Under-load Tap Changer Research Articles

A Framework for Volt-VAR Optimization in Distribution Systems

The possibility of leveraging the data provided by smart meters to understand the load characteristics is studied in this paper. The loads are modeled as voltage-dependent elements to increase the accuracy of volt-VAR optimization (VVO) techniques for distribution systems. VVO techniques are part of the distribution management system and may be used for purposes such as loss reduction, voltage profile improvement, and conservation voltage reduction. A deterministic framework is proposed that formulates the VVO problem as a mixed-integer quadratically constrained programming problem, which is solved efficiently using advanced branch-and-cut techniques. The proposed framework is capable of optimally controlling capacitor banks, voltage regulators, and under-load tap changers (ULTCs) for day-ahead operation planning. The results indicate that loss reductions of up to 40% and a total demand reduction of up to 4.8% are achievable under some loading conditions in a radial test system. The effect of the load voltage dependence is also demonstrated through analytical simulations.

Real-Time Deployment of a Novel Synchrophasor-Based Voltage Stability Assessment Algorithm

This paper proposes a Real-Time Voltage Stability Assessment (RVSA) algorithm based on Classification and Regression Trees (CARTs) for the prediction of the state of the power system and the system’s margin to voltage collapse. A novel RVSA index based on the Effective Generator Reactive Power Reserve (EGRPR) using the field current from synchronous generators is used by the proposed RVSA algorithm. Wide area synchrophasor measurements obtained from Phasor Measurement Units (PMUs) using various scenarios involving the long-term voltage stability dynamics of transformer Under-Load Tap Changers (ULTCs), generator Over-Excitation Limiters (OXLs), and credible contingencies are used in creating the knowledge base for training the CARTs. The trained CARTs are afterwards deployed online in a testbed incorporating a Programmable Logic Controller (PLC) for real-time assessment/prediction. The performance of the proposed algorithm is tested and validated on an equivalent 10-bus multi-machine network using the Real Time Digital Simulator® (RTDS) in a ‘hardware-in-the loop’ architecture with the PLC. Comparisons and analyses made from the results obtained verify the simplicity and accuracy of the proposed RVSA index and algorithm. Their effectiveness for various scenarios such as load variation, topology change, ULTC and OXL actions, are illustrated.

Novel Multiagent-Based Scheme for Voltage Control Coordination in Smart Grids

AbstractIn this paper, a new multiagent-based voltage control scheme is proposed. The algorithm dynamically adapts the settings of control devices, such as the automatic voltage regulator of generators, transformer with under-load tap changer, and flexible alternating current transmission systems (FACTS), to optimize operational objectives. The objectives are controlling the generator excitation current, reducing transmission power loss, maintaining voltage stability, and improving voltage profile. The constraint is to keep the technical capability limits of system equipment and the transmission line power-flow limits. Furthermore, the effect of load model on voltage control dynamics is addressed. Peer-to-peer device communication is developed to facilitate information exchange among agents. The proposed scheme coordinates the operation of installed voltage control devices on both low voltage and high voltage levels. A simulation test model is implemented using a parallel computing arrangement for a 30-bu...

Comparative study of STATCOM and SVC performance on Dynamic Voltage Collapse of an Electric Power System with Wind Generation

One of the major problems of voltage stability is the reactive power limit of the system. Improving the system's reactive power handling capacity via Flexible AC transmission System (FACTS) devices is a solution for prevention of voltage instability. In this paper it is studied the influence of Static Synchronous Compensators (STATCOM) and Static Var Compensators (SVC) in dynamic Voltage Stability during Low Voltage Ride Through (LVRT), in wind farms. The wind turbines are equipped with pitch control coupled with a Fixed Speed Induction Generator (FSIG). Due to the nature of asynchronous operation, system voltage instability of wind farms based on FSIG is largely caused by the excessive reactive power absorption by FSIG after a fault due to the large rotor slip gained during fault. Wind farm models based on FSIG and equipped with either STATCOM or SVC are developed in EUROSTAG. The Automatic Voltage Regulators (AVR) of the generating units and the turbine speed governors were modeled in detail. Different load models were used and the Under Load Tap Changers (ULTC) were also taken into account. Finally, some conclusions that provide a better understanding of the dynamic voltage stability of a system with FSIG model during LVRT, using various capacities of STATCOM and SVC are pointed out.

분산전원이 연계된 배전계통에 있어서 선로전압조정장치(SVR)의 전압제어 개선방안

This paper presents the novel voltage control method in power distribution system with distributed generators. The voltage in distribution systems is regulated by Under Load Tap Changer(ULTC) of substation and pole transformer of primary feeders. Recently, Step Voltage Regulator(SVR) is getting located at distribution feeders to regulate effectively voltage of primary feeders. But the effectiveness of SVR decreases due to independent operation between SVR and ULTC, and also the existing Line Drop Compensator(LDC) method considering the distributed generators may be not able to regulate the proper voltage in a permissible range. Thus, this paper presents a optimal voltage control algorithm of SVR by using the secondary voltage data of main transformer in substation.

Online Tracking of Voltage-Dependent Load Parameters Using ULTC Created Disturbances

This paper presents a field-test verified method for continuous tracking of voltage-dependent load model parameters. The basic idea is to use the natural or automatic movement of the under load tap changer (ULTC) as the source of voltage disturbance. Load parameters are estimated from each taping activity. While the idea of using manual ULTC tapping disturbances for load parameter estimation is not new, this paper has demonstrated, through laboratory tests and field measurements, that load parameters can be estimated from the automatic ULTC movements. Since a ULTC often has many movements in any given day, the load parameters can be collected in a consistent, frequent, and predictable manner and, consequently, online estimation and tracking of load parameters becomes feasible. The paper presents the implementation of the proposed method and its field measurement experiences, especially the characteristics of tap movements and the associated load responses. Experiments and a large number of field measurements presented in this paper have shown that the proposed method is a practical and effective method for determining load parameters online.

Coordinated Control of a DG and Voltage Control Devices Using a Dynamic Programming Algorithm

This paper presents a new control method, in which a distributed generator (DG) actively participates in steady-state voltage control, together with an under-load tap changer (ULTC) and shunt capacitors (Sh.Cs). In the conventional DG control method, the integration of DGs into a distribution power system increases the number of switching operations of the ULTC and the Sh.Cs. To solve this problem, this paper proposes that the DG output voltage be dispatched cooperatively with the operation of the ULTC and the Sh.Cs, based on load forecasts for one day in advance. The objective of the proposed method is to decrease the number of switching device operations, as well as to reduce the power loss in the distribution lines, while maintaining the grid voltage within the allowed range. The proposed method is designed and implemented with Matlab, using two different dynamic programming algorithms for a dispatchable and a nondispatchable DG, respectively. Simulation studies demonstrate that the objective can be achieved under various grid conditions, determined by factors such as the DG output power characteristics, the location of the DG-connected bus on the feeder, and the load profile of the feeder containing the DG.

Coordination Control of Voltage Between STATCOM and Reactive Power Compensation Devices in Steady-State

This paper proposes a new coordinated voltage control scheme between STATCOM (Static Synchronous Compensator) and reactive power compensation devices, such as shunt elements(shunt capacitor and shunt reactor) and ULTC(Under-Load Tap Changer) transformer in a local substation. If STATCOM and reactive power compensators are cooperatively used with well designed control algorithm, the target of the voltage control can be achieved in a suddenly changed power system. Also, keeping reactive power reserve in a STATCOM during steady-state operation is always needed to provide reactive power requirements during emergencies. This paper describes the coordinative voltage control method to keep or control the voltage of power system in an allowable range of steady-state and securing method of momentary reactive power reserve using PSS/E with Python. In the proposed method of this paper, the voltage reference of STATCOM is adjusted to keep the voltage of the most sensitive bus to the change of loads and other reactive power compensators also are settled to supply the reactive power shortage in out range of STATCOM to cope with the change of loads. As the result of simulation, it is possible to keep the load bus voltage in limited range and secure the momentary reactive power reserve in spite of broad load range condition.

Interactive multiobjective daily volt/var control of distribution networks considering wind power and fuel-cell power plants

This paper deals with a multiobjective daily volt/var control (MDVVC) for radial distribution feeders integrated renewable energy sources (RES) by means of the tap position of the under load tap changer (ULTC) transformers, shunt capacitors, and active and reactive power of RES. The multiple objective functions to be minimized are the electrical energy losses, the voltage deviations, and the total emissions of RES and substations. Discrete behavior of equipments in the distribution systems and nonlinear power flow equations change the VVC problem into a mixed integer non-linear programming (MINLP). Hence, a new optimization method based upon the shuffled frog leaping algorithm (SFLA) is presented to solve the optimization problem. The SFLA is modified for resolving the disadvantages of the original algorithm. Besides of accurately passing local optima, the MSFLA takes less time to achieve the optimal response. Furthermore, the tribe-MSFLA is proposed through using the concept of the tribe. Dealing with the multiobjective optimization problem, an interactive fuzzy satisfying method is used while the objective functions are formulated by a fuzzy set theory. An 85-bus radial distribution system is used to test and assess the performance of the proposed algorithm.

분산전원이 연계된 배전계통에서 ULTC와 분산전원의 전압제어

LDC(Line Drop Compensation) is widely used in controlling ULTC(Under Load Tap Changer) output voltage at distribution substation. However, LDC may experience some difficulties in voltage control due to renewable energy resources and distributed generations. Therefore, more advanced voltage control algorithm is necessary to deal with these problems. In this paper, a modified voltage control algorithm for ULTC and DG is suggested. ULTC is operated with the voltages measured at various points in distribution system and prevents overvoltage and undervoltage in the distribution feeders. Reactive power controller in DG compensates the voltage drop in each distribution feeders. By these algorithms, the voltage unbalance between feeders and voltage limit violation will be reduced and the voltage profile in each feeder will become more flat.

Coordinated Voltage Control Scheme for SEIG-Based Wind Park Utilizing Substation STATCOM and ULTC Transformer

This paper presents a coordinated voltage control scheme for improving the network voltage profile and for minimizing the steady-state loading of the STATCOM to effectively support the system during contingencies. The paper addresses implementation issues associated with primary voltage control and optimal tracking secondary voltage control for wind parks based on self-excited induction generators which comprise STATCOM and under-load tap changer (ULTC) substation transformers. The voltage controllers for the STATCOM and ULTC transformer are coordinated and ensure the voltage support. In steady-state operation, the voltage is controlled by only stepping the tap changer when the voltage is outside the deadband region of the ULTC to minimize the number of taps changes. Thus, the STATCOM will be unloaded and ready to react with higher reactive power margin during contingencies. In the paper, the effects of the short circuit ratio of the interconnection and the inherent communication delay between the wind park and the remote bus on the performance of the controllers and the maximum critical clearing time of fault are considered. Simulation results are presented to demonstrate the performance of the controllers in steady-state and in response to system contingency situations.

정상상태 전압제어를 위한 UPFC와 조상설비의 협조

Abstract - This paper presents a new method of local voltage control to achieve coordinative control among UPFC(Unified Power Flow Controller) and conventional reactive compensation equipments , such as switched-shunt and ULTC(Under-Load Tap Changing) transformer. Reactive power control has various difficult aspects to control because of difficulty of system analysis. Recently, the progress of power electronics technologies has lead to commercial availability of several FACTS(Flexible AC Transmission System) devices. The UPFC(Unified Power Flow Controller) simultaneously allows the independent control of active and reactive power flows as well as control of the voltage profile. When conventional reactive power sources and UPFC are used to control system voltage , the UPFC reacts to the voltage deviation faster than the conventional reactive power sources. Keeping reactive power reserve in an UPFC during steady-state operation is always needed to provide reactive power requirements during emergencies. Therefore, coordination control among UPFC and conventional reactive power sources is needed. This paper describe the method to keep or control the voltage of power system of local area and to manege reactive power reserve using PSS/E with Python. The result of simulation shows that the proposed method can control the local bus voltage within the given voltage limit and manege reactive power reserve. Key Words : Coordinative control, UPFC, ULTC, Switched-shunt, PSS/E, Python† 교신저자, 정회원 : 경북대 전기공학과 시간강사․공박E-mail : pjh@ee.knu.ac.kr* 정 회 원 : 경북대 대학원 전자전기컴퓨터학부 박사과정** 펠로우회원 : 경북대 전기공학과 교수․공박***정 회 원 : 거제대 선박전기과 조교수․공박 접수일자 : 2011년 3월 2일 최종완료 : 2011년 4월 26일

Predictive control for voltage collapse avoidance using a modified discrete multi-valued PSO algorithm

Voltage stability is one of the most challenging concerns that power utilities are confronted with, and this paper proposes a voltage control scheme based on Model Predictive Control (MPC) to overcome this kind of instability. Voltage instability has a close relation with the adequacy of reactive power and the response of Under Load Tap Changers (ULTCs) to the voltage drop after the occurrence of a contingency. Therefore, the proposed method utilizes reactive power injection and tap changing to avoid voltage collapse. Considering discrete nature of the changes in the tap ratio and also in the reactive power injected by capacitor banks, the search area for the optimizer of MPC will be an integer area; consequently, a modified discrete multi-valued Particle Swarm Optimization (PSO) is considered to perform this optimization. Simulation results of applying the proposed control scheme to a 4-bus system confirm its capability to prevent voltage collapse.

Volt/Var control in a distribution system by a fuzzy optimization approach

This paper presents a fuzzy optimization approach for solving the Volt/Var control problem in a distribution system with uncertainties. Wind turbines are being considered in the study distribution system. The main purpose is to find an optimum combination of tap position for the main transformer under load tap changer (ULTC) and on/off status for switched capacitors in a day to minimize the voltage deviation on the secondary bus of the main transformer, reactive power flow through the main transformer and real power loss on feeders. When performing the Volt/Var control problem in conventional methods, the hourly load and wind speed must be forecasted to prevent errors. However, actually there are always errors in these forecasted values. A characteristic feature of the proposed fuzzy optimization approach is that the forecast hourly load and wind speed errors can be taken into account using fuzzy sets. Fuzzy set notations in the load demand, wind speed, voltage deviation on the secondary bus, reactive power flow through the main transformer and total real power loss on feeders are developed to obtain the optimal dispatching schedule under an uncertain environment. To demonstrate the effectiveness of the proposed method, the Volt/Var control problem is performed in a distribution system within the service area of Yunlin District Office of Taiwan Power Company (TPC). The results show that a proper dispatching schedule for ULTC position and capacitor switching operation can be reached using the proposed method.

A Lab Setup Illustrating Thyristor-Assisted Under-Load Tap Changers

This paper proposes a simple enough yet challenging lab setup illustrating the application of power electronics to power system devices through a thyristor-assisted under-load tap changer. After a preliminary analysis of the special switching concepts involved in this application, a thorough description of the main steps comprising the proposed experiment is carried out. This lab setup has been included in the electrical engineering curriculum (major in power engineering) of most senior undergraduate students (fifth year in several European countries).

Real-Time Voltage Regulation in Power Distribution System Using Fuzzy Control

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Two different fuzzy approaches to voltage control in electric power distribution systems are introduced in this paper. The real-time controller in each case would act on power transformers equipped with under-load tap changers. Learning systems are employed to turn the voltage-control relays into adaptive devices. The scope of this study has been limited to the power distribution substation, and the voltage measurements and control actions are carried out on the secondary bus. The capacity of fuzzy systems to handle approximate data, together with their unique ability to interpret qualitative information, make it possible to design voltage-control strategies that satisfy the requirements of the Brazilian regulatory bodies and the real concerns of the electric power distribution companies. Fuzzy control systems based on these two strategies have been implemented and the test results were highly satisfactory. </para>

Fuzzy Control System for Voltage Regulation In Power Transformers

A fuzzy control strategy for voltage regulation in electric power distribution systems is introduced in this article. This real-time controller would act on power transformers equipped with under-load tap changers. The fuzzy system was employed to turn the voltage-control relays into adaptive devices. The scope of the present study has been limited to the power distribution substation, and both the voltage measurements and control actions are carried out on the secondary bus. The capacity of fuzzy systems to handle approximate data, together with their unique ability to interpret qualitative information, make it possible to design voltage control strategies that satisfy both the requirements of the Brazilian regulatory bodies and the real concerns of the electric power distribution companies. A prototype based on the fuzzy control strategy proposed in this paper has also been implemented for validation purposes and its experimental results were highly satisfactory.

Minimization of voltage deviation and power losses in power networks using Pareto optimization methods

Voltage regulation is an important task in electrical engineering for controlling node voltages in a power network. A widely used solution for the problem of voltage regulation is based on adjusting the taps in under load tap changers (ULTCs) power transformers and, in some cases, turning on Flexible Alternating Current Transmission Systems (FACTS), synchronous machines or capacitor banks in the substations. Most papers found in the literature dealing with this problem aim to avoid voltage drops in radial networks, but few of them consider power losses or meshed networks. The aim of this paper is to present and evaluate the performance of several multi-objective algorithms, including hybrid approaches, in order to minimize both voltage deviation and power losses by operating ULTCs located in high voltage substations. In particular, a well-known multi-objective algorithm, PAES, is used for this purpose. PAES finds a set of solutions according to Pareto-optimization concepts. Furthermore, this algorithm is hybridized with simulated annealing and tabu search to improve the quality of the solutions. The implemented algorithms are evaluated using two test networks, and the numerical results are analyzed with two metrics often used in the multi-objective field. The results obtained demonstrate the good performance of these algorithms.

The On-Line Voltage Management and Control Solution of Distribution Systems Based on the Pattern Recognition Method

This paper proposes an on-line voltage management and control solution for a distribution system which can improve the efficiency and accuracy of existing off-line work by collecting customer voltage on-line as well as the voltage compensation capability of the existing ULTC (Under Load Tap Changer) operation and control strategy by controlling the ULTC tap based on pattern clustering and recognition. The proposed solution consists of an ADVMD (Advanced Digital Voltage Management Device), a VMS (Voltage Management Solution) and an OLDUC (On-Line Digital ULTC Controller). An on-line voltage management emulator based on multi-thread programming and the shared memory method is developed to emulate on-line voltage management and digital ULTC control methodology based on the on-line collection of the customer's voltage. In addition, using this emulator, the effec- tiveness of the proposed pattern clustering and recognition based ULTC control strategy is proven for the worst voltage environments for three days.

Volt/VAr Control in Distribution Systems by Fuzzy Multiobjective and Particle Swarm

This paper presents the optimal dispatch for the under load tap changer (ULTC) of a substation transformer, substation capacitors, and feeder capacitors for volt/VAr control in a distribution system. Three objectives of interest in the problem consist of energy loss, total capacitor kVAr to be switched on, and total number of daily switching operations of the ULTCand all capacitors. The optimization problem is subjected to power °ow equations, voltage limits, and maximum switching operations for the ULTC and the capacitors. All the objectives are fuzzi¯ed using a trapezoidal membership function and are integrated to represent the fuzzy decision value. Fuzzy multiobjective and particle swarm optimization are employed to determine the optimal dispatch schedule that provides the best compromise among all the objectives. The methodology is demonstrated by a 29-bus distribution system of Provincial Electricity Authority (PEA), Thailand.