Transient stability refers to the ability of a power grid to maintain synchronism during severe disturbances. Methods to improve transient stability include increasing generator rotor size, reducing transmission line reactance, using dynamic braking resistors, independent pole operation of circuit breakers, single pole switching, fast excitation control, fast governor action, generator and load tripping, regulated shunt compensation using static VAR devices, HVDC transmission, and increasing the short circuit ratio. The inertia constant of generators also impacts stability but increasing it is not practical due to increased machine size and cost.

1. TRANSIENT STABILITY
ENHANCEMENT
VIPIN CHANDRA PANDEY

2. Transient stability
“Transient stability is the ability of the
power grid system to maintain synchronism
when subjected to severe disturbances.”
Short circuit on transmission line.
 removal of large transmission loads,
cascaded failure operations,

3. Swing Equation
푀푑2훿
푑푡2 = 푃푎 = 푃푚 − 푃푒 = 푃푚 − 퐸푉
푋12
M=Inertia constant (if speed constant depend on
size)
δ= Power angle of the machine in rad
푃푎=Accelerating power
Pm=Mechanical input to synchronous gen.
Pe=Electrical output of synchronous gen.
E=Generator voltage
V=Bus voltage
X12=Reactance of the line

4. Transient stability improvement
method:
Rotor size and transfer reactance of line
 Dynamic braking resistor
Independent-pole operation of circuit
breaker
Single- pole switching
Fast excitation Control
Fast governor action

5. Cont.
Generator tripping (For large systems)
Load Tripping
Regulated shunt compensation
SCR (Short Circuit Ratio)
HVDC

6. Effect of M
An increase in the value of inertia
constant M reduces the angle through
which the rotor swings farther during a
fault. However, this is not a practical
proposition
Increasing M means
Increasing the dimensions of the machine,
which is uneconomical.
Note: Not feasible

7. Effect of X12
Reduction of transfer reactance
Use of parallel lines instead of single line
use of bundle conductors.

8. Dynamic braking resistor

9. Dynamic Braking resistor
It is one of the most efficient and widely used external
control method.
BR is a artificial (dummy) load added at the terminal
of the synchronous generators for short duration of
time to reduce the generator speed and then remove
from the system so as to maintain synchronism.
It open under fault condition and absorb the
accelerating energy during fault condition.
Control can be done by power electronics switch.

10. Dynamic braking resistor
Shunt resistor energy dissipated α Voltage
Series resistor energy dissipated α Current
Note:
Preferred in hydro station due remote
location from load Centre

11. Independent- pole operation of
circuit breaker
Use of separate mechanism for each
phase .
Each phase open and close individually.
Fault of any phase will not affect the
other phase.
Relaying system is normally arrange to
trip all the pole for any type of fault.

12. Single pole switching
Use of separate mechanism for each
phase.
For single line to ground fault relay
design to trip only fault phase
Reclosing operation followed after certain
time.
Used where single line connect a
generator connected to rest of systems.

13. Fast excitation Control
Generator excitation controls are a basic
stability control. provide powerful and
economical means to ensure stability for large
disturbances.
Automatic voltage regulators
 Detect the decrease in the voltage
 Response by increase in excitation voltage
Power system stabilizer
 Fast excitation due to transient lead to degrading of damping of
local plant mode oscillations
 It produce the damping torque component

14. Fast governor action
Change in power angle during disturbance
can be mitigated by varying the prime mover
output with the help of fast acting governor
Acceleration energy of rotor can be controlled
It operate after about 15 cycle (i.e.0.30sec for
50Hz system). Which can cause stability
problem with severity of the faults.
It is not adequate for hydro power station due
to difficulty in control of water flow.

15. Generator tripping
(For large systems)
Tripping of generator units for severe
transmission system.
Power transferred reduced over the
transmission line.
Generator can be tripped rapidly
So accelerating energy greatly reduced to
maintain the synchronism
Note: Due to tripping of generator power
transferred is reduced and available energy kept
in idle state.

16. Load Tripping
Similar to generator tripping
Tripping at load end
 To reduce the decelerating of receiving end generation
 Tripping of some part of system rather than large system

17. Regulated shunt compensation
Synchronous condenser
 Synchronous machine without mechanical load.
 It can operate in leading, unity and lagging on requirement.
 Control the lagging and leading reactive power.
Static VAR compensator
 Shunt connected generator or absorber with control device.
 Its output can be varied to control the specific parameter of
electrical power system.
 It is static means no moving parts.

18. FACTs DEVICES
TCR TSC

20. Static VAR devices (FACTs Device)
Saturated reactor
Thyristor control reactor(TCR)
Thyristor switched capacitor(TSC)
Thyristor switched reactor (TSC)

21. HVDC
Thyristor control employed.
DC link is asynchronous
 No synchronization is required
 Power transfer can be easily controlled
 No risk of a fault in one system causing loss of stability in the
other system.
Note: high cost of converter and inverter.

22. References
Power system stability and control by Prbha
Kundur.
Power system analysis by Prof. P.S.R.
Murthy.
Power system engineering by D.P.Kothari
and I.J.Nagrath.
Power system analysis operation and control
by Abhijit Chakrabarti and Sunita Halder.

23. Thank You

24. SCR (Short Circuit Ratio)
Ratio of the field current required for the
rated voltage at open circuit to the field
current required for rated armature current
at short circuit.
퐼푓푂퐶
퐼푓푆퐶
SCR=
SCR=
1
푋푑푝.푢.푠푎푡

25. Cont.
Lower the SCR ratio
Reduction machine air gap
Saving machine mmf, size weight and
cost
Reduction in size of rotor reduce inertia
constant lowering thereby stability margin