PD Measurements Research Articles

Diagnosis of abnormal temperature rise observed on a 275 kV oil-filled cable surface — A case study

This paper presents a case study on a 275 kV oil-filled cable. The condition assessment and diagnosis are based on analysis of cable surface temperature in relation to its current load and insulation dielectric loss. The work was initiated by a local abnormal temperature rise of 5.2 °C in cable surface temperature, which was observed during a routine inspection. The temperature rise occurred at bend area with a length of approximately one metre in the Blue phase. No PD activity was identified using on-line PD measurement. The relation between cable surface temperature, cable core temperature and cable insulation condition was then simulated based on the thermal model of power cables. According to simulation analysis, poor condition of cable insulation or oil from an oil duct penetrating a region under the cable surface were identified as possible reasons for the problem observed. An in service X-ray scanning technique was employed for further investigation and to aid diagnosis. The X-ray images revealed a slight distortion of the PVC sheath and the presence of multiple voids between cable insulation paper and the lead sheath. It was concluded that an oil leakage from the oil duct to the voids under the cable lead sheath was responsible for the local cable surface temperature rise. The result removed the concern of incipient cable breakdown, and a potential unplanned outage.

Partial discharge characteristics at ultra-short voltage risetimes

This paper describes continuation of author's works on the impact of PWM waveforms characterized by ultra-short rise times on PD characteristics in high voltage insulation systems. The implications of this kind of voltage exposure is becoming increasingly important as with rapid development of silicon carbide (SiC) and gallium nitride (GaN) based power electronic components the rise times used in machine control circuits keep getting shorter and the carrier frequencies higher. There is an imminent need to further develop diagnostic methods for PD detection at such extreme conditions. A PD measurement circuit based on capacitive decoupling and suitable for ultra-short rise times is presented here. Functionality of the circuit is demonstrated for a twisted pair test object insulated by enamel containing chromium oxide (Cr2O3) filler for rise times between 15 and 800 ns. The impact of voltage steepness on the PD extinction voltages (PDEV) and on other measurable parameters is reported. Specifically, the total number of PDs and their amplitude per modulated cycle are shown. The PD magnitude keeps increasing with the gradually decreased rise time, while the PDEV decreases. However, when compensating for voltage overshoot, less difference is noted. It is also demonstrated that the used PD detector allow for measurements at carrier frequencies well exceeding 1 kHz and rise times well below 100 ns as well as how the PD characteristics changes due to voltage overshoots inflicted by reflections.

P wave dispersion and cardiac troponin i concentration in canine visceral leishmaniasis*

Canine Visceral Leishmanisis (CVL) is mostly fatal chronic systemic disease caused by Leishmaniainfantum (L. infantum) in the Old World countries. In infection, dogs plays both host and reservoir role. By this context, infected dogs are a threat to both people and other dogs. In dogs infected with L. infantum, cardiac involvement in addition to the other affected organs and systems were also reported in the updated literatures. In the present study, the aim was to clinically evaluate the probable cardiac damage in dogs with CVL according to its stage via measuring atrial conduction time by use of Pd determined within computerized electrocardiography and cardiac troponin I (CTnI) level. A total of 24 dogs, of both sexes and various ages, referred with one or more of the clinical findings such as hypertrichosis, periocular alopecia, weight loss, onychogryphosis, skin lesions (severe scaling, exfoliative dermatitis compatible with alopecia) and/or anorexia, lymphadenopathy, hepatosplenomegaly. In the healthy control group for comparison and in dogs with CVL to determine presence, nature and level of cardiac damage, the evaluations were performed with computerized 12-lead ECG device [(1 mV/cm amplitude in resting and 50 mm/sec) (Pd measurement)] and serum CTnI concentrations were measured by using species specific commercial test kit. Eighteen dogs infected with CVL (IFAT positive) with/without different clinical sign were enrolled into three different groups (n=6); of the above mentioned clinical sign, to those of which with single clinical sign (oligosymptomatic) were included in the group I; those with multiple or several clinical signs (polysymptomatic) were included in the group II; and asymptomatic dogs were included group III. CVL-negative healthy dogs without any disease were left in group IV. High levels of cTnI concentration were detected in 10 of 18 dogs infected CVL with all polysymptomatic dogs. In all cases of the control group, cTnI levels were in the reference range [<0.03 ng/dL]. Even the comparison of each groups, no statistically significance (p>0.05) was found between CVL positive and control dogs. Mean ± standard deviation of Pd values were 22.76±3.12, 22.03±0.80, 22.73±0.80 and 25.67±1.41 in the control group, asymptomatic group, oligosymptomatic group, and polysymptomatic group, respectively. In comparison between groups, polysymptomatic group was significantly different than control (p = 0.026), asymptomatic (p = 0.012) and oligosymptomatic (p = 0.027) groups. Although a statistically significant difference was not found between CVL positive and control dogs in the present study, it was suggested that the individual increase may be associated with myocarditis due to disease. Besides, it may be claimed that the mean Pd values determined in especially polysymptomatic dogs was higher compared to the control group, whereas this may be accepted in the reference ranges based on mean Pd values reported in healthy dogs. However considering infected dogs population as 6 in each group, it may be safely claimed that further investigations regarding greater number of cases the may be warranted.

Partial discharge measurement at DC voltage — Evaluation and characterization by NoDi∗ pattern

In this paper the requirements on PD measurement systems for DC voltage are summarized and the experience gained about the PD behavior of some typical defects in HVDC GIS and GIL are discussed. Some physical effects at different voltage levels are shown and a method for the representation and characterization of the PD pulse sequences is introduced. Special test cells with different types of defects such as a protrusion, a floating electrode, a free moving particle and a void are used for the investigations. The procedure for PD measurement at DC voltage is improved and PD pulse sequences at different gas pressures and voltage levels were recorded. In a further step the PD behavior up to the breakdown voltage was determined. The NoDi∗ pattern was developed for the visualization and interpretation of the measured PD pulse sequences. This pattern supports the visualization and the physical interpretation of the pulse sequences and may allow the characterization of the typical defects in HVDC GIS and GIL by a human expert.

New diagnosis method of unknown phase-shifted PD signals for gas insulated switchgears

In many cases of on-site PD measurement, it is difficult to acquire the perfect phase angle of the PD signal synchronized with the applied voltage. This paper describes a new method of PD diagnosis which can classify the types of defects for unknown phase-shifted PDs using decision tree method. 327 cases of on-site PD data were collected in the field. 5 valid parameters were extracted and the decision tree-based diagnostic algorithm was used to classify types of 6 PDs and 4 noises clearly. Finally we confirmed the new method of PD diagnosis can be applied to on-site GIS diagnosis.

Determining insulation condition of 110kV instrument transformers. Linking PD measurement results from both gas chromatography and electrical method

Working methods for on site testing of insulations:Gas chromatography (using the TFGA-P200 chromatographer);Electrical measurements of partial discharge levels using the digital detection, recording, analysis and partial discharge acquisition system, MPD600.First performed, between 2000–2015, were the chromatographic analyses concerning electrical insulating environments of:102 current transformers, 110kV. Items in operation, functioning in 110/20kV substations.38 voltage transformers, 110kV also in operation, functioning in 110/20kV substations.Then, electrical measurements of partial discharge inside instrument transformers, on site (power substations) were made (starting in the year 2009, over a 7-year period, collecting data until the year 2015) according to the provisions of standard EN 61869-1:2007 „Instrument transformers. General requirements”, applying, assimilated to it, type A partial discharge test procedure, using as test voltage the very rated 110kV distribution grid voltage.Given the results of two parallel measurements, containing:to this type of failure specific gas amount (H2) andthe quantitative partial discharge’ level,establishing a clear dependence between the quantity of partial discharges and the type and amount of in oil dissolved gases inside equipments affected by this type of defect: partial discharges, was expected.Of the „population” of instrument transformers subject of the two parallel measurements, the dependency between QIEC (apparent charge) and (H2) (hydrogen, gas amount dissolved within their insulating environment) represents a finite assemblage situated between the two limits developed on an empirical basis.

Level dependent partial discharge signal de-noising using stationary wavelet transform

PD monitoring is an effective tool to evaluate the insulation condition of power electrical equipment. However, the major challenge during PD measurement at site is that PD signals are severely affected by external noises and disturbances like white noise, random noise, Discrete Spectral Interferences (DSI), which are generated due to broadcasting stations, stochastic noise and pulses from power electronics at site conditions. Extracting PD signals from these noises is a challenging task.This paper proposes a new method for selecting the mother wavelet based on the energy of the approximation coefficients. The coefficients are obtained using SWT by decomposing the extracted noisy signal to the maximum decomposition level which depends only on the length of the noisy signal. Hard thresholding is used as the threshold function and range dependent threshold estimator is used for obtaining the threshold value. For reconstruction of de-noised signal, the last level approximation coefficient and the thresholded ‘details coefficient’ are used. As most of the lower level details coefficients comprise of noises it can be discarded during reconstruction. A method for discarding noises during reconstruction is also proposed in this paper.

Comparison of capacitive and inductive sensors designed for partial discharges measurements in electrical power apparatus

In the paper results of simultaneously conducted measurements achieved using capacitive and inductive sensors are presented according to different PD model sources immersed in a mineral transformer insulation oil. All measurements are preceded under laboratory conditions using typical measurement set up commonly applied for on-site PD detection: measuring impedance and capacitor and high frequency current transformer (HFCT) are used respectively. Measuring frequency and voltage level influence as well as phase resolved PD patterns analysis are investigated in the research. Various fundamental PD signal descriptors assigned for selected frequencies are also proposed and compared for chosen sensors. The main purpose of the presented research is to compare PD measurement results achieved using selected type of sensors during laboratory measurements and to point the best application areas in fields of PD detection in high voltage apparatus under normal operating conditions. Furthermore a proper measurement results interpretation coming from different sensors as well as measurement conducting problems and achieved patterns disparities are also discussed in the paper.

Bushing End Screen Failure – Case Study

Paper deals with two 400 kV bushing failures which happened in a relatively short period of time, after about ten years of transformer service. Failure appearance was practically identical in both cases - breakdown between bushing end screen and turret wall followed by a small oil leak through mechanically damaged bushings. Failure happened during medium load, temperature and voltage ordinary conditions. Analysis of the periodic (off-line) and continuous (on-line) diagnostic results revealed that up to the moment of failure no any signs of possible problem had been recorded: no bushings capacitance change, no DGA rise and no any significant overvoltage. All measured values correspond to new equipment. It seems that observed bushing failures were totally unpredictable despite of applied on-line and off-line diagnostic methods. Several end screens were taken from service for further research in HV laboratory. End screen electrode is casted with a thick layer of resin. El. field inside and around end screen electrode was calculated and based on the results special test setup was developed which simulates dielectric stresses as in service. PD measurement revealed that some end screens suffered from harmful PD even on (simulated) service voltage. It was found that PD inside bushing end screen thick epoxy coating can be hidden despite of bushing and transformer testing according to relevant standards.

A Single-Arm PHASE 2A Study of NM-IL-12 (rHu-IL12) in Patients with Mycosis Fungoides-Type CTCL (MF) Undergoing Low-Dose TOTAL Skin Electron BEAM Therapy (LD-TSEBT)

Multiple treatment modalities are available for MF, but most result in inevitable relapse. Therefore, new treatment strategies that improve response rate and prolong response duration are greatly needed. TSEBT is a highly effective therapy in MF. LD-TSEBT (12 Gy) is much more tolerable than the conventional 36+ Gy dose, thereby allowing for re-treatment; however, LD-TSEBT has a less favorable complete response rate and response duration.Combining LD-TSEBT with immunostimulatory modalities in MF has a strong biological rationale, since radiation-induced exposure of cancer-specific antigens should be synergistic with concomitant stimulation of anti-cancer immune responses. Interleukin-12 is a robust candidate for radioimmunotherapy, as IL-12 has significant anti-MF activity as monotherapy, is very well tolerated without overlapping toxicity with TSEBT, and is a potent stimulator of innate and adaptive immunity.We report on a single-arm open-label phase 2a trial of combination of LD-TSEBT and NM-IL-12. Ten patients are planned for enrollment. Eligibility includes MF-type CTCL stages IB-IIIB and patients must be eligible for LD-TSEBT. TSEBT is started on study day 1 (fractionated 4 Gy/week, up to 12 Gy). NM-IL-12 is administered subcutaneously at 150 ng/kg on days 2 and 15, followed by 6 maintenance doses q4w at 100 ng/kg. The primary endpoint is safety with secondary endpoints being response rate and PFS.Currently, 6 patients are enrolled, 5 evaluable for response; 4 male; median age 55; three have stage IB, one IIB and one IIIB. Median number of previous therapies is 2 (0-6). The treatment was well-tolerated with only grade 1 or 2 AEs; most common AEs include grade 1 headache and chills.One patient achieved CR, 2 PR, and 2 SD. Median follow-up is so far 15 weeks and 5 patients remain on study. One patient has been withdrawn from the study due to development of a suspected PLC (pityriasis lichenoides chronica)-like skin reaction requiring topical steroid therapy. PK and PD analysis was completed in the first 4 patients. It demonstrated measurable drug levels in all patients studied, Cmax being 10.8-56.1 pg/ml achieved at 5-24 hours post injection. Interferon-γ and IP-10 (hallmark PD markers of IL-12 activity) were measurable after the first and the second injections in all patients. Levels of the inhibitory cytokine IL-10 were generally measurable after NM-IL-12 injections, but were very low (<1 pg/ml up to 12 pg/ml) - much lower than previously reported in IL-12 studies, thus unlikely to be associated with negative immunosuppressive feedback. There was no clear correlation between PK and PD measurements and either efficacy or toxicity, but the number of patients fully evaluated is small.Total RNA was extracted from patient blood samples and subjected to qRT-PCR. Gene specific primers were used to analyse intracellular expression of the IL-12 receptor and immune cell markers. Ongoing work investigates an activated cellular phenotype at week 15 of LD-TSEBT and NM-IL-12 treatment compared with the respective baseline samples.Overall these early results demonstrate that NM-IL-12 can be safely administered together with LD-TSEBT in CTCL patients. Encouraging clinical activity is observed including a CR. Enrollment is currently ongoing and planned PD and correlative biomarker studies are in progress.A phase 2B randomized trial of NM-IL-12 and low dose TSEBT compared against low-dose TSEBT alone in patients with MF is being developed based on the apparent benign AE profile as compared to approved systemic therapies. DisclosuresKim:Takeda: Membership on an entity’s Board of Directors or advisory committees, Research Funding; Eisai: Membership on an entity’s Board of Directors or advisory committees, Research Funding; Tetralogic: Research Funding; Neumedicine: Research Funding; Merck: Research Funding; Kyowa Kirin Pharma: Membership on an entity’s Board of Directors or advisory committees, Research Funding; Portola: Consultancy; miRagen: Research Funding; Innate: Research Funding; Horizon Pharma: Consultancy, Membership on an entity’s Board of Directors or advisory committees; Forty Seven Inc: Membership on an entity’s Board of Directors or advisory committees; Seattle Genetics: Membership on an entity’s Board of Directors or advisory committees, Research Funding; Soligenix: Research Funding. Ma:Neumedicines: Employment. Kha:Neumedicines: Employment. Lawrence:Neumedicines: Employment, Patents & Royalties. Vainstein:Neumedicines: Employment. Basile:Neumedicines: Employment, Equity Ownership, Membership on an entity’s Board of Directors or advisory committees, Patents & Royalties.

Geometrical Shapes Impact on the Performance of ABS-Based Coreless Inductive Sensors for PD Measurement in HV Power Cables

In this paper, a series of investigations on sensitivity and bandwidth for four types of Rogowski coil (RC) sensors was demonstrated. The proposed RC sensor design using acrylonitrile butadiene styrene material carries the advantages, such as lighter weight, robust, and higher temperature resistance. The prototype development first was carried out with the assistance of Solidwork 3D computer aided design software and MakerBot 2X 3D printer. Subsequently, the sensors were modeled and simulated using electromagnetic transient program-alternative transient program software environment based on the lumped parameter identification. A single-end measuring technique was used to locate the PD signal on the three-phase medium voltage underground cross-link polyethylene insulated power cable with a 240-mm2 nominal area copper conductor. In order to verify the results achieved from the simulation, an experiment was carried out. This experiment was repeated concurrently for each design of the RC sensors. In order to achieve the best PD detection, the precise selection for the best sensor is paramount. In this case, the results indicated that certain geometrical shape performed better with regard to the detection of the PD signal.

Population PK-PD analyses of CD25 occupancy, CD56bright NK cell expansion, and regulatory T cell reduction by daclizumab HYP in subjects with multiple sclerosis.

Daclizumab high yield process (HYP) is a humanized IgG1 monoclonal antibody that binds to the α-subunit of the interleukin-2 receptor and is being developed for treatment of multiple sclerosis (MS). This manuscript characterized the pharmacokinetic-pharmacodynamic (PK-PD) relationships of daclizumab HYP in subjects with MS. Approximately 1400 subjects and 7000 PD measurements for each of three biomarkers [CD25 occupancy, CD56bright natural killer (NK) cell count, regulatory T cell (Treg) count] from four clinical trials were analyzed using non-linear mixed effects modelling. Evaluated regimens included 150 or 300mg subcutaneous (s.c.) every 4weeks. CD25 occupancy was characterized using a sigmoidal maximum response (Emax ) model. Upon daclizumab HYP treatment, CD25 saturation was rapid with complete saturation occurring after approximately 7h and maintained when daclizumab HYP serum concentration was ≥5mgl-1 . After the last 150mg s.c. dose, unoccupied CD25 returned to baseline levels in approximately 24weeks, with daclizumab HYP serum concentration approximately ≤1mgl-1 1L. CD56bright NK cell expansion was characterized using an indirect response model. Following daclizumab HYP 150mg s.c. every 4weeks, expansion plateaus approximately at week 36, at which the average maximum expansion ratio is 5.2. After the last dose, CD56bright NK cells gradually declined to baseline levels within 24weeks. Treg reduction was characterized by a sigmoidal Emax model. Average maximum reduction of 60% occurred approximately 4days post 150mg s.c. dose. After the last dose, Tregs were projected to return to baseline levels in approximately 20weeks. Robust PK-PD models of CD25 occupancy, CD56bright NK cell expansion and Treg reduction by daclizumab HYP were developed to characterize its key pharmacodynamic effects in the target patient population.

Testing and Diagnosis of Extra High Voltage Power Cables Using Damped AC Voltages Combined with Distributed Partial Discharge Measurement

State assessment method of cables with extruded insulation and their accessories, DAC withstand test combined with a diagnostic test (such as PD measurement) has two shortcomings, the output voltage is not high enough for EHV cables, it cannot detect and locate PD effectively when defects are far away from signal acquisition point. This paper focus on DAC voltages up to 250kV and a kind of distributed measurement of PD. Defects existing in arbitrary of tested cable can be accurately measured in theory by installing sensor at intervals along the tested cable. This method has been validated on a long extra high voltage cross-linked polyethylene (XLPE) insulated underground cable circuit and the filed PD test has been completed successfully.

Application of the UHF technology to detect and locate partial discharges in liquid immersed transformer

The paper explains theoretical aspects of PD signal propagation in the UHF range inside a transformer. The influences of inner structures inside the transformer and the sensor characteristics on UHF measurement results are discussed. Based on practical examples measured on transformers ranging from 100 MVA to more than 1000 MVA the sensitivity of UHF measurements is described with respect to the measurement frequency used and the propagation path between PD spot and UHF sensor. Another discussed aspect is a comparison of PD pattern recorded with a UHF system and pattern of the same fault acquired by conventional PD equipment. The application of UHF signals triggering an acoustical PD localization system is shown. Found limitations of the UHF method are discussed, too. Finally, an outlook on foreseeable evolutions and developments in the field of UHF PD detection, measurement and monitoring is given.

New Drug Reduced Parkinson's Psychosis

A novel selective 5–HT2A inverse agonist in a phase III trial significantly improved psychotic symptoms in Parkinson's disease patients without the worsening of motor symptoms that usually occurs with antipsychotic treatment. The drug is now in a phase II feasibility trial in Alzheimer's patients, according to the manufacturer. Clive Ballard, PhD, professor of age–related diseases at King's College London, reported at the Alzheimer's Association International Conference last year that, in Parkinson's disease, pimavanserin benefited patients and eased caregiver burden. “By 4 weeks into the study, the benefit for caregivers began to appear, and it continued to increase” until the study ended at 6 weeks, said Dr. Ballard. About 50% of Parkinson's patients experience psychosis, “the leading cause of nursing home placement,” he said. The atypical antipsychotics quetiapine and clozapine are the only well–tolerated options, he said. “Quetiapine is ineffective, though, leaving only clozapine. It's well tolerated from a motor point of view, but it has other safety and tolerability issues that limit its use in clinical practice.” As a selective 5–HT inverse agonist, pimavanserin is in a unique drug class. According to Acadia Pharmaceuticals, the company developing the drug, it has the benefits of a 5HT agonist but doesn't affect the dopaminergic, histaminergic, adrenergic, or muscarinic systems. Investigators in the phase III study made a number of study design changes to compensate for a high placebo response in earlier trials. In order to qualify for the trial, for example, patients had to have high baseline minimum scores on the Neuropsychiatric Inventory and the Scale for the Assessment of Positive Symptoms (SAPS). The primary endpoint was antipsychotic efficacy as measured by the SAPS–PD, a nine–item scale adapted from the hallucinations and delusions domains of SAPS. Secondary endpoints included the entire 20–item SAPS, the SAPS hallucinations and delusions subscores, the Clinical Global Impression (CGI) score, Unified Parkinson's Disease Rating Scale (UPDRS) domains I and II, and a measure of caregiver burden. At 6 weeks, patients in the active–treatment group had significantly greater improvement on the SAPS–PD, the primary endpoint. By 15 days, both groups had experienced a significant improvement from baseline. Thereafter, the curves separated, with the placebo group becoming stable. The pimavanserin group, however, continued to improve. By the study's end, there was a mean SAPS–PD decrease of 5.79 points in the active–drug group, significantly better than the mean 2.73–point decrease in the placebo group. This translated to a 37% improvement for the study drug, compared with a 14% improvement for placebo. The scores translated into a meaningful clinical benefit, Dr. Ballard added. By the SAPS–PD measurement, response rates were 42% and 65%, respectively—a significant difference. Changes on the CGI subscores were also significantly different. Patients in the placebo group experienced a mean decrease of less than half a point on the improvement subscale, compared with a decrease of about 1 point in the pimavanserin group. On the severity subscale, the placebo group stayed close to baseline, while the pimavanserin group decreased by about 1 point. About 27% of those taking placebo and 49% of those taking pimavanserin were considered responders. In an exploratory analysis examining pimavanserin's effect on sleep, the drug was associated with improvements in both nighttime sleep and daytime wakefulness on the Scale for Outcomes in Parkinson's Disease sleep measure, Dr. Ballard said. Based on the favorable results, Acadia announced that it had discontinued its work on a planned confirmatory phase III study. A New Drug Application for pimavanserin is in preparation, Dr. Ballard said. Dr. Ballard disclosed that he has received honoraria and consulting fees from the company.

Direction Finding Using Rotated Long Baseline Interferometer

To reduce the hardware cost of traditional multichannel array or interferometer, a direction finding method using rotated long baseline interferometer (RLBI) has been developed. To overcome the initialization problem, a multiple hypothesis Gauss-Newton (MHGN) algorithm is proposed. It first chooses multiple initial pairs from a pair of PD measurements and then applies each value to the GN algorithm and meanwhile obtains a cost for this initial value. Finally, the ultimate estimate is then picked with the minimum cost. Simulation shows that the proposed method can approach to the Cramer-Rao lower bound (CRLB).

Risk assessment on defects in GIS based on PD diagnostics

The paper, prepared by CIGRE WG D1.03 (TF 09), presents the guidelines for risk assessment procedure on defects in GIS based on PD diagnostics. The procedure, described in detail in CIGRE Technical Brochure 525, starts with sensitive PD measurement to detect the critical defects and follows with identification of the type of the defect and its location inside the GIS. This information taken together with other essential data from laboratory measurements, manufacturer's experience, design aspects and trend analysis of the PD activity, are the base for the estimation of the criticality of the defects. Finally, the risk assessment is performed based on the estimated dielectric failure probability and failure consequences that can be different in case of on-site testing or in service activity.

Partial discharge diagnostics for mixed-technology switchgear (MTS) in outdoor substations

The application of the UHF method to PD measurements in MTS substations presents technical challenges due to the much higher external noise in outdoor substations. To adapt the UHF technique to handle these challenges, a series of tests was performed in the laboratory and at selected substations. Different PD sources were inserted in a test rig, and the proposed UHF detection system was evaluated. On-site measurements using this system showed that the spectra of external and internal PD sources differed considerably, suggesting that the system is capable of yielding reliable indications of the presence of PD in MTS.

Tuned medium-band UHF PD measurement method for GIS

SUMMARY In order to replace the lightning impulse test, a very sensitive PD-measurement is required for onsite tests of GIS [1]. The most sensitive UHF-PD-measurement technique consists of low-noise high –gain broadband amplifiers applied directly to the PD-sensors combined with the manual selection of possible resonant frequencies in the frequency spectrum for narrowband signal extraction with a bandwidth of some MHz. Further a correlation with the test voltage will lead to phase resolved signal display. The only disadvantage is the time consuming visual selection of the optimal centre frequency of the narrowband measurement system done sensor by sensor compared to broadband or fixed band measurement techniques. The broadband system design has the disadvantage of significantly reduced measurement sensitivity as soon as interfering frequencies are located within the measurement bandwidth. Beside this, in principle, the signal to noise ratio is lower for a broadband measurement system compared to a narrowband system. The narrowband system with fixed frequencies show poor or no sensitivity when the GIS resonant frequency caused by a PD source do not correspond with the narrow band measurement frequency or when interfering frequencies appear at the fixed measurement frequencies. The tuned medium band UHF PD measuring system design consists of one or more manually pretuned band-pass filters with a bandwidth of 50 ... 150 MHz applied in a frequency range of approx. 100 ... 2000 MHz. The selection of the centre frequencies should be based on the individual resonant frequencies of the PD-sensors determined by the CIGRE sensitivity check [2] on site. The medium bandwidth allows to integrate the individually shifted resonant frequencies of a PD-signal at a PDsensor within the measurement band, caused by different PD locations. Due to the wider bandwidth, the probability of missing PD-signals at a specific centre frequency is much lower than with the narrow band technique [3]. The evaluation of measurements carried out in different environment and different types of GIS showed a high possibility that a medium bandwidth is still narrow enough to avoid fix frequency disturbances by the use of a suitable centre frequency. Using several tuned frequency bands the probability of missing the resonance frequencies of a PD-signal is even lower and additionally allows to conduct a first coarse localization of the PD-source based on the frequency dependent damping of the PD-signal. The main advantage of the proposed design is the combination of simultaneous measurement with optimal sensitivity based on selective avoiding of fixed band interfering frequencies together with tuning into the most sensitive frequency band of each individual PD sensor of a GIS.

A time-dependent direct current potential drop method to evaluate thickness of an oxide layer formed naturally and thermally on a large surface of carbon steel

This study describes the use of a time-dependent spring-loaded four-point-probe technique to measure the direct current potential drop (DCPD) on oxidized test surfaces for different spring force values, with the purpose of evaluating the thickness of the oxide layer. The force of the spring attached to the current probe was reduced by inserting spacers with different thicknesses under the supporting legs of the sensor block. The PD measurement was first performed on machined sample exposed to the atmosphere for approximately 6months. The sample was made of carbon steel (SS400). Then, the thickness of the oxide layer formed on the surface of the sample was increased by heat treatment. The PD was also measured on the samples subjected to heat treatment. The measurements were performed at different locations on the test surface under the same experimental conditions. The experimental results establish a relationship between the spring force and the time required for the current probe to penetrate the oxide layer. This article also proposes a procedure for evaluating the thickness of the oxide layer. Finally, the proposed method was used to evaluate the thickness of the oxide layer formed on a large surface. The method was performed to evaluate thickness of single phase monolayer oxide in the range of 3.5–8.7μm. To demonstrate the validity and accuracy of the proposed method, the obtained thickness was compared to the thickness measured directly using a field emission scanning electron microscopy.