generate active voltage vectors with device T5 controlled to generate zero voltage vector to freewheel the machine output current. This is juts one example of many dc/dc converter topologies that can be employed with proposed machine topology. With such dc/dc converters, full four quadrant operation can be achieved for motoring applications. Fixed Polarity DC Bus Bi -polar Variable DC V o ltage To Proposed Machine Topology T1 T2 T3 T4 T5 To fixed Voltage DC System + -
Fig. 1.2 DC/DC Converter Topology for Interfacing to Fixed DC Systems
1.5
Electrical Machines
Electrical machines can be classified into two main categories, asynchronous and synchronous machines. With asynchronous machines, the asynchronous nature of operation comes from the slip (speed difference required for torque production) be- tween the rotational speed of the stator field and that of the rotor field. On the other hand synchronous machines have special rotor construction that enables rotor field to rotate at the same speed i.e. in synchronization — with the stator field. Single fed (squirrel cage type) and doubly fed induction machines fall into the asynchronous machine category and feature machine excitation via the machine stator winding. Single fed asynchronous machines require converters rated to deliver rated torque and are widely used in industry [27]. Doubly fed asynchronous machines are rarely used nowadays with the exception of wind applications where Doubly Fed Induction Generators (DFIGs) are still used. DFIGs require smaller converter kVA rating which depends on the required machine speed control range. Owing to direct grid connection
1.5 Electrical Machines
of the machine stator and a fractionally rated converter, DFIGs have inherent poor grid fault ride through capability [28].
Switched reluctance, synchronous reluctance, permanent magnet, wound rotor syn- chronous machines fall into the synchronous machine category and all rely on separate rotor excitation. Switched reluctance machines have inherent disadvantages of high torque ripple, acoustic noise, vibration and low overload capability[29]. Synchronous reluctance machines suffer from torque ripple and poor power factor [30],[31]. For these reasons, both switched and synchronous reluctance machines will not be con- sidered in this work. Wound rotor synchronous machines are in wide use today for high power generation and motoring applications [32],[28]. Permanent magnet syn- chronous machines are gaining attention in low speed direct drive wind generators [33],[34]. However, the main drawbacks of permanent magnet machines are the pres- ence of cogging torque, the cost of permanent magnets and also the risk of permanent demagnetisation of the rotor magnets under fault conditions.
For purposes of highlighting the machines being considered in this work, the electrical machines have been grouped into two further categories depending on whether mechanical means of current commutation is employed or not, as depicted in figure 1.3. Brushed electrical machines include DC, wound field synchronous and wound rotor induction machines [35]. Brushless machines include squirrel cage induction, permanent magnet and reluctance machines which do not require mechanical means of current commutation [36]. The machine topologies proposed in this work are derivatives of the synchronous machine category highlighted in bold red in figure 1.3 and aim to address the key drawbacks of brushed electrical machines for high power applications by employing electronic means of current commutation as well as utilising multi-phase stator phase windings. Permanent magnet means of excitation is also applicable to this machine and converter topology.
1.5.1
Multiphase Electrical Machines
Conventional three phase machines are standard in industry and power generation units mainly driven by the need to connect to the grid power supply. However, in applications such as dc power systems where the generating units are decoupled from
1.5 Electrical Machines Electric Machines Brushed Brushless DC Wound Field Synchronous Wound Rotor Induction Squirrel cage Induction Permanent Magnet Synchronous reluctance Switched reluctance Other = Synchronous Machines = Asynchronous Machines
Fig. 1.3 Electrical Machine Categories
the grid, there is no requirement for adhering to the standard three phase machine topology as highlighted earlier. Increasing the machine phase number beyond three can yield significant benefits such as; (a) increased machine and converter fault tolerance, (b) reduced per phase VA rating for a given machine power, (c) reduced torque ripple and machine noise and vibration signature, (d) improved torque density by harmonic injection, (e) increased efficiency owing to the reduced lower order space harmonics [37], [38]. By increasing the machine phase number beyond three, machine active material utilisation factor is increased and machine harmonic performance is enhanced by the consequential decrease in detrimental low order space and time harmonics that give rise to undesirable torque pulsations and mechanical resonance issues [39], [40],[41], [42]. When stator phase number increases, the unwanted harmonics are pushed to higher orders where their amplitudes diminish in inverse proportion to their
order. The capacity of a machine can be expressed asS=N.V.I, where N= number
of phases, V & I are phase voltages and currents. It therefore follows that for a given machine rating, increasing the number of phases N, reduces the phase VA rating. Consequently, the paralleling and seriesing of power electronic devices to achieve higher phase VA ratings can be avoided, thereby eliminating problems associated with static and dynamic voltage and current sharing of the power electronics devices. Recent comprehensive reviews of multiphase machines are given in [43],[37]
Recently, there has been an upsurge of interest in multiphase machines for applica- tions such as electric ship propulsion, hybrid electric vehicles and aircraft generating systems [44], [45], [46], [47] to exploit these desirable characteristics. However, most of these converters are based on VSC/VSI power electronic topologies and suffer the afore mentioned fault current limitation issues when applied to dc power generation
1.5 Electrical Machines
and delivery systems. This research seeks to explore the use of current source based multiphase machine topologies for dc power generation and delivery.
For dc power generation and delivery systems to be fully accepted as an alternative to ac power generation and delivery, research efforts on this subject have to go beyond the generators and power electronics topologies. They must also address the knowledge gap and challenges in system level integration issues in dc systems. This knowledge gap is evidenced by the current lack of international standards on dc generation and delivery systems. Research on system architecture design and optimisation, energy flow control and system level modelling for such distributed systems with a large penetration of multiple power electronic converters is imperative. For example, the ability to assess the impact of issues such as; system response to dynamics caused by interactions between the power electronic converters (subsystems), system response to changes in power demand, system behaviour during non-linear events such as switching, load rejections, fault conditions and impact of negative incremental impedance characteristic of certain loads on the dc delivery system is paramount [48],[49].
A number of approaches to address these system level challenges have been published in [50] and others. Recent studies have shown that system level modelling and simulation provides a convenient means of studying the behaviour of these systems in great detail [51], [52]. A variety of methods have been reported in literature including detailed device level models of power electronic systems, small signal average models, large signal average models, generalised average models, exact models just to name a few [53],[48], [54],[55], [56]. Most of these can be implemented in continuous and discrete time formulations and other in frequency domain. Each of these modelling approaches has its own characteristic advantages and drawbacks depending on its intended purpose as summarised in [57]. For example, transient simulations using detailed switch models of power electronic systems will represent the converter PWM switching harmonics but such simulations require significant computer resources and long simulation times. When many power electronic converters are used in power systems, this simulation approach becomes impractical. On the other hand, small signal modelling techniques are useful in designing closed loop control schemes for power electronic converters. However, they do not lend themselves well to the full behavioural characterisation of a system for example during large system