Power system voltage stability
- 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