Autor: Radomir GoNO”;~~ and Stanislav RusEK* ~Department of Electrical Power Engineering, Faculty of Electrical Engineering and Computer Science, VSB-Technical University of Ostrava, 17, Iistopadu 15, 708 OO Ostrava, Czech Republic E-mail .’ radomir.gono@vsb.cz
The paper deals with analysis of data about distribution network device failures and electric power supply outages from distribution companies. There are also suggestions how to improve reliability using new materials. Since each company has different structure of data monitoring the uniform methodology was created to enable comparison of processed results. Data structure, the way of distribution network device reliability evaluation and some of the preliminary results of the analyses are presented in this paper. There are also described the basic principles of the Reliability Centered Maintenance-RCM and its application to electric distribution network devices. Its aim is more effective maintenance program of equipment. The inputs of such analyses are databases of outages, maintenance, equipment condition and financial flow. The last part of the paper consists of introduction of fuel cell systems reliability in context of the grid-1inked operation.
1 INTRODUCTION
Institutional changes associated with market liberalization taking place all over the world change drastically the approach to power supply quality. It develops towards a purely commercial matter between suppliers and their customers. The supply that does not comply with agreed qualitative parameters will lead to trade disputes and financial settlements. So-called undelivered energy, including its valuation, arrives on the scene.
Electricity market liberalization has started in both countries. It is a little bit faster in the Czech Republic than in Japan. The market will be opened completely for all consumers from I st January 2006 in the Czech Republic.
Current trends toward deregulation and heightened competition make distribution companies provide a stable and reliable supply of electricity. That is why they have to seek suppliers offering the highest quality, competitively priced products and services available in the global marketplace today.
The two following aspects of supply quality may be considered:
- Supply reliability-relating the availability of electricity in the given location, by another name lower number of interruptions.
- Voltage quality-relating the purity of characteristics of the voltage waveform, including the absolute level of voltage and frequency.
This paper deals in more detail with the former point.
Lowest price in fact means reasonable, balanced price of electricity as a compromise between costs and certain reliability level.
Since this is journal of materials engineering we would like to point out new materials that are used for improving reliability of electric power network.
2 ANALYSIS OF FAILURES AND OUTAGES
The principal goal of monitoring events occurring in the distribution networks is to secure the reliable supply of electrical energy to consumers in accordance with the Distribution Network Grid Code Ref. [1] . The rate of reliability may be determined from databases of events by means of global indices of supply reliability or reliability indices of particular elements.
The world centers of reliability analyses provide electronic databases of information about availability of electronic and no electronic components and distribution function of kind of failure. They contain not only resultant failure rate but we can even get to the producer, operation conditions, etc. It is possible to use these databases for prediction of availability of complicated systems. Unfortunately, databases do not contain data about electric power equipments operated in our conditions.
In the former Czechoslovakia exclusive database of failures, outages and damaged equipments in whole electric power system began to rise in 1975. Unfortunately, the database filling has been stopped since 1990 because of political and social changes. Distribution companies obtained independence and started introducing their own systems.
Later, according to the new world trends, distribution companies decided to uniformly monitor global indices of reliability and also reliability of chosen pieces of equipment. That necessary data for analyzes are stored centrally at our research workplace where item reliability is also processed Ref. [2] . Data are hand on and process since the year 2000.
In Japan There is The Federation of Electric Power Companies that stores this data.
2.1 Databases of pieces of equipment
The majority of utilities create statistics for the reliability of network components, including lines, transformers, etc. They are especially collected to identify an unreliable piece of equipment and to be used as the input into probability calculations of system behavior.
Basic data on the reliability of equipment and elements of systems is as follows:
- Failure rates of particular pieces of equipment and elements.
- Outages of the piece of equipment due to maintenance and inspections.
- Outages of the piece of equipment due to operating works on the piece itself and labor safety securing in the vicinity of live parts of the distribution system.
For consequential evaluation of reliability it is necessary to have data on the number and range of the examined piece of equipment.
Thus it is not only the case of data on e.g. the number of failures and the mean duration of a failure of the line of the certain type and voltage level, but also the case of data on the total range of the observed piece of equipment, i.e. here on the total length of the line of the certain type and voltage level.
The result of analyses is the determination of the failure rate and mean failure duration for particular items of equipment or for group of equipments. With more detailed databases, other pieces of information may be found that are important for operators, such as the most frequent cause of failures, networks with the greatest amounts of undelivered energy, etc.
This data also serves to evaluate properties of pieces of equipment already operated (or a piece of equipment of the certain type of the selected supplier) , to select new pieces of equipment, to assess the time suitable for restoring pieces of equipment at the end of their lives, to choose the operating mode of the HV network node, and others. Unfortunately, some distribution companies monitor only failures of major units because they do not interested in determination of item reliability at all.
The concrete reliability indices of the item of equipment are calculated according to following methodology Ref. [3] .
For the failure rate the following relation may be written:
N the number of failures
Z the number of elements of the certain type in the network
X the considered period (year)
For the mean duration of the failure the following is valid:
Nr the number of failures of the element of the certain type
Ti the duration ofthe failure of the element ofthe certain type (h)
2.2 Data Structure and Range
Eight distribution companies from the Czech Republic and one from the Slovak Republic furnish Technical University of Ostrava with the databases of outages and failures. Individual companies release the data different formatted in various data files; thus it is necessary to choose different approaches to data conversion to unified database. Thirty one attributes were chosen in amount attributes for the data analysis. These attributes are depicted in Table 1.
Today database contains more than 400 thousand records on voltage levels 110 kV, MV and partially LV.
We use an n-ary relation, where n=31, to model the data. If queries for values of all attributes will be defined we may apply a multidimensional data structure for indexing the relation. We can apply R-tree or Ub-tree. The data structures provide better efficiency to querying many-attribute relation than classical B-tree applied in mainline DBMSS (database management system) . In the case the indexed space is space of dimension 3 1 .
2.3 Some results
The determination of item reliability from total database has been so far provided for the first level structuring (according to type of equipment) : distribution transformer station MV/LV, cable line, transformer station MV/MV and switching station MV and overhead line.
The graphic tendency of the failure rate Eq. (1) and the mean failure duration Eq. (2) interpreted from data mentioned above is displayed for 22 kV cable in the Figure. I .
Great differences between particular periods with one company are evident as well as those between particular companies. These results are to be accepted with reserve, because the statistical reliability of data from the five-year period is insufficient in the area of electrical power engineering and the method of event observation is still different with particular companies. From this the necessity of the long-term observation of failures and outages and the need to unify observation methodologies follow. With more detailed databases, other pieces of information may be obtained that are important for operators. In Figure. 2 there is an analysis of causes of failures.
2.4 How we improve parameters
There are some possibilities as for how to improve reliability:
- Maintenance free equipment-eliminate planned outages. The certain element is not capable to perform its function because it is unavailable during maintenance.
- Indirect live-wire operations shorten working interruptions-new insulating materials for working tools.
- Remote controlled disconnecting switches that can cut off nominal currents-eliminate manipulating time and money. They are also maintenance free.
- Distribution companies in the Czech Republic started to install insulated overhead lines in forest areas.
- Sulfur Hexafluoride or vacuum is used for circuit breakers instead of oil.
- Paper-insulated cables were replaced by polyethylene-insulated cables. There is a comparison of databases 1975-1990 and 2000-2004 in Table 2.
Figure 1 Tendency of reliability indices
Figure 2 Failures according to their causes
3 RELIABILITY CENTERED MAINTENANCE
This part of the paper is related to lifetime estimation of used materials and effective spending of resources.
Maintenance is, from the point of view of reliability, such state of the piece of equipment when the certain element (or a group of elements) is not capable to perform its function because it is unavailable due to maintenance. Then each maintenance outage means, in principle, a decrease in the certain system reliability.
If an element is under maintenance, it is neither in operation, nor is available. In the case of the series reliability system any maintenance outage causes the outage of the whole system. In the case of the parallel reliability system, any maintenance outage causes a decrease in the total reliability of the system; thence it affects the intensity and the mean duration of outages.
RCM is a decision-making tool that makes it possible to control or improve the maintenance schedule. RCM provides underlying data for adequate and logical decisions and is applied outside existing control maintenance systems. Views obtained by means of the RCM method are then utilized in modifying or redefining the existing maintenance schedules. If this method is employed properly, RCM may make the existing maintenance schedules more efficient and optimized.
The goal of reliability-centered maintenance is to formulate such a maintenance strategy so that the total operating costs may be minimized at keeping the necessary degree of the reliability, safety and environmental soundness of equipment operated.
It is necessary to take many steps first that can be briefly summarized in the following points Ref. [4] :
- The determinations of all items of equipment that are subject to maintenance and thus take part in the RCM process itself.
- The determination of functions of these items of equipment.
- The determination of a resultant model of equipment aging.
- The determination of equipment importance.
- The identification of equipment failures and their consequences.
- The setting up of the equation of total operating costs of equipment and the finding of the most suitable forms of maintenance.
First step at the RCM implementation is decision for all maintained equipment what type of maintenance will be applied to them. Generally it is possible to choose from following:
- To keep existing system of maintenance (periodical according to Preventive Maintenance Order) .
- Operation to failure (Corrective Maintenance) -regular inspections and necessary measurements required will be performed for recognition if the equipment is safe operating.
- Periodical RCM.
- On condition RCM.
From the point of view of the technical condition determination of equipment and also its reliability, we can differentiate two kinds of RCM-periodical (optimization of the maintenance cycle) and on condition (determination of the order of components for maintenance) Ref. [5].
In the area of transmission and distribution networks such items of equipmefit as transformers, outdoor and cable lines, switching elements, protection devices, etc. will be included into the RCM system.
In the case of these elements, the foundation for RCM applications is to determine the aging model and the so-called importance of the element. This is mostly expressed by the costs of element maintenance, the costs of element repair and costs of element outage.
At the Department of Electrical Power Engineering at Technical University of Ostrava we have already been concerned with the development of a methodology for RCM for some years. Our main objective is its practical utilization and inclusion into the system of maintenance of the electrical power company. Because the RCM system utilizes many information sources and will optimize the maintenance of several thousand components, it was necessary to design a software tool for useful processing just the same amount of data.
After studying the theory of the RCM system we chose two basic approaches to RCM implementation in the framework of distribution power networks. One approach leads to the optimization of the maintenance cycle for all components of the certain type or groups of components of the same type. The other approach leads to the optimization of condition-based maintenance (on-condition maintenance) , i.e, to the determination of optimum order of maintenance of particular components of the same type (Ref. [6] ) .
The approaches will be applied according to a specific component of the distribution network. The comparison of the approaches is as follows:
- Optimization of the maintenance cycle-the number of components ofthe certain type is high; generally, each component of the certain type has low importance, costs of the specific component of the certain type cannot be obtained, at the analysis of the event (failure, outage) the specific component cannot be found.
- Determination of the order of components for maintenance-the boundary must be defined from when performing maintenance is reasonable not only from the economical point of view, equipment monitoring is possible (e.g. on-1ine monitoring) , we must be able to determine the condition and importance of equipment.
The block diagram of the program is given in Figure. 3 . The basic inputs are databases of the distribution company (technical information system-TIS and financial information system-FIS), from which required data will be read. The inputs, which will be entered by the program operator, are mainly criteria for the determination of the condition of the component (weights of particular influences) and criteria that will serve the determination of importance of particular components. Furthermore, control on the part of authorities must be taken into account, such as penalties imposed for not obeying the standards prescribed for electricity supplies.
For the first group of types of components the output of the program is the optimum maintenance cycle, for the second group of types of components then the optimum
maintenance (coordinates of components condition and importance of them) .
3.1 Optimization of the maintenance cycle
The determination of the optimum maintenance interval is based on the cost function. For each item of equipment, an equation of the total operating costs as function of maintenance rate is necessary to be set up and its local minimum is to be found. The simplified cost equation will consist of three basic parts and will express the total operating costs per year of operation of the piece of equipment:
- Maintenance costs Nu
- Repair costs No
- Outage costs Nv
Additional costs are omitted. The cost function has the following form:
Nc=Nu+Na+Nv (CZK. year~*) (3)
The main simplifying assumption for the quantification of particular cost items is the fact that these particular cost items will not change with time, or that their percentage increases will be roughly equal.
For the sake of simplification it is possible to say that the maintenance costs and repair costs depend on the maintenance and repair rates, and thus on the kind and the condition of the certain element (on its aging model). The outage costs depend on the kind and condition of the element and, in addition, on the location of the element in the electrical power system (depending upon system configuration) ; i.e. on the importance of the element.
Detailed information about particular parts of cost function is introduced in Ref. [7] .
With reference to the fact that any “importance” cannot be assigned to any specific component (neither FIS, nor TIS divides data up to a specific piece of equipment) , it is necessary to proceed to data division into groups. Then, maintenance intervals of the groups are different. Input data for the division of components into groups by importance are as follows:
- For all components of the certain type-coefficient for consumer evaluation, the number of groups for division and their limits and the type of component.
- Separately for each component-identification number, the number of connected consumers by type, possible another division of the component.
The result of the division of components into groups by importance is the determination of the amounts of components in particular groups and the assignation of a group number to each component.
Figure 3 Block diagram of the program
Input data for the RCM analysis itself are maintenance costs, repair costs, failure rate, total time of failures, time of scheduled outage, number of all consumers, including their types, number of outages at not obeying the standards, penalties, price of undelivered electrical energy for specific types of consumers, relationship between costs of undelivered energy by particular types of consumers, relationship between costs of outage by specific groups, maintenance rate and the average power passing through the certain component. The given data are related to the period under consideration of one year.
Sources of these input data are exports from technical records, failure databases and financial databases, or the data are entered directly by the keyboard and are stored in a special file.
There is created maintenance schedule for all equipments of particular group on the basis of the calculated optimal maintenance rate. For example if the optimal maintenance rate is at the 0.2 (Figure. 4), it means once every five years, maintenance will be performed every year approximately for one fifth from all equipments.
3.2 Condition-based maintenance
By means of monitoring systems and various diagnostic methods the condition of a piece of equipment is determined. On the basis of the condition one may assess how long a piece of equipment will be probably run until a functional failure occurs. With reference to the fact that this is a rather expensive matter, this type of maintenance will be applied especially to expensive and operationally important pieces of equipment, such as EHV, HV transformers, etc.
The structure of input data depends on the specific component. Generally, they may be divided into three groups. For instance, for 110 kV power circuit breakers the structure of input data is as follows:
a) Identification of the specific component-identification of the specific circuit breaker, electric station, field/outlet, year of putting into service, type of circuit breaker, extinction medium, serial number of circuit breaker, year of circuit breaker manufacture, kind of drive, type of drive, serial number of drive, year of drive manufacture .
Figure 4 Graphical Representation of the Cost Function
b) Data determining the condition of this component-condition of circuit breaker, date of the last action, tightness of extinguishing chamber, date of the last overhaul of contacts, number of engine hours of compressor after overhauling the contacts, number of close/open (CO) cycles after the overhaul of contacts, date of the last overhaul of compressor (drive) , number of engine hours of compressor after overhauling the compressor, number of CO cycles after overhauling the compressor (drive) , date of diagnostic tests, evaluation of diagnostic tests of circuit breaker, date of technical condition evaluation, climatic conditions, CO number, number of compressor engine hours, condition of metal parts, earth wire condition (protection against dangerous contact with non-live parts) , condition of insulators.
c) Data determining the importance of this component-importance of circuit breaker, circuit breaker location, type of line, possibility of backup, importance of consumption, energy transmitted per year.
The result of maintenance by condition and impedance is a graph with the layout of particular pieces of equipment (Figure. 5) . On the basis of this graph, the optimum order of components for maintenance is then determined.
4 FUEL CELL SYSTEMS
In the so called “White Report” the EU member states have committed themselves to roughly double the share of renewable energy sources in covering their gross domestic energy consume by 20 lO. This strategic target is based on the premise that each nation shall contribute by its own input in accordance with the national conditions. According to EU agreement the Czech Republic will produce 896 of electricity from renewable sources in 2010. But there is a great problem with continuity and reliability of supply from most of renewable energy sources. Our university research focuses on applications of fuel cell. We would like them to support increasing share of renewable energy sources as a back up and some kind of energy store.
The reliability of the current generation of fuel cell power plants is one of the most critical issues. Analyses of the reasons for the reduced reliability of many fuel cell power plants in US and Japan have shown that by far the vast of forced shutdowns have been caused by the failure of the balance of plant components.
Figure 5 Maintenance by condition and importance
Ultimately, fuel cell plants can have on-stream availabilities of 98-99%, values that are currently achieved in conventional hydrogen plants (Ref. [8] ).
The high reliability of a fuel cell system will largely result from the modularity of the stacks (their ease of maintenance-only partial shutdown is necessary) , but should also be attributable to their lack of highly stressed moving parts operating under extreme conditions and to their ease of maintenance. The use of modular units permits a site layout that that can be designed to permit the replacement of complete modules, which not only allows for a more economical use of spare parts but also minimizes lost output. Also, by replacing spare modules, a plant could be operated at full power during periods of routine maintenance, should be this necessary. Even without spare parts, plants could be designed so that only partial shutdown will be necessary in the event of failure.
The following conditions are required for grid-connected plants:
- To maintain high quality of electricity (higher harmonics, voltage regulation, frequency change, and so on) .
- To provide suitable protection in the event of system faults.
In the case of “no supply of electricity to the grid” the plant should be equipped with a “reverse power protection system,” which may be a dummy load or other appropriate equipment.
Fuel cell generators are being considered for grid-connected (grid-parallel) , grid-connected is landing (provide power to local loads at the time of grid outage), grid-independent (stand-alone) as well as grid linked (allowing power import but not export to grid) operations (Ref. [9] ) .
Although the utility grid is generally a reliable power source, its design allows substantial voltage surges or sags, and allows momentary interruptions to clear faults. Recently, the grid outages are occurring more frequently. When a combination of a fuel cell power plant and grid as a further back up are used, unprecedented availability of the power supply can be achieved.
Fuel cell systems have high-energy efficiency, zero or near-zero emission of harmful exhaust gas (using hydrogen) . However, there are several problems that keep FC from commercial use.
4.1 Appropriate type of FC
For transportation and residential power-output up to 100 kW-are used low temperature FC and for power plants-output more than I MW-are used high temperature FC.
There are mentioned only main material requirements for FC in this paper. For all FC types sulfhr substances have poisonous impact.
- PEMFC (Proton Exchange Membrane Fuel Cells-solid perfluorinated-sulfonic acid) -require polymer electrolyie with a good conductance of protons, very pure hydrogen as the fuel, input gases must be humidified, expensive platinum catalyst, they have high performance degradation and there is a poisonous impact of CO.
- FC (Alkaline Fuel Cells-liquid potassium hydroxide) -require pure hydrogen and pure oxygen as the reactants and there is a poisonous impact of CO and C02.
- AFC (Phosphoric Acid Fuel Cells-1iquid phosphoric acid dispersed on Teflon-bonded silicon carbide) -use expensive noble metals catalysts, Iiquid electrolyte that can migrate, and there is a poisonous impact of CO.
- CFC (Molten Carbonate Fuel Cells-molten alkali (potassium or sodium) carbonate retained in a lithium aluminium oxide (LiAI02) matrix) -have high temperature liquid electrolyie that can migrate, properly engineered stainless steel.
- OFC (Solid Oxide Fuel Cells-solid yttria-stabilized zirconia) -require high temperature components with the same thermal expansion coefficient and a good seal management. Unfortunately, all ceramic construction cause higher internal resistance.
4.2 Fuel
The fuel cell reaction requires hydrogen, but there are several methods ofproducing hydrogen from fossil resources. If electricity is generated by natural energy (solar power, wind power, etc) , water could be electrolyzed to obtain hydrogen gas.
- Hydrogen-only water emissions (depends of production) .
- Methanol-1iquid fuel (concentrated energy) but requires reforming heating to about 250~C.
- Gasoline-widespread available but requires reforming heating to about 800 “C . If gasoline includes sulfur, it cannot be reformed easily.
- Biodiesel is similar like gasoline but does not include sulfur, is not toxic and is biodegradable.
- Natural gas-internal reforming in high temperature FC, the sulfur compounds must be removed first.
In area of Ostrava with heavy industry we would like to utilize bio-gas from waste-water treatment or farming and methane from coalmines.
4.3 Storage of hydrogen
There are several methods of hydrogen storage but also with their problems:
- High-pressure hydrogen tank-high pressure (e.g., 35MPa) requires robust tank that are heavy.
- Adsorption Storage-metal hydride and carbon nanotubes alloy-has a very high storage capacity but those storages are quite heavy and very expensive.
- Liquid-hydrogen requires heat-insulating tank-extremely low temperature (-253’O cause material shortness.
4.4 Price
The price is one of the most critical issues facing the commercialization of this technology today. In case of today price of fossil fuels, FC do not have a chance to compete with other sources.
5 CONCLUSION
Electric power distributors have to be prepared that they will be penalized for unsupplied electricity. Monitoring of failures and outages in the transmission and disinbution of electrical energy is necessary for determination of the reliability of network elements and the supply of electrical energy to consumers. Incorrect input data leads, of course, to false results even if the correct computing method is used. For consequential evaluation of reliability it is necessary to have data on the number and range of the examined piece of equipment. We have been canying out the observing and analyzing of failures at the research workplace of the Department of Electrical Power Engineering since the year 2000. With reference to a rather small number of failures in the area of electrical power engineering, the results will be reliable only after several years. The larger data range, the more accurate will be statistical results. That is why we would be glad to have also a possibility to enhance the databases by data from other countries. Knowledge of the item reliability allows finding out defective production runs of the equipment of distribution network, oftenest cause of failures, how long outage causes particular equipment in certain year, areas of the greatest amounts of unsupplied energy, cost of outage etc. Knowledge of the item reliability is necessary for reliability calculations for connection of wholesale consumer that are more and more requested today.
Distributors also have to spend their resources effectively to be able to compete in the electricity market. This is impossible without application of new materials that increase equipment reliability, extend time between inspections and reduce eventually completely cancel time required for maintenance. Without the item reliability is impossible to introduce Reliability centered maintenance system. In this contribution, the RCM theory is summarized in a nutshell. The main problem is always to find reliable updated input data. Thus the primary task is a change in the existing structures of particular databases in the certain regional power distribution companies. The program was developed with the aim to be universal, so that it may solve both the approaches the optimization of the maintenance cycle and determination of the order of components for maintenance. All variables of the program may be entered from input databases and edited by means of the keyboard. The novelty of our approach is its applicability to real data from distribution company. This is verified because one distribution company included our results to maintenance schedule for several types of equipment.
The first experience with fuel cells shows that islanding option allows power delivery to the local critical load during grid outage.
This capability also allows a stand-alone ftiel cell system to operate at the base load and draw peak power from the grid. Most fuel cell systems are being designed to provide an availability of >959i~. Several of these plants can be connected in parallel to achieve an even higher level of availability. It is necessary to develop new cheaper materials that ensure reliable operation of FC.
Acknowledgement
This work is supported by The Ministry of Education, Youth and Sports of the Czech Republic. Project MSM6198910007.
References
[1] Distribution companies of the Czech Republic, “Distribution Network Grid Code”, Appendix No. 2-Methodology of reliability determination of electric power supply and distribution network equipments, Prague, (2001) . [2] Rusek, S., Proch~zka, K., “Metodika ur~0v~ni spolehlivosti dodavky elektrick6 energie a prvku disinbu~n~rch soustav (Methodology for the determination of reliability of elecincal energy supply and elements of distribution systems) ,”Conference ~K CIRED, pp. 4/16-4120, Tabor, (1999) . [3] Rusek, S. “Spolehlivost elektrick~ych siti (Reliability of electric networks) ,” V~B-TU Ostrava, ISBN 80-7078-847-X, (200 1 ) . [4] Moubray J “Reliability centred Maintenance”, Butterworth-Heinemann, (1997). [5] Skog, J., “Maintenance Task Interval Determination,” Maintenance and Test Engineering Co. USA ( 1999) . [6] Rusek, S Raska T , “Zuverlassigkeitsorientierte Wartungsplanung im Verbundnetz,” Jubilaumsforum Netzbau-und Betrieb, Potsdam, (2002). [7] Rusek. S.. Gono. R., “Reliability Centered Maintenance and its Application to Distribution Networks,” International Scientific Conference EPQU 2003, Cracow, pp. 529-536, ISBN 83-914296-7-9, (2003). [8] Blomen, L.J.M.J., Mugelwa, M.N., “Fuel cell systems,” Plenum Press, 614 p., ISBN 0-306-44158-6, (1993)9] Farooque, M., Maru, H.C., “Fuel Cells-The Clean and Efficient Power Generators,” Proc. IEEE, vol. 89, No. 1 2. p 1819-1829, (2001)
