Numerical relays, field applications

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Numerical Relays, Field Applications 
Protection, Control & Automation 
 By: Dr M . Ghezelayagh 
 
 
 
 
 
 
 
First Edition 2017: All Copy rights reserved. Single copy of this book may be filed or printed for personal non-commercial use 
and must include this copyright notice but may not be copied or displayed for commercial purposes or multi personal use 
without the prior written permission of author. 
 
 
 
 
Summary 
 
This book provides practical applications of numerical relays for protection and control of various 
primary equipment namely distribution and transmission networks , HV and EHV transformers and 
busbars, reactive and active power plants. 
Unlike other books attempts have been made to address the subject from practical point of view 
rather than theoretical one which can otherwise be found in most of other text books. The setting, 
design and testing philosophy of numerical relays as discussed in this book have been successfully 
applied in the fields on various projects and consequently can be used as a practical guideline for 
implementation on future projects. 
The book covers the followings subjects: 
• Fundamental concepts in the field of power system protection and control; 
• Required system modelling and fault level analysis for the design and setting of protection 
and control devices; 
• Setting and design philosophy of numerical relays of different primary equipment; 
• Practical application of anti-Islanding schemes for two different systems namely distribution 
generation (DG) and transmission generation (TG); 
• Challenges and solutions which are encountered during secondary equipment 
refurbishment/replacement in brown field substations with inclusion of two practical case 
studies; 
• Required tests for factory acceptance tests (FAT), site acceptance tests (SAT), and 
commissioning tests of numerical relays in conventional and digital substations; 
• Causes, analysis and proposed mitigation techniques of more than 100 worldwide 
disturbances which have occurred in different type of primary equipment which have resulted 
to major system black out or plant explosion or even fatality and; 
• New and future trend of application of numerical relays including application of super IED for 
protection and control of multi-primary equipment, implementation of digital substation 
,remote integrations ,self and remote testing of IED , distribution networks fault location 
techniques and fault locators using travelling waves, synchro phasors and time domain line 
protection using travelling waves. 
 
The main objectives of the book are: 
• To familiarize engineers/technical officers in the field of power system protection and control 
to all daily practical and essential issues; 
 
• To provide guideline for preparation of standards, technical specification, business case, 
functional scope, test and commissioning plan for replacement of secondary equipment and; 
 
• To provide adequate information to relay’s manufacturers and contractors about the 
requirement of electricity companies and end users. 
 
About the author 
Maty Ghezelayagh received his B.Sc. from Sharif University of Technology (Iran), M.Sc. from 
University of Manchester (UK) and PhD from University of Wollongong (Australia) all in 
electrical power systems. He has also worked and did research in the area of optimization 
techniques and adaptive controllers at Stockholm Royal Institute of Technology (Sweden) and 
Ohio University (USA). 
For the last 30 years he has worked with five different companies in different states of Australia 
and before that for four years for the main 400 kV transmission company in Iran. While working 
in industry, he has also been part time university lecturer. He is the author of more than 50 
papers in the area of power systems protection, control and planning. 
His major work experiences have been in the area of setting calculation, design and testing of 
protective relays applicable to different primary equipment namely distribution networks, EHV 
transmission lines, substations and power stations. 
 
Introduction 
 
 
 
 
INTRODUCTION 
 
Today, reliability and security of power systems depends heavily on protection and control devices. 
The role of protection is to detect different type of faults or abnormal system conditions on power 
systems and to isolate the faulty section from the system. The main role of control is to be able to 
open/close circuit breaker, isolator and earth switches locally or remotely from system control centre 
in order to restore the system to normal condition. 
For the last two decades there has been a significant decrease in the number of power system 
courses in technical colleges worldwide. This has been mainly due to increase in market demand in 
other fields such as computer engineering, electronic and information technology which have 
attracted most of the talented electrical engineering students. In addition it has been observed that 
due to financial restrictions, power industries do not provide sufficient or suitable training courses for 
new graduate engineers. The consequences of these factors have resulted to increase in hidden 
costs to power industries in terms of inefficient operation of systems and more frequent power supply 
interruptions. 
To overcome the above issues USA government allocated $4.5 billion for implementation of smart 
grid with $100 million designated for workforce training. Projects under this program facilitate the 
development of a trained and skilled workforce capable of implementing a national clean-energy 
smart grid and providing the next generation of skilled technicians, engineers and managers for the 
electric power industry. The program also raises awareness and interest in careers in the electric 
power industry, helping to address predicted labor shortfalls as an aging utility workforce moves 
toward retirement. The workforce training projects address three subject areas: 
• Developing and Enhancing Workforce Training Programs for the Electric Power Sector 
• Strategic Training and Education in Power Systems 
• Smart Grid Workforce Training 
 
Based on above facts and bearing in mind that most of abstract thinking experienced engineers have 
retired or soon to leave the industry, it is necessary their knowledge and experiences to be well 
documented for the use of new generation of engineers. With this philosophy in mind, this book 
reflects more than 30 years practical experiences of the author as obtained in working with different 
electricity companies. 
A summary of topics covered in each chapter of the book have been outlined below: 
Chapter 1 discusses the fundamental concepts in the field of power system protection and control. It 
covers the mostly common definitions namely dependability, selectivity, sensitivity, stability , speed, 
protection zone, time grading, blind spot, source to line impedance ratio (SIR). Most common type of 
faults on primary plants and general setting and design philosophy applicable to all type of numerical 
relays are discussed. This includes programming of watchdog contacts for critical and non-critical 
device failures, design of anti-pumping relays and tap-changer runaway prevention. Required 
I 
 
Introduction 
 
specifications of current and voltage transformers are discussed and clear elaboration is made 
regarding to assignment of CT polarity, its correct labelling/markings on drawings and direction of 
current flow on secondary winding. Fundamental concepts of IEC61850 (station and process bus) and 
its associate definitions such as GOOSE, MMS, Sample values (SV) and Merging Units (MU) are 
given. Generalfunctionality requirements of IED, required documentations and typical design of 
AC/DC schematic drawings are given. In addition clauses of Australian National Electricity Rules 
(NER) which specifies the required redundancy and fault clearing time by secondary equipment are 
provided. 
Chapter 2 discusses the required system modelling and fault level analysis which is essential for the 
design and setting of protection and control devices. Equivalent circuit modelling of all type of primary 
equipment namely lines, transformers, active and reactive power plants in both phase domain and 
sequence impedances for fault level analysis are discussed. These practical equivalent models are 
widely used in different large commercial computer programs for power system analysis. 
Equations to calculate self-impedance, zero sequence impedance and zero sequence mutual 
coupling impedance for transmission lines are given. 
Based on the concepts of distributed fault analysis, the concepts of equivalent sequence impedance 
of generators with different neutral earthing arrangement, equivalent sequence impedance of 
transformers with different vector group, effect of infeed current on apparent impedance and fault 
resistance are elaborated. Applications of large commercial computer programs on real practical 
cases are given in order to elaborate the concept of each of these. 
Methods to obtain sequence impedances of the line by primary injection tests using test equipment 
such as Omicron and factory tests to obtain the impedances of transformers with different vector 
groups are given. 
Chapter 3 discusses the setting and design philosophy of numerical relays applicable to distribution 
network. It also includes design policy of fuses, sectionalizer and reclosers. 
To develop appropriate criteria for setting and design philosophy, first different types of faults which 
may occur on distribution feeder are identified. 
Required protection schemes, functionalities and logic block diagrams applicable to distribution 
feeders are given. This includes setting and design under normal and abnormal operations (live line 
work and bushfire seasons). 
Required time grading coordination between different devices such as overcurrent & earth fault 
relays, reclosers, fuses and sectionalizer are described and the concept is elaborated by application 
on a real substation with two supply transformers. In addition the concepts of single phase switching 
on distribution feeders and loadability limits (maximum safe loading) of overcurrent relays are 
illustrated. 
The design of SWER lines and setting philosophy for single phase SWER recloser are given. In 
addition the design and setting philosophy of remote control gas switches and fault indicators along 
distribution feeders are outlined. 
Required protection and control schemes of HV customers with and without co-generations are 
outlined. The dependability of design based on the size of co-generation and type (synchronous 
generator or inverter power source) are outlined. 
The existing challenges in electricity industry namely detection of fault location on distribution feeders 
and distribution loop automation are described and future methods in these areas are discussed. 
II 
 
Introduction 
 
Chapter 4 discusses setting and design philosophy of numerical relays namely distance and current 
differential relays applicable to EHV transmission lines (above 110KV). Setting and design philosophy 
of High Impedance Earth Fault relays (Hi-Z), synch-check element, sequential auto-reclosing, stub 
protection and switch onto fault protection (SOTF) are given. For this purpose the required protection 
schemes, functionalities and logic block diagrams are provided. 
The application of numerical relays for protection and control of multi-ended EHV lines and the lines 
with tapped loads are discussed. For this purpose setting and design philosophy of distance and 
current differential relays which have been practically applied to four real EHV transmission systems 
with different system configurations are provided. The challenges which arise due to weak infeed, 
zero sequence mutual impedance and appropriate selection of signalling scheme for distance relays 
for each case are discussed. 
The loadability limits of numerical relays namely current differential and distance relays are discussed. 
For distance relays consideration of load encroachment (LE) and power swing blocking (PSB) 
elements are elaborated. It is shown grading between these two elements are required if correct 
functionality of PSB is required. Otherwise conventional loadability of distance relays can be 
calculated based on last forward zone setting rather than PSB characteristics in order to obtain higher 
loadability level at the cost of ineffectiveness of PSB. 
Chapter 5 discusses setting and design philosophy of numerical relays applicable to supply and 
network transformers. It also includes design policy for transformer guard relays (winding, oil 
temperature and Bucholz). 
To develop appropriate criteria for setting and design philosophy, first different types of faults which 
may occur on transformers are identified. 
Setting and design philosophy of common protection and control relays applicable to transformers 
such as overcurrent, distance, current differential, restricted earth fault, tertiary winding earth fault 
protection and automatic voltage regulators are described. 
In addition setting and design philosophy of other protection elements as listed below are discussed: 
• Thermal overload protection based on calculation of the hottest-spot winding temperature and 
the loss of life calculation according to IEC 255-8 standard (cold and hot curves). 
 
• 2nd harmonic inrush current inhibit for different type of modern numerical current differential 
relays. 
 
• Neutral unbalanced voltage relay supplied from delta open secondary winding of transformer 
 
• Application of synch-check for transformer’s circuit breakers 
Different philosophies which have been applied by different utilities regarding to using transformer 
temperature devices for alarms or trips are discussed. The advantages and disadvantage of the 
application of different techniques namely mechanical and microprocessor fibre optic based devices 
for measuring the winding and oil temperature are elaborated. 
Required protection schemes based on the size and voltage level, functionalities and logic block 
diagrams of protection and control of transformers are provided. 
Chapter 6 discusses the setting and design philosophy of different type of numerical relays for busbar 
protection schemes applicable to HV and EHV busbars. 
There are two types of numerical busbar protection namely high (HIBP) and low impedance (LIBP). 
The selection of each type should be based on criteria of fault clearance requirements, selectivity, 
sensitivity, stability requirements, busbar configuration, functionality, cost as well as the primary 
equipment characteristics. The conditions where only HIBP or LIBP should be selected are identified 
and required specifications of each type are given. 
III 
 
Introduction 
 
Bearing in mind that there are many numerical relays from different manufacturers in market for both 
HIBP and LIBP schemes, a method using weighted factor and a merit index (based on the awarded 
scores for each critical factor) is proposed for best selection of busbar protection scheme. 
Low impedance busbar protection schemes (LIBP) are classified into two categories. The first one is 
centralized and the second one is distributed one. The application of both schemes on a real EHV 
double busbar is illustrated. 
For LIBP schemes, the setting and design philosophy of the followings are discussed: 
• slope characteristics based on sensitivity and stabilitycriteria with consideration of CT 
saturation 
• End fault (CT dead zone) protection for various arrangements such as busbar or line side CT 
• Circuit Breaker Failure functionality 
• CT circuit supervision 
For HIBP schemes, the procedure for specification of shunt and series resistors for stability 
requirement under normal loading and fault condition are given. In addition method to calculate the 
required size of metrosil resistors (MOV) based on system parameters are given. 
For HV busbar, application of other types in addition to HIBP and LIBP schemes such as, Arc Flash 
Detection, Sudden Pressure Detection, Over Current Blocking Schemes, Summated busbar 
protection and Frame earth leakage protection are discussed. 
This chapter provides a practical guideline for standardization of busbar protection schemes when 
only single or duplicate busbar protection scheme are required. For this purpose the requirement of 
International and National Electricity Rules and availability of remote back up protection has been 
considered. 
Chapter 7 outlines the setting and design philosophy of numerical relays of different type of reactive 
power plants namely capacitor banks, shunt reactors, series reactors and static var compensator 
(SVC) for HV and EHV application. 
To develop appropriate criteria for setting and design philosophy, first different types of faults which 
may occur on reactive power plants are identified. 
In addition to common protection type such as current differential and overcurrent /earth fault, setting 
and design philosophy of specific protection such as neutral unbalanced current, reactor turn to turn 
fault, harmonic overloading, protection of inrush limiting reactor, thermal overload, detection of fault 
close to neutral of shunt reactor are discussed. 
For setting the neutral unbalanced current protection of the capacitor banks, two methodologies are 
proposed. In the first methodology the alarm setting is selected based on N-1 contingency (e.g. 
Normal system voltage+failed element). Trip setting is selected based on N-2 contingency 
(e.g. voltage under system contingency+failed element). For the second methodology both alarm and 
trip setting is selected based on N-1 contingency (e.g. Normal system voltage+failed element). Based 
on application of these methodologies on a real case, the advantages and disadvantages of each are 
discussed. 
Required protection and control schemes of reactive power plants based on the size and voltage level 
is discussed. This includes the conditions where it is required installation of duplicate protection 
device for redundancy due to importance of the plant. In addition functionalities of different protection 
elements of numerical relay and typical design applicable to reactive power plants are provided. 
Required control functionalities such as inhibit energization of the capacitor bank for 3 minutes after 
tripping, point of wave switching to limit capacitor inrush current and automatic control of capacitor 
IV 
 
Introduction 
 
banks based on voltage or MVAR or power factor are discussed. Depending of the type of parameter 
to control, setting and design philosophy of the controller is given. 
Chapter 8 discusses the application of numerical relays for protection and control of active power 
plants. Active power plants can be classified as conventional or non-conventional sources. 
Conventional plants include synchronous generators and motors. Non-conventional includes solar 
and wind farms power plants. The turbine of the conventional generators can be run by coal, oil, gas, 
hydro or diesel fuels. The turbine of renewable energy type is run by solar or wind energy. 
 
In order to develop appropriate criteria for setting and design philosophy, first different types of faults 
which may occur on generator (stator and rotor windings) are identified. Secondly national and 
international electricity rules and standards related to protection of active power plants are given. 
In addition to common protection type such as voltage restrained overcurrent, distance and current 
differential and windings temperature, setting and design philosophy of specific protection such as 
100% generator stator earth fault by 20HZ voltage injection, split phase differential protection for 
detection of inter-turn fault of the windings, rotor winding earth fault detection by 3 HZ voltage 
injection and loss of excitation are described. It also includes the setting and design philosophy of 
pole slip, anti-motoring, negative sequence current, under/over frequency, overflux, accidental 
energization, auto synchronizer, synch-check and the circuit breaker failure scheme of the generator 
which includes tripping of remote circuit breaker via signalling and excitation system switch for total 
generator shut down. In addition 95% stator earth fault protection for different stator neutral earthing 
arrangement namely not to earth (floating), solidly earthed and earthed via impedance are given. 
Required protection and control schemes of active power plants based on the size and voltage level is 
discussed. This includes the conditions where it is required installation of duplicate protection device 
for redundancy due to importance of the plant. 
Chapter 9 discusses anti-Islanding schemes for two different system configurations namely 
distribution generation (DG) and transmission generation (TG). Some of the main technologies used 
in DG are photovoltaic system, wind power, fuel cells, micro turbines and diesel generators. TG 
generally applies to large generating units which can be a wind farm, thermal or gas turbine power 
station. 
In order to develop appropriate anti-islanding scheme, first the conditions which causes occurrence of 
islanding and secondly its consequences such as safety concerns, end-user equipment damage and 
out of phase reclosing are discussed. 
The most common type of anti-islanding scheme namely passive, communication based technique, 
SCADA based script calculation, synchro phasors and active networks (signal generators) are 
described. Passive methods include under/over voltage and frequency, rate of frequency change, 
harmonic and voltage phase jump detection. 
It is investigated whether the DG’s technique for anti-islanding is also applicable to TG systems? 
What DGs and TGs techniques have practically been implemented? What are the advantages and 
disadvantages of each technique and what field experiences and issues exist for each technique? For 
this purpose the results of application of different methods on several real systems are presented. 
 
Chapter 10 discusses the challenges and solutions which are encountered during secondary 
equipment refurbishment/replacement in brown field substations. One of the main challenges has 
been lack of sufficient secondary copper cables for new numerical relays. The other challenge has 
been installation and commissioning of the new schemes with minimum primary plant outages. 
This chapter discusses how these and other challenges have been solved on a two real EHV 
substations. The first substation is an industrial substation which requires the replacement of 
transmission line protection and complex multi inter-tripping schemes within the substation and with 
remote substation. The second substation involves the replacement of a single old busbar protection 
V 
 
Introduction 
 
scheme with two new numerical schemes in an EHV substation with double busbar arrangement. For 
the first case utilising fibre optic cable and digital transceivers/receivers for provision of trip/close 
circuits have been implemented to solve the issue of insufficient secondary cable. Due to critically of 
the load for this case, careful planning and project coordination was implemented to carry out stage 
by stage the commissioning tests at thetime where some of the customer plants are out of service for 
maintenance. For the second case significant site visits in conjunction of checking cable schedules 
drawings were carried out to identify the available spare cables to be utilized for new numerical 
relays. For this case in order to avoid bus outage during commissioning tests, stability checks of 
busbar protection schemes was performed with the method of ‘on-load switchings’ of bays between 
buses instead of primary injection teste. For each switching test, all trip links of new busbar protection 
scheme were isolated while the old scheme was maintained in service. The results of stability check 
for each test which gives restrained and operating current of differential elements are given. 
 
Chapter 11 discusses the definitions and standard required tests for factory acceptance tests (FAT), 
site acceptance tests (SAT), and commissioning tests of numerical relays for protection and control of 
different primary plants. In addition definitions of black, white box testing and top down, bottom up 
testing are given. Required safety measures and isolation for each type of test is discussed. 
The required standard tests for IEDs at different stages have been classified into two categories. The 
first category is common tests which should be carried out to all types of IEDs. The second category 
of tests is the specific tests which depend on type of protection and control panels (e.g. transmission, 
transformer, busbar, etc.). The tests includes wiring check, CT/VT burden measurement, binary 
input/output, characteristic tests, trip circuit supervision, alarms/LEDs, functional tests and etc. 
The current test methods as used in industry for different types of IEDs particularly distance, 
differential protection and automatic voltage regulator are described. Test procedures for different 
functionalities such auto reclose, synch-check, power swing blocking and interlocking using test 
equipment such as OMICRON/Doble are described. The deficiencies of existing test methods and 
Omicron test plans are identified and improved methods are proposed. 
In addition to tests on conventional IED’s functionalities, required tests of IEDs with respect to SCADA 
and communication functionalities are also given. 
Required tests and techniques for sensitivity and stability check of current differential relay of 
transformer with different vector group by primary or secondary injection tests are given. The 
proposed techniques can be used effectively to develop correct test plan for test equipment such as 
Omicron. 
End-to-end scheme tests for sensitivity and stability check of current differential protection and 
distance relay with signalling scheme for EHV transmission line with actual CB in service by 
secondary injection tests are discussed. For this purpose it is required that the characteristic (slope or 
alpha plane) of current differential or distance relay be constructed on Omicron test equipment in 
order to be able to perform the scheme test.The scheme tests are also involved testing of successful 
and unsuccessful A/R and direct inter-trip with actual CB in service. 
Testing techniques and required tests in a digital substation using IEC61850 and methods of 
isolations when primary equipment is energized are described. This includes testing techniques in 
station and process bus with multi-publishers and multi subscribers. 
Chapter 12 describes the cause, analysis and recommended mitigation techniques of more than 100 
worldwide disturbances which have resulted to system black out or equipment explosion or even 
human fatality. The analysis of the disturbances is classified into three categories namely 
disturbances in passive networks, reactive and active power plants. Passive networks include 
distribution feeder, EHV transmission line, transformers and busbars. Reactive power plants include 
VI 
 
Introduction 
 
capacitor banks, reactors and static var compensator at both HV and EHV level. Active power plants 
includes thermal, hydraulic , nuclear power plants and large industrial loads such as mining 
companies with significant number of large synchronous motors and adjustable speed drives. 
The disturbances include those ones which have occurred during normal loading condition or after 
occurrence of fault such as short circuit fault or during maintenance or commissioning tests of new 
protection and control panels. 
The contributing factors for disturbances such as protective relays maloperation due to design 
deficiency, incorrect setting, configurations, logic programming, relay failures, incorrect programming 
of test equipment or human error during testing, equipment design defects and inadequate plant 
protection schemes are discussed. 
Chapter 13 discusses new and future trend of application of numerical relays. What benefits and 
challenges are ahead for commercial and wide applications of new generation of numerical relays 
with respect to following purposes? 
• Single super IED for protection and control of multi-primary equipment 
• Station and process bus using IEC61850 in digital substation 
• Fibre optic based current and voltage transformer (digital sensor) 
• Remote integrations of IED 
• Self and remote testing of IED 
• Fault location techniques for distribution feeders (with non-homogenous conductors and multi-
lateral) and application of fault locators using travelling waves 
• Synchro phasors or phase measurement unit (PMU) 
• Time domain line protection using travelling waves 
• Adaptive slope percentage restrained differential protection 
• Development of a mini portable busbar protection and control panel for emergency application 
• Distribution network automation 
• Application of IED for protection and control of Micro grids 
• Special protection system (SPS) for the control of wide area network (direct tripping of loads 
or generators) 
• Prevention techniques for loss of reactive power plants and transformers due to severe solar 
storm 
 
Bearing in mind that no IED at present is available in the market to identify accurately the location of 
fault on an EHV transmission line with non-homogenous conductors and multi circuits for each section 
of the line, in this chapter a new method based on following procedure is proposed to rectify the 
deficiency of existing IEDs: 
 
1) Utilize an advanced protection computer protection program to model the relays and network 
and calculate the apparent impedance (Zapp, ohm/prim) for different fault for different fault 
location along the line. Hence the obtained apparent impedance is independent of the relay 
setting and represents correct values. Record apparent impedance (Za) for fault at each 
location (D_Actual). 
 
2) Derive the reactance component (Xa) of the obtained apparent impedances and obtain the 
equivalent distance to fault (D_relay, Km) based on fault location parameters (ohm/km) 
entered in relay. 
 
3) Plot the actual fault location (D_Actual, Km) against the distance to fault as measured by 
relay (D_relay, Km) 
 
4) Use a curve fitting program to obtain the equivalent mathematical equations for obtained plot 
of each line section. 
 
5) Implement the obtained equations in SCADA using script calculation. 
VII 
 
Introduction 
 
 
In order to illustrate the practicality of the above approach, the technique is applied on a real EHV 
transmission line and results obtained for each of the above step are given. 
 
VIII 
 
	Cover page
	Summery
	Y-Inrto
	 Strategic Training and Education in Power Systems