On-load Tap Changer Research Articles

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.

Rekonfiguracija distributivne mreže i otočna kompenzacija uz prisustvo vetro generatora i solarnih panela

Distribution utilities witness many changes nowadays. If the network voltages must be in 0.95-1.05 p.u. limits and grid input power factor greater than 0.85 all three modern strategies of smart distribution network as reconfiguration, capacitor 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 eight realistic operation cases is presented by Simulated Annealing (SA) and by hybrid algorithm of Minimum spanning tree (MST) plus SA that works in shorter time. MST is used for tackling reconfiguration first and SA for capacitor switching afterwards that gives better results compared with SA alone. Implemented transparent graphical 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 69 bus network is analysed which has 69 nodes and 73 branches, all of which can commutate. 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, OLTC and commutations for the period of month and a half.

OLTC Fault Diagnosis Method Based on Time Domain Analysis and Kernel Extreme Learning Machine

<p>Aiming at the problems of limited feature information and low diagnosis accuracy of traditional on-load tap changers (OLTCs), an OLTC fault diagnosis method based on time-domain analysis and kernel extreme learning machine (KELM) is proposed in this paper. Firstly, the time-frequency analysis method is used to analyze the collected OLTC vibration signal, extract the feature information and form the feature matrix; Then, the PCA algorithm is used to select effective features to build the initial optimal feature matrix; Finally, a kernel extreme learning machine optimized by improved grasshopper optimization algorithm (IGOA), is used to handle the optimal feature matrix for classifying fault patterns. Evaluation of algorithm performance in comparison with other existing methods indicates that the proposed method can improve the diagnostic accuracy by at least 7%.</p> <p> </p>

Research on Topology and Parameter Design of a Novel Flexible On-Load Voltage Regulator

Traditional on-load tap charger (OLTC) transformer use mechanical switch or anti-parallel thyristor as on-load tap changers. By changing the position of the tap chargers, the transformation ratio is changed, so as to achieve discrete step voltage regulation function. In this paper a novel Flexible On-Load Voltage Regulator (Flexible OLVR) is proposed, which is integrated design of Power Electronic Converter (PEC) and OLTC transformer. The converter is set at the primary side of the transformer, and it could control the output voltage accurately to relize the stepless voltage regulation when there is grid voltage fluctuation. The topology and operating principle of 10kV/0.4kV Flexible OLVR are given in this paper. Secondly, the main parameter design criteria is discussed through a group of design examples. Finally, the feasibility of the scheme is verified by the simulation waveform.

Reliability Evaluation of Transformer Oil Insulation with H2O Water and Super O2 Water Contaminants Disaster Mitigation Electrical Engineering

Transformer oil insulation failure is one of them that can cause water to penetrate through the mechanical acrylic cap on the OLTC (on load tap changer) which causes contaminants in the transformer insulation oil which affects the reliability of the transformer until there is damage to the transformers widely. For this reason, research was carried out to evaluate the reliability of transformer oil insulation with contaminants of H2O water and Super O2 water. This study is to evaluate the effect of water contaminants with experimental methods using BAUR Oil Tester test equipment, with H2O water and super O2 as samples of water contaminants on transformer oil insulation. According to SPLN 49-1: 1982, the standard value of breakdown voltage in insulating oil is ≥30kV/2.5mm. Then, 1ml of H2O water will be given so that a decrease in the breakdown voltage value of 31.5% is obtained and this experiment is carried out up to 10ml, which means that the more H2O is given, the value of the breakdown voltage is further away from the SPLN standard. Meanwhile, insulating oil contaminated with super O2 water obtained a breakdown voltage of 68% which means that the more super O2 water, the more the reliability of transformer oil insulation decreases, and the breakdown voltage value also decreases which adversely affects the reliability of the transformer.

Two-layer operation optimization of concentrated solar power with thermal energy storage system and soft open point

In order to improve the dispatching and grid-connected capacity of new energy, enhance the comprehensive economic benefits, and reduce the voltage offset and fluctuation of the distribution network, this paper proposes a two-layer operational optimization model of concentrated solar power (CSP) with thermal energy storage system (TESS) and soft open point (SOP). Firstly, according to the demand of load, the upper-layer distribution network formulates the optimal scheduling strategy of photovoltaic (PV), wind power (WP), CSP, energy storage system (ESS) and power purchase through the schedulability of CSP with TESS. Secondly, the upper-layer scheduling results are connected to the lower-layer distribution network, and coordinated control is carried out through SOP, on-load tap changer (OLTC) and capacitor bank (CB). Then, the lower model is converted into a linear model, a quadratic cone model and a rotated quadratic cone model, followed by absolute value removal, so as to rapidly and accurately produce the solution of the model. Finally, the model is analyzed on the improved IEEE33 node system. The results show that the proposed model in this paper not only ensures the economy of the power grid but also improves the stability of the voltage.

Using OPF-Based Operating Envelopes to Facilitate Residential DER Services

The proliferation of residential distributed energy resources (DER), such as solar PV and batteries, is creating opportunities for DER owners to provide system-level services through aggregators. However, if this aggregated response is not adequately managed by distribution companies, the simultaneous active power exports (or imports) can result in voltages and currents well beyond the limits of their networks. But the big barrier is that, often, distribution companies cannot directly manage DER or aggregators. This work proposes the use of operating envelopes—time-varying, meter-level export/import limits that a distribution company issues to aggregators—to ensure network integrity while facilitating residential DER services. The operating envelopes are calculated using a linearised, three-phase optimal power flow (OPF) technique that also exploits controllable network assets (e.g., on-load tap changers). Using a realistic Australian MV-LV network with 4,600+ residential customers, results show that network integrity can be ensured without the need for distribution companies to directly control DER. Furthermore, existing on-load tap changers create additional voltage headroom and, thus, larger operating envelopes (and potential services). Given that distribution companies do not know the ultimate exports/imports of customers providing services, a probabilistic testing of the operating envelopes confirms that they are adequate for most plausible scenarios.

Multi-Agent Deep Reinforcement Learning for Voltage Control With Coordinated Active and Reactive Power Optimization

The increasing penetration of distributed renewable energy resources causes voltage fluctuations in distribution networks. The controllable active and reactive power resources such as energy storage (ES) systems and electric vehicles (EVs) in active distribution networks play an important role in mitigating the voltage excursions. This paper proposes a two-timescale hybrid voltage control strategy based on a mixed-integer optimization method and multi-agent reinforcement learning (MARL) to reduce power loss and mitigate voltage violations. In the slow-timescale, the active and reactive power optimization problem involving capacitor banks (CBs), on-load tap changers (OLTC), and ES systems is formulated as a mixed-integer second-order cone programming problem. In the fast-timescale, the reactive power of smart inverters connected to solar photovoltaic systems and active power of EVs are adjusted to mitigate short-term voltage fluctuations with a MARL algorithm. Specifically, we propose an experience augmented multi-agent actor-critic (EA-MAAC) algorithm with an attention mechanism to learn high-quality control policies. The control policies are executed online in a decentralized manner. The proposed hybrid voltage control strategy is validated on an IEEE testing distribution feeder. The numerical results show that our proposed control strategy is not only sample-efficient and robust but also effective in mitigating voltage fluctuations.

Double-time-scale distributed voltage control for unbalanced distribution networks based on MPC and ADMM

With the increasing penetration of distributed generators (DGs) in the distribution system, the distributed coordinated optimal control of DG and voltage control devices has become imperative. This paper proposes a double-time-scale distributed voltage control scheme for unbalanced distribution networks with distributed generators based on model predictive control (MPC) and alternating direction method of multiplier (ADMM) to regulate the voltage profile across a network. To better coordinate the economical operation and voltage regulation, the voltage control devices with different temporal characteristics are separately adjusted. In the fast-time-scale control, the decision variables are the active and reactive power outputs of DGs. In the slow-time-scale control, the on-load tap changer, step voltage regulators and capacitor banks are optimized. A tailored solution method based on McCormick envelopes and branch-and-bound algorithm is developed to efficiently solve the optimization problems by transforming the mixed-integer nonlinear program (MINLP) into a second order cone program (SOCP). The voltage control scheme is a combinatorial optimization problem whose computational complexity grows exponentially with the number of binary variables. To resolve this issue, a distributed optimal voltage control strategy is developed based on the alternating direction method of multipliers (ADMM). Through the ADMM, the optimization problem is distributed among the zones of the distribution networks, without requiring a central controller. The modified IEEE 123-node system is used to verify the effectiveness of the proposed control scheme as a test system.

Bidirectional field–circuit coupling analysis of converter transformer inter‐tap short‐circuit faults in on‐load tap changers

Converter transformer inter-tap short-circuit faults in on-load tap changers have occurred successively in recent years, resulting in severe accidents and enormous losses. Due to the lack of an effective model and credible arc-in-oil conductance parameters, related fault analysis has remained a challenge. This paper develops a bidirectional field–circuit coupling method for modelling inter-tap faults, in which the electromagnetic field and electric circuit are directly coupled and simultaneously solved. After verification based on on-site arcing tests, the Schavemaker model is applied to calculate the non-linear arc conductance. Taking a ±800 kV UHVDC system as a study case, inter-tap faults in a 415 MVA/500 kV converter transformer are simulated and analysed. Under an inter-tap fault, the flux distribution is significantly distorted, with large radial leakage fluxes near the shorted turns, and the circulating current can reach tens or even hundreds of kiloamperes. Further calculation results for faults between various taps indicate that leakage flux distortion and the change in the transformation ratio are the main factors influencing the short-circuit current. These representative findings effectively reveal the variation characteristics of the leakage flux and the short-circuit current as well as the non-linear relationship between fault severity and tap position under inter-tap faults.

Voltage Profile Analysis in Smart Grids Using Online Estimation Algorithm

Voltage rise is the main obstacle to prevent the increase of distributed generators (DGs) in low-voltage (LV) distribution grids. In order to maintain the power quality and voltage levels within the tolerance limit, new measurement techniques and intelligent devices along with digital communications should be used for better utilization of the distribution grid. This paper presents a real-time sensor-based online voltage profile estimation technique and coordinated Volt/VAR control in smart grids with distributed generator interconnection. An algorithm is developed for voltage profile estimation using real-time sensor remote terminal unit (RTU) which takes into account topological characteristics, such as radial structure and high R/X ratio, of the smart distribution grid with DG systems. A coordinated operation of multiple generators with on-load tap changing (OLTC) transformer for Volt/VAR control in smart grids has been presented. Direct voltage sensitivity analysis is used to select a single DG system for reactive power support in multi-DG environment. The on-load tap changing transformer is employed for voltage regulation when generators’ reactive power contributions are not enough to regulate the voltages. Simulation results show that the reported method is capable of maintaining voltage levels within the tolerance limit by coordinated operation of DG systems and on-load tap changing transformer.

Reactive Power and Voltage Optimization under High Penetration of Distributed Generations and EVs

With the consumption of fossil energy and the deterioration of the environment, both distributed generation (DGs) including photovoltaic (PVs) and electric vehicles (EVs) have ushered in a period of rapid development. However, the DG’s randomness, intermittency, and large-scale EV charging have brought challenges to the safety and economy of the distribution network (DN). This paper comprehensively considers the impact of large-scale EVs and DGs of high penetration on the DN, establishing reactive power and voltage optimization model for minimizing the network loss and voltage deviation by regulating the energy storage (ES) and voltage adjusting devices including on-load tap changer (OLTC), capacitors banks (CB), and static var compensator (SVC). Next, the proposed model is simplified by the method of variable substitution, second-order cone programming (SOCP), and linearization. Finally, the validity of the model is verified by the IEEE 33-node simulation.

ANALYSIS OF EXISTING APPROACHES AND METHODS OF DIAGNOSTICS OF ON-LOAD TAP CHANGER OF POWER TRANSFORMERS

In electrical networks of almost all voltage classes, the main link in the work on the transmission of electrical energy is a power transformer. At the moment, about 60% of them are operated in our country with a significant excess of service life, which increases the requirements for their technical control and diagnostics of all units. Switching devices are an integral part of power transformers. They are necessary to regulate voltage on substation busbars depending on the load of consumers. Not only the reliability of power supply of industrial enterprises, cities and agriculture, but also the quality of electrical energy depends on their operation. At the same time, they are one of the vulnerable nodes of voltage converters. Although the parameters of diagnostic signs for switching devices are not as extensive as the power transformer itself, they nevertheless require special attention from the operating personnel. This is due not only to the variety of switchgear types that exist, but also to the methods used to diagnose them. For this reason in this work an attempt has been made to analyze the existing approaches and methods of diagnosing these mechanical controlled voltage regulators. We propose a new approach in terms of diagnostics of high-voltage equipment, including power transformers – introdiagnostics. It is based on methods of non-destructive (without opening tanks and draining dielectric liquids) control of parameters that characterize the state of electrical equipment. We present an original method of diagnosing the on-load tap changer, based on oscillography and subsequent analysis of operation of the on-load tap changer contact system without draining transformer oil from the diverter switch’s tank.

Measurement and Interpretation of the Effect of Electrical Sliding Speed on Contact Characteristics of On-Load Tap Changers

This paper analyzes the effect of sliding speed on the electrical conductivity and friction properties of the contact pair of an on-load tap changer (OLTC). Reciprocating current-carrying tribological tests were carried out on a rod–plate–copper–tin–copper contact galvanic couple at different sliding speeds in air and insulating oil media. The results show that as the sliding speed increases from 24 mm/s to 119 mm/s, the average contact resistance in air increases from 0.2 Ω to 0.276 Ω, and the average contact resistance in insulating oil also increases from 0.2 Ω to 0.267 Ω. At 119 mm/s, the maximum contact resistance in insulating oil reaches 0.3 Ω. The micro-topography images obtained by scanning electron microscopy show that with the increase in sliding speed, the wear mechanisms in the air are mainly abrasive wear and adhesive wear, and the wear mechanisms in oil are mainly layered wear and erosion craters; high sliding speed and arcing promote contact surface fatigue and crack generation. X-ray photoelectron spectroscopy was used to analyze the surface. The copper oxide in the air and the cuprous sulfide in the insulating oil cause the surface film resistance, and the total contact resistance increases accordingly. In addition, the test shows that 119 mm/s in air and 95 mm/s in insulating oil are the speed thresholds. Below these speed thresholds, the increase in contact resistance is mainly caused by mechanical wear. Above these thresholds, the increase in contact resistance is mainly caused by arc erosion and chemical oxidation processes. Non-mechanical factors exacerbate the deterioration of the contact surface and become the main factor for the increase in contact resistance.

Dynamic behaviour of a suspended bubble and its influence on the distribution of electric fields in insulating oil of an on-load tap-changer within power transformers

A small amount of gas bubbles generated in an on-load tap changer (OLTC) can cause partial discharges in the intense electric field region, which severely threaten the safe operation of the device. It is crucial to clarify the coupling mechanism of moving bubbles together with the electric field distribution in the OLTC. This paper analyzed the internal structure and operating principle of the OLTC switching core and clarified the electric field distribution of each component under working conditions. A 2D two-phase flow model was established based on a practical OLTC structure by using the phase-field method. We investigated the global dynamics of small bubbles in the OLTC under the combined action of flow and electric fields and generalized the mechanism of their impact on the spatiotemporal distribution of the electric field inside the OLTC. Finally, the influence of the characteristic parameters of the bubbles and the insulating oil on the insulation property of the OLTC were clarified. The results showed that the distribution of the electric field was closely related to the shapes of the bubbles and the initial field strength. When the oil quality was poor, there was a risk that the bubbles caused partial discharge, in which case, oil purification and filtration operations should take place. The oil flow velocity should be controlled within a specific range to ensure that the rapid passage of bubbles through the intense electric field area does not render distortions in the field strength significant. The smaller the bubble size is, the steeper the gradient of electric field becomes at the gas-oil interface, and the greater the field strength inside the bubble, more likely to cause the occurance of accidental discharges in the OLTC.

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.

An Offline and Online Approach to the OLTC Condition Monitoring: A Review

Transformer failures have a significant cost impact on the operation of an electrical network. In many utilities, transformers have been operating for many years past their expected usable life. As power demand has surged, transformers in some areas are being loaded beyond their rated capacity to meet the demand. One of the vital components in a transformer is the on-load tap changer (OLTC), which regulates the voltage in the distribution network. This study aims to review several condition-monitoring techniques (online and offline) that can monitor the health of the OLTC and assure the safety of the transformer’s OLTC from irreparable damage by detecting the defect at an earlier stage, which is preceded by the specification of typical faults. This paper also discussed the common faults of the OLTC and the root causes of these faults. The OLTC is prone to mechanical faults due to its frequently changing mechanism in the tap operation. The OLTC are also prone to oil as well as thermal faults. As a result, it is critical to monitor OLTC conditions while they are in use. Proper management of condition monitoring (CM) for the OLTC is useful and necessary to increase availability and achieve optimised operating. Condition monitoring (CM) and diagnostics methods (DM) have been developing since the 1950s. CM and DM have been implemented to diagnose and detect an incipient fault, especially for the OLTC. Many techniques, online and offline, are being used to monitor the condition of the OLTC to prevent failure and minimize outages. These DM and CM will prolong the operational cycle and avoid a major disaster for the OLTC, which is an unfavorable scenario.

Coordinated Volt/Var Control of PV and EV Interfaced Active Distribution Networks Based on Dual-Stage Model Predictive Control

This article presents a dual-stage coordinated control approach for voltage regulation and congestion management of active distribution networks (ADN) in the presence of photovoltaic (PV) generators and electric vehicle stations (EVS). The proposed scheme operates on rule-based model predictive control (RBMPC) to optimally manage the settings of the regulating devices, i.e., on-load tap changer (OLTC), distribution static synchronous compensator (DSTATCOM), PV generators, and EV inverters that possess different temporal characteristics. The first (hourly time-scale) and the second (one-minute time-scale) stages of the dual-stage coordinated control mechanism are designed to correct the long-term and short-term voltage fluctuations, respectively. The objectives of the proposed approach are to minimize the number of OLTC operations (first stage) and changes in set-points of PV and EV inverters, and DSTATCOM (second stage) in addition to minimization of slack variables, energy loss, and voltage error at EV stations. In both the stages, predefined rules are set for MPC to optimize different objectives on the basis of the magnitudes of bus voltages. Simulations are performed on 33-bus and 38-bus distribution networks to test the efficacy of the control approach. It is observed that the proposed approach could mitigate the voltage variations as well as line congestion for different scenarios.

Distributed methodology for reactive power support of transmission system

This paper deals with the provision of reactive power support from distribution grids to the transmission system. To this end, a new distributed control scheme is proposed that coordinates the reactive power output of distributed generation (DG) units to meet a predefined reactive power set-point at the point of interconnection of the distribution grid with the transmission system. The coordination process is implemented in an optimal way minimizing the network losses, while also satisfying the technical limits of the distribution grid. Furthermore, the proposed distributed control scheme is supplemented with a new control algorithm for the on-load tap changer (OLTC) of the high-/medium-voltage transformer to unlock additional reactive power flexibility in case the voltage limits at the distribution grid are reached. Time-series simulations on the IEEE 33-bus network are performed to evaluate the performance of the proposed method against existing solutions.

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.