Q4Restructured Power System
Question
Q.4. Discuss the ATC, TTC, TRM, CBM.
Answer
ATC (Available Transfer Capability) is the remaining transmission capacity available for additional commercial transactions beyond already-committed uses; TTC (Total Transfer Capability) is the maximum power that can be transferred reliably; TRM (Transmission Reliability Margin) reserves capacity for uncertainty; and CBM (Capacity Benefit Margin) reserves capacity for generation reliability sharing between areas.
These four related quantities together define how much additional transmission capacity is available for new commercial power transactions across a transmission interface, in a restructured, open-access transmission system.
Total Transfer Capability (TTC)
TTC is the maximum amount of electric power that can be transferred reliably across a specific transmission interface (interconnection between two areas/zones) under specified system conditions, while meeting all applicable thermal, voltage stability, and transient stability reliability criteria without violating any operating limits.
Transmission Reliability Margin (TRM)
TRM is a reserve amount of transmission capability deliberately set aside from TTC to account for the inherent uncertainties in system conditions — such as forecasting errors in load, unscheduled (loop) power flows, and variations in generation dispatch — ensuring the transmission system can be operated reliably even when actual conditions deviate somewhat from the forecasted/planned conditions used to calculate TTC.
Capacity Benefit Margin (CBM)
CBM is a further reserve amount of transmission capability set aside to enable load-serving entities in one area to access generation capacity reserves located in another (interconnected) area during emergency conditions, supporting the generation reliability/capacity-sharing benefits that interconnection between areas is intended to provide, ensuring this emergency-access transmission capacity remains available even during normal, non-emergency commercial transaction scheduling.
Available Transfer Capability (ATC)
ATC is the actual transmission capability remaining available for additional new commercial transactions, calculated as:
ATC is the key quantity published by transmission system operators (typically on an hourly or daily basis, for various future time horizons) in a restructured open-access transmission regime, since it directly determines how much additional new transmission service can be granted to market participants wishing to schedule new power transactions across a given interface, without compromising system reliability margins already reserved via TRM and CBM or violating the already-committed capacity represented by existing transactions. Together, these four quantities provide the essential quantitative framework that transmission system operators use to fairly, transparently and reliably manage third-party open access to the shared transmission network in a restructured electricity market environment.