CHAPTER 3 REVIEW OF POWER TRANSFORMER PROTECTION SCHEMES

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Cornelia Charles
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1 CHAPTER 3 REVIEW OF POWER TRANSFORMER PROTECTION SCHEMES 3.1. Introduction Power Transformer is the nerve centre of any power distribution system. The capacity of power transformers is generally decided from the data of the load survey. The power transformers are the main equipment of the substation, while other equipments are to associate with the functional aspects of the transformers. Any fault on the transformer will operate Buchholz relay and differential relay. On action of these relays, the transformer should not be charged. It should be isolated and all the tests are to be conducted. The unit should be put into service only if tests confirm the healthiness. Generally the following are the possible faults in power transformers: Faults within the transformer tank Over heating Faults external to the transformer zone(through fault) Incipient faults Internal faults (To overcome these kind of faults Differential protection schemes are provided ) Over excitation (To overcome this kind of fault over fluxing relay is provided) Turn to turn fault,arcs within the Oil(To overcome these kind of faults sudden pressure relay schemes are provided ) Generally, for the purpose of discussion, transformer faults can be divided into two main categories. They are: (i) Through faults, i.e., overloads and external short circuits. (ii) Internal faults, i.e., Faults in the transformer windings and connections. Internal faults can be sub divided into incipient faults and heavy faults. Incipient faults The incipient faults are not serious in nature. But it should be deducted and cleared as soon as possible before it develops into a major fault. These incipient faults

2 can be detected by periodical dissolved gas analysis test and then with the help of portable partial discharge analyzer. The major causes for incipient faults are: Poor electrical connection of conductors or core fault which causes limited arcing under the oil. Coolant failure (clogged oil flow) which will cause a rise in temperature even when the load is below the rated capacity. In the recent trend, power transformer incipient faults are detected by using Dissolved Gas analysis (DGA) method. So, in this research work investigations are made on this method. Heavy faults This can be sub divided as: Multi phase and phase to earth faults within the transformer and outside the transformer at HV, LV and tertiary terminals Short circuit between turns in HV and LV winding. Heavy electrical faults will be detected by unit protection like Differential relays and so investigations are made in transformer differential schemes. Generally the following types of protection schemes are used for large power transformers: High speed differential protection Restricted earth fault protection. Back up over current protection Buchholz protection Temperature protection Depending upon the location, importance and rating of the transformers a selection among the above protection schemes are made. Out of these, conventionally differential protection and over current protection are used in most of the cases. Therefore in this research work focus is made on incipient fault protection, overcurrent protection and differential protection Incipient Fault Protection Abnormal conditions can occur within a transformer due to several reasons. Some of the possibilities are lightning, switching transients, mechanical flaws, and 20

3 chemical decomposition of oil or insulation. Some of them are incipient faults. The incipient faults may be due to one or more of the causes shown in table 3.1. Incipient faults of power transformers can be classified as Overheating of oil (OHO), Overheating of cellulose (CD), Electrical arcing (HEDA) and Electrical corona (LED). Table 3.1. Correlation between power transformer incipient faults and causes Faults Causes Arcing Corona Overheating of Cellulose Winding turn-turn short circuit X - X - Winding open circuit X - X - Operation of build -in LTC X Winding Distortion - X X - Loose connection to bushing terminals, tap leads, terminal boards Overheating of Oil X X X - Free water or excessive moisture X in oil X - - Floating metal particles X X - - Loose connection to corona shields - X - - Loose collars, spacers, core ground straps, core hold down angle (Braces) - X - - Through fault - - X Overloading - - X X Damaged yoke bolt insulation X Rust or other damage on core X The life and service quality of the transformer gets increased with the preventive and corrective maintenance, carried out at the appropriate time. The solid insulation materials, used in the manufacture of transformer, are basically from cellulose material and are hygroscopic in nature. The ageing or deterioration of the solid or liquid insulation system is very much dependent on the operating temperature and the level of oxygen, moisture and dust particles present in the air, breathed in by the transformer during its service-life. Hence, it is imperative that the maintenance schedule for the upkeep of the transformer needs to be focused on preservation of its insulation system. Generally all types of faults in a transformer result on the localized heating and breakdown of the oil. Some degree of arcing will always take place in a winding fault and the resulting decomposition of the oil will release gases such as hydrogen, 21

4 carbon monoxide and light hydro carbons. When the fault is of a very minor type such as a hot joint, gas is released slowly. But a major fault involving severe arcing may cause rapid release of large volume of oil as well as vapour. Hence to protect from this kind of faults Buchholz relay in conjunction with any pressure relief device are conventionally used. In the recent past focus is made on preventive maintenance action. As a result of various attempts to develop a technique that will detect the transformer fault in its very incipient stage, Dissolved Gas Analysis (DGA) method [37] was developed. The DGA test on oil of transformer in service, periodically, reveals the healthiness of transformer and prediction of development of fault at the initial stages. So, at present, a most widely used method is incipient fault protection based on Dissolved Gas Analysis (DGA). Dissolved Gas Analysis (DGA) methods Dissolved Gas Analysis is a powerful diagnostic technique for detecting the incipient faults, in oil-filtered equipments, particularly Power Transformers. An oil filled transformer in operation is subjected to various stresses like thermal and electrical, resulting in liberation of gases from the hydrocarbon mineral oil. The components of solid-insulation also take part in the formation of gases, which are dissolved in the oil. An assessment of these gases, both qualitatively and quantitatively, would help in diagnosing the internal faults in the transformers. In this method, fault diagnosis is done according to the concentration of the dissolved gases and gas ratios. In this research work, certain AI techniques such as ANN and ANNEPS are used to develop relaying algorithms for power transformer incipient fault protection using DGA method Overcurrent Fault Protection As the fault impedance is less than load impedance, the fault current is more than load current. If a short circuit occurs the circuit impedance is reduced to a low value and therefore large current accompanies a fault. Overcurrent relays sense such fault current and also over-load currents. Overcurrent protection is that protection in which the relay picks when magnitude of current exceeds the pickup level. The basic element in overcurrent protection is an overcurrent relay. Overcurrent protection includes the protection from overloads. Overloading of equipment means that it takes more current than the rated current. Since short circuit currents are generally several 22

5 times (5 to 20 times) of full load current this type of protection also provides shortcircuit protection. Hence Overcurrent protection is widely used for motor protection, transformer protection, line protection and protection of utility equipment. Since most of 11kV, 22kV, 33kV, 66kV and 110kV feeders are radial, normally overcurrent relays without directional feature are employed. For 110kV tie feeders directional overcurrent relays are employed for transformer short circuit protection. Transformers are provided with overcurrent protection against faults when the cost of differential relays cannot be justified. However, overcurrent relays are provided in addition to differential relays to take care of through faults and as back-up to differential protection. While selecting the overcurrent protection of transformers, the following aspects needs consideration: Magnetizing Inrush current Primary full load current CT requirements These overcurrent devices are installed at strategic places in electric systems to sense conditions that are considered to be abnormal. The sensitivity of an overcurrent device is related to the ability to sense a fault when it really exists. The selectivity of an overcurrent device is related to the ability of the system to select the section of the power system that has a problem, and remove service from only that section.this situation demands the proper and optimal coordination of overcurrent relays in a power system. Proper settings of protective relays are essential for the reliable operation of electrical power systems, during both fault and normal system operating conditions. The ideal relay operating characteristics can also be influenced by parasitic phenomena, such as CT saturation. Protection coordination requires serial steps. One suggested approach is to first determine the fault conditions critical to evaluating coordination: the fault types, the fault locations, and the system contingencies in effect. The next step is to determine which devices need to be coordinated (i.e. the devices which offer primary protection, and the devices which are acting in a backup mode for the given fault are to be determined) for a given fault. The knowledge base would need data on system topology and device locations and characteristics. The expert system would have to call on an algorithmic procedure to determine device operating times for these critical 23

6 faults. A final step would look for possible corrective actions when a miscoordination occurs between devices for a particular fault. Some alternatives might be to change the device s pick up, reach, or timer, or possibly even to upgrade the protective device. In some protection coordination situations, human experts have different opinions on the correct action to take. In such cases, it would be desirable to apply the expert system for alternative solutions. Today protection coordination is a design and planning problem rather than an operational problem. Electro mechanical relays do not have provision for remote setting during operation. Their settings cannot usually be changed for different load conditions or changes in configuration. With the customer driven power system and changes in system configuration, the protection coordination can be viewed as an operational problem of adaptive protection device setting. Emerging technologies such as digital and microprocessor based relays will have provision for real time remote setting and can be employed in adaptive protection coordination. The domain of protection coordination involves heuristics and experience and is well suited for AI approach [10]. Hence, optimal coordination of directional overcurrent relays using the advanced Evolutionary programming is done in this research work. Owing to the huge variety of protective relays from different manufacturers and technologies, the approach must be flexible. Moreover the approach must cater for the old, electromechanical relays, static relays as well as the modern digital protective relays. The developed method of optimizing coordination of DOCR satisfies these requirements. Moreover existing conventional Electro mechanical inverse overcurrent relays exhibit limited flexibility and poor accuracy and cannot cater to the modern complex relaying demands. Hence microprocessors controlled relay shall be designed with features like CT ratio selection, plug setting multipliers etc. Also variety of relay characteristics, viz. IDMT, very inverse, extremely inverse and earth fault relay can be realized from a single relay unit. On experimental basis, in this research work the AI techniques such as Fuzz logic and neural approaches are used for modeling conventional IDMT characteristics of overcurrent relaying system Differential Protection Transformer internal faults are very serious since there is always the risk of fire. These internal faults can be classified into two groups. 24

7 (i) Electrical faults which cause immediate serious damage but generally detectable by unbalance of current of voltage. (ii) Incipient faults which are initially minor faults causing slowly developing damage. These are not detectable at the winding terminals by unbalance. It is important that the faulted transformer be isolated as quickly as possible after the fault has occurred. The reason is not only to limit the damage to the transformer, but also to minimize the length of time of low voltage in the system. A prolonged period of low voltage may result in loss of synchronization between rotating machines. This may cause other relays to operate and initiate sequential and necessary tripping. Mostly the universal protective scheme for the faults within the transformer uses transformer differential relay. This relay is the principle form of fault protection for transformers rated 5 MVA and above. Differential relaying usually involves the detection of an imbalance in current flow into and out of a protected area. These relays, however, cannot be as sensitive as differential relays used in generator protection, because they are subject to several factors not ordinarily present for generators that cause mal-operation. Problems Associated With the Conventional Transformer Differential Protection The advances in the transformer differential protection have involved many disturbing compromises between the basic requirements of service protection and limitation of equipment damage. These limitations have become increasingly felt as the art of system protection has progressed, and the standards of service have been raised. Major factors that can cause mal-operation of transformer differential relays are: Different voltage levels including taps, which result in different primary currents in the connecting circuits. Possible mismatch of ratio among different current transformers A 30 degree phase-angle shift introduced by transformer Wye-delta connections. Magnetizing inrush currents and over excitation due to over fluxing which a differential relay considers as internal faults. 25

8 Transformer protection is further complicated by a variety of equipment requiring special attention, multiple winding transformer banks, zigzag transformers etc. Moreover, in a transformer differential relay, the current comparison is complicated by the following factors: Current transformer ratios connecting to the power transformer mismatch. There may be phase shift between the power transformer primary and secondary windings. The power transformer may have a tap changer on one of its winding. The CT can saturate under through fault conditions, giving an effective ratio error. When a power transformer is energized, inrush current flows for a short time into the energized winding. Over fluxing of the transformer can give rise to exciting current flowing in only one winding. Recent Trends in Transformer Differential Protection Since the magnetizing inrush current phenomenon is transient, stability can be maintained by providing a small time delay. In 1960s, an instantaneous relay shunted by a fuse (kick fuse) was introduced. This kick fuse was chosen so as to carry the inrush current without blowing. Only in the event of an internal fault the fuse may blow and permit the relay to operate. In 1970s, induction pattern relays of the IDMT type which provided suitable time delay during switching conditions was developed. The prime drawback of using this low-set relay was the low speed operation under fault conditions. Gradually the need for quicker operation of a relay made way to develop relays with immunity to magnetizing inrush currents. Later in 1980s, the following technique was employed i.e. the current curve during the magnetizing inrush current contains pronounced harmonics, whereas internal fault current is sinusoidal. A relay was designed to operate under fault conditions, restraining all the harmonic frequencies when fundamental frequency was predominant. However, over excitation resulting from over voltages due to sudden tripping of major loads or under frequencies caused heavy magnetizing currents which cause inadvertent relay tripping. To overcome this difficulty in early 1990s, a reputed manufacturer came out with the solution of detecting over excitation by measuring fifth harmonic component 26

9 of differential current, and a fixed percentage of fifth harmonic restraint was introduced as an added feature to the relay. At present, the whole idea of development of transformer differential protection is focused on tackling the magnetizing inrush current phenomenon and the restraint feature provision to various harmonics. Hence recent basic methods to stabilize differential relay during magnetic inrush condition are: Harmonic restraint / blocking Wave shape identification Voltage restraint Traditionally second harmonic is used to block the relay from operation during inrush condition. But this resulted in a significant slowing of the relay operation during heavy internal faults. To overcome this in mid 1990s, one manufacturer came up with a new waveform recognition technique to detect magnetic inrush. In that case, inrush current waveform is characterized by a period of each cycle, simultaneously in all the three phases, where its magnitude is very small (nearly zero). By measuring the time of this period of low current, an inrush condition was identified. In spite of this relays, unwanted tripping occurred during switching on load. For several decades power transformer protection schemes had experienced many changes and arrived to employ fully numerical technology in late 1990s. With the advent of technological advancements in the digital applications, it is now possible to provide powerful protection algorithms within the cost effective hardware modules for dedicated differential protection applications. The features and advantages provided by these devices have proved beneficial in quicker isolation of faults resulting in stability of power system under abnormal operating conditions. In conventional digital technology a transformer differential protection is a biased differential protection to which restraints and high-set threshold must be added in order to obtain correct operation in all circumstances. The restraint is therefore vital when the transformer is energized and when it is used in an over fluxed situation. The most popular solution is to measure second and fifth harmonics of the differential current. The restraint threshold for these harmonics can be adjusted in most cases. Actually these adjustments make the user to determine the compromise between 27

10 stability (on inrush) and sensitivity (on pre-existing faults). However technical progress should allow protection manufacturers to do this kind of adjustment. [38]. Hence, in this research work, the new approaches with AI techniques are developed to achieve stability with much consistent and lower operating times, yet retaining a high degree of through fault stability. This modern relay shall provide the events with real time and recording of disturbance recordings. Also it provides total accessibility for protection, data acquisition and control Conclusion This chapter provided a bird s eye view of the various existing schemes which have ample applications for power transformer protection. In the subsequent chapter, detailed description of the AI techniques proposed for power transformer protection in this research work is discussed. 28

Transformer Protection

Transformer Protection Nature of transformer faults TXs, being static, totally enclosed and oil immersed develop faults only rarely but consequences large. Three main classes of faults. 1) Faults in Auxiliary