Load Tap Changer Research Articles

Voltage Control of Distribution Networks to Increase their Hosting Capacity in South Korea

This paper presents the results of a study of a voltage management technology called parameter determination of voltage regulator (PDVR) implemented in a 22.9 kV distribution network. Korea Electric Power Corporation has developed this PDVR technology for increasing the hosting capacity without causing any voltage problems in LV (220/380 V) networks. PDVR calculates and provides the operation settings of voltage regulators such as on load tap changer, pole mounted automatic voltage regulator, and distributed generation (DG). Subsequently, these settings are applied to the voltage regulators, which further optimize the 22.9 kV network voltage without the control of the distribution management system (DMS). A study of the operations and performance of PDVR through the analysis of test results is presented in this paper.

Study on switching principle of thyristor assisted arc extinguishing hybrid OLTC

Thyristor assisted arc extinguishing hybrid on load tap changer has the advantages of both mechanical switch and thyristor switch. It has strong ability to withstand overload, over-current and overvoltage, high reliability, small maintenance workload and frequent voltage regulation operation. It is a promising on load tap changer of transformer. This paper introduces the principle of passive trigger of thyristor module, gives the switching principle and process of thyristor assisted arc extinguishing hybrid on load tap changer, and analyses the characteristics of hybrid on load tap changer.

Automatic Voltage Stabilization Using IGBT Based on Load Tap Changer With Fault Consideration

In this work, a voltage regulation scheme to stabilize voltage variation in the consumer side is proposed. The voltage regulation scheme uses IGBT-based onload tap changer (OLTC) and nonlinear hysteresis control algorithm. The proposed switching algorithm establishes the commutation of the taps without redundant short circuit operation of the taps. The experimental results of the designed system show accurate voltage regulation at the consumer side and high ability of the designed system to fix voltage sag and voltage swell in maximum of one cycle period. The paper considers the consequences of fault in the drive circuit of the IGBT tap changer and discusses its reliability. The analysis and the experimental results of the faults in the drive circuit show that the loss of one drive signal results in a large circulating DC current in the primary circuit and may result in fault current of value more than three times the operated current, which leads to unavailability of the system.

Development of 110kV Thyristor Assisted Arc Extinguishing Hybrid OLTC

On the basis of formulating the technical conditions of 110kV thyristor assisted arc extinguishing hybrid on load tap changer, this paper carries out the mechanical structure design, gives the design scheme of switching switch, selection switch and other connecting parts, and introduces in detail the adaptive improvement design of switching core structure and the coordination of switching process and sequence after adding thyristor module, and then completes the mixing The factory test and the type test of 110kV hybrid OLTC have been completed.

Application of Equilibrium Optimizer Algorithm for Optimal Power Flow with High Penetration of Renewable Energy

In recent decades, the energy market around the world has been reshaped to accommodate the high penetration of renewable energy resources. Although renewable energy sources have brought various benefits, including low operation cost of wind and solar PV power plants, and reducing the environmental risks associated with the conventional power resources, they have imposed a wide range of difficulties in power system planning and operation. Naturally, classical optimal power flow (OPF) is a nonlinear problem. Integrating renewable energy resources with conventional thermal power generators escalates the difficulty of the OPF problem due to the uncertain and intermittent nature of these resources. To address the complexity associated with the process of the integration of renewable energy resources into the classical electric power systems, two probability distribution functions (Weibull and lognormal) are used to forecast the voltaic power output of wind and solar photovoltaic, respectively. Optimal power flow, including renewable energy, is formulated as a single-objective and multi-objective problem in which many objective functions are considered, such as minimizing the fuel cost, emission, real power loss, and voltage deviation. Real power generation, bus voltage, load tap changers ratios, and shunt compensators values are optimized under various power systems’ constraints. This paper aims to solve the OPF problem and examines the effect of renewable energy resources on the above-mentioned objective functions. A combined model of wind integrated IEEE 30-bus system, solar PV integrated IEEE 30-bus system, and hybrid wind and solar PV integrated IEEE 30-bus system is performed using the equilibrium optimizer technique (EO) and other five heuristic search methods. A comparison of simulation and statistical results of EO with other optimization techniques showed that EO is more effective and superior and provides the lowest optimization value in term of electric power generation, real power loss, emission index and voltage deviation.

Coordinated Countermeasures Against Overvoltage Instability in Autonomous Power Systems

Autonomous power systems with a single power station and a high-voltage (HV) network may be susceptible to overvoltage instability, especially during light-load conditions. Moreover, in low inertia systems, considerable frequency excursions may be observed after disturbances. To deal with voltage and frequency stability problems in such isolated power systems, several measures should be taken, such as rules for adequate automatic voltage control of the synchronous generators and load-tap-changers (LTCs), and/or adequate design of actions applied during emergency system conditions such as reactors switching, and redesigning underfrequency load shedding taking into account overvoltage problems. The effectiveness of the proposed countermeasures against overvoltage instability in autonomous power systems is illustrated in this paper. In 2016, the autonomous power system of the Greek island of Rhodes experienced similar voltage/frequency problems leading to a blackout. This event has been accurately reconstructed in this paper. The application of the suggested countermeasures during the simulation of the 2016 blackout event in Rhodes, further highlighted the effectiveness of these measures, providing a guideline for similar cases.

Coordinated voltage control scheme for transmission system considering objectives and constraints of network and control devices

Need for sufficient reactive power reserve for fast reaction against large disturbances necessitates developing an integrated and coordinated voltage control scheme including controlled and mechanically switched control devices. This paper proposes a day-ahead optimal voltage control scheme for transmission system considering static var compensators (SVCs), capacitor banks, load tap changers(LTCs) transformers and synchronous machines. The scheme is based on defining an optimization problem which includes two objective functions for reactive power reserve and voltage regulation precision of SVCs, two objective functions related to number of capacitors switching and LTCs tap changing, and two objective functions for improving the network performance. The multi-objective optimization problem is solved subject to a complete set of constraints related to the network and voltage control devices. The solution algorithm of the proposed problem is based on defining a single objective function by applying appropriate weighting coefficients to the normalized indices. Performance of the proposed scheme is investigated on IEEE 30-bus system through various studies. Particularly, the effectiveness of the scheme is demonstrated to realize the objectives for conditions where the load profiles at transmission buses are considerably modified due to structuring changes in the downstream networks.

Emergency Support of Transmission Voltages by Active Distribution Networks: A Non-Intrusive Scheme

Non-negligible amounts of Distributed Generating Units (DGUs) are already connected to power systems, predominantly to medium- and low-voltage networks. The interaction between distribution and transmission systems is gaining the attention of system operators. Given the high controlability of DGUs, the active distribution networks can potentially support the transmission system during emergency situations. Previously proposed voltage emergency controls involving distribution network assets focused on preserving the transmission system integrity, even if it implies affecting the distribution system operation (intrusive schemes). More attention needs to be devoted to supporting transmission voltages, while preserving the distribution network operation (non-intrusive schemes). This paper introduces a new adaptive and non-intrusive voltage emergency control scheme based on the synchronization of DGUs and the load tap changer of the corresponding distribution transformer. The conclusions are derived from time-domain simulations using an extended version of the IEEE Nordic test system for voltage stability assessment.

Enhanced wind turbine generator fault ride‐through capability by using hybrid on‐load tap changer

Low-voltage ride-through (LVRT) compliance of wind energy conversion systems is one of the significant concerns for both the wind industry and the system operator. In this study, power electronic-assisted on load tap changer (OLTC) is proposed as a pragmatic solution to enhance LVRT performance of wind turbine generator (WTG). Furthermore, a coordinated control strategy between OLTC and WTG controller is proposed to mitigate post-fault over-voltage issues that are encountered due to the relatively slow dynamics of hybrid OLTC. The results suggest that employing hybrid OLTC does not only improve LVRT compliance of WTG but also significantly improve voltage dip-induced delayed active power recovery of WTG.

Automatic Tap-Changer with TRIAC Switch for Constant Voltage and Current Measurements

This research article demonstrates how to protect the power transformer from over load time. It is to controlling our desired voltage on various load times but the research gives some solution to avoid over voltage problem by automatic tapping technique. In this research tapping of transformer is selected automatically with the help of Peripheral Interface Controller (PIC) microcontroller. Effective use of potential transformer is to find out the precision in the system. The load voltage is measured by potential transformer which is given to precision rectifier. It will produce the accurate voltage levels. Here the things are using PIC microcontroller which is a flash type reprogrammable device in which system was already programmed. The Model was fabricated with Triode for Alternating Current (TRIAC) switches just as a switching devices and Peripheral Interface Controller microcontroller just as a control unit. In this research automatic control sections by using tap changer. In conventional methods industrialist used mechanical on load tap changer which is highly risk, because it does not produces constant output voltage and can create voltage dips. In this research it will be overcome above problem by using automatic electronic tap changer. It will produce constant voltage regulation and to measure variation of current.

Design and Implementation of Thyristor based Automatic on Load Tap Changer Using ANN-IOT

Design and Implementation of Thyristor based Automatic on Load Tap Changer Using ANN-IOT

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.

Distributed Solution of Stochastic Volt/VAr Control in Radial Networks

This paper presents a fully distributed algorithm for the stochastic Volt/VAr control (VVC) problem in active distribution networks. Exact convexification of the VVC problem is achieved through the use of second-order cones on the continuous relaxation of the original optimization structure. The global optimum solution of the relaxed problem is obtained through the application of the alternating direction method of multipliers (ADMM). In order to minimize the effect of rounding off on the final solution, an adaptive threshold discretization technique is used. A two-stage control strategy is adopted where the discrete controllers, like load tap changers and switched capacitors, are dispatched at the beginning of the optimization interval and the continuous controllers, like distributed generation (DG) inverters, are adjusted in real time according to an optimized decision rule. The superiority of the proposed algorithm is demonstrated through numerical simulations on the UKGDS-95 and the IEEE-123 bus systems.

Study on the Influence of Measuring AE Sensor Type on the Effectiveness of OLTC Defect Classification.

The principal objective of this study is to improve the diagnostics of power transformers, which are the key element of supplying electricity to consumers. On Load Tap Changer (OLTC), which is the object of research, the results of which are presented in this article, is one of the most important elements of these devices. The applied diagnostic method is the acoustic emission (AE) method, which has the main advantage over others, that it is considered as a non-destructive testing method. At present, there are many measuring devices and sensors used in the AE method, there are also some international standards, according to which, measurements should be performed. In the presented work, AE signals were measured in laboratory conditions with various OLTC defects being simulated. Five types of sensors were used for the measurement. The recorded signals were analyzed in the time and frequency domain and using discrete wavelet transformation. Based on the results obtained, sets of indicators were determined, which were used as features for an autonomous classification of the type of defect. Several types of learning algorithms from the group of supervised machine learning were considered in the research. The performance of individual classifiers was determined by several quality evaluation measures. As a result of the analyses, the type and characteristics of the most optimal algorithm to be used in the process of classification of the OLTC fault type were indicated, depending on the type of sensor with which AE signals were recorded.

Remote Laboratory Testing Demonstration

The complexity of a smart grid with a high share of renewable energy resources introduces several issues in testing power equipment and controls. In this context, real-time simulation and Hardware in the Loop (HIL) techniques can tackle these problems that are typical for power system testing. However, implementing a convoluted HIL setup in a single infrastructure can be physically impossible or can increase the time required to test a smart grid application in detail. This paper introduces the Joint Test Facility for Smart Energy Networks with Distributed Energy Resources (JaNDER) that allows users to exchange data in real-time between two or more infrastructures. This tool enables the integration of infrastructures, exploiting the synergies between them, and creating a virtual infrastructure that can perform more experiments using a combination of the resources installed in each infrastructure. In particular, JaNDER can extend a HIL setup. In order to validate this new testing concept, a coordinated voltage controller has been tested in a Controller HIL setup where JaNDER was used to interact with an actual On Load Tap Changer (OLTC) controller located in a remote infrastructure. The results show that the latency introduced by JaNDER is not critical; hence, under certain circumstances, it can be used to expand the real-time testing without affecting the stability of the experiment.

Security-Constrained ACOPF: Incorporating Worst Contingencies and Discrete Controllers

The work reported in this paper aims at developing an algorithm for system operators to solve a robust security-constrained AC optimal power flow (R-SC-ACOPF) problem, which also optimizes the settings of discrete controllers including load tap changers (LTCs) and shunt capacitor banks (SCBs). An iterative algorithm involving a master problem and a number of sub-problems is proposed. The master problem selects the optimal discrete controllers' settings by taking into account primal SC-ACOPF constraints based on the single-component worst contingencies identified by the sub-problems. Each sub-problem is a bi-level max-min problem used to find the current worst contingency. The overall objective is to minimize the total cost, including generation and load shedding cost, while satisfying all relevant constraints. The master problem and sub-problems allow representing LTC tap positions, SCB status, and contingencies using binary variables. Two case studies are presented as applications of this novel technique.

System Unbalance Analyses and Improvement for Rooftop Photovoltaic Generation Systems in Distribution Networks

This paper studies the system unbalance caused by rooftop Photovoltaic Generation Systems (PVGSs) in distribution networks and proposes an improved method. The voltage and current unbalance studies for three extreme cases considering all rooftop PVGSs being connected to one single phase and then the stochastic analyses for the integration cases of rooftop PVGSs are considered. These three extreme cases lead to severe system unbalance problems. An improving method that combines the On Load Tap Changer (OLTC) and the optimal phase arrangement of distribution transformers is proposed in this paper to mitigate the system unbalance. The OLTC-based improving method is applied first. If the system unbalance is still out of range, the optimal phase arrangement of distribution transformers is further used to mitigate the system unbalance problem. The objective function of optimal phase arrangement is to minimize the system unbalance with the constraints of voltage limits, line flow limit, etc. Test results show that the proposed method can improve the system unbalance significantly even under very extreme cases.

Dynamic Direct Voltage Controller (D2VC) for grids with intermittent sources

This work proposes a voltage regulator to solve quality issues at the point of common coupling (PCC) of distribution grids affected by the intermittency of renewable energy sources. The proposed solution is a voltage compensator based on single-phase voltage source converter (VSC) units, called Dynamic Direct Voltage Controller (D2VC). The three-phase D2VC topology comprises three single-phase VSC back-to-back units. For each unit, one converter is connected in series with the grid at the PCC and the other converter is connected in shunt similar to an UPFC (Unified Power Flow Controller). To avoid coupling transformer, the series converter is directly connected in series to the existing distribution transformer thus no extra transformer is necessary. The D2VC can regulate the three-phase voltage profile at the PCC and balance all phases independently. The control strategy for each series converter is based on the estimation of the instantaneous symmetrical components of the voltage. Simulation results using PSCAD/EMTDC and an experimental platform of the D2VC are shown to confirm all the D2VC advantages in grids with resistive lines (R >> X). The proposed solution is much lighter and effective than the conventional indirect voltage control (reactive power control) and more robust than on load tap changer transformer.

Secure High DER Penetration Power Distribution via Autonomously Coordinated Volt/VAR Control

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.

A Voltage Smoothing Algorithm Using Energy Storage PQ Control in PV-Integrated Power Grid

Changes in solar irradiance cause variations in photovoltaic (PV) power generation and thus affect customer voltage. Load tap changers in substations are used to mitigate voltage variations, but they are relatively slow and cannot regulate the voltage adequately. Furthermore, abrupt voltage changes induced by PVs could greatly cause mechanical wear and reduce their lifetime. This letter develops a novel voltage smoothing control algorithm for distributed energy storage systems to reduce the impact of PV generation on voltage quality. Different from other works, the proposed control can improve the performance by having both active and reactive power control and monitoring voltage directly. The technique's performance is verified through simulation and modeling of the IEEE 4-bus test system. The proposed technique can effectively smoothen the voltage and reduce the number of tap operations by approximately 50%.