Time-Current Characteristics, also known as TCC curves or time-current curves, play a significant role in relay protection coordination within electrical power networks. These curves illustrate the response time of a specific protective device, such as a circuit breaker or a protective relay, to different levels of faults or abnormal conditions within the power system.
The primary purpose of TCC curves is to ensure that the protective devices within a power system selectively coordinate with each other, so that only the device closest to the fault opens or operates in the event of an abnormal condition. This coordination is vital to avoid unnecessary tripping of healthy sections of the network, reducing the impact on power supply reliability.
TCC curves typically consist of a horizontal time axis and a vertical current axis. The time axis represents the time it takes for a protective device to operate, while the current axis represents the magnitude of the current flowing through the device. The curves plot this relationship for different levels of fault currents.
The slope of the TCC curve indicates the time required for the protective device to operate as the current increases. A steeper slope represents a faster operating time. The curve’s shape depends on the characteristics and settings of the protective device, including the type of protection, the trip unit, and the relay settings. By carefully selecting and coordinating the TCC curves of protective devices, engineers can ensure that protection operates in a sequential and coordinated manner during fault conditions.
Relay coordination involves analyzing and adjusting the TCC curves of protective devices to achieve proper coordination, where upstream devices operate slower than downstream devices. This coordination aims to minimize the network’s downtime by allowing the nearest protective device to isolate the fault while the rest of the system remains unaffected.
To illustrate the concept of TCC curves in a practical scenario, consider a high-voltage transmission system with multiple circuit breakers and protective relays. Suppose there are two circuit breakers, A and B, connected in series with their respective protective relays, RA and RB.
For proper coordination, the TCC curve of protective relay RA should lie above that of protective relay RB. This ensures that RB operates faster than RA for fault conditions downstream of B. By adjusting the settings of RA or the trip unit of circuit breaker A, engineers can achieve the desired coordination.
Let’s say the TCC curve for relay RA has a slope of 0.2 s/kA, meaning it operates in 0.2 seconds for every kiloampere increase in fault current. The TCC curve for relay RB has a slope of 0.1 s/kA, indicating a faster response time. Suppose a fault occurs downstream of B, causing a fault current of 10 kA.
Using the TCC curves, engineers can determine that relay RB, with its faster slope, will operate in 1 second (0.1 s/kA * 10 kA) while relay RA will operate in 2 seconds (0.2 s/kA * 10 kA). This coordination ensures that relay RB isolates the fault, allowing the rest of the system to remain operational.
In summary, Time-Current Characteristics (TCC) curves are crucial in relay protection coordination for electrical power networks. They represent the operating time of protective devices based on the current magnitude, allowing engineers to ensure the sequential and coordinated operation of protective devices during fault conditions. Proper TCC coordination reduces the impact on power supply reliability and minimizes downtime in high-voltage transmission and distribution systems.