08-01-2013, 03:26 PM
Protection and Relay Schemes
Protection and Relay Schemes.ppt (Size: 604.5 KB / Downloads: 62)
What are Relays?
Relays are electrical switches that open or close another circuit under certain conditions.
Relay Purpose
Isolate controlling circuit from controlled circuit.
Control high voltage system with low voltage.
Control high current system with low current.
Logic Functions
Relay Types
Electromagnetic Relays (EMRs)
EMRs consist of an input coil that's wound to accept a particular voltage signal, plus a set of one or more contacts that rely on an armature (or lever) activated by the energized coil to open or close an electrical circuit.
Solid-state Relays (SSRs)
SSRs use semiconductor output instead of mechanical contacts to switch the circuit. The output device is optically-coupled to an LED light source inside the relay. The relay is turned on by energizing this LED, usually with low-voltage DC power.
Microprocessor Based Relays
Use microprocessor for switching mechanism. Commonly used in power system monitoring and protection.
Advantages/Disadvantages
Electromagnetic Relays (EMRs)
Simplicity
Not expensive
Mechanical Wear
Solid-state Relays (SSRs)
No Mechanical movements
Faster than EMR
No sparking between contacts
Microprocessor-based Relay
Much higher precision and more reliable and durable.
Improve the reliability and power quality of electrical power systems before, during and after faults occur.
Capable of both digital and analog I/O.
Higher cost