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Overcurrent Relays for Transmission Line Protection

The use of overcurrent relays is the simplest and cheapest type of line protection. Three types of overcurrent relays are used:

  1. Inverse time delay overcurrent (TDOC) relays, 
  2. Instantaneous overcurrent relays, 
  3. Directional overcurrent relays.

In general, overcurrent relays are difficult to implement where coordination, selectivity, and speed are important. They usually require changes to their settings as the system configuration changes. 

Also, they cannot discriminate between load and fault currents. Hence, when they are implemented only for phase fault protection, they are only useful when the minimum fault current exceeds the full load current. 

However, they can effectively be used on subtransmission systems and radial distribution systems. This is due to the fact that faults on these systems usually do not affect system stability and therefore high-speed protection is not needed.

Inverse Time Delay Overcurrent Relays

The main use of TDOC relays is on a radial system where they are used for both phase and ground
protection. Basic complements of such relays are two phase and one ground relays. 

This can protect the line for all combinations of phase and ground faults using the minimum number of relays.

According to Horowitz, adding a third phase relay can provide complete backup protection, having two relays for every type of fault, and is the preferred practice. 

These relays are usually applied on subtransmission lines and industrial systems due to the low cost involved.

Instantaneous Overcurrent Relays

Since the closer the fault is to the source, the greater the fault current magnitude but the longer the
tripping time, the TDOC relay cannot be used all by itself. 

As shown in the figure, the addition of an instantaneous overcurrent relay makes such system of protection possible. 

However, there must be considerable reduction in fault current as the fault moves from the relay toward the far end of the line. In this manner, the instantaneous relay can be made to see almost up to, but not including, the next bus. 

The relay will not operate for faults beyond the end of the line but still provide high-speed protection for most of the line.

Directional Overcurrent Relays

When it is important to limit tripping for faults in only one direction in multiple-source circuits, the
use of directional overcurrent relays becomes necessary. 

The overcurrent relaying is made directional to provide relay coordination between all the relays that can see a given fault. Otherwise, the coordination is often too difficult if not impossible.

The directional relays require two inputs that are the operating current and a reference(or polarizing) quantity that does not change with fault location. For phase relays, the polarizing quantity is the system voltage at the relay location. 

For ground directional reference, the zero-sequence voltage (3Va₀ ) is used. However, it is often that the current in the neutral of a wye-connected/delta power transformer is used as a directional reference instead.

Neutral Grounding (Earthing) and Equipment Grounding

The term Grounding or Earthing refers to the connecting of a conductor to earth. The neutral points of generator and transformer are deliberately connected to the earth.

In 3 phase a.c. systems the earthing is provided at each voltage level. 

If a neutral point is not available, a special Earthing Transformer is installed to obtain the neutral point for the purpose of earthing. 

Neutral points of star connected VTs and CTs are earthed. The neutral earthing has several advantages.

Advantages of Neutral Grounding

The neutral earthing has several advantages.

  • Freedom from persistent arcing grounds. The capacitance between the line and earth gets charged from supply voltage. During the flash-over the capacitance get discharged to the earth. The supply voltage charges it again. Such alternate charging and discharging produces repeated arcs called Arcing Grounds. The neutral grounding eliminates the problem of 'arcing grounds'. 
  • The neutral grounding stabilizes the neutral point. The voltages of healthy phases with respect to neutral are stabilized by neutral earthing. 
  • The neutral earthing is useful in discharging over-voltages due to lightning to the earth.
  • Simplified design of earth fault protection. 
  • The grounded systems require relatively lower insulation levels as compared with u. grounded systems. 

The modern power systems are 3 phase a.c systems with grounded neutrals.

Equipment Grounding

The Equipment Grounding refers to the grounding of non-current carrying metal parts earth. It is used for safety of personnel. 

If a metal part is grounded, its voltage with respect earth does not rise to a dangerously high value and the danger of a severe shock to personnel avoided. 

Video : How Does a Tesla Electric Car Work?

Electric cars are making big waves in the automobile world. These noise-free, pollution-free and high-performance vehicles are expected to make their I.C. engine counterparts obsolete by 2025. 

This video will unveil the hidden technologies behind the Tesla Model S, which recently became the world’s fastest accelerating car. 
We will see how electric cars have achieved superior performance by analyzing the technology behind the induction motor, inverter, lithium ion battery power source, regenerative braking and above all, the synchronized vehicle mechanism, in a logical, step-by-step manner. 

The working and features of Tesla car is explained here with help of animation.

Voltage and Power Equations of DC Motor | Condition of Maximum Power

Voltage Equation of a DC Motor

The voltage V applied across the motor armature has to
  1. overcome the back emf Eb and
  2. supply the armature ohmic drop IaRa .

This is known as voltage equation of a motor.
Back EMF and Its Significance in DC Motor
Working Principle of DC Motor

Power Equation of DC Motor

Now, multiplying both sides of voltage equation by Ia , we get

Hence, out of the armature input, some is wasted in I2R loss and the rest is converted into mechanical power within the armature.

The gross mechanical power developed by a motor is

It may also be noted that motor efficiency is given by the ratio of power developed by the armature to its input

Obviously, higher the value of Eb as compared to V, higher the motor efficiency.

Condition for Maximum Power

The gross mechanical power developed by a motor is

Differentiating both sides with respect to Ia and equating the result to zero, we get

Thus maximum efficiency of a dc motor occurs when back EMF is equal to half the applied voltage.

Important Points 

Thus gross mechanical power developed by a motor is maximum when back EMF is equal to half the applied voltage.

This condition is, however, not realized in practice, because in that case current would be much beyond the normal current of the motor.

Moreover, half the input would be wasted in the form of heat and taking other losses (mechanical and magnetic) into consideration, the motor efficiency will be well below 50 percent.

5 DC Generator Problems with Solution - Part 1

1. A shunt generator delivers 450 A at 230 V and the resistance of the shunt field and armature are 50 Ω and 0.03 Ω respectively. Calculate the generated e.m.f?

The Generator Circuit is as shown in the figure,

2. A four pole generator having wave-wound armature winding has 51 slots, each slot containing 20 conductors. What will be the voltage generated in the machine when driven at 1500 rpm assuming the flux per pole to be 7.0 mWb ?

For a simplex wave wound generator,

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.

Skin Effect in Wires, Cables and Transmission Lines

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.

In short, current  flow over the surface of the conductor is called SKIN EFFECT

Due to Skin effect   

  Rac > Rdc  

because skin effect increases the conductor resistance.

Causes for skin effect:

  • A solid conductor, assumed to be consisting of large number of strands each carrying a small part of the current.
  • The inductance of each  part of the strand varies depending on its position.
  • Thus strands near the centre are surrounded by a greater magnetic flux having large inductance than at the surface.
  • The high reactance of linear strands cause the AC current to flow near the surface of the conductor.
  • The effective area of the cross-section of the conductor is reduced due to this skin effect.
  • The skin effect will be higher with

    • The frequencies more than 50Hz.
    • The size of the conductor is more than 1cm2

Factors Affecting Skin Effect in Transmission Lines

The skin effect in an ac system depends on some factors like

(1) Diameter of the wire
When the diameter of the conducting wire increased the skin effect will increase drastically.
The skin effect is negligible when the diameter of the wire is less than 1cm.

(2) Frequency
The skin effect is directly proportional to the supply frequency ( ie, increases with the frequency).
The skin effect is negligible when the frequency is less than 50Hz.

(3) Shape of wire
To minimize the skin effect, the shape of the  wire should be less for stranded conductor than that of solid conductor.
To reduce the skin effect, stranded conductors are used in transmission and distribution lines.
(4) Nature of material
This is another factor that affect skin effect in transmission lines.

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;
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 ("");
lcd . setCursor (0,1); //Move coursor to second Line
lcd . print (" Irrigation ");
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
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
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.


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.


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 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.
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