Insulation Measurement and Supervision in Live AC and DC Unearthed Systems

193 Pages·2013·1.7 MB·English

by  Piotr Olszowiec (auth.)

Lecture Notes in Electrical Engineering 167
Forfurthervolumes:
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Piotr Olszowiec
Insulation Measurement and
Supervision in Live AC and
DC Unearthed Systems
ABC
Author
PiotrOlszowiec
ElpoautomatykaPolaniec
Staszow
Poland
ISSN1876-1100 e-ISSN1876-1119
ISBN978-3-642-29754-0 e-ISBN978-3-642-29755-7
DOI10.1007/978-3-642-29755-7
SpringerHeidelbergNewYorkDordrechtLondon
LibraryofCongressControlNumber:2012937999
(cid:2)c Springer-VerlagBerlinHeidelberg2013
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Foreword
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.
Similarly 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 attention
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 final 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. Main causes of electrical insulation
deterioration are ageing, mechanical and thermal stresses, overvoltages, humidity,
chemical factors, oil, radiation etc. Continuous monitoring is an indispensable tool
for preventive maintenance which allows to avoid possible faults caused by
insulation condition deterioration. Information on 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 insulation
monitoring. As result substantial qualities offered by these systems can be
exploited. As 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,
VI Foreword

− 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
important 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 calculation are explained. Some of them were proposed by the
author. Practically all formulas are derived. For the text understanding merely
basic knowledge of electrical circuits theory is required. Review 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 it will supplement very scant information on the
subject 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 .................. 5
1.2.1 Single-Phase Systems .................................................................... 5
1.2.2 Three-Phase Systems ..................................................................... 7
1.3 Ground Fault and Leakage Currents Calculation ...................................... 9
1.3.1 Single-Phase Networks ................................................................. 9
1.3.2 Three-Phase Networks ................................................................. 13

2 Ground Insulation Measurement in AC IT Systems ................................. 17
2.1 General Information ................................................................................ 17
2.1.1 Spatial Distribution of Insulation Resistance – Network
Insulation Equivalent Circuit ....................................................... 17
2.2 Insulation Parameters Determination in Single-Phase Networks............ 18
2.2.1 De-energized Networks ............................................................... 18
2.2.2 Live Networks.............................................................................. 20
2.3 Insulation Parameters Determination in Live Three-Phase Networks .... 22
2.3.1 Insulation Equivalent Resistance and Capacitance Values
Determination .............................................................................. 22
2.3.2 Insulation Resistance and Capacitance Determination
for Single Phases .......................................................................... 24

2.4 Unconventional Measurement Methods ................................................. 30
2.4.1 Periodical Measurement of Insulation Parameters ....................... 30
2.4.2 Devices and Systems for Ground Fault, Earth Leakage
and Shock Currents Measurement ............................................... 33
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
2.5.3 Electric Shock Hazard Assessment .............................................. 37
2.6 Ground Fault Current Compensation ...................................................... 39
References ...................................................................................................... 42

3 Insulation Monitoring Systems .................................................................... 43
3.1 Visual Signalization Systems .................................................................. 43
3.2 Other Systems of Continuous Insulation Monitoring ............................. 43
VIII Contents

3.2.1 Phase Voltages Monitoring .......................................................... 43
3.2.2 Zero-Sequence Voltage Component Monitoring ......................... 47
3.2.3 Residual Current Monitoring ....................................................... 49
3.2.4 Underimpedance System ............................................................. 54

4 Systems of Insulation Resistance Continuous Measurement .................... 57
4.1 Measurement Circuits with Test Direct Current ..................................... 57
4.1.1 Measuring Circuit with Diode Bridge Rectifier ........................... 58
4.1.2 Other Rectifier Based Measuring Circuits ................................... 60
4.2 Measurement Method with an Auxiliary Rectangular Voltage
Source ..................................................................................................... 62
4.3 Measurement with Use of Auxiliary AC Voltage ................................... 65
References ...................................................................................................... 67

Part II: DC IT Systems

5 Equivalent Circuit Diagrams of DC Networks ........................................... 71
5.1 DC Network Simplified Circuit Diagram ............................................... 71
5.2 Equivalent Circuit Diagrams of Batteries ............................................... 73
Reference ........................................................................................................ 77

6 Insulation Resistance Measurement Methods ............................................ 79
6.1 Traditional Methods of Insulation Resistance Measurement
in Live Networks .................................................................................... 79
6.2 Other Analytical Methods ....................................................................... 82
6.3 Unconventional Methods of Insulation Resistance Measurement .......... 84
6.4 Evaluation of Errors of Analytical Methods ........................................... 86

7 Devices and Systems for Insulation Deterioration Alarming .................... 89
7.1 Visual Signaling of Insulation Resistance Level .................................... 89
7.2 Simple Systems of Continuous Insulation Monitoring ........................... 90
Reference ........................................................................................................ 95

8 Modern Methods of Continuous Insulation Measurement ....................... 97
8.1 Measurements with Superimposed AC Test Voltage ............................. 97
8.2 Commutation Method ............................................................................. 99
8.2.1 Determination of Insulation Equivalent Resistance
of DC Network and Its Single Lines .......................................... 101
8.3 “Pulse” Test Voltage Method ............................................................... 104
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 ............................................................................. 110

References ........................................................................................................... 112
Contents IX

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 ........................................................................... 116
9.3 Earth Leakage Currents determination ................................................. 120
9.3.1 Calculations ............................................................................... 120
9.3.2 Electric Shock Hazard Assessment ............................................ 121
9.4 Leakage Current Measurements ........................................................... 124
9.4.1 Periodic Measurements .............................................................. 124
9.4.2 Continuous Measurements ......................................................... 125
9.5 Earth Fault and Shock Currents Measurement ..................................... 126
9.5.1 Earth Fault and Shock Currents Measurements
in Network Models .................................................................... 126
9.5.2 Earth Fault and Shock Currents Measurements
in Live Networks ....................................................................... 127
9.6 Network-to-Ground Capacitance Determination .................................. 128
References .................................................................................................... 131

Part III: AC and DC IT Systems

10 Effects of AC and DC It Systems Insulation Failures ............................ 135
10.1 Reasons of Insulation Failures .......................................................... 135

10.2 Effects of Network Insulation Failures .............................................. 136

10.3 Misoperation of Devices ................................................................... 137

10.4 Prevention of Devices Misoperation ................................................. 145

10.4.1 Device Coil Shunted by Resistor .......................................... 145
10.4.2 Device Coil Shunted by Other Elements .............................. 147
10.4.3 Disconnection of Both Terminals of Device Coil ................. 148
10.4.4 Coil Shorting by NC Contact ................................................ 148
10.4.5 Limitation of Total Conductor-to-Conductor
and Conductor-to-Ground Capacitances ............................... 149
10.4.6 Insulation Resistance Control by Grounding
through Resistors .................................................................. 149
Reference ............................................................................................................. 150

11 Insulation Monitors Settings Selection .................................................... 151
11.1 General .............................................................................................. 151

11.2 Regulations Requirements for DC Systems ...................................... 152

11.3 Modified Approach for DC IT Networks .......................................... 153

11.3.1 Shock and Fire Hazard Assessment ...................................... 153
11.3.2 Misoperation of Devices in DC Circuits ............................... 155
11.3.3 Examples of Practical Checking of Insulation
Condition Assessment Criteria.............................................. 157
11.3.4 Graphical Illustration of Insulation Conditions
in DC IT Networks ............................................................... 159
X Contents

11.4 AC Insulation Monitors Settings Selection ....................................... 160

11.4.1 Simplified Approach ............................................................. 160
11.4.2 Electric Shock and Fire Hazard Assessment ......................... 161
11.4.3 Misoperation Risk for Devices in AC IT Auxiliary
Circuits ................................................................................. 164
11.4.4 Insulation Monitors Application for Devices
Misoperation Risk Detection ................................................ 166
References ........................................................................................................... 167

12 AC/DC IT Systems .................................................................................... 169
12.1 Conductor-to-Ground Voltages ......................................................... 169
12.2 Earth Fault and Leakage Currents ...................................................... 173
12.3 Misoperation of Devices in “Mixed” Systems ................................... 176
12.4 Insulation Measurement and Monitoring in AC/DC IT Systems ....... 180
12.4.1 Insulation Equivalent Resistance Determination ................... 181

12.4.2 Pulse Voltage Method ........................................................... 182

12.4.3 Auxiliary AC Voltage Method .............................................. 184

13 Earth Fault Location in It AC/DC Systems ............................................ 185
13.1 General .............................................................................................. 185
13.2 Test Current Measurement in Fault Locating Systems ..................... 187
13.3 Traditional Earth Fault Location Systems ........................................ 188
13.4 Modern Insulation Fault Location Systems ...................................... 189
References .................................................................................................. 193

About the Author ............................................................................................... 195
Part I

AC IT Systems