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Difference between Dielectric Testing & Insulation Resistance Measurement


All electrical installations and equipment comply with insulation resistance specifications so they can operate safely. 

Whether it involves the connection cables, the sectioning and protection equipment, or the motors and generators, the electrical conductors are insulated using materials with high electrical resistance in order to limit, as much as possible, the flow of current outside the conductors.

The quality of these insulating materials changes over time due to the stresses affecting the equipment.

These changes reduce the electrical resistivity of the insulating materials, thus increasing leakage currents that lead to incidents which may be serious in terms of both safety (people and property) and the costs of production stoppages.

In addition to the measurements carried out on new and reconditioned equipment during commissioning, regular insulation testing on installations and equipment helps to avoid such incidents through preventive maintenance. 

These tests detect aging and premature deterioration of the insulating properties before they reach a level likely to cause the incidents described above.

Dielectric Testing and Insulation Resistance Measurement

At this stage, it is a good idea to clarify the difference between two types of measurements which are often confused: 
  • Dielectric Withstand Testing 
  • Insulation Resistance Measurement

Dielectric Testing (Breakdown Testing)

Dielectric strength testing, also called "breakdown testing", measures an insulation's ability to withstand a medium duration voltage surge without sparkover occurring. 

In reality, this voltage surge may be due to lightning or the induction caused by a fault on a
power transmission line. 

The main purpose of this test is to ensure that the construction rules concerning leakage paths and clearances have been followed.

This test is often performed by applying an AC voltage but can also be done with a DC voltage. This type of measurement requires a hipot tester. 

The result obtained is a voltage value usually expressed in kilovolts (kV). 

Dielectric testing may be destructive in the event of a fault, depending on the test levels and the available energy in the instrument. 

For this reason, it is reserved for type tests on new or reconditioned equipment.

Insulation Resistance Measurement

Insulation resistance measurement, however, is non destructive under normal test conditions. 

Carried out by applying a DC voltage with a smaller amplitude than for dielectric testing, it yields a result expressed in kW, MW, GW or TW. 

This resistance indicates the quality of the insulation between two conductors.

Because it is non-destructive, it is particularly useful for monitoring insulation aging during the operating life of electrical equipment or installations. 

This measurement is performed using an insulation tester, also called a megohmmeter.

What is Skin Effect in Wires and Cables?


When dealing with low current dc hobby projects, wires and cables are straight forward they act as simple conductors with essentially zero resistance.

However, when you replace dc currents with very high-frequency ac currents, weird things begin to take place within wires.

As you will see, these “weird things” will not allow you to treat wires as perfect conductors. Skin effect is one among them. We are discussing about this phenomenon in this article.

When DC current is Flowing

First, let’s take a look at what is going on in a wire when a dc current is flowing through it.


A wire that is connected to a dc source will cause electrons to flow through the wire in a manner similar to the way water flows through a pipe. 

This means that the path of any one electron essentially can be anywhere within the volume of the wire (e.g., center, middle radius, surface).

With High Frequency AC Current

Now, let’s take a look at what happens when a high- frequency ac current is sent through a wire.

An ac voltage applied across a wire will cause electrons to vibrate back and forth. In the vibrating process, the electrons will generate magnetic fields. 

By applying some physical principles (finding the forces on every electron that result from summing up the individual magnetic forces produced by each electron), you find that electrons are pushed toward the surface of the wire. 

As the frequency of the applied signal increases, the electrons are pushed further away from the center and toward the surface. 

In the process, the center region of the wire becomes devoid of conducting electrons.

What is Skin Effect?

The movement of electrons toward the surface of a wire under high frequency conditions is called the skin effect. 

At low frequencies, the skin effect does not have a large effect on the conductivity (or resistance) of the wire. 

However, as the frequency increases, the resistance of the wire may become an influential factor.

One thing that can be done to reduce the resistance caused by skin effects is to use stranded wire the combined surface area of all the individual wires within the conductor is greater than the surface area for a solid core wire of the same diameter.

Automatic Irrigation System Using Arduino

Automatic Irrigation system monitors the soil moisture and depending on set points turns on/off the pump which is connected to the relay. This way you can keep soil moisture to a set point.

This system automatically waters the plants when we are on vacation. As we are setting the soil moisture level, we need not to worry about too much of watering and the plants end up dying anyway.

The project is designed to develop an automatic irrigation system which switches the pump motor ON/OFF on sensing the moisture content of the soil.

In the field of agriculture, use of proper method of irrigation is important. The advantage of using this method is to reduce human intervention and still ensure proper irrigation.

Components Required for this Project

You need the following items to complete this project.

1. Arduino Uno
2. Soil moisture sensor
3. 16x2 LCD
4. 1K, 220E Resistors
5. 12V Relay
6. BC548 Transistor
7. Switches

Automatic Irrigation System Circuit



LCD displays the current moisture level and set point. Set point can be adjusted using push buttons. Connect relay output to water pump. Put soil moisture tip in soil where you want to maintain soil moisture.

Working of Automatic Irrigation System

The soil moisture sensor senses the moisture present in the soil and gives the output to Arduino. The output of the sensor will be from 0-1023. The moisture is measured in  percentage; therefore we should map these values to 0-100.

Whenever the moisture value is lower than the value we have set inside our code as the threshold value, the relay will turn ON and the valve will turn open and whenever the moisture value is lower than this threshold value, the relay will relay will turn OFF and the valve will get closed.

 Automatic Irrigation System Arduino Code


/*
Automatic Irrigation System
*/
#include <LiquidCrystal.h>
// initialize the library with the numbers of the interface pins
LiquidCrystal lcd (9, 8, 7, 6, 5, 4);
const int LED_RED =10; //Red LED
const int LED_GREEN =11; //Green LED
const int RELAY =12; //Lock Relay or motor
//Key connections with arduino
const int up_key=3;
const int down_key=2;
int SetPoint =30;
//=================================================================
// SETUP
//=================================================================
void setup (){
pinMode ( LED_RED , OUTPUT );
pinMode ( LED_GREEN , OUTPUT );
pinMode ( RELAY , OUTPUT );
pinMode ( up_key, INPUT );
pinMode ( down_key, INPUT );
//Pull up for setpoint keys
digitalWrite ( up_key, HIGH );
digitalWrite ( down_key, HIGH );
// set up the LCD's number of columns and rows:
lcd . begin(16, 2);
// Print a message to the LCD.
lcd . print ("studyelectrical.com");
lcd . setCursor (0,1); //Move coursor to second Line
lcd . print (" Irrigation ");
delay(1000);
lcd . setCursor (0,1);
lcd . print (" System ");
digitalWrite ( LED_GREEN , HIGH ); //Green LED Off
 digitalWrite ( LED_RED , LOW ); //Red LED On
digitalWrite ( RELAY , LOW ); //Turn off Relay
delay(2000);
}
//=================================================================
// LOOP
//=================================================================
void loop (){
double WaterLevel = ((100.0/1024.0) * analogRead ( A0 )); //Map it in 0 to 100%
lcd . setCursor (0,0);
lcd . print ("Water :"); //Do not display entered keys
lcd . print ( WaterLevel );
lcd . print ("% ");
//Get user input for setpoints
if( digitalRead ( down_key)== LOW )
{
if( SetPoint >0) //Not less than zero
{
SetPoint --;
}
}
if( digitalRead ( up_key)== LOW )
{
if( SetPoint <99) //Not more than 100%
{
SetPoint ++;
}
}
//Display Set point on LCD
lcd . setCursor (0,1);
lcd . print ("Set Point:");
lcd . print ( SetPoint );
lcd . print ("% ");
//Check Temperature is in limit
if( WaterLevel > SetPoint )
{
digitalWrite ( RELAY , LOW ); //Turn off water pump
digitalWrite ( LED_RED , HIGH );
digitalWrite ( LED_GREEN , LOW ); //Turn on Green LED
}
else
{
digitalWrite ( RELAY , HIGH ); //Turn on water pump
digitalWrite ( LED_GREEN , HIGH );
digitalWrite ( LED_RED , LOW ); //Turn on RED LED
}
delay(100); //Update at every 100mSeconds
}
//=================================================================

What Are Current, Resistance and Voltage?

Voltage, current, and resistance are three properties that are fundamental to almost everything you will do in electrical and electronics engineering. They are intimately related. 

In this article we used water in a river analogy to explain them. This method will help the beginners to imagine and understand these fundamentals much easily.

Current

The problem with electrons is that you cannot see them, so you just have to imagine how they do things. I like to think of electrons as little balls flowing through pipes.

Any physicists reading this will probably be clutching their heads in disgust now. But it works for me.

Each electron has a charge and it’s always the same — lots of electrons, lots of charge, few electrons, and a little bit of charge.

Current, rather like the current in a river, is measured by counting how much charge passes you per second.

Resistance

A resistor’s job is to provide resistance to the flow of current. So, if we keep thinking about our river, it is like a constriction in a river.

The resistor has reduced the amount of charge that can pass by a point. And it doesn’t matter which point you measure at (A, B, or C) because, if you look upstream of the resistor, the charge is hanging around waiting to move through the resistor. Therefore, less is moving past A per second. In the resistor (B), it’s restricted.

The “speed” analogy does not really hold true for electrons, but one important point is that the current will be the same wherever you measure it.

Imagine what happens when a resistor stops too much current from flowing through an LED.

Voltage

Voltage is the final part of the equation (that we will come to in a minute). If we persist with the water-in-a-river analogy, then voltage is like the height that the river drops over a given distance.

As everyone knows, a river that loses height quickly flows fast and furious, whereas a relatively gently sloped river will have a correspondingly gentle current.

This analogy helps with the concept of voltage being relative. That is, it does not matter if the river is falling from 10,000 ft to 5,000 ft or from 5,000 ft to 0 ft. The drop is the same and so will be the rate of flow.

Introduction to Gas Insulated Substations / Switchgears (GIS)


Gas insulated substations (GIS) have been used in power systems over the last three decades because of their high reliability, easy maintenance, small ground space requirement etc.

Gas Insulated Substation (GIS) also called SF6 Gas Insulated Metalclad Switchgear is preferred for 12kV, 36kV, 72.5kV, 145 kV, 245 kV, 420 kV and above voltages.

In a GIS substation, the various equipment like Circuit Breakers, Bus bars, Isolators, Load break switches, Current transformers, Voltage transformers, Earthing switches etc. are housed in separate metal enclosed modules filled with SF6 gas. The SF6 gas provides the phase to ground insulation.


As the dielectric strength of SF6 gas is higher than air, the clearances required are smaller. Hence the overall size of each equipment and the complete sub-station is reduced.

The various modules are factory assembled and are filled with SF6 gas. Thereafter, they are taken to site for final assembly.

SF6 Gas Insulated Substations are compact and can be installed conveniently on any floor of a multi storeyed building or in an underground sub-station.  As the units are factory assembled, the installation time is substantially reduced.

Such installations are preferred in composition cities, industrial townships, hydro stations where land is very costly.

Higher cost of SF6 insulated switchgear is justified by saving to reduction in floor-area requirement.

SF6 insulated switchgear is also preferred in heavily polluted areas where dust, chemical fumes and salt layers can cause frequent flashovers in conventional outdoor sub-stations.

The GIS require less number of lightning arresters than a conventional one. This is mainly because of its compactness.

Why SF6 is Used?

SF6 is used in GIS at pressures from 400 to 600 kPa absolute. The pressure is chosen so that the SF6 will not condense into a liquid at the lowest temperatures the equipment experiences. 

SF6 has two to three times the insulating ability of air at the same pressure. SF6 is about 100 times better than air for interrupting arcs.

Inside a Gas Insulated Substation (Video)

Watch the video below to understand how different components are arranged in a GIS substation.

Advantages of Gas Insulated Substations

The following are the main advantages of Gas Insulated Substations over Air Insulated Substations and Hybrid Substations.

Compactness of GIS

The space occupied by SF6 installation is only about 10% of that of a conventional outdoor substation. High cost is partly compensated by saving in cost of space. 

Protection from pollution

The moisture, pollution, dust etc., have little influence on SF6 insulated sub-stations. However, to facilitate installation and maintenance, such substations are generally housed inside a small building. 

The construction of the building need not be very strong like conventional power houses. 

Reduced Switching over voltages

The over voltages while closing and opening line, cables motors capacitors etc. are low. 

Reduced Installation Time

The principle of building-block construction (modular construction) reduces the installation time to a few weeks. Conventional sub-stations require a few months for installation. 

Superior Arc Interruption

SF6 gas is used in the circuit-breaker unit for arc quenching. This type of breaker can interrupt current without overvoltages and with minimum acing time. Contacts have long life and the breaker is maintenance free. 

Gas Pressure

The gas pressure (4 kgf/cm2) is relatively low and does not pose serious leakage problems. 

Increased Safety

As the enclosures are at earth potential, there is no possibility of accidental contact by service personnel to live parts. 

Demerits of GIS

The following are the main disadvantages of Gas Insulated Substations over Air Insulated Substations and Hybrid Substations.

(a) High cost compared to conventional outdoor sub-station. 

(b) Excessive damage in case of internal fault. Long outage periods as repair of damaged part at site may be difficult. 

(c) Requirements of cleanliness are very stringent. Dust or moisture can cause internal flashovers. 

(d) Such sub-stations generally indoor. They need a separate building. This is generally not required for conventional outdoor sub-stations. 

(e) Procurement of gas and supply of gas to site is problematic. Adequate stock of gas must be maintained. 

Multiple Choice Questions (MCQs) on Alternator



1. In an alternator, voltage drops occurs in




2. The magnitude of various voltage drops that occur in an alternator, depends on




3. In an alternator, at lagging power factor, the generated voltage per phase, as compared to that at unity power factor




4. The power factor of an alternator depends on





5. Which kind of rotor is most suitable for turbo alternators which arc designed to run at high speed ?




6. Salient poles are generally used on




7. The frequency of voltage generated in an alternator depends on





8. The frequency of voltage generated by an alternator having 8 poles and rotating at 250 rpm is




9. An alternator is generating power at 210 V per phase while running at 1500 rpm. If the need of the alternator drops to 1000 rpm, the generated voltage per phase will be




10. A 10 pole AC generator rotates at 1200 rpm. The frequency of AC voltage in cycles per second will be




11. The number of electrical degrees passed through in one revolution of a six pole synchronous alternator is





12. Fleming's left hand rule may be applied to an electric generator to find out





13. If the input to the prime mover of an alternator is kept constant but the excitation is changed, then the





14. An alternator is said to be over excited when it is operating at





15. When an alternator is running on no load the power supplied by the prime mover is mainly consumed




Multiple Choice Questions (MCQs) on Synchronous Motors


1. Synchronous motor can operate at




2. An unexcited single phase synchronous motor is




3. The maximum power developed in the synchronous motor will depend on





4. In case the field of a synchronous motor is under excited, the power factor will be




5. A synchronous motor is switched on to supply with its field windings shorted on themselves. It will




6. When the excitation of an unloaded salient pole synchronous motor gets disconnected




7. The damping winding in a synchronous motor is generally used





8. The back emf set up in the stator of a synchronous motor will depend on




9. A synchronous motor is a useful industrial machine on account of which of the following reasons ?
I. It improves the power factor of the complete installation
II. Its speed is constant at all loads, provided mains frequency remains constant
III. It can always be adjusted to operate at unity power factor for optimum efficiency and economy.






10. Which of the following is an unexcited single phase synchronous motor ?




How Multicolor (Red Green Yellow) LEDs Work?

A LED that emits one color when forward biased and another color when reverse biased is called a multicolor LED.

Related Article: Working of Light Emitting Diode (LED)

One commonly used schematic symbol for these LEDs is shown below.

Working of Multicolor LEDs

Multicolor LEDs actually contain two pn junctions that are connected in reverse-parallel i.e. they are in parallel with anode of one being connected to the cathode of the other.

If positive potential is applied to the to terminal as shown below, the pn junction on the left will light.
Note that the device current  passes through the left pn junction.

If the polarity of the voltage source is reversed as shown in figure below, the pn junction on the right will light.

Note that the direction of device current has  reversed and is now passing through the right pn junction.

How Colors are Formed?

Multicolour LEDs are typically red when biased in one direction and green when biased in the other. 

If a multicolour LED is switched fast enough between two polarities, the LED will produce a third colour. 

A red/green LED will produce a yellow light when rapidly switched back and forth between biasing polarities.

Working of Light Emitting Diode (LED)


A light-emitting diode (LED) is a diode that gives off visible light when forward biased.

Light-emitting diodes are not made from silicon or germanium but are made by using elements like gallium, phosphorus and arsenic. By varying the quantities of these elements, it is possible to produce light of different wavelengths with colours that include red, green, yellow and blue.

For example, when a LED is manufactured using gallium arsenide, it will produce a red light. If the LED is made with gallium phosphide, it will produce a green light.

Working Theory of LED

When light-emitting diode (LED) is forward biased as shown in figure below, the electrons
from the n-type material cross the pn junction and recombine with holes in the p-type material. 

We know that these free electrons are in the conduction band and at a higher energy level than the holes in the valence band. 

When recombination takes place, the recombining electrons release energy in the form of heat and light. In germanium and silicon diodes, almost the entire energy is given up in the form of heat and emitted light is insignificant. 

However, in materials like gallium arsenide, the number of photons of light energy is sufficient to produce quite intense visible light.

The schematic symbol for a LED is shown in the above figure. The arrows are shown as pointing away from the diode, indicating that light is being emitted by the device when forward biased. 

Although LEDs are available in several colours (red, green, yellow and orange are the most common), the schematic symbol is the same for all LEDs. There is nothing in the symbol to indicate the colour of a particular LED. 

This is a graph between radiated light and the forward current of the LED. It is clear from the graph that the intensity of radiated light is directly proportional to the forward current of LED.

LED Voltage and Current

The forward voltage ratings of most LEDs is from 1V to 3V and forward current ratings range from 20 mA to 100 mA. 

In order that current  through the LED does not exceed the safe value, a resistor Rs is connected in series with it. The input voltage is Vs and the voltage across LED is Vⅆ.


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