What is an Intelligent Electronic Device?


The term Intelligent Electronic Device or an IED refers to a broad definition of any electronic device that possesses intelligent function in . The term came into existence to describe a device that has versatile electrical protection functions, advanced local control intelligence, monitoring abilities, and the capability of extensive communications directly to a SCADA system.

The functions of a typical IED can be classified into five main areas, namely: 

  • Protection
  • Control
  • Monitoring
  • Metering
  • Communications

Some IEDs may be more advanced than others, and some may emphasize certain functional aspects over others, but these main functionalities should be incorporated to a greater or lesser degree.

The protection functions of the IED evolved from the basic overcurrent and earth fault protection functions of the feeder to more complex protection requirements in the generator system. The protection functions are typically provided in discrete function blocks, which are activated and programmed independently.

The following is a guideline of protection-related functions that may be expected from the most advanced IED's: 
  • Non-directional earth fault protection (low-set, high-set and instantaneous function blocks) 
  •  Directional three-phase overcurrent (low-set, high-set and instantaneous function blocks, with low-set selectable as long time-, normal-, very-, or extremely inverse, or definite time)
  •  Directional earth fault protection (low-set, high-set and instantaneous function blocks) 
  • Phase discontinuity protection 
  • Three-phase overvoltage protection 
  • Residual overvoltage protection 
  • Three-phase undervoltage protection 
  • Three-phase transformer inrush/motor start-up current detector 
  • Auto-reclosure function 
  •  Under frequency protection 
  • Over frequency protection 
  • Synchro-check function 
  •  hree-phase thermal overload protection.

Typical IED Configuration | Source: General Electric

The control function includes a local and remote control and is fully programmable. 
  • Local and remote control of up to twelve switching objects (open/close commands for circuit breakers, isolators, etc.) 
  • Control sequencing 
  • Bay level interlocking of the controlled devices  such as status information and information of alarm channels
  • HMI panel on the device.

Monitoring of the following functions
  • Circuit-breaker condition monitoring, including operation time counter, electric wear, breaker travel time, scheduled maintenance 
  • Trip circuit supervision
  • Internal self-supervision
  • Gas density monitoring (for SF6 switchgear)
  • Event recording 
  • Other monitoring functions, like auxiliary power, relay temperature, etc. 

IED Metering may include the following:
  • Three-phase currents
  • Neutral current
  • Three-phase voltages
  • Residual voltage
  • Frequency
  • Active power
  • Reactive power
  • Power factor
  • Energy
  • Harmonics
  • Transient disturbance recorder

The communication capability of an IED is one of the most important aspects of modern electrical and protection systems, and is the one aspect that clearly separates the different manufacturers’ devices from one another regarding their level of functionality.  

Modern substation automation uses IEC 61850 to harmonize all IED and other components in the system. 

IEC Network Interface 

IED in Substation Automation 
Automation in a substation is considered as the provision of new generation intelligent electronic devices (IEDs), programmable logic controllers (PLCs) and computers to monitor and communicate. It is always simple to incorporate these components in new substations at the design stage itself. 

"To automate the substation, it is important that an engineer know how to do data modeling as defined by ".

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