Derrick Holliday

Quasi Two-Level Operation of Modular Multilevel Converter for Use in a High-Power DC Transformer With DC Fault Isolation Capability

DC fault protection is one challenge impeding the development of multiterminal dc grids. The absence of manufacturing and operational standards has led to many point-to-point HVDC links built at different voltage levels, which creates another challenge. Therefore, the issues of voltage matching and dc fault isolation are undergoing extensive research and are addressed in this paper. A quasi two-level operating mode of the modular multilevel converter is proposed, where the converter generates a square wave with controllable dv/dt by employing the cell voltages to create transient intermediate voltage levels. Cell capacitance requirements diminish and the footprint of the converter is reduced. The common-mode dc component in the arm currents is not present in the proposed operating mode. The converter is proposed as the core of a dc to dc transformer, where two converters operating in the proposed mode are coupled by an ac transformer for voltage matching and galvanic isolation. The proposed dc transformer is shown to be suitable for high-voltage high-power applications due to the low-switching frequency, high efficiency, modularity, and reliability. The dc transformer facilitates dc voltage regulation and near instant isolation of dc faults within its protection zone. Analysis and simulations confirm these capabilities in a system-oriented approach.

Thermal Modeling of a Segmented Stator Winding Design

This paper presents a thermal analysis of a segmented stator winding design. As the thermal performance is one of the main factors limiting a machines output capability, a thermal test on a complete prototype machine is an essential part of the design process. However, for the segmented stator winding design, a test-informed thermal analysis on a single stator tooth can be performed prior to the manufacture of the full machine. This approach allows for a rapid and inexpensive assessment of the thermal performance of the complete machine and early identification of design modifications needed. The research has been applied to the design of a highly efficient and compact permanent-magnet traction motor. A thermal model for a single tooth was developed and supported by tests to identify key heat transfer coefficients. A number of winding assemblies were compared, and the most promising was selected for the final motor prototype. The results from the approach are compared with thermal test results from the complete machine.

Analysis and Design of a Modular Multilevel Converter With Trapezoidal Modulation for Medium and High Voltage DC-DC Transformers

Conventional dual-active bridge topologies provide galvanic isolation and soft-switching over a reasonable operating range without dedicated resonant circuits. However, scaling the two-level dual-active bridge to higher dc voltage levels is impeded by several challenges among which the high dv/dt stress on the coupling transformer insulation. Gating and thermal characteristics of series switch arrays add to the limitations. To avoid the use of standard bulky modular multilevel bridges, this paper analyzes an alternative modulation technique, where staircase approximated trapezoidal voltage waveforms are produced; thus, alleviating developed dv/dt stresses. Modular design is realized by the utilization of half-bridge chopper cells. This way the analyzed dc-dc transformer employs modular multilevel converters operated in a new mode with minimal common-mode arm currents, as well as reduced capacitor size, hence reduced cell footprint. Suitable switching patterns are developed and various design and operation aspects are studied. Soft-switching characteristics will be shown to be comparable to those of the two-level dual-active bridge. Experimental results from a scaled test rig validate the presented concept.

Thermography-based virtual MPPT scheme for improving PV energy efficiency under partial shading conditions

This paper proposes a new thermography-based maximum power point tracking (MPPT) scheme to address photovoltaic (PV) partial shading faults. Solar power generation utilizes a large number of PV cells connected in series and in parallel in an array, and that are physically distributed across a large field. When a PV module is faulted or partial shading occurs, the PV system sees a nonuniform distribution of generated electrical power and thermal profile, and the generation of multiple maximum power points (MPPs). If left untreated, this reduces the overall power generation and severe faults may propagate, resulting in damage to the system. In this paper, a thermal camera is employed for fault detection and a new MPPT scheme is developed to alter the operating point to match an optimized MPP. Extensive data mining is conducted on the images from the thermal camera in order to locate global MPPs. Based on this, a virtual MPPT is set out to find the global MPP. This can reduce MPPT time and be used to calculate the MPP reference voltage. Finally, the proposed methodology is experimentally implemented and validated by tests on a 600-W PV array.

Hybrid Cascaded Modular Multilevel Converter With DC Fault Ride-Through Capability for the HVDC Transmission System

A new hybrid cascaded modular multilevel converter for the high-voltage dc transmission system is presented. The half-bridge cells are used on the main power stage and the cascade full-bridge (FB) cells are connected to its ac terminals. The main power stage generates the fundamental voltages with quite low switching frequency, resulting in relatively low losses. The cascaded FB cells only attenuate the harmonics generated by the main power stage, without contribution to the power transfer. Thus, the energy-storage requirement of the cascaded FB cells is low and the capacitance of FB cells is reduced significantly. Due to the dc fault reverse blocking capability of the cascaded FB cells, the proposed topology can ride-through the pole-to-pole dc fault. In addition, the soft restart is achieved after the fault is eliminated, without exposing the system to significant inrush current. Besides, the average-value model of the proposed topology is derived, based on which the control strategy is presented. The results show the feasibility of the proposed converter.

Analysis of Shaped Pulse Transitions in Power Electronic Switching Waveforms for Reduced EMI Generation

Consideration of the higher order time derivatives of voltage and current transitions in power semiconductor devices enables the specification of “S-shaped” switching waveforms which offer an improved tradeoff between high-frequency EMI generation and switching losses. In comparison with the widely used first-order derivative trapezoidal switching waveform approximation, Fourier analysis of the proposed “S-shaped” waveform shows that it exhibits a 20 dB/dec steeper spectral gradient at high frequencies, resulting in a 20 dB greater reduction in high-frequency spectral content per decade increase in rise time. Numerical analysis of the proposed waveform shows that both peak and total RF power, employed as indicative EMI metrics, are reduced significantly with no increase in overall switching time. Experimental investigation of the effect of introducing a frequency-selective EMI transmission path shows that the overall trends in the relationships between time-domain waveform parameters and high-frequency spectral content are maintained, while the values of the waveform timing parameters which minimize the two EMI metrics are changed.

New natural observer applied to speed-sensorless DC servo and induction motors

A simple observer design technique with parameter adaptation is proposed for bounded-input bounded-output nonlinear systems. In this technique, no feedback is used in the observer but parameter estimations are considered as if they are observer inputs. The proposed technique is successfully applied to speed-sensorless dc servomotors and speed-sensorless induction motors with load torque adaptation schemes. The observer is robust to noise and parameter uncertainty. Excellent experimental and simulation results have been obtained.

A Hybrid Modular Multilevel Converter With Novel Three-Level Cells for DC Fault Blocking Capability

A novel hybrid, modular multilevel converter is presented that utilizes a combination of half-bridge and novel three-level cells where the three-level cells utilize a clamp circuit which, under dc side faults, is capable of blocking fault current thereby avoiding overcurrents in the freewheel diodes. This dc fault blocking capability is demonstrated through simulation and is shown to be as good as the modular multilevel converter which utilizes full-bridge cells but with the added benefits of: lower conduction losses; fewer diode and semiconductor switching devices, and; fewer shoot-through modes. The semiconductor count and conduction loss of the proposed converter are reduced to around 66.5% and 72% of that of modular multilevel converter based on the full-bridge cells respectively, yielding lower semiconductor cost and improved efficiency. Dc fault ride-through operation is realized without exposing the semiconductors to significant fault currents and overvoltages due to the full dc fault blocking capability of the converter.

A computationally efficient iron loss model for brushless AC machines that caters for rated flux and field weakened operation

This paper presents a simple and computationally efficient approach for predicting iron loss within a field orientated controlled brushless AC permanent magnet machine which can cater for both rated flux and field weakened operation. The proposed method is readily incorporated as part of the design process and is based on two discrete time step 2D magnetostatic finite element field solutions describing the open circuit and short circuit operation of the machine. Parameters obtained from these analyses are used alongside the standard d-q equivalent circuit to generate a map for the iron loss across the entire machine working envelope. Test results taken from a concentrated wound brushless AC traction motor are used to validate the technique.

Compensation of Inverter Nonlinear Distortion Effects for Signal-Injection-Based Sensorless Control

A simple analytical technique, which uses readily available datasheet parameters, is developed to model the low-current switching characteristics of insulated-gate bipolar transistors (IGBTs). Simulation and experimental results obtained using three differently rated inverters are presented to demonstrate the accuracy of the technique. The model is applied to compensate the nonlinear distortion, introduced by IGBT switching action in a three-phase inverter, of high-frequency injected voltage and current signals used in sensorless control of a permanent-magnet brushless ac machine. Experimental results show that the compensation technique improves rotor position estimation by up to 20° electrical.