Fundamental Voltage Research Articles

Size Estimation of Bulk Capacitor Removal Using Limited Power Quality Monitors in the Distribution Network

With a large number of distributed generators (DG) and sensitive power loads connected to the distribution network, power quality issues have increasingly become the focus of users’ attention. Accurate and quick estimation of the amount of bulk capacitor removal that causes voltage sag is helpful to maintain power quality management equipment in time. This paper presents a novel size estimation of bulk capacitor removal using a limited power quality monitor (PQM) in the distribution network, including PQM deployment optimization, feeder localization, and capacitor removal amount calculation. The PQM placement is optimized by taking the estimated capacitance removal sizes of all buses as a constraint. The change of reactive power consumption before and after removing the capacitor at each power line is adopted to determine the feeder where the disturbance is located. Based on the impedance characteristics of the power grid components, the steady estimation method (SEM) is deduced using the fundamental voltage and current. Applying the sampling points of instantaneous voltage and current waveform, the transient estimation method (TEM) is constructed by data fitting. Case studies and index analysis for the IEEE 13 bus test work are presented to verify the reasonableness and accuracy of the proposed method for disturbed bus, capacitor size, load symmetry, disturbance duration, and DGs. SEM shows more stability and accuracy, while TEM performs faster and is more robust. The new methods provide a reliable and acceptable disturbance size estimation with several limited PQMs.

Battery Energy Support to Cascaded H-Bridge Converter-Fed Large-Scale PV System During Unbalanced Power Generation

The cascaded H-bridge (CHB) multilevel converter is an attractive solution for integrating photovoltaic (PV) generators with the ac grid. However, the power generated in the PV panels may be unequal due to partial shading, temperature change, and dust layer accumulation leading to unequal and unbalanced power between the phases and possible violation of power quality constraints. Fundamental frequency zero-sequence voltage (FFZSV) injection can be applied, but this method can produce overmodulation in conditions with high PV power mismatch. A new FFZSV is proposed in this article assuming a CHB with a battery energy support (BES) to compensate for the effects of unbalanced PV power generation. A barycentric coordinate representation helps to determine the capacity of the BES required. A coordinated control between the PV system and BES is also proposed in the article. Additionally, a novel approach to deliberate FFZSV injection is discussed in this article by which the maximum charge and discharge rate for the BES can be obtained along with balanced current injection into the grid. The effectiveness of the proposed technique is verified in numerical simulations and with experimental results.

Performance Investigations of Novel PWM Control Strategies for Fundamental Fortification in Single-Phase Matrix Converter for Cyclo-Conversion Operation and FPGA Implementations

Matrix converter is a class of AC–AC power converters, which facilitates variable voltage and frequency control without intermediate conversion like pulse width modulated [PWM] inverters. Traditional control strategies for variable frequency control for AC–AC conversion can be extended for matrix converters. This research work proposes two PWM strategies with an objective of enhanced fundamental voltage and reduced THD for low-frequency operation. The concept behind the approach is concatenating both fundamental switching and PWM pulses. The fundamental switching reduces switching losses and enhances fundamental output voltage which is equal to the instantaneous values of fundamental component in that particular segment. On the other hand, the variation of pulse width using PWM has a control in the harmonic content which reduces THD. The simulation and experimental results prove the competencies of the proposed strategies in real-time application.

Induction Motor Supplied with Voltage Containing Symmetrical Subharmonics and Interharmonics

Sinusoidal voltage fluctuations can be considered a specific result of the occurrence of voltage subharmonics and interharmonics, which are components of low frequency or not being an integer multiple of the frequency of the fundamental voltage harmonic. These components—symmetrical subharmonics and interharmonics—are of the same magnitude, while their frequencies are symmetrical with respect to the fundamental frequency. Depending on their phase angles, various kinds of voltage fluctuations can be distinguished: amplitude modulation, phase modulation and intermediate modulation. In this study, the effect of phase angles on noxious phenomena in induction motors was analyzed. Additionally, torque pulsations and vibrations of an induction motor under sinusoidal voltage fluctuation and a single voltage subharmonic or interharmonic were compared. The investigations were performed with the finite element method and an experimental method. Among other findings, it was found that for some phase angles torque pulsations could be about ten times higher than for other angles, roughly corresponding to the amplitude modulation.

Parallel-Hybrid Converter Based STATCOM: Operating Principles and Capacitor Voltage Control Using Fundamental Frequency Zero-Sequence Voltage

Parallel-Hybrid Converter Based STATCOM: Operating Principles and Capacitor Voltage Control Using Fundamental Frequency Zero-Sequence Voltage

A Novel Fault-Tolerant Operation Approach for the Modular Multilevel Converter-Based STATCOM With the Enhanced Operation Capability

The operation capability of the modular multilevel converter (MMC)-based static synchronous compensator (STATCOM) is limited under submodule (SM) failures. When running in the STATCOM mode, the floating dc-link voltage of the MMC is adjustable. Therefore, this article proposes a novel fault-tolerant operation approach to enhance the operation capability of the MMC under severe SM failures. First, the dc-link voltage is dynamically regulated under SM failures, where the capacitor voltages of all SMs are slightly increased. The modulation ranges of the faulty and healthy phases can be enlarged, and the even voltage stress distribution can be realized. Then, a low-magnitude fundamental frequency zero-sequence voltage (FF-ZSV) is injected to balance the line-to-line voltages. Finally, the virtual energy idea is proposed, where the capacitor voltages of faulty SMs are considered to be clamped at the reference value. As a result, the arm energy reference will be the same in all six arms, and the arm energy balancing control is simplified. Detailed comparisons show that the proposed approach has an enhanced operation capability with lower capacitor voltage increment. Simulation and experimental results verify that even under severe SM failures, the proposed approach can still guarantee the fault-tolerant operation within the specified security region.

Predictive Current Control and Field-Weakening Operation of SPMSM Drives Without Motor Parameters and DC Voltage

The prior model-parameter-free predictive current control (MFPCC) applies a 1-D lookup table (1-D LUT) to store current variation, where the index input is the number of the basic voltage vector. However, current variation is not only related to the input voltage but also depends on the rotor position. As the influence of rotor position is not considered in the prior MFPCC, high current spikes may occur during normal operation. Aiming to improve the steady-state performance, the proposed MFPCC utilizes a 2-D LUT to backup current variations, where the second-dimensional input is the discretized rotor position. The proposed method keeps good robustness and eliminates current spikes in the prior MFPCC. Moreover, a field-weakening (FW) method is developed to extend the speed range. As no modulator is employed in MFPCC and only discrete voltage vectors are available, the conventional voltage magnitude-based FW control scheme cannot be directly applied. To address such issue, a complex-coefficient filter is employed to calculate the modulation index of the fundamental voltage, which is then fed to the FW controller to generate demagnetizing current. The proposed MFPCC can achieve satisfactory performance without motor parameters and the measurement of dc voltage. Experimental results confirm the effectiveness of the proposed method.

Modified Multiverse Optimizer Selective Harmonic Elimination for Five-Level Modified Packed U-Cells Inverter and Analysis of Motor Torque Ripple Suppression

A selective harmonic elimination method of modified packed u-cells (MPUC) inverter fed induction motor built on modified multiverse optimizer (MMVO) algorithm is put forward. The proposed method adopts the MMVO algorithm to search the optimal solution in the SHEPWM equations through the two processes of exploring the optimal solution distribution and exploiting the optimal solution within it. On this basis, the influence of the different orders of power supply harmonics eliminated selectively on the motor’s torque ripple is compared and analyzed by using the Simulink and MAXWELL. From the results of simulation and experiments, it can be proved that the method can achieve the required fundamental voltage and selectively suppress the harmonic with particular order. At the same time, it is verified by ANSYS MAXWELL that the working quality of the motor can be improved by selectively eliminating the 5th, 7th, 11th, and 13th harmonics in the power source to effectively suppress the 6th and 12th periodic torque ripples that arise from these harmonics.

Compressed Sensing Technique for the Localization of Harmonic Distortions in Electrical Power Systems.

The present work proposes to locate harmonic frequencies that distort the fundamental voltage and current waves in electrical systems using the compressed sensing (CS) technique. With the compressed sensing algorithm, data compression is revolutionized, a few samples are taken randomly, a measurement matrix is formed, and according to a linear transformation, the signal is taken from the time domain to the frequency domain in a compressed form. Then, the inverse linear transformation is used to reconstruct the signal with a few sensed samples of an electrical signal. Therefore, to demonstrate the benefits of CS in the detection of harmonics in the electrical network of this work, power quality analyzer equipment (commercial) is used. It measures the current of a nonlinear load and issues its results of harmonic current distortion (THD-I) on its screen and the number of harmonics detected in the network; this equipment acquires the data based on the Shannon–Nyquist theorem taken as a standard of measurement. At the same time, an electronic prototype senses the current signal of the nonlinear load. The prototype takes data from the current signal of the nonlinear load randomly and incoherently, so it takes fewer samples than the power quality analyzer equipment used as a measurement standard. The data taken by the prototype are entered into the Matlab software via USB, and the CS algorithm run and delivers, as a result, the harmonic distortions of the current signal THD-I and the number of harmonics. The results obtained with the compressed sensing algorithm versus the standard measurement equipment are analyzed, the error is calculated, and the number of samples taken by the standard equipment and the prototype, the machine time, and the maximum sampling frequency are analyzed.

Shunt active power filtering with reference current generation based on dual second order generalized integrator and LMS algorithm

For reducing the carbon emissions from utility sector, it is important to improve the efficiency of network and electrical loads. Current harmonics are responsible for the degradation of the system as well as load efficiency. The propagation of current harmonics into the grid can be prevented with shunt active power filters (SAPFs). Reference current generation (RCG) is critical for SAPF operation. This paper presents the control of SAPF with RCG based on least mean square (LMS) and dual second order generalized integrator (SOGI). The two SOGIs separately process grid voltage and load current to determine the respective fundamental in-phase component (FPC) and fundamental quadrature component (FQC). FPC and FQC of grid voltage are used to determine the phase angle of fundamental grid voltage. With LMS algorithm the distorted current can be decomposed into fundamental active (FA), fundamental reactive (FR) and harmonic components. However, the dynamic response of LMS based estimation is poor when the same learning rate is used for all components. FQC of load current, estimated with SOGI, can be processed by LMS algorithm to separate FA and FR components. RCG can be implemented with the computed phase angle and FA component. In LMS algorithm, as only two components are to be separated, two different learning rates can be easily specified. This results in faster dynamic response for LMS algorithm and has consequent positive impact on RCG and SAPF operation. Also, the computational complexity is considerably reduced. The performance of SAPF with the proposed extraction algorithm is analyzed for different operating conditions and comparative analysis is carried out.

A novel design of circulating current control target to minimize SM capacitance in MMC

In an MMC station, the submodule (SM) capacitor constitutes a high proportion in both the footprint size and construction cost, which greatly limits the application of MMC in some scenarios, such as offshore wind power transmission system. Since the SM capacitor voltage ripple has a close interaction with the phase circulating current in MMC, an appropriate management of circulating current is widely acknowledged to reduce the SM capacitance. This paper proposes a novel design of circulating current control target to minimize the requirement of SM capacitance in MMC. Firstly, the phase angle of circulating current is designed to significantly reduce the fundamental and second-harmonic capacitor voltage ripple at the same time. Secondly, the magnitude of circulating current is determined to achieve the optimal phase angle or magnitude coordination between the components of SM capacitor voltage ripple. Thus, under the proposed design method, the peak-peak capacitor voltage ripple is minimized and the requirement of SM capacitance can be effectively reduced. Finally, an MMC-HVDC case system connecting to the offshore wind farm is built on PSCAD/EMTDC to validate the effectiveness of the proposed design method.

Phase Angle-Based Sub-Cycle Algorithm for High-Speed Digital Relaying of Transmission Lines

Sub-cycle algorithms aid in quick fault detection and isolation by digital relays. A new sub-cycle phase angle-based algorithm, to achieve 3 cycle Circuit Breaker (CB) operation, is proposed in this letter. Half-cycle DFT (HCDFT) is used to estimate the Instantaneous Phase Angles (IPAs) of the fundamental current and voltage. Modified IPAs (MIPAs) are derived from the IPAs. A Sub-Cycle Similarity Index (SCSI), obtained from the MIPAs, is used for Fault Detection (FD) and Fault Classification (FC). A Directional Index (DI) based process, which runs in parallel to FD and FC, is also proposed to distinguish internal and external faults. Simulation results and comparison with previous methods reveal the efficacy of the proposed algorithm in achieving high-speed protection of transmission lines.

Maximum Torque Operation of Open-Winding Induction Motor Dual Drives Using a Floating Capacitor Bridge in the Field Weakening Region

A field weakening controller is presented that extends the speed range of a dual inverter drive using a floating capacitor bridge. The controller utilizes the stator current reference frame to decouple the motor demand into active and reactive components. This is used so that the main bridge supplies the real power and the floating bridge supplies the reactive power until the floating bridge reaches its maximum voltage. The main bridge is then used to supply some of the reactive demand, which in turn adds additional voltage boost that is used to extend the drive’s constant power region, improve the motor speed acceleration performance, output power, and torque. It is found that the presented controller can increase the drive speed extension ratio to 9.2 times the base speed compared to 5 when always operating the main bridge at unity power factor. The motor maximum fundamental voltage is 1.82 p.u when compared to that of a single inverter drive. This controller performance is demonstrated experimentally using both transient and steady-state analysis. The controller is also compared with other two field weakening controllers using the same regulator tunings and experimental setup.

Injected current regulation based stator ground fault arc suppression method for large hydro-generators

Generator stator ground faults are common, and a reliable ground fault arc suppression method is required to ensure generator safety and avoid unnecessary impact on the power system. A fault arc suppression method is proposed in this paper by regulating the neutral injected current and controlling the fault point voltage to a low level. Considering the required injected current is different for different fault positions on the stator winding, a fault location method and a fault winding potential calculation method are proposed. In addition, a fault type discrimination method based on the neutral fundamental voltage variation is proposed to distinguish temporary and permanent faults. In this paper, the grounding transformer impedance and irregular distribution of the stator grounding capacitance are considered. The PSCAD/EMTDC simulation results verify that the proposed method can effectively realize arc suppression at different fault scenes and the ground fault type can be distinguished effectively.

Submodule Fault-Tolerant Control of Modular Multilevel Matrix Converters With Adaptive Optimum Common-Mode Voltage Injection

The modular multilevel matrix converter (M3C) is an attractive topology for high-voltage and high-power direct ac-to-ac power conversion applications. SM fault-tolerant control is of great importance for the M3C. This letter proposes an optimum common-mode voltage injection (CMV) method which can maximize the input and output voltage ranges of the M3C after nonredundant submodules (SMs) are failed. The injected optimum CMV is calculated in real-time according to the relationship between the fundamental reference voltage and maximum available voltage of each branch, which can be adaptive to different load and fault conditions. The operating area of the M3C after the fault when employingthis method is also discussed. Experimental results implemented on an M3C prototype with three SMs in each branch are presented to validate the proposed method and analysis.

Estimation of Harmonics in Partly Monitored Residential Distribution Networks With Unknown Parameters and Topology

The generalized adoption of power-electronic-based loads in residential networks is leading to increasing concern about harmonic distortion levels. At the same time, residential distribution systems are insufficiently monitored to provide a reliable analysis of harmonic limits compliance at the point of common coupling of customers. To address this problem, this paper proposes a practical method to estimate voltage harmonic levels in real time at all connection points of residential consumers. The proposed methodology assumes unknown network parameters and topology according to the limited data availability which is usual in low voltage residential networks. The proposed harmonic estimation method uses information gathered from conventional smart meters typically installed in the network (which register fundamental voltage and power) and from a reduced number of installed power quality monitors. A penetration of power quality meters below 10% has proved to provide accurate estimations of voltage harmonic levels at the whole network. The paper also proposes a methodology to optimally locate these power quality meters in the network. Validation of the methodology is performed in two different European distribution test networks with 55 buses and 471 obtaining satisfactory results. The method’s robustness and the parameters affecting accuracy are also analyzed in the article.

Comparative analysis of dual second-order generalized integrator-phase locked loop based series hybrid filter and static synchronous compensator for load voltage compensation and power quality improvement with grid synchronization in islanded microgrid

PurposeThe purpose of this study is to eliminate voltage harmonics and instantly measure the positive sequence fundamental voltage during unbalanced grid conditions, the dual second-order generalized integrator-phase locked loop used in series hybrid filter structures is often used in grid synchronisation in three-phase networks. The preferred series active hybrid power filter simultaneously compensates for voltage balancing and current harmonics generated by non-linear loads.Design/methodology/approachThis paper examines the use of renewable energy–based microgrid (MG) to support linear and non-linear loads. It is capable of synchronising with both the utility and the diesel generator unit. Power is transferred from the grid throughout a stable grid situation with minimum renewable energy generation and maximum load demand. It synchronises with diesel generator set to supply the load and form an AC MG during outages and minimum renewable power generation. In islanded and grid-connected mode, the voltage and power quality issues of the MG are controlled by static synchronous compensator and series hybrid filter.FindingsBecause of the presence of non-linear loads, reactive loads in the distribution system and the injection of wind power into the grid integrated system result power quality issues like current harmonics, voltage fluctuations, reactive power demand, etc.Originality/valueThe voltage at the load (linear and non-linear) is regulated, and the power factor and total harmonic distortions were improved with the help of the series hybrid filter.

Red deer algorithm based selective harmonic elimination for renewable energy application with unequal DC sources

This study aims to find the optimal switching angles in a cascaded H-bridge multilevel inverter (CHB-MLI) fed by photovoltaic (PV) panels, keeping the output voltage constant regardless of the panel voltage change and eliminating low-order harmonics. For this purpose, Red Deer Algorithm (RDA), a newly developed algorithm, has been applied to solve selective harmonic elimination (SHE) equations. Calculated switching angles are simulated by applying three-phase 7-level CHB-MLI in Matlab/Simulink environment. RDA optimization has been compared in performance to evolutionary algorithms such as particle swarm optimization (PSO), genetic algorithm (GA) and pigeon inspired optimization (PIO). The results show that in the case of unequal DC voltage, RDA can suppress selected harmonics accurately and quickly while controlling the fundamental voltage with a 0.02% error. The proposed RDA optimization can be used for multilevel inverters fed by any number of variable input voltages to eliminate selected harmonics and reduce total harmonic distortion (THD) without complex analytical calculations.

Temperature rise and DC current capability tests of star-point reactor used in HVDC transmission

Star-point reactors are grounding devices installed in HVDC stations between the converter transformer secondary side and AC side of converter arms to provide a reference to ground. Such reactors are used as high-impedance grounding on the converter side of power transformers, providing high impedance path for the fundamental harmonic (i.e. 50 or 60 Hz) and a low impedance path for DC current, eliminating DC current flowing through the transformer windings. Temperature rise and DC current capability tests of 420 kV star-point reactor are presented in this paper. The purpose of temperature rise test is to verify that temperatures that can damage the insulation of star-point reactor will not be reached with the specified service conditions. The temperature rise test was carried out according to the requirements of the IEC 61869-3 standard and client’s request which included simultaneous application of fundamental, 3rd harmonic voltage and DC excitation. The inclusion of 3rd harmonic excitation of an amplitude up to 15% simulates voltage harmonic distortion which may appear in the power system at the location of star-point reactor installation. Prior to temperature rise test, DC current capability test was performed. The goal of this test is to determine the value of DC current at which the saturation point is reached. DC current is injected through star-point reactor while AC voltage is applied. Two different cases are considered regarding AC voltage: test with fundamental voltage harmonic and test with fundamental voltage harmonic with superimposed third voltage harmonic. Test circuit is proposed which is suitable for generation of complex voltages composed of fundamental harmonic and superimposed third harmonic with amplitudes up to 15% of the applied fundamental harmonic. The proposed test circuit was also used during the temperature rise test and it is applicable for testing of HV equipment with rated voltage up to 420 kV.

Coupled Inductors for High-Frequency Drives With Parallel-Connected Inverter Legs

In this article, coupled inductors with cross-coupled windings are described and designed for voltage source converters with parallel-connected inverter legs in each phase. The nature of the cross-coupled windings produces a very low effective series output inductance, related to the intralimb leakage inductance rather than the higher interlimb leakage. The effective inductance between the parallel inverter output terminals can be made high to reduce circulating currents and is related to inductor magnetizing inductance. These two features make the use of parallel inverters attractive for generating multilevel high-frequency fundamental pulsewidth modulation output voltages with a very low fundamental voltage drop across the coupled inductors. Sample magnetic designs are presented for a three-phase system using three inverter legs connected in parallel in each phase. The effective output inductance of the cross-coupled winding arrangement is compared with that of the coupled windings on separate limbs without cross-coupling and is shown to be 97% smaller. The performance of the two inductor winding arrangements is demonstrated with simulation and experimental results of an 11 kW (300 Vdc, 208 Vac/30 A) laboratory prototype operating with fundamental frequencies in the kilohertz range up to 5 kHz.