Unbalanced Operating Conditions Research Articles

Simulation of protection against unbalanced high-voltage asynchronous drive of recycle compressor in fuel hydrotreating unit

In the oil and gas industry, continuous processes such as oil and gas refining play a great role and are sensitive to many external factors. Such processes require special procedures for stopping and restarting. In order to maintain a sustainable process, the entire system needs to be cleaned by removing of unreacted components. Rejected raw materials are often dumped into a flare leading to tangible environmental problems and significant economic disadvantages. Electrotechnical systems (ETS) play an important role in ensuring continuous technological processes in oil and gas industry. Electric motors are one of the key elements of ETS. The majority of the electrical machines used in industry today are Asynchronous motors (AM) – no less than 80 %. Ensuring their trouble-free operation is one of the key factors in the design, simulation, and analysis of asynchronous motor relay protection systems, including unbalanced conditions of their operation. These conditions can occur due to unbalanced AM connection circuits, supply voltage unbalance, or as any faults in a machine itself. Operating a motor under these conditions will result in shorter motor life, reduced power, wear and aging of the insulation. The study subject was an Asynchronous motor drive of a recycle compressor of a gasoline hydrotreating unit at the fuel hydrotreating integrated unit at the Astrakhan Gas Refining Plant (AGRP). The authors used Matlab simulations to study the facility and its protection systems/devices operation. The method of symmetrical components was selected as the main theoretical method. The authors developed a model of an asynchronous motor drive of a recycle compressor. This involved establishing a set of relay protections (RP) and developing the models of the following protections: sequence filter (symmetrical component filter) (SF), negative sequence (nps) O/C protection, and overload protection. It was demonstrated that the specified relay protection complex fully protects the motor from unbalanced operation conditions. The authors conducted a study of the protection complex operation under different supply voltage unbalances, with different motor loads. They formed a conclusion about the performance of the developed protection complex, and gave recommendations for its technical implementation in a business environment. The study findings can be used as a basis for the development and testing of relay protection components of the entire electrical system of the fuel hydrotreating unit at the Astrakhan Gas Refining Plant.

Optimal Selection among Various Three-Phase Four-Wire Back-to-Back (BTB) Converters with Comparative Analysis for Wave Energy Converters

Wave energy converters are attracting attention as an energy source that can respond to climate change. In order to increase the energy efficiency of the wave energy converters, efficient power converters are also required. The efficient converters require operation at a low switching frequency, which increases the weight and volume of the passive components. Therefore, in this paper, the performance of various types of topologies is compared to select the optimal power converter for wave energy converters. In order to cope with the unbalanced operation and unbalanced load of renewable energy, in this paper, the topology of the four-leg type is analyzed centrally. In addition, the analysis was performed by applying the model predictive control that can quickly respond to the rapid energy change of wave energy. In addition, model predictive control was applied to the four-leg converter analyzed in this paper because it is suitable for application to atypical topologies. For performance analysis of various types of topology, the loss and efficiency of each converter were analyzed by applying a loss analysis model, and output current harmonics and leakage current characteristics, capacitor voltage fluctuation rate, etc., were additionally analyzed at various switching frequencies. In conclusion, the three-level four-leg converter showed up to 2.28% and 2.7% higher efficiency under balanced and unbalanced operating conditions.

Multi-objective control strategy for multilevel converter based battery D-STATCOM with power quality improvement

Differences in battery state-of-charge (SOC) in a cascaded H-bridge multilevel converter (CHB-MLC) based distribution static synchronous compensator (D-STATCOM) in battery storage system (BSS) applications could result in undesirable efficiency and performance reductions, and even system failure. Moreover, faults occurring on batteries and/or H-bridges (modules) of CHB-MLC severely increase the risk of failure. Power quality at the output of CHB-MLC is reduced as results of those faults. Therefore, active power differences that are introduced by SOC balancing among modules and bypassing the faulty modules yield to undesired harmonics at the output of CHB-MLC. This paper proposes a multi-objective control strategy for CHB-MLC based BSS D-STATCOM. The proposed control strategy includes a bidirectional power flow controller with unity power factor operation (UPF) feature, a fault-tolerant (FT) controller, a SOC balancing scheme that is designed to work on fault conditions, and a harmonic minimization strategy to enhance the power quality at the output of the CHB-MLC. Performance of the proposed method is validated through different operating conditions including faults, unbalanced SOCs and dynamic load changes. Results indicate that charging and discharging of the batteries with 5 kW rated power are held successfully along with maintaining UPF operation under both normal and fault conditions. Moreover, grid currents are balanced with the help of the FT controller and SOC balancing is properly operated even in fault conditions. Negative impacts of faults and unbalanced operating conditions on the power quality are also mitigated with the help of the harmonic reduction controller with significant decreases on total harmonic distortion reaching up to 47% and 69% during normal operation and faulty operations, respectively.

Multiobjective Model-Free Predictive Control for Motor Drives and Grid-Connected Applications: Operating With Unbalanced Multilevel Cascaded H-Bridge Inverters

A multiobjective model-free predictive control (MO-MFPC) strategy is proposed in this article for multilevel cascaded H-bridge (MLCHB) inverters with unbalanced conditions. The compensated current variation (CCV), which allows the inclusion of the proportional and integral terms into the cost function, is generalized to improve the accuracy of MFPC over a wide range of applications. The new extended CCV is used to define the voltage control objective without involving the output voltage model of MLCHB. This voltage control objective is used to evaluate all state candidates to achieve a suitable subset for the current control objective. To achieve a better tradeoff between the current accuracy and the injected common-mode voltage (CMV), CMV is added to the cost function. Simulations and experimental evaluations show that, compared to existing MFPCs and classic model predictive control for MLCHB inverters, MO-MFPC achieves a better current accuracy over a wide range of applications and unbalanced MLCHB operating conditions.

Application of the Generalized Normal Distribution Optimization Algorithm to the Optimal Selection of Conductors in Three-Phase Asymmetric Distribution Networks

This article addresses the problem of the optimal selection of conductors in asymmetric three-phase distribution networks from a combinatorial optimization perspective, where the problem is represented by a mixed-integer nonlinear programming (MINLP) model that is solved using a master-slave (MS) optimization strategy. In the master stage, an optimization model known as the generalized normal distribution optimization (GNDO) algorithm is proposed with an improvement stage based on the vortex search algorithm (VSA). Both algorithms work with discrete-continuous coding that allows us to represent the locations and gauges of the different conductors in the electrical distribution system. For the slave stage, the backward/forward sweep (BFS) algorithm is adopted. The numerical results obtained in the IEEE 8- and 27-bus systems demonstrate the applicability, efficiency, and robustness of this optimization methodology, which, in comparison with current methodologies such as the Newton metaheuristic algorithm, shows significant improvements in the values of the objective function regarding the balanced demand scenario for the 8- and 27-bus test systems (i.e., 10.30% and 1.40% respectively). On the other hand, for the unbalanced demand scenario, a reduction of 1.43% was obtained in the 27-bus system, whereas no improvement was obtained in the 8-bus grid. An additional simulation scenario associated with the three-phase version of the IEEE33-bus grid under unbalanced operating conditions is analyzed considering three possible load profiles. The first load profile corresponds to the yearly operation under the peak load conduction, the second case is associated with a daily demand profile, and the third operation case discretizes the demand profile in three periods with lengths of 1000 h, 6760 h, and 1000 h with demands of 100%, 60% and 30% of the peak load case. Numerical results show the strong influence of the expected demand behavior on the plan’s total costs, with variations upper than USD/year 260,000.00 between different cases of analysis. All implementations were developed in the MATLAB® programming environment.

Experimental Research of Flow Distribution at Reactor Core Inlet of ACP100

A 1/3 scaled-down mock-up reactor of ACP100 was used to simulate the flow field in the prototype reactor and to carry out the experiment of core inlet flow distribution. In the mock-up reactor, the major flow path of the reactor is linearly scaled except for the core region which is replaced by 57 dummy fuel assemblies. In addition, the experiment measures core inlet flow rates by a specially designed turbine installed in each dummy fuel assembly. In the experiment, both four-loop balanced and unbalanced operating conditions are tested. The experimental results show that the flow in the major flow path of the mock-up reactor has entered the self-modeling zone and the flow field characteristics are the same as the prototype. The core inlet flow rates of different tests show similar distributions, which all meet the hydraulic performance requirements. Moreover, the effect of impeller rotation on the flow field of the core inlet has also been studied. The study indicates that the flow distribution is almost unaffected by the swirl flow.

Harmonic Performance Analysis of Static Var Compensator Connected to the Power Transmission Network

The static var compensator (SVC) is a device which is designed to compensate reactive power, increase voltage stability and to reduce voltage fluctuations. Thyristor controlled reactors (TCRs) are composed of reactors in series with bidirectional pair of thyristors. Current through reactors can be continuously controlled by changing the firing angle of thyristor valves, thus
 the inductive power can be easily controlled. Typical applications of TCRs in AC systems are voltage stabilization and temporary overvoltage reduction, stability improvement, damping of power oscillations and load balancing. In this paper, harmonic performance analysis of SVC equipped with TCRs is presented. SVCs utilizing TCRs generate harmonic currents and therefore it is necessary to determine the effect of harmonics generated by the SVC on the power system and its elements. This includes interaction of the SVC with the system, the SVC performance under balanced and unbalanced operating conditions and finally, evaluation of countermeasures such as installation of harmonic filters. In order to carry out these analysis, it is necessary to determine harmonic characteristics of the network at the point of SVC connection, existing levels of harmonics, and to know appropriate standards regarding acceptable harmonic levels in the power system. Since harmonic distortions in the system are caused by the interaction between SVC and the system, all system contingencies which may affect system’s frequency response should be evaluated. Detailed power system model should be considered to make sure that parallel resonance points of system do not directly coincide with characteristic harmonics from the SVC. Harmonics generated by SVCs are largely dependent on the operating point within the SVC characteristic. A conservative approach is to use the maximum values of harmonics generated within the spectrum irrespective of the operating point. The results of harmonic performance analysis are important for appropriate design of SVC. Harmonic performance analysis related to SVC application which are presented in this paper include the determination of: frequency response of the transmission network impedance required for the specification and design of filters; the effects of SVC generated harmonics on the power system; the overall filter requirements and countermeasures to reduce harmonics to acceptable levels.

Optimal Phase-Shifted Modulation Method for Suppressing the Current Ripple of Modular Multilevel Converter-Bidirectional DC–DC Converter Under Unbalanced Operation

Modular multilevel converter-bidirectional dc–dc converter (MMC-BDC) can be used in the second generation of vessel integrated power system (IPS) to interconnect the medium-voltage dc (MVdc) bus and distributed energy storage elements (ESEs) for its outstanding characteristics. When MMC-BDC operates with submodule (SM) power well balanced, traditional phase-shifted modulation (TPSM) reduces the current ripple at the MVdc side of MMC-BDC for the frequency multiplication effect. However, when it comes to the unbalanced operation conditions, TPSM can result in large current ripple. In this article, the relationship of the MVdc-side current ripple and the switching signal patterns is revealed. An optimal phase-shifted modulation (OPSM) method based on offline genetic algorithm (GA) is proposed to suppress the MVdc-side current ripple by adjusting the phase-shifted ratios of the driving signals of each SM. Combining the proposed modulation method with a proposed online stepwise energy management strategy, all unbalanced conditions can be covered with limited sets of optimal phase-shifted ratios. Simulation and experiment results show that the MVdc-side current ripple of MMC-BDC is remarkably reduced with the proposed OPSM.

Control of grid side converters under constraints in unbalanced operating conditions. A Lyapunov framework

This work presents a Lyapunov-based control of a grid side converter of distributed energy systems. The analysis focuses on the constraints imposed by unbalanced operation and thermal limits. On one side, the proposal, which considers symmetrical components, allows controlling the converter in such a way that it is able to deliver constant real power to the grid in the presence of highly unbalanced voltages. On the other side, to solve the operation under thermal constraints, the references in control loops are adapted. Both issues, unbalanced operation but also thermal constraints, are considered in a sole Lyapunov function, formed by the union of a set of Lyapunov ones. This ensures the stability of the whole system by using simple PI controllers. Finally, different scenarios, strong disturbances and reaching current limits are explored in simulation results.

Analysis of unbalanced fault operating conditions of ship electric networks via Millman’s theorem

ABSTRACT The aim of this paper is in continuation with previous works of other authors, to demonstrate a methodology of analysing unbalanced operating conditions of ship electric energy systems via Millman’s theorem. Its advantages consist in being simple in implementation (either in time domain or in phasor mode) especially by non-electric power engineers, in contrast to the more complicated method via the symmetric components (or sequence networks). Furthermore, the method can be well applied to most ship electric systems, the configurations of which are not as complicated as in inland grids, e.g. with meshed loop double-circuit transmission networks, etc. The analysis covers most unbalanced case studies, i.e. single- and two-phase short-circuit faults as well as single- and two open-circuit faults.

Машинный расчет несимметричных режимов трехфазных цепей с динамической нагрузкой

A new approach to analyzing three-phase circuits in the phase coordinates under unbalanced normal and emergency operating conditions is proposed, in which the information about the three-phase circuit to be analyzed by means of software is entered in a simplified manner. The equivalent circuits of three-phase generators, power lines, and static and dynamic loads are aggregated and considered in a generalized form. With such presentation, the work with a three-phase circuit diagram is significantly simplified even if it contains unbalanced loads, a few faulty sections, and control links in the equivalent circuits of electrical machines. The labeling of three-phase circuit nodes is proposed that allows three-phase and single-phase parts of the circuit to be distinguished. The topologic list of branches intended for computer-aided calculations of currents and voltages and currents is compiled for three-phase branches in a generalized form. The obtained list is compact and retains a clear representation of the three-phase circuit. The analogy between the basic electrical equations written for electrical circuit three-phase and single-phase branches is shown. Thus, the voltages and currents in a three-phase element are interrelated by equations similar to the generalized Ohm’s law, while Kirchhoff's current law is written for three-phase nodes and has the same form as for single-phase circuits. The analogy of drawing up the incidence matrix and the matrix of nodal equations is shown. Submatrices of dimensions 3 × 3, 1 × 3, or 1 × 1 depending on the node label appear as entries in the incidence matrices and nodal admittance matrices of a three-phase circuit. The nodal equations used for carrying out the subsequent analysis of the circuit in the phase coordinates are written in a standard way as in single-phase circuits. In analyzing emergency operating conditions, it is proposed to keep the simplicity and clarity of the approach by representing the circuit faulty section of as a corresponding branch embedded into the three-phase circuit. The developed approach is illustrated by calculation of unbalanced and emergency operating conditions in a complex three-phase unbalanced circuit containing two synchronous generators, one dynamic load, and one static load. The calculation has been carried for four- and three-wire three-phase circuits.

Power Balancing in Cascaded H-Bridge and Modular Multilevel Converters Under Unbalanced Operation: A Review

Multilevel Voltage-Source Converters (VSC) based on modular structures are envisioned as a prominent alternative for grid and industry applications. Foremost among these are the Cascaded H-Bridge (CHB) and the Modular Multilevel Converter (MMC). In this context, depending on the application and the power conversion structure, unbalanced operating conditions can be asked to the converter. Previous investigations regarding the operation and the solutions for modular structures under unbalanced conditions have already addressed this topic, but information is dispersed over a wide number of sources. This paper identifies, classifies, and analyzes the intercluster active power balancing strategies for the adequate operation of the most commonly used modular structures in some typical unbalanced operating scenarios: the Static Synchronous Compensator (STATCOM) under unbalanced voltage and/or current conditions, the unequal power generation in large-scale photovoltaic (PV) power plants, and the uneven power distribution in a battery energy storage system (BESS). Each of the applications has been independently studied so as to provide a comprehensive analysis of the alternative techniques found in the specialized literature, clearly explaining their respective strengths and drawbacks. Several future challenges have been identified during the study, which will involve greater research effort in this key research topic.

Учёт несинусоидального/несимметричного режима работы электрической сети коммунально-бытового назначения при расчете уровня тока нулевого проводника

During the operation of 0.4 kV electric networks, asymmetry of currents and voltages is unavoidable. As is known, unbalanced operation of 0.4 kV networks is stemming from nonuniform distribution of loads between the phases. This results in that a fundamental frequency zero-sequence current will flow through the neutral conductor in a four-wire three-phase electric network. On the other hand, during balanced operation of a four-wire electric network containing loads with a nonlinear volt-ampere characteristic, higher harmonic currents multiple to three are summed in the neutral conductor. Therefore, it is of relevance to analyze unbalanced operating conditions of a four-wire electric network containing loads with a nonlinear volt-ampere characteristic. The article presents the characteristics of a municipal load for analyzing unbalanced/nonsinusoidal conditions of its operation. For carrying out full-valued assessment of unbalanced operating conditions of a four-wire electric network, a simulation model of a 0.4 kV network section containing a nonsinusoidal load is developed. It is shown that during unbalanced operation of an electric network containing electric loads with a nonlinear volt-ampere characteristic it is recommended to take into account zero-sequence current higher harmonic components that are not multiple to three. An algorithm for calculating the neutral conductor current harmonic component for determining the cable temperature rating is developed.

Measurement of Simplified Single- and Three-Phase Parameters for Harmonic Emission Assessment Based on IEEE 1459-2010

This article investigates the feasibility of using a simplified approach, based on the measurement of power ratio parameters, for harmonic emissions assessment at the point of common coupling (PCC). The proposed approach comes from the common concept of power factor correction and the definitions of the IEEE Std. 1459-2010, where line utilization and harmonic pollution levels are evaluated by means of ratios between the power quantities of the apparent power decomposition. In addition to the IEEE Std. 1459-2010 indicators, in this article, the behavior is studied of additional parameters that are conceptually similar to those defined by the IEEE Std. 1459-2010. The suitability of such parameters is discussed, for both single- and three-phase balanced/unbalanced cases, taking into account both their behavior in different scenarios and their effectiveness when the measurement uncertainty is taken into account. The study is supported by some simulation results that have been obtained on an IEEE benchmark power system, which allows reproducing linear and nonlinear load conditions, balanced and unbalanced operating conditions, and the presence of capacitors for power factor correction.

On control and balancing of rigid bipolar MMC-HVdc links enabling subsystem-independent power transfer

High voltage direct current transmission schemes equipped with modular multilevel converter technology are considered in several realized and planned bulk power transmission projects. Especially in the case of dc cable links with high system lengths, the rigid bipolar configuration presents a suitable compromise between power rating, cost-efficiency and converter redundancy. Caused by the absence of a metallic return path, system degrees of freedom are reduced in contrast to the bipolar configuration with dedicated metallic return. Therefore, the dc currents in positive and negative bipolar subsystem are not independent. This leads to new challenges and motivates the development of advanced control and balancing concepts for rigid bipolar schemes. This paper focuses on balanced and unbalanced ac grid operating conditions with subsystem-independent active power set points in an ac split-busbar scenario, which has not yet been covered in literature. In order to validate the presented methodology, case studies are performed in electromagnetic transient software.

Parameter estimation of a synchronous generator model under unbalanced operating conditions

This paper proposes a new method to estimate parameters of a synchronous generator using measurements taken under unbalanced operating conditions. The new method is based on a combination of the application of the Unscented Kalman Filter (UKF) and Trajectory Sensitivity techniques. The method adopts a three-phase model for the synchronous generator that is typically available in EMT-based commercial software and evaluates the sensitivity of the response to changes in the model parameters, in order to determine if it is possible to estimate a certain parameter based on a particular measured output. This enables the user to choose the best output to estimate the parameter of interest, which is important when the only available measurements are responses to small disturbances. After that, a UKF technique is applied to the three-phase model in 0dq coordinates and, differently from other approaches in the literature, the zero-sequence circuit is taken into account to enable the estimation under unbalanced operating conditions, which is the main contribution of the paper. The results showcase the efficiency of the proposed method even in the case where the available measurements are taken from highly unbalanced conditions.

Linear Time-Periodic Theory-Based Modeling and Stability Analysis of Voltage-Source Converters

Stability assessment and control design of future power electronic dominated grids require efficient and adequate modeling methods. This article deals with modeling and stability analysis of a three-phase grid following voltage-source converters (VSCs). First, a general complex domain model is presented for the VSC considering both balanced and unbalanced operation conditions. Then, a linear time-periodic (LTP) model for the small-signal stability analysis is derived based on the Wirtinger calculus. Next, the space transformation analysis (i.e., modal analysis) for the linear time-invariant (LTI) system is generalized to the LTP system. It is revealed that the frequency-coupling effects of power electronic systems result from the nonholomorphic control units and the periodic operation trajectory. Finally, generalized eigenvalue and transfer function analysis methods are adopted to evaluate how the interaction between the advanced phase-locked loop (PLL), the current controller, and the network impacts the stability of a single VSC test system. Simulation results verify the proposed modeling and stability analysis method.

New phasorial oriented single‐PI loop control for industrial VSC‐PFC rectifiers operating under unbalanced conditions

Due to several reasons, the three-phase voltages of power grids cannot be balanced. The performance of AC–DC converters using dq0-based controls can be severely affected by the presence of unbalanced input AC voltages. Different to these proposals, this study presents a new easy-to-implement control scheme based on a single PI loop algorithm for VSC-PFC rectifiers using a phasorial approach. This new scheme has various significant advantages: (i) fast counteracting of large unbalanced voltage and current conditions; (ii) power factor = 1 at any unbalanced sag operating conditions; (iii) negligible current harmonic distortion and (iv) low-ripple DC voltage. All these features are concurrently obtained. The proposed single-PI loop VSC-PFC rectifier control strategy is theoretically and experimentally validated. A revision of the main results and characteristics of various proposed techniques that are similar to the one proposed in this study is also carried out, qualitatively indicating the main advantages featured by the proposed control strategy for VSC-PFC rectifiers.

A Unified Control Structure for Grid Connected and Islanded Mode of Operation of Voltage Source Converter Based Distributed Generation Units Under Unbalanced and Non-Linear Conditions

This manuscript develops a unified control structure for Distributed Generation (DG) units based on Voltage Source Converters considering unbalanced and non-linear operating conditions. This control structure works for both the Islanded and the Grid-connected modes of operation of the Micro-Grid (MG). The objective of this control scheme is to regulate the line currents of the DG unit in such a manner that the voltage at the Point of Common Coupling (PCC) remains balanced despite the line currents of the DG unit being unbalanced and distorted. Multiple adaptive P-R controllers have been proposed for the current control loop of the Voltage Source Converter (VSC). These controllers have been implemented with resettable integrators so as to limit the DC components in the post fault current of the VSC. The Battery Energy Storage System (BESS) is interfaced to the DC link of the VSC through bi-directional dc-dc converters. An improved control structure for the bi-directional dc-dc converter has been developed. The effectiveness of these control structures have been presented and tested in PSCAD/EMTDC in an IEEE 34 node distribution system model being fed by two identical DG units.

Analysis and Control of Neutral-Point Deviation in Three-Level NPC Converter Under Unbalanced Three-Phase AC Grid

This paper presents a neutral-point deviation and ripple compensating control method applied to a three-level neutral-point-clamped (NPC) converter. The neutral-point deviation and its harmonic components are analyzed with a focus on the average current flowing through the neutral-point of the dc link. This paper also proposes a control scheme compensating for the neutral-point deviation and dominant harmonic components under generalized unbalanced grid operating conditions. The positive and negative sequence components of the switching functions and ac input currents are employed to accurately explain the behavior of three-level NPC converter. Simulation and experimental results are presented to verify the validity of the proposed method.