Yingfeng Zhu, Ms. Yawen Li, Mr. An Zhao, Mr. Zhengyu Ye, and Mr. Ruizhe Wang, for developing the soft‐switching technology for the three‐phase converters. We acknowledge the tireless efforts and assistance of Wiley Press editorial staff.
Dehong Xu
Rui Li
Ning He
Jinyi Deng
Yuying Wu
Nomenclature
Subscripts
aphase Abphase Bcphase Cggate of switchmmodulationrresonancesswitching cycle
Superscripts
Variables
vvariable voltageVconstant voltage
1 Introduction
In this chapter, an overview of soft‐switching technology for three‐phase power electronics converters and its evolution are briefly introduced, and the challenges and trends in the soft‐switching three‐phase converters are discussed.
1.1 Requirement of Three‐phase Power Conversions
Three‐phase converters are widely used as grid connecting inverters for renewable energy systems, rectifiers, or inverters for uninterruptible power supply (UPS) for data centers, rectifiers for electrical vehicle (EV) fast charging stations, inverters for EV, inverters for industrial motor drives, etc. For these applications, power flow is quickly and accurately controlled because the power converter is composed of power semiconductor devices, which can be turn on or off within less than a microsecond. With the application of the converters, we can realize high efficiency power conversion between sources and loads or vice versa. In addition, if a converter system operates at high frequency, its volume or weight is reduced due to size reduction of passive components such as inductors, capacitor, electric motors, etc. The higher the switching frequency, the smaller are the passive components. Thus the power density, processing power per liter, of the converter is increased [1, 2]. In addition, the dynamics of converter systems are enhanced with increased switching frequency.
1.1.1 Three‐phase Converters
Three‐phase converters are used as either grid converters to connect the utility or inverters to drive a motor or supply high‐quality alternating current (AC) power to the load as shown in Figure 1.1. When a grid converter is used to convert the utility AC voltage into direct current (DC) voltage, it is usually called a rectifier. When it converts DC voltage to the grid AC voltage, it is usually called an inverter. Actually, it is the same entity, but it has two names. It is sometimes confusing for new learners. In most applications such as battery storage systems, the grid converter is required to control power flow bidirectionally. It can operate in either rectifier mode or inverter mode according to the system requirement. When a three‐phase converter is used to drive a motor or supply AC power to a load, it is usually called as an inverter since it converts the DC bus voltage into three‐phase AC voltages. In the book, a general name “converter” is used, which covers names such as rectifier, inverter, bidirectional converter that swaps between the inverter mode and the rectifier mode according to operation requirement.
Figure 1.1 Three‐phase converters: (a) grid converters; (b) inverter.
The three‐phase converter is one of the most important power conversion building blocks in power electronic systems. It is widely used in various applications due to its distinct advantages as follows:
It has the simplest converter topology that can realize DC/AC or AC/DC conversions with minimum number of power switches. It has lower cost.
It has lower voltage stress on the power devices because the maximum voltage on them is capped by the DC bus. It has lower current stress on the device for AC loads/sources to operate at current continuous mode (CCM).
There are well‐established pulse‐width‐modulation (PWM) control methods such as sinusoidal PWM (SPWM), third harmonics injected SPWM, space vector modulation (SVM), etc.
It has well‐established system control methods for the converter under abc static frame, αβ static frame, and dq synchronous rotating frame.
Because of these advantages, the three‐phase converters have been used almost everywhere from low power to high power such as disk drives, inverters for pumps, inverters for EV, drive inverters for bullet trains, solar inverters, wind turbines, UPS, etc. For these applications, there is an ever increasing demand for the performance of the converter. In addition to basic functions such as AC to DC or DC to AC conversion and output power quality, following demands are critical for the converter.
First, it is expected that the converter has higher efficiency, which has become a more stringent requirement than ever before due to increasing public concern of impact of energy consumption on the environment. Besides, high efficiency can also bring economic benefits to the users. A Photovoltaic (PV) inverter with high efficiency can harvest more electricity and also improves the utilization of the PV panel. A UPS with high efficiency can save the operating cost of data centers. It also can reduce the footprint of the power supply due to lower energy loss. An EV power train with high efficiency increases the utilization of the kWh of the battery and extends mileage at the same time.
Second, we hope that the converter has small size. It is especially critical for moving vehicles such as electric vehicles, electric railway, boats, and airplanes or aerospace applications. Smaller size means less use of material, copper, iron, isolation material, etc., which cuts down the cost. For users, it can reduce space, which is expensive in many large cities. Usually, you may hear the word power density. It means the ratio of power capacity to the volume or weight of the converter. It represents the ability of a converter to process the power at a given size. For a given power capacity of a converter, the higher the power density, the smaller
