TRANSFORMERS, COOLING METHODS AND WINDINGS Group 12 /

Transcription

1 TRANSFORMERS, COOLING METHODS AND WINDINGS Group 12 / Power transformers are being used to transfer electric energy between the generation stations and primary distribution networks. They are used to step up voltage level at the generating stations before being transferred and further stepped down at sub transmission networks. These power transformers are big in size and they generally have high insulation levels. In addition, another type of transformers found in power systems is the distribution transformers which are used to distribute energy throughout distribution networks for local consumption. These transformers step voltage down to the domestic consumer voltage level [usually 400 V or less]. Distribution transformers have different sizes and application areas; for example, pad mount transformers are designed for use at shopping centers, schools, and office buildings. Also, medium-size distribution transformers are used to step down three phase high voltage to low voltage for energy distribution mainly in metropolitan area and industrial applications, while small-size distribution transformer are mainly used in country side or low populated areas. Pole mounted transformers are generally single phase and oil immersed. They are suitable for residential loads, light commercial or industrial loads. Potential or current transformer are used as instrumentation transformers to accurately and safely represent voltage, current or phase position of high voltage or high power circuits. Cooling arrangements There are several factors contributing to power losses in transformers. These are copper losses, which represent the major source of losses in a transformer, and core losses; namely hysteresis and eddy current losses. These losses components are produced in the form of heat energy which should be dissipated in a quick and rather efficient manner. If the transformer has otherwise failed to get rid of such heat generated, many problems could arise and in some cases severe consequences may occur. In fact, the improperly dissipated heat would further accumulate and thus cause the transformer temperature to increase. This process may lead to failure of paper insulation and liquid insulation medium of the transformer. Furthermore, excessive heat may result in damage of the transformer windings, the matter which, in particular, is considered as a catastrophe for expensive high power rating transformers. Therefore, numerous ways are introduced to keep the temperature within acceptable limits which in turn would help to maintain a long lifetime of the transformer. Various types of cooling arrangements have been used in transformers. These different cooling schemes are identified in electrical standards and are given the following naming convention. For Oil-Immersed Transformers ONAN Cooling of Transformer This method stands for Oil Natural Air Natural. The cooling purpose is achieved by that the hot mineral oil is naturally circulated throughout loaded transformer through convection. In this simple cooling method, cold oil replaces the hot oil already flown to the top part of the transformer tank. This heat carried by the insulating medium is dissipated out to the ambient atmosphere through walls of transformer tank. All heat transfer mechanisms [i.e. conduction, convection, and radiation] are employed in this process. This implies the dissipation- process dependence on the tank surface area. That is why radiators or tubes/fins are usually

2 used to effectively solve the tradeoff between the required increase in surface area available for heat transfer and the undesired increase in tank size. While these radiator banks are utilized for larger transformers, integrated fins are however used for smaller ones. Generally, this method of transformer cooling is used with low rating distribution-type transformers. ONAF Cooling of Transformer This method stands for Oil Natural Air Forced. For larger transformers units, electric fans can be installed as a mean for forced air cooling. This method depends in principal on blowing air on the cooling dedicated surface. This way it can thus accelerate the rate of dissipation of heat generated. This is due the increased amount of air pushed across the surface, which makes this cooling method offer improved cooling performance compared to ONAN method. The transformer loading capacity is therefore allowed to be increased by about % without being at risk exceeding the temperature limits. It is worth noting in this arrangement that, similar to ONAN, natural convectional flow still govern the circulation of heated oil. This cooling method best suits large step up and step down outdoor transformers used in transmission power networks. OFAF Cooling of Transformer This method stands for Oil Forced Air Forced. It had been found that the underlying principal of increasing the heat removal rate could be further improved when an accelerating force is applied to the circulation path of oil in the transformer tank. In this regard, oil pumps are used in large transformers where the radiators and fans do not suffice to meet their cooling requirements. In addition, from an economic perspective, it may be advantageous to benefit from this method s much lower space requirements when compared to those imposed by simple radiator batteries. These compact circulation pumps have their motors already submerged into oil. Noise levels produced are deemed to be very low in comparison to that produced by the transformer equipment itself. OFWF Cooling of Transformer This method stands for Oil Forced Water Forced. Water can be considered a better substitute for air as a medium for heat exchange. For same climate conditions, this is due to water temperature is much lower than that of air. Although oil is forced toward the heat exchanger using pumps similar to OFAF method; however, this time the heat exchanger used is an oil to water one. The cooling process is achieved through cold water being sprayed over the hot oil flowing through the exchanger s piping system. Two coolers are used although

3 only one is sufficient for the transformer operation; however, a tripping action should be activated if both fails. The water coolers are positively compact in size yet they can be negatively affected by the transformer environmental operating conditions. This is especially true in very low temperature surroundings where water may freezes and in very hot climate where abundance of water should be secured for proper operation. Moreover, the problem of water tube materials subject to corrosion may call for utilization of more complex materials like titanium during cooler manufacturing process. More importantly, ensuring totally sealed and isolated systems is crucial to avoid risky leakage into oil. This type of cooling is used for very large power transformers [above 500 MVA] ODAF Cooling of Transformer This method stands for Oil Directed Air Force. This transformer cooling scheme is the modified version of OFAF. To ensure higher rate of heat clearance, guided flowing paths between the insulated windings inside the transformer are provided for the oil coolant to pass through. This cooling method is specifically appropriate for transformers with considerably high power ratings. ODWF Cooling of Transformer This method stands for Oil Directed Water Forced. This cooling system is similar to that discussed for ODAF except for the fluid used for cooling down the hot oil. Instead of air, water is forced in the cooler as a cooling system. If Pumps and fans suffer breakdown for some reason and need to be inspected, this should be done without affecting the service continuity of the transformer. Therefore various valves are used to make it possible, if necessary, to temporarily disconnect individual oil circuits for maintenance or replacement. In addition, a given transformer can have a combination of cooling types, for example ONAN/ONAF, to allow a change in the type of cooling. For Dry-type transformers Self-air cooled (for transformers up to 3 MVA) This method depends on the transformer surrounding air flow to naturally cool down the unit. Forced Air Cooled (for transformers up to 15 MVA) Air is pushed blowers to circulate through the transformer windings. This cause the air to heat up and then it starts to be cooled by ambient natural air

4 Three-phase winding arrangements and their purpose Transformers utilize windings to transfer electrical power between separate circuits. These windings can be connected in several different ways, depending on the requirement. A three phase transformer has three sets of primary and secondary windings. Each side can be connected in three different configurations, in star (Y), delta (D) or zig-zag (Z). A combination of letters are used to indicate which winding arrangement is used in a transformer, for example, Dyn11. The first letter, which is capitalized, refers to the winding arrangement of the high voltage primary side. While the second letter specifies the secondary sides winding configuration. If there is a letter N/n after the winding letter, the winding is connected to neutral in the case of zig-zag or star winding. Moreover, the phase displacement between the primary and secondary windings are expressed as a clock hour number, ranging from 0 to 12 o clock. Each hour represents 30 degree lag between the secondary and primary windings phase. In other words, the previously mentioned Dyn11 transformer has a delta-star configuration, with a neutral connected secondary winding, and the low voltage side is lagging 330 degrees. Star (wye), delta and zig-zag (interconnected star) windings Vector groups The different winding arrangements are divided into vector groups. They are separated according to their phase differences between the primary and secondary winding. It is important to know the vector group when connecting transformers in parallel. Since parallel connecting transformers with different vector groups leads to large current flows between the transformers, consequently, damaging the transformers. Thus, the transformer vector groups is generally indicated on the rating plate. Additionally, winding polarity should be considered, since reversing the connections across a set of windings affects also the phase-shift. However, the vector group does not affect the performance of the transformer. Usage of different winding types Different winding types are chosen according to the demands of the application. If there is a need to remove specific harmonics, phase displacement should be considered and if there is a need for parallel transformers, the vector group should be the same in both transformers. If there is a nonlinear load, Dyn11 might be a solution. Neutral point can be used in star or zig-zag windings, if the neutral point needs to be grounded or

5 there is an unbalanced load. The neutral point in star winding without delta is not stable. If the application has higher voltage in comparison to current, in star connection voltage over one winding is 57,7 % from the mains voltage and current in the winding is the phase current. Then again in D-winding the current is 57,7 % and voltage is the phase voltage. Delta-star and star-delta (Dy, Yd) Dyn11 is widely used in the distribution networks as a step-down transformer with high rated power, it is commonly used in commercial, industrial and high density residential locations. The delta-part reduces the third harmonics, and the star provides a neutral point. For example when using Dyn11, 11 kv system has zero phase shift compared with the 400 kv system. In delta-star the transformer does not require 4-wire input. In delta-star 3-wire primary feeder supplies a 4-wire secondary circuit, and the secondary coil is connected to neutral. The Dy system also reduces zerosequence fluxes and associated currents. YNd1 in can be used as step-up or generator transformers, especially when using Dyn11. In this case YNd1 will neutralize the phase angle created by Dyn11. Star-star (Yy) Star-star-connected transformers can be used as a system tie-up transformer. Under a balanced load the star-star connection operates like three separate single phase transformers. The configuration is simpler than most of the connection types, and provides 0 phase shift. The system is not, however, useful when the load is unbalanced and the zero sequence component is not eliminated, and in case of a line-to-earth fault, the fault flow through the transformer. This can be eliminated by adding a tertiary winding in delta. The transformer YY0 can be used when connecting for example two delta sides, so that both sides can be grounded. Yy-transformers can also be converted to autotransformers. Delta-delta (Dd) The advantages of using Dd connection is, that if one winding fails, it can still be operated in reduced capacity (57,7 %). Using delta-delta connected transformers is practical in remote locations where the replacement service is not necessary always available. References: Sen, P.C. (1997). Principles of Electric Machines and Power Electronics. Second Edition. New York. John wiley & sons Elovaara, Harla; Sähköverkot II: verkon suunnittelu, järjestelmät ja laitteet s

6 The J & P Transformer Book, 12th edition p.18