Transformer Protection Progress - Coursework Example

Running Head: TRANSFORMER PROTECTION PROGRESS REPORT Transformer Protection Progress Report Name Institution Table of Contents Abstract 3 Introduction 4 Background 6 Plan 7 Proposed Approach 14 Preliminary Results and Discussions 18 Conclusions 25 References 26 Abstract The purpose of this project progress report is to explore the possibility of protecting transformer devices by means of digital relays. On several instances, electrical equipment have ended up malfunctioning due to lack of proper measures to prevent them against such situations. In the long run, malfunctioning of such equipment may lead to great losses especially with regard to the financial implications in the event that such equipment needs replacement or repairs. Transformers are some of the most common machines that have a significant effect in both high voltage transmission systems as well as in electronic devices. Considering the fact that these devices find applications in virtually all spheres of the electrical world, it would be rather illogical not to consider assessing the possibility of improving their performance to enable them be of better service. As stated, high voltage transformers find applications in high voltage transmission lines, which are essentially three phase systems. However, as per the scope of this project, implementation of this project using the three phase model may not fall within the constraints of the project. As a result, the proposed model will involve the use of a single phase transformer to demonstrate the protection mechanisms of digital relays as far as transformers are concerned. This model will provide a basis that may be modified in the future for use in high voltage applications. Protection of transformers using digital relays may take quite a number of dimensions. However, in this case, only a few of these will be explored as the scope of the project will allow just a basis by which further modification can be done to achieve the intended purpose, whether it is related to false tripping or any other aspect that may be useful in the particular choice of application. This project will therefore offer a heads up for scholars and individuals in the engineering profession to appreciate the need of protecting transformer devices for effectiveness in their various applications. Introduction It is a mandatory electrical practice to ensure protection of electrical equipment in electrical substations so as to minimize damages, and at the same tome control malfunctioning that may arise as a result of unwanted voltages and currents. Every kind of equipment thus used is required to have standard short-time ratings which are indicative of limits that the equipment are able to withstand. Given this situation, it becomes inevitable that the fault conditions be done away with as soon as possible in order to see to it that coordination of protective devices at any point in the system is maintained. It is, therefore, required that though more than one protective device may detect a fault, only one is supposed to respond. There are two generally accepted means that can be used to achieve this purpose. One can opt to use fuses, which in most cases acts either directly, or by means of a device that causes mechanical tripping, attached to it. Such a device works by opening a 3-phase load-break switch which is attached to it. On the other hand, the methods that is progressively gaining popularity and acceptance is the use of relays, which, as opposed to fuses, are independent from the circuit breaker coil, and hence, act indirectly to the coil of the circuit breaker. Transformer protection When it comes to transformers, there are quite a number of faults from which the transformer needs to be protected to make it achieve its proper functioning capacity. To begin with, transformers face stresses which may emanate from the supply network. These are usually in form of voltage surges, which may either be atmospheric or operating voltage surges. Atmospheric ones in most cases occur from natural sources such as lightening strokes that occur close to transmission lines. In the case of operating surges, they mainly result from spontaneous changes in the normal functional conditions of given electrical networks leading to transients within the system. The high frequency voltage surge wave that occurs in this scenario hampers the normal functioning of the transformer. The most effective methods of protecting transformers from such kinds of surges has been found to be the use of varistors and they do not affect the switchgear much. A part from voltage surges, there may also be stresses that arise from the load itself. In most cases, the reason for overloading is usually as a result of simultaneous power need of small loads connected to the network. At times, it arises when the need for apparent power (kVA) increases, especially when the facility (in this case the factory) grows and expands leading to an extension that causes an increase in the power of demands. An increase in the load leads to a subsequent increase in the temperatures of the wirings and insulations. These eventually reduce the lifespan of the equipment. There is no specific place to place overload protection devices as they can always be either on the low voltage or high voltage side of the transformer. The most effective way of protecting a transformer against overloading is by means of a digital relay, which serves to trip the circuit-breaker located on the secondary side. The term ‘thermal relay’ is interchangeably used with the digital relay considering its function on the transformer. Their mode of working usually involves the artificial simulation of temperature based on the time constant of the particular transformer in use, with some of them having the ability to incorporate the impact of harmonic currents resulting from non-linear loads. Such loads are the likes of variable speed drives and computer drives among others. The relays are useful in projecting and estimating the time required for overload tripping as well as the wait time required in the event that tripping has occurred. Such a control measure is a very useful step in the control of the shedding operations, which can either be coordinated with thermostats or heat sensors depending on whether they are being used for oil-immersed or dry-type transformers respectively. Background For the recent past that transformers have been in use, it has been noted that in most cases both internal and external faults tend to occur within the transformer. Such faults have led to excessive damages and losses related to transformer devices. There thus emanated a need to devise a way of identifying such internal faults within a transformer putting into consideration very high levels of sensitivity. Mechanisms employed in such cases should be able to result in incidental de-energization. These should be able to shield the transformer from external faults. Such a kind of setting with sensitive mechanisms of detecting faults and bringing about de-energization consequently leads to the enabling of fault damage, which goes a long way in limiting unavoidable repairs that may result from the faults. Despite this, the protection mechanism is also supposed to offer an effective back-up protection in instances when through faults occur in the system. This is necessitated by the fact that such faults are significant contributors in the impairment of the transformer. They also speed up the rate of aging of the transformer as well as the possibility of failure of the insulation windings caused by massive heat generated during the operation of the transformer. The other cause of failure is the effects of forces within the windings as a result of high fault currents. It is not just the internal faults that cause malfunctioning of the transformer; rather, there may also be unnatural system conditions that may come up. Such include over-excitation and over-voltage. These, together with loss of cooling, have drastic effects to the normal functioning of the transformer as they likewise cause internal failure and speedy aging of the transformer. It is thus essential that such be given priority in the overall protection of transformers. Transformer protection can be either through sensing of current, voltage and frequency, which is the electrical approach, or sensing the various operational parameters, in this sense making use of mechanical protection. In this case, the parameters concerned include the level of oil or pressure and the evolution of gas as well as the winding temperature. In the context of this project, electrical protection emerges to be the method that will be considered. Worth noting is the fact that the choice of protective devices is highly dependent to the economy of the adopted approach as well as the probability of a particular failure being experienced. Another aspect that is usually considered is the operational costs incurred in the replacement of certain parts of the equipment and its effect to adjacent equipment. The various failure costs in the case of a transformer are composed of the direct and indirect costs related to that particular failure. The cost of a particular protection scheme majorly encompasses the cost of the protective device and to a large extent the cost of the disconnecting device such as the circuit breaker and other significant devices that are needed to work alongside it. In addition, it is important to put into consideration the life cycle cost of the devices involved. Plan As earlier explained, this project is going to adapt the electrical protection approach. In this regard, it will involve transformer differential protection, in which a number of factors will be put into consideration. It is worth noting that with differential protection, the highest level of protection is achieved. Much as this is the case, at times ground fault protection may not be possible, especially in the event that there is no grounding or the existent one is of high impedance. This kind of protection finds applications in transformers with a rating of at least 10MVA and the criticality of the transformer plays a significant role. The factors to be considered in regard to the differential currents in transformers are very critical in the use of differential protection as a means of protecting the transformer. This is because such factors are much capable of leading to differential currents even at times when the transformer has balanced power conditions. These factors include; i. Magnetizing inrush Currents – Normally, the accepted levels of this current is about 2-5 percent of the rated current of the transformer, and this at times may go as high as well over 20 times the rated current in the event of magnetizing inrush currents. This may be persistent for up to 10 cycles and is usually dependent on the transformer and system resistance. ii. There is also over excitation, which mainly affects generator-transformer units. It is also critical in a number of transmission transformers in which line capacitance dominates causing high voltages in the event of light load conditions. In general design practice, transformers are meant to work in conditions immediately under the flux saturation level. This is such that a slight increase in the maximum allowed voltage-frequency level causes core saturation, thereby making the excitation current drawn to increase by significant margins. iii. CT saturation is also another factor that needs to be considered, and it mainly arises from external fault currents. The reason as to why this is an important factor is the fact that some times when the saturated CT current is distorted it leads to flow of relay operating current. At other times, the resultant harmonic components usually lead to a delay in the way the differential relay operates in instances of internal faults. This therefore calls for appropriate choices of CT ratios in order to reduce errors that may occur from the saturation. iv. At times there may exist varying primary and secondary voltage levels such that the CTs on the low voltage and high voltage sides tend to be of varying types and ratios. v. Another consideration is the phase displacement that exists in the delta-star transformers vi. There is also need to consider the different transformer voltage control taps vii. The phase shift and voltage taps in regulating transformers also play a significant role Putting all the above considerations into perspective, there is a need to use percentage Differential Relays whose characteristics are in the range of between 15 percent and 60 percent. Also, when using present day microprocessors and numeric relays, it is possible to incorporate harmonic restraints. It has been noted over time that the second harmonic takes most dominance as far as the magnetic inrush current is concerned. In this sense these relays employ a second harmonic restraint so as to minimize the chances of the relay being operational at the time of the inrush. About 25 percent of the third harmonic component and 11 percent of the fifth harmonic component (which are odd harmonics), characterize the excitation current. These, therefore offer a framework for the detection of malfunction. Specifically, the fifth harmonic component acts as a sensor of over-excitation. When this happens, the fifth harmonic signal acts as a blocker for the differential trip signal. This, in the long run, gives room for easy fault discrimination at the time of the trip analysis. When that is not the case, its work is to apply restraints to the operation of the relay. There are also variable percentage relays that are useful such that the through current increase in the transformer causes a subsequent increase in the percentage restraint. In the long run, the inauspicious effects of CT saturation are greatly minimized. In the connection of transformer differential relays, a number of rules become inevitable. To begin with, it is mandatory that all currents that get into and out of the differential zone need to be explained for one unit for every phase. Under this fundamental rule, it is important that; The design procedure should be such that it maintains equality between the numbers of restraint windings to the transformer windings, if not more. For every fault source that exists, there must be at least one restraint winding A lot of caution needs to be placed in the event that the feeder-side CTs are paralleled. Putting this into consideration ensures that equality of phases for the currents through the relay restraints, and that the current difference is as small as possible, tending towards zero in instances of load and through fault conditions (Dash & Rahman, 1987)). This is possible when the steps below are considered; i. Either star or delta CT units are able to give an assurance of proper phasing between primary and secondary currents within the relay restraint ii. The ratio adjustment may be by ensuring the relay has the least possible operating current, putting into consideration the relay tap, CT connections and ratios. For purposes of illustration, the following circuit of a transformer may be considered: The diagram illustrates a connection with a 138/69KV, 75 MVA, star-delta connected transformer. In this scenario, phasing will involve the connection of the transformer in such a manner that currents in the restraint windings are in phase, and this comes about in two ways. First, the CTs on the delta side in ∆ and those on the Y side in Y would be a useful method. The only thing that needs to be watched in this sense is the fact that in the event that a through ground fault occurs; there will be a circulation of the zero sequence currents in the restraint windings. As a result, since there are ∆ connections in the HV primary windings, such currents would flow in the delta connections and it will be difficult for it to be sensed by the primary CTs as well as the restraint windings on the same side. In the long run, current difference arises in the relay, causing a through fault in the operation of the relay. This, evidently, is not the best option to employ in this case. The other approach for phasing would be that of connecting the ∆ side CTs in star while those on the Y side are connected in delta. What happens in this case is that the emergent zero sequence currents tend to be confined on the delta connected CT on both sides of the transformer, consequently ensuring that no zero sequence flows in the restraint winding. This contributes significantly to the maintenance of balance in the circuit. The next thing that happens is to connect the CTs in such a manner to ensure that all the currents within it are in phase. The general assumption in this case is that current flowing in the transformer is balanced, and it may flow in any direction. One however notes that considering the Y side first makes the whole process much easy. The first assumption is that Ia, Ib and Ic flow out of the marked polarity, insinuating that the currents will be (Ia-Ib), (Ib-Ic) and (Ic-Ia) for the delta side windings respectively. At the same time, they will be going in the direction of the indicated polarity. Taking a look at the ABC CT current for the phase ‘ABC’, we can conclude that the currents in the restraint winding are (Ia–Ib), (Ib–Ic) and (Ic–Ia), respectively and their general direction of flow is from the left to the right, just as indicated in the diagram. There is a need to ensure that the phase is maintained, and involves making the currents in the ‘abc’ restraint windings are the same as those in ‘ABC’, and their direction of flow should be from the left side to the right. This is achieved by making a delta connection in the abc side CTs as is evident in the diagram. The next step after phasing would be to determine the CT ratio and the tap selection. It is important to note that for differential relay restraint windings, there are usually taps that allow for the setting of the restraint current difference using the ratios of 2:1 or 3:1 which are useful in the determination of restraint current mismatch is possible. The mismatch is given by the relation: This will assist in the analysis of the currents in the aforementioned example Picking a CT ratio of 400:5 yields If the CT ratio was 700:5 then Assuming the relay taps were selected as follows: TH=1 AND TL=2 Consequently, By relating the mismatch formula We get that The mismatch factor is thus given as 3.2% Considering the fact that the percentage characteristics of the transformer relays come somewhere between 20 and 60 percent, it follows that the mismatch factor gotten in the above case is okay, given the fact that a substantial tolerance is given to take care of the unanticipated mismatch that may occur as a result of CT saturation as well as other errors that may arise. In essence, the taps in power transformers are useful in the process of adjusting the nominal voltage ratio by values within ±10 percent. Taking this particular case into consideration, one realizes that the steps involved are similar, and the only exception is the fact that calculations are done using the nominal values of voltages. Considering this fact, there is addition of the above calculated mismatch factor to the mid value of the adjustable range so as to come up with the final value of the mismatch percentage. Taking the example above, the adjustable range was found to be ±10%, meaning that to get the final mismatch value, we would add 3.2 to 10 so as to obtain 13.2 percent as the final value. Proposed Approach Considering the scope of this project at the moment, some of its features may not be implemented based on the present constraints of time, and finances among others. The entire project will, therefore, take advantage of the available algorithms to simulate a number of elements considered in the project proposal. The choice of algorithms will be mainly to assume the roles of the differential relays, though they may not be able to take care of the utterly complex features that may be associated with the same. Worth considering, as earlier stated, was the fact that attaining digital protection occurs at different levels of accuracy and speed. Based on the standards put forward by the IEEE, there is a need for the time limit of protection of transformers to be about 100 milliseconds. However, the design of algorithms in the present times is such that their response is much faster speeds as high as 10 milliseconds (Herman, 2011). The MATLAB/SIMULINK environment was proposed as the most convenient for the simulation. However, noting the complexity of the same, there may be a need to explore other simulation programs that exist in the field, such as PSPICE and PSIM among others. The key objective is to focus on the protection of the power transformers from faults that arise as a result of internal currents. Such a mode of protection is equally helpful in the prevention of interruptions emanating from excitation currents. In coming up with the algorithm for this kind of project; much emphasis was put in the idea of harmonic current restraint. As such, a description of the case scenario presence of large harmonic components in inrush current, or better still, magnetizing excitation current is highly considered, which, in most cases, cannot be noticed in fault currents. Given the fact that the iron core in the transformer is made up of a soft magnetic material, the saturation levels it has are based on a limited time frame. In the long run, the inrush current in the transformer is highly distorted. As a result, the amplitude of the harmonic, when compared to the fundamental value, may be about 45% and that of the third harmonic comes in at around 20%. Looking at the progressive harmonics gives evidence of continual decrement in the value. For the implementation of this protective mechanism, the Fast Fourier Transform approach is used, whereby the periodic function is generally reduced to the sine and cosine components as per the formula: Where, a0 – DC component of f (t); CK – Cosine coefficient; SK – Sine coefficient CK and SK may be expressed as follows: (Hamming, 1973) As per the proposal, this project the approach was based on the typical behavior of a 3-phase transformer circuit with digital relays shown below: The consideration was a transformer with 250MVA and 60Hz, with a rating of (735/315) kV, having a star-delta connection (Y/∆) as per the simulation model shown: The differential relay We also have the comparator block which may be as shown below: The contents of the amplitude and harmonic comparator blocks are as shown in the figure below Preliminary Results and Discussions Based on the MATLAB/SIMULINK simulation model above, it is expected that different cases will suffice for the different results required. However, as earlier stated, taking the simulation as it is, with the indicated circuit as it is proves somewhat challenging. This is based on the fact that such a venture would be beyond the constraints of the scope of the project. In essence, it requires the knowledge of a number of hidden coefficients that significantly affect the behavior of the circuit. In addition, one simulation environment may not be sufficient to give the valid transformer protection operation of the above circuit. To avert this challenge, there will be a need to use a regular single phase, which will be amended so as to fit the specification of the transformer in use. Another alternative would be to adopt current measurement, though in such an approach the problems of current transformers may not be simulated. All these approaches were stated in the project proposal, and are indicative of the appropriate course for this project. On successful completion of the project, within experimental errors, the results should indicate some bit of consistency with the proposed findings. It should be able to give an effective solution in consideration of the magnetizing inrush current at load and no-load conditions, as well as the fault conditions that may exist. The graphical representations of the expected relations are shown below: a. Magnetizing inrush current (Herman, 2011) Phase A current Phase B current Phase C current The trip signal The harmonic comparator The amplitude comparator b. Magnetizing inrush current with load (Herman, 2011) Below are the results for the second harmonic and the fundamental component c. Three phase to ground fault at loaded transformer (Herman, 2011) The second harmonic and fundamental component Conclusions This report is about the design and simulation of power transformer protection using differential digital relays. The stepwise design is outlined after a thorough analysis of theoretical background information, which validates the need to have transformer protection in place. References Dash, P., & Rahman, M. (1987). A New Algorithm for Digital Protection of Power Transformer. Canadian Electrical Associatio n Transactions, 1-8. Hamming, R. (1973). Numerical Methods for Scientists and Engineers. New York: McGraw-Hill. Herman, S. L. (2011). Electrical Transformers and Rotating Machines. New York: Delmar Cengage Learning. Yabe, K. (1997). Power Differential Method for Discrimination between Fault and Magnetizing Inrush Current in Transformers. IEEE Transactions on Power Delivery, 23-34. Read More

Given this situation, it becomes inevitable that the fault conditions be done away with as soon as possible in order to see to it that coordination of protective devices at any point in the system is maintained. It is, therefore, required that though more than one protective device may detect a fault, only one is supposed to respond. There are two generally accepted means that can be used to achieve this purpose. One can opt to use fuses, which in most cases acts either directly, or by means of a device that causes mechanical tripping, attached to it.

Such a device works by opening a 3-phase load-break switch which is attached to it. On the other hand, the methods that is progressively gaining popularity and acceptance is the use of relays, which, as opposed to fuses, are independent from the circuit breaker coil, and hence, act indirectly to the coil of the circuit breaker. Transformer protection When it comes to transformers, there are quite a number of faults from which the transformer needs to be protected to make it achieve its proper functioning capacity.

To begin with, transformers face stresses which may emanate from the supply network. These are usually in form of voltage surges, which may either be atmospheric or operating voltage surges. Atmospheric ones in most cases occur from natural sources such as lightening strokes that occur close to transmission lines. In the case of operating surges, they mainly result from spontaneous changes in the normal functional conditions of given electrical networks leading to transients within the system.

The high frequency voltage surge wave that occurs in this scenario hampers the normal functioning of the transformer. The most effective methods of protecting transformers from such kinds of surges has been found to be the use of varistors and they do not affect the switchgear much. A part from voltage surges, there may also be stresses that arise from the load itself. In most cases, the reason for overloading is usually as a result of simultaneous power need of small loads connected to the network.

At times, it arises when the need for apparent power (kVA) increases, especially when the facility (in this case the factory) grows and expands leading to an extension that causes an increase in the power of demands. An increase in the load leads to a subsequent increase in the temperatures of the wirings and insulations. These eventually reduce the lifespan of the equipment. There is no specific place to place overload protection devices as they can always be either on the low voltage or high voltage side of the transformer.

The most effective way of protecting a transformer against overloading is by means of a digital relay, which serves to trip the circuit-breaker located on the secondary side. The term ‘thermal relay’ is interchangeably used with the digital relay considering its function on the transformer. Their mode of working usually involves the artificial simulation of temperature based on the time constant of the particular transformer in use, with some of them having the ability to incorporate the impact of harmonic currents resulting from non-linear loads.

Such loads are the likes of variable speed drives and computer drives among others. The relays are useful in projecting and estimating the time required for overload tripping as well as the wait time required in the event that tripping has occurred. Such a control measure is a very useful step in the control of the shedding operations, which can either be coordinated with thermostats or heat sensors depending on whether they are being used for oil-immersed or dry-type transformers respectively.

Background For the recent past that transformers have been in use, it has been noted that in most cases both internal and external faults tend to occur within the transformer. Such faults have led to excessive damages and losses related to transformer devices. There thus emanated a need to devise a way of identifying such internal faults within a transformer putting into consideration very high levels of sensitivity.

Read More