Reciprocating engines

Aircraft engines, marine engines, automotive engines and stationary engines are examples of reciprocating engines. Engine can be designed on the basis of the otto cycle where fuel is mixed with air before compression. In otto engine, combustion takes place with no excess air. In diesel engines, fuel is injected near the end of the compression and combustion takes place with much excess air. Spark ignition is employed in otto engines where volatile fuels are used. Low volatile fuels are used for its operation in diesel engines where ignition is by compression. Depending upon the number of piston strokes required for the complete combustion cycle, there exist two varieties in the engine- two stroke and four stroke. In two stroke engines, piston uncovers exhaust ports in the cylinder wall near the end of the expansion stoke and permits the exhaust gases to flow out. The various events that occur in four stroke engine are stroke, exhaust, suction and compression. According to the arrangement of cylinders with each other and the crankshaft, there exist three types of engines. They are three-cylinder in-line engine, five-cylinder in-line engine and V-type engines. V-type engine has two banks of in-line cylinders connected to a single crankshaft.

Protecting a transformer

Mainly there are two faults that can occur in transformers. They are internal faults and external faults. Internal faults are of three types. They are incipient faults, winding faults and terminal faults. The main incipient faults are fault due to the breakdown of core lamination, breakdown of bolts and clamps, earthing fault, coolant failure, oil flow fault etc. Terminal faults are associated with high voltage or low voltage terminals and winding faults are associated with windings. External faults include the appearance of overcurrent for short duration, short circuit outside the transformer and overloading. Different methods employed for the protection of transformers are electromagnetic protection, differential protection, percentage biased differential protection, tank leakage protection, restricted earth fault protection and digital protection.

Protecting a generator

The major faults that can happen in a generator are overloading, winding faults such as stator winding insulation failure, rotor winding insulation failure, field winding failure, unbalanced loading, overvoltage, overspeeding, overheating, prime mover failure, loss of synchronism and underfrequency. The various protective schemes employed are differential protection, inter-turn fault protection, stator earth fault protection, rotor earth fault protection, pole slipping protection, overload protection, overvoltage protection, over current earth fault protection, underfrequency protection, negative phase sequence protection, field failure protection, reverse power protection and back-up impedance protection. There are three classes of protection schemes for synchronous generators. They are class A, class B and class C schemes.

Circuit-breakers

During overloading and short circuits, we require circuit-breakers to safeguard electric circuits. Circuit-breakers are automatic devices capable of making and breaking of electric circuits. Insulating fluids are used here to extinguish the arc produced due to the fault current interruption and to provide total insulation between the contacts and from each contact to earth. Some examples of insulating fluids are air, compressed air, hydrogen producing oil and sulphur hexafluoride gas. Insulating materials used in circuit-breakers should have high dielectric strength, thermal and chemical stability, high thermal conductivity, low dissociation temperature, should be non-flammable, should not produce carbon during arcing and should be commercially available. The arc between the circuit-breaker consists of ionized gas particles. By deionizing, we can execute arc interruption. This can be done by high pressure development, arc splitting or forced convention and turbulence. Switchgear can be used to execute current interruption.