The reliability of transmission and distribution equipment by Keith J. Ralls

Keith Ralls has given the opening or keynote address at several of the IEE's conferences held during his year in office. As an extended Editorial, we produce here the text of his keynote address a t the IEE's 2nd International Conference on 'The reliability of transmission and distribution equipment' held at the University of Warwick in March. A summary of the conference appears on p. 142.

Keith 1. Ralls. Chairman, IEE Power Division, and Managing Director, GEC Alsthom Transmission & Distribution Systems Group

For all of us in the business of transmission and distribution equipment, reliability is of paramount importance. Customers flick a switch and take it for granted that the light comes on. Supply companies provide services poised to correct any interruption originating from weather or equipment. Transmission companies apply fault criteria to their designs; one, sometimes two, major elements can be absent and still everyone receives their expected share of power. Generation companies provide back-up to any loss and strive to maintain services which are interruption-free. Equipment companies build in margins and redundancies to ensure that equipment works when it should and, even more important, that it does not make the system in which it works unreliable.

Reliability cannot be added to a product or system at a late stage. It has to be incorporated from the start. During the development of a product, when a design is being produced and prototypes built and tested for function, I have noted that there is often such a state of euphoria (that the product works at all) that questions of reliability take a back seat. During prototype

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testing, malfunctions, which a jaundiced eye might ascribe to prospective unreliability, may well be dismissed 'because this is only a knife and fork prototype'. At this development stage, the 'reliability antennae' of the application engineer have to be vigilant. Reliability has then to be pursued throughout the specification, design, manufacturing, testing and commissioning phases as well as operation.

What influences reliability?

clearly the degree of reliability required for the product - how it is defined and the conditions under which this applies. The specification should, however, leave it to the equipment designer to decide how the reliability is best achieved. Unnecessarily detailed specifications often, however, militate against reliability by calling, either explicitly or implicitly, for conflicting requirements.

which often conflict:

0 proven reliability performance-with

The product specification should spell out

I have listed a number of requirements

new technology

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1 Sellindge convertor 0 highest achievable reliability - radical station design changes compared with previous

products survival in rugged environment - low energy consumption shortest possible delivery- with lowest price.

It is clearly important to understand the environment in which the product will have to work, to know to what extent it would be based on proven technology and benefit from past experience, and t o what extent it will employ new, unproven technologies.

Reliability frequently suffers because of inevitable compromises between these competing priorities. This is perhaps to be expected, because reliability is one of the few parameters that is 'invisible' at the design and construction stage.

Predicting system reliability at the design stage may be difficult and often inaccurate because of the inadequate data on component reliability. If operators are to profit from reliability, however, then reliability has to be a key parameter in the design. In this respect it wodld be useful to equipment designers if specifications from utilities include some data from the experience within the utility for particular, or typical, or conventional items of plant, such as statistical failure rates of circuit breakers or

2 CIGRE statistics transformers.

Sample Frequency of Duration of Availability (Oh)

1989-1 992 outage outage allowing for inclusive (events per (hours) forced outages

pole per year)

30 thyristor HVOC schemes 7 47 8 69 98 52 300 pale-years

Crass-Channel (4x500MW at 270 kV) 4 5 3 44 99 65 16 pole-years

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Condition monitoring

of papers on condition monitoring. Redundancy of components to avoid single point failure modes is commonplace, although occasionally the classical error of not monitoring, and therefore not highlighting failures, is still made. For example, an undetected failure of low-cost auxiliary equipment, even if duplicated, could eventually cause an outage of the complete system.

the monitoring equipment is far higher than the equipment being monitored. Although this seems obvious, there still seems to be too many occasions in practice when the reliability of equipment is reduced by poor monitoring, resulting in numerous entries in the service log of 'no fault found'.

Training for reliability Not one of the papers at this conference

has 'training' in its title! I hope I am wrong to deduce that you are putting in systems and equipment and just assuming that the installer, the operator and the maintainer know how to deal with them. In my power electronics business we expect that a team of customer engineers will not only receive formal classroom training but also participate in the factory testing, installation and commissioning, so that they are fully familiar with the equipment they are purchasing before i t goes into service. After all, they shouldn't have any opportunity to become familiar with it afterwards, should they? So perhaps every equipment supplier should be asked 'What training is necessary with his equipment? How familiar are, or were, those responsible for running the equipment?

Customers too, need to take a responsible attitude to training. We have recently had an overseas customer who has sent his most promising engineers to train with us. They have done well but, soon after their return, many were promoted to jobs far away from the equipment they had studied. They were obviously the wrong personnel to train and I worry whether such customers are able to operate and maintain the equipment satisfactorily when we leave site.

Cost of reliability And what about cost? As a managing

director, responsible for businesses in the transmission and distribution field, it is a question I am always asking. I notice that only two paper titles at the conference mention the word 'cost', yet in today's competitive world cost is all important! Do we know what cost is incurred to achieve the reliability that a utility demands, or an equipment supplier provides? Do you specify duplication or even triplication throughout a control system when onlya certain part needs such treatment? Surplus equipment and unnecessary duplication adds cost and you will find no sympathy when a competitor

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I notice the conference has a large section

It is essential to ensure that the reliability of

has taken the market. I commend the authors of the two papers which include costs in their subjects, and I ask others - authors and questioners - what is the total life-cycle cost in your case? By that I mean the aggregate of capital costs, training costs and operating costs.

Feedback of reliability performance I find that feedback of information on

performance is extremely useful. As an equipment supplier I hear soon enough if something goes wrong with one of my products; sometimes it seems even before my customer knows! But seldom do I hear if things go right. And when I am asked for failure statistics the full picture is seldom available.

HVDC reliability and availability records Fortunately in the case of high-voltage

direct-current transmission I can turn to CIGRE, which publishes data on a regular basis.

Fig. 1 is an aerial view of Sellindge, the British convertor station for the ZOOOMW cross-Channel HVDC link to France. The convertor station includes switchgear, transformers, harmonic filters, auxiliary systems and lots of power electronics and controls. The link has four 500MW poles, each operating between earth and 270kV DC.

Fig. 2 is a sample of information from ClGRE statistics showing averages of frequency and duration of forced outages due to station faults: in addition to the reliability data or frequency of outage, this Figure also gives the average outage time. For such systems the availability, which is indicative of the time during which the system is able to perform its intended function, may be just as important as its reliability. The cross-Channel link is about in line with the first quartile of HVDC statistics, with one-quarter of the 30 links reporting performances better and three-quarters reporting performances worse.

This feedback is valuable: it keeps me, the equipment supplier, on my toes, and the specification writer, in the utility, with his feet on the ground. It provides the reliability data to keep the aspects of the design in their proper relationship with respect to capital cost, training cost and operating cost.

And how do we profit from high reliability?

The gains from high reliability can be very large. The quantifiable profits include the savings in direct costs of forced outages that do not happen, the savings in reserve equipment which is not needed, and the savings in unscheduled repairs which are not required. There may also be unquantifiable profits associated with the satisfaction of the customer, which in the long run may be crucial to an equipment supplier. I would like to illustrate this with an example from my experience by describing how reliability

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3 Nelson River HVDC scheme, Manitoba, Canada

4 Dorsey's 4000MW Convertor station - largest in North America

5 Mercury arc valves - VG11

1 1 1

6 Thyristor bi-valves - VG13

considerations have affected one of the major HVDC transmission links which my company has built.

The example in Figs. 3 and 4 is the Nelson River Scheme in Manitoba, the centre of Canada, where vast hydro resources exist in the North of the Province and the great majority of the population live in the South. Two HVDC links, containing 4000MW of conversion capability, are in place making this the largest HVDC scheme in North America. The first scheme, Bipole 1, was installed in the early 1970s and used mercury arc valves. The second Bipole was installed during the late 1970s and early 1980s and used thyristor valves. The HVDC links provide the central electrical artery for some 80% of the power of the Province. It is a case of

7 Thyristorsemi-tier nearly ’all the eggs in one basket!’You can

imagine that availability and reliabilityare crucially important to Manitoba Hydro.

The order for the first link was won on the basis of using mercury arc convertor valves of a higher voltage rating than had been previously made by any supplier (Fig. 5). During the contract development stage however, initial estimates revealed that these valves, operating at a direct voltage of 167kV and direct current of 1800A, would perhaps have a maximum life of only five years. However, detailed attention to design and quality, during the contract design stage, manifested itself in delivering an actual life appreciably longer than estimated.

Mercury arc valves require, however, skilled maintenance attention to ensure reliable operation and, 15 years after entering service, Manitoba Hydro was finding it difficult to attract qualified personnel to work in the frozen North on obsolescent technology where some of the valves required frequent maintenance. Accordingly, Manitoba Hydro decided in 1989 to replace mercury arc valves with thyristor valves.

shows a six-pulse valve group which brings the HVDC voltage up to 500kV DC. The six mercury arc valves have been replaced by three thyristor bi-valves which are much smaller in size although each thyristor valve uses 84 series-connected thyristors (Fig. 7).

With the ability to include three redundant thyristors, the mean-time-between-failures caused by the valves has been extended from a few months for mercury arc valves to a predicted ten years for thyristor valves.

Having kept vigilant records of the performance of individual mercury arc valves, Manitoba Hydro was able to identify and scrap the poorly performing ones. And, although mercury arc is an obsolete technology, Manitoba Hydro is keeping the rest of its well-performing mercury arc valves as spares and half the link is expected to use mercuryvalves until well into the next century. Thus, Manitoba Hydro has been able to extend the life of the scheme at minimum cost and, overall, the reliability of the HVDC link has improved appreciably; in fact it has ‘revitalised’ part of the project. My customer is profiting from this higher reliability by being able to undertake additional power supply contracts with the utilities to the South in the USA.

has been an unquantifiable gain in being associated with a customer who is fully satisfied, and whose representatives, through their pride and enthusiasm, have thus effectively become an extension to my sales team.

As you will recognise, I am not an expert in reliability but, as a manager of businesses in the transmission and distribution field, may I suggest that your utility planners and equipment designers concentrate on the cost-benefit of equipment and system reliability, both to utilities and their suppliers.

Again my company was selected and Fig. 6

From my company’s point of view there

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