The new BuZz - Power Electronics

The hotcake of Electrical Engineering is Power Electronics. Ever wondered! How the Satellite or the space power stations are powered? The answer is obvious the solar energy (incident energy of the sun). But how does a solar array converts that unregulated DC power to a regulated DC power or how does it provides AC power to systems that are driven by alternating current? Well now from space to daily life ever wondered what is the mechanism of a SMPS ?  or the battery chargers that we use for our smartphone also houses some finest of electrical controlled switches.

Picture Courtesy:- Wikipedia

Power electronics is around us, we are surrounded by it, air conditioning, cooking, lighting, refrigerators, electric-door openers, dryers, fans, personal computers, vacuum cleaners, washing machine, food mixers. Talking of the technicalities , It is a vast vast subject which is expanding new researches are going on. It blends three major aspects of electrical engineering i.e. Power, Electronics and Control. The more we control an energy flow more efficient is the flow and we can actually prevent wastage of energy. Same is true for electrical power we need power modulator as Load performance is far superior under controlled condition.It has revolutionized how power is converted and controlled.  Power conversion is known to human beings since long times ago, but conversion was done at the expense of huge energy losses. In the modern world with limited fuel sources human being has started to realize the importance of efficiency, power electronics has enabled the availability of wide range of "CONTROLLED" power converters.

 Brief History of power electronics

Power conversion in the early twentieth century was carried out through the use of vacuum tubes ( such as gas discharge valves , thyratrons and mercury arc rectifiers). The breakthrough in power electronics began during 1947-48 period. Three American scientist John Bardeen, William Shockley and Walter Brattain invented a germanium transistor in Bells Lab in 1947. Later in 1956 they were awarded the Nobel prize for this invention in semiconductor physics. Until mid 70's semi conductor semi-conductor engineering was considered to be of low power engineering. The semi conductor devices were limited up to some tens of volts and milliamperes . Then subsequently in 90's the invention of Insulated Gate Bipolar Transistor the power handling capacity of the solid state devices kept increasing , as of today we have Thyristors that can handle many kilo amperes of current and kilo volts of voltage.

Timeline of semiconductor switches

Here is a picture showing the power handling capacity of modern day power conversion diodes/switches/valve.

 

So what is the definition of power electronics?

Power Electronics is the study of switching electronic circuits in order to control the flow of electrical energy.

How is power systems related to power electronics and why power systems engineers be concerned with it?

High voltage DC Transmission, Excitation systems, static VAR compensation, Static circuit breakers, fans and boiler feed pumps, supplementary energy systems(solar, wind)
Due to the developments in power electronics field, the power transmission and distribution field faces tremendous changes such as HVDC transmission, Smart grid system. Conventional power transmission happens based on AC supply. It has a lot of disadvantages comparing with DC supply based power transmission. Few of the features of High voltage DC Transmission (HVDC) are  given below:
HVDC requires only two connectors. In case of AC transmission three connectors are mandatory. It leads to huge saving in the installation charges.
When the transmission happens at high voltage, the power loss(I²R) in the conductor is less in DC supply.But we can not step up or step down DC voltage so we need High power inverters as well that can convert that DC power of the grid back to AC power. So HVDC systems needs converters at the sending end and inverters at the receiving end and the whole system is an HVDC system.

Pros and Cons of Power-Electronic converters

Pros:-

1. High Efficiency due to low loss in power semi conductor devices.
2. Less maintenance and long life due to absence of any rotating/moving part.
3. Thus high reliability of power electronic converter systems.
4. Small size thus lower floor space needed.
5. Lesser weight  lower transportation cost.
6. Fast response as compared to electro-mechanical systems of power conversion.
7. Higher quantity production of of power semiconductor equipments resulted in lower cost of the converter equipment.

Cons:-

1. Switching generates harmonics, thus power electronic converters introduce a huge harmonics in the power system. Harmonics results in greater loss in the transformers connected in the power system and also reduces the stability of the grid.In supply systems distorted sinusoids influences the performance of other electrical equipments. It also causes radio interference.
2. Load circuit is also affected by the chopping or switching of the semi conductor devices. If a commutating machine is being fed through a converter and has large harmonic contents in it, then commutation problem will be increased , there will be sparking due to poor commutation, more motor heating and undesirable mechanical noise in the machine. So there are steps that are taken to eliminate this harmonics actively or passively.
3. Check on the power factor, "Input" power factor of AC to DC and AC to AC converter systems are always low/poor. Input power factor is the ratio of  mean AC input  power to that of total rms Volt-Amperes.
4. Low overload capacity of the semi-conductor switches. To increase momentary over-load capacity we have to go for higher rating switches which increases the cost of the equipment.

Where power flow is to be regulated, semi conductor converters are employed as the Pros out weigh the Cons of the power electronic converters.

The power conversion systems can be classified according to the type of the input and output power.

• AC to DC (rectifier)
• DC to AC (inverter)
• DC to DC (DC-to-DC converter)
• AC to AC (AC-to-AC converter)

Basically there are four conversion modes. At the heart of a semi conductor converter equipment is a semi conductor device( Diode, SCR, BJT, MOSFET) , which is being controlled by a controller circuit.Power electronic converters are also called static converters. Let us elaborate the four types of power conversion.

1. AC to DC converter(Rectifier)                                                                                                               It can be classified into two types. Fixed DC type and Phase controlled type. A diode is a uncontrolled switch, it conducts when it is forward biased and stops conducting when reverse biased. So these converts AC to fixed value DC voltage. Diode rectifiers are single phase or three phase, they are widely used in UPS systems, battery charging, Traction systems. Now the Phase controlled rectifier , these can provide variable DC voltage to the load. These are line commutated i.e, when the line voltage polarity becomes negative the switch turns off. These may be fed by single phase supply or three phase supply, It depends on the power rating. These are used in speed control drives, chemical industries, excitation systems and systems where according to load the voltage needs to be regulated.