Inspection, Testing and Commissioning of Electrical … · 2019. 2. 4. · 7 Cabling Commissioning...

Inspection, Testing and Commissioning of Electrical Switchboards, Circuit Breakers, Protective Relays, Cables and PLCs

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Presents

Inspection, Testing and Commissioning of Electrical Switchboards,

Circuit Breakers, Protective Relays, Cables and PLCs

Revision 1

Website: www.idc-online.com E-mail: [email protected]

IDC Technologies Pty Ltd PO Box 1093, West Perth, Western Australia 6872 Offices in Australia, New Zealand, Singapore, United Kingdom, Ireland, Malaysia, Poland, United States of America, Canada, South Africa and India Copyright © IDC Technologies 2011 All rights reserved. First published 2011

ISBN: 978-1-921716-86-7 All rights to this publication, associated software and workshop are reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher. All enquiries should be made to the publisher at the address above. Disclaimer Whilst all reasonable care has been taken to ensure that the descriptions, opinions, programs, listings, software and diagrams are accurate and workable, IDC Technologies do not accept any legal responsibility or liability to any person, organization or other entity for any direct loss, consequential loss or damage, however caused, that may be suffered as a result of the use of this publication or the associated workshop and software.

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Trademarks All logos and trademarks belong to, and are copyrighted to, their companies respectively. Acknowledgements IDC Technologies expresses its sincere thanks to all those engineers and technicians on our training workshops who freely made available their expertise in preparing this manual.

Contents 1 Fundamentals of Switchgear 1

1.1 Single Line Diagram 1 1.2 Typical Construction – LV/MV and HV 2 1.3 Active and Passive Network Components 5 1.4 Circuit Breaker Utilisation 5 1.5 Fuse Switches 6 1.6 HV Fuses in Combination with and as an Alternative to Circuit Breakers 6 1.7 Auto-reclosers and Auto-reclose operations 7

2 Specifications of Switchgear 9 2.1 Switchgear ratings – Highest System and Impulse Withstand Voltages, Load and Short Circuit Currents 9 2.2 Simple and Complex Protection Schemes 14 2.3 Switchgear Ancillaries, Measurement CTs, VTs and Relays 14 2.4 Cable Terminations 17 2.5 Indoor and Outdoor Operation of Switchgear 19

3 Short Circuit Testing 21

3.1 Symmetrical and Asymmetrical Ratings 21 3.2 Make and Break Operations 22 3.3 Understanding Test Oscillograms 25 3.4 Case Study – Specification for a 132kV Switchboard 26

4 Switchgear Diagnostics, Testing and Maintenance 41

4.1 Asset Records 41 4.2 Condition Based Maintenance (CBM) 42 4.3 Reliability Centered Maintenance (RCM) 43 4.4 Switchgear Inspection Methodologies 45 4.5 Diagnostic Techniques 47 4.6 Principles of Circuit Breaker Maintenance 59 4.7 Oil Testing 65 4.8 Maintenance of Vacuum Circuit Breakers and SF6 69 4.9 Switchgear Defects and Defect Control 71 4.10 Switchgear Installations 73

5 Power System Protection Principles and Relays 79

5.1 Principles of Protection and Need for Protective Apparatus 79 5.2 Types of Faults 81 5.3 Types of Protection Systems 84 5.4 Types of Protective Relays 85 5.5 Electromechanical and Static Relays 86 5.6 Numerical Relays 95

6 Configuration of Numerical Relays 103 6.1 Introduction 103 6.2 Setting Approach in Conventional Relays 103 6.3 Configuring Numerical Relays 104 6.4 Configuration Security Through Passwords 109 6.5 Protection Settings as Part of a Configuration 110 6.6 Methods Adopted for Setting Numerical Relays 116 6.7 Examples of Front Panel Configuration Sequence 117 6.8 Configuration Exercises for Typical Relays/Simulation Software 122

7 Cabling Commissioning and Periodic Testing 125

7.1 Review of Codes for Testing 125 7.2 Drum Length Checks 127 7.3 Post Installation Checking 128 7.4 Pre-commissioning and Periodic Tests 129 7.5 Tests as Tools for Condition Monitoring and Early Failure Alarm 129 7.6 HV Tests Using DC and Very Low Frequency AC 130 7.7 Partial Discharge Tests and Mapping of Results 133 7.8 Dielectric Dissipation Factor Measurements 137 7.9 Micro Destructive and Non-Destructive Tests for Life Assessment 140 7.10 Operation and Maintenance of Cables 140

8 Cable Failure Modes and Fault Detection 141

8.1 Types of Failures 141 8.2 Reasons for Failures 142 8.3 Fault Location 146 8.4 Electrical Tests for Detection of Cable Faults 147 8.5 Safety Issues in Fault Detection 151 8.6 Analysis of Failures 153

9 Switchboard installation, inspection and commissioning 157

9.1 Receiving, Handling and Storage 157 9.2 Inspection 158 9.3 Installation 159 9.4 Type, Routine, Acceptance and Pre-comissioning Tests 161 9.5 High Voltage Equipment Test Techniques 162 9.6 Commissioning Procedures 163

10 Interface to Control Equipment (PLCs) 165

10.1 Overview of PLC 165 10.2 PLC I/O Modules 167 10.3 Pre-Commissioning Tests 170 10.4 Commissioning Procedures 171 10.5 Typical Faults 173

11 Case Studies of Typical Problems 175 11.1 Improper Circuit Breaker Trip Unit and Relay Settings 175 11.2 Motor Overload Protection 176 11.3 Medium Voltage Distribution System Installation Problems 177 11.4 Switchboard Metering Problems 178 11.5 Emergency Distribution Problems 178 11.6 Noisy Transformers 179 11.7 Incorrect Earthing and Neutrals 180

1

Fundamentals of Switchgear

In this chapter, we will learn about one of the basic, but most important components of any electrical system, namely the Switchgear. As a preparatory step towards the understanding of switchgears, we will review the use of electrical single line diagrams and the types of components that make up electrical systems. We will also study the various applications of switchgear in the later part of this chapter.

Learning objectives • Single line diagrams • Typical construction – LV/MV and HV • Active and passive network components • Circuit breaker utilization • Fuse switches • HV fuses in combination with and as alternatives to circuit breakers • Auto-reclosers and auto-reclose operations

1.1 Single line diagram

Electrical systems are mostly made up in the form of three phase configuration. However trying to represent three phase systems in drawings as actual three phase connections may not only be laborious but also make the drawing quite difficult to understand. This is where the concept of single line diagram comes into being. The term 'Single line diagram' simply means that the three phase nature of the circuits and components is ignored. Figure 1.1 shows a typical example of a high voltage distribution network, represented as a single line diagram, complete with transformer infeeds, cable and overhead circuits, distribution substations, circuit breakers and other network switchgear.

Inspection, Testing and Commissioning of Electrical Switchboards, Circuit Breakers, Protective Relays, Cables and PLCs 2

Figure 1.1 Typical network as a single line diagram

1.2 Typical construction – LV/MV and HV

Circuit breakers come in a variety of types. The prominent types being:

• Bulk oil circuit breaker • Minimum oil circuit breaker • Air blast circuit breaker • Vacuum circuit breaker • SF6 circuit breaker

Figure 1.2 shows the construction of a typical bulk oil type circuit breaker. Such breakers used oil as the insulation medium and were very heavy and bulky. They also posed serious fire hazards and oil

Fundamentals of Switchgear 3

pollution problems. With the advancements in technology such breakers have over a period of time been gradually replaced with more compact and versatile vacuum and SF6 breakers.

Figure 1.2 Bull oil circuit breaker

Figure 1.3 shows the typical construction of a minimum oil circuit breaker. Similar to bulk oil breaker, oil is used as the insulation medium in this breaker. However, as the name implies, much lesser quantity of oil is used in this type of breaker. Porcelain is used for the construction of the body and minimum oil circuit breakers have much smaller foot prints than the bulk oil breakers.

Figure 1.3 Small oil volume circuit breaker (one phase shown)

Figure 1.4 shows the typical construction of an air blast circuit breaker. High pressure air is used as insulation and arc extinguishing medium. Disadvantages of this type of breaker are the need for a compressed air plant and associated pipe lines.


Figure 1.4 Basic air blast circuit breaker

Figure 1.5 shows the construction details of a vacuum interrupter. Vacuum is used for interrupting and extinguishing the arc at the time of the breaker operation. The ceramic cartridge contains the fixed and moving electrodes maintained under high vacuum conditions. Owing to the sealed nature of the interrupter, the vacuum breaker is impervious to external dust and pollution conditions.

Figure 1.5 Vacuum interrupter

Figure 1.6 depicts the construction details of a typical SF6 gas insulated breaker with a vacuum interrupter. SF6 gas under pressure is used as the insulation medium and this enables the breaker to be constructed quite small and compact. The pressure of the gas needs monitoring on a continuous basis and the purity of the gas has to be checked periodically to maintain the effectiveness of the breaker.


Figure 1.6 Vacuum interrupter in SF6 enclosure switchgear, with rated three position disconnector (isolator)

1.3 Active and passive network components

Network components may be divided into two broad categories, those that are continuously in use whenever electricity is being supplied, called the ACTIVE components and those called upon to function only when required to do so, the PASSIVE components.

ACTIVE components comprise transformers, cables, overhead lines and metering equipment and may be considered the main revenue earning elements. PASSIVE components compose mainly switchgear in its various forms, together with switchgear's ancillaries, current and voltage transformers and protection relays. These components perform no revenue earning function and may be considered as an added cost, and to be minimized wherever possible.

1.4 Circuit breaker utilisation

In an ideal network having perfect reliability and no maintenance or repair requirement, there is no need to break the electric circuits; however, this is impossible; therefore switchgear has to be installed. However, we should be careful not to install more switchgear than is absolutely required and the switchgear that is installed should have no more functionality than is necessary. The two most common forms of medium voltage switchgear are automatic circuit breakers and non-automatic, load breaking, fault making switches. The main function of a circuit breaker is to automatically interrupt fault current, although an important secondary function is to close onto a fault and thereby make fault current. This function may be required during for example, fault location. The automatic disconnection of faults by circuit breakers allows the remainder of the network to continue in operation, after the faulty branch has been disconnected. Without this feature, network operation would be 'all or nothing'. In theory all the switchgear shown in Figure 1.1 could be implemented by circuit breakers, but this choice would be very expensive and result in an excessive maintenance burden. In practice, circuit


breakers are normally located only at the beginning of the cable, overhead line or mixed circuit, where they serve to disconnect faults either phase to phase or phase to neutral/earth (or both).

1.5 Fuse switches

Historically, circuit breakers were also utilized at each distribution substation, to control and protect the transformer, although fuse switches may nowadays implement this function. Figure 1.7 shows the cross section of a typical switchfuse unit.

Figure 1.7 Fuse switch in cross section

1.6 HV fuses in combination with and as alternative to circuit

breakers

Medium voltage fuses are available up to a current rating equivalent to a three phase rating of approximately 1.5 MVA and offer a cost-effective means of transformer protection. Generally they are combined with a three phase disconnector and a 'trip all phases' device, so that if one fuse fails, the disconnector trips and the supply to the transformer are fully disconnected. If this device were not fitted, the transformer may be subjected to 'single phasing' and a corresponding low LV side voltage. The use of fuses over circuit breaker has the advantage of lower cost of installation and very fast disconnection (typically less than half a cycle) in the event of a fault. However the disadvantage is the amount of time needed to replace the blown fuse and inability to close or open the circuit from a remote location.


Where fuses are used in conjunction with reclosing type circuit breakers, the satisfactory operation of a circuit breaker in association with fuses depends upon the degree of co-ordination obtained, requiring that the time/current characteristic of the re-closed circuit breaker be graded with those of the fuses to give optimum discrimination. The first trips must be short enough that the fuses are not degraded, whilst the second and third must be long enough to allow the fuses to operate.

1.7 Auto-reclosers and auto-reclose operations

In networks comprising substantial lengths of overhead line, many of the faults that occur are transient in nature. These are caused for example by wind blown debris bridging conductors, insulator flashover, etc. The reliability of the supply may be considerably improved by arranging for the controlling circuit breaker to re-close automatically after a fault occurs. This ensures that for transient faults, the restoration time is measured in seconds, rather than the hours it might take for an engineer to travel to the substation and re-close the circuit breaker manually. On overhead line networks, most transformers and spur lines are protected by medium voltage expulsion type fuses and it will be necessary for the protection/re-close relay of the circuit breaker controlling the circuit to grade with these fuses. This ensures that where a fault on a connected transformer or spur line is permanent, the circuit breaker will close for long enough to allow the fuse to blow, before the circuit is re-energized. Fitting the circuit breaker with a timing mechanism to provide, for example, two instantaneous trips and two delayed trips fulfils these requirements. The timing sequence adopted varies according to local conditions, but Figure 1.8 shows a typical arrangement.

Figure 1.8 Typical auto re-close sequence

The timing sequence is described as follows. At the instant when the fault occurs, the circuit breaker trips immediately and remains open for 1 second. This period is intended to allow for any ionized gases to clear. The circuit breaker then attempts a first re-closure. If the fault is still present, the circuit breaker trips again and remains open for a further 1 second. The second re-closure then occurs, but this time, if the fault is still present, tripping is delayed. This is because the system assumes that the fault lies beyond a fuse or fuses and it therefore allows sufficient time for the fuse(s) to rupture. If fault current still persists, a third re-closure is attempted, again with delayed tripping. After this, the system assumes that the fault is permanent and 'locks out'. It must be re-set either manually or over a telecontrol system. If the fault is cleared at any time during the sequence, the system re-sets itself automatically, ready for the next fault and re-closure sequence.

It should be appreciated that whilst re-closing is not a problem for vacuum and SF6 circuit breakers, oil circuit breakers have a very limited re-closing capability, depending upon the fault level. Normally, the


number of re-closures that may be performed by an oil circuit breaker is programmed into the controlling auto re-close relay and when that limit is reached, further auto-reclosing is prevented until the circuit breaker has been maintained. The advice of manufacturers should be obtained when deciding upon the number of re-closures a particular design of oil circuit breaker may perform. A disadvantage of re-close type circuit breakers is that, where the controlled network comprises both overhead line and cable, the supply to cable supplied customers will be adversely affected by re-closing operations caused by faults on the overhead network. In practice, this means that the customers on the cable network will experience momentary losses of supply, which they would not experience if the network were not re-closed. In addition, in the case of cable and plant faults, the additional fault closures may well cause additional damage. This effect can be reduced by good network design, where the cabled part of the network is controlled by a circuit breaker not fitted with auto-reclose, whilst the overhead line part of the network is controlled by the pole mounted auto recloser. Originally, oil filled pole-mounted reclosers were used for this purpose, but vacuum and SF6 designs are now available. The type of recloser shown in Figure 1.9 has great operational advantages in that the maintenance requirements are low and the unit may be regarded as almost maintenance free, at least so far as the user is concerned, being limited to simple replacement of the primary battery at perhaps 5-year intervals. The vacuum interrupters will provide hundreds of reclosers; eventually, replacement will be required but this is a return to factory operation. A further advantage is that the bushings are elastomeric and less readily damaged than porcelain, either during installation or in service. In these units the control system is normally sited in a lockable metal box close to ground level, remotely from the recloser proper and connected to it by a multicore (umbilical) cable. Because the protection system is microprocessor controlled, any number of possible reclosure strategies may be implemented, exactly as the operator requires and in addition, the unit is easily re-programmed to suit changing operational circumstances.

Figure 1.9 Vacuum interrupter in SF6 auto-recloser

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