Transmission lines are a vital part of the electrical distribution system, as they provide the path to transfer power between generation and load. Transmission lines operate at voltage levels from 69kV to 765kV, and are ideally tightly interconnected for reliable operation. Transmission protection systems are designed to identify the location of faults and isolate only the faulted section. The key challenge to the transmission line protection lies in reliably detecting and isolating faults compromising the security of the system.
The physical construction of the transmission line is also a factor in protection system application. The type of conductor, the size of conductor, and spacing of conductors determines the impedance of the line, and the physical response to short circuit conditions, as well as line charging current. In addition, the number of line terminals determines load and fault current flow, which must be accounted for by the protection system.
2. What is Protection?
Protection is the art and science of detecting problems with power system components and isolating these components. Problems on the power system include:
1. Short circuits
2. Abnormal conditions
3. Equipment failures
3. Purpose of System Protection
1. Protect the public
2. Improve system stability
3. Minimize damage to equipment
4. Protect against overloads
5. Employ relay techs and engineers
4. FAULTS ON Transmission Line (Short Circuits)
Some causes of faults:
3. Animals (birds, squirrels, snakes)
4. Weather (wind, snow, ice)
5. Natural Disasters (earthquakes, floods)
6. Faulty equipment (switches, insulators, clamps, etc.)
Faults “Faults come uninvited and seldom go away voluntarily.”
4.1. Fault Types:
1. Single line-to-ground fault
2. Line-to-line fault
3. Three Phase fault
4. Line-to-line-to-ground fault
4.2. How Do We Protect Transmission Lines?
1. Over current
2. Directional Over current
3. Distance (Impedance)
5. Distance Protection:
Distance relays have been successfully used for many years as the most common type of protection of transmission lines. The development of electromechanical and solid state relays with mho characteristics can be considered as an important factor in the wide spread acceptance of this type of protection at different voltage levels all over the world. A distance relay measures the impedance of a line using the voltage applied to the relay and the current applied to the relay.
When a fault occurs on a line, the current rises significantly and the voltage collapses significantly. The distance relay (also known as impedance relay) determines the impedance by Z = V/I. If the impedance is within the reach setting of the relay, it will operate. When a fault occurs on a transmission line, the current increases and the angle of the current with respect to the voltage changes to a lagging angle, usually between 60 to 85 degrees.
5.1. Basic Principle
A distance relay has the ability to detect a fault within a pre-set distance along a transmission line or power cable from its location. Every power line has a resistance and reactance per kilo meter related to its design and construction so its total impedance will be a function of its length. A distance relay therefore looks at current and voltage and compares these two quantities on the basis of Ohm’s law.
Since the impedance of a transmission line is proportional to its length, for distance measurement it is appropriate to use a relay capable of measuring the impedance of a line up to a predetermined point (the reach point). Distance relay is designed to operate only for faults occurring between the relay location and the predetermined (reach) point, thus giving discrimination for faults that may occur in different line sections.
The basic principle of distance protection involves the division of voltage at the relaying point by the measured current. The calculated apparent impedance is compared with the reach point impedance. If the measured impedance is less than the reach point impedance, it is assumed that a fault exists on the line between the relay and the reach point.
5. Zones of Protection:
Careful selection of the reach point settings and tripping times for various zones of measurement enables correct coordination between distance relays on a power system. A distance relay has to perform the dual task of primary and back up protection. The primary protection should be fast and without any intentional time delay. Back up protection should operate if and only if corresponding primary relay fails. Distance relays are provided with multiple zones of protection to meet the stringent selectivity and sensitivity requirements. Typical reach and time settings for a 3Zone distance protection are shown below:
- Zone 1: this is set to protect between 80% of the line length AB and operates without any time delay. This “under-reach” setting has been purposely chosen to avoid “over-reaching” into the next line section to ensure selectivity since errors and transients can be present in the voltage and current transformers. Also manufacturing tolerances limit the measurement accuracy of the relays.
- Zone 2: this is set to protect 100% of the line length AB, plus at least 20% of the shortest adjacent line BC and operates with time delay t2. (≈0.5s) It not only covers the remaining %20 of the line, but also provides backup for the next line section.
- Zone 3: this is set to protect 100% of the two lines AB, BC, plus about 25% of the third line CD and operates with time delay t3. (≈1.5s)