Balance Scheme for DC Capacitor Voltage Between Different Phase
in Y-type Cascaded H-bridge STATCOM
Jiao-Tong Wang*, Lian-Cheng Xiu**, Jian Lin†, Di Liu***,
Hao Liang*, Lian-Song Xiong***
Abstract –The cascaded H-bridge STATCOM suffers the severe DC capacitor voltage imbalance
under asymmetric operating conditions, and the DC side voltage among phases is hard to keep
well balanced. The classic hierarchical control strategy, including the total active power control,
the total DC voltage balance control in different phase and the individual capacitor voltage balance
control in each phase, is introduced to address it. However, for the Y-type cascaded STATCOM, if
it is tied to the transformer and loads with ungrounded neutrals, the power exchange in different
phase to obtain the total DC voltage balance cannot be achieved naturally. In view of this problem,
this paper proposed that the unbalanced power in the system can be exchanged between the three
phases by injecting the zero-sequence voltage (ZSV) without affecting the external power output
characteristics, to ensure that the DC side voltage of each phase can keep balanced. This paper
calculated and compared the power in different phase when the phase voltage contains ZSV or not,
and hence we deduced the mathematical expression of ZVS for perfectly achieving the total DC
voltage balance in different phase. Finally, its detailed implementation in a simulation model was
also provided. Simulation results show that the proposed method is correct and effective.
Keywords: Cascaded H-bridge STATCOM, Hierarchical Control Strategy, Zero-Sequence
Voltage, DC Capacitor Voltage Balance Scheme
1. Introduction
The irrational allocation of reactive power will aggravate
the loss of electrical equipment and power cables, and the
impact of reactive power will also cause substantial changes
in the voltage of transmission and distribution network [1-3].
Cascaded H-bridge STATCOMs can compensate the reactive
power in-situ in medium and high voltage power distribution
systems, to improve the voltage stability of the Point of
Common Coupling (PCC) and the operation performance of
power distribution systems, which have received a great deal
of concern from academia and industry [3-6].
The DC side of the cascaded H-bridge STATCOM is
connected by several capacitors and each DC capacitor of
different H-bridge modules are isolated from each other.
Additionally, the cascaded STATCOM includes two main
modes of connection, i.e. Y type and Δ type [7]. At the same
voltage level, the Y-type cascaded STATCOM requires fewer
H-bridge modules and lower system costs, making them the
preferred topology for medium- and high- voltage
applications [8-10]. However, due to the difference of
switching loss, parallel loss, modulation ratio and pulse delay
of each H-bridge unit, there is a problem of DC capacitor
voltage imbalance among each H-bridge module. These
problems affect the quality of the inverter output waveform,
the active and reactive power output of the unit module, the
voltage and current stress of the switching device, as well as
the steady state accuracy and dynamic response speed of the
system, which greatly affects the performance of cascaded H-
bridge multi-level inverters. In the transient process,
especially during the asymmetric operation condition, the
STATCOM is very prone to the DC capacitor voltage
imbalance [11-14].
Currently, the DC capacitor voltage control strategy is mainly
divided into three categories: hardware, software and hybrid
methods. The hardware approach mainly provides additional
active power channels by additional hardware circuit that
compensates for the active power required by the modules.
The active power exchange is realized by using the PWM
rectifier, the diode rectifier, the isolation transformer and so
on [11-12]. The hardware method has good performance of
DC capacitor voltage balance, but its need of additional
† Corresponding Author : School of automation, Nanjing Institute of
Technology, Nanjing, China (591928389@qq.com)
* State Grid Ankang Power Supply Company, Ankang, China
** School of Mechanical and Electrical Engineering, Sichuan
Agricultural University, Chengdu, China
*** School of automation, Nanjing Institute of Technology, Nanjing,
China
1019
24th International Conference on Electrical Engineering June 24-28, 2018 | Seoul, Korea
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