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Presentation 7.1 Diagnostic of CT Failure – Same Location, different Phase Anton Junaidi / Agus Latief, PT. PLN (Persero), Indonesia Constant, PT. Citra Wahana Sekar Buana, Indonesia Abstract The Current Transformer (CT) is one of the most important equipment in high voltage substations and it is expected to operate maintenance-free for over 25 years. The reliability of protection systems and the accuracy of measurement instruments rely on the performance of current transformers. A failure of the current transformer can lead into a catastrophic damage of the Current Transformer itself and to the equipment placed nearby. So almost happened to two CTs installed in substations in Panakukang. The first failure occurred on October 13, 2010 on phase T and the second failure occurred on April 17, 2011 on phase S. This paper will attempt to determine the possible cause of the failure by analyzing the test results of the third CT installed on phase R which has not failed yet and which is the same type and manufacturing year from the same brand. Keywords Current Transformer, Dielectric Response Analyzer, CT Analyzer, Dissipation Factor Test, Partial Discharge Measurement 1. Introduction PLN AP2B South Sulawesi (Indonesia) operates a transmission line system with voltage level of 150kV, 70kV and 30kV. It consists of more than 2,200 kilometer transmission line circuits and 34 substations. The total transformer capacity is 1,350 MVA, with an average availability of the system up to 99% on a annual basis. Primary Current: Secondary Current: Rated Burden: Class: Frequency: 800 A 5A 30 VA 5P20 50 Hz Table 1 Current Transformer Data 2. Failure Phenomenon on Phases T and S The first CT failure occurred on October the 13, 2011 on phase T and resulted in a black out in the Panakukang area. This incident has caused the total energy loss of 60 MVA. Visual inspection showed normal temperature conditions at the top of the current transformer (upper core), and field investigation did not reveal the presence of any open secondary circuit. The second failure occurred on April 17, 2011 on phase S. The substation operator has seen smoke coming out from the top of the current transformer and took immediate action to de-energize the current transformer. Thanks to this reaction a further damage and a possible black out could be avoided. The temperature measurement using thermal infrared camera was observed and showed no hot spot on the top of the current transformer. The wiring verification on the secondary side of the current transformer could exclude an open-circuit condition. PLN AP2B South Sulawesi has comprehensive testing standards. All high voltage equipment is tested and the test results thoroughly analyzed to help to determine possible cause of failures. The total quantity of current transformers installed is more than 600 units where most of them are oil immersed type. This paper describes the failure that occurred on two identical current transformers and tries to diagnose the cause of the failures by analyzing the test results of a similar unit that has not failed yet. Fig. 1 Water inside membrane on Phase S CT © OMICRON electronics GmbH 2011 – Instrument Transformer Measurement Forum Presentation 7.2 3. Failure Modes and Effects Analysis (FMEA) Fig. 2 Membrane Phase S CT (left) and membrane Phase T CT (right) Failure modes and effects analysis (FMEA) is a procedure in product development and operations management for analysis of potential failure modes within a system for classification by the severity and likelihood of the failures. A successful FMEA activity helps a team to identify potential failure modes based on past experience with similar products or processes, enables the team to design those failures out of the system with the minimum of effort and resource expenditure, thereby reducing development time and costs. Failure modes are any errors or defects in a process, design, or item, especially those that affect the customers, and can be potential or actual. Effects analysis refers to studying the consequences of those failures. PT. PLN has implemented the FMEA for CT failure consists of: Fig. 3 CT Phase S inner cover melting • Defining the system (equipment) and its function • Determining the sub-systems and functions of each subsystem • Determining the functional failure of each subsystem • Determining the failure modes of each subsystem • With FMEA we can design the proper operation and maintenance guideline according to PLN standard SE 114 for CT such as: • In service inspection (visual check) • In service measurement (Thermo vision) • Offline measurement (Insulation resistant, tan delta, oil characteristic, DGA, Oil characteristic, excitation and grounding. • Offline treatment. 4. CT Testing on Phase R Fig. 4 CT Phase T inner cover melting The two current transformers which failed have been disassembled to observe the type of damage that occurred on the inside of the current transformers. It was found that both failed CTs had loose connectors at the inner cover of the CT (the top cover body with the cover secondary winding). The phase S CT contained water in the membrane thus decrease the insulation level. After the failure of this two CTs (phase S and phase T), the management decided to remove the CT on phase R from the system as this CT is the same type and brand and from the same manufacturing year. Several tests had been done on this CT to analyze and check for a possible failure cause of the other two CTs. © OMICRON electronics GmbH 2011 – Instrument Transformer Measurement Forum Presentation 7.3 4.1 CT Analyzer Several tests can be obtained using OMICRON’s CT Analyzer such as, ratio, saturation, burden, polarity and winding resistance. The assessment according to IEC standard is also possible. The test result using CT Analyzer of phase R: • Ratio: OK • Excitation curve: OK • ALF (Accuracy Limit Factor): OK the location of possible fault cause is the cellulose paper or the oil. So transformer failures can be avoided and the preventive maintenance can be on schedule. New methods are developed for determining the water content by using the dielectric response measurements as Polarization Depolarization Current (PDC) or Frequency Domain Spectroscopy (FDS). The Dielectric Response Analysis has been done to CT phase R. The test results using OMICRON DIRANA with FDS and PDC method as shown below: Table 2 ALF CT Phase R Fig. 7 CT Measurement Configuration Fig. 5 Excitation Curve CT Phase R The DIRANA assessment software consists of a database containing data from measurements of real oil-paper-insulation systems, under various states of moisture content and oil / paper ageing. Through this basis, the measurement results are automatically analyzed and the moisture content then classified according to IEC 60422 between "dry" and "extremely wet." Fig. 6 Ratio Error Graph CT Phase R 4.2 Dielectric Response Analysis One ageing indicator is the water content in the solid part of the insulation (paper, pressboard). Water is an ageing product and accelerates the further deterioration of cellulose through depolymerization. Incremental of water content in oil may cause bubble formation and lead to an electrical breakdown. Method and tools are needed to detect and measure the water content in the cellulose paper. Furthermore with the right tool to diagnose the transformer it can be easily determined as where Fig. 8 CT Phase R DIRANA Assessment Graph The software compensates for conductive ageing by products in the oil and it is well suited for older Instrument transformers. The DIRANA test result for Panakukang CT Phase R indicated: Oil Conductivity: 11pS/m (good) Cellulose Moisture: 3 .3% (moderately wet) © OMICRON electronics GmbH 2011 – Instrument Transformer Measurement Forum Presentation 7.4 4.3 Dissipation Factor and Capacitance Test Another usual approach when performing dielectric evaluation is tan delta. This test uses AC and pursues to know loss angle of the tested element. This measurement technique is off-line measurement and includes information of the moisture, carbonization, and other forms of contamination of winding, bushing, and liquid insulation in Instrument transformers and emulates (greater voltage) the behavior and voltage aggressions similar to service ones. It is important to take note of Instrument transformer temperature and environmental moisture (surface leakage). Apparatus insulation system has measurable electrical parameters, such as capacitance, dielectric loss, and power factor in addition to other less well known characteristic. By detecting changes in these important electrical characteristics, failure hazards can be revealed, thereby preventing loss of service by permitting orderly repair or reconditioning of defective insulation. The CT phase R was tested for the dissipation factor test using Hot Collar method since the measuring tap is not available. The test was done using OMICRON CPC 100 + CP TD1. The dissipation factor test was done in two different measurements. First measurement with fixed frequency of 50 Hz and sweep voltage from 2 kV up to 12 kV. The second measurement test with fixed voltage and frequency sweep from 15 Hz up to 400 Hz. The test results are as follows: Table 5 Tan Delta Test Result (as function of Voltage) Table 6 Tan Delta Test Result (as function of Frequency) 4.4 Partial Discharge In electrical engineering, partial discharge (PD) is a localized dielectric breakdown of a small portion of a solid or fluid electrical insulation system under high voltage stress, which does not bridge the space between two conductors. While a corona discharge is usually revealed by a relatively steady glow or brush discharge in air, partial discharges within solid insulation system are not visible. PD can occur in a gaseous, liquid or solid insulating medium. It often starts within gas voids, such as voids in solid epoxy insulation or bubbles in transformer oil. Protracted partial discharge can erode solid insulation and eventually lead to breakdown of insulation. Table 3 CT DF variable voltage fix frequency Fig. 10 Void and discharge Table 4 CT DF variable frequency fix voltage Fig. 9 Graph correlation between DF and Frequency PD measurements can be taken continuously or intermittently and detected on-line or off-line. PD results are used to reliably predict which electrical equipment is in need of maintenance. Not only do partial discharge levels provide early warning of imminent equipment failure, but partial discharge also accelerates the breakdown process. The excessive arcing between ground and conductor within the insulation will, in time, compromise the dielectric strength and mechanical integrity of the winding insulation. Once this happens, a ground fault or a phase-to-phase fault is inevitable. The Partial Discharge test on phase R CT was done using OMICRON MPD 600. The 120 kV coupling capacitor being used as PD sensor since the measuring tap is not available on CT. The HV- © OMICRON electronics GmbH 2011 – Instrument Transformer Measurement Forum Presentation 7.5 AC hipot test system up to 100kV being used as HV source to inject the HV to test object. The PD measurement being repeated several times since the noise level from HV source and measurement test setup is very high. During PD measurement, the corona activities also observed using DayCor corona camera. It was observed that the corona discharge on the CT bushing is small. The PD test was done by increasing the voltage from 2 kV as starting point for voltage calibration for MPD 600 test set and voltage increased up to 86.6 kV as nominal voltage of 150 kV. The MPD 600 test result shown PRPD as below. The figure 11 shows PRPD from phase R CT when injection voltage up to 86 kV is applied. The figure 12 shows ORPD from HV source only. The figure 11 shows that there are PD activities which are very high. 4.5. Insulation Resistance Testing The purpose of this measurement is to detect the condition of insulation between winding - ground or between two winding. Common method provided the DC voltage and represents the condition of insulation to the units in mega ohm (M). It is also seen from the measurement of the polarization index. The aim of the polarization index test is to ensure the proper equipment to be operated or even transient overvoltage test. Polarization Index is the ratio between the 10th minute result to the 1st minute result, with a constant voltage supply. Table 7 Polarity Index Result CT Phase R 5. Conclusion 1. Fig. 11 PD result CT Phase R Test at 86kV 2. 3. 4. It was observed by disassembling the failed CTs (phase T and S) that both CTs had loose connection on its body-ground cable connection in the CT. It was suspected that this was caused by aging process or manufacture defect design. It was also found that a lot of water trapped inside the membrane of CT phase S. The tan delta test for bushing insulation indicated normal bushing. No suspect on the bushing failure. Partial discharge test on phase R CT indicated high PD activities (over 10 nC). This might indicated the degradation of insulation material and/or contact problem inside the CT. High moisture content (3.3%) was measured on phase R CT. The moisture content may decrease the insulation of the material and lead to CT explosion. References Fig. 12 PD result for HV source only at 86kV [1] V.K.Prasher, P.J.Thakkar Power Grid India, EHV Current Transformers - Failure Mode Analysis & Evaluation Leading to Corrective and Preventive Actions for Realibility Enhancment, CIGRE. [2] OMICRON, DIRANA, CPC 100 / CP TD1. CT © OMICRON electronics GmbH 2011 – Instrument Transformer Measurement Forum Analyzer and Presentation 7.6 [3] Michael Krüger, Maik Koch, Alexander Kraetge, Kay Rethmeier, OMICRON electronics GmbH, Austria Alexander Dierks, Alectrix, SA - Diagnostic Measurement on Power Transformer. [4] Root Cause Failure Analysis Current TransformerSer. #811282 John Stead, Altalink Management Ltd. [5] A.K. Datta, S.C. Singh, S.K. Mishra and S. Suresh Power Grid Corporation of India Limited, Dissolved Gas Analysis (DGA) of Current Transformer (CT) oil – A reliable tool to identify manufacturing defects. [6] PLN SE 114: Guidence Maintenance for CT. About the Authors Anton Junaidi, born in Surabaya 1978, College graduated Diploma Degree of Shipbuilding Polytechnic ITS Surabaya Majoring in Electrical, then continued Bachelor Degree at ITATS Engineering Faculty Majoring Power Electrical. 1999 - 2005 worked at PT. Siam Maspion Polymers as Electrical & Planning Eng. Started working at PLN (2005/2006) first position as Transmission Line Protection PT. PLN (Persero) Wilayah Sulsel, Sultra dan Sulbar AP2B Sistem Sulel, and recent position as Manager Transmission and Substation Panakukang PT. PLN (Persero) Wilayah SulSelraBar AP2B Sistem SulSel. (7806046 Z) E-Mail: [email protected] Agus Latief, Agus Latif (F), was born in Makassar in 1980, graduated high school SMKN makassar majoring in industrial electrical construction, began working at PLN (2002/2003) as a substation operator panakukang PT PLN (Persero) Tragi Panakukang and recent position as substation maintenance staff PT. PLN (Persero) Wilayah SulSelraBar AP2B Sistem Sulsel. (8003044 F) © OMICRON electronics GmbH 2011 – Instrument Transformer Measurement Forum