Low Voltage Side Research Articles

Reinforcement learning for robust voltage control in distribution grids under uncertainties

Traditional optimization-based voltage controllers for distribution grid applications require consumption/production values from the meters as well as accurate grid data (i.e., Line impedances) for modeling purposes. Those algorithms are sensitive to uncertainties, notably in consumption and production forecasts or grid models. this paper focuses on the latter. Indeed, Line parameters gradually deviate from their original values over time due to exploitation and weather conditions. Also, those data are oftentimes not fully available at the low voltage side thus creating sudden changes between the datasheet and the actual value. To mitigate the impact of uncertain line parameters, this paper proposes the use of a deep reinforcement learning algorithm for voltage regulation purposes in a distribution grid with PV production by controlling the setpoints of distributed storage units as flexibilities. two algorithms are considered namely TD3PG and PPO. A two-stage strategy is also proposed, with offline training on a grid model and further online training on an actual system (with distinct impedance values). The controllers’ performances are assessed regarding the algorithms’ hyperparameters, and the obtained results are compared with a second-order conic relaxation optimization-based control. The results show the relevance of the RL-based control, in terms of accuracy, robustness to gradual or sudden variations on the line impedances, and significant speed improvement (once trained).

Modeling and Closed-Loop Control of Three-Port Isolated Current-Fed Resonant DC–DC Converter

The power decoupling three-port isolated current-fed resonant dc–dc converter realizes the power decoupling of each port at the topology level, reduces the control complexity, and ensures the zero instantaneous power of the standby port and high operation efficiency. In this article, the closed-loop control of the transfer power of each port is researched for the first time. First, a simplified single closed-loop control strategy of low-voltage side (LVS) port current is proposed to realize the closed-loop control of the transfer power. Then, the small-signal model is established and the parameters of the closed-loop controllers are designed. In addition, the transient time-domain characteristics of the voltage of the inner dc capacitor and the resonant current are analyzed and it is proven that there is no issue of transient overshoot or oscillation even when these variables are not under closed-loop control. The correctness and feasibility of the proposed closed-loop control strategy and parameters design are verified by the experimental results.

Coupled Inductor Based Soft Switched High Gain Bidirectional DC-DC Converter With Reduced Input Current Ripple

Bidirectional dc–dc converter (BDC) is an integral part of energy storage interface, where high efficiency, high voltage transfer ratio and small input ripple current are essential for application, such as dc microgrid and electric vehicle. In this article, a new approach is proposed to achieve reduced input ripple current at the low voltage side, which potentially replaces the necessity of using interleaved circuit structure. Parallel coupled inductor and capacitor circuit are utilized for charging and discharging the two parallel paths in a complementary fashion using a <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> . The proposed solution helps to increase the voltage conversion ratio unlike interleaved structure. Additionally, in proposed BDC, zero voltage switching turn <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> operation of all active switches are possible utilizing synchronous rectification principle in both the operating modes. Parallel path structure not only reduce the input current ripple, but also helps to share the input current, thereby reducing the coil size and associated losses. A 250 W BDC is designed to validate performance in 1kW dc microgrid system

Fault location discrimination and protection strategy for DC distribution network based on PET

After the bipolar short circuit fault occurs on the low-voltage DC side of the low-voltage DC distribution network with a power electronic transformer, the fault current amplitude is high and the rise speed is fast, which leads to the rapid locking of power electronic devices, and the loss of fault information of line protection. Based on the multi-branch DC distribution network with CHB-type PET, this paper proposes a fault current limiting strategy. PET starts current limiting measures after detecting the DC side fault, which can provide stable and continuous fault transient information for line protection while providing a small amount of short circuit current. On this basis, using the fault current at both ends of the line, longitudinal polarity protection without strict communication synchronization is proposed. The simulation analysis based on PSCAD/EMTDC software shows that the scheme can realize fast and reliable fault line selection and has strong resistance to transition resistance.

Power supply reactive power factor control as a function of consumer power and losses in transformers

The system of automatic control of the reactive power factor of the enterprise's power supply system is considered, in which one part of the transformer substations is equipped with adjustable capacitor units, and the second is not. Its advantage in comparison with the power factor control system is shown, firstly, lower requirements for its error and, secondly, the calculation of the consumer with the supplier for electricity is made taking into account the ratio of reactive energy to active energy. An estimate of the error of regulation of the reactive power factor on the higher voltage side of the transformers of the main step-down substation, where settlement meters are installed, is given, with stabilization of the reactive power factor on the low voltage side of these transformers. The synthesis of the system of automatic control of the reactive power factor, built as a function of the power of consumers and power losses in transformers, and the estimation of its error are considered. The proposed control system is intended for power supply systems with a combined load, the use of which will allow the consumer to avoid penalty coefficients to the tariff for active electricity, and ultimately reduce losses in the transmission of electricity and increase the transmission capacity of the electrical network.

Backup protection method based on multi-interval information in low voltage distribution network of high proportion of renewable energy system

With the large number of renewable energy power sources such as wind power and photovoltaic power connected to the grid, the power system is becoming more electronic. The fault characteristics of renewable energy power sources and synchronous generators are obviously different, mainly manifested as the characteristics of short circuit current amplitude limitation and frequency offset, etc. The traditional protection based on the fault characteristics of synchronous generators may have adaptability problems. For a high proportion of renewable energy system, the short circuit current in the low voltage distribution network is greatly affected by the operation state of the renewable energy power sources after the short circuit fault occurs, which leads to the difficulty of the traditional three-stage overcurrent protection setting and coordination. In order to ensure the safe and stable operation of the system, this paper proposes a new backup protection method for low voltage distribution network. This method uses the information of the electric parameters on the high voltage side and low voltage side of the transformer. Firstly, the modulus sum of the voltage sampling value, the equivalent distance between the fault point and the protection installation place, and the modulus sum of the current sampling value are calculated. Secondly, the direction is judged by the criterion of the voltage direction element and the criterion of the distance protection element based on the multi-interval information. Finally, the maximum current branch of the low voltage distribution network is selected as the fault branch by the overcurrent element based on multi-interval information, so as to trigger the circuit breaker trip and isolate the fault. The low voltage distribution network model of the high proportion of renewable energy system is built on the PSCAD/EMTDC electromagnetic simulation platform. The simulation results show that the proposed protection method can reliably achieve fault removal by using multi-interval information, and has strong adaptability to the high proportion of renewable energy system.

Research on Control Strategy of Solid State Transformer Based on Improved MPC Method

As the core equipment of smart grid, solid-state transformer (SST) needs to have good power quality regulation capability under grid side faults and load side faults. To ensure that the high-voltage side converter works at unity power factor and smooth DC voltage. At the same time, the three-phase output voltages need to have high steady-state accuracy and strong anti-interference capability. In this paper, an improved model predictive control scheme was proposed. First, a theoretical modeling analysis of the high and low voltage side of a solid-state transformer was performed. Then, power and voltage prediction models are developed for the high-voltage side converter and low-voltage side converter of the SST, respectively. Three voltage vectors are selected for prediction by the cost function, and the equivalent voltage vector in the two-phase stationary coordinate system is synthesized and modulated to control the converter. Finally, simulation was conducted to compare and analyze various operating conditions of SST under different control schemes. The control effect of solid-state transformer under improved model predictive control is better than traditional finite control set model predictive control and proportional integral control, which can better regulate power quality.

Distributed Energy Resources Electric Profile Identification in Low Voltage Networks Using Supervised Machine Learning Techniques

Increasing integration of distributed energy resources (DER) in the electrical network has led distribution network operators to unprecedented challenges. This issue is compounded by the lack of monitoring infrastructure on the low voltage (LV) side of distribution networks at residential and utility sides. Non-intrusive load monitoring (NILM) methods provide an opportunity to add value to conventional electric measurements and to increase the observability of LV networks for the implementation of active management network techniques and intelligent control of DER. This work proposes a novel implementation of NILM methods for the identification of DER electrical signatures from aggregated measurements taken at the LV side of a distribution transformer. The implementation evaluates three machine learning algorithms such as k Nearest Neighbours (kNN), random forest and a multilayer perceptron under 100 scenarios of DER integration. A year of minutely reported values of electric current, voltage, active power, and reactive power are used to train and test the proposed model. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$F_{1}$ </tex-math></inline-formula> scores achieved of 73% and 93% for Electrical Vehicles (EV) and rooftop photovoltaic (PV) respectively and processing times below <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$314~\mu \text{s}$ </tex-math></inline-formula> on an Intel Core i7-8700 machine. These results confirm the relevance of the NILM method based on low frequency electric measurements from the real-time identification of DER.

An Approach to Performing Stability Analysis for Power Transformer Differential Protection: A Case Study

Differential protection normally detects short circuits and ground faults in the windings of a power transformer and its terminals. Inter-turn faults refer to flashovers among the electrodes that arise only in a similar type of physical winding. Inter-turn faults can be examined when the adequate sum of turns is served as short-circuited. In electrical protection, it is difficult to detect inter-turn faults. An inter-turn fault of small magnitude is based on the limited number of turns that resultantly provide a large quantity of current. Due to this reason, protection that comes from the differential scenario possesses a higher degree of sensitivity without causing unwanted operations during external faults. In this paper, a protection-based stability method is proposed whereby external voltages are applied at the low-voltage (LV) side of the transformer while keeping the high-voltage (HV) side short-circuited. This was conducted using a three-phase power transformer (rating: 100 MVA, 380 kV/13.8 kV) at SWCC Shoaiba Power Plant, Saudi Arabia. In this work, differential protection (87T) and high-impedance differential protection for HV-restricted earth faults (REFs) were verified by creating In-Zone and Out-Zone fault conditions to ensure current transformer (CT) circuits and tripping logic. All of the IEDs, protection, and control schemes involved were designed by ABB. This method verifies protection stability for power transformers by implementing differential protection (87T) and high-impedance restricted earth fault (64HV) schemes through creating In-Zone and Out-Zone fault conditions.

Identification Method of Dynamic Parameters of Distribution Network Lines

Accurate network parameters are of great importance for the accurate control of the power distribution network (PDN). In fact, the line parameters of the PDN are always affected by external operating conditions. However, most of the line parameters in the PDN account are static parameters. In order to obtain the dynamic parameters that reflect the line operating condition, this study presents a method that uses only the RMS voltage of the first section of the line and the RMS voltage and power at the low-voltage side of the transformer. This study introduces the processing method of abnormal measurement data, constructs a derivative-free identification equation represented by a matrix, and uses the designed loss function combined with a heuristic method to solve the equation. An actual feeder is used in the experimental part. The experimental data show that the method has some antipower noise ability, and the identification accuracy of this method is better than that of the genetic algorithm and random search algorithm.

Study on Short-circuit current interruption characteristics of Double-break fast vacuum circuit breaker within the minimum arcing time

It is significant for circuit breakers to expel a short-circuit fault in a short time for the new power grid with high proportional photovoltaic or wind power plants to achieve high transient operating stability. Compared with a traditional circuit breaker, a fast vacuum circuit breaker (FVCB), operated by an electromagnetic repulsion actuator, has an extremely shorter opening time and lower deviation in their opening times. This potentially contributes the FVCBs to interrupting a short-circuit fault current in the first half-wave when introducing a controlled switching technology. Meanwhile, it is beneficial to improve the short-circuit current breaking capacity of the FVCBs by adopting series-connected double-break circuit breaker technologies. This paper aims to explore short-circuit current interruption characteristics of a double-break FVCB within the minimum arcing time. We performed a series of short-circuit current interruption tests in a Weil synthetic test circuit. Two series-connected FVCBs and independent single-break FVCBs were used. Three different kinds of opening velocities were adopted for each of the FVCBs. Results showed that the increase in opening velocity resulted in both a decrease in the minimum arcing time and the critical arc extinguishing contact gap, whether for the single-break or the double-break interruption tests. The ratio of the shared voltage on each of the series-connected FVCBs was about 54 (for the break on the high-voltage side) and 9 (for the break on the low-voltage side). Taking the break with a shorter opening time as a break in the high-voltage side could reduce the minimum arcing time and the critical arc extinguishing contact gap. At the same opening velocity, the sum of the critical arc extinguishing contact gap of the two breaks in double-break interruption tests was significantly higher than twice the critical arc extinguishing contact gap in the single-break interruption tests. This could be caused by the fact that in double-break interruption tests, after the breakdown of one break, even if the break restored insulation, could not share the transient recovery voltage (TRV) before the break broke down. In addition, the sudden drop of TRV without breakdown occurred for the break on the high-voltage side might correlate with the micro-discharge of the vacuum gap. The interaction of the charged particles with the electrode surface would be responsible for the formation and evolution of the micro-discharge in some successful interruption cases.

Family of Nonisolated Fully Soft Switched Bidirectional Converters With Single Switch Auxiliary Circuit

In this article, a family of nonisolated zero voltage switching (ZVS) bidirectional dc–dc converters (BDCs) with simple auxiliary circuit and continuous current at low voltage side source ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$V_L$</tex-math></inline-formula> ) is proposed. The auxiliary circuit is only composed of a single switch, a capacitor, and a small inductor which is applied to all proposed nonisolated BDCs to provide soft switching condition for all semiconductor elements. Thus, in terms of components count, the presented converters are superior to other counterpart BDCs. In addition, the continuous current at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$V_L$</tex-math></inline-formula> is maintained, while the main and auxiliary switches in both operating modes are turned <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> and <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> at ZVS. Moreover, all diodes turn <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> at zero current switching which mitigate the reverse recovery problem. Hence, a suitable overall efficiency is achieved in all proposed BDCs. The proposed bidirectional buck/boost converter is analyzed for both step-up and step-down operating modes in details and a 100 W converter prototype is implemented to verify the main converter features and the theoretical analysis.

Novel Bidirectional Isolated DC/DC Converter with High Gain Ratio and Wide Input Voltage for Electric Vehicle Storage Systems

This study proposes a novel bidirectional isolated DC/DC converter with a high gain ratio and wide input voltage for electric vehicle (EV) storage systems. The DC bus of an EV can be used to charge its battery, and the battery pack can discharge energy to the DC bus through the bidirectional converter when the DC bus lacks power. The input voltage range of the proposed converter is 24 to 58 V on the low-voltage side, which meets the voltage specifications of most servers and batteries on the market. The proposed topology is verified through design, simulation, and implementation, and voltage gain, voltage stress, and current stress are investigated. The control bidirectional converter is simple. Only one set of complementary signals is required for step-up and step-down modes, which greatly reduces costs. The converter also features a continuous current at the low-voltage side, a leakage inductance function for energy recovery, and zero-voltage switching (ZVS) on certain switches, which can prevent voltage spikes on the switches and increase the efficiency of the proposed converter. A bidirectional converter with a total power of 1 kW is used to verify the topology’s feasibility and practicability. The power at the low-voltage side was 24–58 V, and the maximum efficiency in step-up mode was 94.5%, 96.5%, and 94.8%, respectively; the maximum efficiency in step-down mode was 94.4%, 95.4%, and 93.7%, respectively.

Fully Soft Switched Interleaved High Step-Up/Down Bidirectional Converter With no Pulsating Current at Low Voltage Source

In this article, a fully soft switched interleaved bidirectional buck/boost converter with high voltage conversion ratio (VCR) and no pulsating current at low voltage source is proposed. Winding cross-coupled inductors technique is employed in order to increase VCR and mitigate current ripple at low voltage side. Moreover, soft switching conditions for all semiconductor devices are provided and the leakage inductances energy of coupled inductors are absorbed in both high step-up/down operating modes. Furthermore, the reverse recovery problem of all diodes is removed due to zero current switching turn <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> . All the mentioned advantages are achieved for a bidirectional converter with two interleaved phases in both high step-up/down operating modes by only one clamp circuit consisting of a clamp capacitor and two active switches. Also, the body diodes of the main switches are utilized as the main diodes to reduce the element count. The converter performance is validated by theoretical analysis and an 800 W prototype circuit in both operating modes.

Analysis of imbalanced voltage on 35kV side after single phase replacement of 500kV transformer

After the B-phase of the 500kV transformer was replaced, the three-phase voltage on the 35kV low-voltage side was imbalanced and the neutral point displacement voltage was relatively high during the start-up test. For this problem, firstly, the calculation method of neutral point displacement voltage is given through the method of vector analysis and equivalent circuit, and the reasons for the imbalance of the three-phase voltage are analyzed. According to the measured parameters, the PSCAD/EMTDC power system simulation software is used to establish a model, and the steady-state operating characteristics of the system are simulated and calculated. Theoretical analysis and simulation calculation have determined that the cause of the imbalance of the three-phase voltage is the inconsistency between the ground capacitance of the low-voltage winding and the capacitance between the high-voltage and low-voltage windings of the B-phase transformer than that of the A-phase and C-phase. The impact of the capacitance between the high-voltage and low-voltage windings of the transformer is far greater than the capacitance of the low-voltage windings to ground on the imbalance of three-phase voltage to ground on the low-voltage side of the transformer.

Research on power electronic transformer with hybrid energy storage system

Power electronic transformer is a new type of power equipment for building smart grids. However, when the grid voltage drops deeply, it will cause its output voltage to be distorted and affect the stable operation of the load. The hybrid energy storage system composed of lithium battery and super-capacitor through bidirectional half-bridge DC/DC converter and dual active bridge DC/DC converter is proposed to be connected to the low-voltage DC side of power electronic transformer, so as to stabilize the output voltage of the power electronic transformer. The dual active bridge DC/DC converter and the bidirectional half-bridge DC/DC converter use phase-shift control and droop control, respectively. The result of the simulation shows that the addition of the hybrid energy storage system can enable the power electronic transformer to have the ability to compensate for the deep voltage drops, thereby improving the reliability of the system.

A comprehensive evaluation method of DC voltage sag in medium–low voltage DC distribution system

The development of the DC distribution system (DCDS) has attracted increasing attention, due to its technological superiorities in integrating new energy sources and DC power loads more friendly and effectively. In this paper, a comprehensive evaluation method is proposed for the medium–low​ voltage DCDS to handle the voltage sag assessment problem. Firstly, the topology structure and control strategy of the DCDS are stated, and the pragmatic characteristics of DC voltage sag on the medium-voltage and low-voltage sides based on the dual active bridge (DAB) converter are analyzed. Then, considering the difference in the evaluation indexes of the low-voltage and medium-voltage sides of the DCDS, a comprehensive evaluation system is established. Further, the analytic hierarchy process (AHP) and criteria importance through intercriteria correlation (CRITIC) weighting method are applied to achieve comprehensive weighting, and the TOPSIS model is used to describe the severity of DC voltage sag. Using MATLAB, a 10 kV/ 750 V DCDS with the DAB is built, and comparative assessments are performed. The results show that the proposed method can reasonably evaluate the DC voltage sag of the low-voltage and medium-voltage sides. Compared to the evaluation schemes based on the single index, the proposed method can more effectively reflect the waveform features of the DC voltage sag, and it is very appropriate for the voltage dip evaluation of the medium–low voltage DCDS.

An Asymmetrical DAB Converter Modulation and Control Systems to Extend the ZVS Range and Improve Efficiency

This article presents a new optimized hybrid modulation and control systems based on symmetrical and asymmetrical operations of a dual-active-bridge converter to minimize the transformer root-mean-square (RMS) current and extend the zero-voltage-switching (ZVS) range. Various modulation modes are analyzed, and their corresponding power, RMS current equations, and soft-switching conditions are derived. The RMS equations are minimized using the multivariable optimization method, and the corresponding parametric equations are obtained. A hybrid control system has been proposed to regulate the battery current, to ensure smooth inductor current transients, and eliminate the need for a blocking capacitor on the low-voltage side. Using asymmetrical-extended-phase-shift and conventional symmetrical modes, a wider ZVS range is achieved compared to advanced modulations, such as triple phase shift. Moreover, in the low-power region, an optimized RMS current is achieved by applying an optimum dc voltage on the blocking capacitor at the HV side. Hybrid modulation that extended ZVS range and improved RMS current makes the proposed approach a suitable modulation for high-frequency applications and also high-voltage or high-current applications with high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$C_{\text{oss}}$</tex-math></inline-formula> losses. The efficiency, ZVS operation, and control system performance are validated by a 5 kW converter.

Research on Neutral Grounding Mode of Interconnecting Transformer in Renewable Energy Plant

With the increasing number of renewable energy plants connected to the power system, the gap grounding mode is often adopted by the high-voltage side neutral point of the interconnecting transformer of the renewable energy plants in order to improve the performance of the zero sequence protection. In practical engineering applications, the low-voltage side of the interconnecting transformer is connected with the renewable energy source (RES). It is necessary to analyze and evaluate the possible overvoltage at the high-voltage side neutral point when the power grid fails, as well as the damage brought to the stable operation of power grid. According to the operation characteristics of RES, the calculation method of neutral point overvoltage is proposed and the influence of different types of RES on neutral point overvoltage is analyzed when a single-phase grounding fault or a single-phase break-line fault (incomplete phase operation state) occurs in the power grid. Then the suggestions to prevent neutral overvoltage are proposed. The correctness of correlation analysis is verified by digital simulation and the conclusion can provide an important reference for the reasonable choice of the neutral grounding mode of the interconnecting transformer in renewable energy plant.

A bidirectional two‐input step‐up DC–DC converter with high gain, continuous input current, and common‐ground for EVs and renewable energy systems

Here, a new two-input step-up DC–DC converter for electric vehicles (EVs) and renewable energy systems (RESs) is proposed. It requires few components and provides high step-up voltage gain and high efficiency. The topology includes two input channels connected to two energy sources based on the “switch-mode coupling” method. Sources supply the load via the proposed converter. Each source can charge another if it is rechargeable. The regenerative output side energy is also transmitted to the rechargeable source thanks to the bidirectional power transferring capability. Current sensitive resources can be employed as the converter operates with the continuous input current. The topology comprises only one main power switch located on the low-voltage side; therefore, it is exposed to low voltage stress and can be readily picked up from low voltage rating semiconductors with small on-resistance. Thanks to this feature, the conduction loss decreases considerably. The input and output terminals have a common ground. This paper demonstrates the theory and operating principle of the proposed topology. Experimental results are also presented and examined to verify the converter. For this purpose, a 48/400 V/1 kW prototype has been designed and built. Results confirm the converter and its operation.