Johan I. Guzman

Selective harmonic elimination and current/voltage control in current/voltage-source topologies: a unified approach

This paper presents a unified approach for generating pulsewidth-modulated patterns for three-phase current-source rectifiers and inverters (CSR/Is) that provides unconstrained selective harmonic elimination and fundamental current control. The approach uses the chopping angles or gating patterns developed for voltage-source rectifiers and inverters in combination with a logic circuit to generate the gating patterns for CSR/Is. The circuit also includes naturally and symmetrically distributed shorting pulses. Thus, the approach avoids the hassle of positioning the shorting pulses and defining and solving a set of nonlinear equations dedicated to CSR/Is. Moreover, the approach can eliminate an even or odd arbitrary number of harmonics (e.g., fundamental current control and elimination of the 5th, 7th, and 11th harmonics). This is an improvement over existing techniques and a new approach to pattern generation. Simulated and experimental results for both static and dynamic operating conditions are presented in order to validate the effectiveness of the approach.

Performance Evaluation of a Multicell Topology Implemented With Single-Phase Nonregenerative Cells Under Unbalanced Supply Voltages

The analysis of a multicell topology that is implemented with single-phase nonregenerative cells under an unbalanced ac mains is presented. The study shows that the topology naturally compensates most of the voltage unbalance; for instance, for a 100% voltage unbalance in the ac mains, just 32% reaches the load. For critical applications, a feedforward control technique is proposed in order to compensate the remaining unbalance at the load side. The resulting topology, in combination with the proposed strategy, reduces near to zero the load fundamental voltage unbalance, while the input current unbalance and distortion are also improved. A theoretical analysis that is based on symmetrical components and the experimental results confirm the theoretical considerations.

Selective Harmonic Elimination in Multimodule Three-Phase Current-Source Converters

Modular current-source converter (MCSC) structures can enhance the current capacity and improve the power quality of converters in medium-voltage pulsewidth-modulated applications. An MCSC uses individual three-phase modules in a shunt connection to share the total power in a symmetrical manner. The modulation of these units is normally done using optimized patterns such as selective harmonic elimination patterns. However, the optimization is usually performed at the module level. This paper proposes a modulation technique that optimizes the operation of a complete MCSC taking into account all P modules. Thus, from the injected ac current, we remove P times the number of harmonics as compared with the conventional approach using the same switching frequency. A complete mathematical formulation and experimental results validate the proposed approach.

Selective harmonic elimination and current/voltage control in current/voltage source topologies: a unified approach

This paper presents a unified approach for generating pulse-width-modulated (PWM) patterns for current source rectifiers and inverters (CSR/Is) that provide (a) unconstrained selective harmonic elimination (SHE), and (b) fundamental current control. The approach uses the chopping angles or gating patterns developed for voltage source rectifiers and inverters (VSR/Is) in combination with a logic circuit to generate the gating patterns for CSR/Is. The circuit also includes naturally and symmetrically distributed shorting pulses. Thus, the approach avoids the hassle of (a) positioning the shorting pulses, and (b) defining and solving a set of nonlinear equations dedicated to CSR/Is. Moreover, the approach can eliminate an even or odd arbitrary number of harmonics (e.g., fundamental current control and elimination of the 5th, 7th, and 11th harmonics). This is an improvement on existing techniques and a new approach to pattern generation. Simulated and experimental results are presented to validate the effectiveness of the approach.

Digital Implementation of Selective Harmonic Elimination Techniques in Modular Current Source Rectifiers

Modular current source converters (MCSCs) have been proposed as an alternative method for increasing the power range of medium voltage PWM AC drives. MCSCs are built by stacking parallel current source converters. Two advantages that make MCSCs attractive are: (a) extended current/voltage ratings beyond the device ratings and (b) simplicity in balancing the DC link current in each module. This can be accomplished using two optimized modulating patterns that have been proposed for such topologies: (a) multilevel selective harmonic elimination (MSHE) and (b) displaced selective harmonic elimination (DSHE). Both techniques are based on SHE patterns but there are slight differences in their digital implementation due to the way they generate the harmonic cancellation. Furthermore, when these techniques are used in the rectifier stage, it is reported that MSHE and DSHE do not eliminate all the unwanted harmonics due to practical issues such as changes in the modulating indexes to control the DC link currents. This work compares the operation of DSHE and MSHE when used in rectifiers of an MCSC in terms of execution time, robustness to poor sampling frequency, and quality of harmonic profiles on AC input currents under different operating conditions. Experimental results are presented to validate the theoretical considerations.

Selective Harmonic Elimination in Multi-Modules Three-Phase Current-Source Converters

Modular current source converters (MCSC) can enhance the current capacity and improve the power quality at medium voltage PWM motor drives. A MCSC uses individual three-phase modules in a shunt connection to share the total power in a symmetrical manner. The modulation of these units is normally done using optimized patterns as selective harmonic elimination (SHE). However, the optimization is usually performed at each module. This paper proposes a modulating technique that optimizes the operation of a MCSC built up of P modules. Thus, for the same switching frequency, P times the number of harmonics are removed from the injected ac current as compared to the regular approach. A complete mathematical formulation as well as experimental results validate the theoretical considerations

A novel multi-level converter based on current source power cell

A novel N-level converter based on current-source power cells that is suitable for medium to high-voltage power applications is proposed. The topology is mainly the dual of the conventional topology based on voltage-source power cells. As such, it inherits all the advantages of the standard topology. But more importantly, the second current harmonic injected back by the normal operation of the single-phase current-source inverters are naturally cancelled out at the DC side of the power cells. This feature is assured by sharing the same magnetic circuit among the inverters connected to different load phases. As a result, the DC link filter required by the operation of the cell needs to be designed just to filter out the switching harmonics. Differently to voltage-source power cells based N-level converters, this ensures a small filter size, an important feature for medium to high power applications. Moreover, standard modulating techniques can be used such as SPWM, space vector, and Selective Harmonic Elimination without any penalties on the aforementioned advantages. Preliminary results confirm the theoretical considerations.

Decoupled and Modular Harmonic Compensation for Multilevel STATCOMs

A modular and decoupled approach to achieve harmonic cancellation in a multilevel Static Compensator (STATCOM) is presented in this paper. This work shows that it is possible to split the compensation tasks depending on the frequency components present on the line current that is intended to be compensated by using the superposition principle and the modular features of an H-bridge based multilevel STATCOM. This approach allows the implementation of the topology with dedicated modules in order to decouple and simplify the control algorithms. The H-bridge modules can be implemented with two different kinds of semiconductors: (i) slow switches for fundamental frequency compensation modules and (ii) fast switches for harmonic frequency compensation modules. As the modules meant for harmonic cancellation can self-regulate its dc voltage, they can follow the load requirements and thus operate with minimum power. The theoretical analysis is validated in a laboratory prototype.

Modeling Issues in Three-Phase Current Source Rectifiers that use Damping Resistors

The model of a three-phase current source rectifier (CSR) is a complex mathematical representation as it is nonlinear, multi-variable, and coupled. In order to work with a simplified but still accurate model, a representation in a rotating reference frame dq0 in combination with an average model are normally used. The result is a model that operates with ripple free inputs, outputs, and state variables that should be the accurate average value of the actual variables. However, the average model of these topologies could lead to errors if damping resistors are used. A case study shows as much as 10% error in the DC link current between the actual value and the predicted by the average model. This paper finds and shows systematically the source of the error and proposes a method for the accurate average modeling of CSRs with damping resistors. Simulated results show the validity of the theoretical considerations

Current-source cascaded multilevel converters based on single-phase power cells

Nowadays the control of medium voltage motors has two main alternatives for its development: voltage source inverters (VSI); or current source inverters (CSI). The cascaded multilevel inverters derive from high voltage, high power quality and high reliability requirements in both development alternatives. Up to now, the cascaded multilevel converters have been implemented using voltage source power cells. However, they can also be implemented using current source units. One factor against CSIs is the large size of the inductive filters needed in the DC links, especially when single-phase inverters are considered. This paper presents a current-source cascaded multilevel converter (CS-CMC), based on single-phase power cells and their implementation alternatives, which enable a reduction in the sizes of the DC-link inductors. It is demonstrated that these sizes can be reduced using magnetic couplings between the DC links of the power cells. These magnetic couplings can eliminate second harmonic currents, as well as other even harmonics of current in the DC link of the power cells. This work asserts that the implementation of a current-source cascaded multilevel converter, based on single-phase power cells, must consider magnetic couplings between its DC links. Design results and the simulation of a development alternative of the system, aim to demonstrate the feasibility of implementing the cascaded multilevel converter.