Partial Discharge Calibration Research Articles

Development of an Industrial Partial Discharge Calibrator and Its Performance Tests

Development of an Industrial Partial Discharge Calibrator and Its Performance Tests

Methods for Partial Discharge Calibration in Gas-Insulated Substations for HVDC Power Grids and Charge Evaluation Uncertainty

Methods for Partial Discharge Calibration in Gas-Insulated Substations for HVDC Power Grids and Charge Evaluation Uncertainty

New Synthetic Partial Discharge Calibrator for Qualification of Partial Discharge Analyzers for Insulation Diagnosis of HVDC and HVAC Grids.

A synthetic partial discharge (PD) calibrator has been developed to qualify PD analyzers used for insulation diagnosis of HVAC and HVDC grids including cable systems, AIS, GIS, GIL, power transformers, and HVDC converters. PD analyzers that use high-frequency current transformers (HFCT) can be qualified by means of the metrological and diagnosis tests arranged in this calibrator. This synthetic PD calibrator can reproduce PD pulse trains of the same sequence as actual representative defects (cavity, surface, floating potential, corona, SF6 protrusion, SF6 jumping particles, bubbles in oil, etc.) acquired in HV equipment in service or by means of measurements made in HV laboratory test cells. The diagnostic capabilities and PD measurement errors of the PD analyzers using HFCT sensors can be determined. A new time parameter, "PD Time", associated with any arbitrary PD current pulse i(t) is introduced for calibration purposes. It is defined as the equivalent width of a rectangular PD pulse with the same charge value and amplitude as the actual PD current pulse. The synthetic PD calibrator consists of a pulse generator that operates on a current loop matched to 50 Ω impedance to avoid unwanted reflections. The injected current is measured by a reference measurement system built into the PD calibrator that uses two HFCT sensors to ensure that the current signal is the same at the input and output of the calibration cage where the HFCT of the PD analyzer is being calibrated. Signal reconstruction of the HFCT output signal to achieve the input signal is achieved by applying state variable theory using the transfer impedance of the HFCT sensor in the frequency domain.

Magnetic and electric antennas calibration for partial discharge charge estimation in gas-insulated substations

There are no accepted procedures that quantify the apparent charge of partial discharge (PD) in gas-insulated substations (GIS). This paper proposes a calibration method for PD charge estimation using unconventional electromagnetic sensors: a magnetic loop antenna (inductive coupler) and an electric antenna (capacitive coupler.) The calibration procedure is intended for the voltage double integral method, which is reviewed for magnetic antennas and extended for electric antennas. By injecting low-frequency sinusoidal signals, the calibration constants are determined for two different test setups: the first one being a testbench where the characteristic impedance is matched and the second one a full-scale 420 kV GIS. The calibration method is validated in three ways: with a calibrated pulse in the testbench, a calibrated pulse in a full-scale GIS, and PD defects in the full-scale GIS. The calibration procedure revealed a frequency limit range dependent on the GIS length and the sensor’s signal-to-noise ratio. The three validation methods showed low charge estimation errors for the magnetic and electric antennas, demonstrating that the PD calibration method applies to any electric/magnetic detector with a low-frequency derivative response. This research paves the way for better GIS insulation monitoring and PD sensor harmonization.© 2017 Elsevier Inc. All rights reserved.

Impedance Matching Circuit Design with Partial-Discharge Calibration and Power Supply Modeling for Atmospheric-Pressure DBD Plasma

Impedance Matching Circuit Design with Partial-Discharge Calibration and Power Supply Modeling for Atmospheric-Pressure DBD Plasma

A New Application of Rogowski Coil Sensor for Partial Discharge Localization in Power Transformers

This article deals with a new high efficient and cost effective lattice-Rogowski-coil sensor for partial discharge monitoring of power transformers. The sensor is a thin flat-shape printed circuit board type Rogowski-coil, which can be installed on the internal surface of the transformer tank with minimum disturbance to the normal operation of the transformer. Thanks to the accurate and desirable geometry of the sensor, precise monitoring can be carried out. Also, it was designed to handle the common trade-off between the low resonant frequency and high mutual inductance. An experimental setup for measuring the lumped model parameters of the sensor is made and it is shown that the desired resonance frequency of approximately 10 MHz is obtained. In order to evaluate the performance of the sensor, a specially prepared distribution transformer (20 kV/0.4 kV, 500 kVA) was considered. Partial discharge calibration pulses were injected into different locations of the winding of this transformer, and the ability of the sensor was verified in defect localization. Moreover, the effects of some practical aspects, like the value of the terminating resistor of the proposed special Rogowski-coil, its distance from transformer winding as well as the capability of the extracted features from detected PD signals, on the accuracy of PD localization are studied.

A novel wideband partial discharge measuring circuit under fast repetitive impulses of static converters

Nowadays fast switching power electronic devices are used for driving high voltage electric rotating machines. These power supplies apply sharp transient voltage at the time of switching. To study the effect of these stresses on the insulation system, investigating Partial Discharge (PD) during voltage rapid changes is of special interest. Although, the implemented coupling capacitor in a conventional PD measuring circuit provides a low impedance pass for the PDs, but imposes a transient current caused by sharp changes of voltage at the time of switching; hence, the measuring impedance of the conventional partial discharge equipment cannot distinguish an actual PD pulse from the mentioned transduced capacitive current pulses. Currently, two options are available in order to cope with this issue, including UHF technique and implementation of band pass filters. On the one hand, calibration of the PD pulses in the UHF method is not possible; hence, the PD test results of different manufacturers of machines under such stresses cannot be compared, which is the main drawback. On the other hand, one has to remove a wide range of frequency components of PD pulses that makes PD calibration procedure to be invalid. In this paper, a resonant full bridge topology is implemented to cancel out the capacitive pulse shaped current, where the PD pulses are captured directly via electrical method in a symmetric introduced configuration. The resulting inrush current has drastically reduced and a wideband measuring circuit with a true calibration process has become possible.

Precision Calibration System for Partial Discharge Calibrators

Partial discharge calibrators need to be periodically calibrated traceable to national standards. VTT MIKES, the National Metrology Institute (NMI) of Finland, has official expertise to calibrate the apparent charge of PD calibrators from 1 pC to 100 nC according to IEC 60270. Recently, VTT MIKES has developed techniques to measure charges as low as 0.01 pC with 3 % uncertainty (k = 2). VTT MIKES has participated to an intercomparison of four NMI’s in 2018 and the results indicate that VTT MIKES can both expand their official measurement capabilities and lower their uncertainties. This paper presents VTT MIKES’s precision calibration system for PD calibrators. Additionally, an overview of commercial PD calibrators’ performance is provided and compared to the requirements of IEC 60270. Data is based on the calibrations performed during the last 10 years.

Application of Charge-Sensitive Preamplifier for the Calibration of Partial Discharge Calibrators Below 1 pC

Modern partial discharge (PD) calibrators often have their lowest range below 1 pC. However, traceable calibration services below 1-pC charges are not widely available, or their uncertainties are relatively high because of low signal levels. We have applied a commercial charge-sensitive preamplifier for the low-level calibration of PD calibrators. This calibration method both improves the signal-to-noise ratio and performs the analog integration of the PD pulse. The drooping response of the preamplifier is corrected using software. Comparison with a reference PD calibrator supports an expanded uncertainty less than 1% down to 0.1 pC and 3% for 0.01 pC.

Localization of Partial Discharges Inside a Transformer Winding Using a Ladder Network Constructed From Terminal Measurements

Precise localization of partial discharge (PD) inside a power transformer winding is a challenging task. Previously, researchers have used internal calibration or reference signals to locate the PD source. The location of any arbitrary (test) PD source is ascertained from the maximum correlation between reference and test signals. However, in practical transformer windings, internal tappings or design details are usually unavailable to generate reference signals. The proposed work employs terminal measurements to construct a physically realizable ladder network. Simulated responses obtained from the ladder network for signals of known pulse-widths at all locations are used as reference data. The terminal responses of the test PD signals are obtained from a laboratory-scale winding by applying signals of arbitrary pulse-widths and shapes at various locations. The PD test signals are generated using a function generator, a PD calibrator, and real discharges. To predict the location of the PD source, the simulated reference data are then correlated with the test data. The position corresponding to the maximum correlation indicates the PD location. The proposed methodology is verified using experimental investigations carried out on two different laboratory-scale transformer windings.

Possibility of PD calibration on power transformers using UHF probes

Partial Discharge (PD) measurement on power transformers using Ultra-High Frequency (UHF) techniques has great advantages on IEC method including possibility of online application, localization and greater immunity against external noises. But lacking a method to calibrate or at least determine sensitivity of UHF method for power transformers is the main disadvantage of this method. Thus severity of PD detected by UHF method cannot be determined. In this paper, main parameters that affect the relation between severity of PD, defined by apparent charge of IEC method, and received signals by UHF method are investigated in detail using laboratory measurements and simulation tools. The purpose of paper is to investigate possibility of PD calibration using UHF probes by evaluation of amount of calibration error caused by each of the above parameters. Then, a method to estimate the minimum possible ratio between UHF signal and corresponding apparent charge is proposed based on the results obtained within investigation of calibration parameters.

A New Directional Coupler Type Partial Discharge Sensor Installed on the Power Lead of Rotating Machine

For on-line partial discharge (PD) monitoring of rotating machines, a novel sensor is proposed, which can be installed on the power lead inside the terminal box of the machine. The sensor has been designed to have high capacitance, and minimal reflection of measured pulses. As a sensitivity of the sensor, transfer impedance Zt has been measured and compared to conventional coupler-type sensors. A simple method is presented for measuring Zt of coupler sensors, using a vector network analyzer and a practical lead-cable of rotating machine. Through this method, it became possible to measure the Zt of coupler sensors including the installation environment of them. The Zt of the proposed sensor is higher than that of same sized other conventional couplers at frequencies between 30 and 92 MHz. Another sensitivity test has been performed using a PD calibrator as a test pulse source. The proposed sensor has higher measured peak voltage than the conventional coupler type sensors when the same charges were input.

DEVELOPMENT AND CALIBRATION OF A MICROCONTROLLER BASED PARTIAL DISCHARGE CALIBRATOR

Partial discharge is an instantaneous energy release depending on a charge accumulation caused by an electric field localized in any section of insulation materials. In this phenomenon, there is no observation of full discharge but electrical perforation and damage in insulation materials can be occurred. Traditional measurement method of partial discharge is based on partial discharge detectors which are defined in IEC 60270 standard. This standard also defines partial discharge calibrators which are used for calibration of partial discharge detectors. This paper explains the design, construction and calibration of a simple partial discharge calibrator. Main idea of the design is based on generating fast rising and slow falling voltage pulses from digital to analog converter of an ARM7 based microcontroller. These voltage pulses are applied to a capacitor where they become charge pulses. Complete design was constructed in an aluminium case and the calibrator’s performance was evaluated. Results showed that the designed calibrator is fully compatible with IEC 60270 standard.

Application of a Programmable Partial Discharge Calibrator

This paper presents the application of a programmable partial discharge calibrator, which used to test the performance of four partial discharge measuring systems by injecting a program controlled known charge sequence into the device under test. Performance of linearity, response to opposite polarity pulses, pulse resolution time and response to random pulse sequence are compared and the results obtained show that the tests prescribed by the standards do not fully characterize the partial discharge measuring system behavior. A random partial discharge pulse sequence generated by this calibrator can help to evaluate those systems.

Considerations on the Long-term Reliability of On-line Partial Discharge Ceramic Sensor for Thermal Power Generators and its Demonstration in the Field

The present study describes the considerations on the long-term reliability of the on-line partial discharge (PD) ceramic sensor for thermal power generators. Voltage acceleration aging tests were carried out under continuous and impulsive thermal aging at more than <TEX>$100^{\circ}C$</TEX>, considering the practical service environment. Experimental results show that the sensors have a life that could last for more than 100 years, excellent dielectric characteristics, and insulation strength. In addition, the ceramic on-line PD sensors were installed in a thermal power generator in Korea for demonstration. The results of the PD calibration and test voltage application prove that the on-line ceramic sensors have satisfactory performances for on-line PD measurement.

Setup of standard PD calibrator and its uncertainties

The present paper describes the setup of standard partial discharge calibrator for measuring partial discharge and estimating uncertainties. The standard PD calibrator was designed and set up, consisting of a digital pulse generator, capacitor modules, and a digital oscilloscope controlled by software developed in the laboratory. Using this software, averages of charges and rising times and their standard deviations in the measured pulses can also be calculated. The standard PD calibrator generates five types of pulses: single, double, random, oscillating, and long-rising. The coefficient sensitivities to estimate the uncertainties of pulses were extracted in the model circuit of the standard PD calibrator. The uncertainties of charges and rising times in pulses of the standard PD calibrator were estimated with single pulses. These values were 0.3%-1.4% in charges and 1.9%-7.0% in rising time; however, these values are lower than the limit values in IEC 60270.

Development and Application of a Programmable Partial Discharge Calibrator

A programmable partial discharge (PD) calibrator composed of an arbitrary waveform generator that is series connected to a capacitor is described. The voltage waveform supplied by the generator is software controlled. The characteristics and capabilities of the calibrator are presented, and the procedures used for its characterization are discussed. The calibrator can reproduce different types of pulses, which include both the corona and PDs of random amplitudes and the predetermined phase relationship with respect to the high-voltage (HV) supply. Examples of the actual discharges measured in the laboratory are compared with the corresponding pulse sequences generated by the calibrator. Finally, by means of this developed instrument, the performances of four partial-discharge-measuring systems equipped with analog and digital measuring instruments are investigated and compared by generating suitable pulse charge sequences.

Partial discharge localization in transformer windings using multi-conductor transmission line model

Multi-conductor transmission line model (MTLM) of transformer winding has been used to find partial discharge (PD) location in transformers. To decrease computational complexity of the model, two methods for calculation of model parameters were studied and the most suitable method was chosen based on comparison between measured and calculated transfer functions of a 132 kV/20 kV transformer. To investigate the accuracy of MTLM in PD localization, PD pulses produced by a PD calibrator were injected into different points of a research 20 kV/0.4 kV transformer and measured at both ends of the winding. The sectional winding transfer functions computed by the model were used to refer measured PD signals to possible PD locations. Then by comparison of referred signals, PD location in the winding was estimated.

On-line partial discharge calibration and monitoring for power transformers

Partial discharge (PD) is one of the major factors causing insulation damage of power transformers. This paper describes the development of on-line partial discharge monitoring system for transformers. The developed set can monitor up to four power transformers continuously. It uses various methods to reject interference and enhances monitoring sensitivity of PD effectively. The optical cable is applied to communicate between measurement system near transformers and the upper computer in the main control building. On-line calibration of partial discharge quantity is requisite for on-line detection or monitoring. This paper describes an on-line calibration method using pulse injection through tap of high voltage transformer bushings. It also discusses the influence of limit inductance and bushing capacitance, from tap of bushing to ground, in on-line calibration circuits. Influences of multi-grounding of the transformer tank for monitoring and calibration are also discussed. Simulation in laboratory and site test shows that the method is practically valid for on-line PD calibration of power transformers within permissible engineering error range under certain defined conditions. This system has been in operation steadily in 500 kV substations for more than 2 years.

A method for calibration of partial discharge calibrators

A method for calibration of partial discharge calibrators has been developed based on passive integration of the induced charge. The voltage over a known capacitor is measured by a calibrated digital sampling voltmeter and the charge is calculated as the voltage multiplied by the capacitance. The best achieved uncertainty is /spl plusmn/2% with a coverage factor k=2.