Power electronic devices: from thermionic valves to field effect transistors

Power semiconductor devices are as follows:
p-n junction diode - conventional semiconductor-semiconductor  junction diode
schottky diode - faster switching, low forward voltage drop diode
bipolar junction transistor (BJT) - conventional where both polarity charges operate (electrons & holes)
field effect transistor (FET) - a unipolar transistor
metal oxide semiconductor FET (MOSFET)
Insulated gate bipolar transistor (IGBT)
silicon controlled rectifier (SCR)
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

Active power, reactive power, frequency and voltage

Then the golden rule is,P is directly related with f.
Q is directly related with V.

Even certain experienced staff think that parameters such as line voltage depends on the active power produced and frequency may drop if there is not enough reactive power. This is Totally WRONG.

In other words,
If you are going to change the frequency of the supply you must increase the amount of fuel (eg. steam/diesel/HFO/waterflow) which in turn increases the prime-mover (eg. diesel engine/ gas turbine/ steam turbine/ hydro turbine) speed. But speed governors are meant to regulate this type of frequency variation and to maintain the speed of the prime mover.

If you are going to change the terminal voltage of the supply you must increase the excitation given to the alternator.

But keep in mind that the above are purely applicable to generators running isolated/islanded. Parallel operation and infinite grid operation are bit different and certain parameters can not be independently controlled (for eg. voltage in infinite grid). 

If multiple generators are running in parallel, only by increasing the excitation of all generators - the voltage can be increased, and vice versa. If not, only the reactive power share will change, not the output voltage.

The other important aspect of operating a power system is to do with the power factor. People confuse with whether the power factor must lead or lag. There is another belief, improving the power factor means we try to make the 0.9 to 0.8 lagging. In almost every practical power system (there are few exceptions) , the power factor should be lagging, BUT not necessarily each and every generating set. In other words, even when the system power factor is lagging - one machine may be running with a leading power factor, at the expense of another.

The power system frequency and voltage: are those constants?

We all know that power systems have a frequency of 50Hz (or 60 Hz) and say the transmission voltage is 33kV(or may be 66kV). We fix these variables quite nicely and forget about these when problems arise. After all they believed to be constant!

What if these variables are manipulated to gain advantage at the expense of consumers? why a utility might want to manipulate those?

For example, instead of keeping the system frequency at 50 Hz (keeping in mind the natural tolerance limits, for e.g. 49.5Hz ~ 50.5Hz), what if we can reduce the frequency intentionally, say to 47Hz? Here we assume that the under-frequency tripping of circuit breakers are below 47Hz so that we can keep the system running without tripping. at the same time we have manual controls so that we can adjust the speed governing so that it does not automatically settle at 50Hz.

What will be the effects of running the system at lower than 50Hz. As we all know that in most cases , a power system is inductive in load. Thus remembering the relationship Reactance X=L*w=L*2pi*f. This ends up in saying that if the system frequency is reduced- inductance will become lower in magnitude. Which means we can MANIPULATE the reactive power requirement. But the active energy supplied will not change.(no change in energy units)

This has additional effects in electrical apparatus of consumers as they are designed to operate in standard 50Hz frequency.

The same case is applicable to the line-line volatge. If we can manually reduce the line voltage of a 33kV line to about 32kV, what will happen. Surely we know that certain loads can not operate because of the voltage drop such as ACs, UPSs. but what can be gained from this manipulation. normally if the voltage of each generator is adjusted a bit lower than standard voltage, the power factor indicated in each generator will improve. This means that now each generator is supplying less reactive power requirement - and this in turn reduces the overall line voltage. This has implications in fuel efficiencies in case of a diesel generator. in case of hydro generators too this has certain FAVOURABLE effects to the operators of the power system.