INTERPHASE TRANSFORMER

n number of batteries can only be connected in parallel when their terminal voltages are equal , as otherwise there will be internal circulating current.
Let us look at the above figure , the terminal voltage of each three batteries are V1, V2 and V3 respectively. Let us assume V1 > V2> V3 ,then V1 will drive current into both the load resistor R1 and the two batteries V2, V3.

Same is the case with multiple rectifiers operating in parallel. Rectifier units can be thought of as a battery whose terminal voltage can be varied from zero to rated voltage. Thus parallel operation of two rectifier units is a bit complex as their direct voltages are constantly fluctuating.

Such systems can be operated without circulating current if at all instant of time, their direct voltage are equal(Considering rectifier outputs have ripple contents). Thus, not only the average DC voltage of the two systems must be equal but also their ripples must coincide.

However, it is desired that the ripple voltages instead of coinciding are so displaced that their superimposition results in higher pulse numbers. Note that higher pulse numbers mean smoother output DC and lesser amount(Cost and weight) of filter circuitry required. Through INTERPHASE 

TRANSFORMER or IPT , two rectifier systems with displaced ripple voltages are paralleled. The parallel connection does not affect the working of each individual group. It absorbs, at any instant the difference between the direct voltage of the individual systems and must be designed for the time integral of this voltage.

The figure shows two rectifier units connected in parallel via IPT. It shows the direct current through the IPT. This transformer absorbs at any instance, the voltage difference between the individual groups and thus maintains independent operation of these groups.With respect to the voltage difference to be absorbed, the two windings of the IPT are in series connection. Thus the voltage difference can be balanced by the emf induced in these windings-just as in normal transformer. The voltage impressed on the primary winding is balanced by the induced emf. However, inducing the balancing emf needs a changing magnetic flux and exciting ampere turns, which is the difference of the direct currents to be combined (since these currents pass in opposite direction through the window of the IPT). 

If these currents are well balanced, the core will not be driven into saturation by the dc ampere turns, even without an air gap. The time integral of the voltage to be absorbed by the IPT is a function of the dc voltage, of the operating conditions and of pulse number. For a regular transformer, operating at 50 Hz, the maximum flux density is taken near 1.5 Tesla. For an interphase transformer a lower value near to about 0.9 Tesla , is taken, since the magnetic flux is alternating with three times the supply frequency, if the two three pulse systems are combined.

In all electro-chemical, electro-metallurgical and traction applications, several converters are operated in parallel in order to meet very high current requirements.

Design Features

An IPT can be either a wound type or a bar mounted type depending on the voltage to be absorbed and the direct curent rating.

Wound type

The wound type interphase transformer is like a single phase transformer with the windings on the two legs of the core.
The total number of turns in each winding is divided equally and accommodated on the two limbs.

Bar mounted type

In this type of interphase transformer, the busbar from the neutrals pass through rectangular cores forming a single turn winding.

ESP - Basic principles of Electrostatic Precipitator

Electrostatic Precipitator Principles

Electro Static Precipitator or "ESP" is a globally used system for effective filtration/control of particulate emission. It is a filter that removes minute dust particles from factory exhausts. ESPs can collect dust particles of size 0.1 to 10 micron effectively. They are efficient than scrubbers and cyclones for collecting particulate suspended matters from air or gaseous content.

     1.PRINCIPLE OF OPERATION

Electrodes at high voltage create a corona effect (Ionized atmosphere around them)

This charges the passing particles. Once charged the particles are subjected to a transverse electrostatic force that pulls them towards the collecting plates.

Plates are periodically hammered, it causes mechanical vibrations and makes the collected dust particles to fall into collecting hoppers.

     2.BACK CORONA

High resistivity dust is stubborn. Let us try to understand back corona with the following schematics.

Normally, dust in the range of 104-1011 Ω ･ cm can be collected easily.
Having been easily charged by the negative ions from the discharge electrode.

The dust is attracted to the collecting plate by Coulomb force.

Collected dust is easily separated from the collecting plate by rapping.







High resistivity dust (1011 Ω ･ cm or greater) is hard to remove because

Having been easily charged by the negative ions from the discharge electrode.

the dust is attracted to the collecting plate. Up to this point, it is pliant and controllable.Due to its strong adhesion force, the high resistivity dust is not easily separated by the shock of rapping.

Moreover, this dust layer starts releasing positive ions that cancel the negative ions from the discharge electrode, making the charging process unstable (back corona). This results in the deterioration of dust collection efficiency.

In case of high resistive dust, the dust layer creates insulation between the positively charged collecting plates and negatively charged dust particles.

In such condition spark/arc within the layer of dust particles is formed with the increase of kV(DC). This phenomenon is known as back corona.

To avoid back corona, the field voltage kV has to be reduced sufficiently, such measurer finally reduce the collection efficiency of the field.

     3.FIELD SHORT

Dry and high resistivity dust causes back corona while with wet and low resistive dust, field shorting occurs. The wet dust layer gets positively charged easily.In such condition whenever the gap between positively charged dust particles and negatively charged electrodes gets reduced due to accumulation of dust layer, spark gets emitted from emitting electrodes to collecting plates.This may cause the failure of the high voltage winding if transformer is not switched off immediately after field short.

Volt-Ampere characteristics

Let us graphically try to visualize the actual voltage-ampere condition within the esp equipment in case of normal running condition, back corona and field short.

a)Normal Operating zone(Linear operation) :- This operation mode is shown in green colour in the graph.With the increase in field voltage(kV dc) the field current increases linearly (mA).
b)Back corona zone:- Sparking due to reduced resistivity of the dust , causing decrease in field voltage and increase in field current.
c)Field short point:- The point where the voltage in the field reduces to zero short circuiting the field and the current raises to a dangerous value in the secondary of the high voltage transformer.

     4.PARAMETERS THAT AFFECT THE PERFORMANCE OF THE ESP

a)Gas temperature: Normally ESP  is designed to operate in the temperature range of 200-300 degree centigrade. At high temperature the quality of insulation deteriorates and flash over voltage limit decreases. In such condition operating voltage has to be brought down to avoid back corona that results in lower dust collection efficiency. At temperatures below the acid dew point , deposition of acid in the structure leads to faster corrosion.

b)Moisture content: moisture content has a large influence on the performance of esp. moisture content increase the ionization tendency and decreases the resistivity of the dust particles. As an effect of these factors dust collection efficiency increases with reduced back corona tendency.

c)Dust Particle size: The collecting efficiency increases with increase in particle size, since larger particle receive charge more quickly and attain migration velocity. Migration velocity is directly proportional to diameter. Hence collection efficiency decreases with the increase in fineness of the dust particle.

d)Dust resistivity: Dust resistivity increase with the increase in dryness of dust and quality of fuel. At higher dust resistivity, internal spark over between two layers of dust takes place as a result of potential difference created by the resistance of the dust. This phenomenon is called back corona. Once back corona starts, field intensity start reducing with reducing of field current. This reduced the collecting efficiency of the ESP.

e)Rapping frequency: Whenever the electrode surface is subjected to hammering or rapid shock, the re-entertainment of particles take place in the main flow path and carried away by the gas causing increase in emission level.
 

<a href="https://www.urbanpro.com/ichapur/aritra-sir/9679365?_r=widgets|UrbanProBadge|View My Profile|336x132|1189230" target="_blank" style="display: inline-block;"><img src="https://www.urbanpro.com/assets/new-ui/badges/View My Profile@336x132.png" alt="View My Profile" style="max-width: 100%;"></a>

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.