Читать книгу Power Flow Control Solutions for a Modern Grid Using SMART Power Flow Controllers - Kalyan K. Sen - Страница 15

Both authors have been involved in exploring various power flow controllers since the early 1990s. Kalyan Sen developed power electronics inverter‐based Flexible Alternating Current Transmission Systems (FACTS) models while working at Westinghouse where pioneering development of FACTS products took place. Note that a forced‐commutated inverter with a DC link capacitor is also referred to as a Voltage‐Sourced Converter (VSC). Being an active contributor through patents, publications, design, and commissioning of much‐advertised FACTS controllers since its inception in the 1990s, Kalyan has a first‐hand knowledge of specific applications where the inverter‐based controllers are the desirable solutions and where these solutions are not suitable at all. He has written an award‐winning technical committee paper on the modeling of Unified Power Flow Controller (UPFC) in the IEEE Transactions on Power Delivery. Mey Ling Sen explored an alternate approach to the VSC‐based FACTS Controllers that is cost effective while meeting functional requirements for most utility applications. This effort led to the concept of the Sen Transformer (ST). The ST is fundamentally different from the conventional transformer, in a sense that it uses three primary windings and nine secondary windings to create a compensating voltage that modifies the line voltage to be a specific magnitude and phase angle, whereas the conventional transformer only modifies the magnitude of the line voltage. As a result, by using an ST, the active and reactive power flows in the line can be regulated independently to maximize the revenue‐generating active power flow and minimize the reactive power flow while maintaining the stability of the line voltage.

Since 2002, Kalyan has traveled around the world as an IEEE Distinguished Lecturer and lectured in more than 150 places in 15 countries. When he gives a presentation on power flow controllers, his approach is to start from the basics and lead up to the advanced concept of VSC‐based FACTS Controllers and ST. His emphasis is based on real‐world experience in modeling, simulation, design, and commissioning. He was requested in many places to compile his lecture material in the form of a book, which resulted in the publication of Introduction to FACTS Controllers: Theory, Modeling, and Applications in 2009. At the inception of the FACTS development in the 1990s, the main concerns were the high installation and operating costs of the FACTS Controllers. Over the decades, the list of drawbacks has expanded to include component obsolescence, costly maintenance, lack of trained‐labor, impracticability of relocation and lack of interoperability. A desired feature of a Power Flow Controller (PFC) is that it is easily relocatable to wherever it is needed the most, since the need for power flow control may change with time due to new generation, load, and so on. Interoperability is desired so that components from various suppliers can be used, resulting in a global manufacturing standard, ease of maintenance, and ultimately lower cost to consumers.

The utilities are searching for a suitable power flow controller that offers its inherent features: simplicity, operational reliability, cost‐effectiveness, component non‐obsolescence, high efficiency, low maintenance, ease of relocation, and interoperability to meet their immediate needs to relieve grid congestion due to overload, peak load demand, and integration of renewable energy sources into the grid. The ST combines the best features of the FACTS controllers in terms of the ability to independently control active and reactive power flows while using time‐tested and reliable transformer/Load Tap Changers (LTCs) technology that are familiar to the utilities worldwide for almost a century. More on LTCs can be found in the book, titled On‐Load Tap‐Changers For Power Transformers: Operation, Principles, Applications and Selection, by A. Krämer, Maschinenfabrik Reinhausen, 2000.

Power transformers are the workhorses that make transmission and distribution of AC electric power possible. Transformers step up the generator voltage (e.g. 25 kV) to the transmission level (e.g. 345 kV) and step down to distribution level (e.g. 13.8 kV) and, finally, to household utilization voltage (e.g. 120/240 V). With the addition of an LTC under load, transformers can easily regulate voltage. Specialty transformers, such as Phase Angle Regulators (PARs), can also regulate phase angle of the line voltage. The ST can regulate both the voltage magnitude and the phase angle simultaneously; as a result, the active and reactive power flows through the line can be controlled independently as desired.

The primary goal of this book is to present the fundamentals so that readers can retain the information clearly in their minds and provide a meaningful input in the selection process of adopting a particular solution. The book describes various concepts that are applicable to electric power industries. The concepts can be applied using traditional non‐power electronics‐based solutions and modern power electronics‐based solutions or some hybrid of traditional‐modern solutions. The reason for the primary goal is that a particular solution becomes obsolete as time progresses; however, the fundamental concepts remain the same.

Early power flow controllers employed basic technologies, such as transformers, capacitors, and reactors for the compensating voltage injection into the line. Later designs used power electronics to achieve much greater flexibility and optimization through an independent control of active and reactive power flows. When the first generation of power flow controllers, based on power electronics VSCs, were built in the 1990s, the Gate‐Turn‐Off thyristor (GTO) was the forced‐commutated semiconductor switch of choice because of its availability in high power rating (4500 V, 4000 A) and its low forward voltage that resulted in low conduction loss. Early FACTS Controllers used VSCs with GTOs, switching once‐per‐cycle that resulted in the lowest switching loss and the lowest overall loss of about 1% of the rating of the VSC. These VSCs used special transformers to employ harmonic‐neutralized techniques and produced high‐quality AC waveforms without using filters. The inherent nature of a GTO is its relatively longer turn‐on and turn‐off times. More commonly used modern Pulse Width Modulation (PWM) techniques are based on instantaneous turn‐on and turn‐off of a switch. A voltage waveform that is created with a PWM technique consists of a fundamental component of interest and harmonic components, the dominant of which is related to the ratio of the switching and the fundamental frequencies. A higher switching frequency is desirable, because the higher dominant frequency requires a reduced‐sized filter. To keep the sum of turn‐on and turn‐off times of a GTO to be less than 1% of the switching period, it would result in only several hundred Hz of switching frequency. This would require a fairly large‐sized output filter to eliminate the unwanted low‐order harmonic components, generated by a force‐commutated inverter.

About a decade later, the VSC of choice started to use Insulated Gate Bipolar Transistor (IGBT)‐based PWM techniques. An IGBT offers shorter turn‐on and turn‐off times, which is less than 1% of the switching period that results in a switching frequency of several kHz. A lower switching period means a higher switching frequency and higher order harmonic components that are not of significant interest, since they do not generate significant amount of harmonic currents for two reasons; first, higher order voltage harmonic components are lower in magnitudes and second, the higher order voltage harmonic components “see” higher reactances for a given inductance. However, some filtering may still be needed, since switching frequency could not be increased to the desired level in some cases due to generation of excessive losses (3–4% of the rating of the VSC) as a function of the increased switching frequency. Another decade later, the topology of choice has become multilevel VSCs that do not need any harmonic filtering. While the topologies of VSCs are changing, so are the semiconductor switching devices. The upcoming switches are based on silicon carbide (SiC) and gallium nitride (GaN) for desirable reasons, such as high‐speed operation, which results in lower turn‐on and turn‐off times, thus lower switching loss, high‐temperature operation, lower cooling requirement, and smaller circuits for the gate drive and the snubber. A higher switching frequency creates a higher Electro‐Magnetic Interference (EMI), which requires the use of an additional EMI filter. The fact is that with various advances in the power electronics technology and semiconductor switches, the FACTS controllers become obsolete in a relatively few years and as a result, a one‐to‐one component replacement becomes impossible in 10–15 years. In the utility world where 45–50 years of equipment life is the norm, this means the entire power electronics inverter‐based FACTS installation may need to be replaced several times in those 45‐ to 50‐year period. In addition, simple maintenance requires highly skilled personnel that are not readily available. The global standard and interoperability do not exist due to a limited number of manufacturers. This is a highly expensive proposition perhaps two orders of magnitude more than a long‐lived and easily maintained transformer/LTCs‐based technology, such as ST.

Today’s power grid has evolved into integration of inverter‐based, renewable‐sourced, electricity generation from solar and wind farms, which are intermittent in nature. Therefore, traditional steady‐state power flow controllers, such as series‐connected reactor/capacitor concepts, need to be updated with an improved dynamic response. Additionally, increasing installation of roof‐top solar and its integration into a low‐voltage distribution network has altered the traditional voltage profile in the distribution network and increased the need for a bidirectional power flow controller when the renewable generation is not available. Therefore, all available solutions need to be considered for future needs, which has led to the concept of SMART Controllers.

A considerable amount of effort has been put into modeling various controllers. Modeling is the only approach, before any hardware construction, for the verification of the performance of any concept. The book includes models of many controllers, developed using a freely available Electro‐Magnetic Transients Program (EMTP) software package.

The book is divided into six chapters and three appendices. Chapter 1 presents the origin of power flow controllers and guides the reader to the selection process of a SMART Power Flow Controller (SPFC).

Chapter 2 is for anyone who would like to become familiar with the subject. It discusses various topics of the book in simple electrical engineering terms and corroborates the theory with relevant mathematics. The characteristic equations of various power flow controllers, including their equivalent compensating impedances, are developed. Using these equations, a set of example problems is given, which gives a quick back‐of‐the‐envelope calculation results without much effort. A figure of merit, called Sen Index, is defined for all the Power Flow Controllers (PFCs).

Chapter 3 presents the fundamentals of modeling in EMTP and explains the basic differences of modeling various PFCs, such as the Voltage‐Regulating Transformer (VRT), Phase Angle Regulator (PAR), Unified Power Flow Controller (UPFC), and Sen Transformer (ST). Following the Rough‐Order Magnitude (ROM) calculations performed in Chapter 2, using simple equations to characterize a power flow solution, the ROM results may need to be refined by employing the modeling techniques developed in Chapter 3. An example simulation of a series‐compensating voltage is shown to emulate a VRT, a PAR, and an Impedance Regulator (IR).

Chapter 4 presents the transformer‐based PFCs and gives some baseline examples for comparison with other PFCs in the following chapters. It is shown how a VRT and a PAR may be modeled by using a series‐compensating voltage.

Chapter 5 presents some early PFCs that use mechanical switches and set some baselines for comparison in the following chapters. It is shown how to model a virtual impedance that is equivalent to shunt‐connected and/or series‐connected inductive and/or capacitive compensators.

Chapter 6 presents the evolution of an ST and its wide variety of applications. The most up‐to‐date advancements in ST are described in this chapter. This includes various forms of two‐core designs. Also included is a new factory‐test method under full power.

Appendix A describes the operation of various items, such as (1) three‐phase balanced and unbalanced voltage, current, and power; (2) symmetrical components; (3) d‐q transformation; and (4) Fourier analysis. The reader will find it useful to see the industry techniques and the relevance of the theory and applications.

Appendix B presents the power flow control equations in a lossy line and compares the derived results from those in Chapter 2 for lossless lines. Simpler versions of these equations are derived in Chapter 2, considering the line resistance (R) is zero. These examples will be used as future references for those involved with PFCs. For the readers to recognize the importance of the equations and example solutions presented in Chapter 2, a list of all the “Examples” is placed at the end of Appendix B. Using the information received from Supervisory Control And Data Acquisition (SCADA) about the sending‐ and receiving‐end voltages (V_s and V_r) and active and reactive power flows (P_r and Q_r), other power flow variables, such as the power angle (δ), can be calculated for a known line impedance (Z = R + jX).

Appendix C presents a load flow study of the Chilean network, integrated with Sen Transformer, performed by Siemens PTI and sponsored by New York Power Authority.

Pittsburgh, Pennsylvania

Kalyan K. Sen

Mey Ling Sen