P.C. Loh

Operational analysis and comparative evaluation of embedded Z-Source inverters

This paper presents various embedded Z-source (EZ-source) inverters broadly classified as shunt or parallel embedded Z-source inverter. Being different from the traditional Z-source inverter, EZ-source inverters are constructed by inserting dc sources into the X-shaped impedance network so that the dc input current flows smoothly during the whole switching period unlike the traditional Z-source inverter. This feature is interesting when PV panels or fuel cells are assumed to power load since the continuous input current flow reduces control complexity of dc source and system design burden. Carefully analyzing the operational principles for shoot-through and nonshoot-through switching, it is first revealed that the shunt-embedded Z-source inverters with capacitors partially or fully replaced by dc sources can appropriately perform buck-boost operation with rear-end current source inverter (CSI) circuitry connected instead of the generic voltage source inverter (VSI) circuitry. Further proceeding on to the topological variation, parallel embedded Z-source inverters are presented with the detailed analysis of topological configuration and operational principles showing that they are the superior options among Z-source variants for clean energy harnessing without any loss of voltage buck-boost capability and with the inserted Altering capability for dc source. All theoretical findings are finally verified by PLECS simulations and constructed laboratory prototypes.

Model predictive control for Z-source power converter

This paper presents Model Predictive Control (MPC) of impedance-source (commonly known as Z-source) power converter. Output voltage control and current control for Z-source inverter are analyzed and simulated. With MPCs ability of multi-system variables regulation, load current and voltage regulations, impedance network inductor current, capacitor voltage as well as switching frequency fixation, transient reservation and null state penalization are all regulated as subjecting to constraints of this control method. The quality of output waveform, stability of impedance-network, level constraint of variable switching frequency as well as robustness of transient response can be obtained at the same time with a formulated Z-source network model. Operating steady state and transient state simulation of MPC are going to be presented, which shows good reference tracking ability of this control method.

Voltage Sag Compensation With Z-Source Inverter Based Dynamic Voltage Restorer

The dynamic voltage restorer has been gaining acceptance as an effective device for voltage sag compensation. The compensation capability of a dynamic voltage restorer (DVR) depends primarily on the maximum voltage injection ability and the amount of stored energy available within the restorer. A new topology based on Z-source inverter for the DVR is proposed in order to enhance the voltage restoration property of the device. Z-source impedance network along with shoot-through capability of the proposed inverter would ensure a constant dc-voltage across the dc-link despite dwindling voltage in the storage devices connected in the dc-link during the process of voltage compensation. Even when the dc-link energy is supplied through a shunt connected auxiliary supply, the voltage rating of the shunt converter, shunt transformer and the dc-link capacitor can be kept smaller with the proposed topology. The proposed converter topology and control methods are validated by simulation and laboratory tests carried out on a proto-type of the restorer

Digital carrier modulation and sampling issues of matrix converters

Although the modulation of ac-ac matrix converters using space vector theory has long been established, their carrier-based modulation principles have only recently attracted some attention. Reasons commonly stated for evaluating the carrier-based alternative include simpler converter control because of its inherent auto-sequencing process, and easier implementation using fast on-chip timers embedded in most modern digital signal processors. Motivated by these likely merits, which have previously been proven for dc-ac inverters, an investigation is now pursued here to develop appropriate digital carrier modulation schemes for controlling conventional and sparse matrix converters with minimized semiconductor commutation count and smooth sextant transitions with no erroneous states produced. For guaranteeing the latter two features, correct digital sampling instants and state sequence reversal must be chosen appropriately, as demonstrated in the paper for the two different topological options, which to date, have not yet been discussed in the existing literature. To validate the concepts discussed, experimental testing on the implemented conventional and sparse matrix laboratory prototypes was performed with their respective results captured and presented in the paper for visual confirmation.

Evaluation of resonant damping techniques for Z-source current-type inverter

For the renewable energy sources whose outputs vary continuously, Z-source current-type inverter has been proposed as a possible buck-boost alternative for grid-interfacing. With a unique X-shaped LC network connected between its dc power source and inverter topology, Z-source current-type inverter is however expected to suffer from compounded resonant complications in addition to those associated with its second-order output filter. To improve its damping performance, this paper proposes the careful integration of Posicast or three-step compensators before the inverter pulse-width modulator for damping triggered resonant oscillations. In total, two compensators are needed for wave-shaping the inverter boost factor and modulation ratio, and they can conveniently be implemented using first-in first-out stacks and embedded timers of modern digital signal processors widely used in motion control applications. Both techniques are found to damp resonance of ac filter well, but for cases of transiting from current-buck to boost state, three-step technique is less effective due to the sudden intermediate discharging interval introduced by its nonmonotonic stepping (unlike the monotonic stepping of Posicast damping). These findings have been confirmed both in simulations and experiments using an implemented laboratory prototype.

Topological and Modulation Design of a Buck-Boost Three-Level Dual Inverter

To date, buck-boost inverters have mostly been reported with two-level inverter topologies. For improved output waveform quality, this paper presents the design of a buck-boost three-level dual inverter supplied by a single DC source. The proposed inverter is implemented by cascading two three-phase voltage-source bridges at their AC outputs with only an additional switch added to their common DC network for inductive energy boosting. The designed inverter can be controlled using a carefully formulated modulation scheme, which ensures the proper sequencing and placement of three distinct states in a switching cycle, while at the same time, avoids any short-circuiting of capacitive elements. If desired, the modulation scheme can also be fine-tuned to allow the proposed inverter to operate with reduced common mode voltage generated, hence minimizing bearing and ground leakage currents commonly reported to have caused motor malfunctioning. All findings presented in the paper have been confirmed experimentally using an implemented laboratory prototype

Modulation Schemes of Multi-phase Three-Level Z-Source Inverters

This paper investigates the modulation schemes of three-level multiphase Z-source inverters with either two Z-source networks or single Z-source network connected between the DC sources and inverter circuitry. With the proper offset added for achieving both desired four-leg operation and optimized harmonic performance, the proposed modulation schemes of four-leg three-level Z-source inverters can satisfy the expected buck-boost operation under unbalanced modulation conditions. Except of the modulation complexity hidden in the four-leg inverters, five-phase three-level Z-source inverters show totally different modulation requirement and output performance. For clearly illustrating the detailed modulation process, time domain analysis instead of the traditional multi-dimensional space vector demonstration is assumed which reveals the right way to insert shoot-through durations in the switching sequence with minimal commutation count. Lastly, the theoretical findings are verified in Matlab/PLECS simulation and experimentally using constructed laboratory prototypes.

Reduced semiconductor three-level interline dynamic voltage restorer

The previously proposed Interline Dynamic Voltage Restorer (IDVR) system enhances its capability to compensate for long-term and deep voltage sags from propagating to critical loads in power system without requiring a bulky energy storage device. By having multiple DVRs connected to different transmission feeders, the common energy storage in IDVR system can be dynamically replenished from an unaffected feeder so as to maintain the rated power of critical loads in another feeder where voltage sag appears. However, despite its enhancement in compensation capability, the drawbacks faced by original IDVR system are mainly attributed to its complexity in topology and underutilizations of semiconductors. Aimed to improve the system in these aspects, the paper proposes a new IDVR topology with 25% reduction in semiconductors totally used. The theoretical analysis and its operating principles are also presented. In addition, some constraints faced by this new system are also noted. Finally, simulation results are given to validate the feasibility of proposed topology.

Modelling of Three phase Z-Source Boost Buck Rectifiers

The Z-source rectifier is a recently proposed converter topology which has a unique X-shaped impedance network on its DC side. In the process of designing the controller circuits for the Z-source rectifier, knowledge of transfer functions relating the dynamics of various variables is essential. This paper deals with the modeling of Z-source rectifier with the intent of developing a robust controller. Modelling is carried out using a set of state equations obtained by state space averaging the circuit equations in shoot through and non shoot through states of the rectifier. The derived model is verified with both simulation and experimental results.

A Z-source Inverter Based Flexible DG System with P+resonance and Repetitive Controllers for Power Quality Improvement of a Weak Grid

In the present power systems, nonlinear loads connected at distribution level have increased immensely. This results in reduction of voltage quality for distribution level customers. Significantly, this is an outcome of the concerted action of small loads drawing nonlinear currents. However, the utility has an obligation to deliver a quality supply to consumers. Interestingly, with the increase in energy demand and penetration of renewable sources, generation units popularly known as distributed generators (DGs) are increasingly connected at the distribution level. Most of these sources are connected to the grid using power electronic converters. The power delivered from these sources depend on many factors like energy availability and load demand etc. In many occasions, the converters used in power conversion can be left with some excess capacity. This could be used to provide some ancillary functions of the power distribution like harmonics and unbalance mitigation etc. Moreover, some of these DG sources have large operating ranges demanding special converters with wide operating range. Being a single stage buck-boost inverter, recently proposed Z-source inverter is a good candidate for future DG systems. Considering these factors, this paper presents the controller design for a Z-source inverter based DG system to improve the power quality of distribution systems. To improve the reference tracking and to eliminate harmonics, a p+resonance and repetitive controller designed using a simple time delay is employed. When the system is running at full capacity, the proposed controller improves the quality of the injecting current. Subsequently, the duality of this internal model based control structure is exploited to improve the voltage quality at the connection point of the inverter when the system is not operated in the full power capacity. Proposed control method is tested with simulation results obtained using Matlab/Simulink and PLECS. Subsequently, it is experimentally validated using a laboratory prototype.