Decentralized Control Using Wireless Signal
Communication for Solid-state Transformer
with Cascaded Chopper Cell
Keita Ohata
Dept. Science of Technology
Innovation
Nagaoka University ofTechnology
Nagaoka, Japankeita_ohata@stn.nagaokaut.ac.jp

Masakazu Adachi
Dept. Electrical, Electronics, andInformation Engineering
Nagaoka University ofTechnology
Nagaoka, Japanm_adachi@stn.nagaokaut.ac.jp

Shunsuke Takuma
Dept. Science of Technology Innovation
Nagaoka University of Technology
Nagaoka, Japan
takuma_s@stn.nagaokaut.ac.jp

Keisuke Kusaka
Dept. Electrical, Electronics, and Information
Engineering
Nagaoka University of Technology
Nagaoka, Japan
kusaka@vos.nagaokaut.ac.jp

Abstract— This paper proposes a novel decentralized control
method and controllers on cells, which communicate with the main
controller using Bluetooth communication, for a medium voltage
solid-state transformer (SST) with multiple chopper cells. SST
requires a considerable number of signal lines between the main
controller and each cell in order to communicate the amount of
information such as detected DC voltage, current, fail, andcommands. The proposed control system with wirelesscommunication is effective in reducing the number of signal lines.
Moreover, the proposed control method allows only low-speed
communication and uses only a few parameters to be exchanged
between the main controller and each cell controllers because theinstantaneous values are not required for the control in the
proposed method. The validity of the proposed method isdemonstrated with a 3-kW prototype. From the experimented
results, the input-current imbalance is reduced from 24.6 % to5.46 %, where total harmonic distortion is less than 3%.
Keywords— solid-state transformer, multi-port converter,balancing control, wireless signal communication

I. INTRODUCTION
Solid-state transformers (SSTs) have been attracted in thefield of DC distribution systems as a front-end converter for data
centers. SST typically consists of multiple small-capacity
converters connected in series or parallel [1–6]. The advantagesof the multiple cell topology are; availability of use of lowvoltage devices, scalability for power capacity and voltage.
The multiple cell topology requires a large number of thesignal lines between the main controller and each cell. The
signal lines among cell controllers and the main controller arenecessary to control the voltages, currents of each cell, andXXX-X-XXXX-XXXX-X/XX/$XX.00 ©20XX IEEE

Tetsunori Kinoshita
Dept. Electrical, Electronics, andInformation Engineering
Nagaoka University ofTechnology
Nagaoka, Japankinoshita_t@stn.nagaokaut.ac.jp

Koki Yamanokuchi
Dept. Electrical, Electronics, andInformation Engineering
Nagaoka University ofTechnology
Nagaoka, Japan
koki_yamanokuchi@stn.nagaokaut.ac.jp

Jun-ichi Itoh
Dept. Science of Technology Innovation
Nagaoka University of Technology
Nagaoka, Japan
itoh@vos.nagaokaut.ac.jp

overall systems coordinately. A great deal of wire connection
typically decreases the maintainability of SSTs.
In order to reduce the number of signal lines, decentralized
control methods in which each cell independently operates, havebeen proposed in the previous study [7]. These studies have
improved the maintainability of SSTs. However, there are still
many signal lines between the cells and the main controller.
Thus, a further decrease in the signal lines is required formaintainability and workability.
In order to solve the above problems, a distributed control
method with wireless signal communication has been proposed[8–9]. References [8–9] show that high-speed feedback is not
required for signals such as detected signal lines, control signallines, and gate drive signal lines from the cell controllers to themain controller. However, the decentralized control has notexperimentally demonstrated [8–9].
This paper experimentally clarifies the proposed
decentralized control using the wireless signal communicationfor SST with cascaded chopper cells. The new contribution ofthis paper is that the validity of the Bluetooth wirelesscommunication is demonstrated for the cell controller in SST
system even if the Bluetooth wireless communication has long
delay time. In the rest of the paper, first, the configuration andadvantages of the SST with the proposed decentralized control
methods are explained. Second, the operation of thedecentralized control is described. Finally, the experimental
results of the operating SST by the wireless communication are
mentioned. Furthermore, the practical test for the wirelesscommunication under the magnetic field from the power
converters is verified, as an appendix.

PFC converter
fsw = 20 kHz
iL_u

Resonant DC-DC converter
f0 = fsw = 50 kHz

Lu
Vdc_u
Cs

+

vin_u

U phase conv.

Ls

Cout +

Vout_u

Cconv

Cell

Lm

Rout

V phase conv.
Vout
W phase conv.

Fig. 1. Solid-state transformer with simple cascaded chopper cells.

the role of the main controller, the main controller is only forcalculating the averaged value and communicating with eachcell. Thus, it is possible to employ slow speed microcomputer,which is desirable from a cost point of view. The averaged
current I0 is given by

II. CIRCUIT TOPOLOGY
Figure 1 shows the circuit configuration of SST with thecascaded chopper cells. The SST is operated as three singlephase AC-DC converters. Each chopper cell consists of the PFC
converter and the resonant DC-DC converter. The PFC
converters have a role to obtains the sinusoidal input current andunity power factor. The resonant DC-DC converter is employedto achieve galvanic isolation and reduction of switching loss by
zero current switchings (ZCS). The zero current switchings areachieved by resonance between the leakage inductance Ls in thehigh-frequency transformer and the resonant capacitor Cs
connected to the primary side of the transformer. Each MOSFETof the high-frequency inverter in the primary side switches at thezero-cross of the current by adjusting the resonant frequency andthe switching frequency.

I0 =The function of the cell controller in communication with themain controller and secondary voltage control with input phasecurrent control in the minor-loop. Besides, the secondary
voltage control scheme employs the droop control and current
balancing control in order to prevent diffusion of input current
and power-sharing.

Figure 2 shows the block diagram of the proposed
decentralized control with wireless communication [7]. The
control system is separated into the main controller and the cell
controllers.

A. Droop control
Figure 3 (a) and (b) show the equivalent circuit models of
SST, focusing on the output of the cells, which are connected inparallel at a DC bus. Each cell connected to the DC bus
individually controls the output voltage with feedback control.
However, the interactions of the voltage feedback control
between each cell lead a weak system performance or may drive

The main controller calculates the averaged current based on
the detected current of each phase received from the detection
circuit. Afterward, the main controller transmits a output voltage
command Vo* and averaged current I0 to each cell. To summarize
Vout*

Iv +

Iu
Vout*

+

+

+

1

+

3

I0

Iw

Iu, Iv, Iw

I0

Vd_u

+
Vc_u

PI
-

ZK

PI

0

+

+

+
Vd_comp

Iu

Vout

+
-

-

Vin

：Wir elesscommunica tion

Iu
Current balance
control

×
abs

+
-

PI

iLu

sin
PLL

Vout*

Fig.1

Iu

Main Circuit

Iu
Cell U

vin_u+

Main controller(1),

where I0 is the averaged current of the three-phase current
calculated every few milliseconds.

III. PROPOSED CONTROL SCHEME

Vout*

Iu + Iv + I w
3Iv

Droop control

Cell V

Cell controller(U phase )Cell controller(V phase )Iw

Cell controller(W phase)

Fig. 2. Block diagram of proposed decentralized control with wireless communication.

Cell W

Rout

Vout

the system to instability. The imbalance of the output voltage
causes a rush current. The difference in voltage is caused by two
reasons; The first reason is a gain error of amplifier for a current
sensor. The second is the temperature drift of the current sensor
and the detection circuit.

Iu
Vout_uIn order to solve the above problems, the virtual resistors inorder to eliminate inrush current are implemented in series toeach output voltage source by the droop control, as shown in Fig.
3 (b). In other words, the virtual resistors mitigate the mutualinterference of the voltage feedback control of each cell in
exchange for the high-dynamic response and the steady-state
deviation on the output voltage. The drop voltage of virtual
resistor Vd_u, Vd_v, Vd_w are given by

Iw

Vout_w

Vout

Rout

Vout

Rout

(a) Without a droop control.

Vd_u

ZK

Vd_v

Vd_w

ZK

Iu
Vout_uV
Vd _ u = in * Z K I u
Vout

Iv
Vout_v

Iv
Vout_v

ZK
Iw

Vout_w(2),

ZK :Virtual resistance(b) With a droop control.
Fig.3. Equivalent circuit models of SST focusing on the output of the cells.

Vd _ v =Vin
Z K Iv
Vout *(3),

Vd _ w =Vin
ZK Iw
Vout *(4),

where Vin is the input rectified voltage, Vout* is the output voltage
command. Thus, the controller adds feedforward to this value
as Vd_comp.
B. Current balance control
The droop control prevents uncontrollable current, as
explained in the previous section. Note that implementing only
the droop control might result in imbalanced of the input phasecurrent in each cell. Thus, the proposed control method is
employed the input phase current balance control strategy toeliminate the current imbalance. As shown in Figure 2, themanipulated variables Vc_u, Vc_v, Vc_w are added to the output
voltage command in order to make the detected peak value ofthe phase current following the averaged current in the input
phase current balance control. The notable feature of the strategyof the input current balance control is that a fast response is not

required. In other words, high-speed communication and thehigh computing power of the main controller are not necessary.
Besides, a conventional decentralized system controls thecurrent of each cell by the main controller. The conventional
method requires a fast response. For this reason, only the
proposed control scheme accepts the inclusion of wirelesscommunication.
IV. STRUCTURE OF EXPERIMENTAL CIRCUIT
Figure 4 shows the experimental configuration with wirelesscommunication. The proposed control method does not require
high-speed communication between the main controller and thecell controllers, as mentioned in chapter III. Note that thiscommunication standard is not only Bluetooth specification but
also several communications technologies such as Wi-Fi,
ZigBee, or sub-GHz wireless can be used as the wirelesscommunication method in cells.
A. Main controller
Figure 5 (a) shows a photograph of the main controller forthe experimental circuit, including decentralized control using awireless module. The main controller consists of DSP, FPGA, abus controller, and three Bluetooth modules (Microchip,
Input voltage Vu, Vv, Vw,
Input current Iu, Iv, Iw Output voltage Vo

Micro Tx
controllerRxselector

Bus controller

Vout* ,I0
Iu
Vout* ,I0
Iv
Vout* ,I0

Iw
Main controller

Bluetooth
ModuleBluetooth
ModuleBluetooth
ModuleVout* ,I0
Iu
Vout* ,I0
Iv
Vout* ,I0
Iw

Bluetooth V * , Io0ModuleIu
BluetoothVo* , I0
ModuleIv
BluetoothVo* , I0
ModuleIw

Detection
circuitController

U-phaseConverter

Controller

V-phaseConverter

Controller

W-phaseConverter

Cell controller

Wireless
Fig. 4.Experimental configuration with wireless communication.

B. Cell controller
Figure 5 (b) shows a photograph of the cell controller for theexperimental circuit. The cell consists of a Bluetooth module, adetection circuit, a controller, and the main converter. The
controller controls the output voltage and input current based on
the received data Vo* and I0.
C. Structure of wireless communication
A data structure of transmitted and received data is five bytes
per parameter. The proposed method uses two parameters in atransmitting process from the main controller to the cell
controller and one parameter in another transmitting processfrom the cell to the main. Thus, the number of byte per control
cycle is 45 bytes. The measurement of required time for thecommunication cycle is 270 ms in the condition of section V.
The cycle is decided by the communication time, the delay time,
and the processing overhead in the wireless module.
V. EXPERIMENTAL RESULTS
In this chapter, the experimental results of the proposed
control scheme is introduced. Table II lists the experimental
conditions.
A. Steady-state
Figure 6 shows the input current iu, iv, and iw waveforms andthe output voltage Vo waveform at the steady-state with orwithout the proposed method when Vo* = 400 V, and P = 1.5 kW.
The phase current is high-distorted, and power-sharing is
ununiformed when the droop control and the current balance

control are disabled in Fig. 6(a). Whereas, each phase current is
controlled in a sinusoidal shape, and the amplitudes are matchedin Fig. 6(b). The input current THD of each phase is 3 % or less.
The input-current imbalance rate is reduced from 24.6 % to5.46% compared to without the proposed method. Here, theimbalance rate of the input current is defined as I − I0 Iv − I0 I w − I0 ,, 100
I0
I0 
 I0

 current_err [%] = max  u

TABLE I. Specification of wireless module.

Corporation

Microchip

Communication standard

Bluetooth ver. 4.1

Product name

RN4020Module size

13.4 mm × 25.8 mm × 2 mm

Maximum speed

240 kbps

Allowable transmission distance

20 m

TABLE II. Experimental Conditions.
Quantity
Input voltage

SymbolVin

Rated output powerP

DC-link capacitance

Cconv

Output capacitanceCout
L(%Z =2.3 % )fsw

20 kHzfo

50 kHzwACR
wAVR

6000 rad/s
50 rad/s

Kc

2.0

Input inductance
Switching frequency (PFC)
Resonant frequency(Resonant DC-DC converter)
Angular frequency of ACR
Angular frequency of AVR
Proposal gain of current
balance control

Main Controller (DSP + FPGA)(Replaceable a micro processors)

Value
283 V

(200√3 V)

3 kW
48 mF
2040 mF
3 mH

Outputcapacitor

WirelessmoduleBluetooth module(Microchip,
RN4020)Bus
controller(5).

B. Transient-response
Figure 7 shows the input current iu, iv, and iw waveforms andthe output voltage Vo waveform at the transient-state when the
output voltage command Vo* changes from 250 to 400 V. The
transient response has a delay about one second by wirelesscommunication. The overshoot occurs at the change of output
voltage command on each cell controller. After 1.2 s, the input

Cell
Converter
Cell
controller(a) Main controller.
(b) Cell controllers and cell converters.
Fig. 5 Photograph of experimental circuit including decentralized control using wireless module.

To Electric Load

RN4020). The controller transmits output voltage command Vo*
and the average value of phase current I0. Moreover, the main
controller receives each phase current Iu (or Iv or Iw), the main
controller calculates the average value of phase current by (1).
The wireless module accepts only one to one communication.
Hence, three modules are installed on the main controller. The
number of wireless communication modules on the main
controller side is reduced when using a wireless communication
module capable of 1-to-N communication. The communication
is based on Universal Asynchronous Receiver / Transmitter(UART), 115,200 bps. In the proposed control method, ageneral-purpose controller can be applied as the main controller
because the calculation of the main controller is only (1). Table
I lists the specification of the wireless module. The main
controller is applied to the same controller of the cell for
simplicity.

current converges by the proposed control with wirelesscommunication.
VI. CONCLUSION
In this paper, the decentralized control method using
wireless communication is employed for the control system ofSST. The proposed control system is characterized by the droop
control and the current balance control. These controls are theouter loop of the feedback voltage control. As a result, a simple
configuration and wireless communication are available when
the circuit scale is increased by using the cascaded chopper cell.
In the experimental results of the 3-kW prototype, the inputcurrent imbalance rate is reduced from 24.6 % to 5.46 %, andthe input current THD is 3% or less. The compensation for
delays of the wireless communication will be considered when
the number of cells further increases.

1.5 kW output (0.5 p.u.)0

iu 5 A/div

Vo 250 V/div

iv 5 A/div

iw 5 A/div0

4 ms/div(a) Without proposed balance method.
1.5 kW output (0.5 p.u.)

APPENDIX
VII. PLACTICAL TEST FOR WIRELESS COMMUNICATION IN
POWER CONVERTERS
In this section, the effect of radiated noise generated by thepower converters on the wireless communication module, e.g.,
the communication error rate, effective speed, and transmission
distance, is evaluated. This evaluation is necessary because thecommunication between the main controller and the cell
controller uses the wireless communication module, which islocated near the power converter in the proposed circuit. From
the aforementioned discussion, the wireless communication
module is placed near the three types of power converters, andChanging output voltage reference from 250 V to 400 V
Delay of wireless communication
Vo 250 V/div0

Vo 250 V/div

iu 5 A/div

iv 5 A/div

iw 5 A/div(THD: 1.94%)(THD: 2.79%)(THD: 2.43%)

04 ms/div(b) With proposed balance method.
Fig. 6 Waveform at the steady-state with or without the proposed balancemethod through Bluetooth communication.
a
Vo 250 V/div00

iu 5 A/div

iv 5 A/div

iw 5 A/div

00

Vo* = 250 V

iu 5 A/div

iv 5 A/div iw 5 A/div
Vo* = 400 V

Fig. 8 (b) Fig. 8 (c)2 s/div

10 ms/divaFig. 8 (d)(a) Outline.

(b) Extended Fig.7(a): Before changing output voltage command.

Vo 250 V/div

Vo 250 V/div
0

0

0

0

iu 5 A/div iv 5 A/div

iw 5 A/div(c) Extended Fig.7(a): Overshoot.

10 ms/div

iu 5 A/div iv 5 A/div

iw 5 A/div

10 ms/div(d) Extended Fig.7(a): After changing output voltage command.

Fig. 7. Waveform at the transient response when change output voltage command from 250 V to 400 V

Induction motor (3.7 kW)

Communication conditions
Induction motor driving (3.7kW)
r = 2.0, 8.0 m
Grid-connected Inverter (1kW)
r = 2.0, 8.0 m
Wireless power transfer system (3.2kW) r = 1.0, 1.2, 1.6 m
Wireless
Cell Distance 20 cm
Noise occurs
communication

Bluetoothmodule
20cm

…

Main
controller

Transmitting command

Bluetooth module

Wire communication
Transmission distance r(RN42, Microchip)

Load

r

Cell

Fig. 8. Outline of experimental environment. Three type of converters areplaced away the Bluetooth module from 20 cm as noise source.
TABLE III. Specification of wireless module at the practical test.
Manufacture

Microchip

Communication standard

Bluetooth ver. 2.1 + EDR

Product name

RN42

Module size

13.4 mm × 25.8 mm × 2 mm

Maximum speed

240 kbps (Slave), 300 kbps (Master)

Allowable transmission distance

20 m

To PC

(a) Induction motor (3.7 kW).
Bluetoothmodule20cm

the effective communication speed and error rate ofcommunication are evaluated.

Grid-tied
inverter (1 kW)r

Figure 8 shows the outline of the experimental environment
for measuring the effect of radiated noise from the power
converters on the Bluetooth module. Note that the experiment
was conducted using the Bluetooth module RN42 (Microchip).
Table III lists the specification of wireless module RN42.

To PC

Figure 9 shows the photograph for the experimental
environment. The three types of converters are placed awayfrom the Bluetooth module from 20 cm as a noise source. In
addition, the Bluetooth communication distance is changed from
2 m to 8 m to evaluate the effective speed against thetransmission distance.

REFERENCES
[1]

[2]

[3]

C. Liu et al., “An Isolated Modular Multilevel Converter (I-M2C)
Topology Based on High-Frequency Link (HFL) Concept,” IEEE
Transactions on Power Electronics, vol. 35, no. 2, pp. 1576-1588, Feb.
2020.
J. E. Bosso, M. Llomplat, G. G. Oggier and G. O. García, “Evaluation of
DC-AC Single-Phase Solid-State Transformers”, in IEEE Latin America
Transactions, vol. 17, no. 05, pp. 708-717, May 2019.
H. Chen, A. Prasai, R. Moghe, K. Chintakrinda and D. Divan, “A 50-kVA
Three-Phase Solid-State Transformer Based on the Minimal Topology:
Dyna-C,” IEEE Transactions on Power Electronics, vol. 31, no. 12, pp.
8126-8137, Dec. 2016.

Transmission coil (3.2 kW)transmitterrBluetoothmodule(c) Wireless power transfer system (3.2 kW).
Fig. 9. Photograph of the experimental environment for measuring the effectof radiated noise from some power converters on the Bluetooth module.

400
400
400

Wireless power transfer system (3.2kW)

350
350
350

Ideal (measured in shield room)

Effective speed [bit/s]

Figure 10 shows the relationship between the effective speed
and transmission distance. Bluetooth communication operates at
a communication speed of 250 bit / s to 400 bit / s near power
converters. Moreover, in the comparison between transmitted
data and received data, the error rate is 0% under all conditions.
The experimental results indicate that the communication errors
do not occur even when the wireless communication module isoperated nearby 3.2-kW wireless power transfer system, whichemits much radiation noise, a 1-kW grid-tied inverter, and a 3.7kW induction motor. From experimental results, the Bluetoothmodule is suitable for the communication tool of some power
converters.

(b) Grid-tied inverter (1 kW).

300
300
300

Grid-tied
inverter (1kW)

250
250
250

Using module
Induction motor (3.7kW)
RN42 (Microchip), Bluetooth
200
200
200
00 0
11 1
22 2
33 3
44 4
55 5
66 6
77 7
88 8
99 9
Transmission distance r [m]Fig. 10. Relationship between effective speed and transmission distancewhen the wireless communication module is operated nearby power
converters.

[4]

[5]

[6]

[7]

Mizuki Nakahara, Yuki Kawaguchi, Kimihisa Furukawa, Mitsuhiro
Kadota, Yuichi Mabuchi, Akihiko Kanoda, “Development of a Control
Method for LLC Converter Utilized for Input-Parallel-Output-Series
Inverter System with Solid-State Transformers”, IEEJ Journal of Industry
Applications, vol.8, no. 5, pp. 652-659, Jul 2019.
Yusuke Hayashi and Tamotsu Ninomiya”Highly Scalable SensorlessMulticellular AC-DC Transformer (ADX) for the DC Distribution
System in Data Centers,”IEEJ J. Industry Applications, vol. 7, no. 6, pp.
479-487, 2018.
Jun-ichi Itoh, Kazuki Aoyagi, Keisuke Kusaka, Masakazu Adachi,“Development of Solid-state Transformer for 6.6-kV Single-phase Grid
with Automatically Balanced Capacitor Voltage”, IEEJ Journal ofIndustry Applications, vol.8, no.5, pp. 795-802, Sep 2019.
J. Nie, L. Yuan, Q. Gu, J. Sun and Z. Zhao, "A Coordinate and Distributed
Control Scheme for Multilevel and Multi-Stage Medium Voltage Solid

[8]

[9]

State Transformer," 2018 International Power Electronics Conference(IPEC-Niigata 2018 -ECCE Asia), Niigata, Japan, 2018, pp. 2963-2968.
Masakazu Adachi, Keisuke Kusaka, Jun-ichi Itoh, “Multi-Modular
Isolated Three-Phase AC-DC Converter for Rapid Charging with
Autonomous Distributed Control”, EVS 31 & EVTeC 2018, Kobe, Japan,
2018
K. Ohata, M. Adachi, K. Kusaka and J. Itoh, "Three-phase AC-DC
Converter for EV Rapid Charging with Wireless Communication for
Decentralized Controller," 2019 10th International Conference on PowerElectronics and ECCE Asia (ICPE 2019 - ECCE Asia), Busan, Korea(South), 2019, pp. 3033-3039.