Excitation System in Generator

What is Excitation System?

Excitation is the technique of creating a magnetic field using an electric current. An excitation system’s primary job is to supply direct current to the synchronous machine field winding. The excitation system regulates the field voltage and field current to carry out control and protective tasks necessary for the power system to operate satisfactorily.

• Voltage and
• Reactive power flow

can both be controlled, and system stability can be improved.

An excitation system is a sort of system that is used to supply the required field current to the synchronous machine’s rotor winding. In other terms, an excitation system is described as a system that generates flux by running current through a field coil. An excitation system’s primary requirements include reliability under all operating situations, simplicity of control, ease of maintenance, stability, and quick transient response.

The quantity of excitation necessary depends on the machine’s speed, load current, and load power factor. When the load current is high, the speed is lower, and the system’s power factor lags, the system requires more excitation.

Each alternator has its own exciter, which takes the shape of a generator, in the excitation system, which is a single unit. The bus-bar is fed by two or more exciters in the centralised excitation system. The central system is very affordable, but it has a flaw that negatively impacts the power plant’s alternators.

Generator excitation system function:

1.The primary function of a generator’s excitation system is to provide the rotor field coil with power.

2. The rotational field in the armature is excited under control by the excitation system.

Requirement for an excitation control system:

The synchronous generator and the power system are taken into account while determining the performance requirements of the excitation system.

The primary need is that the synchronous generator’s field current be supplied by and automatically adjusted by the excitation system to maintain the terminal voltage while the output varies within the generator’s continuous capabilities.

The excitation system should help with effective voltage management, system stability improvement, and modifying the generator field to improve small signal stability. The excitation system must meet the following criteria in order to satisfactorily perform the aforementioned roles:

1. Meet the required response standards.

2. Implement the necessary level of redundancy and internal fault detection and isolation capability to achieve the desired dependability and availability.

3.As applicable, perform limiting and protective functions to guard against harm to the generator, the equipment, and the device itself.

4.Secure operating flexibility in accordance with defined standards.

Components of an excitation system:

The structural layout of a typical excitation control system for a big synchronous generator.

The various subsystems shown in fig. are briefly described below:

• EXCITER: Supply dc power to the synchronous machine field winding, which serves as the excitation system’s power stage.
• REGULATOR: The exciter is controlled by processing and amplification of the input control signal to the necessary strength and form.
• POWER SYSTEM STABILIZER: Provides an additional input signal to the regulator to control and dampen oscillations in the power system.
• LIMITERS AND PROTECTIVE CIRCUITS: These comprise a broad range of control and protective functions that make sure the synchronous generator’s and exciter’s combined capacity limit is not exceeded.
• TRANSDUCER OF TERMINAL VOLTAGE AND COMPENSATOR FOR LOAD:
• detects generating voltage, filters and rectifies it to produce dc, and compares it to a reference voltage that represents the desired voltage.

Excitation System Types:

 There are 3 types of Excitation System:

1. DC Excitation System
2. AC Excitation System and
3. Static Excitation System

DC Excitation System:

• The main exciter and pilot exciter are the two different types of exciters that form the DC Excitation system.
• The automatic voltage regulator modifies the exciter output to regulate the voltage at the alternator’s output terminals.
• When the field breaker is open, the field discharge resistor is wired across the field winding. In the direct current system, these two exciters can be driven by a motor or the main shaft.
• About 400 V is the main exciter voltage rating.

• A DC commutator exciter supplies direct current to the main generator field through slip rings in the DC Excitation System.
• An amplidyne is used to control the exciter field.
• A typical dc excitation system with an amplidyne voltage regulator is simplified in the schematic.

AC Excitation System:

• A thyristor rectifier bridge and alternator that are directly coupled to the main shaft make up the AC excitation system.
• An alternating current system’s primary exciter can either be self- or separately stimulated.
• The primary excitation power for the alternator is provided by an AC excitation system.
• Rectifiers rectify the exciter’s ac output to create the direct current required for the generator field.
• The rectifier system can be grouped into:
  1. Stationery rectifier system
  2. Rotational rectifier system

Static Excitation System:

• Rectifier transformers, the SCR output stage, excitation start-up, field discharge, regulator, and operational control circuitry are all components of the static system. Since there is no rotating component in this system, there are no rotational or windage losses.
• These systems only have static or stationary components.
• Through slip rings, static rectifiers whether controlled or uncontrolled supply the excitation current to the main synchronous generator’s field.
• The main alternator’s three-phase output is transmitted to a step-down transformer in this system because it is more cost-effective to use tiny alternators with outputs under 500 MVA.
• One of the following categories describes these systems:
  1. System with Potential-source rectifiers
  2. System with compound-source rectifiers
  3. System for compound-control rectifier excitation

Application of Excitation System:

• The cutting-edge analysis of excitation systems in Application of Excitation Systems covers traditional hydro-turbines, pumped storage units, steam turbines, and nuclear power plants.

• It examines the characteristics and operation of the 700 MW hydro-turbine installed at the hydropower plant’s excitation system, as well as the operation and start-up process of the static frequency converter (SFC) of pumped storage units.

• In addition to the performance characteristics of the excitation systems of conventional 600/1000MW turbines and the excitation system of the 1000MW nuclear power unit, it elaborates on the composition of the excitation transformer, automatic excitation regulator, power rectifier and de-excitation equipment.

Some Useful Questions:

1. What is Automatic Voltage Regulator?

An electronic device called an Automatic Voltage Regulator (AVR) keeps the voltage level of electrical equipment connected to the same load constant. To produce a steady, dependable power supply, the AVR controls voltage changes.

2. What is Controlled Rectifier?

A Controlled Rectifier rectifies AC to provide a controllable DC voltage and/or current using SCRs, possibly in combination with diodes. By regulating the firing angle alpha, control is gained. These rectifiers can be used in

• DC motor controllers,
• Battery chargers, and
• Electrochemical rectifiers.