﻿﻿﻿﻿ November 2016 – APSEEE

## Power Factor

### Concept of Power Factor

#### Most of loads are inductive loads (such as induction motors). The current taken by inductive motors or inductive circuit consists of two components. One is called magnetizing component active components transformed energy into useful work and magnetizing component which is often termed as idle components. Magnetizing component is called wattles component. Active component being in phase with the voltage accounts for the useful work done and magnetizing component being in quadrature with the voltage does not do any work in circuit and is mainly responsible for the creation of magnetism. More magnetizing current causing low power factor.

What is Power Factor?

### DISADVANTAGES OF LOW POWER FACTOR

#### Low power factor has following disadvantages.

• Large KVA rating of equipments

#### The electrical machinery is always rated in KVA. The KVA rating of the machine is inversely proportional to the power factor. If the power factor of the load is low the drawn by the machine will be high and vice versa and large KVA rating of the machine is required.

• Greater conductor size

#### The large current at low power factor causes greater voltage drops in electrical machine (such as alternators, transformer etc.) Which drop the voltage at the supply end which results in poor voltage regulation. In order to keep the supply end voltage with in permissible limits, extra equipment is required.

To Avoid these limitations power factor correction is necessary.

## Rectifiers

Rectifiers

We know that electrical energy is generated at power plants in the form of ac due to many reasons such as ac system is economical but for the operation electronic devices or circuits, dc supply is required cells and batteries cannot meet the demands. So there is conversion of ac into dc is necessary. In Article we will study about, Half Wave Rectifiers and Full wave rectifiers.

What is Rectifier?

Rectifier is a electronic device which converts alternating current (ac) into direct current (dc).

According to their operation rectifiers divided into two types

• Half wave rectifier
• Full wave rectifier

Full wave rectifier further divided into two types

• Centre-tap full wave rectifier
• Full wave bridge rectifier

Half wave rectifiers

In half wave rectifier only one half of the supply of the alternating current is conducts. At output we have only one half cycle the other half of the supply is skipped.

Construction of Half wave rectifier

In half wave rectifier a semiconductor diode is connected in series with load RL. A transformer is used. Basically step down transformer is used. Secondary side diode and load is connected and Primary side of transformer is connected to the ac source.

Operation or working

When primary side of the transformer connected to ac source of induced in secondary side.

• During positive half cycle of the supply end B become negative. At this time diode D become forward biased and starts conduct and current start flowing through the load RL.

• Path of current during positive half cycle is A – Load – B.
• During negative half cycle of the supply end A of the transformer become negative and end B of the transformer become positive. At this time, the diode D become reverse biased and it does not conduct current and diode remains always in non-conducting state.

Output wave form

Centre-tap full wave rectifier

In full wave rectifier both half cycle of the alternating supply conducts.

Construction

In such type of rectifier two diodes are used. These diodes are D1 and D2 a centre tapped transformer is used. The secondary of the transformer is centre tapped. The load RL is connected in such a way that it carry the current in one direction.

Operation or working

The ac supply is connected to the transformer. During positive half of the supply end A of the secondary winding become positive and end B become negative. At this time D1 become forward biased and diode D2 become reverse biased. The diode D1 conduct while diode D2 does not. The current through D1 and load RL. During negative half of the supply end A of the secondary winding become negative and end B of the secondary winding become positive. At this time, diode D1 become reverse biased and diode D2 become forward biased. Now diode D2 conducts while diode D1 does not. The start flowing through diode D2 and load RL. The direction of flow of current is same, so the output is unidirectional. In this rectifier when one diode is conducts the other diode remains off condition means does not conduct.

Output wave form

Full wave bridge rectifier

In this rectifier four diodes are used. This type of circuit also conduct both half cycle of the alternating current.

Construction of full wave bridge rectifier

In such type of circuits the four diode (D1 D2 D3 and D4) connected in such a way the two diodes are conduct during positive half cycle of the supply and other two diodes are conduct negative half cycle of the supply. In this rectifier ordinary two winding transformer is used.

Operation or working

Ac supply is connected to the primary side of the transformer. By mutual induction e.m.f is induced in secondary winding of the transformer.

• During positive half cycle of the ac supply end terminal A of the secondary winding become positive and  B become negative. At this time, diode D1 and D3 conduct while diode D2 and D4 does not. The current will flow through D1, load and D2. The current flows in one direction in same direction as the positive half cycle of the supply.
• During negative half cycle of the ac supply terminal B of the secondary winding become positive and  A become negative. At this time, diode D2 and D4 conduct while diode D1 and D3 does not. The current will flow through D2, load and D4. The current flows in one direction in same direction as the positive half cycle of the supply.

Output wave form

## LINE SUPPORTS

Line supports are used to support the conductors of transmission lines and distribution lines. Line support must be capable of carrying the load due to insulators and conductors. The line supports may be wood, steel and reinforced concrete poles and steel towers.

The lines support should have the following properties.

• The line supports should be high mechanical strength to withstand the weight of the conductors and wind load etc.

• The line supports should be light in weight without the loss of mechanical strength.
• The line supports should be cheap in cost.
• The line supports should be longer life.
• The maintenance cost of line support should be low.

## WOODEN POLES

Wooden poles are cheapest in cost, easily available, provide insulating properties. These are made of seasoned wood (sal or chir). The use of wooden poles are limited to low voltage upto (22kv) and suitable for lines of relatively shorter spans upto 50 meters. These are extensively used for the distribution purposes in rural area. The cost of wooden poles is low. To present decay owing to snow and rain, the wooden poles are protected by Aluminium, zinc or cement cap at the top. The wooden poles usually tend to rot below the ground level, causing foundation failure. In order to prevent this, the portion of poles below the ground level treated with creosote oil or any preservative compound. The wooden poles well impregnated with creosote oil or any preservative have life from 25 top 30 years. Double pole structures of ‘A’ or ‘H’ type are often used to obtain a higher transverse strength than that could be economically provided by poles in normally is 10m to 12m.

The disadvantages of wooden poles are given below

• The life of wooden poles is comparatively smaller than other poles.
• The wooden poles cannot be used above the voltage rating higher than 20kv.
• The mechanical strength of wooden is less.
• Wooden poles are required periodic maintenance.

## STEEL POLES

The steel poles are generally used in place of wooden poles. The steel poles has longer life, permit longer span and it possess greater mechanical strength. The span between two poles is 50 to 80 metre in case of steel poles. The cost of steel poles is higher than the cost of wooden poles. Steel poles are generally used for distribution purposes in cities. The life of steel poles is increased by regular painting. The steel poles are three types (i) tabular poles (ii) rail poles and (iii) rolled steel joist. The tabular poles rolled steel joist. The tabular poles are of round cross-section, the rail poles are of the shape of track and rolled steel joist are I cross-section. The rail poles are used in railways for electrification purposes. The average life of steel poles is more than 40 years.

• The weight of steel poles is lighter than wooden poles.
• Steel poles are easy to install.
• It does not require special equipment for its erection.
• Steel poles gives good appearance.

## RCC POLES

These poles are made of reinforced cement concrete and usually it is known as concrete poles. Concrete poles are extensively used for low voltage and high voltage distribution lines upto 33kv. In recent years RCC poles becomes most popular as a line supports. RCC poles have greater mechanical strength, longer life and permit longer span than steel poles. RCC poles give good appearance, require very little maintenance, have good insulating properties than RCC poles and resistance against chemical actions. RCC poles can be used for longer span between 80m to 200m. Such poles are most suitable for work logged situations where other types of poles like wooden poles and steel poles will not be at all suitable. The disadvantage of these poles, is the high cost of transportation because these poles are very bulky and heavy. Therefore, RCC poles are manufactured at site itself to avoid heavy cost of transportation.

## STEEL TOWERS

For long distance transmission lines at higher voltage steel towers are used. Steel tower have greater mechanical strength. Due to this property steel tower can permit the used of longer span. Steel tower can withstand most severe climatic conditions steel tower have longer life comparatively wooden poles and concrete poles. Steel towers also known as lattice steel tower. There are two types of lattice steel towers. (i) Narrow base lattice steel towers and (ii) broad base lattice steel towers are used for transmission at 11kv and 33kv and broad base lattice steel towers are used for transmission purposes at 66kv and above. The cost of steel tower very high but for longer spans steel towers is more economical. Lightning troubles is minimized by using lightning conductors at the top of the steel towers. Tower footings are usually grounded by driving rods into the earth. For protection against periodically painted. However, at a moderate addition cost, double circuit steel towers can be used because the double circuit steel towers give the insurance against discontinuity of supply. In the event of breakdown to one circuit it is possible to carry out repairs while maintaining the continuity of supply on the other circuit.

Single Circuit Tower

## Fuse and Types of Fuses

### INTRODUCTION

Electrical Fuse is simple and cheapest device which are used for interrupting an electrical circuit under abnormal conditions (such as short circuit conditions, overload conditions etc). Fuses are used upto 66kv called high voltage fuse and low voltage fuse upto 400V. In this article, we discuss about definition of fuse, working principle and fuse, fuse element materials and types of fuse.

### Definition of Fuse

What is fuse?

A fuse is small piece of wire, inserted in the circuit which melts when the value of current plows more than the permissible limit through it and thus break the circuit. Fuse is always connected in series with the circuit.

The fuse element is usually made of materials having low melting point, high conductivity etc. The action of a fuse is based on the heating effect of the electric current when flows through a fuse element. When the current flows through a fuse is in safe value, the heat developed in fuse element rapidly dissipated into the surrounding air and therefore fuse element remains at a temperature below its melting point. However when fault occurs in the electric circuit, the current exceeds the safe value or limiting value. The heat is generated due to this excessive current cannot be dissipated fast enough and fusible element gets heated, melts and breaks the circuit. If the value of current is large enough, the more rapidly the fuse will blow i.e. the fuse has inverse time current characteristics.

It is cheapest and simple form of protection.

It needs no maintenance.

It interrupts very large short circuit current without any noise, flame gas or smoke.

It operation is inherently completely. It means there is no required any more equipment for automatic action.

Its inverse time current characteristics enable. Its use for over load protection.

The fuse has following disadvantages: –

Considerable time is waste n rewiring or replacing a fuse after operation.

The current time characteristic of a fuse cannot always be correlated with that of the protected apparatus.

### DESIRABLE CHARACTERISTICS OF FUSE ELEMENT

The fuse elements should have low melting point like tin and lead.

It should have high conductivity like silver and copper.

It should be free from deterioration due to oxidation. Silver material has such properties.

The fuse element should be cheap in cost like lead and tin.

### FUSE ELEMENT MATERIALS

The materials used for fuse elements must be of low melting point, low ohmic loss, high conductivity, low cost and free from deterioration. A low melting point is available with a high specific resistance metal. The most commonly used materials for fuse element are lead, tin, copper, zinc and silver. For smaller value of currents tin or an alloy of lead and tin (lead 37%, tin 63%) is used for making the fuse element. For exceeding the current 15A this alloy is not used as the diameter of the wire will be lager so that beyond 15A rating circuits Copper wire fuses are employed. Zinc fuse is good if a fuse with considerable time-lag is required i.e. One which does not melt very quickly with a small overload.

The present trend is to use silver despite. Its high lost due to the following advantages.

It does not oxidized. It means it is free from oxidation.

It does not deteriorate when used in dry air. There is no effect of moist on the silver fuse element, when the silver surface is attacked and a layer of silver sulfide is formed at the surface of a fuse element which shield the silver fuse element from further attack.

The conductivity of silver fuse element is very high. Therefore, for a given rating of fuse element, the area of cross section of silver metal is required is smaller than that of other materials.

The conductivity of silver does not deteriorate with oxidation so that life of silver fuse element is long.

Owing to its high conductivity the mass of molten fuse element to be handled is minimum due to this property the operating speed is fast.

### IMPORTANT TERMS OR DEFINITIONS

CURRENT RATING

It is the r.m.s value of current which the fuse element can carry without overheating. It depends upon the temperature rise of contacts of the fuse holder, fuse material and the surrounding area of the fuse.

FUSING CURRENT

It is defined as the minimum value of current at which the fuse element melts. It means the value of fusing current is always more than the value of rated current of the fuse element.

Approximated value of fusing current is given by

I = kd3/2

I = current

K is constant

d is diameter of the fuse element

The fusing current is depends upon the following factors: –

The fusing current the fuse element depend upon the type of materials used for making fuse element.

Cross-section of area of the fuse element whether it is round or rectangular.

Length the shorter length of the fuse element the grater the current can conduct heat easily.

Diameter of fuse wire.

Type of enclosure used.

FUSING FACTOR

It is the ratio of minimum fusing current to the current rating of the fusing element is called fusing factor. It is always greater than unity.

For a semi-enclosed or rewirable fuse which employs copper wire as the fuse element, fusing factor is equal to 2.0 and for cartridge fuses fusing factor is equal to 1.45.

BREAKING CAPACITY

Breaking capacity of a fuse is the r.m.s value of the AC component of the maximum prospective current with at rated system voltage.

PRE-ARCING TIME

When a fault occurs, the fault current rises sharply and heat is generated in the fuse element. At the fault current attains it cut-off value the fuse melts and an arc is initiated.

ARCING TIME

This is the time between the instant of arc initiation and the instant of arc being extinguished.

TOTAL OPERATING TIME

The sum of Pre-arcing time and arcing time is called total operating time.

### TYPES OF FUSES

A fuse unit usually consists of a metal fuse element, a set of contacts between which it is fixed and a body to support the fuse element and isolate them.

How does a Fuse works?

Fuse is the simplest current interrupting device for protection of the electrical circuit against short circuit or overloads which results excessive current flow through the electrical circuit. Many types of fuses also incorporate means for extinguishing the arc that appears when the fuse element melts.

Different Fuse Types are given below

Low voltages fuses

High voltages

### LOW VOLTAGE FUSES

Low voltage fuses are further divided into three types namely semi-closed or rewirable type and cartridge type.

REWIRABLE FUSES

Rewirable fuse is most commonly used fuse in house wiring and in small current circuits. It is simplest form of protection available. Semi-enclosed or Rewirable fuse is also sometimes called kit-kat type fuse. It consist a base and a fuse carrier. The fuse element is placed in fuse holder. Both are made of porcelain material.

When fault occurs, the value of current flowing through fuse element is increased which generated heat in the fuse element and fuse element melts.

CARTRIDGE FUSE

In case of cartridge fuse the fuse element is totally enclosed in a enclosed container and is provided with metal contacts on both sides. The cartridge fuse divided into two types (i) D-type and (ii) Link type.

## SUBSTATIONS

INTRODUCTION

The electrical power is generated at generating station (such as hydro electric power plant, thermal power plant and nuclear power pant) which is located far away from the load centre. Electrical power is transmitted through transmission lines and distributed through distribution lines. For economical reasons, voltage is transmitted at high voltage and distributed at low voltage 400/230 volts. These voltage transformations are carried out at substation, located at suitable places. Thus, it form the most important part of power system. In this article, we will study about the its various types and  various function.

SUBSTATION

It is an assembly of various apparatus (such as, circuit breaker, protective relays etc) which are installed to control transmission and distribution of the electric power.

The various function are given below

It is used to transformation of voltage level from higher level to lower level or vice-versa.

The function of substations is switch ON and OFF the power lines.

In some cases these required to convert DC power into AC power or vice-versa. For some electric traction system DC convert AC power into DC power. In electrolysis process DC power is used.

We can change the frequency from higher level or lower level to higher at the substations.

To improve power factor by installing capacitor banks at the receiving end of the line.

CLASSIFICATION OF SUBSTATION

These are classified in several ways. However, the most important ways of classifying the substations are given below

1. Service requirement
2. Operating voltage

ACCORDING TO THE SERVICE REQUIREMENT

TRANSFORMER SUBSTATION

The substations where voltage level is changed is called transformer substation. These substations receive electric power at some voltage level and deliver it at some other voltage. The main component in this substation is transformer. Maximum number of substations in power system is this type of substations.

SWITCHING SUBSTATION

In switching substation voltage level is not changed. It means incoming line voltage and outgoing line voltage is same. It performs only switching operation connect or disconnect the power lines.

POWER FACTOR CORRECTING SUBSTATION

In these types of substation power factor is improved. Such substation is located at the receiving end of the transmission lines. These substations are equipped with synchronous condensers and capacitor banks. But usually synchronous condenser is used.

CONVERTING SUBSTATION

In these types of substations are installed to change characteristics of the electrical power (such as converting ac into dc or vice versa). We know that electrical energy is generated in ac form. But most of application dc power is required such in electric traction system, in electrolysis processes. These substations receive ac power and converted into dc power. In these types of substations frequency level is also changed. We can change frequency from higher level to lower level and vice versa.

CLASSIFICATION ACCORDING TO THE OPERATING VOLTAGE

HIGH VOLTAGE SUBSTATION

These are used for the voltage from 11kv to 66kv. These types of substations are basically located in industries or near the consumers.

EXTRA HIGH VOLTAGE SUBSTATION

The operating voltage in this substation is varies from 132kv to 400kv. The electric power is generated at low voltage but transmitted at high voltage to reduce transmission losses. But electric power sends to consumer at low voltage. The of voltage is changed in EHV substations. These substations also located at generating station.

ACCORDING TO THE DESIGN

INDOOR SUBSTATION

In this type of substation, the apparatus is installed with in the substation building, so that why these substations are called indoor substation. These substations are designed for voltage upto 11kv but can be erected for 33kv. These are located where the surrounding atmosphere is contaminated with such impurities which may damage the  equipment. These substations are installed in industries. The extension is not possible. This is the limitation of indoor substations.

The auxiliaries of the indoor type of substations are

STORAGE BATTERIES

Storage batteries are used to operate the relays in substations. Batteries are heart of the substations.

Fire extinguishers.

Emergency lighting in substations in case of failure of supply.

OUTDOOR SUBSTATION

For the voltage above 66kv, outdoor substation is used. In outdoor substations, equipment is installed outdoor. It is because for high voltages, the spacing between conductors and the space required for the switches, circuit breakers and other equipment becomes so great that it is not economical to install the equipment indoor. The outdoor substations are divided into two types.

Pole Mounted Substation

Foundation Mounted Substation

POLE MOUNTED SUBSTATION

These types of substations are erected for mounting distribution transformer of capacity about upto 200KVA. The substation is located near the consumers and are cheapest, simple and small in size. All  equipment are mounted on the supporting structures and these equipment are outdoor types. The H type pole is used for supporting structure. In early days, the supporting structure is made from wood but now RCC structure is used. Triple pole mechanically operated (TPMO) switch is used for switch ON and OFF of HT transmission line. HT fuse is installed for protection purposes. Lighting arrester are also used to protect the transformer against the surge. Earthling is used to protect the maintenance cost of such transformer is low.

FOUNDATION MOUNTED SUBSTATION

These substations are built in open place and in this type of substations all the equipment is assemble into one unit enclosed by a fence from the point of view of safety. These substations on above 200KVA. To select the site for these substations, there should be good access for heavy transportation. The extension is possible in foundation mounted substations.

ADVANTAGES OF OUTDOOR SUBSTATION OVER INDOOR SUBSTATION

Fault can be located easier than indoor substations.

The extension of installation is easier, if required.

Building materials required for the outdoor substation is smaller than indoor substation.

Outdoor substation can be easily repaired.

The construction work required is comparatively smaller and cost of the switch gear installation is low.

DISADVANTAGES OF OUTDOOR OVER INDOOR SUBSTATION

The space required for the outdoor substation is more.

In case of outdoor substations more protection devices are required for the protection against lightning surges.

The length of control cables required is more.

UNDERGROUND SUBSTATION

In sophisticated areas or thickly populated cities, there is scarcity of land as well as the prices of land are very high these are installed. The equipment are placed underground. The design of requires more careful considerations as compared to other types of substations.

EQUIPMENT IN A TRANSFORMER SUBSTATION

BUS BARS

Bus Bar is a conductor carrying an electric current to which many connections may be made. It is a bar of conducting material such as Aluminum, copper etc. The Bus Bars are usually of Aluminum.

INSULATORS

The insulators performed two functions, first function is to support the conductor such as bus bar and other function is to confine the current to the conductors. The most commonly used material for the manufacture of insulators is porcelain. The insulators may be pin type insulator, suspension type insulators, strain type insulators etc.

ISOLATORS

Isolators are the switch which used to disconnect a part of system for maintenance and repair. The isolator is designed to open circuit at no load.

CIRCUIT BREAKER

A circuit breaker is a equipment which can open or close a circuit under normal as well as abnormal conditions. It is designed so those which can open the circuit manually under normal conditions and automatically under fault conditions. Circuit breaker may be air blast circuit breaker, oil circuit breaker vacuum circuit breaker and SF6 circuit breaker.

TRANSFORMER

Transformer is a static device which transfer electrical energy from one circuit to another at different voltage level, frequency remains same. A transformer used in a substation to step up or step down the voltage.

INSTRUMENT TRANSFORMER

The large current and high voltage cannot be measured with ordinary instruments. To measure these quantities at this level transformer are connected with these instruments. These transformers are called instrument transformer. The transformer connected with ammeter to extend the range of ammeter is called current transformer (CT) and transformer which is connected with voltmeter to extend the range of voltmeter is called potential transformer (PT).