Power Electronics

power semiconductor devices are as follows:

1. p-n junction diode - conventional semiconductor-semiconductor  junction diode
   
2. schottky diode - faster switching, low forward voltage drop diode
3. bipolar junction transistor (BJT) - conventional where both polarity charges operate (electrons & holes)
   
4. field effect transistor (FET) - a unipolar transistor
   
5. metal oxide semiconductor FET (MOSFET)
6. Insulated gate bipolar transistor (IGBT)
7. silicon controlled rectifier (SCR)
8. triode for AC (TRIAC) - a bidirectional thyristor
   

Junction Diodes:
A p-n junction is the basic building block of a semiconductor. p indicates positive charges and n indicates negative charges.
Zener diode is a special diode which is utilised in the reverse-biased mode in voltage regulating apllications.
Schottky diode has very low forward voltage drop but at the same time weak reverse bias withstand capability.
High Amp power diodes are type of p-n jucntion diodes designed to handle high currnet with proper heat dissipation and easy mounting arrangements.
Rectificaion of AC to DC wave can be performed by diodes. A single diode can rectifiy half of an AC waveform, but a full bridge rectifier which contains 4 diodes in a special arrangement can result in fully-rectified DC waveform.

A simple junction diode

  

Transistors:
Bipolar transistors can be either pnp transistor or npn transistor.
In  operation they employ both polarities of charges (i.e. electrons and holes) so the name bi-polar transistors. These are the conventional transistors.
A transistor can be used in a circuit as a switch which turns ON or OFF in specific conditions, or, as an amplifier which amplifies the signal fed into the transistor. Other evolutions of bipolar transistors are IGBTs, FETs, MOSFETs which have certain advantages depending on the application for which it is used for. For e.g. some of the above are fast-switching.

Symbol of transistor with 3 legs & model of a BJT

   
Thyristors :
Firing angle is the main aspect in the operation of a thyristor. A thyristor will start to conduct when a certain threshold current flows through its secondary circuit. These are mostly applied in high current switching.

Thyristors: with mounting arrangement and heat sink

 HVDC -
Not forgetting the historical fact that there was an AC/DC war between Westinghouse & Edison , AC became predominant in transmitting power to long distances due to the inherent features of AC. Saying that, nowadays for ultra-long distance power transmission DC has been found more beneficial than AC. HVDC refers to high voltage direct current (dc) system. As the power generation is easy in AC form, in this HVDC system, it is then inverted into DC, transmitted over very long distance, and then converted back to conventional AC.

AC power--->Converter--->DC transmission line--->Inverter---->AC power

Here comes the ultimate importance of REAL high current low loss Power Electronic Converters and Inverters.




A 2000A 250 kV high voltage direct current (HVDC) thyristor valve rated 2000 A,250 kV dc at Manitoba Hydro's Henday converter station: Source unknown  

















FACTS -
Flexible alternating current (ac) transmission system is defined by the IEEE as "a power electronic based system and other static equipment that provide control of one or more AC transmission system parameters to enhance controllability and increase power transfer capability."


To summarise the confusing AC/DC below provided is an application example.

1. AC to DC: rectifiers e.g. full bridge rectifier
2. DC to AC: inverters e.g. UPS in battery mode
3. AC to AC: transformers e.g. a 33/11kV power transformer
4. DC to DC: switching/chopping e.g. switch mode power supply

Internal Combustion (IC) Engines

Internal combustion engines have been a compact method to drive a generator to produce mechanical energy. IC engines can be mainly classified into two.
1. Diesel engines (Compression-ignition)
2. Petrol/gasoline engines (Spark-ignition)
Diesel and Gasoline engines are reprocating engines too as 1/more pistons move laterally to generate rotational energy which in turn used as a prime mover for either generators or simply as marine engines to drive the propeller.

A 3D cut-away model of an internal combustion engine, showing cylinder, piston, crankshaft, camshaft, valves etc.

The injection of fuel (either diesel or gasoline) can be direct or indirect.

1. Direct injection is done by the help of specially designed fuel injectors. This is applicable mostly to diesel engines.
2. Indirect injection is where fuel is not directly INJECTED. Actually this suits to gasoline engines. A carburrettor helps to DRAW the fuel rather than injecting it to the engine.This is applicable mostly to petrol engines. In modern cars, a carburettor is replaced by ELECTRONIC FUEL INJECTION (EFI), where injection is controlled by electronic means normally by a dedicated controller.

4-stroke cycle:

1. Intake stroke
2. Compression stroke
3. Power stroke
4. Exhaust stroke

4 strokes in a 4 cylinder engine cross-section, note the relative timing

Main Parts of a diesel generator set:

Parts of a diesel engine, showing main mechanical parts  

Mechanical components:

1. cylinder liner
2. piston
3. connecting rod
4. crankshaft
5. camshaft
6. push rod
7. valve spring
8. air inlet valve
9. exhaust valve
10. injector nozzle
11. fuel pump
12. ac pump
13. fuel filter
14. oil filter
15. air filter
16. oil sump
17. fuel tank
18. flywheel
19. rocker arm
20. governor

A very detailed exploded view of a diesel engine, showing cylinder head, block, valve set, shafts, bearings, gaskets etc.

Electrical components:

1. starting motor
2. battery
3. charging alternator
4. gen stator
5. gen rotor
6. windings
7. MVR/ AVR/ DVR
8. DC generator/PM Generator - exciter
9. rotating diode set
10. oil pressure sensor
11. water temperature sensor
12. speed sensor
13. gen control panel (GCP)
14. gen circuit breaker (GCB)                 
    

Power Quality

 Is there an international quality standard ISO 100001 for Quality of Power? Not yet. But quality of the important parameters of Power, namely, continuity of service, frequency, voltage, sinusoidal waveshape are to be within acceptable limits.

Tree limbs create a short circuit in electrical lines during a storm. This will result in a power outage around these lines

Continuity of Power:

1. momentory interruption (caused by transient faults)
2. temporary interruption (a bird getting burnt)
3. outage (sustained faults in Line/Station)
4. blackout (outage in a wider area, might take days to weeks to recover)

Voltage variations:

1. voltage dip/sag (momentary): rms voltage is below the nominal voltage - but within 10% drop
2. voltage swell: rms voltage is above the nominal voltage - but within 10% limit
3. flickering : rms voltage goes above & below the nominal repetitively in lights - but within 10% variation
4. undervoltage : rms voltage drop is below 10% of nominal voltage
5. brownout: term similar to undervoltage

Other notable power quality issues:

1. Under-frquency/ over-frequency
2. Harmonics - Total harmonic distortion (waveshape distortion caused by non-linear loads such as SMPS and CFLs)
3. Transients (by switching, lightning or network faults)

Harmonics: From a pure sine wave to square wave (click for animation)

High Voltage (HV) Engineering

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.

A patterned arc is formed when a worker tries to close an Air break switch which connects to a shunt line reactor

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.

Increasing the creepage path can reduce insulator flashovers

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 glow around a HV coil

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

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

Light pollution caused by high pressure sodium vapour lamps: courtesy - wikipedia

-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

A colour temperature diagram

Power outlets/sockets:
Ratings - 5A, 13A, 15A

A typical 13A switched socket outlet

Terminals: L, N, E

Routing of power conductors:
1. Cable trench

A typical cable tray within a cable trench

2. Cable tray
3. Cable ladder

A typical cable tray in a form of ladder

4. Cable duct
5. Cable trunking

A typical 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

Parts of a typical domestic lightning protection system

Telecommunication: 

 1. Internet

2. Telephone

RJ11 and RJ45 connectors - RJ means Registered Jack

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 fire

Types of Extinguishers:

1. Water

2. Foam

3. CO2

A typical CO2 fire extinguisher

4. Dry chemical powder

5. Wet chemical powder

6. Sand buckets

Fire detection:

1. Heat detectors

2. Smoke detectors - Infrared

3. Smoke detectors - Photoelectric

A typical smoke detector installed on the ceiling

4. Duct detectors

5. Manual call points

6. Emergency fire exit and gathering (meeting) points

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

Rotor and wound stator of a small motor disassembled

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

Principle of operation of a DC motor: How direction is commuted?

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.

A servo motor

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.

A stepper motor can rotate in step angles,e.g. 15'

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.

A cross-sectional model of a synchronous generator

Permanent Magnet Generator (PMG)
A PMG's either rotor/ stator is a permanent magnet used for excitation. 

A permanent magnet generator disassembled, showing a wound stator and PM rotor

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.

Wind-turbine-generator system: image source unknown