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All the electrical equipments are designed to operate as per the specification under normal system conditions within the tolerance level of voltage, current and frequency. However they are subjected to abnormal situation quite often. This will lead to gradual deterioration of the health of the equipment finally leading to its failure. Conventional protection systems operate under system fault conditions and protect the equipment from severe damage. The faster the fault is cleared the less will be the cost of repair of the faulted equipment.
However, the system will be subject to un-scheduled outage due to the faulted equipment and result in large down times affecting the reliability of power supply.
It is therefore desirable to monitor the health of the equipment either continuously or at specified period of intervals specially designed techniques and instrumentation which provide a warning signal if there is any deterioration in the health of the equipment such fore-warning can prevent major disturbances because proactive maintenance and corrective measures can be initiated at the appropriate time by which the cost of repair and system down time that can be reduced with improved reliability.
Following techniques available to assess the condition of power transformer used to quantify their state of health.
Tan-delta and cap measurement.
Polarizing index and recovery voltage
Dissolved gas analysis.
Frequency response analysis.
Acoustic detection of partial discharges.
(A) Tan-delta & Cap measurement
Deterioration of any insulation system is detected by measuring the Tan delta and Cap of the bulk
insulation through the capacitor taps provided in the bushing or direct energisation of the equipment and measuring the leakage current. For an ideal capacitor the shunt resistance 'R' is infinite and correspondingly tan delta is zero. But due to dielectric losses practical insulation system are represented by C and R is parallel as shown in figure, loss in R(V2/R) corresponds to the dielectric loss.
If the insulation deteriorates due to internal faults or thermal stress, then losses increases and tan delta increases. Similar variation can also be observed if tan delta and C at a particular voltage are plotted as a function time. Tan delta increases with voltage because of the gradual inception of internal discharges. The sudden change in the gradient of tan delta is called the tan delta tip up and the voltage where it occurs is an indicating of the quality of insulation. Similarly capacitance also changes with voltage. This change is because of the shorting of internal voids in the insulation system as the voltage increases, wide variation in these parameters indicate deterioration of the insulation.
Results obtained during testing:
Rating: 16.9/6.9KV,15/18MVA, 3-phase, 50 Hz
1] HV V/s LV GROUNDED
2] HV V/s LV UNGROUNDED
3] LV V/s HV GROUNDED
Tan-delta and capacitance measurements obtained during before and after temperature raise test
Rating: 50MVA, 132 / 33KV power transformer
Transformer - 1
Before temperature rise:
HV v/s LV
HV v/s LV
LV v/s HV
After temperature rise:
Before temperature rise:
HV v/s LV
HV v/s LV
LV v/s HV
After temperature rise:
Then two parameters reasonably signify the overall quantity of complete insulation system comprising of oil, press boards, paper, ceramic material etc. Hence, the measurement of tan delta & C for assessing the bulk insulation system quality is often used for transformers, generators, motors and bushings.
Before and after temperature raise test shows between LV to HV and HV to LV is showing variation in tanÎ´ and capacitance due to thermal ageing effect in the cellulose material (craft paper).
Interpretation of test results:
Correct interpretation of test results require equipment knowledge and characteristics of the insulation used. Changes in the in the capacitance of an insulation indicates presence of moisture layer, short circuit or open circuits in the insulation system. Dissipation factor measurements may indicates the following defects in the insulation of electrical equipments.
1) Cellulosic deterioration due to time and temperature.
2) Ingress of water, contamination by carbon deposits, bad oil, dirt and other chemicals.
3) Severe leakage over surfaces and through cracks.
An increase in dissipation factor in an oil filled bushing may indicate that oil is contaminated whereas increase in both capacitance and tan delta indicates that the ingress of moisture. For a condenser type bushing which has shorted layer, the capacitance value may increase where as the dissipation factor may remain constant.
(B). Polarizing index and recovery voltage measurement:
Insulation deterioration take place because of moisture penetration and deposition of insulation deterioration takes place because of moisture penetration and deposition of conducting impurities. Enough care should be taken to ensure that the insulation system of the equipment is protected from these two agents through hermetically sealing the equipment or by frequent drying and clearing through physical means. Both Polarizing Index (P.I) Recovery Voltage (R.V) given an estimate of the moisture content absorbed by the insulation and provide a good assessment of its quality.
(1). Polarizing Index = IR value after 10min
IR value after 1min
(2). Absorption Index = IR value after 60sec
IR value after 15sec
The insulation sample is charged with a voltage Un by closing switch t1 and insulation being a loss capacitor gets charged to Us. Then t1 is opened and the sample is short circuited through t2. The current I flowing through t2 is plotted as a function of time. The delay of the current I show the quality of insulation.
The ratio of I2/I1 as shown in the figure above is defined as polarized index. Insulation resistance and polarizing index AVO makes meggar 5KV applied voltage.
Rating: KVA: 7250, KV: 5500 / 3160
H.V TO L.V
H.V TO EARTH
L.V TO EARTH
IR after 10min / 1min
14500 / 5750
5290 / 3900
10600 / 4910
Rating: KVA 15000 / 18000 KV 16.5 / 6.9
H.V TO L.V
H.V TO EARTH
L.V TO EARTH
IR after 10min / 1min
18300 / 5020
7130 / 5210
12800 / 6220
The higher the ratio the drier the insulation is. In the case of transformer a PI value not less than 1.3 is desirable. Higher PI values are obtained by subjecting the transformer to drying operations. Too much of drying of insulation becomes brittle. Measurement of PI is done periodically for all equipment with composite insulation to ascertain the health of the equipment.
Un<=36KV 36<=Un<<70 70 Un<=170 170CUn
Limiting values of the water 40 35 30 20
Content for all (ppm)
Limiting values of tan Î´ 1.5 0.8 0.3 0.2
For all at 90 deg C & 50Hz
Like PI, RV (Recovery Voltage) also gives a measure of the moisture content in the insulation system. Insulation specimen is charged with voltage Ue by closing switch t1 for a period tc. Then switch t1 is opened and switch t2 is closed short circuiting the charged specimen. Switch t2 is closed for a duration t2 and then is opened. The open circuit voltage V is plotted as a function of time. The voltage across the specimen recovers, goes to a maximum value Um at tm and decays thereafter as shown in fig above. The polar compounds in the insulation whose time constant lies in between te and td are responsible for the appearance of recovery voltage across the specimen.
Time T where Um/Uc peaks gives an indication of the moisture content of the insulation. Recovery voltage method (also called as return voltage) applied to dielectric material is based on the polarization and relaxation process taking place inside the insulation when the specimen is electrically simulated. The range of tc used in this measurement range from 1 msec to 1000 seconds.
The insulation system with higher moisture content has a relatively fast polarization response. Consequently the insulation will be almost completely polarized and depolarized at along charging (tc) and short circuiting (td) times. Almost no residual energy is available after the short circuit phase. Hence low recovery voltage valves will be measure for enough charging times as shown in fig.5 only at shorter charging time will the relationship between the polarization and depolarization be such that maximum Um value can appear.
Hence, for higher moisture content, the global maximum of the polarization moves to lower time constant regions. This method is very often is applied to assess the insulation quality of power transformer. The RV method indicates the moisture contained in the composite insulation.
In transformer, moisture migrates from paper to oil as the temperature increases. Therefore, under light load conditions, the oil can show low water content. Hence, measurement of ppm level of moisture in the oil alone will not give the correct status of insulation. Hence Rv measurement is preferred since it indicates the global quality of the composite insulation.
C) Dissolved gas analysis:
Mineral oil is used in many transformers both as insulation to the live parts and as a coolant for heat dissipation. Contamination of this all due to moisture impregnation greatly affects its insulation properties. This moisture contamination can be due to leakage through the enclosure of the moist air from outside or from the paper insulation used for the live parts.
Particularly in transformers the paper insulation is hydroscopic and when the temperature of the insulation is high moisture will migrate to the surrounding all. Detection of this moisture in all is very important since it affects its BDV.
During operation, due to over loading, thermal cycling and electrical stress under abnormal voltage conditions the insulation deteriorates results in internal discharges. These are micro discharges usually referred to as partial discharges because these do not amount to a total flashover which represents a fault. The effect of micro discharges is that they gradually weaken the insulation finally leading to a major fault.
Detection of these discharges at the beginning will lead to preventive maintenance and will reduce the cost of damage due to catastrophic failure of the equipment.
Dissolved gas analysis (DGA) is the most popular technique used to analyzing the quality of internal insulation in transformer. Depending on the intensity of internal discharges and the hot spots developed due to overloading or any other faults such as short circuits in magnetic core etc.
The dissociation of oil leads to evolution of different gases which get absorbed in the oil. These gases are analyzed using gas chamotograph and based on the concentrating of these dissolved gases the internal insulation problems can be identified.
Table gives the permissible concentration in ppm of the major gas dissolve d in the oil. These limits increase with the age of the transformers.
Less than years
Age of Trf 4-10 years
More than 10 years
Rating: 8MVA, 33 / 700V
Propylene + propane (C3H6 + C3H8)
Similarly it is possible to monitor evolved gases specifically hydrogen which gets collected in the conservator. Hydrogen detector is placed in the pipe connecting the tank to the conservator conventional Bucholz relay is also located in the same pipe.
After one year:
Transformer 1 indicates electrical discharges.
Transformers 2nd and 3rd are found normal.
Hydrogen gas evolved for all types of faults in the transformer. It has low solubility in all and most of the generated gas goes out and in the process actuates the Hydrogen detector. The on line Hydrogen sensing is more effective and faster than the bucholz relay.
Deterioration of cellulose material with time is due to deploy merization of its molecular structure. This degradation is accelerated by chemical, mechanical, thermal and electrical stress that take place inside the transformer. Degree of polymerization is measure on sample paper insulation and their value is less than 150 the paper is not fit for further uses as insulation. So an alternative method to assess the quality of cellulose insulating used in transformer is the estimate the content of furanic compound dissolved in the oil.
Using higher performance load chromatograph (HPLC) the furanic content can be estimated from the oil sample and there is a direct relationship between DP and the level of furanic compound (2 Furfural). Any value less than 10 ppm of 2 Furfural can be considered as non acceptable for the transformer to continue to work.
C) Frequency response analysis:
Rating: 16.9/6.9KV,15/18MVA, 3-phase, 50 Hz
When there are more than one winding in the equipment their will be coupling between the windings giving rise to mutual inductance and capacitance. Any fault in the equipment will result in short circuit between turns to ground or in deformation of windings due to electro magnetic forces resulting from the flow of short circuit currents. Such changes in the windings will result in the windings variation of the inductance Ls and M capacitance Cs and Cg.
When a ladder network representing a winding is excited by an impulse, each node of the windings a.b.c exhibition oscillatory voltages corresponding to the natural frequency of the network. There will be a number of such frequencies. Each coil of the winding resonates at its natural frequency 1/âˆš (LC). A winding of transformer has a large number of natural frequencies. If such a winding is exited by a variable frequency constant voltage source. Then as the source frequency is changed, the current I through the winding will go through a number of maximum and minimum values as shown in the fig11. The peak and troughs correspond to the natural frequency present in the windings.
The peak take place when the source frequency coincides with any of the series resonant frequencies of the windings 1/âˆš (CgLs) and a trough take place where the source frequency coincides with the parallel resonance frequencies of the winding 1/âˆš(CsKs) when the winding is healthy and its mechanical structural is not atltered these resonant frequencies which are basically function of L & C will remain invariant and if the above experiment as shown in fig is repeated identical results will be obtained. In fig the current I is a function of voltage applied.
In order to make the result independent of outside variables but totally dependent on winding parameters only, the plot is made between impedance (Z=V/I) and the frequency. In this case wherever current I is maximum Z will be minimum and when I is minimum Z will be maximum.
However the nature of plot will be remain same and the variation of Z indicates the various natural frequencies of the winding under consideration. As long as the winding remains same the variation of Z with I will be identical. Hence this plot is considered as an electrical signature of the winding.
If due to any fault, the winding gets displaced or experience a few short circuits either between coils or between coils to ground then due to variations are L and C natural frequencies get changes and so there will be distortion in the variation 'Z'. It can be said the electrical signature of the winding changes.
Probable fault detection by FRA:
5 Hz to 2KHz
Shorted turns, open circuit, residual magnetism or core movement
50 Hz to 20 KHz
Bulk movement of winding relative to each other
500 Hz to 2 MHz
Deformation with in a winding
25 Hz to10 MHz
Problems with winding leads and/ or test lead placement
The main advantage of this method of condition monitoring is that FRA test can be conducted at site where equipment is situated
E) Measurement of partial discharges by acoustic detection:
Measurement of partial discharges is one of the important diagnostic techniques for assessing the condition of power equipments. The conventional electrical method for measurement of partial discharge is well established and widely accepted. However, the electrical partial discharge method has certain limitations which have resulted in development of alternating techniques.
Acoustic Emission (AE) detection technique is one among them. Development of high frequency sensors high speed instrumentation and necessary software has led to the success of the AE techniques internal partial discharge create acoustic waves in the supersonic range from 50 to 150 KHz .this is because of sudden collapse of void due to internal discharges causing pressure waves.
This accoustic signals travel through the insulation medium which includes both solid(paper) and liquid(oil) phases and reach the metallic enclosure(tank) acoustic sensors having frequency band from 50 to 150 KHz mounted on the outer surface of the tank. These sensors pick up the signal which is generated inside the equipment due to internal discharges.
It is possible to move the accoustic sensor over the tank to different places and picked up the resulting signal, through triangulation method it is possible to locate this source of this signal inside the equipment (transformer) by mounting these sensors at three different places on the tank (on the side of the tank) and simultaneously recording the picked up signals.
The acoustic detector is a portable it can be taken to site; the normal audible noise of the environment and that produced by the equipment will not affect the sensor as these sensors picked up signals in range 50 to 150 KHz which is above audible range. The electromagnetic waves generated in the substation due to corona and arcing also will not affect the sensors because the electromagnetic noise is above 500 KHz.
Wave form shows AE signal and frequency spectrum that measured by using VS30-V
Applied voltage (KV)
Amplitude of Electrical PD (max), pc
Magnitude of acoustic PD (max), db
Acoustic PD and electrical PD simultaneously measured as a function of applied voltage:
At the time of commissing of the equipment ex. power transformer, at specified location on the tank the acoustic signals are recorded and kept as reference signatures. Periodically these signals are measured at the same rotations and any major variation (increasing amplitude) in the signal picked up will indicate some abnormal operation. Based on the further investigation be taken to rectify the problem.
The main advantage of this method is that equipment is kept energized while conducting the test .hence this is on line conditional monitoring.
LATEST CONDITION MONITERING TECHNIQUES:
Latest techniques in condition monitoring and diagnostics which can be powerful tool in future for detecting transformer problems can be summarized as follows:
Frequency Response Analysis (FRA) to check for system resonance condition and dynamic movements and detection of winding mechanical distortion.
PD measurement and acoustic localization of faults.
Furfuraldehyde (FFA) analysis of oil (HPLC chromatography) to detect ageing of cellulosic material without taking paper samples.
On-line Tan Delta Monitoring of H.V. bushings: Signals from transducers connected to Voltage tap of bushings are collected and transmitted to user for processing the data by software and converted to dielectric loss angle and leakage current values.
Polarization spectrum or Recovery voltage measurement (RVM) gives indication of moisture in insulation and possible paper ageing and oil condition
On-line Gas Monitors:
On line hydrogen monitors(e.g. HYDRANS) provide earliest possible detection of gas build up and alert the user to the need for detailed laboratory analysis
On line moisture content measurement (e.g. DOMINO, AQUAOIL etc.) provides continues monitoring of water content in oil, indicating the status of solid insulation. The stored data can be down loaded for analysis using software. The software gives trend analyses, generate graphs and reports.
On -line temperature monitoring -Direct measurement through fiber optic sensors for continuous monitoring of hot spot temperature to control loading pattern and thermal ageing of the transformer...
CONDITION MONITORING PARAMETERS:
Dissolved Gas Analysis (DGA) provides an early warning of various incipient faults in transformer winding and core.
Oil parameters testing: Low BDV indicates moisture or particle in the oil, Moisture or acidity indicate oil condition, fusion phenol and cresol indicate the occurrence of solid ageing in the paper barrier insulator or packing material.
C and Tan delta measurement of bushing and winding assesses the condition of insulation (dry or wet) of bushing and winding.
Winding resistance measurement detects problem in broken sub-conductors, winding contact joints and OLTC connections.
IR measurement indicates the presence of contamination (dirt, moisture, etc.) and stress degradation of insulation.
Turn ratio test indicates problem in winding and verifies wrong tap changer connections
Excitation/ Magnetization current test locates faults in the magnetic core structure such as shorted laminates or core bolt insulation breakdown or shorted turns due to insulation failures, which have resulted in conducting paths between winding turns.
The benefits of condition monitoring can be summarized as below:
Reduced maintenance cost.
Results provide a quality control feature.
Limits the probability of destructive failures, leading to improvements in operator safety and quality of supply.
For assessing possibility and severity of any failure and consequential repair activities.
Provide information on the plant operating life, enabling business decisions to be made either on plant refurbishment or replacement.
Using condition assessment techniques, we have been able to detect:
Overheating of conductor / deficiency in thermal design of transformer
Dislocation of stress shield at the bottom of bushings
Loose magnetic wall shunts
Insulation failure between core bolts and coil support structures
OLTC problems like loose nut and bolts and connections.
Lost/floating potential connections to shielding rings
Partial discharges between discs or conductors
Overheating of tank part, bolt etc.
OLTC problems like loose nut and bolts and connections
Failure of core bolt lamination /shorting of core lamination burrs