Power Engineering

Advances and Challenges Part B: Electrical Power

Viorel Badescu, George Cristian Lazaroiu, Linda Barelli

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9780367780586
• eBook ISBN: 9780429843532
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Biology

Traditionally, power engineering has been a subfield of energy engineering and electrical engineering which deals with the generation, transmission, distribution and utilization of electric power and the electrical devices connected to such systems including generators, motors and transformers. Implicitly this perception is associated with the generation of power in large hydraulic, thermal and nuclear plants and distributed consumption. Faced with the climate change phenomena, humanity has had to now contend with changes in attitudes in respect of environment protection and depletion of classical energy resources. These have had consequences in the power production sector, already faced with negative public opinions on nuclear energy and favorable perception of renewable energy resources and about distributed power generation. The objective of this edited book is to review all these changes and to present solutions for future power generation.

Future energy systems must factor in the changes and developments in technology like improvements of natural gas combined cycles and clean coal technologies, carbon dioxide capture and storage, advancements in nuclear reactors and hydropower, renewable energy engineering, power-to-gas conversion and fuel cells, energy crops, new energy vectors biomass-hydrogen, thermal energy storage, new storage systems diffusion, modern substations, high voltage engineering equipment and compatibility, HVDC transmission with FACTS, advanced optimization in a liberalized market environment, active grids and smart grids, power system resilience, power quality and cost of supply, plug-in electric vehicles, smart metering, control and communication technologies, new key actors as prosumers, smart cities. The emerging research will enhance the security of energy systems, safety in operation, protection of environment, improve energy efficiency, reliability and sustainability.

The book reviews current literature in the advances, innovative options and solutions in power engineering. It has been written for researchers, engineers, technicians and graduate and doctorate students interested in power engineering.

CHAPTER 1

New Developments of Electrical Machines

Qingsong Wang¹ and Shuangxia Niu²

¹ HJ810, The Hong Kong Polytechnic University. Email: [email protected]

² CF623, The Hong Kong Polytechnic University. Email: [email protected]

1. Introduction

Electrical machines can be divided into DC machines and AC machines, which are fed with DC current and AC current, respectively. Since brushes or slip rings are needed in DC machines, their relatively low reliability makes the DC machines less competitive. The AC machines can be further divided into induction machines and synchronous machines. Although induction machines are cheap and easy to manufacture, synchronous machines are more attractive due to their high efficiency and precise speed control. Currently, synchronous machines have a wide range of applications and have been extensively investigated by both academics and within the industry. In a synchronous machine, there are two rotary magnetic fields, namely the excitation field and the armature field. The electromagnetic torque is generated through the interaction of these two fields. The armature field can only be excited with the armature winding current, while the excitation field can be produced in various ways. When field winding is used to generate the excitation field, which is referred to as electrically excited machine (EEM), the air-gap field can be easily regulated by controlling the field current and therefore the EEM can operate over a wide speed range. However, since the field current will inevitably introduce additional copper loss, the efficiency of the EEM is reduced and the heat generated by the copper loss may cause a problem of heat dissipation. Permanent magnet (PM) machines can solve the aforementioned problems, in which the excitation field is generated by the PMs. High torque density and high efficiency can be achieved in PM machines when high-magnetic-energy-density PM materials are used. The major drawback of the PM machines is that the air-gap flux is difficult to control due to their fixed excitation and their speed range is limited accordingly. In order to achieve high torque density and high efficiency while still maintaining good flux regulating capability, hybrid excited machines (HEMs) are proposed these can be regarded as combinations of EEMs and PM machines (Wang and Niu 2017). The excitation field in the HEM is provided by a primary PM excitation and a secondary field coil excitation source. Since HEMs theoretically have good overall performances, they are being widely studied by researchers.

Although electrical machines were invented more than 100 years ago, their design, manufacturing, testing, condition monitoring, and control techniques are still in permanent evolution. One of the hot research topics in the field of electrical machines is to develop novel machine concepts with better performances. The purpose of this chapter is to present the latest developments of electrical machine concepts. Each of them has some advantages in certain aspects. It should be based on the real application requirements to determine which kind of machine to be used.

2. Magnetic-geared Machine

The co-axial magnetic gear (MG) was first proposed in (Atallah and Howe 2001) and shown in Fig. 1, which has three components, namely the inner rotor and the outer rotor, both of which are surface mounted with PMs, and the middle modulation ring comprises of ferromagnetic segments. The middle modulation ring is the key component of the MG, which can provide a flux modulation effect with the uneven magnetic reluctance distribution. The number of modulation segments N_s, the pole-pair number (PPN) of inner rotor PMs p_i and the PPN of outer rotor PMs p_o are governed by

$N_s=p_i+p_o$

Keeping either one of the three components stationary, the other two components can work well as a MG. If we keep the middle modulation ring stationary, and let the inner rotor be the driving input while letting the outer rotor be the driven output, the gear ratio G_r can be expressed as

Fig. 1: Configuration of the co-axial MG (Atallah and Howe 2001).

$G_r=\frac{p_o}{p_i}$

Since all the PMs are involved in the torque transmission, the MG can achieve high torque density. Meanwhile, as there is no mechanical contact between the input and the output, the MG can operate with reduced acoustic noise, less vibration, and free lubrication except with the ball bearings. By working in a slip mode, the MG also features an intrinsic overload protection, which can avoid further damage to the transmission system.

By directly connecting the outer rotor of a PM machine and the inner rotor of a MG, a MG integrated PM machine is invented (Chau et al. 2002), as shown in Fig. 2(a). The electromagnetic torque generated by the PM machine can be amplified, and the inner cave space of the MG can be fully utilized, and therefore this MG integrated PM machine can achieve a very high torque density. The applications in EV propulsion and wind power generation are separately reported in (Chau et al. 2002) and (Jian et al. 2009), respectively. The major drawback of this MG integrated PM machine is its complicated mechanical structure. As can be seen from Fig. 2(a), there are three air-gaps in this MG integrated PM machine, which makes it very difficult to assemble. Figure 2(b) shows another configuration of magnetic-geared machine (Wang et al. 2009), in which the outer rotor of PM machine and inner rotor of MG in Fig. 2(a) are eliminated. The armature field is directly coupled with the PM excitation field and modulated by the modulation ring. This magnetic-geared machine has just one PM layer, and is therefore referred to as a single-layer magnetic-geared PM (SL-MGPM) machine, and it can also be regarded as replacing the inner rotor of MG with a stator. Compared with the MG integrated PM machine shown in Fig. 2(a), the SL-MGPM machine in Fig. 2(b) has much simpler structure and is easy to manufacture.

Fig. 2: Configurations of magnetic-geared machines. (a) MG integrated PM machine (Chau et al. 2002). (b) SL-MGPM machine (Wang et al. 2009).

The concept of MG can be further extended to linear machines. Through integrating a tubular linear PM (TLPM) machine and a tubular linear MG (TLMG), a TLMG integrated PM machine (Du et al. 2010) is constructed as shown in Fig. 3(a). The principle of this TLMG integrated PM machine is very similar to that of the magnetic-geared machine shown in Fig. 2(a), and can be used in tidal energy conversion. The TLPM machine can be designed with high power density, and its thrust force can be amplified by the TLMG. The major drawback of this TLMG integrated PM machine is its complicated structure. Figure 3(b) shows another configuration of the tubular linear magnetic-geared machine with sandwiched stator, which is referred to as a sandwiched stator tubular linear magnetic-geared (SS-TLMG) machine (Ho et al. 2015). The stator housed with windings is sandwiched between the high-speed mover and low-speed mover. The high-speed mover and the stator work as a linear PM machine, while the high-speed mover, the stator and the low-speed mover function like a linear MG. Compared with the TLMG integrated PM machine shown in Fig. 3(a), the structure of the SS-TLMG machine shown in Fig. 3(b) is more compact and simple, and thus improved force density.

Fig. 3: Configurations of linear magnetic-geared machines. (a) TLMG integrated PM machine (Du et al. 2010). (b) SS-TLMG machine (Ho et al. 2015).

3. Stator-PM Brushless Machine

PM machines are predominant in the industry applications due to their high torque density and high efficiency features. Currently, most of the PM machines locate the PMs on the rotor, which are referred to as rotor-PM machines, that can achieve good overall performances. However, the rotor-PM machines may suffer from thermal problems since the heat generated by the rotor is difficult to dissipate, and cause demagnetization of PMs. Meanwhile, when the rotor-PM machines run at high speeds, the PMs should be protected from the centrifugal force by employing a retaining sleeve. Stator-PM machines can solve these problems, in which the PMs are located on the stator and the rotor is robust with only salient poles. The PMs will not suffer from demagnetization risk since the stator is easy to cool. Basically, stator-PM machines can be categorized into three types, namely, doubly salient PM (DSPM) machine, flux switching PM (FSPM) machine and flux reversal PM (FRPM) machine.

A. DSPM Machine

DSPM machine is oriented from the switched reluctance machine, in which the PMs are inserted into the stator back iron and the armature coils are concentrated wound on the stator teeth. One typical configuration of the DSPM machine is shown in Fig. 4 (Lin et al. 2003). The stator has 12 slots and the rotor has 8 poles. The PM flux linkage increases when the rotor pole rotates close to the corresponding stator tooth, and decreases when the rotor pole rotates away. By specifically design of the arc of stator teeth and rotor poles, the PM flux linkage can linearly varies with the rotor position, as shown in Fig. 5(a). In this case, trapezoidal back-EMF waveforms can be obtained and the DSPM machine is suitable for brushless DC control. A rotor-skewing method is reported in (Ming et al. 2003), which can obtain quasi-sinusoidal back-EMF waveforms as shown in Fig. 5(b) and the DSPM machine can operate in brushless AC mode.

As the PMs are located on the stator, DSPM machines can easily achieve hybrid excitation function. Figure 6 shows several hybrid excited DSPM (HE-DSPM) machines, in which the flux excited by the PMs and field coils are in series. The PMs are radially magnetized in the primary topology shown in Fig. 6(a) (Leonardi et al. 1996). A large field current is needed to realize a good flux regulation, because the flux generated by the field current should pass directly through the large reluctance PMs, which may also result in demagnetization of the PMs. HE-DSPM machine with magnetic...

Table of contents

• Cover
• Title Page
• Copyright Page
• Table of Contents
• Preface
• 1. New Developments of Electrical Machines
• 2. Onshore and Offshore Wind Energy Applications
• 3. Failures and Defects in PV Systems: Review and Methods of Analysis
• 4. Tidal and Wave Power Systems
• 5. Electrical and Electrochemical Energy Storage Applications
• 6. Modern Substation Technologies
• 7. Power System Stability
• 8. Electricity Markets Operation with Renewable Energy Sources
• 9. Integrating Community Resilience in Power System Planning
• 10. Developments in Power System Measurement and Instrumentation
• 11. High Capacity Consumers Impact on Power Systems
• 12. Grid-Edge Voltage Control in Utility Distribution Circuits
• 13. Large Data Analysis for Advanced Grids
• 14. Power Electronic Application for Power Quality
• 15. Voltage Sag Reporting and Prediction
• 16. Domotics
• 17. Demand Response
• 18. Smart Agent and IoT Towards Smart city
• Index