Читать книгу Power Electronics-Enabled Autonomous Power Systems - Qing-Chang Zhong - Страница 17

As shown in Figure 1.1, this book contains this introductory chapter (Chapter 1) and five parts: Part I: Theoretical Framework (Chapters 2 and 3), Part II: First‐Generation VSMs (Chapters 4–14), Part III: Second‐Generation VSMs (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 , to physically construct the ghost of a (sinusoidal) signal and, further, the ghost of a system with sinusoidal inputs. Moreover, the reactive power of an electrical system is shown to be the real power of the ghost system with its input being the ghost of the input to the original system. This is then applied to define the reactive power for mechanical systems, completing the electrical‐mechanical analogy, and, furthermore, generalizing to any dynamic system that can be described by a port‐Hamiltonian (PH) system model, establishing a significantly simplified instantaneous power theory, referred to as the ghost power theory. This can be applied to any dynamic system, single phase or poly‐phase, with or without harmonics.

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. In addition to the conventional voltage regulation mode (or the ‐mode in short) and the direct control mode (or the ‐mode in short), a third operation mode, i.e. the voltage droop control mode (or the ‐mode in short), is introduced to the operation of the STATCOM. This allows parallel‐operated STATCOMs to share reactive power properly and provides more control flexibility. Chapter 13 presents an improved 1G VSM to make sure that its frequency and voltage always stay within given ranges. Furthermore, its stability region is analytically characterized according to system parameters so that the improved synchronverter can be always stable and converges to a unique equilibrium as long as the power exchanged at the terminal is kept within this region. Chapter 14 explains that the concept of inertia has two aspects of meaning: the inertia time constant that characterizes the speed of the frequency response and the inertia constant that characterizes the amount of energy stored. It is then shown that, while the energy storage aspect of the virtual inertia of a VSM can be met by storage units, the inertia time constant that can be provided by a VSM may be limited because a large inertia time constant may lead to oscillatory frequency responses. A VSM with reconfigurable inertia time constant is then introduced by adding a low‐pass filter to the real power channel. Moreover, a virtual damper is introduced to provide the desired damping ratio, e.g., 0.707, together with the desired inertia time constant. Two approaches are presented to implement the virtual damper: one through impedance scaling with a voltage feedback controller and the other through impedance insertion with a current feedback controller. A by‐product from this is that the fault ride‐through capability of the VSM can be designed as well.

Part III: Second‐Generation VSMs contains six chapters related to the 2G VSM. First, Chapter 15 reveals that the widely adopted droop control mechanism structurally resembles a PLL or the synchronization mechanism of synchronous machines, making droop control a potential technical route to implement SYNDEM smart grids. A conventional droop controller for inverters with inductive impedance is then operated to behave as a PLL, without a dedicated synchronization unit. Chapter 16 reveals the fundamental limitations of the conventional droop control scheme at first and then presents a robust droop controller to achieve accurate proportional sharing without these limitations for R‐, L‐ and C‐inverters. The load voltage can be maintained within the desired range around the rated value. The strategy is robust against numerical errors, disturbances, noises, feeder impedance, parameter drifts, component mismatches, etc. The only error comes from measurement, which can be controlled by using sensors with required tolerance. Chapter 17 shows that there exists a universal droop control principle for impedance having a phase angle between rad and rad and it takes the form of the droop control for R‐inverters. In other words, the robust droop control for R‐inverters is universal and can be applied to inverters with different types of impedance having a phase angle from rad to rad. Chapter 18 removes the PLL from the universal droop controller to achieve self‐synchronization without a PLL. Chapter 19 presents a general framework based on the universal droop control for a rectifier‐fed load to continuously take part in the regulation of grid voltage and frequency without affecting the operation of the DC load. As a result, such a load can provide a primary frequency response, excelling the FERC requirement on newly integrated generators to provide primary frequency response. It can automatically change the power consumed to support the grid, without affecting the normal operation of the load. This is a critical technology that prevents local faults from cascading into wide‐area blackouts via releasing the full potential of loads to regulate system frequency and voltage. Chapter 20 presents a current‐limiting universal droop controller to operate a grid‐connected inverters under both normal and faulty grid conditions without damage by adopting an advanced nonlinear control strategy. This is another critical technology that help prevent local faults from cascading into wide‐area blackouts, via maintaining connection without trip‐off when there is a fault unless itself is faulty.

Part IV: Third‐Generation VSMs contains Chapter 21, which briefly touches upon the third generation VSMs that are expected to be able to guarantee the stability of a power system with multiple power electronic converters. A generic control framework is presented to render the controller of a power electronic converter passive by using the PH systems theory and the ghost operator. The controller consists of two symmetric control loops and an engendering block. With the critical concepts of the ghost signal and the ghost system introduced in Chapter 3, the engendering block is augmented as a lossless interconnection between the control block and the plant pair that consists of the original plant and its ghost plant. The whole system is then passive if the plant pair is passive. Moreover, some practical issues, such as controller implementation, power regulation and self‐synchronization without a dedicated synchronization unit, are also discussed.

Part V: Case Studies contains four chapters. Chapter 22 describes a single‐node system implemented with a SYNDEM Smart Grid Research and Educational Kit, which is reconfigurable to obtain over 10 different topologies, covering DC/DC conversion and single‐phase/three‐phase DC/AC, AC/DC, and AC/DC/AC conversion. Hence, it is ideal for carrying out research, development, and education of SYNDEM smart grids. It adopts the widely used Texas Instrument (TI) C2000 ControlCARD and is equipped with the automatic code generation tools of MATLAB®, Simulink®, and TI Code Composer Studio™ (CCS), making it possible to quickly turn computational simulations into physical experiments without writing any code. The single‐node system is equipped with 2G VSM technology and additional functions so that it can autonomously blackstart, regulate voltage and frequency, detect the presence of the public grid, self‐synchronize with the grid, connect to the grid, detect the loss of the grid, and island it from the grid. Chapter 23 presents a 100% power electronics based SYNDEM smart grid testbed with eight nodes of VSMs connected to the same AC bus to demonstrate the operation of a SYNDEM smart grid. Experimental results are presented to show that the SYNDEM smart grid framework is very effective and all the VSM nodes, including wind power, solar power, DC loads, AC loads, and an energy bridge, can work together to collectively regulate the SYNDEM grid frequency and voltage, without relying on ICT systems for control. Chapter 24 presents a practical home grid based on the SYNDEM framework. It consists of four 3 kW solar inverters, one 3 kW wind inverter, and one 3 kW energy bridge for interconnection with the public grid. The home grid can be operated in the islanded mode or the grid‐tied mode if needed. Chapter 25 discusses the Texas Panhandle wind power system, which suffers from the severe problem of exporting the wind power generated to load centers far away. It is shown that the SYNDEM smart grid architecture and its underpinning technologies could remove the export limit imposed on the wind farms in Panhandle so that they can export the wind power generated at full capacity without causing problems to the grid.