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An improved hybrid unified power flow controller (IHUPFC) is developed in this paper to control the power flow and increase the utilization rate of the existing transmission lines. The IHUPFC is composed of an improved “Sen” transformer (IST) and a small capacity unified power flow controller (UPFC). After a detailed introduction of the structure and voltage regulation performance of the IST, this paper derives the capacity relationship between the IST and the UPFC. Then, this paper introduces the technology of the IHUPFC in power flow regulation and transmission capability improvement. Besides, the advantages of the IHUPFC in performance and structure are analyzed. The IST has simpler structure and larger power flow control area than a “Sen” transformer, and the IHUPFC provides a continuous active and reactive power flow control. Simulation and experiment studies are used to verify the practicability and superiority of the IHUPFC. [ABSTRACT FROM AUTHOR]

This paper proposes a new power decoupling control method for a direct ac-to-ac converter. The ac-to-ac converter consists of two half-bridge converters, two input filter capacitors, and a series-resonant circuit, which enables it to convert the single-phase line-frequency ac input to the high-frequency ac output directly. The proposed power decoupling control method stores input power pulsation at double the line frequency in the input filter capacitors. Thus, the proposed control method realizes a unity power factor in the line-frequency ac input and a constant amplitude current in the high-frequency ac output without any additional switching device or energy storage element. This paper theoretically discusses the principle and operating performance of the proposed control method and confirms the effectiveness of the proposed control method in experiments using an isolated ac-to-dc converter based on the direct ac-to-ac converter. As a result, the proposed power decoupling control method effectively improved the displacement power factor of the line-frequency ac input current to more than 0.99 and reduced the voltage ripple in the dc load to 4%. [ABSTRACT FROM AUTHOR]

Centralized electricity markets currently do not optimize reactive power and voltage in the market clearing software. Voltage and local reliability (VLR) commitment requirements are mostly identified through out of market operational procedures. Failure to maintain VLR may incur voltage collapse, generation, transformer loss, or even blackout. Midcontinent Independent System Operator (MISO) employs binary constraints, minimum/maximum generation constraints, interface constraints, and manual commitments to address VLR requirement in the market clearing process and ensure adequate commitment for reliability. This paper integrates binary VLR constraints in day-ahead security constrained unit commitment to improve market efficiency. However, VLR constraints may cause uplift cost, and sometimes the associated uplift cost can be significant. An uplift cost allocation method with the consideration of resources commitment reasons is developed in this paper. The proposed approach is considered for implementation in MISO market clearing process and a MISO test case is used to validate the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

In this paper, a dc–ac buck converter is proposed as a grid-side converter in high voltage dc (HVdc) transmission systems for high-power renewable energy source grid integration. The proposed architecture consists of multimodules of half-bridge voltage source converters (HB-VSCs). The dc terminals of the HB-VSCs are connected in series across the entire dc link (i.e., capacitor tapped), whereas their ac outputs are connected to multiwinding transformers to provide the three-phase terminals for ac grid integration. The proposed grid-connected capacitor-tapped multimodule converter (CT-MC) architecture, inspired from the HVdc shunt tap proposed by ABB, provides a dc–ac conversion with a relatively moderate voltage rating of semiconductor devices. It also provides operation with a lower number of semiconductor devices, gate driver circuits, voltage sensors, and lower total MVA rating of semiconductor devices (67%) compared with the conventional three-phase HB modular multilevel converter, which positively affects the system cost and reduces the computational burden of the employed controller. In this paper, the operational concept and control of the CT-MC are presented along with a capacitor voltage balancing approach. Simulation results are presented during normal and abnormal conditions to show the viability of the proposed architecture. Finally, a scaled down prototype for the CT-MC is employed to validate the converter operation. [ABSTRACT FROM AUTHOR]

SUMMARY: This paper presents parallel multipopulation differential evolutionary particle swarm optimization (DEEPSO) for voltage and reactive power control (VQC). The problem can be formulated as a mixed integer nonlinear optimization problem and various evolutionary computation techniques have been applied to the problem including PSO, differential evolution (DE), and DEEPSO. Since VQC is one of the online controls, speed‐up of computation is required. Moreover, there is still room for improvement in solution quality. This paper applies parallel multipopulation DEEPSO in order to speed up the calculation and improve solution quality. The proposed method is applied to IEEE 30, 57, and 118 bus systems. The results indicate that the proposed method can realize fast computation and minimize more active power losses than the conventional evolutionary computation techniques. [ABSTRACT FROM AUTHOR]

Aiming at effective power management in microgrids with high penetration of renewable energy sources (RESs), this paper proposes a simple power control for the so-called second-generation, single-phase electric springs (ES-2), which overcomes the shortcomings of the existing ES control methods. By the proposed control, the unpredictable power generated from RESs is divided into two parts, i.e., the one absorbed by the ES-2 that still varies and the other injected into the grid that is controllable by a simple and accurate signal manipulation that works both at steady-state and during RES transients. It is believed that such a control is suitable for the distributed power generation, especially at domestic homes. In this paper, the proposed control is supported by a theoretical background. Its effectiveness is at first validated by simulations and then by experiments. To this purpose, a typical RES application is considered, and an experimental setup is arranged, built up around an ES-2 implementing the proposed control. Testing of the setup is carried out in three steps and proves not only the smooth operation of the ES-2 itself, but also its capability in running the application properly. [ABSTRACT FROM PUBLISHER]

Essential imbalance and fluctuation of the mobile train load draw some problems, e.g., three-phase unbalance, reactive power, voltage and load fluctuation, harmonics, etc. in both public and train supply networks (TSNs). Railway power flow controller (RPFC)'s good performance on dealing with those problems has been convinced in practice these years. However, as the primary task in commercialization, how to qualitatively determine RPFC's design capacity with high cost efficiency and how to comprehensively assess its performance in the design stage have perplexed engineers for many years. To solve those problems in both planning and real-time executing processes, this paper focuses on an RPFC design capacity optimization strategy for grid-connection compatibility and operating performance improvement in terms of both the utility and the TSN. The strategy not only makes a railway substation satisfy the grid-connection standards with high calculating efficiency, but also renders the RPFC to have the minimum design capacity, the excellent feeder voltage control ability, the good fixed capacitor installing prospect, and the high operating efficiency on the targets of reducing the investment and enhancing the transportation ability. The generic models, the industry-oriented design and the quantitative analysis, discussed in this paper give an insight for those concerned based on a balance transformer. Future RPFC projects on serviced railway power systems will benefit from the proposed methodology. [ABSTRACT FROM PUBLISHER]

This paper proposes a novel approach to TSO–DSO coordination in day-ahead operation planning for the procurement of ancillary services (AS) such as congestion management and voltage control. The approach overcomes two major issues hindering practical applicability of existing TSO–DSO coordination mechanisms, namely the computational complexity due to the large number of times the TSO and DSO problems are resolved as well as the inability to map analytically the cost and amount of flexibility at DSO level into TSO problem. To this end, a two-fold novelty of our approach consists of the decomposition of the overall AS problem into two sequential stages, which seek the procurement of: (i) "active power related" AS (i.e., for congestion management) and then (ii) the "reactive power related" AS (i.e., for voltage support), respectively as well as a fast approximation of the cost of aggregated active and reactive power flexibilities of active distribution systems (ADSs). Unlike existing mechanisms, the proposed approach considers real-world challenging aspects such as the N-1 security at TSO level and operation uncertainties at both TSO and DSO levels. Accordingly, the approach relies on tailored versions of stochastic multi-period AC security-constrained optimal power flow (S-MP-SCOPF) at TSO level and stochastic multi-period AC optimal power flow (S-MP-OPF) at DSO level, which are formulated as non-linear programming (NLP) problems. The importance and performance of the proposed approach are illustrated on a power system connecting a 60-bus transmission system (under 33 N-1 contingencies) with five 34-bus ADSs. • This paper proposes a two-stage approach to TSO–DSO coordination in day-ahead operation. • The approach procures optimally ancillary services for managing congestion and voltages. • A new mapping cost vs amount of aggregated flexibility in distribution grid is proposed. • The paper considers real-world challenges such as operation uncertainties and the N-1 security. • The approach relies on tailored versions of stochastic multi-period AC optimal power flow. [ABSTRACT FROM AUTHOR]

Value based services are the new emerging area in Indian Electricity Market. Technically, such value added services in the electricity market are known as Ancillary Services. Voltage Control Ancillary Service (VCAS) deals with local level voltage control problem of power sector. According to IEEE standards 1159:1995, 1250:2011, only ±5% voltage variation is allowed with almost 0 deviations in frequency (±0.03% maximum variation limit) to interconnect any new generating plant with a grid. The utilization of squirrel cage induction generators (SCIG) as wind operated micro grid may be in a fix without adequate availability of reactive power. Besides SCIG, load changes also cause voltage varying problems and so, this paper investigates how load change can be coped up with fast acting reactive power compensator for such arrangements. To demonstrate this real time Simulink models for fixed capacitor and STATCOM are developed using MATLAB software. The SCIG starting and load change conditions are tested with fixed capacitor, six and 12 pulse STATCOM as reactive power compensators. The SCIG performances in terms of rotor speed, voltage, and frequency control characteristics during starting of the machine as well as at the time of load perturbation are compared. [ABSTRACT FROM AUTHOR]

Reactive power reserves are important for grid stability and have a variety patterns depending on fault location as power systems become larger and complex. This paper presents the clustered effective reactive reserve (CEQR), an indicator that is suitable for identifying the risk of dynamic voltage stabitlity in terms of power system operation. This indicator provides the following features: i) It calculate the local realistic reactive reserve to recognize dynamically changing system conditions; ii)it derive clustered areas to identify valid regions for system control. The proposed method uses the sensitivity between dynamic reactive resources or load buses and a particular bus to obtain the correlation. And since this indicator is calculated using only the system topology, current and maximum outputs of the generators, there is less computational burden. This study verified the features of clustered effective reactive reserve (CEQR) by analyzing the practicable dynamic voltage collapse scenarios in the Korean electric power (KEPCO) system. In addition, as a countermeasure against the dynamic voltage collapse, the control performance of blocking tap changer scheme using the proposed indicator is described. [ABSTRACT FROM AUTHOR]

Summary: Tariffs for ancillary services like reactive power are a significant tool for raising energy efficiency in the distribution systems. In this paper, a new method for the valuation of reactive power support provided by photovoltaic (PV) systems is proposed. As a valuation approach, it is assumed that the voltage regulation service can be individually provided by other sources like shunt reactors or demand response programs whose service prices have been formerly regulated. The proposed voltage control method is modeled by a network partitioning approach and the distributed algorithm called the alternating direction method of multipliers (ADMMs). Moreover, the uncertain nature of PV generation and load demand is modeled by a stochastic method. The proposed method is tested on a real rural distribution system and 123 bus IEEE distribution test system. The result evidence that the proposed method has a lower computation time compared to the centralized method in addition to having the same accuracy. For distribution companies, also, the proposed method of reactive power valuation will be a basis for pricing reactive power service provided by PVs. [ABSTRACT FROM AUTHOR]

In recent years, with the large‐scale access of distributed generation (DG), the line voltage of the distribution network will be offset, which will adversely affect users. This paper aims at the voltage adjustment of the distribution network. Firstly, the reactive power control methods of distributed static synchronous compensator (DSTATCOM) and electric spring (ES) are studied separately, and then a layered cooperative control strategy based on them is proposed, which combines centralized compensation and distributed compensation to stabilize the entire line voltage. The upper layer aims at optimizing the comprehensive factors including voltage offset, user comfort and cost, and applies the dynamic adaptive particle swarm optimization (DAPSO) algorithm to study the optimal scheme of voltage adjustment. The lower layer designs the control loops of the compensation devices in response to the reactive power instruction. Finally, simulation results show that the voltage of the distribution network is effectively controlled. © 2020 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC. [ABSTRACT FROM AUTHOR]

This paper proposes an optimal voltage regulation scheme (OVRS) for distribution systems with rich photovoltaic (PV). Various regulation devices are optimally controlled in a coordinated manner: PV inverter, D-STATCOM, step voltage regulator (SVR), and on-load-tap-changer (OLTC). A data structure algorithm is proposed to split the distribution system into layered zones considering the radial structure of the system. The solution process of the proposed scheme is accomplished by a meta-heuristic optimizer. OVRS addresses the voltage violations while yielding a coordinated operation of the various control devices. The proposed OVRS involves three control levels to completely prevent voltage violations. In the first control level, the PV inverter and D-STATCOM mitigate rapidly the local voltage deviation through injecting/absorbing optimized reactive power. The second control level is a decentralized-based control scheme that utilizes the voltage control devices in each zone to handle the voltage violations if any. For each zone, the control devices in the upper-stream zones (parent zones) are managed by the third control level to ensure cooperative control actions. The simulation results on the 119-bus distribution system, with clear, low fluctuation, and high fluctuation of solar radiation profiles, demonstrate the effectiveness of the proposed OVRS. [ABSTRACT FROM AUTHOR]

Since intermittent nature of distributed generation and uncertainty in load behavior, the existing secondary control are not sufficient to deal with robustness and fast response. In this paper, attacked from a cooperative based control perspective inspired to the tracking consistency paradigm, a robust nonlinear distributed secondary voltage controller is proposed to achieve fast compensation for the islanded microgrid. Employing sparse communication, the proposed control strategy can achieve a compromise between the conflicting goals of voltage regulation and reactive power sharing. As the distributed architecture only requires the information of its own and its neighbor, the use of centralized controllers is avoided and the flexibility and scalability of system can be enhanced. Finally, the stability analysis is provided and the simulation results are presented to verify control performance under rapid load changes, robustness under communication failure, and flexibility during plug-and-play operation. [ABSTRACT FROM AUTHOR]

Different from voltage-source converter (VSC), reactive power absorption of line-commutated converters (LCCs) changes depending on DC voltage level. Therefore, the amount of AC voltage fluctuations at inverter side are different according to I – V droop constants in hybrid multi-terminal high-voltage direct current (MT-HVDC) system. This paper proposes a method for calculating the I – V droop constants in a hybrid MT-HVDC system to suppress these AC voltage fluctuations, based on the sensitivity of AC and DC networks. Simulations performed using PSCAD demonstrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper discusses the implementation of a transient stability model of Compressed Air Energy Storage (CAES) systems in a power system analysis package. A block-diagram based model of a two-machine CAES system is proposed, including specific controls for active power, reactive power, and State of Charge (SoC), which consider limits associated with the cavern pressure. As an application, the model is implemented in Powertech's TSAT software connected to the 9-bus WSCC benchmark power system, which is then used to study the impact of a CAES facility in the transient and frequency stability of the system. Several contingencies are simulated comparing the CAES performance to a gas turbine and a base-case without storage, demonstrating that the CAES system improves the system transient stability due to its charging stage, controls, and additional inertia. Finally, the CAES model is used to study the effects of cavern sizes in the frequency of the system. It is shown that CAES systems have certain special characteristics that make them attractive as a storage technology to provide stability and regulation services, besides their energy arbitrage capabilities. [ABSTRACT FROM AUTHOR]

This paper presents a novel day-and-night control of large PV solar farm as STATCOM, termed PV-STATCOM, for FIDVR alleviation. The proposed control involves dynamic modulation of both reactive and real power during day, and of reactive power during night. The active and reactive power output of solar farm are based on the sensitivity of solar plant voltage to active and reactive power injections. Enhanced voltage control up to utility Transient Overvoltage Limit is utilized. Eigenvalue analysis is done to study the sensitivity of proposed controller performance to various power system and PV plant parameters. Extensive PSCAD simulation studies of FIDVR are performed in a realistic power transmission system with large-scale PV plant and comprehensive IM loads. It is shown that the proposed PV-STATCOM control: (i) mitigates FIDVR even if solar farm is located more than 100 km from motor loads, (ii) is more effective than reactive power support required by German Grid Code, (iii) is equally effective as a STATCOM connected locally at motor loads, and (iv) stabilizes motors at night which is beyond Grid Code requirements. The PV-STATCOM potentially provides significant cost savings to utilities and new revenue stream for solar plants in providing 24/7 FIDVR mitigation service. [ABSTRACT FROM AUTHOR]

This paper considers the voltage control problem in wind farms, with the goals of maintaining feasible voltage profile and adequate reactive power (VAR) reserves by coordinating the reactive power outputs of wind turbines (WTs) and static synchronous compensators (STATCOMs). Rather than a conventional centralized and offline fashion, we propose a distributed online voltage control scheme based on dual decomposition. First, the optimal voltage control problem is formulated based on the branch flow model. Then, thanks to the sparsity of the constraints in this problem, a distributed solution framework is developed based on the dual decomposition and its online implementation is achieved through voltage feedback. To pursue fast convergence of the distributed algorithm and consequently facilitate the online implementation, the fast gradient method is generalized and then applied to solve the dual problem. The effectiveness of the proposed voltage control scheme is numerically validated under both static and dynamic cases. [ABSTRACT FROM AUTHOR]

This paper presents a technique based on the novel bat algorithm (NBA) for the design of the parameters of power system stabilizers and the generalized Unified power flow controller-power oscillation damping in a multimachine power system. The objective is to ensure minimal damping rates to low-frequency electromechanical oscillation modes. Simulations were carried out on the New England system to compare the performance of the NBA with three other techniques: particle swarm optimization, bacterial foraging optimization oriented by particle swarm optimization, and the bat algorithm. For the cases analyzed in this paper, the NBA produced results superior to other techniques. Moreover, the proposed methodology was able to obtain solutions with high damping levels, which showed to be robust when a wide range of loading conditions of the power system were considered. [ABSTRACT FROM AUTHOR]

In this paper, we study the optimal location planning of renewable distributed generation (RDG) units by taking into account the random uncertainties of renewable generation and load demand. In presence of the random uncertainties, location planning problem is naturally a two-stage stochastic mixed integer nonlinear programming problem, which is hard to solve efficiently. Instead of using traditional sampling methods or robust optimization methods, we propose a novel analytical approach in this paper to solve the problem efficiently and optimally. In particular, analytical expressions are derived for efficiently evaluating the performance of a candidate RDG placement decision. In this way, the stochastic mixed integer nonlinear programming problem is equivalently transformed into a deterministic integer problem, which can be solved efficiently using off-the-shelf tools. Numerical results show that the optimal RDG placement can save up to $4.2\%$ of the long-term average cost and $80.59\%$ of the line losses on the IEEE 13-bus test feeder. In addition, our proposed approach effectively reduces the computational time by $99.51\%$ on the IEEE 123 node test feeder compared with other traditional sampling-based metaheuristic approaches. [ABSTRACT FROM PUBLISHER]

Low-voltage (LV) feeder voltage magnitude and unbalance are often the constraining factors on a feeder's capacity to absorb rooftop photovoltaic (PV) generation. This paper presents a new probabilistic method for voltage management in distribution networks through the placement of distribution static compensators (dSTATCOMs) and on-load tap changers (OLTCs) considering the reactive capability of PV inverters in multiple LV and medium-voltage distribution networks. The method uses a modified particle swarm optimization. In this paper, several scenarios for the placements of multiple dSTATCOMs with and without embedded energy storage systems using both reactive and real power compensation are investigated in combination with an OLTC equipped with independent per-phase tap-changing control. The voltage constraints in the proposed method are statistically defined using three duration curves. These are the voltage unbalance, maximum voltage, and minimum voltage duration curves. The method is comprehensively tested for varying load and PV generation based on data from a real Australian distribution network with considerable unbalance and distributed PV generation. The results show that PV hosting capacity increases where the proposed approach is applied. [ABSTRACT FROM PUBLISHER]

Subsynchronous control interaction (SSCI) is the interaction between the power-electronics control and the series-compensated transmission system that occurs at frequencies below the system nominal frequency. SSCI may occur between the doubly-fed induction generator (DFIG) control system and the series-compensated transmission line, to which the wind park (WP) is connected. Not only do the DFIG control system parameters, but also the WP operating conditions have a significant impact on SSCI. In this paper, the impact of WP operating conditions and DFIG control system parameters on SSCI is analyzed in detail. Guidelines are presented for modifying the DFIG control system parameters to ensure safe operation and acceptable transient responses due to faults. This paper also examines the accuracies of various analytical tools used for SSCI problem identification and proposes a new frequency scan analysis approach for accurate prediction of potential SSCI problems. [ABSTRACT FROM PUBLISHER]

This paper examines distribution systems that have a high integration level of distributed energy resources (DERs), and addresses the design of local control methods for real-time voltage regulation. Particularly, the paper focuses on proportional control strategies wherein the active and reactive power output of DERs are adjusted in response to (and proportionally to) local changes in voltage levels. The design of the voltage-active power and voltage-reactive power characteristics leverages suitable linear approximations of the ac power flow equations and is network-cognizant; that is, the coefficients of the controllers embed information on the location of the DERs and forecasted noncontrollable loads/injections and, consequently, on the effect of DERs power adjustments on the overall voltage profile. A robust approach is pursued to cope with uncertainty in the forecasted noncontrollable loads/power injections. The stability of the proposed local controllers is analytically assessed and numerically corroborated. [ABSTRACT FROM AUTHOR]

Sheppard–Taylor power factor correction (ST-PFC) rectifier could obtain a high power factor due to its capability of overcoming the control detuning issue. However, it needs a bulky electrolytic capacitor at the load side to buffer the double-frequency ripple power (DFRP), which reduces the reliability and power density significantly. This paper proposes a control method to divert the DFRP to the small energy transfer capacitor. Consequently, the bulky electrolytic capacitor is replaced with a small film capacitor. The proposed method is carried out by introducing the freewheel state (one switch is turned on and the other is turned off) into the control. So no extra hardware is added, which makes the proposed method cost-effective. A low electromagnetic interference emission is also achieved due to the continuous input–output currents. In addition, the proposed method can be extended to other topologies easily. This paper first gives the detailed analysis of the proposed control method, and then introduces the controller design. The selection of the passive components is also briefly discussed. Finally, the simulation and experimental results verify the effectiveness of the proposed control method. [ABSTRACT FROM AUTHOR]

This paper proposes a decentralized control scheme for controlling active and reactive power of grid-tied ac-stacked photovoltaic (PV) inverter architecture using single member phase compensation. Reactive power control is required for the next generation of grid-tied smart PV inverter systems in power networks with high PV penetration. The decentralized control scheme proposed in this paper allows for a fully distributed architecture, both in terms of active and reactive power control and physical implementation of a PV system. This will result in higher reliability and potentially lower cost with minimum communications requirements. A decentralized controller enables higher switching frequencies that can shrink passive components. Therefore, string voltage variations due to voltage drop across passive components will be negligible and the system will be controlled with minimum communications requirement. The relative gain array approach has been used to study the feasibility of the decentralized control scheme. Detailed modeling and analysis are provided to show the effectiveness of the proposed decentralized approach and the effect of this approach on control implementation. Finally, effectiveness of the proposed decentralized approach is verified using mathematical modeling, simulation, and a lab-scale experimental setup in different operation conditions. [ABSTRACT FROM AUTHOR]

This paper presents a method for small-signal dynamic studies of power systems including utility-scale PV energy systems considering the effects of dc-side components. Existing methods commonly model a PV energy system as a dc source connected to a grid-tied inverter. This paper shows that the dc components of a PV energy system can also contribute in the dynamics of the power system in the range of small-signal studies. The paper first develops the model of a PV energy system by merging the submodels of dc- and ac-side components. Then, a method to obtain the augmented model of the power system including a PV energy system will be explained using a study system. The study system consists of a PV energy system in parallel with a synchronous generator connected to a grid. Then, based on eigenvalue analysis, verified by time-domain simulations, it is shown that the dc-link capacitor, front-end converter, and its controller can introduce oscillatory modes in subsynchronous range of frequencies. Furthermore, it is shown that the damping of the sensitive modes can significantly vary with respect to the parameters of the dc-side components. [ABSTRACT FROM PUBLISHER]

The wind-diesel-based isolated hybrid power system requires dynamic reactive power compensation for the fast recovery of voltage under load and input changes. Dynamic compensation techniques give a better voltage response but at a high cost; in contrast, the static compensator reduces the compensation cost by compromising on the voltage response. The use of, both, static as well as dynamic compensators together may give cost-effective reactive power compensation for predefined transient limits of voltage response. This paper presents the pricing of reactive power compensation under steady-state and dynamic conditions of a system with fixed capacitor (FC) and STATCOM. The main contributions of the paper are: (1) evaluation of reactive power compensation using FC as a static and STATCOM as a dynamic compensator, (2) fast recovery of voltage response using genetic algorithm-based tuning of the STATCOM controller's gain constant, (3) evaluation of reactive power compensation cost for steady and dynamic conditions due to change in load and/or input demand, and (4) comparison of responses obtained for the optimized case with a pre-existing reference compensation method. [ABSTRACT FROM AUTHOR]

This paper presents a methodology to control a two-staged ac/dc converter, which is a cascade combination of a three-phase ac–dc boost power factor correction (PFC) rectifier and a phase-shifted full-bridge (PSFB) dc/dc converter. This paper explains the issues of instability and losing unity power factor operation, i.e., high reactive power transfer with conventional PI compensator due to the sensitive variation of output impedance of the PFC stage at different loads. In this paper, this instability issue is resolved by the proposed state-feedback control methodology, which shows that there exists a load-insensitive range of controller parameters, providing unity PFC. As an additional benefit of this control, total number of sensors are reduced by three (an input phase current, a dc/dc primary current, and an output dc voltage), in comparison to the conventional PI-based control. This reduction is achieved by introducing two augmented state variables followed by their reduced state observer design. A 6-kW laboratory prototype of the integrated three-phase PFC and PSFB dc/dc is developed and designed to validate the proposed control algorithm. The experimental results show a conversion efficiency of 95.4% at full load, input total harmonic distortion of 4.1%, power factor of 0.998, and output voltage ripple of ±1%. [ABSTRACT FROM PUBLISHER]

Traditionally voltage control in distribution power system (DPS) is performed through voltage regulating devices (VRDs) including on load tap changers (OLTCs), step voltage regulators (SVRs), and switched capacitor banks (SCBs). The recent IEEE 1547-2018 from March 2018 requires inverter fed distributed energy resources (DERs) to contribute reactive power to support the grid voltage. To accommodate VAR from DERs, well-organized control algorithm is required to use in this mode to avoid grid oscillations and unintended switching operations of VRDs. This paper presents two voltage control strategies (i) static voltage control considering voltage-reactive power mode (IEEE 1547-2018), (ii) dynamic and extensive voltage control with maximum utilization of DER capacity and system stability. Further, effective time-graded control is implemented between VRDs and DER units to reduce the simultaneous and negative operation. The proposed voltage control strategies are tested in a realistic 140-bus southern California distribution power system through extensive time-domain simulation studies. The results show that voltage quality in a distribution system is effectively achieved through the proposed voltage control strategies with a significantly reduction in the number of switching operations of VRDs. In addition, proposed voltage control strategies increase reliability and security of the DPS during unexpected failures. [ABSTRACT FROM AUTHOR]

This paper focuses on power system stability issues arising under adverse network conditions requiring support from wind farm (WF) electronic converters. The specific instability mechanism investigated refers to a WF participating in emergency reactive support to enhance long-term voltage stability of a weak transmission network. During this support the WF converter may be operating under maximum current limit, in which case it is shown that the WF can become unstable. An aggregate WF equivalent model for voltage stability studies is derived, so as to simplify power system modeling. Protection schemes are proposed to avoid the above-mentioned instability, taking into account the WF converter and pitch control operation. Finally, a typical feeder connecting a WF taken from the Hellenic Interconnected System is simulated as an illustrative case study and the efficiency of the protection schemes proposed is determined. [ABSTRACT FROM AUTHOR]

This paper analyzes the control and operation of offshore wind farms connected with diode rectifier based HVDC (DR-HVDC) system. A small-signal state-space model of the offshore wind turbines (WTs) connected with DR-HVDC system is developed to design the WT Q–f droop control. The use of WT P–V and Q–f control during individual WT active power variation is clearly clarified. In order to reduce the interaction between WT active power and reactive power, an angle feedforward control is proposed where an additional phase shift is directly added to the WT output voltage based on the WT's active power output. The effectiveness of the proposed control on improving dynamic response and reducing active and reactive power interaction is verified by frequency-domain analysis and time-domain simulations in PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

This paper presents a dynamic coordination control strategy to enhance the reactive power capability of wind power plants (WPPs) which deploy doubly-fed induction generator-based wind turbines (DFIG-WTs). The proposed control architecture seeks to maximize reactive power availability during grid faults without violating the stable and thermal operational limits of the generator. To achieve this objective, the presented control topology calculates the maximum available reactive power that can be provided by the DFIG-based WPP under any disturbed operation of a power system. The reference value of the required reactive power compensation is thereafter calculated using an adaptive proportional-integral (PI) regulator which outperforms the traditional PI controller. The computed reference value is directly applied to the outer control of the rotor side converter (RSC) to regulate reactive power generation of the stator circuit. Furthermore, the $P-V$ droop characteristics of the induction generator is employed to adjust the active power reference of the RSC converter to increase the reactive power capability during grid faults. Meanwhile, de-rating the active power generation of the DFIG-based WT is accompanied with modifying the pitch angle of the rotor blades to balance the mechanical and electromagnetic torques and to avoid any stresses excreted on the mechanical parts of the WT. If rotor current limits are reached and more reactive power support is still required, the responsible control loop of the grid side converter (GSC) will be activated and augmented to enhance the overall reactive power capability of the DFIG-based WPP. Finally, the proposed control strategy is verified using Two area four machines and IEEE 68 bus test systems. [ABSTRACT FROM AUTHOR]

In this paper, a new control method for a line-commutated converter-based (LCC) high-voltage direct current (HVDC) system to support the frequency and AC voltage was proposed. For coordinated control of the inverter and rectifier, droop-based controllers are adopted for the outer loop of conventional controllers, and DC voltage is used to transfer the frequency information so that communication is not required. A method to determine the droop constants of the proposed controller is presented to emulate the conventional $f$ / $P$ droop characteristic and suppress the AC voltage fluctuations. Furthermore, a small-signal state-space model of the LCC HVDC system with the proposed scheme is derived, and root locus analysis is carried out to evaluate its stability. The effects of the proposed method are verified based on case studies, using the well-known CIGRE BENCHMARK model. The advantages of the proposed method can be summarized as follows: 1) system reliability is enhanced because there is no risk of communication failure during frequency information transfer and 2) voltage stability at the inverter side is enhanced as AC voltage fluctuation during frequency support is minimized. [ABSTRACT FROM AUTHOR]

Recently, the low-frequency oscillation (LFO) frequently happens in the vehicle-grid system of high-speed railways when electric multiple units (EMUs) are put into operation. As a possible solution, a single-phase cascaded H-bridge static synchronous compensator (CHB-STATCOM) directly connected to the traction network is applied to dynamically compensate the reactive power of vehicle-grid system. This paper focuses on the impedance modeling and stability analysis in the vehicle-grid system with CHB-STATCOM. Firstly, the vehicle-grid system is simplified to a single-phase rectifier with large grid inductance. The accurate dq-frame impedance of the rectifier system with CHB-STATCOM is derived in detail. Secondly, through the generalized Nyquist stability criterion (GNSC), the influence of CHB-STATCOM and its different control parameters on the stability of rectifier system is revealed. Thirdly, the excellent performance of CHB-STATCOM in suppressing the LFO and the accuracy of theoretical analysis are verified via time-domain simulations. Finally, to further verify the effectiveness of CHB-STATCOM in real applications, a dSPACE based controller hardware-in-the-loop (HIL) simulation platform is established, which includes the physical control circuits of eight EMUs. The experimental results show that the CHB-STATCOM has a good dynamic compensation effect and can efficiently suppress the LFO. [ABSTRACT FROM AUTHOR]

This paper proposes a battery charger (BC) for electric vehicles based on sinusoidal-ripple-current (SRC) method. The SRC method is used as an advanced charging-discharging method. On the one hand, the sinusoidal current variations alternately change output active power of the BC, negatively affect AC-side terminal current of the BC, and therefore, cause a power quality problem. On the other hand, the output active and reactive powers must be regulated at constant values without any variations for participation in vehicle-to-grid technology. Thus, a particular topology is suggested and then a control system is precisely designed in order to implement SRC charging-discharging method and adjust the output active and reactive powers for vehicle-to-grid technology without the power quality problem. SRC method can also be used in online electrochemical impedance spectroscopy (EIS) for state of charge and state of health estimation in battery management. The proposed control system can also generate the SRC signals, suitable for online EIS, in a controlled way unlike the recently proposed BCs. The simulation and experimental results demonstrate the effectiveness of the proposed solution. [ABSTRACT FROM AUTHOR]

This paper presents a microgrid composed of photovoltaic (PV) array‐battery energy storage (BES) and a diesel generator (DG) set. A bidirectional DC–DC converter (BDDC) is used to integrate the BES to the DC link of the voltage source converter. The BDDC controls the voltage at the DC link to the output voltage of the maximum power point tracking (MPPT) controller, thereby PV array always operates at its maximum power point (MPP). Another function of the BDDC is to block the second‐order harmonic from the BES, when the microgrid is feeding with unequal loads in the three phases. The magnitude of DG set terminal voltages is regulated by an electronic automatic voltage regulator (AVR) that controls the excitation to the synchronous generator. A fast, accurate and oscillations‐less modified variable step size least mean square based adaptive control is used here for improving the power quality. The main features of this microgrid topology are the operation of the PV array at its MPP in all operating scenarios, regulation of DG set voltages, harmonics elimination, balancing of DG set currents and reactive power compensation for the unity power factor operation. [ABSTRACT FROM AUTHOR]

Increase of photovoltaics (PV) penetration and concentration of electric vehicle (EV) charging may cause voltage violation in distribution lines. One measure for voltage violation in distribution lines is reactive power control. In this research, to reduce additional installation cost of reactive power control equipment, reactive power control by use of power factor correction converters in consumer loads was proposed. In this paper, voltage control ability of the proposed method was evaluated from the point of additional increase of PV generation and EV charging. Distribution voltage distortion caused by harmonic currents from consumer loads to which the proposed method were applied was also evaluated from the point of maximum reactive power output from loads within allowable voltage distortion level. [ABSTRACT FROM AUTHOR]

The increasing participation of wind turbines based on doubly fed induction generator (DFIG) in the power system around the world is motivating the investigation of novel strategies to enhance the reliability of DFIG and power system connection. This paper proposes the use of a new hybrid multilevel back-to-back converter on DFIG wind turbine to improve its fault ride-through capability and power system transient stability. The proposed topology is built adding single-phase full-bridge inverters in series with each phase of traditional three-phase inverter. This converter is able to generate higher voltages in rotor circuit whenever it is required, which can effectively protect power converter and at the same time contribute with grid transient performance. Simulation results, using PSCAD®, are presented for a 2 MW DFIG machine submitted to symmetrical faults in single machine—infinite bus and three-bus system. Results show that the proposal is effective as a protection and works well as an alternative to traditional solutions, limiting currents and maintaining power control during voltage dips. It is also effective in contribution with the improvement of transient stability margin by the possibility of reactive power injection. [ABSTRACT FROM AUTHOR]

This paper aims to investigate a full value energy chain using a grid-connected PV (GCPV) generator and a full value water chain meaning by three pumping units, the basic substructures of water-energy nexus. A dynamic study is exploited to elucidate the different operating modes that can be encountered and more enlighten the interdependence in water and energy by exploiting the sustainability in energy-providing to save water. In the side of the GCPV generator, Voltage Oriented Control (VOC) strategy is used to maintain the stability of the voltage DC bus and control the reactive power between the grid and the PV array. The water system is composed of three identical pumping units which make them useful whatever the irradiance. Here, Power-Field Oriented Control (P-FOC) is used in each pumping unit in order to control the absorbed power and the magnetic state of the induction motor coupled to the used centrifugal pump. On that account, by amending the intermittence of the solar irradiance and the needed water volume, the water-energy nexus concept is being more substantial and makes more practically the used system in rural areas and agriculture zones, which in turn represents the most belief ideology in the developed policies. [ABSTRACT FROM AUTHOR]

The conventional voltage observer based distributed control can make a tradeoff between voltage restoration and reactive power sharing. However, it suffers from estimation error when there exist input disturbances, such as initial values and communication delays. To address this problem, this paper proposes a novel distributed secondary control based on PI consensus algorithm. A voltage controller and a frequency controller are designed separately to realize the restoration of voltage/frequency and also accurate reactive/active power allocation. Moreover, a detailed small-signal model with PI consensus algorithm is established to show the impact of the proposed control strategy on the system stability. Compared with the conventional distributed control, the proposed control scheme is simpler with higher control accuracy under input disturbances. Simulations are conducted to verify the effectiveness of the proposed control strategy in an isolated ac microgrid. [ABSTRACT FROM AUTHOR]

In this paper, we develop a data-driven framework for controlling distributed energy resources (DERs) in a balanced radial power distribution system with the objective of regulating voltages across the whole system. The objective is to determine optimal DER power injections that minimize the voltage excursions outside a desirable voltage range without knowing a complete model of the power distribution system. To this end, we approximate the nonlinear relationship between the voltage magnitudes and the power injections by a linear model. The parameters of this linear model—referred to as the voltage sensitivities—can be computed using information on the network topology and the line parameters, the values of which will be estimated. Assuming the knowledge of feasible network topology configurations and distribution line resistance-to-reactance ratios, we propose a framework for identifying the true network topology configuration and the corresponding line parameters using only a few measurements of voltage magnitudes and power injections. Utilizing the estimated voltage sensitivities, the optimal DER power injections can be readily determined by solving a convex optimization problem. Due to its data-driven nature, the proposed framework is intrinsically adaptive to changes in system conditions such as unknown topology reconfiguration. The effectiveness of the proposed framework is validated via numerical simulations on the IEEE 123-bus distribution test feeder. [ABSTRACT FROM AUTHOR]

In this paper, we address the problem of setting the tap positions of load tap changers (LTCs) for voltage regulation in power distribution systems. The objective is to find a policy that maps measurements of voltage magnitudes and topology information to LTC tap ratio changes so as to minimize the voltage deviation across the system. We formulate this problem as a Markov decision process (MDP), and propose a data and computationally efficient batch reinforcement learning (RL) algorithm to solve it. To circumvent the “curse of dimensionality” resulting from the large state and action spaces, we propose a sequential learning algorithm to learn an action-value function for each LTC, based on which the optimal tap positions can be directly determined. By taking advantage of a linearized power flow model, we propose an algorithm to estimate the voltage magnitudes under different tap settings, which allows the RL algorithm to explore the state and action spaces freely offline without impacting the system operation. The effectiveness of the proposed algorithm is validated via numerical simulations on the IEEE 13-bus and 123-bus distribution test feeders. [ABSTRACT FROM AUTHOR]

Sudden start of an IM load and frequent change of nonlinear load in a standalone distributed generation system (DGS) cause the dip in ac voltage and frequency. Moreover, these loads distort DGS currents. Therefore, in this paper, an improved-reweighted zero-attracting quaternion-valued least mean square (I-RZA-LMS)-algorithm-based voltage source converter (VSG) control is proposed for voltage and frequency regulation, and power quality improvement in DGS. Moreover, the control algorithm of dc–dc bidirectional converter (BDC) is used for dc link voltage regulation and MPPT of the solar photovoltaic array at IM starting and nonlinear loading. In this DGS, the I-RZA-LMS-based control algorithm estimates the active and reactive component currents of distorted load currents for effective harmonics mitigation, reactive power compensation, and point of common coupling voltages regulation. The proposed control algorithm rejects the dc-offset component from load currents and gives the fundamental load component. The dc link voltage of VSC is regulated using BDC with a solar feedforward term, which is used to enhance the capability of dc link voltage control under solar power variation, IM starting, and unbalanced loading. The effectiveness of the proposed DGS is validated through the simulation as well as experimental results obtained on a developed prototype of DGS. [ABSTRACT FROM AUTHOR]

Widely existing circuit topologies and inverter control strategies for photovoltaic (PV) systems allow customer flexibility but also introduce different kinds of interharmonics into the grid. A complete understanding of interharmonics from PV systems, with reasons behind their origin, remains needed. In addition, the time-varying nature of interharmonics and the potential impacts on other equipment are yet to be understood. In this paper, laboratory and field measurements of seven different inverter types at multiple locations are presented. A comprehensive analysis is performed to understand the existence, persistence, and propagation of interharmonics in PV systems on the dc side as well as grid side for different power levels. The origins of the interharmonics are established with experimental evidence and through a comparative analysis. A rural low voltage six customer network, with two different impedance profiles caused by the installation of PV, is considered to show the potential impact on customer voltage. To address the time-varying nature of interharmonics, a sliding window ESPRIT method is preferred over fast fourier transform (FFT)-based methods. [ABSTRACT FROM AUTHOR]

The ever-increasing penetration of intermittent energy sources introduces new demanding operating regimes for the bulk power generation in the power grid. During a worst-case power system disturbance scenario, the generator needs to operate beyond its limits to maintain stable operation. Therefore, a new online low-order thermal model of a hydrogenerator has been recently proposed, where the periodic extension of the long-forgotten capability diagram of the machine was in-depth investigated. An increased performance can be obtained if the total thermal capacity of the generator is exploited by applying optimal control theory in the Automatic Voltage Regulator (AVR). This paper proposes a Non-linear Model Predictive Controller (NMPC) combined with an Unscented Kalman Filter (UKF) with a modeling framework geared for use in a supervisory control structure for the conventional control system. The method provides maximum utilization of the machine's thermal capacity by providing the controller with an Enhanced Capability Diagram (ECD). Case studies on a single-machine system and a 16-bus multi-machine system were investigated. The results show that a satisfying controller action during different long-term voltage instability scenarios is realized. [ABSTRACT FROM AUTHOR]

Modern grid codes demand the integration of voltage support capability with photo-voltaic (PV) generators to ensure a secure operation of power systems. On the other hand, short-term voltage instability (STVI) of distribution networks (DNs) is one of the key issues to be addressed. It can occur due to high proportion of induction motor loads. However, the existing literature lacks a thorough analysis of short-term voltage stability (STVS) of DNs following an asymmetrical fault. Furthermore, an effective voltage-support strategy for PV units, which can improve the STVS while mitigating the excessive voltage swell, is essential for secure operation. Hence, firstly this paper thoroughly investigates the STVS of DNs subjected to asymmetrical faults. It is perceived that voltage support through conventional methods can increase the risk of STVI and excessive voltage swell. Secondly, a new voltage-support strategy is proposed based on the negative sequence voltage at point of common coupling (PCC) aimed at mitigating the STVI and regulating the phase voltages within the margins. The key features of the proposed strategy are, fast and accurate estimation of the network's impedance at PCC is not required and it can be easily re-designed considering the network's dynamic behaviors. The efficacy of the proposed method is validated on two IEEE benchmark test systems, and the results demonstrate the effectiveness in improving the STVS and alleviating over voltage issues. [ABSTRACT FROM AUTHOR]

This paper presents a hierarchical-hybrid architecture for Volt/Var control of distribution grids in the presence of high penetration of distributed generators (DGs). The architecture includes three layers with specific operational goals associated with each layer based on data resolution, communication network, and control device response time. The top layer carries out central optimization and seeks to minimize power losses via optimal 15-minute scheduling of load tap changers (LTCs), voltage regulators (VRs), and capacitor banks (CBs), and DGs. The middle layer ensures voltage regulation using a distributed approach for reactive power provisioning by fast response DGs. The bottom layer carries out local decision making that enlists edge intelligence to operate LTCs, VRs and CBs to cope with fast and real-time changes in DGs and loads. The top layer generates overall references for the lower layers optimized over a 24-hour horizon, updated at 15-minute intervals, which are then suitably corrected at faster time-scales of seconds in the middle and bottom layers. Appropriate coordination is introduced between each layer so as to meet combined goals of optimization and accommodation of load and generation uncertainties in near real-time. The proposed method is validated using a modified IEEE 34 bus test feeder and at 80% penetration of DGs. [ABSTRACT FROM AUTHOR]

Under unbalanced grid voltage conditions, an ac–dc matrix converter (MC) induces ripple on the dc side, as well as severe grid current harmonics. To overcome these problems, an enhanced control strategy for an ac–dc MC based on an independent control scheme for the active and reactive powers is proposed in this paper. The generated current reference is directly synthesized from instantaneous power analysis in a stationary frame, and a constant dc voltage and current and sinusoidal grid current are obtained. Moreover, the input power factor (IPF) becomes almost unity due to the reactive power control, in spite of unbalanced grid voltage conditions. There is no need to extract grid voltage components such as the positive and negative sequences, and a controller for the grid current is not required, in contrast to previous methods. Thus, the proposed control method can be implemented easily without a large storage requirement. Simulation and experimental results are presented to verify the effectiveness of the proposed control strategy. [ABSTRACT FROM AUTHOR]