Pin Type insulators; Pin type insulators failure problem

Insulators are used for transmission of High voltages as during transmission of higher voltages bare conductors are used and to hold these bare conductors insulators are used. Pin Type insulator is one of the type. Which will be discussed in brief below:-

 

Pin Insulator

Pin Insulator is the most earliest developed overhead insulator and is still popularly used in power network up to 33KV system.

Pin type insulator Types:-

(i)                 One part

(ii)               Two parts

(iii)             Three parts type depending upon application voltage.

 Generally for 11KV voltage system we generally use one part type insulator where whole pin insulator is one piece of properly shaped porcelain or glass.

Pin type insulator is show above:-

 As the leakage path of insulator is through its surface, it is desirable to increase the vertical length of the insulator surface area for lengthening leakage path. In order to obtain lengthy leakage path, one, two or more rain sheds are provided on the insulator body.

These sheds serve another purpose that these rain sheds are so designed, that during raining the outer surface of the rain shed becomes wet but the inner surface remains dry and non-conductive. So there will be discontinuations of conducting path through the wet pin insulator surface.

In higher voltage like 33KV and 66KV manufacturing of one part porcelain pin insulator becomes difficult. Because in higher voltage, the thickness of the insulator become more and a quite thick single piece porcelain insulator cannot manufactured practically. In this case we use multiple part pin insulator, where a number of properly designed porcelain shells are fixed together by Portland cement to form one complete insulator unit. For 33KV two parts and for 66KV three parts pin insulator are generally used.

Designing Consideration of Electrical Insulator

The live conductor attached to the top of the pin insulator is at a potential and bottom of the insulator fixed to supporting structure of earth potential. The insulator has to withstand the potential stresses between conductor and earth. The shortest distance between conductor and earth, surrounding the insulator body, along which electrical discharge may take place through air, is known as flash over distance.

1. When insulator is wet, its outer surface becomes almost conducting. Hence the flash over distance of insulator is decreased. The design of an electrical insulator should be such that the decrease of flash over distance is minimum when the insulator is wet. That is why the upper most petticoat of a pin insulator has umbrella type designed so that it can protect, the rest lower part of the insulator from rain. The upper surface of top most petticoat is inclined as less as possible to maintain maximum flash over voltage during raining.

2. To keep the inner side of the insulator dry, the rain sheds are made in order that these rain sheds should not disturb the voltage distribution they are so designed that their subsurface at right angle to the electromagnetic lines of force.

Causes of Insulator Failures

Insulators are required to withstand both mechanical and electrical stresses.

 The latter type is primarily due to line voltage and may cause the breakdown of the insulator. The electrical breakdown of the insulator can occur either by flash-over or puncture. In flashover, an arc occurs between the line conductor and insulator pin (i.e., earth) and the discharge jumps across the air gaps, following shortest distance. Figure shows the arcing distance (i.e. a + b +c) for the insulator. In case of flash-over, the insulator will continue to act in its proper capacity unless extreme heat produced by the arc destroys the insulator. In case of puncture, the discharge occurs from conductor to pin through the body of the insulator. When such breakdown is involved, the insulator is permanently destroyed due to excessive heat. In practice, sufficient thickness of porcelain is provided in the insulator to avoid puncture by the line voltage. The ratio of puncture strength to flashover voltage is known as safety factor.

Capacitor color coding; Capacitor coding; How to find capacitor value

Most of the capacitor values has been written on them especially on electrolytic capacitors, like 10μF that stands for 10 micro-Farads. For them, the value is straight-forward. There are some smaller types of capacitors with a coded 2 or 3 digits number on them. If there are 2 digits printed on them, then this is the exact value in pico-farads. For example, 21 means 21 Pico-Farads. For the 3 digits code, the first 2 digits are the significant digits (in pico-Farads) and the third is the multiplier. The multiplier digit is as follows:

Multiplier digit	Multiplier value
0	1
1	10
2	100
3	1.000
4	10.000
5	100.000
6
7
8	0,01
9	0,1

A capacitor with code 322 means 32*100 pico-Farads = 3200 pF=3,2 nano-Farads.

Sometimes, a tolerance letter is also added at the end of the code. These letter tolerances are as follows:

Tolerance letter	Tolerance value
D	+/- 0.5
F	+/- 1%
G	+/- 2%
H	+/- 3%
J	+/- 5%
K	+/- 10%
M	+/- 20%
P	+100% ,-0%
Z	+80%, -20%

So a 103J is a 10,000 pF with +/-5% tolerance

Capacitance is measured in “farads.” There are picofarads (pF),nanofarads (nF), and microfarads (μF).

Color coded capacitors always have 4 bands, and are measured in picofarads – the smallest unit of measurement.

Equivalents: 1,000,000pF (picofarads) = 1,000nF (nanofarads) = 1μF (microfarad)

Capacitors color code

Things may get a little more complicated in this section so make sure that reading a value from a carbon resistor is clear for you.This color code is mostly applied to ceramic capacitors, but could be also met to other types of capacitors too. It is not very popular, but is good to know how it works. And here is how it goes:

Same as a resistor, a capacitor's color code is read in even way. Notice that on the left side as you see the cap there mast be an thicker strip or the strip must be at the edge of the capacitor. If you have a disc type capacitor then there is no problem of course. The first strip is the temperature coefficient. You can ignore this strip. The second and the third strip are the significant digits and the fourth strip is the multiplier (result in pico Farads). Use the same methodology as reading a resistor to get the value of the capacitor. For example:

Lets choose a capacitor having color coding as below:-
Second strip: Orange (3)
Third strip: Red (2)
Fourth strip (Multiplier): Orange (x 1.000)

This capacitor's value should be 32*1000=32.000 pF=32nF 

The tolerance is different for capacitors less than 10pF and for capacitors more than 10pF. For the previous example, the capacitor is more than 10pF. If the last strip was green, the tolerance should be +/-0.5%.

Above given table for capacitors is self explanatory.

KAVR into Farad; Capacitor KVAR conversion into Farad

Below we study how to Convert Farads into kVAR and Vice Versa. It usually comes in the mind that capacitor used for motors or higher capacity inductive load is in KVAR but capacitor rating is in farad . Then how we can convert KVAR into Farad.

Enjoy reading

This can be explained by some very simple examples as below:-

Example 1:

A Single phase 400V, 50Hz, motor takes a supply current of 50A at a P.F (Power factor) of 0.6. The motor power factor has to be improved to 0.9 by connecting a capacitor in parallel with it. Calculate the required capacity of Capacitor in both kVAR and Farads.

Solution.:

(1) To find the required capacity of Capacitance in kVAR to improve P.F from 0.6 to 0.9 (Two Methods)

Solution #1 (By Simple Table Method)

Motor Input = P = V x I x Cosθ

                              = 400V x 50A x 0.6

                              = 12kW

From Table, Multiplier to improve PF from 0.60 to 0.90 is 0.849

Required Capacitor kVAR to improve P.F from 0.60 to 0.90

Required Capacitor kVAR = kW x Table Multiplier of 0.60 and 0.90

= 12kW x 0.849

= 10.188 kVAR

Solution # 2 (Classical Calculation Method)

Motor Input = P = V x I x Cosθ

                              = 400V x 50A x 0.6

                              = 12kW

Actual P.F = Cosθ1 = 0..6

Required P.F = Cosθ2 = 0.90

θ1 = Cos-1 = (0.60) = 53°.13; Tan θ1 = Tan (53°.13) = 1.3333

θ2 = Cos-1 = (0.90) = 25°.84; Tan θ2 = Tan (25°.50) = 0.4843

Required Capacitor kVAR to improve P.F from 0.60 to 0.90

Required Capacitor kVAR = P (Tan θ1 - Tan θ2)

= 5kW (1.3333– 0.4843)

= 10.188 kVAR

(2) To find the required capacity of Capacitance in Farads to improve P.F from 0.6 to 0.9 (Two Methods)

Solution #1 (Using a Simple Formula)

We have already calculated the required Capacity of Capacitor in kVAR, so we can easily convert it into Farads by using this simple formula

Required Capacity of Capacitor in Farads/Microfarads

C = kVAR / (2 π f V2) in microfarad

Putting the Values in the above formula

 = (10.188kVAR) / (2 x π x 50 x 4002)

= 2.0268 x 10-4

= 202.7 x 10-6

= 202.7μF

Solution # 2 (Simple Calculation Method)

kVAR = 10.188 … (i)

We know that;

IC = V/ XC

Whereas XC = 1 / 2 π F C

IC = V / (1 / 2 π F C)

IC = V 2 F C

= (400) x 2π x (50) x C

IC = 125663.7 x C

And,

kVAR = (V x IC) / 1000 … [kVAR =( V x I)/ 1000 ]

= 400 x 125663.7 x C

IC = 50265.48 x C … (ii)

Equating Equation (i) & (ii), we get,

50265.48 x C = 10.188C

C = 10.188 / 50265.48

C = 2.0268 x 10-4

C = 202.7 x 10-6

C = 202.7μF

Good to Know:

These are the main Formulas to Convert Farads into kVAR and Vice Versa

Required Capacity of Capacitor in Farads/Microfarads

C = kVAR / (2 π f V2) in microfarad

Required Capacity of Capacitor in kVAR

kVAR = C x (2 π f V2)  

Multimeter functions; How to use multimeter

Multimeter is every electrical and electronic person requirement. There are so many parameters that multimeter provides.

Below picture help you to measure various electrical parameters.

There are there are three measuring points at bottom of multimeter as
1. COM means common
2. V, Resistance, mA
3. 10A DC

Ranges for multimeter are as shown above. Above that ranges multimeter doesn't work and even multimeter get faulty.
For measurement of mA connect Red wire to center point and for measuring current upto 10A DC connect Red wire at 3rd terminal.
Black wire should be always connected at Common Point.

Resistance color coding

Resistance value can be easily determined from the color coding of resistance. The value of the resistor is marked on the body using colors. Every color is different number and you can remember these numbers or you can just use the table on next step. Below is interpretation for the same.
Colors

Here is the table with the colors and numbers. As you can see they are:

·         BLACK:     0

·         BROWN:   1

·         RED:           2

·         ORANGE:   3

·         YELLOW:   4

·         GREEN:       5

·         BLUE:          6

·         VIOLET:      7

·         GREY:          8

·         WHITE:        9

But this is not for all colors. From right to left the second color is multiplier. Digits from the first colors must be multiplied with the number of this color.

·         BLACK:     1

·         BROWN:   10

·         RED:           100

·         ORANGE:   1000

·         YELLOW:   10000

·         GREEN:       100000

·         BLUE:          1000000

·         GOLD:          0.1

·         SILVER:       0.01

Above Diagram is self explanatory in itself. 

Last color represents the tolerance. Tolerance is the precision of the resistor and it is given as a percentage. For example a 390 resistor with a tolerance of ±10% will have a value within 10% of 390, between 390 - 39 = 351 and 390 + 39 = 429 (39 is 10% of 390).

·         BROWN:   1%

·         RED:           2%

·         GOLD:        5%

·         SILVER:     10%

·         NOTHING:  20%

Capacitor working principle

Capacitors are used to store charge so you can say that capacitor is a little like a battery. Although they work in completely different ways, capacitors and batteries both store electrical energy. Inside the battery, chemical reactions produce electrons on one terminal and absorb electrons on the other terminal. A capacitor is much simpler than a battery, as it can't produce new electrons -- it only stores them.

Capacitor consist of two metal plates separated by a non-conducting substance, or dielectric. When DC voltage is applied across capacitor then +ve charge will get accumulated at one plate and –ve charge at another plate. +ve and –ve Charges are of equal amount.

A parallel plate capacitor consists of two conducting plates of same dimensions. These plates are
placed parallel to each other. Space between the plates is filled with air or any insulating material (dielectric). One plate is connected to positive terminal and other is connected to negative term-
inal of power supply. The plate connected to positive terminal acquires positive charge and the
other plate connected to negative terminal acquires equal negative charge .The charges are stored between the plates of capacitor due to attraction.

In theory, the dielectric can be any non-conductive substance. However, for practical applications, specific materials are used that best suit the capacitor's function. Mica, ceramic, cellulose, porcelain, Mylar, Teflon and even air are some of the non-conductive materials used. The dielectric dictates what kind of capacitor it is and for what it is best suited. Depending on the size and type of dielectric, some capacitors are better for high frequency uses, while some are better for high voltage applications.

Capacity of charge storage inside a capacitor. Here is your formula.
C=Q/V.

·         Air as dielectric medium are most  Often used in radio tuning circuits

·         Mylar- Most commonly used for timer circuits like clocks, alarms and counters

·         Glass- Good for high voltage applications

·         Ceramic - Used for high frequency purposes like antennas, X-ray and MRI machines

·         Super capacitor - Powers electric and hybrid cars

Capacitor storage capacity is measured in Farads. A 1-farad capacitor can store one coulomb  of charge at 1 volt. A coulomb is 6.25e18 (6.25 * 10^18, or 6.25 billion billion) electrons. One ampre presents a rate of electron flow of 1 coulomb of electrons per second, so a 1-farad capacitor can hold 1 amp-second of electrons at 1 volt.

A 1-farad capacitor would typically be pretty big. It might be as big as a can of tuna or a 1-liter soda bottle, depending on the voltage it can handle. For this reason, capacitors are typically measured in microfarads

Basic electrical & Electronic Circuit Foumulas

Below are very important formulas which are applicable for both electrical and electronic circuits designing:-

Above formulas are used in electrical and electronic circuits.