Technology and Challenges

DC Fast Charging Stations TECHNOLOGY AND CHALLENGES

SFO TECHNOLOGIES R&D SOLUTIONS Dr. Pratheesh K, S. S. Ghosh, N. Mangal, R. Poulose & R. Narayanan CONTENTS

Introduction...... 3 Types of Electric Vehicles...... 3 The Global EV Outlook...... 4 The Electric Vehicle Charging Infrastructure ...... 4 Standards for EV Charging ...... 5 EV Chargers: Classi cation...... 6 Charger Connector Types...... 7 Charging Network Protocols...... 8 Converters for DC Fast-Charging Stations...... 8 Topologies for PFC Stage...... 9 Topologies for DC-DC Stage...... 10 Technical Challenges...... 12 Technical Speci cations...... 12 The Modular Architecture...... 12 A 50kW DC fast-charging station developed by SFO Technologies ...... 13 Eciency Considerations...... 13 Communication System and Controllers...... 14 Environmental and Thermal Management...... 14 Conclusion...... 16 References...... 16

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2 INTRODUCTION The energy demand has increased exponentially over the past decades, due to industrializa- tion and globalization. It is predicted that world energy consumption will increase beyond 50% by 2030. At present, the world relies heavily on fossil fuels which provide almost 80% of the global energy demands. The increase in retail prices of fossil fuels and the side eects on the environment have been a motivation to look for cheaper, environment-friendly, and ecient alternative options. The major contributor to these energy-related issues is the automotive industry. As such, there is an increased focus on alternative energy sources to drive the vehicles. While this focus has been there for the last few years in other countries, in India, this is on the way to becoming an important focus area in present times. The most common and successful form of alternate source of energy is electricity that has been utilized in electric and hybrid vehicles due to the recent enhancements in battery and its charging technology. However, the fear of becoming stranded with a discharged battery and long charging times caused reluctance in the adoption of electric vehicle (EV) technologies. To overcome these issues, EV manufacturers have come up with larger and improved battery technologies. Parallelly, the charging solution providers have moved to DC fast-charging stations (DCFCS). The DC fast-charging is necessary since the ever-increasing battery in the vehicles needs to be charged over a very short time for public convenience. This whitepaper is primarily focused on the DC fast-charging technology and its challenges. Dierent types of EV chargers, charging requirements and standards, power converters for DCFCS, technology enhancements for eciency improvement, and environmental aspects will be discussed in the rest of this paper.

Types of Electric Vehicles

Hybrid Electric Vehicles (HEV): These vehicles are powered mainly by an internal combustion engine or other propulsion sources that can run on conventional or alternative fuel and an electric motor that uses the stored energy in the battery. The primary purpose is to combine the bene ts of high fuel economy and low emissions with the power and range of EVs. A HEV uses both regenerative braking and an internal combustion engine to recharge the batteries since it cannot be plugged into o-board chargers. HEVs can be further classi ed into mild/micro and full hybrid vehicles. Mild/micro hybrids use a smaller battery and electric motor, which can power the vehicle at rest. This allows the engine to shut o and save fuel. Mild hybrid vehicles cannot drive using electricity alone. Full hybrids vehicles have more powerful electric motors and larger batteries that can drive for short distances at low speeds. Plug-in Hybrid Electric Vehicles (PHEV): These vehicles can charge their batteries from an o-board charger. PHEVs have larger battery packs than HEVs, which enables them to drive moderate distances using electricity only (referred to as all-electric range). The internal combustion engine usually powers the engine when the battery is mostly depleted, during accel- eration or when intensive air-conditioning is required. All Electric Vehicles: These are also called battery electric vehicles (BEVs), which use only the stored energy in the battery to power the vehicle. BEVs have large batteries to support running with electricity only. These batteries are charged by an external electrical power source, the nature of which can determine its charging time. Because BEVs use only electricity, there is no associated pollution.

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3 The Global EV Outlook In 2020, despite the Covid-19 pandemic, the global electric car stock has hit the 10 million mark, a 43% increase over 2019. BEVs contributed to two-thirds of new electric car registra- tions and two-thirds of stock in 2020. One particular reason is the availability of a wide range of BEV models with a higher range in the market as in Fig. 1 (a). The charging infrastructure goes hand-in-hand with the electric vehicle market. Publicly available chargers reached 1.3 million units in 2020, of which 30% are fast chargers. Publicly available fast chargers facilitate longer journeys, while the increase in their deployment will enable longer trips and faster adoption of EVs. Even then, the ratio of public chargers per EV is remarkably less for the majority of the countries as shown in Fig. 1(b).

Figure 1: (a) EV stock available (b) Public chargers available per EV; Source: Global EV outlook 2021 by IEA

As evident from the above discussions, there is certainly a need for more fast-charging infrastructure to enable the growth of EV adoption. Charging levels above 22kW are normally classi ed as fast-chargers. DC fast-chargers use high power DC (>22kW) to charge the EV battery directly up to 80% in 20-40 minutes. DC fast-chargers are usually rated at 50kW which are mainly targeted for city use. Highway DC fast-chargers are rated as high as 150kW. For supercars, trucks, and buses, 350kW DC fast-chargers are used. THE ELECTRIC VEHICLE CHARGING INFRASTRUCTURE

Delivering the electrical energy from an electrical source to the electric vehicle requires EV supply equipment (EVSE) often called EV chargers. They will have cables, connectors, and inter- faces between the utility power and the electric vehicle. This entire system contributes to EV charging infrastructure. Countrywise, speci cations and con gurations of the EVSE could be dierent. The most important part of the charging infrastructure is its con guration, which is well de ned through various international standards. The Society of Automotive Engineers (SAE) and the International Electrotechnical Commission (IEC) are two popular associations that deal with various standards for EV and EV charging.

WWW.SFOTECHNOLOGIES.NET 4 Standards for EV Charging

Although there are several guidelines followed in various countries/ regions, IEC covers all of them in its dierent series of standards. These are de ned for connectors, EVSE, EV, and communication interface between EV and EVSE. Table 1 shows the IEC standards related to EV charging infrastructure. Other notable standards SAE J1772: This covers general physical, electrical, communication protocol, and performance requirements for the EV conductive charge system and coupler. In SAE terminology, dierent charging solutions are de ned in levels, which are reviewed in consecutive sections. DIN SPEC 70121: De nes digital communication between a dc charging station and an EV for control of dc charging in the combined charging system (CCS).

Table 1 Applicable Standards in EV Charging Infrastructure

No. Standards Scope 1 IEC 61851 Conductive charging systems (a) IEC 61851-1 Defines cables and plug setup (b) IEC 61851-21 EV requirements for conductive connection to an AC/DC supply (c) IEC 61851-22 AC electric vehicle charging station requirements (d) IEC 61851-23 Electrical safety, harmonics, grid connection and communication architecture for DCFCS (e) IEC 61851-24 Digital communication for DC charging control 2 IEC 62196 Plugs, socket-outlets, vehicle connectors and inlets (a) IEC 62196-1 General requirements of EV connectors (b) IEC 62196-2 Coupler types for different charging modes in AC charging (c) IEC 62196-3 Connectors and inlets for DCFCS and its configurations 3 IEC 60309 Plugs, socket-outlets and couplers for industrial purposes 4 IEC 60364 Electrical installations for buildings (a) IEC 60364-7-22 Low voltage electrical installations- requirements for special installations or locations- supply of electric vehicles 5 IEC 61439-7 Low voltage switchgear and control gear assemblies 6 IEC 60038 IEC standard voltages 7 IEC 61000 Electromagnetic compatibility (EMC) (a) IEC 61000-4-4 Testing and measurement techniques- electrical fast transient/burst immunity test (b) IEC 61000-4-5 Testing and measurement techniques- surge immunity test (c) IEC 61000-4-6 Testing and measurement techniques- immunity to conducted disturbances (d) IEC 61000-4-11 Testing and measurement techniques- voltage dips, short interruptions and voltage variations immunity tests (e) IEC 61000-6-1 Generic standards- Immunity for residential, commercial, and light industrial environments 8 IEC 61557-8 Electrical safety in low voltage distribution systems up to 1000VAC and 1500VDC- equipment for testing, measuring, or monitoring of protective measures- Insulation monitoring devices for IT systems 9 IEC 60529 Degrees of protection provided by enclosures (IP Code) 10 IEC 15118 Road vehicles: Vehicle to grid communication interface (a) IEC 15118-1 General information and use case definition (b) IEC 15118-2 Network and application protocol requirements (c) IEC 15118-3 Physical and data link layer requirements (d) IEC 15118-4 Network and application protocol conformance test 11 IEC 61850 Communication networks and systems in substations

WWW.SFOTECHNOLOGIES.NET 5 Table 2 Indian Standards for e-mobility

No. IS Number IS Title 1 IS/ISO 15118: Part 1: Road vehicles: Vehicles to grid communication interface: Part 1- 2013/ISO 15118-1 general information and use: Case definition 2 IS/ISO 15118: Part 2: Road vehicles: Vehicles to grid communication interface: Part 2- 2014/ISO 15118-2 network and application protocol requirements 3 IS/ISO 15118: Part 3: Road vehicles: Vehicle to grid communication interface: Part 3: 2015/ISO 15118-4 physical and data link layer requirements 4 IS/ISO 15118: Part 4: Road vehicles: Vehicle to grid communication interface: Part 4- 2019/ISO 15118-3 network and application protocol conformance test 5 IS/ISO 15118: Part 5: Road vehicles: Vehicle to grid communication interface: Part 5- 2018/ISO15118-5 physical and data link layer conformance test :2018 6 IS/ISO 15118: Part 8: Road vehicles: Vehicle to grid communication interface: Part 8- 2018/ISO 15118-8 physical and data link layer requirements for wireless communication 7 IS 17017: Part 1: 2018 Electric Vehicle Conductive Charging System Part 1 General Requirements 8 IS 17017: Part 2: Sec 1: Electric Vehicle Conductive Charging System Part 2 Plugs, Socket- 2020 Outlets, Vehicle Connectors, and Vehicle Inlets Section 1 General requirements 9 IS 17017: Part 2: Sec 2: Electric Vehicle Conductive Charging System Part 2 Plugs, Socket 2020 AC Outlets, Vehicle Connectors and Vehicle Inlets Section 2 dimensional compatibility and interchangeability requirements for ac pin and contact-tube accessories 10 IS 17017: Part 2: Sec 3: Electric Vehicle Conductive Charging System Part 2 Plugs, Socket 2020 AC Outlets, Vehicle Connectors and Vehicle Inlets Section 3 Dimensional compatibility and interchangeability requirements for DC and AC/DC pin and contact-tube vehicle couplers 11 IS 17017 : Part 21 : Sec Electric Vehicle Conductive Charging System Part 21 1 : 2019/IEC 61851-21- Electromagnetic Compatibility ( EMC ) Requirements Section 1 On- 1:2017 board chargers 12 IS 17017 : Part 21 : Sec Electric Vehicle Conductive Charging System Part 21 2 : 2019/IEC 61851-21- Electromagnetic Compatibility ( EMC ) Requirements Section 2 Off- 2:2018 board chargers

Indian Standards for e-mobility The national standards body of India, known as the Bureau of Indian Standards (BIS) has published various Indian Standards (IS) related to e-mobility, which is summarized in Table 2. ARAI Standards Automotive Research Association of India (ARAI) has published standards for EV, HEV, and charger standards in its e-mobility standards. Charger standards as per ARAI are: (a) AIS-138 Part 1: Electric vehicle conductive AC charging system (b) AIS-138 Part 2: Electric vehicle conductive DC charging system The charging station has to comply with the applicable standards to be approved for installa- tion at public charging locations. EV Chargers: Classiﬁcation

The EV chargers can be classi ed mainly based on their charging power levels. The Society of Automotive Engineers (SAE) categorizes charging levels as AC Level 1. AC Level 2, DC Level 1, and DC Level 2, along with the subsequent functionality requirements and safety systems. A summary of classi cation based on SAE de nitions is provided in Table 3.

WWW.SFOTECHNOLOGIES.NET 6 Table 3 EV charger levels classification as per SAE

Level Voltage & Charger Maximum Charging Time in Hours Current Location Power (kW) (30kWh Battery) AC Level 1 120VAC-16A, (1φ) On-board 1.92 15 AC Level 2 240VAC-32A (1φ) On-board 7.7 4 400VAC-64A (3φ) 25.6 1.2 DC Level 1 50-1000V- 80A Off-board 80kW 0.4 (25 minutes) DC Level 2 50-1000V- 400A Off-board 400kW 0.075 (5 minutes)

IEC also de nes four modes of EV charging namely AC Mode 1, AC Mode 2, AC Mode 3, and DC Mode 4 as per IEC 61851-1. This is de ned with respect to the possible connections between EVSE and EV which are Case A, Case B, and Case C. In Case A, the cable is permanently connected to the EV end but removable at the EVSE end. In Case B, both cable ends are removable while in Case C, the cable is permanently connected to the EVSE end. The summary of dierent modes is listed in Table 4.

Table 4 Charging modes as per IEC

Charging Maximum Type of Charge Protections Mode Current Mode 1 16A/ 3.7kW- Slow in AC (1φ or • Requires earth leakage and circuit 7.7kW 3φ) breaker. Standard power connection. Mode 2 32A/ 3.7kW- Slow in AC • Requires earth leakage and circuit 15kW (1φ or 3φ) breaker. • Special cable with intermediate electronic device with pilot control function and protections Mode 3 250A (as per the Slow/semi-quick • Included in the dedicated EVSE. connector used) in AC (3φ) Mode 4 400A (as per the Fast, DC • Included in the dedicated EVSE. charger) • Cable is fixed to EVSE

Dierent types of charger connectors available are shown in Fig. 2. The same is discussed below. Type 1: Single-phase vehicle coupler: Follows SAE J1772/2009 automotive plug speci cations and can be used for charging levels up to 7kW. Type 2: Three-phase vehicle coupler: This can be used for single-phase or three-phase connections with power levels up to 22kW. CCS Type 1: Combined charging system (CCS) connector is used to provide either single-phase AC or DC from the same connector. It is speci ed as con guration EE in IEC 62196-3. CCS Type 2: In this con guration, the top part will be of a Type 2 connector while the bottom part will be dedicated DC charging pins. It is speci ed as con guration FF in IEC 62196-3.

WWW.SFOTECHNOLOGIES.NET 7 Figure 2: EV connector types; Source: EnelX

CHAdeMO: This is a fast charge coupler with DC-only connections. It can deliver up to 400kW with 1000V/400A DC. In IEC 621916-3, it is speci ed as con guration AA. GB/T: This is DC charging standard developed in China. IEC 62196-3 speci es this as con guration BB. Tesla superchargers: It uses the same design as Type 2, but they will be able to charge only tesla vehicles, with DC pins capable of delivering much higher power. Charging Network Protocols

Another important EV charging infrastructure requirement is the application protocol for communication between EVSE, EV, and a central management system (CMS)/ charging station network. Open Charge Point Protocol (OCPP) is an open application protocol that allows EVSE and CMS from dierent vendors to communicate with each other. It oers bene ts to EV owners in terms of competitive pricing, service, and product features. Although OCPP 2.0 is the latest version, only OCPP 1.6 is fully tested and certi ed through the OCPP certi cation program to date. Protocols for the management of EV charging infrastructure are under development and will be published through IEC 63110.

CONVERTERS FOR DC FAST- CHARGING STATIONS

DC Link

DC Link AC-DC DC-DC DC Link AC-oDnCv erter DC-oDnCv erter DC Link CAoCn-vDeCrt er CDoCn-vDeCrt er Input Filter DC Link ACCo-nDvCe rter DCCo-nDvCe rter Three-phase ACCo-nDvCe rter DCCo-nDvCe rter AC Source Converter Converter EV Battery

Figure 3: Basic architecture of a DC fast charging station.

The DCFS can deliver power in the range of 50kW to 200kW, which can charge the EV batteries to 80% in 20-40 minutes. A modular approach is used wherein multiple converter modules are stacked together to obtain such high power output of DCFS. These charging stations receive

WWW.SFOTECHNOLOGIES.NET 8 three-phase AC input from the utility grid and convert it into DC output. The basic architecture of the power converter module in EVSE has one AC-DC converter and one DC-DC converter. Galvanic isolation between the inputs to output is required from a safety point of equipment as well as operating personnel. Each converter module consists of power semiconductor switches, gate drivers, sense circuits, and a microcontroller. Such a power module is generally designed for 10kW. Five numbers of such modules can be stacked in parallel to form a 50kW charging station. The basic structure of a DCFS is shown in Fig .3. The AC-DC converter, also known as the power-factor-correction (PFC) converter is the rst stage of power conversion in the EVSE. It converts incoming AC power from the grid (370-440VAC) into a stable DC link voltage, typically around 800V. The PFC converter is respon- sible for meeting the power factor requirement (>0.99) and the input current THD requirement (<5%). There are multiple topologies available for PFC converter, choice of which depends mainly on the energy density, cost, eciency, and reliability. The DC-DC converter in the second stage converts the 800V DC link voltage to a lower voltage to charge the EV battery. The output voltage range of this DC-DC converter can vary from 200V -500V or 500V to 800V to charge the batteries of BEV/PHEV. The converter will be able to operate in either constant current (CC) mode or constant voltage (CV) mode, depending on the battery state of charge (SOC). The galvanic isolation between the input and output can also be provided at this stage through a high-frequency transformer. Topologies for PFC Stage The active PFC recti er systems widely used for DCFS are two-level six switch converter, Vienna recti er, three-level NPC converter, and three-level T type converter. +Vdc Three-phase Two-level PFC Three-phase two-level PFC, as shown in Fig. 4, is a boost type recti er, which consists of six controlled switches. It can achieve a high power factor (close to unity) with good A B C eciency. The converter is suitable for bidirectional power ow. The devices in a two-level PFC are rated for the full DC bus voltage. Additionally, a large lter-choke is required in -Vdc this converter compared to the other multilevel topologies. Figure 4: Three-phase The EMI performance compared of the two-level converters Two-level PFC converter. are poor compared to multilevel-level topologies. Three-phase Vienna Recti er The Vienna recti er, as depicted in Fig. 5, is a three-phase three-level recti er with six controlled switches and six +Vdc diodes. Two controlled switches are connected in series for each phase. These switches are connected to the mid-point of the DC link and hence need to block only half of the DC link A N B voltage whereas, full DC bus voltage appears across the C diodes during blocking. The EMI performance is better compared to a two-level converter. The lter-choke requirement is lesser in Vienna recti er due to the multilevel switching. The -Vdc major downside of the Vienna recti er is its inability to oper- Figure 5:. Three-phase ate as a bidirectional converter due to the presence of diodes. Vienna Recti er. Additionally, DC link neutral point voltage control is required for balancing the DC link voltage of Vienna recti er.

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9 Three-phase Three-level NPC PFC Fig. 6 shows the three-phase three-level NPC (neutral point clamped) topology, which has 12 switches, with four in each

phase connected in series. Additionally, the DC link neu- +Vdc tral-point (or mid-point) is connected to two series diodes (also termed as clamping diodes) in each phase. All the device in this topology needs to block only half of the DC link volt- N age. The NPC topology has a lower current ripple which can A B C further reduce the lter-choke size. This topology oers bidirectional power transfer and is suitable for higher switching

frequencies. Eciency and power density of the NPC can be -Vdc greatly improved by using wide-bandgap (SiC/GaN) devices. Figure 6. The main drawback of the NPC is the higher number of power Three-phase Three-level NPC converter. semiconductor devices which results in a higher cost of the system. Three-phase Three-level NPC PFC

The three-level T-type converter (also called TNPC), as depicted in Fig. 7, is a modi ed version of the Vienna recti er. In TNPC active switches replace the diodes of the Vienna recti - +Vdc er. Thus, TNPC is suitable for bi-directional power ow opera- tions. During switching state transitions, there are no chances of unequal blocking voltage distribution in TNPC. Hence, N DC-link neutral point voltage control is not required in this A B C case. Although TNPC has overall lower conduction losses, the switching losses of the DC link blocking devices are high. These devices are also prone to peak voltage stress during -Vdc their operation. Although, the conduction losses in the Figure 7: DC-link mid-point connected devices are high, it usually Three-phase Three-level T Type remains under acceptable limits. TNPC oers better eciency, PFC converter. good power density, and the capability of bidirectional power ow when compared to the other discussed topologies. Topologies for DC-DC Stage The most suitable and popular topologies used in DCFS are interleaved buck converter, phase shift full bridge converter (PSFB), LLC resonant converter, dual active bridge (DAB) converter, and CLLLC Converter. Interleaved Buck Converter Interleaved buck converter or multiphase buck converter is shown in Fig. 8. It is used in high-power DCDC step down + + conversion systems where high-frequency galvanic isolation

is not required. In DCFS, where interleaved buck converter is Vdc Vo used in the DC-DC stage, a line-frequency isolation transform- - er is used at the grid-side for isolation between input and - output. Although, the line-frequency transformer is not Figure 8: considered as a part of the EVSE, Interleaved buck converter.

WWW.SFOTECHNOLOGIES.NET 10 this makes the overall system very bulky. Hence, this system is usually adopted for very high power chargers which are rated for several hundred kilo-watts. Phase Shift Full Bridge Converter The phase-shifted full-bridge (PSFB) converter, as depicted in + + Fig.9, has one H-Bridge with active switches at the primary

side connected to the DC link, followed by a high-frequency Vdc Vo transformer. The secondary waveform of the transformer is - - recti ed using a diode bridge. It is a buck-derived topology where the power transfer between the primary and second- Figure 9: Phase Shift Full Bridge Converter. ary is controlled by the phase-shift (or overlap) between the two legs of the primary bridge. The longer the phase-shift, the higher the energy transfer. It is possible to employ soft-switching techniques in PSFB converter. Thus, zero-voltage switching (ZVS) or zero-current switching (ZCS) can be achieved in the PSFB converter. Hence, it is possible to achieve very high eciency and power density using the PSFB converter. A very high power rating can be achieved by paralleling the PSFB converters at the output. The secondary diodes can be replaced with active switches to achieve bidirectional power ow capability - it also improves the overall eciency. LLC Resonant Converter The LLC resonant converter topology, as shown in Fig. 10, consists of the same elements as that of PSFB discussed earlier. The voltage gain of the LLC converter

depends on the switching-bridge gain, resonant tank + + gain, and transformer turns-ratio. The resonant tank is Vo comprised of a resonant inductor, a resonant capacitor, Vdc and the magnetizing inductance of the transformer. The - - amount of energy to be transferred mainly depends Figure 10: upon the resonant circuit’s impedance at a given LLC Resonant Converter. frequency for a given load impedance. Hence, the switching frequency is used as the controlling parameter in resonant converters. The output voltage regulation is achieved through the switching frequency variation of the converter. It is possible to achieve ZVS turn-on and ZCS turn-o of the devices with a proper selection of resonant components and switching frequency of the converter which, yields the highest eciency for this type of converters. It is dicult to parallel multiple LLC converters which, often requiring some additional control complexities. Hence, it is dicult to achieve a very high power level using LLC resonant converters only. Dual Active Bridge Converter + + The dual active bridge converter (Fig. 11), as the name

suggests, has two active full bridges located on either Vdc Vo side of a high-frequency isolation transformer. The - - power transfer principle in a dual-active bridge is similar to the power ow principle between two voltage sourc- Figure 11: es at either end of an inductive transmission line in a Dual Active Bridge Converter. power system. The power transfer between the primary and secondary bridges can be controlled by adjusting the phase-shift between the two bridges. Power transfer takes place from the leading bridge to the lagging bridge.

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11 The primary and secondary bridges are controlled simultaneously where all the switches operate at 50% duty cycle. It is possible to achieve ZVS/ZCS in all the devices throughout the operational range in DAB based converters with proper choice of modulation techniques. The DAB-based converter tends to operate with very high r.m.s. current, which results in higher conduction loss. However, this can be optimized using a suitable selection of the leakage inductance and modulation schemes to achieve very high eciency and energy density for the DAB-based converters. CLLLC Resonant Converter

The CLLLC incorporates all the functionalities + + of the LLC converter with the added advan- Vo tage of using active switches in the secondary Vdc to get bidirectional power transfer capabili- - - ties. The CLLLC converter, with the symmetric Figure 12: resonant tank structure as shown in Fig. 12, CLLLC Resonant Converter. oers better control on the gain and switching frequency in bidirectional operation. It oers both ZVS and ZCS with the highest eciency compared to other topologies. However, the resonant converters, in general, are not suitable for wide range of output voltage regulation with xed input voltage. Additionally, it is dicult to parallel the CLLLC converter for higher power levels since it requires highly symmetrical resonant tank components. The synchronization between multiple modules is also cumbersome due to the above reason. TECHNICAL CHALLENGES The demand for DCFS will only grow in the future as both the Government and the market is push- ing for more EVs in more sectors and cities. Although the DC fast-charging technology is still evolv- ing, it is the best solution that exists now to meet the EV charging infrastructure. However, a DC fast charging station is not a simple product that can be readily developed using a suitable converter for the desired power level. These charging stations use advanced technology and communication to work reliably over the long term. Technical Speciﬁcations Before going into the details of the technical challenges, it is important to know the basic speci cations of a DCFS. Most of the charging solution manufacturers publish their product brochures with technical speci cations on their websites. The Modular Architecture Manufacturers opt for a modular architecture where a 50kW system will be typically made of ve 10kW units stacked in parallel. It is more important in terms of redundancy and uptime. A single module failure in such a system will be still good enough to support 40kW charging resulting in reduced or no downtime for the charging station owners or users. However, simply stacking the modules in parallel does not guarantee a reliable operation. The charging station needs to use the available module wisely to its advantage whenever an EV is availing its services. The EV demands a charging current to charge its battery as per its SoC. This demand is governed by the battery management system (BMS) available in the EV. Now the charging station has to determine how many of its available modules are required to provide the requested charging current. Once that is nalized, the charging station can divide the charging current requirement among the modules. This process ensures uniform stress on every module. The charger can also decide to rotate the active modules in between the charging sessions or even inside the charging session itself since not many EVs would need the full charging power of 50kW or 100kW.

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12 A 50kW DC fast-charging station developed by SFO Technologies A 50kW DC fast-charging station based on modular architecture has been developed by SFO Tech- nologies is shown in Fig. 13 below. The charging station uses modular architecture where an intelli- gent controller dynamically optimizes module utilization based on the charging power requirements. The speci cation of the same charging system is given in Table 5.

Table 5 Speci cations of the 50kW DC fast-charging station developed by SFO Technologies.

Technical Specification Charging type DC fast-charging Outlet options CCS Type 2 DC Output Power 50kW DC Output Voltage Range 200 – 500 V DC Maximum charging current 125A Max AC voltage 415V RMS (L-L) ± 10 % Frequency 50/60Hz Input Current THD <5% Power Factor >0.99 Efficiency >95 % Charging protocol OCPP 1.6, upgradable to OCPP 2.0 General Specification Application Outdoor Humidity <95%, non - condensing Ingress Protection IP54 System Weight Approx. 150kg Operating Temperature -20degC to +55degC Cooling Built in Fans Compliance ARAI (as per AIS 138 Part 2) User Interface Figure 13: A 50kW DC Fast-Charger developed Display 7” TFT LCD Touch display by SFO Technologies. Language English Emergency Stop Mushroom Headed Push button Local Authentication RFID Visual Indication Mains available, Charging status, System Error

Eﬃciency Considerations Eciency is one of the most important aspects of a high-power charging station. Even a 0.1% loss means100Watts of power is lost for a 100kW DC fast-charging station. This has to be taken care of both during the design phase and in the operational phase. The rst part is to select the suitable converter topology for the individual power module. The topology selection and design process shall consider converter power rating, load-range, voltage-gain range, regulation, soft-switching, passive-components, eciency, device-stress, ease of control, compactness, weight, costs, etc.

WWW.SFOTECHNOLOGIES.NET 13 Next comes the device selection and switching frequency. The higher the switching frequency, the higher the power density is possible to achieve. However, this comes at a cost of increased switching losses. Wide bandgap devices, such as Silicon Carbide (SiC) or Gallium Nitride (GaN), can be used to tackle these issues. These devices are suitable for operation at very high switching frequencies with extremely low losses. These devices require special considerations in their gate drive circuits concerning the drive voltage levels for turn on and turn o. During the operation, the converter modules in the charging station mostly operate in the same operating point as discussed before. While it is easy to divide the current request for EV among the modules proportionally, this also has to consider the optimal operating point of the modules. Once the module is designed, its optimum operating range is determined for a given output voltage and current to achieve maximum eciency. This information is utilized to use the optimum number of modules in every charging session maximizing the overall charging eciency. Communication System and Controllers

So far the discussion was focused on the hardware part of the charging station. The missing piece is the communication system. There needs to be a master controller for the entire charging station. This master controller has many functions to support namely communication with EV, communication with the power modules, fault management, communication with display module/HMI, payment system, and overall system management. The most obvious requirement is that the master controller needs to understand the charging protocols (CCS/CHAdeMO) intended to be supported by the charging station. The controller will act as a slave to the commands from the EV during the charging session, unless in the case of emergen- cies. The EV request the current to charge its battery and then the EVSE controller decides which of the power modules needs to be operated, considering the eciency criteria as discussed above. It also tells the power modules the required voltage and current to satisfy the charging session demand. Another function of the EVSE controller is fault management and logging, which includes safety interlocks and emergency stop functions. Additionally, the EVSE controller monitors individual power modules containing the power converters. It also includes system-level monitoring of fans, circuit breakers, relays, etc. This information will be accessible to the EVSE owner either through the display/HMI module or in the SD card installed in the controller. The HMI is also used as an input where the user can start a charging session for a time or money-based charging. The user can also monitor the charging progress through the HMI. An RFID reader is required for payment service, which is integrated with the controller. Supports for the OCPP network, credit card transactions, NFC smartphone payment, etc. have to be taken care of in the EVSE controller. Rigorous testing and validation are essential to incorporate all these control aspects in the EVSE controller.

Environmental and Thermal Management

The public charging station is exposed to extreme weather and climate conditions, seismic activity, theft, and even vandalism. A rugged enclosure is required to protect all the sensitive components inside the charging station under harsh environmental conditions. Typical charging station enclosures are rated with IP54/55. This enclosed form warrants proper thermal management inside the station as well as the power modules. Starting with the power modules, all the major devices and magnetic components are the primary sources of loss in the form of heat. At full load, the case temperature of the devices may go as high as 100°C. Thermal is managed by cooling fans at both system and module levels.

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14 The fan cooling system can be a nuisance to the public since the charging station is installed in places where people are shopping, working, or living. Excessive noise also contributes to vibration, wear, and tear which, causes premature failure eventually. Hence these fans should be controlled according to the temperature inside the station/module. PWM-based fan control is the best option in this case. It also reduces unnecessary power consumption. Hence, each power module will have its fan control, and the EVSE controller will control the fans at the system level.

CONCLUSION

The DC fast-charging stations have an important role in the sustainable future of the electric vehicle infrastructure. This is underlined by the concerted eort from the authorities to evolve and update the standards required for the infrastructure. EVSE manufacturers have to comply with all the applicable standards to use the product in the public domain. The targeted power levels, eciency, and power density gures are ever-increasing with the advancement of technology. The choice of converter topology needs to satisfy the bidirectional operation as the interest in the vehicle to grid (V2G), and vehicle to home (V2H) are gaining popularity. With the developments in the eld of semi- conductors, digital control, and power electronics in general, the challenges in the implementation of DC fast-charging stations will always have a solution. REFERENCES

1. Salman Habib et. Al. “Contemporary Trends in Power Electronics Converters for Charging Solutions of Electric Vehicles”, CSEE Journal of Power and Energy Systems, VOL. 6, NO. 4, December 2020

2. Global EV Outlook 2021, IEA. https://www.iea.org/reports/global-ev-outlook-2021

3. The Electric Vehicle (EV) Landscape - A Deep Dive, Team BHP; https://www.team-bhp.com/forum/electric-cars/171370-electric-vehicle-ev-landscape-deep-dive.html

4. “Power Topology Considerations for Electric Vehicle Charging Stations”, Texas Instruments, September 2020. https://www.ti.com/lit/pdf/SLLA497

5. TIDM-1000 Vienna Recti er-Based Three Phase Power Factor Correction Reference Design Using C2000 MCU | TI.com

6. TIDA-01606 10-kW, bidirectional three-phase three-level (T-type) inverter and PFC reference design | TI.com

7. TIDM-PSFB-DCDC Phase-Shifted Full Bridge DC/DC Power Converter Reference Design | TI.com

8. TIDA-010054 Bi-directional, dual active bridge reference design for level 3 electric vehicle charging stations | TI.com

9. An evaluator’s guide to DC fast charging stations, Whitepaper, ABB Inc. Electric Vehicle Charging Infrastruc- tures.

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15 About SFO Technologies Pvt Ltd The agship arm of the diversi ed conglomerate, the NeST Group provides end-toend design-engineering-software-manufacturing solutions to clients across geographies such as the USA, Canada, Europe, Middle East, South East Asia, Japan, Australia, and India. SFO has invested in building competence, scale and standards compliant process framework, in PCBA, bre optics, Cable & wire Harness, Magnetics, High Level Assembly, VLSI design, embedded software development, etc. SFO’s capabilities transcend the plain vanilla “Build-to-Spec or Build-to-Print” EMS and our ODM+ solutions are rapidly rede ning standards for the OEMs across Aerospace & Defence, Communications, Transportation, Healthcare and Energy & Industrial domains.

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16 Ph: 91- 484 - 661 - 4300

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