IEEE Electrification - September 2020 - 30

The dc bus system
is naturally
simpler than its
ac counterpart
thanks to the
absence of frequency,
phase angle, and
reactive power.

The propulsion drives in the ac bus
IEP systems are equipped with power
electronics-based converters, which
can be digitally controlled. The drive
comprises ac-dc and dc-ac converters
to convert fixed-frequency and voltage ac power to variable-frequency
and variable-voltage ac power for
speed and torque control of the propulsion motors. The diode front end
(DFE), consisting of a large transformer and diode bridge, is commonly
adopted as the ac-dc rectifier for propulsion drives. However, the uncontrollable DFE produces large current
harmonics thus dedicated harmonic
filters, in the case of larger line reactors, and a number of
capacitor are incorporated to meet the grid code because
the hotel loads are fed by the ac bus directly.

G

DG

System Schematic

The schematic of the dc bus IEP system is shown in Figure  2. The ac
power generated by the DG is immediately rectified into dc power for the
dc bus. The bus voltage is controlled
by the power electronic-based ac-dc
rectifiers on the DG side.
All loads need to be connected to
the dc bus via a converter. For ac
drives, only dc-ac inverters are
employed to convert the bus dc voltage to variable-voltage and variablefrequency ac voltage to control the
electric motors. The hotel loads of the dc system are fed by
dc-ac inverters in accordance with an isolation transformer for fixed-voltage and fixed-frequency ac supplies. The

G

G

CB

G

690-V, 60-Hz, 3-ph ac Bus
Propulsion
Transformer

Harmonics Filter

Bus Tie

400 V

DC Bus System Architecture

Analog Bus
Ferromagnetic Power System
(High Steel and Copper)

ac/dc

ac/dc

Hotel Loads

ac/dc

ac/dc

Digital Drive
Semiconductor Devices
(Silicon)

Propulsion
Motor

dc/ac

dc/ac

dc/ac

dc/ac

M

M

M

M

Figure 1. The ac bus IEP system schematics, using more steel and copper for ferromagnetic components. The control of ac bus and hotel
loads is analog. M: motor; G: generator.

30

I E E E E l e c t r i f i cati o n M agaz ine / SEPTEMBER 2020



IEEE Electrification - September 2020

Table of Contents for the Digital Edition of IEEE Electrification - September 2020

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IEEE Electrification - September 2020 - Cover1
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IEEE Electrification - September 2020 - Contents
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