Basic Question and Answers for Power system analysis

EE 1352 -POWER SYSTEM ANALYSIS QUESTION AND ANSWERS

1. What are the main divisions of power system?

The generating system, transmission system, and distribution system

2. What is the adv. Of interconnected power system?

1. Less no. of generators are required as are serving for operation at peak loads. Hence the reserve capacity of the generating station gets reduced.

2. Less no. of generators which are running without load are required for meeting the sudden unexpected increase in load.

3. It allows the use of most economical sources of power depending on time.

3. What are the problems of interconnection?

1.it increase the amount of current which flows when a short circuit occurs on a system and thereby requires the installation of breakers which are able to interrupt a larger current‘2. Synchronism must be maintained between of all the interconnected systems.

4. Define one line diagram.

A simplified diagram by omitting the completed circuit through the neutral and by indicating the components of the power system by standard symbols rather than by their equivalent

5. What is meant by impedance diagram?

The equivalent circuit of all the components of the power system are drawn and they are interconnected is called impedance diagram.

6. What is meant by reactance diagram?

Omitting all static loads, all resistance. The magnetizing components of each transformer and the capacitance of the transmission line are reduced from the impedance diagram is called reactance diagram.

7. Define per unit value.

Per unit of any quantity is defined as the ratio of the quantity ti its base value is expressed as a decimal.

8. Need for per unit value

1. The per unit impedance referred to either side of a single phase transformer is the same.

2. The chance of confusion b/n line and phase quantities in a three phase balanced system is greatly reduced.

3. The manufacturers usually produce the impedance values in per unit.

9. Define base current.

Ratio of base MVA to base KV

10. What is the need for short circuit study.

To determine the current interrupting capacity of the circuit breakers so that the faulted Equipments can be isolated. To establish the relay requirements and settings to detect the fault and cause the circuit breaker To operate when the current flowing through it exceeds the max. Value.

11. Define stability study.

Stability studies are performed in order to ensure that the system remains stable Following a severe fault or disturbance.

12. What are the elements of y bus matrix?

Short circuit driving point admittance. Short circuit transfer admittance.

13. What are the elements of Z bus matrix.

Open circuit driving point impedance, open circuit transfer impedances.14. What are the methods to determine the Ybus and Z bus matrices. Primitive n/w, n/w graph theory, incidence matrix

15. What is primitive n/w

Primitive network is a set of unconnected elements which provides information regarding the characteristics of individual elements only.

16. What is meant graph of a network.

Graph shows the geometrical interconnection of the elements of a n/w.

17. Define sub graph?

A sub graph is any subset of elements of a graph.

18 What is meant by path of a n/w?

Path is a sub graph of connected elements with not more than two elements Connected to any one side.

19. What is meant by connected oriented graph?

A graph is connected if and only if there is a path b/n every pairs of nodes. If each Element of the connected graph is assigned a direction it is called oriented graph.

20. What are the properties of a graph.

Tree is a sub graph connecting all the nodes of the oriented graph. Tree is a connected sub graph.

21. Define basic cutest?

Acutest is the minimum set of elements in the graph, which when removed, divide a Connected graph into two connected sub graph..

22. What are the quantities whose base values are required to represent the power system By reactance diagram.

The base values of voltage, current, power and impedance are required to represent the Power system by reactance diagram. Selection of base values for any two of them determines the base values of the remaining two.

23. What is the need for base values?

The components of power system may operate at different voltage and power levels. It will be convenient for analysis of power system if the voltage, power, current ratings of the components of the power system is expressed with reference to a common value Called base value.

24. What is impedance and reactance diagram?

The impedance diagram is the equivalent circuit of power system in which the various Components of power system are represented by their approximate equivalent circuits. The impedance diagram is used for load flow studies. The reactance diagram is the simplified equivalent circuit of the power system in which the various components are represented by their reactance. The reactance diagram can Be obtained from impedance diagram if all the resistive components are neglected.

25. What are the approximations made in impedance diagram?

The neutral reactances are neglected. The shunt branches in equivalent circuit of induction motor are neglected.

26. What are the approximations made in reactance diagram?

The neutral reactances are neglected. The resistances are neglected. All static loads and induction motors are neglected.

27. What is a bus?

The meeting point of various components in a power system is called a bus. The bus is a conductor made of copper having negligible resistance. The buses are considered as Points of constant voltage in a system.

28. What is bus admittance?

The matrix consisting of the self and mutual admittances of the network of a power System is called bus admittance matrix.

29. Name the diagonal and off diagonal elements of bus admittance matrix.

The diagonal elements of bus admittance matrix are called self admittances of the Buses and off diagonal elements are called mutual admittances of the buses.

30. What is bus impedance matrix?

The matrix consisting of driving point impedances and transfer impedances of the Network of a power system is called bus impedance matrix.

31. Name the diagonal elements and off diagonal elements of bus impedance matrix.

The diagonal elements of bus impedance matrix are called driving point impedances Of the buses and off diagonal elements of bus impedance matrix are called transfer Impedances of the buses.

32. What are the methods available for forming bus impedance matrix.

1. Form the bus impedance matrix and then take its inverse to get bus impedanceMatrix.

2. Directly form the bus impedance matrix from the reactance diagram. This Method utilizes the techniques of modifications of existing bus impedance Matrix due to addition of new bus.

32. Write the four ways of adding an impedance to an existing system so as to modify bus impedance matrix.

1. Adding a branch of impedances from a new bus p to the reference bus.2. Adding a branch of impedance Zb from a new bus p to an existing bus.3. Adding a branch of impedance Zb from an existing bus q to the referenceBus.4. Adding a branch of impedance Zb between two existing buses h and q.

33.What are symmetrical components?

An unbalanced system of N related vectors can be resolved into N systems of Balanced vectors. The N sets of balanced vectors are called symmetrical Components.

Types of transmission conductors

Remarkable changes have occurred in the utility industry since Thomas Edison began the commercial sale of electricity more than 100 years ago. One area that has undergone extensive change has been in the types of conductors available to transmit and distribute electricity.

Copper was the first metal used to transmit electricity during the development of the electrical industry in the early 1880's.

A review of the selection criteria for transmission and distribution conductors, prior to the extensive use of aluminum, suggests copper conductor sizes were being determined primarily on the basis of mechanical considerations because of the disproportional high conductivity of copper relative to its strength-to-weight ratio. Conductors were, therefore, generally larger than required from the standpoint of efficient electrical conductivity. Because of the weight, span lengths were short, thus increasing the overall cost of the transmission line.

Shortly before the turn of the century, aluminum began to replace copper as the metal of choice for transmission and distribution conductors. The first transmission line using aluminum conductors was constructed in California in 1895, quickly followed by a second line in 1898.

The first transmission line using a stranded (7-strand) aluminum cable was constructed by the Connecticut Electric Light Company in 1899 and remained in daily operation for more than 50 years. Starting with these early installations, the use of aluminum electrical conductors has increased steadily until it is the material of choice by transmission line design engineers today. For more than 90 years aluminum has been used by electric utilities for the transmission and distribution of electrical power. Although it’s almost completely replacing copper for overhead applications. Of all the known non precious metals, aluminum ranks second only to copper in volume conductivity. Aluminum possesses a conductivity-to-weight ratio twice that of copper and its strength-to-weight ratio is 30% greater than copper.

When aluminum conductor came into relatively wide use in the early 1900's, experience indicated the need for a conductor with a greater strength-to-weight ratio. Thus, in 1907 a new aluminum-steel composite cable was introduced. This new conductor combined the light weight and high current carrying capacity of aluminum with the high strength of a galvanized steel core. ACSR, as this aluminum conductor, steel reinforced, cable became known, gained rapid acceptance and was used almost exclusively throughout the world until 1939.

The excellent conductivity of ACSR, coupled with its excellent strength-to-weight ratio and ease of handling made it the dominant conductor for rural electrification in the United States that began during the early 1920's.

In 1939 a new all aluminum-magnesium-silicon alloy cable was introduced. The new all-aluminum alloy cable (AAAC) was developed to retain the mechanical and electrical properties of ACSR while improving weight and corrosion resistance characteristics. The introduction of the all-aluminum alloy cable and the subsequent development of the composite aluminum conductor, aluminum-alloy reinforced cable provided new alternatives to ACSR. As with most new products, particularly in applications as critical as electrical transmission and distribution, acceptance of the new alloy conductor was slow. In recent years, however, the recognized electrical improvements of alloy conductors over ACSR has led to an increasing trend of usage in aluminum alloy and composite aluminum-aluminum alloy cables.

More recently, many innovative conductor designs have been developed to address the changing needs of the electrical utility industry. New alloys have been developed to provide thermal stability, increased conductivity, vibration resistance and other specific characteristics. With each change there is a compromise. With each compromise there is a new design opportunity.

Conductor design and/or selection for transmission and distribution lines has become a science. The selection of the optimum conductor type and size for a given transmission or distribution line design requires a complete understanding of the characteristics of all the available conductor types. This understanding must encompass more than just the current carrying capability or thermal performance of a conductor. It must include a systems approach to conductor selection: line stability versus current loading; economic operation versus thermal loading; conductor creep and resultant sag under high temperature and adverse mechanical loading; conductor strength as determined by component metal stress-strain performance and metal fatigue characteristics are just a few of the system design parameters to be evaluated.

Types of Conductors:
There is no unique process by which all transmission and/or distribution lines are designed. It is clear, however, that all major cost components of line design depend upon the conductor electrical and mechanical parameters.
There are four major types of overhead conductors used for electrical transmission and distribution.

• AAC - All Aluminum Conductor
• AAAC - All Aluminum Alloy Conductor
• ACSR - Aluminum Conductor Steel Reinforced
• ACAR - Aluminum Conductor Aluminum-Alloy Reinforced

The various combinations and modifications of these conductor types provide a wide variety of possible conductor designs.

AAC - All Aluminum Conductor, sometimes referred to as ASC, Aluminum Stranded Conductor, is made up of one or more strands of 1350 Alloy Aluminum in the hard drawn H19 temper. 1350 Aluminum Alloy, previously known as EC grade or electrical conductor grade aluminum, has a minimum conductivity of 61.2% IACS. Because of its relatively poor strength-to-weight ratio, AAC has had limited use in transmission lines and rural distribution because of the long spans utilized. However, AAC has seen extensive use in urban areas where spans are usually short but high conductivity is required. The excellent corrosion resistance of aluminum has made AAC a conductor of choice in coastal areas.

ACSR - Aluminum Conductor Steel Reinforced, a standard of the electrical utility industry since the early 1900's, consists of a solid or stranded steel core surrounded by one or more layers of strands of 1350 aluminum.

The inner-core wires of ACSR may be of zinc coated (galvanized) steel, available in standard weight Class A coating or heavier coatings of Class B or Class C. Class B coatings are about twice the thickness of Class A, and Class C coatings about three times as thick as Class A. The inner cores may also be of aluminum coated (aluminized) steel or aluminum clad steel.

Conclusion:
The selection of the optimum conductor type and size for a given line consists of finding that conductor which results in the lowest present net worth cost spread over the life of the line. The transmission line design engineer is confronted with choosing a conductor type from among this bewildering assortment. This choice must be based on basic conductor parameters.
It is clear that all the major cost components of a transmission line depend upon conductor physical, mechanical and electrical parameters. A list of these basic parameters are:

• conductor diameter
• weight per unit length
• conductivity of material(s)
• crossectional area(s)
• modulus of elasticity
• rated breaking strength
• coefficient(s) of thermal expansion
• cost of material(s)
• maximum unloaded design tension
• resistance to vibration and/or galloping
• surface shape/drag coefficient
• fatigue resistance

These basic parameters are not necessarily independent of one another. However, certain parameters can be varied independently over a range of design considerations.

It is the hope of this writer that a better understanding of available conductor types and materials will provide a better base for future conductor selections.

REFERENCES:
1. Douglass, Dale A., Economic Measures of Bare Overhead Conductor Characteristics, IEEE Paper 86 TD 502-9 PWRD.
2. Kennon, Richard E., Douglass, Dale A., EHV Transmission Line Design Opportunities for Cost Reduction, IEEE Paper 89 TD 434-2 PWRD.