05-10-2012, 12:28 PM
Numerical Relaying Fundamentals :
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Why Numerical Relaying?
To improve dependability as well as security
Self checking facility
Immune to variation in parameters of individual
components
Very low burden
More flexibility because of programmable capability
Fiber optical communication with substation LAN
Adaptive relaying schemes
Permit Historical data storage
Allow GPS (Geographical Positioning System) Time
stamping
Relay Hardware Blocks
Digital relay consists of:
Analog input subsystem
Digital input subsystem
Digital output subsystem
A processor along with RAM (data scratch pad), main
memory (historical data file) and power supply
Non-Simultaneous Sampling Scheme: MUX
Multiplexer is a collection of analog switches, which can be
selected by supplying binary code
Typically, power system applications involve more than one
analog input
To reduce the cost of the hardware, multiple channels are
multiplexed through analog multiplexer to a single ADC
Accuracy of the analog multiplexer depends on load
impedance at the output terminal
As S & H circuit has impedance in the range 108 to 1012,
no problem is encountered
Simultaneous Sampling Scheme
All S & H amplifiers are set to hold state simultaneously
This preserves the relative phase information between
multiple analog signals
Successive ADC which have a conversion time of 15-30
sec
An Anti-Aliasing filter is a low pass filter (LPF) used to cut
off the high frequency content in the input signal,
connected after signal conditioning hardware