10. Dependability of an electrical installation

78310. Dependability of an electricalinstallationPublication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, translation and reproduction, either wholly or partly, of this document are not allowed without our written consent.Industrial electrical network design guide T & D 6 883 427/AE

78410. DEPENDABILITY OF AN ELECTRICAL INSTALLATIONThe voltage at the terminals of a load is affected by phenomena originating either from theutility's network or the disturbed user's electrical installation.Abnormal operation and the cost of damage undergone depend on the type of loads and thecritical level of the industrial process. Thus, a momentary critical load cut can have seriousconsequences on the industrial process operation without the load itself being affected. Inother cases, the same disturbance can damage loads and be tolerated by the industrialprocess.For the design of an electrical network, a precise analysis of the effects of failures anddisturbances which may occur must be carried out. Provisions must generally be taken in orderto limit their consequences.Harmonic effects, flicker, voltage unbalance, frequency variations and overvoltages can bereduced by implementing network structures and equipment suitable to each case (see § 3;§ 5 and § 8).Power cut immunity also requires specific equipment such as uninterruptible power suppliesand electrical energy generator sets (see § 4). This equipment is generally not sufficient tosolve all the problems. The network structure, the automatic supply restoration systems, thereliability level of equipment, the presence of a control and monitoring system, as well as themaintenance policy, all play an important role in reducing and eliminating power cut times. Acomplete analysis of the problem requires dependability studies taking into account all of thesedata as well as the occurrence and duration of power cuts allowed by the industrial process.These studies can be used to determine the structure and equipment most suitable for theneeds of the plant. They generally require loads to be classed in relation to their sensitivitylevel and the following distinctions to be made:- loads allowing prolonged power cuts: 1 hour or more- loads having to be re-supplied after several seconds- loads unable to withstand any power cuts.Publication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, translation and reproduction, either wholly or partly, of this document are not allowed without our written consent.Industrial electrical network design guide T & D 6 883 427/AE

785Figure 10-1 shows a network example for which this distinction has been made:- vital loads unable to tolerate any power cuts are fed by an uninterruptible power supply- essential loads are re-supplied several seconds after utility network loss once the voltageand frequency of the generator set have been stabilised- priority loads are reconnected once the essential loads have been fully started again(generator set cannot instantaneously accept the whole load)- non-priority loads allow long power cuts and are only re-supplied when the utility networkhas been restored.Replacement source sizing with respect to operating requirements can be optimised if the rightstructure and automatic controls are chosen.Dependability studies can be used to determine the "minimum" structure meeting the powersupply requirements of critical loads.It must also be remembered that the choice of neutral earthing system is also an importantelement. Thus, for loads requiring high availability, it is strongly advisable to choose anisolated neutral system (see § 2).Publication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, translation and reproduction, either wholly or partly, of this document are not allowed without our written consent.Industrial electrical network design guide T & D 6 883 427/AE

786utility supplynetworkGmains /standbycan be shedcannotbe shedcannotbe shedcan be shedfor severalminutesUPS. non priority loads. power cut ofseveral hours. essential loads. power cut ofseveral seconds. vital loads. no power cut. priority loads. loads that can be shedfor several minutesFigure 10-1: network with loads classified in relation to their sensitivity to power cutsPublication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, translation and reproduction, either wholly or partly, of this document are not allowed without our written consent.Industrial electrical network design guide T & D 6 883 427/AE

78710.1 definition of terms relating to dependability (according to IEC 50 § 191)FailureThe terminaiton of the ability of an item to perform a required function.Note: - after failure the item has a fault- "failure" is an event, as distinguished from "fault", which is a state- this concept as defined does not apply to items consisting of software onlyAvailabilityThe ability of an item to be in a state to perform a required function under given conditions at a giveninstant of time or over a given time interval, assuming that the required external resources are provided.Note: - this ability depends on the combined aspects of the reliability performance, themaintainability performance and the maintenance support performance- required external resources, other than maintenance resources, do not affect the availabilityperformance of the itemReliability (performance)The ability of an item to perform a required function under given conditions for a given time interval.Note: it is generally assumed that the item is in a state to perform a required function at the beginning ofthe given time intervalMaintainability (performance)The ability of an item under given conditions of use to be retained in, or restored to, a state in which itcan perform a required function, when maintenance is performed under given conditions and usingstated procedures and resources.Maintenance support performanceThe ability of a maintenance organization, under given conditions, to provide upon demand, the resourcesrequired to maintain an item, under a given maintenance policyNote: The given conditions are related to the item itself and to the conditions under which the item isused and maintained.Preventive maintenanceThe maintenance carried out at predetermined intervals or according to prescribed criteria and intendedto reduce the probability of failure or the degradation of the functioning of an item.Corrective maintenanceThe maintenance carried out after fault recognition and intended to put an item into a state in which itcan perform a required function.FaultThe state of an item characterized by inability to perform a required function, excluding the inabilityduring preventive maintenance or other planned actions, or due to lack of external resources.Note: a fault is often the result of a failure of the item itself, but may exist without prior failure.Publication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, translation and reproduction, either wholly or partly, of this document are not allowed without our written consent.Industrial electrical network design guide T & D 6 883 427/AE

78810.2 application areas of dependability studiesThe studies can be carried out on all network types and on their control and monitoringsystem:- single fed network (see fig. 1-17)- dual fed network (see fig. 1-18 and 1-19)- loop network with automatic reconfiguration system (see fig. 1-20a)- double busbar network (see fig. 1-14, 1-15 and 1-16)- network with uninterruptible power supplies (see fig. 1-30, 1-31, 1-32, 1-33, 1-34 and 1-35)- network with automatic changeover systems (see § 12.2.1) and load-shedding andrestoration systems (see § 12.2.3)- ...They can be customised in relation to the requirements:- detail of the study- types of dependabiltiy criteria to be assessed- type of analysis• detail of the study (see table 10-1)- a rapid study or pre-study is generally used to make a rapid structure choice- a very detailed study takes into account as many factors as possible; all the operatingconfigurations, detailed analysis of possible faults and their consequences, network modelas close as possible to its behaviour during a fault.Detail of hypothesesDetail of modelPre-studyA single type of fault with averageduration and occurrenceThe consequences of faults aregrouped into large familiesDetailed studyFaults divided into familiesdepending on their effects on theinstallationThe consequences of faults areanalysed in detailTable 10-1: example illustrating the differences between a pre-study and a detailed studyPublication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, translation and reproduction, either wholly or partly, of this document are not allowed without our written consent.Industrial electrical network design guide T & D 6 883 427/AE

789• types of dependability criteria to be assessed- the average number of operating hours before the first failure(MTTF: Mean Time To Failure)- the average number of hours between two failures (MTBF: Mean Time Between Failures)- the availability of power supply to a load. This is the probability of correctly supplying a load.This probability takes into account the frequency of faults and the duration of repairs- the average number of faults per year- the average repair time (MTTR: Mean Time To Repair)- the average time during which the system is unavailable during a fault (MDT: Mean DownTime). This includes the fault detection time, the maintenance service travel time, the faultyequipment procurement time and the repair time- the optimum frequency of preventive maintenance operations and the availability of theinstallation during these operations- the optimisation of replacement sets for maintenance.For example, for the electrical network in figure 10-1, we can calculate:- the probability that the generator set does not start on loss of utility supply- the optimum frequency of generator set tests- the average number of priority load power cut minutes per year- the average number of hours before vital loads undergo a power cut.Publication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, translation and reproduction, either wholly or partly, of this document are not allowed without our written consent.Industrial electrical network design guide T & D 6 883 427/AE

790• types of analysis- help with structure design by assessing dependability criteria (see fig. 10-2).simpleststructuredependabilitycriteria assessmentdependabilitycriteria metnonewstructureyeschosenstructure- comparison of structures (see fig. 10-3)Figure 10-2: help with structure designstructureAstructureBdependabilitycriteria assessmentdependabilitycriteria assessmentstructure Adependabilitycriteria nearerto objectivesnostructure Bchosenyesstructure AchosenFigure 10-3: comparison of two structures- determination and analysis of structure dependability criteria.Publication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, translation and reproduction, either wholly or partly, of this document are not allowed without our written consent.Industrial electrical network design guide T & D 6 883 427/AE

79110.3 carrying out a dependability studyCarrying out a study requires a profound knowledge of the dependability area. Abnormaloperation analysis software (Adelia, Sofia) and modelling software (Supercat, MOCA-RP) helpexperts to assess chosen dependability criteria.10.4 exampleA drinking water production plant supplies 100 000 m 3 /day at low power 300 days/year and200 000 m 3 /day at high power 65 days/year.The water production is ensured by four production sections, each able to supply100 000 m 3 /day.Six types of load L1, L2 , L3, L4, L5and L 6 (pumps, disinfectors, etc.) which must operatesimultaneously to ensure production, are associated with each section.We can thus mark the loads ensuring operation of each section as follows:L1a, ... , L6a→ section n° 1L1b, ... , L6b→ section n° 2L1c, ... , L6c→ section n° 3L1d, ... , L6d→ section n° 4Publication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, translation and reproduction, either wholly or partly, of this document are not allowed without our written consent.Industrial electrical network design guide T & D 6 883 427/AE

792• current networkThe current wiring diagram of the network supplying the loads used for production is shown infigure 10-4.Figure 10-4: current wiring diagram of the network supplying the loads used for prodcutionPublication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, translation and reproduction, either wholly or partly, of this document are not allowed without our written consent.Industrial electrical network design guide T & D 6 883 427/AE

793During normal operation, the switchboard MV1 is fed by "utility 1", the switchboard MV2 by"utility 2".In case of loss of one utility incoming feeder, the other feeder ensures the entire power supply.In case of loss of both utility incoming feeders, the generator sets start and are able to ensurethe entire power supply.In case of loss of the transformer T1 , the transformer T2 ensures back-up and vice versa.In case of loss of transformers T3, T4or T5, the transformer T6 ensures back-up of the nonsuppliedbusbar.To ensure low power operation (100 000 m 3 /day), a single section is necessary, and for highpower operation (200 000 m 3 /day), two sections are necessary.The plant has four production sections with numerous redundancies in the electrical supply. Itshould thus have a sufficient level of availability.Now, electrical installation faults have occurred, causing insufficient production.Indeed, the distribution of load power supply is such that availability is not very good. Thus, afault on the busbar BB3 puts all L6 and L5 type loads out of service and no productionsection can operate.Similarly, a fault on busbar BB4 or BB5 puts the entire production system out of service.• proposed solutionTo improve availability, we propose to group the loads differently so that the loss of a lowvoltage busbar only puts one production section out of service.Thus, the solution proposed is the network shown in figure 10-5.Publication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, translation and reproduction, either wholly or partly, of this document are not allowed without our written consent.Industrial electrical network design guide T & D 6 883 427/AE

794utility 1 G1 G2 G3utility 2switchboard MV1switchboard MV220 kV5.5 kVT120 kV5.5 kVT25.5 kV400 Vsection n°1. . .L1a L2a L6a5.5 kV400 Vsection n°2. . .L1b L2b L6b5.5 kV400 Vsection n°3. . .L1c L2c L6c5.5 kV400 Vsection n°4. . .L1d L2d L6dFigure 10-5: wiring arrangement proposed to supply loads used for productionPublication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, translation and reproduction, either wholly or partly, of this document are not allowed without our written consent.Industrial electrical network design guide T & D 6 883 427/AE

795However, this requires investment in equipment as eight extra 5.5 kV switches and two extra5.5 kV circuit-breakers are necessary (6 LV circuit-breakers are taken away).This extra cost will be largely compensated by the very considerable gain in availability, asshown by what follows.The dependability study carried out has allowed us to compare the fault frequency and theavailability of the new network and the old network.Furthermore, two specific studies have been carried out on the new network to optimise thefrequency of generator set tests and compare the contribution of the MV part and LV part tofaults.o comparison of availabilities and fault frequencies of the two networksThe results are presented in the form of a comparison between the two networks. The oldnetwork serves as a reference and the availabilities and fault frequencies associated with it aretaken to be equal to 1.The comparison of fault frequencies is shown in table 10-2.Old networkNew networkLow power op. 1 1/22High power op. 1 1/21In total 1 1/21Table 10-2: comparison of the fault frequencies of the two networksOverall, the faults causing insufficient production will be 21 times less frequent with the newnetwork.The comparison of availabilities is shown in table 10-3.Old networkNew networkLow power op. 1 110High power op. 1 55In total 1 105Table 10-3: comparison of availabilities of the two networksPublication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, translation and reproduction, either wholly or partly, of this document are not allowed without our written consent.Industrial electrical network design guide T & D 6 883 427/AE

796Overall, the availability is 105 times better with the proposed new network.The parameters determining availability are the fault frequency and repair time. A fault causesa reduction in availability which is all the greater the longer the repair time is. Non-availabilitythus reflects the loss of production.The gains made on availability are much greater than on the fault frequency since:- the fault repair times for the new network are shorter- during preventive maintenance operations on the new network, the availability is lessdegraded.We can also see that the availability difference is much greater for low power operation thanfor high power operation as the probability of losing the four sections on the new network isvery low. However, the contribution of the low power operation availability is preponderant overthe overall result as the plant operates mostly at a low power operation (300 days to 65 days).o optimisation of generator set testing frequencyThis study has only been carried out on the new networkThe curve in figure 10-6 shows the impact of the generator set testing frequency on theprobability that the generator sets are unavailable when they are required.This probability takes into account all of the events which may occur during starting. It hasbeen fixed at random at 1 for a test frequency of one month.The curve in figure 10-6 shows that there is a minimum unavailability point for a test frequencyof 6 months.We can see that a higher test frequency does not improve the availability of the sets but, onthe contrary, degrades it as, during testing, the generator sets are not available.We can thus advise the customer to carry out tests on the sets every 6 months.Publication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, translation and reproduction, either wholly or partly, of this document are not allowed without our written consent.Industrial electrical network design guide T & D 6 883 427/AE

797generator set unavailabilityprobability54.543.532.54.64.921.510.50.91.5test frequency1 month 6 months 1 year 2 years 5 yearsFigure 10-6: probability that the sets are unavailablein relation to the testing frequencyo calculation of the MV part and LV part contribution to fault eventsThese calculations have only been carried out on the new network. They have shown that theMV part has a much greater contribution than the LV part, roughly 99.9% to 0.1 %, and asmuch for low power operation as for high power operation.Preventive maintenance must therefore be particularly thorough on the MV part of the network.Finally, it is advisable to implement a control and monitoring system on the MV part in order tomonitor the state of the installation and check electrical equipment maintenance.Publication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, translation and reproduction, either wholly or partly, of this document are not allowed without our written consent.Industrial electrical network design guide T & D 6 883 427/AE

798CHAPTER 10 BIBLIOGRAPHY• standardso IEC 50 (191) (1990): international electrotechnical vocabulary. Chapter 191: dependabilityand quality of serviceo IEC 812 (1985): analysis techniques for system reliability - Procedure for failure mode andeffects analysiso IEC 863 (1986): presentation of reliability, maintainability and availability predictions• Schneider cahiers techniqueso Introduction to dependability design, Cahier Technique n° 144, P. Bonnefoio High availability electrical power distribution, Cahier Technique n° 148, A. Longchamp,G. Gatineo Dependability of MV and HV protection devices, Cahier Technique n° 175, M. Lemaireo Industrial approach to dependability, Cahier Technique n° 134, H. KrotoffPublication, traduction et reproduction totales ou partielles de ce document sont rigoureusement interdites sauf autorisation écrite de nos services.The publication, translation and reproduction, either wholly or partly, of this document are not allowed without our written consent.Industrial electrical network design guide T & D 6 883 427/AE