Why are power engineers interested in Very High Voltages? Can't they do it in low voltage?
According to the Ohm's theorem;
Power loss in a conductor, P' = I^2 * R
=> So the power loss is directly proportional to the square of the line current.
=> Therefore to reduce losses, 'I' should be minimised.
But,
The Power requirement is fixed. Then by looking at the equation -
Apparent power, P = V * I
=> To minimise I;
=>V should be maximised.
That leads to the conclusion that transmission voltage should be increased to the MAXIMUM possible value.
Can we do that?
Even if we produce a three phase voltage of 1,000,000,000 V for argument's sake, can our equipments withstand it?
starting from the power cable of transformers, transformer terminals, power conductors, line towers, insulators, switchgear, and numerous things in a line SHOULD be made to withstand that same 1,000,000,000 V - Which is nearly impossible in present conditions.
So it boils down to simply an issue of trade-off.
=> bigger the voltage better; smaller the current better. for that it means a VERY big investment. So let us reduce some voltage. Then the current flow is going to increase. So as the line losses!
Three types of matter:
As matter is present in 3 kinds, so as the electrical insulation is of following types:
1. Solid
2. Liquid
3. Gas
HV breakdown:
High voltage naturally consists of an electric field pattern, and it depends on the voltage. It implies that certain insulation is capable of catering a certain maximum voltage. Beyond that, it will exhibit signs of leakage. That may sometimes lead to complete breakdown of that insulation medium. This value for still air is about 30 kV per cm.
HV insulators are of special design in order to create a longer creepage path. This avoids flashovers to some extent. But when dust particles and particulate matter get deposited on the insulator surface, and when rain water falls on that dirty surface, flashovers become frequent. Proper maintenanance of Hv insulators are thus essential for power quality.
Corona:
We can say corona is a phenomenon which occurs near a HV conductor. This effect may have been observed by many people residing under HV lines and especially near line supports.
An electrical field (stress) will be present around any electrical conductor. In HV conductors, the field strength would be higher. When this value increases beyond a certian limit, it starts to ionise the sorrounding air molecules. When sufficient ions are formed around that line, a portion of air becomes conductive (or called leakage). This is called 'corona effect'.
Corona has the following features, which are clearly visible in night time.
1. Hissing sound
2. Bluish/violet glow
This certainly means there is a power loss due to corona effect.
It should be noted that the electrical field strength will become more in bent/sharp conductors. Near the pole supports conductors need to bent for spur connections/ tensioning. That's why we observe corona near pole supports.
Terms associated with corona voltages are:
-corona inception voltage
-disruptive critical voltage
Friday, February 26, 2010
Thursday, February 25, 2010
Design of Electrical Installations
Lighting/ Illumination:
Types of light fixtures-
1. Incandescent lamps
2. Fluorescent lamps
3. Compact fluorescent lamps
4. Halogen lamps
5. High pressure mercury vapour lamps
6. Metal Halide lamps
6. Low pressure Sodium vapour lamps
7. Neon indicator lamps
-Illumination level unit - Lux, Recommended lighting levels
-Color temperature - Warm white, cool white, daylight white
-Color rendering
-Ambient lighting & task lighting
-Illumination Simulation tools, e.g. Dialux
Power outlets/sockets:
Ratings - 5A, 13A, 15A
Terminals: L, N, E
Routing of power conductors:
1. Cable trench
2. Cable tray
3. Cable ladder
4. Cable duct
5. Cable trunking
Airconditioning & Ventilation:
1. Exhaust fans
2. Split type AC (Indoor & outdoor unit)
3. Package type AC
4. ACCU
5. PACU
6. AHU
Lightning protection and Earthing:
1. Earth rods
2. Earthing tape
3. Down conductors
4. No of Earthing points
4. Dry chemical powder
Types of light fixtures-
1. Incandescent lamps
2. Fluorescent lamps
3. Compact fluorescent lamps
4. Halogen lamps
5. High pressure mercury vapour lamps
6. Metal Halide lamps
6. Low pressure Sodium vapour lamps
7. Neon indicator lamps
-Illumination level unit - Lux, Recommended lighting levels
-Color temperature - Warm white, cool white, daylight white
-Color rendering
-Ambient lighting & task lighting
-Illumination Simulation tools, e.g. Dialux
Power outlets/sockets:
Ratings - 5A, 13A, 15A
Terminals: L, N, E
Routing of power conductors:
1. Cable trench
2. Cable tray
3. Cable ladder
4. Cable duct
5. Cable trunking
Airconditioning & Ventilation:
1. Exhaust fans
2. Split type AC (Indoor & outdoor unit)
3. Package type AC
4. ACCU
5. PACU
6. AHU
Lightning protection and Earthing:
1. Earth rods
2. Earthing tape
3. Down conductors
4. No of Earthing points
Telecommunication:
1. Internet
2. Telephone
3. IP TV
4. PABX
5. Satellite TV
Fire protection:
Types of Fire-
1. Paper/cloth/wood etc
2. Flammable liquids
3. Flammable gases
4. Electrical faults
Types of Extinguishers:
1. Water
2. Foam
3. CO2
4. Dry chemical powder
5. Wet chemical powder
6. Sand buckets
Fire detection:
1. Heat detectors
2. Smoke detectors - Infrared
3. Smoke detectors - Photoelectric
4. Duct detectors
5. Manual call points
6. Emergency fire exit and gathering (meeting) points
Tuesday, February 16, 2010
Electrical Machines: Motors & Generators
Motors convert electrical energy into mechanical energy, and generators do the opposite. Both can be either connected in three-phase or single-phase.
Note that: 1 horse power (hp)= 750 W = 0.75 kW approximately.
Above 10hp, single phase motors are very rare, which are replaced by 3-phase motors because of the size and efficiency.
Common terminology associated with rotating machinery are,
Rotor:
The rotating part of a machine.
Stator:
The stationary part of a machine.
Armature:
This is the part from which useful output is taken. In the case of a motor, it is usually the rotor , and in the case of an alternator it is the stator.
Field:
This is the part to which exciting input is given. In the case of a motor, it is usually the stator , and in the case of an alternator it is the rotor.
Motors
Classified into AC motors and DC motors
AC Motors:
Induction Motor (IM)
This is the mostly used motor in the world. This works on the principle of induction. That is, when an AC is applied to stator, it produces a rotaing magnetic field in the rotor. These two fields interact, and as a result it yields a rotating motion according to Faraday's Law.
3 Phase induction motor technology was invented by not-so-popular Austrian scientist Nicola Tesla.
Three phase induction motors are classified as;
1. Squirrel cage IM : rotor is made up of laminated steel bars, not by coil windings.
2. Wound rotor IM : rotor has coil windings.
In order to produce torque, an IM needs a slip - a deviation from the nominal synchronous speed. Slip is defined as;
Slip, s= (Ns-Nr) / Ns
The main difference between a power transformer and an Induction motor is , a power transformer has its rotor virtually locked. i.e. a transformer is equivalent to an IM with its rotor locked.
An IM does not need any special starting mechanism. The main disadvantage of an IM is the lack of control of its rotating speed. Even if we control its speed, it is in a very narrow range.
Synchronous Motor
In a synchronous motor, stator is supplied by AC, and rotor is supplied by DC. This makes the motor to rotate in a speed called 'synchronous speed'.
Synchronous speed, Ns = (120*f )/ p;
where f = supply frequency such as 50 Hz
p= no of poles
DC Motors:
Conventional DC Motor (DCM)
DC motors have brushes to supply a rotating magnetic field to obtain continuous rotation. But the main advantage of them is the ability to control their speed over a wide range. They are classified as;
1. Series excited motors
2. Shunt excited motors
3. Compoundly excited motors
BrushLess DC Motor (BLDCM)
This type of motors are of modern nature which utilise the semi-conductors for providing excitation without the use of brushes. This has resulted in lesser maintenance.
Servo Motor
These are precise position control applications. This motor has an inbuilt arrangement for a closed loop position control of the rotor.
Stepper Motor
This is special type of DC motor which can rotate in steps (not continuous). Normally seen in open loop positioning such as in computer CD ROMs.
Permanent Magnet Motor (PMM)
These are widely found in toys where less power is enough. Instead of a DC winding, a permanet magnet is used to create a fixed field.
Alternators:
DC Generator
This generator gives a DC output. Normally limited in applications.
Synchronous Generator (SG)
This is the most widely used AC generator, such as hydropower plants and thermal power plants.
Here the exciting DC is provided to the rotor windings and, AC output is taken from the stator windings.
Permanent Magnet Generator (PMG)
A PMG's either rotor/ stator is a permanent magnet used for excitation.
Induction Generator (IG)
When an induction motor is rotated more than the synchronous speed, it enters the zone of generator. IG can be used in wind-turbine-generators. As an IG needs external excitation, it has to be supplied by other means.
Note that: 1 horse power (hp)= 750 W = 0.75 kW approximately.
Above 10hp, single phase motors are very rare, which are replaced by 3-phase motors because of the size and efficiency.
Common terminology associated with rotating machinery are,
Rotor:
The rotating part of a machine.
Stator:
The stationary part of a machine.
Armature:
This is the part from which useful output is taken. In the case of a motor, it is usually the rotor , and in the case of an alternator it is the stator.
Field:
This is the part to which exciting input is given. In the case of a motor, it is usually the stator , and in the case of an alternator it is the rotor.
Motors
Classified into AC motors and DC motors
AC Motors:
Induction Motor (IM)
This is the mostly used motor in the world. This works on the principle of induction. That is, when an AC is applied to stator, it produces a rotaing magnetic field in the rotor. These two fields interact, and as a result it yields a rotating motion according to Faraday's Law.
3 Phase induction motor technology was invented by not-so-popular Austrian scientist Nicola Tesla.
Three phase induction motors are classified as;
1. Squirrel cage IM : rotor is made up of laminated steel bars, not by coil windings.
2. Wound rotor IM : rotor has coil windings.
In order to produce torque, an IM needs a slip - a deviation from the nominal synchronous speed. Slip is defined as;
Slip, s= (Ns-Nr) / Ns
The main difference between a power transformer and an Induction motor is , a power transformer has its rotor virtually locked. i.e. a transformer is equivalent to an IM with its rotor locked.
An IM does not need any special starting mechanism. The main disadvantage of an IM is the lack of control of its rotating speed. Even if we control its speed, it is in a very narrow range.
Synchronous Motor
In a synchronous motor, stator is supplied by AC, and rotor is supplied by DC. This makes the motor to rotate in a speed called 'synchronous speed'.
Synchronous speed, Ns = (120*f )/ p;
where f = supply frequency such as 50 Hz
p= no of poles
DC Motors:
Conventional DC Motor (DCM)
DC motors have brushes to supply a rotating magnetic field to obtain continuous rotation. But the main advantage of them is the ability to control their speed over a wide range. They are classified as;
1. Series excited motors
2. Shunt excited motors
3. Compoundly excited motors
BrushLess DC Motor (BLDCM)
This type of motors are of modern nature which utilise the semi-conductors for providing excitation without the use of brushes. This has resulted in lesser maintenance.
Servo Motor
These are precise position control applications. This motor has an inbuilt arrangement for a closed loop position control of the rotor.
Stepper Motor
This is special type of DC motor which can rotate in steps (not continuous). Normally seen in open loop positioning such as in computer CD ROMs.
Permanent Magnet Motor (PMM)
These are widely found in toys where less power is enough. Instead of a DC winding, a permanet magnet is used to create a fixed field.
Alternators:
DC Generator
This generator gives a DC output. Normally limited in applications.
Synchronous Generator (SG)
This is the most widely used AC generator, such as hydropower plants and thermal power plants.
Here the exciting DC is provided to the rotor windings and, AC output is taken from the stator windings.
Permanent Magnet Generator (PMG)
A PMG's either rotor/ stator is a permanent magnet used for excitation.
Induction Generator (IG)
When an induction motor is rotated more than the synchronous speed, it enters the zone of generator. IG can be used in wind-turbine-generators. As an IG needs external excitation, it has to be supplied by other means.
