The document discusses various methods for improving power system stability, including automatic voltage regulators (AVR), load frequency control (LFC), and power system stabilizers (PSS). AVR works to maintain generator terminal voltage at a preset value by adjusting excitation current. LFC maintains system frequency and power exchange between areas at scheduled values. PSS adds damping to generator oscillations to stabilize the grid by modulating voltage regulator setpoint based on speed.

1. IMPROVING POWER
SYSTEM STABILITY
By Ghulam Rasool

2. Methods of Improving Power System Stability:
There are Basic three methods
• Automatic Voltage Regulators (AVR)
• Load Frequency Control (LFC)
• Power System Stabilizer (PSS)

3. Automatic Voltage Regulators (AVR):
• Automatic voltage regulator (AVR) maintains the Generator
terminal voltage at a given value automatically by changing the
excitation current to the Generator field.
• The AVR supplies the required D.C. to the Generator field
depending on the load, power factor etc. to maintain a constant
terminal voltage.

4. Types of AVR:
Single
Channel
AVR
Dual
Channel
AVR
Twin
Channel
AVR

5. Function of AVR:
• The major function of AVR is to
continually adjust the terminal
voltage to maintained at
preset value.
• Compares the Generator
terminal voltage with a preset
reference voltage.
• If the Generator terminal
voltage is less than the
reference voltage, the
AVR increases D.C. voltage
across the Generator field.
• Maintaining the constant
voltage as per the setting.

6. Functions of AVR:
• Challenges
The normal feedback control actions of (AVR) & Speed Governor
introducing negative damping which can cause undamped modes of
oscillations (Critically stable).
• Solution
Switching (AVR) from Auto/Manual.
However, removing AVR from service is not realistic solution to the
problem.
Supplementary feedback signal introduced to AVR as PSS.

7. Load Frequency Control:
• For large scale electric power systems with interconnected areas,
Load Frequency Control (LFC) is important to keep the system
frequency and the inter-area tie power as near to the scheduled
values as possible.
• The input mechanical power to the generators is used to control
the frequency of output electrical power and to maintain the
power exchange between the areas as scheduled.
• The change in frequency is sensed when the rotor angle ∂ is
• changed.

8. LFC cont.…..
• The change in frequency is sensed when the rotor angle ∂ is changed.
• The error signals are transformed into real power command signal, which is sent to
prime mover to call for an increment in the torque.
• The prime mover then brings change in the generator output by an amount which will
change the values of within the specified tolerance.
• Load frequency control is basic control mechanism in the power system operation.
Whenever there is variation in load demand on a generating unit, there is a
momentarily an occurrence of unbalance between real-power input and output. This
difference is being supplied by the stored energy of the rotating parts of the unit.
• Load Frequency Control (LFC) is being used for several years as part of the Automatic
Generation Control (AGC) scheme in electric power systems. One of the objectives of
AGC is to maintain the system frequency at nominal value (50 Hz).

9. Block diagram Load
frequency control:
• The frequency is sensed by frequency
sensor.
• The change in frequency and tie line real
power can be measured by change in rotor
angle δ.
• The load frequency controller amplify and
transform error signal, i.e., (Δfi and ΔPtie)
in to real power command signal ΔPci which
is sent to the prime mover via governor
(that control the valve mechanism).
• To call for an increment or decrement in
torque the prime mover balances the output
of governor which will compensate the
value of error signal that is Δfi and ΔPtie.
• The process continues till deviation in form
of Δfi and ΔPtie as well as the specified
tolerance.

10. Reasons for constant frequency:
• The speed of the alternating current motors depends on the
frequency of the power supply. There are situations where speed
consistency is expected to be of high order.
• If the normal frequency is 50 Hertz and the system frequency falls
below 47.5 Hertz or goes up above 52.5 Hertz then the blades of
the turbine are likely to get damaged so as to prevent the stalling
of the generator.
• Due to the subnormal frequency operation the blast of the
Induced Draft (ID) and Force Draft (FD) fans in the power stations
get reduced and thereby reduce the generation power in the
thermal plants.

11. AGC Automatic Generation
Control:
• Automatic generation control (AGC) is
defined as, the regulation of power
output of controllable generators
within a prescribed area.
• When system frequency is on
schedule, each area is expected
automatically to adjust its generation
to maintain its net transfer with other
areas on schedule, thereby absorbing
its own load variations.
• When system frequency is off-
schedule, because one or more areas
are not fulfilling their regulating
responsibilities, other areas are
expected automatically to shift their
respective net transfer schedules
proportionally to the system
frequency deviation and in direction
to assist the deficient areas and
hence restore system frequency.
Block diagram of a synchronous generator with
basic frequency control loops. AUTOMATIC
GENERATION CONTROL

12. Power System Stabilizer:
• The power system stabilizer (PSS) is a device that measures
improvements in system stability when added to a generator’s
automatic voltage regulator (AVR). Therefore, compared to
system reconstruction or enhancement, it offers
overwhelmingly superior cost performance.
• The Power System Stabilizer (PSS) is a supplementary excitation
controller used to damp generator electro-mechanical oscillations
in order to protect the shaft line and stabilise the grid. It
also damps generator rotor angle swings, which are of greater
range in frequencies in power system.

13. The Main Function of PSS:
• The objectives of PSS is to add damping to rotor oscillations. This
is achieved by modulating the voltage regulator set point to
produce torque variations in phase with speed.
• The PSS will produce variations in electrical torque in phase with
speed and acts to damped out oscillations.

14. Basic PSS Theory:
• A synchronous generator
working on the network is
principally an oscillating
structure.
• In order to produce a
torque the rotating
magnetic fields of the rotor
and the stator must form a
certain angle (the so called
load angle δ).

15. Basic PSS Theory: cont…….
• The electrical torque (Te) increases, as the angle (δ) increases,
just similar to a torsion spring. Because during steady-state
operation the electrical torque (Te) of the generator and the
mechanical driving torque (Tm) from the turbine are in
equilibrium, the load angle δ remains in a given position
• The dynamic of the mechanical power (Tm) , electrical power (Te)
, and rotor angular speed (ω) of the synchronous machine is an
origin for theoretical consideration and justifications of the PSS
processing signal.