Distance protection schemes are an integral part of modern electrical power networks. These schemes provide quick and reliable fault detection and isolation by measuring the distance to a fault point. They are widely used in both transmission and distribution systems to safeguard equipment and ensure the continuous and safe operation of the power network.
The main principle behind distance protection is based on the observation that fault impedance in a power system changes as a function of the distance between the relay and the fault point. By providing adequate settings and coordination, distance protection schemes can effectively locate and isolate faults, minimizing disruption to the power supply.
One of the key advantages of distance protection schemes is their ability to operate independently of the fault type. Whether it is a phase-to-phase or phase-to-ground fault, distance protection can adapt to different fault conditions. This flexibility ensures robust and reliable operation in a variety of fault scenarios.
There are various types of distance protection schemes, each with its own specific application. The most commonly used schemes include:
Impedance Distance (21): The impedance distance scheme is the simplest and most widely used distance protection scheme. It compares measured impedance with pre-set impedance limits to determine the distance to the fault point. It is particularly suitable for long-distance transmission lines.
Reactance Distance (24): The reactance distance scheme takes into account the reactive component of fault impedance. It is mainly used for lines with a significant reactance-to-resistance ratio, such as long underground cables.
Mho Distance (67): The mho distance scheme operates on the principle of a mho circle, which represents the impedance seen by the relay. This scheme provides fast and accurate operation and is commonly used for short and medium transmission lines.
Permissive Overreaching Transfer Trip (POTT): The POTT scheme allows remote-end relays to trip adjacent relays if a fault is detected within their zone of protection. This scheme ensures fast and coordinated fault clearing and is commonly used for transmission lines.
To illustrate the practical application of a distance protection scheme, let’s consider a transmission line with a length of 100 km. The relay at the sending end of the line is set with a reach setting of 80% of the line length. This means that the relay will trip for any fault within 80 km of the sending end.
If a three-phase fault occurs at a distance of 50 km from the sending end, the impedance seen by the relay can be calculated using the formula:
Assuming the voltage at the relay location is 345 kV and the fault current is 20 kA, the impedance seen by the relay is:
The measured impedance is then compared against the pre-set impedance boundary to determine if the fault lies within the protection zone. If the measured impedance falls within the expected range, the relay initiates a trip command to isolate the faulted section of the network.
It’s worth noting that proper coordination and settings are crucial for effective distance protection. This includes consideration of fault resistances, line constants, and network configuration. Coordination between relays is also essential to avoid misoperation and ensure efficient fault clearing.
Overall, distance protection schemes offer reliable and efficient fault detection and isolation in electrical power networks. They play a vital role in maintaining the stability and integrity of the network, minimizing downtime, and ensuring the safe delivery of electrical power. Proper design, settings, and coordination are essential for the successful implementation of distance protection schemes in transmission and distribution systems.