OHM'S LAW IN AC CIRCUITS

Many ac circuits contain resistance only. The rules for these circuits are the same rules that apply to dc circuits. Resistors, lamps, and heating elements are examples of resistive elements. When an ac circuit contains only resistance, Ohm's Law, Kirchhoff's Law, and the various rules that apply to voltage, current, and power in a dc circuit also apply to the ac circuit. The Ohm's Law formula for an ac circuit can be stated as

Remember, unless otherwise stated, all ac voltage and current values are given as effective values. The formula for Ohm's Law can also be stated as

The important thing to keep in mind is: Do Not mix ac values. When you solve for effective values, all values you use in the formula must be effective values. Similarly, when you solve for average values, all values you use must be average values. This point should be clearer after you work the following problem: A series circuit consists of two resistors (R1 = 5 ohms and R2 = 15 ohms) and an alternating voltage source of 120 volts. What is Iavg?

The alternating voltage is assumed to be an effective value (since it is not specified to be otherwise). Apply the Ohm's Law formula

The problem, however, asked for the average value of current (I avg). To convert the effective value of current to the average value of current, you must first determine the peak or maximum value of current, Imax.

HVDC Transmission

INTRODUCTION

Electric power transmission was originally developed with direct current. The availability of transformers and the development and improvement of induction motors at the beginning of the 20th Century, led to greater appeal and use of a.c. transmission. Through research and development in Sweden at Allmana Svenska Electriska Aktiebolaget (ASEA), an improved multi-electrode grid controlled mercury arc valve for high powers and voltages was developed from 1929. Experimental plants were set up in the 1930’s in Sweden and the USA to investigate the use of mercury arc valves in conversion processes for transmission and frequency changing.

D.c. transmission now became practical when long distances were to be covered or where cables were required. The increase in need for electricity after the Second World War
stimulated research, particularly in Sweden and in Russia. In 1950, a 116 km experimental transmission line was commissioned from Moscow to Kasira at 200 kV. The first commercial HVDC line built in 1954 was a 98 km submarine cable with ground return between the island of Gotland and the Swedish mainland.

Thyristors were applied to d.c. transmission in the late 1960’s and solid state valves became a reality. In 1969, a contract for the Eel River d.c. link in Canada was awarded as the first application of sold state valves for HVDC transmission. Today, the highest functional d.c. voltage for d.c. transmission is +/- 600 kV for the 785 km transmission line of the Itaipu scheme in Brazil. D.c. transmission is now an integral part of the delivery of electricity in many countries throughout the world.

WHY USE DC TRANSMISSION

The question is often asked, “Why use d.c. transmission?” One response is that losses arelower, but this is not correct. The level of losses is designed into a transmission system
and is regulated by the size of conductor selected. D.c. and a.c. conductors, either as overhead transmission lines or submarine cables can have lower losses but at higher expense since the larger cross-sectional area will generally result in lower losses but cost more. When converters are used for d.c. transmission in preference to a.c. transmission, it is generally by economic choice driven by one of the following reasons:

1. An overhead d.c. transmission line with its towers can be designed to be less costly per unit of length than an equivalent a.c. line designed to transmit the same level of electric power. However the d.c. converter stations at each end are more costly than the terminating stations of an a.c. line and so there is a breakeven distance above which the total cost of d.c.transmission is less than its a.c. transmission alternative.The d.c. transmission line can have a lower visual profile than an equivalent a.c. line and so contributes to a lower environmental impact. There are other environmental advantages to a d.c. transmission line through the electric and magnetic fields being d.c. instead of ac.

2. If transmission is by submarine or underground cable, the breakeven distance is much less than overhead transmission. It is not practical to consider a.c. cable systems exceeding 50 km but d.c. cable transmission systems are in service whose length is in the hundreds of kilometers and even distances of 600 km or greater have been considered feasible.

3. Some a.c. electric power systems are not synchronized to neighboring networks even though their physical distances between them is quite small. This occurs in Japan where half the country is a 60 hz network and the other is a 50 hz system. It is physically impossible to connect the two together by direct a.c. methods in order to exchange electric power between them. However, if a d.c. converter station is located in each system with an interconnecting d.c. link between them, it is possible to transfer the required power flow even though the a.c. systems so connected remain asynchronous.

Thermal Power Plant

What are the main circuits in the Thermal Power Plant?

Answer:Thermal Power plant consists of four main circuits, they are:

·                     Feed water and steam flow circuit

·                     Coal and ash circuit

·                     Air and gas circuit

·                     Cooling water circuit

Steam power plant works on which cycle?

Answer: Steam power plant works on the principle of Rankine Cycle

Why Thermal Power Plants are always situated by the side of rivers or lakes?

Answer: Thermal Power Plants are situated close to the rivers or lakes because to meet the large quantity of the water requirement. Water is required in the steam power plants

·                     steam in the boiler

·                     for cooling purposes such as in condensers

·                     as carrying medium such as in disposal of ash

·                     for drinking purpose

What is the purpose of deaerator in steam power plant?

Answer: The presence of dissolved gases like oxygen and carbon dioxide in water makes the water corrosive, as they react with metal and forms iron oxide. The solubility of the gases in the water decreases with increase in the temperature and becomes zero at boiling point or saturation point.The purpose of the deaerator is to reduce the dissolved oxygen content in the condensate ( feed water) by increasing its temperature. These gases are removed in the deaerator  by heating the feed water to saturation temperature through the bled steam from the turbine.

Why cooling towers require in the Steam power plant ?

Answer: Cooling towers cool the warm water discharged from the condenser and feed the cooled water back to the condenser. There are two types of cooling methods provided in the cooling towers, they can be either wet type or dry type

Wet type Cooling towers: In wet cooling towers warm water from the condenser are made to spray on slats or horizontal bars and air is passed from bottom, as the water splashes down from one slat to other due to gravity air and water mixes and heat is rejected in to the air

Dry type Cooling tower:

 Dry type towers are employed where the cooling water is not available in plenty. In dry type warm water from condensate is made to flow through the finned tubes over which cooling air is passed. Heat is rejected to air as water is cooled

Why are feed water heaters are used?

Answer:Regenerative Feedwater heaters are employed in steam power plants to improve the cycle efficiency. Also it increases the steam flow rate and reduces the steam flow to the condenser. They raise the temperature of the feed water before it enters the economizer

What is the Thermal efficiency of steam power plant?

Answer: Thermal efficiency of steam power plant is defined as the ratio of heat equivalent of mechanical energy transmitted to the turbine shaft to the heat of combustion. Generally Thermal efficiency of the steam power plant will be in the range of 30-35%

What is the overall efficiency of the Thermal Power Plant or Steam Power Plant?

Answer: Overall efficiency of the system is defined as the ratio of heat equivalent of electrical output to the heat of combustion. Generally Overall efficiency of the steam plant will always be less than the thermal efficiency of the steam plant, it will be of the order of 29-33%

Overall efficiency of steam plant is determined by multiplying the thermal efficiency of the plant with efficiency of the generator (electrical efficiency)

Why the Thermal efficiency of the steam power plant is quite low?

Answer: In Steam power station, more than 50% of the total heat of combustion is lost as heat rejected to the condenser and the loss is unavoidable as the heat energy cannot be converted in to mechanical energy with out a drop in temperature. Steam in the condenser is at lowest temperature. This is the reason that the thermal efficiency of the power plant is quite low.

On what factors efficiency (thermal) of the steam plant depends?

Answer: Efficiency of the thermal plant depends on three factors, they are

1.             pressure of steam entering the turbine

2.             temperature of the steam entering the turbine

3.             pressure in the condenser

Thermal efficiency increases with increase in temperature and pressure of the steam entering the turbine. For this reason high temperature and pressure are used. Thermal efficiency is effectively increased by decreasing the pressure in the condenser, so pressure in the condenser is kept as low as possible.Thermal efficiency also increases by reheating the steam between turbine stages

Indian Engineering Services (IES) Objective Questions and Answers:

1) A self excited d.c shunt generator, driven by its prime mover at the rated speed fails to build up the voltage across its terminals at no load. What reason can be assigned for this? (IES 2006) 

a) The initial shunt field mmf does not assist the residual magnetism

b) The field circuit resistance is higher than the critical resistance

c) One of the inter-pole connection is removed

d) Brush axis slightly shift from the geometrical neutral axis of the machine

2) Wave winding is employed in a dc machine of: (IES 2006)

a) High current and low voltage rating

b) Low current and high voltage rating

c) High current and high voltage rating

d) Low current and low voltage rating

3) The resultant flux density in the air gap of a synchronous generator is the lowest during: (IES 2006)

a) Open circuit b) Solid short circuit c) Full load d) Half load

4) If the load of an Induction motor is increased from no load to full load, its slip and the power factor will respectively (IES 2006)

a) decrease, decrease

b) decrease, increase

c) increase, decrease

d) increase, increase

5) A single phase Induction motor is running at N rpm. Its synchronous speed is Ns. If its slip with respect to forward field is 's', what is the slip with respect to the backward field is: (IES 2006)

a) s  b) –s c) (1-s)  d) (2-s)

6)  Match the following: (IES 2003)

List I                                                List II 

A. DC Motor                        1. Circle Diagram

B. DC Generator                  2. V-Curves

C. Alternator                        3. Open circuit characteristics

D. Induction Motor                4.Speed-Torque characteristics

Codes:

         A          B          C         D

a)      4           3          1         2

b)      3           4          2         1

c)      4           3          2         1

d)      3           4          1         2

7)  A smaller air gap in a polyphase induction motor helps to: (IES 2004)

a) reduces the chances of crawling

b) Increases the starting torque

c) reduces the chances of cogging

d) reduce the magnetising current

8) If the supply voltage of the induction motor is reduced by 10%. By what percentage approximately will the maximum torque decreases? (IES 2004)

a) 5% b) 10% c) 20% d) 40% 

 Answers:

(1) b (2) b (3) b (4) d(5) d (6) c (7) d (8) c

 1) A self excited d.c shunt generator, driven by its prime mover at the rated speed fails to build up the voltage across its terminals at no load. What reason can be assigned for this? (IES 2006) 

a) The initial shunt field mmf does not assist the residual magnetism

b) The field circuit resistance is higher than the critical resistance

c) One of the inter-pole connection is removed

d) Brush axis slightly shift from the geometrical neutral axis of the machine

2) Wave winding is employed in a dc machine of: (IES 2006)

a) High current and low voltage rating

b) Low current and high voltage rating

c) High current and high voltage rating

d) Low current and low voltage rating

3) The resultant flux density in the air gap of a synchronous generator is the lowest during: (IES 2006)

a) Open circuit b) Solid short circuit c) Full load d) Half load

4) If the load of an Induction motor is increased from no load to full load, its slip and the power factor will respectively (IES 2006)

a) decrease, decrease

b) decrease, increase

c) increase, decrease

d) increase, increase

5) A single phase Induction motor is running at N rpm. Its synchronous speed is Ns. If its slip with respect to forward field is 's', what is the slip with respect to the backward field is: (IES 2006)

a) s  b) –s c) (1-s)  d) (2-s)

6)  Match the following: (IES 2003)

List I                                                List II 

A. DC Motor                        1. Circle Diagram

B. DC Generator                  2. V-Curves

C. Alternator                        3. Open circuit characteristics

D. Induction Motor                4.Speed-Torque characteristics

Codes:

         A          B          C         D

a)      4           3          1         2

b)      3           4          2         1

c)      4           3          2         1

d)      3           4          1         2

7)  A smaller air gap in a polyphase induction motor helps to: (IES 2004)

a) reduces the chances of crawling

b) Increases the starting torque

c) reduces the chances of cogging

d) reduce the magnetising current

8) If the supply voltage of the induction motor is reduced by 10%. By what percentage approximately will the maximum torque decreases? (IES 2004)

a) 5% b) 10% c) 20% d) 40% 

 Answers:

(1) b (2) b (3) b (4) d(5) d (6) c (7) d (8) c

Transformer

Why Low Voltage (LV) winding is placed near to the transformer core?

Anwer: For a given conducting material insulation required depends on the voltage. Hence if High Voltage (HV) winding is placed near to the transformer core, more insulation is required to insulate between the transformer core and the High Voltage (HV) winding. This results in increase in the cost of the insulation material and also size of the transformer increases significantly. Thus Low Voltage (LV) winding is placed near the core which requires less insulation between the core and LV winding.

What are the different insulation materials used in power transformers?

Answer: In power transformers the primary insulation medium and cooling medium is transformer oil. It serves the purpose of both insulation and cooling. Apart from transformer oil different insulation materials employed are oil impregnated paper, press board, wood, mica, and asbestos.

What is Transformer Breathing?

Answer: When transformer under full load, power transformer oil present in the transformer heats up and gets expand. During this process gas at the top of the oil gets expel out in to the conservator present at the top of the power transformer along with hot oil and cool oil from conservator comes down. This process is called breathing out of the transformer.

When the load on the power transformer is removed or during no load condition, transformer oil cools and air is drawn in to the transformer. This is called breathing in of the transformer. The incoming air may consists of moisture which should be removed else results in the deterioration of the dielectric strength of the transformer oil. Hence the air entering the power transformer is made to pass through the breather where moisture is removed from air through silica gel

Transformer Testing methods

Transformer Tests:

Tests on the transformers are carried for verifying the capabilities of transformer to withstand Thermal stresses, Dielectric stresses, short circuit electro-dynamic stresses and environmental stresses.

Routine tests:

These are carried out on every transformers before dispatch to ensure that it is in accordance with the specifications.Some of the tests come under routine test include:

·                     Measurement of the winding resistance

·                     Verifying the polarity of the windings

·                     Measurement of load losses and the impedance voltages

·                     Measurement of no load losses and no load no load current.

·                     Electrical tests at the power frequency

Type Tests:

Type tests are performed on the first transformer of one type and are intended to check the design characteristics. It is presumed that every transformer would also comply with the type test, since its design is identical.

·                     Temperature rise test

·                     Tests of ability to withstand full wave impulse

·                     Tests for switching impulse withstand

·                     Noise level test

Special Test:

Special tests are conducted in the presence of the purchaser or his representatives as specified in the tender

·                     Partial discharge test

·                     Checking the level of Radio interference voltage

·                     Vibration test

·                     Test on ability to withstand Short -circuit current

·                     Measurement of Noise level

Voltage Regulation of Transformer

Voltage Regulation:

When the secondary of the transformer is loaded (from no load to full load), the secondary voltage changes even though primary supply voltage is held constant at rated value.

% Voltage Regulation = (V_{2(no load)} - V_{2 (full load)})*100 / (V_{2(no load)})

With the rated voltage applied to the primary winding, the secondary  terminal voltage varies with the load current and power factor even the primary voltage is kept constant. This variation is called voltage regulation.

Voltage regulation depends on the voltage drop in the impedance of the transformer, load current and load power factor.

Leading and Lagging power factor regulation:

·                     For lagging power factor load the secondary voltage decreases with increase in the load current (when the transformer is loaded). Thus for lagging power factor loads, the regulation is positive ( voltage drop observed as the load current increases). 

·                     For leading power factor load, the secondary voltage increases slightly with increase in the load current. This type regulation. Thus for leading power factor loads, the regulation is negative (raise in voltage as load current increases) 

For better performance of the system voltage regulation should be as minimum as  possible i.e,  Change in secondary voltage with load should be as minimum as possible when primary voltage is kept constant.