Protection and Relay SchemesSalman Rajwani 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. Control high voltage system with low voltage. Logic Functions . Control high current system with low current.Relay Purpose Isolate controlling circuit from controlled circuit. Relay Types Electromagnetic Relays (EMRs) Solid-state Relays (SSRs) EMRs consist of an input coil that's wound to accept a particular voltage signal. usually with low-voltage DC power. The relay is turned on by energizing this LED. 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. Commonly used in power system monitoring and protection. Microprocessor Based Relays Use microprocessor for switching mechanism. 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. . 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. Higher cost . during and after faults occur. Capable of both digital and analog I/O. It is neither practical nor economical to build a ‘fault free’ system. lighting. .Why A System Needs Protection? There is no ‘fault free’ system. etc. Usually faults are caused by breakdown of insulation due to various reasons: system aging. Electrical system shall tolerate certain degree of faults. Electrical Faults majority are phase-to-ground faults phase-to-phase phase-phase-phase double-phase-to-ground . . Mitigating the effects of failures after they occur. Minimize risk of fire.Advantages for Using Protective Relays Detect system failures when they occur and isolate the faulted section from the remaining of the system. danger to personal and other high voltage systems. Protective Devices Comparison Relays Circuit Breakers Activation Fuses Actuation Acquisition Detection . Protective Devices Comparison Circuit Breakers V. Based the sending signals circuit breakers will open/close. Relays Relays are like human brain. .S. circuit breakers are like human muscle. Relays send signals to circuit breakers. Relays ‘make decisions’ based on settings. Fuses cannot be reset but replaced if they blow. Fuses only have one specific characteristic for a individual type. Relays Relays have different settings and can be set based on protection requirements. .S.Protective Devices Comparison Fuses V. Relays can be reset. Protection and Relay Schemes Motor Protection Transformer Protection Generator Protection . Motor Protection Timed Overload Locked Rotor Single Phase and Phase Unbalance Other . Motor Protection Timed Overload Solution: Thermal overload relays Plunger-type relays Induction-type relays . Motor Protection Timed Overload Protection Timed Overload Definition: Continuously operate motor above its rated value will cause thermal damage to the motor. . Require certain reaction time Inverse time/current relationship .Thermal Overload Relays Use bimetallic strips to open/close relay contacts when temperature exceeds/drops to certain level. Thermal Overload Relays . Plunger-type Relays Fast reaction time Use timer for time delay Such as oil dash pot. Inverse time/current relationship . Plunger-Type Relays . Induction-type Relays Most frequently used when AC power presents Change taps to adjust time delay . Induction-Type Relays . This will cause thermal damage to the motor winding and insulation. . Effects: this will result in excessive currents flow given the same load. It is also called locked rotor. but motor rotor is not rotating.Motor Protection Stalling Some Definitions… Motor Stalling: It happens when motor circuits are energized. Motor Protection Stalling Similar types of relays that are used for motor timed overload protection could be used for motor stalling protection. . Motor Protection Single Phase and Phase Unbalance Some definitions… Single Phase: three-phase motors are subject to loss of one of the three phases from the power distribution system. . Otherwise. .Motor Protection Single Phase and Phase Unbalance Some definitions… Phase Unbalance: In a balanced system the three lineneutral voltages are equal in magnitude and are 120 degrees out of phase with each other. the system is unbalanced. Motor Protection Single Phase and Phase Unbalance These conditions will cause Motor winding overheating Excessive vibrations Cause motor insulation/winding/bearing damage . Motor Protection Single Phase and Phase Unbalance These conditions will cause Motor winding overheating Excessive vibrations Cause motor insulation/winding/bearing damage . Motor Protection Single Phase and Phase Unbalance . Motor Protection Other Instantaneous Overcurrent Differential Relays Relays Undervoltage Electromagnetic Ground Fault Differential Relays . Transformer Protection Gas and Temperature Monitoring Differential and Ground Fault Protection . . Temperature Monitoring Relays: These relays are used to monitor the winding temperature of the transformer and prevent overheating. A tiny little amount of gas will cause transformer explosion.Transformer Protection Gas Monitoring Relays: These relays will sense any amount of gas inside the transformer. ground fault can be detected from the grounded neutral wire. .Transformer Protection Ground Fault For a wye connection. Transformer Protection Ground Fault and Differential Relay . Generator Protection Differential and Ground Fault Protection Phase Unbalance .