AC Line Research Articles

Highly stretchable integrated system for micro-supercapacitor with AC line filtering and UV detector

In-plane micro-supercapacitors (MSCs) possess higher volumetric energy density and are thus more compact compared with traditional aluminum electrolytic capacitors (AECs). As a result, developing in-plane MSCs with AC line filtering function is desired to replace the AECs and integrate with other electronic devices. Here, highly stretchable integrated system for in-plane MSCs with AC line filtering and UV detector were fabricated by designing buckled micro-electrodes based on SWCNT film and TiO2 NPs. Although gel electrolyte was used in these MSCs, they still show an ultrafast frequency response with a phase angle of -75.2° because of their unique structure. Such MSCs thus can function as the AC line filtering devices. Furthermore, their AC line filtering behavior remains almost unchanged even stretched up to 200%. Further coating TiO2 NPs on the buckled SWCNT micro-electrodes is able to endow the MSCs with another ability of UV photodetection with the sensitivity of 6.2. Importantly, such integrated device can still show stable photocurrent response and capacitance behaviors at different stretching times and even repeated stretching 100 times. The rational design of such stretchable integrated system for MSCs with AC line filtering and UV photodetector will pave the way for the applications in assembling supercapacitors and other electronics into highly stretchable integrated devices.

Coordinated voltage regulation of hybrid AC/DC medium voltage distribution networks

In a hybrid AC/DC medium voltage distribution network, distributed generations (DGs), energy storage systems (ESSs), and the voltage source converters (VSCs) between AC and DC lines, have the ability to regulate node voltages in real-time. However, the voltage regulation abilities of above devices are limited by their ratings. And the voltage regulation efficiencies of these devices are also different. Besides, due to high r/x ratio, node voltages are influenced by both real and reactive power. In order to achieve the coordinated voltage regulation in a hybrid AC/DC distribution network, a priority-based real-time control strategy is proposed based on the voltage control effect of real and reactive power adjustment. The equivalence of real and reactive power adjustment on voltage control is considered in control area partition optimization, in which regulation efficiency and capability are taken as objectives. In order to accommodate more DGs, the coordination of controllable devices is achieved according to voltage sensitivities. Simulations studies are performed to verify the proposed method.

A Boost PFC Stage Utilized as Half-Bridge Converter for High-Efficiency DC–DC Stage in Power Supply Unit

The half-bridge (HB) LLC converter is one of the most attractive dc–dc converters for medium power supplies due to its soft switching capability. However, its conversion efficiency is considerably degraded in wide-link-voltage applications because of a small magnetizing inductance and wide switching frequency variation for a high voltage gain. In this paper, a boost power factor correction (PFC) stage, which can also play an important role during the hold-up time, is proposed for a high-efficiency HB LLC converter. In the proposed PFC stage, the boost PFC converter can be effectively utilized as a HB converter by replacing a boost diode and inductor with a synchronous switch and transformer, respectively. After the ac line is lost, the proposed PFC stage can operate as the HB converter and regulate the output voltage instead of the HB LLC converter. Thus, it enables the HB LLC converter to be designed with a large magnetizing inductance and narrow switching frequency variation. As a result, the proposed PFC stage can enhance the overall efficiency of the PSU by improving the efficiency of the HB LLC converter. To confirm the validity of this paper, a prototype with 180–264-Vrms ac line, 250–400-V link voltage, and 48 V/480 W output is tested.

An Efficient Buck/Buck-Boost Reconfigurable LED Driver Employing SIN2Reference

An efficient buck/buck-boost reconfigurable LED driver based on peak current control is introduced. The driver supports pulse-width modulation (PWM) and pulse-frequency modulation (PFM) operations. The use of a combination of rectified sin and sin2 functions in the reference is introduced for the purpose of improving the power factor (PF) and total harmonic distortion (THD) of buck and buck-boost converters. The optimal reference waveform shape for buck and buck/boost modes can be set externally. The design ensures that the peak of the inductor current maintains a constant level that is invariant for different ac line voltages. The LED driver has been implemented in a 130-nm CMOS process. PF and THD are improved when the proposed reference is employed, and the peak PF and lowest THD values are 0.995/0.983/0.996 and 7.8/6.2/3.5% for the buck (PWM), buck (PFM), and buck-boost (PFM) cases, respectively. The corresponding peak efficiency for the three cases is 88/92/91%, respectively.

Method for controlling voltage and frequency of the local offshore grid responsible for connecting large offshore commercial wind turbines with the rectifier diode‐based HVDC‐link applied to an external controller

This study introduces a method for controlling a system that rectifies electric current where power is transferred from several offshore wind power generation systems through a common offshore AC line, into a diode-based bridge. This modular rectifier comprises one or more uncontrolled modules (such as diode rectifiers). According to the study, the wind power supply systems receive a reactive power reference from a central external regulator to maintain the common AC-line frequency. Although there are other control methods for this topology, the proposed external control uses commercial wind turbine with no changes in their internal control. The power plant control systems use their own algorithm to select the active and reactive power reference, maintaining the original wind turbine control unaltered. This method for controlling the voltage and the frequency of local offshore grid allows the operation of the different rectifiers end of the HVDC link. PSCAD ® simulations are used to prove the technical control and the design concept.

Efficient integration of (n − 1)-security into probabilistic network expansion planning

This paper presents a new efficient way to integrate the (n−1)-security into modern probabilistic network expansion planning. Therefore, a method is presented for an (n−1)-secure probabilistic load flow based on AC distribution factors. For the simulation of multiple segment outages, a new mathematical description is presented. The presented method is able to handle the islanding of single nodes within the calculation. Afterwards, a linearization is presented using the superposition theorem and Newton Raphson load flow calculations. A new linearization of the power flow is used to consider reactive power flows. Based on this, AC line outage distribution factors can be calculated using the normal operation state of the grid. The analyzed sensitivity of the distribution factors to the load and generation scenario is low. Therefore, the linearized method shows a good congruency. Additionally, the proposed method identifies correctly the worst case conditions in massively reduced computation time. A comparison between the (n−1)-secure probabilistic method and today’s deterministic planning methods show that the high penetration of renewable energy sources leads to an under dimensioning of the grid.

A Line Impedance Conditioner for Saturation Mitigation of Zigzag Transformer in Hybrid AC/DC Transmission System Considering Line Unbalances

Hybrid ac/dc transmission extends the power transfer capacity of existing long ac lines closer to their thermal limit, by superposing the dc current onto three-phase ac lines through a zigzag transformer. However, this transformer could suffer saturation under unbalanced line impedance conditions. This paper introduces the concept of hybrid line impedance conditioner (HLIC) as a cost-effective approach to compensate for the line unbalance and therefore avoid saturation. The topology and operation principle are presented. The two-level control strategy is described, which enables autonomous adaptive regulation without the need of system-level control. Design and implementation are also analyzed, including dc-link capacitance as one of the key line conditioner components, HLIC installation, and protection under fault conditions. The cost study on this HLIC-based hybrid system is also performed to reveal the benefits of the solution. Simulation results and experimental results based on a down-scaled prototype are provided to verify the feasibility of the proposed approach.

Low-Switching-Frequency Operation of a Modular Multilevel DSCC Converter With Phase-Shifted Rotating-Carrier PWM

This paper provides a theoretical discussion on a new phase-shifted rotating-carrier (PSRC) pulsewidth modulation (PWM) method for a modular multilevel double-star chopper-cell (DSCC) converter. The new method enables the DSCC converter to operate at a low switching frequency in a range of 50 to 100 Hz where the ac grid or line frequency is 50 Hz. The phase shift of each carrier signal is rotated to enable capacitor-voltage balancing in such a low-carrier-frequency range while maintaining the same voltage in each arm as that of the original phase-shifted-carrier PWM. Theoretical analysis of harmonic-voltage spectrum at the arm side of a chopper cell is conducted on the basis of double-variable Fourier series. Then, the analytical results are compared to numerical ones. A three-phase grid-connected DSCC converter rated at 66 kV, 50 Hz, and 100 MW with 64 chopper cells per leg is used in circuit simulation to confirm the validity of the proposed PSRC-PWM method. The simulation results show that the PSRC-PWM can maintain stable operation of the DSCC converter even if the carrier frequency goes lower than 100 Hz. Several performance criteria conclude that the optimal carrier frequency is 75 Hz.

Resonance Reduction for AC Drives With Small Capacitance in the DC Link

Pulse-width modulated ac drives equipped with a small dc-link capacitor are becoming an attractive solution for electric drive applications with moderate requirements for shaft dynamic performance. However, when these drives are fed from a weak grid, a resonance between the line side impedance and the dc-link capacitor appears. Due to this resonance, the total harmonic distortion and the partially weighted harmonic distortion of the line currents are increased, which may raise compatibility problems with the ac line harmonic standards. This paper proposes a novel dc-link voltage compensation method, which can reduce the amplitude of the aforementioned oscillation, while the large dc-link voltage variation, caused by the rectification, can be eliminated from the motor current. The method has been experimentally validated and the results show excellent performance.

Impact of converter losses on the optimal power flow solution of hybrid networks based on VSC-MTDC

This paper studies the impact of voltage source converter (VSC) losses on the solution of the optimal power flow (OPF) of hybrid AC/DC systems with a multi-terminal configuration. The motivation of this analysis is that the expected development of high voltage direct current (HVDC) systems will entail an increase of the number of converter stations, and consequently, converter losses might not be negligible when compared with conventional transmission losses of AC and DC lines. Towards this end, an extended OPF model is proposed considering a combination of VSC based multi-terminal HVDC grids (VSC-MTDC) and AC systems. The OPF model represents converter losses according to the state-of-the art modeling where different expressions are used when the converter functions as an inverter or as a rectifier. Three simpler alternative approaches are also implemented and the obtained OPF solutions are assessed. In order to compare the OPF solutions, a comparison metric is proposed. Obtained results show that modeling the converter losses in a simplified way could lead to very different power flow solutions, especially for the DC branches.

Development of 2.8 V Ketjen black supercapacitors with high rate capabilities for AC line filtering

Supercapacitors are generally more compact than conventional bulky aluminum electrolytic capacitors (AECs). Replacement of AECs with supercapacitors can lead to miniaturization of electronic devices. However, even state-of-the-art supercapacitors developed in laboratories are superior to or competitive with AECs only in low voltage applications (<∼40 V). In order to improve the voltage limits of current supercapacitors, we have incorporated Ketjen black (KB) as an electrode material. Utilizing the open pore structure and the graphitic nature of KB, we demonstrate that the voltage limit can be extended to 53 V. The KB supercapacitor exhibits excellent areal capacitance, cell voltage, and phase angle values of ∼574 μF cm−2, 2.8 V, and ∼−80°, respectively. In addition, we demonstrate that an AC line filtering circuit with three supercapacitors connected in series can extend the application voltage without significant sacrifice in rate capability (ϕ ∼ −77° at 120 Hz). On the other hand, KBs are much less expensive than carbon materials previously demonstrated for AC line filtering and hence are very attractive for practical applications. We believe that this demonstration of high-performance supercapacitors made from low-cost carbon materials is both scientifically interesting and important for practical applications.

Stability enhancement of an interconnected AC/DC power system through VSC-MTDC operating point adjustment

This paper investigates the improvement of dynamic and transient stability in interconnected AC/DC power systems through operating point adjustment in an embedded VSC-MTDC network. Hopf Bifurcation (HB) is known to result in oscillatory instability in the power system, therefore a sufficient margin to HB has to be maintained to enhance the dynamic stability and ensure a secure operation of the system. The enhancement of dynamic stability is based on the adjustment of the MTDC converters’ controller setpoints to drive the system away from HB point and increase the damping of oscillatory modes. The variation of each setpoint is calculated in proportion to the eigenvalue sensitivity to the respective setpoint. On the other hand, transient stability enhancement is based on operating point adjustment according to Power Transfer Distribution Factors (PTDFs) with the aim to reduce inter-area power flow over AC lines. The proposed method is applied to a VSC-MTDC network interconnecting two IEEE 14-bus sections. The results show an improvement in the system's stability margin in case of various disturbances such as load increase and short circuit fault. Time-domain simulations in DIgSILENT are used to corroborate the results obtained through small signal stability analysis.

Graphene‐Based Organic Electrochemical Capacitors for AC Line Filtering

AbstractLess‐defective graphene oxide sheets with a small average size of 0.7 µm are electrochemically reduced to form a hydrogel film with highly oriented porous structure. It is applied as the electrode of organic electrochemical capacitor (OEC) after solvent change with organic electrolyte and deep reduction in this organic medium. At 120 Hz, the typical OEC exhibits a high areal specific energy density of 472 µF V2 cm−2 with a wide workable voltage window of 2.5 V, a phase angle of −80.5°, a resistor‐capacitor time constant (τRC) of 0.219 ms, and an excellent electrochemical stability. Thus, it is promising to replace aluminum electrolytic capacitors for AC line filtering. Furthermore, two identical OECs connected in series keep the performance of single device, making them practically applicable in electronics.

The Upgrading Renovation of an Existing XLPE Cable Circuit by Conversion of AC Line to DC Operation

The development of new technologies in power generation and application, together with the demand for the reliability and capability of power supply has drawn increasing attention to dc distribution and microgrid technology. This paper introduces the renovation project for a single-circuit XLPE cable line being converted into dc operation after its second failure under 35 kV ac. This is the first time ever in China for this type of system conversion, in which the existing XLPE cable line was put into operation in a bipolar voltage-source converter (VSC) dc system of 10 kV voltage and 150 A current to maintain its 3 MW capacity. The test methods for cable performance that were checked under the designed dc scheme were put forward and conducted on sectioned cable samples. After the cable test and equipment installation, the line was put into operation under the VSC dc topology in 2011. It has now been running for four years without any interruption. Compared with other alternatives for ac operation, this proposal shows advantages in aspects of economy, reliability and engineering. Besides, the theoretical evaluation suggests that the system capacity can be further increased by four times, and the first step is underway for that goal.

Dynamic Optimal AC Line Current Regulation Method for Three-Phase Active Power Conditioners

The main feature of the three-phase back-to-back active power conditioner (APC) is to improve the power quality of the microgrid. During the large power variation transient, the severe voltage surge/sag on the dc-link of the APC may cause its malfunction. Therefore, the dynamic optimal ac line current regulation (DOCR) with flexible dc-link voltage slope (FDVS) is proposed in this paper. The proposed DOCR can determine the accurate input current reference value to regulate the dc-link voltage under the large power variation condition. On the other hand, the proposed FDVS can further mitigate the dc-link voltage variation during the large power variation transient. The mathematical equations of the proposed control strategy are derived thoroughly. Furthermore, procedures to determine the dc-link capacitance with the proposed DOCR are developed. Finally, the experimental results obtained from a 5-kVA back-to-back APC verify the feasibility and the performance of the proposed line current regulation strategies.

Modeling and analysis of a smart grid monitoring system for renewable energy sources

The observing and metering processes are necessary in renewable energy conversion systems as applied in smart grid applications of conventional grid. In this study, the requirements of the renewable energy sources examined with a solar microgrid model that is developed via Matlab/Simulink. The dc-ac conversion system utilized in this paper contains three solar power plants with maximum power point tracking (MPPT) system and a multilevel inverter to create three-phase ac line voltages. The transmission line is modeled at the output of the inverter with a length of 25km by using real line parameters. The infrastructure of power line communication (PLC) is managed by binary phase shift keying (BPSK) modems that are located in different places. The usage of the proposed energy monitoring technique eliminates additional monitoring costs due to the fact that the power lines are not only exploited to carry the generated voltage, but also utilized to convey the drawn power rate of loads at the back-end of the microgrid.

A simple input current wave-shaping technique for three-phase diode and SCR converters

ABSTRACTThree-phase converters using diode or silicon-controlled rectifier (SCR) are widely employed to convert the commercial AC supply to DC. Such converters inject harmonics into the power supply system and thereby distort supply system voltage waveform. A simple input current wave-shape improvement technique using a shunt-connected harmonic current compensator is presented in this work, intended to reduce the total harmonic distortion (THD) of input current of three-phase diode and SCR phase-controlled rectifiers operating with inductive loads, by matching them to the specific converter as a combined package. The compensator proposed here comprises of a three-limb voltage source converter using insulated-gate bipolar transistor, working on instantaneous current and voltage measurements of the compensator only and not of the load. The technique uses a simple feedforward control for AC source current harmonic compensation of rectifiers without monitoring the AC line currents, i.e. use of online computation. The proposed system is simulated and tested on a laboratory prototype. The measured input current THD values without additional line filters are found to be below 8.3%, which is within acceptable limits, proving that the new technique is capable of compensating predetermined current harmonics of diode or SCRs.

Commutation Failure Influence on Power Converse in UHVDC Power System

For UHVDC transmission systems, the commutation failure of the DC system may cause the AC side power converse and may further lead to the AC line protection misoperation. This paper has analyzed power converse of AC system caused by commutation failure of DC system, and establishes the transmission line of UHVDC transmission line by using PSCAD/EMTDC simulation software. Using this simulation model to analyze how transition resistance in fault impact on power converse. The simulation results show: With the transition resistance increases, power converse presented a V shape change. By reducing positive sensitivity of sudden change relay or set a certain delay time (usually 40 ms) to start blocking mode pilot protection can prevent line protection misoperation caused by power converse.

A Case Study of Wind-PV-Thermal-Bundled AC/DC Power Transmission from a Weak AC Network

Wind power generation and photovoltaic (PV) power generation bundled with the support by conventional thermal generation enables the generation controllable and more suitable for being sent over to remote load centre which are beneficial for the stability of weak sending end systems. Meanwhile, HVDC for long-distance power transmission is of many significant technique advantages. Hence the effects of wind-PV-thermal-bundled power transmission by AC/DC on power system have become an actively pursued research subject recently. Firstly, this paper introduces the technical merits and difficulties of wind-photovoltaic-thermal bundled power transmission by AC/DC systems in terms of meeting the requirement of large-scale renewable power transmission. Secondly, a system model which contains a weak wind-PV-thermal-bundled sending end system and a receiving end system in together with a parallel AC/DC interconnection transmission system is established. Finally, the significant impacts of several factors which includes the power transmission ratio between the DC and AC line, the distance between the sending end system and receiving end system, the penetration rate of wind power and the sending end system structure on system stability are studied.

Research on Safety Protection for Live Working on 1000kV AC Transmission Line with AC and DC Multi-circuit Transmission Lines

In recent years, in order to save line corridors and reduce the scope of demolition, some new transmission modes for high voltage AC and DC power transmission line are proposed. According to the characteristics of the parallel transmission line, this paper studies the distribution characteristics of the electric field intensity of multi circuit parallel transmission lines and the single circuit transmission line. Comparing and analysing the simulation results of the electric field strength, the safety measures for live working of 1000kV AC transmission line with multi circuit parallel transmission lines is put forward. That is the electric field protective equipment and measures used in the live line work of the 1000kV UHV AC single circuit line can meet the requirements of the live operation of the 1000kV UHV AC double circuit line.