Y型串联H桥STATCOM中不同相间直流电容器电压的平衡方案。资源-CSDN文库

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Y型串联H桥STATCOM系统中相间直流电容器电压的平衡是一个关键的技术难题，尤其是在不对称操作条件下，直流电容器电压不平衡会严重影响设备的运行稳定性。本文主要讨论了如何通过注入零序电压(ZSV)来实现Y型串联H桥STATCOM相间直流电容器电压的平衡，同时保证不影响外部电力输出特性。文章介绍了级联H桥STATCOM的背景，它能够在中高压配电系统中进行无功功率的现场补偿，提高公共耦合点(PCC)的电压稳定性，并提升配电系统的运行性能。在这些系统中，直流侧的电压平衡是至关重要的，因为直流侧电压的不平衡会导致电容器寿命缩短、设备过热以及系统性能下降等问题。传统的级联H桥STATCOM面临的主要问题是，在不对称操作条件下，由于直流侧电压不平衡，各相间的直流电容器电压难以保持平衡。为了解决这一问题，提出了分层控制策略，包括总有功功率控制、各相间的总直流电压平衡控制以及每相的电容器电压平衡控制。但是，对于Y型连接的级联STATCOM而言，由于变压器和负载中性点未接地，各相间自然无法进行功率交换以实现总直流电压平衡。为解决这一难题，本文提出了一种新的方法：通过注入零序电压(ZSV)来交换系统中的不平衡功率。这种方法能够在不影响外部电力输出特性的情况下，在三相之间交换不平衡功率，以确保每相的直流侧电压保持平衡。文章详细计算了相电压包含ZSV与否时各相的功率，并推导出完美实现总直流电压平衡的ZSV数学表达式。最终，文章提供了一个仿真模型详细实施方法，仿真结果表明所提出的方法是正确且有效的。关键词包括级联H桥STATCOM、分层控制策略、零序电压以及直流电容器电压平衡方案。文章引言部分强调了无功功率合理分配的重要性。无功功率的不合理分配会增加电气设备和电力电缆的损耗，并且无功功率对电压的影响也会导致输配电网络电压发生显著变化。此外，无功功率的补偿可以有效减少电气设备的损耗，提高电力系统的功率因数和效率。在Y型级联H桥STATCOM的设计和操作中，需要特别考虑直流侧电容器电压平衡的问题。这不仅关系到设备的长期稳定运行，也影响到整个电力系统的性能和稳定性。通过本文提出的零序电压注入方法，可以实现Y型级联H桥STATCOM的直流侧电容器电压平衡，进而为电力系统提供更为可靠的无功功率支持。

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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

hardware circuit, high cost and lower efficiency make it less

applied. The software method uses feedback closed-loop

control and does not add additional hardware circuits [2-3].

However, it suffers from complex calculation and slow

response. To compromise the hardware cost and control

performance, the hybrid methods are also proposed [14-15].

Xiong et al [14] proposed a chopper controller-based DC

voltage control strategy for cascaded multilevel STATCOM,

whose major advantages are that the PI regulator quantity has

been reduced remarkably and accordingly the blindness of

system design and debugging also reduces obviously.

Generally, both the hardware and hybrid methods require

hardware circuits, which increase the cost and reduce the

reliability. The software method such as the classic

hierarchical control strategy can reduce the cost and the

volume size, which makes it become the focus of current

research [2-3]. However, for the Y-type cascaded STATCOM,

if it is tied to the transformer and loads with ungrounded

neutrals, the active power between phases cannot be

exchanged, and the average active power flowing into and

out of each phase is not zero, which lead to a gradual increase

in the sum of the capacitor voltages of one phase among the

three, and the system will exit owing to over voltage.

Therefore, the Y type cascaded STATCOM must consider the

capacitor-voltage balance control during asymmetric

operation, especially the capacitor voltage balance between

phases.

From the perspective of active power exchange between the

devices and the grid, this paper proposes a DC capacitor

voltage control method based on the classic hierarchical

control for the Y-type cascaded H-bridge STATCOM. The

simulation and experimental results show that the cascaded

STACOM can not only respond quickly to the reactive load

demand but also control the balance of DC capacitor voltage

swiftly and effectively, to ensure the safe and stable operation

of the devices.

2. Configuration of Cascaded STATCOM

The three-phase cascaded STATCOM comprise of Cas

caded Multilevel Inverter (CMI) in series with a linking

inductor L

(μ=a, b, c) [14], as illustrated in Fig.1.

Several identical H-bridge inverters compose the CMI and

the carrier phase shifted SPWM method is utilized [16]. In

each H-bridge, the DC-side capacitance is C

and the DC

capacitor voltage is u

dc_μk

(k=1, 2,…N). Ideally, the DC

voltage of each power unit is equal. R

dc_μk

represents the

equivalent loss of the power unit. Besides, u

sμ

, i

sμ

, i

lμ

and i

cμ

represent the grid voltage, the grid side current, the load

current and the output current of STATCOM. As shown in

Fig. 2, the double closed-loop decoupling control strategy in

d-q frame is selected as the basic control frame of the

developed control strategy in this paper. This paper focuses

the DC capacitor voltage control. Besides, the typical current

decoupling feedback control in Fig. 2 is shown in Fig. 3 [17-

18]. The real-time phase of the grid voltage is obtained by the

Synchronous Reference Frame based Open-loop Phase

Locking (SRF-OPL) scheme proposed in [19-24], which can

effectively cope with the grid phase disturbances. The

obvious advantages of this method are simple structure, easy

implementation and fast response.

_1dc a

_2dc a

_dc aN

_1dc b

_2dc b

_dc bN

_2dc b

_1dc b

_1dc c

_2dc c

_dc cN

_2dc c

_1dc c

Fig. 1. Layout of cascaded H-bridge STATCOM

Fig. 2. Double closed-loop decoupling control.

Fig. 2. Decoupling feedback control of balanced system.

1020

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