Voltage Support Research Articles

Hierarchically Coordinated Voltage Control in Seaport Microgrids Considering Optimal Voyage Navigation of All-Electric Ships

The uncoordinated scheduling and control of all-electric ships (AESs) will impose great challenges on voltage profiles in seaport microgrids. This article proposes a three-stage hierarchically coordinated voltage control strategy in seaport microgrids considering the optimal voyage navigation and scheduling of AES. In stage one, we first propose an arrival time estimation method for AES to reduce the potential voltage violation risk in different seaport microgrids. The optimal power flow (OPF) is then performed in each microgrid to determine the day-ahead dispatch of on-load tap changer (OLTC) and inverter-based renewable energy resources (RESs). In stage two, the optimal route planning is carried out based on the sea states and the Dijkstra algorithm. The voyage and generation scheduling of AES are formulated as optimization problems and solved by CPLEX solvers. In stage three, a real-time local voltage control strategy is performed to mitigate fast voltage violations. To fully exploit the voltage support capability of energy storage systems (ESSs) on berthed-in AES, both constant charging mode and voltage control mode are designed to regulate voltages while ensuring sufficient SOC levels of ESS when leaving the seaport. The simulation results verify the effectiveness of the proposed method both in saving the generation cost of AES and improving voltage profiles in seaport microgrids.

Distributed Control of Behind-the-Meter Energy Resources for Multiple Services

Behind-the-meter battery energy storage is used to mitigate non-dispatchability of customer-owned renewable sources. While serving their owners’ objectives, their underutilized capacities can be used to provide grid services to justify their cost. However, the integration of these resources extends the size and complexity of the network operation problem, imposing a processing burden on the central control system. In this paper, an agent-based distributed control framework is proposed that enables utilizing of the underutilized behind-the-meter capacities. In this framework, while the prosumer agents initially schedule their resources for daily cost management, a local customer aggregator agent performs day-ahead active and reactive power control on the underutilized behind-the-meter batteries to provide multiple grid services. A novel grouping algorithm is developed based on voltage sensitivity and a modified <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$k$</tex-math></inline-formula> -means clustering method to assign customer groups to the local agents and establish the network subsystems. Mixed-integer linear programming is used to model and solve multiple optimization problems. The proposed framework is applied to the 123-bus distribution network. The simulation results show its effectiveness in terms of complexity and optimality, compared to the central control approach in allocating the unused behind-the-meter resources for two common industry needs: network bottleneck congestion relief and grid-edge voltage support.

The effects of betamethasone on the amplitude integrated EEG of infants born at 34- or 35-weeks gestation.

Assess if maternal betamethasone administration at 34-35 weeks accelerated neonatal amplitude integrated EEG (aEEG) maturation. Nested, observational cohort in 7 centers participating in the Antenatal Late Preterm Steroid randomized trial. Up to 2 aEEGs were obtained in neonates born from 340-356 weeks gestation before 72 h (aEEG 1) and at 5-7 days (aEEG 2) if hospitalized. Personnel and aEEG central readers were masked to the intervention. The primary outcome was maturation reflected by cycle frequency; secondary outcomes were border voltage, span, and discontinuity. 58 neonates were enrolled (betamethasone, 28, placebo, 30). On aEEG 1, cycle frequency did not differ, but betamethasone exposed infants had a greater lower border voltage and a broader span. On aEEG 2, both groups displayed increases in lower border voltage. Betamethasone associated changes in lower border voltage support accelerated electrical activity. Further investigation is needed to understand the broader span.

Using PV inverters for voltage support at night can lower grid costs

Areas with sparse transmission lines are common in regions with high solar energy potential and need voltage support. This may require installing expensive voltage compensators, such as static synchronous compensators (STATCOMs). This expense can increase the cost and decrease the acceptance of large-scale adoption of solar power. Unlike current photovoltaic (PV) inverter controllers, which provide voltage support only during the day, commercially available augmented voltage controllers can provide voltage support at night. We examine whether PV inverters improve nighttime voltage on the grid and how much such an operation would cost compared to a STATCOM. We ran grid contingency analyses on a model for West Texas within the Electric Reliability Council of Texas (ERCOT) jurisdiction under spring and summer conditions to determine if PV inverters can support nighttime voltage under varying reactive power demand. The cost of reactive power has not been defined previously, especially in the context of the United States. Our methods and application provide a way to determine the cost of reactive power for both PV project developers and system planners. Allowing PV inverters to provide reactive power can reduce system costs by millions of dollars, or 4–15 times less costly than installing a STATCOM. We determined inverter voltage support costs by calculating the cost of earlier inverter replacements due to increased reactive power output and voltage controllers. The net system savings argue for ERCOT changing their voltage support policies to incentivize PV plants to provide voltage support at night.

Research on integrated control strategy of doubly‐fed induction generator‐based wind farms on traction power supply system

With the large-scale construction of high-speed railway and wind farms, the traction power supply system (TPSS) and wind farms are connected through the power system. The three-phase unbalance of grid voltage caused by the TPSS will seriously affect traditional wind farms’ operation control. Considering the TPSS and wind farms are connected to the power system's public point of common coupling (PCC), the wind farm's current control and voltage support strategy are studied. Firstly, aiming to suppress the influence of harmonics and the unbalanced power grid caused by the TPSS, the repetitive controller (RC) is introduced to the current control of the doubly-fed induction generator (DFIG). The vector control strategy based on the combination of proportion integral (PI) and RC is adopted, where the PI controller is used to adjust the average value of DFIG output power and the RC regulator controls DFIG output sinusoidal stator current. Secondly, the wind farm's voltage support strategy is designed to reduce the voltage fluctuation caused by the TPSS's loads. By coordinating three measures of putting in reactive power compensation devices, DFIG working in voltage control mode, and limiting DFIG output active power, the voltage of wind farm and traction station is effectively supported. Finally, a simulation model with the TPSS and wind farm is built to verify the proposed control strategy's effectiveness.

A Two-Stage Simultaneous Control Scheme for the Transient Angle Stability of VSG Considering Current Limitation and Voltage Support

Since the wide application of virtual synchronous generators (VSGs), the power grid faces great challenges in the safe and stable operation due to their limited thermal capacity and weak anti-disturbance ability. During transient period, for example, a fault occurs in the transmission line, the VSG may lose the transient angle stability and provoke the current hard limit. Even if the fault is cleared by tripping of line, it still faces the problem of instability and voltage dips. To address this problem, in this paper, the post-fault large-signal model of VSG is derived first via the travelling waves based fault information acquisition. Subsequently, with the effect of both active and reactive power loops taken into account, a two-stage simultaneous control scheme is proposed for improving the transient stability of VSG, while considering the current limitation during fault state and voltage support after fault clearance. This method is fulfilled by mode switching and an additional feedback control based on the fault signal. Finally, the effectiveness of the proposed method under both symmetrical and asymmetrical faults is verified. Moreover, the application of the proposed method in a multiple VSGs system is also verified. Besides, the robustness to parameter mismatch and the feasible operating region for the method are discussed.

Adaptive Power Control Strategy for Smart Droop-Based Grid-Connected Inverters

Grid-connected inverters play an important role in the integration of renewable energy sources such as solar and wind. However, due to the unneglectable grid impedance value seen by the inverters at the point of common coupling (PCC), especially in the weaks and resistive low voltage distribution networks, there is an inherent strong coupling between active and reactive power flow. This power coupling causes significant power quality problems including 1) voltage fluctuation of the common AC bus resulted from high penetration of intermittent renewable generation systems, 2) non-optimal control of neither power flow nor power factor of the delivered power by inverters to the common AC bus, and 3) unintended/uncompensated transmission losses, where the flow of active power through the transmission/distribution lines will cause unintended reverse reactive power flow from the grid-side and vice versa with the reactive power flow. To solve these issues, this paper proposes an adaptive mechanism for droop-based grid-connected inverters to decouple the power flow by compensating the associated unintended active and reactive power losses flowing through the transmission line (or any desired segment of it). This control strategy relies on modifying the power command provided to the frequency and voltage droop loops by considering the effects of both the transmission line resistance and inductance components on the power flow between the inverter and the grid. It uses only the local current and voltage measurements to first perform an online estimation of the transmission line resistance and inductance and then to calculate the proposed adaptive power terms. The performance of the proposed control is validated in MATLAB/Simulink and HIL experiment for a 350 kW droop-based grid-connected inverter system. The proposed control strategy can be utilized to provide ancillary services to the grid such as accurate frequency and voltage support at the location of interest.

On the Factors Affecting Battery Unit Contributions to Fault Currents in Grid-Connected Battery Storage Systems

This article investigates factors affecting the contributions of battery units to fault currents in grid-connected battery storage systems (BSSs). The work in this article is intended to examine the effects of the state-of-charge (SOC) on battery currents that are drawn due to faults. This article also examines the impacts of charger controller actions on the currents drawn from battery units to faults in grid-connected BSSs. The impacts of the SOC and charger controller on battery currents due to faults are examined for the lead-acid, lithium-ion, and nickel–cadmium battery units. Examination results show that the battery currents due to faults are directly dependent on the SOC. Moreover, these results show that actions of charger controller can support the battery terminal voltage, thus preventing the fast reduction of the SOC. The support of the battery terminal voltage helps in limiting the currents drawn from battery units during faults. The effects of the SOC and charger controller are verified using a 1-MW, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$3\phi$</tex-math></inline-formula> grid-connected BSS, which has lead-acid battery units. Several faults have been created during charging and discharging operations, and at different values of SOC. Test results confirm the direct dependence of battery currents (due to faults) on the SOC. In addition, obtained results demonstrate the ability of charger controller to limit the currents drawn from battery units due to faults in different parts of a grid-connected BSS.

Consensus-Based Distributed Secondary Frequency Control Method for AC Microgrid Using ADRC Technique

To ensure safe and reliable operation, the ability to ride through various disturbances is vital for a microgrid with multi-inverters. As the voltage and frequency support comes from the power-electronic-based inverters, it is necessary to find a proper control strategy to improve the rejection ability of the DG inverter against disturbances. In this regard, this paper proposes a new distributed secondary frequency control approach for islanded microgrids, in which the main purpose is to remove the frequency deviation under droop control method with better disturbance rejection performance. Unlike many traditional approaches which rely on a detailed control model, the proposed one needs little model information thanks to the model-independent characteristic of active disturbance rejection control (ADRC) technique. A linear extended state observer is introduced to estimate the useless model dynamics (including unknown disturbances, unmodeled dynamics and nonlinear dynamics) which are then compensated in the control input. After the active compensation procedure, the nonlinear frequency control model can be converted into a quasi-linear model, based on which a proportional distributed control algorithm is established to restore the frequency and equalize the active power among the DGs. Simulation results based on a four-inverter-based microgrid show that the proposed approach achieves frequency restoration, active power sharing, as well as satisfactory disturbance rejection performance.

Data-Driven Fast Voltage Control in Non-DPMU Distribution Networks With Microgrids

Traditional voltage control methods for distribution networks assume perfect knowledge of the power system model. Nevertheless, the extensive scale of future distribution networks makes it unrealistic to acquire the overall operation state monitoring. Moreover, with the deregulation of distribution networks, partial controllable resources belong to independent systems, such as microgrids, causing distribution system operators unable to force them to provide voltage support directly. To cope with the previously mentioned problems, a data-driven fast voltage control method for distribution networks with MGs is proposed in this article. First, voltage sensitivity matrices are estimated indirectly by identifying line parameters in a regression approach, without using measurement data of distribution phasor measurement units (DPMUs) in distribution networks. Then, an incomplete information game model is proposed to motivate MGs to provide ancillary services of voltage control. To guarantee privacy, only a little key information is shared among MGs and distribution system operators. Moreover, MGs make voltage control strategies autonomously based on the data-driven deep reinforcement learning algorithms, while maximizing their own profits. Finally, we test the method on the modified IEEE 33-node networks and IEEE 123-node networks. The results demonstrate that the proposed method can provide an accurate voltage estimation in electricity markets with non-DPMU measurement data and increase energy and asset utilization.

A Unified Inner Current Control Strategy Based on the 2-DOF Theory for a Multifunctional Cascade Converter in an Integrated Microgrid System

Wider applications of integrated microgrids have been significantly restricted by converters with the sole function of power regulation. In microgrids with distributed generation and energy storage equipment, it is crucial for converters to be capable of reactive power compensation, harmonic suppression, voltage support, and other functions simultaneously. To achieve this, it is essential to modify the discrepancy between different control modes, both in control architectures and input signals. However, previous researches have focused on the stability and robustness of the system’s operation, rather than the instantaneous tracking performance of instructions during the mode switching. Instead, a unified control strategy based on the two-degree-of-freedom theory is conceived in this paper, to impose no reconfiguration of the inner current loop, so that the inherent stability can be guaranteed. In the proposed strategy, mode transition is replaced by the readjustment of referring instructions, and the complex tuning of filter parameters is abandoned. Thus, a desirable performance in a wide range of operating conditions for the microgrid system is provided and the effects of the disturbances associated with the mode transitions are eliminated. The simulations studied in MATLAB and experimental evaluations of the prototype both corroborate the simplicity and effectiveness.

Distributed Secondary Control in Microgrids Using Synchronous Condenser for Voltage and Frequency Support

A high share of distributed energy resources (DERs) in power distribution grids has posed many challenges for system operation and control. Microgrid (MG) application with different distributed control approaches for DERs has been drawn a lot of attention from the research community to provide more flexibility, reliability and resilience for the system. This paper develops a distributed secondary control for DERs in MGs and on top of that using synchronous condenser (SC) participating in the secondary control for voltage support. The proposed distributed secondary control framework of MGs is designed to obtain four objectives as follows: (i) frequency restoration, (ii) average voltage restoration, (iii) arbitrary active power sharing among SGs and BESSs and (iv) arbitrary reactive power sharing among all SGs, BESSs and SCs. The comparison results under different scenarios show that with SC participating in the distributed secondary control in MGs, the system frequency and voltage response are much improved and quickly recovered to the nominal values thanks to the natural inertia response and fast reactive power control of SC sharing with other DERs in the MGs. Additionally, a multi-agent system is implemented to realize the proposed control method in hardware environment.

An LVRT strategy with quantitative design of virtual impedance for VSG

Grid-forming inverters controlled as Virtual Synchronous Generators (VSGs) can provide voltage and frequency support for the power grid, which enhances the stability of the grid. However, VSGs face three challenges under grid voltage sags, i.e., maintaining power angle stability, avoiding overcurrent, and providing reactive power support, while previous virtual impedance-based LVRT strategies for VSGs cannot meet those three requirements simultaneously. In this paper, an LVRT strategy with frozen power loops and quantitatively designed virtual impedance is proposed. By freezing the power loops under grid voltage sags, the power angle stability is maintained. Meanwhile, the virtual impedance is quantitatively designed taking both the tolerable current of the inverter and the reactive power requirement of the grid code into account, which limits the overcurrent and fulfills the requirements of the grid code simultaneously. The experimental results verify that the proposed LVRT strategy not only meets the above three requirements but also makes full use of the maximum output capacity of the inverter during grid voltage sags.

Soft open points based load restoration for the urban integrated energy system under extreme weather events

With frequent occurrences of extreme natural disasters such as typhoons in urban integrated energy systems (UIES), it is of great significance to cope with different kinds of disturbances. This paper proposes a load restoration method under typhoon weather for urban distribution and natural gas networks based on soft open points (SOP). Firstly, the typhoon wind speed model is introduced and the line fault rates of distribution networks under typhoons are calculated. Secondly, the gas turbine and electric-driven compressor are considered as the coupling units of the integrated electric–gas energy system, and related models are constructed. The fault analysis method of the natural gas network is proposed considering the faults in the distribution network lead by typhoons. Thirdly, SOP installed in the distribution network with the V/f control mode is applied to restore electrical loads and provide voltage support for the loads on the fault side. After that, the loads of the gas network could also be restored because of the restoration of the coupling units. Optimal energy flow is applied to determine the output of the power and gas sources, coupling units and also the loads to be restored. Finally, the fault rate of each line under typhoon disaster is analysed and the correctness and effectiveness of the resilience improving method based on SOP are verified with test systems UIES E33-G14 and UIES E123-G48.

Improved generator circuit breaker failure protection method combined with voltage quantities

Safe and reliable generator circuit breaker (GCB) failure protection can ensure effective isolation between generator and system under different fault conditions to avoid generator damage. In this paper, aiming at the existing problems of the GCB failure protection criterions based on current quantities, the voltage quantities are combined to constitute the improved GCB failure protection. Through fault sequence network analysis, it is concluded that there are voltage phasor differences between the two sides of GCB for the normally separated phases, while the values are zero for the rejected phases. On this basis, a GCB failure protection criterion based on the voltage phasor difference is proposed for the generators equipped with PTs at the two sides of GCB. After a generator fault occurs, in the process of de-excitation, the terminal voltage drops quickly except GCB three-phase rejection because of the voltage support from the system side. Moreover, under the GCB three-phase rejection situation, the loads cannot be effectively shed and no terminal voltage increase is caused compared with other GCB operation situations. Thus, the protection criterions based on the voltage variation are proposed to distinguish GCB three-phase rejection situation, including the voltage changing rate criterion and the increased voltage mutation criterion. They are used for generators equipped with only one terminal PT. Simulation results of PSCAD/EMTDC verify the effectiveness of the proposed methods.

A New Approach for the Incorporation of the End-User’s Smart Power–Electronic Interface in Voltage Support Application

Technology advancement in power–electronic interfaces and their evolution open an opportunity to end-users to benefit from their newfound capability. For the end-users, power–electronic interfaces can act as Distributed Energy Resources (DERs) for reactive power injection and absorption. If these power–electronic interface capabilities can be properly integrated into traditional utility system operations, they can be used as beneficial tools for distribution management and voltage profile enhancement. Considering the present distribution system, it is not possible to communicate to all DERs. In this paper, we considered two proposed residential-control and droop-control methods. The multi-criteria decision-making technique (MCDM), along with fuzzy theory, was used to prioritize candidate buses for their participation in the Volt-VAR program. In this paper, the contribution of active DERs in reactive power compensation was evaluated.

Enhancement of Line-to-Line Voltage Support During Asymmetrical Microgrid Faults Using a Four-Leg Three-Level Inverter

Recent research is focused not only to keep a three-phase inverter connected to the grid but also to support the voltage at the point of common coupling (PCC) during grid faults. The line-to-line PCC voltage can be enhanced by increasing the amplitude of positive and negative sequence current supplied to the grid for a given inverter rating. 3-leg inverters can supply only positive and negative sequence current to the grid. However, 4-leg inverters have the ability to supply zero-sequence current along with positive and negative sequence currents. This extra degree of freedom of control remains un-utilized during low voltage ride through operation. This paper presents a novel method to select the amplitude and phase angle of the zero-sequence current in such a way that the peak current amplitude of the 4-leg inverter remains minimum. Hence, it is possible to supply more positive and negative sequence currents to the grid. The proposed strategy can be applied to any asymmetrical grid fault condition. Experimental verification for two specific fault conditions has been carried out and results are presented.

Multi-stage supply restoration of active distribution networks with SOP integration

Supply restoration from outages is essential for improving the reliability of active distribution networks (ADNs) after fault isolation. Soft open point (SOP) can adjust the power flow among feeders and provide voltage support for power outage areas. Considering the sequential coordination of switching operation and SOP control mode selection, a multi-stage supply restoration method with SOPs is proposed for ADNs. First, the sequential energization is formulated, in which the impact of SOP control mode on restoration sequence is analyzed. By providing voltage support, the coordination of SOPs will rapidly energize the outage area and improve the voltage profile. Then, a multi-stage restoration model with SOPs is proposed, in which reconfiguration of switches and control mode selection of SOPs are coordinated in sequence to maximize the load recovery level of ADNs. Through the switching action-time mapping, secure operation is ensured during the entire supply restoration process. Finally, the effectiveness of the proposed method is validated on a modified IEEE 33-node distribution system and practical distribution networks with four-terminal SOP. Results show that the proposed method can fully exploit the potential benefits of SOPs and effectively enhance the load recovery level of ADNs.

A hybrid architecture for volt-var control in active distribution grids

Modern active distribution grids are characterized by the increasing penetration of distributed energy resources (DERs). The proper coordination and scheduling of a large numbers of these small-scale and spatially distributed DERs is necessary, and warrants the use of novel distributed approaches. In this paper, we propose a hybrid volt-var control architecture for the distribution grid, which leverages existing centralized and local approaches to planning, decision making, and control, and augments it with distributed optimization and distributed control for DER management. First, we propose a convex model to describe the power physics of distribution grids of meshed topology and unbalanced structure, based on current injection and McCormick Envelopes. Second, we employ the distributed proximal atomic coordination (PAC) algorithm to coordinate DERs to provide voltage support. We implement volt-var optimization by optimally coordinating DERs including PV smart inverters and demand response. We present results using the IEEE-34 bus network, using real data from a distribution feeder in Hawaii, to model load and PV generation. Different levels of DER penetration and objective functions are simulated. Our results show the need for the coordination of DERs to improve voltage profiles, even in networks with existing voltage control devices. Further, we show the need for flexible reactive power capabilities to achieve desired grid performance.

Dynamic interactions in large scale photovoltaic power plants with frequency and voltage support

The increasing penetration of Large-Scale Photovoltaic Power Plants in the power system is raising the concern of potential stability issues due to dynamic interactions of power electronics components. In particular, Power Plant Controllers are starting to provide a fast dynamic response for specific services, such as fast frequency response, which could trigger unexpected unstable interactions with the PV inverters or other grid components. Also, an eventual reduction of synchronous generation in the grid could lead to a power system with high penetration of power electronics, which presents unexplored stability problems. This paper presents an interaction and stability study of a Photovoltaic Power Plant connected to the grid. This study is based on a small-signal analysis employing eigenvalues and participation factors to identify potential unstable modes. A linear model of the Photovoltaic Power Plant is implemented to apply a small-signal analysis. This linear model is validated with the equivalent non-linear model based on time-domain simulations in Matlab Simulink.