Protection schemes are an integral part of power systems as they ensure the safe and reliable operation of electrical networks. These schemes employ various relays, devices, and equipment to detect and isolate faults, minimizing the potential damage and disruptions caused by electrical faults. In this section, we will delve into some practical examples and case studies to demonstrate the application and effectiveness of protection schemes in real-world scenarios.
One commonly used protection scheme is the overcurrent protection scheme. In this scheme, relays are set to trip the circuit breaker when the current flowing through a particular section of the power system exceeds a predetermined threshold. This scheme safeguards against faults such as short circuits, overloads, and excessive current flows due to equipment failures. Let’s consider a case study to illustrate the application of overcurrent protection.
Case Study 1: Overcurrent Protection Scheme in a Distribution System
Consider a distribution system supplying power to multiple residential areas. The system consists of a step-down transformer, overhead lines, and several distribution transformers. Faults in this system can occur due to various reasons, including insulation failures, equipment malfunctions, and lightning strikes.
To protect this system from faults, an overcurrent protection scheme can be deployed. An example of such a scheme could involve installing overcurrent relays at strategic locations, such as near the distribution transformers and along the overhead lines. These relays would be connected to trip the respective circuit breakers when the current exceeds a predetermined threshold for a specified time.
Let’s assume that the distribution system has an overcurrent relay installed at the input side of each distribution transformer. The relay is set to trip the circuit breaker if the current exceeds 150 A for more than 0.5 seconds. This setting allows for temporary overloads, but ensures that the circuit breaker operates promptly when a fault occurs. The fault analysis can then be carried out to determine the source of the fault and plan for necessary maintenance or repair.
Another commonly employed protection scheme is the distance protection scheme. This scheme is based on the principle that faults in a power system cause changes in voltage and current waveforms. By measuring the impedance, i.e., the ratio of voltage to current, the distance to the fault point can be estimated. If the impedance exceeds a predetermined threshold, it indicates a fault beyond a certain distance, and the relay activates the circuit breaker to isolate the faulted section. Let’s explore a case study to illustrate this concept.
Case Study 2: Distance Protection Scheme in a Transmission System
Consider a transmission system consisting of multiple substations connected by long-distance transmission lines. The system is designed to transmit high voltages, typically ranging from 110 kV to 765 kV. Faults in this system can occur due to various reasons, including lightning strikes, insulation failures, and equipment malfunctions.
To protect this system from faults, a distance protection scheme can be implemented. For example, each transmission line can be equipped with a distance relay connected to the respective circuit breaker. The relay would be set to trip the circuit breaker if the impedance exceeds a certain threshold, indicating a fault beyond a specified distance.
Let’s assume that a 220 kV transmission line has a distance relay installed at both ends. The relay settings can be adjusted based on the characteristics of the transmission line. For instance, if the line is 150 km long, the relay can be set to trip the circuit breaker if the impedance exceeds 0.35 ohms, indicating a fault beyond 75 km from the relay location.
In this case, if a fault occurs at a distance of 100 km from the relay location, the relay would detect the fault, activate the associated circuit breaker, and isolate the faulted section. Fault analysis can then be carried out to identify the type and location of the fault and plan for necessary maintenance or repair.
These case studies illustrate the practical application of protection schemes in real-world scenarios. Overcurrent protection schemes are commonly used in distribution systems, ensuring the safe and reliable supply of electricity to residential areas. On the other hand, distance protection schemes play a vital role in safeguarding larger transmission systems, which transmit bulk power across long distances. By employing appropriate protection schemes, power systems can operate efficiently, providing reliable electricity supply while minimizing disruptions and potential damages caused by electrical faults.