IEEE Power Electronics Magazine - June 2017 - 60

MV
Side

Unit
Unit

dc
Feeder

ac Feeder 1

CM and DM

Phase B

Phase C

Filter
ac Feeder 1

Phase B

Phase C

CM and DM

Filter

dc
Feeder

(a)

(b)

FIG 3 An ST architecture considering two dc/dc solutions: (a) DAB and (b) QAB. CM: common mode; DM: differential mode.

QaB and DaB Converter Comparison
Since the DAB and QAB converters are the most promising
topologies, they are compared in this article in terms of cost,
efficiency, complexity, and reliability. Both converters are
designed assuming the specifications and parameters shown
in Tables 1 and 2. A phase shift modulation is considered [15],
with a nominal phase angle of 35° and switching frequency of
fs = 20 kHz. The detailed values obtained from the QAB
design and DAB design, used in the comparative analysis of
these converters, are presented in Table 5. A comparative
analysis of both converters appears in Table 6, where the
main parameters of the design are summarized, and the components quantity and cost are presented.
The QAB converter has fewer LV cells, as well as fewer
transformers. Consequently, the employed semiconductors
and auxiliary components, such as gate-driver units, auxiliary power supplies, and control and communication systems
are also reduced compared to the DAB solution. Although
higher current rating devices are required to implement the
LV cell of the QAB (Table 6), the individual device cost does
not differ much from the cost of the devices required by the
DAB solution, as can be seen in Table 3. Consequently, the

QAB converter presents the most effective solution both economically and practically, since it uses fewer components. By
using QAB instead DAB, a cost saving of US$2,350.08 (only
in semiconductors) is achieved; this value can be higher if
the auxiliary components are considered. Figure 4 shows a
qualitative comparison between the QAB and DAB converters. In a balanced condition, both converters present the
same performance from the efficiency viewpoint [15]. On the
other hand, the control of the QAB presents a higher complexity than the DAB one. Despite this fact, the QAB solution
presents several advantages over the DAB solution; thus, we
chose to implement the dc/dc stage of the ST.

Proposed Grid-Tailored Approach
ST design is a complex matter: the peak load consumption
is difficult to evaluate, and it is limited in time to a few
hours per year. The actual procedure for sizing of the conventional transformer is quite conservative and is based on
the peak load. As highlighted in [16], this results in a transformer oversizing in most cases (63% in the study mentioned). Only a few transformers have been adequately sized
or undersized.

Table 2. The main parameters of the MV side of the ST for different numbers of power units.
number of Units

Unit Power Level (kw)

QaB

DaB

QaB

DaB

number of
CHB Cells

MV dc
Link (kV)

IgBt Voltage
rating (kV)

333.33

111.11

3.4

6.5

Mean
IgBt Current
Current (a)
rating (a)
17.6

Cost
(US$)

150

12,402

166.67

55.56

1.7

3.3

19,404

111.11

37.04

1.13

1.7

6,480

83.33

27.78

0.85

1.2

5,760

66.67

22.22

0.68

1.2

7,200

IEEE PowEr ELECtronICS MagazInE

z June 2017

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - June 2017