Soft-Switching Technology for Three-phase Power Electronics Converters. Rui Li

      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 is the size of the converter.

      Third, the converter is required to have good dynamics. The dynamics of a converter mainly depends on bandwidth of the close loop control systems. It is mainly constrained by the switching frequency of the converter. Generally speaking, power electronic converters have high dynamics since they use power semiconductor devices as the switch. In many applications such as ultra‐high‐speed pumps and compressors in industrial applications, power generation for aeronautics, EV, etc., it is required that the converter drives the motor to reach ultra‐high speed from tens to hundreds of thousands rpm. In some applications such as moving vehicles, the size of the electric machines can be reduced by increasing their operating frequency. To increase the fundamental frequency of the converter, it is natural to increase switching frequency. What is the limit of the switching? We will discuss it in detail later.

      Another demand is lower cost. The cost of a converter or a converter system should be optimized. A converter system is generally composed of power semiconductor devices and passive components. Size of the passive components depends on operating frequency. If we can increase the frequency, their size can be reduced.

      Actually, switching frequency is a critical parameter for the converter. It plays an important role in efficiency, power density, dynamics, and cost reduction.

      1.1.2 Switching Frequency vs. Conversion Efficiency and Power Density

In this section, we will discuss the effects of the switching frequency on the converter. First, an inverter used for UPS is taken as an example. The main circuit of UPS is back to back (BTB) converter as shown in Figure 1.2. It is composed of three‐phase grid converter as the rectifier cascaded with a three‐phase converter as the inverter to provide high‐quality power to the load. To satisfy the load requirement, output voltage of UPS needs to be an almost sinusoidal waveform. Its output voltage quality is usually described by total harmonic distortion (THD). Filters are installed in load sides. In addition, to satisfy the grid standard, filters in utility side are also installed. Filter cost ranges from 15 to 30% of total material expenditure in the UPS (excluding the battery cost). Besides, it also occupies a large footprint. Size of the passive components depends on operating switching frequency. If we can increase the switching frequency, their size will be reduced.

Figure 1.2 Circuit diagram of UPS.

      It is assumed that a UPS equipment has rated power 100 kVA at load power factor PF = 0.8. Both the input grid phase voltage and output phase voltage are 220 Vrms. Internal DC bus voltage is 750 Vdc. THD of output voltages is less than 5%. Inductor‐capacitor filters are