Some precautions for the field users of PDC measurement for transformer insulation condition assessment

Abstract

Abstract--The need for economic, reliable and effective delivery of electric power has lead to the search for new, efficient and effective methods for diagnosing the insulation of the HV equipments in the industries all over the world. One of the methods currently being investigated as a potential nondestructive diagnostic tool for condition monitoring of the oilpaper insulation of power transformers is the Polarisation and Depolarisation Current (PDC) measurement. This paper starts with a theoretical and physical description of the PDC technique. Several practical aspects of the technique have been discussed in the paper along with corresponding experimental and field test results. These physical considerations can be used as 'precautions' for the potential field users of the PDC technique for insulation condition assessment of transformers. Index Terms-Condition I. INTRODUCTIONOISTURE and ageing strongly influence the dielectric properties of oil/paper insulation system of power transformer. To assess the reliability of insulation it is necessary to know the condition of the oil and the paper separately.In recent years, new methods to assess insulation systems have been promoted in addition to the classical insulation resistance and power frequency loss factor measurements. Dielectric diagnostic measurements based on polarisation and depolarisation current measurements and return voltage measurements have gained significant importance over the last several years [1].The Polarisation and Depolarisation Current (PDC) analysis is a non-destructive dielectric testing method for determining the conductivity and moisture content of insulation materials in a transformer [2][3][4][5]. PDC can provide information about the Response Function of the dielectric and the conductivities of oil and paper. On the basis of this analysis it is possible to take further actions like oilrefurbishment or drying or replacement of the transformer. This paper presents a description of the PDC technique with the physical and mathematical explanations. The different physical parameters and practical considerations of the technique are discussed along with supporting experimental and field test results on transformers. These parameters have been identified by the authors from their experience with the actual on-site measurements over the years. It is recommended that the field-users of the PDC technique should carefully address these issues while conducting the test and also during analysis of the results. II. PDC MEASUREMENT A. Theoretical BackgroundWhen a step voltage of magnitude U 0 is applied to an initially discharged dielectric, the polarisation current flowing through it is given by (1). The process is summarised in Fig.1. (1)Where σ is the DC conductivity, ε 0 is the permittivity of vacuum, f(t) is the response function and C 0 is the geometric capacitance (measured capacitance divided by relative permittivity) of the dielectric material.Once the step voltage is replaced by a short circuit, a depolarisation current is built up. ...

Investigation of non-linearity in dielectric response measurements for transformer insulation diagnosis

“…In PDC measurements, a DC voltage of magnitude U 0 is applied to a test object for a considerable long period [10]. During this charging period, the polarisation current following through this object can be expressed as:…”

“…The voltage source is then replaced with short circuit to leave the object discharge [10]. The resultant depolarisation current is expressed as:…”

Understanding the impacts of moisture and thermal ageing on transformer's insulation by dielectric response and molecular weight measurements

“…In practice however, these initial transients are not recorded. Details of first version of the measurement system are described in [6,27]. Several new features have been added to the latest version of the RV and PDC measuring equipment developed by the research group at the School of Information Technology and Electrical Engineering, University of Queensland.…”

“…It was expected that the polarization and depolarization current will entirely pass through the oil impregnated paper insulation between the two electrodes, with minimum leakage through the oil. Such a test system, used in our previous studies [1,4,6,11,14,[25][26][27]29] was able to demonstrate the general trend of oil-paper insulation system's performance to dielectric tests. However, such a test sample, with a small capacitance between the two electrodes was suspected to have some leakage current between the electrodes through the surrounding oil, Perspex assembly, and stray capacitances.…”

“…techniques for insulation condition assessment. These include Recovery Voltage Measurement (RVM) [2][3][4], and more recently, the Polarization and Depolarization Current (PDC) measurement technique [5][6][7][8] in time domain and Frequency Domain Spectroscopy (FDS) [9][10]. These studies have shown that dielectric response measurements could be used as an effective tool for transformer condition assessment.…”