Three-phase Distribution Grid Research Articles

Compensation of Systematic Errors for Improved PMU-based Fault Detection and Location in Three-Phase Distribution Grids

Compensation of Systematic Errors for Improved PMU-based Fault Detection and Location in Three-Phase Distribution Grids

CoLinFlow: An Iterative Solution Approach for the Scenario-Based AC OPF in Active Meshed Distribution Grids With Batteries

Distribution grids are characterized by an increased installation of renewable energy sources (RESs) and of batteries. Both the uncertainty of RESs and the controllability of batteries should be considered when optimizing their operation. This leads to large-scale, non-convex Optimal Power Flow (OPF) problems. Existing solution approaches either refer to radial grids or do not work under uncertainty. However, many Distribution System Operators operate their grids in meshed topologies. In this paper, CoLinFlow, an efficient solution approach for a scenario-based AC OPF in meshed balanced three-phase distribution grids with intermittent RESs and controllable batteries is proposed. First, a particular power flow form in rectangular coordinates that is accurate for meshed grids is applied. Then, Generalized Linearized Power Flow (GLPF), a novel linearization of the power flow equations based on replacing non-linear terms with constants is developed. Based on GLPF, we design CoLinFlow, a heuristic iterative scheme that at each iteration solves a convex OPF and updates GLPF until convergence. The solution of CoLinFlow at convergence is proven to be feasible with respect to the original non-convex OPF and convergence is reached after very few iterations. Also, it is shown that CoLinFlow achieves almost the same optimal objective as Ipopt, while being faster.

Unified variable regularization factor based static compensator for power quality enhancement in distribution grid

A unified variable regularization factor based strategy for harmonic filtering at the supply side of the distribution system is presented in this paper. For this purpose, a three-phase distribution grid along with a distribution static compensator and a rectifier based nonlinear load is considered. A combined regularization affine projection sign algorithm-based control technique is used to calculate the weight values that denote the active power and reactive power segments of the load current. The proposed algorithm contains a variable mixing factor of two adaptive filters with different regularization parameters. The variable mixing factor enhances the tracking performance of the algorithm and hence it offers quick convergence and less steady-state error when compared with other affine projection-based filtering schemes. The control algorithm is evaluated under varying load patterns to derive its robustness and stability. Empirical assessment is carried out through real time simulation analysis aided by dSPACE-1202 controller. The performance of the applied control technique is found good-enough in mitigating the supply side harmonics and the total harmonic distortion levels rendered by the experimentation are in-line with the IEEE-519 standard.

Multi-Stage Volt/VAR Support in Distribution Grids: Risk-Aware Scheduling With Real-Time Reinforcement Learning Control

The ever-increasing penetration of intermittent renewable resources in low-voltage power grids necessitates efficient operational strategies for voltage regulation as well as power scheduling of the available resources. In this paper, a risk-aware Volt/VAR support framework followed by a real-time reinforcement learning controller is presented for three-phase distribution systems. In the risk-aware stochastic scheduling stage, the legacy voltage regulating assets along with inverter-based photovoltaics (PVs) and energy storage system (ESS) are optimized considering day-ahead and intra-day markets. Moreover, demand response (DR) and voltage reduction plans are included in the proposed scheduling framework. By incorporating voltage-dependent load modeling in this study, the implementation of the voltage reduction plan reduces energy consumption in feeders by running the network at lower permissible voltage limits. The shiftable loads under the DR program are employed for peak shaving and to reduce operational costs. The result shows that DR implementation also reduces dependencies on the operation of traditional devices. The stochasticity of abrupt changes in PV generations is represented as the Gaussian Mixture Model (GMM), indicating a non-unimodal probability distribution in day-ahead PV forecasting errors. The scenario sets for uncertain variables are then reduced using a fuzzy clustering technique. Decisions made in the scheduling, associated with PV inverters and ESS operation, are revised with a real-time controller, i.e., Deep Deterministic Policy Gradient (DDPG) reinforcement learning. The DDPG is adopted in the control stage of the framework considering the detailed modeling of unbalanced three-phase distribution grids to minimize the voltage deviation and power ramping of ESS. The performance of the proposed multi-stage scheme is verified using a three-phase active distribution grid under different scenarios.

Battery Storage System as Power Unbalance Redistributor in Distribution Grids Based on Three Legs Four Wire Voltage Source Converter

This article discusses the application of battery energy storage systems (BESSs) as power redistributors in three-phase distribution grids as an add-on functionality to typical BESS applications, such as congestion management and energy arbitrage. Combining those ancillary services into a single power unit is not yet performed in practice but may constitute an emerging business opportunity to increase the BESS revenues. The unbalanced operation of the BESS voltage source converter (VSC) leads to the circulation of low-frequency current harmonics in the dc-link through the capacitors and the battery cells. Therefore, it is particularly interesting whether relatively large 50- and 100-Hz currents can safely circulate within these components. Analytical modeling and design guidelines for the dc-link of a three-leg four-wire two-level VSC operating under unbalanced loads are detailed. Furthermore, a low-power VSC prototype is used to demonstrate the working principle of the BESS, providing power unbalance redistribution and symmetric power exchange. Additionally, the ICR18650-26F Lithium-ion cells are cycled to reach end-of-life with different current profiles and C-ratings. The analysis shows that charging with a 100 Hz ripple superimposed to the dc current leads to a 10% increment in degradation.

Identification of the phase connectivity in distribution systems through constrained least squares and confidence-based sequential assignment

This paper addresses the customer-phase identification problem in three-phase distribution grids including three-phase customers characterized by aggregated energy measurements. The proposed technique first solves a relaxed problem, in which the binary nature of the variables is ignored, which leads to a constrained, least-squares estimation, using as inputs the active and reactive energy readings provided by the smart meters, along with the energy delivered by each phase at the head of the feeder. With the estimated values of the decision variables, and their corresponding variances, a confidence-based selection technique is then applied for the sequential assignment of the customer with the highest joint probability of being connected to one of the three phases but not to the other two. The performance of the proposed procedure is assessed with five different scenarios in terms of accuracy for increasing number of loads and measurement errors. The robustness of the algorithm is additionally tested in the presence of model errors, and its performance is compared to that of existing methods.

Control of line voltage unbalance factor in three-phase distribution grids caused by single-phase photovoltaic systems

This study investigates the use of photovoltaic systems to regulate the voltage amplitude of electric power networks and to analyze their impact on the stability of Line Voltage Unbalance Factor (LVUF). It will be shown that instability of the LVUF can occur when single-phase domestic photovoltaic systems equipped with a voltage controller are connected to the three-phase low voltage network. The degradation of the LVUF is one of the problems to be avoided in three-phase electrical networks. To address this problem, this paper presents a complementary algorithm to be applied on the existing controller of the photovoltaics system. This algorithm calculates the optimum reference voltage for the controller, to keep the voltage amplitude within the permitted range, and at the same time prevent the degradation of the LVUF.

A Cascaded Generalized Integral Control for Multiobjective Grid-Connected Solar Energy Transfer System

A cascaded generalized integral control algorithm is presented in this article for a solar energy transfer system (SETS), connected to the grid. The control algorithm introduces a two part fundamental component extraction such as, primary harmonics filter with higher order harmonic reduction and dc-offset rejection, and secondary fine fundamental component tuning with fractional order and lower harmonics rejection property. This SETS transfers electrical energy converted from solar radiant energy, efficiently to the three-phase distribution grid. The two-stage system consists of a boost type dc–dc converter on the primary side. Moreover, the incremental conductance algorithm-based operation of the boost converter ensures maximum power output from the photovoltaic (PV) array. This system is designed to meet load demands and feeds remaining power to the grid. It mitigates the reactive power and harmonics requirements of the load at any irradiance level, and thereby keeps the balanced sinusoidal grid currents, in-phase with the respective grid voltages. As the PV array power and load power are varying, performance of SETS with adverse operating conditions is validated in this work. A prototype is developed and tested under varying irradiance conditions as well as nonlinear unbalanced load. The steady-state and dynamic behaviors of the system during transient operating conditions are expressed to substantiate the acceptability of the control technique for multiobjective grid-connected SETS.

An Optimal Power Flow Algorithm for the Simulation of Energy Storage Systems in Unbalanced Three-Phase Distribution Grids

An optimal power flow algorithm for unbalanced three-phase distribution grids is presented in this paper as a new tool for grid planning on low voltage level. As additional equipment like electric vehicles, heat pumps or solar power systems can sometimes cause unbalanced power flows, existing algorithms have to be adapted. In comparison to algorithms considering balanced power flows, the presented algorithm uses a complete model of a three-phase four-wire low voltage grid. Additionally, a constraint for the voltage unbalance in the grid is introduced. The algorithm can be used to optimize the operation of energy storage systems in unbalanced systems. The used grid model, constraints, objective function and solver are explained in detail. A validation of the algorithm using a commercial tool is done. Additionally, three exemplary optimizations are performed to show possible applications for this tool.

Control of the Transformer Phase Powers Using a Single-Phase Voltage Source

Power flow in three-phase distribution grids containing single-phase prosumer voltage sources strongly depends on the RMS value of the voltage of these sources and their phase shifts in relation to the grid voltage. The ideal way to control single-phase prosumer sources should guarantee no return active power to the MV grid through a distribution transformer and no additional reactive power flows in the LV grid. This means that the active power of the one-phase voltage source is consumed by other single-phase customers (in the same phase or in other phases) and the reactive power of this source is equal to zero. The paper presents the results of the investigations of the dynamic control system of a single-phase voltage source that allows meeting these conditions. On the basis of steady-state calculations, the static characteristics of the above-mentioned control, needed to determine of the proper working point of a prosumer source were also obtained. The control process involves the control of the RMS value and phase angle of the voltage source against the phase voltage of the LV grid, to which the source is connected, with simultaneous control of the current phase angle issued by the power source against voltage. The result of the research is the confirmation of the necessity of using a zig–zag connection of the secondary side of distribution transformers. The developed control system of the prosumer voltage source does not fully control the active power of individual phases of the distribution transformer. The paper shows that the power losses in a distribution transformer strongly depend not only on the active power of the prosumer source, but also on its effective voltage and phase in relation to the transformer voltage.

Метод идентификации одиночных утечек тока в фазах распределительной сети, контролируемой автоматизированной системой учета

The paper dwells upon a 0.4-kV four-wire three-phase distribution grid (DG), where energy consumption is monitored by an automated metering system. Phase wires are assumed to have equal unknown non-neutral resistance in each inter-customer section of the DG; the metering system measures the operating voltage, current, and phase angles between the customers as well as at the DG entry point. The paper dwells upon finding the values of, and locating the leakage currents resulting from unauthorized power outtake. It analyzes an earlier proposed method that relies on phase (linear) wire voltage increments to simulate disturbed and desired state of the DG; analysis reveals the disadvantages of this method. The paper further presents a novel solution based on calculating the grid parameters (resistances) and currents in real time. The paper also proposes an algorithm for calculating the grid parameters, which samples data from two different modes of DG operation. Mathematical expressions are shown herein that implement this method and have been tested by computational experiments. The results could be of use in the design of non-process electricity loss metering systems for distribution grids.

Application of nonlinear Kalman filters to the identification of customer phase connection in distribution grids

This paper presents a state estimation approach to address the problem of identifying the phase to which single-phase customers are connected in three-phase distribution grids. The proposed method performs Kalman filtering on the information provided simultaneously by the smart meter of every customer and the aggregated energy consumption measured at each phase of the secondary substation feeding the set of customers. Different nonlinear formulations of the Kalman filter are tested and their performance compared, showing that the ensemble Kalman filter provides better estimation results when the system size increases. The accuracy, robustness and limitations of the estimator are also tested when measurement errors are considered.

Optimal Dispatch of Battery Energy Storage System Using Convex Relaxations in Unbalanced Distribution Grids

This paper presents second-order cone programming (SOCP) and semidefinite programming (SDP) models to solve the multiperiod optimal control problem of unbalanced three-phase distribution grids with battery energy storage systems. The decision variables are the active and reactive power of the battery energy storage system. The objective is to minimize the power loss and energy purchase cost from the distribution substation. The optimal dispatch problem requires solution by volt/VAR optimization. The SOCP and SDP models ensure global optimum of original nonconvex nonlinear programming problem. Multiobjective volt/VAR optimization is performed and the benefits of reactive power support from battery energy storage systems are explored. A simulation-based heuristic to extract rank-one solution from the higher rank solution of SDP model is also proposed. Empirical results show the numerical stability and exactness of the proposed three-phase SOCP and SDP models.

不平衡接続された単相分散型電源による三相系統の高調波電流抑制

As a result of the progress in renewable energy technologies, distributed generation (DG) with harmonic compensation capability has become an attractive topic. If photovoltaic (PV) systems have harmonic compensation functionality, they can help suppress the harmonic currents produced by multiple harmonic sources like energy-saving household appliances. This paper proposes harmonic circulate control, which is a harmonic suppression method that uses DG units. Using the proposed method, the current distortion in three-phase distribution grids can be suppressed by a smaller number of harmonic compensators than with conventional methods. In order to verify the validity of the proposed control strategy, experimental results using a laboratory system are presented.

96/03901 A low-cost method of evaluationg saving from lighting retorfits

Photovoltaic (PV) power generation is an important component for the future energy system. High penetration of PV power in a power distribution system might however lead to problems with overvoltage and overload. In this study, a method for PV power curtailment and placement of decentralized energy storage is developed to control voltage, feeder currents and distribution substation overloading. The method determines an individual feed-in power limit for each PV system owner based on a voltage-power relationship. Measured data from a 10 kV/400 V three-phase distribution grid in the Swedish municipality of Herrljunga with more than 5000 end-users and simulated PV electricity production data are used for a case study to verify the model. The method is evaluated for yearly PV electricity production of up to 100% of the yearly electricity consumption. The results show that the method is able to prevent overvoltage for all penetration levels in the studied distribution grid, reduce the number of feeders affected by overcurrent and lower the maximum load on the two substations.