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23.1 Goals of the lesson
In this lesson, we shall study two winding ideal transformer, its properties and working principle under no load condition as well as under load condition. Induced voltages in primary and secondary are obtained, clearly identifying the factors on which they depend upon. The ratio between the primary and secondary voltages are shown to depend on ratio of turns of the two windings. At the end, how to draw phasor diagram under no load and load conditions, are explained. Importance of studying such a transformer will be highlighted. At the end, several objective type and numerical problems have been given for solving. Key Words: Magnetising current, HV & LV windings, no load phasor diagram, reflected current, equivalent circuit. After going through this section students will be able to understand the following.
1. necessity of transformers in power system.
2. properties of an ideal transformer.
3. meaning of load and no load operation.
4. basic working principle of operation under no load condition.
5. no load operation and phasor diagram under no load.
6. the factors on which the primary and secondary induced voltages depend.
7. fundamental relations between primary and secondary voltages.
8. the factors on which peak flux in the core depend.
9. the factors which decides the magnitude of the magnetizing current.
10. What does loading of a transformer means?
11. What is reflected current and when does it flow in the primary?
12. Why does VA (or kVA) remain same on both the sides?
13. What impedance does the supply see when a given impedance Z2 is connected across the secondary?
14. Equivalent circuit of ideal transformer referred to different sides.
23.2 Introduction
Transformers are one of the most important components of any power system. It basically changes the level of voltages from one value to the other at constant frequency. Being a static machine the efficiency of a transformer could be as high as 99%. Big generating stations are located at hundreds or more km away from the load center (where the power will be actually consumed). Long transmission lines carry the power to the load centre from the generating stations. Generator is a rotating machines and the level of voltage at which it generates power is limited to several kilo volts only a typical value is 11 kV. To transmit large amount of power (several thousands of mega watts) at this voltage level means large amount of current has to flow through the transmission lines. The cross sectional area of the conductor of the lines accordingly should be large. Hence cost involved in transmitting a given amount of power rises many folds. Not only that, the transmission lines has their own resistances. This huge amount of current will cause tremendous amount of power loss or I 2 r loss in the lines. This loss will simply heat the lines and becomes a wasteful energy. In other words, efficiency of transmission becomes poor and cost involved is high. The above problems may addressed if we could transmit power at a very high voltage say, at 200 kV or 400 kV or even higher at 800 kV. But as pointed out earlier, a generator is incapable of generating voltage at these level due to its own practical limitation. The solution to this problem is to use an appropriate step-up transformer at the generating station to bring the transmission voltage level at the desired value as depicted in figure 23.1 where for simplicity single phase system is shown to understand the basic idea. Obviously when power reaches the load centre, one has to step down the voltage to suitable and safe values by using transformers. Thus transformers are an integral part in any modern power system. Transformers are located in places called substations. In cities or towns you must have noticed transformers are installed on poles – these are called pole mounted distribution transformers. These type of transformers change voltage level typically from 3-phase, 6 kV to 3-phase 440 V line to line.
In this and the following lessons we shall study the basic principle of operation and performance evaluation based on equivalent circuit.
23.2.1 Principle of operation
A transformer in its simplest form will consist of a rectangular laminated magnetic structure on which two coils of different number of turns are wound as shown in Figure 23.2. The winding to which a.c voltage is impressed is called the primary of the transformer and the winding across which the load is connected is called the secondary of the transformer.