1、 1 Types and Construction of Transformer A Transformer is a device that changes ac electric energy at one voltage level into ac e1ectric energy at another vo1tage level through the action of a ma8netic field It consists of two or more coil of wire wrapped around a common ferromagnetic core These coi
2、ls are (usually) not directly connected The only connection between the coils is the common magnetic flux present within the core. One of the transformer windings is connected to a source of ac electric power，and the second (and perhaps third) transformer winding supplies electric power to loads The
3、 transformer winding connected to the power source is called the primary winding or input winding， and the winding connected to the loads is called the secondary winding or output winding. If there is a third winding on the transformer， it is called the tertiary winding Power transformer are constru
4、cted on one of two types of cores One type of construction consists of a simple rectagu1arlaminated piece of steel with the transformer windings wrapped around two sides of the rectangle. This type of construction is known as core form The other type consists of a three-legged laminated core with th
5、e windings wrapped around the center leg This type of construction is known as shell form. In either case， the core is constructed of thin laminations electrically isolated from each other in order to reduce eddy currents to a minimum The primary and secondary windings in a physical transformer are
6、wrapped one on top of the other with the low-voltage winding innermost Such an arrangement serves two purposes： 1.It simplifies the problem of insulating the high-voltage winding from the core 2.It results in much less leakage flux than would be the two windings were separated by a distance on the c
7、ore Power transformers are given a variety of different names, depending on their use in power systems. A transformer connected to the output of a generator and used to step its vo1tage up to transmission levels is sometimes called a unit transformer The transformer at the other end of the transmiss
8、ion line， which steps the voltage down 2 from transmission levels to distribution levels is called a substation transformer. Finally， the transformer that takes the distribution voltage and steps it down to the final vo1tage at which the power is actually used is called a distribution transformer A1
9、1 these devices are essentially the same the only difference among them is their intended use In addition to the various power transformers， two special-purpose transformers are used with electric machinery and power systems The first of these special transformers is a device specially designed to s
10、ample a high vo1tage and produce a low secondary vo1tage directly proportional to it. Such a transformer is called a potential transformer A power transformer also produces a secondary vo1tage directly proportional to its primary voltage； the difference between a potential transformer and a power tr
11、ansformer is that the potential transformer is designed to handle only a very small current. The second type of special transformer is a device designed to provide a secondary current much smaller than but directly proportional to its primary current This device is called a current transformer. The
12、Directional Protection Basis Early attempts to improve power-service reliability to loads remote from generation led to the dual-1ine concept Of course， it is possible to build two 1ines to a load， and switch the load to whichever line remains energized after a di51urbance But better service continu
13、ity will be available if both lines normally feed the load and only the faulted line is tripped when disturbances occur Fig.1-l shows a sing1e-generator，two-1ine,single-load system with breakers properly arranged to supply the load when one line is faulted For the arrangement to be effective it is n
14、ecessary to have the proper relay application Otherwise， the expensive power equipment will not be able to perform as p1anned Consider the application of instantaneous and/or time delay relays on the four breakers.Obvious1y the type of the relay cannot coordinate for a11 1ine faults For example, a f
15、ault on the line terminals of breaker D.D tripping should be faster than B, however , the condition reverses and B should be faster than D. It is evident that the relay protection engineer must find some characteristic other than time delay if relay coordination is to be achieved 3 The magnitude of
16、the fault current through breakers B and D is the same， regardless of the location of the fault on the line terminal of breaker B or D Therefore relay coordination must be based on characteristics other than a time delay that starts from the time of the fault. Observe that the direction of current f
17、lowing through either breaker B or D is a function of which line the fault is on. Thus for a fault on the line between A and B, the current flows out of the load bus through breaker B toward the fault At breaker D the current flows toward the load bus through breaker D In this case breaker B should
18、trip, but breaker D should not trip This can be accomplished by installing directional relays on breakers B and D that are connected in such a way that they will trip only when current flows through them in a direction away from the load bus. Relay coordination for the system shown in Fig 1-l can no
19、w be achieved by the installation of directional over current time delay relays on breakers B and D Breakers A and C can have non directional over current time delay relays They may also now have instantaneous relays applied The relays would be set as follows：The directional relays could be set with
20、 no intentional time delay. They will have inherent time delay The time delay over current relays on breakers A and C would have current settings that would permit them to supply backup protection for faults on the load bus and load equipment faults The instantaneous elements on breakers A and C wou
21、ld have current settings that would not permit them to detect faults on the load bus Thus the lines between the generator and the load would have high-speed protection over a considerable portion of their length It should be observed that faults on the line terminals of breakers A and C can co11apse
22、 the generator vo1tage The instantaneous relays on breakers A or C cannot clear the circuit instantaneously， because it takes time for power equipment to operate During this period there will be little or no current flow through breakers B and D Therefore， B or D cannot operate for this fault condition until the appropriate breaker at the generating station has operated This is known as sequential tripping. Usually, it is acceptable under such conditions
