Transient enhancement technique

TRANSIENT STABILITY ENHANCEMENT

VIPIN CHANDRA PANDEY

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,

Swing Equation

M=Inertia constant (if speed constant depend on size)

δ= Power angle of the machine in rad

=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

Transient stability improvement method:

Rotor size and transfer reactance of line Dynamic braking resistorIndependent-pole operation of circuit breaker

Single- pole switchingFast excitation ControlFast governor action

Cont.Generator tripping (For large systems)Load TrippingRegulated shunt compensationSCR (Short Circuit Ratio)HVDC

Effect of MAn 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 meansIncreasing the dimensions of the machine, which is uneconomical.

Note: Not feasible

Effect of X12

Reduction of transfer reactanceUse of parallel lines instead of single lineuse of bundle conductors.

Dynamic braking resistor

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.

Dynamic braking resistorShunt resistor energy dissipated α VoltageSeries resistor energy dissipated α Current

Note:

Preferred in hydro station due remote location from load Centre

Independent- pole operation of circuit breakerUse 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.

Single pole switchingUse 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.

Fast excitation ControlGenerator 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

Fast governor actionChange 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 controlledIt 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.

Generator tripping (For large systems)Tripping of generator units for severe transmission system.

Power transferred reduced over the transmission line.

Generator can be tripped rapidlySo 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.

Load TrippingSimilar to generator trippingTripping at load end

To reduce the decelerating of receiving end generation Tripping of some part of system rather than large system

Regulated shunt compensationSynchronous 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.

FACTs DEVICES

TCR TSC

Static VAR devices (FACTs Device)

Saturated reactor Thyristor control reactor(TCR)Thyristor switched capacitor(TSC)Thyristor switched reactor (TSC)

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.

ReferencesPower 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.

Thank You

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.

SCR=

SCR=

Cont.Lower the SCR ratioReduction machine air gapSaving machine mmf, size weight and cost

Reduction in size of rotor reduce inertia constant lowering thereby stability margin