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Title A NEW LOOK AT THE DETECTION OF INTERRUPTED AND

DOWNED CONDUCTORS IN THE MV OVERHEAD LINES

Registration Nº: (Abstract)

Company E-SENZOR d.o.o., Maribor, Slovenia

Authors of the paper Name Country e-mail

Marjan Bezjak Slovenia marjan.bezjak1@guest.arnes.si

Goran Ambrožič Slovenia goran.ambrozic@gmail.com

Matjaž Finc Slovenia matjaz.finc@gmail.com

Rafko Bertoncelj Slovenia raff.bertoncelj@gmail.com

Gregor Bezjak Slovenia gregor@e-projekt.si

Key words

overhead lines, interrupted conductors, voltage measurements, sensors

Conductors on power network are often interrupted (broken) due to various reasons. Causes for the interruption or breaking of a conductor can be listed as external (mechanical overloads, lightning surges, falling trees, interference by a third party) or internal (inadequate materials, inadequate technological solutions etc.). In most cases, such interruptions of one or more conductors are detected by electrical protection which is based on detection of grounding current that after a programmed process disconnects a downed power line.

Still, during cases of broken conductor fault, the downed conductor does not allow sufficient grounding current to be detected by the electrical protection (e.g. long lines, poor ground contact due to isolation on the conductor or damp terrain). In all of these cases, the interrupted conductor with high voltage represents a high risk of death or a serious injury until failure is detected and the overhead line is switched off, which can take a few hours or more.

The problem of often interrupted conductors in Slovenia in recent years is particularly characteristic of the covered conductors in MV overhead lines. Therefore there were a lot of research projects to discover the causes of conductor interruption and to develop the protective device which can determine interruption and location of fault.

A few years ago we have developed and installed a protective device, called LiSa®, which measures voltage conditions in the MV line using capacitive divisors and is equipped with modern communication. The devices are installed along the three-phase MV lines, in order to determine the location of fault.

These devices are part of a complex system for remote control of substations and MV lines. Due to high complexity of such systems, requirements have emerged for the development of simpler and lower cost devices for contactless detection of interrupted conductors, which are easy installed on poles along the three-phase lines and equipped with GSM communication.

1

A NEW LOOK AT THE DETECTION OF INTERRUPTED AND DOWNED

CONDUCTORS IN THE MV OVERHEAD LINES

Marjan Bezjak, Goran Ambrožič, Matjaž Finc, Rafko Bertoncelj, Gregor Bezjak

marjan.bezjak1@guest.arnes.si, goran.ambrozic@gmail.com, matjaz.finc@gmail.com,

raff.bertoncelj@gmail.com, gregor@e-projekt.si

ABSTRACT

This paper presents a method for contactless

detection of downed conductor faults in overhead

lines. The results of theoretical analysis, simulations

of multi-phase electrical fields, measurements on an

MV test field and also on real MV lines are

presented, compared and interpreted. A model is

introduced based on a variation of electrical

parameters. This model is based on measuring

different parameters of the electric field and can be

applied to networks with different conductor

geometries (horizontal, triangular…). The

developed prototype sensor devices, which can be

implemented along a three-phase radial line and

integrated into a protection system (SCADA) via

GSM communication, are described.

Keywords: overhead lines, interrupted

conductors, voltage measurements, sensors

1 INTRODUCTION

Interrupted and downed conductors are

hazardous when energized. Therefore, they have to

be quickly detected and repaired. When a downed

line creates a high-impedance fault (HIF) e.g.

conductor falls onto soil, asphalt, foliage, dry snow,

rocks or, in the case of covered conductors or long

lines, the fault current is so low that conventional

protection devices are often unable to detect it.

The nature of HIF has been widely studied

since the early 1960s with the aim of finding a

practical method for detecting such disturbances.

Various detection techniques have been studied [1,

2, 3]. In general, two approaches are the most

common, based on sensing changes in:

• current with its harmonic and non-harmonic

frequency components; protection is at the

beginning of the line, together with feeder

protection [4, 5, 6, 7, 8],

• voltage: protection devices are installed along

the line for determining the locations of failures

and at the end of the line [2, 9, 10, 11, 12, 13,

14, 15].

Current protection of HIF operates in the

same way and in the same position as conventional

protection, but determining the location of the failure

is difficult and false alarms cannot be ruled out.

Monitoring voltage changes along the radial

lines serves as a relatively simple and reliable

detection of energized open conductors. Voltage

detectors can be easily connected to the Medium

Voltage (MV) lines via voltage transformers or

capacitive dividers on the pole [13] or can be

integrated into a power quality monitoring system at

MV/LV transformer stations [12, 14, 15]. Beside

using wired communication infrastructure [9, 10],

modern low-power autonomous wireless technology

enables easier distribution of voltage detectors and

independent integration into the SCADA systems.

Contactless voltage monitoring sensors with

wireless communication would make fault detection

systems radically easier to install and cheaper to

maintain as long as they provide robust and reliable

fault detection. We introduce an approach based on

contactless voltage monitoring using electrical field

sensors.

2 THEORY

2.1 Single Probe System

Three conductors 1, 2 and 3 (spacing d, height

H) with identical properties and geometry are placed

above earth. A small conductive probe 4 is placed at

distance h below the middle line. The configuration

is illustrated in Figure 1.

Figure 1 – Single probe configuration

Due to symmetry, the line configuration can

be represented by the substitute capacitive circuit as

illustrated in Figure 2.

Figure 2 – Capacitive circuit

2

By applying a symmetrical three-phase

source to the conductors, we can derive the

following model from the introduced circuit:

�� � ��� ∙ sin�� sin�� � ∙ sin��� �1�

� �����

2�� �� ��

�2�

� ���

��

�3�

The model shows that, measuring the

amplitude can be much more volatile (in relation to

generator voltage amplitude and geometry i.e.

substitute capacitances), while the phase angle is

only defined by a simple constant ratio η,

independent of generator voltage and can be

interpreted more easily. Different situations can be

displayed on a phasor diagram as in Figure 3.

Figure 3 – Single probe diagram

When all phase voltages are present

(reference condition), the result is a phasor:

�� � ���1 � �� ∙ sin��� �4�

By changing the location of the probe (h),

ratio η varies and therefore the ratio of amplitudes

during different scenarios (i.e. reference condition

and faults) can be interpreted from the diagram.

2.2 Dual Probe System

If we upgrade the model to dual probes,

spaced for x from the central line through the middle

conductor as in Figure 4, we can approximate the

new model by using different ratios for potentials V4

and V5:

�� � ���� ∙ sin�� sin�� �� ∙ sin��� �5�

�� � ���� ∙ sin�� sin�� �� ∙ sin��� �6�

Figure 4 – Dual probe configuration

If we display the model on the phasor

diagram, as in Figure 5, we can see that in the

reference condition, the phase angle difference

between V4 and V5 is wider than during faults. For

open conductors, single phase presence or fault

modulation through the end-of-line symmetrical

load (e.g. transformer), the difference in phase angle

is similarly different from the reference condition.

Figure 5 – Dual probe diagram

For other symmetrical line configurations

(e.g. delta), the model still applies, but with different

K and ratio η, meaning the probes must be positioned

at different locations for similar phase angle

behavior.

3 MEASUREMENTS

3.1 Sensors

We have experimented with different

configurations of sensor architecture and probe

shapes for contactless measurements. We can divide

the sensor types into two groups:

• single probe type (Figure 6),

• dual probe type (Figure 7).

Both types are autonomous battery powered

electronic devices, housed within a protective case.

Each external voltage sensing probe is connected to

a dedicated amplifier, which measures the potential

between the probe and the printed circuit board

(PCB) reference plane. Data is processed within an

internal processing unit and stored using internal

storage. The data can be collected remotely (e.g. sent

3

via GSM) and/or downloaded by local wired

connection.

Figure 6 – Single probe amplitude sensors

Single probe sensors were used to measure

only the voltage amplitudes. We experimented with

floating and with grounded PCB references. In order

to maximize the collected charge (i.e. capacitance),

the probes were shaped in the forms of discs or caps,

installed on top of the sensor.

Figure 7 – Dual probe amplitude/phase sensor

Dual probe sensors were used to measure

voltage and phase differences between the probes.

We experimented with different distances between

probes, installed on an isolated spacer in the form of

a tube.

3.2 Simulation

In order to evaluate the practicality of the

introduced model we set-up exact equations for an

electrostatic model of the system and have

numerically displayed the solved results within with

the use of spreadsheet software (MS Excel). In order

to verify the results, we used special software for

modelling electromagnetic systems (Ansoft

Maxwell).

For example, we took a dual probe

configuration with three conductors at height H =

8m, spacing between conductors d = 0.5m, dual

probe at h = 2m below central conductor and spacing

between probes 2·x = 1m. The different fault

scenarios are displayed in Figures 8 to 11 and the

results are summarized in Table 1.

Figure 8 – Reference condition

Figure 9 – U1 = 0V

Figure 10 – U2 = 0V

Figure 11 – U1 = U2 = 0V

fault |U4| (norm.) |φ(U4-U5)|

Excel Maxwell Excell Maxwell

U1=U2=U3=20kV 1 1 92° 88°

U1=0V, U2=U3=20kV

4.5 5 8° 6°

U2=0V,

U1=U3=20kV 5,1 5,1 18° 16°

U1=U3=0V, U2=20kV

4.4 4.4 0° 0°

Table 1 – Summary results of simulated faults

The simulation showed that the amplitude

and phase angle had drastically changed, as the

model predicted.

3.3 Preliminary Experiments

In order to find optimal configuration of the

sensor hardware (layout, amplifier gain etc.),

algorithm and probe shapes for optimizing the

sensitivity and range, we set-up a test field with a

replica of a horizontal three-phase overhead line.

The pole was configurable, with a maximum height

of 5.8m, allowing for different line configurations

(e.g. horizontal, delta, etc.). The lines were made of

standard covered conductors. Controlled low voltage

AC generators were routed through three single

phase variable transformers, stepped up via MV

measuring transformers and connected to one end of

the lines. The maximum test voltage used on the

polygon was 30kV. The other end was left open. The

actual polygon set-up is shown in Figure 12.

4

Figure 12 – Actual polygon setup

We tested all the aforementioned types of

sensors (single probe floating, single probe grounded

and dual probe).

The list of tests was extensive and was out of

scope of this paper. It took a number of iterations and

different scenarios to optimize the sensor’s response.

U1U2U3 U1U2 U1U3 U2U3

Ho

rizo

nta

l

DU

T1 |φ| 169° 12° 49° 15°

Up1 34 60 84 81

Up2 17 59 56 49

DU

T2 |φ| 138° 5° 21° 7°

Up1 28 82 100 100

Up2 11 84 82 82

Del

ta

DU

T1 |φ| 83° 27° 55° 2°

Up1 68 133 185 175

Up2 39 72 75 61

DU

T2 |φ| 26° 13° 20° 1°

Up1 88 129 194 189

Up2 71 100 137 137

Tri

angu

lar

DU

T1 |φ| 112° 13° 60° 17°

Up1 70 89 146 136

Up2 36 76 75 52

DU

T2 |φ| 51° 7° 27° 7°

Up1 81 91 152 147

Up2 37 100 110 102

Table 2 – Example test results (dual probe, 20kV)

The results showed (examples for 20kV tests

with dual probe type in Table 2), that although

voltage on a probe increases when faults are present,

its value is sensitive to probe location and the

environment (e.g. terrain configuration, vicinity of

poles, humans etc.). Also, the voltage levels on the

probes and, additionally, the voltages between the

probes in the dual type significantly vary according

to different line configurations (i.e. horizontal, delta,

triangular). All this made voltage measurements

more scattered and the margins between the

reference and the faulty conditions were narrower.

Therefore, it was not trivial to universally define the

thresholds for the detection of faults. Our conclusion

was, that sensing faults based only on voltage

detection is possible but would require adaptation

and tuning of sensors for different configurations. In

contrast, measuring the phase angles between probes

proved to be much more stable and distinct

regardless of the line configurations and was more

resilient to external factors. Since K is dependent on

more factors than η (Ug – voltage fluctuations; C7 –

geometry/terrain), this behavior can be justified.

3.4 Real Lines

A field test was carried out on a real MV line

(20kV) between Novokračine and Jelšani, which is

managed by a Slovenian utility Elektro Primorska.

The sensor set-up is shown in Figure 11. We were

monitoring it with a floating single probe sensor

(DUT1), a grounded single probe sensor (DUT2) and

two dual probe sensors (DUT3, DUT4).

Figure 13 – Real line field test

We performed two types of tests:

• long term tests,

• line interruptions.

The purpose of the long-term tests was

monitoring the sensor operation under different

environmental conditions and testing

communication reliability. Sensors DUT1 and

DUT2 were actively measuring and sending data

daily via GSM. DUT1 was down after a few months

due to an installation error, DUT2 was running

reliably for almost a year.

As we had already found during preliminary

experiments, the measured voltage was quite

sensitive to environmental conditions. An excerpt

from the measured data of floating (DUT2) and

grounded (DUT1) single probe sensors is provided

in Figure 14. The measured voltages on both sensors

followed the ambient temperature, which was

measured using internal on board temperature

sensors. Obviously, the voltage inside the floating

sensor was much more unstable, due to the

measurements not being made against a fixed

reference.

5

Figure 14 – Long term voltage measurements1

The purpose of line interruption tests was to

verify the sensor detection in a real installation. All

the interruption tests were performed under

supervision of trained utility personnel, who also

performed the controlled interruptions. During the

tests, the lines were interrupted a few kilometers

before the location of the sensors. Interruptions were

carried out in sequences of 5 minutes to avoid any

transients and to have stable conditions for testing.

The interruption sequence was as follows: 1.

interrupt line U1, 2. interrupt line U3. The measured

results of a floating single probe sensor (DUT1), a

grounded single probe sensor (DUT2) and two dual

probe sensors (DUT3, DUT4) are given in Tables 3

and 4 for open transmission lines at the far end and

in Tables 5 and 6 for loaded lines.

fault DUT1 DUT2 DUT3

Ugrounded Ufloating Uprobe1 Uprobe2 |φ|

all on 1027 1157 320 330 90°

U1 off 6500 9100 900 900 10°

U1,U3 off 7100 12400 970 970 0,2°

Table 3 – Open lines' measurements1

fault DUT4

Uprobe1 Uprobe2 |φ|

all on 53 51 124°

U1 off 329 191 8°

U1,U3 off 327 190 8°

Table 4 – Open lines' measurements (cont.)1

fault DUT1 DUT2 DUT3

Ugrounded Ufloating Uprobe1 Uprobe2 | φ |

all on 1012 1130 320 320 90°

U1 off 6400 8200 900 900 12°

Table 5 – Loaded lines' measurements1

fault DUT4

Uprobe1 Uprobe2 |φ|

all on 51 51 124°

U1 off 295 168 5°

Table 6 – Loaded lines' measurements (cont.)1

The simulation example, given in Table 1,

shares the same configuration as DUT3. The

increase of voltage amplitude during faults in DUT3

was less than during simulation, while the phase

angle matched more closely. The reasons for this had

1 Units of measured voltage are scaled relative to internal

amplifier range and serve as amplitude indication only;

units in different sensor types are not in the same scale

already been established in preliminary experiments

(the effects of terrain configuration, pole proximity

and grounded PCB planes etc.).

4 CONCLUSION

In this article a theoretical model was

introduced as the basis for interpreting amplitude

and phase angles below the overhead lines. It

showed that besides monitoring voltage, detecting

the phase difference between two probes not too far

apart, it could serve as good and even more stable

indicator for detecting interruptions.

Based on the model, different types of sensors

were introduced and their feasibilities studied using

preliminary experiments. Single probe sensors are

more compact and cost less, but they only rely on

voltage amplitude measurements. It turned out, that

such devices are sensitive to environmental effects.

Therefore it is not trivial to use them for reliable

interruption detection. Furthermore, floating sensors

are less demanding for installation and maintenance

than the grounded type, but they are much less

reliable. Dual probe sensors have provided much

more reliable results, especially based on phase

angle measurements.

The suitability of the introduced sensors was

verified on a real line configuration with forced

interruptions. The tests showed that the introduced

dual probe principle could provide a solid basis for

implementing stable and reliable indicators of

conductor interruptions.

In comparison to standard voltage monitoring

indicators, connected via a capacitive or transformer

interface, the introduced contactless sensor

architecture is incomparably easier to install and

maintain. Due to smaller size and lower cost, they

can be more feasible for utilities to cover a wider

network area.

By using modern GSM infrastructure

effectively, it is rather straightforward to integrate

such sensors into other software platforms (e.g.

SCADA or IoT systems). The long-term tests

showed that with the use of an adequate battery pack,

running a sensor for a period of 5-10 years is viable.

While GSM is feasible mostly in urban areas, other

wireless technologies could be exploited.

Future work would relate to more in depth

analysis of environmental impact on measurements

and measurement optimization in order to maximize

battery life. In order to evolve measuring sensors

into reliable real-time indicators of faults, optimized

decision algorithms need to be implemented in the

sensors.

6

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Paper 1405

AUTHORS

Marjan Bezjak received his M.Sc. in 1995 in

Electrical Engineering at University of Zagreb,

Croatia. He is currently working as Senior

Researcher and as Project Developer at E-SENZOR.

Goran Ambrožič received his B.Sc. in 2000 in

Electrical Engineering at University of Ljubljana,

Slovenia. He is currently working as Engineer at

Iskra Sistemi. His current work is related to testing

PQM equipment and software, customer consulting

and project designs in the field of industrial and

energy sector.

Matjaž Finc received his M.Sc. and Ph.D. in 2004

and 2006 in Electrical Engineering at University of

Ljubljana, Slovenia. He is currently working as

Senior Researcher at RC ENEM and as Project

Developer – Specialist at Iskra Sistemi. His current

work is related to research and development of new

communication equipment, systems and products in

the field of industrial and energy sector.

Rafko Bertoncelj received his B.Sc. in 2002 in

Electrical Engineering at University of Ljubljana,

Slovenia. He is currently working as Senior

Developer at RC ENEM and as Senior Developer at

Iskra Sistemi. His current work is related to

development of new electronic measuring

instruments in the field of industrial and energy

sector.

Gregor Bezjak received his B.D. in 2001 as

graduate of Commercial Tourism School, Koper,

Slovenia. He is currently working as Manager of

E-SENZOR.