Protection and Relay Schemes

Agenda  Introduction of Protective Relays  Electrical System Protection with Protective Relays  Conclusion

What are Relays?  Relays are electrical switches that open or close another circuit under certain conditions.

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.

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.

How a Relay Works

Sold-State Relay

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.

Electrical Faults  majority are phase-to-ground faults  phase-to-phase  phase-phase-phase  double-phase-to-ground

Protective Devices Comparison RelaysCircuit BreakersFuses  Acquisition  Detection  Activation  Actuation

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.

Transformer Protection  Gas and Temperature Monitoring  Differential and Ground Fault Protection

Transformer Protection Gas Monitoring Relays:  These relays will sense any amount of gas inside the transformer. A tiny little amount of gas will cause transformer explosion. Temperature Monitoring Relays:  These relays are used to monitor the winding temperature of the transformer and prevent overheating.

Transformer Protection Ground Fault  For a wye connection, ground fault can be detected from the grounded neutral wire.

Transformer Protection Ground Fault and Differential Relay

Conclusion  Relays control output circuits of a much higher power.  Safety is increased  Protective relays are essential for keeping faults in the system isolated and keep equipment from being damaged.

Reference:  IEEE Red Book (IEEE Recommended Practice for Electric Power Distribution for Industrial Plants)  Principles of power system by V.K Mehta