Insulation Measurement and Supervision in Live AC and DC Unearthed Systems

182 Pages·2014·5.56 MB·English

by  Piotr Olszowiec (auth.)

Lecture Notes in Electrical Engineering 314
Piotr Olszowiec
Insulation
Measurement
and Supervision
in Live AC and DC
Unearthed Systems
Second Edition
Lecture Notes in Electrical Engineering
Volume 314
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Piotr Olszowiec
Insulation Measurement
and Supervision in Live
AC and DC Unearthed
Systems
Second Edition
1 3
Piotr Olszowiec
Elpoautomatyka Polaniec
Staszow
Poland
ISSN 1876-1100 ISSN 1876-1119 (electronic)
ISBN 978-3-319-07009-4 ISBN 978-3-319-07010-0 (eBook)
DOI 10.1007/978-3-319-07010-0
Springer Cham Heidelberg New York Dordrecht London
Library of Congress Control Number: 2014939050
1st edition: © Springer-Verlag Berlin Heidelberg 2013
© Springer International Publishing Switzerland 2014
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Preface
Low voltage unearthed AC and DC networks have been for long commonly applied
for supply of power and control circuits in industry, transportation, medical objects,
etc. The main reasons for their use are high reliability and numerous advantages
offered by isolating the networks against ground.
Similar to other electrical systems, also in unearthed (IT) networks insulation
level, is a decisive factor for operational reliability and safety. Electrical networks
insulation fulfills the following main functions:
• establishing path for current flow,
• elimination of various hazards including electric shock and fire risks.
The basic technical parameter determining condition and quality of electrical
insulation is its resistance. Insufficient level of this parameter can cause various
disturbances. According to statistic data ground faults are the most frequent type
of failures in AC and DC networks. Other common defects include line-to-line
faults, breaks (broken wires), voltage loss, or its abnormal deflections. Ground
faults in IT systems do not make networks operation impossible, however, they
may cause severe problems with their safe functioning. Therefore, special atten-
tion should be paid to these abnormal conditions, possible threats created by them,
and ways for their detection and elimination.
It has been proved that many insulation breakdown cases do not happen
suddenly but are the nfi al stage of a long degradation process. This fact is a strong
argument for conducting continuous supervision over insulation condition to ensure
timely detection of possible problems. The main causes of electrical i nsulation
deterioration are aging, mechanical and thermal stresses, overvoltages, humidity,
chemical factors, oil, radiation, etc. Continuous monitoring is an i ndispensable tool
for preventive maintenance, which allows to avoid possible faults caused by insula-
tion condition deterioration. Information about current insulation level helps users
to achieve high reliability and safety of electrical systems.
AC and DC IT systems, as isolated against ground under normal operation,
allow—in distinction from TN and TT systems—to fulfill continuous insula-
tion monitoring. As a result, substantial qualities offered by these systems can be
v
vi Preface
exploited. Compared to TN and TT systems, electrical unearthed networks are
featured by:
1. high safety and reliability of operation, namely
• insulation-to-ground monitoring is possible only in networks isolated against
ground,
• networks can operate with a single ground fault,
• it is possible to conduct preventive maintenance due to on-line insulation
monitoring in live network,
• insulation breakdowns can be detected without delay,
• insulation monitoring can be fulfilled both in live and in de-energized
networks,
2. smaller fire and explosion hazards,
3. lower shock currents and touch voltages,
4. higher permissible resistance of devices protective earthing.
Utilization of these advantages is dependent on conducting correct insulation
monitoring. Importance of electrical networks insulation monitoring has been
known for long, but only rapid development of electronic and microprocessor
technologies has led to implementation of sophisticated methods and systems.
However, for their proper application adequate knowledge of electrical systems
operation is indispensable. Therefore, in this book there are described most impor-
tant issues concerning normal operation and ground fault phenomena occurring
there. Theoretical basis of these subjects is delivered in concise form. Numerous
methods of insulation parameters measurement in live circuits are presented. Few
other procedures of the parameters determination based on measurement and cal-
culation are explained. Some of them were proposed by the author. Practically
all formulas are derived. For the text understanding merely a basic knowledge of
electrical circuits theory is required. Overview of selected insulation measurement
devices as well as fault locating systems is included. This book is addressed to
electrical engineers, technicians, and students of this specialty. The author hopes
that its extended second edition will supplement scant information about the sub-
ject available in existing publications.
Contents
Part I AC IT Systems
1 General Characteristics ...................................... 3
1.1 AC IT Systems Circuit Diagrams ............................ 3
1.2 Phase-to-Ground Voltages Determination in AC IT Systems ....... 4
1.2.1 Single-Phase Systems ............................... 4
1.2.2 Three-Phase Systems ............................... 6
1.3 Ground Fault and Leakage Currents Calculation ................ 8
1.3.1 Single-Phase Networks .............................. 8
1.3.2 Three-Phase Networks .............................. 11
2 Ground Insulation Measurement in AC IT Systems ............... 15
2.1 General Information ...................................... 15
2.1.1 Spatial Distribution of Insulation Resistance: Network’s
Insulation Equivalent Circuit ......................... 15
2.2 Insulation Parameters Determination in Single-Phase Networks .... 16
2.2.1 De-energized Networks ............................. 16
2.2.2 Live Networks .................................... 19
2.3 Insulation Parameters Determination in Live
Three-Phase Networks .................................... 21
2.3.1 Insulation Equivalent Resistance and Capacitance
Values Determination ............................... 21
2.3.2 Insulation Resistance and Capacitance Determination
for Single Phases .................................. 24
2.4 Unconventional Measurement Methods ....................... 29
2.4.1 Periodical Measurement of Insulation Parameters ......... 29
2.4.2 Devices and Systems for Ground Fault, Earth Leakage
and Shock Currents Measurement ..................... 32
2.5 Influence of Insulation Parameters on Possible Ground Fault,
Electric Shock and Ground Leakage Currents Levels ............ 35
2.5.1 Assessment of Ground Fault and Ground
Leakage Currents .................................. 35
2.5.2 Assessment of Power Losses in Insulation ............... 36
vii
viii Contents
2.5.3 Electric Shock Hazard Assessment .................... 37
2.6 Ground Fault Current Compensation ......................... 38
References .................................................. 41
3 Insulation Monitoring Systems ................................ 43
3.1 Visual Signalization Systems ............................... 43
3.2 Other Systems of Continuous Insulation Monitoring ............ 45
3.2.1 Phase Voltages Monitoring ........................... 45
3.2.2 Zero-Sequence Voltage Component Monitoring .......... 46
3.2.3 Residual Current Monitoring ......................... 48
3.2.4 Underimpedance System ............................ 52
4 Systems of Insulation Resistance Continuous Measurement ........ 57
4.1 Measurement Circuits with Test Direct Current ................. 57
4.2 Measuring Circuits with Diode Rectifier ...................... 58
4.2.1 Star Connected Diode Rectifier ....................... 58
4.2.2 Diode Bridge Rectifier .............................. 62
4.2.3 Other Rectifier Based Measuring Circuits ............... 64
4.3 Measurement Method with an Auxiliary Rectangular
Voltage Source .......................................... 67
4.3.1 Examples of Application ............................ 68
4.4 Measurement with Use of Auxiliary AC Voltage ................ 69
4.4.1 Application Example ............................... 71
References .................................................. 71
Part II DC IT Systems
5 Equivalent Circuit Diagrams of DC Networks .................... 75
5.1 DC Network Simplified Circuit Diagram ...................... 75
5.2 Equivalent Circuit Diagrams of Batteries ...................... 77
Reference ................................................... 81
6 Insulation Resistance Measurement Methods .................... 83
6.1 Traditional Methods of Periodical Measurement of Insulation
Resistance in Live Networks ............................... 83
6.2 Other Analytical Methods ................................. 86
6.3 Unconventional Methods of Insulation Resistance Measurement ... 87
6.4 Evaluation of Errors of Analytical Methods .................... 90
7 Devices and Systems for Insulation Deterioration Alarming ........ 93
7.1 Visual Signaling of Insulation Resistance Level ................ 93
7.2 Simple Systems of Continuous Insulation Monitoring ........... 94
Reference ................................................... 98
Contents ix
8 Modern Methods of Continuous Insulation Measurement ........ 99
8.1 Measurements with Superimposed AC Test Voltage ........... 99
8.2 Commutation Method .................................. 100
8.2.1 Example of Application .......................... 101
8.2.2 Determination of Insulation Equivalent Resistance
of DC Network and Its Single Lines ................ 102
8.2.3 Example of Application .......................... 104
8.3 “Pulse” Test Voltage Method ............................. 105
8.3.1 An Example of Application ....................... 106
8.4 Unconventional Methods of Insulation Resistance Monitoring ... 108
8.4.1 Insulation Supervision with Insulation Leakage
Resistance Control .............................. 108
8.4.2 Method of Auxiliary Voltage “Triangle” Pulses ........ 109
8.4.3 System of Automatic Insulation-to-Ground
Capacitance Compensation ....................... 109
References ................................................. 111
9 Ground Fault, Leakage and Electric Shock Currents
in DC IT Systems .......................................... 113
9.1 Time Function of Ground Fault Current .................... 113
9.2 Measurements of Maximum and Steady-State Magnitudes
of Earth Fault Current .................................. 115
9.3 Earth Leakage Currents ................................. 118
9.3.1 Calculations ................................... 118
9.3.2 Electric Shock Hazard Assessment ................. 120
9.4 Leakage Current Measurements ........................... 122
9.4.1 Periodic Measurements .......................... 122
9.4.2 Continuous Measurements ........................ 123
9.5 Earth Fault and Shock Currents Measurement ................ 124
9.5.1 Earth Fault and Shock Currents Measurements
in Network Models .............................. 124
9.5.2 Earth Fault and Shock Currents Measurements
in Live Networks ............................... 125
9.6 Network-to-Ground Capacitance Determination .............. 126
References ................................................. 128
Part III AC and DC IT Systems
10 Effects of Insulation Failures ................................ 131
10.1 Reasons of Insulation Failures ............................ 131
10.2 Effects of Network Insulation Failures ..................... 132
10.3 Misoperation of Devices ................................ 133
10.4 Prevention of Devices Misoperation ....................... 139
10.4.1 Device Coil Shunted by Resistor ................... 139