rel="nofollow" href="#u2e2e15ec-6d18-5a44-9501-7082e37f53ab">Chapters 15–20), Part IV: Third‐Generation VSMs (Chapter 21), and Part V: Case Studies (Chapters 22–25). Most of the chapters include experimental results or real‐time simulation results, as indicated with a large or small triangle tag at the bottom‐right corner of the corresponding chapter box in Figure 1.1, and, hence, the technologies can be applied in practice with minimum effort.
Figure 1.1 Structure of the book.
In this introductory chapter, in addition to the outline of the book, the evolution of power systems is briefly presented to set the stage for the following five parts.
Part I: Theoretical Framework contains two chapters. Chapter 2 presents the SYNDEM theoretical framework for next‐generation smart grids – power electronics‐enabled autonomous power systems, covering the concept of SYNDEM smart grids, the rule of law that governs SYNDEM smart grids, the legal equality for all SYNDEM active players to equally take part in grid regulation, the architecture of SYNDEM smart grids, a brief description of potential technical routes, and the roots of the SYNDEM concept. Chapter 3 introduces a new operator, called the ghost operator
Part II: First‐Generation VSMs contains 11 chapters related to the first‐generation VSMs (synchronverters). Chapter 4 presents the synchronverter (1G VSM) technology to operate an inverter to mimic a synchronous generator (SG) after directly embedding the mathematical model of synchronous generators into the controller. The real and reactive power delivered by synchronverters connected in parallel can be automatically shared with the well‐known frequency and voltage droop mechanism. Synchronverters can also be easily operated in the standalone mode. Chapter 5 describes a control strategy to operate a PWM rectifier to mimic a synchronous motor. Two controllers, one to directly control the power exchanged with the grid and the other to control the DC‐bus voltage, are discussed. At the same time, the reactive power can be controlled as well. Chapter 6 applies the synchronverter technology to control the back‐to‐back PWM converters of PMSG based wind turbines. Both converters are operated as synchronverters. Different from the common practice, the rotor‐side converter (RSC) is controlled to maintain the DC‐link voltage and the grid‐side converter (GSC) is controlled to inject the maximum power available to the grid. Since both converters are operated with 1G VSM technology, the whole wind power system is able to support the grid frequency and voltage when there is a grid fault, making it friendly to the grid. Chapter 7 applies the 1G VSM to control the speed of an AC machine in four quadrants, via powering the AC machine with a VSM that generates a variable‐voltage‐variable‐frequency supply. This is a natural and mathematical, rather than physical, extension of the conventional Ward Leonard drive systems for DC machines to AC machines. If the rectifier providing the DC bus for the AC drive is controlled as a virtual synchronous motor according to Chapter 5, then an AC drive is equivalent to a motor–generator–motor system. This facilitates the analysis of AC drives and the introduction of some special functions. Chapter 8 takes a radical step to improve the synchronverter as a self‐synchronized synchronverter by removing the dedicated synchronization unit, which is often a phase‐locked loop (PLL). It can automatically synchronize itself with the grid before connection and maintain synchronization with the grid after connection without using a PLL. Experimental results show that this improves the performance of frequency tracking by more than 65%, the performance of real power control by 83% and the performance of reactive power control by about 70%. Chapter 9 discusses the removal of the dedicated synchronization unit from synchronverter based loads (rectifiers). Two controllers are presented: one is to directly control the real power exchanged with the grid and the other is to control the DC‐bus voltage. At the same time, the reactive power can be controlled as well. Chapter 10 reveals the analogy between differential gears and DFIG and then operates a DFIG based wind turbine as a VSM. An electromechanical model is presented to represent a DFIG as a differential gear that links a rotor shaft driven by a prime mover (the wind turbine), a virtual stator shaft coupled with a virtual synchronous generator and a virtual slip shaft coupled with a virtual synchronous motor. Moreover, an AC drive, which consists of an RSC and a GSC, is adopted to regulate the speed of the slip virtual synchronous motor. Then, the whole DFIG–converter system is operated as one VSG with the stator shaft synchronously rotating at the grid frequency, even when the rotor shaft speed changes, through controlling the virtual slip shaft to synchronously rotate at the slip frequency. Following the concept of the AC Ward Leonard drive systems discussed in Chapter 7, the RSC is controlled as a virtual synchronous generator to generate an appropriate slip voltage, via regulating the real power and reactive power sent to the grid. In order to facilitate this, the GSC is controlled as a virtual synchronous motor to maintain a stable DC‐bus voltage without drawing reactive power in the steady state. A prominent feature is that there is no need to adopt a PLL – either on the grid side or on the rotor side. The system is not only able to provide the kinetic energy stored in the turbine/rotor shaft as inertia to support the grid frequency, but it is also able to provide reactive power to support the grid voltage. Chapter 11 applies the 1G VSM to a transformerless PV system, which consists of an independently controlled neutral leg and an inversion leg. The presence of the neutral leg enables the direct connection between the ground of PV panels and the neutral line of the grid, removing the need to have an isolation transformer. This significantly reduces leakage currents because the stray capacitance between the PV panels and the grid neutral line (ground) is bypassed. Another benefit is that the voltage of the PV is only required to be higher than the peak value of the grid voltage, which is the same as that required by conventional full bridge inverters. The resulting PV inverter is also grid‐friendly. Chapter 12 applies the 1G VSM to operate a STATCOM as a synchronous generator in the condenser mode, without using a dedicated synchronization unit, such as a PLL.
