Aveneu Park, Starling, Australia

Underground method but transmission line by underground

Underground
cables have been widely used in power distribution networks due to the advantages
of underground connection, involving more security than overhead lines in bad
weather, less liable to damage by storms or lightning. It is less expensive for
shorter distance, eco- friendly and low maintenance. To improve the reliability
of a distribution system, accurate identification of a faulted segment is
required in order to reduce the interruption time during fault, i.e. to restore
services by determining a faulted segment in timely manner. 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 LG, LL, LLL faults. Hardware implementation of
both methods will be carried out to determine the effectiveness of both the
methods for fault detection.

 

 

 

 

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 a simple
method to localize cable fault testing. This test is performed for the location
of either an earth fault or short circuit fault in underground cable. In these
tests the resistance of fault does not affect the results obtained except when
the resistance of fault is very high. There are two loop tests usually used and
are known as Murray Loop and Varley Loop Test. This test works on the principle
of Wheatstone bridge. The test is used to find the fault location in an
underground cable by making one Wheatstone bridge in it and by comparing the
resistance we shall find out the fault location. the faulty cable is connected
with sound cable by a low resistance wire, because that resistance should not
affect the total resistance of the cable and it should be able to circulate the
loop current to the bridge circuits without loss 4.

 

2-      OHM’S
LAW METHOD

 

In this method simple OHM’s law
is used to locate the short circuit fault. A DC voltage is applied at the
feeder end through a series resistor, depending upon the length of fault of the
cable current varies. The voltage drop across the series resistor changes
accordingly, this voltage drop is used in determination of fault location.

 

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

 

Accurate identification of a
faulted segment is required in order to reduce the interruption time during
fault in an underground distribution network. 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
the methods use voltage convertor, microcontroller and potentiometer to find
the fault location under LG, LL, LLL faults. 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-      Experimentation

6-      Discussion
based on results

7-      Conclusion