Transformer Tap Positions Research Articles

Fuzzy-logic method for adjustment of variable parameters in load-flow calculation

The paper presents a new approach using fuzzy logic to adjust variable parameters so as to meet constraints during load-flow study. Parameters to be adjusted include transmission-line impedance, phase angle and transformer tap position, and the constraints are transmission-line power flow and bus-voltage magnitude. Versatile apparatus which are used to tune these parameters include the thyristor-controlled series compensator, thyristor-controlled phase shifter, unified power-flow controller and on-load tap changer. The proposed approach is particularly aimed at assisting convergence in load flow programs. The parameters to be adjusted are decoupled from the main body of the load-flow formulation. The methodology for parameter adjustment is conducted by using fuzzy logic.

Optimizing generator reactive power resources

A method is described for selecting generator step-up and auxiliary transformer tap positions to provide the desired reactive power requirements in a coal burning power plant. It is shown how a generator could be made to absorb reactive power, when previously unable to do so, by the adjustment of tap positions on the generator step-up and auxiliary transformers. The approach described provides an indication of the range of reactive power outputs that are available for different tap positions. This information would facilitate the alteration of transformer tap positions to make more effective use of generator reactive power resources.

Optimal reactive power dispatch using an adaptive genetic algorithm

This paper presents an adaptive genetic algorithm (AGA) for optimal reactive power dispatch and voltage control of power systems. In the adaptive genetic algorithm, the probabilities of crossover and mutation, p c and p m, are varied depending on the fitness values of the solutions and the normalized fitness distances between the solutions in the evolution process to prevent premature convergence and refine the convergence performance of genetic algorithms. The AGA applied for optimal power system reactive power dispatch is evaluated on an IEEE 30-bus power system in which the control of bus voltages, tap position of transformers and reactive power sources are involved to minimize the transmission loss of the power system.

ALLEVIATION OF VOLTAGE VIOLATIONS AND LINE OVERLOADS IN POWER FLOW SOLUTIONS

ABSTRACT This paper describes a method to alleviate bus voltage violations and line overloads by minimizing a weighted quadratic performance index. The performance index consists of a weighed sum of active and reactive power mismatches together with system variable increments. Violations of voltages and line overloads are identified from the base case of the power flow solution. The performance index is minimized to alleviate these violations. Different weighting factors are assigned to the variables in order to control the amount of change in these variables. The variables are bus angles and voltages, transformer tap positions and active power generation. Generator voltages and active power generation together with transformer tap positions are then modified and the power flow problem is solved. The process is repeated until convergence is achieved. Simulation results of IEEE 30 and 118 bus test systems are given. The results demonstrate the ability of the method to alleviate bus voltage violations and line overloads in the solution of the constrained power flow problem.

A non-incremental model for optimal control of reactive power flow

Based on the decoupled principle, a non-incremental quadratic programming model for optimal control of reactive power flow in power systems is presented in this paper. In the model, the reactive powers through all lines are regarded as the unknown variables to be evaluated, as well as the transformer tap positions, generator terminal voltages and switchable injected reactive powers; and a characteristic linear approximation of power flow is formulated. Because of the characteristics of the non-incremental variables of the model, execution time and memory requirements and convergence properties are satisfactory; thus, it is particularly fit to improve voltage magnitudes and to minimize system losses under operating conditions. Real power economic dispatch is accomplished by standard techniques. Numerical results show the practicality of the model.

Methodology for alleviation of voltage excursions in large power systems

This paper presents a method for minimizing the sum of the square of voltage deviations by a least-square minimization technique, and thus improving the voltage profile in a given system by adjusting control variables, such as tap position of transformers, reactive power injection of VAR sources and generator excitations. The control variables and dependent variables are related by a matrix J whose elements are computed as the sensitivity matrix. Linear programming is used to calculate voltage increments that minimize transmission losses. The active and reactive power optimization sub-problems are solved separately taking advantage of the loose coupling between the two problems. The proposed algorithm is applied to IEEE 14-and 30-bus systems and numerical results are presented. The method is computationally fast and promises to be suitable for implementation in real-time dispatch centres.

Transformer model for application in load‐how analysis

AbstractThis paper gives a closed formula which expresses the variation of the leakage reactance of transformers as a function of the transformer tap position. The derivation of the formula is based on the analysis of the distortion of the leakage field as portions of the winding are cut out by means of tappings. Three cases are examined, depending on whether taps are cut out: ‐ asymmetrically at the end of a winding, ‐ at the electrical centre of a winding or ‐ symmetrically at the end of a winding. The validity of the derived model is tested by comparison with existing transformer impedance correction data. Finally, test results from the incorporation of the model in a load‐flow program are presented.

Transformer tap position estimation and bad data detection using dynamic signal modelling

Tracking estimation of a transformer tap position is considered. An estimation algorithm incorporating additional information about the dynamic behaviour of the measurements and tap positions is developed. It is based on the simultaneous state and tap position estimation method augmented by measurement prefiltering based bad data detection and incorporation of a tap position dynamics model into the estimation algorithm. Increased bad data identification reliability and the robustness of the estimates to the measurement noise is obtained. The performance of the algorithm has been examined using the IEEE 30 and 118-node systems. >

State estimation of voltage and phase-shift transformer tap settings

A transformer tap estimation technique is presented that overcomes earlier limitations because of a direct algorithmic approach. The procedure provides for turn ratio and phase angle measurements, and treats each transformer tap setting as an independent state variable. The problem of discrete tap positions is addressed, and an appropriate integer variable technique is presented. The method is general and is compatible with state estimation techniques which are based on network measurements and the Jacobian matrix. Because of the simultaneous solution of taps with complex bus voltages, the method is not predicated on the presence of transformer tap position, terminal voltage, or reactive flow measurements, although the method exploits such measurements if available. The method allows for bad data detection of anomalous tap measurements. Test results for an actual 300-bus network demonstrated the tap estimation capability. >

Overload and voltage control of power systems by line switching and generation rescheduling

Presents of fast mathematical technique for solving security related problems in the steady-state operation of a power system. Both thermal overloads of transmission elements and nodal voltage violation problems are dealt with. In order to minimize the increase in operating costs, priority is given to the rescheduling of the network topology; however, the real and reactive power generation outputs and tap position of voltage transformers are also considered. A new network topology is determined by a set of distribution factors that relate the violating constraints to the transmission elements. The remaining control variables are determined by sensitivity analysis techniques. The violating constraints are reduced in steps so that, at any point during the computation, the most recent operating point is better than the previous one. The proposed algorithm has been implemented and tested on 24- and 118-bus power systems.

An elegant approach to optimal real and reactive power dispatch

This paper presents a new reactive VAR and real power dispatch method for the economic operation of a power system. The analysis is twofold, involving successive evaluation of the optimum voltage and the real power generation. The former is obtained by reactive VAR control, avoiding non-linear programming techniques, and the latter by a closed-form expression of λ, a linear function of the penalty factors. Transmission losses are considered as a function of the system voltage and are minimized by adjusting the transformer tap positions, the reactive power injection of VAR sources, and the generator voltage magnitude dependent on excitation. The feasibility of the technique is demonstrated by means of a standard system.

An Efficient Technique for Reactive Power Dispatch Using a Revised Linear Programming Approach

Abstract This paper presents a mathematical formulation of the reactive power operation/planning problem. The outstanding features of the proposed method are represented by the fact that it requires no matrix inversion, will save computation time and memory space, and hence can be implemented on very large-scale power systems. The method employs a linearized objective function and constraints, and its approach is based on adjusting control variables which are tap positions of transformers and reactive power injection. Linear programming is used to calculate voltage increments which minimize transmission losses, and adjustments of control variables are obtained by a modified Jacobian matrix. This approach would greatly simplify the application of decomposition methods in power systems planning and operation. The proposed algorithm is applied to a six-bus system and the IEEE 30-bus system and numerical results are presented. These results verify the superiority of the proposed method over the existing ones.

Network parameter estimation using online data with application to transformer tap position estimation

This paper deals with the estimation of parameters involved in the classical steady-state modelling of network elements within the context of power system state estimation. A general method is proposed for the identification and correction of the real-time data, based on the sensitivity relationship between the measurement residuals and the parameter error; this relationship is treated as an estimation problem, yielding a parameter error estimate. Special emphasis is put on the real-time correction of erroneous transformer tap positions, the most frequent parameter errors during the real-time operation of state estimators. The proposed method is very simple and efficient, since it merely requires a single substitution in the state estimation linear system. Relationships with other methods are also discussed. Simulation results on a real-life system are reported.

Real-time processing of transformer tap positions

Reviews the methods for network parameter estimation and correction in power system state estimation and proposes a sequential-type method for real-time processing of transformer tap positions. The method exploits the information contained in the measurement residuals to estimate a better tap position; it uses a linearized sensitivity model to relate the measurement residuals to the tap position error. Based on part of the Belgium HV system, several testing results are reported.

A New Approach for Minimizing Power Losses and Improving Voltage Profile

This paper presents a new method for minimizing transmission line losses and improving voltage profile in a given system by adjusting control variables, i.e., tap position of transformers and reactive power injection of VAR sources. Transmission losses are considered as a function of voltage increments. The control variables and voltage increments are related by a modified Jacobian matrix. Linear Programming (LP) is used to calculate the voltage increments which minimize the transmission losses, and the adjustments of control variables are obtained by a modified Jacobian matrix. Since this method does not need any matrix inversion, it will save computational time and memory space.

Transformer Tap Position Estimatiion - Field Experience

This paper presents the field experience with a simplified tap estimation algorithm implemented at the Union Electric Load Dispatch System. The technique enhances the capabilities of the State Estimator by generating estimates for non-telemetered transformer tap positions. Various test results indicate that the method is simple, fast and highly suitable for real-time applications. Actual field data are compared with the calculated tap positions. The paper presents some background information followed by algorithmic descriptions and test results on the Union Electric system.

Transformer Tap Position Estimation

The estimation of voltage transformer tap position is a desirable feature that can be appended to most state estimation algorithms. The primary requirement to estimate transformer tap position is the availability of a three-phase reactive-power measurement through either the fixed or tap side of the transformer. These ``measured'' flows can also be calculated, depending on the availability of reactive injection and reactive transmission line flow measurements around the fixed or tap side buses. The gain matrix of the state estimator is not modified so only the tables used to calculate real and reactive residual mismatch need to be updated during the state estimator solution. The computational cost of including transformer tap position estimation is extra iterations of the state estimator. A number of tests were conducted using the IEEE 118 bus system as a model. The results of one of these tests are presented along with the algorithm to modify the tap positions while the State Estimator solution proceeds. A practical stopping criterion for the algorithm is also presented. The extension of the algorithm to line parameter estimation is also discussed in the Appendix.

Transformer Tap Position Estimation

The estimation of voltage transformer tap position is a desirable feature that can be appended to most State Estimation algorithms. The primary requirement to estimate transformer tap position is the availability of a three phase reactive power measurement through either the fixed or tap side of the transformer. These "measured" flows can also be calculated, depending on the availability of reactive injection and reactive transmission line flow measurements around the fixed or tap side buses. The gain matrix of the State Estimator is not modified so only the tables used to calculate real and reactive residual mismatch need to be updated during the State Estimator solution. The computational cost of including transformer tap position estimationis extra iterations of the State Estimator.

The Han Powell Algorithm Applied to the Optimization of the Reactive Power Generation in a Large Scale Electric Power System

In this paper the Han-Powell algorithm is proposed as a means for optimizing the reactive power generation in a large scale system. The algorithm is based upon the iterative resolution of quadratic programming subproblems. Other approaches based upon recursive resolution of linear programs seem not to be suitable for the strongly non linear characteristics of the problem.In the adopted model, a reduced and compact problem is solved with only inequality constraints.The equality constraints (load flow equations) are utilized to eliminate a large number of variables (dependent variables). In this way the optimization model is expressed only in terms of the control variables (reactive power generations or generator terminal voltages and LTC transformer tap positions). This approach allows ns to perform the reactive power scheduling of the Italian electric production and transmission system (with about 500 nodes) with reasonable computer requirements (CPU time and central core). The main advantages of the Han-Powell method are:-it is robust and reliable-it shows a sure and fast convergence-it is easy to implement provided a good quadratic programming subroutine is available.

Optimal Control of Reactive Power flow for Improvements in Voltage Profiles and for Real Power Loss Minimization

A mathematical formulation of the optimal reactive power control (optimal VAR control) problem and results from tests of the algorithm are presented in this paper. The model minimizes the real power losses in the system. The constraints include the reactive power limits of the generators, limits on the load bus voltages, and the operating limits of the control variables, i.e., the transformer tap positions, generator terminal voltages and switchable reactive power sources. Real power economic dispatch is accomplished by standard techniques.