Electrical Engineering Tutorials

Types of Hydro Turbines

2010-09-29T07:13:00.000-07:00

Impulse Turbine (Pelton wheel) for high heads.
Reaction Turbine (Francis) is used for medium heads.
Propeller Turbine (Kaplan) is used for low head.

In impulse turbine high velocity water from nozzles creates impulse on the buckets of pelton wheel.
In reaction turbine (Francis) the water pushes the turbine-blades by reaction.
In propeller turbine (Kaplan) the blades are pushed by propelling action of the water.

Earthing Switch

2010-09-27T08:56:00.000-07:00

Earthing switch connect the live parts/ line conductors and earth. This switch is normally open.

Earthing switch is used to earth the live parts during maintenance and during testing. During maintenance although circuit is open still there are some voltages on line , due to which capacitance between line and earth is charged. Before proceeding to maintenance work the voltage s discharged to earth, by closing the earth switch.

1. Maintenance Earthing Switch: These are two or three pole units with a manual operating mechanism.

2. High Speed Earthing Switch: These are operated by spring energy. Spring is charged by motor-mechanism.

Circuit Breaker

2010-09-26T08:41:00.000-07:00

Circuit Breaker: It is a device which is capable of making and braking a electric circuit under normal and abnormal condition such as short circuit. Circuit Breaker can be manually or remotely operated.Circuit Breaker are of following types.:

Minimum Oil
Air blast
Air break
Vacuum
SF6 (Sulphur Hexaflouride)

Energy Efficiency of Electrical Machines.

2010-09-26T08:21:00.000-07:00

In the generation of electrical power; electrical machinery, converting mechanical energy into electrical energy is found to be quite efficient. Therefore, for overall improved efficiency of the energy system, electrical energy conversion to other useful forms(mechanical, thermal etc) must be carried out as efficiently as possible. Since major portion of the generated power is consumed in electrical drives, emphasis must be laid on efficient use of these drives. High efficiency of electrical drives can be achieved by

(a) Judicious design of electrical machinery.

(b) Proper matching of motor-load combination.

1. Judicious Design:-

For electrical drive,

Power Efficiency = Power (Power output + losses)

Energy Efficiency = Power Output * H(number of hours or days in service)

Isolators

2010-09-26T06:08:00.000-07:00

They are the disconnecting switches, which are used to disconnect the circuits under no load conditions. Isolators are very important for maintenance purpose because it provides safety as no live part is connected to the part of circuit under maintenance.

Megawatt ( Electrical Power Plant)

2010-09-26T05:28:00.000-07:00

Power plants whether thermal or hydro are rated in Megawatt or Kilowatt.

This rating denotes their power or in lay man language it tells the strength of power plant. More the strength more is the electricity generated by the plant.

Electrical Energy generated by a 100 MW Power plant:

We know we consume electrical energy in terms of Electrical Unit. 1 Electrical unit = 1 KWH= 1 KW * 1 H where KW=kilowatt and H=Hour Therefore a plant of 1 KW rating/capacity will produce one unit of electrical energy in one hour.

For Plant of 100 MW the electrical energy generated is as follows

Electrical Energy in one hour = 100 MW * 1 H = 100000 KW * 1 H = 100,000 KWH = 100,000 Units

Electrical Energy in one day = 100,000 Units * 24 = 2400,000 Units

Electrical Energy in one year = 2400,000 Units* 365 = 87,60,00,000 Units

Suppose rate of selling one unit = Rs 3

Annual Income of 100 MW power plant = Rs 3 * 876000000 = Rs.262,80,00,000

= Rs 263 crore

Instrument Transformer

2010-09-26T05:25:00.000-07:00

Current Transformer: Current transformers are used for measuring,control and protection purpose. There current ratio of these transformer is high (e.g. 500A/5A) and voltage . Their location is decided by protective zone measurement requirement.

Voltage Transformer: Basically are of two types

1. Electromagnetic Voltage Transformer.
2. Capacitive Voltage Transformer.

The voltage ratio of these transformer is high. Used for measuring, control and protection purpose The secondary sides of these transformer are connected to protective relays

Substation Equipments

2010-09-26T05:16:00.000-07:00

Power Transformers
Current Transformers
Voltage Transformers
Busbars
Circuit breaker
Isolator
Earthing Switch
Shunt Capacitor
Shunt Reactor
Surge Arrestor (Lightning Arrestor)
Line Trap
Line
Power Cables
Control Cables
Series Capacitor
Metering, Control and Relay Panel
Insulators
Coupling Capacitor

Corona

2010-09-26T02:40:00.000-07:00

During cloudy, rainy weather the high voltage gradient at the surface of conductor may cause ionization of air surrounding the surface of conductor giving audible, violent visible, intermittent discharge corona. Corona losses are high on a new transmission line and n rainy season. Corona is accompanied by hissing sound, vibration, power loss and radio interference (R.I.). The phenomena of corona is accompanied by predominant third harmonic currents which cause radio interference in the adjacent communication circuits.

Properties of Electric Current

2010-09-26T02:02:00.000-07:00

Electric Current posses some properties which make it as most useful form of energy.

Heating Property: Whenever current flows through any conducting material it heats the material.
Magnetic Property: When current flows through a conductor produces magnetic field.
Electro-Chemical Property: Whenever current passes through electrolyte it produces chemical reaction.

Advantages of Electrical Energy over other types of energy

2010-09-26T01:41:00.000-07:00

Its most common form of energy used in houses and industry. And some reasons for its popularity are listed below:-

Its called "Clean and Green energy" . Clean because it does't have any byproducts and green because it doesn't cause any kind of pollution neither any of the resources of mother earth are exhausted when we use this form of energy.
It can be easily converted to other form of energy.
It is much cheaper than other forms of energy.
It can be easily transmitted to various location very conveniently and efficiently.
This form of energy can be controlled and monitored easily.

Methods to Decrease earth resistance

2010-09-24T08:34:00.002-07:00

Clean the surface of the electrode, mix the soil in the earthing pit with charcoal, salt and soft soil. Pour sufficient water in the earthing pit. Connect the earthing rod with the equipment by low resistance copper strip. Use more number of electrodes, larger mesh.

Gas Insulated Switchgear

2010-09-24T08:34:00.000-07:00

SF6 Gas Insulated Switchgear (GIS) are preferred for voltage rating of 36 kV, 72.5 kV, 145 kV, 245 kV, 420 kV and above. In such a substation, the various equipments like Circuit Breakers, Busbars, Isolators, Load Break Switches, Current Transformers, Voltage Transformers, Earthing Switches, etc are housed in metal enclosed modules filled with SF6 gas. The SF6 gas provides the phase to ground insulation and phase to phase insulation. As the dielectric strength of SF6 gas is higher than air, the clearance required are smaller. The overall size of each equipment and the complete substation is reduced to about 10% of convention air-insulated substation. GIS are installed indoor.

The various modules of GIS are factory assembled and are filled with SF6 gas at a pressure of about 3kgf/cm2 . Thereafter, thy are taken to site for final assembly. Such substations are compact and can be installed conveniently on any floor of a multistoried building or in an underground substation. The installation time s substantially reduced. Such installation are preferred in cosmopolitan cities , industrial townships, hilly terrains etc., where cost of land is very high and higher cost of gas insulated switchgear is justified by saving due to reduction in floor area requirement. They are also preferred in heavily pollutes areas where dust, chemical fumes and salt layers cause frequent flashovers in conventional outdoor air-insulated switchgear.

The SF6 Gas Insulated Switchgear (GIS) contain the same component as the conventional outdoor substations. All live parts are enclosed in metal housings filled with SF6 gas. The live parts are enclosed in metal housings filled with SF6 gas. The live parts are supported on cast resin insulators. Some of the insulators are designed as barriers between neighboring modules such that gas do not pass through them. The entire installation is sub-divided into compartments which are gas tight with respect to each other. Thereby the gas monitoring system of each compartment can be independent and simpler.

The enclosures are non-magnetic material such as aluminum or stainless steel and are earthed. The gas tightness si provided by static ‘O’ seals placed between machined flanges. The ‘O’ rings are placed in the grooves such that after assembly, the O ring get squeezed by about 20%. Quality of material and dimension of grooves and O-seals are important to ensure gas tight performance.

The GIS has gas monitoring system. The gas density in each compartment is monitored. If pressure drops slightly, the gas is automatically tapped up. With further gas leakage, the lo pressure alarm is sounded or automatic tripping or lock out occurs.

Creep in Hydro Turbines

2010-09-22T01:30:00.000-07:00

Creep is the situation in which there is slow rotataion (below 20 rpm) due to leakage of water through wicket gates which is always there. Creep situation is totally unwanted and undesirable for the m/c life. To know its presence we usually apply an instrument known as Creep Indicator. This assembly consists of solenoid and brushes. In ususal case when unit is not runing brush has contact with shaft and for any movement of shaft, brush tilts actuating a micro switch which give an alarm signal. Creep inidcator is designed such that it remains insensitive during Start Up, Running and Stopping. During this period solenoid get energised and removes the brush contact from the shaft.

Major Components of a Hydro Power Plant

2010-09-22T01:13:00.001-07:00

1. Concrete Dam
2. Head Race tunnel
3. Surge Shaft
4. Pressure Shaft
5. Power House
6. Penstock
7. Main Inlet valve(MIV) & Bypass Valve
8. Turbine
9. Generator
10. Cooling Water System
11. Fire Fighting System
12. Ventilation System
13. Drainage System.
14. Auxillary Supply System
15. DC Supply System
16. Transformer Gallery
17. SwitchYard
18. Tailrace Tunnel (TRT)

Advantages of A.C. over D.C. / Why generation is done in A.C.

2010-09-20T08:35:00.000-07:00

Electric energy is generated and used is AC because it offers much advantages then DC. The few are listed below:-

AC can be generated at high voltages, but DC cannot be generated at high voltages because sparking starts at the commutator at high voltage, due to which commutator gets damaged.
High voltages AC generators are much simpler and cheaper than DC generators of the same range. It is because in AC generators there i no commutator which is costly part and is damaged.
Alternating current can be stepped up or stepped down with a static device called transformer. When voltages is stepped up current decreases to a small value. Small current produces less heat and can be transmitted through a thin conductor.Thus it is possible to transmit AC at high voltages. This reduces the size of conductor, transmission losses ans increases transmission efficiency. At the receiving station, voltages can be stepped down to the required value by using step down transformer. This is most important reason for generating and using electrical energy as AC.
A.C. induction motors are simplest in construction, cheaper in cot and require less maintenance whereas D.C. motors are complicated.

Voltage Transformer

2010-09-16T09:12:00.000-07:00

The VT is useful in communication systems. VTs in combination with wave traps are used for filtering high frequency communication signals from power frequency. This forms a carrier communication network throughout the transmission network.

Shear Pin

2010-08-13T07:40:00.000-07:00

Shear Pin is a safety device which breaks in case of mechanical overload, and prevents further damage to machinery, it is sacrificed to protect or prevent damage to bigger and more critical component of machinery. Its mechanical analogous to electrical fuse.

In a hydro power plant this pin is used to couple lever and lever handle, if the two loses synchronism between each other due two some mechanical fault the pin breaks, and air pressure decreases.

Resistance and its Unit

2010-07-29T21:40:00.000-07:00

Resistnace
Is is the peoperty of substnace by virtue of which it opposes the flow of electrons(electricity) through it. Metals,acid and salt solutions are good conductors of electricity. Amongst metals Silver, Copper and Aluminium ae very good conductors of electricity. this is due to presence of a large number of free or loosely -atttached electrons in their atoms. These Vagrant electrons assume a directed motion on the application of an electric potential difference. These electrons while flowing pass through the molecules or the atoms of the conductor, collide and other atoms and electons, thereby producing heat.

Those substance which offer relatively greater diffculty or hindrance to the passage of these electrons are said to be poor conductors of electricity like bakelite, mica dry wood etc.They are also called as insulators

Unit
The practical unit of resistance is ohm. A conduator is said to have a resistance of one ohm if it permitsone ampere current to flow through it when one volt is imptressed across its terminals.

Energy Efficiency of Power and Distribution Transformers

2010-07-16T08:36:00.000-07:00

Power Transformer:- these are used at the sending and receiving end of a long, high voltage transmission line for stepping up or stepping down the voltage,they are manipulated to operate almost at or near their rated capacity (KVA rating). In view of this a power transformer is designed to have maximum efficiency at or near full load.(rated KVA). Hence choice of transformer should be based on full load efficiency.

Distribution Transformers are the one which are installed at consumer premises, where they are use to step down the voltage. The secondary winding of distribution transformers are directly connected to consumer terminals. The load on this transformer varies wide range during 24 hours day due to varying requirement ranging from zero to a maximum of its rated capacity. Note the primary of distribution transformer is constantly energized therefore core losses occur continuously.So these transformer are designed to have minimum core losses, but for reduced core loss the maximum efficiency may occur at about one-half of its rated capacity. therefore choice of distribution transformer must be calculated as follows
Energy efficiency of a transformer is defined as the ratio of total energy output for a certain period to the total energy input for the same period

Relationship between Magnetism and Electricity

2010-07-15T09:31:00.000-07:00

Whenever current flows through a conductor magnetic field is produced in the area surrounding the conductor (this phenomena was discovered by scientist Orested). So concluded that whenever the electrons are in motion magnetic field is produced vice versa is also true whenever magnetic field is in motion with respect to a given conductor electrons start flowing in that conductor, hence the current comes into existence (as proved by Faraday) .

This phenomena whereby an e.m.f and hence current is induced in any conductor which is cut across of is cut by magnetic flux is known as electromagnetic Induction.

Electromagnetic Induction is the basic phenomena on which every electrical machine works whether DC or AC including Transformers.

* intersecting fact

While the production of magnetic field due to flow of electrons was accidental discovery.

It took nine long years for Faraday to convert magnetic energy into electrical energy and all through these he use to carry magnets in his pockets so as to constantly remind him of the problem.

Rating of Transformers (why KVA and not KW)

2010-07-14T06:46:00.000-07:00

Transformers have a name plate fixed on them on which rated output, rated voltages, the rated frequency, cooling method are recorded. The interesting fact is that its output rating is expressed in KVA (kilovolts ampere)rather then KW (kilowatt). This is due to the fact that the transformer rated output is limited by heating and hence losses in the transformer. The two type of losses in a transformer are core loss and ohmic losses.These losses depend on transformer voltage and current and are almost unaffected by the load pf, therefore transformer rated output is expressed in VA or in KVA and not in KW

Transformers Noise

2010-07-13T11:04:00.000-07:00

Transformers usually with ferromagnetic core produce a constant hum which leads to noise which can be very annoying especially in residential areas.The major cause of this noise is a phenomena called as magnetostriction. When the ferromagnetic core of transformers is magnetize, the core length along the alternating flux decreases and increases alternatively, with an alternative decrease and increase of its cross section. this phenomena of small change in the core dimensions is known as magnetostriction, alternatively the ferromagnetic core vibrates and humming is produced. this humming is further transfered to oil, transformer tank and then surroundings. The level of sound will increase with core flux density.

Transformers

2010-07-13T10:34:00.000-07:00

The transformer is a device that transfers electrical energy from one circuit to another circuit through the medium of magnetic field and without change in the frequency. The electrical circuit which receives energy from the supply mains is called primary winding and the other circuit which delivers electric energy to the load is called the secondary winding.

This device is a electromagnetic energy conversion device. The electrical energy present in the primary winding is changed into magnetic energy and this magnetic energy is again converted back to electrical energy in secondary circuit. The two circuits primary and secondary are magnetically coupled. Energy is transfered by this magnetic coupling between two cicuits and no mechanical device is utilized therefore efficiency of transformers is as high as 98%.

Step Up Transformers: If the secondary windings has more number of turns then primary winding then the secondary voltage is higher then the primary voltage and the transformer is called as step up transformer.

Step Down Transformers: If the number of turns in the secondary side is less than the primary side then the voltage in secondary side is lower then the primary voltage and the transformer is called as step down transformer.

There is a third kind of transformer where number of windings in primary is equal to the number of windings in secondary these transformers are used for isolation purpose.

A.C. Machines

2010-07-12T09:23:00.000-07:00

These are further classified as transformers, synchronous machines, induction machines and ac commutator machines.

1. Transformers. :- A transformer is not an electromechanical converter. It is used in almost all device energized from ac source. It usually steps up or step down electrical energy from one voltage level to another voltage level. However, it is also employed with unity turns ratio for isolation purposes.

In small sizes, single-phase are used. for large ratings, 3-phase transformers are more common.

2. Synchronous machines.:-In these machines, the field poles may be on stator or rotor. In bigger sizes, field poles are always on the rotor.The cross sectional view of a two pole synchronous machine is given below.

Field winding on rotor is excited with dc whereas stator winding handles three phase ac power. Rotor run at synchronous speed N(s). Frequency f of the emf generated in armature is given by

f = PN(s)/120 Hz

where P =number of field poles.

A synchronous machine is a doubly excited machine. It is employed as an alternator for the generation of three-phase power at all the generating stations. It is also used as a synchronous motor for special purpose.

3. Induction machines.:-These are of two types, three phase induction machines and single phase induction machines. Both types are singly excited in the sense that stator of three-phase ac source whereas stator of single phase induction motor is energized from 1-phase ac source.

(i) Three phase induction machines:-The stator winding of three phase induction motor, connected to 3 phase mains, carries both load current and field producing exciting current.the field produced rotates at synchronous speed in the air gap but rotor speed is always less then synchronous speed in the air gap but rotor speed is always less then the synchronous speed. Two type of 3 phase induction motors are as under:-

1. SCIM Squirrel cage induction machine.:- Rotor carries copper or aluminium, bars embedded in rotor slots. These bars are short circuited by two end rings, one at each end. SCIM has the most rugged construction and is used where control of starting torque or speed is not needed.

2. SRIM Wound rotor induction machine.:- Its stator is similar to the stator of SCIM, but rotor carries 3-phase winding with three leads connected to three slip rings mounted on shaft. Brushes pressing on the slip rings allow the insertion of external resistance in the rotor circuit.SRIMs are used where speed, or torque, control is required.

(ii) Single phase induction machines.:-In addition to 3 phase induction machines discussed above, single-phase induction motors are used where single phase low voltage(230 V, 50 Hz) is available, as in homes, offices, classrooms, shops etc.

4. AC Commutator machines.:-As the name suggest these machines are fed from ac source and are fitted with commutators.