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POWER SYSTEM PROTECTION

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Current Transformers :

Distinction between CT and conventional
transformers; Equivalent circuit and phasor diagrams;
errors; ALF; Voltage developed across CT secondary ; CT ratio; Short time withstand current; Bar
and wound primary CTs Classification based on application. (Metering, Protection and PS class.) ; CT
specification.

Potential Transformers :

Errors; voltage factor; connections (star, delta, open delta and ‘V’ connections); PT ratio; Insulation
level ; Classification based on application ( Metering, Protection class ) ; Protection of PT; PT
specification.

Fault Level Calculations :

Impedance diagram; per unit and percentage impedance; sequence components; impedance to be
considered for different network elements like generator, transformer, transmission line, motor, cable,
etc; default figures where data not available; fault calculation procedures; step by step fault level
evaluation exercise for phase fault and ground faults.

HT Motor Protection :

Major protection elements; thermal over load; over current vs overload; critical review of motor and
relay thermal characteristics; major short comings; locked rotor; speed switch; negative sequence;
unbalance vs negative sequence; short-circuit; ground fault (zero sequence); use of residual
connected CT and CBCT; differential protection; under voltage protection; under current protection;
special protection requirement for wound motor; rotor unbalance; additional protection for
synchronous motor; loss of field.

LT Motor Protection :

Causes for LT Motor damage, Bimetal overload relay, HRC fuses, cost effective Motor Protection
relay for contactor controlled motor, co-ordination between fuse and relay, Application of thermistors.

Generator Protection :

Protection elements; Stator over current; Stator earth fault (95% and 100% ); Differential High
impedance and low impedance schemes; Over voltage protection; Over fluxing protection; Rotor
earth fault; Field over heating; Loss of excitation; Bearing vibration; Motoring (reverse power and low
forward power) protection; Unbalanced armature current protection; Voltage controlled over current
protection; Voltage restrained over current protection; Under frequency protection, withstand times for
steam, hydro, DG and gas units; Comparison of protection practices from different references.
The protection of the electrical system is a branch of electrical power engineering that deals with the protection of electrical systems from faults through the insulation of parts damaged from the rest of the electrical network. The purpose of a protection scheme is to keep the power supply system stable, isolating only those components that are in fault, while leaving as much of the network as possible still in operation. Therefore, protection schemes should be applied with a very pragmatic and pessimistic approach to compensation system failures. Devices used to protect faulty power systems are called protection devices.

Components

Protection systems generally comprise five components:
• Current and voltage transformers to lower the high voltages and currents of the electrical system to levels suitable for relays
• Protection relays to detect the fault and initiate a trip, or disconnection, order;
• Circuit breakers to open / close the system based on relay and automatic closing commands;
• Batteries to provide power in case of power disconnection in the system.
• Communication channels to allow the analysis of current and voltage on remote terminals of a line and allow remote tripping of equipment.

For parts of a distribution system, the fuses are capable of detecting and disconnecting faults. Faults may occur in each part, such as insulation failure, dropped or broken transmission lines, malfunction of circuit breakers, short circuits, and open circuits. The protective devices are installed with the aim of protecting the assets, and ensuring the continuous supply of energy.

The switch is a combination of electrical switches, fuses or circuit breakers used to control, protect and isolate electrical equipment. Switches can be opened safely under normal load current, while the protective devices can be safely opened under fault current.