This chapter presents the summaries of the publications and their scientific contribution.
Publications [P1]–[P2] present the modulation scheme and Publications [P3]–[P4] present the analysis of the distortion migration in matrix converters (MCs). Those publications are the basis for the comparison between the direct MC (DMC) and the indirect MC (IMC) presented in Publications [P2] and [P4]–[P7]. Next, the publications are summarised and what is new in them emphasised in italics.
Publication [P1]
Publication [P1] presents the conventional space vector modulation (CSVM) method using the output phase voltage space vector approach. It also presents the vector control of a cage induction motor (IM) in a rotor-flux-oriented reference frame, which is applied in the IMC prototype supplying a 2.2-kVA IM drive. The measurement results confirm that the IMC operates in motor drives as assumed: the currents are sinusoidal, but the nominal operating point of the standard motor cannot be achieved because of the limited voltage transfer ratio of the MCs. Incomplete experimental results in field-weakening speed-range operation show the importance of parameter variation when the MCs are operating near voltage limits. To the best of the author’s knowledge, Publication [P1] is the first publication presenting the CSVM applying the output phase voltage vector approach and the first publication presenting experimental results of the vector-controlled IM drive, supplied by the IMC. In addition, Publications [P1], [RP2] and [RP3] are the first publications in which a digitally implemented space vector modulator is applied in the IMC and in which a modulation scheme of the DMC is directly applied in the IMC.
Publication [P2]
Publication [P2] presents two comparative analyses concerning common-mode voltages. First, four different space vector modulation schemes of MCs are considered. Second, the DMC, the IMC and the three-level voltage source inverter with a supply-side diode bridge (3LVSC) are compared with the different MC modulation schemes. The analysis is verified in the Matlab Simulink simulations and partly in combined Matlab Simulink and Simplorer simulations.
The basic methods were also tested with both the MC prototypes and a 3LVSC apparatus, which confirmed the simulated results. As assumed, the maximum common-mode voltages of the converters can be minimised using a suitable modulation method. With a suitable method an MC produces a lower common-mode voltage maximum, but the voltage steps in the 3LVSC are always lower. No noticeable differences were found between the common-mode voltages produced by the DMC and the IMC. To the best of the author’s knowledge,
Publication [P2] is the first publication to compare space vector modulation methods for MCs to each other and to 3LVSC by minimising the common-mode voltage.
Publication [P3]
Publication [P3] presents the space-vector-based analysis of the migration of symmetric supply voltage harmonics to the output voltage in MCs. It also presents four computationally simple control methods based on the CSVM with and without supply voltage magnitude measurement and with and without load current closed-loop control. The methods are tested both by simulating with Simplorer models or models combining Simplorer and Matlab Simulink and by the measurements with the IMC prototype. The simulation and experimental results confirm the analysis and the analytical comparison of the control methods: the methods with measured supply voltage based modulation index calculation mitigate the distortion migration from the supply to the output more effectively than methods assuming ideal supply despite output current control. To the best of the author’s knowledge, Publication [P3] is the first publication to analyse the migration of the supply voltage distortion in MCs simply with space vectors and compares different methods extensively. It also introduces the IMC with distorted supply voltage and considers for the first time the capability of output current closed-loop control alone to mitigate the distortion migration.
Publication [P4]
Publication [P4] presents the analysis of migration of load current distortion to supply current in MCs in space vector form. The analysis also includes the passive elements of the main circuits. The analyses are verified by the simulations and experiments with the DMC and IMC prototypes, which are also the basis for the comparison between the DMC and the IMC. It is shown that the supply current distortion of the MC depends on the output current distortion, as assumed, and the real dc link of the IMC increases supply current distortion compared to the DMC. To the best of the author’s knowledge, Publication [P4] is the first publication to analyse the migration of the load current distortion in MCs with space vectors. It also compares the supply current quality between the DMC and the IMC for the first time.
Publication [P5]
Publication [5] presents an analysis of the voltage transfer characteristics of both the CSVM modulated DMC and IMC. The analysis is based on the modelling of the non-ideal characteristics caused by real semiconductor devices and safe commutation methods. The analysis is verified by the numerical solving of the average equations derived in Matlab Simulink and the experimental results of the permanent magnet synchronous motor drive supplied by both the DMC and IMC prototypes. As assumed, the DMC follows the output voltage reference more accurately than the IMC. Thus, the DMC is also found to be a more suitable converter solution for sensorless motor drives if no methods of compensating voltage inaccuracies are used. To the best of the author’s knowledge, Publication [P5] is the first publication to analyse the IMC voltage transfer characteristics and to compare these characteristics to the DMC.
Summaries and Contribution of the Publications 103
Publication [P6]
Publication [P6] presents the analysis and comparison of total semiconductor power losses in the DMC and the IMC. The average total loss models are derived on the basis of the semiconductor characteristics and the CSVM. The replacement of conventional IGBTs by reverse blocking IGBTs in the DMC is also considered. The calculated results of the loss models developed are compared to both the simulations with real semiconductor models in combined Matlab Simulink and Simplorer models and to the power loss measurements. The numerical solving of models gives more accurate results than the simulations. As assumed, the DMC losses depend mainly on the load current magnitude and the IMC losses depend on the active load power. Thus, the DMC has lower losses than the IMC in most cases. To the best of the author’s knowledge, Publication [P6] is the first publication to analyse and model the DMC and the IMC losses using simple but still accurate average loss models. It also presents the first comparison between DMC and IMC power losses.
Publication [P7]
Publication [P7] presents a comparison of the CSVM-modulated DMC and IMC in IM drive.
It also describes the implementation of the IM control system introduced in [P1] and introduces briefly the basis of the analyses in [P5]–[P6]. The operational simulations are performed with an ideal MC model in Matlab Simulink and their results are compared to the experimental results of both the DMC and IMC prototypes, which all also show satisfactory operation in field-weakening speed range unlike [P1]. The non-ideal behaviours are modelled as in [P5]–[P6]. As assumed, the effects of non-ideal characteristics are more severe in the IMC than in the DMC with most loading situations tested. To the best of the author’s knowledge, Publication [P7] is the first publication to compare the DMC and the IMC supplying an IM drive. For the first time, it also illustrates clearly the requirement to operate in field-weakening speed range with MC drives when it is required to attain the rated speed and load torque of the IM.
104
In this thesis, space-vector-modulated direct and indirect matrix converters have been studied.
Matrix converters are direct frequency converters, which are considered a solution for the input power quality, regeneration and dc link capacitor problems of the conventional diode-bridge-supplied two-level voltage source inverters. However, there are some problems with matrix converters: they contain more semiconductor components than conventional solutions and due to the lack of a dc link they are considered sensitive to supply and load disturbances.
The regeneration and the input power quality problems can also be solved with the active voltage source PWM rectifier bridge supplying the dc link capacitor instead of the diode bridge. In addition, an indirect matrix converter, which can be used to replace a conventional direct matrix converter, has not been studied much and has never been compared with the direct matrix converter.
One of the main objectives of the thesis, presented in Section 1.3, was to compare direct and indirect matrix converters. The comparison was based on the review of the matrix converter technology in Chapter 2 and on the space vector modulation in Chapter 3 based on [P1]–[P2].
Chapter 3 also presented a brief comparison of different space vector modulation methods concerning mostly common-mode voltages, which are a severe problem in PWM converter drive applications and could not be ignored [P2]. The general modelling of the matrix converters was presented in Chapter 4. The simulation results confirmed that in the ideal case there are no differences between direct and indirect matrix converters. The experimental setup, i.e. the prototypes of the direct and indirect matrix converters and measurement conditions, were presented in Chapter 5, in which the correct operation of both prototypes was also confirmed.
The second main objective of the thesis, presented in Section 1.3, was to study the effects of supply and load disturbances on matrix converter operation and how these effects could be mitigated. That was presented in Chapter 6, which comprised a summary of [P3]–[P4].
Chapter 6 presented the space-vector-form analyses of supply-to-load and load-to-supply distortion migrations and also included a comparison of computationally easy methods for mitigating the migration of supply voltage distortion to the MC output. It was found that the distortion migrates from supply voltages to load voltages and from load currents to supply currents as assumed by the space vector analyses. Thus, the distorted circumstances affect the capacity of matrix converters to generate undistorted output voltages and currents. It was also found that the migration of voltage distortion from the supply to the load can be mitigated significantly using measured supply voltage magnitudes in the calculation of the modulation index.
Conclusions 105 The third main objective of the thesis, presented in Section 1.3, was to analyse and compare the non-ideal characteristics of direct and indirect matrix converters. These were presented in Chapter 7, which comprised a summary of [P4]–[P7]. Firstly, the supply current distortions of direct and indirect matrix converters were compared and the reason for the difference was studied [P4]. Secondly, the average voltage transfer characteristics of direct and indirect matrix converters were analysed and compared [P5], [P7]. Thirdly, the semiconductor power losses of direct and indirect matrix converters were modelled and compared, taking into account both the effects of the modulation and the characteristics of the semiconductors [P6], [P7]. The main results of the comparison between the non-ideal direct and indirect matrix converter were
• The indirect matrix converter has lower supply current quality due to the small dc link capacitor set at the terminals of the load-side bridge in the indirect converter, whose voltage collides with the supply filter capacitor voltage in the switchings of the supply-side bridge [P4].
• The indirect matrix converter has higher voltage losses than the direct matrix converter in most operation conditions because real semiconductor components have different effects due to the different main circuit structures and safe commutation methods [P5].
• The indirect matrix converter has higher power losses than the direct matrix converter in most loading conditions, but when the output active power is low and output current is near the rated value, the indirect matrix converter has lower losses. However, the situation changes, depending on the semiconductors used in each case [P6]–[P7].
As presented above, the direct matrix converter was found to be more suitable than the indirect matrix converter for most operation conditions. However, the differences can be considered significant only in the case of semiconductor power losses. This thesis did not consider the possible economic advantages obtainable from using conventional voltage source inverter bridges in indirect matrix converters.
For future research, extended studies on matrix-converter-supplied machine drives are recommended. Above all, the question of the total optimum solution with a matrix converter and a machine should be solved. After that, it would be possible to design an optimum matrix converter drive for each case, which could then be compared with an optimum converter drive incorporating conventional technology. Thus, the optimum drive system for each specific case could be found. Future research in the field of semiconductor technology can also be expected to provide improvements on the current state-of-the-art. For example, reverse-blocking IGBT components could be an optimum semiconductor device for matrix converters, but they are still immature technology and need to be developed.
106
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