Low Voltage Side Research Articles

Electromagnetic interference and the effect of low-voltage protective measures at electric vehicle charging stations

The nominal power of electric vehicle charging stations or charging parks is constantly increasing. Most of the users are ordinary persons and handle such equipment with a rated power of several 100 kW. Until now, equipment with such power ratings was only common in electrical operating facilities such as industrial plants where the users are at least instructed and protective measures are specified. If ordinary persons handle equipment with such power ratings in the field, the question arises as to whether the conventional safety goals are met in the event of an electrical fault. The consideration is: If the power increases so much, it can be assumed that the short-circuit power and thus the fault current increase and so does the risk of a dangerous electric shock. In this contribution, calculations of line-to-earth short-circuits on the low-voltage AC side of the three-phase system and their effects in typical configurations of charging stations are carried out. Considering the electromagnetic interference, the calculations provide the fault current and its distribution to determine the electrical potentials during the fault. From this, the (partial) fault voltages and active fault voltages are calculated. Based on the active fault voltage, the expected body impedance and consequently, the body current can be determined. With the body current and the break time of the protection device the risk of electric shock using international standards as guidelines is evaluated. As a result, recommendations for the planning, installation and safe operation of charging stations are given. It turns out that considering certain aspects like the conductor cross-sections or the electromagnetic interference, the risk of electric shock can be reduced to a conventional level. Periodic testing of the electrical system is necessary for safe and reliable operation. For example, follow-on faults due to unintended, improper use by ordinary persons can be prevented. Also, regular inspection of the electrical system is necessary for safe and reliable operation to prevent hazards due to aging or wear. However, it seems challenging to define an installation guideline that applies to all configurations as the boundary conditions vary depending on the type of system, installations in the area of interference and environmental influences.

Analysis of the Effect of Capacitor Placements at High and Low Voltage Transformer Sides as Voltage Profile and Total Cost in Distribution Systems

This study mentions the problem of determining optimal placements and sizes for capacitor banks by comparing different schemes that allow implemented buses at both high and low voltage side, only high voltage side or low voltage side of transformers in distribution systems. Criterions evaluated for each scheme are active and reactive power losses in whole system, voltage value at all buses, installation and operation cost and accumulative profit saving. Standard with and without transformer branches in distribution systems and Newton-Raphson method are introduced to make clear about the way to analyze power flows in whole system. Optimal placements and sizes for capacitor banks are determined by using a proposed algorithm and optimal capacitor placement tool in ETAP software. Simulation results can help to show the comparisons, evaluations and make decision about optimal capacitor placements and sizes. The contribution shows the best scheme that install capacitors at low voltage side of transformers because it has the lowest implemented capacity, highest economic factors while voltage quality is always in allowable range. Achieved results can be applied in any distribution system, help managers have overall review and decide optimal placements and sizes for capacitor banks. Keywords: Capacitor bank, OCP Tool, ETAP software, Optimal placement, Optimal size, Voltage quality, Optimal cost.

Improved Duty Compensation Control-Based Bidirectional Resonant DC−DC Converter With Reduced Input-Current Ripple for Battery Energy Storage Systems

This paper proposes an improved duty control-based resonant bidirectional dc–dc converter (BDC) to remarkably minimize input-current ripple for battery energy storage systems. The proposed converter offers a resonant-boost operation using a pulse-width-modulation (PWM) full-bridge circuit with a push-pull transformer under the forward-mode. Whereas, in the backward-mode, a resonant-buck operation is implemented with a PWM neutral-point-clamped (NPC)-type circuit. Unlike conventional current-fed BDCs, the most appealing characteristic of the proposed converter is the NPC-type structure with a push-pull transformer, which can significantly mitigate the input-current ripple without using extra input-inductors. Moreover, the proposed BDC minimizes the conduction-loss of the low-voltage side bottom switches by alleviating unbalanced DC offset-currents via proposed duty-compensation control under mismatched magnetizing inductances of the transformer's primary-windings. The designed circuit operation is analyzed in detail and its practicability has been fully confirmed with the proportional-integral and spider-monkey-optimization control techniques-based experimental verifications using a prototype 1-kW test-bed.

A hybrid DC-DC modular multilevel converter with capacitors parallel connectivity for arm energy balancing

The integration of renewable energy sources in Medium-/High-voltage DC grids has called up continuous research to develop DC-DC conversion systems. This paper presents a hybrid modular multilevel DC-DC converter topology which successfully overcomes the issue of capacitors voltage imbalance during DC-DC conversion. In the proposed topology, single MMC leg is employed with half-bridge submodules in the upper arm and full-bridge submodules in the lower arm along with soft-switched high-voltage valves. The proposed hybrid modular multilevel converter has inherited arm energy equalization with the help of the parallel connectivity of involved arms’ capacitors. During the equalization period, the series-connected upper arm capacitors are connected in parallel to the series-connected lower arm capacitors to transfer the energy between the involved arms which results in operating with balanced capacitors voltages. The proposed modular DC-DC converter is a bidirectional converter which allows power flow from high-voltage side to low-voltage side and vice versa, i.e., it can be operated successfully as a DC transformer. The operational concept of the proposed configuration, control strategy, and parameters design are presented. Simulation results are presented along with experimental results of a scaled-down prototype to validate the proposed approach. Simulation and experimental results show the viability of the suggested approach.

The early detection of short-circuit fault in the active low-voltage AC system

The distributed power supply decentralized access to the low-voltage side of the distribution network transforms the existing low-voltage AC system into an active low-voltage AC system. The original low-voltage AC short-circuit detection methods are no longer applicable. In this paper, a short-circuit early detection method suitable for different branch types in the active low-voltage AC system is proposed. Based on the analysis of the transient characteristics of the active low-voltage AC short- circuit, a short-circuit time-varying vector theory and a fast method for solving its modulus length are proposed. Then, we construct a branch state variable containing time-varying vector modulus length difference information of current and voltage. The constructed branch state variable can reflect the characteristics of short-circuit state in real-time, which greatly improving the reliability of fast short-circuit detection. Finally, the early detection of short-circuit fault can be realized by comparing the branch state variable with the set detection threshold. The physical experiment system experiments show that the proposed short-circuit early detection method is effective and reliable. Compared with the existing short-circuit fast detection method, the detection time is reduced by 3–5 ms. Moreover, the short-circuit breaking and current limiting can be achieved by combining with the fast protection mechanism, and the current limiting ratio is as low as 0.33.

Transformerless bidirectional switched-inductor switched-capacitor converter

ABSTRACT This research presents a unique bidirectional converter based on the active switched-inductor and switched-capacitor networks of the ASL-SU2C converter. The converter’s primary advantages are its high voltage gain and reduced switch stress. This was accomplished by the converter using three inductors, two switched-capacitors, two switches, and diodes. Switches are used in place of diodes to enable bidirectional power flow in the proposed work, and synchronous rectification is used to boost efficiency by reducing conduction losses. Two inductors and two switches on the low voltage side of the converter aid in lowering the average switch current and, consequently, the current stress. The control operation is made simpler by the fact that two switches on the high voltage side (HVS) have one duty cycle and the other two switches on the low voltage side (LVS) have a different duty cycle. Consequently, two switches will always have identical duty cycles. In order to validate the theoretical findings, the performance of the transformerless bidirectional switched-inductor switched-capacitor (TBSISC) topology is examined in steady state, compared to that of other bidirectional converters, its efficiency is calculated, and experiments with a 500 W prototype are conducted.

Active disturbance rejection control of photovoltaic microgrids based on disturbance compensation and feed-forward correction strategy

Photovoltaic microgrid (PV) is a promising direction of new energy generation technology. However, due to the performance of low-voltage side interface affected by complex disturbance, PV microgrid has the problem of output voltage fluctuation, which will make the power quality not guaranteed. In order to stabilize the low-voltage side load supply, the active disturbance rejection control strategy with disturbance compensation and feed-forward correction (ADRC-DCFC) is proposed. By expanding the conventional state observer (ESO) dimension, a feed-forward correction link for disturbance tracking compensation is added. In contrast, the internal deviations extracted from the system model are actively added to the disturbance compensation link. In addition, a theoretical analysis of its disturbance observation performance and noise suppression performance is presented. Experimental results on a 300-watt test platform verified the accuracy and effectiveness of the ADRC-DCFC.

Theory and simulation analysis of parallel operation after main transformer replacement

As one of the most important main equipment in the power system, the safe and stable operation of transformer plays a vital role in ensuring the reliability of power supply. Because the three-phase parameters of the main transformer are not the same, the parameters of each phase and the three-phase load of the power grid are also different, resulting in a certain degree of asymmetry between the three-phase current and voltage of the transformer during normal operation. There will be negative sequence and zero sequence current components. The degree of asymmetry is generally small for the high-voltage power grid, but it has a large impact on the low-voltage side of the main transformer. In this paper, the parameters of two main transformers in a substation are quite different, but when they need to run in parallel, whether they can run in parallel, what are the conditions of parallel operation, these problems have been studied in theory and simulation. The results show that the two main transformers can be operated in parallel under suitable conditions. The actual operation result of a substation’s main transformer is normal, which further verifies the correctness of theoretical calculation and simulation.

A Phase-Shift-Modulated Resonant Two-Switch Boosting Switched-Capacitor Converter and Its Modulation Map

In this paper, a Phase-shift-modulated (PS) Resonant Two-switch Boosting Switched-capacitor Converter (RTBSC) and its modulation map are proposed. Compared with the traditional RTBSC, all diodes are replaced by active switches and then phase-shift modulation is applied. The advantages of the modified topology are obtained as follow. 1) The voltage gain can be regulated efficiently either below or above the nominal value even at light-load condition, making it suitable for applications with voltage fluctuations; 2) The conduction loss is reduced by replacing all diodes in RTBSC with low turn-on resistance transistors, especially for high-order and high-gain configuration; 3) All switches achieve the zero-voltage-switching (ZVS) turn-on operation; 4) The soft-charging property smooths the current profile. Aside from the topology modification, a comprehensive analysis of operation principle, voltage gain, component stress and ZVS region is given. On the basis, a modulation map with the optimal operation region is proposed. It provides a guideline to operate and design such converters in the target gain range, low component stress and ZVS region, making up for the lack of modulation map and design guideline in aforementioned literatures. A 120W prototype with low-voltage side 43-60V and high-voltage side 150V was designed to verify the above analyses.

Asymmetric Trapezoidal Wave Modulation of Modular Multilevel Resonant DC/DC Converter for Current Stress Optimization

A modular multilevel resonant dc/dc converter (MMRDC) possesses high power and high voltage rating, which is adopted for high step-ratio interconnection in dc grid applications. However, the current stress and reactive power of MMRDC are high with the conventional quasi-square-wave modulation scheme when in a wide voltage range and light load conditions. Hence, this article proposes an optimized asymmetric trapezoidal wave (ATW) modulation scheme that reduces current stress over wide load and voltage ranges by separately modulating the duty cycles of string output voltage rising and falling processes. The time-domain mathematical model of MMRDC with the ATW modulation scheme is established. Then, based on the soft switching characteristic and reactive current analysis, the optimized operating point of MMRDC can be derived while considering low current stress and zero voltage switching constraint of low voltage side full-bridge switches. On this basis, the numerical solutions of optimized control variables are obtained, and a control block diagram is presented for the practical implementation of the optimized ATW modulation scheme. Finally, a 4kW MMRDC prototype is constructed, and the experimental results verify the correctness and effectiveness of the analysis and the optimized ATW modulation scheme.

Design of a simple and low-cost solid-state ultra-fast high-voltage switch.

Ultra-fast high-voltage switches (UFHVSs) are a core component of time-of-flight mass spectrometers for realizing high accuracy ion acceleration, deceleration, and temporal focusing. The desirable features of high performance UFHVSs include a large range of adjustability of pulse width, a high maximum output amplitude, and minute rising and falling times. Besides the simplicity of the driver circuit, the total cost of the whole device is also critical to its practical applications. In this work, we present a low-cost and easy-fabrication 5000V bipolar solid-state UFHVS for a high-resolution mass spectrometer. A double-pulse transformer isolates the circuit's high- and low-voltage sides and synchronously drives series-connected cascode SiC FETs to form its push-pull topology. This scheme allows transmitting drive signals with long widths but without the magnetic saturation of the transformer. Testing results show that output pulses reach a maximum voltage of 5000V and a width of 150 µs, with rising and falling times of 8.5 and 18.3ns, respectively. More importantly, they have nearly no voltage decay.

Auxiliary Power Supply for Input-Series Output-Parallel Medium-Voltage Solid State Transformers

Auxiliary power supply (APS) is an essential component of medium voltage (MV) solid state transformers (SSTs) feeding the auxiliary units, such as gate drivers, controllers and sensors. Usually APSs are obtained from the distributed bus, which may cause the input voltage imbalance before the main power transmission. This paper proposed a new design for isolated APS of an input-series output-parallel (ISOP) SST. The designed distributed MV APSs are galvanically isolated by flybacks, and the centralized APS on the low voltage (LV) side is supplied from the MV APSs via a decoupling four-port medium frequency transformer (MFT) with high isolation voltage. To solve the issue of input voltage imbalance posed by the ISOP system, a dual-loop control scheme based on APS-fluctuating is proposed, which can realize output voltage regulation (OVR) and input voltage sharing (IVS) simultaneously. The small-signal model, start-up strategy and parameter design of the proposed system are investigated. Finally, a 500 V/2.2 kW lab prototype of SST with APS has been built and the overall system efficiency reaches 95.1%.

Online Monitoring Method of Medium Voltage Energy Meter Based on Load Characteristic

There are some problems in the operation state of online electric energy meters, such as monitoring blank spots and loss caused by inaccurate measurement, which will reduce the economic benefits of power grid enterprises. This paper presents an online monitoring method suitable for the medium voltage side, which determines whether the voltage at the low voltage side of the transformer changes with the load and conforms to the external characteristic law of the transformer. The measured data collected by the metering device are used to calculate the voltage value of the first three-phase line at the operation of the transformer area. The three-phase balance rate and voltage fluctuation rate are used as multi-objective functions to solve the correction coefficient of current deviation.

A SOGI-based adaptive controller design for second-harmonic currents suppression of variable frequency loads in DC buildings

DC buildings with integrated DC transformers (DCTs), as a typical representative of photovoltaics, energy storage, direct current, and flexibility (PEDF) building systems, are a crucial approach to reducing carbon emissions. Heating, ventilation, and air conditioning (HVAC) systems are typical loads in DC buildings that primarily aim to enhance the air quality and thermal comfort. However, the variable frequency drives (VFDs) used in HVAC systems can generate second-harmonic currents (SHC), which decrease equipment reliability and system power efficiency. To prevent SHC on the low-voltage side from flowing to the high-voltage bus, even in the grid, a second order generalized integrator (SOGI)-based adaptive controller for suppressing the SHC of variable frequency loads in DC buildings is proposed. The controller employs a phase-locked loop (PLL) to provide feedback on the VFD output frequency to the DCT control loop's notch filter, enabling adaptive suppression of the variable frequency SHC. Firstly, an accurate small-signal model of DCT based on Fourier series is established, which considers the inductor current. Then, the formula for DCT equivalent impedance is derived based on the model, and the feasibility of suppressing the SHC by increasing the equivalent impedance at variable frequency is analyzed. To enhance the stability of DC buildings, the parameters of the controller are designed to improve dynamic performance. Finally, the simulation results and experimental results verify the effectiveness and feasibility of the proposed adaptive controller compared to other suppression strategies.© 2017 Elsevier Inc. All rights reserved.

Consumer-feeder relationship identification method for low-voltage distribution station area considering feature fusion of voltage variation and spatial distribution

Consumer-feeder relationship is an important basis for the fault location, outage management, load adjustment and power quality management in low-voltage distribution station area (LVDSA). The current identification method of the consumer-feeder relationship based on electrical characteristics analysis is single and lack of reliability. Besides, the consumers of different feeders which locate near the bus at the low voltage side of distribution transformer have high voltage variation similarity, which makes it difficult to distinguish reliably. The spatial distribution of consumers can directly reflects the feeder trend. Therefore, a consumer-feeder relationship identification method for LVDSA considering feature fusion of voltage variation and spatial distribution is proposed in this paper. Firstly, the electrical characteristics based on voltage variation and spatial distribution characteristics based on geographical location are analyzed, and the correlation between the characteristics above and consumer-feeder relationship is explained. Secondly, a semi-supervised spectral clustering (SSC) algorithm based on the feature of voltage variation and spatial distribution is proposed to identify the consumer-feeder relationship. Finally, the proposed method is tested with the real data of LVDSAs in China. The results indicate that the feature fusion of voltage variation and spatial distribution can effectively improve the identification accuracy of consumer-feeder relationship.

Optimal substation capacity planning method in high-density load areas considering renewable energy

With the increasing penetration of renewable energy, the adaptability of the existing substation planning model in terms of capacity and quantity of transformer needs to be further studied when preferring large-capacity substations. Considering the variations of renewable energy penetration rate and load, this paper proposes a method to optimize the total capacity of substations in distribution networks. This paper introduces the influence of renewable energy access on power supply reliability and introduces the idea of partitioning for economic analysis of reliability. An economic analysis model for simultaneously optimizing the capacity and quantity of substation transformer in the distribution network is constructed, taking into account the effects of reducing net load and enhancing the reliability of the distribution feeders resulting from renewable energy access to the medium and low voltage side of the substation. Various wiring means of the distribution network are retained and the impact of renewable energy access on the reliability of the network power supply is quantified. The optimization model is solved by the multivariate universe optimizer(MVO) algorithm with stronger optimality finding capability and short solving time. Finally, the case study results of a regional distribution network are employed to demonstrate and verify the validity and rationality of the method.

Fault Segment Location for MV Distribution System Based on the Characteristic Voltage of LV Side

The voltage and current measurement of the medium-voltage (MV) side are used for the traditional fault location method, which leads to a high installation cost of the measurement and complicated post-operational and maintenance work. A fault location method is proposed based on the distributed measurement at the low-voltage (LV) side. On the analysis of voltage distribution rules and the influence of the distribution transformer on voltage transmission, obvious differences are found in the variation of voltage (phase voltage and sequence voltage) at the LV side for different faults—specifically, the detection sensitivity of the voltage to different faults varies. Therefore, a fault location method for the MV distribution network based on characteristic voltage at the LV side is proposed. Firstly, the characteristic voltage is selected adaptively according to the fault type. The suspected fault path is then determined by the characteristic voltage amplitude of measuring points. Finally, the fault segment is located via the characteristic current of each segment. This method can be applied in combination with the widely used LV measurement equipment such as the power consumption information acquisition system and the intelligent fusion terminal to acquire LV side voltage without adding new measurement devices. The distribution difference of the characteristic voltage at the LV side is applied for accurate fault finding, which is an economical and practical new idea for the fault location of the distribution network. The correctness and effectiveness of the method were verified by the simulation of the IEEE 34 system built in PSCAD/EMTDC and a real distribution network.

Research on Short-Circuit Impedance Analysis Method of Auto-transformer for Side Column Voltage Regulation

In order to realize the accurate analysis of the impedance of transformers with complex connection relations, this paper derives the impedance calculation method of low-voltage excitation side column linear voltage regulating autotransformer by using the power method. The transformer network with different taps is analyzed, and the calculation formulas of transformer HV-MV impedance, HV-LV impedance and MV-LV impedance are derived. The analysis is simplified to the solution of concentric column coil impedance and magnetic potential of each coil. By comparing the calculated data of actual products with the measured data, the calculation deviation of this method is controlled within 2.03%; By using the finite element method, the impedance of the transformer with the measures to control the magnetic shielding is analyzed and calculated. By comparing the impedance of the transformer with different measures, it can be confirmed that the magnetic leakage measures have an impact on the impedance of the transformer.

Analysis of Typical Faults and Defects and Testing Methods of Complete Set of Turns-Regulating Arc Suppression Coil

In recent years, many automatic tracking compensation turns-regulating arc suppression coils have been installed in substations to compensate for the increasing capacitive current. However, the frequent occurrence of the fault defects of the arc suppression coil itself has caused a series of new problems, such as causing abnormal bus voltage at the low-voltage side of the substation. Through the analysis and classification of typical faults and defect cases of the complete set of turns-regulating arc suppression coil, it is found that the most vulnerable parts of the device are the on-load tap changer, damping resistance cabinet and control unit. Aiming at the problem that the detection means of the complete arc suppression coil device are insufficient, this paper proposes a new test method, including a compensation characteristic test and relevant accessories test. Based on the new test method, a simulation test platform was built to test 2 sets of turns-regulating arc suppression coil devices. The results show that the new test method has great application value.

ANALISIS GANGUAN PERANGKAT HUBUNG BAGI TEGANGAN RENDAH PADA GARDU DISTRIBUSI DI PLN UP3 BEKASI

This analysis focuses on device interference to connect low or short voltage (PHB-TR) at substation distribution and overcome it at PT PLN (Persero) UP3 Bekasi UID West Java. PHB-TR is an electrical device or equipment in the form of a connecting device made of conductive and non-conductive materials mounted on a frame or cabinet and equipped with electrical equipment and electrical safety and is part of a distribution substation on the low voltage side. The disturbances in the PHB-TR are usually caused by human error or environmental factors that refer to electrical devices and equipment. Therefore, based on the results of the analysis of PHB-TR disturbances in the distribution of substations at PLN UP3 Bekasi, they found disturbances and looked for solutions to these problems so that the distribution network installation operates reliably, performance operates well and the continuity of electric power services is maintained properly. activities that include a series of stages from planning, implementation, to controlling and evaluating PHB-TR installation maintenance work.