What is an Excitation System : Types and Its Elements

The first excitation system is developed in 1971 by Kinte Industrial Co. Ltd. Some of the excitation systems and exciter suppliers are Acoustical surfaces, Spincore Technologies, Mitsubishi Electric Power Products, DirectMed Parts, Basler Electric Co., etc. This system is used to provide dc supply or DC to the synchronous machines. The dc exciters, ac exciters, signal sensing or processing circuits, electronic amplifiers, rectifiers, and the excitation system stabilization feedback circuits are the basic elements of the different excitation systems. In this article, the different types of excitation systems, elements, advantages, and disadvantages are explained.

What is an Excitation System?

Definition: The system which provides DC to the synchronous machine field winding to perform protective & control functions of the power system. This system consists of exciter, PSS (Power System Stabilizer), AVR (Automatic Voltage Regulator), processing unit, and measuring elements. The current provided by this system is excitation current. This system input values are obtained by using the measuring elements, for the field winding of generator exciter is the source of electrical power and the autonomic voltage regulator circuit performs controlling the exciter current, the PSS stabilizer is used to produce additional signals in control loop.

Types of Excitation System

The classification of the excitation system is shown in the below figure.

DC Excitation System

The DC (Direct Current) system consists of two types of exciters they are main exciter and pilot exciter. The exciter output is adjusted by the automatic voltage regulator to control the alternator output terminal voltage. Across the field winding, the field discharge resistor is connected when the field breaker is open. These two exciters in the direct current system can be driven either by motor or by the main shaft. The main exciter voltage rating is about 400 V. The DC system figure is shown below.

Advantages

The advantages of the DC system are

• More reliable
• Compact in size

Disadvantages

The disadvantages of the DC system are

• Large size
• Voltage regulation was complex
• Very slow response

AC Excitation System

The AC (Alternating Current) system consists of a thyristor rectifier bridge and alternator which are connected directly to the main shaft. The main exciter in an alternating current system is either be separated excited or self-excited. This system is classified into two types they are rotor system or rotating thyristor system. The classification of the ac system is shown in the below figure.

Rotating Thyristor System

The rotating thyristor or rotor system figure is shown below. The rotating portion of this consists of alternator field rectifier, a rectifier circuit, power supply, and an alternating current exciter or AC exciter. The controlled triggering signal is generated by the power supply and rectifier control.

Advantages

The advantages of the rotating thyristor system are

• Fast response
• Simple
• Low cost

Disadvantages

The main disadvantage is the response rate of the thyristor is very low

Brushless System

The stator and rotor are the main components of the brushless alternator system. The stator body consists of the main stator and an exciter stator similarly the rotor assembly consists of the main rotor and the exciter rotor along with a bridge rectifier assembly mounted on a plate that is attached to the rotor.

The exciter stator has residual magnetism when the rotor starts rotating AC (Alternating Current) output is generated in the exciter rotor coils and this output is passed through a bridge rectifier. The output passed through a bridge rectifier is converted into DC (Direct Current) and given to the main rotor. The moving main rotor generates AC in the stationary main rotor coils.

The exciter plays a key role in controlling the output of the alternator. The DC magnetization current supplied to the rotor, which is the field of the main alternator thus if we increase or decrease the amount of current to the stationary exciter field coils, the output of the main alternator can be varied. The brushless system is shown in the below figure.

To the synchronous generator, the brushless system provides field current without using the slip ring and carbon brushes. The brushless exciter system coupled with a rotor shaft with 16 PMG (Permanent Magnet Exciter) and a three-phase main exciter with a silicon diode rectifier. The permanent magnet exciter produces 400 Hz, 220 V AC supply.

The alternator main rotor shaft coupled to the brushless exciter circuit through no brushes, no slip rings, and through rotor leads. The main output of exciter is connected to the SCR bridge in hallow shaft while permanent magnet exciter and the main exciter are connected to the solid shaft.

Advantages

The advantages of the brushless system are

• Reliability is excellent
• The flexibility of operation is good
• System responses are good
• There is no moving contact in the brushless system, so maintenance is low

Disadvantages

The disadvantages of the brushless system are

• Response is slow
• There is no fast de-excitation

Static System

This system consists of rectifier transformers, SCR output stage, excitation start-up, and field discharge equipment, and regulator and operational control circuits. In this system, there is no rotating part, so there is no windage losses and no rotational losses. In this system, the three-phase output of the main alternator is transferred to step down transformer and the system is cheaper in small alternator below 500 MVA. The static system is shown in the below figure.

Advantages

The advantages of the static system are

• Reliability is good
• The flexibility of operation is very good
• System responses are excellent
• Small in size
• Low loss
• Simple
• High performance

Disadvantages

The main disadvantages of the static system are, it requires a slip ring and brush

Elements and Signals of Excitation System

The general block diagram for the synchronous machine control system is shown in the below figure. The figure consists of five blocks they are control elements block, exciter block, terminal voltage transducer, and load compensator, synchronous machine and power system, and power system stabilizer and supplementary discontinuous excitation control.

Where EFD is the synchronous machine field voltage or exciter output voltage, IFD synchronous machine field current or is the exciter output current, IT is the synchronous machine terminal current phasor, VC is the terminal voltage transducer output, VOEL is the overexcitation limiter output, VR is the voltage regulator output, VS is the power system stabilizer output, VSI is the power system stabilizer input, VREF is the voltage regulator reference voltage, and VUEL is the under excitation limiter output.

FAQs

1). What is the excitation voltage?

It is an amount of voltage required to excite field coil and the voltage varies by the rectifier control. The alternating voltage and direct voltage are the two types of excitation voltage.

2). Why DC is used for excitation?

The electric current is produced only when the wire rotates in a constant magnetic field obtained by only direct current (DC) voltage, so dc voltage is applied to a coil to get the constant magnetic field.

3). Why do generators need excitation?

The excitation is needed for the generator to create a magnetic field and to provide a constant or fixed or stationary rotating magnetic field.

4). What happens when generators loss excitation?

The rotor current decreases when the generator loss excitation and by the field time constant the field voltage decays as well.

5). Why do we need an excitation system for alternators?

This system is needed for an alternator to control the voltage and reactive power of the synchronous alternator or generator.

In this article, the different types of excitation systems, advantages, and disadvantages of the system are discussed. Here is a question for you what is the pilot exciter in the dc excitation system?