1.1 STRESSES ACTING ON POWER EQUIPMENT

Transcription

1 Chapter 1 Dielectric Diagnosis of Stator winding insulation 1.0 INTRODUCTION Electric Power System comprises of a large number of Power equipments like high voltage generators, motors, transformers, bushings, cables etc. which are quite expensive and form a significant portion of plant assets. More importantly they are vital components for reliable delivery of electric power. However, the reliability of these equipments depends to a large extent on the healthy condition of the insulation. Failure of the insulation either directly or indirectly will result in failure of power equipment which in turn may result in forced outage, reduced reliability, and increased maintenance costs. Insulation systems of power equipments are a complex combination of materials and have undergone changes like non-synthetic to synthetic in the last few years. Insulating materials generally comply with the required performance at the beginning of their life, but during their course of operation, ageing phenomena and deterioration may occur due to the effects of various stresses. The ageing processes are complicated and take place under various stresses simultaneously or sequentially which may result in deterioration of physical / chemical / dielectric properties. Thermal ageing is a chemical process leading to molecular decomposition and oxidation of organic materials resulting in changes in dielectric response. The dielectric response at low frequencies involves phenomena like direct conduction, quasi dc conduction (low frequ ency dispersion), and alpha dipolar relaxation mechanisms. 1.1 STRESSES ACTING ON POWER EQUIPMENT The major stresses acting on the power equipment either sequentially or in combination are electrical, thermal, mechanical / vibrational stresses and environmental factors (temperature, humidity, pollutants etc.) and any one of them may dominate depending on the type of power equipment. The mechanical stresses acting on the winding of power transformer (between the 1

2 conductors, leads and windings) are due to fault currents caused by system short circuits. Thermal stresses arise due to local overheating and over load currents. Dielectric stresses arise due to system over-voltages and transient impulse conditions. The stator windings of a rotating machine experience thermal, electrical, vibrational stresses which is dependent on the load output, type of duty cycle and ambient conditions like humidity & contamination. The stresses acting on other power equipments like cables (oil filled cable, extruded cables and gas filled cables) depend on the type of cable. Thermal stresses arise due to operating current under normal or abnormal conditions. Mechanical stresses arise due to bending of cable; thermo mechanical stresses are due to cyclic or short-circuit behavior. Depending on environmental conditions, deterioration occurs due to parameters like corrosivity etc. Also electrical stresses arise due to operating and abnormal voltage conditions. In case of large power capacitors the dielectric stresses are predominant. Also stresses arising due to over voltages like switching and impulse result in deterioration of capacitor insulation. Therefore dielectric diagnosis plays a vital role in assessment of insulation condition of the power equipment and helps in trend analysis of degradation of insulation to ensure reliability of power equipment. 1.2 DIELECTRIC DIAGNOSIS Dielectric diagnosis is the application of suitable procedures and measurements to evaluate insulation degradation and deterioration caused during service conditions. The deterioration criteria is either tailored to the insulation material or to the equipment and may be classified as direct or indirect. The direct criteria are connected with properties like electrical strength, flexural strength etc. while indirect criteria have a relationship with electrical properties like loss angle, insulation resistance, partial discharge, moisture content, non-ohmic behavior etc. which may change during service life. Several diagnostic tools both off line and on line techniques are used to assess the insulation condition of the power equipment when they are in service. 2

3 1.3 DIELECTRIC DIAGNOSTIC TECHNIQUES Electrical, Thermo analytical, Chemical and Spectroscopic techniques are widely used for dielectric diagnosis of power equipment Electrical techniques: Some of the electrical techniques are 1. Insulation resistance / polarisation index to check for index of dryness in case of transformers, rotating machines, cables, power capacitors. 2. Tan measurement & capacitance measurement to assess the dielectric loss in case of transformers, generators, motors, cables, CT s, PT s, capacitors. 3. Partial discharge measuring techniques to determine discharge activity in cables, rotating machines, capacitors, transformers etc. On line acoustic emission techniques for PD activity in power transformers, gas insulated substations. 4. Recovery or return voltage measurements (RVM) are being used to evaluate the polarization and discharge characteristics of an insulation system since these processes are strongly influenced by the quality and condition of the dielectric materials. This technique is widely used for determination of moisture present in paper insulation of transformers and oil filled cables. 5. Winding resistance for determination of bad joints, poor connection in case of transformer windings, rotating machines etc. 6. Dielectric spectroscopy for dielectric response studies for oil filled transformers, cables, etc. 7. Low frequency bridge methods for measuring tan, PD etc. for cables and transformers Thermo Analytical techniques (For Physical and chemical properties of materials) 1. Differential scanning calorimeter (DSC) for investigation of phase transformations in crystalline, nanocrystalline and amorphous materials. 3

4 2. Differential thermal analyser (DMA) for high temperature investigation 3. Thermo gravimetric analysis (TGA) 4. Thermo mechanical analysis (TMA). 5. Hot spot detection using infra red cameras Spectroscopic techniques (Equipments for investigation of structure and chemical analysis of materials) 1. Fourier Transform Infrared spectrometer (FTIR) 2. Ultraviolet spectroscopy 3. Scanning Electron microscope (SEM) for structural characterization 4. Transmission Electron microscope 5. Nuclear magnetic Resonance spectroscopy 6. X-ray Fluorescence spectroscopy 7. Raman spectroscopy 8. X-ray diffractometer 9. Optical microscopes Chromatographic / Chemical methods 1. Gas Chromatography Mass spectrometry 2. Analysis of dissolved gases in oil in case of power transformers, oil filled cables 3. Degree of polymerization in paper oil insulation. 4. Chemical composition, oxidation induction time to study the polymer morphology as in case of extruded cables, epoxy insulator, rotating machines etc. A variety of diagnostic tests representing a broad spectrum of electrical, thermo analytical, physico chemical and spectroscopic techniques are available as either on-site or laboratory tests for insulation condition assessment. These diagnostic tools are mainly used for trend analysis to know the condition of the power equipment insulation. However, determination of remaining life is the most difficult part of analysis because of lack of well defined deterioration models, multiplicity of failure mechanisms which are not well understood in quantitative form and are highly dependent 4

5 on environmental and operating conditions. Due to various degradation processes and during ageing, insulation material undergoes structural changes which can be monitored and studied by some analytical and spectroscopic techniques. However correlation of structural changes with dielectric parameters obtained by diagnostic tests is not fully explored for improving the accuracy of diagnostics and prediction of remaining life. This correlation is very much necessary to assess the reliability of the system. CIGRE Working group 33/15.08 [1] has emphasized that there is need to apply physical / chemical tools like structural, morphological and spectroscopic procedures for value addition to dielectric diagnosis. Though Electric Power System comprises of a large number power equipments, high voltage rotating electrical machines represent the most vital components of power generation and industrial application fields. They are quite expensive and form significant portion of plant assets. However, the reliability of these machines depends to a large extent on the healthy condition of the stator winding insulation. The insulation systems used in stator winding of high voltage rotating machine, the deterioration mechanisms due to ageing and the electrical evaluation techniques are discussed in the succeeding sections. 1.4 STATOR WINDING OF HIGH VOLTAGE ROTATING MACHINE The insulation systems of high voltage (HV) rotating machines are a complex combination of materials and have undergone considerable changes in the last few decades. In the past, natural binding materials were employed as stator winding insulation for HV rotating machine. However, with the advent of synthetic materials ( polyester, epoxy, silicone resins, etc.) the development of generator with larger capacity became possible. Epoxy-mica is used as a main ground wall insulation (electrical insulation that separates the copper conductors from the grounded stator) of stator coils of large generators since 1960 [2]. To fully understand and predict the electrical behavior of insulation and ageing characteristics, one must have knowledge of the chemistry of materials used: 5

6 the atomic-molecular arrangement and the nature of the chemical bonds. The stator winding of rotating machine usually comprises of mica with organic reinforcing, bonding and impregnating materials [2]. Mica paper / synthetic resin combinations (polyester, epoxy, silicone resins, etc.) are employed almost exclusively in the form of tapes, which are wrapped around the stator coil (form wound), impregnated and cured. Mica s unique combination of physical, thermal and electrical properties and its ability to be split into very thin, incompressible sheets while maintaining flexibility, toughness and high tensile strength find wide applications in high voltage machines. The two classes of mica most commonly used in electrical applications are muscovite [KAl 2 (Si 3 Al)O 10 (OH,F) 2 ] and phlogopite [KMg 3 (Si 3 Al)O 10 (OH,F) 2 ] respectively [3]. Mica minerals comprise of layers of silicates separated by alternating layer of metal oxides and metal ions. Figure 1.1 shows the cross section where every four silicon atoms is replaced by an aluminum atom. Each three layer structure is separated by a layer of potassium ions. Epoxy resins provide high strength, good adhesion to most materials including metals, resistance to moisture, solvents and other chemicals. Epoxies take their name from the epoxide functional group (three -membered oxygen containing ring) which forms part of the epi-chlorohydrin molecule, one of the Figure 1.1 Molecular Structure of muscovite mica [3]. two reactants to make the resin. The molecular structure of basic epoxy resin pre-polymer [3] is shown in Figure 1.2. The other component which is most commonly used is diphenyl propane, often called Bis-phenol A. Many different curing agents are used to bring about cross linking of thermosetting epoxy resins. 6

7 Figure 1.2. Molecular structure of basic epoxy resin pre-polymer Stator winding insulation do comply with the required dielectric performance at the beginning of their life, but due to occurrence of various stresses during service condition, ageing and deterioration DETERIORATION MECHANISMS OF STATOR WINDING INSULATION During machine operation, the stator windings are mainly subjected to stresses like electrical, thermal, mechanical etc. and undergo ageing. Some of the major deterioration mechanisms are: Electrical Ageing: The various causes of electrical ageing are high dielectric stress during over voltages, effect of operating dielectric stress over a long period of time, electrical tracking due to surface build up of moisture, oil, carbon dust etc., treeing, corona, and insufficient insulation on leads. The electrical stress distribution at the operating voltage has an important role in ageing of insulating materials. Enhanced electrical stresses can occur at certain local points due to voids, imperfections and defects. Since the insulation is composite in nature, it is very likely that cavities are present within the system. The partial discharges in these cavities are always a starting point of degradation resulting in surface erosion as shown in figure 1.3 [4]. The occurrence of PD in rotating machine stator insulation is a very common phenomenon and the origin of failure in many instances has infact been traced to this. In addition, two other types of degradation phenomena can occur in the insulation, namely the slot discharges and discharges in the end region which are called end discharges. 7

8 Figure 1.3 Surface erosion of epoxy resin Partial Discharges Partial discharges are caused by local breakdown in voids in insulating media. As a result of high local electric field, charge carrier swarms are generated in the voids and may produce local decomposition of the insulation. Partial discharges generate short duration current pulses which propagate from the origin towards both ends of the winding, and can take place in a series mode like a traveling wave on a transmission line with frequency dependent attenuation & reflections and in a parallel mode mainly by inductive and capacitance coupling between the windings. The most significant locations of PD occurrence are Voids enclosed in the slot insulation material or at its boundary with the copper conductor Between the semi-conducting paint on the bar and the iron core (slot discharges). At the damaged spot of the paint i.e. where the electric field along the surface becomes too high. Where the stator bar emerges from the slot and no special stress control is present. Between the non-linear and highly resistive stress grading paint at slot entry and the low resistance coating on the bar. In HV electrical rotating machines three types of discharges can be identified. 8

9 a) Internal discharges that occur in voids occluded in the bulk volume of the winding insulation. b) Slot discharges that occur in the air gaps between the core laminations and adjacent coil sides in the slots. c) End winding discharges that occur at the extremity of the conducting coating outside the end of the slot where there is an interface on the coil surface between ground and high voltages Slot Discharges The discharge between the main ground insulation of high voltage coil and the slot wall is termed as slot discharges. One can easily distinguish between mechanical slot discharges caused by the stator bar movement in the slot, and electrical slot discharges caused by poor contact between the semiconducting layer of the stator bar insulation and the stator iron core. A view of the slot portion of the stator winding is shown in Figure 1.4 [4] Causes of Damage to Insulation Due to Discharges Various causes responsible for damage to insulation are: Slot discharge sites may occur as the result of certain semiconductive coating conditions that either are present when the machine is new or develop in operation. These conditions include discontinuities in the semiconducting slot coating, high resistivity values of the coating such that it does not function as intended, stator bar vibration levels due to wedge looseness or migration, coil damage, clearance between coil side and slot wall, loss of effective electrical contact of coil surface to ground, level of vibration, thermal cycling etc. Each factor has its own effects and finally the combined effects lead to damage of insulation. The following are the common symptoms and failure modes: Damage to bar armour and insulation surface, white or brown discoloration and powdering due to corona specially between phases, dark or black discoloration & powdering due to arcing, burning along creapage paths and along stress control coatings, higher temperature due to increased dielectric losses resulting in puncture and ground fault. 9

10 Figure 1.4. A view of Slot portion of the stator winding Thermal Ageing The thermal stress on the insulation occurs due to the normal continuous I 2 R loss in the conductor and sometimes due to thermal shock in the event of a short circuit. The insulation inside the slot is surrounded by large amounts of iron whose heat capacity is very large as compared to air. However in deep slotted machines, the temperature in the middle of the slot could be C because of low thermal conductivity of the surrounding insulation and localized runaway condition may occur. The various effects & causes of thermal ageing are Chemical changes in binder and backing materials due to operating temperature and with time, Ingress of moisture and contaminants during periods of shut down, overhaul, relative motion between conductor and core due to thermal cycling, inadequate cooling due to dead spots, poor distribution and reduction in heat transfer, loss of volatiles The common symptoms and failure mode due to thermal ageing are: Burning smell, change in colour and texture, strand separation, powdering, puffiness, embrittlement, flaking and delamination of insulation, tape separation, cracks and sponginess, flow of insulation in case of bitumen, increase in PD activity, flashover along surfaces or gaps under electrical stress due to nearby arcing or extremely high metal temperatures in the presence of contaminated gas. 10

11 Mechanical Ageing During operation of the machine, the firm contact of the bar with the slot walls is disturbed due to core vibrations, thermal expansion and contraction of the winding and forces imposed on the winding due to sudden short circuits. The mechanical stresses occur due to the load changes. These stresses may be amplified many times during a short circuit. In very large capacity generators the conductor overhangs (in spite of being rigidly supported) are subjected to enormous vibrations and embrace each other upon severe short circuit. Due to the fact that the heat capacity of the surrounding medium (air) near the conductor overhangs is very low, the insulation in the conductor overhangs [Figure 1.5] is subjected to tearing stress due to differential thermal expansion and contraction. As load changes, the temperature of the conductor increases or decreases rapidly giving rise to thermal expansion and contraction. The insulating materials over the conductors may not sustain these sudden expansions or contractions and cracks on the surface of the insulation may be formed, which may become a site of PD. Performance under mechanical stress is one of the most important aspects in deciding the materials for rotating machine insulation. Figure 1.5 Overhang portion of the stator winding The probable causes of insulation ageing due to mechanical stresses are End winding vibration due to magnetic forces between phase belts, resonance of mechanical support, inadequate support and axial 11

12 restraint of end winding, deterioration of radial support in slots, wedges, springs and packing. The common symptoms and failure modes are Loosening of slot wedges, strand separation and cavitations, rubbing and looseness at end windings blocking, higher temperature due to bar bouncing and consequent reduction in heat transfer especially at cross over resulting in flashover and ground fault Ageing due to Environmental factors Machines operating in polluted atmosphere e.g. cement and chemical plants, rubber factories etc. would suffer rapid deterioration of insulation system if not designed with proper care. Also majority of stator windings may get affected by surface deposition of chemicals, dust, etc. and ingress of moisture. Electrochemical failure will occur if the electrolyte concentration within the insulation is high enough Causes of failure of windings during manufacture Quality deficiencies of the insulating materials (mica tapes, varnishes, enamels etc.) Improper storage, defective method of application (loose taping, causing voids and formation of wrinkles), non-uniform pressing of insulation during baking & curing, non-adherence to established procedures of maintaining temperature and curing time, inclusion of foreign particles during application of insulation, development of cracks, sharp corners, mechanical damage caused while inserting the winding into the slots, sharp edges / burrs in the stator slots, mishandling of the machine during manufacture, overheating of insulation during brazing of end joints, loose core causing vibration & damage, magnetic particles inside stator, inadequate corona protection, defective inter-turn insulation, lack of or inadequate quality control at different manufacturing stages are the causes of winding failure during manufacture. 12

13 1.4.2 EVALUATION TECHNIQUES FOR STATOR WINDING The main features of the electrical methods of evaluation of electrical insulation are summarized below Evaluation of Electrical Parameters IEEE standards [5-10] and procedures are widely used by motor and generator manufacturers and utilities during commissioning of windings for new machines and as well as to evaluate the condition of the winding insulation in operating machines. The tests can be broadly classified as A. Over Voltage tests 1 Proof tests a) Power frequency ac voltage (at twice rated voltage+1) in kv b) Direct Voltage at 2.5 times rated voltage c) 0.1 Hz d) Half cycle e) Impulse B. Direct Current tests 1. Insulation resistance 2. Polarization Index 3. Polarization / depolarization versus time C. High Voltage AC bridges 1. Tan delta, Capacitance and tip-ups 2. Integrated Partial discharge energy (DLA) 3. Product of Resistance and Capacitance (R*C) D. Pulse and HF Measurements 1. Partial Discharge 2. Neutral signal analysis 3. R F Slot probe, manual / automatic Over Voltage tests Over voltage tests are carried out on full stator windings and individual phases. Proof tests like power frequency ac voltage (at twice rated voltage+1) in kv and dc voltage (at 2.5 times rated voltage) in kv are normally recommended. These proof tests simulate two conditions namely, the permanent stressing of the insulation by power frequency and the action of 13

14 switching surges. IEEE standard 56 [5] is an extensive guide for various tests on stator windings and also discusses the maintenance ac Hipot test. A Hipot test is application of a high potential applied to the winding and is normally higher than what the winding experiences in service (Details described in the succeeding sections ). Any gross flaws in the windings are detected during the test and if the winding does not fail during the over voltage test, then the winding is not likely to fail when put into service. High KVA rating transformer is required to test the winding of a high capacity machine at power frequency because the capacitance of these windings will be quite high which can result in high capacitance current at power frequency. DC tests were introduced to overcome the difficulty of portability of high KVA rating test transformer. The objective of many investigators [11] was to determine the ratio between dc/ac test voltage which would produce the same probability of insulation failure, or the same ability to detect weak spots DC Method DC methods range from the simple withstand or proof tests to more refined ramped voltage test in which the dc high voltage is applied as a linearly increasing ramp function and the current response is recorded. IEEE standard 95 [6] gives the guidelines for conducting dc test. The 2002 version standard highlights a new variation of the dc Hipot test called DC Ramp test Hipot method for Evaluation of winding insulation The dc Hipot tests are performed by several methods. DC test sets being lower in capacity are portable and power consumption is much less. The ratio between dc and ac tests in terms of applied voltage varies between 1.2 to 2.5. Usually a value of 1.6 is chosen for the machines in operation. (a) Conventional dc Hipot In this method suitable high voltage is applied quickly to stator winding terminal which is maintained for either 1 or 5 minutes and then the voltage is gradually reduced to zero. If the insulation is healthy there will be no high 14

15 current surge. If the power circuit breaker trips, then it is an indication that, puncture has occurred in insulation. (b) Step stress Hipot In this method the applied dc voltage is increased in steps of 1 kv with each voltage level being held for I minute. The dc current is measured after the end of each step and current plots are obtained with voltage. The trend is generally a curve with slight upward slope. An abrupt increase in current is an indication of weakened insulation. (c ) DC Ramp Hipot In this method, the dc voltage is smoothly and linearly increased at a constant rate, usually 1 or 2 kv/minute and there are no discrete steps in voltage or current. The current vs voltage plot is graphically displayed. The advantage is that it is a very sensitive method to detect defects when current instability occurs since the capacitive charging current is not changing with time. However the disadvantage is that it does not experience the voltage stresses applied across the insulation when it is in operation [12] Insulation Resistance Insulation Resistance (IR) tests are made to determine current leakage through insulation and over its surfaces under specific conditions of voltage and time. DC insulation resistance also provides information on humidity, conductive contamination, degree of cure, cracks and certain types of mechanical damage of insulation. The test voltages are chosen such that it does not overstress the insulation and cause failure and must be restricted to a value appropriate for both the voltage rating of the system and basic insulation level. Guidelines for performing insulation resistance measurements are given in IEEE std , Recommended practice for testing Insulation Resistance of Rotating Machinery [6] which recommends a dc test voltage of 5 to 10 kv for insulation system rated for 11 kv. 15

16 Polarisation Index [PI]: For determination of current to earth and its rate of change with time, the commonly used characteristic is the PI. Polarisation index is the ratio of observed leakage currents at two instants i.e. 1 min and 10 min. PI is a recognized criterion for judging the insulation dryness or humidity, contamination, cure and physical integrity of a new winding or after a service interruption. A low value of PI indicates a humid insulation. IEEE std [6] recommends a polarization index greater than or equal to 2 for class F insulation. The variation of polarization or depolarization current with time follows a power law in most of the cases. The value of the exponent might provide useful information on the state of the insulation or contaminated with oil, dirt, etc Surge comparison Surge comparison test is used to determine condition of inter turn insulation of the stator winding [9]. A steep front voltage of suitable magnitude is applied to any two winding sections under test. In each cycle, a capacitor is charged to an appropriate voltage, then discharged by means of a suitable switch (such as a spark gap, thyratron, or a solid-state device) into a circuit that includes the windings. Voltage and current then oscillate at the natural frequency of the circuit. The resultant damped oscillatory superimposed waves are displayed on an oscilloscope. The two waveforms will be identical if both the phase windings are electrically identical and free from faults. Any discrepancy in the two waveforms indicates inter turn fault in one of the windings. A variety of dc testing techniques have been devised to monitor and assess the condition of stator winding insulation systems. It was an international practice to subject the windings to direct current (dc) over voltage tests to avoid damage due to overheating and intense partial discharges of the insulation during the high voltage ac tests. However, it was realized that failures occurring during ac and dc testing respectively were not always of the same kind; end winding failures were more frequent in dc tests. This is due to 16

17 the fact that the electrical stress distribution over a winding is governed by resistivity in the case of direct voltage and permittivity in the case of ac voltage. In order to obtain a realistic stress distribution over the winding and the influence of the dielectric parameters, very low frequency [13] tests were introduced and the effect of frequency on the stress distribution was analysed. Another approach is the power frequency half cycle test. Alternatively a resonant power frequency voltage generator is used in place of portable 50 Hz, voltage test equipment. The advantage is that when an insulation failure occurs, the limited available power causes much less destruction to the insulation. The voltage wave has a very low harmonic distortion, which is extremely important for proper evaluation and analysis of partial discharge. Impulse voltage testing of multi-turn windings will produce a realistic simulation of the stresses produced by surge voltages, but damping will confine stresses to a small portion of the winding near the energized terminal Non-linear Analysis Non-linear analysis test is carried out by means of application of ac voltage at predetermined voltage levels up to a maximum of rated phase voltage on the stator winding. The voltage & the current flowing through the insulation is monitored by capturing several waveform cycles of the voltage and the current on a digital storage oscilloscope [14]. The instantaneous admittance of the insulation is calculated and the admittance patterns analysed for specific harmonic patterns. The extent of harmonics, predominance of odd or even harmonics, high or low frequency harmonics is analysed to provide information on insulation ageing and or degradation status and the ionic activity intensity inside the slots caused by the presence of voids between the stator core and the winding. This current spectrum tends to be displaced from the fundamental frequency to higher harmonics along the time indicating the presence of ionic activity [14]. 17

18 Dielectric loss measurements by HVAC bridge The dielectric loss is the power dissipated in a dielectric as heat when the dielectric is subjected to an electric field. The power loss occurs due to the conduction processes in a dielectric that cause a leakage current Dissipation factor tip-up (Delta tan ) Dissipation factor, also called tan is a measure of dielectric losses in the insulation. It is the property of the insulation system used. IEEE [8] gives the guidelines for measurement of power factor tip-up of electric machinery coil insulation. Power factor tip-up is the increment in dielectric dissipation factor (tan of the insulation measured at two designated voltages. Tip-up is an indirect way of determining if partial discharges (PD) are occurring in a high voltage stator winding [8]. For a given insulating system the tan shall be as small as possible. An AC bridge such as a Schering bridge or transformer ratio arm bridge is used to measure tan and capacitance of the stator winding. Dissipation factor is determined at several levels of test voltage. The first test voltage is usually 0.2 times the rated phase to phase voltage V ph where PD is just below inception level and is taken as the reference value (tan δ o ). The inception voltage may decrease due to reversal of charges or presence of homopolar charges at the interface or on the system or due to a single large void. Tan δ and capacitance measured at low voltage are a function of the state of the curing of the resin, the presence of moisture or contamination in the windings, loss of contact of the coil outer surface with the core due to erosion of adhesive coating used for preventing slot discharges, non linear effect of slot end stress grading systems, the influence of inter winding capacitances and losses, besides other factors [8]. The higher levels often in steps of 0.2 V ph are interpreted as revealing the gradual ignition of PD in voids and imperfections [11]. These measurements are performed by balancing the bridge with appropriate earthing and guard circuitry. The settings of a balanced bridge give values of the capacitance of the test object as well as the dissipation factor. Information about PD activity is contained not only in 18

19 tan δ tip-up ( tan ) but in capacitance tip-up as well. Different shapes of the voltage wave will at constant amplitude produce different values of tan for a particular test object. Capacitance tip-up is another important insulation diagnostic tool. A change in capacitance may occur due to change in the size, shape or distance between the two conductors. As insulation system cures or ages, the dielectric constant may change causing a change in the measured capacitance Partial Discharge measurements Discharge measurements provide a sensitive, nondestructive means of detecting minute defects in insulation. The successful application of discharge technique depends on the following; the conditions at which discharges occur, factors which affect the discharge magnitude, recurrence frequency, the various mechanisms of deterioration and breakdown by discharges. The Schering bridge and modifications of HVAC bridge circuit (PD detectors) represents the new development which reveals the extent of PD activity. Partial discharge test is another important diagnostic test for HV machines as it is capable of revealing incipient faults in the stator winding structure. Partial discharges in stator windings are manifest by positive and negative current pulses of nanoseconds duration. IEEE std [10], Guide to the Measurement of Partial Discharges in Rotating Machinery describes typical systems for conducting off-line partial (PD) measurements on individual formwound coils. The magnitude of a particular PD pulse is proportional to the size of the void in which PD occurred. Consequently the bigger the detected PD pulse, the larger is the defect the originated that discharge. Smaller defects tend to produce smaller PD pulses. The key measurement in a PD test is the peak magnitude, Qm. i.e the magnitude of the highest PD pulse. The PD magnitude depends on several factors like size of the defect, the capacitance of the winding, the inductance between the PD site and the PD detector [10]. The PD test is thus a comparison test and one can compare the PD from the 19

20 same winding over time during ageing. There are no specified limits in the standards. The slot and end winding discharges are known to be more detrimental to the insulation than internal discharges. The internal discharges cause slow but gradual deterioration of the insulation in the course of service. The slot and end winding discharges are severe and can cause deterioration and eventual breakdown of the insulation with in the span of a few months or later. The PD evaluation involves energizing the individual phase winding to earth from an external source. The coupling or blocking capacitor C b blocks the power frequency high voltage and allows the high frequency current pulses of PD to be coupled to the discharge detector. The magnitudes of PD are calibrated in pico coulombs. The ac applied voltage is raised gradually until PD pulses are observed on the detector. The voltage at which PD starts occurring is called discharge inception voltage (DIV). The test voltage is increased up to the maximum of phase to earth voltage and magnitude of the PD pulses is noted down. The test voltage is decreased until PD pulses disappear and the voltage is called discharge extinction voltage (DEV) which is lower than the DIV Integrated Partial Discharge Energy Another method of deriving more direct information on PD activity is by dielectric loss analyzer (integrating capacitance bridge) developed in UK [ 15]. This bridge is balanced at a voltage below PD inception. A signal proportional to the test voltage is connected to the X-Plates of an oscilloscope and the signal across the bridge detector to Y-Plates. When the test voltage is increased above PD inception, the Lissajous figure on the oscilloscope screen opens up to a loop typically in the form of a parallelogram. Information is derived from the dimensions and shape of this figure as a function of the test voltage. An overall judgment is expressed in terms of dissipated power loss per microfarad of winding capacitance and per ac cycle. With advancements in technology and new measuring instruments, this method is sparingly used. 20

21 Ohm-Farad R*C As insulation system cures or ages, the permitivity may change causing a change in the measured capacitance. The product of the insulation resistance measured after one minute of application and the capacitance of the winding at 0.2 times line voltage (V L ) is another diagnostic tool. It has the dimension of time, the insulation time constant. It decreases with time in service and this is sometimes related to the ongoing processes of deterioration of insulation On line monitoring techniques Of late, a new PD sensor called the stator slot coupler (SSC) is developed by the Canadian Electrical association [16-19] which is able to differentiate between the PD in the winding and all types of electrical interference and permit on-line test for turbo generators. The new sensor requires no HV connection to the winding, and is easily installed in the stator slot, underneath the wedges. The SSC is essentially a directional electromagnetic coupler and consists of a ground plane and a sense line with co-axial output cables at each end. The SSC yields an output pulse from each end whenever an electromagnetic wave propagates along the SSC near the sensor line. The dual port nature of the SSC permits determination of the direction of PD pulse travel using instrumentation that can indicate which end of the SSC has detected the signal first. The Passive Rotor Temperature Sensor (PRTS) to measure the temperature at specific locations on the rotor of a turbine generator [18,19] has been developed. The PRTS uses an optical temperature measurement technique. The surface of the rotor is painted with a special fluorescent paint, when illuminated with ultra violet (UV) light (via a fibre optic cable in the stator that focuses the UV light on the rotor), will fluoresce with a temperature dependent decay time. The higher the temperature, faster is the decay time. Electronic Rotor Temperature Sensor (ERTS) is an alternative means of measuring rotor temperature at specific locations [19]. Other online method devices like thermal life indicator, insulation sniffer, global motor monitor are used for insulation diagnostics of stator winding of generators [19]. 21

22 1.5 Summary: This chapter summarizes the various deterioration mechanisms of stator winding insulation due to ageing under service conditions and presents a bird s eye view of the various electrical test methods for diagnosis of stator winding insulation of high voltage motors and generators. Several off-line and on-line electrical techniques for condition assessment of stator winding insulation of Generators and motors have been enumerated. The techniques like Dielectric Spectroscopy and Recovery voltage measurement (RVM) for dielectric response studies at low frequencies are described in chapter 2. The theoretical basis on which these techniques are based is briefly described and Spectroscopic techniques like Fourier Transform Infra Red (FTIR) spectrometry, Dielectric spectroscopy, Scanning Electron microscopic techniques and thermal analysis techniques like Thermo gravimetric analysis, Differential Scanning Calorimeter are also described in chapter 2. 22