In the context of electrical protective relays, specifically overcurrent relays, TCC stands for Time-Current Characteristics.
Understanding TCC in Relays
TCC, or Time-Current Characteristics, is a fundamental concept that describes how a protective relay responds to different levels of fault current over time. It dictates the relationship between the magnitude of the current flowing through the relay and the time it takes for the relay to initiate an action, typically tripping a circuit breaker.
As stated in the provided reference, "Overcurrent relays are sensitive to the measured current from current transformers in a way that if the current is higher than preset threshold value, then the relay will operate either instantaneously or with a delayed time based on time-current characteristics (TCC) curve as depicted in Fig."
This means:
- Current Sensitivity: Relays monitor the current level.
- Threshold: A preset current value must be exceeded for the relay to consider operating.
- Operation Time: If the current exceeds the threshold, the time it takes for the relay to operate depends on its Time-Current Characteristics.
How TCC Influences Relay Operation
The TCC curve is essentially a graphical representation of this relationship. Plotting current (usually on the x-axis) against time (on the y-axis), the curve shows how quickly the relay will operate for any given fault current magnitude.
- Inverse Relationship: For most overcurrent relays, there's an inverse relationship – higher currents cause faster operation times, and lower currents cause slower operation times. This is known as inverse time or definite minimum time characteristics.
- Instantaneous Operation: For very high fault currents, the TCC might show the relay operating nearly instantaneously.
- Delayed Operation: For lower fault currents (above the threshold), the relay operation is intentionally delayed.
Importance of TCC
Understanding and setting the correct TCC for relays is crucial for:
- Selective Coordination: Ensuring that only the protective device nearest to the fault trips, isolating the problem area while leaving the rest of the system energized.
- Equipment Protection: Allowing equipment to withstand temporary overloads while tripping quickly during dangerous short circuits.
In summary, TCC is the core principle defining how an overcurrent relay responds to varying fault current magnitudes over time, enabling effective and coordinated protection of electrical systems.
