变压器--三相变压器工作原理

变压器--三相变压器工作原理 三相变压器工作原理,变压器的基本工作原理是电磁感应原理。当交流电压加到一次侧绕组后交流电流流入该绕组就产生励磁作用,在铁芯中产生交变的磁通,这个交变磁通不仅穿过一次侧绕组,同时也穿过二次侧绕组,它分别在两个绕组中引起感应电动势。这时如 果二次侧与外电路的负载接通,便有交流电流流出,于是输出电能。 用三只单相变压器或如图的三相变压器来完成.三相变压器的工作原理和单相变压器 是的. 在三相变压器中,每一芯柱均绕有原绕组和副绕组,相当于一只单相变压器.三相变压器高压绕组的始端常用A,B,C,末端用X,Y,Z来表示.低压绕组则用a,b,c和x,y,z来表示.高低压绕组分别接成星形或三角行.在低压绕组输出为低电压,大电流的三相变压器中(例如电镀变压器),减少低压绕组的导线面积,低压绕组亦有采用六相星行或六相反星行 接法 我国生产的电力配电变压器均采用Y/Y-12或Y/三角形-11这两种结线方法.数子120 和11表示原绕组和副绕组线电压的相位差,也所谓变压器的结线组别.在单相变压器运行是,结线问题不为人们所重视,然而,在变压器的并联运行中,结线问题却具有 意义. 三相变压器设计 (一) Sn=10KVA,U1=380V,U2=220V。 短路损耗Pd(W),空载损耗Po(W),空载电流Io(%) 按三相双圈计算,I1=Sn/U1*?3=10000/380*1.7321=15.2(A) I2=Sn/U2*?3=10000/220*1.7321=26.2(A) , D=K*4?S柱,4?S柱,柱容量开4次方。S柱=Sn/3,K经验系数,铜线,冷轧硅钢片,K=54-60,热轧硅钢片K=58-64 所以D(柱芯直径)=60*4?3.33=80(采用热轧硅钢片K取60) 柱芯截面积=40^2*3.14?5000平方毫米,50平方厘米。 园柱芯制作工艺复杂、价格高,采用F型或E型 方形截面柱芯7X7厘米。 每匝伏电压,磁通密度Bm取16000高斯 et=2.22Bm*Sc*10^-6=2.22*16000*49*10^-6=1.74(伏/匝) 高压则匝数W1=380/1.74=218(匝),低压则匝数W2=220/1.74=126(匝) 匝数最后确定,W1=250(匝),W2=145(匝) 电磁线的选择,电流密度取2.5A/mm^2 高压绕组导线,15.2/2.5=6.08平方毫米,选用φ2.8mm玻璃丝包漆包线 低压绕组导线,26.2/2.5=10.48平方毫米,选用4X2.62玻璃丝包扁漆包线 Three-phase transformer circuits Since three-phase is used so often for power distribution systems, it makes sense that we would need three-phase transformers to be able to step voltages up or down. This is only partially true, as regular single-phase transformers can be ganged together to transform power between two three-phase systems in a variety of configurations, eliminating the requirement for a special three-phase transformer. However, special three-phase transformers are built for those tasks, and are able to perform with less material requirement, less size, and less weight than their modular counterparts. A three-phase transformer is made of three sets of primary and secondary windings, each set wound around one leg of an iron core assembly. Essentially it looks like three single-phase transformers sharing a joined core as in Figure below. Three phase transformer core has three sets of windings. Those sets of primary and secondary windings will be connected in either Δ or Y configurations to form a complete unit. The various combinations of ways that these windings can be connected together in will be the focus of this section. Whether the winding sets share a common core assembly or each winding pair is a separate transformer, the winding connection options are the same: , Primary - Secondary , Y - Y , Y - Δ , Δ - Y , Δ - Δ The reasons for choosing a Y or Δ configuration for transformer winding connections are the same as for any other three-phase application: Y connections provide the opportunity for multiple voltages, while Δ connections enjoy a higher level of reliability (if one winding fails open, the other two can still maintain full line voltages to the load). Probably the most important aspect of connecting three sets of primary and secondary windings together to form a three-phase transformer bank is paying attention to proper winding phasing (the dots used to denote “polarity” of windings). Remember the proper phase relationships between the phase windings of Δ and Y: (Figure below) (Y) The center point of the “Y” must tie either all the “-” or all the “+” winding points together. (Δ) The winding polarities must stack together in a complementary manner ( + to -). Getting this phasing correct when the windings aren't shown in regular Y or Δ configuration can be tricky. Let me illustrate, starting with Figure below. Inputs A, A, A may be wired either “Δ” or “Y”, 123 as may outputs B, B, B. 123 Three individual transformers are to be connected together to transform power from one three-phase system to another. First, I'll show the wiring connections for a Y-Y configuration: Figure below Phase wiring for “Y-Y” transformer. Note in Figure above how all the winding ends marked with dots are connected to their respective phases A, B, and C, while the non-dot ends are connected together to form the centers of each “Y”. Having both primary and secondary winding sets connected in “Y” formations allows for the use of neutral conductors (N and N) in each power 12 system. Now, we'll take a look at a Y-Δ configuration: (Figure below) Phase wiring for “Y-Δ” transformer. Note how the secondary windings (bottom set, Figure above) are connected in a chain, the “dot” side of one winding connected to the “non-dot” side of the next, forming the Δ loop. At every connection point between pairs of windings, a connection is made to a line of the second power system (A, B, and C). Now, let's examine a Δ-Y system in Figure below. Phase wiring for “Δ-Y” transformer. Such a configuration (Figure above) would allow for the provision of multiple voltages (line-to-line or line-to-neutral) in the second power system, from a source power system having no neutral. And finally, we turn to the Δ-Δ configuration: (Figure below) Phase wiring for “Δ-Δ” transformer. When there is no need for a neutral conductor in the secondary power system, Δ-Δ connection schemes (Figure above) are preferred because of the inherent reliability of the Δ configuration. Considering that a Δ configuration can operate satisfactorily missing one winding, some power system designers choose to create a three-phase transformer bank with only two transformers, representing a Δ-Δ configuration with a missing winding in both the primary and secondary sides: (Figure below) “V” or “open-Δ” provides 2-φ power with only two transformers. This configuration is called “V” or “Open-Δ.” Of course, each of the two transformers have to be oversized to handle the same amount of power as three in a standard Δ configuration, but the overall size, weight, and cost advantages are often worth it. Bear in mind, however, that with one winding set missing from the Δ shape, this system no longer provides the fault tolerance of a normal Δ-Δ system. If one of the two transformers were to fail, the load voltage and current would definitely be affected. The following photograph (Figure below) shows a bank of step-up transformers at the Grand Coulee hydroelectric dam in Washington state. Several transformers (green in color) may be seen from this vantage point, and they are grouped in threes: three transformers per hydroelectric generator, wired together in some form of three-phase configuration. The photograph doesn't reveal the primary winding connections, but it appears the secondaries are connected in a Y configuration, being that there is only one large high-voltage insulator protruding from each transformer. This suggests the other side of each transformer's secondary winding is at or near ground potential, which could only be true in a Y system. The building to the left is the powerhouse, where the generators and turbines are housed. On the right, the sloping concrete wall is the downstream face of the dam: Step-up transfromer bank at Grand Coulee hydroelectric dam, Washington state, USA. Three Phase Transformers Introduction: Three phase transformers are used throughout industry to change values of three phase voltage and current. Since three phase power is the most common way in which power is produced, transmitted, an used, an understanding of how three phase transformer connections are made is essential. In this section it will discuss different types of three phase transformers connections, and present examples of how values of voltage and current for these connections are computed. Three Phase Transformer Construction: A three phase transformer is constructed by winding three single phase transformers on a single core. These transformers are put into an enclosure which is then filled with dielectric oil. The dielectric oil performs several functions. Since it is a dielectric, a nonconductor of electricity, it provides electrical insulation between the windings and the case. It is also used to help provide cooling and to prevent the formation of moisture, which can deteriorate the winding insulation. Three-Phase Transformer Connections: There are only 4 possible transformer combinations: 1. Delta to Delta - use: industrial applications 2. Delta to Wye - use : most common; commercial and industrial 3. Wye to Delta - use : high voltage transmissions 4. Wye to Wye - use : rare, don't use causes harmonics and balancing problems. Three-phase transformers are connected in delta or wye configurations. A wye-delta transformer has its primary winding connected in a wye and its secondary winding connected in a delta (see figure 1-1). A delta-wye transformer has its primary winding connected in delta and its secondary winding connected in a wye (see figure 1-2). Figure 1-1: Wye-Delta connection Figure 1-2: Delta-Wye connection Delta Conections: A delta system is a good short-distance distribution system. It is used for neighborhood and small commercial loads close to the supplying substation. Only one voltage is available between any two wires in a delta system. The delta system can be illustrated by a simple triangle. A wire from each point of the triangle would represent a three-phase, three-wire delta system. The voltage would be the same between any two wires (see figure 1-3). Figure 1-3: Wye Connections: In a wye system the voltage between any two wires will always give the same amount of voltage on a three phase system. However, the voltage between any one of the phase conductors (X1, X2, X3) and the neutral (X0) will be less than the power conductors. For example, if the voltage between the power conductors of any two phases of a three wire system is 208v, then the voltage from any phase conductor to ground will be 120v. This is due to the square root of three phase power. In a wye system, the voltage between any two power conductors will always be 1.732 (which is the square root of 3) times the voltage between the neutral and any one of the power phase conductors. The phase-to-ground voltage can be found by dividing the phase-to-phase voltage by 1.732 (see figure 1-4). Figure 1-4: Connecting Single-Phase Transformers into a Three-Phase Bank: If three phase transformation is need and a three phase transformer of the proper size and turns ratio is not available, three single phase transformers can be connected to form a three phase bank. When three single phase transformers are used to make a three phase transformer bank, their primary and secondary windings are connected in a wye or delta connection. The three transformer windings in figure 1-5 are labeled H1 and the other end is labeled H2. One end of each secondary lead is labeled X1 and the other end is labeled X2. Figure 1-5: Figure 1-6 shows three single phase transformers labeled A, B, and C. The primary leads of each transformer are labeled H1 and H2 and the secondary leads are labeled X1 and X2. The schematic diagram of figure 1-5 will be used to connect the three single phase transformers into a three phase wye-delta connection as shown in figure 1-7. Figure 1-6: Figure 1-7: The primary winding will be tied into a wye connection first. The schematic in figure 1-5 shows, that the H2 leads of the three primary windings are connected together, and the H1 lead of each winding is open for connection to the incoming power line. Notice in figure 1-7 that the H2 leads of the primary windings are connected together, and the H1 lead of each winding has been connected to the incoming primary power line. Figure 1-5 shows that the X1 lead of the transformer A is connected to the X2 lead of transformer c. Notice that this same connection has been made in figure 1-7. The X1 lead of transformer B is connected to X2, lead of transformer A, and the X1 lead of transformer B is connected to X2 lead of transformer A, and the X1 lead of transformer C is connected to X2 lead of transformer B. The load is connected to the points of the delta connection. Open Delta Connection: The open delta transformer connection can be made with only two transformers instead of three (figure 1-8). This connection is often used when the amount of three phase power needed is not excessive, such as a small business. It should be noted that the output power of an open delta connection is only 87% of the rated power of the two transformers. For example, assume two transformers, each having a capacity of 25 kVA, are connected in an open delta connection. The total output power of this connection is 43.5 kVA (50 kVA x 0.87 = 43.5 kVA). Figure 1-8: Open Delta Connection Another figure given for this calculation is 58%. This percentage assumes a closed delta bank containing 3 transformers. If three 25 kVA transformers were connected to form a closed delta connection, the total output would be 75 kVA (3 x 25 = 75 kVA). If one of these transformers were removed and the transformer bank operated as an open delta connection, the output power would be reduced to 58% of its original capacity of 75 kVA. The output capacity of the open delta bank is 43.5 kVA (75 kVA x .58% = 43.5 kVA). The voltage and current values of an open delta connection are computed in the same manner as a standard delta-delta connection when three transformers are employed. The voltage and current rules for a delta connection must be used when determining line and phase values of voltage current. Closing a Delta: When closing a delta system, connections should be checked for proper polarity before making the final connection and applying power. If the phase winding of one transformer is reversed, an extremely high current will flow when power is applied. Proper phasing can be checked with a voltmeter at delta opening. If power is applied to the transformer bank before the delta connection is closed, the voltmeter should indicate 0 volts. If one phase winding has been reversed, however, the voltmeter will indicate double the amount of voltage. It should be noted that a voltmeter is a high impedance device. It is not unusual for a voltmeter to indicate some amount of voltage before the delta is closed, especially if the primary has been connected as a wye and the secondary as a delta. When this is the case, the voltmeter will generally indicate close to the normal output voltage if the connection is correct and double the output voltage if the connection is incorrect. Overcurrent Protection for the Primary: Electrical Code Article 450-3(b) states that each transformer 600 volts, nominal or less, shall be protected by an individual overcurrent device on the primary side, rated or set at not more than 125% of the rated primary current of the transformer. Where the primary current of a transformer is 9 amps or more and 125% of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, the next higher standard rating shall be permitted. Where the primary current is less than 9 amps, an overcurrent device rated or set at not more than 167% of the primary current shall be permitted. Where the primary current is less than 2 amps, an overcurrent device rated or set at not more than 300% shall be permitted. Example #1: What size fuses is needed on the primary side to protect a 3 phase 480v to 208v 112.5 kVA transformer? * Important when dealing with 3 phase applications always use 1.732 (square root of 3). To solve: P / I x E 112.5 kVA X 1000 = 112500 VA 112500 VA divided by 831 (480 x 1.732) = 135.4 amps Since the transformer is more than 9 amps you have to use 125 %. 135.4 X 1.25 = 169 amps. Answer: 175 amp fuses (the next higher standard, Electrical Code 240-6). Example #2: What size breaker is needed on the primary side to protect a 3 phase 208v to 480v 3kVA transformer? To solve: P / I x E 3kVA X 1000 = 3000 VA 3000 VA divided by 360 (208 x 1.732) = 8.3 amps Since the transformer is 9 amps or less you have to use 167%. 8.3 X 1.67 = 13.8 amps Answer: 15 amp breaker (preferably a 20 amp breaker) Electrical Code Article 450-3(b)(2) states if a transformer 600 v, nominal, or less, having a an overcurrent device on the secondary side rated or set at not more than 125% of the rated secondary current of the transformer shall not be required to have an individual overcurrent device on the primary side if the primary feeder overcurrent device is rated or set at a current value not more than 250% of the rated primary current of the transformer. Overcurrent Protection for the Secondary: Electrical Code Article 450-3(b)(2) states that a transformer 600 v, nominal, or less, shall be protected by an individual overcurrent device on the secondary side, rated or set at not more than 125% of the rated secondary current of the transformer. Where the secondary current of a transformer is 9 amps or more and 125% of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, the next higher standard rating shall be permitted. Where the secondary current is less than 9 amps, an overcurrent device rated or set at not more than 167% of the secondary current shall be permitted. Example: What size breaker is needed on the secondary side to protect a 3 phase 480v/208v 112.5 kVA transformer? To solve : P / I x E 112.5 kVA x 1000 = 112500 VA 112500 divided by 360 (208 x 1.732) = 312.5 amps 312.5 X 1.25 = 390.6 amps Answer: 400 amp breaker