Active Charging Stations for Electric Vehicles Charging
P. Simonik, A. Havel, M. Hromjak, and P. Chlebis
VSB – TU Ostrava, Katedra elektroniky, 17. listopadu 15, 708 33 Ostrava, http://fei1.vsb.cz/kat430/
Abstract- This paper deals with the issues of active charging stations for electric vehicles. In the
introduction are proposed general presumptions for the electric vehicle’s diffusion in the
environment of the Czech Republic. Paper also deals with conceptual issues of the charging stations’
structure and analyses some requirements on its function. In conclusion is proposed the solution of
conceptual design of the resonant ZVS converter with resonant DC circuit. In the paper are shown
simulation results and results acquired on physical laboratory model of the converter.
1. INTRODUCTION
Currently is the electromobility one of the most discussed issues in the automobile technology field. The
intent of electric vehicles’ diffusion assumes that electromobiles will relive extreme ecological situation in cities
and especially in their centres and also enables occasional connection between distant locations as well.
Possibilities and the interest in electric vehicles’ diffusion in business environment, both as an adequate
transport vehicle and the part of so-called green marketing strategy, had been investigated by the marketing
research held in 2010 [2]. The results of this research confirmed, among other things, that most then half of
companies (56%) use the vehicles above all in interurban traffic, which occasionally require to overcome longer
distances (up to approximately 300km). Most large firms (only 3.6% of respondents) use vehicles in the cities,
which meets the expected concentration of charging stations in cities. Another important finding is that 81% of
respondents is not able to define where and how to recharge these vehicles at their current purchase. These and
other problems resulting from lack of awareness and negative attitude of potential users toward the infrastructure,
prevent further spread of the technology. If accessible network of charging stations providing an operation in
vehicles’ operational areas and also occasional travelling to medium distances (about 300km) was not be built,
entrepreneurs do not have the interest in electric cars. This assumes deployment of charging stations along major
roads within a maximum 50km.
The Czech Republic is in this branch still distinctive, because electromobilism is rather activity of amateur
enthusiasts and the corresponding number of operating electric vehicles is still low. The low number of electric
vehicles corresponds to the low density of charging network, which is run mostly by amateurs.
Therefore, the expansion of electric cars goes along the claim to build an adequate network of charging
stations and the group of users will create a significant market segment for suppliers of electricity.
2. CONCEPT OF ACTIVE CHARGING STATION
The concept of active charging station has to ensure, that the energy needed for electric vehicles charging
will be available regardless network options, e.g. at the time of vehicle shut down period and its charging also at
the time of maximum power consumption. Therefore, the station has to be equipped by own accumulation source,
which is able to cover the reductions of network supply by accumulated energy and also use energy from various
types of renewable sources attainable in given location.
The characteristic feature is usage of more types of energy sources, which significantly decreases primarily
the peak load of distribution network. Quite typical is the usage of unconventional and renewable sources, whose
timing of energy supply to network together with the accumulation of energy in the charging station creates a
stabilizing element.
The involvement of one or more charging stations, in addition to their stabilizing function, has to perform
many other functions. These include:
 minimizing reactive energy consumption, EMC, and high frequency disturbance
 maximizing the energy conversion efficiency for all types of converters
 optional remote control of charging modes from the dispatching centre, e.g. discontinue charging the vehicle
from the network and switch for charging from the accumulator of the charging station
 ensuring communication for billing the electricity in terms of electric car user, a charging station owners,
and suppliers (distributors) of electrical energy
If all mentioned conditions were met, the active charging stations represent high sophisticated technical device.
2.1. STRUCTURE OF ACTIVE CHARGING STATIONS
It is necessary to distinguish whether the electric vehicle will be charged by conventional way, i.e. the
long-term charging of the accumulator by low current, or by fast charging. For the conventional charging vehicle
usually uses own charging device connected directly to the AC distribution network. Input power usually does
not exceed 10 – 15 kW. But this simple charging method generally brings problems with increased outputs for
3th and 5th harmonics, which is given by the conception of a switching power supply in the charging device. By
the mass operation of electric vehicles and their charging from the current distribution network without earlier
remedies, could this kind of charging lead to significant problems with electricity quality, especially in regions
with a higher number of operated electric vehicles. Concerning common power current protection, this
conception does not enable fast charging.
In this paper we will further discuss the concept of active charging stations with AC output. The block
diagram of this concept is shown in the figure 1 below.
Fig. 1 Structure of the charging station for the electric vehicles charging
Proposed conception of charging station uses parallel connection of accumulation part to distribution
network output. Power blocks create reversible inverter, which allows bidirectional flow of energy between the
network and the accumulator. This arrangement allows separate charge of the vehicle only from the distribution
network, common charging partly from the accumulator of the charging station and partly from the network, and
charging of the accumulator from the network when an electric vehicle is not connected to an electric outlet. The
special function of the module could be filtration of line harmonics and reactive power from the car charging
device to the network.
The system is controlled by measuring and control unit, which ensures control and setting of above
mentioned operation modes, collects and processes data of output energy on unit’s output and input for billing
the energy, and ensures communication with the superior dispatching and billing system. The advantage of that
connection is significant variability in operating conditions, thus ensuring high operational reliability.
2.2. CIRCUIT CONCEPTION OF ACTIVE CHARGING STATION
As is apparent from the foregoing analysis, the power part of the active charging station must meet the
following requirements:
 high dynamic control of operating conditions
 four-quadrant operation due to network
 wide range of charging and compensation current
 active filtration of line harmonics and reactive power
 high efficiency conversion of electric energy
To achieve these requirements, particularly with regard to the increased efficiency of energy conversion
processes in the inverter of the charging station, the following chart was created. In the figure 2 is the main
charging connection presented by the network sources usa, usb, usc and network reactances Ls. By the standard
configuration of the electric vehicle is charger of the vehicle connected to the power network (distribution
network) through a cable with connector. As the majority of electric vehicles use accumulators with low nominal
voltage up to 400 V, the conception with input diode rectifier, filtering capacitor and step-down converter has to
be used. The input circuit of this conception is shown also in figure 2.
Fig. 2 Scheme of the higher efficiency active charging station connection
Problematic feature of this otherwise very simple concept is the output/consumption of the 5th and 7th line
harmonics from the network, which, except increased current consumption in peak time, present also additional
deformation power load. Proposed solution enables not only to cover the consumption by drawing power from
the auxiliary/additional accumulator of the active charging station, when the network has lack of energy, but also
minimize the reactive effects of vehicle’s charging device on the charging/power network. The advantage of this
solution is also that in the lack of energy in the auxiliary/additional accumulator, the station can only operate as a
parallel active filter with the structure as shown below. In the case of inaction of the active parallel part,
recharging of the electric vehicle continues, but with all the above-mentioned deficiencies.
To increase efficiency of the active part of charging station, is in the figure 2 used conception with resonant
DC circuit consisting of resonant capacitor Cr and resonant coil Lr. Resonant DC circuit is switched between
output of the accumulator and input of the bridge inverter. For a limitation, resp. definition of maximum voltage
of resonant pulses is used Clamp circuit consisted of switch Sclamp and capacitor Cclamp. The energy source Caccu
on the picture simply represents the involvement of additional accumulator of the active station.
As shown in figure 3, by the resonant circuit activity occur controlled oscillations (see lower waveform),
which ensures switching of switching combinations in inverter by zero voltage value. This eliminates much of
switching power losses of the inverter. The form of modulated pulses on the inverter output is in this case
created by the sequence of resonant pulses with required polarity (see upper waveform) and by switching the
switch combination by the zero voltage (Zero Voltage Switching – ZVS).
Fig. 3 Waveforms in resonant DC circuit of active charging station
In order to active charging station could principally contribute to the network, the actual voltage value of
resonant pulse has to be in every moment greater than the actual AC voltage value in compensated line phase of
the charging network. The whole system of converter has to be able to work in the inverter mode to supply
energy to the network, and in the pulse rectifier mode to recharge the station accumulator. Results of the numeric
model by the energy supply in the conditions of DC resonant circuit are shown in Figure 3.
2.3. LABORATORY MEASUREMENT RESULTS
According to positive results of computer simulations, with some courses mentioned in previous chapter,
was realized laboratory model of low power converter with the control unit based on the signal processor
TMS320F2812. The control unit ensures correct function and stability of converter processing and further
correct function of parallel filter as a whole. The basic task of the control unit is to:
• realize correct control to keep resonant oscillation
• realize correct control of the Clamp circuit of resonant voltage impulses
• realize correct sampling of inverter IGBT switches in resonant circuits by zero voltage
• compute referent current curve using Fast Fourier Transformation (FFT)
• control output currents by appropriate modulating method, e.g. hysteresis modulation
• on the basis of voltage sensing on the DC side of the converter regulate this voltage
• in case of emergency conditions ensure quick switching off the active part of the charging station
Examples of results obtained on laboratory model are shown in figure 4 below.
Fig. 4 Detail of active station current waveforms. Above – current in electric vehicle charging device
(C = 2200 µF). Middle – compensation current of active part. Below – current from AC line
3. CONCLUSION
In the paper was given the possibility to use soft switching converters in active charging stations for electric
vehicles, which could significantly increase efficiency of electric vehicles operation considering the charging
network. In real conditions the benefits of soft switching could occur by the decrease of high frequency
disturbance, because by the application of this method are reduced dV/dt by the switching of active parts. For
further application of the principally proved method is necessary to optimize the design of parts in resonant DC
circuit and minimize losses in resonant coil.
ACKNOWLEDGEMENT
The research described in this paper was supported by SGS project SP2011/103: Research of accumulation
subsystem structures.
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