Inadvertent Interchange

Inadvertent interchange is not an instantaneous power deviation (MW) but an energy quantity (MWh) that integrates over time. It represents the net sum of all Area Control Error (ACE) deviations that were not fully corrected by Automatic Generation Control (AGC) within the same clock-hour. A persistent, uncorrected offset of just a few megawatts can accumulate into a significant inadvertent energy balance over a month.

The root causes are operational discrepancies between scheduled and actual flows:

• Frequency Bias Contribution: A balancing authority's generation responds to interconnection frequency deviations, creating unscheduled flows that accumulate as inadvertent interchange.
• Schedule vs. Actual Mismatch: Differences between the confirmed interchange transaction tag values and the real-time telemetered power flow on tie-lines.
• Dynamic Schedule Errors: Latency or scaling errors in the telemetered signal from a dynamically scheduled resource, causing its output to be incorrectly reflected in the ACE equation.
• Ramp Rate Limitations: The physical inability of generators to follow the AGC regulation signal instantaneously, leaving a residual uncorrected ACE.

Inadvertent interchange can be categorized by how it is measured:

• Unilateral Inadvertent: Calculated independently by a single balancing authority by integrating its own ACE over time. This is the primary value tracked for NERC compliance.
• Bilateral Inadvertent: The algebraic difference between the inadvertent interchange calculated by two adjacent balancing authorities for a shared tie-line. Discrepancies often arise from metering errors, time skew in SCADA polling, or different dynamic schedule accounting methods.

NERC Operating Policy 1 requires balancing authorities to manage inadvertent interchange by scheduling a corrective offset in the future. This is known as Inadvertent Interchange Payback. The payback is executed by adding a unilateral, offsetting component to the net interchange schedule, effectively forcing the ACE in the opposite direction to unwind the accumulated energy debt. Payback is typically scheduled during off-peak hours to minimize operational stress.

Inadvertent interchange is meticulously logged in hourly, daily, and monthly accounts. The NERC Inadvertent Interchange Accounting Standard requires balancing authorities to archive this data for auditing. Key accounting practices include:

• Monthly Netting: Accumulated inadvertent for the month is netted to a single value.
• On-Peak/Off-Peak Separation: Inadvertent is often tracked separately for on-peak and off-peak periods to value the energy appropriately for payback.
• Time Error Correction Interaction: Periods of intentional time error correction can create large, artificial inadvertent accumulations that must be isolated in the accounting logs.

Inadvertent interchange is an unintended byproduct of continuous regulation. It is distinct from Emergency Energy, which is a deliberate, operator-initiated transfer of power requested by a balancing authority facing a capacity or energy deficiency. Emergency energy is governed by bilateral contracts and NERC E-Tags, whereas inadvertent interchange is an automatic accounting consequence of interconnected synchronous operation.