Job interview Topics for Electrical and electronic Engineers (Part-2).

Electrical Engineering interview question and answers:Job interview or viva Topics for Electronics and Electrical Engineers (Part-2).

In this topic we discuss about Load Curve,Demand Factor and Load Factor.

4. Load Curve:

The curve which shows the variation of load on the electrical power station with respect to time is known as load variation curve or simply load curve.

5. Importance of Load Curve:

                  

• The Daily Load Curve gives the information of load on the power station during different running hours of the day.
• The number of unit’s generation per day is found from the area under the daily Load Curve.
• Average load is found from the Load Curve.

Average load= [Area (KWh) under daily load curve/24 hours]

• The maximum demand of the station on that day is found from the highest point of the daily Load Curve.
• The size and the number of generating units can be determined from the load curve.
• This Load Curve helps to determine the operation schedule of the station. In that case when all the units or the less units needs to running is found.

6. Demand Factor:  

The ratio of maximum demand and connected load on the power station is called as demand factor.

Demand Factor= [Maximum Demand/Connected Load]

• In the power station Maximum demand < Connected Load.
• Demand Factor < 1
• Demand Factor is very important in determining the capacity of the plant equipment.

7. Load Factor:  

The ratio of Average load and the Maximum demand in a given period of time in a power plant is known as load factor.

Load factor = [Average load/ Maximum demand]

• In a power plant Average load < Maximum demand
• Load factor<1.

The Load factor is very important for determining the cost of per unit generation of power.

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Electrical engineering interview

Electronics and Electrical Engineering interview Question and Answers for fresh engineers (Part 9)

Electronics and Electrical Engineering interview Question and Answers for fresh engineers (Part 9)

In this topic we will know about motor protection

In the nameplate of the motor we see IP XY . i.e. IP 55
Example: With the IP rating IP 23, 

• 2 describes the level of protection from solid objects and 
• 3 describes the level of protection from liquids.

IP stands for 

Ingress Protection Ratings

 IP rating normally has two (or three) numbers:

1. Protection from solid objects or materials
2. Protection from liquids (water)
3. Protection against mechanical impact 

specifying the environmental protection the enclosure provides.

Ingress Protection (IP) ratings are Introduced by the European Committee for Electro Technical Standardization .

 NEMA IEC 60529 Degrees of Protection Provided by Enclosures - IP Code,

IP First number - Indicates the Protection against solid objects. Which we indicates as X in the example

0	No special protection
1	Protected against solid objects over 50 mm, e.g. accidental touch by persons hands.
2	Protected against solid objects over 12 mm, e.g. persons fingers.
3	Protected against solid objects over 2.5 mm (tools and wires).
4	Protected against solid objects over 1 mm (tools, wires, and small wires).
5	Protected against dust limited ingress (no harmful deposit).
6	Totally protected against dust.

IP Second number -Indicates the Protection against liquids.Which we indicates as Y in the example 

0	No protection.
1	Protection against vertically falling drops of water e.g. condensation.
2	Protection against direct sprays of water up to 15o from the vertical.
3	Protected against direct sprays of water up to 60o from the vertical.
4	Protection against water sprayed from all directions - limited ingress permitted.
5	Protected against low pressure jets of water from all directions - limited ingress.
6	Protected against temporary flooding of water, e.g. for use on ship decks - limited ingress permitted.
7	Protected against the effect of immersion between 15 cm and 1 m.
8	Protects against long periods of immersion under pressure.

Example - IP 35

An electrical socket rated IP22 is protected against

• Protected against solid objects over 2.5 mm (tools and wires).
• Protected against low pressure jets of water from all directions - limited ingress.

IP stands for 

Ingress Protection Ratings

IP Third number - Indicates the Protection against mechanical impacts .

Its not used in normal cases. 

the third number is not available in IEC 60529

0	No protection.
1	Protects against impact of 0.225 joule (e.g. 150 g weight falling from 15 cm height).
2	Protected against impact of 0.375 joule (e.g. 250 g weight falling from 15 cm height).
3	Protected against impact of 0.5 joule (e.g. 250 g weight falling from 20 cm height).
4	Protected against impact of 2.0 joule (e.g. 500 g weight falling from 40 cm height).
5	Protected against impact of 6.0 joule (e.g. 1.5 kg weight falling from 40 cm height).
6	Protected against impact of 20.0 joule (e.g. 5 kg weight falling from 40 cm height).

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Electrical engineeringinterview

Job interview or viva Topics for Electrical Engineers (Part-6)

Electrical engineering interview question and answers:

Job interview or viva Topics for Electrical Engineers (Part-6)

21. Swing equation: 

During the transient state of the synchronous machine, the relation between the accelerating power and angular acceleration is called swing equation.

M (d²δ/dt²) = Ps – Pe

Here,

M = angular momentum of the rotor,

Ps = mechanical power input,

Pe = electrical power output,

δ= load angle,

t = time,

22. Swing Curve: 

The graph which shows the relation between δ versus t is called swing curve. These graphs provide the stability information. This swing curve information is useful for understanding the adequacy of relay protection in the power station. This information helps to solve the faults before one or more machines become unstable.

23. Unsymmetrical faults: 

The unbalanced conditions of the system are called unsymmetrical faults. If an unbalanced connection is attached at any point of the balanced system, then this point is called the fault point of the system. This unsymmetrical fault has normally two types:

1. Shunt fault. 

2. Series fault.

24. Shunt Fault:

An unbalanced shunt fault is an unbalanced condition between phases and ground or, an unbalance between phases.

25 Classification of shunt fault:

In a 3-phase system the shunt fault are as follows,

·        Single line to ground fault (LG).

·        Line to line fault (LL).

·        Double line to ground fault (LLG).

·        Three phase short Circuit fault (LLL).

·        Three phase to ground fault (LLLG).

Here,

·        LG, LL, LLG ---- unsymmetrical faults.

·        LLL, LLLG ---- symmetrical faults.

26. Series fault:

The unbalance in the line impedances is known as series fault.

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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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Job interview or viva Topics for Electrical Engineers (Part-5).

Electrical Engineering interview question and answers:Job interview or viva Topics for Electronics and Electrical Engineers (Part-5).

In this topic we will describe Transient Stability,Stability Limit,Steady State Stability Limit,Transient Stability Limit,Power-angle Equation.

 16. Transient Stability: 

In the system the ability of maintaining synchronous operation to reach a stable condition or state or the one close to it after a large disturbance is called Transient Stability.

For the Transient Stability there are some important assumptions. Such as,

·        System resistance(R) may be neglected in comparison with reactance(X).

·        The System may be reduced in form of two-machine equivalent system.

·        Each machine may be considered to supply an infinite bus.

·        Direct Axis Reactance= Quadrature Axis Reactance.

·        The rotor shape can be cylindrical.

·        For Machine representation Direct Axis Reactance can be used.

·        For few seconds after occurrence of disturbance, the input power of the shaft may be assumed constant.

17. Stability Limit: 

In the system, when the maximum amount of power can be transferred between source and loads in a network, without the loss of synchronism is called Stability Limit.

18. Steady State Stability Limit:

Under Steady state condition, when the load is gradually increased but there is no unstable condition in the system and the maximum power can transferred is called Steady State Stability Limit.

19. Transient Stability Limit: 

The amount of maximum power that can be transferred in the system without making the system unstable when a sudden or large disturbance occurs. This is called Transient Stability Limit.

20. Power-angle Equation: 

Here, Pe= Power, δ= load angle.

Pe=Pemax Sin δ

The maximum steady state power transferred, when δ = 90°.

The value of Pemax often called pull-out power or steady state limit.

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Job interview Topics for Electrical and electronic Engineers (Part-3).

Electrical engineering interview topic:

Job interview Topics for Electrical and electronic Engineers (Part-3).

8. Diversity Factor:  

The ratio of Sum of individual maximum demand and Maximum demand in power station is called Diversity Factor.

Diversity Factor= [Sum of individual maximum demand/ Maximum demand in power station]

• Sum of individual maximum demand <Maximum demand in power station.
• Diversity Factor> 1

9. Load Duration Curve: 

In the Load Duration Curve the load elements are shown with respect to time. When the Load elements are arranged in the descending magnitude order in the Curve, thus the curve is known as Load Duration Curve.

10. Plant Use Factor:

The ratio of Station Output in KWh with respect to Plant Capacity and hours of use is called Plant Use Factor.

Plant Use Factor= [Station Output in KWh/ Plant Capacity * hours of use]

11. Node Equation: 

In the electrical circuit when a junction is formed with two or more pure elements (The elements can be Resistive, Inductive, Capacitive or an ideal source of current or voltage) and are connected to each other at their terminals are called nodes. The process of calculating this nodes and check their working basis is known as node equation. In the Node Equation usually the major nodes are considered. In the nodes, where more then two pure elements are connected in a junction is known as major nodes.

In the Node Equation there are two types of admittance. They are Self Admittance and Mutual Admittance. Some times the Self Admittance is called Driving point & Mutual Admittance is called transfer admittance.

12. Plant Capacity Factor: 

The ratio of Actual Energy Produced and the Maximum Energy that could have been produced in the power station is called Plant Capacity Factor.

Plant Capacity Factor= [Actual Energy Produced/ Maximum Energy that could   have been produced in the power station]

        = [Average Demand/Plant Capacity]

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Job interview or viva Topics for Electrical Engineers (Part-7).

Electrical Engineering Interview Questions and answers:Job interview or viva Topics for Electrical Engineers (Part-7).

In this topics Fault analysis procedure is described.

27. The symmetrical designed generator only generates the voltage of positive phase sequence. Whether, the system is faulted or not there is only positive sequence voltage.

28. Fault analysis procedure: 

Basic fault analysis procedures are as follows,

• Circuit diagram.
• Boundary condition.
• Transformation.
• Sequence currents and sequence voltages.
• Interconnections of sequence networks.
• Information from sequence networks.

• Circuit diagram: 

If all the fault and phase connection are shows in the circuit diagram from that the fault analysis is possible.

• Boundary condition: 

The relation between the known phase current and Voltage at the fault point of the line is known as the boundary condition.

• Transformation: 

From the boundary condition we found symmetrical components of voltages and currents by solving the equation. This process is called Transformation from the a-b-c to 0-1-2 system. The relation for the transformation is,

V012=Aˉ¹Vabc

In the transformation step we found that, for single line to ground fault the sequence currents are equal.
I012=Aˉ¹Iabc

Ia0=Ia1=Ia2=(Ia/3)
Here,
Ia0 = zero sequence current.
Ia1= positive sequence current.
Ia2= negative sequence current.

• Sequence currents and sequence voltages: 

For determining the interconnection of sequence networks, the sequence currents and sequence voltages which is found in the transformation step is examined. The impedances which are found may be added in the sequence network.

• Interconnections of sequence networks:

 The sequence networks are interconnected in such a way by this the equation describing the fault conditions are satisfied. The interconnection shows the constraints impressed on the system by the fault.

• Information from sequence networks:

 From the known angular relation of the balance sheet the voltages of phases and currents are found sequentially.


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Electrical engineering interview

Fortescue’s Theorem

Fortescue’s Theorem

Fortescue’s Theorem mainly worked for the unbalanced System in Electrical Engineering. This theorem follows the zero phase sequence system.

This zero phase sequence indicates that in a 3-phase system all phasors are stated in identical angels & their magnitude are equal.

In a brief the Fortescue’s Theorem say’s that,

In a unbalanced set of unbalanced phasors the system may be resolved in (n-1) balanced system for different phase sequence & non zero phase sequence.

Fortescue’s give this theorem in 1918 & this system is mention as “Three component method”. This method is perfect for setting symmetrical components.

In this Fortescue’s Theorem, The direction of the rotation is always as anticlockwise .Whether it is “+” or “-”. The “+” indicates the original phase system .In other “-” signs oppose on the other hand. This system is voltage defined. The system can be defined by the help of arbitrary label  “A, B, C”.

When this is In “ABC ”indicates positive.

When this is In “ACB” indicates negative.

 The phasor can be defined by the α-operator. This operator is used for the phasors rotates in 120 degree.

The operator minus alpha does not produce rotation through -120 degree.

This theorem has some basic concept's .They are:

Positive Phase Sequence Components : When a set of phasors are equal in magnitude, displaced from each other by 120 degree in phase and they have the same phase sequence as the original unbalanced phasors.

Negative Phase Sequence Components : When a set of phasors are equal in magnitude, displaced from each other by 120 degree in phase and they have the opposite  phase sequence as the original phasors.

Identical Phase Sequence Components : When a set of three phasors are  equal in magnitude with zero displacement from each other then the components of this set are all identical.

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