Power Electronics-Enabled Autonomous Power Systems. Qing-Chang Zhong

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.

1.3 Evolution of Power Systems

      Electricity is the workhorse of the modern world and has been in existence for over 100 years.

      1.3.1 Today's Grids

A power system today typically consists of facilities to generate, transmit, distribute, and utilize electrical power (Karady and Holbert 2004). Power plants generate electricity from energy sources, such as fossil fuels, hydro, nuclear, etc., and are often far from load centers because of associated pollution and risks or geographical limitations. In order to reduce losses and investment costs, the generation of electricity is currently dominated by centralized large power plants and the electricity generated is transformed into high or ultra‐high voltage for transmission and then transformed to low voltage for utilization at load centers. At the transmission level, there are often interconnections in order to form a strong grid, to which massive power plants are connected. Distribution networks are often radial. Generation facilities and loads are generally separated by transmission and distribution networks and electricity normally flows unidirectionally from generation