Online Drying Systems with Moisture Monitoring, IEC 61850 Integration and Online OLTC Filtration Systems for Unmanned Substations Yogesh Sood Advisor (Application Technology), www.ptssglobal.com INTRODUCTION HISTORY A number of Electrical Utilities in India are trying to meet the growing demands of power by adding more overhead lines and adding substations. Substantial cost savings result in deploying unmanned substations as compared to the conventional manned substations. Once the substation becomes unmanned, there will not be any need for posting maintenance staff and it can be operated remotely. We all know the 3 main ageing accelerators of transformer are DISSOLVED MOISTURE, OXYGEN & HEAT. ‘If it doesn’t break, fix it’ and ‘distinguishing the broken from the un-broken’ is the issue which has to be tackled smartly and intelligently addressed, these expressions in the old days were neglected because of limited resources and inadequate knowledge. In the era of today, one cannot avoid any situation when the transformer and oil results are not in international acceptable norms. This could be very critical to the health of the grid all together. It also introduces new risks. The failure rate of significant assets like Transformers is expected to grow to 6-7%. There are, of course, different opinions, including more pessimistic views, but most agree that the future “is not what it used to be”. Things will definitely get worse, much worse. The average replacement cost of, say, a 100 MVA transformer can total about 4-5 Crores (and can take approx. 12 months to build or replace). Similar costs could incur heavy loss for different rating of transformers annually in future; therefore, saving the transformer health is very important. The good news is that ‘Most Transformer Failures (>50%) Are Preventable’ using the latest preventive maintenance techniques. “Only real-state (real time) eliminates all disadvantages”. Online Transformer Drying System and OLTC Filter System are the best preventive maintenance equipment’s. The unmanned substation concept is the successful working model of the Power Grid Corporation India Limited (PGCIL) and slowly being adopted by few state utilities who have initiated pilot projects of various capacities to study the viability of the Substations. Substation Automation is the Key and many Advances and Innovations has taken place in the recent years which has helped the Power Sector to integrate the Substation at all levels using Intelligent Electronic Devices (IED’s), control and automation capabilities within the substations as well as from remote users using SCADA to control power-system (switchyard) devices. Online drying system plays a very important role in the Substation automation and specially in the Substations which are unmanned or expected to be unmanned in the near future, where we not only get the real time RS(Relative Saturation) & Moisture PPM values with real temperature but have an integrated system to remove moisture and conductive contaminants from the Transformer. The latest IEC 61850 communication protocol is the pre-requisite in the PGCIL specifications to supply Online Drying Systems in any of their Substations. Hundreds of ODS are in operation in PGCIL where the key data from the HMI of the Online Dryout Systems have been integrated with the SCADA from various Contractors like Siemens, GE, ABB, Schneider etc. How does water affect solid insulation in transformers????? The Kraft paper used for transformer insulation is similar to the brown paper used in grocery bags. “Imagine setting your paper bag of soup cans and a dozen eggs down in a water puddle, then picking it up five minutes later. What do you think will have happened to the paper’s tensile strength?” Operating experience with transformer insulation over many years has shown that moisture in microscopic amounts – not liters – is the cause of more electrical breakdowns than any other impurity. Recognition of the importance of extremely small amount of moisture has grown immeasurably with the increase in voltage stress, load and reduced BILs (Basic Insulation Levels). weight of cellulose paper. The %age of moisture in the transformer can be accessed using ppm, temperature equilibrium curves and more than 4% moisture is dangerous for transformer operating at 90DegC. Indeed, moisture ‘constitutes a hazard not only to the dielectric performance of the oil itself but also to insulations that are immersed in the oil’. Thus, we should all agree that water is “enemy number one” What is not known or generally accepted is that water problem may begin before what is laid down/revealed in any national or international standard. There are 5 processes practiced to achieve the complete drying of wet Power Transformers 1. Vapour phase system (off line) • Can be used in workshop environment only. • Lot of piping, gas cutting and bolting works required. • Required to dismantle the transformer, transporting to workshop from site and back to site for re erection. • Down time and interruption. • Manpower required. 2. Conventional off line dry out system (stream line filter) •Interruption required. • Long time multiple circulations with high temperature and vacuum will remove the aromatic fractions from the transformer oil (i.e.) Natural oxiditation inhibitors. • Winding shrinkage. • Reduction of winding compression pressure after multi circulations. • For many transformers jacking of winding at site poses certain risks. • Manpower required. • IR & Tan Delta of insulation cannot be achieved to the desired level. 3. Low frequency heating (offline dry out) • Service interruption. • Winding shrinkage. • Cumbersome process • Manpower required. 4. Oven heating dry out • This option requires shipping and untanking. • Drying core and coil in vacuum oven. • Insulation may be in the end of its useful life. • Oven drying in oven would cause the insulation to become more brittle. • Increase the risk of insulation damage during re-tanking. 5. Heating the winding by circulating current (short circuiting secondary winding) • Applicable only for small transformers. “The electrical user …. recognized the electrical hazard which is presented by the presence of water but rarely does he recognize the equally disastrous effect which traces of moisture have on the longtime usefulness of the insulated equipment, even when the amount of moisture is not sufficient to cause dielectric difficulties” as stated by one of the experts. Therefore, the moisture content of a transformer’s solid insulation plays a major role in determining a transformer’s length of life. Every time the moisture content of the solid insulation doubles, the expected life of the transformer is cut by half. Throughout a transformer’s operating life, moisture will accumulate in the solid insulation. This moisture originates either from: outside the transformer or from within, as the liquid and solid insulation age and oxidize over time. Cellulose insulation has the affinity to absorb the moisture up to 8-9% moisture per dry weight (%M/DW). International standards say it should be less than 1% for ideal healthy operating transformer. Who knows how much the manufacturer has supplied? What is the present %age?? Keep in mind that just an extra half percent for a total of 1.0% M/DW, will cut your transformer’s expected operating life to half. “Taking out the moisture out of the transformer insulation is as difficult as to dry the telephone directory kept in the bucket full of water for more than a week and taken out for drying all pages without opening.” Rule of thumb says that the weight of the cellulose paper in transformer is 0.3 times the KVA rating of the transformer in pounds and it has to be maintained to less than 1%moisture/dry SOLUTIONS FOR REMOVING TOO MUCH WATER FROM THE CELLULOSE • 1/3 of name plate KVA and impedance voltage is required at site. • Not suitable for EHV transformers To avoid the above costly and cumbersome processes and to protect your transformer investment through the most use of a safer, more effective and less costly equipment, automatic online moisture removal systems without heat and vacuum process should be practiced. MOISTURE DETERMINATION The present method of determining the dryness of transformer insulation is to measure the moisture content of an oil sample and use the well-established moisture equilibrium characteristics between oil and paper insulation. However, the dynamics of the moisture movement between the paper and the oil during temperature cycling is significant and, unless taken into consideration, can cause significant errors in moisture assessment. In addition, the actual determination of the moisture content of the oil is full of uncertainties. Although Karl Fischer method is the standard practiced for measuring the water content of the oil, human error and a number of uncertainties associated with the oil sampling procedure reduce the reliability of measurements and could lead to incorrect conclusions. Transformer insulation system comprises of several different grades, and physical conditions of oils vary considerably from one unit to another. Even without these complications, moisture migration between cellulose and oil is a complex, temperature dependent process: time constants for the exchange of moisture between cellulose and oil are different in each direction; moisture in the cellulose is not evenly distributed; and not all the moisture in the cellulose is available for transfer in the oil. In addition, dissolved moisture in oil can form precipitate during rapid cool-down periods and become free water, which may not re-dissolve. Therefore, any method of moisture determination based on a single measurement without regard to those co-founding factors may provide a false indication to the insulation's integrity. The results of moisture saturation measurements by capacitive probes eliminate the frequent discussions that question the validity of Karl Fischer titration (which suffers from severe errors resulting into a poor accuracy) in aged oils specifically. HOW DOES THE TRANSFORMER? WATER ENTER THE The main source of water contamination is atmospheric moisture. 1. Residual after processing: • Manufacturing • Installation • Maintenance 2. Leaks, through weak points of Transformer 3. Transformer prevention system • Ineffective dryers breathing conservators • Ruptured Bladders/diaphragm – sealed conservators 4. Byproduct of cellulosic degradation. 5. Water Ingress Points: Cooler plugs, Cooler Gaskets and valves, Lid Gasket, Electrode Shaft, Manhole Gaskets, Bushing Gaskets, Gauges and plugs, Valves. Large amounts of rainwater can be sucked into a transformer in a very short time (several hours), when there is a rapid drop of pressure (after a rapid drop of temperature that can be induced by rain) combined with insufficient sealing. Some valuable moisture interpretation tips to access the transformer condition and to take predictive maintenance actions for life extension of large transformers: - Interpretation of absolute saturation of solid insulation: Percent M/DW of Cellulose Classification <0.5% New Transformer >0.5-1.5% Dry Insulation >1.5-2.5% Medium Wet Insulation >2.5-4% Wet Insulation >4% Very Wet Insulation Interpretation based on top oil relative saturation:Condition of Cellulose Insulation RS of water in oil after three days at GOOD <5 FAIR >5<8 PROBABLY WET >8<12 WET >12 60-70DegC, %age. PARTICLE CONTAMINATION The particles in oil range from microscopic to visible range. Large particles usually settle down. Time constant of particle sedimentation depend on oil viscosity. Hence an oil sample taken from a transformer at high temperature may contain only small suspended particles. Suspended particles are usually those above 0.45 µm. The visible range starts at about 50 µm. Manufacturing contaminants: Cellulose fibers, iron, aluminum, copper and other particles resulting from manufacturing processes are naturally present in the transformer’s oil. Non conductive mode particles presumable would be present in a 5 to 50 micron range – easily removable with 0.5 micron filters (filter selection is very important) IMPORTANT The fewer the particles, the weaker the effect of water on the dielectric strength of the oil. Hence removing particles could be a task of priority to maintain dielectric safety margin of insulation having an excessive level of moisture contamination. Efficient processing shall incorporate drying and filtering procedures simultaneously. The most important parameter, which determines effectiveness of the online process, is relative rate of contaminant removed per one pass, namely: Ratio of input and output water, Ratio of particles, Ratio of oil aging characteristics (neutralization number, interfacial tension, PF, resistivity) SAFETY ISSUES Recommendations for some safety measures: Ensure automatic tripping of the system with automatic shutdown controls by integrating electro valves, pressure sensor, flow sensor, gauges, air bleed valves, moisture sensor,etc. ADVANTAGES Benefits of the Energized processing :- Transformer remains in service No switching scheduling or costs. No transformer downtime. No auxiliary power equipment needed. Reduced transformer drying expense Less costly compared to heat and vacuum. Minimal energy consumption. Unattended operation saves labor expense. Improved Transformer Condition IR/TanDelta values improvement after drying. Increased oil dielectric strength. Increased protection from hot spot damage. Increased safety margin for higher transformer loading. Oil power factor and acidity improvement. Transformer Life extended Removing moisture retards insulation degradation. Transformer oil life is enhanced Since online processing through specialized catridges is a vacuum less process and no external heating is applied during circulation, it will not destruct the oil, inherent oxhidation inhibitors (aromatic fractions) will not be removed. It retains the colour of the oil. INSTALLATION AND IMPLEMENTATION Installing the Online moisture removal systems is very quick and easy. It can be installed both on old wet Transformers as well as new Transformers. Just take the unit near the transformer and simply connect the discharge hose to the top of the Transformer and the suction hose to the bottom drain port of the Transformer. Now, you are ready to start the online dry out process on the energized Transformer. Few of the Power Utilities like PGCIL, NTPC, BBMB, PSEB, HPSEB etc. have used this online processing for drying their wet Transformers. Various case studies are available. MOISTURE MONITORING Monitoring of the moisture ppm values along with the online processing gives an Engineer a confidence about the safe and healthy condition of the Transformer. Moisture ppm should be restricted to less than 8ppm and RS less than 5% preferably at 600C oil temperature. Thus a perfect online processing equipment should deliver the best of dehydration i.e removal of dissolved moisture to less than 5ppm with simultaneous removal of contamination of 0.3/0.5/1.0microns from the OLTC/Main tank coupled with continuous monitoring of moisture. SCHEMATIC FLOW of ODS Connections of Online Drying Systems and Online OLTC Filtration Systems IEC 61850 DATA From ODS Pre and Post Results of a Reactor SN Particulars Before After 01. Moisture (PPM) 12ppm 6ppm 02. RS 8% 5% 03. %M/DWi 2.0% 1.6% 04. Insulation Tan Delta/Capacitance 0.621/8587 0.402/8562 05. Oil Tan Delta/PF 0.02296 0.0143 06. IFT (Interfacial Tension) 28.8Dynes/cm 31.3Dynes/ 07. Sp. Resistivity @90DegC 0.819x1012 Ohm-cm 1.9x1012 Oh cm 08. Acidity (NN) 0.039407 0.024 09. Weight Cartridges 47.45Kgs 66.8Kgs OLTC FILTER SYSTEM of Power Factor of Press Board(Before & After) Using Above results. 3. 4. CASE STUDIES 1. More Case studies are available on request and can be requested at info@ptssglobal.com 2. References:1. SD Myer’s Guide for Transformer maintenance 2. Ben Taylor’s article on Online processing of High Voltage Transformers 3. Doble Laboratory Diagnostics