21-08-2013, 04:37 PM
CSP to Minimize the Failures of Distribution Transformer
Minimize the Failures.docx (Size: 1.06 MB / Downloads: 23)
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.
Distribution transformer:
A distribution transformer is a transformer that provides the final voltage transformation in the electric power distribution system, stepping down the voltage used in the distribution lines to the level used by the customer.
The above are discussed below in greater details:
Over load:
Over load for a short duration for transformer in service are unavoidable. To regularly monitor 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 achieved through completely self protected (CSP) transformers manufactured by M/s. Vijay electrical 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.
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 preferred solution is to use several single phase transformers which can ensure good quality of power supply in place of a single high capacity three phase transformer.
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 monitor, 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.
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.
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.
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.
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.
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.
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.
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.
COMPLETELY SELF PROTECTIVE TECHNOLOGY
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.
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 transformer’s own impedance. Is taken as the only current limiting impedance.
Coordination is achieved by selecting the expulsion fuse’s minimum melt curve and the circuit breaker’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.