Under-load Tap Changer Research Articles

Optimal Voltage Control Method for a Step Voltage Regulator Considering the Under-Load Tap Changer in a Distribution System Interconnected with a Renewable Energy Source

The voltage in distribution systems is controlled by the under-load tap changer of the substation and the pole transformer of the primary feeders. Recently, as one of the main countermeasures, a step voltage regulator is being introduced to solve voltage problems such as overvoltage phenomena in a distribution feeder interconnected with a renewable energy source and under voltage in a long-distance feeder. However, the tap of the step voltage regulator may be frequently operated due to its interdependent relationship with the under-load tap changer in the distribution system. Furthermore, given the existing operating characteristics of the step voltage regulator, it is difficult to perfectly maintain the customer voltage within an allowable limit using existing methods such as the line drop compensation method for a step voltage regulator. In addition, the existing line drop compensation method, considering the distributed generators, may be not able to control the proper voltage within an allowable limit. Therefore, in order to solve such voltage problems, this paper proposes a voltage control method for a step voltage regulator by considering the output voltage of an under-load tap changer that is operated via the line drop compensation method. In other words, to overcome the limitations of existing voltage control methods for step voltage regulators, this paper proposes an optimal control method to determine the optimal compensation rate for a step voltage regulator by considering the reverse power flow from a renewable energy source and the output voltage of the under-load tap changer of the main transformer.

Impact Assessment and Mitigation Techniques for High Penetration Levels of Renewable Energy Sources in Distribution Networks: Voltage-control Perspective

The integration of renewable distributed generation (RDG) into distribution networks is promising and increasing nowadays. However, high penetration levels of distributed generation (DG) are often limited as they may have an adverse effect on the operation of distribution networks. One of the operation challenges is the interaction between DG and voltage-control equipment, e. g., an under-load tap changer (ULTC), which is basically designed to compensate for voltage changes caused by slow load variations. The integration of variable DGs leads to rapid voltage fluctuations, which can negatively affect the tap operation of ULTC. This paper investigates the impact of high penetration levels of RDG on the tap operation of ULTC in distribution networks through simulations. Various mitigation techniques that can alleviate this impact are also examined. Among these techniques, constant power-factor mode is regarded as the best trade-off between the simplicity and effectiveness of minimizing the number of tap operations. Simulations are performed on a Canadian benchmark rural distribution feeder using OpenDSS software.

Supervisory Controller Design for Power System Stability using Hybrid Automata Modeling

Power systems are categorized as nonlinear dynamical systems, and the importance and complexity associated with their stability have dramatically increased. Accordingly, the behavior of power systems can be characterized by interactions between continuous and discrete-event dynamics. This paper proposes a systematic approach to the design and analysis of a supervisory control scheme for power systems using the hybrid automata (HA) model. The proposed model for optimal controller application is derived, and the power system's overall behavior is modeled using HA to enhance its stability. Hybrid systems' formulation incorporates continuous dynamics as well as discrete switching behavior into a modeling and control framework, thus allowing a complete system description while crystallizing the concepts of safety into system design criteria. This study uses a power system HA model as a discrete event system (DES) plant and controller. In the proposed method, to present the hybrid model, the discrete events used include the presence and absence of disturbances and voltage control elements, fault, sudden load increase, capacitor bank, and under-load tap changer (ULTC) transformer. Voltage stability and control are investigated by the generators’ rotor angle, bus voltage, eigenvalues, and the stability theory of the switched linear systems. Applications in voltage control, stability, and dynamic service restoration are presented on two benchmark power systems with 12 discrete states. The simulation results reveal the effective performance of the proposed supervisory controller model to enhance voltage stability in power systems.

PV system reactive power coordination with ULTC & shunt capacitors using grey wolf optimizer algorithm

The presence of PV systems increases rapidly in distribution systems to improve reliability and quality of supply. This will influence the performance of under load tap changing (ULTC) transformer and related reactive power devices. Therefore, many researchers are working on this area. This paper main objective is to reduce switching operations of reactive power devices (ULTC and Shunt capacitors) together with system power loss. Distribution system load and solar system power will predict one day in advance and grey wolf optimizer (GWO) algorithm proposed to solve the objective function. Reactive power of solar system is coordinated together with ULTC and shunt capacitors (SCs) with the aid of forecasted load. Distribution system losses and switching operations of ULTC and SCs converted into objective function in terms of cost. The proposed method is applied on practical 10KV system and the results are compared with conventional and particle swarm optimization (PSO) methods considering grid conditions.

Optimal Power Flow with Renewable Generation: A Modified NSGA-II-based Probabilistic Solution Approach

The rapid expansion of renewable generation has drastically increased the planning complexity of modern power systems as additional uncertainties, environmental concerns, and technical–economic issues should be accounted for. Within this context, the best operation performance of contemporary power system operators (SOs) depends not just on tractable realistic optimal power flow (OPF) formulations, but also on powerful optimization approaches. In this work, a tractable life-like multi-objective probabilistic OPF-based model for the SO’s medium-term operation considering high penetration of renewable resources is proposed. This model includes an explicit formulation of the operation of dispatchable and non-dispatchable generation, shunt reactive power sources, and under-load tap-changing (ULTC) transformers. The resulting model is a large-scale probabilistic multi-objective non-convex nonlinear mixed-integer programming (NLMIP) problem with continuous, discrete, and binary variables. To ensure tractability, uncertainties are modeled through a fast and efficient 2m probabilistic approach. To handle the nonlinearities and non-continuous variables that characterize the problem, a modified non-dominated sorting genetic algorithm (NSGA)-II solution approach is proposed and effectively tested.

PSO Based Reactive Power Coordination of Multiple Dispatchable Distributed Generations & Voltage Controlled Devices

The presence of multiple distributed generations on radial distribution system increases switching operations of voltage controlled devices (VCDs). Reduction of switching operations (SOs) of VCDs in the presence of multiple dispatchable distributed generations (MDDGs) is a demanding research area now a days. This paper proposes a novel method for the reduction of SOs of VCDs together with system power loss, in which we estimated the load one day in advance which makes MDDGs to dispatch the reactive power cooperatively with VCDs and particle swarm optimization (PSO) method used for solving multiple objective function. Multiple objective function (MOF) is formulated with cost of power loss and cost of SOs. The proposed method is tested on 16 nodes practical system under different operating output patterns and locations considering under load tap changers (ULTCs) and shunt capacitors (SCs) as VCDs. Simulation results clearly depicts that this method is effectively achieved the goals compared with existing conventional optimization techniques..

Multi‐objective linearised optimal reactive power dispatch of wind‐integrated transmission networks

This study develops a multi-objective approach to pursue an optimal reactive power dispatch (ORPD) of power systems equipped with wind farm (WF) integrations. The preliminary control variables include shunt compensators, synchronous condensers, and under-load tap changers (ULTCs). Innovatively, reactive power provision capability of WFs, beyond their active power contributions, is scheduled besides the variables above. Seemingly, a double operating area is adopted for WF within which it injects active power towards the main grid. Concurrently, it could inject or absorb reactive power, which impacts technical metrics of the power system operation. The main purpose is then establishing a compromise between the power loss and voltage deviation minimisation and also reducing the number of ULTC tap variations and operations. To handle the multi-objective feature of the proposed model, the global criterion method is deployed. The established ORPD model represents a multi-objective non-linear mathematical problem, which is converted to a mixed-integer linear programming format. By this way, the computational burden of the problem is reduced to maintain an acceptable accuracy of the obtained results. Detailed discussions are provided to assess the performance of the proposed model.

Control of Parallel ULTC Transformers in Active Distribution Systems

This paper proposes a two-level control scheme for controllable devices in a smart grid, enabled with various types of distributed energy resources (DERs) and the other sources of flexibility provision. The main focus is on under-load tap changer (ULTC) control of parallel transformers in active distribution systems. The first-level control that can be viewed as a function of distribution management system, minimizes the system operation cost, considering the voltage limits, maximum line current, maximum circulating current in parallel transformers, and controllers’ capacity as the optimization constraints. The second-level control holds the voltage stability in emergency conditions. The control rules in this level are intended to restore the system voltages as soon as possible, minimize the total number of costly control actions, and prevent conflicts between different controllable devices. In the second-level voltage control, the solution speed is of utmost importance. Therefore, the formulation of the control problem is first linearized using node analysis technique and capacitor banks, ULTC, DERs and voltage regulators are coordinated based on their priority and their effect on the bus voltages.

D-FACTS Cooperation in Renewable Integrated Microgrids: A Linear Multiobjective Approach

This study proposes a multiobjective approach for daily scheduling of microgrids (MGs) contributing to a higher penetration of photovoltaics (PVs). In this process, the existing control devices including distribution flexible ac transmission system and under-load tap changer (ULTC) are integrated in Volt/VAr control process. Besides, the substantial role in enhancing the service quality, a proper support of reactive power by distribution static compensator annihilates the overvoltage/undervoltage issues and increases the power provision share of PVs. Thus, a higher monetary saving is attained. Moreover, a moderated tap changing operation of ULTC is noticed and the dispatchable distributed generations mainly participate in active power processes. In addition, the hourly reconfiguration of MG assists to a proper feeding path to higher dispatch of cost-effective resources and enhancing the technical metrics. The proposed approach is formulated within a mixed integer linear programming being tackled by an augmented ϵ-constraint method. Efficient linearization techniques are deployed to lower the computational burden of the problem. Detailed simulation studies are conducted to assess performance of the proposed approach.

Model predictive control scheme for coordinated voltage control of power systems at the presence of volatile wind power generation

This study proposes an optimal voltage control scheme to deal with long-term voltage stability of power systems. The control of voltage is accomplished by model predictive control (MPC) scheme. The control objective function considers the control efforts and the difference between the predicted and reference voltages. Also, this study considers the detailed non-linear dynamic model of the system including doubly-fed induction generator wind turbines, over excitation limiter and under-load tap changer, which are important elements, should be considered in voltage stability evaluation of power systems. The proposed approach composed of the following two major stages at each time instance: first, the power system non-linear dynamic equations are linearized and optimal control laws are obtained by MPC technique;in the second stage, the system dynamic behavior is investigated via time-domain simulations by applying the attained optimal control signals at the first step. The proposed MPC-based voltage control scheme is implemented on a well-known test system, under variable wind speed and fault conditions. Also, this method's performance is compared with state feedback control technique. The obtained numerical results validate the capability of the proposed control scheme to preserve voltage stability at the presence of stochastic wind speed variations and severe disturbances.

Multiobjective Scheduling of Microgrids to Harvest Higher Photovoltaic Energy

Photovoltaics (PVs) and plug-in electric vehicles (PEVs) could instigate voltage violation issues, if not controlled properly. Besides, these components highly contribute to economic efficiency of microgrids (MGs). Being motivated to enhance technical and economic issues, this study develops an energy management system that is able to coordinate voltage control devices, say under-load tap changers (ULTC), PVs, PEV aggregators, and dispatchable distributed generations (DDGs). Active and reactive power provisions of PEVs and DDGs along with voltage control of ULTC lessen the plausible violations. So, higher renewable energy is contributed by PVs and extra monetary saving is reflected. In this manner, augmented Epsilon-constraint method followed with fuzzy decision making is applied to operation cost and voltage deviation minimizations. The proposed approach is formulated as a mixed-integer nonlinear multiobjective problem and tested on a modified IEEE 33-bus medium-voltage MG. Results are discussed in depth.

Multiobjective capacity planning of photovoltaics in smart electrical energy networks: improved normal boundary intersection method

Incentive-based regulations, toward higher performance networks, are the main driver for minimising losses in distribution systems. On the other hand, more renewable generation is needed to achieve environmental targets. Hence, a multiobjective model is introduced in this study seeking to minimise energy losses as well as maximise renewable generation in radial distribution systems (RDSs). Two alternative control strategies of future smart grids such as reactive power management using adaptive power factor control and coordinated voltage control are considered in the optimisation problem. The problem is subjected to the various technical constraints such as voltage limits, thermal limits and reactive capability limits of photovoltaic (PV) penetration, power factor regulations and underload tap changer adjustment. Also, the uncertainties of load and renewable generation are considered, too. Then, the obtained non-linear programming problem is relaxed and reformulated as a well-suited and computationally efficient second-order cone programming problems. To obtain more efficient and evenly distributed Pareto set to help decision-making process, a modified normal boundary intersection method is introduced for solution methodology. The implementation of the proposed framework on IEEE 33-bus RDSs shows the gains that the flexibility provided by innovative control strategies can have on energy loss reduction and PV capacity.

Techno-Economic Collaboration of PEV Fleets in Energy Management of Microgrids

This paper develops a generalized energy management approach for smart microgrids (MGs) with the collaboration of plug-in electric vehicle (PEV) charging facilities in both active and reactive powers exchange. In addition to the monetary saving and economical benefits, this feature reveals a promising opportunity for power quality improvement in modern MGs. To do this, an efficient multi-objective framework is proposed for the optimal operation scheduling of MGs. The optimization model incorporates the full potential benefits of PEV fleets in balancing both active and reactive powers supply and demand. The multi-objective formulation lies within a mixed-integer linear programming fashion, which is tackled by $\varepsilon $ -constraint method. A quadruple linear operating area is envisioned for PEV fleet inverters caped with the maximum apparent power limit. A suitable voltage-dependent load model, with negligible error, is deployed to tailor the proposed approach for real-world applications. The new methodology is successfully applied to a 33-bus test MG with illustrative case studies. The obtained results are reported in terms of voltage profile improvement, released capacity of generating units, moderated operation of under-load tap changers, and MG operation cost reduction.

Coordinated voltage control of wind-penetrated power systems via state feedback control

This paper presents an optimal coordinated voltage control scheme for preserving long-term voltage stability of power systems. Linear quadratic integral (LQI) controller is employed to construct this scheme. Also, this paper considers the detailed dynamic model of doubly-fed induction generator (DFIG) wind turbine, synchronous generators, over excitation limiter (OEL) and under-load tap changer (ULTC) system, which are important elements influencing voltage stability of power systems. The proposed approach at each time instance involves following two major steps: First, the power system nonlinear equations are linearized and optimal controllers are obtained by LQI technique. Then, in the second step the system dynamic behavior is investigated via time-domain simulations by applying the attained optimal control signals at the first step. The impact of the proposed coordinated voltage control scheme is evaluated by time domain simulations on a well-known test system, under variable wind speed and fault conditions.

A new sequential AC–DC power flow algorithm for multi‐terminal HVDC systems

This paper presents a new algorithm based on the sequential method for power flow calculation in integrated multi‐terminal, high‐voltage, direct current (HVDC) systems. Unlike similar studies in the literature, a real equivalent circuit model is considered for under‐load tap changer (ULTC) transformers of the DC converters, for the first time. So, new DC equations are obtained. Thus, exact and accurate results can be obtained for practical applications by the proposed algorithm. Adjustment effects of the DC converters' ULTCs tap values are included in the Jacobian matrix instead of the bus admittance matrix in the sequential AC power flow algorithm as well as other ULTCs in AC system. To this aim, new equations for the calculation of power and Jacobian matrix elements are obtained for the AC system. The proposed approach is tested on the modified IEEE 17‐bus AC–DC test system. Numerical results show that the proposed approach is accurate and reliable in convergence. © 2017 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

A new sequential power flow algorithm for AC/DC systems including independent multiterminal DC subsystems

This paper presents a new algorithm based on the sequential method for power flow calculation in multiterminal AC/DC systems. The proposed approach differs from other similar studies in the literature by handling two main properties. Firstly, a real equivalent circuit model is considered for the under-load tap changer transformers (ULTCs) of DC converters for the first time in the literature. Hence, new DC equations are obtained. Thus, exact and accurate results can be obtained for practical applications using the proposed algorithm. Tap values adjustment effects of the ULTCs connected to DC converters are included into the Jacobian matrix instead of the bus admittance matrix in the sequential AC power flow algorithm, as well as other ULTCs in the AC system. For this aim, new equations for calculating power and Jacobian matrix elements are obtained. Secondly, more than one independent multiterminal DC subsystem in terms of DC connection is included in the study. DC power flow calculation is performed for each DC subsystem independently in the algorithm. Thus, modularity and flexibility are provided for including or removing any DC subsystems without needing to change the proposed algorithm. The proposed approach has been tested on a modified IEEE 20-bus AC/DC test system. The proposed AC/DC power flow algorithm was written and performed in MATLAB. The results show that the proposed approach is accurate and reliable in convergence.

Reactive power coordinated optimisation method with renewable distributed generation based on improved harmony search

The reactive power coordinated optimisation problem for distribution network (DN) with the integration of renewable distributed generation (DG) is investigated in this study. By actively participating in reactive power control together with an under-load tap changer and shunt capacitors, DG can operate more effectively in the DN. Non-dispatchable DG is considered in the coordinated optimisation method. The objective of the proposed reactive power coordinated optimisation method is to minimise active power loss and to decrease the number of switching device operations while maintaining the grid voltage within the allowable range. The optimisation problem is to solve a variable combination including integers and continuous variables, which will formulate a mixed integer non-linear programming (MINLP) problem. The control variable includes operations of traditional reactive power control devices and the reactive power adjustments of DGs. To solve the MINLP problem, a heuristic searching method, an improved harmony search algorithm is proposed with adaptive parameters process. Numerical tests on a standard 10-kV distribution system and a real distribution system demonstrate the applicability of the proposed reactive power coordinated optimisation method.

Coordinated Voltage Control With Online Energy Margin Constraints

Based on data available from phase measurement units (PMUs), this paper proposes an energy margin constrained online coordinated voltage control (CVC) algorithm. First, the energy function model is established, which is based on detailed device models that include the effects of shunt capacitor banks, reactive power output of generators and under-load-tap-changers (ULTCs). In order to improve the computational efficiency, a Lie derivative-based method is used to map the energy change into the system trajectory along the energy manifold. Then, the change of energy margin based on the Lie derivative is used as the objective function and the reactive power margin is used as a constraint to formulate a linear programming problem. A coordinated strategy is proposed to help achieve the optimal solution when the linear programming problem is solved. In comparison with the other control modes, the proposed method improves the stability margin and the control effectiveness of voltage profile. Numerical results are provided for a three-machine system, the IEEE-30 bus system and the New England-39 bus benchmark system.

PV-Installable Capacity in Medium-Voltage and Low-Voltage Distribution Networks with Optimal Line-Drop Compensation Parameters

AbstractTo reduce greenhouse gases, the high penetration of photovoltaic (PV) generation has been greatly anticipated in Japan. However, local distribution networks face voltage-rise problems when numerous PV systems are connected. In particular, rooftop-type PVs on detached houses can be widely installed in Japan. In that case, the voltage rise in low-voltage wires or service wires is significant because of the high resistance of these wires. In terms of network operators, spatial distribution scenarios for PV installation cannot be prespecified because each PV installation is determined by the particular customer. This paper evaluates PV-installable capacity from a systematic perspective under voltage constraints in medium- and low-voltage networks with a consideration of voltage control equipment [under-load tap changer (ULTC) and step voltage regulator (SVR)]. Through the use of optimal parameters for ULTC and SVR, PV-installable capacity can be improved. In particular, PV-installable capacity around ...

A Trust Region SQP Method for Coordinated Voltage Control in Smart Distribution Grid

This paper investigates the coordinated voltage control problem for smart distribution grid with the integration of distributed generation (DG). By actively participating in voltage control together with under-load tap changer and shunt capacitors, DG can operate more effectively in the distribution network. The objective of the proposed control method is to minimize the active power loss in the distribution system and to decrease the number of switching device operations while maintaining the grid voltage within the allowable range. Nondispatchable and dispatchable DG are both considered in the control method. To solve the mixed integer nonlinear programming problem, the trust region sequential quadratic programming method is integrated with the branch and bound approach to iteratively approximate the optimization with trust region guidance. Numerical tests on a standard 10-kV distribution system, a real 10-kV distribution system in the Sichuan province of China, and the IEEE 13-bus demonstrate the applicability of the proposed coordinated voltage control method.