Electric power system (EPS) is the infrastructure to generate, transmit, and distribute electricity. The existing EPS is the largest and most complex man-made system. The bulk electricity is generated in large power plants in the form of alternating current (ac) using synchronous generators (or alternators). The ac voltage is then boosted using transformers and transmitted via long transmission lines. After transmission, the voltage levels are decreased and the electricity is used for various industrial, commercial, and residential applications. Electrification has been the greatest engineering achievement of the twentieth century [1].
The existing EPS has a hierarchical structure with central power plants generating the electricity and sending to users through the transmission system. The power flow is unidirectional from Generation to Distribution via Transmission.
The EPS is highly interconnected: many generation and transmission systems are connected together to form a large pool of energy resulting in a highly reliable system. Maintenance of such a large interconnected ac system in terms of synchronized, stable operation, and protection of all components while preventing cascading failures is the everyday challenge of electric utility companies.
According to US Energy Information Administration (EIA) [2], about kWh of electricity was generated in United States in the year 2016.1 This caused a total emission of kg.2 Every kWh of electricity caused about 0.44 kg of emission. An average house in United States consumes about 900 kWh of electricity per month which corresponds to release of about 13 kg of per day, about 400 kg per month and about 4800 kg per year. This is a major problem with the existing EPS among several others summarized as follows.3
Renewable energy4 (from sources such as sun, wind, moving or stored water, etc.) can be converted to electricity. Only a small portion of the total energy in the solar rays reached to the earth is sufficient to supply our total energy demand [4, 5].
Dispersed or distributed generation systems are small generators interfaced with the distribution (low-voltage) or sub-transmission (medium-voltage) lines.
Geothermal generators tap to the earth heat at locations that are susceptible for it.
Biomass generators burn biological materials from nature to generate steam.
Fuel Cell technology uses chemical reactions to generate electricity (dc).
Technical challenges arise when high level of distributed generation is integrated.
There are also the regulatory and policy-related challenges which are not discussed here. For example, the regulation of possible ancillary services that the distributed generators can provide to the grid, given the wide range and variety of services that can be possible, is an ongoing challenge.
A microgrid (G) is a cluster of distributed energy resources (DERs) and loads that is connected to the grid at a single location [6]. The G can operate in grid-connected (GC) and in islanded mode. It may also operate in isolated mode without a grid connection. From our technical discussions, islanded and isolated conditions are often similar. Thus, the term standalone (SA) is used to describe this mode.
DER includes distributed generator, distributed storage, and distributed load [7].7 The G concept may be the key concept to address the aforementioned challenges.
Low rotating inertia of distributed generators is a possible concern. This will reduce the total inertia of the EPS and makes it susceptible to larger frequency swings. Low over-current capability and low over-load limit of power electronic switches are other issues that must be respected. For instance, a conventional induction motor requires high level of current to start.
In order to smooth down the variable generation of renewable sources, certain amount of nonrenewable distributed generation (such as diesel and gas turbine generators) and distributed storage resources should be included in a G. Distributed storage technologies include battery, ultra-capacitor, flywheel, pumped hydro, stored hydrogen, etc. Electronically controlled loads (e.g. active rectifiers and motor drives) may be considered among DERs as they can actively participate in the G performance control.
DERs are either directly coupled or use a power electronic converter (PEC) as the interface with the grid. A PEC converts the form of power (e.g. dc to ac) and controls the flow of power. Use of a PEC is an efficient and convenient way of converting and controlling the power extracted from the renewable sources [8, 9].
The existing grid is an ac grid. However, with the proliferation of PV and BES technologies and given the fact that many residential applications need dc power, a paradigm of dc distribution system or a hybrid grid (comprising both ac and dc distribution) has been taken into serious consideration lately [10].
Figure I.1 shows the general diagram of a DER which is interfaced using a PEC. The circuit breaker (CB) disconnects the local grid from the main grid when faults occur in the local grid. The G should not energize the local grid when CB is open. During the short-term transitory faults, the G must remain connected and support the local area grid. This is called the fault ride through capability of the G.
The Primary Side of the DER may generally be one of the following cases.
Responsibilities of the PEC may be listed as follows.
The PEC is an interface between the source side (primary source) of the system to the grid side. It is responsible for making this interface efficient and strong. It must be able to minimize the adverse impacts of source side disturbances on the grid side and also to minimize the adverse impacts of grid side disturbances on the source side. Source side disturbances are those such as fast and unexpected changes in the input power due to intermittent nature of the renewable source.8 Grid side disturbances are those such as grid voltage faults, distortions, and its frequency swings. In a more effective scenario, the DER provides grid-supporting and grid-stabilizing functions such as reactive power support and inertial response to reduce the grid transients.
Inverter9 (single-phase or three-phase) is often a voltage source converter (VSC) where its dc side is a voltage. Therefore, a capacitor is used to support the dc side voltage. Output Filter is responsible for smoothing the switching ripples and noises. In order to minimize losses, it should avoid (or minimize) using dissipative elements (such as resistors). Inductors and capacitors are used.
First Stage Converter is a dc/dc converter or simply a dc converter in PV applications. In this case, its job is normally to boost or buck the voltage and also to perform maximum power point tracking (MPPT). In Wind and Micro-turbine applications, it is an ac/dc converter (or rectifier) to convert an ac variable to dc. The dc converter allows the possibility of high-frequency galvanic isolation between the source side and the grid side as well.
Two-stage topology breaks down the control objectives so they can be addressed more efficiently by the two converters. It, however, requires more hardware which means higher cost and also lower total efficiency. The control objectives are basically the same as discussed above with the addition of controlling the dc link variable. dc Link is a capacitor (in a VSC).
Some DERs use a low-frequency transformer (after the output is generated) to adjust the generated voltage level to the grid level. Low-frequency transformers are bulky and not very efficient and the advanced converters avoid them by integrating their function inside the converter.
This book's overall objective is to bridge the gap between the power and the control aspects of a DER application. The power domain includes the primary source; the converter including its possible multiple stages, its dc link and its filters; the G; and the grid. The control part consists of the entire control system on the DER that controls the interaction of the DER with the rest of the system.
The book approaches its objective by deriving simple yet efficient models that describe the power and the control. The emphasis is placed on deriving models that lend themselves well to analysis and design. After deriving the models, the control objectives and specifications are comprehensively and clearly formulated. Finally, optimal and robust controllers are designed to address them.
The book reviews the fundamentals of power electronics including the introduction of the main power electronic elements, introduction of major converters mainly the VSC, and derivation of switching and control models for such converters. In addition to that, the control theory principles in both classical Laplace domain and in modern state space time domain are reviewed and some advanced optimal control design methods are explained which will form the basis for subsequent DER control designs.
The main body of the text is devoted to system analysis and control design of DERs. In order to respect the learning curve of the readers, the book starts off with simple cases where the control models and control objectives are not much demanding. The reader is walked through the analysis and design stages gradually while more and more control objectives are pulled in and addressed. Toward the end of the book, the reader will have a deep understanding of the full requirements pertaining to a desirable interaction of a DER with the grid. Advanced topics such as fault ride-through, grid support, weak grid conditions, grid forming versus grid following controllers, and inertial response are mathematically formulated and addressed.
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