Load Tap Changer Research Articles

Methods to mitigate voltage-rise issues in transmission systems

This paper proposes some novel methods for mitigating undesired voltage rises in modern-day power systems. Some older approaches are also revisited in the light of new developments including the proliferation of power-electronic interfaced equipment. The analysis is performed using both power flow technique as well as long-term simulation employing quasi-steady-state approximation of short-term dynamics. Guidelines and mitigation countermeasures that may be utilized during either system planning or operation are investigated, such as optimal placement of new reactor installations, special transformer load-tap-changer (LTC) controls and reactive power absorption by renewable energy sources, such as wind farms connected through power converters in distribution feeders. Several case studies are investigated in two test systems: a sub-transmission area of the French Transmission Grid, and the Hellenic transmission power system, for which a number of different operating points is considered.

Development of calibration device for on-load tap changer tester of transformer

Abstract More and more transformer on load tap changers are being tested using the AC method, which is closer to the actual operating status of the transformer on load tap changers. From the analysis of the switch operating status, the test results are closer to the actual value. However, the existing calibration devices for transformer on load tap changer testers are all aimed at DC testers, and traditional calibration methods are difficult to achieve synchronous calibration of transition resistance, transition time, and synchronization under AC test signals. Therefore, this article designs a three channel variable AC resistor module based on FPGA controlled DAC to simulate different resistors. The variable resistor module can achieve fast switching of resistors through frequency compensation, and achieve synchronous verification of analog conversion resistance, conversion time, and synchronization. After testing, the maximum allowable error of the calibration device for simulating transition resistance is ± (0.3% RD+5 m Ω), and the absolute error of transition time and synchronization is less than 20us, which fully meets the needs of laboratory calibration and solves the calibration problem of the transformer on load tap changer tester.

An adaptive heuristic approach for Volt-VAr optimization in distributed generation integrated system

Volt-VAr optimization (VVO) is an important tool in improving the efficiency of distribution system by simultaneously coordinating voltage and reactive power both. Today, loads are largely dependent on voltage and thus, this voltage-dependence affects both real and reactive power. So, minimizing apparent power is more important than minimizing the energy. VVO enables distribution system utilities to run their network on comparatively lower voltage which ultimately increases energy savings. Devices such as, on load tap changers (OLTC), and voltage regulators (VRs) are commonly adopted techniques to regulate/boost the voltage. Capacitive banks (CBs) are also used to inject reactive power (VAr) into the system keeping the operational constraints and system constraints specified and satisfied. This paper presents a new methodology of Volt-VAr optimization using Genetic algorithm considering a variety of constraints. The test is performed on IEEE-69 bus system and results validate the efficacy of the proposed algorithm.

A coordinated voltage control scheme based on linearized power flow functions for non-optimal and optimal problems

This paper proposes a coordinated voltage control scheme for transmission systems based on linearized power flow functions to determine the set points of local control voltage devices. The scheme considers models and interactions between different devices to attain the appropriate performance (non-optimal or optimal) of the whole power system. The sensitivity analysis is used to formulate a partly linearized model that relates the voltage variations of buses to the reactive power variation of synchronous machines, settings of static VAr compensators (SVCs), shunt capacitors susceptance, and tap position of load tap changers (LTCs) to determine the set of the settings of the voltage control devices. Two formulations are proposed by considering the susceptance of SVCs or the slope and reference voltage of characteristics of SVCs as control variables. The developed formulation based on the linearized power flow functions provides the possibility of the practical implementation of the coordinated voltage control scheme, especially the optimal one, by reducing the model complexity and solving time. The performance of the proposed formulations is investigated by some studies of voltage control and an optimally coordinated problem on the IEEE 30-bus and IEEE 118-bus test systems.

A supervisory Volt/Var control scheme for coordinating voltage regulators with smart inverters on a distribution system

This paper concentrates on the efficient utilization of smart inverters for Volt/Var control (VVC) within a distribution system. Although new smart inverters possess Var support capability, their effective deployment necessitates coordination with existing Volt/Var schemes. To address this, a novel VVC scheme is proposed to facilitate such synchronization. The proposed scheme bifurcates the issue into two levels. The initial level involves utilizing Load Tap Changer (LTC) and Voltage Regulators (VRs), coordinating their control with smart inverters to regulate the circuit's voltage levels within the desired range. The subsequent level determines the Var support required from smart inverters to minimize overall power loss in the circuit. The supervisory control results are communicated to the respective devices equipped with their local controllers. To minimize frequent dispatch, smart inverters are supervised by adjusting their Volt/Var characteristics as necessary. This approach enables the smart inverters to operate near their optimal control while meeting the limited communication prerequisites in a distribution system. A case study employing the IEEE 34 bus system illustrates the efficacy of this supervisory control scheme in contrast to traditional Volt/Var schemes.

A comprehensive optimization mathematical model for wind solar energy storage complementary distribution network based on multi-regulatory devices under the background of renewable energy integration

In the context of global energy transformation and sustainable development, integrating and utilizing renewable energy effectively have become the key to the power system advancement. However, the integration of wind and photovoltaic power generation equipment also leads to power fluctuations in the distribution network. The research focuses on the multifaceted challenges of optimizing the operation of distribution networks. It explores the operation and control methods of active distribution networks based on energy storage and reactive power compensation equipment. The stable operation of the distribution network is analyzed under the conditions of wind and photovoltaic integration, with a particular focus on precise regulation to address the limitations of existing methods. Afterwards, the study proposes an improvement plan that combines on load tap changer transformers and reactive power compensation equipment to solve complex power balance problems through second-order cone programming relaxation method. The results of numerical analysis show that the constructed mathematical model maintains a stable voltage of 1 to 1.1 pu at distribution network nodes within 24 h. Especially during peak hours from 15:00 to 24:00, it remains normal without any abnormal fluctuations when the control equipment is not added. These results confirm that precise regulation of multiple devices ensures voltage stability and avoids low or high voltage issues.

Pengaruh Panjang Jaringan Terhadap Drop Tegangan Pada Penyulang Duku dan Kurma PT. PLN (Persero) ULP Rumbia

Voltage drops in distribution systems can occur in medium voltage networks (JTM), distribution transformers, low voltage networks (JTR) and house lines. Voltage drops in the network cause suboptimal distribution and increase power losses. PT. PLN (Persero) ULP Rumbia has two feeders with pure radial network characteristics sourced from the Seputih Banyak Main Substation. The length of the Duku Feeder network has a voltage of 96.8 KMS with a load of 150 A while the length of the Kurma Feeder network is 207.21 kV with a load of 155 A. The network reconfiguration of the Dente Teladas Substation was carried out to shorten the network length. This research aims to calculate the voltage drop that occurs at the Duku and Kurma feeders before and after network configuration. This research uses the quantity method to compare the calculation results with the voltage drop standards set by PT. PLN (Persero). After reconfiguring the Duku and Kurma feeders, there was an increase in tip voltage which indicated improvement. From the calculation results before adding the network to the Duku feeder, the voltage at the receiving end was 19.5 kV with a voltage drop of 7.1%, and at the Kurma feeder the voltage at the end of the line was 17 kV with a voltage drop of 19%. This is clearly no longer in accordance with the local TMP, and requires further study to overcome this problem. The voltage on the two feeders goes through a step up process using a Medium Voltage OLTC (On Load Tap Changer) so that the voltage on each feeder becomes between 19.86 kV to 20.16 kV.

Dynamic Voltage Stability of an Electric Power Network with Double Fed Induction Wind Power Generators

In recent years, the large penetration of wind power poses new challenges for the voltage stability analysis of an electric power network. In this paper is studied the effect of large scale integration of wind power in the dynamic voltage stability of a power network under a fault condition. The automatic voltage regulators of the generating units, the turbine speed governors and the double fed induction generator were modeled. Different load models were used and the under load tap changers were also taken into account. The simulation results were obtained using the EUROSTAG software package. Finally, some conclusions that provide a better understanding of the dynamic voltage stability in a system with a large amount of wind power generation are pointed out.

Multi-Stage Operation Optimization of PV-Rich Low-Voltage Distribution Networks

The high expansion of a variable and intermittent nature of distributed generation, such as photovoltaics (PV), can cause technical issues in existing distribution networks (DN). In addition to producing electrical energy, PVs are inverter-based sources, and can help conventional control mechanisms in mitigating technical issues. This paper proposes a multi-stage optimal power flow (OPF)-based mixed-integer non-linear programming (MINLP) model for improving an operation state in LV PV-rich DN. A conventional control mechanism such as on load tap changer (OLTC) is used in the first stage to mitigate overvoltage caused by PVs. The second stage is related to reducing losses in DN using reactive power capabilities from PVs, which defines the optimization problem as a fully centralized observed from the distribution system operator’s (DSO) point of view. The optimization problem is realized under the co-simulation approach in which the power system analyzer and computational intelligence (CI) optimization method interact through an interface. This approach allows keeping the original MINLP model without approximations and using any computational intelligence method. OpenDSS is used as a power system analyzer, while particle swarm optimization (PSO) is used as a CI optimization method in this paper. Detailed case studies are performed and analyzed over a single-day period. To study validation and feasibility, the proposed model is evaluated on the IEEE LV European distribution feeder. The obtained results suggest that a combination of conventional control mechanisms (OLTC) and inverter-based sources (PVs) represent a promising solution for DSO and can serve as an alternative control method in active distribution networks.

Distribution Automation: Enhancing Efficiency and Reliability in Power Distribution Systems

This paper investigates the importance of distribution automation in power distribution systems. The introduction highlights the challenges faced by traditional distribution systems, such as high losses, inefficient processes, and outdated infrastructure. Distribution automation, referred to as smart grid technology, is a transformative solution that integrates advanced technologies and automation devices to enhance power distribution, operational efficiency, and system reliability. This paper provides a comprehensive examination of various distribution automation devices, such as remote fault indicators, smart relays, automated switches and reclosers, automated capacitors, voltage regulators and load tap changers, feeder monitors, and transformer monitors. The importance of distribution automation is emphasized, including enhanced reliability, improved operational efficiency, enhanced customer satisfaction, and the integration of renewable energy resources. Opportunities for distribution automation, such as enhanced reliability, improved operational efficiency, enhanced data collection and analysis, integration of distributed energy resources, and demand response programs, are highlighted. Furthermore, the challenges associated with distribution automation, such as the cost of implementation, technical complexity, cybersecurity risks, workforce training and transition, and regulatory and policy framework, are discussed. Addressing these challenges requires collaboration among utilities, regulators, policymakers, and technology providers. The successful implementation of distribution automation can revolutionize power distribution, leading to more efficient, reliable, and sustainable electricity supply.

A Modular Algorithm Based on the Minimum-Cost-Path Problem for Optimizing LTC Operations in Photovoltaic Integrated Distribution Systems

This paper presents a novel modular voltage control algorithm for optimal scheduling of a distribution system’s load tap changers to minimize the number of tap changes while maintaining a voltage deviation (VD) around a desired target. To this end, a bi-objective optimal voltage regulation (OVR) problem is addressed in two distinct stages. First, the operational constraint on the load tap changer is removed to form a single-objective OVR problem relating to the voltage. The solution obtained in this stage is ultimately utilized to determine the penalty value assigned to the distance from the optimal (solely in terms of voltage) control value. In the second stage, the optimal scheduling problem is formulated as a minimum-cost-path problem, which can be efficiently solved via dynamic programming. This approach allows the identification of optimal scheduling that considers both the voltage-related objective as well as the number of load tap changer switching operations with no added computational burden beyond that of a simple voltage optimization problem. The method imposes no restriction on the load tap changer’s operation and is tested under two different target functions on the standard IEEE-123 test case. The first attains a nominal voltage with a 0.056 p.u. voltage deviation and the second is the well-known conservation voltage reduction (CVR) case with a 0.17 p.u. voltage deviation. The method is compared to an evolutionary-based algorithm and shows significant improvement in the voltage deviation by a factor of 3.5 as well as a computation time acceleration of two orders of magnitude. The paper demonstrates the effectiveness and potential of the proposed method as a key feature in future cutting-edge OVR methods.

Coordinated Use of Smart Inverters With Legacy Voltage Regulating Devices in Distribution Systems With High Distributed PV Penetration—Increase CVR Energy Savings

This paper studies the coordinated use of smart inverters with legacy voltage regulating devices to help increase the energy savings from conservation voltage reduction (CVR) in distribution systems with high photovoltaic penetration. Two voltage optimization algorithms are developed to evaluate the CVR benefit by considering two types of smart inverter functions: aggregated reactive power control and autonomous volt/VAR control. In each algorithm, smart inverters are coordinately controlled with the operation of legacy voltage regulating devices including load tap changers and capacitor banks. Both algorithms are tested on representative utility distribution system models. Simulation results demonstrate that the proposed algorithms can achieve around 1.8-3.6% energy savings when only using legacy voltage regulating devices and an additional 0.3-0.9% when adding smart inverters.

Machine Learning Accelerated Real-Time Model Predictive Control for Power Systems

This paper presents a machine-learning-based speed-up strategy for real-time implementation of model-predictive-control (MPC) in emergency voltage stabilization of power systems. Despite success in various applications, real-time implementation of MPC in power systems has not been successful due to the online control computation time required for large-sized complex systems, and in power systems, the computation time exceeds the available decision time used in practice by a large extent. This long-standing problem is addressed here by developing a novel MPC-based framework that i) computes an optimal strategy for nominal loads in an offline setting and adapts it for real-time scenarios by successive online control corrections at each control instant utilizing the latest measurements, and ii) employs a machine-learning based approach for the prediction of voltage trajectory and its sensitivity to control inputs, thereby accelerating the overall control computation by multiple times. Additionally, a realistic control coordination scheme among static var compensators (SVC), load-shedding (LS), and load tap-changers (LTC) is presented that incorporates the practical delayed actions of the LTCs. The performance of the proposed scheme is validated for IEEE 9-bus and 39-bus systems, with ±20% variations in nominal loading conditions together with contingencies. We show that our proposed methodology speeds up the online computation by 20-fold, bringing it down to a practically feasible value (fraction of a second), making the MPC real-time and feasible for power system control for the first time.

Design of Hybrid Electromechanical Switch for Automatic Voltage Control of on Load Tap Changer Transformer

Design of Hybrid Electromechanical Switch for Automatic Voltage Control of on Load Tap Changer Transformer

Embedded control system for reactive power control in distributed energy resources for voltage regulation in the distributed power system

Since more distributed energy resources (DER) are being linked to the electrical grid, the current distributed power system (DPS) is encountering additional voltage regulation challenges. Traditionally, on-load tap changers, step voltage regulators, and switched capacitor banks have been used for voltage regulation in DPS. However, these sources are insufficient for voltage regulation in current DPS. As a result, reactive power assistance from a DER unit based on power electronics is intended to adjust the voltage in the DPS. In terms of flexibility, security, reliability, and availability, embedded control systems are now being investigated by researchers for use in power converter-fed DER units. This paper presents a method for constructing an embedded controller unit based on XynergyXS for the power converter controller, which includes reactive power regulation in the DER unit. Furthermore, it proposes dynamic reactive power control in the DER unit for enhanced DPS voltage regulation. The suggested voltage control approach is tested in a MATLAB/Simulink model of a practical 85-bus distributed power system located in the northern Tamilnadu region, India as well as a modified IEEE 33-bus system. The new control approach investigates all potential power grid disruptions. The results reveal that the proposed reactive power control method in DER units enhances network voltage regulation while reducing the number of switching operations performed by static voltage regulating units such as on load tap changers and switched capacitor banks.

Rekonfiguracija i otočna kompenzacija u prisustvu distribuiranih izvora u razgranatoj distributivnoj mreži

Distribution utilities witness many changes nowadays. If the network voltages must be in 0.9-1.1 p.u. limits and grid input power factor greater than 0.85 all three modern strategies of smart distribution network as reconfiguration, capacitors switching and presence of distribution generation (DG) ought to be applied. Even in this case On Load Tap Changer (OLTC) in the supplying substation has to be used. In this paper, detailed technical analysis of 21 realistic operation cases is presented by hybrid algorithm of Minimum spanning tree (MST) plus Simulated Annealing (SA). MST is used for tackling reconfiguration first and SA for capacitor switching afterwards. Implemented transparent grafical Monte Carlo method for locating of distributed generators and capacitor banks is unique and simple. Assumption is that DG is already present. Gauss and Weibull changing nodal loads and wind generators output, respectively as well as daily load curves for working and weekend day with insolation of solar panels are included in analysis. The objective function comprising of the cost of power losses, price of capacitors and undelivered energy is minimized. IEEE 118 bus network is analyzed which has 118 nodes and 132 branches (15 ties), all of which can commutate, that is considered to be large scale. The basic switching logic of uniform distribution of wind generators to the nodes and capacitors in accordance with objective function, that changes every hour is unrealistic. More realistic one was issued with fixed nodes for allocation of DG and capacitors (the most frequently visited nodes). The programme final objective function indicates the price of peak power losses, losses of delivered electrical energy, of the banks, of undelivered electrical energy and commutations for the period of month and a half (1008 network operating hours).

Fast Solving Method for Two-Stage Multi-Period Robust Optimization of Active and Reactive Power Coordination in Active Distribution Networks

This paper considers constraints on the cycle number of energy storage systems (ESSs), travel distances of switchable capacitors reactors (SCRs), on load tap changers (OLTCs) and step voltage regulators (SVRs). The characteristics and operational constraints of these equipments are properly formulated with binaries and big-M method, obtaining desirable linear descriptions. The branch flow equations of distribution systems for active and reactive power coordination are constructed. Considering physical constraints, a multi-period mixed-integer deterministic second-order conic programming (SOCP) to minimize network losses is formulated. Then, considering uncertainties of loads and active power of renewable distributed generators (DGs), a two-stage robust model is developed. A directly iteratively solving method between the first and second stage models is proposed. In contrast to the traditional column-and-constraint generation (CCG) method, increases to variables and constraints are not needed to solve the first stage model. To solve the second stage multi-period model, only the model of each single period needs to be solved first. Then their objective function values are accumulated, and the worst scenarios of each period are concatenated. As a result, the solving complexity is greatly reduced. The capabilities of the proposed method are validated by three simulation cases. It is found that the computational rate using the proposed method is significantly enhanced with much less computer storage. Specifically, the simulation results of 4, 33 and 69-bus systems indicate that the computing rate of the proposed method is about 28 times higher than CCG method when 8 iterations are performed. Meanwhile, the precision of optimization results is also improved.

Centralized Control Method for Voltage Coordination Challenges With OLTC and D-STATCOM in Smart Distribution Networks Based IoT Communication Protocol

This paper proposes a coordination method for the voltage control devices based on optimal settings of the D-STATCOMs, on the load tap changer (OLTC) transformer and the distributed generations (DGs)-based renewable energy resources (RERs). The central controller is introduced to handle the active smart distribution network (SDN) problems to maintain the voltage profile within its permissible limits, minimize power losses in different operating conditions, and minimize the energy wastage from the distributed renewable energy resources. These problems are formulated as a multi-objective optimization problem. With increasing load demand and RERs in the distribution system, voltage coordination threatens real-time efficiency. In this research work, central controller-based Gorilla Troops optimization (GTO) algorithm is proposed to detect the optimum solutions for the voltage coordination problem. The load demand uncertainty and the stochastic nature of power generated from RERs (PV panels and wind turbines) are considered in the voltage coordination problem due to their significant effects on the operation and planning of the SDN. The proposed SDN has been represented based on the Internet of Things (IoT) communication protocol. It enhances the data and information transfer between the system-connected agents. A practical test system, NDEDC-24 bus radial distribution network from the North Delta Electrical Distribution Company and IEEE-33 node system are used to test and evaluate the proposed method. The results are compared with other well-known evolutionary methods. The proposed method obtains more accurate results than the other methods.

Optimal Capacitor Placement and Rating for Large-Scale Utility Power Distribution Systems Employing Load-Tap-Changing Transformer Control

Significant opportunity for savings in energy and investment through improved performance of power distribution systems exists in the optimal placement and rating of capacitors, a conventionally cost-effective and popular reactive power compensating technology. A novel optimal capacitor planning (OCP) procedure is proposed for large-scale utility power distribution systems, which is exemplified on an existing utility circuit of approximately 4,000 buses. An initial sensitivity analysis is employed to intelligently reduce OCP computation time and maintain quality of optimal configurations. Three optimization objectives are considered, including the minimization of total system active power losses, standard deviation of node voltages, and investment in total capacitor power rating. Eight multi-objective optimization methods that employ the non-dominated sorting algorithm III (NSGA-III) concept are compared to determine individual merits. Differences between the methods include the incorporation of a penalty constraint for voltage violations and the automatic readjustment of load-tap-changing (LTC) transformer tap settings for proposed capacitor re-configurations concurrently within the optimizer, which ensures peak system performance and fair comparison to the reference case. A multi-step model conversion process was developed with OpenDSS to enable the OCP procedure to be generally applicable to real large-scale utility circuits. OCP is performed for three example sub-circuits served by a substation with a 48MW, 9Mvar peak load, which represents the most extreme case and offers the best opportunity for savings. Example configurations from the resulting Pareto sets through a pseudo-weight vector approach are also analyzed through a systematic procedure of comparison between the most extreme configuration types to inform configuration selection.

Practical Maintenance Model of Power Transformer in Thai Transmission System by Using FMECA

Failure Modes Effects and Criticality Analysis of the maintenance model of risk management for power transformers in Thai transmission system is proposed and measured by severity, occurrence, and detection in order to establish a risk priority number or criticality. The severity involves two criteria: susceptibility and efficiency. The susceptibility-severity concerns four sub-criteria: service life of transformer components, system stability, social aspect, and public image, while the efficiency-severity focuses on the percentage of component condition index determined by the electrical and insulating oil tests, and visual inspection. The weight and score technique is also applied. The occurrence specifies the frequency of component failure, while the detection refers to the capability to detect failures. Furthermore, the scores of severity, occurrence, and detection are compared to their limitation with five levels: one for low and five for high risks. Subsequently, the criticality to plan and prioritize the maintenance model is established into two approaches: susceptibility-criticality and efficiency-criticality. The maintenance action is categorized into four models: low for acceptable, medium for tolerable, high and very high degrees for unacceptable risks. In the analysis, 78 transformers rating 115 kV and 33 transformers rating 230 kV in Thailand are studied. Available major failures on winding, on load tap changer and bushing are investigated. The results show that all the windings are first prioritized with refurbishment or replacement action due to unacceptable risk with frequent failure and very low detectability. Consequently, the effective maintenance model can be achieved with higher system reliability and reduced failure risk.