Electrical power systems are susceptible to faults caused, for example, by storm, pollution, vandalism, lightning, salt spray, etc. The unscheduled interruption in the supply of electricity to consumers, whether industrial, residential, or commercial, entails severe fines for the transmission utility and/or electricity distributor, imposed by the regulatory agency. Thus, the EPS must have a well-dimensioned protection system, capable of identifying the fault, which is characterized by a single-phase, two-phase, three-phase short circuit, among others, and interrupt the missing section in the minimum time so that the effects of this lack are as small as possible for the SEP, especially with regard to its integrity and operational security.
Top1. Introduction
In this work, an Electric Power System (EPS) composed of four buses will be presented, with the respective levels of short circuit, as shown in Figure 1.
Figure 1. Basic scheme of an electricity transmission system
An unscheduled interruption in the supply of electricity can result in excessive costs due to the interruption of a process in the industry, for example, as well as social costs that are difficult to measure. Therefore, it must be ensured that the EPS is reliable and safe with regard to the regular supply of electricity to final consumers. Notwithstanding, the EPS is subject to failures, caused by numerous factors, making it necessary to design an effective protection system with a prompt response to these unpredictable events.
The EPS illustrated in Figure 1 is composed of two load buses (Power Utilities) with nominal voltages of 50 kV, interconnected by electric power transmission lines. Circuit breakers and protection relays, with the respective current transformers (CTs), are also shown in this figure.
It should be noted that for a fault occurring in the CD section of the EPS above, the relay R2 must trip the circuit breaker and de-energize the transmission line corresponding to that missing section. Thus, only the load connected to bus D will be without power supply for a certain period of time, until the transmission concessionaire's maintenance team moves to the defect location and corrects the cause of the defect (corrective maintenance).
From the above, if relay R1 operates before R2, the supply of electricity will be interrupted for a greater number of consumers, by de-energizing the transmission line corresponding to the BC section, after “trip” on the circuit breaker commanded by relay R1. This, in turn, characterizes a failure in the coordination / selectivity of the protection relays.
Therefore, it is up to the EPS Protection Engineer to carry out protection studies in the system in order to ensure the coordination and selectivity of the protection relays. A well-dimensioned protection system has the advantage of guaranteeing continuity in the supply of electricity, thereby avoiding the extrapolation of continuity indicators imposed by the regulatory agency, as well as increasing the operational safety of the system and, therefore, the levels of quality of the electric energy delivered to the final consumers.
Top2. Methodology For Adjusting 51F / 50F Functions (Phase Overcurrent)
Figure 2 below illustrates a block diagram showing the basic arrangement of a protection system, indicating the transducers of electrical quantities (CTs and PTs), as well as the protection relay responsible for the trip in the circuit breaker.
Figure 2. Basic arrangement of a protection system
In addition to the task of identifying and “clearing” the fault, the protection also has resources capable of locating the fault, making it possible to quickly restore the system once the corrective maintenance team will be moved to the defect location with the maximum speed, avoiding inspecting the entire length of a transmission line, considering routes that in Brazil reach 2,000 km in some cases (connection of the 500 kV line connecting the plants of the river Madeira complex with the country's Midwest) (G.V.D.B et al, 2019).