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WordGirl's Word of the Week: Hilarious

Prepare to laugh while explaining WordGirl’s new word of the week.
on July 30, 2013

Word of the Week: HILARIOUS

Definition: (Adjective) extremely funny and causing a lot of laughter

Your children have been laughing their entire lives. They probably had their first laugh when you tickled their feet or played a game of peek-a-boo. As they get older, your methods of entertainment become more sophisticated, with knock-knock jokes and maybe even some slapstick humor. Well, now it’s time to introduce your children to definition dynamo WordGirl’s ‘word of the week’: hilarious.

Ask your children what makes them laugh. Perhaps it is when they watch WordGirl fight villains like The Butcher, who butchers the English language and hurls meat at his enemies by saying things such as “Sausage Clone!” It could be when they see Captain Huggy Face break-dance at the end of each WordGirl episode. Or maybe they have a favorite book that always makes them chuckle. Find out what they find to be “hilarious”, and be  sure to try these fun learning activities below.

Activity 1: Write your own joke. Knock, knock! Who’s there? Boo! Boo who? Don’t cry! It’s me! I’m sure, by now, your children have heard all the cliché jokes in the book. So, why not get their creative juices flowing by asking them to write their own original jokes? You can stick to traditional formats, like knock-knock jokes, or try to write original punch lines. If your children need some help, try reading 101 Wacky Kids Jokes, by Jovial Bob Stine, for inspiration.

Activity 2: Read a hilarious book. Books can be a source of some great jokes. So, to teach your children all about their new vocabulary word, find a title that’s hilarious and read it aloud together. You can check out these book lists to find some fantastic reads by Mo Willems and Dav Pilkey. The books that still make me laugh no matter how old I get are those in the Captain Underpants series, written by the latter of these two authors. These hilarious books follow two elementary school students, George and Harold, as they accidentally turn their school principal into the greatest super hero on earth, Captain Underpants. Sit down to read these books with your children and you’ll have them laughing from beginning to end.

Activity 3: Watch a funny movie together. You should never need an excuse for a family movie night, but celebrating your word of the week is just a really good one. Find a movie the whole family can enjoy, pop the popcorn, gather up the whole crew, and watch your favorite family-comedy. Try to make sure the movie is appropriate for all ages -- the jokes shouldn’t go over the heads of your little ones, but your oldest child shouldn’t be bored, either. After the movie, talk about what made it enjoyable, and why you found the flick funny.

Check out the Parent Guide to WordGirl for printables, books, games, and more.

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vol.100 número12Setting priorities for health in 21st-century South AfricaThe SAPIT trial provides essential evidence on risks and benefits of integrated and sequential treatment of HIV and tuberculosis índice de autoresíndice de assuntospesquisa de artigos
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SAMJ: South African Medical Journal

versão On-line ISSN 2078-5135
versão impressa ISSN 0256-9574

SAMJ, S. Afr. med. j. vol.100 no.12 Cape Town Dez. 2010

Can disease control priorities improve health systems performance in South Africa?

L C Rispel; P Barron

Improving health systems performance in order to achieve good health care outcomes and meet the Millennium Development Goals (MDGs) has received increased global attention. Using the World Health Organization (WHO)'s framework on health system strengthening, an overview is presented of key aspects of performance of the South African (SA) health system that are likely to impact on the Disease Control Priorities (DCP) initiative.

SA is gripped by a complex disease burden, consisting of the twin epidemics of HIV and tuberculosis, as well as non-communicable diseases and injuries. Despite an enabling legal and policy framework, health system challenges include sub-optimal leadership, insufficient resources for many national policies, lack of a broad public health approach to service delivery, and poor utilisation of existing information for decision-making.

Provided that these and other health systems issues are addressed, cost-effectiveness studies and interventions may be beneficial in improving the functioning of the health system in SA and in getting better value for money.

In the past few decades, the combination of biomedical and technological advances, a substantial increase in global knowledge to improve population health, and improved access to primary health care, essential drugs, water and sanitation has resulted in aggregate worldwide improvements in the health of individuals and communities.1-2 However, this progress has been marred by a multiplicity of factors, including globalisation, changing burden and complexity of disease profiles, inequalities between and within countries, and inadequate or poorly performing health systems.1 Common shortcomings of contemporary health systems include fragmented, unsafe and misdirected care, which mitigates against a comprehensive and balanced response to health needs.1

Improving health systems performance to achieve good health care outcomes and meet the MDGs has received increased global attention, especially in the last decade.1,3-9 In SA, the current health political leadership has committed itself to a substantial overhaul of the public health sector in order to address the complex burden of disease; improve health outcomes, access and affordability; and ensure responsiveness to the needs of the population.10 The DCP project in SA aims to determine which effective interventions should be included in a package of care that offers the greatest gain in health (or averted disease burden) per SA rand spent.11 The DCP can contribute to health policy changes and improve and prioritise health resource allocation and spending provided that it takes account of the existing issues and challenges in the health system and pays attention to process and those stakeholders who have the potential to take forward, block or challenge policy change or implementation.

Using the WHO's framework on health systems strengthening, we present an overview of key aspects of performance of the SA health system that are likely to impact on DCP-SA. Critical issues are suggested that must be taken into account in the execution of DCP-SA.

Measuring health systems performance

The measurement of health systems performance is not straightforward, as health systems are complex, consisting of all organisations, people and actions whose primary intent is to promote, restore or maintain health.6-8 Measurement tends to focus on the provision of health services and is often hampered by data problems, the difficulty of defining measurable objectives in a reliable and valid manner, and the challenge of capturing social determinants of health and community experiences.12-13 The WHO health systems strengthening framework, despite critique of its limitations, is useful in focusing attention on the performance of the health system by linking system building blocks, performance elements and overall outcomes, including population health status.8

An adaptation of the WHO framework is shown in Fig. 18 and consists of the following:

Health system building blocks. These include six building blocks of service delivery; human resources (health workforce); finances; medical products, vaccines and technology; information; and leadership and governance.

Health delivery platforms include the district health system; hospitals; and the private sector (both for-profit and non-profit organisations).

Health system performance includes equitable access, coverage, quality and safety.

System outcomes include improved health (level and distribution); social and financial risk protection; responsiveness; and improved efficiency.

Assessing the South African health system

System building blocks

The democratic SA government inherited a highly fragmented health system in 1994, with wide disparities in health spending and inequitable distribution of health care professionals. There were inequities in access to and quality of care between and within provinces; between black and white; between urban and rural areas; and between the public and private health sectors.3,6,14-16 Transformation efforts in the health sector spanning more than 15 years include numerous structural, legislative and policy changes, overcoming apartheid in health services, implementation of health programmes for priority conditions, and improvements in access to health care services.6,17 There have been numerous positive developments and improvements in the lives of South Africans since the country's democratic transition.15,18-19 However, urban/rural and public/private inequities remain acute, and are exacerbated by numerous health system challenges.19-22

An enabling legal and policy framework is in place, and there have been many areas of progress (Table I). At the same time, significant health system challenges for each of the six health system building blocks need to be addressed.10,20,22

Delivery platforms

Resources are being inequitably and inefficiently used in the SA health system. Specific examples of primary care, district and central hospitals follow.

Table II shows the district primary health care spending trends.23 The districts are categorised from highest to the lowest spending per capita. Paradoxically, from an equity perspective, some of the largest percentage increases occur in districts that are already spending higher than the average per capita.23 The data highlight the marked differences in spending on primary health care.

Fig. 2 shows the cost of keeping an average patient in a district hospital for one day, the patient-day equivalent (PDE), an indicator showing on average how much it costs for one patient to spend one day in the hospital. This figure illustrates the wide differences between districts, which conceal the even greater differences between individual hospitals. Even in the same province there is a wide range. For example, in the Eastern Cape the cost per PDE in Nelson Mandela Bay Metro is twice as high as in Chris Hani.24

As district hospitals consume over 40% of total district resources, the wide ranges in PDE are of great concern. Costs at the high end may indicate lack of efficiency or leakage out of the system, while costs at the low end may indicate poor quality of care.

Table III shows selected indicators in a sample of tertiary hospitals.22 Focusing on the cost per PDE indicates a wide range of potential inefficiencies in the system. Some central, academic hospitals in KwaZulu-Natal and the Free State are more expensive than those in the Western Cape and Gauteng, partly because the institutions are not working at optimal capacity.22

These differences point to poor monitoring and accountability at all levels of the system.

How healthy are South Africans?

Health outcomes in SA are poor and not commensurate with the level of health expenditure in the country. Scientists have described a quadruple burden of diseases in SA, comprising HIV and AIDS, poverty-related diseases, chronic diseases of lifestyle and high rates of injury.15,20,25

SA has an estimated 5.5 million people living with HIV, with an HIV prevalence of close to 30% of all public sector antenatal clinic attendees, and wide geographical differences.26 The rise of HIV prevalence has been followed relentlessly by a threefold increase in the incidence of tuberculosis (TB) from 1996 to 2006 (Fig. 3).27 Mortality statistics show an increase in deaths due to TB, from 13.1% of all deaths in 1997 to 25.5% in 2005, and deaths due to pneumonia increased from 4.8% in 1997 to 8.7% in 2005.28 These large increases are almost certainly due to the classification of AIDS-related illnesses into these categories.

After HIV and AIDS deaths (29.8%), cardiovascular disease (16.6%), infectious and parasitic diseases (10.3%), malignant neoplasms (7.5%), intentional injuries (7.0%) and unintentional injuries (5.4%) were the leading cause of death in 2000.25 Hence, noncommunicable diseases (NCDs) and injuries constitute a growing public health problem, which must be addressed at the same time as HIV and TB.29

Implications for DCP in South Africa

This assessment paints a picture of a society that is gripped by a complex disease burden: the twin epidemics of HIV and TB, coupled with non-communicable diseases and injuries. The health system is inordinately complex. Visible and decisive leadership is needed to contextualise and prioritise the interventions required to improve the health system and the health status of South Africans.22

Although considerable resources are being spent on health and there have been massive improvements in reducing inequitable spending, there are still large disparities exemplified by the per capita expenditure on non-hospital primary health care.23-24

Data from district hospitals point to large-scale inefficiencies among individual hospitals and also among different provinces. Costeffectiveness studies and interventions may improve the functioning of the health system in SA and get better value for money. However, key issues must be taken into account in any DCP initiative that is taken forward:

• The risk of emphasising selective interventions in health care delivery, inherent in the cost-effectiveness approach. Hence, any DCP project should take into account the complex disease burden and existing challenges in the health system, and aim to analyse the cost-effectiveness of integrated services.

• The technical complexity and enormous data inputs required for cost-effectiveness analyses. The project should ensure that due emphasis is placed both on building local capacity at universities, and on building capacity within government to utilise the information.

The reality that budgeting and planning is not zero-based, i.e. future planning must take into account existing services and systems. Therefore the information gathered in the cost-effectiveness exercise must reflect the costs of adjusting the supply of a particular intervention upwards or downwards from its current level.

• The DCP should assist with the development of appropriate staffing models.

• The DCP should assist with discontinuation of health care interventions/activities that add no value to health outcomes.

• The DCP must recognise that society often places a disproportionate value on certain sorts of treatment, including expensive life-saving care. The approach should take into account public values and professional opinion and pay due attention to the context and processes of decision-making.

• Few developing countries have a comprehensive monitoring and evaluation system, existing systems suffer from lack of co-ordination and are often paper-based, and information generated has problems of quality, completeness, timeliness and duplication. The DCP should facilitate the development of a streamlined system, rather than exacerbate existing data requirements

• Lastly, the DCP should recognise that a technical approach to health sector priorities based on burden of disease and cost-effectiveness analysis should not be a rigid prescription for all health system ailments, but is only one input to the policy process.4

We would like to thank Professors Karen Hofman and Steve Tollman for their encouragement and support. This paper is based on a longer report entitled: 'Overview of health care and the health system in South Africa: Implications for the Disease Control Priorities (DCP) project', done as part of the launch of the Priority Cost Effective Lessons for Systems Strengthening (PRICELESS) South Africa initiative, August 2009. The report was funded by the Fogarty International Center at the US National Institutes of Health and the Bill and Melinda Gates Foundation.

1. World Health Organization. The World Health Report 2008: Primary Health Care, Now More Than Ever. Geneva: WHO, 2008.         [ Links ]

2. Beaglehole R, Bonita R. Global public health: a score card. Lancet 2008;372:1988-1996.         [ Links ]

3. Barron P, Roma-Reardon J, eds. South African Health Review 2008. Durban: Health Systems Trust, 2008.         [ Links ]

4. Murray CJ, Kreuser J, Whang W. Cost-effectiveness analysis and policy choices: investing in health systems. Bull World Health Organ 1994;72:663-674.         [ Links ]

5. Rispel L, Setswe G. Stewardship: Protecting the public's health. In: Harrison S, Bhana R, Ntuli A, eds. South African Health Review 2007. Durban: Health Systems Trust, 2007.         [ Links ]

6. Schneider H, Barron P, Fonn S. The promise and practice of transformation in South Africa's health system. In: Buhlungu S, Daniel J, Southall R, Lutchman J, eds. State of the Nation: South Africa 2007. Cape Town: Human Sciences Research Council, 2007.         [ Links ]

7. World Health Organization. World Health Report 2000: Health Systems: Improving Performance. Geneva: World Health Organization, 2000.         [ Links ]

8. World Health Organization. Everybody's Business. Strengthening Health Systems to Improve Health Outcomes: WHO's Framework for Action. Geneva: World Health Organization, 2007.         [ Links ]

9. Travis P, Bennett S, Haines A, et al. Overcoming health system constraints to achieve the Millenium Development Goals. Lancet 2004;364:900-906.         [ Links ]

10. Department of Health. National Department of Health Strategic Plan 2010/11-2012/13. Pretoria: Department of Health, 2010.         [ Links ]

11. Hoffman K. Scope and expectations of the South Africa Country Study. Paper presented at the launch workshop entitled 'Setting priorities for health: Crafting a South Africa-specific strategy', 10 - 12 August 2009, Mount Grace Country Hotel, Magaliesberg.         [ Links ]

12. Loeb JM. The current state of performance measurement in health care. Int J Qual Health Care 2004;15:i5-i9.         [ Links ]

13. Navarro V. The new conventional wisdom: An evaluation of the WHO report Health systems: Improving performance. Int J Health Serv 2001;31:23-33.         [ Links ]

14. Chopra M, Lawn JE, Sanders D, et al. Achieving the health Millennium Development Goals for South Africa: challenges and priorities. Lancet 2009;374:1023-1031.         [ Links ]

15. Coovadia H, Jewkes R, Barron P, Sanders D, McIntyre D. The health and health system of South Africa: historical roots of current public health. Lancet 2009;374:817-834.         [ Links ]

16. Shisana O, Rehle T, Louw J, Zungu-Dirwayi M, Dana P, Rispel L. Public perceptions on national health insurance: Moving towards universal health coverage in South Africa. S Afr Med J 2006;96:814-818.         [ Links ]

17. Health Systems Trust. South African Health Reviews. Durban: HST, 1995-2008.         [ Links ]

18. Buhlungu S, Daniel J, Southall R, Lutchman J, eds. State of the Nation: South Africa 2007. Cape Town: Human Sciences Research Council, 2007.         [ Links ]

19. Statistics South Africa. Achieving a Better Life for All: Progress between Census 1996 and 2001. Pretoria: Statistics South Africa, 2005.         [ Links ]

20. Development Bank of South Africa. Health Sector Roadmap. Johannesburg: DBSA, 2008.         [ Links ]

21. Gilson L, McIntyre D. South Africa: The legacy of apartheid. In: Evans T, Whitehead M, Diderichsen F, Bhuiya A, Wirth M, eds. Challenging Inequities in Health: From Ethics to Action. New York: Oxford University Press, 2001.         [ Links ]

22. Integrated Support Teams. Review of Health Over-spending and Macro-assessment of the Public Health System in South Africa: Consolidated Report. Pretoria: ISTs, 2009.         [ Links ]

23. Bletcher M, Day C, Dove S, Cairns R. Primary health care financing in the public sector. In: Barron P, Roma-Reardon J, eds. South African Health Review 2008. Durban: Health Systems Trust, 2008.         [ Links ]

24. Day C, Barron B, Montecelli F, Sello E, eds. District Health Barometer 2007/8. Durban: Health Systems Trust, 2009.         [ Links ]

25. Bradshaw D, Nannan N, Groenewald P, et al. Provincial mortality in South Africa 2000 - priority setting for now and a benchmark for the future. S Afr Med J 2005;95:496-503.         [ Links ]

26. Department of Health. National HIV and Syphilis Antenatal Prevalence Survey, South Africa 2008. Pretoria: Department of Health, 2009.         [ Links ]

27. Department of Health. Tuberculosis Strategic Plan for South Africa, 2007-2011. Pretoria: Department of Health, 2008.         [ Links ]

28. Statistics South Africa. Mortality and Causes of Death in South Africa, 2003-2004. Pretoria: Statistics South Africa, 2006.         [ Links ]

29. World Health Organization. 2008-2013 Action Plan for the Global Strategy for the Prevention and Control of Non-communicable Diseases. Geneva: World Health Organization, 2008.         [ Links ]

Laetitia Rispel is adjunct Professor in the Centre for Health Policy, School of Public Health, University of the Witwatersrand, Johannesburg, current President of the Public Health Association of South Africa (PHASA), and the principal investigator of a large research programme on Human Resources for Health (HRH). She was Executive Director of the Social Aspects of HIV/AIDS and Health Research Programme at the Human Sciences Research Council, and was head of the Gauteng provincial health department from 2001 until 2006.
Peter Barron is a medical doctor, public health specialist and previous accountant with more than 30 years of public health sector experience, both as a clinician and manager in local and provincial government. The main focus of his recent work has been around improving the quality of care at primary care level and implementation of a district health system in South Africa. He has published widely on immunisation, maternal and child health, district health, quality of care, health information systems and health service delivery.

Corresponding author: Laetitia Rispel (

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vol.25 número2Performance of a compression ignition engine operated with sunflower ethyl ester under different engine loadsGeneral aspects of carbon dioxide as a refrigerant índice de autoresíndice de materiabúsqueda de artículos
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Journal of Energy in Southern Africa

versión On-line ISSN 2413-3051
versión impresa ISSN 1021-447X

J. energy South. Afr. vol.25 no.2 Cape Town may. 2014

A review on protective relays' developments and trends

Abdelkader Abdelmoumene; Hamid Bentarzi

Signals and Systems Laboratory, IGEE, Boumerdes University, Algeria

One of the most complex disciplines in electrical engineering is power system protection which requires not only the proper understanding of the different components of a power system and their behaviours but also a good knowledge and analysis of the abnormal circumstances and failures that can occur in any element of a power system. Moreover, the rapid changing and development in relays principles as well as in their technologies are additional factors that oblige those people working in the field to expand and update continuously their knowledge. In this paper, we shed light in the evolution of protective relays since the onset of electrical energy to currently. We try also to foresee the future prospects and trends in this area.

Keywords: digital / numerical relay, protective relay, developments, trends, reliability

1. Introduction

Protective relaying is an integral part of any electrical power system. The fundamental objective of system protection is to quickly isolate a problem so that the unaffected portions of a system can continue to function. Protective relays are the decision-making devices in the protection scheme. These relays underwent, through more than a century, important changes in their functionalities and technologies. Each change brings with it odds and improvement in both technical and financial aspects. In this paper, we shed light in the evolution of protective relays since the onset of electrical energy to currently. We try also to foresee the future prospects and trends in this area.

2. Historical background

Since the early days of the onset of electrical power the need of devices to prevent or limit the undesirable events in power system have been prescribed. The history of protective relays refers to more than a century ago. Some literatures say that the first protective relay was produced in 1902 (Singh, 2007; Pathirana, 2004), others refer to 1905 (Lundqvist, 2001; Rebizant et al., 2011). But whatever the date, the hard fact is that protective relays knew an important revolution since the beginning of the twentieth century. In 1909, induction disk type inverse time current relays came into practice and the concept of directional discrimination of faults was incorporated in these protective relays (Singh, 2007).

Differential relay was developed using pilot wires for conveying information from one end to the other end of the line (Pathirana, 2004). In 1923, distance relay appeared in the form of impedance. Later, the induction type mho relays with very high precision came into practice. After that, polarized dc relays with better accuracy and sensitivity were developed in 1939 (Singh, 2007; Warrington, 1968).

All of the relays developed until the 1940's were electromechanical relays. These devices achieved very high precision and sensitivity in the form of induction cup mho relays and perform well for the missions attributed to them.

The early 1940's showed the way into the development of relays using electronic devices (Singh, 2007). These relays are known as static relays or solid state relays because they didn't contain a moving parts. The advent of transistor circuits opened the door to the development of several new protection concepts like block spike comparator, phase comparator, etc.

The major advantage of these relays was that no moving parts were needed for performing their intended functions. The operating speeds of these relays were also more than the speed of their electromechanical counterparts and, their reset times were less than the reset times of electromechanical protective relays. In addition to these benefits, the solid-state relays could be set more precisely (Sachdev et al., 2009).

This generation of static relays became quickly very popular and found a large place in power system protection.

During the peak famous period of these solid state relays, another generation of protective devices was being set in way to see the light; it was the digital technology.

The use of Digital computers and microprocessors for protective relaying purposes has been engaging the attention of research since the late 1960s (Singh, 2004). The first serious proposals for using digital computers came from Rockefeller in 1969 (Rockefeller, 1969; Singh, 2004). Much literature reported digital relays shortly afterwards. But the first microprocessor based relays offered as commercial devices was only in 1979 (Sachdev, 1979). In that era, the efforts were concentrated to obtain a very high speed fault clearance. Different techniques and algorithms were proposed for achieving this objective. These include common hardware platforms, configuring the software to perform different functions (Sachdev et al., 2009).

In the late 1980s, Multifunction digital relays were introduced to the market (Sachdev et al., 2009). These devices reduced the product and installation costs drastically and has converted microprocessor relays to powerful tools in modern substations.

In 1988, the Virginia Tech research team developed the first prototype phasor measurement unit (PMU) based relay (Phadke, 2002). This technique allows measuring, beside the magnitudes of the electrical entities, the phase angles (Zhang, 2010) and could offer new information that can be used to improve the functional logic of protective relays.

In the 1990s, the notion of integrated protection and control became very popular and benefited full advantage of microprocessor technology, for protection, monitoring, control, disturbance and event handling, and communication. The relays' volumes as well as wiring were significantly reduced due to the integration of functions and the use of serial communication.

Perhaps the most attracting feature of these numerical relays was the ability of communication that offered new horizons for protection and protection related applications.

At the present time, there are many advanced communication techniques which can be used to improve protection, control, speed outage restoration, operation analysis, maintenance functions and planning. This communication facilities allow engineer operating, testing, maintenance and accessing real-time and historical relay information to the neighbours (Wang et al., 2002; Eissa, 2002).

The information is the basic constituent in the protection scheme. The important issue is communicating and processing information in an efficient and economical way. The key element in a communication system is the physical medium used in conveying information through the system. There are many different types of communication media such as twisted pair cable, coaxial cable, fibre optic cable and wireless communication (Ali et al., 2007).

The wireless networks are by far the most popular choice for new network algorithm.

Nowadays, modern digital relays draw on the experience and technical resources of the previous series and also featured compactness and less power consumption along with support for remote operation based on enhanced communication functions. The use of global positioning system (GPS) for digital measurement, especially for overhead line protection, gives very encouraging results. They are more accurate than distance relaying algorithms which are affected by inadequate modelling of transmission lines and parameter uncertainty due to line aging, line asymmetry and environmental factors (Bo et al., 2000). The use of GPS technique allows providing time synchronization to± 1µs accuracy (Radojevic' & Terzija, 2007) a thing which proves the high precision character of this technique.

In fact, the accuracy of relays depends not only on their hardware components but also on the manner of information processing to evolve the decision signal; this is what is called the data processing algorithm or the processing method.

So, the research of the optimal method to obtain the most accurate decision in the fastest way is one major challenge in the numerical protective relay design.

Over the past two decades, the application of the artificial intelligence methods on power protection relaying (ANN, Fuzzy logic, genetic algorithms...) is under investigation. Perhaps the most wonderful aspect in artificial intelligence techniques is the ability to learn by training any complex input/output mapping and recognize the noisy patterns.

These techniques have been quite successful but are not adequate for the present time varying network configurations, power system operating conditions and events (Babu et al., 2011).

3. Performances evaluation

Electromechanical relays are still the most predominant relays in almost all countries through the world including the USA (EPRI, 2004) and Russia (Gurevich, 2012), especially for HV and EHV. Truth proves, without doubt, the effectiveness of this generation of protective relays. However, during the past 15-20 years, there has been a widespread displacement of EMR by microprocessor-based relay protection devices (Gurevich, 2012). This transition from the electromechanical to the numerical relays can be justified by the large flexibility and the wonderful features such as:

  • Multifunction
  • Compactness
  • Communication
  • Reduced volume and wiring
  • The low cost.

While all these functionalities and characteristics of numerical relay make it a magic solution for protection system; some specialists and experts expressed their anxieties about the reliability and the lifetime of these new relays. Thus, the lifetime of old electromechanical relays can exceed 40 years, some of them were performed in service for more than 50 years. Whereas that of the numerical relay is estimated between 10 - 15 years (Gurevich, 2012). This is a result of the ageing of their electronic components, bringing on changes in their parameters, during their lifetime. For example, the service life of electrolyte capacitors, which are widely used in microprocessor based relays, does not exceed 7-10 years (Gurevich, 2009), and this is under favourable conditions of temperature and humidity.

From the viewpoint of reliability, a statistical study (between 2000 and 2009), posted by A.N.

Vladimirov., Deputy Head of Relay Protection Department of Central Dispatch Service of UES of Russia, showed that the reliability of microprocessor based relay is about 60% less than that's of electromechanical relay (Vladimirov, 2009; Gurevich, 2010).

The high accuracy and precision of numerical relays (NR) compared with electromechanical relay (EMR) are counterbalanced by its weak immunity against electromagnetic perturbations.

Table 1 shows a comparison between the different generation relays.

The previous cited problems of NR are the important titles' debated in this field. But whatever the matter, we guess that these problems are issues of risk control. And once the risk is well identified and properly assessed, it is just enough to take it into account and to plan making the necessary actions in the good instants. For example, if we are sure enough that the lifetime of a digital relay is 10 years, we must foresee its change after ten years minus epsilon , we should, also, be careful in its last years of service.

So, after identifying the weak points of the protective relay; it will be clear where one must focus the efforts to reduce the failure risk and, consequently, improve the reliability of the relay (Abdelmoumene & Bentarzi, 2012). The use of the redundancy technique (back up element) allows a significant reduction of failure rates, and hence improving the reliability of the considered protection system (Abdelmoumene & Bentarzi, 2012).

Nuclear power plants (NPPs) traditionally relied upon analogue instrumentation and control systems for monitoring, control, and protection functions. With a shift in technology from analogue systems to digital systems with their functional advantages (e.g., fault-tolerance, self-testing, signal validation, and process system diagnostics), plants have begun such replacement.

However, digital systems have some unique characteristics, such as using software, and may have different failure causes and/or modes than analog systems; hence, their incorporation into NPP probabilistic risk assessments (PRAs) entails special challenges (U.S.NRC, 2011).

At any rate, it is inevitable that power plants will slowly move toward newer technology. In fact, this movement toward the use of digital/numerical type relays has already begun. In 2004, some U.S. nuclear plants had already installed digital relays for selected applications (EPRI, 2004). Japanese and French power plants had also begun testing these new technologies in their nuclear power plants (NPPs) (IAEA, 1999; Hashemian, 2011).

4. Concluding remarks

Throughout this paper, an historical background of protective relays has been presented. The latest developments and trends have been also introduced and discussed whether for hardware and technology aspect or software and method aspect. The performance evaluation and comparison between the different relays generations have been done in order to bring out the strong and the weak points of each relay type. The relatively low reliability level of new numerical relays constitutes the major worry expressed by many experts in the protection field. In our opinion, the wonderful achievement in performance and functionalities enhancement brought out in numerical relays must be completed by the research to increase significantly the reliability level and mastering the risk assessment.

This paper is the result of great efforts during a long time of research in specialized literatures to establish an exhaustive document which resumes the main highlights in protective relay's developments and trends.

The authors acknowledge the immense help received from the scholars whose articles are cited and included in the references of this manuscript. The authors are also grateful to authors / editors / publishers of all those articles, journals and books from where the literature for this article has been reviewed and discussed.

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Received 23 July 2013
Revised 21 March 2014

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