Medium Voltage Power System Research Articles

Measurement of medium-voltage AC air arc temperature and particle number density based on dual-wavelength Moiré deflection technology

AC air arcs are generated in medium-voltage (MV) power systems under the effect of harsh weather conditions, equipment aging, and high penetration of distributed generation, threatening equipment and public safety. The arc current and temperature are low due to the wide application of arc suppression devices. In this scenario, the MV AC air arc does not satisfy the local thermodynamic equilibrium (LTE) condition. In addition, the repeated arcing and extinguishing processes further complicate the arc discharge mechanism, which bring challenges in the modeling and detection of MV AC air arcs. Experimental methods are a direct and efficient approach to determine the properties of arc plasmas. In this study, a dual-wavelength Moiré deflection diagnostic system was established to determine the time evolution of the particle density and radial distribution of the temperature in an MV AC air arc without relying on the LTE assumption. The electron number density and heavy particle number density change transiently during the arc discharge process and change gradient along the radial direction. The heavy particle temperature and electron temperature were then calculated based on the measured particle number density. During the arcing stage, the temperature of the electrons exceeded that of the heavy particles significantly, and the arc deviated from LTE. Finally, the limitations of the traditional single-wavelength Moiré deflection method are analyzed. The classic single-wavelength Moiré deflection method, while capable of estimating heavy particle temperature in plasma, exhibits a significant error in electron density estimation compared to the dual-wavelength Moiré deflection method.

Identification of Intermittent Ground Fault Lines in Medium Voltage Shipboard Power Systems

To locate increasingly frequent intermittent ground faults in medium-voltage shipboard power systems, this study proposes a fault branch selection method using wavelet analysis. By analyzing transient fault current characteristics in ship medium-voltage grids, this method utilizes wavelet analysis tools to extract features and obtain modulus maxima of branch currents. Based on a dual threshold detection strategy, the fault branch is then identified. The feasibility and validity of the proposed method are verified through simulations on the Matlab/Simulink platform. The simulation results demonstrate the capability of the wavelet analysis approach to accurately locate intermittent ground faults in ship medium-voltage power systems. This research provides an effective solution for fault branch selection of intermittent grounding faults in ship medium-voltage grids.

Evolutionary algorithm application to medium voltage power system restoration

The voltage quality is one of the most important issue in power system. The distribution network has to keep up with the growing consumption and increasing reliability requirements. In many cases supply disturbance is caused by faults on medium voltage distribution networks. Throughout system restoration many switching operation are needed, figuring out the optimal ones is a demanding task. In this paper a genetic algorithm based method is proposed, which can reduce the outage time in those areas where supply can be restored while the repairing is carried out on the failed section

Static state power smoothing and transient power quality enhancement of a DC microgrid based on multi-function SMES/battery distributed hybrid energy storage system

Using hybrid energy storage system (HESS) to solve the issues of power regulation and fault protection of DC microgrids has attracted extensive attention. To realize multiple functions of power flow control and power quality enhancement of the DC microgrid, a concept of distributed HESS based on superconducting magnetic energy storage (SMES) and battery energy storage (BES) is introduced in this paper. A SMES-based energy-storage-composited DC transformer (ESDCT) is designed to interconnect the low-voltage DC microgrid into a medium-voltage DC power system. It is with the functions of DC voltage transformation, and grid fault isolation. The BES is selected to regulate the unstable output power of the LVDC microgrid caused by renewable energy generators and load variations. The modeling of the SMES-based ESDCT and the cooperative control of the BES and SMES is analyzed. Finally, the performance of the SMES/BES distributed HESS is verified via a simulation, and the feasibility of the proposed SMES-based ESDCT is demonstrated via a scaled-down experiment.

Fault diagnosis and location of independent sub-module of three-phase MMC based on the optimal deep BiD-LSTM networks

Modular multilevel converter (MMC), which is highly modular, easy to be expanded, and has good output voltage waveform, is especially suitable for medium and high voltage power system applications. The MMC bridge arm inductance fault and the independent sub-module fault have the greatest influence on the transmission quality of transmission systems. However, there is few research on fault diagnosis and location for multiple sub-modules(SM) in a single arm at present. A deep bidirectional Long Short Term Memory (Bid-LSTM) network is proposed in this paper, which is used to diagnose and locate the faults of different sub-modules. which can deal well with the prediction problem with sequence dependence. In addition, because the Bid-LSTM model contains many hyperparameters, which seriously affects the prediction performance of BiD-LSTM, an optimal fruit fly optimization (FOA) algorithm based on the composite chaotic mapping of 3-dimensional Logistic-Sine (3d-CCFOA), which searches the optimal value of related hyperparameters to ensure the accuracy of classification. 3d-CCFOA has good chaos characteristics, which enables FOA to avoid falling into local optimal solution effectively and obtains an effective global optimal solution. Then, the MMC simulation system of RT-TAB is used to simulate the open circuit faults of different independent sub-modules of one bridge arm and collect relevant fault data. The analysis process and experimental results adequately indicate that this method can effectively and accurately diagnose and locate the faults of independent sub-modules. Meanwhile, this method also provides a reference diagnosis strategy for multi-bridge arm fault diagnosis and multi-sub-module location.

Python-PowerFactory co-simulation for the optimal location of electric vehicle charging stations

This paper presents a genetic algorithm-based methodology for optimal placement of Electric Vehicles Charging Stations using co-simulation between Python and DigSILENT PowerFactory. The medium voltage power system is modelled in DigSILENT PowerFactory, a powerful software widely used by electric network operators. On the other hand, the genetic algorithm is implemented in Python, one of the most used software in engineering. The objective function considers the cost of EVCS power losses and their construction costs, and it is solved using a genetic algorithm. A method to communicate the two software is proposed. The methodology presented is evaluated using a 33-node power network for different numbers of EVCS location in the medium voltage grid. Results show that the Python-PowerFactory co-simulation is extremely useful when analysing multiple cases of location of EVCS on the network, which could help Network Operators analyse the impact of including EVCS to the network.

Channel Characterization and SC-FDM Modulation for PLC in High-Voltage Power Lines

Digital communication over power lines is an active field of research and most studies in this field focus on low-voltage (LV) and medium-voltage (MV) power systems. Nevertheless, as power companies are starting to provide communication services and as smart-grid technologies are being incorporated into power networks, high-voltage (HV) power-line communication has become attractive. The main constraint of conventional HV power-line carrier (PLC) systems is their unfeasibility for being migrated to wideband channels, even with a high signal-to-noise ratio (SNR). In this scenario, none of the current linear/non-linear equalizers used in single carrier schemes achieve the complete compensation of the highly dispersive conditions, which limits their operation to 4 kHz channels. In this paper, a new PLC-channel model is introduced for transmission lines incorporating the effects of the coupling equipment. In addition, the use of the single-carrier frequency-division modulation (SC-FDM) is proposed as a solution to operate PLC systems in a wide bandwidth, achieving transmission speeds above those of the conventional PLC system. The results presented in this paper demonstrate the superior performance of the SC-FDM-PLC over conventional PLC systems, obtaining a higher transmission capacity in 10 to 30 times.

An impact of the line resistance on the power flow calculations with installed phase-shifting transformer in different voltage levels power systems

In modern power systems, phase-shifting transformers are installed not only in the extra high voltage lines, but some of these devices can be built for lower voltage levels (e.g., 40 kV). Due to big voltage difference, in the power flow analyses, not only phase-shifting transformer properties, but also line parameters, including resistance, should be considered. For high voltage power systems, the line reactance is much greater than its resistance (the ratio is even greater than 10), while it is smaller in lower voltages. This fact should be taken into account during any calculations of power flows in such systems. This article considers 400 kV, 220 kV, and 70 kV transmission lines with installed asymmetrical phase-shifting transformers to present the line resistance impact on the final power flow calculations. Also, the IEEE 5-bus power system is tested to prove the dependency of line resistance on power flow calculations and bus voltage values. The highest voltage levels calculation results show that the assumption of omitting line resistance is correct (the relative error is equal to 2%), while for the 70 kV system, the same error results even 30%. What is more, the line resistance for medium voltage power systems cannot be omitted. What is more, the resistance effect is intentionally not taken into account in some power flow calculations presented in the literature.

Comparative Analysis of Partial Discharge Detection Features Using a UHF Antenna and Conventional HFCT Sensor

This article presents a design of ultra-high frequency (UHF), ultra-wide band (UWB) antenna used for partial discharge (PD) detection on high voltage and medium voltage power system equipment. The proposed UHF antenna has a working frequency band of 1.2GHz-4.5GHz, covering a total bandwidth of 3.3GHz with a return loss less than -10dB in the entire antenna’s operating frequency. The computer simulation technology (CST) Microwave Studio software was used to design, simulate and optimize the proposed antenna. Upon simulation and optimization process, the antenna prototype was fabricated on the FR-4 substrate of 1.6 mm thickness and dielectric permittivity of 4.4. This antenna has a compact size of 100mm x100mm.The radiating patch and the ground plane of this antenna are made of annealed copper whose thickness is 0.035mm. The simulations and measurement results for the proposed antenna are in a good agreement, and the return loss of this antenna is less than -10dB with voltage standing wave ratio, VSWR, < 2 within the frequency range of interest. The proposed antenna performance in PD sensing is compared with a commercial high-frequency current transformer, HFCT. To validate the sensitivity performance of the designed antenna, experimental PD measurements were carried out by using an epoxy slab inserted between two parallel plates electrode model, in order to generate surface discharge on the insulator, and using a needle-plate electrode configuration to generate corona discharge in transformer oil. Based on PD measurement results, it was shown that the designed antenna has a high sensitivity, which make it a suitable candidate for UHF partial discharge monitoring on high voltage and medium voltage power assets.

Study On Resistive SFCL for Fault Ride-Through Fulfillment of Power Electronic Transformer Interconnecting MV and LV Power Systems

Power electronic transformers (PETs) can be used to interconnect different voltages and forms of electrical systems, and enable to offer higher operation efficiency and flexible power flow controllability. This paper focuses on the fault ride-through (FRT) problem of a PET interconnecting medium voltage (MV) and low voltage (LV) power systems, and the solution of a resistive superconducting fault current limiter (SFCL) is proposed. Firstly, theoretical modeling of the PET and the SFCL is addressed, and a suitable control strategy is applied to realize reliable power exchange between the LV and MV systems. Then, in regards to a fault at the PET's output-terminal, the fault developing process is probed, and considering the SFCL's quench behavior, the PET's current variations are clarified. Further, a 3 MW PET with a SFCL is modeled in MATLAB/Simulink, and through changing the fault severity and the SFCL parameter, different simulations are implemented. The results reveal that the SFCL stabilizes the PET from multiple aspects. A visible current-limiting function is achieved, and the voltage dynamic fluctuations at the AC and DC sides of the PET are effectively alleviated. Meanwhile, the power delivery from the LV to the MV system is smoothed, and a successful FRT fulfillment is gained.

Application of Modern Non-Linear Control Techniques for the Integration of Compressed Air Energy Storage with Medium and Low Voltage Grid

Compressed air energy storage is a well-used technology for application in high voltage power systems, but researchers are also investing efforts to minimize the cost of this technology in medium and low voltage power systems. Integration of this energy storage requires a robust control of the power electronic converter to control the power injection due to the dynamic behavior of the system. The conventional linear control design requires a thorough knowledge of the system parameters, but the uncertain disturbances caused by the mechanical properties of the energy storage is neglected in the design and the system fails in presence of such instances. In this paper an adaptive control-based boost converter and sliding mode control-based three phase inverter for a grid integrated compressed air energy storage system of up to 1 kW has been presented that can mitigate any uncertain disturbances in the system without prior knowledge of the system parameters. The experimental results along with the simulation results are also presented to validate the efficiency of the system.

Application of Decision Trees for Optimal Allocation of Harmonic Filters in Medium-Voltage Networks

The paper proposes a solution for the problem of optimizing medium voltage power systems which supply, among others, nonlinear loads. It is focused on decision tree (DT) application for the sizing and allocation of active power filters (APFs), which are the most effective means of power quality improvement. Propositions of some DT strategies followed by the results have been described in the paper. On the basis of an example of a medium-voltage network, an analysis of the selection of the number and allocation of active power filters was carried out in terms of minimizing losses and costs keeping under control voltage total harmonic distortion (THD) coefficients in the network nodes. The presented example shows that decision trees allow for the selection of the optimal solution, depending on assumed limitations, expected effects, and costs.

Review of Standards on Insulation Coordination for Medium Voltage Power Converters

The increasing viability of wide band gap power semiconductors, widespread use of distributed power generations, and rise in power levels of these applications have increased interest and need for medium voltage converters. Understanding the definitions of insulation coordination and their relationship to applications and methodologies used in the test environment allows system engineers to select the correct insulation materials for the design and to calculate the required distances between the conductive surfaces, accessible parts and ground accurately. Although, design guidelines are well established for low voltage systems, there are some deficiencies in understanding and meeting the insulation coordination requirements in medium voltage, medium frequency applications. In this study, an overview on standards for insulation coordination and safety requirements is presented to guide researchers in the development of medium voltage power electronic converters and systems. In addition, an insulation coordination study is performed as a case study for a medium frequency isolated DC/DC converter that provides conversion from a 13.8 kV AC system to a 4.16 kV AC system.

Detection of Ferroresonance Oscillations in Medium Voltage Networks

This paper presents a new algorithm for the detection of ferroresonance oscillations in medium voltage (MV) power systems. The proposed method is based on an analysis of estimated flux resulting from the voltage in open delta voltage transformer (VT) connection. The testing results proved that the new solution is capable of identifying quickly and reliably ferroresonance oscillations and it does not maloperate during faults. In addition, this method is simple for hardware implementation. The developed solution has been thoroughly tested on simulation runs (from the MATLAB2019b/Simulink program), as well as recorded signals.

Fault diagnosis of shipboard medium-voltage DC power system based on machine learning

This study proposed a fault diagnosis method of a shipboard medium-voltage DC (MVDC) power system based on Noise-Assisted Multivariate Empirical Mode Decomposition (NA-MEMD) and Multilevel Iterative – LightGBM (MI-LightGBM), which overcomes the limitations of the existing fault diagnosis methods in this regard, such as relies heavily on the relay or slow training process. MI-LightGBM is proposed to solve the problem of unbalanced training samples caused by the difficulty in obtaining fault samples in practical engineering. First, NA-MEMD was adopted to pre-process the voltage signals, which were decomposed into a set of components called Intrinsic Mode Functions (IMFs) according to the local characteristic time scales of the original signals. The energy moment of each order IMF was calculated as fault feature vector to train the MI-LightGBM model, which led to the development of a high-precision fault classifier. A model of a shipboard MVDC power system was established using the AppSIM Real-Time Simulator. Simulations were performed on earth fault and short-circuit fault at the generator output and DC cable. Compared with the existing fault diagnosis methods, the proposed method is simple to use and save more than half of the training time while maintaining high diagnostic performance, which is more suitable for engineering applications.

Simulation of the pre-magnetization technology for transformer on ship power system

The no-load closing of transformer in the medium-voltage ship power system will cause a large inrush current and threaten the security of the system. This paper aims to provide reference for the pre-magnetization technology of the transformer in medium-voltage ship. Firstly, the basic principles of the inrush current and the series small-capacity transformer based pre-magnetization technology for the transformers are analyzed. Based on the analysis, the typical medium-voltage engineering ship and power generation ship are taken as examples to perform the simulation of the series small-capacity transformer based pre-magnetization scheme, and the result shows that the pre-magnetization technology can suppress the inrush current effectively. Besides, the factors of the switch closing time and the number of generators operating in parallel are considered in the research and their effects of on the inrush current as well as the transient voltage drop are analyzed. Finally, the influence of the pre-magnetization scheme on the differential protection is studied and the operation suggestions are given.

Experimental Investigation of Drawing Vacuum Arc Under Different TMF Contacts in Vacuum Interrupter

Vacuum interrupter is widely used in the medium-voltage power system. Controlling the arc by the magnetic field is an effective method to improve the breaking capacity. The transverse magnetic field (TMF) forces the constricted arc to rotate to reduce local overheating. In this paper, three kinds of TMF contacts, which are Swastika-type contact, cup-shaped contact, and double-TMF contact that combines Swastika-type and cup-shaped contacts coaxially, are investigated in a demountable vacuum chamber with a high-speed camera. The arc voltage and current are also recorded. The experiments were conducted with circuit currents from 5 to 20 kA (root mean square) with a frequency of 50 Hz. There are four current levels in the experiment, which are 5, 10, 15, and 20 kA, and at least three tests are taken for each current level. The results showed that under low current (I = 10 kA), the arc of the three TMF contacts did not rotate. Under high current (I = 20 kA), it was observed that the arc of Swastika-type and double-TMF contacts rotated in the gap. The arc speed of double-TMF contacts is higher than that of Swastika-type contacts. The cup-shaped contacts have the highest arc voltage and noise among the three TMF contacts. The Swastika-type contacts were eroded most seriously. The simulation results show the force distribution of the arc on different locations. The resultant force makes the arc rotate counterclockwise.

DC Marine Power System: Transient Behavior and Fault Management Aspects

DC marine vessels with medium-voltage compact dc power systems are dominated by a significant amount of active loads and a finite number of generation sources. In such scenarios, the network configuration of the dc power system is expected to get dynamically altered to fulfill the required generation and load demands for the desired marine mission. Such varying network configurations make the transient responses significantly different from the conventional ac grids and the prospective dc grids. In this regard, this paper performs systematic transient studies to devise fault management strategies for the dc marine vessels. A platform supply vessel (PSV) is taken as an example of the marine vessel, due to its complex operating scenarios and wider applicability in the marine industry. Pole-to-pole short-circuit faults are considered owing to its severity. A novel current-only directional protection for the dc PSV is proposed based on the directional zonal interlocking and short-time Fourier transform. The efficacy of the proposed method is substantiated by confirming against a range of fault impedances initiated at the generator terminals, load terminals, lines, and buses of the dc PSV. All the analyses are conducted in the real-time simulation model of the dc PSV.

Analysis of Dynamic Arc Parameters for Vacuum Circuit Breaker Under Short-Circuit Current Breaking

The vacuum circuit breaker (VCB) is an important vacuum switch widely used in generator protection systems and medium-voltage distribution power systems. For observing the vacuum arc behavior variation and analyzing the dielectric recovery characteristics and interrupting capability of VCBs, an experimental platform for a 40.5-kV/31.5-kA/2500-A VCB was established. Under the condition of typical short-circuit current breaking, the short-circuit current breaking test was carried out and the vacuum arc image was collected to obtain the curve of voltage versus current during the breaking process and the arc resistance distribution. By analyzing the variation trend of dynamic arc resistance, a physical and mathematical description of dynamic arc resistance was obtained by the curve fitting method. By comparing and analyzing the experimental data of arc resistance, it is shown that the physical and mathematical description can appropriately describe the spatiotemporal variation of arc resistance in the arcing interval.

Passive Filters Design Applied to an Electrical Submersible Pump System

This paper shows modelling, studies and analyses of an electrical submersible pump system (ESP) and considers several characteristics of key elements in this system, highlighting system's start-up and also proposing solutions to mitigate some problems by projecting and designing passive power filters. In the study cases, it was considered a medium voltage power system with a 3835HP three phase squirrel cage induction motor which is connected with a centrifugal pump that results in a high inertia coefficient. Rotational friction model is considered in the studies to characterize the peculiarities that occur at the initial seconds of the system's start-up and also to evaluate the performance of the implemented control strategy. In the same way that occurs in real systems, it is considered an open-loop V/Hz control and accomplished a method to estimate and compensate voltage drop that exists between the terminals of the converter and motor input.