Keywords: Multi-phase Drives Matrix converters Multilevel converters Renewable Energy Field oriented control of ac machines Diagnosis of Electrical Drives

- Control techniques for AC machines for operation in a wide speeed range.
- Control techniques for multiphase machines.
- Modulation techniques for multilevel converters of dual two-level topology.
- Modulation techniques for matrix converters.
- Wound rotor synchronous machines for industrial traction
drives

- Diagnostic technique based on rotor modulating signals signature analysis for doubly fed induction machines in wind generator systems.

1. Control techniques for AC machines for operation in a wide speeed range

When the induction motors are used for applications at high speed, it is desirable to retain the maximum torque capability in the field-weakening region. The torque capability of an induction motor is limited by the maximum current and the maximum voltage the inverter can apply to the motor. Several papers were presented in order to achieve the maximum torque capability of the machine over the whole field-weakening region. According to these field-weakening algorithms, the optimal flux value of the motor should be updated by means of look-up tables or explicit expressions containing the motor parameters and quantities such as the motor speed, the motor currents, the dc-link voltage and the requested torque. However, the performance of these algorithms is strictly related to the accuracy by which the parameters are known. For these reasons the stator-flux-oriented drive, more insensitive to parameter variations than the rotor-flux-oriented one, has received an increasing attention for field weakening applications. In particular, a robust method for field weakening operation of DTC induction motor drives has been developed, where the flux reference is adjusted on the basis of the torque error behavior. In fact, a suitable method for robust field weakening is to determine the optimal flux level using closed-loop schemes that analyze the motor behavior, rather than look-up tables or explicit expressions containing the motor parameters.

A further control method has been analyzed, in which the flux is adjusted on the basis of the supply voltage requested by the regulators. If this voltage is greater than the available one, the field-weakening algorithm reduces the flux. Furthermore, employing a suitable voltage control strategy allows the motor to exploit the maximum torque in the whole speed range. In the proposed rotor-flux-oriented control scheme the main control variables are the stator flux components instead of the stator current components. This basic choice simplifies the control scheme, exhibits a fast torque response and reduces the number of PI regulators. In addition, the proposed scheme allows the motor to exploit the maximum torque capability in the whole speed range.

2. Control techniques for multiphase machines

The conventional approach for a variable-speed drive is to supply a three-phase motor with a three-phase inverter. However, since variable-speed drives are invariably supplied from power electronic converters, the number of phases does not have to be equal to three any more and it can be considered as a design variable. The use of multiphase inverters together with multiphase ac machines has been recognized as a viable approach to obtain higher power ratings with current limited devices, by reducing the stator current per phase without increasing the voltage per phase. Furthermore, multiphase motor drives have several advantages over the traditional three-phase motor drives such as reduction of the amplitude and increase of the frequency of torque pulsations, and an improvement of the fault tolerance.

To fully exploit the potential of M-phase motor drives, a suitable and flexible modulation strategy for M-phase voltage source inverters (VSIs) has been defined using the duty-cycle space vector (DCSV) representation. Basically, this representation describes the state of the switches by means of complex variables. Using the DCSV representation, it is possible to combine the multiple space vector representation, useful in modeling multiphase machines, with traditional carrier based PWM principle, suitable for the modulation of multiphase VSIs.

The research activity has been also focused on the problem of the inverter output voltage limit, or in other words the capability of a multiphase inverter to generate simultaneously the required magnitudes of the multiple space vectors. The determination of the output voltage limit in multiphase inverters represents a difficult task, owing to the large number of inherent degrees of freedom involved in the description of a multiphase system of voltages. An explicit solution for the voltage limit of a seven-phase inverter has been determined, leading to a set of inequalities that link the magnitudes of the multiple voltage space vectors. These inequalities are valid for any value of the phase angle of the voltage vectors. The effectiveness of the proposed modulation strategy and the validity of the voltage limit analysis have been verified by experimental tests.

3. Modulation techniques for multilevel converters of dual two-level topology

The multilevel inverter technology has been widely recognized as a viable solution to overcome the voltage limits of power switching converters in the area of high power medium-voltage drive systems. The research activity has been focused on the dual two-level inverter configuration, which has received large attention due to the simplicity of the power stage. It is based on two standard three-phase voltage source inverters supplied by two separate dc sources. The presence of two insulated dc supplies inherently eliminates common-mode currents and makes it possible to achieve the maximum output voltage without the need of a common-mode reactor. Furthermore, the dual inverter topology has a high reliability because in case of fault in one inverter, its output terminals can be short-circuited, and the system can operate using the healthy inverter as a standard three-phase two-level inverter. The proposed dual two-level inverter configuration can be usefully implemented when using a battery supply system because, in this case, it is very simple to split the dc supply into two electrically separated dc sources. Being the converter supplied by two distinct sources, in several applications, it is necessary to regulate the power flow from the two sources. This requirement can be demanded in order to equalize the state of charge of two banks of batteries

or to exploit the different characteristics of two sources. A new modulation technique has been developed, which is able both to perform multilevel operation and to regulate the load power sharing between the two dc sources within each switching period.

4. Modulation techniques for matrix converters

The research on matrix converters has been carried out either from a theoretical or experimental point of view, and in cooperation with research groups of Aalborg (DK) University and Nottingham (UK) University. The analysis has been focused on SVM control, improvement of the input current quality and identification of critical operating conditions for matrix converters.

A new approach for the analysis of matrix converters has been proposed, which leads to a general solution for the determination of the optimum modulation technique. This approach is based on the use of space vectors for the representation of the matrix converter duty-cycles.

In addition, a method to analyse the behaviour of system including a matrix converter has been developed to study possible unstable operating conditions.

Two other topics have been developed, which are interesting mainly from an applicative point of view:

- determination of a new commutation strategy for matrix converters, named "3-step commutation", based on the measurements of the input voltages and the output currents. It allows to avoid the uncertainty concerning the commutation instant of the current between two switch and therefore guarantees a better linearity of the voltage transfer ratio;

- comparison between the performance of a matrix converter and a back-to-back converter, in terms of maximum output power delivered to the load and maximum output power per switch, taking the thermal stress of the switches into account due to the switching and conducting losses.

5. Wound rotor synchronous machines for industrial traction drives

An electric drive based on the wound rotor synchronous machine (WRSM) has been developed and proposed for the use in traction systems of heavy-duty electric vehicles. While the industry in this field was massive introducing induction machine IM for the traction system of industrial vehicles, the scientific community was proposing to use the most elegant permanent magnet synchronous machines PMSM for its best performance (efficiency and torque density). The problem was that all the experimentations with PMSM failed due to the high cost or to reliability problems of this drive system. The solution based on the WRSM could match industry needs with a product, which is simple and economic to be manufactured but having performance comparable to that of a PMSM. This result was obtained through the development of a completely new motor control algorithm, which maximize the performance of this machine in any operating conditions.

6. Diagnostic Technique based on Rotor Modulating Signals Signature Analysis

In a wind generator turbine, the doubly fed induction machine (DFIM) operating in variable-speed constant frequency mode is widely used mainly because of the lower rating of the power converter connected to the rotor side. The vector control strategy is based on the regulation of the rotor currents by a back-to-back converter connected between the mains and the rotor. In power-generation systems, it is interesting to detect incipient faults as soon as possible in order to minimize maintenance cost and to prevent unscheduled downtimes by using advanced online diagnostic techniques for electrical and mechanical faults. In fact, in distributed generation, predictive maintenance has become an unavoidable tool in order to improve the gain in energy market. Many diagnostic systems have been presented for wind turbines using DFIM. However, up to now, it is very difficult to find a fault-detection procedure that is really suitable for DFIM without adding sensors or digital hardware. With reference to closed-loop induction-machine drives with a digital control system, as the control itself affects the behavior of external variables, new diagnostic procedures must be adopted to perform the machine monitoring. A new technique has been developed, which is based on the analysis of the modulating signals generated by current regulators of the back-to-back converter connected to the three-phase induction machine rotor side. These modulating signals give spectra containing all the necessary information required by an effective diagnostic index. Moreover, simulations and experimental results have shown that the analysis of rotor modulating signals yields a higher sensitivity with respect to the current signature analysis performed directly on three-phase stator currents. The signals used for diagnostic purpose are generated inside the control system of the back-to-back converter. Therefore, the proposed diagnostic system is particularly interesting, since it can be effectively embedded in the control board of the back-to-back converter connected to the induction-machine rotor side.