********** Knowledge Area Made by Mahabub ***********: August 2018

Sunday, August 12, 2018

The objectives of Maintenance Program

Maximizing the production or increasing facilities availability at the lowest cost and at the highest quality and safety standards.
Reducing breakdowns and emergency shutdowns.
Optimizing resources utilization.
Reducing downtime.
Improving spares stock control.
Improving equipment efficiency and reducing scrap rate.
Minimizing energy usage.
Optimizing the useful life of equipment.
Providing reliable cost and budgetary control.
Identifying and implementing cost reductions.

What is zero sequence current?

The unbalanced current flows in the circuit during the earth fault is known as the zero sequence current or the DC component of the fault current.
The zero phase sequence means the magnitude of three phases has zero phase displacement.
The zero sequence currents are produced due to the existence of zero sequence voltage.

Voltage variation effect on transformer?

Under-voltage operation does not have significant impact on transformer, but due to under-voltage condition current drawn by the loads increases. If this increase in load current can create an over current condition. Transformer can be operated upto 110% at no load and 105% at full load without any harm. Further increase in voltage can results in the saturation of magnetic core thereby increasing the magnetizing current and losses. Continued operation in over excited region can result in failure of unit.

Voltage variation impact on induction motor?

Induction Motor voltage below name plate rating will result in decrease in the starting torque of the motor and increase in the full load temperature rise.
On the other hand, increase in the supply voltage above the specified voltage will results in increase in the starting torque of the motor, increase in the inrush currents during starting and decrease in the power factor.
Increase in the starting torque to above supply voltage results in acceleration of the coupling or driven equipment. Increase in the starting currents results in voltage drop in the bus connected to the motor. This effects the performance of the other loads connected to the bus.
Supply voltage above the rating will have less impact on the induction motor compared to under voltage condition but if voltage above 110% of the rated voltage will saturate the stator iron (flux is proportional to voltage / frequency) thereby damaging the magnetic properties and increase in losses.

Why reactive power cannot be transmitted to long distances?

If reactive power is to be transmitted through long distances in the power system It requires larger size of the power system equipment (transformers and cables).
It is inefficient during the high real power transfer and also requires higher voltage magnitude gradients.
It causes high real and reactive power losses if reactive power is transmitted through long distances.
It can lead to damaging temporary over voltages following load rejections. When the receiving end circuit breaker opens with transmission line still energized from sending end, then over voltages occurs as the voltage magnitude depends upon the reactive power in the system. Over voltage will damage the insulation of the conductor and wingdings.