Nonregenerative pulsewidth-modulated (PWM) rectifiers are increasingly being considered for applications, where the power flow is unidirectional, such as power supplies for telecommunications, X-ray, ...the machine-side converter for wind energy conversion systems, etc. They use fewer active switches, which increase their power density and reduce cost. This paper proposes a novel reduced switch topology for a multilevel (five-level or higher) nonregenerative PWM rectifier. It uses only four controlled switches and eight diodes per phase for a five-level rectifier. Half of the diodes are naturally commutated (zero current switching) at the line frequency, which reduces switching losses. This topology has several other advantages compared to similar topologies reported in the literature, such as minimum voltage stress across the devices, elimination of transient voltage-balancing snubbers, no extra hardware for balancing the flying capacitors, the dc-link mid-point voltage, etc. In this paper, switching cycle average modeling and the carrier-based modulation strategy for this rectifier are also presented to maintain a balanced dc link and to regulate flying capacitor voltages, while achieving unity displacement factor at the rectifier input terminals. The overall performance of the rectifier is verified by experimental results.
The VIENNA rectifier is increasingly being considered as the active front-end converter in many applications that do not require regeneration or controllable input reactive power. In such ...applications, this rectifier is normally operated at unity source displacement factor, which derates its nominal voltampere (VA) rating. Moreover, in the "source voltage oriented" reference frame that is normally used for control, this rectifier has complex operating limits on the rectifier terminal voltage (imposed by the operating conditions), which needs to be computed online to avoid the line currents from going through zero-crossing distortions. Also, the source voltage has to be either measured or estimated, which further increases the cost and/or computational burden. This paper proposes a control method that eliminates the requirement of the input voltage sensors (or estimators) and makes the limits on the rectifier terminal voltage far simpler to compute. The control method is implemented in the input line current oriented reference frame, considering unity displacement factor at the rectifier input terminals, which minimizes the VA rating of the rectifier (for a given rated load power). The proposed sensorless current-oriented control approach is compared with the conventional voltage-oriented control strategy through experimental results.
This paper proposes a system design and control technique for a newly developed brushless and permanent magnetless synchronous generator-based variable-speed wind energy generation system, ...transferring power to a constant voltage dc grid via a three-level Vienna rectifier (VR). The recently established generator named Brushless Induction excited Synchronous Generator (BINSYG) is a wound field synchronous generator (WFSG), whose excitation is developed by controlling an Induction Machine fitted to the same machine structure and sharing the same magnetic core. A new controller is proposed that ensures the stable operation of BINSYG for a wide variation of shaft speeds. VR achieves sinusoidal input current and can control the power factor at its input, which is particularly suitable for wind energy applications. The top and bottom capacitor voltages of the VR are balanced using redundant switching combinations. The system with its proposed control algorithm is modelled in MATLAB/Simulink for a 5 kW rated BINSYG feeding power to a 750 V dc grid. The steady-state and dynamic state simulation results are presented and the controller performance is verified for a wide range of wind speeds. Further, real-time results using the OPAL-RT testbed are presented for the same system to verify the effectiveness of the overall control strategy.
An integrated generator-rectifier architecture is a promising candidate for harvesting energy from offshore wind. The most attractive feature of this architecture is that the majority of the power is ...processed by reliable, efficient, and inexpensive diodes operating at the generator frequency. The use of dc collection grids (medium-voltage or high-voltage dc) is an emerging trend in a wind farm. This article proposes a dc grid interface circuit for the integrated generator-rectifier and a control strategy for maximum power point tracking. Using a 10-MW illustrative design, it is shown that the proposed architecture reduces the total switch volt-ampere rating requirement by 22.8%, which translates into overall loss reduction ranging between 28.3% and 71.7%, depending on the extracted power. The maximum power point tracking algorithm is implemented based on the relationship between the active rectifier <inline-formula><tex-math notation="LaTeX">d</tex-math></inline-formula>-axis current and the generated power. Experimental results obtained from a laboratory prototype validates the effectiveness of the proposed converter architecture and its control strategy. This approach opens up opportunities for integrating dc collection networks with offshore wind farms through an efficient, reliable, and cost-effective energy conversion system.
In applications, where bidirectional power flow is not required, unidirectional pulsewidth modulated (PWM) rectifiers are advantageous in terms of power density, hardware simplicity, and reliability ...compared to the conventional PWM rectifiers. For high-power medium-voltage applications, a discontinuous pulsewidth modulation (DPWM) strategy is useful in reducing the switching loss. This article proposes a DPWM strategy for a recently proposed five-level unidirectional rectifier. The rectifier is analyzed in the space vector (SV) domain to determine the switching sequences for DPWM. Two such DPWM sequences (i.e., DPWMA and DPWMB) are identified. Implementation of these DPWM sequences in the SV domain is known to be very computation intensive. Therefore, carrier-based implementation of both the DPWM strategies is proposed, which are derived following the SV approach. The proposed strategies are found to be very simple with very few computations. A new voltage-balancing method for the dc-link capacitors is also proposed in this article. Performances of these DPWM strategies over the entire operating range are predicted analytically and verified experimentally. This article is accompanied by MATLAB files demonstrating various performance indexes of the proposed DPWM sequences.
This paper proposes a three-phase, five-level, non-regenerative pulsewidth modulated rectifier using only two active switches (minimum required) per phase, which drastically reduces gate driver ...requirement and hardware complexity. It draws sinusoidal input current at close to unity power factor. All the semiconductor devices are rated at only one fourth of the dc-link voltage, and none of them requires any transient voltage balancing snubber. A total of 8 out of the 14 diodes per phase undergo soft switching transition under all operating conditions, which increases its efficiency. No extra hardware circuitry for balancing the flying capacitors (FCs) or the dc-link mid-point voltage are required, which further reduces hardware complexity and increases the conversion efficiency. The proposed topology does not need any sophisticated startup procedure for charging the FCs either, which solves the problem of semiconductor overvoltage during starting. A 3-kW laboratory prototype is built to experimentally verify the proposed topology. The maximum efficiency obtained from the prototype is 98.7%, and it is always more than 96% for the load range from 15% to its rated.
This paper proposes an improved space vector pulse width modulation (SVPWM) based DC link voltage balancing control of a three-phase three-level neutral point clamped (NPC) centralised inverter ...supplying the generated power from photo voltaic (PV) array to a three-phase utility grid. Two possible schemes have been developed based on the power conversion stage between PV array and the utility grid namely, two-stage (three-level boost converter three-phase three-level NPC inverter) and single-stage (three-phase three-level NPC inverter alone). The comparison between these two schemes has been thoroughly discussed in terms of the control strategies employed, power loss analysis and efficiency. The performance of the centralised inverter under different modes of operation has been investigated by developing the required control strategies for smooth operation. Using the proposed control strategy, the centralised inverter can be operated as a static synchronous compensator (STATCOM) during night time, if needed. The power loss incurred in the power-electronic converters has been analysed for constant and also for variable ambient temperature. The effectiveness of the centralised inverter as an active filter (AF) has also been verified when a three-phase non-linear load is considered in the system.
A brushless doubly-fed reluctance machine requires partially-rated power converter, reduced maintenance, and operates without permanent magnets making it an attractive option for many applications ...such electrified propulsion and wind energy conversion system. However, BDFRMs have inherently poor torque density and high torque ripple. Since the flux modulation in a BDFRM is carried out by the rotor, the rotor structure plays an important role in torque production. This article proposes an analytical approach to model the effect of the rotor flux-barriers on the mean and ripple torque. The method shows that the instantaneous torque is highly sensitive to the location of these flux-barriers. A rotor with two flux-barriers per pole is used to illustrate the effectiveness of the proposed approach. The proposed modeling framework is used to optimize the rotor geometry, stator geometry and excitations to maximize mean torque and minimize ripple. One of the candidate designs obtained from the pareto front is manufactured. The analytical torque profile is validated using both the finite element analysis (FEA) and experimental tests.
This paper presents the variable speed applications of a recently developed generator named Brushless Induction Excited Synchronous Generator (BINSYG) for offshore wind power applications. The ...machine is basically a Synchronous generator (SG) with its excitation fed from a three-phase Induction Machine (IM). Both the machines are embedded in the same machine frame and share the same magnetic core. The SG feeds power to the dc grid via a five-level Unidirectional Dual Flying Capacitor Active Neutral Point Converter. The converter is controlled such that the Total Harmonic Distortion (THD) of the current flowing through the stator of SG is below 5% and at unity power factor at SG terminal. The excitation developed by IM-type exciter is adjusted using a new control strategy that is proposed for Voltage Source Inverter, such that BINSYG can operate at wide speed range necessary for Wind Energy Systems (WES). The complete system is controlled without using ac voltage sensors and speed encoders. The proposed WES with its control algorithm is first modelled in MATLAB/Simulink. Further, rapid control prototyping verification in the form of real-time results using OPAL-RT is presented for a small scale (5.5 kVA) BINSYG feeding power to a 750 V dc grid.
The three-level, three-switch Vienna rectifier is fast emerging as the preferred topology for the high-power factor front end converter in many applications. Conventional control algorithms for the ...Vienna rectifier requires sensing of both the supply voltage and the line current. However, in many applications, where an electrical generator feeds the rectifier, the ‘supply voltage’ is not available for measurement. Even for grid-connected applications the supply voltage may contain significant harmonic distortion which, in turn, increases the harmonic distortion of the rectifier input current. To mitigate these disadvantages, this study presents a novel voltage sensorless control algorithm for the Vienna rectifier. Voltage sensorless control algorithms reported in the literature for this rectifier, so far permit only unity rectifier terminal power factor operation. In contrast, the proposed control algorithm can operate the rectifier at programmable power factor. For the verification of the algorithm, experiments were conducted on a laboratory prototype of the Vienna rectifier using both voltage sensorless and sensor-based control algorithms. Performances of both these algorithms were found to be similar under transient conditions. The voltage sensorless controller achieved better input line current total harmonic distortion in steady state.
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