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The charging model of the plug-in electric vehicles and the design

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					             The charging model of the plug-in electric vehicles and the
           design of a universal charging station for the electric vehicles
                    He Weiguo & Du chenggang (SMEPC)
Abstract:The electric vehicle is a green means of transportation , which is an
important means of solving the energy and environmental issues . The electric vehicle
charging station provides the energy supplies for the plug-in electric vehicles , which
is an important supporting infrastructure for the development of electric vehicles . In
this paper, we analyze the charging demand for electric vehicles, and meanwhile, put
forward the requirements by charging technology and the building of recharging
stations with the development of electric vehicles. Based on this , we do an
preliminary study on the design of the charging equipment .
Key words: electric vehicles, charging system, charging model, charging station

I   Introduction
   The electric vehicles are powered by electricity , which can achieve ‘zero-emission’
and low noise . It’s an important means of solving the energy and environmental
issues . With the increasing shortage of petroleum resources and the continuous
development of the battery technologies , the electric vehicle is getting close to the
traditional fuel-powered vehicles in performance and economy , and begins to
promote into wide use gradually around the world .
   The charging system can provide the energy supply for the operation of electric
vehicles , which is an important fundamental supporting system for the electric
vehicles and also the key link of the commercialization and industrialization , with
great influence on the development of the electric vehicles industry .The construction
of the public energy supply system of electric vehicles need to depend on the charging
demand of the electric vehicle . With the consideration of the characteristics of power
load , it’s necessary to adopt different charging service models and plan charging
infrastructure layout so as to realize the target of improving the comprehensive
utilization efficiency of electrical equipment , regulating the balance of the peak and
the valley load of the power grid , improving the characteristics of the load , raising
the portion of power consumption at terminal energy facilities .
II Guiding to charge with the valley electric power to regulate the load of
power grid
   The development of electric vehicle industry will have a positive impact on the
increasing demand of electric power and the balance of the peak and the valley load
of the power grid . Taking the instance of Shanghai , the growth rate of power load of
the power grid is faster than the growth rate of the power consumption , the gap
between the peak load and the valley load is enlarging year by year , the utilization
hours are dropping year by year, and the utilization rate of the power generation
equipments and the power transmission and distribution equipments are going down.
   In 2007, the maximum peak load of Shanghai power grid amounted to 21,208 MW
and the gap between the peak load and the valley load was 8,240 MW. Due to the
imbalance between the supply side and the demand side caused by the load gap, if
only to increase the investment in the power equipment to meet the maximum peak
load will lead to excess electricity .
   Because of the current battery technology restrictions , it is still hard to finish the
charging of the battery unit of electric vehicle as fast as the traditional fuel-powered
vehicles’ gas filling do. For instance , the charging time by the use of the way of C/3
for a wholly discharged battery will be at least 3 hours to finish . Even by the use of
1C quick charger of the constant-current, it still will take more than 1 hour to finish
the battery charging[1]. Thus, the power make-up pattern for the electric vehicles
should be that the valley power at night will be used for the charging of the electric
vehicles at regular time and the quick charging facilities will be used for the charging
of the electric car at day time for an urgent need . Through appropriate guidance, 80
percents of the electric vehicle users will choose the valley power at night for their
vehicles charging . So the charging power demand on the increase can be met on the
premise of not increasing a further investment for the fundamental power equipments.
   China's automobile industry is at a stage of rapid development , according to an
annual growth rate of 6-8 per cent. In consideration of some limitation factors for
roads and energy resources, by 2030, the total number of using automobiles for China
will be about 200 millions by a conservative estimate, in which 20% will be
renewable-energy-fueled vehicles. According to the relevant model calculations, the
total number of BEV and PHEV in Shanghai will be 2.4millions by 2030, which will
make an additional valley power load demand of 1050kW and an increase of the load
factor for the power distribution facilities by 30%.
 So, it will be of a great significance for us to expand the power terminal users market,
to narrow the gap between the peak and the valley load on the demand side, and to
improve the balance between the power supply and demand and the load factor of the
power grid facilities by promoting the development of the electric vehicles industry
and guiding the users to choose the valley power at night for their vehicles’ charging.

III Electric vehicle charging demand service and its energy supply model
   Compared with the traditional fuel-powered vehicles, there are some problems for
the electric vehicle such as with a long time used for power charging, a short mileage
for its driving for one time and a high cost for its use. Among them, battery
technology is the main bottleneck that hampered development of electric vehicles
industries . The energy density of the gasoline is high up to 10,000 – 12,000Wh/kg,
and currently the energy density of the latest lithium ion battery is only 150 Wh/kg.
The advances in battery technology will become the main factors that affect the
electrical energy supply service model vehicle . So, as far as the power utility is
concerned , to promote the development of the electric vehicle industry, it needs to
closely follow up of the development of the battery technology ,to plan the oriented
implementation and construction of the fundamental power charging facilities by
different charging technologies .
    There are different requirements for the continuous going mileage and the
charging time once to the different types of the electric vehicles, which will decide the
construction model of the charging station and the power capacity for that. The
following Table 1 shows the analysis of the charging power demand for the different
types of the electric vehicles:
Table1:The classification of users and the model of power supply service for the
electric vehicles
                      Vehicles for the       Vehicles for the         Vehicles for the
                       public service         government &                 private
                                            enterprise service
Going features       Quite Smooth and      Smooth and steady         Free and flexible
                           steady
  The way to         Charging without       Charging without         Charging without
    make-up          carry on replacing          carry on                carry on
     power                 battery                                  Charging with carry
                                                                            on
 Connection
 interface to              DC                     DC                      DC
    power                                                        AC(220V or 380V)
                                          Charging at night in    Charging at night by
                  Regular charging at      regular, and quick       valley power in
     Charging      parking lot of the     charging at parking      regular, and quick
      model            employer           lot of the employer      charging at public
                                             in case of need     parking lot in case of
                                                                    need in day time
Requirements                               Special charging
for     charging Special charging unit           unit              Universal charging
facilities                                Universal charging              unit
                                                 unit
    Charging             Big capacity     Medium capacity            Small capacity
     power              (over 60kW)        (30kW or so)             (below 5kW)
    capacity
  Dependence
  on the public          Low degree            Middle degree          High degree
   service for
 power supply
The sensibility
 for the cost of         Not so much               Much                Very much
       use
   As showed in the Table 1 above, for the vehicles for the public service in operation
by the regular stops and the regular routes, the charging facilities can be located at
some stops within their regular operation range, with a little dependence on the public
power supply service facilities. And for the vehicles of government , enterprises and
individuals in operation without regular stops and routes, the charging facilities only
can be located at parking lots of their own, and the public power supply service
facilities are still needed for them additionally.

IV    The design of the universal charging station for the electric vehicles
    In the production chain of the industries, the charging station has become an
intersection of the power industry and the auto industry.
IV.1     Design requirements
(1) Practice requirements
    Confronted with the different kinds of the electric vehicles and the different using
habits by the customers, the charging station should be made universal for charging
and intellectual for charging process to meet the actual needs in maximum, which can
be considered in the following aspects:
     Under the background of the coexistence of multiple modes of the electric
     vehicles and power batteries in the market, the charging station should adapt to
     the different charging voltage degrees and different battery units;
     To meet the need of self-service charging, the charging station should have a
     function for a simple communication between the operator and the charging
     machine and a smart operational performance, without a need of some more
     interference by the operators;
     To combine the demand side management and the centralized control of the load
     The functions of charging at night and the interrupted charging also should be
     taken into consideration ;
     Due to the high efficiency of the energy transferring , the charging station should
     enhance a high power factor by the use of relevant technology to low its pollution
     effect on the harmonic of the power distribution grid and other power supply
     equipments;
(2) Safety requirements
      To serve the public, the safety of the charging station is of a great importance.
When the station is charging for an electric vehicle, the safety measures should be
taken to ensure that the operational personnel and the charged battery units are safe
even under the circumstances such as mistakes, faults and bad weathers. The
measures are as follows:
     The charging station should be designed to completely avoid operational staff
     touching any electrics with the facilities while working;
     The design of the charging station should take into account for the requirements
     for its outdoor uses. The charging unit, special jacks, connectors, plugs and
     sockets etc. should meet the requirements of the protection degree by
     IP54(outdoor), some parts, if necessary, should raise the protection degree
     according to the stipulations by GB4208;
     The charging station should be against lightning, be able to avoid happening any
     harm to the facilities caused by a lightning;
     The charging facilities should have a good performance for heat radiation and
     over-heating prevention;
     All the plugs used by the charging facilities should be with an interlock unit or a
     holding unit, and be well interlocked;
     Make sure that only after the engine of the electric vehicle stopped working, the
     charging work can start, and the vehicle should be interlocked to be not able to
     start operation during the charging time;
(3) Reliability requirements
    For the hardware design, it needs to full consider the actual working environment,
select the key parts with a stable and reliable performance, for which referring to the
compatible standard for electric and magnetic fields, adopting the reasonable layout
and the shield measures, raising interference-against level of the system and using
quake-resistance technology for the charging station, a good performance of the
stability for a whole hardware system has be guaranteed.
    For the software assembling, considering the actual working conditions and
analyzing the existing possible unstable factors, the contingency measures will be
made to guarantee the reliability for software system to be in stable and regular
operation.
(4) Extension and easy maintenance requirements
   To meet the requirement for generalization and to adapt the various kinds of
charging service models, the hardware design of the charging station should be made
in module file to guarantee the standardization of the function and the module for
their easy quick maintenance and replacement and for supporting the additional
function plus with flexibility. Meanwhile, the design also should be made with some
redundant connection interface for the direct addition of the function modules in case
of need for new function plus and the regulation of layout required by the customers,
which makes the hardware of the charging station extensible.
   In the process of the software developing, the adoption of the modular subprogram
has made the program quite readable, transplantable and generalized, which makes
the revision of the control program for the charging station easy. The maintenance and
upgrading work are also easy to do.
IV.2   The modular design of the charging station
     On the basis of the requirement above, the charging station design is shown in
the Figure 1 below.




                    Figure 1: a block diagram for the charging station
    The whole system is comprised of three parts: power supply module, control
module and charging module. The power supply module from the AC power grid
going through AC/DC transformer, being rectified and transformed into 700V DC
current, and after going through the control module, it delivers the charging voltage
for the charging module. The charging module gets the charging reference
information from the carry on energy management system for the electric car by the
way of CAN bus. After the DC/DC transformation, the charging voltage and current is
out in ready to charging the power-fueled batteries.
(1) Power supply module: Based on the numbers of users and the power demand
capacity, the charging power can be supplied by AC380V after AC/DC rectification or
by DC bus after the big capacity rectification.
(2) Control module: to complete the following functions
     Identity distinguishing function, to control the charging module for power supply
     based on the distinguishing result;
     Obtaining the information of the working status of the charging module;
     Billing function in different ways and the charging, and the receipt printing
     billing function;
     On/ off control for the charging terminal jacks;
     Statistical record for the charging and relevant faults information;
     Providing the connecting jacks with the grid for a remote control and monitoring
     by wire or wireless.
(3) Charging module: communication with the vehicle carry on BMS, the realization
     of DC/DC transformation and the output of DC charging voltage and current with
     the relevant power based on the orders.

V Conclusion
    The electric vehicle is a major direction for future renewable energy fueled
vehicles to develop, which is of a great significance for the power supply enterprises
to increase the power sale and balance the gap between the peak and the valley load.
And the erection of the public charging systems is the premise and the basis for the
electric vehicle to develop. The paper is to provide the thought for the power supply
enterprises to participate in the construction of the charging system for the electric
vehicle, by the study of the charging model and the erection of the charging station for
the electric vehicle and also the preliminary research of the power charging system for
the electric vehicles.

Reference
[1] Chen Quanshi , <The advanced technologies for the electric cars>,     The chemical
industry press, August, 2007

				
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