Medium Voltage Distribution System Research Articles

Analytical Study Based Optimal Placement of Energy Storage Devices in Power Systems to Support Voltage and Angle Stability

Larger penetration of Distributed Generations ( DG ) in the power system brings new flexibility and opportunity as well as new challenges due to the generally intermittent nature of DG . When these DG are installed in the medium voltage distribution systems as components of the smart grid, further support is required to ensure a smooth and controllable operation. To complement the uncontrollable output power of these resources, energy storage devices need to be incorporated to absorb excessive power and provide power shortage in time of need. They also can provide reactive power to dynamically help the voltage profile. Energy Storage Systems ( ESS ) can be expensive and limited number of them can practically be installed in distribution systems. In addition to frequency regulation and energy time shifting, ESS can support voltage and angle stability in the power network. This paper applies a Jacobian matrix-based sensitivity analysis to determine the most appropriate node in a grid to collectively improve the voltage magnitude and angle of all the nodes by active/reactive power injection. IEEE 14, 24, and 123-bus distribution system are selected to demonstrate the performance of the proposed method. As opposed to most previous studies, this method does not require an iteration loop with a convergence problem nor a network-related complicated objective function.

Ensuring System Reliability: Distributed Energy Resources and Bulk Power System Considerations

Rooftop solar energy, storage batteries, and electric vehicles are changing the way people use and produce electricity. These are among the growing set of technologies called distributed energy resources (DERs) because they are typically connected to the utility at the medium-voltage distribution system. In general, the utility distribution system connects substations to homes and businesses. The effect of DE Rs on operating the power grid has been dramatic and exciting, and they provide both challenges and benefits to the entire grid: spanning utilization, distribution, and transmission voltage ranges. The zero-carbon resources among the broader family of DE Rs are critical in complying with state and federal energy policies. This, in turn, has required grid operators, utilities, regulators, consumer advocates, and lawmakers to reach out across traditional domains to engage in the robust dialog and coordination needed to integrate distribution-level resources to the larger high-voltage grid and the networked bulk power systems (BPSs).

A decomposition approach for integrated planning of primary and secondary distribution networks considering distributed generation

This paper presents a new model for optimal integrated planning of medium and low voltage distribution systems with penetration of distributed generation (DG) in the low voltage network. The proposed bilevel model takes into account in the upper and the lower levels, the medium and low voltage network planning respectively. This approach considers as conflict between these two agents (upper and lower levels), the size and location of the distribution transformers (DT), i.e. the incidence in both networks of the power flow circulating from the primary to the secondary system. The main objective of this approach is to find a joint global solution that establishes a balance to benefit the planning of both networks, by decomposing the problem in two subproblems (or levels). The upper and lower levels involve the costs of installing and upgrading the new and existing elements (branches, DT, substations and DG) and the cost of the energy technical losses. This problem is formulated as a mixed integer non-linear model, and is solved using a tabu search algorithm (TSA). To verify the efficiency of the proposed methodology, three cases of study are compared: (i) traditional integrated planning, (ii) bilevel integrated planning and (iii) bilevel integrated planning with DG in the LV network. The obtained results show the importance of considering both networks in a simultaneous way in the electric distribution system planning, which allows finding answers of lower global costs.

Evaluation of the Mitigation Effect of the Shield Wires on Lightning Induced Overvoltages in MV Distribution Systems Using Statistical Analysis

Lightning overvoltages are one of the most dangerous factors affecting the power quality of a distribution grid. In medium voltage (MV) overhead distribution systems, lightning transients can be caused by either direct or indirect strikes. Indirect strikes are much more frequent than direct strikes and can cause flashovers, especially when the line insulation level is low. The most widely employed protection measures against lightning overvoltages are the use of the shield wires, and/or surge arresters, and increasing the line insulation level. In particular, many studies have shown a significant reduction in the induced voltage magnitudes, thanks to the use of the shield wires in both transmission and distribution systems. This paper deals with the possibility of adopting the shield wires to mitigate lightning induced overvoltages in MV networks, and proposes an analysis on the main parameters that influence their performances. This aim is pursued by means of a probabilistic and statistical analysis based on the response surface method (RSM), which permits to obtain the equation of the surface that best approximates a given model. The application and efficiency of the proposed RSM-based approach have been illustrated by applying it to a realistic situation of a rural distribution line.

Dynamic Analysis and Improved LVRT Performance of Multiple DG Units Equipped With Grid-Support Functions Under Unbalanced Faults and Weak Grid Conditions

Due to the increased integration of multiple distributed generation (DG) units into the distribution network, riding through short-term faults and supporting the host grid have been requested by the new grid codes in many countries. However, the literature lacks the detailed dynamic analysis and control coordination of multiple grid-connected converter-based DG units, equipped with advanced controllers in the synchronous reference frame. To fill this gap, this paper initially presents a detailed small-signal modeling framework for typical medium-voltage multi-bus power distribution systems with multiple DG units equipped with grid-support functions to operate under the unbalanced conditions. This model encompasses the positive and negative sequences of the current and voltage and is developed for three stages of the fault (i.e., before, during, and after the fault) to cover a wide range of system operating points. In addition, to precisely study the interactions among DG units, four different control modes in DG units are considered to study the system dynamics under low-voltage and unbalanced conditions and at different grid strengths. Using the proposed detailed state-space models and based on the small-signal stability analyses, different control parameters are redesigned using the eigenvalue analysis on the complete multi-DG system. As a second contribution, sensitivity analyses are performed to study the effects of different system parameters, such as line characteristics, loading levels, and unbalanced fault characteristics, on the stability of the multi-DG system under unbalanced faults. Comparative simulation and experimental results are also reported to show the accuracy and effectiveness of the theoretical analyses.

Dynamic Performance of Doubly Fed Hydroelectric Machines Under Voltage Unbalance—A Relative Electrothermomechanical Analysis

Deployment of renewable energy sources in power grid causes an undesirable characteristic in the voltage profile due to its stochastic nature. Such electrical perturbations may impact the performance of equipment or machines connected to the grid, especially large rated machines (e.g., 250 MW). This paper investigates the electrical, thermal, and mechanical stresses on large rated doubly fed synchronous (fixed speed) and doubly fed asynchronous (variable speed) machines used in pumped storage power plants under voltage unbalance. Also, the lifespan of the drive train considering thermal stress is estimated. In this paper, considering the medium voltage distribution system (15.75 kV), the magnitude of voltage unbalance is opted as 2.5% and 5% to test the performance of doubly fed machines. With respect to these considerations, the impacts on machine parameters such as rotor current, stator current, input power, torque, and losses are comprehensively investigated. To observe the performance of large rated doubly fed machine drive system under voltage unbalances, the MATLAB/Simulink environment is adopted. Also, to realize the real-time impact on doubly fed machines, a prototype rig (with 2.2-kW synchronous and asynchronous machines) are experimented. The practical significance of this relative analysis is to minimize voltage collapse in the system and for developing the robust system with countermeasures.

A Hybrid State Estimator Based on SCADA and PMU Measurements for Medium Voltage Distribution System

With the increasing importance of renewable energy and flexible loads, the operation of the distribution system is becoming more stochastic and complex, and it is necessary to monitor the power system in real-time. Considering the gradual applications of intelligent electronic devices in the distribution systems, a hybrid state estimator based on supervisory control and data acquisition (SCADA) and phasor measurement unit (PMU) measurements is proposed in this paper, which consists of the improved robust estimation and linear state estimation. At the time of SCADA data acquisition, the improved robust estimation combining the SCADA measurements with PMU measurements is performed. To eliminate the effect of bad data, the internal student residual method is introduced, and the robust thresholds are adjusted adaptively. Then the linear state estimation is performed at the time of PMU data acquisition based on the results of the previous estimation time and the PMU measurements, which can quickly correct the robust estimation results and track the changes of the distribution system. Finally, the effectiveness and performance of the proposed method are verified in a modified IEEE 33-bus distribution system and a real distribution system in China.

VSI Based DSTATCOM for Compensating Nonlinear and Unbalanced Loads

This paper presents study of distribution static compensator (D-STATCOM) for compensation of neutral current, source&PCC current harmonic distortion, PCC voltage regulation and compensation of unbalanced current waveform for the three phase four wire nonlinear and unbalanced load medium voltage distribution system. The proposed control algorithm is developed based on synchronous reference frame theory with PI controller. The obtained reference current signal from control algorithm is compared in hysteresis band current controller for better switching of D-STATCOM. The performance of system without & with D-STATCOM is also analysed and compared. The proposed control method is implemented on 11/0.4kv medium voltage distribution system and it is provided effective compensation for reactive power and harmonic distortion mitigation. The simulation results are obtained using MATLAB/SIMULINK.

Energy Storage Characteristic Analysis of Voltage Sags Compensation for UPQC Based on MMC for Medium Voltage Distribution System

The modular multilevel converter (MMC), as a new type of voltage source converter, is increasingly used because it is a distributed storage system. There are many advantages of using the topological structure of the MMC on a unified power quality controller (UPQC), and voltage sag mitigation is an important use of the MMC energy storage system for the power quality compensation process. In this paper, based on the analysis of the topology of the MMC, the essence of energy conversion in a UPQC of voltage sag compensation is analyzed; then, the energy storage characteristics are calculated and analyzed to determine the performance index of voltage sag compensation; in addition, the simulation method is used to verify the voltage sag compensation characteristics of the UPQC; finally, an industrial prototype of the UPQC based on an MMC for 10 kV of medium voltage distribution network has been developed, and the basic functions of UPQC have been tested.

Impacts of the model uncertainty on the voltage regulation problem of medium‐voltage distribution systems

In this paper, a novel framework is proposed in order to evaluate impacts of the uncertain models of the system components on the voltage regulation problem of the medium-voltage distribution systems. The investigation focuses on the model uncertainty associated with voltage dependency of loads, power factor of loads, thermal dependency of lines, shunt admittances of lines and internal resistance of substation transformer. To this end, firstly, voltage constraints are managed using a centralised voltage control algorithm (VCA) by relying on the simplified models of the system components. The system loads and lines as well as the substation transformer are then modelled with the uncertain variables which are bounded in the predefined ranges. Monte Carlo (MC) simulations are used to create wide series of scenarios that cover the possible values that the parameters of the system components can take due to their uncertain nature. The model uncertainty impacts on the voltage regulation problem are finally evaluated by the load flow calculations considering the scenarios created by the MC simulations and the set-point obtained by the VCA. The proposed investigation brings useful information regarding the possible deviations that the node voltages can have due to the uncertain models of the studied components.

Multi-agent based distributed voltage regulation scheme with grid-tied inverters in active distribution networks

Due to the increasing penetration of distributed generation, large voltage fluctuations are becoming a problem to the distribution network operators. In this paper, the voltage problem is mitigated using smart grid technologies. A multi-step distributed voltage control method is proposed that utilizes the coordination of reactive power control (RPC) and active power curtailment (APC) methods of inverters, while using node voltages, loading and photo voltaic distributed generation (PVDG) output power as the decision making entities. Multi-agent system (MAS) is utilized to keep the method fully distributed and autonomous, in order to improve the latency of response to voltage violations. The proposed scheme incorporates the least data for voltage control strategies. The effectiveness of the proposed technique is authenticated through numerical simulations on a typical Finnish medium voltage distribution system. The results show that the proposed MAS based coordinated RPC and APC of inverters will keep the voltage value of nodes within the statutory limit, with least curtailment possible.

A Novel Method for Detection of Covered Conductor Faults in Medium Voltage Overhead Line Systems

Although there are many reasons why medium voltage (MV) overhead lines with covered conductors (CCs) are used instead of aluminum-conductor steel-reinforced cables, the operation of CCs takes into account a difficult detection of faults, e.g., the contact of a tree branch with a CC. In this paper, we continue with the previous work on an electric voltage-based method enabling the detection of the CC faults; we introduce a new indicator of the faults based on the frequency of peaks in the signal of partial discharges. We mainly demonstrate the fault detection in a real MV distribution system, including a number of fault detectors; a fault is detected in 99.2% of occurrences within a long-term testing, although the cost of the detector is minimized in comparison with other similar fault detectors.

Impact of Data Quality in Home Energy Management System on Distribution System State Estimation

As the coupling between home energy management system (HEMS) and distribution system state estimation (DSSE) becomes stronger for smart distribution grid operations, unexpected HEMS data change can, through the distortion of the HEMS solution, deteriorate the DSSE performance. In this paper, we investigate the impact of data changes in the HEMS for multiple homes on three-phase unbalanced DSSE. We develop a two-level sensitivity analysis framework based on the perturbed Karush–Kuhn–Tucker conditions from the HEMS and DSSE optimization formulations. The developed framework is used to assess the impact of the HEMS data changes in a low-voltage (LV) distribution network (at the first level) on the DSSE solutions in a medium-voltage (MV) distribution network (at the second level). Using the sensitivity framework, system operators can quantify the sensitivity of DSSE to changes in various types of HEMS data (e.g., demand response signals, appliance parameters, and consumer comfort). Along with the HEMS data impact analysis, the proposed sensitivity approach is tested under different measurement redundancy for DSSE in an IEEE 13-bus MV distribution system with 12 smart households in a radial LV network.

Optimization of MV Distribution System Designs

This paper will present a new approach that is advantageous to medium-voltage (MV) distribution design configurations for large industrial and power distribution system configurations [specifically main-tie-main (MTM)] for large industrial and power station installations. Sixty years of traditional low-cost design approaches (typical MTM and operated with open tiebreaker) are re-evaluated to demonstrate that traditional low capital cost design may miss improvement opportunities. Those opportunities can be major capital expenditure reduction, power quality improvement, MV motor soft starter deletion, and improved system stability to name a few. Many designers may not have considered the component interrelationships and/or their tradeoffs when in an integrated system. The designer and/or user should consider the advantages of higher short-circuit capacity (SCC) systems by integrating fault current limiters and MV switchgear. Utilizing higher SCC systems permits power system designer optimization opportunities not possible before using conventional design methods. This paper will provide documentation of substantial savings of capital investment and energy under higher SCC conditions. The savings in one system component will offset another's increased cost. An obvious benefit of stiffer systems is ease of direct-on-line starting of large motor-driven compressor trains. The future need for motor soft starters minimizing system impact will be challenged. This paper may change fundamental system design methodology for system designers by removing several performance tradeoffs. This paper, in its original version, was previously presented at the 2010 IEEE/IAS/Petroleum Chemical Industry Conference. It was expanded with new sections for the 2017 IEEE/Industry Applications Society (IAS)/Pulp, Paper Forestry Industries Conference to include pre-emptive prefault adjustable power quality and multisourced networked power systems.

Harmonics compensation in industrial power network using Hybrid Active Power Filter (HAPF) in d-q frame

In AC power networks, the use of harmonic and reactive power compensations through active power filters has become emerging aspect of today’s commercial market. With the advancement of new technologies, loads based on power electronics are regularly being used in low and medium voltage distribution systems, which eventually cause the harmonics in the system currents and voltages. The most dangerous harmonics are the later i-e, 3rd , 5th , 7th and so on, which are basically result of current harmonics. They cause degradation in the performance and working of other important power system equipment. In order to mitigate these daunting harmonics, the best option is to use the hybrid power filters which combine the characteristics of both active and passive harmonic filters al-together. The main idea of this research is to model an industrial network and to perform an analysis over the Total Harmonic Distortion (THD) in the power system. The use of Hybrid Active Power (HAPF) kept the THD value well below 8%, as recommended by European Standard EN50160. With the implementation of HAPF, an additional advantage of power factor improvement was also achieved. PQ theorem was used for calculating parameters of hysteresis control used for filter implementation. Results presented in the research paper confirm the efficiency of designed filter because the THDs of both voltage and current were restricted to 2% and the system power factor was improved too. The major contribution of this research is that power quality was improved most economically using P-Q technique through HAPF.

A Multi-Stage Restoration Method for Medium-Voltage Distribution System With DGs

A fast and effective service restoration method is proposed for medium-voltage distribution system with distributed generators by combining intentional islanding of distributed generators with network reconfiguration to maximize restoration of out-of-service loads. The whole process is divided into four stages: 1) matching islanding schemes; 2) restoration of network connectivity and distributed generators; 3) network reconfiguration; and 4) optimization of load shedding. Whether the last three stages are executed or not depends on violations of operational constraints. In accordance with the topological feature of distribution system, a new concept of fundamental loop matrix is presented and applied in detecting out-of-service areas, restoration of network connectivity, and network reconfiguration. Simulation studies on the PG&E 69-bus system shows the effectiveness of the proposed approach.

A Multi-Platform Lab for Teaching and Research in Active Distribution Networks

Today's electricity paradigm requires that the notion of active distribution systems be introduced at both undergraduate and graduate curricula. This involves not only the customary theoretical foundations but also a suitable power engineering laboratory, where flexible enough and affordable resources allow students and researchers to carry out hands-on experiments reinforcing the concepts explained in the classroom. This paper describes the smart grid laboratory of the Power Engineering Group at the University of Seville, composed of a scaled-down distribution system, along with the required control and monitoring equipment, designed to help its users easily grasp the major influence that distributed generation and storage devices exert in the operation of medium-voltage distribution systems.

Optimal siting and sizing of renewable energy sources, storage devices, and reactive support devices to obtain a sustainable electrical distribution systems

This paper presents an integrated planning framework to optimally determine the location and allocation of renewable-based distributed generation (DG) units, energy storage systems (ESSs), and capacitor banks (CBs). This planning aim at improving the performance of electrical distribution systems (EDSs). In the proposed model, the cost of energy delivered by the substation and the investment costs are minimized. The environmental aspects are taken into account to obtain an efficient environmentally committed plan. The uncertainties due to PV generation and demand profile are considered via external uncertainty indexes in a deterministic environment. The proposed model is a mixed-integer nonlinear programming (MINLP) problem, which is recast to a mixed-integer linear programming (MILP) problem using appropriate linearization techniques. This approximated MILP model is implemented in the mathematical language AMPL, while the commercial solver CPLEX is used to obtain the global optimal solutions. The proposed model is validated by testing on a medium voltage distribution system with 135 nodes under different conditions and topologies.

MV grids development and automation

The study is focused on the development of current medium-voltage (MV) distribution systems. There are increasingly stricter demands on these distribution grids, especially in the area of grid reliability, connection of renewable energy sources or high industry parks, and targets of The Smart Grid Concept. The primary aim of this study is to propose a gradual development of existing MV equipment with expectation of partially automated grid in the future.

Optimal allocation of capacitor devices on MV distribution networks using crow search algorithm

This study aims at enhancing the performance of medium-voltage distribution systems. Improving the performance of distribution networks is employed through optimally reducing the energy loss, minimising of the loading level for the transformer substation, and economical penetration of shunt capacitors. Moreover, the impacts of changing the switching tie-line in the distribution network on the optimal allocation of the capacitor devices are discussed. A novel recent powerful swarm intelligence optimiser called crow search algorithm (CSA) is developed for optimal allocations of capacitors into electrical distribution networks. Achieving the technical and economical objective functions is subjected to various operational limits. The proposed procedure is applied on East Delta Network (EDN) as a real distribution network in the Unified Egyptian Network. Significant technical and economical merits are satisfied which prove the capability of the proposed allocation procedure.