DC Current Injection Research Articles

Dual-Loop-Based Current Controller for Transformerless Grid-Tied Converters with Improved Disturbance Attenuation against Voltage Measurement Errors

To eliminate current harmonics and to obtain enhanced dynamic behaviors, the voltage feedforward scheme is normally adopted in the control process of transformerless grid-tied converters. However, voltage measurement errors caused by parameter variation of sampling resistance and zero-voltage drift in analog devices will result in DC current injection and distorted grid currents in grid-tied systems. Conventional compensation strategies based on plug-in repetitive controllers and multiple resonant controllers are considered to be effective solutions to solve this problem. Even though undesired components in grid currents can be partially mitigated with these conventional compensation schemes adopted, degraded reference-tracking process with oscillations and slow dynamics are inevitably caused by the controller coupling. To obtain decoupling between reference tracking and disturbance rejection, this paper proposes a dual-loop controller to achieve current regulation and to suppress the disturbances caused by voltage measurement errors. With the proposed dual-loop controller adopted, DC current injection and distortion of grid currents can be effectively attenuated while excellent transient performance with negligible overshoot and fast step response can be simultaneously guaranteed. Frequency-domain analysis and experimental validations are both conducted to verify the effectiveness of the proposed strategy.

Investigation of the Effects of DC Current Injected by Transformer-Less PV Inverters on Distribution Transformers

Rooftop transformer-less photovoltaic inverters (TLPVIs) have the potential to inject a very small dc current into the distribution network. The dc current quantity can be very high in a residential power distribution system where several TLPVIs are installed. Distribution transformers (DTs) supplying power to the customers in the networks can be adversely saturated with this high dc current accumulated from the small dc current injected by individual TLPVI. In this paper, a systematic investigation is carried out on the dc current injection potential of commercially available TLPVIs. Furthermore, the influence of this aggregated dc current on the DTs are analyzed through modelling, simulation, and laboratory testing. A three-phase three-leg transformer is used to analyze the effects of the high dc injection from the TLPVIs. A finite element analysis is carried out in COMSOL Multiphysics environment to explore the dc current injection effect. The results show that the injected dc current from TLPVIs highly affect the DTs and a solution is needed to overcome this challenge before the problem spreads widely and causes a barrier to the growth of the integration of renewable resources to the power grids.

DC Injection Suppression in Transformer-Less Grid-Connected Inverter using a DC-Link Current Sensing and Active Control Approach

There is increasing interest in the use of transformer-less systems for grid-connected photovoltaic applications. Compared to transformer-coupled solutions, transformer-less systems offer a typical efficiency increase of 1–2%, reduced system size and weight, and a reduction in cost. However, the removal of the transformer has technical implications. In addition to the loss of galvanic isolation, dc current injection into the grid is a potential risk. While desirable, complete mitigation of dc current injection via convention current control methods is known to be particularly challenging. For this reason, this paper proposes an active dc suppression method, in which the dc current injection is accurately determined by extracting the line-frequency component from inverter dc-link current measurements, and then, mitigated with an active closed-loop controller. Experimental results from a laboratory grid-connected inverter system are presented to demonstrate the high performance of the proposed technique.

Mitigation of DC Current Injection in Transformer-Less Grid-Connected Inverters Using a Voltage Filtering DC Extraction Approach

Transformer-less grid-connected PV inverters are an attractive technology choice due to their high-efficiency performance and relatively small volume. However, in the absence of galvanic isolation, dc current can potentially flow into the grid and adversely affect the grid performance. In addition, as the sinusoidal ac output is significantly larger than the dc injection component, accurate dc current sensing at the inverter output remains a technical challenge. As a result, this paper proposes a two-stage dc voltage detection technique that employs an isolated RC attenuation circuit and a software dc-component filtering algorithm. The principle of the proposed technique can be applied in many switch-mode power converter topologies. The simple and cost-effective combination facilitates dc component extraction within the accuracy of 1 mV; moreover, this can be further used as digital dc feedback for active current control purposes. Experimental results from a laboratory prototype demonstrate the high performance of the proposed dc extraction and suppression approach.

Detection of Ground Insulation Faults in Ungrounded DC Power Supply Systems by Using a DC Current Injection Method

This paper proposes a DC current injection method for detecting ground insulation faults. When the ratio of the voltage of the positive bus to that of the negative bus is lower than a threshold value, the DC current is injected into the ungrounded DC power supply systems. DC power supply systems have essentially some grounded stray and external capacitors, so some of the injected DC current will flow into the grounded capacitor, increasing the voltage of the fault bus and the leakage current. The proposed method increases the leakage current by the injection of DC current, so the current sensor can detect the leakage current from which the insulation resistance can be calculated. Experimental results demonstrate that the DC current injection method can significantly improve the detection of ground insulation faults in ungrounded DC power supply systems.

Multi‐functional distributed generation control scheme for improving the grid power quality

Renewable sources are expected to be a key element of future power systems. With many challenges ahead, thinking out of the box is necessary in order to equip the power system with additional advanced functionalities and capabilities to combat the anticipated challenges. The associated equipment of distributed generation can be seen as a grid asset upgrade because of the increased number of elements connected to the grid, such as grid side converters (GSC) of photovoltaics and battery storage systems. It therefore makes sense to think of ways to diversify the role of distributed renewables in a way that benefits the grid, consumers, and prosumers in terms of power quality. The work proposes an advanced, multi-function GSC control technique for improving the distribution grid power quality by enabling the injection of asymmetric, DC and harmonic currents in case it is needed. Injecting for example such currents locally through the GSC of a residential photovoltaic system can compensate undesired characteristics of the prosumer loads in order to allow the flow of symmetric and high quality currents between the prosumer and the distribution grid. Experiments and simulation results are presented to validate the performance of proposed control technique.

An intelligent dc current minimization method for transformerless grid-connected photovoltaic inverters

Due to the scaling and zero-drift of current sensor errors, unbalanced grid voltages, tolerance of power switching devices, and asymmetry of PWM gate driving pulses, transformerless grid-connected inverters usually have certain amount of dc components injected to the ac grid. Therefore, power quality of the grid is degraded. Many efforts, such as using blocking capacitors, a dc current feedback control method, and a voltage dc component feedback control method, etc., have been introduced to minimize the dc injection. This paper proposes an intelligent control strategy of dc current injection suppression to the grid by utilizing adaptive-back-propagation (ABP) neural network PID controller. The performance of the proposed scheme is evaluated and compared with the traditional and existing method. Finally, the control scheme is verified on a 2-kW three-phase grid-connected inverter in the laboratory.

A Novel DC Current Injection Suppression Method for Three-Phase Grid-Connected Inverter Without the Isolation Transformer

Due to current sensor errors, tolerance of power switching devices, and asymmetry of PWM gating driving pulses, grid-connected inverters without isolation transformer usually have certain amount of dc components injected to the ac grid. Many efforts, such as using a blocking capacitor, a current dc component feedback control, and a voltage dc component feedback control have been introduced to try to suppress the dc injection to the grid. This paper proposes a novel control strategy of suppressing dc current injection to the grid for a three-phase inverter by accurately sensing the dc component of line voltages of three-phase inverter and adding a dc component control loop. A dc suppression control scheme is presented, and the suppression performance of the proposed dc rejection method is evaluated and compared with the traditional method. Finally, the control scheme is verified on a 20-kW three-phase grid-connected inverter.

Application of voltage multiplier in 12‐pulse rectifier for sinusoidal input current

A DC current injection circuit of a trifurcated autotransformer (TAT)-based 12-pulse diode bridge rectifier (DBR) is proposed which can help reduction of harmonics in the AC mains. The circuit at the DC side employs a voltage multiplier (VM) circuits namely voltage doubler, voltage tripler and voltage quadrupler and an interphase transformer which provides the required circulating current. This induced circulating current shapes the input AC line current near to a sine wave. The output of VM circuits is connected across the load and thereby reuses the harmonic energy absorbed by the system. Thus, improves the energy conversion efficiency of the proposed system. The TAT-based 12-pulse DBR has the implicit ability to prevent the zero sequence components thus expels the necessity of zero sequence blocking transformer. The proposed configuration is analysed, simulated in MATLAB Simulink and the simulation results are presented, which confirms the improvement in power quality parameters in the input AC line current. Further, the viability of the proposed configuration is verified by experimental results which confirm the suitability of the proposed configuration in AC–DC applications.

Simulation of electrically induced vortical flows

In this work, we study electrically induced flows numerically using open-source software and experimentally. Two systems are considered - single and multiphase (free surface) flows driven by axisymmetric DC and AC current injection. We investigate characteristic velocity and free surface deformation dependence on the injected current and validate it with experimental data. Results show that maximum axial velocity is a linear function of injected current, but free surface deformations are approximately proportional to current squared.

A Reconfigurable Solid-state Plasma Dipole Antenna Based on SPiN Diodes

This paper proposed an innovative reconfigurable antenna concept based on the dynamic definition of metal-like conductive plasma in high-resistivity silicon that are activated by the injection of dc current. Based on investigations of the solid-state plasma characteristics, the metallic properties of solid plasma were verified. And a demonstration prototype solid-state plasma frequency reconfigurable antenna containing 36 SPiN diodes was designed, fabricated, and tested. This antenna can work at three different states by turning SPiN diodes on or off selectively, in turn, working at the desired frequency state. The results show that this type of antenna posseses the unique characteristics compared with the conventional one.

Room temperature continuous-wave nanolaser diode utilized by ultrahigh-Q few-cell photonic crystal nanocavities

Few-cell point-defect photonic crystal (PhC) nanocavities (such as LX and H1 type cavities), have several unique characteristics including an ultra-small mode volume (Vm), a small device footprint advantageous for dense integration, and a large mode spacing advantageous for high spontaneous-emission coupling coefficient (β), which are promising for energy-efficient densely-integratable on-chip laser light sources enhanced by the cavity QED effect. To achieve this goal, a high quality factor (Q) is essential, but conventional few-cell point-defect cavities do not have a sufficiently high Q. Here we adopt a series of modified designs of LX cavities with a buried heterostructure (BH) multi-quantum-well (MQW) active region that can achieve a high Q while maintaining their original advantages and fabricate current-injection laser devices. We have successfully observed continuous-wave (CW) lasing in InP-based L1, L2, L3 and L5 PhC nanocavities at 23°C with a DC current injection lower than 10 μA and a bias voltage lower than 0.9 V. The active volume is ultra-small while maintaining a sufficiently high confinement factor, which is as low as ~10-15 cm3 for a single-cell (L1) nanocavity. This is the first room-temperature current-injection CW lasing from any types of few-cell point-defect PhC nanocavities (LX or H1 types). Our report marks an important step towards realizing a nanolaser diode with a high cavity-QED effect, which is promising for use with on-chip densely integrated laser sources in photonic networks-on-chip combined with CMOS processors.

Soft Switching of Three-Phase AC to DC CIHRC with Wireless Power Transfer (WPT) Function

<span lang="EN-US">This paper presents the verification of soft switching condition for three-phase AC to DC current injection hybrid resonant converter (CIHRC) with wireless power transfer (WPT) function. Details on the operation of current injection technique with the lossless zero voltage switching (ZVS) condition on shaping the high power factor of supply current waveforms are presented. With a suitable high switching frequency operation, the proposed resonant converter is capable to operate with ZVS conditions, thus, allowing reduction in the size of inductive and magnetic components. Selected results are also presented to verify the lossless ZVS condition for three-phase AC-DC CIHRC with WPT function.</span>

Mitigation of DC and Harmonic Currents Generated by Voltage Measurement Errors and Grid Voltage Distortions in Transformerless Grid-Connected Inverters

The dc offset and scaling errors in the voltage measurements cause the injection of undesired dc and harmonic currents into the three-phase output currents of the grid-connected inverter. This study proposes an enhanced current control scheme to eliminate the dc and harmonic currents caused by voltage distortion and voltage measurement error in three-phase grid-connected inverters. The proposed current controller is designed in the synchronous ( d-q ) reference frame and composed of a proportional resonant (PR) controller and a repetitive controller (RC). The role of the RC is to regulate the grid current follow the reference value as well as compensate all $6{\rm{n}}\omega _{{\rm{s}}}$ and $(6{\rm{n}}\pm 2)\omega_{{\rm{s}}}\,(n= 1,\,2,\,3,\ldots)$ harmonic components caused by distorted grid voltage and scaling errors. Meanwhile, the PR controller helps to compensate the dc current generated by dc offset measurement errors to guarantee that the three-phase grid currents are balanced and sinusoidal with extremely low dc component. Since the RC employed in the proposed current controller reduces to time delay by four times compared to the conventional RC, the proposed control system ensures good steady-state performance of the grid current without deteriorating its fast dynamic response. The effectiveness of the suggested solution is verified by various experimental tests.

Compensation of DC Offset and Scaling Errors in Voltage and Current Measurements of Three-Phase AC/DC Converters

The dc offset and scaling errors in the voltage and current measurements cause the injection of undesired dc and unbalanced currents into the three-phase input currents and subsequently lead to voltage ripple at the dc output voltage of the ac/dc converter. This paper proposes a compensation scheme for current measurement error where the dc offset and scaling errors in the current measurement are estimated from the voltage ripple characteristics of the dc output voltage ripple combining with simple band-pass and low-pass filters. Meanwhile, the dc offset and scaling errors in the voltage measurement can be rejected by using an advanced current controller developed with a proportional resonant plus a repetitive controller designed in the synchronous ( d–q ) reference frame. The proposed control algorithm is able to effectively reject the impact of both voltage and current measurement error so that the three-phase input currents are regulated to be balanced and sinusoidal with ultralow dc current component. Meanwhile, the dc output voltage is also well regulated to be pure dc with a negligibly small voltage ripple. The proposed compensation method is developed without the need of extra hardware circuit, sensor, or precise information of system parameters so that it can be considered as more cost effective and robust solution. The effectiveness of the proposed solution is verified by experimental results.

Research of the SPiN diodes for silicon-based reconfigurable holographic antenna

Silicon-based solid state plasma antennas were characterized by its wide radiation range, good stealth characteristic, and dynamic reconfigurability, which have broad application prospects in the future. In this paper, investigations of surface PiN diodes developed for silicon-based reconfigurable holographic antennas have been demonstrated for using millimeter-wave communication systems. The SPiN diodes have been extensively discussed, and the obtained results (simulations and experiments) confirm the applicability of these devices for dynamically reconfigurable antennas. A carrier concentration of 1018–1019 cm−3 has been achieved within the optimized SPiN diode. Reconfigurable holographic antennas based on SPiN diodes that were activated by the injection of dc current were demonstrated in this paper. The resonance frequencies at 60 GHz and 64.5 GHz have been easily achieved by turning on or off different sections of the reconfigurable dipole antenna, while the radiation efficiencies were 85.1% and 83.8%, respectively. A double-layer holographic structure was also investigated in this paper. The study reveals that a novel reconfigurable antenna with SPiN diodes has been formed in a single system and has numerous advantages over the traditional antenna.

Coupled-Inductor-Based DC Current Measurement Technique for Transformerless Grid-Tied Inverters

Grid-tied photovoltaic inverters must fulfill several requirements, including high efficiency and reduced cost and complexity of the overall system. Hence, transformerless operation is advantageous in order to achieve the prior requirements. Meanwhile, such operation results in several demerits, such as the dc current component injection into the grid. This component should be effectively mitigated in order to avoid some impacts, such as the saturation of the transformers in the distribution network. On the other hand, limiting this component up to few milliamperes is a challenging issue due to the various measurement errors. Accordingly, different blocking and measurement techniques have been proposed and studied to overcome this issue, where some demerits are seen behind each technique such as the implementation complexity, the common-mode voltage problems, and the high filter requirements. Moreover, none of them measures the dc component directly, but predicts its value using different approaches. Hence, this letter proposes a new technique to measure this dc current component with high accuracy using a coupled inductor combined with a small-range Hall effect current sensor in order to achieve the lowest possible cost with the highest possible accuracy. The proposed technique is introduced, analyzed, and tested experimentally to verify its principle of operation. Also experimental measurement of the dc current component using a 5-kVA transformerless grid-tied voltage-source inverter is introduced with and without the proposed technique in order to validate its operation.

Improvement in Harmonic Reduction of a Zigzag Autoconnected Transformer Based 12-Pulse Diode Bridge Rectifier by Current Injection at DC Side

In this paper, to improve the harmonic reduction ability in the input line current of a 12-pulse diode bridge rectifier using zigzag autoconnected transformer with a dc side current injection technique is presented. Conventional 12-pulse rectifier does not meet the IEEE-519 standard and therefore current injection technique at dc side is used to shape the converter current to minimize the harmonic distortion in the input line current. The proposed converter configuration uses a zigzag autoconnected transformer with inherent ability to block zero-sequence components, which eliminates the requirement of a zero-sequence blocking transformer, and thereby reduces the overall rating of the converter. The current injection technique at dc side enables to alter the dc side currents, which further shapes the input line currents to near sinewave with total harmonic distortion of less than 1% and power factor close to unity. The proposed converter configuration is simulated and analyzed in MATLAB/Simulink. The effects of variation in the magnitude of injected current on the input line current harmonics in terms of power quality indices are analyzed. A prototype of the proposed converter configuration is developed, and experimental results are presented to validate the theoretical design, analysis, and simulation results.

Droop Control of Solar PV, Grid and Critical Load using Suppressing DC Current Injection Technique without Battery Storage

This paper presents design of a sustainable solar Photo voltaic system for an Indian cities based residential/community house, integrated with grid, supporting it as supplementary sources, to meet energy demand of domestic loads. The role of renewable energy sources in Distributed Generation (DG) is increasingly being recognized as a supplement and an alternative to large conventional central power supply. Though centralized economic system that solely depends on cities is hampered due to energy deficiency, the use of solar energy in cities is never been tried widely due to technical inconvenience and high installation cost. To mitigate these problems, this paper proposes an optimized design of grid-tied PV system without storage which is suitable for Indian origin as it requires less installallation cost and supplies residential loads when the grid power is unavailable. The energy requirement is mainly fulfilled from PV energy module for critical load of a city located residential house and supplemented by grid/DG for base and peak load. The system has been developed for maximum daily household demand of 50kWp and can be scaled to any higher value as per requirement of individual/community building ranging from 50kWp to 60kWp as per the requirement. A simplified control system model has been developed to optimize and control flow of power from these sources. The simulation work, using MATLAB Simulink software for proposed energy management, has resulted in an optimal yield leading efficient power flow control of proposed system.

A Fixed-Frequency Sliding Mode Controller for a Boost-Inverter-Based Battery-Supercapacitor Hybrid Energy Storage System

The boost-inverter-based battery-supercapacitor hybrid energy storage systems (HESSs) are a popular choice for the battery lifetime extension and system power enhancement. Various sliding mode (SM) controllers have been used to control the boost inverter topology in the literature. However, the traditional SM controllers for the boost inverter topology operate with a high and variable switching frequency which increases the power losses and system components design complexity. This can be alleviated by a pulse width modulation (PWM)-based fixed-frequency SM controller proposed in this paper. The SM controller is implemented using variable amplitude PWM carrier signals generated using the output capacitor voltage and inductor current measurements, thus eliminating the requirement of the output capacitor currents measurement. The battery-connected inductor reference currents for the SM controller are generated by a supercapacitor energy controller which is responsible for the HESS power allocation. First, the theoretical aspects of the SM controller, the operation and parameter selection of the supercapacitor energy controller, and the supercapacitor sizing for the HESS are discussed in the paper. Then, the proposed control system is experimentally verified, and it is shown that the HESS is able to satisfy the HESS output power requirements, while allocating the ripple current and the fast power fluctuations to the supercapacitor while maintaining operation of the supercapacitor within predefined voltage limits. The main advantage of the proposed SM controller, as compared with the traditional double-loop control method, is in eliminating possible DC current injection into the grid when the equivalent series resistance values of the boost inductors become unequal due to the tolerances and temperature variations.