Power:Introduction to Electrical Relays

Electrical Relays

Figure 1 Protection Relay (courtesy of ABB)

Electrical relays are compact analog networks that are connected throughout the power system to detect intolerable or unwanted conditions within an assigned area. They are, in effect, a form of active insurance designed to maintain a high degree of service continuity, and limit equipment damage. They are "Silent Sentinels". While protective relays are more populat, other types of relays are applied on a more limited basis or used as part of a total protective relay system.

Classification of Electrical Relays

Relays can be divided into five functional categories:

Protective Relays

Detect defective lines, defective apparatus, or other dangerous or intolerable conditions. These relays can either initiate or permit switching or simply provide an alarm.

Monitoring Relays

Verify conditions on the power system or in the protection system. These relays in- clude fault detectors, alarm units, channel-monitoring relays, synchronism verification, and network phasing, Power system conditions that do not involve opening circuit breakers during faults can be monitored by veri- fication relays.

Programming Relays

Establish or detect electrical sequences, Programming relays are used for reclosing and synchronizing.

Regulating Relays

Activated when an operat- ing parameter deviates from predetermined limits. Reg- ulating relays function through supplementary equip- ment to restore the quantity to the prescribed limits.

Auxiliary Relays

Operate in response to the open- ing or closing of the operating circuit to supplement another relay or device. These include timers, contact- multiplier relays, sealing units, receiver relays, lock-out relays, closing relays, and trip relays.

Functional Categories

In addition to these functional categories, relays may be classified by input, operating principle or structure, and performance characteristic:

• Input
  • Current
  • Voltage
  • Power
  • Pressure
  • Frequency
  • Temperature
  • Flow
  • Vibration.

• Operating Principle or Structure
  • Percentage
  • Multi-restraint
  • Product
  • Solid state
  • Electromechanical
  • Thermal.

• Performance Characteristic
  • Distance
  • Directional-overcurrent
  • Inverse time
  • Definite time
  • Undervoltage
  • Ground or phase
  • High- or slow-speed
  • Phase comparison
  • Directional comparison
  • Segregated phase.

Reference

IEEE Std C37.90 - IEEE Standard for Relays and Relay Systems Associated with Electric Power Apparatus