CSP Technology
Complete Self Protection Technology for High Performance Distribution Transformers

Introduction
The high rate of failure of secondary distribution transformers in power systems may perhaps be described as one of the tragedies of distribution system management of present times, especially in developing countries like India.
The advent of CSP technology has encouraged progressive manufacturers to go in for high performance distribution transformers which mitigate the operation and maintenance problems associated with conventional transformers.

Why do Distribution Transformers fail in such large numbers ?
Every year distribution transforerms worth nearly 200 crore 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 failurerate at over20% compared to less than '-2% in many utilities in advanced countries.
Why should these simple, static, silent and efficient pieces of electric equipment fail in such large numbers causing enormous loss to electric utilities ?
The reasons are not far to seek.
Distribution transformers in rural areas form the bulk of the transformers in service.They are very much exposed to changing weather conditions and more dangerously to lightning. Distribution transformers in low load density rural areas feed lengthy low voltage lines which are themselves prone to faults, not merely because they are with bare conductors and in exposed environment but are also carelessly constructed.
Faults on low voltage lines constitute considerable menace to distribution transformers. Short circuits caused on account of clashing of loosely strung LVlines and high impedance faults where bad tree conditions exist are typical faults. Small is beautiful where LV system is concerned since we have lesser loop resistance to contend with and protective gear operates positively.
Overloading of transformers without looking into their overload capacity is another reason for early failure. It has become the practice to connect additional loads on the basis of maximum demand recorded at some point of time without reference to seasonal variations and assuming unrealistic diversity factors. Unauthorised loads result in unforeseen overloading. Periodical checks are not made for overloading and corrective measures are not taken.
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 interchange of air brings oxygen from the atmosphere into contact with oil. It is well known that moisture weakens the dielectric strength of oil to form sludge and finally causes a deposit to form on the windings. The deposit may in time be sufficient to obstruct the ducts placed in the windings for the purpose of oil circulation resulting in temperatures higher than those for which the transformers are designed. Ultimately the insulation of the winding may become carbonized to such an extent as to cause failure. The dehydrating breather is more often than not in a deteriorated condition to be of any use for want of timely check and reconditioning / replacement which is not practicable because of the ever increasing number of these transformers in the distribution network.

CSP Technology
► CSP Technology shows the way out of this distressing situation. Unfortunately, the advantages of CSP Technology are 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 protected from persistent overloads 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 consumer served by the transformer are affected.
► Protection from lightning is most effective with the surge arrester mounted to the transformer tank and directly connected to the HV bushing,reducing to the minimum the impedance of the ground connection.
► The transformer is completely sealed.Thereis no scopefor ingress of moisture and pilferage of oil which is very common, with disastrous consequences. The space above the oil level is filled with nitrogen. The volume of the space above the oil level 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.

Components of the CSP System
CSP System has essentially three components. They are:
► Primary Fuse: Internal expulsion fuse (other than oil filled) for system protection.
► Secondary CireuitBreaker: forover - load and secondary fault protection. Signal Light, The Emergency Control, Magnetic trip.
► Surge Arrester: for lightning protection.

Primary (High Voltage Fuse)
Power Utilities in India provide a HG Fuse on primary side of a distribution transformer for system protection. It has the following demerits.
► It is exposed to wind and rain and becomes mechanically weak very soon and blows frequently.
► It is vulnerable to tampering by eager consumers especially in rural areas who would replace a blown fuse, with the available fuse wire.
► It is not ensured that it does not blow for secondary faults and inrush current surges.
Ideally the primary side fuses for an outdoor for distribution transformer should be internally mounted (tamper proof) and the rating is determined on the basis that it should not blow for secondary faults and exciting current surges. British Electricity Authority have found from experience that when the fuse is rated to stand 12 times the full load current for 10ms it meets the requirement.
In a CSP transformer, the primary fuse which fulfills the above requirement 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 ahead of the transformer from faults which occur inside of the 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 meit open and clear the circuit. In this way, the fault is 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 fault exists, the transformer is no longer usable and must be removed from service for repair. Any fault ahead of the transformer will not be seen by any of the transformer's internal protective devices and will have to be cleared by some other protective device upstream from the transformer.

Secondary (Low Voltage) Circuit Breaker
The low voltage circuit breaker is the central component of the CSP pro¬tection package. It is this circuit breaker which provides the entire overcurrent 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.

Thermal Protection of the 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, these will be or 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 predetermined 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 these can be the limiting thermal parameters (instead of average winding temperature) in certain transformer designs.
The CSP circuit breaker in order to be universally applicable to all trans¬formers and all thermal constraints, has protective characteristics which are sensitive to the same thermal inputs as the transformers.

Secondary Fault Protection: The Other Important Function 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 will prevent any thermal damage occurring to the transformer. This feature is particularly important for the installations where un-insulated Secondary distribution and service lines are used. The use of bare conductors increases the risk of faults especially in areas where there is large growth of trees and vegetation.
If the circuit breaker does trip in response to even a temporary secondary fault, 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 benefit of CSP technology is apparent.

Constructional Features
For alt that it does the circuit breaker is of relatively simple con-struction. It is an electro mechanical 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 into the breaker such that the load current flows through them.
The circuit breaker is mounted inside the transformer, so that these bimetallic strips are within 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 and tripping functions of the circuit breaker are carried out with an assembly of parts quite simi¬lar to those used in industrial type air circuit breakers. Other features that are built into the latching and tripping functions are
► The signal fight 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 element consists of copper carrying parts plus a set of copper tungsten current interrupting contacts. Once the "hold-close" latch is released the contacts spring open and interrupt the circuit.

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 overload and overload duration at least once, and thus alerts the power utility about the need to change out a transformer for a longer size in time.
The signal circuit is mechanically connected to the circuit breaker latching and bimetal systems through an auxiliary contact. The signal light circuit consists of an auxiliary transformer winding (one turn) which generates about 3 volts and signal light contacts set within the circuit breaker. Signal light is mounted on the wall of the transformer tank.
The signal light contacts will dose at a preset thermal condition. This occurs before the main latching system opens the main contacts.
The signal light mechanism does not reset itself when 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 overload has persisted the signal light will re¬light as soon as the operating handle is restored to its normal position indicating the need for a larger size transformer.

The Emergency Control
This device is provided when the power utility wants the facility of immediate restoration of service in an emergency by closing the circuit breaker even when the preset overloa¬ding limit 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 and signal light latches within the circuit breaker which has the effect of requiring more bimetal strip movement to trip the circuit breaker open and more bimetal movement requires higher temperature.

MageneticTrip
Certain circuit breakers are furnished with an instantaneous magnetic trip element 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 conditions. This increased opening speed permits the circuit breaker to interrupt larger values of fault current than would normally be possible. The response of the circuit breaker to thermal activity is unchanged by the addition of the magnetic trip element.

Primary Fuse Vs Secondary 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 the secondary circuit breaker as mentioned earlier. In performing this coordination task, the design engineer must use the minimum melt time current characteristic curves of the primary expulsion fuse and the average clearing time current characteristic curves for the CSP Circuit breaker. 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 that current created by a bolted fault on the secondary terminals of the transformer. Usually, when this calculation is made, an infinite 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 breaker 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 occur when a fault is inside the transformer. When this type of fault 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, the circuit breaker would have interrupted the circuit.
Of course, any fault ahead of the transformer will not be seen by any of the transformer's internal protective devices and will have to be cleared by some other protective device upstream from the transformer.

Surge Arrester
The closer the surge arrester can be mounted to the transformer, the shorter will be the ground lead connection be¬tween the arrester and the transformer. The shorter this connection, the less will be the lightning surge induced voltage stress on the transformer'winding. When the surge arrester is mounted directly to the transformer tank {as in the case of CSP transformer) the ground lead length is effectively zero and maximum transformer protection is obtained.

How CSP Technology Facilitates Optimum use of the Transfor-mer's Capability?
In the case of cyclic loads (containing peaks and valleys) where peak load is of relatively short duration, transformers considerably smaller than the peak loads can be safely installed without any concern for rapid loss of transformer life due to overload. 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 to the power utility. They are
► A change out of the transformer for a larger size can be planned for at a future convenient date.
► The signal light may be reset to determine if it will light again indicating that the overload condition has become a normal load condition at this site and then a change out can be planned.
► Nothing can be done except to wait and see if the breaker itself 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 lineman from the power utility must be sent to the transformer location in order to restore electric service to these customers.
Several courses of action are open now.
► The transformer can be immediately changed out for a larger size.
► if it is not possible to change because of time of day factor and availability of personnel for change over, it may be possible to close the circuit breaker manually and restore service and then plan a change out.
► Not plan a change out and see if the load reaches these levels again and causes 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 pro¬vides several types of warning to the power utility of the existence of overload conditions long before it becomes mandatory to change out the transformer.
To overcome the problem indicated at (4) and to permit the power utility to rapidly restore services without having to perform an immediate transformer change out, most CSP transformers contain the emergency control element.
It is these early warning features which permit the power utility to effectively plan its transformer loading so that maximum use of the transformer's capability is obtained without sacrifi¬cing significantly the life expectancy of the transformer.

To sum up, what then are the benefits of CSP technology?
► Lower Installed Cost: Less external mounting arrangement and connections: There is no need for separate mounting arrangement for primary fuse, surge arrester, low voltage circuit breaker and connecting leads.
► Less time for Installation: A non CSP installation takes twice as long as a CSP installation.
► Easier and Simpler Installation: Less external connections and spacing for electrical clearances. Transformer, surge arrester, H.V.Primary Fuse and secondary circuit breakers are one compact unit.
► Safer 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 temperatures before the fuse operates in response to the flow of overload current. By the time the fuse does operate, not only is the oil very hot, but the transformer'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 lineman inspects the installation for any obvious secondary faults and finding none places a new fuse in the cut out to 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 chances of this kind of failure occurring can be reduced significantly in the CSP transformer because the circuit breaker provides the type of protection which will prevent excessive oil temperatures and/or severe damage to the transformer insulation system. When severe 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 operates only when the transformer itself has been damaged.
► 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 consumers.
In the case of the non CSP transformer installation, there is no early warning 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, trie consumer is suddenly without electric service, generally during peak load time, and a lineman must be sent out to try and restore service.
If the transformer has been severely damaged, the serviceman 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 overload 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.
► 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 transformers to insure the best economic use of each size transformer. This concept is further explored under Benefit - 6.
► Lower Cost of Operation: There are some 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, a point is reached where the best eco¬nomic decision is to replace the smaller size transformer with the larger size
Use of this type of analysis requires continuous knowledge of the loading on individual transformers and a thorough knowledge of the system economics involved.
The CSP technology method of op¬timizing loading on individual transfor¬mer 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 temperature is about 80% of the value which will trip the breaker.
► Lower Maintenance Cost and Time: From the comparative statement of schedules of maintenance for con¬ventional secondary distribution trans¬formers and CSP secondary distribution transformers, it will be seen that CSP 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.
► Neater Appearance: CSP transformer installation pre¬sents a much cleaner and uncluttered appearance. Unlike the non-CSP trans¬former installation with mounting arrangements for externally fixed protective equipment like primary fuse, surge arrester and secondary circuit breaker and electrical connections between them,D J Ristuccia, the Westing House Engineer aptly describes the CSP trans¬formers as "beautiful in concept and in physical appearance".

Maintenance Schedule of Secondary (Substation) Distribution Transformers
Conventional Transformer CSP Distribution Transformer
Clearing of bushings and external surface of 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 Silicagel in the breather and replacement if necessary Monthly not necessary
Checking of H.G Fuses and LT. Fuses and renewing with correct gauges if necessary Monthly not necessary
Checking of vent pipe diaphragm Monthly At the time of other checks
Checking of loose terminal connections if any and tightening the same As and When necessary As and when necessary
Checking for any oil leaks and rectification (including replacement of oil seals if required) Replacement of oil seals does not arise as there is no breather. The transformer is hermetically sealed and ingress of air & moisture affecting oil is ruled out.
Tanking tong tester reading during peak load hours and remedial action whenever 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 temperatures effect of the instantaneous load current plus the steady state oil temperatures as above 80 percent of the value which will trip open the 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 8DV and acidity Quarterly Not necessary
Checking of Surge Arrester and replacement if required Preferably before monsoon Preferably before monsoon
Measurement of earth resistance, checking of earthing system, and rectification if required Half yearly Half yearly
Overhaul of transformer once in 5 years once in 5 years

Conclusion
CSP technology has paved the way to high performance distribution transformers and better distribution system management.
 
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