Substation Equipment's Switchgear Protection and Power Systems By Sunil S. Rao

Substation Equipment's Circuit Breaker Isolator Earthing switch Surge Arrester Current transformer Potential transformer Relay

Circuit Breaker A Circuit breaker is a switching and current interrupting device in a switchgear Function of CB Switching during normal operating conditions for the purpose of operation and maintenance. Switching during abnormal (Faulty condition) to interrupt the fault current

Requirements of Circuit Breaker The normal operating current and short circuit current must be safely interrupted by the circuit breaker. The faulty system must be isolated by CB as quickly as possible keeping minimum delay. It should not operate with the flow of over current during healthy condition The faulty circuit must be isolated without affecting the healthy one

Types of Circuit Breaker (on the basis of arc extinction medium Air break circuit breaker Up to 0.415kV application Current 100 to 4000 A Breaking capacity 80KVA Oil CB Oil is used as arc extinction medium Breaking capacity 500MVA Air blast CB(ABCB) Used force air mechanism breaking capacity 3500 MVA current carrying capacity (241-735)KV.

Types of Circuit Breaker (on the basis of arc extinction medium SF6 circuit breaker Mostly used CB High rate of recovery of dielectric strength Breaking capacity 35000 to 50000 MVA Vacuum CB Most modern and recent technology Voltage rating up to 33 KV Breaking capacity 500 MVA

Isolator, Earthing switch & Surge arrester Function of Isolator Disconnect a part of the system from the live parts under no load condition. Function of Earthing switch Discharge the voltage on the lines to earth after disconnecting them Function of surge arrester Diverting the high voltage surges to earth and maintaining continuity during normal voltage

Opening and Closing sequence of CB, isolator and Earthing switch While opening, CB open------ Isolator Open----- Earthing switch close While closing, Earthing switch open---Isolator close----CB close

Instrument transformer (Current transformer and potential transformer) Function of current transformer Stepping down the current for measurement, protection and control Function of Potential transformer Stepping down the voltage for measurement, protection and control

Relay An automatic device which closes its contacts when the actuating quantity reach a certain predetermined magnitude. Relay senses the fault and circuit breaker clears the fault

Classification of ac faults Unsymmetrical faults Single line to ground fault (Most common) Double line to ground fault Line to line fault Symmetrical fault Three phase fault (most severe but often occurs)

Classification of ac faults The other abnormal conditions in ac system Voltage and current unbalance Under frequency and over frequency Temperature rise Over voltage and under voltage Instability---- Reverse power flow

Classification of ac faults The other abnormal conditions in ac system Voltage and current unbalance Under frequency and over frequency Temperature rise Over voltage and under voltage Instability---- Reverse power flow

Operation of trip circuit

Bus bar Conductor to which a number of circuits are connected Types of Bus bar Open bus bar Enclose bus bar Outdoor bus bar Indoor bus bar Compound immersed bus bar Oil immersed bus bar Compressed gas insulated bus bar and solid-insulated bus bar