On-load Tap Changer Operation Research Articles

Optimal Sliding Speed and Contact Pressure Design of On-Load Tap Changer Based on Multivariate Nonlinear Regression

During the voltage regulation of on-load tap changers (OLTCs), the movement of the contacts can easily cause arcing, which may lead to erosion or malfunction. To reduce the energy and probability of arcing, we focus on designing an optimal range for the sliding speed and contact pressure of the contacts to minimize arc energy. Initially, our research introduces a novel OLTC arc testing platform to simulate the motion of static and dynamic contacts, exploring the relationship between different sliding speeds, contact pressures, and factors like arc voltage waveform, arcing rate, arc resistance, and arc energy. Subsequently, by employing multiple nonlinear regression methods, we establish functional relationships between sliding speed and arc energy, as well as contact pressure and arc energy, evaluating the fit using correlation coefficients. Finally, through analyzing their nonlinear behaviors, we determine the ideal sliding speed and contact pressure. The results indicate that when the OLTC contacts slide at an optimal speed between 89 and 103 mm/s and optimal contact pressure between 1.5 and 1.7 N, the arc energy can be minimized, thereby enhancing the performance and lifespan of the on-load tap changer. This study offers feasible insights for the design and operation of OLTCs, aiding in the improvement of power system regulation.

Impact of OLTC equipped transformer operation on PV installation in urban distribution network

Presented paper addresses the problem of proper large-scale integration of photovoltaic (PV) systems in urban area distribution network (DN). A methodology for minimization of annual energy losses in DN by optimal placement of PV systems, which simultaneously considers rooftop PV potential and time dependent network operation, is presented. Optimal reactive power generation of PV systems and optimal setting of on-load tap changer (OLTC) equipped transformer in secondary substation are determined as well. Furthermore, proposed methodology enables evaluation of the impact of OLTC equipped transformer on the share of PV installation that urban DN can accommodate. Proposed methodology is based on the optimization tool called differential evolution, with the objective function set to minimize network’s annual energy losses, while preventing the voltage violations and thermal overloading of lines. Results of the case study performed on a real urban low voltage distribution network, with consideration of different scenarios of OLTC operation are presented. The results indicate that simultaneous consideration of chosen variables yields greater benefits to DN operation in terms of reduction of annual energy losses and greater PV penetration.

Multi-Objective-Based Charging and Discharging Coordination of Plug-in Electric Vehicle Integrating Capacitor and OLTC

The integration of plug-in electric vehicles (PEVs) in residential distribution networks demands a significant amount of electrical load where random and uncoordinated charging affects the quality and performance of the distribution network. Random and uncoordinated charging may increase the peak demand and can increase stress on critical network assets such as line, transformer, and switching devices. Moreover, the charging of PEVs in a low network reduces the voltage of the system below the lower limit. On the other hand, using PEVs as storage in the V2G mode can improve the network condition. Therefore, it is critical to properly manage the charging and discharging operation of PEVs. This paper proposes a multi-objective-based charging and discharging coordination of PEVs with the operation of the capacitor and on-load tap changer (OLTC). With the proposed strategy, the distribution network is operated safely, and charging is ensured for all PEVs connected to the network. The main consideration of this research is to reduce the daily power loss, operational cost, and voltage deviation of the system. The metaheuristic optimization binary firefly algorithm (BFA) has been applied to coordinate PEV charging and discharging as well as capacitor and OLTC operation in the system. A modified IEEE 31 bus 23 kV distribution system is used to implement the proposed strategy. From the obtained results, it is found that the combined PEV charging and discharging coordination with capacitor and OLTC operation reduces the power loss and cost by 34.16% and 12.68%, respectively, with respect to uncoordinated charging and enhances the voltage condition of the network.

Analysis and Simulation Study on a Fault of Service Duty Test of Vacuum On-load Tap Changer

Aiming at a fault on service duty test of vacuum on-load tap changer (OLTC), the principle of the resonance test method is analyzed. Combined with the voltage and current waveform measured and the standard switching sequence, the cause and process of the test fault are qualitatively deduced. Based on the theoretical analysis, the simulation model of the test circuit and fault is built based on PSCAD. The voltage and current waveforms obtained by simulation are highly consistent with the test waveforms. The error of peak current between the simulation and the test waveform is 0.75%. The simulation results show the accuracy of the theoretical analysis and modeling calculation, they are verified by the follow-up inspection results of OLTC and vacuum tube solution. This paper provides a powerful technical means for fault analysis and fault mechanism research in the follow-up OLTC operation. It also has reference and guiding significance for the quality control, design, operation and maintenance, and monitoring of vacuum OLTC.

Decentralized Control of OLTC and PV Inverters for Voltage Regulation in Radial Distribution Networks With High PV Penetration

The voltage rise problem due to the reverse power flow is one of the main obstacles to expanding the photovoltaic systems (PVSs) in distribution networks. In this paper, a decentralized control method for coordinated control of on-load tap changer (OLTC) transformers and PV inverters, is proposed for the voltage regulation of radial distribution networks. In this method, the allowable voltage range is divided into several zones, and appropriate corrective actions are adaptively taken according to each zone. To reduce the tension on the OLTC, first, the reactive power capability of the PV inverters is employed for the voltage regulation, and if it is insufficient, OLTC is used as the last solution. In this regard, the unused capacity of PV inverters is utilized for the reactive power compensation, considering the grid codes reactive power requirements. Furthermore, decentralized control based on a signaling method is used for coordination between OLTC and PVSs, which eliminates the need for communication links. Finally, the effectiveness of the proposed method is validated by the simulation results for the IEEE 33-bus distribution network, and the impact of different reactive power control methods on the number of OLTC operation is investigated.

Operation Method of On-Load Tap Changer on Main Transformer Considering Reverse Power Flow in Distribution System Connected with High Penetration on Photovoltaic System

The increasing use of photovoltaics (PVs) in distribution systems owing to the low-carbon policy has given rise to the need for various technological changes. In particular, the operation of on-load tap changers (OLTCs) has attracted attention. In traditional distribution systems, the OLTC operates via a line-drop compensator (LDC), which focuses on the load to solve the low-voltage problem; however, the problem of over-voltage caused by PVs persists. Currently, a method for operating an OLTC using the measured voltage is being researched; however, solving the voltage problem for several feeders connected to a main transformer (MT) is not viable. Therefore, this study proposes an OLTC operation method to address the feeder with the largest voltage problem depending on the direction of power flow. The proposed method selects a point where the OLTC operates using the difference between the measured and reference voltages. Setting the reference voltage can solve the problem that occurs due to the direction of power flow. Finally, the effectiveness of the proposed method is verified via case studies. Based on the results, we can conclude that the proposed method effectively solves the voltage problem, and an increase in hosting capacity can be expected.

고밀도 태양광 연계 배전 계통에서의 OLTC 동작 횟수 저감을 위한 협조 전압 제어

This paper proposed coordinated voltage control between Photovoltaics (PVs) and on load tap changer (OLTC) for reducing the number of operation of OLTC based on PV and load forecasting. Nowadays, PV compensates the reactive power for improving voltage problems such as voltage violation and voltage fluctuation due to the high PV penetration in distribution system. Therefore, the direction of current of system is different from the conventional direction. Due to this reason, there is a probability that the number of operation of OLTC using line drop compensator method has increased than conventional system. This problem causes the malfunction of OLTC and financial burden for system operator. Also reactive power compensation of PV increases the line loss of the system which can also increase the financial burden of system operator. To solve these problems, proposed coordinated control which is realized by optimization algorithm for line loss reduction with constraints of OLTC for reducing the number of operation of OLTC based on forecasted data is proposed.

A Substation-Based Optimal Photovoltaic Generation System Placement Considering Multiple Evaluation Indices

The placement of the photovoltaic generation system (PVGS) and operation of the on-load tap changer (OLTC) should have great impacts on the system loss and voltage quality, which are the main concerns of the distribution operator. Considering these multiple evaluation indices and other constraints, this paper proposed a substation-based optimal PVGS placement and OLTC operation model. The objective function that was used to evaluate the optimal PVGS placement and OLTC operation consists of a minimization of system loss and voltage quality. The model’s constraints contain the voltage and line flow limits, voltage deviations, voltage unbalance, etc. Uncertainties of the load and irradiance are also included in the model. A nondominated sorting genetic algorithm II (NSGA II) is used to solve this multi-objective optimization problem. Comparisons of the substation-based and feeder-based planning are also studied in this paper. The test results demonstrate the substation-based planning could obtain a better solution.

Voltage control in low voltage grids with independent operation of on-load tap changer and distributed photovoltaic inverters

This paper aims to find the optimal setups of voltage control devices in different configurations of Low Voltage (LV) grids with strong PhotoVoltaic (PV) diffusion by performing dedicated simulations. Distributed PV inverters and On-Load Tap Changer (OLTC) are simulated without considering their coordination, to avoid large investments in new communication infrastructures. Thus, each device independently works to decrease voltage deviations in the respective grid connection point. PV generation and consumption profiles are measured and used in two simulated LV grids, connected to the Medium Voltage (MV) grid by a MV/LV transformer with rated powers of 400 and 250 kVA, respectively. The calculation of the optimal devices setups is addressed as a multi-objective problem, considering objectives of voltage quality, grid losses, and OLTC lifespan increase. Multiple simulations are performed by varying the setup of the voltage controls, and considering different positioning and sizes of the generators. In the hardest case, the ratio between the maximum PV power generation and the maximum load in the whole grid is ≈70%. Pareto analysis is carried out to find the non-dominated solutions and TOPSIS is applied to rank the solutions. Finally, a sensitivity analysis is performed by changing the weights attributed to each objective function.

Voltage fluctuation mitigation with coordinated OLTC and energy storage control in high PV penetrating distribution network

The surge in the growth of renewable energy resources integration such as solar photovoltaic (PV), wind energy, and tidal energy, etc., is introducing fast fluctuating voltage challenges to the power grid. A battery energy storage system (BESS) can suppress voltage fluctuations up to certain limits that are introduced by intermittency in solar photovoltaic. Although battery energy storage system (BESS) corresponds to faster responses, the lack of coordination between conventional voltage fluctuation mitigation strategies such as On-load tap changer (OLTC) and BESS might lead to an overuse of BESS and underuse of OLTC. This research presents an intelligent and real-time coordinated-control of BESS and OLTC that mitigates the voltage fluctuations, enhances the life of BESS and optimizes the use of existing OLTCs. In the proposed control technique the BESS controls the bus voltage at local level and OLTC controls the voltage at global level. The devised scheme monitors the real-time bus voltages using phasor measurement units (PMU). The scheme assigns respective weights to individual buses that decide the optimal operation of OLTC and BESS. In control of OLTC all the buses are assigned some weights which decide the optimal operation of OLTC and BESS. The proposed scheme is demonstrated on IEEE-13 bus system and results are being compared with the conventional scheme. The proposed control scheme surpasses the voltage regulation by reducing the average voltage violation time to one-fifth compared to conventional control scheme and significant reduction in BESS energy cycle.

Smart transformer-based medium voltage grid support by means of active power control

In the last decades the voltage regulation has been challenged by the increase of power variability in the electric grid, due to the spread of non-dispatchable generation sources. This paper introduces a Smart Transformer (ST)-based Medium Voltage (MV) grid support by means of active power control in the ST-fed Low Voltage (LV) grid. The aim of the proposed strategy is to improve the voltage profile in MV grids before the operation of On-Load Tap Changer in the primary substation transformer, which needs tens of seconds. This is realized through reactive power injection by the AC/DC MV converter and simultaneous decrease of the active power consumption of voltage-dependent loads in ST-fed LV grid, controlling the ST output voltage. The last feature has two main effects: the first is to reduce the active power withdrawn from MV grid, and consequently the MV voltage drop caused by the active current component. At the same time, higher reactive power injection capability in the MV converter is unlocked, due to the lower active power demand. As result, the ST increases the voltage support in MV grid. The analysis and simulation results carried out in this paper show improvements compared to similar solutions, i.e. the only reactive power compensation. The impact of the proposed solution has been finally evaluated under different voltage-dependence of the loads in the LV grid.

Dynamics of high penetration photovoltaic systems in distribution circuits with legacy voltage regulation devices

Concerns for criteria pollutant emissions and air quality, as well as greenhouse gas emissions, have resulted in stringent energy and environmental rules and regulations mandating deployment of renewable resources. Moreover, increased frequency of extreme weather events highlighted the need for increased reliability and resiliency of the grid further encouraging deployment of distributed energy resources. This has resulted in an increasing deployment of solar PV in the distribution system. While many studies focus on analyzing the impact of increased penetration of PV on smart circuits or systems with upgraded infrastructure, it is necessary to assess impacts on existing circuits with legacy devices. In this study, the impacts of high penetration of PV are investigated for two distribution circuits (one residential and one commercial) located in northern California, equipped with legacy voltage regulation devices including on-load tap changer (OLTC) transformers and switched capacitor banks.The dynamics of circuit operation and bus voltage profiles are studied by developing time-resolved three-phase balanced feeder models of the two circuits. Using connectivity and load demand measurements provided by the utility, the models are validated and calibrated to the point of the substation. In order to help generalize the results associated with the two circuits, a set of circuit design metrics are developed to characterize the distribution of the load demand and generation, load center (LC) and generation center (GC). The load center and generation center metrics are weighted averages of the total consumed and total produced bus energy relative to the distance from the substation.Various scenarios are simulated in order to quantify generation limits necessary to conform to various standards including ANSI C84.1 voltage standards while maintaining reliable circuit operations. In these scenarios, location of the PV installation as well as PV penetration (from baseline to 100%) are varied. Simulation results indicate that high PV penetrations have a direct impact on OLTC transformer operation but only marginal impact on capacitor switching. Tap operation is found to increase approximately linearly with PV penetration for both of the circuits and specific Load Drop Compensation (LDC) controller settings greatly affect the sensitivity of OLTC operation to PV penetration. It is also concluded that not only the location of PV installation relative to loads and substation affects the performance, the metrics developed are a powerful tool to describe high PV circuits and to help utilities better plan for deployment of PV in their distribution network.

Exploiting OLTC and BESS Operation Coordinated with Active Network Management in LV Networks

The large number of small scale Distributed Energy Resources (DER) such as Electric Vehicles (EVs), rooftop photovoltaic installations and Battery Energy Storage Systems (BESS), installed along distribution networks, poses several challenges related to power quality, efficiency, and reliability. Concurrently, the connection of DER may provide substantial flexibility to the operation of distribution grids and market players such as aggregators. This paper proposes an optimization framework for the energy management and scheduling of operation for Low Voltage (LV) networks assuring both admissible voltage magnitudes and minimized line congestion and voltage unbalances. The proposed tool allows the utilization and coordination of On-Load Tap Changer (OLTC) distribution transformers, BESS, and flexibilities provided by DER. The methodology is framed with a multi-objective three phase unbalanced multi-period AC Optimal Power Flow (MACOPF) solved as a nonlinear optimization problem. The performance of the resulting control scheme is validated on a LV distribution network through multiple case scenarios with high microgeneration and EV integration. The usefulness of the proposed scheme is additionally demonstrated by deriving the most efficient placement and sizing BESS solution based on yearly synthetic load and generation data-set. A techno-economical analysis is also conducted to identify optimal coordination among assets and DER for several objectives.

Service restoration of active distribution systems with increasing penetration of renewable distributed generation

The increasing penetration of renewable generation poses several challenges to the operation of distribution networks under emergency conditions. This study proposes a service restoration method that exploits active management of the distribution network. The proposed active network management (ANM) method considers the coordinated control of the available switches, distributed generation, and the operation of on-load tap changer (OLTC) to determine the optimal service restoration plan. The objectives of the proposed model are the minimisation of (i) the out of service area considering customer priorities, (ii) the number of switch operations, (iii) the number of tap changes of the OLTC, and (iv) the injected power from the substations considering the variation of load demand and renewable generation. The service restoration problem is formulated as a mixed-integer second-order cone programming problem, which is efficiently solved by commercial branch and bound solvers. Results on a 135-bus distribution system and a 540-bus distribution system highlight the importance and the benefits of incorporating ANM into the solution of the service restoration problem.

Coordinated Voltage Regulation by On-Load Tap Changer Operation and Demand Response Based on Voltage Ranking Search Algorithm

The growing penetration of photovoltaic (PV) systems may cause an over-voltage problem in power distribution systems. Meanwhile, charging of massive electric vehicles may cause an under-voltage problem. The over- and under-voltage problems make the voltage regulation become more challenging in future power distribution systems. Due to the development of smart grid and demand response, flexible resources such as PV inverters and controllable loads can be utilized for voltage regulation in distribution systems. However, the voltage regulation needs to calculate the nonlinear power flow; as a result, utilizing flexible resources for voltage regulation is a nonlinear scheduling problem requiring heavy computational resources. This study proposes an intelligent search algorithm called voltage ranking search algorithm (VRSA) to solve the optimization of flexible resource scheduling for voltage regulation. The VRSA is built based on the features of radial power distribution systems. A numerical simulation test is carried out on typical power distribution systems. The VRSA is compared with the genetic algorithm and voltage sensitivity method. The results show that the VRSA has the best optimization effect among the three algorithms. By utilizing flexible resources through demand response, the tap operation times of on-load tap changers can be reduced.

Failure Detection of On-Load Tap-Changers Using a New Power-Based Algorithm

This paper presents a novel arcing power-based algorithm to identify failures in operations of on-load tap-changers (OLTCs). An OLTC is the solely moving part of power transformers and thus highly prone to failure. Most of the OLTC defects are originated from abnormal arcing times leading to abnormal arcing energies. The proposed algorithm utilizes the power difference between the input and output terminals of transformer to estimate the OLTC arcing power. The integration of arcing power for a tap-changing operation results in an arcing energy. This arcing energy is estimated for each operation and used as an indication of OLTC failure. By this definition, whenever the estimated arcing energy exceeds a predefined value for a tap-changing event, it shows a failure in OLTC operation. The outcomes obtained from extensive simulation studies for different internal failures and external faults prove that any malfunction or failure in the operation of OLTC can be detected by the proposed method in a timely manner.

A new arc-based model and condition monitoring algorithm for on-load tap-changers

An on-load tap-changer (OLTC) regulates the output voltage level of transformer by changing the winding voltage ratio without current interruption. It is one of the most expensive and vulnerable parts of power transformers. Therefore, modeling and condition monitoring (CM) of OLTCs are important for the power system operation. This paper presents a new modeling principle for OLTCs based on the circuit breaker (CB) arc models. There is a sequence of switching events with certain timings during each operation of OLTC. These switching operations are modeled individually and a full model of OLTC operation is derived. Computer simulation studies and the measurements captured from the real-time test studies of OLTC operation verify and validate the performance of the proposed OLTC model.On the other hand, the instantaneous differential power of tap-changing transformer measured from its input and output terminals is used to calculate the arcing power and energy associated with OLTC operation. The cumulative arcing energy is a good indication of the electrical wear of OLTC contacts. Therefore, it is used to propose a novel CM algorithm for OLTCs. The proposed method considers the effects of both arcing voltage and current signals to accurately estimate the arcing energy. The results obtained from computer simulation studies demonstrate that the proposed algorithm accurately evaluates the electrical wear of OLTC contacts and determines the inspection or maintenance schedules of OLTC contacts.

Multi-Timescale Model Predictive Control of Battery Energy Storage System Using Conic Relaxation in Smart Distribution Grids

This paper proposes a multi-timescale volt/var optimization for the optimal dispatch of battery energy storage system in smart distribution grids. It aims to coordinate the substation on-load tap changer operation on slow-timescale (hourly basis) with the photovoltaic inverters and battery storage operations on fast-timescale (15 min basis). This coordination is achieved by using two-stage stochastic programming and implemented via model predictive control. The power loss and energy purchase cost are reduced while maintaining voltages within limits. The forecasting uncertainties are modeled by generating a large number of random scenarios and then subsequently reducing scenario numbers to establish a tradeoff between computational burden and accuracy of the solution. The mixed-integer second-order cone program (MISOCP) is formulated with reduced scenarios, which achieves global optimum. Simulation results demonstrate the effectiveness of proposed MISOCP model in keeping the voltages within limits under forecasting uncertainties.

On Savitzky–Golay Filtering for Online Condition Monitoring of Transformer On-Load Tap Changer

Vibro-acoustic measurement on a transformer's on-load tap changer (OLTC) can provide indications on its mechanical condition. Recently, a joint vibro-acoustic and arcing measurement system has been proposed, which can correlate the vibro-acoustic signal to mechanical events of OLTC's operation. However, there are still considerable difficulties in extracting useful information from both vibro-acoustic signal and arcing signal in a synchronized manner without any distortions in the extracted signals. In this paper, Savitzky–Golay filter is introduced to process the signals acquired from a joint vibro-acoustic and arcing measurement system installed on in-service OLTCs. It proves that the Savitzky–Golay filter can process both vibro-acoustic and arcing signals induced by OLTC, extract essential information without any time delay from both types of signals, and retrieve voltage phase information from the arcing signal. The methodologies developed in this paper can improve the visibility of OLTC's mechanical operation for an effective online condition monitoring.

Управление устройством РПН сетевого трансформатора с учетом режимов работы электросталеплавильного комплекса

The paper presents the main results of developing and researching a new principle for controlling the step-down transformer on-load tap changer (OLTC), which coordinates the OLTC operation with the electrical parameters of the electric-arc steelmaker, compensators installed in the high-power and ultrahigh-power EAF power supplies, and the mains. The proposed principle consists in pre-determining the operating voltage ranges on the mains-transformer primary side for all OLTC tap positions taking into account the existing of both the electric-arc steelmaking facility and the mains. The static VAR compensator (SVC) control system is improved by adding a block for finding the mains-transformer OLTC tap position number so as to use new position-switching algorithms. The problem of optimizing the OLTC tap-position switching in mains-transformers supplying power to electric-arc steelmaking facilities under significant mains-voltage deviations is analyzed on the basis of evidence from three Russian steelworks companies. We herein discuss the particularities selecting the mains-transformer OLTC positions in two SVC control modes, one consisting in keeping zero consumption of reactive power from the mains, and the other one consisting in maintaining a coordinate voltage in the common EAF and SVC connection point. The effect of the proposed solutions is found from the results obtained by means of mathematical modeling of electric-arc steelmaking facilities in MATLAB Simulink. We discuss the options for further improvement of principles for coordinating the operation of automatic transformer-voltage regulators (AVR) with the parameters of steelmakers, compensators, and mains.