On-load Tap Changing Transformers Research Articles

Applying protection principles for controlling distributed generation

In a distribution system, it is essential to maintain the voltage variation within a specified limit for satisfactory operation of connected customers' equipment. Normally, this goal is achieved by controlling the operation of compensating devices, such as load tap changing transformers, shunt capacitors, series capacitors, shunt reactors and static VAr compensators. However, technical and regulatory developments are encouraging a greater number of small generator units, known as distributed generation (DG), and this has the potential to significantly affect voltage control systems. This paper presents an adaptive voltage control technique that incorporates DG systems into the voltage control system. The control scheme uses on-load tap changing transformer (OLTC) and DG for voltage corrections, both are driven by advanced line drop compensators (LDC). At the substation, the LDC is employed to control step-up or step-down decisions of the OLTC, while another LDC will be used at the DG connection point to set DG parameters. Also, for a more cost-effective system, voltage control action coordination is proposed using magnitude and time grading. The control approach is tested on a modified distribution system with load variations that are stochastic in time and location. The results show that the integration of these magnitude and time grading protection principles have considerably reduced the DG energy required to achieve the desired control.

Short and Long-Term Dynamic Voltage Instability

This paper presents a novel approach to capture the development of dynamic voltage instability caused by the dynamics of different power system devices, such as loads, generators, automatic voltage regulators (AVR), overexcitation limiters (OXL), power system stabilizers (PSS), and on-load tap changing (OLTC) transformers using an accurate time-domain analysis. A small power system model is presented which allows one to analyse combinations of these effects, showing how different major forms of long-term and short-term dynamic voltage instability occur. Effects of line tripping, sudden change of load, and fault clearing time on dynamic voltage instability will also be discussed. Finally, advantages of the dynamic analysis over the static analysis will be investigated.

Identification of static load characteristics based on measurements in medium-voltage distribution network

The authors deal with static load (SL) characteristics (P–V and Q–V characteristics) of a medium-voltage distribution network derived from field measurements. The experiments have been performed by changing the tap position of an on-load tap-changing transformer in different seasons, different week days and day periods. P–V and Q–V load characteristics are divided into two classes, transient and steady-state characteristics. The differences between them are discussed and quantified. Both classes are mathematically described by polynomial, linear and exponential SL models. Model parameters are obtained by curve fitting using a least square method. The adequacy of developed load models is critically assessed and discussed. All the static characteristics are grouped according to seasons and day periods and are mutually compared. Finally, the parameters of the SL models for residential load without electrical heating are suggested for different seasons.

Decentralized robust control of uncertain Markov jump parameter systems via output feedback

This paper addresses the problem of decentralized robust stabilization and control for a class of uncertain Markov jump parameter systems. Control is via output feedback and knowledge of the discrete Markov state. It is shown that the existence of a solution to a collection of mode-dependent coupled algebraic Riccati equations and inequalities, which depend on certain additional parameters, is both necessary and sufficient for the existence of a robust decentralized switching controller. A guaranteed upper bound on robust performance is also given. To obtain a controller which satisfies this bound, an optimization problem involving rank constrained linear matrix inequalities is introduced, and a numerical approach for solving this problem is presented. To demonstrate the efficacy of the proposed approach, an example stabilization problem for a power system comprising three generators and one on-load tap changing transformer is considered.

Optimum Reactive Power Dispatch and Identification of Critical On-load Tap Changing (OLTC) Transformers

The voltage stability problem has become a major concern in planning and operation of today's power system as a result of stressed conditions. It is known that the following disturbances, on-load tap changers (OLTCs), act to restore the load bus voltage and thus restore the load power to near pre-disturbance level. But the operation of OLTCs has a significant influence on voltage stability under some operating conditions. This article discusses the effect of OLTCs with different objectives of reactive power dispatch and identify critical OLTCs based on voltage stability criteria. Results obtained on an equivalent system of 24-node EHV system are presented.

Loading Guide for Individual Transformers with On-load Tap-Changer Facility and the Comparison of Calculation Results with Established Loading Guide and the Consideration

4 units differential equations were introduced to propose individual loading guide instead of established loading guide in the previous paper. This paper showed how to determine the coefficients of the equations above mentioned by using the manufacturers examination reports of each transformer and further, to regulate the coefficients by the field measurement data.But, for on-load tap-changing transformers (especially distribution transformers), tap position is continuously changing during load cycle, therefore primary resistances and stray load loss shall also change accompanied with this phenomenon. Therefore tap changing facility should be added to the equations to make individual loading guide more reliable and accurate than the before. To realize mathematical method, impedance test examination in each manufacturer examination reports are also used to assume primary leakage inductances, second leakage inductances and the mutual inductances.In this paper, the method of how to detect the position of tap changer and the comparison between the calculation results of established loading guide and proposing individual loading guide shall be showed, thereafter consider the overall results.

Condition assessment of power transformer onload tap changers using wavelet analysis and self-organizing map: field evaluation

An onload tap changer (OLTC) is the most maintenance intensive subassembly on a power transformer. Vibration monitoring is an effective technique that can be used to assess the condition of an OLTC nonintrusively. The authors have developed a condition monitoring system for common types of OLTCs that enables the condition of tap changer contacts and associated drive system to be inferred from vibration signals. A number of prototype systems have been installed onto OLTCs in distribution zone substations for field trials. Particular emphasis has been given to the detection of faults in a particular type of older tap changer that had been prone to a range of faults associated with the switching contacts and drive mechanism. For this type of tap changer, it has been shown to be possible to determine not only that the tap changer is aging but also to identify the particular part that is degrading.

Condition Assessment of Power Transformer on-Load Tap-Changers Using Wavelet Analysis and Self-Organizing Map: Field Evaluation

An on-load tap-changer (OLTC) is the most maintenance-intensive subassembly on a power transformer. Vibration monitoring is an effective technique that can be used to assess the condition of an OLTC effectively and nonintrusively. The authors have developed a condition monitoring system for common types of OLTCs that enables the condition of tap changer contacts and associated drive system to be inferred from vibration signals. A number of prototype systems have been installed onto OLTCs in distribution zone substations for field trials. Particular emphasis has been given to the detection of faults in a particular type of older tap-changer that had been prone to a range of faults associated with the switching contacts and drive mechanism. For this type of tap-changer, it is possible to determine not only that the tap-changer is aging but also to identify the particular part that is degrading.

Condition assessment of power transformer on-load tap-changers using wavelet analysis

Condition assessment of power transformer on-load tap-changers using wavelet analysis

Condition Assessment of Power Transformer on-Load Tap-Changers Using Wavelet Analysis

The operation of a power transformer on-load tapchanger (OLTC) produces a well-defined series of vibration bursts as its signature. Due to the harmonic and nonstationary nature of the transient vibration signal traditional frequency and time-frequency techniques are on longer effective for characterization of this type of vibration signals as the localized time domain features-such as delays between bursts, the number of bursts, and the strengths of bursts-are essential for the condition assessment of OLTC. A wavelet transform-based technique is developed in this paper to characterize the OLTC vibration signals. This technique gives a simplified format for displaying and representing the essential features of the OLTC vibration signatures. Application results from a selector type OLTC demonstrate that the features extracted in the wavelet domain can be utilized to provide reliable indications of the actual heath of an OLTC.

Condition monitoring of power transformer on-load tap-changers. Part 2: Detection of ageing from vibration signatures

The paper describes a technique for on-line automatic condition assessment of an on-load tap-changer (OLTC) using a self-organising map (SOM). With a condition indicator giving the correct indication of the current condition status, an estimate can be made of the remaining life of the equipment. The condition assessment technique is demonstrated using the signatures collected by on-line monitoring systems installed on selector type OLTCs in distribution substations. Using the real-time fault detection procedure, reliable identification of incipient faults in the equipment can be achieved for the pre-specified false alarming rate.

Condition monitoring of power transformer on-load tap-changers. Part 1: Automatic condition diagnostics

Automatic diagnostics for an on-load tap-changer (OLTC) requires a reliable technique that can classify vibration signals measured using an accelerometer mounted on the tank. In the paper, the authors investigate the automatic classification of OLTC vibration signatures using a self-organising map (SOM) and develop a feature extraction procedure that can extract essential features from the original vibration signature. The proposed SOM signature classifier is evaluated with a database established for one type of distribution class OLTC. The application results reveal the practical advantages of SOM for a number of tasks in OLTC condition diagnostics.

Automatic voltage control relays and embedded generation

Keeping customer voltages within tolerance has always been a priority for distribution network operators. On-load tapchanging transformers and their associated automatic voltage-control relays play a pivotal role in this. Unfortunately, such relays can be affected by the connection of embedded generation. Sometimes this presents an apparent limit to the allowable penetration of embedded generation in a given area. Any such limit is in fact highly dependent on the configuration and settings of the relays and a clear understanding is necessary in order to ensure that the penetration is not unnecessarily restricted. This is the first of two articles that seek to promote such understanding by reviewing the use of voltage-control relays typically in the UK today: master-follower line-drop-compensation, true-circulating-current detection and negative-reactance compounding.

Evaluation of control sequence for improving voltage stability in power systems

This paper addresses an investigation of control sequence for improving voltage stability in power systems. The controllers include transformers (step-up, EHV and on-load-tap-changer (OLTC)), generators, and shunt capacitors. Dynamic simulations are performed first for a typical five-bus system covering the above mechanisms: (1) the time constants for the EHV transformer and the OLTC transformer are evaluated first; (2) the generator voltage and the step-up transformer are utilized to evaluate the performance; (3) the effectiveness of the shunt capacitors at load buses and at EHV buses is assessed; (4) the effectiveness of lock of the OLTC is examined; and (5) finally, the system voltage evolutions with different starting times for both EHV/OLTC taps are compared to ensure a proper control action. Based on the above results, a set of 12 dynamic simulations is used to explore the control sequence for effectiveness in improving voltage stability. It is found that the best control sequence is as follows: generator voltages, shunt capacitors at load buss, EHV transformer taps and then OLTC taps.

Dynamic reactive load model

A new dynamic reactive load model for power system dynamic voltage stability studies is introduced in this paper. The model is measurement-based and incorporates the effect of voltage regulating equipment, specifically, switched distribution capacitor banks and subtransmission substation on-load tap changer transformers. A set of parameters that reflect these delayed-action devices are involved in the proposed model. A parameter estimation scheme is introduced with the aim of minimizing errors between model and collected data at two Pennsylvania Power and Light Company 230/64 kV substations. System simulation is also carried out based on the estimated parameters.

Steady-state model of direct connected generator–HVdc converter units in the harmonic domain

A new twelve-pulse unit connection model is formulated and solved in the harmonic domain. The model takes into account the effects of generator saliency and saturation, interaction of the converter with the DC system, and variation in the firing and end of commutation angles caused by AC voltage and DC current harmonics. The reactive power control by generator excitation in the absence of converter transformer on-load tap-changers is also modelled. The solution of a test system is validated by means of time domain simulations.

Intelligent tap changer duty cycle control for load voltage improvement

Voltage stability of a power system is related to load characteristics and voltage control devices such as on-load tap-changing (OLTC) transformers. This paper discusses the need for enhancing the OLTC operation, and identifies five control requirements for intelligent control of the OLTC duty cycle. The paper also describes an adaptive fuzzy logic controller (AFLC) developed for satisfying these requirements. By simulations on a six-bus study system, the ALFC is shown to be an effective means of enhancing the OLTC operation. The five control requirements are fulfilled by preventing the reverse actions, suppressing voltage transients, reducing control errors, and ensuring fast response and robustness.

Challenging algorithm for optimal reactive power dispatch through classical co-ordination equations

A maiden attempt is made to develop an algorithm based on classical co-ordination equations for optimal reactive power dispatch as an alternative to existing algorithms for real-time application. A set of new loss formulae for both active and reactive power losses is proposed. An innovative approach considering the concept of fictitious reactive powers is used for modelling on-load tap-changing (OLTC) transformers. A maiden attempt is made to consider effectively constraints on bus voltages by expressing these voltages in terms of reactive power generation through distribution factors, which are developed from already available load flow information using a perturbation technique. The proposed model, based on classical co-ordination equations, is tested on IEEE 14- and 30-bus test systems and the results are compared with those obtained by more rigorous methods.

A New Improved Algorithm for OLTC Transformers in the Fast Decoupled Load Flow

This paper describes a new improved version of the fast decoupled load flow that directly incorporates the on-load tap changing transformer ratios into the approximate Jacobian matrix so that the tap ratios are adjusted during solution in the same manner as the bus voltage magnitudes. This algorithm has been found to have superior convergence reliability to the normal fast decoupled load flow, particularly for severe contingency cases where the power system is close to voltage collapse. This algorithm has been found to be particularly suited to on-line applications where convergence reliability takes precedence over solution speed.

Improved Voltage Regulation in Power Systems by Microcomputer Control

The role of on-Load tap Changing transformers to voltage collapses has only recently recived some attention, although this phenomenon has been known for a long time. The use of microcomputer control makes it possible to introduce more elaborate rules of operation, not only limited to a simple voltage modulus comparision on the Load side of the transformer. It seems obvious that a decentralized control could be better turned than a centralized one, based on a somewhat general appreciation of voltage level. Further More local load shedding could also be included. After a short theoretical analysis of the unstable mode of automatic on-Load tap changer operating two critical zones are defined. A control program is proposed to prevent operation near these zones and in this way avoid voltage collapse. This program has been successfull implemented on a laboratory model.