Finite-control-set model predictive control
scheme of three-phase four-leg back-to-back
converter-fed induction motor drive
ISSN 1751-8660
Received on 14th December 2015
Revised on 6th April 2016
Accepted on 3rd May 2016
doi: 10.1049/iet-epa.2015.0617
www.ietdl.org
Dehong Zhou, Jin Zhao
✉
, Yang Liu
The Key Laboratory of Image Processing and Intelligent Control of the Ministry of Education of China, School of Automation, Huazhong
University of Science and Technology, Wuhan, People’s Republic of China
✉ E-mail: jinzhao617@163.com
Abstract: The three-phase four-leg back-to-back converter-fed induction motor drive with only eight switches has the
capability of variable-frequency speed control and bidirectional power flo w. It can provide the be nefit of higher
reliability and less cost in comparison with the full-bridge back-to-back converter. On the other hand, the four-leg back-
to-back converter can be utilised in fault-tolerant control to solve open-circuit fault occurring at both rectifier and
inverter leg in a full-bridge back-to-back converter. However, the deviation of the two capacitor voltages which will lead
to variation of voltage vectors in both amplitude and phase angle hinders its applications. This study proposes a
control scheme based on finite-control-set mo del predi ctive control to remedy this disadvantage. With the proposed
scheme, capacitor voltage deviation is suppressed. Bidirectional power flows and balanced input and output currents
are achieved. The effectiveness of the proposed scheme is verified by the experimental results presented.
1 Introduction
In the high-power industrial application, variable-frequency electric
motor drives are usually fed by voltage-source inverter supplied by
grid-tied rectifier to improve the power efficiency. The full-bridge
converter with 12 switches helps to achieve this goal. However,
the reduced-switch-count converter is desirable in industrial
applications where lower cost, higher reliability and less volume
are of importance [1–4]. The four-leg back-to-back converter,
consisting of an AC–DC and DC–AC converter with an
intermediate DC-link is an alternative one [1]. On the other hand,
this topology with a reduced number of switches can be adopted
in fault-tolerant control to solve open-circuit fault occurring in the
converter by hardware reconfiguration [5, 6].
The four-leg back-to-back converter is well-known for having
several disadvantages compared with full-bridge back-to-back
converter. First, in comparison with a full-bridge converter, the
lower output voltage bound is much higher in the grid side with
the same AC supply [7] while the DC-link voltage utilisation is
halved in the load side [4]. Second, the deviation of capacitor
voltage stemmed from the imbalanced current flowing through the
capacitor may destroy the balanced three-phase current in both
rectifier and load side, and the unbalanced grid and motor currents
degrade the power efficiency and reliability of the converter. Thus,
substantial works have been conducted to improve the
performance of the four-leg back-to-back converter.
Regarding the grid side, the previous works have been focused on
the elimination of the imbalanced input AC current [7, 8]. A
control-oriented model of the rectifier was built in the dq
synchronous frame [7]. However, the proposed transfer function
was developed under the assumption of equal voltage between the
two capacitors. A control strategy accounting for the effect of
capacitor voltage oscillation and deviation on the output voltage
was developed in [8]. However, its over-abundant control
parameters made it hard for industrial application. As for the drive
side, several papers focused on closed-loop control scheme were
published [9, 10]. However, the two capacitor voltages were also
assumed constant in these papers. In the four-leg back-to-back
converter, the grid and motor drive are inherently coupled. The
grid and motor drive should be controlled simultaneously to
achieve global satisfactory performance. An additional voltage
sensor with its signal processing blocks is also required to control
the deviation of capacitor voltage.
To mitigate the deviation of capacitor voltages and get high
current quality of the four-leg back-to-back converter, an
additional balance controller injecting additional current reference
was introduced to compensate the unbalance split capacitor
voltage [11]. As this controller design was based on the
steady-state model, transient performance of the current loop was
poor. Regarding to the potentially increased capacitor current, a
pulse-width modulation (PWM) technique was proposed to
minimise the capacitor current for applications in which the load
frequency was equal to the grid frequency with a similar level [ 12,
13]. However, as load frequency had to be equal with the grid
frequency, the potential applicability of the control scheme was
restricted to a relatively small proportion of the variable speed
drive market.
With the availability of fast computing digital signal processor
(DSP), model predictive control (MPC), especially the
finite-control-set MPC (FCS-MPC) [14], has been introduced into
real-time process, such as motor drives [15, 16], active front-end
rectifier [17, 18], multilevel inverters [19], back-to-back converter
[20], and other power electronic converter configuration [4, 6, 21].
However, there is no evidence of any previous attempt to utilise
FCS-MPC in conjunction with four-leg back-to-back converter-fed
induction motor drive.
This paper proposes a control scheme based on the FCS-MPC to
achieve high performance of the four-leg back-to-back converter-fed
induction motor drive. The deviation of the two capacitor voltages
can be dealt with by a single cost function without additional
controller. Bidirectional power flows and balanced input and
output current are achieved. The developed FCS-MPC scheme for
a four-leg back-to-back converter is described in detail and its
effectiveness is validated by the experimental investigation.
2 Four-leg back-to-back converter-fed induction
motor drive model
The general configuration of four-leg back-to-back converter-fed
motor drive is illustrated in Fig. 1. Two phases of grid and motor
are connected to the converter, respectively, while phase g3 of the
IET Electric Power Applications
Special Issue: Advances in Predictive Control of Variable-Speed Electric Drives
IET Electr. Power Appl., 2017, Vol. 11, Iss. 5, pp. 761–767
761
&
The Institution of Engineering and Technology 2016