Load Tap Changer Research Articles

Anti‐icing measures in the Italian power system

Despite the good operating results of passive anti-icing devices installed by the Italian transmission system operator (TSO) on high-voltage overhead lines (OHLs), the increasing number and consequences of wet snow events have prompted consideration of other anti-icing measures. In particular, attention focuses on the thermal approach, which is intended to achieve – before, or in the early stage of wet snow events – the anti-icing current in OHL phase conductors where snow sleeve formation is foreseen. The authors carried out a specific anti-icing study with reference to a 132 kV critical path in an area of the Apennines, with simulation results illustrated in this study. Anti-icing measures investigated were (i) load shifting actions and (ii) insertion of shunt reactors used as ballast loads. The results show that shunt reactors are the most effective anti-icing measure. The main disadvantage of this innovative solution is the degradation of voltage regulation in the area. However, this downside can be limited by combining the insertion of reactors with control of autotransformers under load tap changers. Similar results are obtained from analyses of other critical areas. As a result of these findings, the Italian TSO is now undertaking the installation of the first ballast reactors.

Distributed coordination control strategy for multiple residential solar PV systems in distribution networks

Distributed rooftop PV generator systems have been increasing significantly in distribution networks. However, due to the intermittent nature of the PV power generation, it is challenging to handle the fast voltage variation of the distribution system through dispatching traditional low response speed devices such as load tap changers and capacitor banks. In this paper, a distributed hierarchical control strategy is proposed to deal with the voltage fluctuation issues through real-time regulating the injection or consumption reactive power of the fast response PV inverters. The proposed control strategy includes a primary droop control level and an agent-based distributed secondary control level. The droop-based primary level control can quickly regulate the voltage of PV inverter with locally measured information to overcome the fast voltage variation. The agent-based secondary level control can guarantee the average bus voltage restoration and proportionally reactive power sharing among the PV inverters through distributed communication between neighboring agents. With the proposed control strategy, both the average bus voltage restoration and proportionally reactive power sharing can be satisfied, which improves the stability of the distribution system. Furthermore, the proposed strategy is a fully distributed method that can distribute the computational and communication tasks among the local controllers through working in parallel, which is more flexible, scalable, and insusceptible to single-point failure.

Assessment of the consumers’ contribution to the deterioration of the electrical power quality

The article presents the rationale for the further development of the methods that uniquely identify consumers making a major contribution to the deterioration of the quality of electricity in power supply systems of enterprises. A new method that gives an unambiguous result is proposed. This feature is a significant difference from the known methods. The given statements are confirmed by the results of computer simulation that are presented in the article. The method is based on the principle of changing the input resistance of the system implemented by varying the impedance of the transformer windings. An on load tap changer can be used for this purpose. The prospects for applying the method to determine the contribution of individual consumers to the changes in the quality of electric power of the enterprises supply systems in terms of percentage are shown.

Enhancement of genetic algorithm optimization on distribution operation control by fuzzy inclusion

The optimal control of distribution system operation is presented in this paper. The objective of the control is minimum real power loss simultaneously achieved with voltage profile improvement. The distribution system equipped with Load Tap Changer (LTC) and switchable Shunt Capacitors is optimally controlled by determining the tap position of LTC and switching status of Shunt Capacitor at every hour for the 24-hour period. Two Genetic Algorithms (GAs) are developed for load curve partition and hourly components status determination. The performance of GA for components optimal scheduling is further enhanced by integrating Fuzzy Strategies in fitness evaluation and constraints satisfaction. The scheme of Fuzzy Convex Decision Making (FCDM) that provides flexible objective achievement and soft constraints is employed. This enables considering more extensive solutions that may lead to better optimal control. The proposed control strategy is implemented on 30-bus distribution system and the system improvements are observed. The enhancement of fuzzy integration is highlighted.

Hierarchical Hybrid Architecture for Volt/Var Control of Power Distribution Grids

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.

Impact of low-carbon technologies on the power distribution network

Over recent decades, there has been a growing concern about how modern technologies would impact on an electrical grid. Whilst the UK is set to meet the target of 15% energy demand from renewable sources by 2020 to ensure energy security and decarbonisation objectives, there has been a greater concern over how the increasing deployment of low carbon technologies (LCTs) would affect an electrical grid, in particular the residential low voltage distribution network. While LCTs can provide clean energy and decreases the dependence on oil and natural gas stocks, their impact on distribution networks is unknown and the operators are visibly blind as they use a ‘fit and forget’ approach. Consumers’ connections are uncertain and stochastic and the LCT uptake poses a potential threat for distribution network operators. A review is presented in this paper, illustrating the potential impacts the LCTs have on a local electrical grid such as voltage regulation, thermal limits, power quality and harmonics. Potential solutions, such as Deregulation, Soft Open Points, On Load Tap Changer, and Active Power Filter currently available for Distribution Network Operators and power system planners before costly network reinforcement work is undertaken.

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..

Innovative Volt/VAr Control Philosophy for Future Distribution Systems Embedded With Voltage-Regulating Devices and Distributed Renewable Energy Resources

Conventional voltage-regulating devices such as load tap changers, voltage regulators, and local capacitor banks are commonly used for voltage-regulation purposes in medium-voltage distribution systems. Although such devices exhibit excellent capability for Volt/VAr control (VVC) support, fine-tuning of their controller parameters and enactment of Volt/VAr support by renewable distributed generation (DG) units is essential for effective and efficient operation of future distribution systems embedded with wind and solar-Photo-voltaic (PV) units. In this paper, an innovative algorithm is conceptualized for control set-point re-adjustment of different VVC devices for coordinated voltage control in distribution systems. The intended VVC strategy is able to minimize the voltage variations including voltage drop and rise cases mainly caused by power output of renewable DG units that cannot be solved by voltage regulating devices due to their time-delayed operation. Moreover, the control algorithm is proposed to be implemented online using advanced distribution management system for effective voltage control. The simulation studies are carried out using the test distribution system derived from an electricity network in New South Wales, Australia. The simulation results show that voltage regulation can effectively be achieved in renewable rich future distribution systems by applying the proposed tuning algorithm, which has been designed based on a new VVC philosophy.

Efficient Distribution System Optimal Power Flow With Discrete Control of Load Tap Changers

Load tap changers (LTCs) are commonly used for voltage control in distribution systems. The mechanical relays on LTCs are controlled in discrete steps; hence, distribution optimal power flow (DOPF) formulations require LTCs to be modeled with integer variables. Integer variables are generally ignored or relaxed and rounded off to reduce the computational burden in DOPF models. However, in this paper, we highlight and overcome complex infeasibility issues caused by rounding methods. Moreover, recent advances in convex optimization have improved the computational tractability of the DOPF problem. In this paper, we develop and analyze numerical efficiency of different but exact formulations for incorporating LTCs as integer control assets in a relaxed second-order cone program (SOCP) version of the DOPF model. The resulting formulation becomes a mixed-integer SOCP (MISOCP), which is computationally tractable compared to mixed-integer non-linear program (MINLP). To recover the exact optimal solution in the proposed MISOCP-DOPF model, a McCormick relaxation is employed within a sequential bound-tightening algorithm. We show that solutions obtained from MISOCP-DOPF are always ac feasible. Thus, the MISOCP-DOPF yields optimal and realistic ac solutions, and is validated in large distribution feeders and is compared to MINLP counterpart.

Classification of Fault and Failure Types Determined by Dissolved Gas Analysis for Transformers

In this study, the locations of fault and failure that can be visually identified during internal inspection are determined by classifying the fault and failure types by dissolved gas analysis (DGA). The fault types are classified by analyzing 195 internal inspected transformers that faults were identified, and the failure types are classified by analyzing 338 failed transformers of Korea Electric Power Corporation. The fault types that affected DGA in oil are classified as winding, core, clamp, bushing, on load tap changer (OLTC), and oil. The faults are classified into 18 types according to subcomponent and fault cause. The failure types that affected DGA in oil are classified as winding, bushing, and OLTC. The failures are classified into 21 types according to subcomponent and failure cause. The failure results are classified as turn-to-turn breakdown, breakdown, and oil leakage. The phenomena caused by faults and failures that can be identified by visual during internal inspection are classified as carbon, discharge, and overheat traces.

Secondary voltage control applied to DFIG-based wind park and its effect on long-term voltage stability

This paper investigates the use of secondary voltage control (SVC) in a wind park based on doubly fed induction generator (DFIG) and its effect on long-term voltage stability. The performance of the SVC applied to wind park is compared with the case when only the primary voltage control (PVC) is adopted. A detailed analysis is conducted with time-domain simulations, considering high and low wind speed regimes, control variable limits of wind generators, static and dynamic loads, and dynamic models of overexcitation limiter (OEL) and load tap changing (LTC) transformer. Based on the results, the use of secondary voltage control in a DFIG-based wind park can postpone long-term voltage instability. Further, some interesting findings were obtained showing that SVC can lead the grid-side converter (GSC) of DFIG to absorb a significant amount of reactive power and lose the capability of injecting additional reactive power. To address this problem, two novel auxiliary control strategies are presented, and the results indicate the effectiveness of the proposed strategies in postponing voltage instability.

Export Transformer Switching Transient Mitigation in HVDC Connected Offshore Wind Farms

An increasing number of offshore wind farms are connected to the public grid with large-scale HVDC links. The electrical transients caused by the energization of the export transformers appear more significant in the converter-driven offshore AC grid with low short-circuit power, than in direct AC grid connections. This may lead to a violation of the 2% voltage dip criterion, thus seriously affecting the production of neighboring wind farms. Additionally, the transformer and the entire insulation system are exposed to mechanical and dielectric stress. This paper investigates the influence of three factors, namely excitation, switching angle, and remnant core magnetization, onto the inrush current. Electromagnetic transient simulations and field measurements show that with a smart use of the on load tap changer, point on wave switching, and a demagnetization of the core by a cable-shunt swing circuit, the three factors above can be used as degrees-of-freedom to effectively mitigate the inrush current phenomenon.

A Novel Dynamic Index of Voltage Instability Expectation with Power System Contingency

This paper proposes a novel dynamic voltage instability expectation index which takes into account dynamic voltage stability characteristics, the long-term phenomena and probability state of power system contingency. This index is a meter to measure the general robust power system under critical cases which are identified by contingency analysis. The critical contingencies are precisely analyzed by steady-state power load flow in perturbations to figure out violation cases, the weakness of the power system. The probability states of each of violation cases are calculated through availability and unavailability values of devices. The degree of voltage instability is estimated newly by the smallest determinant of dynamic load flow Jacobian matrix in evolution time of power system simulation. The variables of dynamic devices such as under load tap changer of the transformer, excitation limiters of generator unit, affected on a long-term characteristic of the power system, are considered in dynamic load flow Jacobian matrix. Combining the voltage instability rate and the probability state, a novel index, namely voltage instability expectation in dynamic index, is proposed. This index reflects the mutual correlation between the voltage instability rate and probable contingency cases which result in either voltage violation, overload flow, or voltage instability. The new index is demonstrated in IEE 9-bus test system and actual Vietnam power system. It should be a new measure to make a decision of transmission system expansion planning.

Optimal Partitioning of Smart Distribution Systems Into Supply-Sufficient Microgrids

This paper presents a systematic procedure for partitioning smart distribution systems into supply-sufficient microgrids. First, renewable distributed generations (DGs) are optimally allocated in the distribution system. A multiobjective performance index including voltage profile and energy losses indices is utilized in this problem as the objective function. Two alternative control approaches of future smart grids, including on load tap changer control and adaptive power factor control are assessed to maximize potential benefits and increase the penetration level of DGs. Then, optimal allocation of protection devices and energy storage systems for constructing supply-sufficient microgrids is presented for a feeder equipped with capacity-constrained DGs. To this end, two different optimization problems are formulated and proper indices are developed for minimizing power exchange between microgrids and minimizing generation-load imbalance within microgrids. Finally, test results of the proposed models on 33-bus IEEE radial distribution system are presented and discussed.

Distributed voltage regulation of smart distribution networks: Consensus-based information synchronization and distributed model predictive control scheme

This paper proposes a distributed voltage control (DVC) scheme for smart distribution networks with high penetration of inverter-based distributed generators (DGs), aiming to optimally coordinate DG units and on load tap changer (OLTC) transformer to regulate the voltages within the feasible range. The proposed scheme consists of two important parts: (1) distributed information synchronization (DIS) framework and (2) distributed model predictive control (DMPC)-based voltage control scheme. The DIS framework is established based on the consensus protocols to synchronize the specific information about the critical bus voltages and potential OLTC actions. The DMPC-based voltage control scheme is presented, in which each DG unit only exchanges information with its immediate neighbors and solves the local optimal control problem. Two control modes are designed to better deal with different operating conditions. In the normal mode, only the reactive power outputs of DG units are optimized to mitigate the voltage deviations. In the corrective mode, both of the active and reactive power outputs of DG units are optimally controlled to correct the severe voltage deviations. To mitigate the mutual interaction between the DGs and OLTC, the potential actions of OLTC are predicted and considered in the optimization problem of each units. The control performance of the proposed scheme was demonstrated using a real medium-voltage (MV) distribution network with two feeders under both normal and large-disturbance conditions.

Real‐time spatio‐temporal trend and level (T&L) filtering scheme for early detection of voltage instability

Discovering early signatures of voltage instability can enhance situational awareness and facilitate emergency control. Such signatures usually appear as decaying trends in load voltages and increase in reactive power output of generators. Discrete control actions such as frequent switching of load tap changer (LTC) and capacitor banks, that are otherwise unmonitored, can also be crucial indicators of a voltage problem. Hence, the authors propose a unified early warning scheme (EWS) for detecting voltage instability. To quantify and correlate spatial, temporal (spatio-temporal) trends and level changes, the authors use a multivariate, adaptive trend and level (T&L) filter. A unique feature of the proposed scheme is the signature authentication, by verifying simultaneity in T&L changes across multivariate time series. The authors show that, by extrapolating trends, limit violations in bus voltages and generator reactive power output can be estimated well in advance. Further, the level-change detection logic is shown to have the ability to detect frequent LTC-actions directly from the bus voltage time series. While the T&L filtering scheme is validated on field phasor measurement unit (PMU) datasets, EWS and emergency control is demonstrated on the classical 10-bus voltage stability test system.

Optimal Tap Setting of Voltage Regulation Transformers Using Batch Reinforcement Learning

In this paper, we address the problem of setting the tap positions of load tap changers (LTCs) for voltage regulation in radial power distribution systems under uncertain load dynamics. The objective is to find a policy to determine the tap positions that only uses measurements of voltage magnitudes and topology information so as to minimize the voltage deviation across the system. We formulate this problem as a Markov decision process (MDP), and propose a batch reinforcement learning (RL) algorithm to solve it. By taking advantage of a linearized power flow model, we propose an effective 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. To circumvent the "curse of dimensionality" resulted 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. The effectiveness of the proposed algorithm is validated via numerical simulations on the IEEE 13-bus and 123-bus distribution test feeders.

A Distributionally Robust Reactive Power Optimization Model for Active Distribution Network Considering Reactive Power Support of DG and Switch Reconfiguration

Abstract In recent years, the rapid development of the Distributed Generation (DG) has brought new challenges to the reactive power optimization in the active distribution network. The traditional stochastic optimization and robust optimization methods may result in over-conservative or over-risky decisions when dealing with the uncertainty of renewable energies. Therefore, this paper proposes a distributionally robust reactive power optimization model for active distribution network, which regards the network loss as the its optimization target, and considers the reactive power support of and the switch reconfiguration. This model includes not only switching, on load tap changer (OLTC), discrete and continuous reactive power compensation devices usually contained in traditional reactive power optimization, but also the detailed model of capacity curve of Double-Fed Induction Generator and Micro-Gas Turbine. After the reactive power support is integrated, the linearization method of active and reactive power output coupling characteristics constraint is specially designed. Then, based on the typical scenario data of wind and photovoltaic power, and the regulation characteristics for different decision variables, a data-driven based two-stage distributionally robust reactive power optimization model is set up, in which the uncertainty probability distribution confidence set is simultaneously constrained by 1-norm and ∞-norm. Finally, by the Columns and Constraints Generation algorithm, the IEEE 33-bus system is applied to verify the effectiveness of proposed.

Effect of stochasticity on voltage stability support provided by wind farms: Application to the Hellenic interconnected system

This paper focuses on the effect of wind farm stochastic power variability on the voltage stability support to a real power system. An emergency maximum reactive support (EMRS) control strategy is considered, with the help of which wind farms can contribute in increasing the power transfer limit in a weak part of a transmission system. In order to generate realistic synthetic wind power data, a Markov model with low complexity is chosen and implemented to simulate wind power generation. Wind farm feeder characteristics and MV/HV substation controls, such as feeder resistance and reactance, switched capacitor banks and load tap changer controls, as well as wind farm converter voltage and current limitations are modeled in detail. Different scenarios, initiating from a historic insecure snapshot in an area of the Hellenic interconnected system, are used to assess the impact of stochastic wind farm generation on the contribution of the maximum reactive support control scheme on voltage stability.

Voltage Stability Of South Sulawesi Power System Using Fuzzy Logic Method Based On The Linguistic Variable

This study aims to apply a new method for controlling voltage stability in electric power systems, using new modeling with linguistic variables of cryptic logic. The method can easily carry out appropriate control measures in accordance with human thinking and acting. The Fuzzy Logic Stabilizer periodically checks the voltage in real time at each bus and maps into linguistic variables, analyzes its control decisions, then calculates how much stability control is sent to all mobile operator numbers that have been registered at the short message service center, so that appropriate control measures are carried out on the On Load Tap Changer, or On Load Voltage Regulator at each substation, according to the contents of the short message it receives. Application of fuzzy logic stabilizer in the 46 bus electricity system of South Sulawesi will increase the quality of bus voltage from an average of 0.08% to 0.5% of the nominal voltage.