Underground underground cable is widely preferred to ensure safety.

Underground
cables have been intensively employed in power distribution systems owing to
their numerous advantages such as; less vulnerability to damage by weather
conditions and lightning, underground connections, aesthetic requirements, eco
friendliness, low maintenance and low costs for shorter distances. Despite
their numerous advantages, identification of fault location in underground cables
is a very complicated issue and poses a serious challenge to the reliability,
stability and security of an underground distribution network. A timely and
accurate identification of a faulted segment on the transmission network is
very critical to reduce circuit interruption time during a fault. In this
research work we present two methods which will be very useful to identify the
exact distance of fault of underground system from base station. One of the
methods is Murray loop method and other one is Ohm’s Law Method. Murray loop
method uses the whetstone bridge to calculate exact distance of fault location
from base station. Whereas in Ohm’s law method, when any fault occurs, voltage
drop will vary depending on the

 

 

length of fault
in cable, since the current varies. Both the methods use voltage convertor, microcontroller
and potentiometer to find the fault location under Line to ground, Line to line,
LLL faults. Hardware implementation of both methods will be carried out to
determine the effectiveness of both the methods for fault detection

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1.     
Introduction:

 

Most of the transmission lines
are laid using overhead line method but transmission line by underground method
also finds its use and application over a large area. In areas like hospitals
or colleges, underground cable is widely preferred to ensure safety.
Underground cable installations are costly as compared to overhead cable but
are more reliable and also the life of underground cable are more as compared
to overhead lines.

Although underground cables are
unaffected by adverse conditions like a storm, rainfall and the chances for
fault in underground cables are less than that of overhead cables but when the
fault happens at undergrounds cables its detection becomes difficult. So it
becomes essential to calculate the distance of fault for an efficient way to
employ underground cable method 1. Locating a faulted segment of underground
cable system requires broader aspects of consideration and analysis 2. Unlike
overhead lines, underground cables have the characteristics of smaller
inductance but larger capacitance. The analysis becomes complicated when various
types of underground cables are used 3.

 

TYPES OF FAULT:

Fault in a cable can be
classified as:

 

A.) Open circuit fault

B.) Short circuit fault

 

Open circuit fault:

This type of fault is caused by
breaking in conducting path etc. Such fault happens when one or more phase
conductor wire break. The value of current in such fault becomes zero and the
load side gets isolated from the Generation side. This fault is less harmful as
no current flows when short circuit fault occurs.

Short circuit fault:

When conductors of different
phases get connected with each other than such fault comes under short circuit
fault. In this type of fault the value of current increases so it becomes
harmful at the load ends.

 

There are basically 2 types of
short circuit fault:-

i. Symmetrical Fault

ii. Unsymmetrical Fault

 

Symmetrical Fault:

The 3-phase fault is called a
symmetrical fault. In this, all 3-phases are short-circuited. In this fault the
phase angles are unchanged but the magnitude of the current can vary.

Unsymmetrical Fault:

In this fault magnitude of the
current is not equal and also not displaced by 120-degree angle. The different
phases are short-circuited with each other.

 

 

 

Two methods presented in this
research work can resolve this problem related to underground system. Both of
these methods are very fast and accurate in finding the fault

Location therefore can be proved
very useful. These methods are explained in detail:

 

1-      MURRAY
LOOP METHOD

 

Murray loop test is an effective
way to locate fault in an underground cable. This test can be carried out for
either a short circuit fault or an earth fault in an underground cable. The key
to this method is that generally resistance of the fault affects the results
only when its magnitude is very high. This test is based on Wheatstone bridge
and includes Murray loop test as well as Varley loop test. To find out fault in
the cable a Wheatstone bridge is made in it and resistances are compared to
determine fault location. The faulty cable is connected with the un faulty
cable through a wire having very low resistance as low resistance does not
affect total resistance of the entire cable and permits loop current to
circulate through the Wheatstone bridge without loss 4.

 

 

2-      OHM’S
LAW METHOD

 

Ohm’s law method is relatively a
very simple method used to locate a short circuit fault. Depending upon length
of the power cable current varies when at feeder end when DC voltage is applied
through a series resistor. The voltage drop in the series resistor varies
accordingly and fault location is determined using voltage drop across the
resistor.

 

The core objective of this
research work is to design and implement hardware modules employing both of the
above mentioned techniques for fault detection and study their effectiveness carrying
out various performance tests.

 

 

2.     
Literature Review:

 

Power system fault location and
identification of the different faults on a underground power cable system for
quick & reliable operation of protection scheme. Fault location estimation
is very important issue in power system in order to clear faults quickly 5.
Different techniques commonly used for underground cable fault detection and
relevant work is summarized below:

 

6 proposes a novel method for
detecting and locating a multicycle incipient fault in a cable. The incipient
fault is modeled as a self-clearing arcing fault. The distortion degree of
calculated voltage is used to detect the occurrence of an incipient fault. The
degree of match between the measured and calculated waveforms is used to guide
the search for the fault distance. The accuracy is further improved by taking
into account the incipient fault angle as seen in the voltage waveform and the
power loss characteristics.

 

A three step process is followed
in 7 with creation of transmission system model using a Matlab/Simulink and
followed by creation of faults in the system. In second step the Fourier
analyzed fault voltages and currents obtained from the SIMULINK model are fed

 

to the training set of artificial
neural network (ANN) in order to detect the type of fault.

In the last step, an independent
software OrCad is used to locate the fault distance from the either ends using
the principle of time domain reflectometry in a simulated practical underground
distribution system.

 

In 8 the feasibility of
applying complex wavelet analysis to fault detection. We combine complex
wavelets with continuous wavelet transform (CWT), and calculate the impedance
from the voltage and current data in the wavelet domain. We then examine the
magnitude and phase distributions of the impedance under various conditions. We
test our analysis approach with measurement data from different types of
cables. The results show that the complex wavelet analysis based approach is
able to provide unique signatures for distinguishing between the cables, thus
very promising for fault detection.

 

9 shows a fault detection
method by monitoring current of sheath. Using MATLAB to analysis the simulation
data, the results show that the method can effectively detect cable main
insulation breakdown. Using this can realize the online monitoring of power
cable fault.

 

10 Presents a new method for
detection of incipient cable failures by using measured current and voltage at
one end of the cable. The incipient faults are detected using an innovation
signal calculated from the measured fault current via a Kalman filter. Upon
change detection by the innovation signal, the change is checked for
discriminating possible incipient fault from other similar conditions.

 

11 Medium voltage underground
cables may exhibit incipient, self-clearing arcing faults prior to failing
permanently. These events typically last one half- cycle and extinguish at the
first natural zero crossing of the current. The magnitude of the half-cycle
event is primarily dependent on the location of the fault on the feeder, but is
also dependent on the point on the voltage waveform that the fault starts.

 

 

3.     
Statement of the Problem

 

Identification of fault location
in underground cables is a daunting task and poses a serious challenge to the
reliability of underground distribution network. A timely and accurate
identification of a faulted segment on the transmission network is very
critical for reduction of circuit interruption time during a fault. Fault
detection using Ohm’s law method and Murray loop method is the most cost
effective method to estimate the distance of the fault from base station. In
this proposed research work it is intended to design prototypes to estimate
fault location and present an in-depth analysis of their performance in a
conventional underground distribution system.

 

 

 

 

 

4.      Research
Methodology

 

In this research work we present two methods which will be very useful to
identify the exact distance of fault of underground system from base station.
One of the methods is Murray loop method and other one is Ohm’s Law Method.
Murray loop method uses the

Whetstone bridge to calculate exact distance of fault location from base
station. Whereas in Ohm’s law method, when any fault occurs, voltage drop will
vary depending on the length of fault in cable, since the current varies. Both
of these methods discussed make use of Microcontroller, Voltage converters and
potentiometer to locate the fault in an underground cable for LL, LG and LLL
fault types. Hardware implementation of both methods will be carried out to
determine the effectiveness of both the methods for fault detection. The
proposed research work will be carried out in the following steps:

 

1-      A
detailed analysis of fault detection methods will be presented

2-      Identification
of design specification

3-      Selection
of design components

4-      Hardware
implementation

5-      Discussion
based on result

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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