Poh Chiang Loh

Nanyang Technological University | ntu · School of Electrical and Electronic Engineering

As the grid incorporates more voltage-source converters (VSCs), the stability issues arising from VSC-grid interactions escalate. While passivity theory offers a promising solution, existing passivity-based controller designs primarily rely on heuristic methods which do not advance stability beyond achieving passivity. To fully exploit the availabl...

Passive property of grid-tied converter’s output admittance is widely recognized as favorable to stability. However, the admittance of active power filter (APF) is dependent on the nonlinear load (NLL) applied. Thus, the passivity-based design of APF is also dependent on the specific NLL. To deal with this problem, this article suggests replacing A...

Indirect-current-controlled active power filter (APF) is preferable compared with the direct-current-controlled APF due to its simpler structure and its direct regulation of the grid current. However, unlike direct current control, indirect current control does not have explicit current reference posing difficulty on current limiting operation. Thi...

Digital proportional-resonant (PR) controllers have proven effective when used with an active power filter (APF) to compensate for nonlinear load harmonics. Their performances may however degrade significantly when implemented with a low-end microcontroller due to coefficient quantization errors. The extent of the degradation depends on the chosen...

By considering time-scale separation characteristics and nonlinear node voltage equation, an improved slow coherency algorithm has been proposed, and for the first time, applied to the slow coherency model widely used for modeling three-phase droop-controlled grid-connected inverters. The method works by combining two coherency clustering results,...

Within a power converter, multiple currents are usually measured for control, protection and / or monitoring. They are therefore important sources of information, which must unquestionably be measured accurately to maintain continuous and reliable operation of the power converter. Accurate current measurement has however become tougher with the int...

To accelerate the broad application of silicon carbide (SiC) power MOSFETs, their short-circuit (SC) robustness and reliability must be thoroughly evaluated. This paper, therefore, presents an overview of related research methods aiming to meet that objective. Topics reviewed include the non-destructive tester (NDT) used to implement SC faults, ext...

Microinverters for photovoltaic (PV) energy conversion typically encounter challenges, including the requirement of a voltage boost and suppression of the high-frequency leakage current. These challenges are usually not met simultaneously if only a standard full ( H )-bridge is used. Such limitation is, however, not applicable to the proposed dual...

Cascaded multilevel inverter produces a step-up output voltage by series-connection of several submodules supplied by isolated dc sources which can be implemented by photovoltaic (PV) sources. The unbalanced power provided by different PV sources may however distort the output voltage. To use a single dc source while retaining the voltage boosting,...

The single-phase T-type converter has an imbalance issue of dc-link voltages to deal with. Instead of active neutral point (NP) control, natural balancing properties are typically preferred owing to their superior spectral performance of output voltage. However, existing techniques to accelerate the natural balancing process inevitably require seve...

Single-phase symmetrical converter with five-level switching and a high-order filter offers remarkably low switching harmonics. It however experiences noticeable DC neutral-point imbalance, worsened by second-order power ripples associated with all single-phase systems. Although the imbalance will naturally rectify, its progress is very slow. It is...

As a direct AC/AC conversion concept, the direct matrix converter (DMC) outperforms its counterparts (e.g. AC-DC-AC) in terms of compactness and lifespan due to omitting the bulky and fragile dc-link capacitors. However, most of off-the-shelf DMC modules suffer from high current spikes due to their degraded reverse recovery characteristics (RRC) ca...

Reliable and affordable AC/AC power conversion plays a key role in supplying various electrical equipment. Among the options available, direct matrix converter (DMC) outperforms its counterparts (e.g. AC/DC/AC), in terms of lifespan and compactness, due to its omission of fragile and bulky dc-link capacitors. However, most off-the-shelf DMC modules...

The three level converters have an inherent problem of the neutral point (NP) voltage unbalance due to the split dc-link capacitors. This NP voltage unbalance degrades the output voltage/current waveform quality. A carrier based discontinuous pulse width modulation (CB-DPWM) with NP voltage balancing strategy- that involves the injection of calcula...

Voltage boosting three-level inverters may present a monotonically increasing or decreasing voltage gain when enlarging the pulse-width modulation (PWM) ratio. To produce a flexible voltage gain, this paper proposes a novel three-phase three-level PWM inverter by cascading a traditional two-level three-leg (B6) inverter with half-bridges. The propo...

A three-phase three-level converter with an LCL filter has been used in the industry because of its low switching harmonics. However, it inevitably suffers from dc neutral-point imbalance, which may unnecessarily stress semiconductors and deteriorate power quality. The imbalance will, nevertheless, naturally rebalance, but it usually takes a long...

Switched-capacitor-based multilevel inverters for boost-type dc-ac power conversions usually exhibit a trade-off between the switch count and switch-voltage rating, i.e., a reduction of one necessitating an increase of the other. Such a dilemma is well addressed in this paper by proposing a novel single-phase nine-level inverter based on a switched...

Modulation schemes play a key role in effectively driving the current source converter (CSC). Among various modulation schemes, the carrier-based PWM (CB-PWM) has gained much attention owing to its inherent auto-sequencing process. However, the fixed switching patterns in the existing CB-PWM methods reduce the available degrees of freedom, which im...

This paper presents a direct carrier-based modulation scheme to mitigate the dc-link current ripple for a current source converter (CSC). Excessive dc-link current ripple increases system losses, causes current distortion, and leads to more electromagnetic interference, so, it should be mitigated. Compared with increasing dc-link inductor or switch...

The performance of a current source converter (CSC) is greatly affected by its adopted modulation schemes. Among various modulation schemes, the carrier-based PWM (CB-PWM) has gained increasing attention owing to its inherent auto-sequencing process. However, existing CB-PWM schemes suffer from either high switching harmonics in ac current, or high...

Reliable current commutation, which is essential for the safe operation of a direct matrix converter, can be implemented with a field-programmable gate array (FPGA) to benefit from its parallelism and pipelining. However, the gating signals that are received in the FPGA from a digital controller (DSP or dSPACE) cannot strictly complement with each...

Split-source inverter (SSI) has been proved to be an attractive single-stage DC/AC converter for its compact structure, continuous input voltage and input current. However, due to the coupling of the boost stage and the inverter stage, SSI suffers from the low DC-link voltage utilization problem, which will increase the system cost and switching lo...

This paper elaborates on comprehensive harmonic analyses of pulse width modulation schemes for the three-phase three-level converter with unbalanced dc-link voltages. The derived harmonics of common-mode (CM) and differential-mode (DM) voltages is established by a precise analytical model. With the developed model, the impacts of DC neutral-point i...

Impedance-source inverters with coupled inductors can provide much higher voltage gains, but occasionally at the expense of discontinuous input currents and large voltage spikes at their dc-links. The former is caused by absence of inductances at their inputs, while the latter is due to unintentional interruptions of leakage currents through their...

The frequency-locked loop based on a second-order generalized integrator has been widely used in grid synchronization systems but featuring unfavorable harmonics filtering performance. To solve this issue, a frequency-locked loop based on multiple second-order generalized integrators was proposed, which is to realize harmonics attenuation and frequ...

Split‐source inverter (SSI) is a topology developed for flexibly stepping up and down its ac output voltage using only a standard inverter bridge. However, when configured as a single‐phase inverter, it is still burdened by common second‐order ripples. This study, therefore, proposes a modified SSI (MSSI) that can perform dual power decoupling. Unl...

Multilevel inverters in renewable applications usually require a high-voltage DC link fed by a front-end DC-DC boost stage. Such a two-stage power conversion however increases the switch count, leading to higher costs and power losses. To lower the number of switches, this letter proposes a six-switch inverter, capable of generating a seven-level v...

A three-winding coupled inductor connected as a Y-shaped impedance network has been used in DC-DC and DC-AC converters for producing a high voltage gain for renewable applications. Its versatility is further extended in this paper by the proposed differential Y-source inverter with two Y-source networks. The proposed inverter is modulated by a half...

Because of their fast switching speeds, current measurements for wide band-gap (WBG) devices have become increasingly challenging. Particularly, the designed current sensor must be nonintrusive and have a small size and a very wide bandwidth. One promising current sensor is the printed-circuit-board (PCB) Rogowski current sensor, whose bandwidth is...

Silicon carbide metal-oxide-semiconductor field-effect transistor has a smaller short-circuit tolerance, and hence, requires faster and more accurate short-circuit protection. One prospective method is to combine fast di/dt detection with an integration circuit. The former is for detecting the extremely fast increase of short-circuit current, whi...

Small-signal stability of three-phase ac grid-connected converters can be analysed by using the impedance-based method in d-q frame. Based on this approach, the d-q impedance models of both source and load subsystems are represented with 2-by-2 matrices, and their ratio (known as the return ratio matrix) is used for stability assessment. This, howe...

Latest wide-bandgap power devices are switching progressively faster compared with existing Silicon devices. Their accurate current measurements for either control or protection have therefore become tougher. One method, that can fulfill the requirements, is to use a Rogowski coil and its accompanied electronics to form a Rogowski current sensor wi...

Coupled-inductor-based impedance-source inverters have been suggested as a solution for improving voltage boost capability. However, large voltage spikes can sometimes appear across the switch bridge, if leakage inductance of the coupled inductor cannot be nullified. Higher voltage-rated switches are therefore necessary for implementing each invert...

Most grid‐connected inverters can operate in both grid‐connected and islanded modes with usually different control schemes adopted in each mode. The control schemes, if noticeably complex and different, can result in inrush currents and voltage distortions at instants of mode transfer. An existing grid‐connected form of droop control has hence been...

Accurate measurement of current through a silicon carbide (SiC) device is especially challenging, because of its fast switching speed. Its accompanied current sensor must hence have a high-bandwidth and a high-noise immunity. It must also not interfere with operation of the device. Due to these, the printed-circuit-board (PCB) Rogowski current sens...

LCL-filtered three-level inverters have been widely used in PV applications because of their high efficiency, high power density, and low cost. In practice, the inverter-side current feedback (ICF) control has dominated the PV market due to its low-cost property. However, the grid-side current becomes more sensitive to the grid distortion owing to...

This paper proposes a three-phase voltage source inverter (VSI), assisted by a Y-source network to perform single-stage DC to AC power conversion with the amplitude of output voltage higher than the DC input voltage. Analysis shows that while the modulation and boost duty ratios can be increased simultaneously, the proposed inverter achieves a high...

Accurate measurement of the switch current through a SiC device is especially challenging due to its fast switching speed and low junction capacitance. Its accompanied current sensor must hence have high bandwidth, strong noise-immunity, as well as a non-intrusive property. For these, the Rogowski current sensor (RCS) is particularly suitable. The...

Single-phase split-source inverter is an attractive single-stage topology, as it can realize buck-boost function in a very compact way. However, this topology suffers from higher voltage stresses on dc-link than two-stage structure, especially under low ac gain conditions. This paper therefore analyzes the substantial reason of this problem. Then,...

In a three-phase three-level grid-connected inverter, the ac output current ripple actually fluctuates four times per switching period with each fluctuation generating some core loss. However, only the two larger fluctuations are considered in most loss estimation methods, resulting in a smaller core-loss evaluation. In this letter, an alternative...

Three-winding coupled inductor, arranged as a Y-source network, has been used for voltage-boosting with a standard inverter bridge and no other active switches. However, its maximum modulation and boost duty ratios are correlated with an increase of one maximum necessitating the reduction of the other. Its voltage quality and DC-bus utilization wil...

A sliding-mode scheme using only a single current regulator for both speed and flux-weakening control has been proposed for permanent magnet synchronous motor. The scheme is simple and can effectively resolve problems faced by conventional flux-weakening control with dual current regulation loops. However, with only a single current regulator, the...

The Y-source inverter uses different turns ratios of a coupled inductor and a controllable duty ratio to vary its gain. However, its input current is discontinuous, and any leakage inductances of its coupled inductor will lead to unintentional dc-link voltage spikes and gain reduction. To solve these problems, an extended quasi-Y-source inverter ha...

The centred space vector modulation scheme is considered to be the best since it produces the least switching harmonics among the modulation techniques developed for neutral‐point‐clamped (NPC) inverters. However, this common belief was concluded in the condition when inverters interfaced to the grid with a simple inductor filter. Nevertheless, to...

This study presents an enhanced method for simultaneously achieving optimised disturbance damping and axis-decoupling in high-power converters operating in the synchronous rotating frame. High-power converters are considered here because they are normally constrained to a low-switching-to-fundamental pulse ratio, and hence experience a large contro...

Impedance-source inverters using coupled inductors have been investigated as alternatives for providing high step-up voltages. However, leakage inductances of the coupled inductors have commonly led to lower overall effectiveness, in addition to generating high dc-link voltage spikes. The latter raises voltage stresses of switches, which in turn, m...

Second-order oscillating power is always generated by a single-phase converter. To decouple this oscillating power from its dc-link then requires a power decoupling circuit to operate continuously. In contrast, ac grid voltages supplying a three-phase converter are usually balanced. There will hence be no oscillating power, if grid currents are bal...

Differential inverters provide a costeffective solution to the second order ripple power issue in single phase systems. Most existing differential inverter based power decoupling methods are for linear loads, which may not work well for nonlinear loads. When supplying nonlinear loads, differential inverters may suffer from harmonics at the AC termi...

By using coupled inductors, impedance-source inverters can greatly increase their voltage step-up abilities at the expense of high voltage spikes at their dc-links due to leakage inductances. These voltage spikes can then cause damages and greater power losses. Therefore, to better clamp their dc-link voltages, various absorbing circuits have been...

Single-phase differential boost inverter, built from two identical DC-DC boost converters, has been suggested as an alternative to the usual connection of a DC-DC boost converter to a standard full-bridge inverter. Some control schemes for controlling it have also been proposed, but mostly for voltage regulation, supplying a standalone load, rather...

The unbalance of the three-phase grid-voltages/loads can cause double-frequency power ripples in the Voltage Source Converter (VSC) system. Such power ripples can further lead to current and/or voltage ripples on the DC link, which may create instability of the system, lower its efficiency and shorten the lifetime of DC sources (e.g. batteries). Ex...

The output currents of high-power Voltage Source Inverters (VSIs) are distorted by the switching harmonics and the background harmonics in the grid voltage. This paper presents an active harmonic filtering scheme for high-power, low-frequency switching VSIs with an additional auxiliary VSI. In the approach, the auxiliary low-power, high-frequency s...

This paper addresses the harmonic stability resulting from the current-control interactions of the multi-paralleled, LCL-filtered voltage-source converters. First, an alternative impedance model is proposed for the single-loop current control. The control output admittance of the converter is decomposed into a passive filter output admittance in se...

This paper comprehensively analyzes the stability of a grid-interfacing inverter with LCL filter in the discrete domain, where the LCL filter, along with the controller, are modeled in a polar coordinate. System open-loop and closed-loop poles are analytically studied and expressed in the z-domain. Through the poles movement and distribution analys...

This paper analyzes the stability of digitally voltage-controlled Voltage-Source Inverters (VSIs) with the linear voltage regulators. It is revealed that the phase lags, caused by using the resonant controller and the time delay of a digital control system, can stabilize the single-loop voltage control without damping of the LC-filter resonance. Th...

This paper investigates the interharmonic generation process in the input current of double-stage Adjustable Speed Drives (ASDs) based on voltage source inverters and front-end diode rectifiers. The effects of the inverter output-side low order harmonics, caused by implementing the double-edge symmetrical regularly sampled Space Vector Modulation (...

A Dual Active Bridge (DAB) converter can achieve a wide high-efficiency range when its input and output voltages are equal, assuming a 1:1 turns ratio for its isolation transformer. If its input or output voltage is doubled, efficiency of the DAB will drop significantly, because of the introduction of hard switching and high circulating power. Thus...

LCL filters feature low inductance; thus, the injected grid current from an LCL-filtered Voltage Source Inverter (VSI) can be easily distorted by grid voltage harmonics. This problem is especially tough for the control system with Inverter-side Current Feedback (ICF), since the grid current harmonics can freely flow into the filter capacitor. In th...

Traditional droop-controlled system has assumed that generators can always generate the powers demanded from them. This is true with conventional sources, where fuel supplies are usually planned in advance. For renewable sources, it may also be possible if energy storage is available. Energy storage, usually as batteries, may however be expensive,...