This presentation is about power system voltage stability. What is voltage stability? How voltage instability occurs? How to improve voltage stability of the system?

1. APRIL30,2019
SIDDHANT MISHRADR. AK. SHARMA
DR. SEEMA AGARWAL

2. What is voltage stability?
Why does voltage instability occur in
mature power system?
How to improve voltage stability?

3. Introduction
Voltage Stability
Relation of voltage stability to
rotor angle stability
Simple two bus system
Tools for voltage stability analysis
Voltage Instability Time frames and
mechanism
Methods of improving voltage stability
Conclusion

4. Present day power systems are being
operated closer to their stability
limits due to economic constraints.
Maintaining a stable and secure
operation of a power system is
therefore very important and
challenging issue.

6. Voltage stability refer to the
ability of power system to
maintain steady voltages at all
buses in the system after being
subjected to a disturbance from
a given initial operating point.
Definition by IEEE
A system is said to be in voltage stable state if at a
given operating condition, for every bus in the
system, the bus voltage magnitude increases as the
reactive power injection at the same bus is
increased.
A system is voltage unstable if for at least one bus in
the system, the bus voltage magnitude decreases as
the reactive power injection at the same bus is
increased.
It implies that if, V-Q sensitivity is positive for every
bus the system is voltage stable and if V-Q
sensitivity is negative for at least one bus, the
system is voltage unstable.

7. VOLTAGE INSTABILITY TIME FRAMES
AND MECHANISM
TRANSIENT VOLTAGE
STABILITY
LONGER TERM VOLTAGE
STABILITY
0 - 10 SECONDS 2 - 3 MINUTES

8. TRANSIENT VOLTAGE STABILITY
COLLAPSE IS
CAUSED BY
fast acting load component
such as large induction
motor and D.C. converters
IMBALANCE IN
REACTIVE POWER
During under frequency load
shedding there is possibility
that system voltage may
collapse
HVDC CIRCUITS
Converters and inverters
require large amount of
reactive power thus causing
stability problems.

9. LONGER TERM VOLTAGE STABILITY
COLLAPSE IS
CAUSED BY
High loads, high power
imports and loss of large
generators or loss of
transmission lines.
HIGH REACTIVE
POWER LOSSES
The disturbance causes high
reactive power losses and
voltage sags in load areas.
RAPID VOLTAGE
DECAY
partial or complete voltage
collapse follows.
50
40
30
20
10
0

10. RELATION OF VOLTAGE STABILITY
TO ROTOR ANGLE STABILITY

11. When asynchronous load connected
to a large system.
Concerned with load areas and
load characteristics.
It is basically load stability.
synchronous machine connected to
infinite bus or a large system.
Normally concerned with
integrating remote power plant
to a large system.
It is basically generator
stability.

13. Voltage
instability in
mature
power
system
HOW INSTABILITY
HAPPENS?
Intensive use of existing
generation and transmission.
Vast use of shunt capacitor
banks for reactive power
compensation. It results into
voltage collapse prone
fragile network.

14. VOLTAGE STABILITY OF SIMPLE
TWO BUS SYSTEM

15. REAL POWER
Real power transfer from bus 1 to bus
2 is given by
P = (EVsinδ)/X (1)
NORMALIZATION
Normalizing the terms in equation (1)
and (2) with
v=V/E
p=(P*X)/E^2
q=(Q*X)/E^2
v^4+v^2(2q-1)+(p^2+q^2)=0 (3)
REACTIVE POWER
Reactive power transfer from bus 1 to
bus 2 is given by
Q = -(V^2)/X+(EVcosδ)/X (2)
SOLUTIONS OF V
Positive real solutions of v from
equation (3) are given by
v={.5 -q±(.25-p^2-q)^.5}^.5 (4)

18. P-V curve method.
V-Q curve method and
reactive power reserve
TOOLS FOR
VOLTAGE
STABILITY
ANALYSIS

19. P-V CURVE
METHOD
Widely used method for voltage stability analysis.
Gives available amount of active power margin before
the point of voltage instability
For a simple two bus system as shown in previous fig.
equation (4) gives real solution of V^2 provided
(1-4*q-4*p^2) >= 0 (5)
Assuming constant power factor load such as q/p=k, the
inequality can be expressed as, p<= .5{(1+k^2)^.5-k} (6)
Equation p <= .5{(1+k^2)^.5-k} determines maximum value of p
Thus representing the load as a constant power factor type
with a suitably chosen power factor, the active power
margin can be computed from above equation.

20. V-Q CURVE
METHOD
AND
REACTIVE
POWER
RESERVE.
Voltage security of a bus is closely
related to the available reactive power
reserve, which can be easily found from
the V-Q curve of the bus under
consideration.
The reactive power margin is the MVAR
distance between the operating point
and the nose point of the VQ curve
Stiffness of the bus can be qualitatively
evaluated from the slope of the right
portion of the V-Q curve. The greater the
slope is, the less stiff is the bus, and
therefore the more vulnerable to voltage
collapse it is.

22. METHOD OF IMPROVING
VOLTAGE STABILITY
The reliability
aspect of supply
can be improved
by sitting
generating plants
in the load areas.
PLANNING OF
GENERATION
SYSTEM.
Over excitation
and under
excitation limiters,
alarm settings, tap
changer settings
need to be verified
and maintained.
MAINTENANCE
OF
GENERATION
SYSTEM
During peak load
period, power
import over the
transmission
network should be
reduced.
OPERATION OF
GENERATION
SYSTEM
EHV transmission
lines requires
shunt reactors for
energization and
under lightly
loaded condition.
These shunt
reactors should be
switched off
during voltage
emergencies.
REACTIVE
POWER
COMPENSATION
Shunt capacitor
banks act as
constant reactive
power sources.
CAPACITOR
BANK

23. THERE ARE MANY ASPECTS OF
VOLTAGE STABILITY AND ALSO
HAS MANY SOLUTIONS
ASSOCIATED TO THE VOLTAGE
STABILITY IN TERMS OF
GENERATION, TRANSMISSION
AND DISTRIBUTION .POWER
SYSTEM ENGINEER JOB IS TO
FIND LOW COST SOLUTION
WHENEVER POSSIBLE WHICH
REQUIRE SPECIAL CONTROL
AND SPECIAL POWER SYSTEM
OPERATION