Tap Changing Transformers Research Articles

Coordinated distribution network control of tap changer transformers, capacitors and PV inverters

A power distribution system operates most efficiently with voltage deviations along a feeder kept to a minimum and must ensure all voltages remain within specified limits. Recently with the increased integration of photovoltaics, the variable power output has led to increased voltage fluctuations and violation of operating limits. This paper proposes an optimization model based on a recently developed heuristic search method, grey wolf optimization, to coordinate the various distribution controllers. Several different case studies on IEEE 33 and 69 bus test systems modified by including tap changing transformers, capacitors and photovoltaic solar panels are performed. Simulation results are compared to two other heuristic-based optimization methods: harmony search and differential evolution. The simulation results show the effectiveness of the method and indicate the usage of reactive power outputs of PVs facilitates better voltage magnitude profile.

Optimal hourly scheduling of distributed generation and capacitors for minimisation of energy loss and reduction in capacitors switching operations

Now-a-days electric power systems all over the world are undergoing a phase of increased size and complexity due to rising load demand expansion. This situation leads to excessive burden on the power distribution systems posing many challenges before the distribution system utilities. Minimisation of real power loss and maintaining voltage profile are the major requirements for a distribution system apart from cost reduction. Capacitors and tap changing transformers are being conventionally used for maintaining voltage profile along a primary distribution network. When a distributed generation (DG) is incorporated in a distribution system, the problem of efficiently controlling a system becomes more complex. In this study, a novel multi-objective optimisation problem of hourly scheduling of voltage control devices in coordination with a DG is attempted. An efficient particle swarm optimisation technique is used to solve the problem. Another novelty of this work is the use of Newton-based power flow method for evaluation of function value. The algorithm proposed is validated on IEEE standard 33 bus and 69 bus radial distribution systems. The results are encouraging.

A new sequential AC–DC power flow algorithm for multi‐terminal HVDC systems

This paper presents a new algorithm based on the sequential method for power flow calculation in integrated multi‐terminal, high‐voltage, direct current (HVDC) systems. Unlike similar studies in the literature, a real equivalent circuit model is considered for under‐load tap changer (ULTC) transformers of the DC converters, for the first time. So, new DC equations are obtained. Thus, exact and accurate results can be obtained for practical applications by the proposed algorithm. Adjustment effects of the DC converters' ULTCs tap values are included in the Jacobian matrix instead of the bus admittance matrix in the sequential AC power flow algorithm as well as other ULTCs in AC system. To this aim, new equations for the calculation of power and Jacobian matrix elements are obtained for the AC system. The proposed approach is tested on the modified IEEE 17‐bus AC–DC test system. Numerical results show that the proposed approach is accurate and reliable in convergence. © 2017 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Thyristor Switched Smart Transformer for Active Power Flow Control in Multi Micro grid System

An intentional active power flow control between microgrids in a multi micro grid system in the grid connected mode is achieved at the point of common coupling. The thyristor controlled on load tap changing transformer at the point of common coupling is equipped with voltage based power flow controller where different tapping are switched according to the required power flow condition to be satisfied and the amount of power to be transmitted. Accordingly, the voltage based droop control associated with the distributed generation units in each of the micro grid changes their active power generation. Thus, without much communication to other elements of the micro grids, the Smart Transformer at the point of common coupling allows active power exchange. The simulations performed in MATLAB/ SIMULINK for both three phase and single phase system follows that active power flow control is achieved in a multi micro grid system using the thyristor switched Smart Transformer. Hardware implementation has also been initiated to analyze the concept of power flow control with thyristor switched Smart Transformer and results are studied. The proposed system shows its operational effectiveness, which can be employed to solve grid emergencies in real time.

REAL POWER LOSS MINIMIZATION BY MUTUAL MAMMAL BEHAVIOR ALGORITHM

This paper presents a Mutual Mammal Behavior (MM) algorithm for solving Reactive power problem in power system. Modal analysis of the system is used for static voltage stability assessment. Loss minimization is taken is taken as main objective. Generator terminal voltages, reactive power generation of the capacitor banks and tap changing transformer setting are taken as the optimization variables. A Meta heuristic algorithm for global optimization called the Mutual Mammal Behavior (MM) is introduced. Mammal groups like Carnivores, African lion, Cheetah, Dingo Fennec Fox, Moose, Polar Bear, Sea Otter, Blue Whale, Bottlenose Dolphin exhibit a variety of behaviors including swarming about a food source, milling around a central location, or migrating over large distances in aligned groups. These Mutual behaviors are often advantageous to groups, allowing them to increase their harvesting efficiency, to follow better migration routes, to improve their aerodynamic, and to avoid predation. In the proposed algorithm, the searcher agents emulate a group of Mammals which interact with each other based on the biological laws of Mutual motion. MM powerful stochastic optimization technique has been utilized to solve the reactive power optimization problem. In order to evaluate up the performance of the proposed algorithm, it has been tested on Standard IEEE 57,118 bus systems. Proposed MM algorithm out performs other reported standard algorithm’s in reducing real power loss.

Ant lion optimizer for solving optimal reactive power dispatch problem in power systems

This paper presents the use of a recent developed algorithm inspired by the hunting mechanism of antlions in nature, called ant lion optimizer (ALO) algorithm for solving optimal reactive power dispatch (ORPD) problem considering a large-scale power system. The ORPD is formulated as a complex combinatorial optimization problem with nonlinear characteristic. The ALO algorithm is inspired from the hunting mechanism of antlions. One of the most interesting things in antlions is that they have a unique hunting behaviour and exhibit high capability of escaping the local optima stagnation. The ALO is used to find the set of optimal control variables of ORPD problem, such as generators terminal voltage, position of tap changers of transformers, and number of switchable capacitor banks. The performance and feasibility of the proposed algorithm are demonstrated through several simulation cases on IEEE 30-bus, IEEE 118-bus power systems and large-scale power system IEEE 300-bus power system. Comparison of obtained results with those reported in the literature shows clearly the superiority of ALO algorithm over other recently published algorithms in regards to real power losses and computational time, and hence confirmation of the efficiency of ALO algorithm in providing near-optimal solution.

A Multiobjective Minimax Regret Robust VAr Planning Model

This paper proposes a risk-based mixed integer quadratically-constrained programming model for the long-term VAr planning problem. Risk aversion is included in the proposed model by means of regret-based optimization to quantify the load shedding risk because of a reactive power deficit. The expected operation and expansion costs of new installed reactive power sources and load shedding risk are jointly minimized. Uncertainty in the active and reactive load demands has been included in the model. An $\epsilon$ -constraint approach is used to characterize the optimal efficient frontier. Also, discrete tap settings of tap-changing transformers are modeled as a set of mixed integer linear equations which are embedded into an ac optimal convex power flow. Computational results are obtained from a realistic South and South–East Brazilian power system to illustrate the proposed methodology. Finally, conclusions are duly drawn.

A review of condition monitoring techniques and diagnostic tests for lifetime estimation of power transformers

Power transformers are a key component of electrical networks, and they are both expensive and difficult to upgrade in a live network. Many utilities monitor the condition of the components that make up a power transformer and use this information to minimize the outage and extend the service life. Routine and diagnostic tests are currently used for condition monitoring and appraising the ageing and defects of the core, windings, bushings and tap changers of power transformers. To accurately assess the remaining life and failure probability, methods have been developed to correlate results from different routine and diagnostic tests. This paper reviews established tests such as dissolved gas analysis, oil characteristic tests, dielectric response, frequency response analysis, partial discharge, infrared thermograph test, turns ratio, power factor, transformer contact resistance, and insulation resistance measurements. It also considers the methods widely used for health index, lifetime estimation, and probability of failure. The authors also highlight the strengths and limitations of currently available methods. This paper summarizes a wide range of techniques drawn from industry and academic sources and contrasts them in a unified frame work.

Using a new modified harmony search algorithm to solve multi-objective reactive power dispatch in deterministic and stochastic models

The optimal reactive power dispatch (ORPD) problem is a very important aspect in power system planning, and it is a highly non-linear, non-convex optimization problem because it consists of both the continuous and discrete control variables. Since a power system has an inherent uncertainty, this paper presents both the deterministic and stochastic models for the ORPD problem in multi-objective and single-objective formulations, respectively. The deterministic model considers three main issues in the ORPD problem including the real power loss, voltage deviation, and voltage stability index. However, in the stochastic model, the uncertainties in the demand and equivalent availability of shunt reactive power compensators have been investigated. To solve them, we proposed a new modified harmony search algorithm (HSA), implemented in single and multi-objective forms. Since, like many other general purpose optimization methods, the original HSA often traps into the local optima, an efficient local search method called chaotic local search (CLS) and a global search operator are proposed in the internal architecture of the original HSA algorithm to improve its ability in finding the best solution because the ORPD problem is very complex, with different types of continuous and discrete constrains, i.e. excitation settings of generators, sizes of fixed capacitors, tap positions of tap changing transformers, and amount of reactive compensation devices. Moreover, the fuzzy decision-making method is employed to select the best solution from the set of Pareto solutions. The proposed model is individually examined and applied on different test systems. The simulation results show that the proposed algorithm is suitable and effective for the reactive power dispatch problem compared to the other available algorithms.

Weighted elitism based Ant Lion Optimizer to solve optimum VAr planning problem

In this paper, a newly proposed Ant-Lion Optimizer (ALO) is applied to solve Optimal Reactive Power Dispatch (ORPD) problem of power system. The ORPD is a VAr planning problem and is a highly non linear, non convex, challenging optimization problem; usually devised as constrained Optimal Power Flow (OPF). This paper also proposes the improvement in the search capability of ALO. A novel weighted elitism concept is introduced in the elitism phase of the original ALO to improve the exploration property of the algorithm. The proposed Modified ALO (MALO) intelligently balances both exploration and exploitation, which enhances the hunting capability of ALO. Both ALO and MALO is used to determine the optimal settings of generator voltages, tap positions of tap changer transformers and VAr output of shunt capacitors to optimize objectives: like, the active power loss, total voltage deviation and voltage stability index. The proposed algorithm is programmed and simulated on MATLB and tested on IEEE-30 and 57 bus systems. The results are compared with ALO and other methods. The effectiveness of MALO is further verified by solving few benchmark mathematical functions. The numerical results reveal that, MALO gives better optimum solutions for the benchmark functions compare to original ALO and outperforms several other methods used in the literature to solve ORPD problems. The t-Test and parametric analysis confirms the robustness and consistency of the MALO over ALO and other similar methods.

Online hierarchical and distributed method for voltage control in distribution smart grids

This study proposes a coordinated and distributed smart grid based method for optimal set points of distributed generators (DGs) as well as on-load tap changer (OLTC) transformer in the unbalanced power distribution grid. In this approach, energy efficiency is improved and voltage profile is kept in an acceptable range under different operating conditions of renewable energy resources and loads. In the first level of proposed hierarchical method, DGs are independently trying to keep voltage profile in an acceptable range in an optimal way. If the first level is not succeeding, at the second level, OLTC cooperates with the first level to eliminate the voltage violation in the system. The rules for updating control setting are derived based on distributed gradient algorithm. The proposed distributed solution presents analogous steady state results in comparison with centralised optimisation solution. Since the processing data is reduced, short-timed response to change operating conditions could be achieved. Simulation results on three distribution test systems with different sizes demonstrate the effectiveness of proposed control solution.

Evolution of Coordinated control strategy for SVC and Fixed Capacitors for Reactive power control

To control the reactive power, voltage regulation is very difficult in power system from aspects of load compensation. Reduce the power losses and voltage improvement as main aim to compute best setting of shunt devices such as shunt capacitors, SVC, tap changing transformers etc., in power system, when load will gradually increased, then automatically voltage is not maintain reference value due inductive load and energy storage elements. In this paper proposed reduce the power losses and improve voltage profile by using SVC and , switched capacitors with co-ordination have been included in the system. The proposed system has been validated to 9-bus test system considered.

Maximising wind generation through optimised operation of on‐load tap changing transformers in active distribution networks

On-load tap changing transformers are the most common control device to regulate and maintain distribution network voltage within required limits. Voltage rise issues on the other hand have become a major factor limiting greater penetration of low carbon generators, particularly in weak distribution networks. Here, the voltage rise problem is addressed through the application of optimised set-point voltage technique that aims to improve network hosting capacity to accommodate high wind penetration. It assesses the effectiveness of the technique on a realistic 289-node UK generic 11 kV distribution network using time-series optimal power flow simulations. The results reveal that when the tap changer is operated at the optimised set-point voltage, it can lead to greater energy yields. It also shows a reduction in the number of tap changing operations when the transformer is operated within the optimised deadband allowing for an improved life-span and minimum maintenance cost.

A new sequential power flow algorithm for AC/DC systems including independent multiterminal DC subsystems

This paper presents a new algorithm based on the sequential method for power flow calculation in multiterminal AC/DC systems. The proposed approach differs from other similar studies in the literature by handling two main properties. Firstly, a real equivalent circuit model is considered for the under-load tap changer transformers (ULTCs) of DC converters for the first time in the literature. Hence, new DC equations are obtained. Thus, exact and accurate results can be obtained for practical applications using the proposed algorithm. Tap values adjustment effects of the ULTCs connected to DC converters are included into the Jacobian matrix instead of the bus admittance matrix in the sequential AC power flow algorithm, as well as other ULTCs in the AC system. For this aim, new equations for calculating power and Jacobian matrix elements are obtained. Secondly, more than one independent multiterminal DC subsystem in terms of DC connection is included in the study. DC power flow calculation is performed for each DC subsystem independently in the algorithm. Thus, modularity and flexibility are provided for including or removing any DC subsystems without needing to change the proposed algorithm. The proposed approach has been tested on a modified IEEE 20-bus AC/DC test system. The proposed AC/DC power flow algorithm was written and performed in MATLAB. The results show that the proposed approach is accurate and reliable in convergence.

Analysis of step-up transformer tap change on the quantities at the point of connection to transmission grid

The analysis of a step-up transformer tap change on the quantities at the point of connection to the transmission grid is presented in this paper. The point of connection of generator TENT A6 has been analyzed, and a detailed model of this generator is available in software package DIgSILENT Power Factory. The comparison between the effect of a step-up transformer tap change on the quantities at the point of connection during automatic and manual operation of voltage regulator has been conducted. In order to conduct the analysis of the manual operation of the voltage regulator, the comparison between the different methods of modeling of these modes has been performed. Several generator operating points, selected in order to represent the need for tap change, have been analyzed. Also, previously mentioned analyses have been performed taking into account the voltage-reactive stiffness at the point of connection.

Impact of Solar Photovoltaic System on Transformer Tap Changer in Low Voltage Distribution Networks

Abstract This paper investigates the impact of solar resource variability on the operation of a low-voltage On-Load-Tap-Changer (OLTC) in a generic distribution network from the Malaysian grid. The OLTC's operation is studiedin two different weather conditions—sunny and cloudy days. The aspects analysed are the OLTC's time delay setting, PV penetration levels and PV installation location. The results suggest that the number of tap changes in a cloudy day is approximately 1.5 times higher than in a sunny day. In addition, at 50% PV penetration level on a cloudy day, the OLTC operation increases by 38% and it is doubled at 100% penetration.

Comparison of Mamdani-Type and Sugeno-Type Fuzzy Inference Systems for Transformer Tap Changing System

<p>Voltage control is considered one of the basic operational requirements of electrical power systems. The most popular voltage control equipment includes On-Load Tap Changer (OLTC) transformer controlled by Automatic Voltage Control (AVC) relay. Recent studies have shown that fuzzy inference systems (FIS) are applicable for transformer tap changing system. In this paper, FIS are developed for transformer tap changing system using Mamdani-type and Sugeno-type fuzzy models. The results of the two fuzzy inference systems (FIS) are compared. The basic difference between the Mamdani-type FIS and Sugeno-type FIS is included in this paper. It also shows which one is a more suitable choice of the two FIS for transformer tap changing system.</p>

Technical assessment of centralized and localized voltage control strategies in low voltage networks

Abstract Both the electric power demand and the penetration of small-scale embedded generators (SSEGs) is rapidly increasing in LV distribution grids. These trends are expected to have significant effects on system performance, efficiency and reliability necessitating the adjustments to the current framework for power supply dispatch and delivery. This paper proposes hybrid voltage control schemes for LV distribution networks combining: (i) centralized control via the on-load tap-changer (OLTC) of secondary distribution transformers and (ii) localized control via both the real and reactive power management from single-phase PV inverters. Specific control algorithms are proposed for both the centralized and localized methods. All the possible operation sequences of the aforementioned methods are examined. The performance of the proposed approaches is investigated on four different test feeders considering the voltage magnitudes, the voltage unbalances, the total losses and the PV power utilization.

Investigation of Phase-wise Voltage Regulator Control Logics for Compensating Voltage Deviations in an Experimental Low-voltage Network

This article investigates the control logics of an on-load tap-changer transformer by means of experimental system validation. The experimental low-voltage unbalanced system consists of a decoupled single-phase on-load tap-changer transformer, a 75-m 16-mm2 cable, a controllable single-phase resistive load, and an electric vehicle that has the vehicle-to-grid function. Three control logics of the on-load tap-changer transformer are described in the study. The three control logics are classified based on their control objectives and control inputs, which include network currents and voltages, and can be measured either locally or remotely. To evaluate and compare the control performances of the three control logics, all of the tests use the same loading profiles. The experimental results indicate the modified line compensation control can regulate voltage in a safe band in the case of various load and generation conditions.

Coordinated Voltage Control of a Wind Farm Based on Model Predictive Control

This paper presents an autonomous wind farm voltage controller based on model predictive control. The reactive power compensation and voltage regulation devices of the wind farm include static Var compensators, static Var generators, wind turbine generators and on-load tap changing transformer, and they are coordinated to keep the voltages of all the buses within the feasible range. Moreover, the reactive power distribution is optimized throughout the wind farm in order to maximize the dynamic reactive power reserve. The sensitivity coefficients are calculated based on an analytical method to improve the computation efficiency and overcome the convergence problem. Two control modes are designed for both voltage violated and normal operation conditions. A wind farm with 20 wind turbines was used to conduct case studies to verify the proposed coordinated voltage control scheme under both normal and disturbance conditions.