Unit 3
Unit 3
Unit 3
4.1 INTRODUCTION
4.2 REACTIVE POWER
4.3 GENERATION AND ABSORPTION OF REACTIVE
POWER
4.3.1 Synchronous Generator
4.3.2 Synchronous Compensator
4.3.3 Capacitance and Inductive Component
4.4 METHODS OF VOLTAGE CONTROL
4.4.1 Reactors
4.4.2 Shunt Capacitor
4.4.3 Series Capacitor
4.4.4 Synchronous Compensator
4.4.5 Static VAR Compensator
4.5 TYPES OF SVC
4.6 APPLICATION OF STATIC VAR COMPENSATOR
4.7 EXCITATION SYSTEM REQUIREMENTS
4.8 ELEMENTS OF EXCITATION SYSTEM
4.9 TYPES OF EXCITATION SYSTEM
4.9.1 Static Excitation System
4.9.2 Brushless Excitation System
4.9.3 AC Excitation System
4.9.4 Excitation System
4.10 RECENT DEVELOPMENT AND FUTURE TRENDS
4.11 MODELING OF EXCITATION SYSTEM
4.12 STEADY STATE PERFORMANCE EVALUATION
4.13 DYNAMIC RESPONSE OF VOLTAGE REGULATOR
CIRCUIT
TECHNICAL TERMS
Regulator: Process and amplifies input control signals to alevel and form appropriate
for control of the exciter. This includes both regulating and excitation system
stabilizing function.
Power Exchange: The entity that will establish a competitive spot market for electric
power through day- and/or hour-ahead auction of generation and demand bids.
Exciter: provides dc power to the synchronous machine field winding constituting the
power stage of the excitation system.
Prime Mover: The engine, turbine, water wheel, or similar machine that drives an
electric generator; or, for reporting purposes, a device that converts energy to
electricity directly (e.g., photovoltaic solar and fuel cell(s)).
Power system stabilizer: Provides an additional input signal to the regulator to damp
power system oscillation;
Revenue: The total amount of money received by a firm from sales of its products
and/or services, gains from the sales or exchange of assets, interest and dividends
earned on investments, and other increases in the owner's equity except those arising
from capital adjustments.
1. Overhead lines and underground cables, when operating at the normal system voltage, both
produce strong electric fields and so generate reactive power.
2. When current flows through a line or cable it produces a magnetic field which absorbs
reactive power.
3. A lightly loaded overhead line is a net generator of reactive power while a heavily loaded
line is a net absorber of reactive power.
4. In the case of cables designed for use at 275 or 400kV the reactive power generated by the
electric field is always greater than the reactive power absorbed by the magnetic field and so
cables are always net generators of reactive power.
5. Transformers always absorb reactive power.
A great advantage of the method is the flexible operation for all load
conditions. Being a rotating machine, its stored energy is useful for riding through transient
disturbances, including voltage drops.
Figure4.4:Synchronous compensator
The term static var compensator is applied to a number of static var compensation
devices for use in shunt reactive control. These devices consist of shunt connected, static
reactive element (linear or non linear reactors and capacitors) configured into a var
compensating system. Some possible configurations are shown in above Figure. Even though
the capacitors and reactors in are shown in figure connected to the low voltage side of a
down transformer, the capacitor banks may be distributed between high and low voltage
buses. The capacitor bank often includes, in part, harmonic filters which prevent the
harmonic currents from flowing in the transformer and the high voltage system. Filters for the
5th and 7th harmonics are generally provided. The thyristor controlled reactor (TCR) is
operated on the low voltage bus. In another form of the compensator illustrated in Figure the
reactor compensator is connected to the secondary of a transformer.
With this transformer, the reactive power can be adjusted to anywhere between 10% to the
rated value. With a capacitor bank provided with steps, a full control range from capacitive to
inductive power can be obtained. The reactor's transformer is directly connected to the line,
so that no circuit breaker is needed.
The primary winding is star connected with neutral grounded, suitable to the thyristor
network. The secondary reactor is normally nonexistent, as it is more economical to design
the reactor transformer with 200% leakage impedance between primary and secondary
windings. The delta connected tertiary winding will effectively compensate the triple
harmonics. The capacitor bank is normally subdivided and connected to the substation bus
bar via one circuit breaker per sub bank. The regulator generates firing pulses for the thyristor
network in such a way that the reactive power required to meet the control objective at the
primary side of the compensator is obtained. The reactor transformer has a practically linear
characteristic from no load to full load condition. Thus, even under all stained over voltages;
hardly any harmonic content is generated due to saturation. The transformer core has non
ferromagnetic .Gaps to the required linearity.
The current flowing in the inductance would be different in each half cycle, varying with the
conduction angle such that each successive half cycle is a smaller segment of a sine wave.
The fundamental component of inductor current is then reduced to each case. Quick control
can be exercised within one half cycles, just by giving a proper step input to the firing angle
control Static var compensators when installed reduce the voltage swings at the rolling mill
and power system buses in drive system applications. They compensate for the average
reactive power requirements and improve power factor.
Electric arc furnaces impose extremely difficult service requirements on electrical
power systems since the changes in arc furnace load impedance are rapid. Random and non
symmetrical. The three phases of a static var compensator can be located independently so
that it compensates for the unbalanced reactive load of the furnace and the thyristor controller
will respond quickly in order to minimize the voltage fluctuations or voltage flicker seen by
the system.
Figure4.8: Application of the static var compensator
Thus, the furnace characteristics are made more acceptable to the power system by the static
var compensator. Above figure shows the application of the static var compensator to an arc
furnace installation for reactive power compensation at the HV bus level.
1. Saturated reactor
2. Thyristor controlled Reactor
3. Thyristor switched capacitor
4. Thyristor Switched Reactor
5. Thyristor controlled Transformer
Exciter: provides dc power to the synchronous machine field winding constituting the
power stage of the excitation system.
Regulator: 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: 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: provides an additional input signal to the regulator to damp
power system oscillation.
Limiters and protective circuits: These include a wide array of control and protective
function which ensure that the capability limits of the exciter and synchronous generator
are not exceeded.
Figure4.9: Schematic picture of a synchronous machine with excitation system with several
control, protection, and supervisory functions.
Today, a large number of different types of exciter systems are used. Three main
types can be distinguished:
• DC excitation system, where the exciter is a DC generator, often on the same axis
as the rotor of the synchronous machine.
• AC excitation system, where the exciter is an AC machine with rectifier.
• Static excitation system, where the exciting current is fed from a controlled
rectifier that gets its power either directly from the generator terminals or from the
power plant’s auxiliary power system, normally containing batteries. In the latter case,
the synchronous machine can be started against an unenergised net, “black start”. The
batteries are usually charged from the net.
When the exciter is operated at rated speed at no load, the record of voltage as function of
time with a step change that drives the exciter to its ceiling voltage is called the exciter build
up curve. Such a response curve is show in Figure.4.14
In general the present day practice is to use 125V excitation up to IOMVA units and
250V systems up to 100MVA units. Units generating power beyond IOOMVA have
excitation system voltages variedly. Some use 350V and 375V system while some go up to
500V excitation system.
4.9.4. DC Excitation System
Dc machine having two sets of brush 90 electrical degree apart, one set on its direct
(d) axis and the other set on its quadrature (q) axis. The control field winding is located on
the d axis. A compensating winding in series with the d axis armature current, thereby
cancelling negative feedback of the armature reaction. The brushes on the q axis are shorted,
and very little control field power is required to produce a large current in the q axis
armature. The q axis current is supplied mechanically by the motor.
The advances in excitation system over the last 20 years have been influenced by
development in solid state electronics. Development in analogue integrated circuitry has
made it possible to easily implemented complex control strategies.
The latest development in excitation system has been the introduction of digital
technology. Thyristor continue to be used for the power stage. The control, protection, and
logic function have been implemented digitally, essentially duplicating the function
previously provided by analog circuitry.
Mathematical model of excitation system are essential for the assessment of desired
performance requirement, for the design and coordination of supplementary control and
protective circuits, and for system stability studies related to the planning and purpose of
study.
The performance of the AVR loop is measured by its ability to regulate the terminal
voltage of the generator within prescribed static accuracy limit with an acceptable speed
of response. Suppose the static accuracy limit is denoted by Ac in percentage with
reference to the nominal value. The error voltage is to be less than (Ac/100)∆|V|ref.
From the block diagram, for a steady state error voltage ∆e;
=1- ∆ [V]ref
∆e=1- ∆ [V]ref
= 1- ∆ [V]ref
= ∆ [V]ref
= ∆ [V]ref
Larger the overall gain of the forward block gain K smaller is the steady state error. But too
large a gain K cans instability.
4.13. DYNAMIC RESPONSE OF VOLTAGE REGULATION CONTROL:
Consider
The response depends upon the roots of the characteristic eqn. 1 + G(S) = o.
As there are three time constants, we may write the three roots as S1, S2 and S3. A typical
root locus plot is shown in Figure
From the plot, it can be observed that at gain higher than Kc the control loop becomes ln
stable.
QUESTION BANK
PART- A
PART-B
1. Derive the relation between voltage, power & reactive power at a node for applications in
power system control.
2. Discuss generation and absorption of reactive power.
3. Discuss about the various methods of voltage control.
4. Explain different types of static VAR compensators with a phasor diagram.
5. Explain the A.C and D.C excitation system.
6. Draw the circuit diagram for a typical excitation system and derive the transfer
function model and draw the block diagram (OR) Draw the diagram of a typical
automatic voltage regulator (AVR) & develop its block diagram representation. (OR)
the typical brushless AVR with static and dynamic performance of AVR loop.
7. Explain the injection of reactive power by switched capacitors to maintain acceptable
voltage profile & to minimize transmission loss in a power system.