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September 11, 2013
September 7, 2013
Useful Electrical Equations!
Useful Electrical Equations:
· For Sinusoidal Current: Form Factor = RMS Value/Average Value = 1.11
· For Sinusoidal Current: Peak Factor = Max Value/RMS Value = 1.414
· Average Value of Sinusoidal Current (Iav) = 0.637 x Im (Im = Max.Value)
· RMS Value of Sinusoidal Current (Irms) = 0.707 x Im (Im = Max.Value)
· A.C Current = D.C Current/0.636.
· Phase Difference between Phase = 360/ No of Phase (1 Phase=230/1=360°, 2 Phase=360/2=180°)
· Short Circuit Level of Cable in KA (Isc) = (0.094 x Cable Dia in Sq.mm) /√ Short Circuit Time (Sec)
· Max.Cross Section Area of Earthing Strip (mm2) = √(Fault Current x Fault Current x Operating Time of Disconnected Device ) / K
K = Material Factor, K for Cu = 159, K for Al = 105, K for steel = 58 , K for GI = 80
· Most Economical Voltage at given Distance = 5.5 x √ ((km/1.6) + (kw/100))
· Cable Voltage Drop (%) =
(1.732 x current x (RcosÇ¾+jsinÇ¾) x 1.732 x Length (km) x 100) / (Volt(LL) x Cable Run.
· Spacing of Conductor in Transmission Line (mm) = 500 + 18 x (P – P Volt) + (2 x (Span in Length)/50).
· Protection radius of Lighting Arrestor = √h x (2Dh) + (2D+L).
Where h= height of L.A, Ddistance of equipment (20, 40, 60 Meter), L=V x t (V=1m/ms, t=Discharge Time).
· Size of Lighting Arrestor = 1.5x Phase to Earth Voltage or 1.5 x (System Voltage/1.732).
· Maximum Voltage of the System = 1.1xRated Voltage (Ex. 66KV = 1.1 × 66 = 72.6KV)
· Load Factor = Average Power/Peak Power
· If Load Factor is 1 or 100% = This is best situation for System and Consumer both.
· If Load Factor is Low (0 or 25%) = you are paying maximum amount of KWH consumption. Load Factor may be increased by switching or use of your Electrical Application.
· Demand Factor = Maximum Demand / Total Connected Load (Demand Factor <1)
· Demand factor should be applied for Group Load
· Diversity Factor =
Sum of Maximum Power Demand / Maximum Demand (Demand Factor >1)
Diversity factor should be consider for individual Load
· Plant Factor (Plant Capacity) = Average Load / Capacity of Plant
· Fusing Factor = Minimum Fusing Current / Current Rating (Fusing Factor>1).
· Voltage Variation (1 to 1.5%) = ((Average Voltage – Min Voltage) x 100)/Average Voltage
Ex: 462V, 463V, 455V, Voltage Variation= ((460 – 455) x 100)/455 = 1.1%.
· Current Variation (10%) = ((Average Current – Min Current) x 100)/Average Current
Ex: 30A,35A,30A, Current Variation = ((3531.7) x 100)/31.7 = 10.4%
· Fault Level at TC Secondary
= TC (VA) x 100 / Transformer Secondary (V) x Impedance (%)
Motor Full Load Current = Kw /1.732 x KV x P.F x Efficiency
August 29, 2013
Sometimes we have seen that a ceiling fan suddenly starts moving in opposite direction with slow speed, what could be the defect?
The starting winding circuit is opencircuited may be due to burnt winding or damaged capacitor or the running winding is damaged. The fan is running only on one winding. The blades and thrust of air in the room accelerated the blades in opposite direction. As we know that the single phase motors are not selfstarting, they will continue to move in the direction in which these are initially accelerated.
July 12, 2013
What are the Functions of Substation Equipment?
Functions of Substation Equipments:
Sl. No.

Equipment

Function


BusBar

Incoming & outgoing circuits Connected to busbar


Circuit
Breaker

Automatic switching during normal
or abnormal conditions


Isolators

Disconnection under noload
condition for safety, isolation and maintenance.


Earthing
switch

To discharge the voltage on deadlines
to earth


Current
Transformer

To stepdown currents for
measurement, control & protection


Voltage
Transformer

To stepdown voltages for
measurement, control & protection


Lightning
Arrester

To discharge lightning over
voltages and switching over voltages to earth


Shunt
reactor

To control over voltages by providing
reactive power compensation


NeutralGrounding
resistor

To limit earth fault current


Coupling
capacitor

To provide connection between high
voltage line & PLCC equipment


Line
–Trap

To prevent high frequency signals
from entering other zones.


Shunt
capacitors

To provide compensations to reactive
loads of lagging power factors


Power
Transformer

To stepup or stepdown the voltage
and transfer power from one a.c. voltage another a.c. voltage at the same frequency.


Series
Capacitor

Compensation of long lines.

What is REF relay?
It is restricted earth fault relay. When the fault occurs very near to the neutral point of the transformer, the voltage available to drive the earth circuit is very small, which may not be sufficient to activate the relay, unless the relay is set for a very low current. Hence the zone of protection in the winding of the transformer is restricted to cover only around 85%. Hence the relay is called REF relay.
What are the errors in Current Transformer?
(a) Ratio error
Percentage ratio error = [(Nominal ratio – Actual ratio)/Actual ratio] x 100
The value of transformation ratio is not equal to the turns ratio.
(b) Phase angle error:
Phase angle =180/Ï€[(ImCos Î´I1Sin Î´)/nIs]
Percentage ratio error = [(Nominal ratio – Actual ratio)/Actual ratio] x 100
The value of transformation ratio is not equal to the turns ratio.
(b) Phase angle error:
Phase angle =180/Ï€[(ImCos Î´I1Sin Î´)/nIs]
What is an arc? Give the two methods of arc interruption?
Arc is a phenomenon occurring when the two contacts of a circuit breaker separate under heavy load or fault or short circuit condition.
Methods of arc interruption:
 High resistance interruption:the arc resistance is increased by elongating, and splitting the arc so that the arc is fully extinguished
 Current zero method:The arc is interrupted at current zero position that occurs 100 times a second in case of 50Hz power system frequency in ac.
What do you mean by current chopping?
When interrupting low inductive currents such as magnetizing currents of transformer, shunt reactor, the rapid deionization of the contact space and blast effect may cause the current to be interrupted before the natural current zero. This phenomenon of interruption of the current before its natural zero is called current chopping.
How to calculate voltage drop?
How to calculate voltage drop:
Singlephase voltage drop calculation:
Threephase voltage drop calculation:
In situations where the circuit conductors span large distances, the voltage drop is calculated. If the voltage drop is too great, the circuit conductor must be increased to maintain the current between the points. The calculations for a singlephase circuit and a threephase circuit differ slightly.
Singlephase voltage drop calculation:
Threephase voltage drop calculation:
OR 3Ã¸VD = (SQRT( 3) *L*R*I) /1000
VD = Voltage drop (conductor temp of 75°C) in volts
VD% = Percentage of voltage drop (VD ÷ source voltage x 100). It is this value that is commonly called "voltage drop" and is cited in the NEC 215.2( A) (4) and throughout the NEC.
L = Oneway length of the circuit's feeder (in feet)
R = Resistance factor per NEC Chapter 9, Table 8, in ohm/kft
I = Load current (in amperes)
Source voltage = The voltage of the branch circuit at the source of power. Typically the source voltage is either 120, 208, 240, 277, or 480 V.
Related Link: How much voltage drop is Acceptable?
Related Link: How much voltage drop is Acceptable?
How much voltage drop is Acceptable?
The national Electrical Code (NEC) recommends that the combined voltage drop of the electrical system (brunch circuit and feeders) not exceed 5% for optimum efficiency. This recommendation not only can improve safety but can ensure proper equipment operation and power efficiency. Foe Example, In a 220 volt 5 ampere circuit, this means that there should be no more than a 11 V drop (109 volts) at the furthest outlet when the circuit is fully loaded.
Related Link: How to calculate voltage drop?
July 5, 2013
What are the Components of the power system?
The various components of power system are:
 Generator (alternator),
 Transformer,
 Transmission line,
 Induction motor,
 Synchronous motor,
 Resistive and
 Reactive loads.
June 8, 2013
Mention The major features of a good protective gear for alternator and transformer
Major
features of a good protective gear
Ã¼ High
speedy
Ã¼ Selectivity
Ã¼ High
sensitivity
Ã¼ Stability
Ã¼ Simplicity
Ã¼ Reliability
Write down the Protective System of Alternator and Transformer
Alternator
protection
Ã¼ Differential
protection system
Ã¼ Balanced
Earth fault protection
Ã¼ Statorinter
run protection
Transformer
protection
Ã¼ Using
Buchholz relay
Ã¼ Combined
leakage and overload protection
Ã¼ Circulatingcurrent
scheme for transformer protection
Write down some major Faults of Alternator and Transformer.
12:49 AM
Alternator, Generator, Power Station, Power System, Switchgear and Protection, Transformer
No comments
Major faults in Alternator
 Ã¼ Failure of primemover
 Ã¼ Failure of field
 Ã¼ Over Current
 Ã¼ Over Voltage
 Ã¼ Over Speed
 Ã¼ Stator Winding faults
 Ã¼ Unbalanced
loading
Major faults in
Transformer
Ã¼ Open
circuits
Ã¼ Over
heating
Ã¼ Earth
Fault
Ã¼ PhasetoPhase
fault
Ã¼ Over
Load
Ã¼ Phase
to ground fault
Ã¼ Incipient
fault
Ã¼ High
voltage surge fault
How can a circuit element absorb power?
By converting electrical energy into
 Heat (resistors in toasters);
 Light (light bulbs);
 Acoustic energy (speakers);
 Storing energy (charging a battery).
June 2, 2013
May 30, 2013
Write down some Elements of Protection System.
Protection System Elements:
¤ Protective relays
¤ Circuit
breakers
¤ Current and
voltage transducers
¤ Communications
channels
¤ DC supply
system
¤ Control
cables
How Do Relays Detect Faults?
¤ When a
fault takes place, the current, voltage, frequency, and other electrical
variables behave in a peculiar way. For example:
o
Current suddenly increases
o
Voltage suddenly decreases
¤ Relays can
measure the currents and the voltages and detect that there is an overcurrent,
or an undervoltage, or a combination of both
¤ Many other
detection principles determine the design of protective relays
February 12, 2013
Write down some common applications of Diodes.
Some common applications of Diodes:
 Power supply applications
 AM (amplitude modulation) detectors
 BackEMF path
 Clamping or DC restoration
 Clipper or limiter
 Nonlinear circuits
 Logic circuits
February 1, 2013
Discuss about Electronic Logic Gates.
Introduction:
A logic gate is an idealized or physical device implementing a Boolean function, that is, it performs a logical operation on one or more logic inputs and produces a single logic output. Depending on the context, the term may refer to an ideal logic gate, one that has for instance zero rise time and unlimited fanout, or it may refer to a nonideal physical device (see Ideal and real opamps for comparison).
Logic gates are primarily implemented using diodes or transistors acting as electronic switches, but can also be constructed using electromagnetic relays (relay logic), fluidic logic, pneumatic logic, optics, molecules, or even mechanical elements. With amplification, logic gates can be cascaded in the same way that Boolean functions can be composed, allowing the construction of a physical model of all of Boolean logic, and therefore, all of the algorithms and mathematics that can be described with Boolean logic.
Logic circuits include such devices as multiplexers, registers, arithmetic logic units (ALUs), and computer memory, all the way up through complete microprocessors, which may contain more than 100 million gates. In practice, the gates are made from fieldeffect transistors (FETs), particularly MOSFETs (metaloxidesemiconductor fieldeffect transistors).
Logic gates are primarily implemented using diodes or transistors acting as electronic switches, but can also be constructed using electromagnetic relays (relay logic), fluidic logic, pneumatic logic, optics, molecules, or even mechanical elements. With amplification, logic gates can be cascaded in the same way that Boolean functions can be composed, allowing the construction of a physical model of all of Boolean logic, and therefore, all of the algorithms and mathematics that can be described with Boolean logic.
Logic circuits include such devices as multiplexers, registers, arithmetic logic units (ALUs), and computer memory, all the way up through complete microprocessors, which may contain more than 100 million gates. In practice, the gates are made from fieldeffect transistors (FETs), particularly MOSFETs (metaloxidesemiconductor fieldeffect transistors).
Logic Gate:
Logic Gates perform basic logical functions and are the fundamental building blocks of digital integrated circuits. These are process signals which represent true or false. Normally the positive supply voltage +Vs represents true and 0V (Zero) represents false. Other terms which are used for the true and false states are shown in the table on the right. It is best to be familiar with them all.
Gates are identified by their function: NOT, AND, NAND, OR, NOR, EXOR and EXNOR. Capital letters are normally used to make it clear that the term refers to a logic gate.
Logic states


True

False

1

0

High

Low

+Vs

0V

On

Off

 Switches in series (AND function)
 Switches in parallel (OR function)
 Combining IC outputs with diodes (OR function)
Logic Gate Symbols:
There are two series of symbols for logic gates:
1. Traditional Symbols &
2. IEC (International Electrotechnical Commission) Symbols.
The Traditional Symbols have distinctive shapes making them easy to recognize so they are widely used in industry and education.
1. Traditional Symbols &
2. IEC (International Electrotechnical Commission) Symbols.
The Traditional Symbols have distinctive shapes making them easy to recognize so they are widely used in industry and education.
The IEC (International Electrotechnical Commission) symbols are rectangles with a symbol inside to show the gate function. They are rarely used despite their official status, but you may need to know them for an examination.
Inputs and Outputs:
The Inverting Circle (o):
Truth Tables:
NOT Gate (inverter):
Inputs and Outputs:
Gates have two or more inputs, except a NOT gate which has only one input. All gates have only one output. Usually the letters A, B, C and so on are used to label inputs, and Q is used to label the output. On this page the inputs are shown on the left and the output on the right.
The Inverting Circle (o):
Some gate symbols have a circle on their output which means that their function includes inverting of the output. It is equivalent to feeding the output through a NOT gate. For example the NAND (Not AND) gate symbol shown on the right is the same as an AND gate symbol but with the addition of an inverting circle on the output.
Truth Tables:
A truth table is a good way to show the function of a logic gate. It shows the output states for every possible combination of input states. The symbols 0 (false) and 1 (true) are usually used in truth tables. The example truth table bellow shows the inputs and output of an AND gate.
Input A

Input B

Output Q

0

0

0

0

1

0

1

0

0

1

1

1

There are summary truth tables below showing the output states for all types of 2input and 3input gates. These can be helpful if you are trying to select a suitable gate.
NOT Gate (inverter):
The output Q is true when the input A is NOT true, the output is the inverse of the
input: Q = NOT A
A NOT gate can only have one input. A NOT gate is also called an inverter.
input: Q = NOT A
A NOT gate can only have one input. A NOT gate is also called an inverter.
AND Gate:
The output Q is true or high if input A AND input B are both true or high:
Q = A AND B
or, Q = A.B
Q = A AND B
or, Q = A.B
An AND gate can have two or more inputs. A dot (.) is used to show the AND operation i.e. A.B.
Keep in mind that this dot is sometimes omitted i.e. AB
(a) Traditional AND gate Symbol
(b) IEC AND gate Symbol
AND gate Truth Table
Input A

Input B

Output Q

0

0

0

0

1

0

1

0

0

1

1

1

OR Gate:
(b) IEC OR Gate Symbol
OR Gate Truth Table
Input A

Input B

Output Q

0

0

0

0

1

1

1

0

1

1

1

1

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