Job interview and viva Topics for Electrical and electronics Engineers (Part-4).

Electrical engineering interview topic:Job interview and viva Topics for Electrical and electronics Engineers (Part-4).

In this topic we know about Power System Stability,Steady State Stability,Static Stability,Dynamic Stability and Steady State Stability Criterion

13. Power System Stability: 

 In the large power system there might be some disturbance always. The system has the tendency for the system to develop forces for bringing the system in a normal or stable condition. The ability of the system to obtain the normal or stable condition after getting disturbed is called Power System Stability.

The stability has mainly two types: 

1. Steady State Stability.

2. Transient Stability.

14. Steady State Stability:  

The ability of being stable after a small disturbance in the system is called Steady State Stability.

The Steady State Stability is concerned with the very small power changes .which are changed gradually.

The Steady State Stability has two types:

1. Static Stability

2. Dynamic Stability.

·        Static Stability: 

In the Static Stability there is a large duration phenomenon with small disturbance. This is the inherent stability that can achieve with out the help of automatic control devices.

·        Dynamic Stability: 

In the Dynamic Stability there is a small duration phenomenon with large disturbance. This Dynamic Stability refers to artificial stability. This artificial stability is given to an inherently unstable system by the help of automatic control devices.

15. Steady State Stability Criterion:

The rate (dP/dδ) of the differential power increased is obtained per differential load angle increased. This are called as electrical stiffness or the synchronizing power coefficient of a synchronous machine. This is considered as the measure of the stability of the system.

The Steady State Stability has basically three Criterion or state.

When,

·        (dP/dδ) > 0 ; Direct State Synchronous Stability.

·        (dP/dδ) = 0 ; Steady State Stability.

·        (dP/dδ) < 0 ; The System is unstable.
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Electronics and Electrical Engineering interview Question and Answers for fresh engineers (Part 7)

Electronics and Electrical Engineering interview Question and Answers for fresh engineers (Part 7)

In this topic we know about the Power Factor, the leading and lagging condition for the power factor, the disadvantage of the low Power Factor and the main causes of the low power factor.

 Question: What is Power Factor?

 Answer: In an a.c. circuit the cosine of angle between voltage and current is called as power factor.

Fig: Power factor angles

 Question: What are the leading and lagging conditions in the power factor improvement?

 Answer: Normally there are two types of circuit, capacitive and inductive circuits.

·        In the capacitive circuits, current leads the voltage and power factor is called as leading.

·        In the inductive circuits, current lags behinds the voltage and power factor is called as lagging.

Question: What are the disadvantages of low power factor?

 Answer: The main disadvantages of low power factor are as follows,

1.     Large size of the Conductor.

2.     Large Kilo Volt Ampere rating of the electrical machinery or equipment.

3.     Poor or unaccepted voltage regulations.

4.     A big amount of Copper loss.

5.     The handling capacity of the system reduced.

Question: What are the main causes of the low power factor?

 Answer: In the supply system normally the power factor is lower then 0.8 for the overall load. Low Power factor is very undesirable, because it hampers the economic parameters for the power generation and distribution. Now, some short important causes are as follows,

·        In the low lagging factor some electrical equipment such as electric discharge lamps, Arc lamps and the industrial heating furnaces are running.

·        The power factor is also depending on the load conditions. The load is varying time to time.  The supply voltage is increasing in the low load period which increases the amount of the magnetization current. This magnetizing current is resulting in the decreased power factor.

·        The single phase and three phase ac motors have low lagging power factor and they are induction type also. The power factor is varying for these types of motor. It can run from 0.2 to 0.9 values depends on load. In the heavy load the power factor can be 0.8 or 0.9 and in the light load the power factor can be 0.2 to 0.3.

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Electrical engineering interview

Electronics and Electrical Engineering interview Question and Answers for fresh engineers (Part 8)

Electronics and Electrical Engineering interview Question and Answers for fresh engineers (Part 8)

 In this topic we know about the substation and its some requirements for setup.

 Question: What do you mean by substation? Give some details.

Answer: The electric power is now generated in the form of alternating current. The electric power is produced in the power station, which is generally very far from the consumer premises. So, the generation, transmission and distribution are facing much difficulty. It is distributed to the consumer or user by the help of a very large interconnected network. In the network it is very important to change some characteristics for transmitting and distributing the electric power properly. Such as, ac to dc conversion (by using thyratron and mercury arc rectifier), voltage up or down (If the generating voltage is 11kv then transfer it to the high voltage range of 220kv), frequency change, power factor correction and improvement. This change needs a setup of some apparatus properly, in short this types of setup is called substation.

          The assembly of electrical apparatus whose purpose is changing the some major characteristics of the electric supply system is called substation.

            The substation is very important for achieving the continuity of supply to the consumers. In some cases the ac is need to convert in dc for running the dc motor, traction, electroplating processes this type of conversion is performed by the sub-station (ignitron).

SUB STATION

Question: What are the basic requirements for setup a substation? Or which conditions are considerable for setup a substation?

Answer: The continuity of electric energy supply system is solely depending on substation. So, enough care should be taken when designing and set upping a sub station in a suitable place.

Some points are as follows,

·        The substation must have the chance of easy operation and maintenance.

·        The place must be proper for setup a substation. It must be setup near the centre of the load.

·        The substation must have enough safety apparatus or safety setup. There must be enough chance for running the maintenance and clearing process. Also there should be setup for fire protection.

·        The system must be reliable. When designing protected gears and other instruments must be included. The construction process also maintained properly for reliable operation.

·        The capital cost must be as low as possible.

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Electrical engineering interview