electric power generation and excitation system

8/6/2019 Electric Power Generation and Excitation System

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CHAPTER THREE:

ELECTRIC POWER GENERATION AND THE EXCITATION

SYSTEM

2.1 Introduction

The relationship between magnetism and electrical current was discovered and

documented by Oerstad in 1819. He found that if an electric current was caused to flow

through a conductor that a magnetic field was produced around that conductor.

In 1831, Michael Faraday discovered that if a conductor is moved through a magnetic

field, an electrical voltage is induced in the conductor.

The magnitude of this generated voltage is directly proportional to the strength of the

magnetic field and the rate at which the conductor crosses the magnetic field. The

induced voltage has a polarity that will oppose the change causing the induction

Lenzs law.

This natural phenomenon is known as Generator Action and is described today by

Faradays Law of Electro Magnetic Induction:

Vind = /t, where Vind = induced voltage, = change in flux density,

t = change in time All rotary generators built today use the basic principles of

Generator Action.


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2.2.0 THE BASIC GENERATOR

2.2.1 The Elementary Ac Generator

The elementary ac generator consists of a conductor (or loop of wire) in a magnetic field

(usually produced by an electromagnet). The two ends of the loop are connected to slip

rings and they are in contact with two brushes.

When the loop rotates it cuts magnetic lines of force, first in one direction and then the

other.

In the first half turn of rotation, a positive current is produced and in the second half of

rotation produces a negative current. This completes one cycle of ac generation.


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2.2.2 Development of the Sine Wave

At the instant the loop is in the vertical position, the loop sides are moving parallel to

the field and do not cut magnetic lines of force.

In this instant, there is no voltage induced in the loop. As the loop rotates, sides will cut

the magnetic lines of force inducing voltage in the loop.

When the loop is in the horizontal position, maximum voltage is induced. The rotation

of the coil through 360 degrees results in an ac sine wave output

2.2.3 Three Phase Voltage

Three phase voltage is developed using the same principles as the development of

single phase voltage.

Three (3) coils are required positioned 120 electrical degrees apart.


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A rotating magnetic field induces voltage in the coils which when aggregated produce

the familiar three phase voltage pattern.

2.3.0 TYPES OF GENERATORS

Essentially, there are two basic types of generators:

- Asynchronous (Induction) generators

- Synchronous generators

2.3.1 Induction Generators

The induction generator is nothing more than an induction motor driven above its

synchronous speed by an amount not exceeding the full load slip the unit would have as

a motor.


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Assuming a full load slip of 3%, a motor with a synchronous speed of 1200 rpm would

have a full load speed of 1164 rpm. This unit could also be driven by an external prime

mover at 1236 rpm for use as an induction generator.

The induction generator requires one additional item before it can produce power it

requires a source of leading VARs for excitation. The VARs may be supplied by

capacitors (this requires complex control) or from the utility grid.

Induction generators are inexpensive and simple machines, however, they offer little

control over their output. The induction generator requires no separate DC excitation,

regulator controls, frequency control or governor.

2.3.2 Synchronous Generators

Synchronous generators are used because they offer precise control of voltage,

frequency, VARs and WATTs.

This control is achieved through the use of voltage regulators and governors.

A synchronous machine consists of a stationary armature winding (stator) with many

wires connected in series or parallel to obtain the desired terminal voltage.

The armature winding is placed into a slotted laminated steel core. A synchronousmachine also consists of a revolving DC field - the rotor.

A mutual flux developed across the air gap between the rotor and stator causes the

interaction necessary to produce an EMF. As the magnetic flux developed by the DC

field poles crosses the air gap of the stator windings, a sinusoidal voltage is developed

at the generator output terminals. This process is called electromagnetic induction.

The magnitude of the AC voltage generated is controlled by the amount of DC exciting

current supplied to the field.

If FIXED excitation were applied, the voltage magnitude would be controlled by the

speed of the rotor (E=4.44fnBA), however, this would necessitate a changing frequency!


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Since the frequency component of the power system is to be held constant, solid state

voltage regulators or static exciters are commonly used to control the field current and

thereby accurately control generator terminal voltage.

The frequency of the voltage developed by the generator depends on the speed of the

rotor and the number of field poles. For a 60 Hz system, Frequency = speed(rpm)*pole

pairs/60.

The synchronous generator units at Nalubaale power station

2.4.0 GENERATOR PARTS AND FUNCTIONS

Generator Frame: Provides the structural strength and rigidity for the generator and

serves as a housing to guide cooling air flow

Inner End Shield: Is a baffle used to form a path for cooled air

Generator Fan: Provides continuous circulation of cooling air

Rotating Field: A magnetic field which induces AC voltage in the stator windings

Collector Rings: Provide a connection and path for DC power into the rotating field

windings


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Main Coupling: Is the connection to the drive shaft Generator Coolers: Remove heat

from the generator cooling air

Stator Core: Houses the stationary windings and forms a magnetic path necessary for

induced voltages

Air Gap: Is the radial clearance between the rotating field and the stator core

Stator Coil End Turns: Formed when coils leave one slot in a stator core and are returned

to a different slot

Terminal Leads: Serve to conduct the three phase voltage and current flow from the

generator stator to the external system

An out put stator terminal for one phase


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2.5.0 THE EXCITATION SYSTEM

In power plant excitation systems has a powerful impact on generator dynamic

performance and availability, it ensures quality of generator voltage and reactive power,

i.e. quality of power plant delivered energy to consumers.

Main functions of the excitation system are to provide variable DC current with short-

time overload capability, controlling terminal voltage with suitable accuracy, ensurestable operation with network and/or other machines, contribution to transient stability

subsequent to a fault, communicate with the power plant control system and to keep

machine within permissible operating range.


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2.5.1.0 Types of Excitation Systems

Brushless excitation systemsWith rotating exciter machines and Automatic Voltage Regulator (AVR).

Static excitation systems (SES)This involves feeding rotor directly from thyristor bridges via brushes.

Among the above types of exciters, Static excitation system plays a very important role

in modern interconnected power system operation due to its fast acting, good response

in voltage & reactive power control and satisfactory steady state stability condition.

For the machines 500 MW & above and fire hazards areas, Brushless Excitation System is

preferred due to larger requirement of current & plant safety respectively.

2.5.1.1 Static Excitation System

In order to maintain system stability in interconnected system network it is necessary to

have fast acting excitation system for large synchronous machines which means the field

current must be adjusted extremely fast to the changing operational conditions.

Besides maintaining the field current and steady state stability the excitation system is

required to extend the stability limits. It is because of these reasons the static excitation

system is preferred to conventional excitation systems.

In this system, the AC power is tapped off from the generator terminal stepped down

and rectified by fully controlled thyristor Bridges and then fed to the generator field

thereby controlling the generator voltage output. A high control speed is achieved by

using an internal free control and power electronic system. Any deviation in the

generator terminal voltage is sensed by an error detector and causes the voltage

regulator to advance or retard the firing angle of the thyristors thereby controlling the

field excitation of the alternator.

This equipment controls the generator terminal voltage, and hence the reactive load

flow by adjusting the excitation current. The rotating exciter is dispensed with and

Transformer & silicon controlled rectifiers (SCRS) are used which directly feed the field

of the Alternator.


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Static exciter panel

2.5.1.2 Description of Static Excitation System

Static Excitation Equipment Consist of;

i. Excitation (Rectifier) TransformerThe excitation power is taken from generator output and fed through the excitation

(rectifier) transformer which steps down to the required voltage, for the SCR bridge and

then fed through the field breaker to the generator field. The rectifier transformer used

in the SEE should have high reliability as failure of this will cause shutdown of nit/power

station.

Dry type cast coil transformer is suitable for static excitation applications. The

transformer is selected such that it supplies rated excitation current at rated voltage

continuously and is capable of supplying ceiling current at the ceiling excitation for a

short period of ten seconds.

Rectifier transformers directly connected to the generator terminals and feeding power

to the field of the machine via thyristor converters, plays an important role in an

excitation system and in turn power generation Reliability of this transformer has to be

ensured in all respects. The selection of the secondary voltage of excitation transformer

depends upon the field forcing voltage. The primary voltage is the same as that of

generator terminal voltage.


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Current rating is dependant on the maximum continuous current in the field winding.

Generally the power rating of the Excitation transformer used in Static Excitation System

is around 1 % of the rating of generator in MVA.

These transformers are of class "F" insulation and indoor type. The cooling method of

the autotransformer is Air Natural (AN).

Excitation transformer

ii. SCR Output stageThe SCR output stage consists of a suitable number of bridges connected in parallel.

Each thyristor bridge comprises of six thyristors, working as a six pulse fully controlled

bridge. Current carrying capacity of each bridge depends on the rating of individual

thyristor. Thyristors are designed such that their junction temperature rise is well within

its specified rating. By changing the firing angle of the thyristors variable output is

obtained. Each bridge is controlled by one final pulse stage and is cooled by a fan.

These bridges are equipped with protection devices and failure of one bridge causes

alarm. If there is a failure of one more thyristor bridges then the excitation current will

be limited to a predetermined value lesser than the normal current. However, failure of

the third, bridge results in tripping and rapid de-excitation of the generator. The above

is applicable for 4 bridges thyristor with (n-1) principle operation.


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The thyristor bridge or power converter

iii. Excitation Start Up and Field Discharge EquipmentFor the initial build-up of the generator voltage, field-flashing equipment is required.

The rating of this equipment depends on the no-load excitation requirement and field

time constant of the generator. From the reliability point of view, provisions for both

the AC & DC field flashing are provided.

The field breaker is selected such that it carries the full load excitation current

continuously and also it breaks the max. field current when the three phase short circuit

occurs at the generator terminals.

The field discharge resistor is normally of non-linear type for medium and large capacity

machines i.e. voltage dependent resistor.


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To protect the field winding of the generator against over voltages, an over voltage

protection along with a current limiting resistor is used to limit the over voltage across

the field winding. The OVP operates on the insulation break over Principle. The voltage

level at which OVP should operate is selected based on insulation level of field winding

of the generator.

Field Discharge

During load condition whenever the Field breaker, opens suddenly there will be a surge

voltage in the rotor which will damage the rotor winding insulation. To avoid this rotor

winding is connected to the earth through field discharge Resistor thereby by passing

the surge voltage to earth and limiting the current to earth. Field discharge greatly

helps to limit the damages. 'Non-linear field discharge resistance is used which helps in

faster field suppression/discharge.

The field discharge resistor


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iv. Regulator & Operational Control Circuits (Control Electronics)Regulator is the heart of the system. This regulates the generator voltage by controlling

the firing pulses to the thyristors.

Error detector & amplifierThe Generator terminal voltage is stepped down by a three phase PT. and fed to the

AVR. The a.c. input thus obtained is rectified, filtered and compared against a highly

stabilized reference value and the difference is amplified in different stages of

amplification.

The AVR is designed with highly stable elements so that variation in ambient

temperature does not cause any drift or change in the output level. Three CTs sensing

the output current of the generator feed proportional current across variable resistors in

the AVR. The voltage thus obtained across the resistors, can be added vectorially eitherfor compounding or for transformer drop compensation. The percentage of

compensation can be adjusted as the resistors are of variable type.

Grid - control unitThe output of the AVR is fed to a grid control unit, it gets its synchronous a.c. reference

through a filter circuit and generates six double, pulses spaced 600 electrical apart

whose position depends on the output of the AVR, i.e. the pulse position varies

continuously as a function of the control voltage. Two relays are provided, by energizing

which, the pulses can be either blocked completely or shifted to inverter mode of

operation

Pulse - amplifierThe pulse output of the ""Grid control unit "' is amplified further at an intermediate

stage amplification. This is also known as pulse intermediate stage. The unit has a d.c.

power supply, which operates from a three phase 38OV supply and delivers

+15V,1 l5V,+5V, and a coarse stabilized voltage VL. A built in relay is provided which

can be used for blocking the 6 pulse channels. In a two channel system (like Auto and

Manual), the change over is effected by energising/ de-energizing the relay.

Pulse final stageThis unit receives input pulses from the pulse amplifier and transmits them through

pulse transformers to the gates of the thyristors. A built in power supply provides the

required d.c. supply to the final pulse and amplifier. Each Thyristor bridge has its own


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final pulse stage. Therefore, even if a thyristor bridge fails with its final pulse stage, the

remaining thyristors bridges can continue to cater to full load requirement of the

machine and thereby ensure (n-1) operation.

Manual control channelA separate manual control channel is provided where the controlling d.c. signal in taken

from a stabilized d.c. voltage through a motor operated potentiometer. The d.c. signal

is fed to a separate grid control unit whose output pulses after being amplified at an

intermediate stage can be fed to the final pulse stage. When one channel is working,

generating the required pulses, the other remains blocked. Therefore a changeover from

""Auto" to "Manual' control or vice versa is effected by blocking or releasing the pulses

of the corresponding intermediate stage.

"A pulse supervision unit detects spurious pulses or loss of pulses at the pulses bus bar

and transfers control from Automatic Channel to manual channel.

Follow-up unitTo ensure a smooth changeover from 'Auto"' to Manual" control, it is necessary that the

position of the pulses on both channels should be identical. A pulse comparison unit

detects any difference in the position of the pulses and with the help of a follow-

up unit actuates the motor operated potentiometer on the "'Manual"' Channel to turn in

a direction so as to eliminate the difference.

However, while transferring control from "Manual"' to "Auto" mode any difference in thetwo control levels can be visually checked on a balance meter and adjusted to obtain

null before change over.

Limit controllersWhen a generator is running in parallel with the power network, it is essential to

maintain it in sychronism without exceeding the rating of the machine and also without

the protection system tripping. Only automatic Regulator cannot ensure this. It is

necessary to influence the voltage regulator by suitable means to limit the over

excitation and under excitation. This not only improves the security of the paralleloperation but makes operation of the system easier. However limiters do not replace

the protection system but only prevent the protection system from tripping

unnecessarily under extreme transient conditions.

The AVR also has a built-in frequency dependent circuit so that when the machine is

running below the rated frequency from the regulated voltage should be proportional


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to frequency. With the help of a potentiometer provided in the AVR, the circuit can be

made to respond proportionally to voltage above a certain frequency and proportional

to a voltage below the certain frequency. The range of adjustment of this cut off

frequency lies between 40 and 60 Hz.

The static excitation system is equipped with three limiters which act in conjunction with

the AVR. These limiters are as under;

-Rotor current limiter

The unit basically comprises an actual value converter a limiter with adjustable PID

characteristics a reference value; dv/dt sensor and a signalisation unit. The field current

is measured on the a.c. input side of the thyristor converter and is converted into

proportional d.c. voitages. The signal is compared with an adjustable reference value,

amplified, and with necessary time lapse fed to the voltage regulator input.Rotor current limiter avoids thermal overloading of the rotor winding and is provided to

protect the generator rotor against excessively long duration over loads. The ceiling

excitation is limited to a predetermined limit and is allowed to flow for a time which is

dependant upon the rate of rise of field current before being limited to the thermal limit

value.

-Rotor angle limiter

This unit limits the angle between the voltage of the network centre and the rotor

voltage or it limits the angle between the generator voltage and the rotor voltage. Itcomprises an actual value converter, a limiting amplifier with adjustable PID

characteristics and a reference value unit. The limiting regulator operates as soon as the

d.c. value exceeds the reference value. For its operation the Unit is given separate power

supply from a d.c. power pack.

It generates a d.c. signal proportional to the load or rotor angle from the stator current

and voltage by means of a simple analog circuit. The device takes over as soon as the

set limit angle is exceeded. By increasing the excitation and ignoring opposite control

signals the unit is prevented from failing out of step.

-Stator current limiter

This unit functions in conjunction with an integrator unit which provides the necessary

dead time and the gradient, which can be adjusted by potentiometers. The regulator

consists essentially of a measuring converter, two comparators, two PID regulators and a

d.c. power pack. A discriminator in the circuit differentiates between inductive and


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capacitive current. The positive and negative signals processed by two separate

amplifiers are brought to the output stage and only that output which has to take care

of the limitation is made effective.

Stator current limiter avoids thermal over loading of the stator windings. Stator current

limiter is provided to protect the generator against long duration of large statorcurrents. For excessive inductive current it acts over the AVR after a certain time lag and

decreases the excitation current to limit the inductive current to the limit value. But for

excessive capacitive current it acts on the AVR without time delay to increase the

Excitation and thereby reduce the capacitive loading. This is necessary as there is a risk

for the machine failing out of step during under excited mode of operation.

-Slip stabilizing units

The slip stabilizing unit is used for the suppression of rotor oscillations of the alternator

through the additional influence of excitation. The slip as well as acceleration signals

needed for the stabilization are derived from active power delivered by the alternator.

Both the signals, which are correspondingly amplified and summed up, influence

the excitation of the synchronous machine through AVR in a manner as to suppress the

Rotor oscillations.

2.5.1.3 Merit of Static Excitation System

Performance

1) Faster Response in voltage & Reactive Power Control

2) Higher Accuracies of Voltage control

3) Faster discharge of field energy with inversion of thyristor bridge output

4) Provision of limiters & stabilizing equipment help to improve dynamic & transient

stability

5) Largely independent of variation in the excitation supply voltage & frequency

Operational Maintenance

1) Redundancies in different Circuits increase reliability and availability

2) Adequate monitoring facilities aid fault finding & reduce down time


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3) Absence of rotating parts enables less maintenance

4) Lower SCR value for larger generators (Reduces weight/compact in size & cost)

General

1) Up rating of the machine can be done by adding additional power circuits/adding

more redundancies

2) Location of the equipment can be planned independent of the machine

thereby increasing the flexibility in the plant layout

3) Retrofitting of Static Excitation Equipment for old slow acting exciter

4) Length of the machine shorter when compare to other excitation equipment

2.5.1.4 Power Supply

The voltage regulating equipment needs an a.c. supply 38OV 3 Phase for its power

supply units which is derived from the secondary side of the rectifier transformer

through an auxiliary transformer. This voltage is reduced to different levels required for

the power packs by means of multi-winding transformers.

A separate transformer supplies the synchronous voltage 3x38OV for the filter circuit ofeach channel and the voltage relay. During testing and pre-commissioning activities

when generator voltage is not available, the station auxiliary supply 3 Phase 415V can be

temporarily connected through an. auxiliary step down transformer for testing purpose

with the help of a regulator test/service switch.

The supply for the, thyristor Bridge fan is taken from an independent transformer which

gets it input supply from the secondary of the excitation transformer. The control &

protection relays need 48V & 24VDC which are delivered from the station battery by

means of the DC/DC converters, which are internally protected against overload.


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2.5.1.5 PROTECTIONS

The following protections are provided in the Static Excitation Equipment;

1) Rectifier transformer over current instantaneous and delayed.

2) Rectifier transformer over Temperature

3) Rotor Over-Voltage

4) Rotor earth fault.

5) Fuse failure monitoring circuit for thyristors

6) Loss of control voltage (48V & 24V)

7) dv/dt protection of SCR by snubber net works

8) Cooling System failure for thyristors


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electric power generation and excitation system

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