Why transmission line 11KV , 33KV, 66KV not in 10KV 20KV?

 Why transmission line 11KV , 33KV, 66KV not in 10KV 20KV?

if we consider 11kV, then the voltage should come 11111V
but the case is we get 11000V.
Think on it guys, 11kV has nothing to do with form factor. 

 Form factor is the ratio of RMS/Average value which 
equals 1.11

If the Form factor 1.11 then the voltage waveform is pure sine wave.

Any AC voltage will have a form factor irrespective of the voltage level. Variation in the form factor value will tell how good the sine wave is. 

if a 440V AC sine wave with form factor 1.11 is stepped down or stepped up to any value, still that waveform have a form factor of 1.11

So form factor is the quality of the sine wave. Its not a deciding factor on how much the voltage should be.

§  The miss concept is Line voltage is in multiple of 11 due to Form Factor.  The form factor of an alternating current waveform (signal) is the ratio of the RMS (Root Mean Square) value to the average value (mathematical mean of absolute values of all points on the waveform). In case of a sinusoidal wave, the form factor is 1.11.

§  The Main reason is something historical. In olden days when the electricity becomes popular, the people had a misconception that in the transmission line there would be a voltage loss of around 10%. So in order to get 100 at the load point they started sending 110 from supply side. This is the reason. It has nothing to do with form factor (1.11).

§  Nowadays that thought has changed and we are using 400 V instead of 440 V, or 230 V instead of 220 V.

§  Also alternators are now available with terminal voltages from 10.5 kV to 15.5 kV so generation in multiples of 11 does not arise.  Now a days when, we have voltage correction systems, power factor improving capacitors, which can boost/correct voltage to desired level, we are using the exact voltages like 400KV in spite of 444KV

Why most of analog o/p devices having o/p range 4 to 20 mA and not 0 to 20 mA?

Why most of analog o/p devices having o/p range 4 to 20 mA and not 0 to 20 mA?

§  4-20 mA is a standard range used to indicate measured values for any process. The reason that 4ma is chosen instead of 0 mA is for fail safe operation.

§  For example: A pressure instrument gives output 4mA to indicate 0 psi  up to 20 mA to indicate 100 psi or full scale. Due to any problem in instrument (i.e) broken wire, its output reduces to 0 mA. So if range is 0-20 mA then we can differentiate whether it is due to broken wire or due to 0 psi.

§  In a digital input, voltage between 1-5V is considered as ON and voltage less than 1V are considered as OFF.  A/D converters and display devices are also working in the 5V range. So 250ohm resistor across the 4-20mA circuit will give a voltage of 1-5V.

Why humming sound occurred in HT transmission line?

 Why humming sound occurred in HT transmission line?

§  This sound is coming due to ionization (breakdown of air into charged particles) of air around transmission conductor. This effect is called as Corona effect, and it is considered as power loss.

Which is more dangerous AC or DC

Which is more dangerous AC or DC

§  Low frequency (50 – 60 Hz) AC currents can be more dangerous than similar levels of DC current since the alternating fluctuations can cause the heart to lose coordination, inducing ventricular fibrillation, which then rapidly leads to death.

§  However any practical distribution system will use voltage levels quite sufficient to ensure a dangerous amount of current will flow, whether it uses alternating or direct current. Since the precautions against electrocution are similar, ultimately, the advantages of AC power transmission outweighed this theoretical risk, and it was eventually adopted as the standard.

What is the difference between MCB & MCCB, Where it can be used?

What is the difference between MCB & MCCB, Where it can be used?

§  MCB is miniature circuit breaker which is thermal operated and use for short circuit protection in small current rating circuit.

§  Normally it is used where normal current is less than 100A.

§  MCCB moulded case circuit breaker and is thermal operated for over load current and magnetic operation for instant trip in short circuit condition. Under voltage and under frequency may be inbuilt.

§  Normally it is used where normal current is more than 100A.

What is Skin Effect and how does it happen?

What is Skin Effect and how does it happen?

§  According to faradays law of electromagnetic induction, a conductor placed in a changing magnetic field induces an emf. The effect of back emf is maximum at the centre because of maximum lines of field there. Hence the maximum opposition of current at inner side of conductor and minimum opposition at the surface. Hence the current tries to follow at the surface. It is due to this reason that we take hollow tube conductors in bus duct.

§  Taking into account the inductance effect, its simple consider the DC current. Since its constant & not varying hence no back emf but if we gradually start increasing the frequency then the flux cutting the conductor goes on increasing, hence greater the frequency greater the alternating flux cutting the conductor & hence greater the back emf & therefore greater the skin effect.

Why we use a resistance to ground the neutral when we need always low resistivity for the grounding?

Why we use a resistance to ground the neutral when we need always low resistivity for the grounding?

§  If we ground the generator directly then whenever a fault will take place at any phase with ground the fault current flowing throw the faulted phase-to ground-to neutral will be very high cause there will be no resistance to limit the value of fault current. Hence we insert a resistance in the neutral circuit to limit this fault current. Also we need to reduce the fault current to such a value that the protection CTs are able to identify the fault current without saturating the CTs. Communicate it to the protection relays & hence the relays can then isolate the system from the fault; so that the system is isolated from the fault before the harm is done by the fault current. That is the reason that all the equipment will be designed for fault KiloAmpere values for 1 sec so that the total operation(CT sensing-relay functioning-circuit breaker operation ) time will be less than 1 sec. hence the Breakers will isolate the fault before 1 sec i.e. within the time period the equipment are designed to carry the fault current. Thus all your objectives of:

§  preventing the arcing.

§  limiting the fault current.

§  isolating the faulted system are achieved

Why Relief v/v is provided in condenser of refeer system.

Why Relief v/v is provided in condenser of refeer system.

Ans- If ur condenser not cooling properly there will be chances of developing excess pressure because of this pressure your hp will activated if hp will not work at that time this relief vlv comes in action and remove excess pressure so it prevent us from serious damage as a secondary arrangement after hp trip.

    Condenser tubes gets excess pressure when there is air in system or the compresser has to do more work when sea water temp is high in certain areas which leads to increase in pressure thus for ensuring safety since HP trip failure chances are very common onboard. 

Which of the following will result in automatic stopping of IG blower

Which of the following will result in automatic stopping of IG blower (Asked in Sept 2014 and Feb 2015 in MMD Kochi,Kolkata and Chennai)
A) low water level in deck seal
B) Oxygen content above 8%
C) Scrubber tower low water level
D) scrubber tower high water level

Ans – D

 In IG system there always the alarm for Opion A,B & C because these all are not that much critical but Option D is very critical related to the operation of IGG plant because if scrubber tower high water level alarm came it means may be your ovb valve is not fully open because it contain two valves in the ovb line one is hydraulic and one is normal butterfly valve and manufacture gives the shutdown for this alarm because if water level keeps on increasing then sea water goes all through the line and from there it will go to the tanks and contaminate the cargo which may degrade the quality of the cargo which is one of the worst scenario.

How LVDTs Work

How LVDTs Work

The LVDT or Linear Variable Differential Transformer is a well established transducer design which has been used throughout many decades for the accurate measurement of displacement and within closed loops for the control of positioning.  So, how does an LVDT work?  In its simplest form, the design consists of a cylindrical array of a primary and secondary windings with a separate cylindrical core which passes through the centre. 

The primary windings (P) are energised with a constant amplitude A.C. supply at a frequency of 1 to 10 kHz.  This produces an alternating magnetic field in the centre of the transducer which induces a signal into the secondary windings (S & S ) depending on the position of the core.

Movement of the core within this area causes the secondary signal to change . As the two secondary windings are positioned and connected in a set arrangement (push-pull mode), when the core is positioned at the centre, a zero signal is derived.

Movement of the core from this point in either direction causes the signal to increase . As the windings are wound in a particular precise manner, the signal output has a linear relationship with the actual mechanical movement of the core.

The secondary output signal is then processed by a phase-sensitive demodulator which is switched at the same frequency as the primary energising supply. This results in a final output which, after rectification and filtering, gives D.C. or 4-20mA output proportional to the core movement and also indicates its direction, positive or negative from the central zero point .

The distinct advantage of using an LVDT displacement transducer is that the moving core does not make contact with other electrical components of the assembly, as with resistive types, as so offers high reliability and long life. Further, the core can be so aligned that an air gap exists around it, ideal for applications where minimum mechanical friction is required.

The LVDT design lends itself for easy modification to fulfill a whole range of different applications in both research and industry.

Some typical variations include:-

·         Complete sealing for part or full submersion in liquids and gases

·         Heavy construction build for tough industrial areas

·         Miniature and low cost models for price-conscious OEM usage

·         Internal electronic circuitry eliminating the need for additional instrumentation

If 200w, 100 w and 60 w lamps connected in series with 230V AC , which lamp glow brighter? Each lamp voltage rating is 230V.

 If 200w, 100 w and 60 w lamps connected in series with 230V AC , which lamp glow brighter? Each lamp voltage rating is 230V.

§  Each bulb when independently working will have currents (W/V= I)

§  For 200 Watt Bulb current (I200) =200/230=0.8696 A

§  For 100 Watt Bulb current (I100) =100/230=0.4348 A

§  For 60 Watt Bulb current (I60) =60/230=0.2609 A

§  Resistance of each bulb filament is (V/I = R)

§  For 200 Watt Bulb R200= 230/0.8696= 264.5 ohms

§  For 100 Watt Bulb R100= 230/0.4348 = 528.98 ohms and

§  For 60 Watt Bulb R60= 230/0.2609=881.6 ohms respectively

§  Now, when in series, current flowing in all bulbs will be same. The energy released will be I²R

§  Thus, light output will be highest where resistance is highest. Thus, 60 watt bulb will be brightest.

§  The 60W lamp as it has highest resistance & minimum current requirement.

§  Highest voltage drop across it X I [which is common for all lamps] =s highest power.

§  Note to remember:

§  Lowest power-lamp has highest element resistance.

§  And highest resistance will drop highest voltage drop across it in a Series circuit

§  And highest resistance in a parallel circuit will pass minimum current through it. So minimum power dissipated across it as min current X equal Voltage across =s min power dissipation

How to identify the starting and ending leads of winding in a motor which is having 6 leads in the terminal box

How to identify the starting and ending leads of winding in a motor which is having 6 leads in the terminal box

§  If it is a single speed motor then we have to identify 6 leads.

§  Use IR tester to identify 3 windings and their 6 leads. Then connect any two leads of two winding together and apply small voltage across the other two terminals and measure the current.

§  Then swap the connections of one coil of same two windings and apply small amount of voltage (same as before) and measure current.

§  Check in which mode you get the max current and then mark it as a1-a2 & b1-b2. You get max current when a2-b1 will be connected and voltage applied between a1-b2.

§  Follow the same process to identify a1-a2, b1-b2, c1-c2.now we will be able to connect it in delta or star.

How to check Capacitor with use of Multi meter.

How to check Capacitor with use of Multi meter.

§  Most troubles with Capacitors either open or short.

§  An ohmmeter (multi meter) is good enough. A shorted Capacitor will clearly show very low resistance. A open Capacitor will not show any movement on ohmmeter.

§  A good capacitor will show low resistance initially, and resistance gradually increases. This shows that Capacitor is not bad. By shorting the two ends of Capacitor (charged by ohmmeter) momentarily can give a weak spark. To know the value and other parameters, you need better instruments

How many types of Neutral grounding system?

How many types of Neutral grounding system?

§  There are primarily three types of grounding system which are:

§  (1)Solid grounding – The neutral point of the system is grounded without any resistance. If the ground fault occurs, high ground current passes through the fault. Its use is very common in low voltage system, where line to neutral voltage is used for single phase loads.

§  (2) Low Resistance grounding (LRG) – This is used for limiting the ground fault current to minimize the impact of the fault current to the system. In this case, the system trips for the ground fault. In this system, the use of line to neutral (single phase) is prohibited. The ground fault current is limited to in the rage from 25A to 600A.

§  (3) High Resistance Grounding (HRG) – It is used where service continuity is vital, such as process plant motors. With HRG, the neutral is grounded through a high resistance so that very small current flows to the ground if ground fault occurs. In the case of ground fault of one phase, the faulty phase goes to the ground potential but the system doesn’t trip. This system must have a ground fault monitoring system. The use of line to neutral (single phase) is prohibited (NEC, 250.36(3)) in HRG system, however, phase to neutral is used with using the additional transformer having its neutral grounded. When ground fault occurs in HRG system, the monitoring systems gives alarm and the plant operators start the standby motor and stop the faulty one for the maintenance. This way, the process plant is not interrupted. The ground fault current is limited to 10A or less.

§  There are other two types such as Corner Grounding (for Delta system) and ungrounded system but they are not commonly used.

How Corona Discharge Effect Occur in Transmission Line?

How Corona Discharge Effect Occur in Transmission Line?

§  In a power system transmission lines are used to carry the power. These transmission lines are separated by certain spacing which is large in comparison to their diameters.

§  In Extra High Voltage system (EHV system ) when potential difference is applied across the power conductors in transmission lines then air medium present between the phases of the power conductors acts as insulator medium however the air surrounding the conductor subjects to electro static stresses. When the potential increases still further then the atoms present around the conductor starts ionize. Then the ions produced in this process repel with each other and attracts towards the conductor at high velocity which intern produces other ions by collision.

§  The ionized air surrounding the conductor acts as a virtual conductor and increases the effective diameter of the power conductor. Further increase in the potential difference in the transmission lines then a faint luminous glow of violet color appears together along with hissing noise. This phenomenon is called virtual corona and followed by production of ozone gas which can be detected by the odor. Still further increase in the potential between the power conductors makes the insulating medium present between the power conductors to start conducting and reaches a voltage (Critical Breakdown Voltage) where the insulating air medium acts as conducting medium results in breakdown of the insulating medium and flash over is observed. All this above said phenomenon constitutes CORONA DISCHARGE EFFECT in electrical Transmission lines.

5)    Methods to reduce Corona Discharge Effect:

§  Critical Breakdown voltage can be increased by following factors

§  By increasing the spacing between the conductors:

§  Corona Discharge Effect can be reduced by increasing the clearance spacing between the phases of the transmission lines. However increase in the phases results in heavier metal supports. Cost and Space requirement increases.

§  By increasing the diameter of the conductor:

§  Diameter of the conductor can be increased to reduce the corona discharge effect. By using hollow conductors corona discharge effect can be improved.

§  By using Bundled Conductors:

§  By using Bundled Conductors also corona effect can be reduced this is because bundled conductors will have much higher effective diameter compared to the normal conductors.

§  By Using Corona Rings or Grading Rings:

§  This is of having no greater significance but i presented here to understand the Corona Ring in the Power system. Corona Rings or Grading Rings are present on the surge arresters to equally distribute the potential along the Surge Arresters or Lightning Arresters which are present near the Substation and in the Transmission lines.

What is electrical corona?

§  Corona is the ionization of the nitrogen in the air, caused by an intense electrical field.

§  Electrical corona can be distinguished from arcing in that corona starts and stops at essentially the same voltage and is invisible during the day and requires darkness to see at night.

§  Arcing starts at a voltage and stops at a voltage about 50% lower and is visible to the naked eye day or night if the gap is large enough (about 5/8″ at 3500 volts).

5)    What are the indications of electrical corona?

§  A sizzling audible sound, ozone, nitric acid (in the presence of moisture in the air) that accumulates as a white or dirty powder, light (strongest emission in ultraviolet and weaker into visible and near infrared) that can be seen with the naked eye in darkness, ultraviolet cameras, and daylight corona cameras using the solar-blind wavelengths on earth created by the shielding ozone layer surrounding the earth.

6)    What damage does corona do?

§  The accumulation of the nitric acid and micro-arcing within it create carbon tracks across insulating materials. Corona can also contribute to the chemical soup destruction of insulating cements on insulators resulting in internal flash-over.

§  The corona is the only indication. Defects in insulating materials that create an intense electrical field can over time result in corona that creates punctures, carbon tracks and obvious discoloration of NCI insulators.

7)    How long does corona require creating visible damage?

§  In a specific substation the corona ring was mistakenly installed backwards on a temporary 500kV NCI insulator, at the end of two years the NCI insulator was replaced because 1/3 of the insulator was white and the remaining 2/3 was grey.

8)    What voltage are corona rings typically installed at?

§  It varies depending upon the configuration of the insulators and the type of insulator, NCI normally start at 160kV, pin and cap can vary starting at 220kV or 345kV depending upon your engineering tolerances and insulators in the strings.