this coefficient correctly ensures that a control area does not fight the natural Primary Frequency Response of its own generators. A Flat Tie-Line mode is functionally equivalent to setting the frequency bias to zero, forcing the area's governors to be overridden by AGC pulses attempting to maintain a constant net interchange.

A NERC reliability metric that a balancing authority operating in Flat Tie-Line Control would almost certainly violate. CPS1 statistically measures the relationship between a balancing authority's ACE and the interconnection's frequency error over a rolling 12-month period. Because Flat Tie-Line Control ignores frequency, its ACE will frequently be in the opposite direction of the interconnection's frequency error (e.g., exporting power when frequency is low), resulting in a non-compliant CPS1 score and potential financial penalties.

The accumulated, unintended difference between a balancing authority's actual net interchange energy and its scheduled net interchange energy over time. While Flat Tie-Line Control rigidly maintains the net schedule instantaneously, it paradoxically can lead to massive inadvertent interchange accumulation for the interconnection as a whole. By refusing to support frequency, the area forces surrounding balancing authorities to absorb the energy imbalance, creating a systemic accounting problem that must be corrected through future Time Error Correction procedures.