26-02-2011, 02:42 PM
presented by:
Rajesh Sharma
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Power System Protection
definition
Overcurrent Coordination
A systematic study of current responsive devices in an electrical power system
Objective
To determine the ratings and settings of fuses, breakers, relay, etc.
To isolate the fault or overloads.
Criteria
Economics
Available Measures of Fault
Operating Practices
Previous Experience
Design
Open only PD upstream of the fault or overload
Provide satisfactory protection for overloads
Interrupt SC as rapidly (instantaneously) as possible
Comply with all applicable standards and codes
Plot the Time Current Characteristics of different PDs
Analysis
When:
New electrical systems
Plant electrical system expansion/retrofits
Coordination failure in an existing plant
Protection vs. Coordination
Coordination is not an exact science
Compromise between protection and coordination
Reliability
Speed
Performance
Economics
Simplicity
Protection
Prevent injury to personnel
Minimize damage to components
Quickly isolate the affected portion of the system
Minimize the magnitude of available short-circuit
Spectrum Of Currents
Load Current
Up to 100% of full-load
115-125% (mild overload)
Overcurrent
Abnormal loading condition (Locked-Rotor)
Fault Current
Fault condition
Ten times the full-load current and higher
Coordination
Limit the extend and duration of service interruption
Selective fault isolation
Provide alternate circuits