02-08-2013, 03:26 PM
Design of a Concept and a Wireless ASIC Sensor for Locating Earth Faults in Unearthed
Electrical Distribution Networks
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Abstract
In this paper, a new concept for using simple wireless
sensors for detecting and locating faults in an unearthed electrical
distribution network is presented. The concept is based on dis-
tributed wireless sensors that are attached to the incoming and
outgoing power lines of different feeders and branches. A sensor
measures only the phase current characteristics of the wire it is
attached to, is not synchronized to other sensors, and does not
need configuration of triggering levels. The main novelty of the
concept is in detecting and locating faults by using the electrical
distribution system aspects and the Bayesian probability theory.
The feasibility and selectivity of the concept is shown with several
simulations done with the Matlab SimPowerSystems environment.
In addition, a new design of a wireless application-specific inte-
grated circuit sensor for the proposed fault-management concept
is presented.
INTRODUCTION
N electrical high-voltage distribution systems (for example,
20 kV), different means to connect the neutral to earth exist.
In the Nordic countries (Finland, for instance), the neutral is
commonly ungrounded. With this approach, the fault current is
primarily composed of the currents flowing through the earth
capacitances of the sound phases. In other countries, espe-
cially in Continental Europe, the neutral is often compensated
(commonly known as a Petersen coil system). The aim of the
compensation is to cancel the system earth capacitance by
connecting an equal inductance to the neutral. Hence, the earth
fault current decreases correspondingly. In the case that the
inductance is tuned to exactly match the system capacitance,
the fault current will contain only a small resistive compo-
nent. In practice, however, the network is slightly under or
overcompensated at 95% or 105%. Solidly earthed distribution
networks are used in the U.S. for example. In these networks,
the single-phase-to-earth fault current varies with the fault
location and the fault resistance.
NETWORKS WITH AN UNEARTHED NEUTRAL
In electrical distribution networks with an unearthed neutral,
the single-phase-to-ground fault causes a current that mostly de-
pends on the phase-to-ground capacitances of the phase lines.
When the fault occurs, the network becomes unsymmetrical as
the capacitance of the faulty phase is bypassed (Fig. 1) [2].
Applying Thevenin’s theory, the system can be simplified to
Fig. 2, where the line impedances have been neglected because
they are small compared to those of the total earth capacitances
( is equal to the phase voltage).
CASE STUDY
The feasibility of the new concept and the characteristics of
the probabilistic approach are illustrated in a case study with the
system shown in Fig. 5. The system has three feeders of over-
head distribution lines connected to a 20-kV source and a funda-
mental frequency that randomly varies in the interval 49.5 Hz–
50.5 Hz (allowed variation in Europe). One feeder is divided
into four zones according to the layout in Fig. 3. Three tests
were designed.
1) The feeders have lengths of 50, 50, and 45 km. The 45-km
feeder is divided into four zones and a fault is applied to
every zone in order. The fault resistance is changed from
0 001 to 1 M .
2) The feeders have lengths of 5, 5, and 25 km. The 25-km
feeder is divided into four zones and a fault is applied to
every zone in order. The fault resistance is changed from
0 001 to 1 M .
WIRELESS ASIC SENSOR FOR THE POWER SYSTEM
The previously developed wireless sensors for power systems
have been built of commercial-off-the-shelf (COTS) compo-
nents. Typically, these sensors have a small microcontroller with
an inbuilt analog-to-digital converter, a short-range radio oper-
ating on a license-free band (433 MHz and 868 MHz in Europe,
900 MHz in the U.S.), as well as active and passive components
for filtering, signal amplification, and system power manage-
ment, see [4]–[11].
CONCLUSION
One of the biggest problems with using wireless sensors as
fault detectors in electrical distribution networks has been the
energy consumption of the sensors. Using batteries as the power
source is not feasible in the harsh environment, and previous so-
lutions have typically used an iron core winding that surrounds
a line conductor. These solutions have been big and heavy and
have thus affected the strength and mechanical characteristics of
the conductors they have been attached to (especially in windy
and winter conditions). To minimize the energy consumption of
sensors, the number of sampled quantities and the sampling fre-
quency must be low, and the sensors must have a very simple
mechanical and logical structure. The inaccuracy of the col-
lected data is instead mitigated using intelligent methods on a
higher system level.