Power System Transient - Introduction.pptx

Introduction to Power System Transient

1. Power System Transient Chapter – 1 Introduction Prepared by Viren Pandya

2. Contents Source of transients Various types of power systems transients Effect of transients on power systems Importance of study of transients for insulation design

3. Source of transients The sudden, short time and large magnitude surges in energy system are termed as transients. If the energy system is operated in static equilibrium, system models are normally nonlinear but algebraic equations e.g. power flow analysis, optimal power flow etc.

4. Source of transients Theory of small-scale dynamic perturbations around the static equilibrium point requires system models to be linear differential equations e.g. ALFC and AVR study under steady state with normal load variations. Large scale disturbances -transients resulting into dynamic state fluctuations in the energy system need models to be nonlinear differential equations.

5. Source of transients Internal Sources (Inside facilities) Capacitor switching Current interruption (motors, etc.) Power electronics operation (SCRs, etc.) Electrostatic discharge (Arc) Welding Copy machines Faulty wiring or circuit breaker operation Contact and relay closure Load startup or disconnect

6. Source of transients Internal sources do not produce surges of large magnitude. Experience shows that transients due to internal sources hardly increase the system voltage to twice the normal value.

7. Source of transients External Sources (Outside facilities) Lightning Capacitor switching Line/cable switching Transformer switching Current limiting fuse operation

8. Types of power systems transients Depending upon the speed of the transients, they can be classified into three categories. Class A: Ultrafast transients – Surge phenomena Class B: Medium-fast transients – Short-circuit phenomena Class C: Slow transients – Transient stability

9. Types of power systems transients Class A: Ultrafast transients – Surge phenomena Due to lightning or switching operation of large component in the energy system Entirely electric in nature and normally involves transmission system only Electromagnetic transient - travelling wave travels almost the speed of the light along the lines, giving rise to reflected wave at the line termination

10. Types of power systems transients A few milliseconds after initiation – then get attenuated due to system R and die out High inductance of transformers in most instances effectively provides protection to generator against such transients. Lightning arrester provide protection against lighting strokes striking on the lines. These transients’ study basically provides a basis for selection of insulation level of the line equipment.

11. Types of power systems transients It is to be noted that if insulation of the system gets destroyed due to such overvoltage transient, this type of transient can lead to a short circuit, which is an abrupt and abnormal structural change of the network which will give rise to a new – slower type of transient that belongs to Class-B.

12. Types of power systems transients Class B: Medium-fast transients – Short-circuit phenomena These are caused by normally abrupt and abnormal structural changes – short circuits in the system. These could be caused due to Class A transients, salt spray on insulators, birds sitting, other mechanical causes etc. LLL/LLLG, LG, LL, LLG, 1CO, 2CsO Important to study for selection of CBs, protection system design, transient stability analysis.

13. Types of power systems transients Class C: Slow transients – Transient stability Short circuit will always be accompanied by an instantaneous collapse (partial or full) of bus voltages throughout the system. Now the generator voltages drop suddenly with sudden reduction in generator output power under short circuit. Since the mechanical/turbine input power to generator at the time of fault stays constant for first instants, each generator will be subjected to a surplus accelerating torque.

14. Types of power systems transients If it is sustained, it can develop into the most dangerous type of transient situation to which a power system can be subjected - mechanical oscillations of the synchronous machine rotors. These electromechanical transients may under unfortunate circumstances pull some or all the of the machines out of synchronism. Such an occurrence constitutes a partial or complete breakdown of the energy system.

15. Types of power systems transients Here the system is said to have reached its transient stability limit. It may take hours to resynchronize a fully blacked-out system. These rotor swings, being mechanical in nature, are relatively slow. Thus such transients' study becomes necessary to assess transient stability.

16. Effect of transients on power systems Effects on insulation Effects on semiconductors Effects on electromechanical contacts Noise generation D owntime or running outages Efficiency of components gets affected.

17. Effect of transients on power systems Under severity, black out of power system will be produced. Lightning transient produces steep fronted wave on transmission line. Travelling wave produced due to transient will shutter the insulations and week poles. Cause damage to windings of transformer and generators.

18. Importance of study of transients for insulation design Failures in insulation cause safety risks for human, fires and system outages. Transient voltage peak value is very much important for insulating material breakdown in the power system. Design of the insulation needs to be done in such a way that it does not breakdown under lighting impulses, switching surges or any other overvoltage transients.

19. Importance of study of transients for insulation design The EMTP has became popular for the calculation of power system transients especially a switching overvoltage from the viewpoint of insulation design and coordination of a transmission line and a substation.