Protective Relays and Their Functions

These flashcards cover key terms and definitions related to protective relays, their functions, types, and operational principles.

Protective Relay

An automatic device that senses an abnormal condition in an electrical circuit and closes its contacts to trip the circuit breaker.

Sensing Element

The component of a relay that responds to changes in the actuating quantity, like current in an overcurrent relay.

Comparing Element

The part of a relay that compares the actuating quantity's action with a preselected relay setting.

Control Element

The relay component that accomplishes sudden changes in the control quantity, such as operating the circuit.

Electromagnetic Attraction Relay

A relay type that operates by the attraction of an armature to electromagnet poles, or a plunger drawn into a solenoid.

Electromagnetic Induction Relay

A relay whose operation depends on the movement of a metallic disc or cylinder due to induced currents and alternating magnetic fields.

Electrodynamic Relay

A relay that consists of a coil moving in an electromagnetic field, typically used for DC applications.

Electro-thermal Relay

A relay that operates based on the heat generated by the current flowing through its elements.

Buchholz Relay

A gas-actuated relay used for protecting oil-immersed transformers, triggering alarms or tripping the transformer on internal faults.

Induction Type Overcurrent Relay

A relay that provides inverse-time operation characteristics and has two electromagnets for current sensing.

Plug Setting Multiplier (PSM)

A measure used to adjust overcurrent ranges in protective relays, defining the fault current setting.

Time Setting Multiplier (TSM)

A factor used to adjust the operable time of a relay based on its initial settings.

Directional Relay

A type of relay that operates based on the direction of the current flow in the circuit.

Impedance Relay

A distance relay measuring the impedance of the line to detect faults, operating if the impedance is below a set value.

Current Differential Relay

A relay that compares primary and secondary currents of a transformer to detect faults.

Voltage Differential Relay

A relay that operates based on the voltage differences between current transformers in a system.

Bias Protection Relay

A relay type that compares primary and secondary currents and adjusts its operation based on the level of detected difference.

Core Balance Method

A method for earth fault detection that uses a ring core to monitor the balance of currents in three conductors.

Residual CT Connection Method

A method of earth fault detection where phase currents are connected in a way that allows for monitoring of any imbalance.