［ New Technologies ］

Development of TransformerlessMulti-Level Medium VoltageInverters
Keywords

Isamu Hasegawa,Shizunori Hamada,
Kenji Kobori,
Yutaka Shoji

Multi-level inverter, PWM, Transformerless

Abstract

Reﬂecting on Climate Change, environmental impact-reducing technologies, such as energy conservation and reduction of CO2, are getting worldwide
attention. As such, the demands for compact and high-efﬁciency medium-voltage inverters have increased in the ﬁeld of power and industrial systems.
In particular, there is growing interest in “transformerless” multi-level inverters because they do not require any multi-phase winding transformers on theinput source side. Thus far, we invented a unique circuit system suitable for adaptation to medium voltages and succeeded in the development of a transformerless a 4kV prototype whereby verifying its effectiveness. Still more, we
recently developed a new circuit system to realize higher voltages and higherperformance.

1 PrefaceReﬂecting on Climate Change, environmental
impact-reducing technologies, such as energy conservation and reduction of CO2, are getting worldwide attention. In the ﬁeld of manufacturing andindustrial activities, motor power accounts for about
70% of the total energy consumption and they tend
to have a higher voltage to realize higher efﬁciencies. Motor operation at a constant speed has conventionally been popular with fans, pumps, andblowers used to control ventilation and water supplies. Currently, however, the application of inverters has become commonplace due to its energy
saving getting high attention.
The demand for medium-voltage inverters thatcan drive medium-voltage motors has increased.
Required performance characteristics of mediumvoltage inverters are as follows:( 1) High efﬁciency( 2 ) Compact and light weight design
In order to meet the above requirements, weproposed a new circuit system in 2012 and havebeen studying the possibility of commercialization
by making several prototypes (1)(2) since. Our new
approach makes it possible to eliminate multi-phasewinding transformers that are generally used in
medium-voltage converters. We continue to makeefforts to decrease the number of capacitors andreduce the total capacitance in order to meet market requirements. This paper introduces the operational principles of the converter and the featuresof the transformerless multi-level medium-voltage
inverter developed our expertise.

2 Operational Principle
2.1 Circuit Conﬁguration
We propose a new circuit system of the ﬂying
capacitor type for converters. The ﬂying capacitor
system is a method where potential switching is
performed for the ﬂying capacitor to generate
multi-level voltage outputs. Fig. 1 shows the circuit
conﬁguration of our proposal. This circuit is composed of two blocks, a phase module for a 3-phase
input and a 3-phase output, and a DC module. The
circuit comes in a Back To Back (BTB) conﬁguration
where the input and output circuits are connected
through the DC module. The 3-phase input circuit
functions as a rectiﬁer and the 3-phase output circuit functions as an inverter. The DC module contains two basic cells directly connected and thesebasic cells are composed of the DC link capacitors,
ﬂying capacitors, and four Insulated Gate Bipolar
Transistors (IGBTs). The phase module is composedof ten IGBTs and four diodes. Since the same capacitors are used in common for the rectiﬁer andinverter, the number of capacitors can be decreasedMEIDEN REVIEW Series No.172 2018 No.125

and the overall capacitance can be reduced. In thiscircuit, the active power to be supplied from thepower source to the rectiﬁer can be fed directly tothe load through the inverter. At that time, sinceincoming current into the capacitor is made low, it
can also reduce capacitance and volume of thecapacitors.

Basic cell 1

CDC1

C1

CDC2

C2Fig. 1

When the DC link capacitors CDC1 and CDC2 arecontrolled at a voltage of 2E and the ﬂying capacitors
C1 and C2 are at a voltage of E, ﬁve potentials (2E,E, 0, −E, and −2E) can be generated. Subsequently,
these ﬁve levels of potentials are outputted by
selecting adequate switching pattern of phase modules. Fig. 2 shows an example of phase moduleswitching patterns. Circles in the ﬁgure indicate
the IGBTs that are currently turned on. The phasemodules have ﬁve switching patterns from which
ﬁve kinds of potential outputs can be generated.
Regarding the modulation system, a carrier comparison type of Pulse Width Modulation (PWM) isadopted and based on the result of modulation, asuitable switched pattern is selected.
Capacitors CDC1 and CDC2 can make voltages
constant by controlling the active power in the samemanner as for conventional power converters.

2.3 Voltage Control for Flying Capacitors

Basic cell 2
Phase module

2.2 5-Level Output Approach

DC module

Phase module

Circuit Configuration of our Proposal

The BTB conﬁguration is shown. It combines the phase modules
and the DC module.

Two kinds of switching patterns are used tocontrol voltage of capacitors C1 and C2. Fig. 3
shows the charge-discharge mode of the ﬂyingcapacitor. When a current ﬂow is carried in thedirection of the arrow along the dotted line, C1 ischarged by selecting Mode1. It is discharged when
Mode2 is selected. By selecting a desired current
path for C1 in such a manner, voltage control
becomes possible. Similarly, C2 is also used for[Legend]: ON2EFig. 2

E0Example of a Phase Module Switching Patterns

Each phase module can generate ﬁve types of potential outputs.

26MEIDEN REVIEW Series No.172 2018 No.1－E－2E: OFFVoltage command value v＊uPhase current iuCarrier

10C1E

CDC12E

CDC12EC1E－1
DC1, the rate of use by C10(1) Mode 1Fig. 3(2) Mode 2

Estimated output current value ofbasic cell 1: icell1

Rate of useby C2

Charge-Discharge Mode of the Flying Capacitor

Charge-discharge control of a ﬂying capacitor is performed bymode changeover in the basic cell.

voltage control by selecting two kinds of switchingpatterns.
When the phase voltage command value isdeﬁned by ( 1) and the phase current detection value
is deﬁned by ( 2 ), the rate of using C1 in regard to thevoltage command value of ( 1) is given by ( 3 ). Since
the current carried in the basic cell is identical withthe phase current ﬂowing during the period of ( 3 ),the amount of this current is given by ( 4 ) that is a
product of ( 2 ) and ( 3 ).
vm sin θuiu ＝ 2 Iu sin (θu −φ)＋1−v＊u (＋ 0.5 ＜v＊u ≦ ＋1.0)(0.0 ＜v＊u ≦ ＋ 0.5)
Du1 ＝
v＊u
(v＊u ≦ 0.0)0iˆcell1 ＝ iu × Du1＊ ＝u( 1)(2)0
Estimated output current value of basic cell2: icell2

Fig. 4

Outline Diagram of Basic Cell Current
Estimation Method

According to the phase voltage command value and the phasecurrent detection value, the currents of the basic cells are
estimated.

Table 1 Switching Pattern for a DC Module
The charge-discharge mode is determined based on a combination of switching pattern and the current polarity of the basic cell.
vc 1 , vc 2

icell1, icell2＜E＜0Mode 2 (C1 and C2 charged)＜E＞0

Mode 1 (C1 and C2 charged)＞E＜0Mode 1 (C1 and C2 discharged)＞E＞0Mode 2 (C1 and C2 discharged)Mode(3)(4)

In the expressions above, Value m denotes themodulation index of the phase voltage, Value Iu
denotes the rms value of the phase current, Valueθu denotes a phase of the voltage command value,and Value φ denotes a phase difference between
voltage and current.
Fig. 4 shows outline diagram of basic cell current estimation method. In Fig. 1, six phase modules are connected through the DC modules. Assuch, all currents are carried in the basic cells. Inthis case, the estimated current value ﬂowing
through the basic cells is given by ( 5 ) below.
iˆcell1_btb ＝ iu × Du1 ＋ iv × Dv1 ＋ iw × Dw1 ＋ ir × Dr1(5)＋ is × Ds1 ＋ it × Dt1
Since the current value ﬂowing in the basic
cells can be estimated based on the detected valuesof voltage command value of the phase module andphase current value, current control is possible without installing additional current sensors.

Table 1 shows the switching pattern for a DCmodule. To control the ﬂying capacitor voltages ofvC1 and vC2 to Value E, such voltage control is carriedout by selecting a suitable switching pattern of theDC module. If Mode2 is selected while the current
polarity is positive or Mode1 is selected while thiscurrent polarity is negative, the ﬂying capacity is discharged. On the contrary, if Mode1 is selected while
the current polarity is positive or Mode2 is selected
while this current polarity is negative, the ﬂying
capacitor is charged. In this way, the ﬂying capacitor
voltage can be controlled to a required voltage valueby making charge-discharge switchover according
to the current polarity and ﬂying capacitor voltage
deﬁned by ( 5 ).

2.4 Downscaled Model Conﬁguration
In order to conﬁrm performance of the proposed circuit, we produced a downscaled model forevaluation. Fig. 5 shows circuit conﬁguration of thedownscaled model. Table 2 shows speciﬁcations ofMEIDEN REVIEW Series No.172 2018 No.1

27

vDC1vC1
LR load oran induction
motor

420V
50Hz

LoadvDC2Fig. 5vC2Circuit Configuration of Downscaled ModelTesting was carried out by adding an input ﬁlter and a load to the proposed circuit conﬁguration.

Table 2 Specifications of the Downscaled Model

System voltage

420V

Rated power

8kVA

System voltage frequency

50Hz

into a reverse rotation from forward 50Hz to 0Hz
and further to reverse 50Hz. After that, the rotation
was stopped. Fig. 6 shows the result of the induction motor test. Even in the case of a 4-quadrant
operation, ﬂuctuation was low in DC and ﬂying
capacitor voltages, and we conﬁrmed that this would
offer stable operation.

Carrier frequency

1kHz

2.4.2 Sudden Load Change Test with a LR Load

Rated output of induction motor

5.5kW

No. of poles of induction motor

4pole

Rated frequency of induction motor

50Hz

This testing was carried out by changing theinverter load into the LR load. Fig. 7 shows theresult of a sudden load change test with an LR load.
The shown waveforms were observed when theload resistance was changed while operation wasmaintained at an output voltage frequency of 50Hz.
A sudden load change was purposely caused in thearea surrounded by the dotted lines in the ﬁgure. Ina moment when a sudden load change was caused,variations can be seen in DC and ﬂying capacitor
voltages. After the lapse of a certain period, however,voltage ﬂuctuations seemed to settle down. We
conﬁrmed that the input current followed up the output load.

Main circuit speciﬁcations of the produced downscaled model
are shown.
Item

Specifications

the downscaled model. The input side is connectedwith an AC power supply of 420V and 50Hz throughan input ﬁlter for the purpose of harmonics removal.
The output side was connected with an induction
motor or a load which was conﬁgured by resistor
and inductor (LR load). In this circuit conﬁguration,
Voltages vDC1 and vDC2 were set up at 360V while vC1and vC2 were at 180V, respectively. In this state, DC
voltage control and input power factor control werecarried out on the input side and frequency control
was conducted on the output side. With the use of
the produced downscaled model, the following twotests were carried out.

2.4.1 4-Quadrant Operation Test with anInduction Motor
For this testing, an inverter load was connected to an induction motor. An output frequency command was given to start acceleration from 0Hz up toforward rotation at 50Hz. This rotation was changed28

MEIDEN REVIEW Series No.172 2018 No.1

3 Application to TransformerlessMulti-Level Medium Voltage Inverters
We are working on product commercializationof transformerless multi-level medium voltage inverters for 6.6kV motor drive, where the above power
conversion circuit is employed. Table 3 shows speciﬁcations of the product and its features are itemized
below.

Preliminary excitation

Inverter
Input
Rectifieroutputcurrent (A) output
voltage (V)voltage (V)Acceleration
1.0s

Forward run
50Hz

Reverse run
Deceleration Acceleration
1.0s
1.0s

50Hz

DC brake
Deceleration
1.0s＋2000－200＋100－10＋2000－200

Inverter
outputcurrent (A)＋100－10vDC1, vDC2 (V)＋400＋200vC , vC (V)
01Fig. 6200ms/div

2

Result of Induction Motor Test

Inverter output
voltage (V)Input
current (A)Rectifier output
voltage (V)As a result of making acceleration and deceleration from the output frequency, four-quadrant operation was attained.

＋2000－200＋100－10＋2000－200

Inverter output
current (A)＋100－10＋400＋2000vDC1, vDC2 (V)vC1, vC2 (V)25ms/div
Load increase (50% → 100%)

Fig. 7

Load decrease (100% → 50%)Result of Sudden Load Change Test with a LR Load

Stabilized operation was attained even though a sudden change in load was purposely caused during operation at the rated speed.

( 1) 98% efﬁciency that is the highest in industrial
ﬁeld (97% for our former product)( 2 ) Smallest size in the industrial world (53% volumetric rate compared with our former product)( 3 ) Power regeneration function as standard speciﬁcation( 4 ) Reduction of input harmonics (IEEE519 compliant)Compared with conventional systems, sincehigh efﬁciency is secured, the effect of energy conservation is better than our conventional model
when applied to ﬂow rate control for machinery such
MEIDEN REVIEW Series No.172 2018 No.1

29

Table 3

Specifications of Transformerless Multi-Level
Medium-Voltage Inverters

Speciﬁcations of transformerless multi-level medium-voltage
inverters under development are shown.
ItemSpecifications

Rated output

1MW

Input/output voltage

6000/6600V (Output voltage is lowerthan input voltage.)

Output frequency

0-75HzMaximum efficiency

98% or above

Operational domain

Four-quadrant operation

Control mode

V/f control
Speed sensorless vector control
Vector control with speed sensor

Input power factor

1

Protective construction

IP21

Maintenance

Front maintenanceCooling configuration

Forced-air-cooled

Approx. dimensions

W2200 × H2100 × D1100mm※ Not including cooling fan size.

4 Postscript
In order to realize medium-voltage inverters
featuring highly efﬁcient, compact, and light weight
design which are high in the market demand, wedeveloped ﬁve-level medium-voltage drive in aunique circuit conﬁguration. In this newly proposedsystem, no multi-phase winding transformer isrequired and the number of ﬂying capacitors to be
used is minimal. It excels in terms of compact andhigh efﬁciency design.
Going forward, we will work on increasing
applications, by utilizing such features.
・All product and company names mentioned in this paper are
the trademarks and/or service marks of their respective owners.
《References》( 1 ) I. Hasegawa, S. Urushibata, T. Kondo, K. Hirao, T. Kodama, andas fans and pumps. This system is also suitable forapplication to machines that make sudden deceleration or repeat frequent acceleration and deceleration. It is also useful if it requires a smaller footprintfor inverter installation.

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H. Zhang, “Back-to-back system for five level converter with common flying capacitors,” International Power Electronics Conference(IPEC Hiroshima2014-ECCE-ASIA), pp.1365-1372, May 2014( 2 ) H. Zhang, W. Yan, K. Ogura, S. Urushibata, “A Multilevel Converter
Topology with Common Flying Capacitors,” Energy Conversion Congressand Exposition (ECCE), p.1274, Sep. 2013