Voltage Limit Violations Research Articles

Effective Volt/var Control for Low Voltage Grids with Bulk Loads

This paper investigates the voltage and reactive power control problem in low voltage grids with connected prosumers and bulk loads. The X(U) local control, which maintains the voltage at the feeders’ ends within a predefined band, and its combination with Q-Autarkic customer plants are the most effective and reliable strategies in grids with high prosumer share. However, these strategies may need adaptations to guarantee voltage limit compliance when bulk loads, such as electric vehicle parking garages and community-owned photovoltaic systems, are connected to the low voltage feeders. This paper extends the X(U) local control concept to involve bulk loads in Volt/var control and investigates the resulting load flows in different real low voltage grids. The results show that the extended control arrangement reliably removes all voltage limit violations by deteriorating the effectiveness of the original X(U) local control arrangement: reactive power flows and equipment loading within the low voltage grids are increased.

A Framework for Analytical Power Flow Solution Using Gaussian Process Learning

This paper proposes a novel analytical solution framework for power flow (PF) solutions in active distribution networks under uncertainty. We use the Gaussian process (GP) regression to learn node voltage as a function of effective bus load or negative net-injection vector. The proposed approximation is valid over a subspace of load and provides an understanding of system behavior under uncertainty via GP interpretability. We interpret the relative variation extent of different node voltages using the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">quality ratio</i> (QR) defined based on the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">hyper-parameters</i> of GP. Further, the application of the proposed framework in calculation of voltage limit violation probability and dominant voltage influencer ranking has also been presented. Through test simulations for 33-bus and 56-bus systems, the proposed method achieves low mean absolute error (MAE) of order E-05 (pu) in voltage magnitude and E-04 (rad) in angle. The discussions on salient features of the proposed method and comparative analysis with large-scale Monte-Carlo simulations, and state-of-art methods is also presented for the proposed applications.

Determination of harmonic contributions using active filter: Theoretical and experimental results

The nonexistence of a methodology feasible in practice to determine the responsibilities (contributions) of a customer and the utility on harmonic voltage distortions at the Point of Common Coupling (PCC) of these systems is a serious problem when there are violations of harmonic voltage limits, since it is impossible to define the participation in expenses with the adoption of mitigating measures (e.g., filter installation). To solve the problem, this paper proposes a methodology for determining these responsibilities. Its development is based on the performance of active filters and meters (voltage and current) connected to the PCC (or in its vicinity). The computational simulation of the methodology was applied to all cases of the IEEE Benchmark System, proving its validity and effectiveness. In addition, a laboratory circuit specially designed and with characteristics like those of real systems allowed its experimental validation, since in this controlled environment the quantities of interest that should be reproduced by the methodology could be accurately determined. One of its important characteristics is the possibility of verifying whether the adopted hypothesis for the validity of its application has been met.

Spatial Model of Optimization Applied in the Distributed Generation Photovoltaic to Adjust Voltage Levels

The main objective of this work is to develop a methodology for analyzing the quality of the voltage level in the distribution power grid to identify and reduce the violations of voltage limits through the proposition of optimal points for the allocation of photovoltaic distributed generation. The methodology uses the geographic location of the power grid and its consumers to perform the grouping and classification in spatial grids of 100 × 100 m using the average annual consumption profile. The generated profiles, including the grid information, are sent to the photovoltaic distributed generation allocation algorithm, which, using an optimization process, identifies the geographic location, the required installed capacity, and the minimum number of photovoltaic generation units that must be inserted to minimize the violations of voltage limits, respecting the necessary restrictions. The entire proposal is applied in a real feeder with thousands of bars, whose model is validated with measurements carried out in the field. Different violations of voltage limits scenarios are used to validate the methodology, obtaining grids with better voltage quality after the optimized allocation of photovoltaic distributed generation. The proposal presents itself as a new tool in the work of adapting the voltage of the distribution power grid using photovoltaic distributed generation.

GRID STATE EVALUATION OF A LVDC TRACTION NETWORK: METHODS FOR THE ANALYSIS OF FORECAST DATA

The paper focuses on the methodical grid state evaluation of a 660 V low-voltage direct current (LVDC) traction network, which primarily serves as a power source for battery-trolleybuses. The replacement of conventional trolleybuses alongside with the integration of stationary storage systems, photovoltaic systems and electric vehicle charging stations within the LVDC traction network is accompanied by the development of a monitoring and control system. The grid state is subject to high volatility due to the dynamically changing power consumption of the battery-trolleybuses. This results in highly volatile current and voltage profiles, making a forecast-based monitoring and control system most suitable. In an initial step, the grid is divided into several zones. The forecast data processing firstly performs an evaluation of the predicted current and voltage time series depending on the permitted limits and time frames in order to detect current overloads and voltage limit violations. The detected problem situations are then sorted according to their criticality for reasons of prioritisation. Finally, depending on the location, time and other physical aspects, the problem situations are combined and summarised as appropriate since related voltage and current problems can often be linked to a specific cause and identified as a single problem situation.

LOW-VOLTAGE GRIDS IN TRANSITION - AUTO-MATIC GRID RECONFIGURATION APPROACH FOR FUTURE SMART GRIDS CHALLENGES

Current trends, such as the increasing spread of electric vehicle charging stations (EVCSs) with and without battery storage combined with heat pumps (HPs) and air conditioning systems, replacing classical heat supply, are challenging the operation of low-voltage (LV) grids. This can result in noteworthy load flow and short circuit problems in traditional existing power LV grids. A crucial example is the overloading of line segments by loads with significant simultaneity (e.g. EVCSs, HPs, air conditioning systems). Moreover, high local infeed caused by decentralised power generators (e.g. photovoltaic (PV) systems, small hydropower plants, battery energy storage systems (BESSs)) can lead to power quality problems (e.g. voltage limit violations, voltage drop, very short interruption). The project ‘Power System Cognification’ (PoSyCo) defines six Use Cases (UCs) to tackle these challenges. It aims to implement a ‘SOFTprotection’ system, which contributes to fault prevention and serves as an add-on for the conventional ‘HARDprotection’ (fuses, circuit breakers (CBs)). This paper presents an overview of PoSyCo&apos;s UCs but is focusing on algorithm for UC4: overload prevention by temporary meshing.

Advanced deep flux weakening operation control strategies for IPMSM

This paper proposes an advanced flux-weakening control method to enlarge the speed range of interior permanent magnet synchronous motor (IPMSM). In the deep flux weakening (FW) region, the flux linkage decreases as the motor speed increases, increasing instability. Classic control methods will be unstable when operating in this area when changing load torque or reference speed is required. The paper proposes a hybrid control method to eliminate instability caused by voltage limit violation and improve the reference velocity-tracking efficiency when combining two classic control methods. Besides, the effective zone of IPMSM in the FW is analyzed and applied to enhance stability and efficiency following reference velocity. Simulation results demonstrate the strength and effectiveness of the proposed method.

Effect of Individual Volt/var Control Strategies in LINK-Based Smart Grids with a High Photovoltaic Share

The increasing share of distributed energy resources aggravates voltage limit compliance within the electric power system. Nowadays, various inverter-based Volt/var control strategies, such as cosφ(P) and Q(U), for low voltage feeder connected L(U) local control and on-load tap changers in distribution substations are investigated to mitigate the voltage limit violations caused by the extensive integration of rooftop photovoltaics. This study extends the L(U) control strategy to X(U) to also cover the case of a significant load increase, e.g., related to e-mobility. Control ensembles, including the reactive power autarky of customer plants, are also considered. All Volt/var control strategies are compared by conducting load flow calculations in a test distribution grid. For the first time, they are embedded into the LINK-based Volt/var chain scheme to provide a holistic view of their behavior and to facilitate systematic analysis. Their effect is assessed by calculating the voltage limit distortion and reactive power flows at different Link-Grid boundaries, the corresponding active power losses, and the distribution transformer loadings. The results show that the control ensemble X(U) local control combined with reactive power self-sufficient customer plants performs better than the cosφ(P) and Q(U) local control strategies and the on-load tap changers in distribution substations.

Ensuring Distribution Network Integrity Using Dynamic Operating Limits for Prosumers

The number of residential consumers with solar PV and batteries, aka prosumers, has been increasing in recent years. Incentives now exist for prosumers to operate their batteries in more profitable ways than self-consumption mode. However, this can increase prosumer exports or imports, resulting in power flows that can lead to voltage and thermal limit violations in distribution networks. This work proposes a framework for Distribution Network Operators (DNOs) to ensure the integrity of MV-LV networks by using dynamic operating limits for prosumers. Periodically, individual prosumers send their intended operation (net exports/imports) as determined by their local control to the DNO who then assesses network integrity using smart meter data and a power flow engine. If a potential violation is detected, their maximum operating limits are determined based on a three-phase optimal power flow that incorporates network constraints and fairness aspects. A real Australian MV feeder with realistically modelled LV networks and 4,500+ households is studied, where prosumers' local controls operate based on energy prices. Time-series results demonstrate that the proposed framework can help DNOs ensure network integrity and fairness across prosumers. Furthermore, it unlocks larger profitability for prosumers compared with the use the 5kW fixed export limit adopted in Australia.

Adaptive Congestion Control for Electric Vehicle Charging in the Smart Grid

This article proposes an adaptive control algorithm for plug-in electric vehicle charging without straining the power system. This control algorithm is decentralized and merely relies on congestion signals generated by sensors deployed across the network, e.g., distribution-level phasor measurement units. To dynamically adjust the parameter of this congestion control algorithm, we cast the problem as multi-agent reinforcement learning where each charging point is an independent agent which learns this parameter using an off-policy actor-critic deep reinforcement learning algorithm. Simulation results on a test distribution network with 33 primary distribution nodes, 1760 low voltage end nodes, and 500 electric vehicles corroborate that the proposed algorithm tracks the available capacity of the network in real-time, prevents transformer overloading and voltage limit violation problems for an extended period of time, and outperforms other decentralized feedback control algorithms proposed in the literature. These results also verify that our control method can adapt to changes in the distribution network such as transformer tap changes and feeder reconfiguration.

Investigation of Impact of Contingencies on Power Plant and Transmission Lines

Contingency analysis are widely applied to predict the effect of outages in power systems, like tripping of equipment in power plants and transmission lines. Using off line analysis to predict the effect of individual contingency is a tedious task on power system containing large number of components. Practically, only selected contingencies will lead to severe conditions in power system, like violation of voltage and active power flow limits. Simultaneously, the value of active power flow before and after severe transmission and power plant contingencies was analysed using Genetic Eigenvalue Analysis Technique. This was achieved by simulating the Simulink of Nigerian 330KV 48 bus power system using m-file programme in MATLAB environment.The result of the simulation for the power flow solution for transmission line outage contingencies shows that the voltage trajectory at bus 11 stood at 0.3934 p.u, at bus 15 is0.4986 p.u, and bus of 23 is 0.4647 p.u while for the contingencies on power plant, there voltage trajectories stood at 0.2342 p.u for bus 11, 0.3987 p.u for bus 23. The result shows that the impact on power plant is higher than that of transmission line by 40%, 20%, 14%for sampled buses 11, 15, and 23 respectively. Keywords: Power Stability- Genetic Eigenvalue, Load flow DOI: 10.7176/JETP/11-1-04 Publication date: January 31 st 2021

A Fair Incentive Scheme for Participation of Smart Inverters in Voltage Control

The increasing trend of renewable energy sources integration in the distribution systems may cause voltage fluctuations and voltage limit violations. The smart inverters with reactive power capability and active power curtailment (APC) control can solve the grid voltage issues. In this article, three-phase optimal power flow (TPOPF) is applied to dispatch the APC and reactive power injection (RPI) of inverters. The TPOPF takes smart meter measurements as inputs. The nonproportional APC and proportional APC are examined. In this article, a fair incentive scheme for APC by the inverters is proposed, appropriate for both utility and renewable energy producers. Similarly, unequal RPI and equal RPI by the inverters to maintain the system voltage are studied. The RPI incentive scheme is analyzed by accounting for the costs incurred in overrating of the inverters, power losses, and life reduction due to reactive power supplied by the inverters. The effective location of inverters for APC and RPI in the distribution network is explored. The incentive schemes are implemented in 27-node distribution system and IEEE European low-voltage distribution system.

Operation of Meshed Hybrid Microgrid During Adverse Grid Conditions With Storage Integrated Smart Transformer

Microgrid faces various challenges such as reverse power flow, peak power demand, voltage limit violations, etc., due to significant installations of renewable energy sources (RESs) and electric vehicles (EVs). For continuous and effective operation of such a system, enhanced and complex control methodologies are needed. Smart transformer (ST) has emerged as a promising solution for overcoming such challenging issues. This paper proposes the operation of battery energy storage system (BESS) integrated ST in a meshed hybrid microgrid. Instead of connecting through ac interconnections with normally open (NO) circuit breakers (CBs), the same line is connected through the medium voltage (MV) dc links of ST. This introduces several paths for controlled power flow in the system. As compared to ac interconnections, this configuration improves the performance of the overall system during various adverse operating conditions such as reverse power flow, peak power demand, and voltage sag. Moreover, the MVDC line voltage is controlled by one ST, resulting in reduced control complexity of other power converters. Detailed simulation and experimental results are provided to show the merits of the proposed system.

Contingency Control Capability of an Optimized HVDC-Based VSC Transmission System

Power network operations are increasingly becoming complex due to upsurge in power transactions, following the ongoing power system deregulation and restructuring. The alteration in dynamics of power operations based on this upsurge, usually results into contingency because power facilities are made to operate in region of their limits. In this regards, high voltage direct current (HVDC) based voltage source converter (VSC) and other flexible alternating current transmission systems (FACTS) imbedded with VSC have established competence for noticeable diverse power network characteristics improvements. Therefore, this study applies optimized VSC-HVDC transmission system for mitigation of bus voltage and line thermal limit violation as a result of n-1-line outage contingency during bilateral, simultaneous and multilateral transactions. Line contingency evaluation through real power performance index (RPPI) is adopted for severe contingency identification purposes. Optimization of VSC-HVDC system is achieved through statically developed sensitivity-based analysis of VSC control parameters, while its performance for both steady state and contingency conditions are verified with IEEE 6 and 30 bus networks. An improvement in power flow, voltage deviation minimization and automatic alleviation of violated thermal and voltage limits during contingency present optimized VSC-HVDC system as a solution for network performance optimization especially during various transactions occasioned by unbundling power processes. Besides, the superiority of the proposed optimization techniques is established through performance comparison with other sensitivity methods involving reactive power (RPSM), active power (APSM) and performance index (PISM), where the proposed sensitivity method (PSM) outperformed these other methods.

Sparse Measurement-Based Coordination of Electric Vehicle Charging Stations to Manage Congestions in Low Voltage Grids

The increasing use of distributed generation and electric vehicle charging stations provokes violations of the operational limits in low voltage grids. The mitigation of voltage limit violations is addressed by Volt/var control strategies, while thermal overload is avoided by using congestion management. Congestions in low voltage grids can be managed by coordinating the active power contributions of the connected elements. As a prerequisite, the system state must be carefully observed. This study presents and investigates a method for the sparse measurement-based detection of feeder congestions that bypasses the major hurdles of distribution system state estimation. Furthermore, the developed method is used to enable congestion management by the centralized coordination of the distributed electric vehicle charging stations. Different algorithms are presented and tested by conducting load flow simulations on a real urban low voltage grid for several scenarios. Results show that the proposed method reliably detects all congestions, but in some cases, overloads are detected when none are present. A minimal detection accuracy of 73.07% is found across all simulations. The coordination algorithms react to detected congestions by reducing the power consumption of the corresponding charging stations. When properly designed, this strategy avoids congestions reliably but conservatively. Unnecessary reduction of the charging power may occur. In total, the presented solution offers an acceptable performance while requiring low implementation effort; no complex adaptations are required after grid reinforcement and expansion.

Contingency Analysis for Improved Power System Stability of the Nigerian 330 Kv, 48-Bus System Using Series Facts Controllers

Stressed electric network, either due to increased loading because of system expansion, or due to extreme contingencies, often result in voltage limit violations and overload of lines leading to extreme degradation in the system's damping torque, metamorphosing into fluctuations and dynamic instability. This paper employs two series, Flexible Alternating Current Transmission System (FACTS) controllers; Static Synchronous Series Compensator (SSSC) and Thyristor Controlled Series Compensator (TCSC) to investigate the system responses during line outages for dynamic stability improvement. The commercial version of Power System Analysis Toolbox (PSAT) was used to model the Nigerian 48-bus power system network with simulation of line outages using breakers on lines 6-8 (Oshogbo to Ikeja West transmission stations) and 35-39 (Ugwuaji to Ikot Ekpene transmission stations). SSSC and TCSC were optimally positioned on line 21-28 close to bus 21 (Jos transmission station) after continuation power flow simulation and computation of voltage sensitivity factor of all the load buses. Results show that the two FACTS controllers effectively improved the system stability by reducing the rotor angle and speed and by damping the post outage oscillations of the bus voltage and real power.

A decentralized optimization method based two-layer Volt-Var control strategy for the integrated system of centralized PV plant and external power grid

The large-scale integration of centralized photovoltaic (PV) plant (CPVP) is encountering severe challenges. On the one hand, the weak static voltage stability (SVS) of the external power grid (EPG) greatly restricts the maximum transmission power provided by CPVP. On the other hand, voltage limit violation is prone to occur at the end of collecting power lines inside CPVP. Driven by the above voltage issues, this paper proposes a novel two-layer Volt-Var control (VVC) strategy with two timescales for the integrated system consisting of CPVP and EPG. In this VVC strategy, besides the interaction between CPVP and EPG is fully considered, the coupling between transmission network (TN) and distribution network (DN) in EPG is also highly valued. The explicit integrated optimization model is constructed. Two decentralized optimization methods are applied for efficiently solving the integrated optimization problems corresponding to the different control layers, respectively. The operations of various VVC devices are coordinated via the proposed VVC strategy. The simulation results obtained indicate that the proposed two-layer VVC strategy can effectively reduce the voltage limit violation risk inside CPVP, and even in the worst case, the average nodes voltage deviation is only 0.024 p.u. Meanwhile, compared with the case without VVC, the value of the selected average SVS margin index will be reduced by about 33.18%, which means EPG is significantly improved.

Optimizing line-voltage-regulators with regard to power quality

The rapid expansion of distributed energy resources in the low voltage grid causes voltage limit violations, especially in rural areas with little electric load. To solve this issue without costly grid reinforcement, one approach is the use of smart devices such as line-voltage-regulators. However, the majority of these devices carry out stepped voltage adjustments, which cause a negative effect on the dynamic voltage characteristic. For this reason, this paper deals with a novel line-voltage-regulator and its power quality optimized control. The novel setup enables a continuous voltage regulation in a robust way by using variable inductors. The control design focuses on mitigating side-effects like harmonic distortion emission and increased grid impedance. Finally, a laboratory analysis evaluates the novel line-voltage regulator in comparison to a stepped one with regard to power quality.

Analysis of interactions between regulated distribution transformers (RDT) and local voltage control Q(V) by means of the harmonic balance method

The increase of decentralized power generation in low voltage grids imposes new challenges on the conventional operating strategies for electrical grids, as power transmission used to predominantly pass from high voltage levels to low voltage levels. Distributed generation in low voltage grids lead to unprecedented voltage limit violations, which can efficiently be solved by implementing new control systems e.g. regulated distribution transformers (RDT) or local voltage control (e.g. Q(V)). Both correspond to closed-loop control systems and thus, are prone to stability issues and possible interactions with other control elements. Furthermore, the nonlinear characteristics of RDTs complicate the systems' stability analysis. This paper introduces the harmonic balance method as an approach to characterize the nonlinear behavior of an RDT and assess the system stability as well as possible interactions with other linear controllers in the electrical grid.

Available Transfer Capability Determination for the Electricity Market using Cuckoo Search Algorithm

In the electricity market, power producers and customers share a common transmission network for wheeling power from generation to consumption points. All parties in this open access environment may try to produce energy from cheaper sources for greater profit margin, which may lead to transmission congestion, which could lead to violation of voltage and thermal limits, threatening the system security. To solve this, available transfer capability (ATC) must be accurately estimated and optimally utilized. Thus, accurate determination of ATC to ensure system security while serving power transactions is an open and trending research topic. Many optimization approaches to deal with the problem have been proposed. In this paper, Cuckoo Search Algorithm (CSA) is applied for determining ATC problem between the buses in deregulated power systems without violating system constraints such as thermal, voltage constraints. The suggested methodology is tested on IEEE 14 and IEEE 24-bus for normal and contingency cases. The simulation results are compared with the corresponding results of EP, PSO, and GWO and show that the CSA is an effective method for determining ATC.