UNIT-III

MODELLING OF EXCITATION AND SPEED

GOVERNING SYSTEM
Elements of 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.
Elements Of Excitation System
• Exciter:
It provides dc power to the synchronous machine field winding constituting the
power stage of the excitation system.

• Regulator:
• It Process and amplifies input control signals to a level and form appropriate for
control of the exciter.

• This includes both regulating and excitation system stabilizing function.

• Terminal voltage transducer and load compensator

• It Senses generator terminal voltage, rectifier and filters it to dc quantity, and

compares it with a reference which represents the desired terminal voltage.
• Power system stabilizer:
It provides an additional input signal to the regulator to damp power system
oscillation
 Types of Excitation System
• The classification of the excitation system is shown in the below figure.
DC Excitation System
• The DC excitation system has two exciters – the main exciter and a pilot exciter.

• The exciter output is adjusted by an automatic voltage regulator (AVR) for controlling
the output terminal voltage of the alternator.

• The current transformer input to the AVR ensures limiting of the alternator current
during a fault.

• When the field breaker is open, the field discharge resistor is connected across the
field winding so as to dissipate the stored energy in the field winding which is highly
inductive.

• The main and the pilot exciters can be driven either by the main shaft or separately
driven by the motor.

• Direct driven exciters are usually preferred as these preserve the unit system of
operation, and the excitation is not excited by external disturbances.

• The voltage rating of the main exciter is about 400 V, and its capacity is about 0.5% of
the capacity of the alternator.

• Troubles in the exciters of turbo alternator are quite frequent because of their high
speed and as such separate motor driven exciters are provided as standby exciter.
• Advantages
The advantages of the DC system are

• More reliable

• Compact in size

• Disadvantages
The disadvantages of the DC system are

• 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 rotor excitation system is shown in the figure below.
• The rotating portion is being enclosed by the dashed line.
• This system consists an AC exciter, stationary field and a rotating armature.
• The output of the exciter is rectified by a full wave thyristor bridge rectifier
circuit and is supplied to the main alternator field winding.
• The alternator field winding is also supplied through another rectifier
circuit.
• The exciter voltage can be built up by using it residual flux.
• The power supply and rectifier control generate the controlled triggering
signal.
• The alternator voltage signal is averaged and compare directly with the
operator voltage adjustment in the auto mode of operation.
• In the manual mode of operation, the excitation current of the alternator is
compared with a separate manual voltage adjustment.
Brushless Excitation System
 Brushless Excitation System
• This system is shown in the figure below.
• The rotating portion being enclosed by a dashed line rectangle.
• The brushless excitation system consists an alternator, rectifier, main exciter
and a permanent magnet generator alternator.
• The main and the pilot exciter are driven by the main shaft.
• The main exciter has a stationary field and a rotating armature directly
connected, through the silicon rectifiers to the field of the main alternators.
• The pilot exciter is the shaft driven permanent magnet generator having
rotating permanent magnets attached to the shaft and a three phase stationary
armature, which feeds the main exciter field through silicon rectifiers, in the
field of the main alternator.
• The pilot exciter is a shaft driven permanent magnetic generator having
rotating permanent magnets attached to the shaft and a 3-phase stationary
armature, which feeds the main’s exciter through 3-phase full wave phase
controlled thyristor bridges.
• The system eliminates the use of a commutator, collector and
brushes have a short time constant and a response time of fewer
than 0.1 seconds.
• The short time constant has the advantage in improved small signal
dynamic performance and facilitates the application of
supplementary power system stabilising signals.
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 Excitation System
• In this system, the supply is taken from the alternator itself through a 3-
phase star/delta connected step-down transformer.

• The primary of the transformer is connected to the alternator bus and their
secondary supplies power to the rectifier and also feed power to the grid
control circuit and other electrical equipment.

• This system has a very small response time and provides excellent dynamic
performance.

• This system reduced the operating cost by eliminating the exciter windage
loss and winding maintenance.
Advantages
• 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.
Functional Block Diagram of Power Generation
and Control
• The voltage of the generator is proportional to the speed and excitation (flux) of
the generator. The speed being constant, the excitation is used to control the
voltage. Therefore, the voltage control system is also called as excitation control
system or automatic voltage regulator (AVR).

• For the alternators, the excitation is provided by a device (another machine or a

static device) called exciter.
• For a large alternator the exciter may be required to supply a field current of as large
as 6500A at 500V and hence the exciter is a fairly large machine. Depending on the
way the dc supply is given to the field winding of the alternator (which is on the
rotor), the exciters are classified as: i) DC Exciters; ii) AC Exciters; and iii) Static
Exciters. Accordingly, several standard block diagrams are developed by the IEEE
working group to represent the excitation system. A schematic of an excitation
control system is shown in Fig2.1.

• A simplified block diagram of the generator voltage control system is shown in Fig.
The generator terminal voltage Vt is compared with a voltage reference Vref to
obtain a voltage error signal ∆V. This signal is applied to the voltage regulator shown
as a block with transfer function KA/(1+TAs).
• The output of the regulator is then applied to exciter shown with a block of transfer
function Ke/(1+Tes). The output of the exciter Efd is then applied to the field
winding which adjusts the generator terminal voltage. The generator field can be
represented by a block with a transfer function KF/(1+sTF). The total transfer
function is

The stabilizing compensator shown in the diagram is used to improve the dynamic
response of the exciter.
The input to this block is the exciter voltage and the output is a stabilizing feedback
signal to reduce the excessive overshoot.
Schematic of a hydroelectric plant

• In a hydroelectric generating station, the potential energy and quantum of water are
utilized to generate electrical power. In other words, hydroelectric schemes
function on flow of water and difference in level of water known as head.

• Due to the difference in head, considerable velocity is imparted to the water,

which is used to drive a hydro turbine. This hydro turbine acts as a prime mover
and is coupled to an electric generator to produce electrical energy.

• Hydroelectric stations depend on the availability of a head of water. As such they

are often sited in mountainous terrain and require long transmission lines to deliver
power to the load centers.
Hydroelectric schemes are classified on the basis of the head utilized to generate
power: high head storage type, medium head pondage type, and run-of-river. In
low head type of hydro generators, both the velocity of water and difference in
levels are used to rotate the turbine. In high head generators, the difference in
levels is used to impart high velocity to the water to run the turbine.
• As the name suggests, in the run-of-river hydro generators, the natural flow of
river water is used to drive the turbines. Figure 1.7 shows a schematic diagram of
the high head storage type hydroelectric scheme.

• The merits of a hydroelectric station are as follows: Minimal operational costs

(since there is no fuel cost involved) 0 No air pollution No waste products 0
Minimal maintenance Quick start-up time (within five minutes) Long life
(minimum fifty years)

• The demerits of a hydroelectric station are as follows:

• High capital costs

• Long gestation period

• Ecological damage to the region

 Governor for Hydraulic Turbine
• Governing system or governor is the main controller of the hydraulic turbine. The

governor varies the water flow through the turbine to control its speed or power

output. Generating units speed and system frequency may be adjusted by the governor.

• The primary functions of the hydraulic turbine governor are as follows:

i) To start, maintain and adjust unit speed for synchronizing with the running

units/grid.

• ii) To maintain system frequency after synchronization by adjusting turbine output to

load changes.

• iii) To share load changes with the other units in a planned manner in response to

system frequency error.

iv) To adjust output of the unit in response to operator or other supervisory
commands.
v) To perform normal shut down or emergency over speed shut down for protection.
In isolated systems the governor controls frequency. In large system it may be needed
for load operation control for the system. A block diagram is shown in figure. Digital
electronic load governor are now employed.
electrical analogue of hydraulic turbine
• Next came the third generation Electro-Hydraulic Governors where speed sensing,

speed/output setting and stabilizing parameters were controlled electrically and the

use of mechanical components was reduced considerably.

• They increased the reliability, stability and life of the equipment and facilitated

more functional requirements. The design of electrical part of the governors kept

changing based on the advancement in electronics and development work by

individual manufacturers. In this type of governor analogue circuitry is used to

develop set point signal that is used to position the control actuators of

hydroelectric units. An electro hydraulic interface is used to connect the electronic

set point signal into a hydraulic oil flow from a hydraulic servo valve system

which determines the position of the turbine control actuators.

Reheat type
There are mainly two configurations and they are:
1.Tandem compound system configuration.
2. Cross-compound system configuration.

These two configurations are further classified into the following types:
1. Tandem compound, single reheat type.
2.Tandem compound, double reheat type.
3. Cross-compound, singlereheat type with two low-pressure (LP) turbines.
4. Cross-compound, singlereheat type with single LP turbine.
5. Cross-compound,
double reheat type.
Tandem compound single reheat system
• All compound steam turbines use governor-controlled valves, at the inlet to
the HP turbine, to control the steam flow. The steam chest, re heater, andcross-
over piping introduce delays. These time delays are represented by

τ = Steam-chest time constant (from 0.1to 0.4s) CH

τ =Reheat time constant (from 4 to11s)

τ =Cross-over time constant (from 0.3 to 0.5s) CO

The fractions of total turbine power are represented by:

F = Fraction of HP turbine power (typical value is HP)

F = Fraction IP turbine power(typical value is 0.3)

F = Fraction of LP turbine power (typical value is 0.4)