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4. Protection Principles
There exists several different protection techniques and principles. Fuses are the
simplest and cheapest technology, however they need to be manually replaced
when they melt (operate). Protective relays are used when fuses are not feasible.
Different relays can have different inputs, and they will treat the information
differently, but their objective is shared; to correctly detect and clear a fault as
soon as possible. A closer look at overcurrent, distance and differential relay
principles will be made in this chapter, as they are the most common protection
applications in power systems today. Other protection principles also exists, e.g.
overvoltage, undervoltage, volts per Hertz, however they are not the scope of this
thesis.
4.1. Fuses
The first type of protection for electrical networks were fuses. As they are simple
and cheap, they are still commonly used for protection purposes today. The most
common type of fuses consists of a short conducting wire inside a casing, capable of carrying the current permitted for the protected zone. The cross-section
and material of the wire decides how much current the fuse can conduct without
melting. The wire will melt if the temperature increase caused by the current
going through the fuse, which has some resistance, becomes higher than the melting temperature of the wire material. The fuse can melt almost instantaneously
or with some time delay as seen in Figure 4.1. A melted fuse will need to be
manually replaced, and this is one of the drawbacks of using fuses for protection.
Overcurrent relays, which will be looked at later in this chapter, can have a
time-current characteristic similar to fuses.[33]
Figure 4.1.: Time-Current Characteristics for a Fuse[19]
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