14-09-2012, 11:37 AM
Protection and Relay Schemes
Protection and Relay.ppt (Size: 276.5 KB / Downloads: 36)
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
Why A System Needs Protection?
There is no ‘fault free’ system.
It is neither practical nor economical to build a ‘fault free’ system.
Electrical system shall tolerate certain degree of faults.
Usually faults are caused by breakdown of insulation due to various reasons: system aging, lighting, etc.
Advantages for Using Protective Relays
Detect system failures when they occur and isolate the faulted section from the remaining of the system.
Mitigating the effects of failures after they occur. Minimize risk of fire, danger to personal and other high voltage systems.
Protective Devices Comparison
Circuit Breakers V.S. Relays
Relays are like human brain; circuit breakers are like human muscle.
Relays ‘make decisions’ based on settings.
Relays send signals to circuit breakers. Based the sending signals circuit breakers will open/close.
Fuses V.S. Relays
Relays have different settings and can be set based on protection requirements.
Relays can be reset.
Fuses only have one specific characteristic for a individual type.
Fuses cannot be reset but replaced if they blow.
Protection and Relay.ppt (Size: 276.5 KB / Downloads: 36)
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
Why A System Needs Protection?
There is no ‘fault free’ system.
It is neither practical nor economical to build a ‘fault free’ system.
Electrical system shall tolerate certain degree of faults.
Usually faults are caused by breakdown of insulation due to various reasons: system aging, lighting, etc.
Advantages for Using Protective Relays
Detect system failures when they occur and isolate the faulted section from the remaining of the system.
Mitigating the effects of failures after they occur. Minimize risk of fire, danger to personal and other high voltage systems.
Protective Devices Comparison
Circuit Breakers V.S. Relays
Relays are like human brain; circuit breakers are like human muscle.
Relays ‘make decisions’ based on settings.
Relays send signals to circuit breakers. Based the sending signals circuit breakers will open/close.
Fuses V.S. Relays
Relays have different settings and can be set based on protection requirements.
Relays can be reset.
Fuses only have one specific characteristic for a individual type.
Fuses cannot be reset but replaced if they blow.