Review Of Modern Diagnostic Techniques For Assessing Insulation Condition In Aged Transformers

Cellulosic paper and oil insulation in a transformer degrade at higher operating temperatures. Degradation is accelerated in the presence of oxygen and moisture. Power transformers being expensive items need to be carefully monitored throughout its operation. Well established time-based maintenance and conservative replacement planning is not feasible in a current market driven electricity industry. Condition based maintenance and online monitoring are now gaining importance. Currently there are varieties of chemical and electrical diagnostic techniques available for insulation condition monitoring of power transformers. This paper presents a description of commonly used chemical diagnostics techniques along with their interpretation schemes. A number of new chemical techniques are also described in this paper. In recent times a number of electrical diagnostic techniques have gained exceptional importance to the utility professionals. Among these techniques, polarisation/depolarisation current measurement, return voltage measurement and frequency domain dielectric spectroscopy at low frequencies are the most widely used. This paper describes analyses and interpretation of these techniques for transformer insulation condition assessment

Investigation Of Polarization And Depolarization: Current Measurements For The Assessment Of Oil-Paper Insulation Of Aged Transformers

Moisture and ageing strongly influence the dielectric properties of oilrpaper insulation system of power transformer. Moisture measurement in oil sample generally gives inconclusive information since oilrpaper moisture equilibrium is temperature dependent and takes a long time to be in equilibrium. Direct moisture measurement of paper sample is not practicable for in-service transformers. The measurement and evaluation of the 'dielectric response'and conductivity is one possible way of diagnosing a transformer insulation condition. In a recent research project, polarization and depolarization current measurement has been used for assessing the condition of oilrpaper insulation. The polarization and depolarization current PDC analysis is a non-destructive dielectric testing method for determining the conductivity and moisture content of insulation materials in a transformer. On the basis of this analysis it is possible to take further actions like oilrefurbishment, drying or replacement of the winding of the transformer. This paper presents a description of the PDC technique with the physical and mathematical background and some results of PDC measurements on several transformers. Analyses and interpretation of the field test data are also presented in this paper

The Reliability and Power Quality Performance of Overhead Lines - with Reference to the Electrical Properties of Wood and Covered Conductors

In the information technology age, users of electricity services require both better reliability (fewer interruptions of supply) and better power quality (fewer disturbances, particularly voltage sags). The paper shows that the best way to reduce the frequency of interruptions and voltage sags is to reduce the number of faults on the distribution utility's low voltage and high voltage lines. Most utilities already use lines with wood poles and crossarms with the wood's electrical properties used to good effect. But a trend to 'maintenance-free' designs will inevitably increase the frequency of faults, particularly those caused by direct and nearby lightning and by wind-effects. The most effective way to reduce the frequency of faults on existing overhead lines would be to develop an acceptable technology for cost-effective retro-fitting of insulating covers on those spans of existing lines near trees

Investigating the impact of Powerformer on voltage stability by dynamic simulation

This paper provides an overview of a dynamic analysis carried out on a modified Nordic test system to determine the impact of the Powerformer on voltage stability. The unique aspects of the Powerformer will be highlighted and the modeling of long-term dynamic elements, especially those pertinent to the study of the Powerformer will be discussed. Overexcitation limiter models created for use in the PTI PSS/E analysis program will be discussed and utilized. The importance of choosing the right value of overexcitation limiter gain will be highlighted and discussed. The impact of the location of the Powerformer and the compensation scheme utilized on the time to collapse following a system contingency will also be highlighted and discussed

Load capability and collapse margin analysis of the large scale Queensland power system

This paper provides an overview of load capability and collapse margin analyses carried out on a modified CIGRE 'Nordic' test system and on the large-scale Queensland Transmission system. The importance of certain key generators in the loading capability of a transmission system is highlighted. Emphasis is also placed on the impact of potential installations of Powerformer (TM) on the loading capability of these two real and representative systems. Powerformer (TM) is a unique form of generator that can be directly connected to transmission, without the need for a step-up transformer. Select system sensitivity parameters are determined for the two systems and used to aid explanation of the relative importance of the key generators and the impact of Powerformer observed

Unveiling the dynamics of canard cycles and global behaviour in a singularly perturbed predator-prey system with Allee effect in predator

In this article, we have considered a planar slow-fast modified Leslie-Gower predator-prey model with a weak Allee effect in the predator, based on the natural assumption that the prey reproduces far more quickly than the predator. We present a thorough mathematical analysis demonstrating the existence of homoclinic orbits, heteroclinic orbits, singular Hopf bifurcation, canard limit cycles, relaxation oscillations, the birth of canard explosion by combining the normal form theory of slow-fast systems, Fenichel's theorem and blow-up technique near non-hyperbolic point. We have obtained very rich dynamical phenomena of the model, including the saddle-node, Hopf, transcritical bifurcation, generalized Hopf, cusp point, homoclinic orbit, heteroclinic orbit, and Bogdanov-Takens bifurcations. Moreover, we have investigated the global stability of the unique positive equilibrium, as well as bistability, which shows that the system can display either 'prey extinction', 'stable coexistence', or 'oscillating coexistence' depending on the initial population size and values of the system parameters. The theoretical findings are verified by numerical simulations.Comment: 26 pages, 13 figure

An Attempt to Correlate Time and Frequency Domain Polarisation Measurements for the Insulation Diagnosis of Power Transformer

The insulation system of power transformers (oil and paper/pressboard) generally change their dielectric properties during the life of the transformer and dielectric measurements have therefore been applied to assess the quality or state of ageing of the system. Both time domain and frequency domain dielectric diagnosis techniques have recently gained popularity in providing information on the condition of transformer oil-paper insulation. The present paper aims at deriving a correlation between these two methods of diagnosis. Simulation results along with actual field test results have demonstrated a direct correlation between the time and frequency domain tools for insulation diagnosis

Separation Of Ageing And Moisture Impacts On Transformer Insulation Degradation By Polarisation Measurements

This paper describes an accelerated ageing experiment of transformer oil/paper insulation with preset moisture levels. These samples were tested with return voltage and polarisation/depolarisation measurements. Molecular weight has also been measured with Gel Permeation Chromatography (GPC). Results suggested the impact of moisture on the ageing of oil/paper insulation. An attempt has been made regarding the separation of ageing and moisture on oil/paper insulation using the dielectric polarisation measurements

Experience with Return Voltage Measurements for Assessing Insulation Conditions in Service Aged Transformers

This paper describes the investigation of the insulation condition of a number of power and distribution type transformers of different manufacturing dates and with different operating histories. Return voltage measurements were conducted for these transformers. Effects of moisture and aging on the oilpaper insulation of these transformers were investigated and results are presented in this paper. Moisture in oil samples at known temperature was measured for these transformers. Previously accelerated aging experiments were performed on paper wrapped insulated conductors in different environments over a temperature range of 115 to 145 C. Results from the measurements on transformers are compared with those of the accelerated aged samples and are described in this paper

Voltage Stability Analysis of Grid Connected Embedded Generators

The increasing costs and stringent environmental regulations are making the construction of large power stations to meet rising energy demands economically unfeasible. Hence, Embedded Generation (EG) is predicted to play a prominent role in the electric power systems of the future. The term 'embedded generation' refers to electricity generation connected at distribution level rather than transmission level. The insertion of EGs presents a new set of conditions to distribution networks. The aim of this paper is to conduct a voltage stability analysis using an iterative power system simulation package, PowerWorld (TM) Simulator, to evaluate the impact of strategically placed EG on distribution systems with respect to the critical voltage variations and collapse margins. This paper concludes with the discussion of EG's excellent options for system reactive power compensation and voltage stability