By Srdjan Srdic and Srdjan Lukic©istockphoto.com/https://www.facebook.com/PlargueDoctor/

Toward Extreme
Fast Charging
Challenges and opportunities in directly connecting to a medium-voltage line.
ith an increasing awareness of the detrimental
effects of the fossil fuel–based transportation sector,
which accounts for 14% of human-made greenhousegas emissions globally, individuals, companies, andgovernment entities have made a concerted push todevelop solutions to provide modes of transportation that are less carbon intensive. Importantly, several countries, including Norway, India,

W

Digital Object Identifier 10.1109/MELE.2018.2889547
Date of publication: 4 March 201922

I EEE E l e c t r i f i c a t i on M a gaz ine / march 20192325-5987/19©2019IEEE

France, and Britain, have decided to end the sale ofinternal combustion engine cars in the near future,
further accelerating the shift to electric transportation.
Many progressive strides have been taken in recent
years to make electric vehicles (EVs) cost competitive
while delivering a driving range of more than 200 mi(see Figure 1). At the same time, recent advances inlithium-ion battery technology promise to deliver vehicles with an even longer range while reducing battery
costs and weight. These new batteries also exhibit
ever-improving charge acceptance, allowing significantly faster charging rates.
To serve this growing fleet of new vehicles, an adequate charging infrastructure is needed. The simplest
recharge method uses a vehicle’s onboard charger,
which connects to the residential single-phase ac supply. The so-called Level 1 and Level 2 ac chargers(defined in SAE J1772) serve as interfaces between the
120-V (Level 1) and 240-V (Level 2) supply lines and theonboard charger and can deliver up to 1.92 kW (Level 1)
or 19.2 kW (Level 2) to the vehicle. The International
Electrotechnical Commission (IEC) classification (IEC61851) allows up to 26.6 kW for Mode 2 home ac chargers. Due to their limited power rating, the onboard
chargers are not capable of replenishing the battery
charge quickly enough to provide a refueling experience
comparable to that of gasoline cars. As seen in Figure 1,
it takes more than 8 h to add 200 mi of range to the battery of an EV when using the standard 208-V or 240-V
plug available within U.S. homes. While this is acceptable for overnight charging, it is not suitable for longer
trips, and it can create range anxiety (the fear that avehicle has insufficient range to reach its destination).
The limited capability of onboard chargers has led tothe development of dc fast chargers, typically rated at
50 kW and, more recently, at power levels up to 350 kW.
These chargers deliver dc power to the vehicle battery via
an isolated power converter located outside the vehicle,
and they have the potential to provide the user with an

Chevrolet Bolt
EV (2018)

258

Tesla Model 3
Long Range

3350State-of-the-art dc fast chargers are installed either as single-stall units or multistall charging stations. Single-stall
units are typically rated at 50 kW and powered by a dedicated service transformer. These chargers are capable ofadding up to 100 mi (161 km) of range in approximately
30 min. Higher-power multistall charging stations, such as
Tesla’s supercharger stations, include multiple chargers
and need additional switchgear and low-voltage (LV)metering circuits, as illustrated in Figure 2. In the case of
Tesla’s stations, each supercharger unit serves two charging stalls and can add up to 170 mi (273 km) of range
in 30 min. When these systems are deployed, all of thesystem components are commonly installed on relatively

500
72

dc Fast (50 kW)
310

Tesla
Model S 100DState-of-the-Art EV dc Fast Chargers

ac HomeCharging (7.2 kW)

238

Hyundai
Kona (2019)

experience similar to that of refueling a gasoline car. A fast
charger rated at 50 kW requires more than 1 h to deliver
enough charge for a 200-mi trip, while a 350-kW extreme
fast charger would only take roughly 10 min to deliver thesame amount of energy, as illustrated in Figure 1. Clearly,
designing and building a system that can deliver thismuch power is quite challenging.
In this article, we present the current state-of-the-art
fast-charging technologies. We discuss the state-of-theart-fast chargers, their design, and their main characteristics. Additionally, we detail how these chargers can be
combined to form fast-charging stations that mimic thefunctionality of a gasoline refueling station. Various
power delivery concepts for these charging stations aredescribed, and we discuss the benefits and shortcomings of the current and proposed approaches. Finally, we
present a novel approach in which the charging station
directly connects to the medium-voltage (MV) line, thus
eliminating a number of power electronics conversion
stages and the low-frequency step-down transformer.
We outline significant benefits of this technology when
considering extreme fast-charging rates, and we identify
the key technology gaps in making this technology more
widely accepted.

Tesla Supercharger(135 kW)27

dc Ultrafast(350 kW)10

50 100 150 200 250 300 350 400
(mi)
(a)

0

100

200

300 400
(min)
(b)

500

600

Figure 1. EVs with more than 200 mi of range and time to add 200 mi of range to the vehicle if it is charged at a constant power equal to acharger’s rated power. The (a) range of EVs in miles and (b) the minutes to add 200 mi.

IEEE Elec trific ation Magazine / ma r c h 201 923large concrete foundations, which adds to the installation
costs. According to a study by the Idaho National Lab, themain cost drivers for charging station infrastructure
installations include electrical service upgrades, the condition of the ground surface under which the electrical conduits were installed, the length of the conduits from thepower source to the service transformer and from thetransformer to the fast charger, material costs, permits,
and administration.
Typical state-of-the-art dc fast chargers have two
power conversion stages: a three-phase ac/dc rectification stage that includes power factor correction (PFC)and a dc/dc stage that provides galvanic isolation. The
simplified block diagram of a state-of-the-art dc fastcharger power stage is illustrated in Figure 3. The ac/dc
rectification and PFC stage converts three-phase input
ac voltage to an intermediate dc voltage and ensures
acceptable power quality on the grid side. The dc/dc
stage converts the intermediate dc voltage into the regulated dc voltage required by the vehicle and provides
galvanic isolation. The galvanic isolation separates thevehicle from the grid and also allows for the charger
output stages to be easily connected in parallel.
A conventional dc fast charger typically uses anactive pulsewidth-modulated (PWM) rectifier to implement the conversion and PFC functions. This single-stage

converter is preferred at higher power levels because of
its high reliability and simplicity. Strict harmonic standards are met by introducing an input filter. The active
PWM rectifier is often based on converter modules usedin motor drives that are typically based on reverse-conducting insulated-gate bipolar transistors and operate at
switching frequencies not higher than 10 kHz. The morecompact and efficient converter can be achieved withsilicon carbide (SiC) power MOSFETs, which have substantially lower total power losses and operate at
switching frequencies of several tens of kilohertz. The
second stage is an isolated dc/dc converter, which provides dc regulation and galvanic isolation between thegrid and the EV. Since the vehicle battery is not grounded, the isolated dc/dc converter stage maintains the isolation of the vehicle battery and the secondary side ofthe dc/dc converter. By maintaining system isolation,
the charging system leaves the protection scheme
designed for the vehicle battery intact.
Table 1 lists the technical specifications of some of thecommercially available fast chargers. The vast majority of
the commercially available units are rated at 50 kW andpowered from a three-phase LV distribution grid. The
chargers are typically built out of identical building blocks
whose power output is combined to meet the required
power needs. For example, Tesla’s 135-kW supercharger

Distribution Main Breaker Incoming
Panelboard and Metering
Tesla Superchargers 1–4
MV/LV Transformer

24 ft 2 in (7.36 m) × 10 ft 3 in (3.12 m)

5 ft (1.52 m) × 5 ft 8 in (1.72 m)

Figure 2. The layout of Tesla’s supercharger station in Madison, Wisconsin (charging stalls and the seclusion composite fence surrounding thesuperchargers and switchgear are not shown). The transformer is located approximately 24 ft (7.3 m) from the superchargers.

A B C
Isolated dc/dc Stage
Input
FilterRectifier(ac/dc)PFC
Circuit(dc/dc)(ac/dc)(dc/ac)
HF
Transformer

Figure 3. A simplified block diagram of a state-of-the-art dc fast charger power stage. HF: high frequency.

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OutputFilterwas made by combining and packaging 12 converters intoa single system.
Commercial dc fast chargers support one or more of
the five existing dc fast-charging systems: CHAdeMO(used globally), CCS Type 1 (in the United States), CCS
Type 2 (in the European Union), GB/T (in China), and theTesla supercharger (used globally). In all cases, the charging process follows a sequence of events defined by the
charging protocol. The charging sequence typically starts
with signal handshaking, insulation testing, and exchanging maximum charging parameters between the vehicle
and the charger. If all of the required criteria are met, thevehicle closes its dc contactor, and charging begins. During
the charging process, the vehicle and the charger
exchange information on the desired current and voltage
reference; in addition, the vehicle battery management
system updates the charger on the battery state of charge(SOC) and other system parameters that may be displayedon the human–machine interface. When the battery
reaches a certain preset SOC, the vehicle signals the charger to terminate charging by reducing the charging current to zero. The vehicle then disconnects itself from thecharger by opening its dc contactor.
Typically, the charging profile follows the wellknown constant current, constant voltage (CC/CV) profile, which is commonly used in charging mostlithium-ion battery variants. As the battery charges in
the CC region, the battery voltage increases with the
SOC. As a result, the power acceptance by the battery
pack slowly increases in the CC region. Once the charge
profile transitions to the CV phase, the power delivered
to the battery reduces since the charge current reduces
to maintain the CV at the battery terminals. As a result,the battery’s rate of charge rapidly reduces during theTable 1. Technical specifications of commercially available dc fast chargers.
EVTEC
espresso&charge

ABB Terra HP

Rated power

50 kW

50 kW

135 kW

150 kW

350 kW

Supported ­standards

CCS Type 1
CHAdeMO 1.0

CCS Types 1 and 2
CHAdeMO 1.0

Supercharger

SAE Combo 1
CHAdeMO 1.0

SAE Combo 1
CHAdeMO 1.2

Input voltage

480 Vac

380–480 Vac
600–900 Vdc

200–480 Vac

400 Vac ± 10%

400 Vac ± 10%

Output dc voltage

200–500 V
50–500 V

200–500 V
50–500 V

50–410 V

170–500 V

150–920 V

Output dc current

120 A

125 A

330 A

300 A

375A

Peak efficiency(charger only)

94%

>92%

92%

93%

95%

Volume

758 L

495 L

1,047 L

1,581 L

1,894 L

Weight

880 lb(400 kg)

364 lb(165 kg)

1,320 lb
(600 kg)

880 lb(400 kg)

2,954 lb
(1,340 kg)

350

410

300

400

250

390

200

380

150

370

100

Battery Current
Battery Voltage

50
0

0

360
350

340
5 10 15 20 25 30 35 40 45 50 55 60 65
Time (min)
(a)

120
110
100
90
80
70
60
50
40
30
20

0

100
90
80
70
60
Charging Power
50
Battery SOC
40
30
20
10
0
5 10 15 20 25 30 35 40 45 50 55 60 65
Time (min)(b)

Battery SOC (%)

Tesla ­
Supercharger

Charging Power (kW)

Tritium Veefil-RT

Battery Voltage (V)ABB Terra 53

Battery Current (A)Manufacturer
and Model

Figure 4. The Tesla Model S85 (a) charging profile and (b) battery SOC and charging power.

IEEE Elec trific ation Magazine / ma r c h 201 925

CV charging period, as illustrated in Figure 4. In Figure 4, the battery was charged from 12 to 94% SOC,
with a peak power of 117.2 kW occurring at 16% SOC(2 min after the charging was started). The CC mode
lasted for approximately 2 min.

system connects to the MV utility supply via a threephase step-down transformer that delivers power at LV(up to 480 V line to line) to all of its subsystems. The subsystems are connected to the transformer via switchgear
cabinets that contain breakers and disconnects. The system may include local storage and generation capabilities to help mitigate demand charges that are incurred
during peak power consumption requirements at thestation. An example of an ac-coupled system is a supercharger station in Mountain View, California, which
includes six superchargers and 200 kW (400 kWh) of battery-energy storage, as illustrated by the simplified oneline diagram in Figure 6.
The ac-connected system is typically used in today’s
multiport charging stations. The advantages of the ac-coupled approach include availability and maturity of the converter technology, switchgear, and protection devices andFast-Charging Station Designs:
dc and ac Distribution Concepts
Since EV fast chargers are meant to operate similarly togas stations, the design of a system with multiple charging nodes is necessary. Such a system would attempt tomimic the design of a conventional gasoline refuelingstation, with the same infrastructure supplying multiple
vehicles from the same reservoir by using multiple
pumps. Figure 5 shows conceptual designs of EV fastcharger stations with ac and dc coupling. In the case ofan ac-coupled system, shown in Figure 5(a), the entire

EnergyStorage

PV
MV
Grid MV/LV

dc/dc

dc/dc

dc/ac

dc/ac

EnergyStorage

PV
MV
Grid MV/LV

dc/dc

dc/dc

ac/dc
ac/dc

ac/dc

dc/dc

dc/dc

dc/dc

dc/dc

(a)

(b)

Figure 5. Charging station concepts: (a) an ac-coupled station and (b) a dc-coupled station. PV: photovoltaic.

1A

200 A
5

3A

3B

4A

4B

5A

Supercharger 4

200 A
5

2B

Supercharger 3

200 A
5

2A

Supercharger 4

200 A
5

1B

Supercharger 3

200 A
5Supercharger 2

200 A
Supercharger 1

Main Breaker
and Metering

5

1,300 A

Distribution Panelboard

CT

1 MVA

Incoming
MV Grid

5B

6A

400-A
Breaker
400-A
Switch4100-kW 100-kW
Inverter Inverter
400-kWh Battery
200-kW
Stationary Storage
6B

Figure 6. A simplified one-line circuit diagram of Tesla’s supercharger station in Mountain View, California. CT: current transformer.

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well-established standards and practices for the ac distribution systems. Having more conversion stages (to interface
dc loads, dc generation, and battery-energy storage to theac system) is the main disadvantage of the ac-coupled system. These conversion stages decrease the system efficiency and increase its complexity. Additionally, ac-connected
systems are more complicated to control than dc-connected systems since they need to deal with reactive power
control, inverter synchronization, and voltage and frequency control during islanded operation of the system.
In a dc distribution concept, shown in Figure 5(b),the charging station connects to the distribution system
via a service transformer and an LV rectifier. The threephase transformer delivers power at LV to a single rectifier
stage, which then distributes the dc power to individual
station subsystems. The main advantages of distributing
dc power are as follows: it eliminates ac/dc and dc/ac conversion stages, it minimizes the number of conversion
stages when delivering power from local storage to a charger, and it simplifies the integration of renewable resources
and battery-energy storage, which outputs dc power.
Due to the variations in power demand for battery
charging as a function of battery size and SOC, there will
be a large variation in the power demand when multiple
EVs charge simultaneously. Including a local storage
capability within the charging station provides significant system-level benefits, as it allows the station ownerto profile the power demand from the station while still
delivering the desired power to individual customers. By
profiling the power demand of the station, the station
owner can be responsive to the needs of the local utility
and avoid high-demand charges, which can account for
more than 90% of the electricity costs for the charging
station. In both presented concepts, the system could
benefit from load diversification by sharing a large common power supply. A number of studies have shownthat when exploiting the load diversification, which
results from varying EV battery capacities and changing
charge acceptance of the battery as a function of the
SOC, the actual system power demand is substantiallylower than the rated value.
In our recent work, we showed that for a bank of 50
Level 2 chargers, the power required is only 60% of the sumof the chargers’ rated power when accounting for diversity.
Similarly, in the case of a dc-coupled system with 10 fast
chargers rated at 240 kW each, we found that the power
requirement is closer to the average value than the peak
power rating and the grid tie can be sized for the average
power demand rather than for the peak demand. If a relatively small storage system is connected to the station,
more than 98% of the power demand can be satisfied withan average charging delay time of less than 10 s.

Grid Impacts of Extreme Fast Chargers
Today, fast chargers are designed to absorb power fromthe grid at a unity power factor. That is a realistic

approach, given that the power demand from fast chargers is a relatively small fraction of the power consumption even in areas with very high EV adoption.
However, a station with multiple extreme fast chargers(XFCs) would require an order of magnitude more
power compared to systems deployed today, with thetotal power rating of the system in the multimegavolt
amperes (multi-MVA) range (individual XFCs are rated
up to 400 kW). A multi-MVA charging station may se­­verely influence the power quality on the feeder that
supplies it. Recent studies show that the additionalload at a single point can lead to feeder overload andvoltage variations along the feeder that are beyond theallowable limits.
One way to mitigate the negative effects of XFCs on
the distribution grid is to allow the utility to control thestation’s demand during hours of peak consumption.
Additionally, if the front end of the charging station iscapable of power factor control and bidirectional power
flow, the station will be able to provide ancillary services
to the grid to help mitigate power quality issues on its
feeder. The capability to inject reactive power from powerelectronics-interfaced loads is becoming more important,as evidenced by the updated IEEE 1547-2018 standard.
Allowing the multi-MVA charging station to inject reactive power would substantially simplify the utility’s voltage control problem, and a charger station’s abilities tocontrol the power factor and provide for bidirectional
power flow are becoming increasingly important from theutility’s perspective.

MV Solid-State Transformers: An Enabling
Technology for Extreme Fast-Charging Stations
Modern EV fast chargers are designed to connect to athree-phase power supply with line-to-line voltage of up
to 480 V, which is usually not readily available in public
installations. Therefore, a dedicated service transformer isused to reduce the distribution system MV and provide
three-phase supply to a single dc fast charger or to a dc
fast-charging station. The service transformer adds cost to
the system and generally complicates installation. Furthermore, distributing high power to a charger or a charging station at LV implies the need for large conductors andbulky LV distribution and switchgear equipment, as illustrated in Figure 2.
An alternative approach is to use a solid-state transformer (SST) to connect directly to an MV line, thus eli­minating the need for a grid-frequency step-downtransformer, as illustrated in Figure 7. The SST would
replace both the step-down transformer and the charger
unit in a conventional single-stall transformer-and-charger
system, or it would replace the step-down transformer andthe ac/dc stage in a dc-coupled station. In a SST, galvanic
isolation is achieved by using a high-frequency transformer (HF TR). The SST-based converter provides either unidirectional or bidirectional power flow, regulates active andIEEE Elec trific ation Magazine / ma r c h 201 927

reactive power, performs PFC function, and provides isolation and protection features. The system uses a common
dc bus to interface renewable energy generation systems
and battery-energy storage to EVs via dc/dc converters
only, ensuring a reduced number of conversion stages andhigh conversion efficiency. Several studies evaluating thebenefits of SST-based EV fast chargers have been reported
in the literature, and several prototypes demonstratingthese benefits are currently in development.

and the fast charger, further increasing installation andpermitting costs. The result can be an expensive systemthat requires significant infrastructure. By eliminating
the service transformer and connecting directly to an MV
line, the system installation costs can be reduced by atleast 40% compared to the conventional solution, as
reported in a study done for 50-kW systems.
Reducing the system size can be particularly beneficial
if the fast charger is installed in densely populated areas
because it enables better site utilization—that is, more
Key Benefits of the SST-Based EV Fast Charger
power in the same system footprint. The higher available
The SST-based approach to EV fast charging is beneficial
power from the station leads to faster charging and lower
to both EV users and charging station owners, as illuswaiting time since more higher-power charging nodes
trated in Figure 8. The key benefits of the SST-based EV
can be served simultaneously. The higher energy efficiencharger that uses wide-bandgap power semiconductor
cy of the system can lead to potentially cheaper charging
devices are reduced system size and higher power condue to the energy savings. For example, increasing theversion efficiency. The reduced system size means lower
system efficiency by 5% (from 92.5% to more than 97.5%)
installation costs and more available power in the sameat a 1-MW power level would mean the reduction ofsystem footprint. In a conventional system, both the fast
power losses from 75 to 25 kW. Assuming a 20% utilizacharger and the transformer have substantial size andtion of the station at full power, this efficiency improveweight, which adds cost and complexity to the installament will result in 87.6-MWh savings over one year. Of
tion of the system, as both the charger and the transcourse, the savings would be higher with a higher utilizaformer are commonly installed on a concrete foundation.
tion rate. Connecting directly to an MV line can further
Additionally, fast-charger installation in seismicallyreduce the electricity costs since many utilities offer
active areas of the United States requires seismicallytime-of-use rates with up to 20% lower demand charges
restrained concrete foundations for both the transformer
when the electricity is purchased on the MV side (primary
service). These savings can be substantial considering that demandcharges account for most of theelectricity costs.
EnergyPV

MV Grid

SST
HF TR
ac/dc

Storage
dc/dc

dc/dc
dc/dc
dc/dc

dc/dc

Figure 7. An EV charging station concept based on SST technology.

Faster Charging

Cheaper to Install
Better Site Utilization

Higher Availability
Reduced
System Size
Vehicle Owners

Station Owners
Higher Efficiency

Cheaper Charging
Figure 8. The key benefits of the SST-based dc fast charger.

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I EEE E l e c t r i f i c a t i on M a gaz ine / march 2019

Higher Revenue

Case Study:
A 50-kW MV Fast Charger
A team at North Carolina State
University developed a 50-kW MV
fast charger (MVFC) that connects
directly to a 2.4-kV single-phase
distribution line and outputs 200–
500 Vdc to charge the vehicle. The
developed system is based on SST
technology, and it uses fewer
relatively expensive SiC devices
compared to other single-phasemodular converter topologies. Itfeatures a simple input rectifier
stage with high-voltage Si diodes,
which improves the system efficiency but also makes the system
unidirectional. The system isdesigned to be modular, using
three identical 16.7-kW dc/dc
modules (shown in Figure 9) as
building blocks to reach thedesired output power and input
voltage levels. The modules’ in­­puts are connected in a series toaccommodate the high input voltage coming from thediode rectifier connected to the MV grid, and their outputs are connected in parallel to increase the output
power. The series connection of the modules’ inputs
allows the 1.2-kV off-the-shelf SiC devices to serve theMV application. As such, the existing module design
and the control approach can be reused and scaled up
to a three-phase system operating at higher voltage andpower levels by simply increasing the number of modules in the system.
Due to the lower switching and conduction losses in
the SiC devices, the system efficiency is significantly
increased compared to the Si-based solution. Additionally,
the ability of SiC devices to operate at higher switchingfrequencies (a few tens of kilohertz as opposed to fewer
than 10 kHz for Si devices) enables a significant reduction
in size of the inductors and the transformers used to provide the required galvanic isolation, thus reducing theoverall system size and weight. The selected topology
enables independent control of the input three-level boost
stage and the output dc/dc stage. It also enables relatively
simple dc bus capacitor voltage balancing and small input
and output inductors due to the interleaving of the input
three-level boost converters and the NPC-based dc/dc converters at the output.
At a 50-kW level, which represents most of theexisting dc fast chargers, the efficiency of the existing
systems is approximately 93% when accounting forthe 98.5% transformer efficiency. The developed MVFC
prototype has a significantly smaller size than theexisting system, and it improves the system efficiency
from 93% to more than 97.5%, a reduction in losses bymore than 60%, as illustrated in Figure 10. Despite thehigher cost of SiC devices, the estimated total cost of
the system (bill of material and installation) is still
acceptable due to the substantial cost savings at thesystem level.

Three-Level Boost +NPC Converter

SST-Enabled MV Extreme Fast Charging
The SST-based MVFC approach can be expanded to an
extreme fast-charging station with a dc distributionbus, as conceptually illustrated in Figure 7. In this case,a multi-megawatt three-phase SST can be designed tosupply the entire charging station. Figure 11 compares astate-of-the-art 675-kW Tesla supercharger station (withan estimated grid-to-vehicle efficiency of 92%) with an
SST-based 2,700-kW MV charging solution (with a gridto-vehicle efficiency of 97%). As shown in Figure 11,
using the MV solution can increase the power delivery
capability from the same station footprint by fourfold.
At the same time, the energy efficiency savings are evenmore significant at these high-power levels. Similar tothe corridor charging stations, the SST-based MV technology can also be applied in high-power community
charging stations, where it can enable a more efficient
integration of local renewables and battery-energy storage systems, as illustrated in Figure 12. Moreover, theutility-owned battery-energy storage system can beused in the system to reduce demand charges and provide ancillary services for improved grid stability.

Adoption Challenges for the
SST-Based EV Fast Chargers
Despite the many advantages of the SST-based solution for EV charging, some challenges still stand in theway of its complete adoption by the electric utilities
today. The key barriers include reliability concerns ofreplacing the passive transformer with a power electronic equivalent; relatively low penetration of EVs inpower systems, which is perceived as not enough toprovide economic justification for using higher-efficiency but costlier power conversion equipment; thelarge inertia still present in the distribution system,
supplied by the legacy generators; and the limited ability to monetize the perfect power quality supplied by

Output Inductor
Output Diode
Bridge + Snubber

dc Link capacitors

HF Power
Transformer

Output Diode
Bridge + Snubber

Three-Level Boost +NPC Converter

HF Power
Transformer

Output Inductor

dc Link capacitors

Figure 9. The MVFC’s 16.7-kW dc/dc module with PFC functionality. NPC: neutral point clamped.

IEEE Elec trific ation Magazine / ma r c h 201 929the SST. Another challenge is the integration of the
SSTs in the existing power systems, which could
require the implementation of additional layers ofcommunication and control in the system and additional customer education.
There are also some technical challenges that need tobe resolved before the wider adoption of SST-based technology can take place. Some of the biggest technical difficulties are the lack of a comprehensive and fast-acting
protection against overvoltages, short circuits, and circuit overloads and, especially, the lack of fast-acting circuit breakers that can be used in these protection
systems. The conventional mechanical MV circuit breakers can interrupt a fault current in several tens of milliseconds, which is too slow to prevent damage to MV
power electronics equipment in the case of a fault. To

protect the MV power electronics systems, the breaker
would need to interrupt the fault current in several hundred microseconds (depending on the system that isbeing protected). These speeds could only be achieved
with solid-state circuit breakers and hybrid breakers,
which are currently under development by several
research groups.
Another significant challenge that needs to be overcome is standardization and certification of the EV charging equipment that connects directly to the MV line.
Currently, the Underwriters Laboratories (UL) category
FFTG, which covers dc fast chargers (with basic requirements used for this category given in the ANSI/UL 2202
standard), does not cover systems supplied from branch
circuits of more than 600 V. Instead, MV power conversion
equipment is listed under the NJIC category, with the basic

Existing Transformer and Charger System

SST-Based MVFC

Efficiency:

93%

Power Losses Halved

97.6%

Volume:4,300 L

30x Volume Reduction

140 L

Total Cost*:

US$75,000

Total Cost Halved

US$35,000

EV Charger
Same Charging
Speed

Service Transformer* Includes Installation
Figure 10. A comparison of the SST-based MVFC to the existing solution.

LV Switchgear
and Metering
Service
Transformer

Tesla Superchargers
675 kW (5 × 135 kW)

675-kW Tesla Supercharger Station(a)Three-Phase SST dc
dc/dc Converters(2,700 kW)
Busbar 3 × (6 × 150 kW)
MV
Switchgear

MV
Metering

2,700-kW SST-Based MVFC Station(b)

Figure 11. A comparison of (a) a state-of-the-art 675-kW Tesla supercharger station (with an estimated efficiency of 92%) and (b) an SST-based
2,700-kW MVFC solution (97% efficiency) with the same footprint.

30

I EEE E l e c t r i f i c a t i on M a gaz ine / march 2019

MV 3φ Distribution

SST
ac

Breaker

dc

dc

ac
dc

dc

dc
dc
DESD
dc

Net Meter

dc

Battery

Utility Owned
Breakerdc

Breaker

Breakerdc

dc
dc
Breaker
Net Meter
dc Busdc

Battery

800 V
dc

400 V
dc
dc

ac BusFigure 12. An SST-based intelligent energy router in a high-power community charging station. DESD: distributed energy storage device.

requirements used for this category given in the UL 347 A
standard. Despite the substantial market potential andmany technical benefits of the SST-based EV fast chargers,
a significant pushback from the industry is expected
before fully adopting this new approach with power electronic converters directly connecting to an MV distribution line. However, there is a wide consensus that a logicalfirst step toward full adoption is to successfully deploy
several pilot charging stations that will demonstrate all of
the potential utility and industry benefits that the technology has to offer.

For Further Reading
Idaho National Laboratory. (2015). What were the cost drivers
for the direct current fast charging installations? Idaho
National Laboratory. Idaho Falls, ID. [Online]. Available: https://
avt.inl.gov/sites/default/files/pdf/EVProj/WhatWereThe
CostDriversForDCFCinstallations.pdf
Y. Du, S. Lukic, B. Jacobson, and A. Huang, “Review of highpower isolated bi-directional DC-DC converters for PHEV/EV
DC charging infrastructure,” in Proc. 2011 IEEE Energy Conversion Congr. Expo, Phoenix, AZ, 2011, pp. 553–560.
C. Nelder, “Rate-design best practices for public electricvehicle chargers,” Rocky Mountain Institute. Accessed on: Jan.
14, 2019. [Online]. Available: https://rmi.org/rate-design-bestpractices-public-electric-vehicle-chargers/

S. Hutchinson, M. Baran, and S. Lukic, “Power supply for anelectric vehicle charging system for a large parking deck,” inProc. 2009 IEEE Industry Applications Society Annu. Meeting,
Houston, TX, 2009, pp. 1–4.
S. Bai, D. Yu, and S. Lukic, “Optimum design of an EV/PHEV
charging station with dc bus and storage system,” in Proc.
2010 IEEE Energy Conversion Congr. Expo, Atlanta, GA, 2010, pp.
1178–1184.
T. Ortmeyer, L. Wu, and D. Dean. (2016). Parameter identification for optimal electric vehicle rate structures. New
York State Energy Research and Development Authority.
Albany, NY. [Online]. Available: https://www.nyserda.ny.
gov/-/media/Files/Publications/Research/Transportation/1632-Parameter-Identification-Optimal-Electric-Vehicle-RateStructures.pdf
A. Maitra, “EPRI’s utility direct DC fast charger–development, testing, demonstration,” in Proc. EPRI IWC Conf., Atlanta,
GA, Mar. 2012. [Online]. Available: http://mydocs.epri.com/
docs/publicmeetingmaterials/1203/JKN2SD37ZWH/D2_PEV_
D2-5_Maitra.pdf

Biographies
Srdjan Srdic (ssrdic@ncsu.edu) is with North Carolina
State University, Raleigh.
Srdjan Lukic (smlukic@ncsu.edu) is with North Carolina State University, Raleigh.
IEEE Elec trific ation Magazine / ma r c h 201 931