Selection of the Circuit Breaker has to be done carefully, so that it does not allow the transformer to exceed the permissible winding temperature. The built-in Bi-metallic strips mounted in the Circuit Breaker, located below the top oil, achieve the temperature sensing function. The circuit breaker thus provides Thermal matching with the transformer & protects it against over loads...
- C.V.Govinda Raju
Power transformers are used in transmission network of higher voltages for step-up and step down applications (400 kV, 220 kV, 110 kV, 66 kV, 33kV), whereas Distribution transformers are used in the Power Distribution net works, to provide the final lower voltage requirements of the end user. (11kV, 6.6 kV, 3.3 kV, 440V, 230V). Distribution transformers are one of the most important purchases any power distribution utility makes & constitute a large percentage of the Utility’s investment each year. The focus has been on improving efficiencies (Lower Losses) & reliability while maintaining an affordable purchase price. In USA & other western countries, the failure rate of Distribution transformers due to all causes have been stated to be around 1% per year whereas in India, the failure rate which was around 22% in 1999-2000 have been brought down to around 17% today, still our country has a long way to go to reach the levels of Western countries. This high rate of failure of Distribution transformers in Power systems perhaps be described as one of the tragedies of the present Power Distribution system management.
Following are some of the causes of failure of transformers in service.
1. PROLONGED OVERLOADING
2. UNBALANCED LOADING
3. FAULTY TERMINATIONS
4. POWER THEFT AND HOOKING OF MAINS
5. PROLONGED SHORT-CIRCUIT
6. VOLTAGE SURGES DUE TO LIGHTNING
7. LACK OF PROPER MAINTENANCE
1. Prolonged Overloading
Overloading of distribution transformers cannot be avoided for short durations, however, continuous overloading will heat up the windings resulting in failure. Hence, utilities have to maintain a history card for each transformer & record the pattern of loading (Using periodical Current measurements during peak hours) which will facilitate taking a decision on changing of the transformer at the proper time with a bigger capacity. These history cards will also facilitate determining the capacity of the new transformer. In India, It has been a general practice by the line men using higher current rated re-wirable fuses to avoid frequent breakdowns of Power supply caused due to overload, thus resulting in ultimate failure of the transformers.
2. Unbalanced Loading
In Distribution transformers feeding domestic power to suburban & Metro cities, it is very difficult to exactly balance all the three phases uniformly. An unbalance of about 10% may not create a serious operational problem. Continuous un-balance exceeding the limit, will result in an additional circulating current on the ‘Delta’ primary winding of the transformer resulting in over current, increase in winding temperature & ultimate damages to winding. A regular measurement & recording of currents in each phase as well as neutral current, can help in proper balancing of the loads on three phases, by proper re- distribution of the residential single phase loads on the three phases.
3. Faulty Terminations
On many occasions, one can observe heavy arcing / sparking on the terminations done at the outdoor transformer centers, near transformer HT / LT bushings, group operating switches, lightning arrestors, HT fuse holders etc., which indicate loose connections. When once an arc is observed it can only increase creating more arc eventually resulting in melting of the conductors / terminals. Normally the line men who attend to such sparking & melting of conductors, carry a piece of aluminum wire & they place the two conductors to be joined, side by side & use the aluminum wire to bind the two conductors manually. This kind of joint does not last long. Use of crimped cable shoes & bolted connections are seldom adopted, resulting in ultimate failure of the transformers.
4. Power Theft And Hooking Of Mains
Stealing of power by hooking on to the Over head power system is a serious national problem, which causes over loading / unbalanced loading resulting in transformer failures. Utilities are not only loosing revenue for the stolen power, but also loose their valuable transformers. Power utilities should seriously look into this area. Facilities of power measurement at substations can throw more light on the magnitude of power theft by calculating the power sold to genuine users & estimating the actual power loss in the transmission system. Presently SEB’s wrongly account all the stolen power as Transmission loss which is not correct. Regular energy audit & surprise raids can solve this problem to some extent.
Figure 1: Single Phase & Three Phase CSP Transformers...
5. Prolonged Short-Circuit
Distribution transformers can withstand occasionally external short circuits of approximately 20/25 times the rated LV current for a period of 2/3 seconds. Prolonged short circuits will result in transformer failures, if proper protections are not foreseen on the HV & LV sides & the line men use over sized fuse wires.
6. Voltage Surges Due To Lightning
Lack of surge arrestors, or if Surge Arrestors are located at a distance from H.V. side of the transformer, then the H.V. Side of the transformer is exposed to voltage surges arising out of Lightning causing failure of the transformer.
7. Lack Of Proper Maintenance
Proper routine maintenance will prolong the life of the transformers. Checking of Transformer oil levels, its Breakdown values, general cleaning of the transformer, prevention of oil leakage, checking of Silicagel in the breather, checking of loose connections etc. will enhance the life of the transformers.
Most of the distribution Transformers are located in remote Rural areas, with long distribution lines etc., where special attention can not be given by the maintenance team as in the case of Metro or cities. Power theft is more in rural areas leading to overloading of Transformers resulting in failures. Power theft can be prevented to some extent using armored cables instead of over head lines. Since the distances are long, cost of cabling is prohibitive.
Completely Self Protected Transformers (Csp Transformers)
The only solution to the above problems, is to look out for transformers equipped with all the protective devices, built-in, to prevent failures against over loading on HT & LT sides, surge protection, as well as a transformer needing the least maintenance. This kind of transformers is ideally suited not only for Rural areas, but also for sub-urban & cities (Fig.1). The design of Completely Self Protected Transformers (CSP Transformers) meet this requirement. These transformers are widely used in USA & other advanced countries in their distribution net work & hence their transformer failure rates have come down drastically to around 1% or even less as against ours which is around 17%. CSP Transformers come also with sealed tanks, totally stopping oil pilferage as well as its contamination. Unfortunately, our Electricity boards are unable to appreciate the positive features of CSP technology & still sticking on to older conventional design, which answers the high rate of failures.
M/s. KAVIKA (Karnataka Vidyuth Karkhane Ltd)., Bangalore, were the first one to bring out CSP transformers, in India, under the Technical Collaboration with M/s. Westinghouse Electric Corporation USA, in the year 1983 (Fig.1).
Special Features Of Csp Transformers Over Conventional Transformers
CSP transformer, is primarily a Conventional transformer built as an integrated package incorporating the following additional built-in protective elements.
a) Load / Temperature sensing built-in Circuit breaker on the LV side of the transformer,
b) Protective expulsion type fuse on the HV side,
c) Tank mounted Lightning Arrestors,
d) An indication lamp providing visual warning against over temperature
e) Sealed tank construction (Without conservator) with Nitrogen filling is also available to prevent pilferage / contamination of Transformer oil.
Reasons for the failure of the distribution transformer listed under points 1 to 7 above, are fully taken care of by the combination of the above protective devices, built into the conventional transformer, thus the design of the CSP Transformers, ensure reliability, continuity of service & prolonged life of the transformer.
L V Circuit Breaker
The specially designed built-in Circuit breaker in a CSP unit, is connected between the L.V. Windings & the L.V. Bushings, & mounted in the oil inside the transformer tank. This forms the heart of the protection against over current. The average temperature of the transformer winding at any moment is given by the Load vs Time curve up to the moment under consideration PLUS the thermal effect of the instantaneous load current flowing at that moment (Fig.2).
For an oil immersed transformer, the average temperature of the winding is given by the average oil temperature PLUS the average winding temperature rise due to the instantaneous load current at the moment under consideration. Permissible average winding temperature is determined by the transformer design based on the thermal qualities of the insulating material used. Hence, selection of the Circuit breaker has to be done carefully so that it does not allow the transformer to exceed the permissible winding temperature. The built-in Bi-metallic strips mounted in the Circuit breaker, located below the top oil, achieve the temperature sensing function. The circuit breaker thus provide Thermal matching with the transformer & protects it against over loads.
H V Protective Fuse Links
The expulsion fuses (Fig.3) are mounted inside the transformer, between the incoming HV leads from the bushings & the HV leads of the transformer primary winding. The function of these fuses are: in the event of an internal winding fault in the transformer, blowing out of the fuse will isolate the defective transformer, thus ensuring isolation of the defective transformer, without affecting the rest of the Electrical distribution network. Proper co-ordination between HV Fuse link & LV Circuit breaker ensures that even in the event of dead short on the LV bushing, the LV circuit breaker will clear the fault without blowing the H V expulsion fuse.
Surge Arrester For Lightning Protection
In a CSP Transformer Surge Arrestors (Fig.3) are mounted directly on the transformer tank thus protecting the transformer from insulation damage caused by Lightning induced surges. Basically the surge arrestor appear as an open circuit to power frequency voltages & as a short circuit to surge disturbances diverting the surge to earth thus protecting the transformer. In the case of conventional distribution transformer, the lightning arrestors are separately mounted on the tower inter connected by a conductor wire. In such a case, the voltage appearing across the transformer winding when the arrester is operating is the SUM of the arrester discharge voltage & the voltage drop in the lead connecting the arrester & the transformer. As a thumb rule, the voltage drop in the connecting lead is around 1.6KV/foot of the lead length. If the lead length is say, 10 feet, this extra voltage will be 1.6 X 10 = 16kV, which will not be there in the case of CSP transformers, since the arresters are directly mounted on the transformer tank.
The purpose of the CSP circuit breaker is to safely permit the safe functioning of the transformer upto the point where the load begin to affect the life of the transformer. At this stage, the signal light provided on the side of the CSP transformer tank, will light up giving the first indication to the operating personnel that the load on the transformer has gone up to a stage where significant insulation deterioration can occur. The operating personnel have to manually reset the signal lamp. If this lamp repeatedly lights up, it is a warning sign to the utility supplier, leaving the following option.
a) Change the existing transformer with a larger capacity.
b) Think of re-distributing part of the existing load of this transformer to other transformers.
If no action is taken by the utility supplier & the signal lamp glows continuously, resulting in excessive winding temperature, the CSP Circuit breaker will trip. Re-closure of this breaker is only possible after the temperature comes down.
Emergency Reclosure Of CSP Breaker (Fig.2)
During emergency if the power has to be restored urgently, an emergency control handle has been provided on the side of the CSP transformer tank which facilitate increasing the load carrying capacity of the breaker. By operating this, one should understand that we are restoring the power supply due to emergency, however the Circuit breaker is no longer thermally protecting the transformer which can result in significant deterioration of the insulation if continuous over loading should occur on this transformer.
Figure 4: CSP Load Time Curves Following 75% Load at 28C ambient 63 kVA, 11/0.433 kV Transformer...
CSP Transformer Performance
CSP transformer performance is defined by a set of Load vs Time curves - one curve for signal light, one for CSP Breaker trip, & one for Emergency operation of the Transformer. (Fig.4) From the curve it can be seen that for a 200% load, the signal light will light up in 18 minutes & the transformer will carry the load for a total duration of 69 minutes before the circuit breaker trips. If under this condition, if Emergency control is activated, the transformer can carry the same percentage of load for further 51 minutes before the CSP breaker trips again.
Advantages Of CSP Transformer Installation
a) Lower Installation cost. If one observe the conventional transformer installation mounted on Two pole Steel / RCC structure, you will observe provision of separate mounting fixtures for mounting of triple pole HT horn gap fuse system, separate mounting fixtures for surge arrestors, Separate Steel enclosure box for L.V. Load break switch / MCB. Cost of all these components & associated hard ware items , associated labour costs etc. have to be added to the cost of the conventional transformer. On the contrary, a CSP transformer unit with all the built-in protective devices, makes the installation simpler, cheaper & presents a better look. If one compare purely the basic cost of the Conventional Transformer & the CSP transformer of the same capacity, outwardly the cost of CSP transformer will be marginally high, however if you add the cost of the protective elements which are already built-in into the CSP unit, the cost difference becomes negligible.
b) CSP transformer installation presents a much cleaner and uncluttered appearance. (Fig. 5) Unlike the non-CSP transformer installation with individual mounting arrangements for externally fixed protective equipment like primary fuse, surge arrester and secondary circuit breaker and electrical connections between them.
c) CSP Transformers are the ultimate solution to prevent expensive transformer burnouts, maintenance free, making it ideally suitable for installations in all locations Rural, Sub-urban, Cities.etc. It is also ideally suitable for use in multi storied Apartment complexes , industries etc. looking for continuous, maintenance free & trouble free services.
Utility’s Concern About CSP Transformers
Though the protective elements provided on CSP transformers, have technical merits, Utilities feel that continuity of service is more important than overload protection & tripping of the breaker is considered as a nuisance. The greatest draw back, perhaps, is the nuisance of internal fuse blow outs, which is a time consuming replacement causing prolonged interruption. This has impacted the use of CSP transformers in favor of Conventional transformers.
Surges Entering Transformer Secondary Windings
The impact of “Secondary side Surges” entering the un protected secondary windings of pole mounted transformers, had gone undiagnosed till recently, because failures due to secondary surge would show up as Primary winding failures resulting in fuse blowouts. Recent investigations have shown that use of High Energy Low Voltage arresters on the Transformer’s secondary terminals, will eliminate failures caused by Secondary induced surges. With the additional provision of High Energy Low Voltage Secondary side Surge protection, the problem of internal fuse blowouts due to the secondary surges is taken care of in the CSP transformers thus making it ideally suitable for power distribution networks, drastically bringing down the transformer failure rates to the minimum. With this modification , CSP would be a solution to transformer failures.
Former Executive Director
Karnataka Vidyuth Karkhane Limited
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