Sunday, 5 November 2017

Static Excitation System - Working Principle

Excitation system in a Generator is essential for the production of working magnetic flux in the air gap. It is usually provided by having filed winding on the rotor in case of Synchronous Generator. Providing field winding on the rotor possess certain advantages over field winding on the stator (Read Why Armature Winding on Stator in Synchronous Machine?). It shall be noted at this point that DC current flows in the field winding to create working magnetic flux. Thus for any excitation system, the main aim shall be to flow DC current in the field winding. Static Excitation system is one such method.



Principle of Static Excitation System


In Static Excitation System, power for providing field excitation is derived from the Generator output terminals. A transformer know as Excitation Transformer, is connected to the output terminals of Generator to step down the voltage to required voltage level usually 415 V AC. As we need DC supply, therefore Transformer output is connected to a Thyristor Full Bridge Rectifier. Figure below shows simplified block diagram of Static Excitation System.

Static-Excitation-System


The firing angle of Thyristor Full Bridge Rectifier is controlled by a Regulator so that required field excitation may be provided. Secondary terminal of CT and PT connected to Generator output terminals is fed to regulator. On the basis of Generator terminal voltage, the regulator adjusts its firing angle. Let’s say, Generator output voltage has increased beyond its rated voltage of 21 kV (say), in that case, field current must be reduced to maintain the terminal voltage. Therefore, regulator increases the firing angle so that average value of DC current may reduce. Similarly, if the Generator terminal voltage goes below its rated value then field current must be increased. Therefore regulator decreases the firing angle to increase the average value of the field current.

As in Static Excitation System, excitation is provided by field winding wound on the rotor therefore Slip Rings and Carbon brushes are used.

How to provided Field Excitation during Startup of Generator?


As in Static Excitation System, field excitation power is derived from the Generator output terminals therefore it can only work during the normal and steady operation of Generator. Suppose we are going to start a Generator, in that case it is not possible to have field excitation using static excitation system as there is no Generator output terminal voltage. In such case, excitation power shall be provided using separate source. As shown in figure above, this is normally made available using a Battery Bank. As soon as the Generator reaches its rated speed, its terminal voltage reaches to rated voltage and hence Static Excitation System comes in picture. Thus as soon as Generator reaches its rated speed, Battery Bank is isolated and excitation power is fed by Static Excitation System.

Advantages of Static Excitation System

  • The excitation system with the use of reliable and high power thyristor, is simple in design and provides fast response characteristics as needed in modern power system.
  • As there is no separate rotating type exciter, the system is free from friction, windage and commutator loss occurring in the exciter.
  • Since excitation energy is directly taken from generator output terminals therefore excitation voltage is directly proportional to the Generator speed. This improves the overall system performance.
But the due to use of slip ring and carbon brushes, sparking and contact resistance loss take place in this system of excitation. To eliminate this, Brushless Excitation System is used these days.

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