25-04-2010, 08:32 PM
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A Mini project On
COMPLETE SELFPROTECTION TECHNOLOGY TO MINIMIZE THE FAILURES OF DISTRIBUTION TRANSFORMER
Presented By :
Mr. P. Vinay kumar 06M21A0257
Mr. K. Susheel kumar 06M21A0251
Mr. G.Manikanta 06M21A0210
ELECTRICAL AND ELECTRONIC ENGINEERING
PROFILE OF VIJAI ELECTRICALS
Vijai Electricals is a 1300 Crore Company with an ISO 9001 Certification. It is the countrys largest manufacturer of Single Phase and Three Phase Oil Filled and Cast Resin Distribution Transformers (primary and secondary) upto 10 MVA, 66 kV, complying with national and international standards. It has world class manufacturing facilities for CRGO & Amorphous Metal Transformers. The EHV Power Transformer Plant with an installed capacity of 10,000 MVA manufactures transformers upto 500 MVA/500kV class. The company is thus manufacturing a complete range of transformers of capacities from 5 KVA to 500 MVA.
Vijai Electricals also manufactures H.V. (Vacuum & SF6) and E.H.V. SF6 Switchgear and Gas Insulated Substations, Instrument Transformers and allied products upto 400 kv.
Vijai Electricals has a Distribution Transformer Plant at Haridwar also and Aluminium Conductor Plant at Roorkee to cater to the requirements of North India.
The company has a Certificate of Accreditation from National Accreditation Board for Testing and Calibration Laboratories (NABL) with testing facilities in accordance with ISO/IEC 17025:1999.
Vijai Electricals main customers are Power utilities both within the country and abroad. The company is a recognized Export House since 1994. Its overseas customers are presently from Abu Dhabi, Bangladesh, Brazil, Cyprus, Dammam, Dubai, Ethiopia, Ghana, Germany, Japan, Kuwait, Kenya, Nepal, Nigeria, Philippines, Oman, Tanzania, Uganda, Vietnam and Zimbabwe.
Vijai Electricals has a Distribution Transformers Plant in Brazil and is in the process of setting up similar plant in Mexico to meet the overseas demand. Ever since its foundation in 1973, Vijai Electricals has made an impact in the industry with quality products. Its products have been inspected and accepted by several internationally reputed third-party Inspection Agencies like Lloyards-UK, Crown Agents-UK, BSI Inspectorate Griffith-UK, Bureau Veritas, OMIC Japan, CAPE-Thailand, Societe Generale de Surveillance (S.G.S.), Tubescope Vecto GmbH-Germany, Robert W. Hunt Organization, RITES, AT Survey & Inspection Company-Bangladesh and so on.
Vijai Electricals is also executing several rural electrification projects under the restigious Rajiv Gandhi Gramina Vidyutikaran Yojana (RGGVY) on turn key basis for PGCIL, NHPC, NTPC and state power utilities, in Uttar Pradesh, Bihar, West Bengal and Karnataka apart from projects for renovation and modernization of distribution systems under Accelerated Power Development and Reform Programme (APDRP). Under this scheme electricity has been extended to 7600 villages till March, 2007. The company is also executing 220 kV transmission projects in Karnataka and Andhra Pradesh.
Vijai Electricals Has its registered office in Hyderabad, Andhra Pradesh, India. The works are located in a 165-acre campus at Rudraram. It has a committed workforce of 4,000+. The company is professionally managed with a mission to create best value for all its stakeholders, society and economy.
COMPLETE SELF PROTECTION TECHONOLOGY TO MINIMISE THE FAILURES OF DISTRIBUTION TRANSFORMER
INDEX
Chapter 1
1.0 Basic Theory of transformer
1.1 Introduction
1.2 Derivation of EMF equation
1.3 Transformer on no load
1.4 Transformer on load
1.5 Transformer
1.6 Applications of transformers
1.7 Single phase transformers
1.7.1 Phase to earth primary
1.7.2 Phase to phase primary
1.8 Three Phase transformers
Chapter 2
2.0 Constructional details and types of transformer and winding
2.1 Core
2.2 Windings
2.3 Tank
2.4 Conservator
2.5 Bushings
2.6 Breather
2.7 Insulation
2.8 Types of Transformer
2.9 Three phase Transformer
2.10 Neutral earthing
Chapter 3
3.0 Installation, operation and maintenance of transformer
3.1 Installation practices
3.1.1 As soon as the transformer are received at site stores the customer should Ensure
3.1.2 When the transformer is taken to site for installation it must be ensure that
3.1.3 Operation and maintenance practices of transformer
3.2 Failures of distribution transformers
3.2.1 Over load
3.2.2 Signal phasing/unbalanced load
3.2.3 Low oil levels
3.2.4 Faulty terminations
3.2.5 Improper earthing
3.2.6 Unclear LT lines fault
3.3 Operation of tap switch on load
3.4 Poor quality of LT cable
3.5 Improper installation & maintenance
3.6 Abnormal operating conditions
3.7 Improper design and manufacture
Chapter 4
4. 0 Completely self protected technology
4.1 Introduction
4.2 CSP technology
4.2.1 Primary (high voltage fuse)
4.2.2 Secondary (low voltage circuit breaker)
4.2.3 Secondary fuse
4.2.4 Surge arrester
4.2.5 Primary fuse vs. secondary circuit breaker
4.3 Constructional features
4.3.1 The signal light
4.3.2 Emergency control
4.3.3 Magnetic strip
4.3.4 Thermal protection of the transformers
4.4 Secondary fault protection: the other important functions of circuit breakers
4.5 Benefits of CSP technology
4.5.1 Lower time Installed cost
4.5.2 Less time installation
4.5.3 Safe operation
4.5.4 More reliable service
4.5.5 Automatic load management
4.5.6 Lower cost of operation
4.5.7 Near appearance
5. 0 Conclusion
ABSTRACT
The high rate of failure of secondary distribution transformer in power systems may perhaps be described as 0ne of the tragedies of distribution system management of present times especially in developing countries like India. The advent of CSP technology has encouraged progressive manifactures to go in for high performance distribution transforms which mitigate the operation and maintenance problems associated with conventional transformers. CSP technology enables a transformer to protect itself from faults CSP system has essentially there components. They are primary fuse which is used as internal expulsion fuse for system protection. Secondary circuit breaker is used for over load and secondary fault protection, signal light, the emergency control magnetic trip and surge arrester used for lightning protection. CSP technology also help in thermal protection to the transformer where maximum oil temperature is limiting constant, the other important function of CSP technology is a use of CSP circuit breaker for secondary fault protection.(i.e.) for the faults external to the transformer. Hence the construction futures consists of circuit breaker, with there major elements of temperature sensing, latching, tripping and current interrupter. One of the most important desired task done by the CSP transformer during engineer is the coordination between the primary fuse and secondary circuit breaker and performing the coordination task. the benefits for CSP technology is low installed cost, less time for installation, safe operation and more reliable service.
Chapter 1
1. 0 BASIC THEORY OF TRANSFORMER
1.1 Introduction of transformer
Transformer is a static piece of apparatus used for transforming power from one circuit to another with out change in frequency. It can raise or lower the voltage with a corresponding decrease or increase in current. In its simplest form, transformers consist of two conducting coils having a mutual inductance linked by a common magnetic flux through a path of low reluctance.
The coil which receives power is called primary winding and the coil delivers power is called secondary winding. Both the coils are wound on a laminated magnetic core material
1.2 Derivation of EMF Equation:
N1 = number of primary turns
N2 = number of secondary turns
Fm = Maximum flux density in the transformer core in Weberâ„¢s
= Bm*A
By = flux density in the transformer core
A =area of cross section of the transformer core
F =frequency of A.C. input voltage
Flux increases from its zero value to maximum value Fm in one quarter of cycle i.e.1/4f second.
Average rate of change of flux=Fm/1/4f=4f Fm web
Now the rate of change of flux per turn means included EMF in volts.
Average EMF/turn=4fFm volts
If flux Fm varies sinusoid ally, then R.M.S value of induced EMF is obtained by multiplying the average value with form factor.
Form factor=R.M.S value/Average value=1.11
The R.M.S value of EMF/turn=1.11*4fFm volts
Now R.M.S value of the induced EMF in the whole primary winding = (induced EMF/turn)*No of primary turns
E1 = 4.44f N1Fm
= 4.44f N1Bm A volts
1.3 Transformer on no load
The primary input current (I0) under no load condition has to supply (i) iron losses in the core i.e. Hysterics loss & eddy current loss and (ii) A very small amount of copper loss in the primary winding.
1.4 Transformer on load
When the secondary is loaded, current (I2) flows in the secondary winding. the secondary current setup its own MMF (N2I2) and hence its own flux f2 which is in apposition to the main primary flux f0 due to I0.the opposing secondary flux f2 weakens the primary flux momentarily and causes more current (I2â„¢)to flow in the primary. The current (I2â„¢ ) is known as load component of primary current .this current is in phase opposition to current I2.
The additional primary MMF (N) sets up (N2Iâ„¢2 ) which opposes (fâ„¢2). The magnetic effects of secondary current (f2) get neutralized immediately by additional primary current (Iâ„¢2). Finally, the flux (f0) flowing in the transformer core throughout its service depends on primary voltage.
So the designed flux density in the transformer core is not dependent on its loading pattern and it depends only on primary voltage.
Fig.1
Fig.2
Fig.3
Fig.4
1.5 Transformers
Bulk power generation takes place generally at far away to the load centers. It is to be transmitted to the load centers effectively. For EX., 250MW generator generating at 13.5kV will have 10692A current. The ( I2R) losses and mechanical constraints will not permit with the high current in order 10692A. Since the power V*I is constant the voltage is stepped up to the operating grid voltage level at generating station it self. Schematic transmission power line sketch shown below.
A transformer feeding power to another transformer is called power transformer.
A transformer feeding power to load is called distribution transformer.
In Vijai Electrical, power supply is received at33kV and stepped down to 400V.
So, the transformer 33kV/400V is a Distribution transformer feeding power to load.
1.6 Applications of transformer
The transformer can be used from either side depending upon the requirement.
The efficiency and performance of transformer are un altered.
For example, the electrical gadgets made in USA to utilize in INDIA, a step up.
Transformer of 110/230V is required and vise versa for the gadgets made in INDIA to use in USA.
1.7 Single phase transformer
1.7.1 Phase to earth primary:
Single phase transformers are mostly used for high voltage distribution system or for small power requirement. Single phase transformers with primary voltage 6.351 KV using one HV bushing as shown below.
Here earthing is very important. If earth terminal got open, the transformer burns as the high voltage appears at the earthing point.Separate earth wire is to be run from the 33KV/11KV substation.
Many transformers are observe red burning due to lighting if the earth resistance is high.
The following figure shows single phase pole mounted distribution transformer:
1.7.2 Phase to phase primary:
Phase to phase primary transformers are also being manufactured. In this case earthing is not important.
These transformers have become popular. It is very convenient for erection with a hook to the pole. No structure is required for erection of these transformers.
1.8 Three phase transformers:
For bulk power requirements, people have tried from single phase to 12 phases and finally, as a techno economical consideration three phase power supply is being adopted all over the world.
The following figure shows the three phase distribution transformer:
Chapter 2
2.0 CONSTRUCTIONAL DETAILS AND TYPES OF TRANSFORMER AND WINDING
Constructional details:
The Important parts of transformer
1. Core
2. Winding
3. Tank
4. Conservator
5. Bushings
6. Breather
7. Radiators
2.1 Core
The transformer core is made of silicon steel or sheet steel; with 4% silicon. In addition to this ,the sheet are laminated and are coated with oxide layer to reduce the iron loss .the thickness of the lamination is 0.35 mm for 50hz .cold rolled grain oriented steel(CRGO) exhibits excellent magnetic properties in the direction of rolling .amorphous alloys are used witch reduced iron loss.
The core provides a magnetic path of low reluctance between the two windings so that whenever one winding is excited, the flux establishment by the winding will page link fully with other winding without appreciable leakage and this facility to absorb minimum magnetizing current from the mains In a magnetic circuit flux = MMF/reluctance =primary no load amperes “turns/reluctance.
2.2 Windings
A transformer will have two windings. The winding which receive electrical energy is called primary winding which delivers the electrical energy to the load is known as the secondary winding. Winding are generally made of high grade copper .for caring high currents, standard conductor s are used. The winding are provide with insulation such that any one turn will not contact into contact with other turns press board or paper insulation is used.
Types of windings
A) Spiral winding
B) Helical winding
C) Cross “over winding
D) Continues disc type winding
A) Spiral winding
Spiral winding is suitable for windings carrying very high currents and generally not used for currents less then 100 A they are almost used for LV windings. These coils are wound on solid insulating former they are mechanically strong.
B) Helical windings
The name itself that these coils are wound in the form of helix. This is used generally for low voltages 11 to 33 KVfor large transformers. Each conductor may consist of a number of rectangular strip wound in parallel radially indicated.
These are useful for intermediate current ratings between spiral windings and continuous disc type windings.
C) Cross “over windings
Cross- over coils is usually wound on former. Each coil consists of several layers and each layer with several turns.
The conductors may be of round wire with paper or cotton insulation as indication they are not suitable for currents exceeding 20a and are generally used for small transformers and HV windings
D) Continuous disc type windings:
These windings consist of number of discs. Each disc consist of number of turns wound racially over one another from inside outwards and out side inwards alternatively as conductors may consist of single or number of rectangular strips and pass continuously from disc when multiple strip conductors are used, transposition of conductors is necessary to ensure uniform distribution of current. For this reason they are expensively used for HV windings of large power transformers.
2.3 Tank
Characteristics of the oil used for cooling
(a) it is must a good insulator with high dialectic strength of 200kv/cm. the dielectric consist of the oil
(b) It should not absorb moisture and dust
© It must be free acide,alkalies and sulphur,
(d) It must have low viscosity and low sludge forming qualities sludge is due to semi solid hydrocarbons due to heat and oxidation it gets deposited on transformer windings and in the bottom of tank. It is a bad conductor of heat and thus reduce the of cooling the transformer
The quantity oil required is 1 liter/KVA for rating 1.6mva
0.8 Liter/KVA for rating. 1.6 to 80 MVA
0.6 Liter/KVA for ratings for more then 80 MVA
DISTRIBUTION TRANSFORMERWITH RADIATORS
DISTRIBUTION TRANSFORMER WITH CORRUGATED TANK FOR EFFECTIVE COOLING
DISTRIBUTION TRANSFORMER WITH COOLING TUBES
2.4 Conservator
When the transformer is oil filled and self cooled the oil, in the tank is subjected to heat and thus will naturally expended and contact due to seasonal the variations, the conservator tank provides space for the oil to settle down by expending under heavy loads. Also without such a tank, very high pressure will be developed inside it and can load to the bursting of the tank.
2.5 Bushings
The purpose of bushing is to provide proper insulation for the output leads to be taken out from the transformer tank. Bushing are used are generally of two types.
(a) Porcelain type which are used for voltage ratings of up to 66kv.
(b) Condenser type which are used for voltage ratings higher then 66kv.
bushing are designed such that the radial stress does not exceed 7.5 KV/mm for synthetic resin bonded paper and 20 KV/ mm for oil impregenarated paper. The axial stress should not exceed 0.4kv/mm for air and 0.65 KV/mm for oil
2.6 Breather
Transformer oil should not be exposed directly to the atmosphere because it may absorb moisture and dust from environment in a vary short time. To avoid this to a breather is provide which contain silica jelly. The breather in fresh condition completely prevent the moisture and dust from coming into with oil in conservator tank when it expends or contacts depending on the variation in the load . Silica jelly is blue in colour when it is fresh and changingâ„¢s its colour of moisture.
2.7 Insulation
Transformer insulation can be divided in two groups one is major insulation and the other is miner insulation
Major insulation is the insulating cylinder between LV and HV windings. Insulation barriers adjust limbs, insulation between the coil and the yoke of the core fails in the category of major insulation. Usually press board or synthetics resin bounded paper can be used for cylindrical with a relative permittivity around 4.mica is also used. The permittivity of transformer oil is almost twice that of the value in cylinders of solid insulation Miner insulation on conductors, between individual turns and between layers paper, cotton or tape can be used for conductor insulation. The windings of the transformer are generally impregnated with varnish in vacuum drying an pressure process cycle
2.8 Types of transformers
Transformers are also classified based on the placement of windings. There are two types of transformer, one is core type and other is shell types The core is made of thin stamping of silicon steel. Eddy currents induced in the core due to cutting of alliterating flux through the iron are minimized by using thin lamination and by insulating adjustment laminated with insulating varnish.Hysteresis loss whish arise due to subjecting of the core material through each cycle of magnetization are minimized by using a special grade CRGO (cold rolled grain oriented)silicon steel lamination.
(a) Core type: Core types of transformers are furtherer divided as core type and the distributed core type. In simple core type there is only single magnetic circuit the two vertical members called the limbs each carry one of the primary and one of the secondary windings the horizontal members of the core are called the yokes. The distributed core type are berry type, on the other hand is similar to a number of cores so arranged that they have radial symmetry like a spokes of the wheel. Again it is only the central limb that carries the windings.
(b) Shell type: In shell type transformers, the iron core surrounds the copper windings coils. The LV and HV windings are alternatively placed on the central limb only and therefore the transformer has two magnetic circuits. In this type of transformers also the windings are from wound and are built up in thin flat section called pancake or inter leaved coils. These pancake coils are sandwiched together with the required insulation between them
2.9 Three phase transformers:
a) Air break switch (external): An air switch is a switch in which the interruption of the circuit occurs in air. Air is used as the insulation medium between the open contacts (air break switch).
b) Horn gap fuse (external): A horn gap is a form of an air switch which is provided with arcing horns.
2.10 Neutral Earthing
¦ Earthing of star connected neutral is very important. If earth is disconnected the unbalanced load will shift the neutral point continuously causing high voltage in one are two lightly loaded phases.
Chapter 3
3.0 INSTALLATION, OPERATION AND MAINTENANCE OF TRANSFORMER:
3.1 Installation practices:
3.1.1 As son as the transformers are received at site store the customer should ensure:
No damage of the tank, radiators, conservators tank during transport.
No damage to the HV& LV bushings.
No oil leaks due to damage gaskets.
No damage to the oil drain & filling values.
Pressure relief devices are in place.
Surge arrester are in place.
Thermometer pocket device are intact
Breather in good condition.
No scope for external air/ moisture to enter especially from conservator/breather.
HV, LV Terminal, washers, nuts are all available and connections are tight.
3.1.2 When the transformer is taken is site to for installation it must be ensured that:
The mounting structure is properly erected.
The high voltage air break switch is properly installed
The HV fuse set is properly installed.
The LV fuse set is properly provided.
The lighting arrester are installed correctly.
The structure is solidly earthed with two independent earth pits.
Earth resistance is within limits.
Fuse wire of adequate size is used for protecting the transformer on high and low voltage side.
The transformer is properly bolted to the structure.
The body of transformer is connected to earth at two different points.
The LV neutral is firmly connected to earth.
The insulation value of transformer HV, LV windings are as per standards.
All the jumper connections on HV, LV side are properly tightened and appropriate capacity of conductor is used.
Oil samples are to be tested for dielectrical strength, acidity, and moisture contents for satisfactory values.
The pressure relief value is tested and ok.
Ensure that lifting hooks provided on the tank body only are used for lifting the transformer.
Please note that the lifting hooks provided on the top cover are meant for inspection of the core and winding only.
There are no fault either temporary or permanent in the LT lines to be connected to the installed transformer.
After the transformer is energized and taken in to regular operation periodic checks as below should be done.
3.1.3 Operation and maintenance practices of transformer:
Load monitoring on LT side and assessment of over loads.
Voltage monitoring to ensure proper voltages on secondary side as per relevant standards.
Cleanliness of bushing and body.
Tightness of HV, LV, LA and earth connections.
Functioning of breather and reconditioning of silica gel and oil seal.
Periodic measurement of insulation resistance of HV, LV windings and temperature.
Periodic testing of transformer oil dielectric and acidity.
3.2Failures of distribution transformers:
The failures of transformers in service are broadly due to:
Over load Single phasing/ unbalanced load
Low oil levels
Faulty termination
Improper earthing
Uncleared LT line faults
Operation of Off load tap switch on load.
Poor quality of LT cables
Improper installation and maintenance
Improper design and manufacture
Abnormal operating conditions Ex: voltage & frequency
The above are discussed below in greater details:
3.2.1 Over load:
Over load for a short duration for transformer in service are unavoidable. To regularly moniter all the distribution transformer against over load is equally impracticable, unless automated. Every transformer will be normally designed to with standard for an over load of 10% against its rating with reduced service life. The transformer must have built in future to externally indicated overloads by means of a visible indication which can to some extent warn the customer to operate the transformer within its rated loading capacity. However the sure and safe way of operation of the transformer against over loads and short circuits, use appropriate capacity of LT circuit breakers with the ability to trip instantaneously high short circuits, or trip with time delay for overloads up to 40% of rated capacity. Built high voltage fuses, with time coordination between HV and LV circuit breaker must be ensure. This can be achived through completely self protected (CSP) transformers manufactured by M/s. vijai electricals Ltd. The field staff must be educated to use the appropriate size of fuses if external fuse protection is adopted. Also adequate quality and quantity fuses must be made available to field staff.
3.2.2 Single phasing by unbalanced load:
Ideally the tree phase transformer should be loaded equally in al three phases. But this is impracticable as the utility caters to several single phase and three phase customers from the same transformers. This unbalanced causes circulating currents in three phase delta star connected transformer in the HV delta winding develops heat and results in faster detoriation of HV winding. The perfered solution is to use several single phase transformer which can ensure good quality of power supply in place of asingle high capacity three phase transformer.
3.2.3 Low oil levels:
if transformer oil leakage occurs in a transformer due to loose tank top cover bolts, are a worn out gasket, this can lead to failure of transformer winding, the solution lies in adopting welded cooling tubes, welded top cover providing leak proof oil level moniter, sealed drain pluge are providing drain pluge using proper pressure relief device with facility to built in LT circuit breaker to trip in case of excessive internal presume/low oil level.
3.2.4 Faulty termination:
Some times it is possible that heavy spark occur on LV terminal of the transformer due to improper connections i.e., loose connections, not using flat washer, spring washer, check nuts etc or connecting the cable with out bi metallic strips, which own result in loose connection between the two surfaces of the joint due to differential coefficient of expansion of copper and aluminum conducting material. It is necessary to ensure a proper connection at the time of insulation and also subsequently during operation and maintenance.
3.2.5 Improper earthing:
The neutral of star connected secondary of three phase distribution transformer must be rigidly connected to earth with a strip of adequate size and quality, similarly the body of the transformer tank should be connected to earth through two separate earth strips. It is possible over years of use, the earth pipe and connection may get rusted due to which increase the resistance and the earth connection may break. The primary purpose of earthing neutral is to ensure appropriate high fault current flowing through the faulted phase to earth when LT line fault occurs. The tank can also attain high potential leading to danger to the public. Hence proper earthing must always be ensure.Earthing of star connected neutral is very important. If neutral is disconnected from earth, the unbalanced load will continuously shift the star point, causing high voltage in one or to lightly loaded phases.
3.2.6 Uncleared LT line faults:
Improper tree clearance can result in high faults which are similar to over loads and can causes failure of equipment. Hence it is necessary to ensure proper maintenance of LT lines to avoid conditions similar to phantom loading on transformers.
3.3 Operation of taps switch on load:
Off load tap switches are provided on every transformer to ensure proper voltages to the consumers. These switches must be operated after insulating the transformer from power supply and should not be operated on load. After every tap change operation a ratio test must be conducted to ensure the correct ratio in all three phases for safe operation of transformer.
3.4 Poor quality of LT cable:
On multiple occasions, it has been observed that the transformer fail due to poor quality and under-rated PVC cables connected at LV terminals of TV as the PVC insulation melts or get charred due to heat, causing dead shirt circuit in the transformer. Poor quality of PVC cable some times effects the insulation resistance of the LV circuit also. User should make a note of it while selecting the size of cable.
3.5 Improper installation & maintenance:
Generally the breather is sent separately in sealed condition. The adhesive tape provided to seal the air passage at the bottom pluge of the breather is to be removed before putting the breather in position. The oil tray inside the breather container should be filled with oil before energizing the transformer. The colour of silica gel should be checked (which should to remain blue).
If rollers are provided for pole mounted transformer, the same should be removed before installing the transformer on the pole are should be locked in their position after installation. After installation the level position of the transformer may be checked.
3.6 Abnormal operating conditions:
Check if entire distribution system itself is suffering due to improper voltages i.e. either momentary high voltage is continuous under voltages. Also check if the system frequency is maintained at beyond the slandered specified limits, such operating conditions lead to insulation failures due to over heat and must be avoided.
3.7 Improper design and manufacture:
In order to complete with the market conditions, the trend of many non standard manufactures is to design a low cost transformer as required by tender analysis procedure of customers and this many result in sacrifice of factor of safety in the design and likely, lead to premature failures. Lake of proper super vision, quality assurance standards also results in premature failures of transformer. All this could over come by procuring transformers from ISO certified manufacturing units and insisting for a higher minimum guaranteed life which can ensure best quality transformers.
Chapter 4
4.0 COMPLETELY SELF PROTECTIVE TECHNOLOGY
4.1 Introduction
The high rate of failure of secondary distribution transformer in power systems may perhaps be described as 0ne of the tragedies of distribution system management of present times especially in developing countries like India.
The advent of CSP technology has encouraged progressive manufactures to go in for high performance distribution transformers which mitigate the operation and maintenance problems associated with conventional transformers.
Every year distribution transformers worth nearly 200 crores rupees fail in power distribution companies in India. The average period before a new distribution transformer comes back to repair shop is estimated to be a mere 3-4 years. Even a conservative estimate puts the failure rate at over 30% compared to less than 1-2% to many utilities in advanced countries.
These simple static, silent and efficient pieces of electric equipments fail in such large numbers causing enormous loss to electric utilities. Due to the reason that
Wide variation in load levels and ambient temperature makes undesirable breathing and ingress of moisture even more intense in the case of rural distribution transformers the interchanged of air brings oxygen from the atmosphere in to contact with oil, it is well known that moisture weakens the dielectric strength of oil to from sludge and finally causes a deposit to from on the windings. The deposit may in times be sufficient to obstruct the ducts placed in the windings for the purpose of oil circulation resulting in Temperature higher than those for which the transformers are designed. Ultimately the insulation of the winding may becomes carbonized to such an extent as to cause failure. The dehydrating breather is more than not in a deteriorated condition to be any use for want of timely check and reconditioning/replacement which is not practicable because of the ever increasing number of these transformers in distribution network
4.2. CSP Technology:
CSP technology shows the way out of these distressing situations. Unfortunately the advantage of CSP technology is yet to be fully appreciated by a majority of power utilities in developing countries.
¢ CSP technology enables a transformer to protect itself form faults.
¢ The transformer is protected form persistent over loads not cleared by conventional protective gear and causing dangerous temperature rise.
¢ The distribution system to which it is connected is protected from a transformer that has failed. The faulty transformer is isolated and only consumers served by transformer are affected.
¢ On an economic over loading on a regular bases is avoided.
¢ Protection from lightning in most effective with the surge arrester mounted to the transformer and directly connected to the HV bushing, reducing to the minimum impedance of the ground connection.
¢ The transformer is completely sealed. There is no scope for ingress of moisture and pilferage of oil which is very common, which disastrous consequences. The space above the oil levels is filled with nitrogen. The volume of the space above the oil levels is not less than 55% of the volume of oil. Thus expansion of oil is taken care of as well as the condition of the oil. A pressure relief device takes care of undue pressure, rises which should be rare.
The below figure shows the circuit diagram for CSP transformer:
COMPONENTS OF CSP SYSTEM:
1. Primary side: 2. Secondary side:
a) Air break switch (external) d) circuit breaker (internal)
b) Horn gap fuse (external) e) Secondary fuse (external)
c) Primary Fuse (internal) f) surge arrester
4.2.1 Primary (high voltage fuse):
Internal expulsion fuses (other than oil filled) for system protection. Power utilities in India provide a HG fuse on primary side of a distribution for system protection. It has the following demerits,
¢ It is exposed to wind and rain and becomes mechanically week very soon and blows frequently.
¢ It is vulnerable to tampering by eager consumers especially in Rural areas who would replace a blown fuse, with available fuse wire.
¢ It is not ensured that it dose not blown for secondary faults and inrush current surges.
Ideally the primary side fuses for out door for distribution transformer should be internally mounted (tamper proof) and the ratings is determined and the basis that it should not blow for secondary faults and exciting current surges.
British electricity authorities have found from experience that when the fuse is rated to stand 12 times the full load current for 10ms in meters in requirements.
In a CSP transformer, the primary fuse which fulfills the above requirements is placed in series with the primary winding. This fuse is normally mounted inside of the primary bushing and is connected via a terminal block to the high voltage winding. The purpose of this expulsion fuse is to protect the part of the electrical distribution system, which is a head of the transformer from faults which occurs inside of distribution transformer. If a fault occurs in the windings or some other part of the transformer, it will cause abnormally large currents to flow and the flow of these currents will cause the fuse to melt open and clear the circuit. In this way, the fault limited only to those customers who are served by this particular transformer and service is maintained on the rest of the system. When this type of faults exists, the transformer is no longer useable and must be removed from service for repair. Any fault ahead of the transformer will not be seen by any of the transformers internal protective devices and will have to be cleared by some other protective device upstream from the transformer.
4.2.2 Secondary (low voltage circuit breaker):
For over load and secondary fault protection signal light, the emergency control magnetic trip. The low voltage circuit breaker is the central component of the CSP protection package. It is this circuit breaker which provides the entire over current protection to the transformer in order to perform this critical function its thermal characteristics and the time response to the thermal changes must match those of the transformer.
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INTERNAL VIEW EXTERNAL VIEW
4.2.3 Secondary fuse:
It is an external fuse, connected in series with phase. It has low resistance with short piece of alluminium wire. If any fault occurs on load side secondary fuse will blown.
4.2.4 Surge arrester:
The closer the surge arrester can be mounted to the transformer, the shorter will be the ground lead connection between the arrester and the transformer. The shorter this connection. The less will be the lighting surge induced voltage stress on the transformer winding. When the surge arrester is mounted directly to the transformer tank (as in the case of transformer) the ground lead length is effectively zero and maximum transformer protection is obtained.
4.2.5 Primary fuse vs. secondary circuit breaker:
One of the most important design tasks which are done by the CSP transformer design engineer is the coordination between the primary fuse and secondary circuit breaker as mentioned earlier. in performing this coordination task, design engineer must use the minimum melt time current characteristics of the primary expulsion fuse and average clearing time current characteristics curves for the CSP circuit breakers. Coordination should be such that the circuit breaker clears the circuit for any fault on the load side of the transformer before the primary fuse melts. In order to achieve this coordination, the calculations are made for the worst case.
The maximum secondary current that can flow under any fault condition is the current created by bolted fault on the secondary terminals of the transformer. Usually, when this calculation is made, an infinity bus is assumed on the primary side of the transformer and the transformerâ„¢s own impedance. Is taken as the only current limiting impedance.
Coordination is achieved by selecting the expulsion fuseâ„¢s minimum melt curve and the circuit breakerâ„¢s average clearing curve so that under this worst case situation. The circuit will clear the circuit without the expulsion fuse melting.
If the coordination is not properly done ,the expulsion fuse can melt when the fault is on the secondary side of the transformer thus bypassing the protective function of the circuit breaker when coordination is properly done, the melting open of the primary fuse, generally, can only occurs when a fault is inside the transformer. When this type of occurs, the transformer is no longer usable and must be removed from service and taken to a repair shop. If the fault had been on the load side of the transformer, would have interrupted the circuit.
Of course, any fault ahead of the transformer will not be seen by any of the transformerâ„¢s internal protective device and will have to be cleared by some other protective device upstream from the transformer.CSP technology facilitates optimum use of the transformerâ„¢s capability
In the case of cyclic loads (containing peaks and valley) where peak load is relatively short duration, transformers considerable smaller then the peak loads can be safely installed without any concern for rapid loss of transformer life over load. The CSP circuit breaker will permit the transformer to function within cyclic loads up to the point where the amount and duration of the peak load begins to cause significant loss of transformer life. When this point is approached, the signal light will light with the first indication that the loads on this particular transformer have grown to the point when significant insulation deterioration can occur.
With the signal light indication mentioned, several options are open the power utility. They are.
¢ A change out of the transformer for a larger size can be planed for at a future convenient date
¢ The signal light may be reset to determine if it will light again indicating that the overhead condition has become a normal load condition at this site and then a charge out can be planed .
¢ Nothing can be done except to wait and see if the breaker it self will trip open at some future date.
If nothing is done, and the load continues to grow at the location, eventually a condition of peak load and load duration will be reached which will cause the circuit breaker to open. At this point the line man from the power utility must be sent to the transformer location in order to restore electric service to this customer.
Several courses of action are open now.
¢ The transformer can be immediately changed out for large size.
¢ If it is not possible to change because of time of day factor and availability of personal for change over, it may be possible to close the circuit breaker manually and restore service and then plan a change out.
¢ Not plane a change out and see if the load reaches these levels again and cause the circuit breaker to trip open. .
¢ Finally, it may not be possible to close the circuit breaker at all because the connected load has not dropped off and the circuit breaker will trip open as soon as it is closed.
¢ The CSP circuit breaker thus provide several types of warnings to the power utility of the existence of over load condition long before it becomes mandatory to change out the transformer.
To over come the problems indicated and permit the power utility to rapidly restore service without having to perform an immediate transformer change out, most CSP transformer contain the emergency control elements .
It is these early warning features which permit the power utility to effectively plan its transformer loading so that maximum use of the transformerâ„¢s capability is obtained without sacrificing significantly the life expectance of the transformer.
4.3 Constructional features:
For all that it does the circuit breaker is of relatively simply constructed. It is an electromechanical device with three major elements. These elements are;
¢ Temperature sensing,
¢ Latching and Tripping,
¢ Current interrupter.
The temperature sensing function is accomplished through the use of bimetallic strips which are built in to the breaker such that the load current flows through them. The circuit breaker is mounted inside the transformer, so that those bimetallic strips are with in the top layer of the transformer. In this way, the critical thermal modeling of the transformer by the circuit breaker is accomplished because the bimetallic strips are responding thermally to the temperature of the transformer oil and also to the temperature changes created by the flow of the load current through them.
The latching tripping functions of the circuit breaker are carried out with an assembly of parts quite similar to those used in industrial type air circuit breakers. Other features that are built in to the latching and tripping functions are
¢ The signal light latch,
¢ The emergency control assembly,
¢ The magnetic trip device.
The last major element of a circuit breaker is the current interruption element. The current interruption consists of copper carrying parts of plus a set of copper tungsten current interruptive contacts once the hold close latched is released the contacts spring open and interrupt the circuit.
4.3.1 The signal light.
A signal light is mounted on the wall of the transformer tank. It gives a visual external indication that the transformer has reached a specified level of over load and over duration at least once and thus alerts the power utility about need to change out transformer for longer size in time.
The signal circuit mechanically connected to the circuit breaker latching and bimetals system through an auxiliary contact. The signal light circuit consists of an auxiliary transformer winding (one turn) which generate about 3volts and signal light contacts set with in the circuit breaker. Signal light is mounted on the wall of the transformer tank.
The signal light contacts will close at present thermal condition. This occurs before the main latching system opens the main contacts.
The signal light mechanism does not resend itself then the load drops off. The signal light remains lighted once the signal light contacts close and can only be turned off by manually operating the external handle of the circuit breaker. However if the over load has persisted the signal light will relight as soon as the operating handle is re stored to its normal position indicating the need for larger size transformers.
4.3.2 The emergency control
This device is provided then the power utility wants facility of immediate restoration of service in an emergency by closing the circuit breaker even when the present over loading limits is reached.
Once the emergency control is activated the circuit breaker is no longer thermally protecting the transformer and significant insulation deterioration can occur if these high loads reoccur.
Once it becomes necessary to activate the emergency control, the power utility should plan to change out the transformer for a larger size as soon as possible. The emergency control linkages can be externally activated to increase the amount of engagement of the main signal light latches within the circuit breaker which has the effect of a requiring more bimetallic strips moment to the trip circuit breaker open and more bimetal moment requires higher temperature.
4.3.3 Magnetic strip.
Certain circuit breakers are furnished with instantaneous magnetic trip elements in addition to the standard bimetallic thermal trip element. The magnetic trip element increases the opening speed of the circuit breaker under high fault current condition. This increased opening speed permits the circuit breaker to interrupt larger values of fault currents than would normally be possible. That the response of circuit breaker to thermal activityâ„¢s unchanged by the addition of the magnetic trip element.
4.3.4 Thermal protection of transformer:
The average temperature of the transformer winding at any time is given by the average oil temperature plus the average winding temperature rise due to the instantaneous load current. In general, this will be are maximum value of average winding temperature which should not be exceeded if the transformer is to function satisfactorily over its normal product life. One of the functions of the circuit breaker is to make sure that this pre determined value of average winding temperature is not exceeded.
Maximum oil temperature could also be the limiting constraint. In many cases oil temperature limits are established recognizing the inflammability of insulating oil and this can be the limiting thermal parameters (instead of average winding temperature) in certain transformers designs.
The CSP circuit breaker in order to be universally applicable to all transformers and thermal constraints has protective characteristics which are sensitive to the same thermal inputs as the transformers.
4.4 Secondary fault protection: The other important functions of the CSP circuit breaker.
The CSP circuit breaker will respond to secondary faults external to the transformer by tripping open, and in most cases, this action prevent any thermal damage occurring to the transformer. This feature is particularly important for the installations where uninsulated secondary distribution and service lines are used. The use of bare conductors increases the risk of faults especially in areas where there is the large growth of trees vegetation. If the circuit breaker does trip in response to even temporary secondary faults, service can be restored easily by clearing the fault and reclosing the circuit breaker.
When the simple action of reclosing the CSP circuit breaker is compared to the action required in the case of a non CSP transformer where either a primary fuse or secondary fuse must be replaced, the benefits of CSP technology is apparent.
4.5 Benefits of CSP technology
4.5.1 Lower installed cost: less external mounting arrangements and connections; there is no need for separate mounting arrangements for primary fuse, surge arrester, low voltage circuit breaker and connecting leads.
4.5.2 Less time installation: A non CSP installation takes twice as long as a CSP installation.
Easier and simpler installation less external connection and spacing for electrical clearance. Transformer surge arrester, HV primary fuse and secondary circuit breaker are one compacts unit.
4.5.3 Safe operation
When a distribution transformer becomes severely overloaded, the temperature of the insulating oil becomes dangerously hot. A non CSP transformer which is protected by fusing can reach excessive oil temperature before the fuse operates in response to the flow of overload current. By the time the fuse does operates, not only is the oil very hot, but the transformerâ„¢s solid insulation has been severely damaged. When the primary fuse finally does operate, service must be restored quickly and safely. One procedure commonly used is to send the lineman to the location. The line man inspects the installation for any obvious secondary fault and finding none place a new fuse in the cut out restore service. If the fuse operates, again, he replaces it and tries again. in some cases there were failures causing harm to person (s) and property. The chance of this kind of failures occurring can be reduced significantly in the CSP transformer because the circuit breaker provides the type of protection which will prevent excessive temperatures and/or severe damage to the transformer insulation system.
When severe a failure does occur within the CSP transformer, the internal primary fuse operates operation of this fuse is a signal to the lineman that a severe fault has taken place and the transformer must be replaced.
There is no provision in the CSP transformer installation for the replacement of any primary side fuse because the fuse operate only when the transformer it self has been damaged
4.5.4. More reliable service
The early warning feature of the CSP transformer via the signal light helps the power utility to increase the reliability of the electrical service which it provides to its consumer.
In the case of the non CSP transformer installation, there is no early warring of increasing load on a particular transformer. The load will increase until either the transformer completely fails or the cut “out fuse operates. When this happens, the consumer is suddenly without electric service, generally during peak load time, and a line man must be sent out to try and restore service,
If the transformer has been severely damaged, the transformer has been severely damaged; the service man must call out repair over to replace the transformer which will create a service outage of several hours duration.
The CSP transformer, as load builds up, will light the signal light and alert the power utility to the potential over load problem at the installation. As stated before, once the potential load problem is identified, it can be corrected on a planned basis through planned transformer change out. a planned transformer change out creates much less of a problem for the consumer because the consumer can be informed of the time of change out, the consumer will be without electric service for a very much shorter period of time and the change out can be scheduled for a time of day when the demand for electric power is minimal.
Maintenance Schedule of Secondary (Substation Distribution Transformers)
Conventional Transformer CSP Distribution Transformer
Cleaning of Bushings and external Surface tank Cooling pipes As and when necessary As and when necessary
Checking of oil levels in the
conservator and gauge glass Monthly Not necessary
Checking of oil Silicagel in the
breather and replacement if necessary Monthly Not necessary
Checking of H.G fases and L.T.Fuses
and renewing with correct gauge if necessary Monthly Not necessary
Checking of vent pipe diaphragm Monthly At the time other checks
Checking of loose Terminal connections if any tightening the same Replacement of oil seals does not arise as there is no breather, the Transformer is Hermetically sealed and ingress of air & moisture effecting oil is ruled out.
Tanking long tester readings during
peak lode hours and remedial Acton when ever load exceeds 80% rated capacity Quarterly The CSP Transformer Method of optimizing
loading of individual Transformers relies upon the information provided by the signal light normally the signal light is calibrated to operate when the temperature effect of the instantaneous load current plus the steady state oil temperatures as above 80% of the value which we trip open circuit breaker.
Noting down the neutral currents and load
balancing in all the three phases Quarterly Quarterly
Measurement of IR Values Half Yearly Half Yearly
Testing of oil for BDV & ACIDITY Quarterly Not necessary
Checking of surge arrestor and
replacement if required Preferable before Monsoon Preferable before Monsoon
Measurement of earth resistance
, checking of earthing system, and rectification if required Half Yearly Half Yearly
Overhaul of Transformer Once in Five years Once in Five years
4.5.5 Automatic load management:
The CSP signal light at each transformer provides information about loading conditions. This can be used by the power distribution company to manage the loading on the transformer to insure the best economic use of each size transformer. This concept further explored under benefits.
4.5.6 Lower cost of operation:
There are very sophisticated analytical techniques available which compare the cost of operating two different sizes of transformers with a given load profile. As the load increases, point is reached where the best economic decision is to replaced the smaller size transformer with a large size.
Use of this type of analysis requires continuous knowledge of loading on individual transformers and a thorough knowledge of the system economics involved.
The CSP technology method of optimizing loading on individual provided by the signal light.
Normally, the signal light is calibrated to operate when the temperature effects of the instantaneous load current plus the steady state oil temperature is about 80% of the value which will trip breaker.
Lower maintenance cost and time from the comparative statement of schedules of maintenance for conventional secondary distribution transformers, it will be seen that CSP secondary distribution transformers offer. Great advantage to power utility in reducing the time and cost of maintenance of the ever increasing population of distribution transformers in power utilities.
4.5.7 Near appearance:
CSP transformer installation presents a much cleaner and uncluttered appearance. Unlike the non with mounting arrangements for externally fixed protective equipment like primary fuse, surge arrester and secondary circuit breaker and electrical connection between them.
5.0 CONCLUSION:
CSP technology has paved the way to high performance distribution transformer and better distribution system management. CSP technology shows the way out of these situations. Unfortunately the advantage of CSP technology is yet to be fully appreciated by a majority of power utilities in developing countries. CSP technology enables a transformer to protect itself from faults. The transformer is to protected from over loads not cleared by conventional protective gear and causing dangerous temperature rise. The distribution system to which it is connected is protected from a transformer that has failed. The faulty transformer is isolated and only consumers served by transformer are affected. On an economic over loading on a regular bases is avoided. Protection from lightning in most effective with the surge arrester mounted to the transformer and directly connected to HV bushing, reducing to the minimum impedance of the ground connection.
BIBLOGRAPHY:
1. B.L. Theraja & A.K. Theraja
2. Dr. S.Kamakshaiah
3. Prof. B.N. Yoganarasimhan
4. Reading materials of Vijai Electricals
5. Manufacturing of plant visit
6. Complete self protection technloogy for high performance distribution transformer by Sri. T. Sugunakara rao advisor Vijai Electricals Ltd
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