AC Voltage Conditions Research Articles

Analysis and Suppression of Capacitor Voltage Ripple for Hybrid MMCs Under Boosted AC Voltage Conditions

Capacitor voltage ripple suppression is significant for hybrid modular multilevel converters (MMCs) to reduce the capacitance, footprint and cost. However, the existing voltage ripple suppression methods are mainly intended for half-bridge submodule (HBSM)-based MMCs, and not applicable to the hybrid MMC due to its special configuration of mixed half-bridge and full-bridge submodules (FBSMs). Therefore, this article aims to reduce the capacitor voltage ripple for hybrid MMCs under boosted ac voltage conditions. The analysis of charging and discharging behaviors of FBSMs and HBSMs with boosted ac voltage is conducted first to obtain the maximum energy ripple in SM capacitors, which is equivalent to peak-to-peak value of voltage ripple. Then, the effect of modulation index and power factor angle on the capacitor voltage ripple is investigated in detail to find the possible ways to suppress the fluctuation. Consequently, the optimal modulation index that minimizes the capacitor voltage ripple is derived. Furthermore, this article proposes a circulating current injection method to suppress the capacitor voltage ripple under various operating points. In this method, the capacitor voltage ripple amplitude is taken as the direct suppression object to ensure the effect of voltage ripple minimization. Finally, the validity of the proposed method is proved under different operating conditions by simulation and experimental results.

A Second-Order Voltage Ripple Suppression Strategy of Five-Level Flying Capacitor Rectifiers Under Unbalanced AC Voltages

Unbalanced ac voltages drastically deteriorate the performance of five-level flying capacitor (FC) rectifiers. Without introducing an extra buffering circuit to absorb the second-order oscillating power, it is an incompatible contradiction for realizing both negative-sequence current reduction and second-order voltage ripple suppression. In this article, based on the idea of using FCs in the rectifier to absorb the oscillating power, a second-order voltage ripple suppression strategy for unbalanced ac voltage conditions is proposed. Reducing the negative-sequence currents and suppressing the dc voltage ripples can be simultaneously realized. Simulations and experimental results verified the effectiveness and performance of the proposed scheme well.

Fluorescent fiber-based identification of incipient discharges in liquid nitrogen

Identification of incipient discharges is a major challenge in liquid nitrogenbased super-conducting power apparatus. In the present study, a variety of fluorescent fibers with different absorption/emission characteristics were used for the identification of corona and surface discharges, and we find that a particular type of fluorescent fiber is more sensitive to identify discharges in liquid nitrogen insulation than the UHF technique. Enhancement in the magnitude of fluorescent signal/UHF signal is observed for an increase in applied voltage beyond the corona inception voltage (CIV). Also, it is observed that while reducing the applied voltage, the corona activity quenches at a much lower voltage than the inception voltage. No characteristic variation in rise time or the fall time of the fluorescent signal is observed on corona inception and due to discharges at higher voltages. The corona inception voltage is least for AC voltage conditions, and slightly higher inception voltage is observed for positive DC and negative DC voltage conditions. Different weight percentages of MgO doping in GFRP nanocomposites are prepared and used in the surface discharge studies. It is observed that the 3 wt% sample exhibits enhanced surface discharge inception voltage (SDIV) compared to samples with other doping concentrations and that the inception voltage is lower for AC voltage when compared with the DC voltage conditions. The characteristic spectral content of the fluorescent signal due to corona and surface discharge activity is observed up to 3 MHz. A phase-resolved partial discharge (PRPD) analysis was also carried out to identify corona and surface discharge activity using fluorescent fiber sensor and with a UHF sensor. From the PRPD studies, it is observed that the corona discharge occurs near the peak magnitude whereas surface discharge is observed at the rising part of AC voltage.

An effective fault management scheme and comprehensive double line‐frequency ripple propagation analysis for MVDC networks

This paper proposes an effective fault management scheme for medium-voltage direct current (MVDC) networks, which includes a virtual-impedance-based fault current limiter (VI-FCL) on the DC side and a positive-negative-sequence (PNS) control scheme on the AC side. Depending on the fault current value, different protection strategies are adopted, which include either isolating the fault by opening circuit breakers or riding through the fault with the proposed VI-FCL. It is known that unbalanced grid voltage can be found in conventional AC distribution feeders due to unbalanced loading conditions among three phases. The unbalanced grid voltage could induce double line-frequency (2ω) ripples on both DC voltage and current, which could thereby trigger protection malfunction in the DC sub-sections. Therefore, the PNS control scheme is used to eliminate the 2ω ripples of both DC voltages and currents caused by unbalanced AC voltages. Furthermore, a detailed 2ω mathematical model of the MVDC network under unbalanced AC voltage condition is derived to investigate how the 2ω ripple propagates across the network. A voltage source converter based multi-terminal MVDC network is built to test DC faults with different fault currents, unbalanced AC voltages, and different control schemes. The test results verify both the proposed fault management scheme and the 2ω ripple propagation analysis.

Analysis and Design of a Double Fuzzy PI Controller of a Voltage Outer Loop in a Reversible Three-Phase PWM Converter

Aiming at the application of a reversible three-phase pulse width modulation (PWM) converter with a wide range of AC side voltage and DC side voltage, a double fuzzy proportional integral (PI) controller for voltage outer loop was proposed. The structures of the proposed controller were motivated by the problems that either the traditional PI controller or single fuzzy PI controller cannot achieve high performance in a wide range of AC and DC voltage conditions. The presented double fuzzy controller studied in this paper is a sub fuzzy controller in addition to the traditional fuzzy PI controller; in particular, the sub fuzzy controller can get the auxiliary correction of PI control parameters according to the AC side voltage and the DC side given voltage variation of PWM converter after the reasoning of the sub fuzzy controller, while the traditional fuzzy PI controller outputs the correction of PI control parameters according to the DC voltage error and its error change rate. In this paper, the traditional fuzzy PI controller can be called the main fuzzy controller, and the adaptive adjustment of PI control parameters of the voltage outer loop is the sum of the PI parameter correction output by the main fuzzy controller and the auxiliary PI parameter correction output by the sub fuzzy controller. Finally, the experimental results show that the reversible three-phase PWM converter can achieve excellent dynamic and static performance in a wide range of AC voltage and DC voltage applications by using the proposed double fuzzy PI controller.

Two-species modeling of electrohydrodynamic pump based on surface dielectric barrier discharge

The electrical and mechanical characteristics of a cylindrical electrohydrodynamic pump are discussed in this paper. The air flow in this pump is produced by the surface dielectric barrier discharge generated between two strip electrodes attached to a hollow dielectric cylinder. The pump characteristics are predicted by simulating the ionized electric field (non-thermal plasma) and the air flow. A quasi-stationary plasma model is approximate and involves two ionic species. However, their full spatial and temporal distributions are calculated by solving the Poisson and charge transport equations using the Finite Element Method. The pump electrical (electric power) and mechanical (body force, velocity distribution, volume flow rate, output mechanical power, efficiency) parameters are presented for different magnitudes of ac voltage, its frequency, pump geometry and operating conditions. Simulations were based on combination of the COMSOL and ANSYS/FLUENT commercial software.

DC and AC electric field analysis and experimental verification of a silicone rubber insulator

This paper presents the electric field analysis as well as the experimental verifications of a silicone rubber insulator by considering DC and AC voltage conditions. As a first step, a 3D electric field simulation of the insulator was performed by using a commercially available software based on finite element method. DC voltage and 50 Hz power–frequency voltage simulations were conducted for the given insulator. Low frequency field distribution along the insulator has been investigated to find threshold frequency values for resistive and capacitive field regions. The effect of the conductivity of air surrounding the insulating system on the electric field was evaluated. For each condition, electric potential and field distribution along the creepage path of the insulator was obtained. In addition to the simulation studies, voltage measurements under AC and DC conditions utilizing a different approach for the silicone rubber insulators were conducted on the insulator by using a non-contact type electrostatic voltmeter to validate the simulation results.

Discharge Characteristics of Non-Cross Linked Polyethylene for a Power Cable

The XLPE (Cross Linked Polyethylene) is widely used to fabricate the sheath of power cable for its excellent electrical characteristics. However, the XLPE may cause environmental disruption and electrical aging due to its thermosetting property and included impurities created during a manufacturing process. A study on the development of a non-cross linked polyolefin material is conducted as a substitute material for XLPE material by the research project, 'Development of non-cross linked polymetric insulators for eco-friendly distribution power cable' supported by Korea Institute of Energy Technology Evaluation and Planning (KETEP). In this study, experiments on the newly developed non cross linked polyolefin material and the conventional XLPE material are conducted in accordance with the standards of ASTM D-149 and the experimental results are compared with each other. Also, dielectric experiments by using several kinds of electrode systems made with stainless steel in silicone oil are conducted under AC voltage condition and the experimental results are analyzed by the Weibull distribution.

High-Performance Three-Phase PWM Converter With a Reduced DC-Link Capacitor Under Unbalanced AC Voltage Conditions

A high-performance three-phase pulse width modulation (PWM) converter with reduced dc-link capacitor under unbalanced ac voltage conditions is proposed in this paper. The unique feature of the converter lies in that sinusoidal symmetrical ac currents and ripple free dc voltage can be achieved simultaneously without using a bulky capacitor, which was thought to be a dilemma in the past. The control scheme is implemented in an α–β frame, so the complex rotate transformations are avoided, and it does not require the extraction of positive and negative sequence currents, which simplifies the control algorithm. A two-quadrant operation active power compensator together with its controller is developed to suppress the voltage ripple in the dc-link. Compared with the passive compensation method, the compensator is equipped with a smaller capacitor to reduce the system size, weight, and cost. Moreover, the compensator could be integrated with the conventional three-phase three-leg PWM converter seamlessly. The effectiveness of the proposed control schemes is verified by the experiment results.

배전용 전력 케이블을 위한 비-가교 폴리올레핀 재질의 절연특성 연구

A cross linked polyethylene (XLPE) material has been widely used to develop a distribution power cable due to its excellent electrical characteristics and mechanical strength. However, several problems such as environmental disruption, electrical aging, thermosetting property, and impurities which cause degradation also arise. Therefore, a novel dielectric material should be developed to substitute for the XLPE. Several kinds of polyolefin materials to substitute for the conventional dielectric material, XLPE are developed and A cylindrical rod to cylindrical rod electrode system made with stainless steel is used to perform the experiments according to the ASTM D-149 protocol under an AC and Impulse input voltage condition. The experimental results are calculated by the Weibull distribution method and analyzed by an Finite Element Method(FEM). Finally, the dielectric characteristics of the conventional XLPE and novel polyolefin are experimented compared with each other in this study.

Effect of repetitive square voltage frequency on partial discharge features

The inverter-fed motor is a key component of environmental-friendly hybrid cars, high-speed trains, and other industrial applications. After the widespread use of inverter-fed motors, the repetitive impulse surges from inverters were found to cause new insulation problems in inverter-fed motors that were linked to premature insulation failure. The partial discharge (PD) induced by the overvoltage inner stator windings was found to be the root cause of insulation reliability problems. To mitigate this problem, PD detection and life testing at repetitive square voltages should be performed on new types of insulation models and entire motor insulation systems. The obtained partial discharge inception voltage (PDIV) and lifetime of insulation in the tests can be used to evaluate the insulation status of inverter-fed motor insulation systems and thus to improve the insulation designs. At AC voltage conditions, the voltage frequency can significantly affect PD activity. At repetitive square voltage with rapid rise time, the influence of the square voltage frequency on PD activity should be investigated (1) to suggest values for the repetitive square voltage frequency selection for the PDIV and lifetime tests, and (2) to increase the signal-to-noise ratio when extracting PD pulses from strong disturbances generated by the operating switch of impulse generators. Therefore, the main purpose of this paper was to investigate the effects of repetitive square voltage frequency on PD events. Insulation models of both Type I (low-voltage inverter-fed motor) and Type II (high-voltage inverter-fed motor) were used to obtain single-PD and multi-PD statistical features, respectively. Experimental results indicated that higher frequencies caused less PD events with lower PD magnitudes and shorter delay times in one square voltage cycle. This phenomenon was attributed to the different surface charge decay ratios and the initial electron emission probabilities resulting from different square voltage frequencies. Finally, a number of considerations for PD measurements on inverter-fed motors were provided to improve the sensitivity of the PD test system and to obtain high objective insulation lifetime test results, which are used to assess the insulation status of inverter-fed motors.

AC 계통 무효전력 제어를 위한 HVDC 시스템 설계

This paper deals with the concept design of HVDC system for controlling AC network reactive power. HVDC system can active power and reactive power and the concept of reactive power is similar to SVC(Static Var Compensator). Reactive power is controlled by adjusting firing angle of HVDC system under the condition that AC filters are switched. Reactive power depends on AC voltage condition, considering the steady-state and transient state to maintain the stable operation of AC network in the viewpoint of voltage stability. Therefore, in the design stage of HVDC, the reactive power required in the AC network must be considered. For the calculation of operation angle in HVDC system, the expected reactive power demand and supply status is examined at each AC system bus. The required reactive power affects the determination of the operation angle of HVDC. That is, the range of control deadband of operation angle should have the capability supplying the required reactive power. Finally, the reactive power concepts is applied to 1GW BTB Pyeongtaek-Dangjin HVDC system.

The Effect of AC Frequency on the Electrowetting Behavior of Ionic Liquids

This paper presents a study of electrowetting of ionic liquids (ILs) under AC voltages, where nine different ILs (including mono-, di-, and tricationic varieties) with three different AC frequencies (60 Hz, 1 kHz, 10 kHz) were experimentally investigated. The main foci of this study are (i) an investigation of AC frequency dependence on the electrowetting of ILs; (ii) obtaining theoretical relationships between the relevant factors that explain the experimentally achieved frequency dependence; and (iii) a systematic comparison of electrowetting of ILs using AC vs DC voltage fields. The frequency of the AC voltage was found to be directly related to the apparent contact angle change (Deltatheta) of the ILs. This relationship was further analyzed and explained theoretically. The electrowetting properties of ILs under AC voltages were compared to that under DC voltages. All tested ILs showed greater apparent contact angle changes with AC voltage conditions than with DC voltage conditions. The effect of structure and charge density also was examined. Electrowetting reversibility under AC voltage conditions was studied for few ILs. Finally, the physical properties and AC electrowetting properties of ILs were measured and tabulated.

Modeling of breakdown voltage of White Minilex Paper in the presence of voids under ac and dc conditions using fuzzy logic techniques

Occluded gaseous cavities within the insulating materials are potential sources of electrical trees which can lead to continuous deterioration and breakdown of materials. To determine the suitability of use and to acquire the data for the dimensioning of electrical insulation systems breakdown voltage of insulators should be determined. In this paper, fuzzy logic (FL) method is used to model breakdown voltages of White Minilex Paper samples based on experimental data generated in the laboratory. Different models are proposed with different shapes of the membership functions for the FL under both dc and ac voltage conditions. The cavities are created artificially with different dimensions. Low values of mean absolute errors of the estimated breakdown voltage of the test data show the effectiveness of such models.

Electrical Insulation of 500-m High-Tc Superconducting Power Cable

Electrical insulation is one of the essential technologies for the electric power apparatus. Determination of testing voltages and design method of the electrical insulation layer are inextricably linked each other, and are critical to developing and realizing a cold dielectric (CD) type high-Tc superconducting (HTS) power cable. The authors had proposed the electrical insulation design method with concepts of partial discharge-free designs for ac voltage condition. This paper discusses the testing voltages for a 77 kV 1000 A HTS power cable with a length of 500 m, and describes results of various voltage withstand test. As a result, it is concluded that the proposed electrical insulation design method is appropriate for the HTS power cable.

Partial discharge characteristics of inverter-fed motor coil samples under ac and surge voltage conditions

This work investigates the partial discharge (PD) characteristics of twisted pair samples for inverter-fed motor coil under ac and surge voltage conditions. It also discusses the difference in the PD inception characteristics of a twisted pair under 60-Hz voltage and surge voltages with a steep wave front. The PDIV under ac voltage was measured and was found to agree well with the theoretical value using electric field analysis and Paschen's law. The PDIV under surge voltage was measured for different repetition rates and the PDIV for 60-pps repetition was found to be lower than those for 6 pps and for the single shot, which suggested that the electrical insulation performance would be influenced by the repetition rate. The PDIV under surge voltages was 1.5 to 1.8 and 2.3 to 2.7 times higher than those under AC voltage application for the used and virgin samples, respectively.

Development of low-permittivity pressboard and its evaluation for insulation of oil-immersed EHV power transformers

The development of a low-permittivity pressboard obtained by blending polymethylpentene fiber with cellulose fiber is described. The permittivity is 3.5, which is lower than that of conventional pressboard. With this pressboard, insulating characteristics of intercoil models were investigated under lightning impulse and AC voltage conditions. It is demonstrated that a spacer fabricated from this pressboard material increases the partial discharge inception and breakdown voltages up to 30% compared with conventional pressboard spacers. >

A Laboratory Study on RI, TVI and an of Insulator Strings for DC Transmission line Under Contaminated Condition

This paper reports the results of a laboratory study on radio interference (RI), television interference (TVI) and audible noise (AN) of contaminated insulator strings for DC transmission line. It is clarified that RIV (Radio Influence Voltage), TVI and AN levels of contaminated insulator under DC voltage condition are almost the same as those under AC voltage condition, although those of clean insulator and conductor are lower under DC voltage condition than those under AC voltage condition. So, in the DC transmission line, the noises from insulator, especially under contaminated condition, are considered more influential than those in the AC transmission line.