Transmission lines transport electrical power from generators to load centers, much like the arteries in the human circulatory system. Damage to or loss of transmission infrastructure can have far-reaching consequences for the electrical system as a whole and the consumers it serves, much like blood arteries in the human body.

Construction of transmission lines

Underground cables and overhead air-insulated wires are the two most common ways that transmission lines are constructed. Overhead lines are the most typical form of transmission circuit; these lines use insulators made of polymer, glass or porcelain to suspend loaded conductors from buildings. Commonly, for reasons of isolation and safety, the line's conductors are positioned dozens of meters above ground, and the distance between them depends on the voltage level being operated.

Underground cables employ a solid dielectric substance to separate the grounded phase conductors from the energized ones, as opposed to lines that run overhead. The phase conductors of older cables were insulated using multiple layers of paper saturated in mineral oil; however, polymer dielectric materials like crosslinked polyethylene (XLPE) are used in contemporary cable technologies.

Why is protection needed in transmission lines?

Protection is crucial in transmission lines to ensure the safety of both equipment and personnel, as well as to maintain system reliability. Transmission lines are prone to faults, such as short circuits or open circuits, which can lead to significant damage if not addressed promptly. Protection systems are designed to isolate faulty sections of the line, preventing the fault from spreading and causing further damage.

Damage to underground cables happens when the dielectric material that acts as an insulator wears down over time. Many factors can contribute to this, including thermal stress from continuous line overloading, electrical stress from both steady-state and transient over voltages, and, most frequently, water seeping into the insulation of the cable.

Different types of protection for transmission lines

Line protection should have certain characteristics such as the ability to trip only the circuit breaker that is physically nearer to the problem location in the event of an outage. In the event that the breaker at the source of the problem does not trip, the breaker immediately adjacent to it will do so as a backup. To keep circuit breakers in working areas of a system from tripping for no reason, make sure the relays in line protection are operating as fast as possible. The following are the details of some of the way of methods of transmission line protection:

Overcurrent protection

  • Overcurrent relays: These devices detect excessive current flow in a circuit and initiate protective actions, such as circuit breaker tripping.
  • Directional overcurrent relays: These relays not only detect overcurrent but also determine the direction of fault current flow, which is essential for selective protection.

Distance protection

  • Impedance relays: These relays measure the impedance between the relay location and the fault point. By comparing this impedance to a preset threshold, the relay can determine the approximate location of the fault.
  • Mho relays: Similar to impedance relays, Mho relays use an Mho characteristic to determine the fault location. They calculate admittance (the reciprocal of impedance) using the ratio of current to voltage and the characteristic is a circle on a complex plane.

Differential protection

  • Differential relays: These relays compare the currents flowing into and out of a protected zone. If there's a significant difference, it indicates a fault within the zone, triggering protective action.

Pilot protection

  • Carrier-current protection: This scheme uses high-frequency signals to communicate between the ends of a transmission line. If a fault occurs, the communication is disrupted, triggering protective action.
  • Power-line carrier communication: This method uses the power line itself as a communication channel to transmit protection signals. However, power lines can be susceptible to noise and interference, which can degrade the quality of the transmitted signals.

Ground fault protection

  • Ground fault relays: These relays detect ground faults and initiate appropriate protective actions to isolate the faulty section.

Overvoltage protection

  • Surge arresters: These devices protect equipment from excessive voltage surges caused by lightning or switching operations.

Underfrequency protection

  • Underfrequency relays: These relays detect a decrease in system frequency and initiate load shedding or other protective actions to prevent system collapse. Its purpose is to identify possible generation-load imbalances by alerting the user if the AC power system's frequency drops below a certain threshold.

Phase balancing protection

  • Phase balancing relays: These relays monitor the balance between the three phases of the transmission line and initiate protective actions if an imbalance occurs. In the event that a phase imbalance is found, the circuit in question is immediately interrupted by the phase protection relays. This protects against imbalance-related faults and harm.

Remote terminal unit (RTU) based protection

  • RTU-based protection: This scheme uses RTUs to collect data from various points on the transmission line and implement protection functions based on predefined algorithms.

Conclusion

Different types of protection are available for transmission lines, including overcurrent, distance, differential, pilot, ground fault, overvoltage, underfrequency, phase balancing and RTU-based solutions. Each protection scheme has its own advantages and disadvantages, and the choice of scheme depends on factors such as the length of the transmission line, the type of load, the fault characteristics and the desired level of reliability. A combination of different protection schemes is often used to provide comprehensive protection for transmission lines.