If you can't explain it simply, then you don't know it well enough. — Albert Einstein

Advertisement:

Difference between Potentiometer and Rheostat

Potentiometer and Rheostat are two terms that are associated with the variable resistors. Technically both these terms represent the two different configurations provided by the same components. After reading this post you'll be able to develop a crystal clear concept regarding both terms.

Introduction to Variable Resistor (VR)

A variable resistor is a three terminal device. It provides a variable value of resistance across electrical circuits. For example, a 9 kΩ V.R will provide resistance ranging between 0-9k.
The most common type of V.R is shown below. It has three terminals a, b, c (We will drive in the details later on). The circular knob can be rotated to achieve variation in output resistance.

variable resistor

As previously mentioned the above type of variable resistor is most common. Meanwhile, it is the oldest one too.
Today's variable resistors are packaged as trimpots (the latest version) with a small bolt on one side. A screw tightener can be used with trimpots for operations.
[Also read: Diodes, Transistors and GTO]

Potentiometer

Let's reconsider the original variable resistor. The potentiometer configuration utilizes all the three terminals in working.
The left side of image displays the circuit diagram for config and right side displays the practical look.

potentiometer

Two blue wires connect to an external circuit for providing a variable voltage to the output. And that is the reason why potentiometer is named so.

Rheostat

This arrangement employs two terminals of a variable resistor in its working. Terminal a connects with the power source, b connects in series with the external circuit and c is left open. The purpose is to achieve a constant value of 'R' so as to achieve a variable current in the connect circuit/device. The left side of image displays the circuit diagram for Rheostat configuration and the right side provides a practical connection for this config.

rheostat

Potentiometer vs. Rheostat : Practical applications

A potentiometer provides variance in voltage at output terminals and is employed in Power industry for controlling the speed of DC Machines. It also finds its application in sound equipment for controlling the audio. The frequency matching on old radio sets utilised repeated principles of both these configurations.
Closing the above discussion, in a nutshell, we can summarise the results:
The potentiometer and rheostat are the two configurations that can be used in electronic circuits and components to achieve a variable voltage and current values.

Author: Guzel Sans completed his bachelor in Power Electrical Engineering. His areas of interest are HF Modelling, Power Systems Protection, and Electronics design Engineering. He loves Programming JS, CSS and playing with HTML5 in leisure hours. He is the founder of online tool Electrical Calculators. Favourite software: MATLAB.
Read More...

Loop Tests for Locating Faults in Underground Cables

We have seen the types of faults in underground cables and how to detect them using a megger in the previous article. This article explains how to locate the exact place where the fault has occurred.
There are various methods for locating the faults in underground cables. Following are some popular methods explained.

Murray loop test for location of faults in underground cables

Murray loop test is the most common and accurate method for locating earth faults and short-circuit faults. However, to perform the Murray loop test, it is necessary that a sound (good) cable runs along the faulty cable.This test employs the principle of Wheatstone bridge for fault location.
To perform the Murray loop test, the alongside sound cable and the faulty cable are shorted with a jumper conductor at the far end. The test side end is connected through a pair of resistors to a voltage source. Also, a null detector or galvanometer is connected between the two conductors at the test end. The circuit diagram is as shown in the image below.
murray loop test for location of faults in underground cables
Once the connections are made as shown in the above circuit, adjust the values of R1 and R2 so the null detector/galvanometer shows zero reading. That is, bring the bridge to the balance. Now, in the balanced position of Wheatstone bridge, we have,
murray loop test equation 1
Now, if r is the resistance of each cable,
then, Rx + Ry + Rg = 2r
Putting this in the above equation,
murray loop test equation 2
We know, the value of resistance is proportional to the length of the cable. Therefore the value of Rx is proportional to the length Lx. Therefore,
murray loop test fault distance location formula
Where L is the total length of the cable under test. (The value of L is proportional to the value of Rg.)

Varley loop test

Varley loop test is also for locating short-circuit and earth faults in underground cables. This test also employs the principle of the wheatstone bridge. However, the difference between Murray loop test and Varley loop test is that, in Varley loop test resistances R1 and R2 are fixed, and a variable resistor is inserted in the faulted leg. If the fault resistance is high, the sensitivity of Murray loop test is reduced and Varley loop test may be more suitable.
Varley loop test for location of underground cable faults
To perform Varley loop test, connections are done as shown in the circuit diagram above. Resistors, R1 and R2 are fixed and the resistor S is variable. In this test, the switch K if first thrown to the position 1. Then the variable resistor S is varied till the galvanometer shows zero deflection (i.e. bridge is balanced). Lets say, the bridge is balanced for the value of S equal to S1 Then,
varley loop test equation 1
Now, the switch K is thrown to the position 2 and the bridge is balanced by varying the resistor S. Say, the bridge is balanced at the value of resistor S is equal to S2. Then,
varley loop test equation 2
Now, putting the result of eq.(ii) in eq.(i),
varley loop test equation 3
Since the values of R1, R2, S1 and S2 are known, Rx can be calculated. When Rx is known, the distance from the test end to the fault point Lx can be calculated as,
Lx = Rx/r
Where, r = resistance of the cable per meter.

Read More...

Faults in Underground Cables : Types and Detection

One of the major limitations of underground cables is the fault detection. Since the cables are laid under the surface (directly or inside pressurized ducts), the visual methods of inspection don’t work effectively. This is not the case in Overhead Lines. In order to identify the faults in the cable, we need to develop special methods, which will be discussed in this article.
Before we discuss fault detection methods, we shall study the various types of faults occurring in Underground cables and their causes. The faults occurring in cables are:
  • Open circuit fault
  • Short circuit fault
  • Earth faults

Causes of Faults in Underground Cables

Most of the faults occur when moisture enters the insulation. The paper insulation provided inside the cable is hygroscopic in nature. Other causes include mechanical injury during transportation, laying process or due to various stresses encountered by the cable during its working life. The lead sheath is also damaged frequently, usually due to the actions of atmospheric agents, soil and water or sometimes due to the mechanical damage and crystallization of lead through vibration.
We shall study various faults and how to detect them.

Open Circuit Fault

As the name suggests, this fault involves an open circuit in the conductors. When one or more cable conductors (cores) break, it leads to discontinuity. This discontinuity also occurs when the cable comes out of its joint due to mechanical stress. This is known as Open circuit fault.

Fault detection

An open circuit is characterized by infinite resistance. This is utilized in fault detection. The conductors at the far end are bunched together (shorted) and earthed. Then the resistance between each conductor and the earth is measured using a megger.

Observations

  • If there’s no fault, megger will read nearly zero.
  • If there’s an open circuit in a conductor, the will read infinite when connected between that conductor and the earth.

Short Circuit Fault

It occurs only in multi-cored cables. When two or more conductors of the same cable come in contact with each other, then this is called a short circuit fault. It is impossible to detect visually without taking the cable apart. A short-circuit fault occurs when the individual insulation of the cables is damaged. It can also be detected using a megger.

Fault detection

A short-circuit is characterized by zero resistance. This is utilized in fault detection. The resistance between any two conductors is measured using a megger. This is done for all the conductors, two at a time.

Observations

  • If the megger reads zero, it indicates that a short-circuit fault has occurred between those two conductors.

Earth Fault

When any of the conductors of the cable comes in contact with the earth, it is called an earth fault. This usually occurs when the outer sheath is damaged due to chemical reactions with soil or due to vibrations and mechanical crystallization. It is somewhat similar to a short circuit fault as the current again takes the least resistive path and flows through the earth. This too can be detected using a megger.

Fault detection

The megger is connected between the conductor and the ground and megger reading is noted. This is repeated for all the conductors of the cable.

Observations

  • If an earth fault is present, the megger will show nearly zero reading.

Hence we can detect faults in underground cables using a megger.
detection of underground cable faults using a megger
Read More...

Grading of underground cables

The electrostatic stress in a cable isn’t uniformly distributed. The potential gradient is inversely proportional to the distance from the center of the cable. Hence it will be maximum (gmax) at the surface of the conductor and goes on decreasing until it becomes minimum (gmin) at the surface of the sheath. That means electrostatic stress in the dielectric of a cable is maximum at the surface of the conductor and minimum at the surface of the sheath.
Obviously, for a safe cable, the dielectric strength of the insulation provided must be more than gmax i.e. maximum value of potential gradient. As the electrostatic stress in a cable isn’t uniformly distributed, the strength of the dielectric required isn’t uniform either. We need maximum dielectric strength only at the surface of the core. The remaining dielectric is unnecessarily strong and hence not utilized properly. This also causes the cable to be unnecessarily thick. A large size of electric equipment is always a disadvantage. Furthermore, the possibility of insulation breakdown is more if the stress distribution is non-uniform. These problems are rectified by Grading of the cables.

Grading of underground cables

Grading of a cable is nothing but the process of achieving uniform electrostatic stress in the dielectric of cable. This is achieved by making potential gradient equal throughout the dielectric layer. It can be done in two ways - (i) capacitance grading and (ii) intersheath grading.

Capacitance grading

Capacitance grading is done by employing various layers of different dielectrics having different permittivities between the core and the sheath. Hence the dielectric insulation provided is no longer homogeneous, but composite. The various layers are arranged so that the permittivity decreases from the surface of the conductor to the sheath of a cable i.e. the permittivity of dielectric is inversely proportional to the distance from the center (just like the potential gradient.)
capacitance grading of underground cables

Let an underground cable consist of three dielectric layers as shown in the above image. The inner conductor core is represented by the circle of radius r. The radii of the three dielectric layers are r1, r2 and R respectively. Similarly, let relative permittivities be ε1, ε2 and ε3 respectively. The relative permittivity values and their distances are ε1 > ε2 > ε3 and r1 < r2 < R. The uniform dielectric stress can be achieved by maintaining the product of permittivity and radius of each dielectric as same, i.e. ε1r1 = ε2r2 = ε3R.
Ideally, the dielectric stress will be uniform throughout the cable if we use infinite layers of dielectric. Practically, two or three layers are used. The chief disadvantage is that we require more number of dielectrics with their permittivities varying over a wide range. These are, of course, costly. An alternative is Inter sheath grading.
[Also read: Capacitance of underground cables]

Intersheath grading

In this method, instead of using various dielectrics and having a composite dielectric, we use a homogeneous dielectric material. However, in order to distribute the stress properly, we use extra metallic sheaths between the conductor and the main sheath. These intermediate sheaths are called ‘intersheaths’. These intersheaths are then held at adequate voltage levels. This method improves voltage distribution in the dielectric of the cable and consequently uniform potential gradient is obtained.
intersheath grading of underground cables

There are certain disadvantages of intersheath grading. The major limitations arise in fixing the intersheath potentials accurately and the losses encountered due to the increased charging currents of the various inter sheaths. For these reasons, this practice is rarely employed.

[Also read: Types of underground cables]
Read More...

Capacitance of underground cables

As we saw earlier in the construction of Underground cables, a cable is basically a set of one (or three) conductors surrounded by a metallic sheath. This arrangement can be considered as a set of two long, coaxial, cylinders, separated by insulation. The current carrying conductor forms the inner cylinder while the metallic sheath acts as the outer cylinder. The sheath is grounded, and hence voltage difference appears across the cylinders. The dielectric fills the space between the charged plates (cylinders), making it a capacitor. Hence, capacitance of the cable becomes a very important aspect, and must be calculated.
We can broadly classify cables as single-cored and three-cored. And the calculation of capacitance is different for both.

Capacitance of single core cable

A single core cable can be represented as shown below.
capacitance of single core cable
Let,
r = radius of the inner conductor and d = 2r
R = radius of the sheath and D = 2R
ε0 = permittivity of free space = 8.854 x 10-12
εr = relative permittivity of the medium
Consider a cylinder of radius x meters and axial length 1 meter. x be such that, r < x < R.
Now, electric intensity Ex at any point P on the considered cylinder is given as shown in the following equations.
Then, the potential difference between the conductor and sheath is V, as calculated in equations below.
After that, capacitance of the cable can be calculated as C= Q/V
calculation of capacitance of single core cable
When the capacitance of a cable is known, then its capacitive reactance is given by Xc = 1/(2πfC) Ω.
Then the charging current of the cable can be given as,
Ic= Vph / Xc        A

Capacitance of three core cable

Consider a three cored symmetric underground cable as shown in the following figure (i). Let Cs be the capacitance between any core and the sheath and Cc be the core to core capacitance (i.e. capacitance between any two conductors).
capacitance of three core cable

In the above figure (ii), the three Cc (core to core capacitance) are delta connected and the core to sheath capacitance Cs are star connected due to the sheath forming a single point N. The circuit in figure (ii) can be simplified as shown in figure (iii). Outer points A, B and C represent cable cores and the point N represents the sheath (shown at the middle for simplification of the circuit).
Therefore, the whole three core cable is equivalent to three star connected capacitors each of capacitance Cs + 3Cc as shown in fig. (iii).
The charging current can be given as,
Ic = 2πf(Cs+3Cc)Vph      A

Measurement of Cs and Cc

In order to calculate Cs and Cc we perform various experiments like:
  1. First, the three cores are connected together and capacitance between the shorted cores and the sheath is measured. Shorting the three cores eliminates all the three Cc capacitors, leaving the three Cs capacitors in parallel. Therefore, if C1 is the now measured capacitance, Cs can be calculated as, Cs = C1/3.
  2. In the second measurement, any two cores and the sheath are connected together and the capacitance between them and the remaining core is measured. If C2 is the measured capacitance, then C2 = 2Cc+Cs (imagine the above figure (iii) in which points A, B and N are short circuited). Now, as the value of Cs is known from the first measurement, Cc can be calculated.

Effects of capacitance in underground cables

We know that capacitance is inversely proportional to separation between plates. Hence, if the separation between the plates is large, capacitance will be less. This is the case in Overhead Lines where two conductors are separated by several meters. The converse, of course, is also true. If the separation is small, the capacitance is more. In Underground cables, obviously, the separation is relatively smaller. Hence capacitance of underground cables is much more than that of Overhead lines.
The most important factor that is affected by this is the Ferranti effect. It is more pronounced in cables than in lines. This induces several limitations.
Also, with increased capacitance, the charging current drawn is also increased. Underground cables have 20 to 75 times the line charging current compared to Overhead lines.
Due to these two conditions, the length of Underground cables is limited.
Read More...