MCB CHARACTERISTIC CURVE

MCB = Miniature Circuit Breaker is a device which protect from electric circuit from over current or short circuit. It is a better alternative of fuse in modern era of electric distribution. MCB are used at lower current circuit & having following specification.

• Current rating
• Short Circuit Rating
• Operating Characteristics - B, C or D Curves  

Types of MCB based on Characteristics Curve:

• B Type Curve
• C Type Curve
• D Type Curve

Type B Curve MCB:

This type of MCB trips between 3 and 5 times full load current. Type B devices are mainly used in residential applications or light commercial applications. The surge current levels in such cases are relatively low.

Type C Curve MCB:

This type of MCB trips between 5 and 10 times full load current. This is used in commercial or industrial type of applications where there could be chances of higher values of short circuit currents in the circuit.

Type D Curve MCB:

This type of MCB trips between 10 and 20 times full load current. These MCBs are use in specialty industrial / commercial uses where current inrush can be very high. Examples include transformers or X-ray machines, large winding motors etc. 

UNDERSTANDING GENERATOR CAPABILITY CURVE

       BASICS OF GENERATOR CAPABILITY CURVE

       Each & Every Synchronous Generator are running on Capability Curve. Capability curve of Generator is provided by the manufacturer & the curve has been decided during the type test of the Machine during manufacturing process. 

—       A Synchronous Generators are rated in terms of the maximum MVA output at a specified voltage & power factor (Usually 0.8 to 0.9 Lagging) which then carry continuously without overheating.—The active power output is limited by the prime movers capability to a value within the MVA rating.The continuous reactive power output capability is limited by three consideration ( - Armature Current Limit , - Field Current Limit, - End Region Heating Limit).

     

      Armature Current Limit: The armature current results in an I2R Power Loss and the energy             associated with this loss must be removed so as to limit the increase in temperature of the                     conductor & its immediate environment. There for one of the limitation on generator rating is the       maximum current that can be carried out by armature without exceeding the heating limitations.         Therefore in P-Q plane the armature current limit appears as a circle with centre at the origin &            radius equal to the MVA rating. The per unit complex power output is 

                               S = P + JQ

         

Armature Current Limit

       Field Current Limit: Because of the heat resulting from the Ifd2Rfd power loss, the field                      current imposes a second limit on the operation of the generator. 

Field Current Limit

       End Region Heating Limit:The localized heating in the end region of the armature imposes a            third limit on the operation of a synchronous machine. This limit affects the capability of the                machine in the under excited condition. When Generator running at Over excited region then              high current induced in rotor field, that's why rotor retaining remains in saturated condition.                However in the under excited region the field current is low & the retaining ring is not saturated;        this permits an increase in armature end flux. Also in the under excited condition, the flux                    produced by the armature currents add to the flux produced by the field current; there fore, the            end turn flux enhances the axial flux in the end region & the resulting heating effect may                      severely limit the Generator output, particularly in the case of a cylindrical rotor machine.

End Region Of a Generator

        

End Region Heating Limit

     

       Capability Curve Of a Synchronous Generator:

CAPABILITY CURVE OF GENERATOR

Detail of Generator Capability curve will be studied in upcoming blogs (RRS)