Analysis of Distribution Substation Topologies for Energy Exchanging between EV and Utility Networks

Analysis of Distribution Substation Topologies for Energy Exhanging between EV and Utility Networks Marek Mägi Tallinn University of Tehnology marek.magi@he.ee Abstrat - This paper desribes the integration of eletri vehile harging stations and energy storage systems with distribution substations for bidiretional energy exhange appliations. Several hallenges our in onneting filling stations to power grid, whih onsist of several harging stations. Substation topology examples for energy exhanging between eletri vehiles and utility networks are desribed and substation layout is presented. I. INTRODUCTION In reent years the rae for manufaturing eletri vehiles (EV) and harging methods for EVs has piked up onsiderably. The main reasons are firstly reduing the dependeny of fossil fuels as oil has limited resoures and oil pries are growing higher, seondly limate hange issues with CO 2 redutions and greener thinking. Eletri vehiles an improve air quality in ities, implement new and innovative tehnial solutions and tehnologies in the eonomy, inrease publi awareness of the importane and benefits of eletri transportation and energy use. As ar manufaturers are fousing on EV design and battery solutions, eletri engineers are fousing on harging EVs with questions of onneting EVs to power grid and harging methods. In partiular, EV and vehile-to-grid (V2G) or vehile-to-any load (V2X) onepts will have great impat to utility networks. With new battery solutions (e.g. High-Density Lithium-Ion batteries) the ruising range of EVs has extended, making EVs more pratial in everyday use, thus making EVs more ompetitive with fuel engines. The major problem with harging EV batteries is time. Charging proedure has to fulfil two main riteria regarding where it takes plae: in publi plaes the harging must be fast (people annot wait for hours), in private homes and offies harging an be slower (fast harging equipment are also very expensive). Charging at homes will take the hole night and does not enable to travel several hundred kilometres with EV during day time. In the urban environment many itizens do not own personal garages nor there is not muh spae to develop single harging station areas for 8 hour harging periods. Charging an be done more effiiently at EV filling stations. In the near future new filling stations will all be built on a onept of fast harging. Preferably a disharged 16 kwh battery should be reharged to 80 % level in 30 minutes [1]. Private homes and offies will partiipate in the future in Smart Grid solutions, where EVs are onneted to harging stations with bidiretional energy flow apability. Several papers have looked into several topologies and ontrol methods that an perform bidiretional power transfer using EVs as energy storing systems. However, there has not been muh tehnial analysis about their appliations in substations. Substation topologies should support Smart Grid priniples with energy storage systems for V2G onnetions aording to IEC 15118-1. In this paper harging priniples and IEC 61851 harging modes are desribed. Later, an example bidiretional onverter is analysed and its onnetion to the power grid is examined through substation topology. II. ELECTRIC VEHICLE CHARGING PRINCIPLES A plug-in eletri vehile (PEV) is any motor vehile that an be reharged from any external soure of eletriity, suh as wall sokets, and the eletriity stored in the rehargeable battery paks drives or ontributes to drive the wheels [2]. Battery eletri vehile (BEV) is one of PEV subategories (other ategories inlude hybrid tehnologies) and is defined as an eletri vehile (EV) that uses eletri motors instead of an internal ombustion engine (ICE) to propel a vehile. The eletri power is derived from a battery of one of several hemistries inluding lead aid, nikel metal hydride (NiMH) and lithium-ion (Li-ion). Inevitably, batteries do disharge and need to be reharged with battery hargers. A battery harger is a devie, where a eletri energy is onverted into d with an appropriate voltage level for harging the battery. Battery hargers ontrol the harging proess and therefore have a great impat to the ondition and health of the battery. Battery harging systems an be integrated either into the vehile (on-board harger) or speially onstruted harging station (off-board harger). On-board systems allow batteries to be reharged anywhere, where there is an eletri outlet in present (e.g. home harging or harging at work with ground protetion). The drawbak with on-board harging systems is the limitation in their power output as their size and weight is restrited with vehile design. Due to these restritions it takes more time to reharge an EV battery ompared to offboard systems. Off-board harging systems enable fast harging, where the vehile is harged in less time. It is possible to harge a battery in 15-30 minutes with inreasing battery's state of harge (SOC) from 20 % to 70-80 %. Offboard harging systems are limited in their power output only by the ability of batteries to aept higher harging urrents. 158

The drawbak with off-board systems is the restrition with flexibility to harge at different loations. As off-board harging systems are big in size, they are quite ostly solutions as investments have to be made into property. Additional harging options inlude ontatless indutive harging or battery replaement servies. III. IEC 61851 CHARGING MODES AND APPLICATIONS Standards for EV harging is a widely disussed topi nowadays. There are existing different types of harging modes, different types of onnetors and protools. Japan, e.g. has the CHAdeMO standard for ultrafast d harging, while in Europe, IEC 61851-x standard is still under disussion for EV harging as well as the IEC 62196-x standard for the harging onnetors. Aording to the IEC 61851-1 standard there are 4 types of harging modes: Mode 1 - slow harging from a household-type soketoutlet; Mode 2 - slow harging from a household-type soketoutlet with an in-able protetion devie; Mode 3 - slow or fast harging using a speifi EV soketoutlet with ontrol and protetion funtion installed; Mode 4 - fast harging using an external harger. The IEC 61851-1 standard douments the pilot signal flagging the harging requirements by using pulse width modulation. The pilot signal is integrated into the IEC 62196 plugs of EV harging equipment for ontrolling higher harging urrents. For Mode 1 the eletri outlet is non-dediated, onventional household plug an be used. Earthing is essential for safety and a residual urrent devie is mandatory. The harging mostly takes plae with Single- Phase 230 V a voltage, with maximum urrent of 16 A per phase, where the harging power is in the range of 3-11 kw. This type of normal a harging has a long harging time (approximately 8 hrs.) and is usually done overnight. This is mainly due to a fat that domesti household plugs are usually designed up to 16 A (moreover the maximum ontinuous urrent is also limited up to 10-13 A). The basi onverter is loated inside ar. This type of harging is relatively simple and heap. For Mode 2 the eletri outlet is non-dediated. An additional inline ontrol box is required, whih must be loated near the plug or in the plug. The supply network side of the able does not require a ontrol pin (it is required only on the side of the EV) and the ontrol funtion is governed by the ontrol box in the able. Cable ontains an intermediate eletroni devie for Control Pilot and residual urrent devie. These provisions allow harging stations to be with low omplexity, while extending the permissible range of harging urrents ompared to Mode 1 harging. The harging mostly takes plae with 3-phase 400 V a voltage, with maximum urrent of 32 A per phase, where the harging power is in the range of 7,4-22 kw. Mode 2 is suitable for semi-publi harging, where EV harging takes plae at offie workplaes (e.g. in/outdoor offie garages or ar parking plaes), ommerial omplex parking garages et. For Mode 3 the eletri outlet is dediated, soket outlet speifi for EV must be used (5 or 7 pins for EV onnetion). The mode is ommonly known as a fast harging. Mode 3 onnetors aording to IEC 61851-1 require a range of ontrol and signal pins for both sides of the able. EV power demand is regulated through the ontrol pilot line modulating a pulse width modulation signal. Protetion is realised with ontrol pilot funtion. The harging station soket is dead, if no vehile is present - the pilot pin in the plug on the harger side ontrols the iruit breaker. The harging mostly takes plae with 3-phase 400 V a voltage, with maximum urrent of 63 A per phase, where the harging power is in the range of 14-44 kw. The ommuniation wire between ar eletronis and harging station allows integration into Smart Grid senarios. This type of a fast harging (semi-fast with harging power 6-10 kw, fast over 22 kw) takes approximately 2 hrs. The drawbak with suh harging is the neessity of an advaned onverter inside the ar, whih has a high weight. Mode 3 is also suitable for semi-publi harging. For Mode 4 the supply network a power is onverted in the harging station to d, thus the mode is ommonly known as d fast harging. The eletri outlet is dediated, soket outlet speifi for EV must be used. The plug type ensures that only a mathing eletri vehile an be onneted to the off-board harging station. Control pilot funtion extends to equipment permanently onneted to the supply. The harging mostly takes plae with 500-600 V d voltage, with maximum urrent of 400 A, where the harging power is in the range of 50-150 kw. This type of d fast harging is ideal for publi harging for quik top-ups of battery power. Charging takes approximately 30 min (e.g. for 25 kw battery). Mode 4 (and also Mode 3) harges the battery only to a ertain degree (typially to 80 %). Fast harging does not allow final harging. The advantages with this type of harging is that only a basi onverter is required inside the ar, whih has low weight. High power onverter is loated outside the ar. Charging powers an go up to muh higher values ompared to the on board harging. The drawbaks inlude expensive osts for high power harger and investments into infrastruture as higher powers are not available in domesti environment. Infrastruture onsists of a filling station, speifi harging hardware (harging station, plug, able) and software (station's panel). Software must be able to identify the user, ollet data from eletriity meter, manage payment and billing, roaming, remote maintenane and load management. With publi fast harging risks are onsiderably higher than with a low voltage home harging. The two main risks are in personnel safety and higher short iruit levels. Also onnetors used in harging must be able to handle higher power levels. Charging has to be omfortable, have an easyto-use human-mahine-interfae for operation, lient must be easily authentified. Many ar manufaturers develop their own eletri vehile models and harging methods. The variety in harging hanges from (e.g. BMW, Meredes, Mitshubishi [3]): 1-phase 230 V a, 13-32 A, harging power 3-7,4 kw for battery apaity 15,3-44 kwh; 159

3-phase 400 V a, 16-36 A, harging power 11-25 kw for battery apaity 16,5-36 kwh; 330-345 V d, 120 A, harging power 50 kw for battery apaity 16-24 kwh. IV. CHADEMO STANDARD "CHAdeMO" is an abbreviation of "CHArge de MOve", equivalent to "harge for moving", and is a pun for "O ha demo ikaga desuka" in Japanese, meaning "Let's have a tea while harging" in English [4]. CHAdeMO is a harging protool for rapid d harging issued from TEPCO (Tokyo Eletri Power Company). Common type of harging is with 50 kw d voltage. Max figures inlude d output 62,5 kw, d voltage 500 V, d urrent 125 A. CHAdeMO does not work for urrent battery hemistries to muh over 90 % SOC. Charging urves are typially 1.2 C - 4 C (up to 10 C). High (ultra) voltage power grid an supply eletriity to quik harger easily. If there are enough quik hargers in publi areas, drivers will satisfy with small size on-board hargers. EV omputer deides optimal harging urrent based on its battery ondition (BMS observation [5]). Charging urrent signal is sent to harger using CAN bus. Additional analogue ommuniation allows fail safe design. Charger supplies d urrent following order from EV omputer. CHAdeMO quik harger an hange harging speed to meet eah batteries harateristis and ondition. When harging speed is well ontrolled there is no negative influene to the lifetime of battery (battery must support fast harging). The more higher urrent the battery an absorb the more higher power it an reeive. Fig. 1 represents a basi off-board harger, whih onsists of: main supply inome (power grid), ground fault interrupter (GFI) between grid and harger, a input filter, input retifier (e.g. diode bridge), d link, full or half bridge inverter, isolation transformer, output diode retifier, output LC filter, ground fault interrupter (GFI) between harger and EV [6]. Filter in a part removes higher harmonis distortion to protet power grid. Power fration orretor improves onversion effiieny and performane. Isolation transformer is neessary for separating battery iruit from grid for operator protetion against eletri shok. Output LC filter redues ripple noise from output urrent to protet battery system. Ground Fault interrupters/earth leakage breakers (GFI/ELB) are for rapid response for earth leakage to protet operator from eletri shok. One GFI/ELB is for monitoring harger's primary side of transformer and other for monitoring seondary side of transformer and vehile. Advaned off-board harger has in addition to basi onfiguration also bidiretional energy flow apability and additional energy storage devie. The harger's power abinet an be separate from the harging station or integrated with the harging station. When separated, there is less visual impat for ustomers. With multi-output topology there an be one ommon input setion for all the harging onverters. Inoming power an be redued, if a simultaneity index is onsidered. Should there be only one harging station the one abinet solution is heaper. V. ANALYSIS OF POWER TRANSFER BETWEEN GRID AND CHARGER For desribing a harger working in 4 quadrants a simplified (ideal) model is represented. Positive urrent diretion from the power grid to the inverter is firstly viewed. Shemati for the model is shown in Fig. 2 [7]. The model parameters are given as follows: v (t) instantaneous harger voltage [V], v s (t) instantaneous grid voltage [V], i (t) instantaneous harger urrent [A], L oupling indutor [H], d phase differene between v (t) and v s (t), q phase differene between i (t) and v s (t), f system frequeny (50 Hz). The grid voltage is assumed to be purely sinusoidal. For simplifiation, high frequeny omponents of inverter output voltage, v (t), is negleted. Following equations an be derivated: v ( t) 2V sin( wt) s = s, (1) v ( t) 2V sin( wt d) =, (2) X = 2π. (3) fl Fig. 1. Topologies of d fast hargers aording to CHAdeMO standard. 160

Fig. 2. Representation of grid and harger. For desribing power transferring from harger to grid in the model, two voltage soures should be viewed as deoupled in Fig. 2 and oupling indutor is to be viewed as a soure. From this model simplifiation line urrent an be written as: i ( t) 2I sin( wt q) =. (4) Sine the default diretion for ative and reative power transfer is from grid to harger, i (t) and v (t) are lagging the grid voltage: V = V + jx I. (5) s P-Q plane shown in Fig. 3 indiates all the different operation modes in whih the system an be working. Ative power is provided by the grid as long as v (t) lags v s (t), and it is sent to grid when v s (t) lags v (t). Sine v (t) and v s (t) are sinusoidal, i (t) is also sinusoidal as shown before. Its phase angle, q, determines the diretion of the reative power flow. If q is positive, reative power is sent to the grid, and if q is negative, reative power is provided by the grid to the harger. VI. BIDIRECTIONAL CONVERTER TOPOLOGY Bidiretional onverters allow energy to flow in two diretions (provide energy from the grid as a power soure to harge the battery and to give power bak to the a power grid with disharging the battery). Several different power eletroni iruit topologies for bidiretional onverters are possible. The evaluation and development of the optimized onverter is still a hallenge. Optimized topology depends on the power rating. Single-phase Power Fator Corretor (PFC) mains interfaes an be used for low harging power levels (P < 7 kw). Fig. 3. P-Q plane showing harger operation modes and vetor diagram for different operation modes. For higher harging power levels 3-phase PFC interfaes have to applied. Voltage and urrent rating as well as the operating frequeny are the main riteria for the seletion of the power semiondutor devies. For d fast harging the most suitable devies are IGBTs together with ultrafast swithing diodes. One optional EV harger topology is desribed in Fig. 4 [8]. The harger is omposed of a-d-d isolated onverter with bidiretional power flow apability. The analysed struture onsists of a 3-phase urrent soure onverter and d-d isolated onverter. In this d-d onverter, the primary side of the high-frequeny transformer is a urrent soure onverter and the seondary side is a voltage soure onverter. Current of the a-side is ontrolled by the voltage of the indutor in the d-side. Current and voltage in d-side an be regulated in a wide range from zero to the rating value. Current and voltage in a-side have low total harmoni distortion and power fator in a wide output power range. 3-phase urrent soure onverter onsists of 6 fully ontrolled swithes (S11-S16) and 6 diodes (D11-D16). Ad onverter is neessary to realise the line-side sinusoidal urrent urve with the pulse width modulation (PWM) ontrol strategy. Fig. 4. Bidiretional onverter topology. 161

Fig. 5. Bidiretional onverters in parallel. Elements S17 and D17 provide a bypass to redue the voltage spike of the swithes, when they are turned on or off. The isolated d-d onverter onsists of high-frequeny transformer with urrent soure onverter (S21-S24 and D21- D24) at its primary side and the voltage soure onverter (S31-S34) at its seondary side. Converter an hange the diretion of the urrent onveniently through the voltage polarity ontrol of the CSC II in the d-d onverter. When harging the battery, input urrent phase is the same as the input voltage phase in the 3-phase a grid system. Converter transfers power from a-side to the d-side. CSC I works as a retifier. When disharging the battery, the voltage phase is reversed to the urrent phase in a 3-phase a grid system. The proposed onverter transfers power from the d-side to the aside. CSC I works as an inverter. The urrent diretion is the same when harging or disharging the battery, but the voltage phase is reversed to the urrent phase at the a-side. For higher harging power levels, the neessary output urrent an by generated by onneting smaller modular hargers in parallel ( Fig. 5 ). One harger will have to be the master and others slaves. Eah individual modular harger has its own protetion fuses, filters and IGBT bridges, whih are ontrolled by using PWM-swithing tehnology. Information from the main ontroller is sent to the IGBTs by means of an optial link. The maximum output urrent depends on the power ratings of individual hargers and also on how many modular hargers an one main ontroller handle. Depending on the topology of hargers, preloading iruit might be neessary to harge apaitors in the d side to ensure a smooth start-up without exessive inrush urrents. VII. SMART GRID In the onept of Smart Grid, entralized power systems (top-down priniple) [9] should hange to distributed power systems in the future ( Fig. 6 ). Smart Grid's goal is to supply and onsume energy in a reasonable way that suffiient energy an be available at all times and with high quality. Smart Grid inludes data ommuniations Network, whih is integrated with the power grid. Communiation Network enables power grid operators to ollet and analyse data about power generation, transmission, distribution, and onsumption - all in near real time. Smart Grid tehnology predits energy onsumption and reommends to suppliers and onsumers the best way to manage power. Fig. 6. Network topology today and tomorrow. Most of entities of Smart Grid beome ative. They are both users and produers of eletri energy (e.g. smart buildings). Users of eletri energy beome prosumers. The EVs and harging stations an be a part of the interating Smart Grid. EVs are mobile energy storing systems, whih an partiipate in energy trading. Vehile-to-grid (V2G) desribes a system, in whih EVs ommuniate with the power grid to sell demand response servies by either delivering eletriity into the grid or by throttling their harging rate. Energy storage systems enable storing energy and transferring energy bak to grid, when neessary. Substation equipped with energy storing or V2G system enables peak load shaving and demand response, whih will redue/postpone the need to make new investments into building new power soures or power grids to meet peak demand. Energy storing systems enhane renewable energy resoures effiieny, ompensate reative power peaks in the grid to redue losses and enhane ative power let through. VIII. DESCRIPTION OF AN EXAMPLE SUBSTATION Utility networks are typially designed for speifi load arrying apability. When EVs are added to the utility network, load patterns will be hanged drastially. This may lead to overloading of the utility in some periods of the day, making eletri iruits and transformers vulnerable. Before integrating a filling station to an existing power grid, alulations have to be made to verify that the power grid an withstand filling station loads. In the onept of smart harging, EV harging is ontrolled with available soures in the power grid. Calulations have to onsider that d fast harging should not take more than 30 minutes. If we look at an EV, whih requires 15 kwh of eletri energy to travel 100 km (rule of thumb in most ases), this will require minimum of 30 kw harging power (15 kwh divided by 0,5 hours). If we onsider that reharging the battery should take as muh time as refilling a petrol ar, e.g. 5 minutes, this requires 180 kw harging power. For simplifiation, assume that a filling station onsists of 10 harging stations (CS). If all 10 harging stations are oupied, the filling station requires minimum 300 kw harging power (30 minute harging time for 10 CS x 15 kwh) or maximum of 1800 kw harging power (5 minute harging time for 10 CS x 15 kwh). The differene between maximum and minimum is 6 times. 162

This differene is a hallenge, when designing a new prosumer onnetion to the power grid, whih inludes a filling station. In 3 phase 400 V a power grid, this will require minimum of 433 A transfer urrent and maximum of 2598 A transfer urrent. These figures indiate learly that a d fast filling station is a burden for the power grid and therefore annot be designed to a low voltage grid (e.g. residential area), but should be designed into a medium voltage (MV) grid. A substation for filling station should distribute and ontrol eletri energy flow from MV grid to harging stations, distribute energy to other a onsumers (e.g. nearby buildings), store eletri energy and be a part of the power grid ommuniation infrastruture for Smart Grid senarios. Substation must enable for harging stations a slow harging (1 phase harging power 3,7 kw, max. harging urrent 16 A, harging voltage 230 V a), AC fast harging (3-phase harging power 22 kw, max. harging urrent 32 A, harging voltage 400 V a) and d fast harging (harging power 50 kw, max. harging urrent 125 A, harging voltage 50-600 V d, power fator 0,97-0,99) [10]. Eletri energy an be stored from nearby renewable resoures, whih an be taken into use e.g. in ase of power grid supply interruption. As several onverters are required in the filling station, filters have to be inluded in the substation to remove higher harmonis distortion to protet the power grid. Substations equipped with supervisory ontrol and data aquisition (SCADA) remote terminal unit (RTU) systems annot be saled and should evolve to support next generation intelligene for Smart Grid senarios. Flexible IEC 61850 ompliant intelligent eletroni devies (IEDs) and utilitygrade rugged IP routers and Ethernet swithes allow IP-based ommuniations. IX. SUBSTATION TOPOLOGIES A. Medium voltage side and low voltage side in the substation An example topology of medium voltage side of a substation for EV filling station is shown in Fig. 7. The medium voltage inoming able from utility network is onneted to the MV swithgear MVSW inoming ubile MVI.The seletion of type of swithgear depends on the medium voltage range. Commonly in Estonia 6-24 kv equipments are used. The MVSW swithgear onsists of modular ubiles equipped with SF6 or vauum air breaking tehnology. Ciruit breaker feeders MVF 1 to MVF n ( Fig. 7. ) are equipped with motor drives, opening/losing ommand oils and with earthing swithes. Mehanial interloks prevent operating earthing swithes and breakers at the same time, guarantee earthing at working with ables. MVF 1 to MVF n ubiles inlude multifuntional protetion devies [11] with IEC 61850 ommuniation protool for protetion, monitoring and ontrolling systems, metering and signalling, ommuniation, auto diagnostis. Swithgear is provided with MVTS ubile, whih inludes MV surge arresters and voltage transformers for relay protetion. Separate ontrol swithboard DCSW is used for transferring supply voltages to different MVSW devies. DCSW inludes ad onverter ADC and d-d onverter DDC. DCSW swithboard supplies different relay and devie iruits DD1 and DD2. DCSW inludes batteries ESD for uninterruptable power supply. For remote ontrol the MV side requires separate RTU (remote terminal unit) swithboard RTU1 ( Fig. 7. ). Multifuntional protetion devies are onneted with fibreopti able to RTU1 fibreopti input terminal FO. Different relay signals REL, position and fault signals of devies DIT, different digital inputs DI, fibreopti input terminal FO are onneted to RTU1 data onentrator and annuniator terminal RTA, whih transfers and reeives data from SCADA entres. RTU1 is equipped with temperature sensor TEMP, whih value is sent to SCADA system. RTU1 transfers ontrol signals (e.g. MV iruit breakers in or out), position signals (e.g. position of breakers and earthing swithes, blown fuses or tripped breakers, battery faults in d swithboard, seurity and fire alarms), measuring values (e.g. urrent, voltage, ative and reative power, short iruit fault loation). Fig. 7. Topology of medium voltage side of the substation. Fig. 8. Topology of low voltage side of the substation. 163

One of the MVSW outgoing ubiles (in this ase MVF 1 ) is onneted with the transformer ( TRANSFORMER on Fig. 7 ). As an option MV metering panel MVM an be inluded before the transformer. Transformer onverts MV voltage to low voltage (LV). Low voltage side of the transformer ( Fig. 8 ) onsists firstly from devies for protetion (ground fault interrupter GFI, surge arresters SP, undervoltage relay VM ), main iruit breaker LVCB, devies for metering LVM, devies for self-onsumption of substation SCS and RTU2 for the low voltage. Substation's self-onsumption iruit onsists of fused main swith FSS and several distribution iruit breakers and swithes for ontrolling lighting and ventilation LIG, heating HT and soket onnetions SOC in the substation. Additional ontrol iruits inlude relay iruits ( REL1, REL2, REL3 ), emergeny swith ut-off iruit ( EMG ), retifiers to onvert 230 V a into 24 V d ( ADC1 ) and 12 V d ( ADC2 ). Main iruit breaker LVCB is equipped with motor drive for opening/losing the breaker automatially. The low voltage side in Fig. 8 onsists of a smart eletri meter LVM, whih is apable of transferring measurement value data with IEC 61850 protool. Smart eletri meter is partiularly neessary to measure higher harmonis in the low voltage grid. IEC 61850 ommuniation links are olleted to one main swith SWITCH1 ( Fig. 8 ), whih supports IEC 61850 protool. The swith transfers data to entral ontroller CONTROLLER1. The ontroller is supplied through uninterruptable power supply UPS1. The entral ontroller proesses data gathered from measuring devies, bidiretional hargers and harging stations (presented later on Fig. 9 and Fig. 10 ). The ontroller runs protetive and ontrolling funtions throughout several funtion stages: start-up stage, normal operation stage, emergeny stage (e.g. supply interruption), restart stage (voltage returns). The entral ontroller determines how many bidiretional onverters have to be onneted in parallel (to ahieve the neessary output power to supply all the harging stations) based on the measuring information from the LVM meter and ontrol links from the bidiretional hargers. The entral ontroller also needs ommuniation links to harging stations to be able to identify the battery bak of the EV. Controller ontrols energy flow to the energy storing system and energy bak from the storing system to the low-voltage grid. Previous parts have desribed how eletri energy is transferred in the substation from medium voltage grid to the low voltage grid. Following parts desribe topologies how eletri energy an be onverted to d voltage, for harging EVs, and stored, whih enables Smart Grid senarios. B. Common d bus topology An example topology for supplying harging stations through substation is shown in Fig. 9. Bidiretional hargers BC 1 to BC n onvert mains a voltage and urrent into d voltage and urrent ( Fig. 9 ). Chargers are proteted with fuses FABC 1 to FABC n and ontrolled through ontators CBC 1 to CBC n. Charging stations CS 1 to CS n, energy storage devie ESD and renewable energy resoures ( WIND POWER and SOLAR POWER ) are onneted to the same d main bus system ( ommon d bus ). Fig. 9. Common d bus topology. 164

LCL filters LCLF 1 to LCLF n must be used before hargers on a side to redue harmoni levels in the a power grid. Common d bus has a bus oupler BCDC, whih allows energy to flow separately into the storing system ESD through bidiretional d-d onverter BCES (e.g. battery pak has low apaity). Energy flow is ontrolled in the d system through d ontators ( CDCS 1 to CDCS n, CES, CTW, CTS ), bidiretional d-d onverters ( BCS 1 to BCS n ) and proteted with ultra-fast ating fuses ( FDBC 1 to FDBC n, FDCS 1 to FDCS n, FES, FDW, FDS ). CONTROLLER2 ontrols BCS 1 to BCS n, monitors ommon d bus voltage and ommuniates with SWITCH2. Charging stations are separately monitored with ground-fault interrupters ( GFID 1 to GFID n, GFIA 1 to GFIA n ). The 400 V a main grid is used for transferring a power to the harging stations for a fast harging (through iruit breakers FACS 1 to FACS n and ontators CACS 1 to CACS n ). The a main grid is also used to transfer energy to other a onsumers ( FEB ) onneted with the substation (low voltage swithboards for buildings, residential areas, street lighting et.). The advantage with ommon d bus topology is a fat that there is only one bidiretional harger in the a power grid. The harger may onsist of several smaller hargers in parallel ( BC 1 to BC n ), but the ontrolling an be made through one ommon ontrol link. In ase the load of harging stations is low, some of the parallel hargers an be disonneted through ontators ( CBC 1 to CBC n ). The disadvantages with ommon d bus topology are mainly related with harging of several EVs at the same time, whih are onneted to the harging stations at different times. Every EV requires different harging voltages for optimal harging and therefore bidiretional d-d onverters ( BCS 1 to BCS n ) are required in the system. Challenges are also with payment related issues as metering has to be done separately in the a and d side and inside the harging stations. C. Individual harging topology An example topology for supplying harging stations through substation is shown in Fig. 10. In individual harging topology every harging station ( CS 1 to CS n ) has its own bidiretional harger ( BC1.1 to BC n ). Chargers are proteted with fuses FABC 1 to FABC n and ontrolled through ontators CBC 1.1 to CBC n. LCL filters LCLF 1.1 to LCLF n must be used before hargers on a side to redue harmoni levels in the a power grid. D energy flow is ontrolled through d ontators ( CDCS 1 to CDCS n, CES, CDES, CTW, CTS ) and proteted with ultra-fast ating fuses ( FDBC 1.1 to FDBC n, FDBE, FES, FDW, FDS ). Charging stations are separately monitored with ground-fault interrupters ( GFID 1 to GFID n, GFIA 1 to GFIA n ). Fig. 10. Individual harging topology. 165

The 400 V a main grid is used for transferring a power to the harging stations for a fast harging (through iruit breakers FACS 1 to FACS n and ontators CACS 1 to CACS n ). The a main grid is also used to transfer energy to other a onsumers ( FEB ) onneted with the substation. When hargers are plaed inside the substation the harging stations take less spae and fewer investments have to be made into property. Charging stations onsist of onnetors for the EV, ontroller for the ommuniation with EV, payment system and interfae panel. Metering for payment an be made inside the substation in the a side ( CSM 1 to CSM n ). One metering system is suitable for both a and d harging. Metered values are sent via IEC 61850 ommuniation link to servers and harging stations for payment and harging information. Chargers for harging stations may onsist of one harger BC n or several smaller hargers in parallel BC 1.1 to BC 1.n. The number of harging stations defines how many harging stations the entral ontroller (in Fig. 8 ) has to ontrol at the same time. The energy storage system ESD requires separate bidiretional onverters BCES1 and BCES2, filter LCLE, a fuses FABE and a ontator CABE. The advantage with individual harging topology is the possibility to harge EVs in a short time and every harging station an determine the most optimum harging algorithm for its onneted EV. This the most ruial riteria for filling stations in publi areas. The disadvantage with suh topology is that it is osts more ompared to ommon d bus topology as more hargers and devies are required in the system. D. Topology with two inoming transformers For ensuring higher reliability, topology with two inoming transformers an be onsidered ( Fig. 11 ). In normal operation, both transformers ( TR1 and TR2 ) distribute energy to harging stations and other a onsumers. Transformers are separated from eah other through bus oupling swith LVBC. In ase of supply interruption, bus oupling swith will onneted all harging stations to one of the operating transformers and disonnets the interrupted iruit's low voltage side main iruit breaker ( LVCB1 or LVCB2 ). The swithing is ontrolled through automati transfer swith system ( ATS ), whih an operate independently from the entral ontroller (in Fig. 8 ). The ATS system an be designed on relay onnetions or smaller logial ontroller. The ATS status signals are onneted with entral ontroller E. Proposed layout of the substation For illustration, an example 500 kva substation layout with individual harging topology is represented on Fig. 12 and Fig. 13. The substation supplies five 50 kva harging stations and 250 kva of other a onsumers. The substation an be onstruted aording to ustomer's speifiations. Substation's enlosure should be made out of reinfored onrete to withstand the total weight of the equipments. Separate base module is neessary for abling. In order to prevent environmental damage, an integrated oil olletor has to be installed in the base. Substation transforms 6 24 kv medium voltage to 400 V a voltage. The appearane (exterior finishing) and overall dimensions of the substation an vary with different requirements presented by the ustomer. In general, the substation onsists of a medium voltage swithgear, low voltage transformer, low voltage swithgear and eletri energy storing system (e.g. battery pak). The length of the swithgear depends of how many outgoing feeders are required in the medium voltage side and in the low voltage side. The substation must have internal air duts and ventilators to extrat heating losses from bidiretional hargers and remove pressure gasses from medium voltage swithgear in ase of a short-iruit. Fig. 12. Inside layout of a 500 kva substation. Fig. 11. Topology with two inoming transformers. Fig. 13. Outside layout of a 500 kva substation. 166

X. FUTURE STUDIES Tallinn University of Tehnology is onstruting a sample mirogrid, from where it is possible to study energy flow and ommuniation movement during EV harging. Mirogrid onsists of amongst several devies from a harging station for EV harging and battery pak for storing eletri energy. The basi funtions and operation modes (inluding protetion algorithms) suh as energy transmission from power grid to energy storing system, EV battery harging, balaning power loads et. have to be developed, tested and analysed. Management softwares have to be fine-tuned. During experimentations data and measurement values will be olleted to server for further analysis. Primary goals in the onstrution of mirogrid is to analyse energy flow quality and harmoni levels during EV harging through mirogrid, eletromagneti ompability related issues. The analysis will indiate were modifiations have to be made in the mirogrid struture for optimization and to improve overall effiieny and power fator levels in the system to ensure quality of eletriity in aordane with International standards. The pratial appliations will show possible drawbak areas in the ommuniation between devies, whih will then have to be solved with different ontrol algorithms. Future studies will fous more on V2G and Smart Grid solutions. In V2G onnetions energy will be firstly saved from EV to energy storing system and used inside the mirogrid. Further studies will look into possibilities to transfer energy to outside power grid with synhronization related issues. XI. CONCLUSIONS This work has given a brief overview of substation topologies. Before onstruting a real life substation a smaller prototype has to be onstruted. This is planned to be realized in onstruting a mirogrid in Tallinn University of Tehnology. Experiments with the mirogrid will give vital data about harging algorithms and ommuniation movement between devies. From these studies it will be possible in the future to onstrut a larger real life substation, whih would be able to supply power to several harging stations and be part in Smart Grid solutions. ACKNOWLEDGEMENT This researh work has been supported by Estonian Arhimedes Foundation (projet Dotoral Shool of Energy and Geotehnology II ). REFERENCES [1] http://www.valitsus.ee/en/news/press-releases/28702/estonia-willpromote-the-use-of-eletri-ars-under-a-green-investment-sheme [2] http://en.wikipedia.org/wiki/plug-in_eletri_vehile [3] T. Wittmann, The General Senario of Eletri Vehile and its Charging Infrastruture in the ECPE Workshop on Power Eletronis for Eletri Vehiles, 2011, 24 p. [4] http://www.hademo.om/ [5] A. Jossen, V. Späth and H. Döring, Reliable battery operation - a hallenge for the Battery Management System, Journal of Power Soures 84, 1999, pp. 283-286. [6] J.M. Magraner, Ultra-Fast DC Charging Stations in the ECPE Workshop on Power Eletronis for Eletri Vehiles, 2011, 23 p. [7] M.C. Kisaikoglu, B. Ozpinei, L.M. Tolbert, Examination of a PHEV Bidiretional Charger System for V2G Reative Power Compensation, 8 p. [8] J.F. Zhao, J.G. Jiang, X.W. Yang, AC-DC-DC isolated onverter with bidiretional power flow apability in IET Power Eletron., Vol. 3, lss. 4, 2010, pp 472-479. [9] S. Shah and S.Shuflat, Energy Storage Tehnologies in Utility Markets Worldwide, USA: SBI Energy, 2010, pp. 183. [10] https://riigihanked.riik.ee/register/hange/127167 [11] http://www.vamp.fi [12] C. Cleveland and C. Morris, Ditionary of Energy (Expanded Edition), Elsevier, 2006, pp. 323. [13] C. Mamay and R.Firestone, Mirogrids: An emerging paradigm for meeting building eletriity and heat requirements effiiently and with appropriate energy quality, European Counil for an Energy Effiient Eonomy, 2007, pp. 1-13. [14] J. Joller, Jõuelektroonika, Tallinn: TTÜ Elektriajamite ja jõuelektroonika instituut, 1996, pp. 216. 167