Fundamentals of Automatic Generation Control
• A change in system load will result in a steady
state frequency deviation, depending on the
governor droop characteristic and frequency
sensitivity of the load.
• All generating unit on speed governor will
contribute to the overall change in generation,
irrespective of the location of the load change.
• Restoration of system frequency to nominal value
requires supplementary control action which
adjust a load reference set up through speed
changer motor.
• Therefore the basic means of controlling prime
mover power to match in system load is through
the control of the load reference set point of the
selected generating unit.
Primary objectives of Automatic
Generation Control
• To regulate frequency to the specified nominal
value
• To maintain and interchange power between
control areas at the scheduled values by
adjusting the output of selected generators
(This is commonly referred to as load frequency
control).
Secondary objectives of Automatic
Generation Control
• To distribute the required change in generation
among the units to minimize operating cost
CONTROL OF VOLTAGE AND REACTIVE POWER CONTN
Tap changing transformer:
By changing the transformer ratio, the voltage in the secondary
circuit is varied.
Combined use of tap changing transformers and reactive
power injection:
VOLTAGE CONTROL IN DISTRIBUTION NETWORK
• Single phase supplies to houses and other small consumers are
tapped off from three phase feeders connected between one phase
and the neutral.
• Loads are not applied at the same time and some unbalance
occurs.
• To offset voltage drops various voltage boosts are employed:
• Main transformer boost (onload tap changer), distribution
transformer boost through and off circuit taps boost
POWER SYSTEM PROTECTION
Protection is all about detecting faults on power
system and automatically removing them.
Such faults results in:
• Abnormal currents and voltages
• Loss of synchronism
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IMPORTANCE OF PROTECTING
SYSTEM
• The main importance of a protection
system is to discriminate between
those items of plant that are faulted
and must be removed from the
system and those that are sound and
should remain in service.
• In summary, protection and
automatic switching has two main
functions:
• To isolate faulty equipment
TYPES OF PROTECTIVE SYSTEMS
• There are many types of automatic protective systems.
1. Relay
This is an automatic device which senses abnormal
condition in an electric circuit and operates. They do not
prevent faults but operates when there is an abnormal
current
• Primary relaying equipment
• Back up relaying equipment
HOW DOES A RELAY WORK?
• When an abnormal condition is sensed, the relay
closes its contact.
• This contact in turn energize the Circuit Breaker
trip-circuits which opens the circuit breaker, and
the faulty part is isolated
Failure of primary relay
• Primary relaying may fail because of failure of any of
the following:
• Moving Mechanism
• DC Voltage Supply
• Current/Voltage Supply to the Relay
IMPORTANT PARAMETERS
• Pick-up level:
This is the value of the actuating quantity which is
on the threshold above which the relay operates
• Reset Level:
This is the value below which a relay opens its
contact and come to the original position.
• Operating Time:
This is the time lapse between the instant when
the actuating quantity exceed pick up and the instant
when the relay contact closes.
IMPORTANT PARAMETERS
• Reset Time:
• The time lapse between the instance when the
actuating quantity becomes less than the reset value to
the instant when the relay contact return to its normal
position.
• Primary Relay:
• The relay which is connected to the circuit to be
protected through current.
• Secondary Relay:
The relay which are connected to the circuit to be
protected through current and potential
transformer.
• Auxiliary Relay:
This operate in response to the opening or
closing of its function. This may be instantaneous or
may have a time delay.
• Reach:
A distance relay operates whenever the impedance
seen by the relay is less than a specified value.
• Under Reach:
The tendency of the relay to restrain at the
set value of the impedance or impedance
lower than the set value
• Over Reach:
The tendency of the relay to operate at
impedances larger than its setting is known
as over reach
COMPONENTS OF
PROTECTION
• Instrument transformers
• Relays
• Circuit Breakers
INSTRUMENT TRANSFORMERS
• There are two basic types of instrument transformers:
• Voltage Transformers (PTs)
• Current Transformers (CTs)
VT
• The VT reduces the primary voltage to much
lower, standardized level suitable for operation
of relays.
• For system-protection purposes, VTs are
generally considered to be accurate, hence
modeled as an ideal transformer
• A standard VT secondary voltage rating is 115V
VT Continues
• Ideally, the VT secondary is connected to a voltage-
sensing device with infinite impedance, such that the
entire VT secondary voltage is across the sensing
device.
• In practice, the secondary voltage divides across the
high impedance sensing device and the VT series
leakage impedances.
• VT leakage impedances are kept low in order to
minimize voltage drops and phase-angle diferences
from primary to secondary.
CTs
• The primary winding of a current transformer usually
consists of a single turn, obtained by running the
power system’s primary conductor through the CT
core.
• The normal current rating of CT secondaries is
standardized at 5 A in the United States, whereas 1 A
is standard in Europe and some other regions.
• Currents of 10 to 20 times (or greater) normal rating
• often occur in CT windings for a few cycles during
short circuits.
• Ideally, the CT secondary is connected to a current-
sensing device with zero impedance, such that the
entire CT secondary current flows through the
sensing device.
• In practice, the secondary current divides, with most
flowing through the low-impedance sensing device
and some flowing through the CT shunt excitation
impedance.
• CT excitation impedance is kept high in order
tominimize excitation current.
• An approximate equivalent circuit of a CT is shown
below
Figure: CT equivalent
circuit
Excitation Curves for a multiratio bushing CT
Example
• CT ratio = 1200:5
• Secondary resistance = 0.5Ω
• Relay burden = 0.5Ω
• For 20 times rated
secondary current (100A)
• V2 = I2 *(ZB* n + Z2)
• Secondary Voltage =
(100) (0.5*number of relays
+0.5)= 400V
Functions of CT & VT
• Current Transformer:
• It produces a scaled down version of its input
current.
• It ensures a consistent output of circuit current
regardless of the state of the system.
• Voltage Transformer:
• It produces voltage much lower than system’s
voltage for application
Types of VTs
• There are two basic types of VTs and they are:
• The wound type (electromagnetic small power
transformer)
• Capacitor type: a tapping is made on the
capacitor bushing and the voltage from the
tapping is stepped down by a small
electromagnetic VT
Figure: A Capacitor Voltage
Transformer Circuit Arrangement
Equivalent circuit-burden-impedance of CVT
Tuning Inductors in Capacitive Voltage Transformer
• The voltage divider action of a capacitor aim to cut
down the cost of a VT and introduces a phase
difference between the network voltage and input
of electromagnetic VT and this is compensated for
by a series inductor.
• The role of the inductor is to compensate for the
voltage drop due to the capacitance divider
Final equivalent circuit of CVT
RELAYS
HOW RELAY OPERATES
• To monitor the state of the system, relay
senses:
• Current through a CT,
• Voltage through a VT.
• Settings are generally reference current or
impedance.
• The trip signal is the trip decision when fault
is detected
RELAY PERFORMANCE
•
•
Example
• The performance of an over current relay was
monitored for a period of three years. It was found
that the relay operated 16 times, out of which 10
were correct trips. If the relay failed to issue trip
decisions on 3 occasions, compute the:
• Dependability,
• Security,
• Reliability of the relay as percentages
Protection Schemes
• The application of relays and other protection
forms is a large and complex subject. They can be
classified as:
• Unit type
• Non unit type
Non Unit Scheme
• Example of non-unit or system protection are:
• Over-Current Protection Scheme: used in
distribution networks and applied to
generators, transformers and feeders
• Directional Over-Current Protection Scheme:
this is used in system with loops or networked
system.
Application of over-current relay for feeder protection
Overcurrent protection scheme
UNIT SCHEME
• Differential Relaying: They are commonly used to protect
generators, buses and transformers
SWITCHGEAR
• For maintenance to be carried out on HV plant, it
must be isolated from the rest of the network.
• There are different designs of switches e.g. Circuit
breakers, earth ground switches and isolators.
• CBs can interrupt fault current while Isolators can
interrupt load current.
• It is important to operate a switchgear within its
capabilities.
CIRCUIT BREAKERS
• High voltage circuit breakers take five basic
forms:
• Oil immersed
• Air blast
• Small oil
• SF6
• Vacuum
