Set 1

Here is the first collection of the most basic concepts you would often encounter in the interviews. Hope it helps...

Plant load factor is often defined as the ratio of average load to capacity or the ratio of average load to peak load in a period of time. A higher load factor is advantageous because a power plant may be less efficient at low load factors, a high load factor means fixed costs are spread over more kWh of output (resulting in a lower price per unit of electricity), and a higher load factor means greater total output. If the power load factor is affected by non-availability of fuel, maintenance shut-down, unplanned break down, or reduced demand (as consumption pattern fluctuate throughout the day), the generation has to be adjusted, since grid energy storage is often prohibitively expensive.


The load, or burden, in a CT metering circuit is the (largely resistive) impedance presented to its secondary winding. Typical burden ratings for IEC CTs are 1.5 VA, 3 VA, 5 VA, 10 VA, 15 VA, 20 VA, 30 VA, 45 VA & 60 VA. As for ANSI/IEEE burden ratings are B-0.1, B-0.2, B-0.5, B-1.0, B-2.0 and B-4.0. This means a CT with a burden rating of B-0.2 can tolerate up to 0.2 Ω of impedance in the metering circuit before its output current is no longer a fixed ratio to the primary current. Items that contribute to the burden of a current measurement circuit are switch-blocks, meters and intermediate conductors. The most common source of excess burden in a current measurement circuit is the conductor between the meter and the CT. The rated secondary current is commonly standardized at 1 or 5 amperes.


Why go for sinusoids in power system?
Electrical power system is usually voltage-driven. Voltage drops across various elements thus modify the load voltage with respect to source voltage. These voltage drops are decided by a scaling of current by resistance value in the case of a resistor. It is decided by derivative of current in the case of an inductor and by integral of current in the case of a capacitor. A time-function retains its waveshape when multiplied by a constant. But, in general, it does not maintain its waveshape on differentiation and integration. Therefore, it follows that, in general, voltages and currents at various locations in an interconnected electrical network will have different waveshape even if all sources in the network have same waveshape.

Thus, sinusoidal signals have gained their pre-eminent position in Electrical Power engineering since they belong to a special class of time functions that preserve their waveshape on differentiation and integration. A linear network excited by sinusoidal sources of a particular frequency will have sinusoidal voltages and currents at the same frequency everywhere in the system in the long run.


Why not DC instead of Sinusoids?
If the customer had to be given 220V at his premises, the generators had to generate 220V and all the interconnection system had to work at that voltage level. As the load level increases, the current flow everywhere become excessive and generation/transmission become inefficient due to resistive losses everywhere in the system. It would have been very convenient if generation could be done at a voltage level economical from the point of view of electrical machine design and operation. Similarly, it would have been convenient if the transmission of power through transmission lines could be done at high voltage level so that the current level and consequently losses in lines would decrease. But this calls for generation at low voltage level, transmission at high voltage level and consumption at low voltage level. An efficient ‘voltage level conversion unit’ is needed for this. Such voltage level conversion equipment for dc at high power levels was simply not available at the initial stages of evolution of power systems in late 19th century and early 20th century.


IMPORTANT: Need for Governors in Turbines

When load on alternator increases, the load angle also increases. Hence the electromagnetic torque acting in opposition to prime mover would increase (being proportional to sin(δ) ) , which causes subsequent decrease in speed! However, speed deviations would cause frequency variations and so have to be controlled fixed. This brings in governors (Turbine speed controls)


When we increase load , current drawn from source also increses why ?
The electrical loads are normally connected in parallel, whenever the load(impedance) is increased means the additional load (or you can say impedance) is connected in parallel to the existing load. The parallel combination always reduces the equivelent impedance. Hence the current is increases.


What is the Difference between MCB & MCCB?
MCB is MINIATURE CIRCUIT BREAKER
MCCB is MOULDED CASE CIRCUIT BREAKER
MCB-A circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city. Rated current not more than 100 A. Trip characteristics normally not adjustable. Thermal or thermal-magnetic operation. Breakers illustrated above are in this category.
MCCB (Moulded Case Circuit Breaker)—rated current up to 1000 A. Thermal or thermal-magnetic operation. Trip current may be adjustable in larger ratings.