TESI DI
“Planning long-term maintenance for electric vehicle charging infrastructure through
RELATORI
_________________________
Dott. Ing. Gino Dini Dipartimento di Ingegneria Meccanica,
Nucleare e della Produzione Università di Pisa
_________________________
Prof. Andrew Starr
Manufacturing Department Cranfield University
Evalu8 Transport
FACOLTÀ DI INGEGNERIA
ESI DI LAUREA SPECIALISTICA IN
INGEGNERIA GESTIONALE
term maintenance for electric vehicle charging through the Reliability Centered Maintenance (RCM)
method”
IL CANDIDATO
_________________________ ___________________
Luigi Lioce Ingegneria Meccanica,
_________________________
Anno Accademico 2010-2011
term maintenance for electric vehicle charging the Reliability Centered Maintenance (RCM)
IL CANDIDATO
___________________
Table of contents
ABSTRACT ... 1
1. INTRODUCTION ... 2
2. SCOPE OF THE CHARGING INFRASTRUCTURE DEPLOYMENT PLANS ... 6
2.1. Electric vehicle charging infrastructures across the UK ... 6
2.2. Electric vehicle charging infrastructures across the world... 8
3. ELECTRIC VEHICLE CHARGING INFRASTRUCTURE: STATE OF ART ... 9
3.1. Charging technologies... 9
3.2. Charging modes ... 10
3.3. Explaining the charging process ... 11
3.4. Components of the infrastructure ... 13
3.5. Software and communication technologies ... 15
3.6. Functional analysis ... 16
3.7. Conclusions ... 18
4. THE RELIABILITY CENTERED MAINTENANCE: A LOGICAL WAY TO PREVENT FAILURES ... 19
4.1. What is reliability and how it is measured ... 19
4.2. The three phases and cornerstones of an RCM Program ... 26
4.3. Why RCM has historically been difficult to implement ... 27
4.3.1. Loss of in-house control ... 28
4.3.2 Incorrect mix of personnel ... 28
4.3.3. Unnecessary administrative burdens ... 29
4.3.4. Misunderstanding fundamental RCM concepts ... 29
4.3.5. Confusion determining system functions ... 29
4.3.6. Confusion concerning system boundaries and interfaces... 30
4.3.7. Divergent expectations ... 30
4.3.8. Misunderstanding hidden failures and redundancies ... 31
4.3.9. Misunderstanding run-to-failure approach ... 31
4.3.10. Inappropriate component classifications ... 31
4.4. Fundamental RCM concepts ... 32
4.5. The RCM first phase ... 37
4.6. The RCM second phase: the PM tasks selection ... 42
4.7. The RCM Living Program ... 48
4.8. Conclusion ... 51
5. MAINTENANCE OF SYSTEMS WITH COMMON FEATURES WITH THE CHARGING INFRASTRUCTURE: CHARACTERISTICS AND REQUIREMENTS ... 52
5.1. Information technologies to support maintenance of infrastructures in the public domain ... 54
6. ELECTRIC VEHICLE CHARGING STATION DESIGN REQUIREMENTS... 55
6.1. Logistics ... 55
6.2. Functionality... 57
6.3. Interface ... 58
6.4. Form ... 59
6.5. Aesthetics and Semantics ... 60
6.6. Safety ... 60
6.7. Durability and Maintenance ... 61
6.8. Production and End of Life ... 62
7. RELIABILITY BLOCK DIAGRAM (RBD): FUNDAMENTAL CONCEPTS ... 63
7.1. Series Components ... 64
7.2. Parallel components ... 64
7.3. K out of N parallel design ... 65
7.4. Other important arrangements ... 66
8. THE ELECTRIC VEHICLE CHARGING STATION RELIABILITY BLOCK DIAGRAM ... 69
9. THE APPLICATION OF THE RCM METHOD TO THE ON-STREET CHARGING STATION .. 73
9.1. Phase 1: component classification ... 73
9.1.1. Step 1: component identification ... 74
9.1.2. Step 2: component description ... 79
9.1.3. Step 3: component functions definition ... 82
9.1.4. Step 4: component functional failures definition ... 86
9.1.5. Step 5: dominant component failure modes identification ... 90
9.1.6. Step 6: failure occurrence ... 95
9.1.7. Step 7: failure effects on the EVCI ... 95
9.1.8. Step 8: effects on the Asset Reliability Criteria identification ... 100
9.1.9. Step 9: component classification ... 100
9.2. Phase 2: preventive maintenance tasks definition ... 102
9.2.1. Step 4: failure cause identification ... 103
9.2.2. Step 5: preventive maintenance tasks selection ... 107
10. RESULT DISCUSSION AND FUTURE DEVELOPMENTS ... 108
11. BENEFITS ... 112
APPENDIX A – COFA WORKSHEET ... 116
APPENDIX B – PM TASKS WORKSHEET ... 122
References ... 129
Ringraziamenti Ringraziamenti Ringraziamenti Ringraziamenti
Cinque anni… Chi l’avrebbe mai detto sarebbero passati così in fretta. Sembra ieri quando sono entrato per la prima volta in un’aula universitaria per seguire la mia prima lezione, con un unico pensiero nella testa: “Cinque anni non passeranno mai”! E invece questi anni sono letteralmente volati e questo è l’ultimo atto della mia carriera universitaria. Ma prima di mettere definitivamente la parola fine a questo percorso mi sento in dovere di porgere alcuni ringraziamenti.
Il ringraziamento più grande va ai miei genitori. I loro sacrifici e la loro fiducia in me mi hanno dato l’opportunità di vivere questa esperienza, che non è stata solo un’esperienza di studio ma anche, e soprattutto, di vita. Se oggi sono arrivato a questo traguardo è grazie a loro.
Un ringraziamento particolare va a mio fratello Donato, costante punto di riferimento e sempre disponibile a consigli e aiuti. La sua saggezza e lucidità mentale mi hanno sostenuto nei momenti di confusione e smarrimento.
Ringrazio anche i miei nonni e i miei zii per la loro costante premura nei miei confronti. Un ringraziamento particolare e “spontaneo” va a zia Tonia: se anche in questi ringraziamenti avessi dimenticato di menzionarla, stare con lei vicino a una lavatrice sarebbe stato troppo pericoloso!
Come non ringraziare i miei compagni di viaggio Danilo, Luca, Gabriele, Giulio, Vincenzo, Federico! Tra mille chiacchierate, mille caffè, mille pensate per cercare di passare un esame, ho imparato a conoscere persone diverse che hanno reso ancor più belli i momenti positivi e mi hanno strappato un sorriso in quelli un po’ meno positivi.
Infine, ma non per importanza, ringrazio la mia fidanzata Valeria che è stata capace di rendere questi ultimi due anni indimenticabili. Tra mini e macro vacanze, tra pranzi e cene prelibate, tra risate e “brevi excursus”, mi ha dato la forza di raggiungere questo obiettivo, convincendomi che non sarebbe stato solo la fine di un percorso ma l’inizio di un percorso ben più lungo, nel quale mi auguro di averla sempre accanto.
1
ABSTRACT
Electric vehicles (EVs) are mainly known for their advantages as emission free, energy efficient and noiseless transport, but electric mobility has never matured in the automotive market and it remains in the shadow of the internal combustion engine (ICE) vehicles. Studies revealed that the EV penetration depends mainly on the availability of the charging facilities. The availability and the performances of the electric vehicle charging infrastructure (EVCI) will have a major impact on the satisfaction of electric vehicle drivers and therefore on the future viability and successful of the technology. In this context, maintenance will play a key role to ensure appropriate levels of availability and reliability and also to keep the expensive infrastructure in good conditions for a long time: it will need to have a long and trouble free life, if it is to persuade the typical car user to change his behavior and choices.
This paper will provide a long-term maintenance plan for this infrastructure, in which the preventive maintenance tasks will be defined basing on the Reliability Centered Maintenance (RCM) principles and logics, starting from the definition of the electric vehicle charging infrastructure and explaining how it works and the components by which it is constituted.
2
1. INTRODUCTION
One of the major problems that most of the governments across the world are facing nowadays is the reduction of the volume of greenhouse gases emissions that are caused by human activities, where current road transport has a major contribution. During decades alternative propulsion methods have been researched, outstanding the electric vehicles (EVs) as a possible near future solution to this problem, which can also help to reduce the external fuel dependency caused by the shortage of fossil resources.
An electric car is an automobile which is propelled by electric motor(s), using electrical energy stored in batteries or another energy storage device. Electric cars were popular in the late 19th century and early 20th century, until progresses in internal combustion engine technology and mass production of cheaper gasoline vehicles led to a decline in the use of electric drive vehicle. The energy crisis of the 1970s and 1980s brought a short lived interest in electric cars, but in the mid 2000s took place a renewed interest in the production of electric cars due mainly to concerns about rapidly increasing oil prices and the need to curb greenhouse gas emissions. As of November 2011 series production models available in some countries include the Tesla Roadster, REVAi, Renault Fluence Z.E., Buddy, Mitsubishi i-MiEV, Tazzari Zero, Nissan Leaf, Smart ED, Wheego Whip LiFe, Mia electric, and BYD e6. The Leaf, with more than 20.000 units sold worldwide by November 2011, and the i-MiEV, with global cumulative sales of more than 17.000 units through October 2011, are the world's top selling highway-capable electric cars.
Electric cars have several potential benefits as compared to conventional internal combustion automobiles that include a significant reduction of urban air pollution as they do not emit harmful tailpipe pollutants from the on-board source of power at the point of operation (zero tail pipe emissions); reduced greenhouse gas emissions from the on-board source of power depending on the fuel and technology used for electricity generation to charge the batteries; less dependence on foreign oil, which for developed and emerging countries is cause of concerns about their vulnerability to price shocks and supply disruption. Also for many developing countries, and particularly for the poorest in Africa, high oil prices have an adverse impact on their balance of payments, hindering their economic growth.
Despite their potential benefits, widespread adoption of electric cars faces several hurdles and limitations. As of 2011 electric cars are significantly more expensive than conventional internal combustion engine vehicles and hybrid electric vehicles (HEVs) due to the additional cost of their lithium-ion battery pack. However, battery prices are coming down with mass production and expected to drop further. Other factors discouraging the adoption of electric cars are the lack of
3
public and private recharging infrastructure and the driver's fear of the batteries running out of energy before reaching their destination (“range anxiety”) due to the limited range of existing electric cars. Several governments have established policies and economic incentives to overcome existing barriers, to promote the sales of electric cars, and to fund further development of electric vehicles, more cost-effective battery technology and their components. The U.S. has pledged 2.4 billion US$ in federal grants for electric cars and batteries. China has announced it will provide 15 billion US$ to initiate an electric car industry within its borders. Several national and local governments have established tax credits, subsidies, and other incentives to reduce the net purchase price of electric cars and other plug-ins.
Despite of the problems just introduced, at the present time electric vehicles have achieved familiarity and acceptance by private consumers and fleets across the world (for example, EV sales in the United States have grown from 9.367 in 2000 to 324.318 through 2007) so that they are currently emerging in the market and they are seen as a promising option towards a less carbon intensive road transport. Sales of plug-in electric vehicles (PEVs) are expected to accelerate rapidly over the next several years, posting an annual growth rate of 43% between 2011 and 2017, with annual sales reaching almost 360.000 vehicles by 2017. Adoption of PEVs will vary significantly by geography. Unsurprisingly, the most populous states will see the highest sales, with California, New York and Florida recording the highest PEV sales over that same period.
EV popularity in government fleets has been driven by mandates to reduce fuel consumption and by the civic communities increasingly careful to environmental issues. The public sector’s reasons are somewhat more varied, ranging from environmental concerns, to image, to fuel savings.
For what the EV charging parameters are concerned the methodology used to establish them, including charge power, charge energy, and charge times, began with an evaluation of typical daily vehicle trips and daily vehicle miles travelled. Additionally, an evaluation of actual EV driver behaviour and an evaluation of charge power requirements (based on experience with charge characteristics of various battery chemistries) were conducted. Based on the results of these evaluations, typical charge infrastructure scenarios were developed, including (1) overnight charging at a home garage, (2) overnight charging at an apartment complex, and (3) opportunity charging at a commercial facility, (4) charging in highways. Each scenario will be described and the infrastructure and on-board power electronic devices will be determined.
There are plenty of initiatives that aim to stimulate the EV adoption, as said before. However, a widespread deployment requires high customer confidence in the use of this technology, who are generally afraid of the short action radius. Therefore, it is necessary to create in advance a large,
4
diverse and distributed charging infrastructure to support them. Currently, there are several plans across the world to install and test a number of charging stations. The functioning of this charging infrastructure in terms of availability and reliability will have a major impact on the future viability and successful deployment of the EV.
Appropriate maintenance, especially for those charging points located on-street, because of being exposed to more severe working conditions (due to their more severe working conditions, considering for example the exposition to bad weather conditions and vandalism), will play a major role to keep the expensive infrastructure in appropriate working conditions with high levels of availability and reliability for a long time. It is also important to ensure safe operation as charging stations are a new type of urban furniture that involves high voltage and current on the streets and nobody wants to do something usual (like recharging the own electric vehicle) in dangerous conditions. In addition, good maintenance can help to increase satisfaction of early drivers and therefore, to favour EV penetration. An initial bad impression of the functioning of the infrastructure may collaborate to discard prematurely the EV technology.
In this context, this work has the purpose to study the typical architecture of the on-street electric vehicle charging points and the several charging technologies and modes with the intention to understand the likely failure modes and prevent them by preventive maintenance strategies. All the maintenance studies that will be accomplished will be based on the Reliability Centered Maintenance (RCM) method, whose aim is to identify preventive maintenance tasks to address those functional failures which could have unwanted consequences for the whole system either for safety, production or protection matters.
Particularly, the list below shows the main objectives of the project:
- identify the typical architecture of the electric vehicle charging infrastructure;
- provide a Functional Analysis of the charging point;
- identify the principles and the logics of the RCM method;
- analyse each infrastructure component in according to the RCM logic;
- provide the maintenance planning for the infrastructure analysed.
Like it was said before the RCM logic will be used for the purpose: this choice has depended on the fact that a first study about the electric vehicle charging infrastructure likely failure modes has already been accomplished one year ago through the Failure Mode Effect Criticality Analysis (FMECA) (Pablo Tola Lorenzo, “Planning long-term maintenance for the electric vehicle charging
5
infrastructure”). The RCM method seemed to have a deeper key lecture of failure modes and their consequences on the whole system than the FMECA. Indeed, literature reveals that the FMECA method could be considered as a tool of the more generic RCM method.
This work has been oriented to the on-street charging stations, because most of them need appropriate maintenance strategy due their more severe working conditions, considering for example the exposition to bad weather conditions and vandalism. It is also important to ensure safe operation as charging stations are a new type of urban furniture that involves high voltage and current on the streets and nobody wants to do something usual (like recharging the own vehicle) in dangerous conditions.
In the next of this report you can find the following macro-sections:
• Electric vehicle charging infrastructure overview: in this section basic information about the state of the art of the electric vehicle (EV) charging infrastructure will be provided. It will be introduced the different technologies available to charge the EVs and the various charging modes. It will be provided also a general description of the components of the infrastructure and the software capabilities, explaining the function of each component and of the three softwares involved.
• Reliability Centered Maintenance introduction: the purpose of this section is to introduce very practically the Reliability Centered Maintenance (RCM) method, explaining the basis on which it is founded, its major objectives and each step by which it is constituted.
• Discussion of the results and future developments: in this final section of the work a brief discussion of the analysis results will be provided in order to resume what the analysis has led to and in order to understand which are the future possible developments.
6
2. SCOPE OF THE CHARGING INFRASTRUCTURE DEPLOYMENT PLANS
2.1. Electric vehicle charging infrastructures across the UK
The Department for Transport of the United Kingdom (UK) has forecasted between 0.8 and 1.5 million HEVs and EVs for 2020 in the UK roads, while 1.7 million is the estimation of the Committee on Climate Change to meet the carbon emission reduction targets by that time. To reach those penetration levels, the UK charging infrastructure needs to be developed.
To date, there are several initiatives which aim to create an initial interoperable electric vehicle charging infrastructure (EVCI) in the UK. One of the most important programs is “Plugged-In Places Plan”, which is funded by the UK government. The results derived from this project will be used to plan the future UK infrastructure. This plan consists of three pilot areas: London, Milton Keynes and North East of England, where 11.000 charging points will be installed between 2010 and 2013. In particular:
- London: outstands as the biggest charging network, where 25.000 charging points are targeted to be available by 2015. In particular 22.500 will be located in workplaces, 2.000 in public parkings and 500 in the streets. It is expected that 7.500 of them will be already operative by 2013.
- North East: 1,300 plug-in points.
- Milton Keynes: recharging units.
In March of 2011, around 2.250 charging posts were already available, being 1.600 installed in London, 600 in the North East and the remaining 50 in Milton Keynes.
The UK government offered funding for a second phase of “Plugged-In Places”, which includes five projects: Midlands, East of England, Greater Manchester, Northern Ireland and Scotland. The scope of this second phase, which extends from 2011 to 2013, is detailed in the following points:
- Midlands: 500 charging units to be mounted in Birmingham, Corby, Coventry, Derby, Leicester, Northampton, Nottingham and Worcester.
- East of England: the “Evalu8 Program” seeks to build an initial EVCI of 1.200 on-street charging points in Bedford, Cambridge, Ipswich, Norwich, Peterborough, Luton and Hertfordshire, Thames Gateway South Essex and London Stansted Airport.
7 - Greater Manchester: 300 recharging stations.
- Northern Ireland: 1.500 plug-in points will be installed in Belfast, Derry, Newry, Armagh, Enniskillen and Larne.
- Scotland: 375 charging points will be placed across the central part of the country.
The Figure 2.1. below resumes the results of the “Plugged-In Places Plan” programs.
Fig. 2.1 - UK deployment plan 2011-2013
The EVCI deployment plans have considered different charging options and locations for the plug- in points to cover the different user needs. The most likely locations for the public and semi-public infrastructure will be on-street, work places, public car parks. The plug-in facilities will allow standard, fast and rapid charging depending on the location, being the majority of them standard and fast. Standards points are more likely to be installed in workplaces, while fast and rapid are more suitable for on-street and public parking, shopping malls, etc. In addition, inductive charging and battery swapping technologies will be also tested in minor scale as part of these deployment programs.
8
2.2. Electric vehicle charging infrastructures across the world
There are plenty of projects across the world to develop an EVCI to support the electrification of road transport. Some of the most important deployment plans are summarized in the following Table. As well as in UK, the majority of the public accessible charging points allow conductive technology, offering standard, fast or rapid charging depending on the location. However, some countries such as Australia, Denmark or Israel have allocated efforts in developing and testing battery swapping or inductive charging technologies.
The meaning and the characteristics of conductive, inductive and battery swapping charging technologies and of standard, fast and rapid charging modes will be introduced in Chapter 3.
Country Charging Points Battery Swapping
Stations Year
Australia 200,000 150 2012
China 200,000 2,300 2015
Denmark 100,000 20 2012
France 400,000 - 2020
Ireland 250,000 - 2020
Israel Thousands 40 2011
Japan 25,000 - 2020
Netherlands 10,000 - 2015
Spain 18,350 - 2014
USA 22,000 - 2013
Tab. 2.1 - Scope of the EVCI deployment plans across the world
9
3. ELECTRIC VEHICLE CHARGING INFRASTRUCTURE:
STATE OF ART
Electric vehicles (EVs) are mainly known for their advantages as emission free, energy efficient and noiseless transport, but electric mobility has never matured in the automotive market and it remains in the shadow of the internal combustion engine (ICE) vehicles. The EV penetration depends mostly on the combination of three factors: electric vehicle capabilities (speed, acceleration, torque), the placement of EVs in the automotive market and the availability of the charging facilities. These three factors can be considered highly interconnected.
If the design of an electric vehicle is relatively simple, the implementation of an effective and efficient charging network is quite more difficult. Furthermore, studies reveal that the availability and the performances of the charging network will have a major impact on the satisfaction of electric vehicle drivers and therefore on the future viability and successful of the technology. There is the need to improve the actual network and expand it, trying to solve the actual criticalities in order to persuade the vehicle users to change their behavior and choices.
3.1. Charging technologies
The exchanging of energy between the EV battery and the grid can be done in three different ways:
- Conductive charging: it requires a physical connection between the vehicle and the charging point.
The link is constituted by a cable which allows the energy transmission, the data exchange and the control of the charging process. In the most fundamental sense, there are 3 functions, 2 electrical and 1 mechanical, that must be performed to allow the charging of the EV battery from the electric supply network. The electric supply network transmits alternating current (AC) electrical energy at various nominal voltages. The EV battery is a direct current (DC) device which operates at a varying voltage depending on the nominal battery voltage, state-of-charge and charge/discharge rate. The first electrical function converts the alternating current to direct current and is commonly referred to as rectification. The second electrical function is that the supply voltage must be controlled or regulated at a voltage level which permits a managed charge rate based on the battery charge acceptance characteristics (i.e. voltage, capacity, electrochemistry, and other parameters).
The combination of these two functions is the embodiment of the charger. The mechanical function is the physical coupling or connecting of the EV to the EVCI and it is performed by the user. The
10
conductive charging system consists therefore of a charger and a coupler. The conductive system architecture is suitable for use with both on-board and off-board chargers.
- Inductive charging: it doesn’t require a physical plugged connection between the two elements but just proximity between them as the inductive charging uses an electromagnetic field to transfer energy between the two items. The major advantage of the inductive approach for vehicle charging is that there is not any possibility of electric shock as there are not exposed conductors, although interlocks, special connectors and residual current devices (RCDs, ground fault detectors) can make conductive coupling nearly as safe. In the same time this charging technology presents two main problems: the significant costs to create and maintain the infrastructure and the big energy losses due to a lower efficiency.
- Battery swapping: it is the most practical solution consisting of the replacement of the empty battery with a charged one in stations similar to the petrol ones. The process takes less than five minutes so it can represent a valid alternative to the fast charging process explained in the following. But in the other hand this technology presents the problem of the significant weight of the battery pack (around 300 kg) so it is not easy to extract and of the damages caused by repeated plugging and unplugging.
Nowadays the most common solution is the first one and studies reveal it will be the most used also in the near future.
3.2. Charging modes
The charging process and its characteristics depend mainly on the battery size, on the percentage of the battery capacity to fill, on the characteristics of the battery (expressed in kW). At present day the most common charging modes are the following:
- Slow: it takes from 6 to 8 hours to replenish the battery from empty. This mode is mainly used in homes and residential areas, and in all the other places where the vehicle is expected to be parked for long time. It is also common in workplaces, as an 8 hours working day is sufficient to fully charge the battery, even if after the way to work the battery is not fully empty.
- Fast: this charging mode is capable to fully charge a battery in 2 hours and to fill half of its capacity in 30 minutes. It requires higher voltage and currents. This solution is particularly used in
11
places like shopping malls, public garages and in those other places where the stay is expected to be comprised between 30 minutes and 2 hours.
- Rapid: it takes from 10 to 20 minutes to recharge a battery from fully discharged, but this mode is possible only for those batteries which have specific architecture to tolerate high currents. In the same time the infrastructure involves expensive equipment and there could be adverse effects to the grid and the battery due to the high currents, and it could have a lower efficiency because of the high heat generated. The rapid charging process could be located in places similar to the petrol stations or in fast food restaurants in highways or main roads so as it could be used during the daily journeys.
At the present time the most used charging modes are the slow one and the fast one: the high costs for the rapid charging infrastructure and the problems and limitations it involves make the other two modes more preferable and more common.
3.3. Explaining the charging process
The functioning of the charging process can be well explained by Figure 3.1 and Figure 3.2, considering the home conductive slow charging process.
Fig. 3.1 – Interconnection between the EV and the grid and its main components
12
Fig. 3.2 – Charging system components on-board the vehicle
The EV is connected to the home wall socket by a cable. In the other end, the cable is attached to the EV inlet. Considering that the battery requires DC, the electricity which comes out of the home wall socket is entirely wrong to charge it directly, because the voltage is too low and it is AC electrical energy: the AC electricity from the home outlet is run through the charger, constituted by a transformer, which changes the voltage from 120 V or 240 V to 300 or more Volts, and a rectifier, which converts the electricity from AC to DC. The amperage is set by other part of the charger.
Once the incoming current has run into the charger, it can recharge the battery which is indirectly connected to the motor through the motor controller. For what the EVSE (Electric Vehicle Supply Equipment) is concerned, the EVSE comprises all the safe devices which permit the safe power transmission once the connection has been verified and they will be more accurately introduced in the following of this Chapter.
13
3.4. Components of the infrastructure
In this section it will be generally presented the main components by which the outdoor plug-in infrastructure is constituted, referring to Figure 3.3.
Fig. 3.3 - Scheme of the main components of the EVCI
- External structure: a charging point can be presented in the form of a bollard, ground-installed.
The bollard is anchored to the ground that gives more stability and resistance.
- Cable: the physical link between the grid and the electric vehicle is constituted by a coiled power cable, which allows the energy transmission and data exchange. The cable can be part of the infrastructure, part of the vehicle or it can be an independent element. If the cable is part of the infrastructure it could be damaged by vandalism and adverse weather conditions even if the main problem is the leak of standardization of the EV connectors so the car manufacturers would need to build vehicles with the same kind of connectors. If the cable is part of the vehicle the problem would be the non-standardization of the infrastructure sockets. The last solution, instead, seems to be the most preferable, because the cable can be carried in the vehicle avoiding damages caused by vandalism and bad weather conditions and the non-standardization problems would vanish because the design of the cable could be done regardless of the connector solutions.
14
The cable is connected both to the grid and the vehicle and the two connections are equipped with an electromechanical interlock system to avoid spontaneous disconnections while charging.
- Electric vehicle supply equipment (EVSE): the EVSE comprises all the safe devices which permit the safe power transmission once the connections have been verified. It includes devices as the residual current device (which protects from ground faults), the contactor (or cut-out, which energizes and de-energizes the connections), the circuit breaker (which protects form overloads and shortcuts).
- Charger: it is an electrical device which adapts the incoming power to the charging algorithms required by the battery. It can be on-board or off-board the vehicle. The off-board charger is required for the rapid charging mode, because there is the need of a bigger charger. Anyway, if an off-board charger can be used for all the charging modes, the on-board one can be used only for the slow and fast charging processes.
- Power supply: the on-street charging stations are connected first to a feeder pillar, in which the metering system is installed, then to the local electricity network by main underground supply cables. The feeder pillar can be located inside or close to the charging point and it is not necessary for indoor wall-mounted stations, as it takes the energy from the electric equipment of the building.
- Electronic equipment: electronic devices are integrated in the infrastructure for allowing the communication between the user and the charging point, between the electric vehicle and the infrastructure and between the network and the back office. It includes a card/RFID (Radio Frequency Identification) reader, a touch screen, LAN (Local Area Network) communication system, GPRS/GMS (General Packet Radio System/Global System for Mobile Communications) communication system and a processor.
- Other surrounding equipment: this category includes elements like signs, warnings, lighting and shelter. Information signal is recommended for publicly available charging stations, helping EV drivers to locate charging stations. Shelter is not typically required for outdoor-rated equipment, but for geographic locations which have significant rainfall or snow, providing shelter over the charging equipment will provide added convenience to EV users.
15
3.5. Software and communication technologies
There are three softwares involved in the charging process, each of one with a specific function.
- Network management software: it manages the charging network to meet the grid capacity and the power demand, trying to avoid charging in peak time. It provides also information like energy consumption, usage and errors to the back office.
- Charging software: it controls the charging process using input parameters like battery algorithms, percentage of the battery capacity to fill, desirable charging time.
- Customer oriented software: it provides information concerning the charging process, the network availability, payments. There is the need of a sophisticated communication system between the four parts involved in the charging process: the user, the charging point, the charging network and the grid. The communication needs to be bidirectional and sometimes remote. The EV user can access into the network by using a Card/RFID and can interact with it by the touch screen. The communication between the vehicle and the charging station is established via pilot wire which is located inside the charging cable. LAN communication system allows communication between the charging stations within a radius of 100 meters. A master station is responsible of communicating with the back office via GPRS/GSM providing information about the station status and failure.
16
3.6. Functional analysis
In this section it will be illustrated the activities needed to charge an electric vehicle by an on-street charging station. As the Figure 3.4 above shows, the charging process is constituted by 9 different activities:
Fig. 3.4 – Process of charging an electric vehicle : (a) client identification, (b) authorization, (c) authorization confirmation, (d) socket access, (e) access protection, (f) EV battery charging, (g) end of charging, (h) transmitting data, (i) confirmation of receipt, (j) client notification, (k) access protection, and (l) release of terminal
a) the EV user requires to have access to the network by his Card/RFID tag;
b) the charging station communicates with the back office and recognizes the customer;
c) authorization confirmation: communication between the back office and the charging point; the customer can use the charging station;
d) the costumer puts the cable in the charging point socket;
e) access protection;
f) the charging point starts to recharge the battery;
17 g) the charging process ends;
h,i) data transmission and confirmation of receipt: communication between the EV and the charging point for ending the charging process;
j) client notification: the user receives via the registered GSM telephone number by SMS service information on the status of the charging;
k) access protection;
l) release of terminal.
All the elements of the electric vehicle charging system are connected within the GSM network, and the exchange of information is bidirectional. It should be noted that data sent to the individual participants of the charging system is controlled by the EV system operator; individual participants do not have the means of direct contact between each other. The exchange of information sent between the system units in the process of charging an electric vehicle takes place on several levels, as it is shown in Figure 3.4.
• Exchange of data: charging terminal – EV system operator. The charging terminal sends to the EV system operator three classes of signals: (a) about the technical status of the terminal, including information on the protection status; (b) authorization, identification of the user on the basis of the magnetic proximity card (access card); (c) information on the status of the charging process and use of electric energy by the authorized user (driver).
• Exchange of data: EV – EV system operator. The electric vehicle transmits to the EV system operator two classes of signal: (a) identification information and current location; (b) information from the internal Battery Management System (BMS) about the charging status (State of Charge – SOC) and the technical State of Health (SOH) of the battery.
• Exchange of information: EV system operator – driver (user). The exchange of information between the user and the EV system operator is transferred via two channels: (a) the system authorizes the charging process by means of a RFID proximity card containing identification information (authorization takes places via the terminal, in which there is located a magnetic proximity card reader) and correct authorization allows access to the charging terminal and the charging process begins; (b) in return, the driver (user) receives via the registered GSM telephone number (by SMS service) information on the status of the charging.
18
3.7. Conclusions
The confluence of battery technology developments, oil price and its volatility and environmental concerns could cause a transition to a transportation system which does not depend on oil. The challenge is building a robust, flexible and low-cost system for electric vehicle energy supply and this should be the core of research programs in the coming years.
If at the present time the slow and fast charging modes are the most common solutions to charge the EVs, it will be necessary to invest also in the rapid charging mode, to stimulate the EV users to use their vehicle during all the day and to stimulate those users who are afraid of running out of battery.
But the investment costs are very high. A valid alternative could be the battery swapping, which permits the replacement of the empty battery in few minutes. Nevertheless, this solution presents some problems: the high weight of the battery pack (around 300 kg) which makes difficult the extraction of the battery; the damages caused by repeated plugging and unplugging; during cold winter the snow and the ice could pile up under the car making the battery change impossible (if the battery is installed on the bottom of the vehicle); the limitation of the space and the high land prices (a battery swapping station should be able to store and recharge dozens of batteries).
For what it was said it is presumable that the slow and fast charging modes will remain the most common in the coming years and the current literature seems to be unanimous about that, but on the other hand, suggests improvements and extensions of the existing charging networks.
19
4. THE RELIABILITY CENTERED MAINTENANCE: A LOGICAL WAY TO PREVENT FAILURES
Reliability Centered Maintenance (RCM) is a qualitative systematic approach for organizing maintenance, originated in the civil aircraft industry in the 1960s with the introduction of the Boeing 747 series, and the need of lower preventive maintenance (PM) costs in attaining a certain level of reliability. The results were successful and the methodology was developed further. In the 1980s, the Electric Power Research Institute (EPRI) introduced RCM into the nuclear power industry. Today RCM is used, or being considered, by an increasing number of electrical utilities.
The main feature of the RCM is its focus on preserving system function, where critical components for system reliability are prioritized for PM measures. Nowadays, RCM method is viewed as a logical way of identifying what equipment in a facility are required to be maintained on a preventive maintenance basis rather than a run-to-failure (RTF) basis. Literature considers the RCM method the most appropriate and the most effective maintenance approach it can be taken to get as close as possible to the 100% reliability threshold that every company hopes to reach. The essence of this method is that every component needs to be treated equally, with the analysis focused on the component functions, where the functions are the explanation of why each component has been installed in the facility: if a component neither has a necessary function nor the failure consequences have an impact on the facility and can be ignored, that means the component is in excess.
4.1. What is reliability and how it is measured
Reliability is a commonly used term during this project work and for an effective and efficient comprehension of the Reliability Centered Maintenance it is necessary to understand what reliability is, how it is measured, whether it encompasses just counting the number of plant trips per year to determine if a facility is reliable or not, whether it is the capacity factor of the facility, if it can be expressed as the mean time between failure (MTBF) for certain components. These generic and rather thin measuring standards, which are the most commonly associated measurements of reliability, can be deceiving and even lull into a false sense of comfort and security if they are used alone. Reliability was defined by Nowlan and Heap as “the probability that an item will survive to a specified operating age under specified operating conditions without failures”. Another very
20
common definition of reliability is the “probability that a device will perform its intended functions during a specified period of time under stated conditions”.
Before defining mathematically the reliability concept, it is necessary to introduce other fundamental concepts strictly connected to the reliability one: these concepts are the failure probability function f(t), which describes the probability of a given equipment to fail at the time t and the cumulative failure probability function F(t), which describes the probability that the given equipment will fail before the time t.
An example of failure probability function is shown in the figure below.
Fig. 4.1 - Example of failure probability function
By watching the curve above (Figure 4.1) you can simply understand that the failure probability of the component is expressed by a Gauss curve and that for example the probability that the component will fail exactly at the time 2,5 is comprised 30% and 40%.
0 1 2 3 4 5
0.10.20.30.4
t
y
21
Each component has a specific failure probability function which can be expressed in the form of a Gauss curve, an exponential curve, a Weibull curve or other more complex curves.
For what the cumulative failure probability function F(t) is concerned, it has been said that it represents the probability that a given component will fail before a given time t. Mathematically, it is defined by this equation:
=
Referring to Figure 4.1 the probability that the component will fail before the time 1 is the graph portion evidenced in Figure 4.2.
Fig. 4.2 – Probability that the component will survive until the time 1
22 Mathematically that probability is thus expressed:
1 =
Comparing the definition of reliability and the definition of the cumulative failure probability function it is simple to understand the equation below, which connects the two concepts:
= 1 −
After having defined the failure probability function and the cumulative failure probability function, it is necessary to introduce another important concept: the failure rate λ(t), which describes the failure frequency of a given equipment at the given time t. Generally the curve which describes how the failure rate changes with the effect of the duration of time is known as “bathtub curve” and it is shown in the following figure.
Fig. 4.3 – Bathtub curve
Once these three concepts have been introduced you can finally specify the relationships between them arriving to define mathematically the concept of reliability.
First of all you can express the probability that a given component fails at the time t as the product of the failure rate and the reliability at that time, because it is assumed that the component doesn’t fail until the time t. Mathematically:
= λ ∗ ⇒λ =
=
1 −
23
At this point it is possible to express f(t) as a function of λ(t) through a formula obtainable with simple steps:
= [1 − ]
= −
= −
= −
Thus you have:
λ =
1 − =
∗ 1
= [1 − ]
∗ 1
= −
∗ 1
Thereby:
λ = −
[]
The result is a first grade differential equation:
+ ∗λ = 0
whose solution is:
= !λ
This is the general mathematical definition of reliability.
In the case of the constancy of the failure rate (λ(t) = constant and failures happen randomly) the equation becomes simpler:
= λ
24
After defining the mathematical equation of reliability it is important to remind that reliability engineering is concerned with four key elements of this definition:
• First, reliability is a probability. This means that failure is regarded as a random phenomenon: it is a recurring event, and it doesn’t express any information on individual failures, the causes of failures, or relationships between failures, except that the likelihood for failures to occur varies over time according to the given failure probability function. Reliability engineering is concerned with meeting the specified probability of success, at a specified statistical confidence level.
• Second, reliability is predicated on "intended function". Generally, this is taken to mean operation without failures. However, even if no individual part of the system fails, but the system as a whole does not do what it was intended to do, then it is still charged against the system reliability. The system requirements specification is the criterion against which reliability is measured.
• Third, reliability applies to a specified period of time. In practical terms, this means that a system has a specified chance that it will operate without failures before time t. Reliability engineering ensures that components and materials will meet the requirements during the specified time.
Units different than time may sometimes be used. The automotive industry might specify reliability in terms of miles, the military might specify reliability of a gun for a certain number of rounds fired. A piece of mechanical equipment may have a reliability rating value in terms of cycles of use.
• Fourth, reliability is restricted to operation under stated (or explicitly defined) conditions. This constraint is necessary because it is impossible to design a system for unlimited conditions. A Mars Rover will have different specified conditions than the family car. The operating environment must be addressed during design and testing. Also, that same Rover, may be required to operate in varying conditions requiring additional scrutiny.
What it has been said is a correct definition in regard to the reliability of a specific item based solely on failures. But how is the reliability of the entire entity measured? What are the precursors to failure that cause concern that the entire facility may not be reliable? Addressing these concerns requires more depth than just looking at reliability as the probability of failure of a given item.
Applying Nowlan’s definition of reliability to the human body, the reliability of a person could be defined as “the probability that an individual will survive to a specified age under specified living conditions without dying.” This is likewise a correct statement. But obviously, there are many
25
subtle precursors to failure that can affect that outcome. For example, how many car accidents was the person involved in and was he or she at fault for most of them? This is an indicator of reckless driving habits and maybe the person won’t live to a very old age if that indicator continues at the same level. How many times was the individual overdue for a physical examination? This is an indication of apathy in regard to one’s health. How many times was the person cited for serious infractions of the law? This is an indication that the person’s behavior may be imprudent and that he or she may not have as many years to live as anticipated. Was the person a smoker or a heavy drinker? Did the person’s activities include high-risk sports like skydiving and bungee jumping?
These are all valid precursors that provide some insight into the individual’s longevity.
Reliability is more than just the probability that an individual item will survive without failures.
Likewise, it is more than merely counting gross numbers of failures or the number of lost production days resulting from some type of equipment failure. It is necessary to go beyond Nowlan and Heap’s definition and view reliability more as a measurement of events, which could be defined as “the cumulative and integrated rate of unwanted aggregate events per unit of time, where the events are not limited to just equipment failures”. The meaning of this is that reliability includes a whole host of unwanted events and occurrences that can be measured as a rate of unit operating hours. Reliability represents a broader spectrum of events than just failures, and thus, reliability measurements can offer much more intuitive insight for determining how well a facility is performing.
26
4.2. The three phases and cornerstones of an RCM Program
The RCM method could be presented as formed of 3 different phases:
1. Identify the equipments in the facility which are deemed important for the system safety, production or protection. These components need to be maintained by a preventive maintenance strategy in order to prevent their failures before they occur. In this phase you define that equipment population which is considered important for preserving the “Asset Reliability Criteria” which have been established.
2. Identify the PM tasks and the relative periodicities for the components defined in the previous step. These tasks must be both applicable and effective.
3. Execute what it has been planned in the second phase and control the work in progress.
In the same time, the RCM method could be presented as based on 3 cornerstones:
1. Know when a single-failure-analysis is acceptable and when it is not acceptable.
2. Know how to identify the hidden failures.
3. Know when a multiple-failure-analysis is required.
Generally, the RCM is a single-failure-analysis approach except when the single failure is hidden: a hidden failure is a failure mode which has not evident occurrence and has not immediate adverse effects on the system and in this case a multiple-failure analysis is required.
All these concepts will be accurately explained in the following of this Chapter.
27
4.3. Why RCM has historically been difficult to implement
It has been estimated that more than 60 percent of all the RCM programs initiated have failed to be successfully implemented. Many of the other 40 percent that were completed were performed quite superficially, making their true value only marginal. Why has it been so difficult? Why has its success been so elusive? There are many reasons. In this section it will be defined what the pitfalls to success are, why they happen, where they happen, and how to avoid them. Literature claims that RCM has become overly complicated in its transfer from the airline industry.
There is the need to state very clearly that RCM is not a PM reduction program. It is a reliability program. The results of an RCM analysis are what they are. There is no bias to either delete work or add work. If a facility is one that is laden with an inordinate number of PM tasks, many of which are believed to be unnecessary, RCM will indeed identify those unnecessary PM tasks and they will become candidates for deletion. On the other hand, if the PM program at the same facility is a superficial one, the process will probably add PM tasks to the program, but they will be PM tasks which weren’t being done and that should have been done. Industry experiences have shown that although it is definitely beneficial to delete unnecessary work, the true benefits of an RCM program are not measured by the work that can be deleted. Instead, it is more accurately measured by some of the tasks added to the preventive maintenance program that were not being done prior to the analysis but should have been. If a facility first established its PM program based on performing every task specified in each vendor manual, it will undoubtedly have too many PM tasks and it will be afforded the opportunity to delete a rather large number of the unnecessary ones. Many facilities base their PM program on the experience of some of their older employees. While this is commendable, it is not, however, in itself, a valid basis on which to stake the reliability of the plant.
Another misunderstanding derives from the Nowlan and Heap RCM application, who wrote the original RCM treatise in aircraft terminology using examples found in commercial aviation. The aircraft language, as well as the process itself, resulted in significant confusion by those trying to transfer it to other industries, and they left out some of the most important aspects along the way.
Some of the more significant reasons for this lack of success in the past include the following, which are not listed in any specific order.
