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Voltage Collapse And Sympathy Trips

Learn about Voltage Stability & Collapse, Dynamic Instability, Surge Impedance Loading, and Generator Excitation Systems in electric power systems. Recognize symptoms and effects preceding a Voltage Collapse.

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Voltage Collapse And Sympathy Trips

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  1. Voltage Collapse And Sympathy Trips

  2. Objectives • Identify the symptoms an electric system displays preceding a Voltage Collapse. • Identify the 2 types of System Load and how each is affected by Low Voltage. • Recognize the definition of Dynamic Instability. • Identify the definition of Surge Impedance Loading (SIL) of a transmission line and the effects of a line loaded above or below its SIL.

  3. Objectives • Define Voltage Stability • Define Voltage Collapse

  4. Recognizing a Voltage Collapse IEEE Definitions: Voltage Stability is the ability of a system to maintain voltage so that when load is increased, power will increase, so that both power and voltage are controllable. Voltage Collapse is the condition by which voltage instability leads to loss of voltage in a significant part of the system.

  5. Recognizing a Voltage Collapse IEEE Definitions: Voltage Security is the ability of a system, not only to operate stably, but also to remain stable (as far as the maintenance of system voltage is concerned) following any reasonably credible contingency or adverse system change. A system enters a state of voltage instability when a disturbance, increase in load, or system changes causes voltage to drop quickly or drift downward, and operators and automatic system controls fail to halt the decay. The voltage decay may take just a few seconds or ten to twenty minutes. If the decay continues unabated, steady-state angular instability or voltage collapse will occur.

  6. Recognizing a Voltage Collapse Surge Impedance Loading The SIL is the loading in MW at which the MVAR from the natural capacitance of a line exactly cancels the MVAR the line needs to support its voltage. At Surge Impedance Loading Source Load MW MW MVAR MVAR

  7. Recognizing a Voltage Collapse Surge Impedance Loading The SIL is the loading in MW at which the MVAR from the natural capacitance of a line exactly cancels the MVAR the line needs to support its voltage. Below Surge Impedance Loading Source Load MW MW MVAR MVAR

  8. Recognizing a Voltage Collapse Surge Impedance Loading The SIL is the loading in MW at which the MVAR from the natural capacitance of a line exactly cancels the MVAR the line needs to support its voltage. Above Surge Impedance Loading Source Load MW MW MVAR MVAR

  9. Recognizing a Voltage Collapse • Loss of Synchronization: The point at which the generating unit loses its electrical bond with the system and begins to over speed and “slip poles”. • Active (MW) Active power is what does the work in the system. • Resistive Loads (MW) During a voltage dip resistive load current will decrease. • Inductive (Motor) Loads (MW & MVAR) During a voltage dip motor load current will increase - the lower the voltage, the more current they draw.

  10. Reactive (MVAR) MVARs are produced by capacitive loads like transmission lines capacitor banks and generators. MVARs are absorbed by inductive loads, like transformers, reactors, motors and generators. MVARs are needed to support voltage and produce magnetic fields in the AC system. Recognizing a Voltage Collapse

  11. Generator Excitation Systems • The excitation system maintains the desired terminal voltage by means of automatic controls that measure the terminal voltage and compare it to the preset (“desired”) value. • The AVR is the device that controls the “set point” automatically to the excitation system. • When the AVR is in manualmode the set point will not be controlled and will not respond to disturbances that require voltage support.

  12. Generator Excitation Systems • As voltage rises above the set point the generator excitation system begins to lower the excitation current and consume MVARs. • The generator will continue to absorb MVARs until it reaches its Under Excitation Limit and at that point will trip to prevent the generator from overheating and damaging rotor windings. • At this point the generator is dangerously close to “slipping a pole” if there is a large voltage fluctuation on the transmission system.

  13. Generator Excitation Systems • As voltage drops below the set point the generator excitation system begins to raise the excitation current and provide MVARs. • The generator will continue to produce MVARs until it reaches its Over Excitation Limit and will trip to prevent damage to the generator.

  14. Generator Excitation Systems • The connection strength of the generator to the transmission system is determined by the strength of the excitation system field on the generator. • The further the generator goes into the lead the weaker the connection becomes. • In order to understand the relationship between the generator and transmission we must discuss power angle.

  15. Power Angle Review δ V1 V2 Sending BUS Receiving Bus XL Power Angle P= V1 V2 sinδ XL Bus 1 Bus 2

  16. Recognizing a Voltage Collapse 1 - Remote Generation Gen Station A Load Center And Gen

  17. Recognizing a Voltage Collapse 2 - Local Load center Gen Station A Load Center And Gen

  18. Recognizing a Voltage Collapse 3 - 345KV Transmission Lines Gen Station A Load Center And Gen

  19. Recognizing a Voltage Collapse Gen Station A Maximum Flow Load Center And Gen F L O W Current Flow 0 30 90 180 Power Angle

  20. Recognizing a Voltage Collapse Gen Station A Load Center F L O W Maximum Flow Current Flow 0 60 90 180 Power Angle

  21. Recognizing a Voltage Collapse Machine Dynamic Stability • During steady state operation the turbine throttle valve is adjusted so that the mechanical power is equal to the mechanical load applied to the generator. • During the dynamic period these two quantities will not be equal and the turbine-generator rotor will over speed. • The change in electrical output results in an immediate change in the mechanical load applied by the generator to the turbine-generator rotor assembly. This results in an imbalance between the mechanical power from the turbine and the mechanical load applied by the generator.

  22. Machine Dynamic Stability • Since the generator electric angle is related to the position of the rotor. Any change in the rotor speed (due to the imbalance) will, in turn, result in changes in the generator output. There will be an interaction between the system and rotor speed. • The term “equal area criteria” is used to describe the comparison of the area that represents the energy stored in the turbine generator during the acceleration period and the area that represents the energy removed from the turbine generator in the deceleration period.

  23. If one line trips: The power angle drops from the initial operating point (A) to point (B). This is because, for an instant, the phase angle is at its original value of 30 deg. The electrical power is now less than the mechanical power. Recognizing a Voltage Collapse Power – Angle Curve Trip of One Line Turbine Mechanical Power (Input) A MW Output – System 1 Generator 1000 MW With 3 Lines (Pre-Disturbance) B With 1 Line (Post-Disturbance) Angle in degrees 30 60 90 120 150 180

  24. The turbine – generator accelerates. The acceleration advances the generator rotor phase angle increasing the δ between busses. As the angle increases from 30 to about 50 the power output increases from point (B) to point (C). Recognizing a Voltage Collapse Power – Angle Curve Trip of One Line Turbine Mechanical Power (Input) A MW Output – System 1 Generator 1000 MW F With 3 Lines (Pre-Disturbance) C B With 1 Line (Post-Disturbance) Angle in degrees 30 60 90 120 150 180

  25. The turbine –generator begins to decelerate from (C) to (D). Once the unit reaches (D) it has increased the phase angle to approx. 60 (E) and continues to output the MW value from pre disturbance. Recognizing a Voltage Collapse Power – Angle Curve Trip of One Line D Turbine Mechanical Power (Input) A MW Output – System 1 Generator 1000 MW F E With 3 Lines (Pre-Disturbance) C B With 2 Line (Post-Disturbance) Angle in degrees 30 60 90 120 150 180

  26. Recognizing a Voltage Collapse • If the system is operating with a δ close to 90 deg. and we attempt to transmit more power across the line by putting more power into the turbine on the sending generator the phase angle will increase beyond 90 deg. and the power transfer would decrease. • If the phase angle exceeds 90 deg. the rotors on the sending end generators will begin accelerating and the rotors on the receiving end generators will decelerate.

  27. Recognizing a Voltage Collapse Gen Station A Load Center F L O W Maximum Flow Current Flow 0 90 180 Power Angle

  28. Recognizing a Voltage Collapse Transmission Tripping The transmission line trips at about 60 degrees. The power angle drops to (B). The turbine is now at 60 Hz and the power angle is at 70 degrees. The turbine continues to decelerate to maintain speed. The turbine reaches maximum deceleration. At (D) the turbine is accelerating again and now has a power angle of close to 120 degrees. C A D Turbine Mechanical Power (Input) B MW Output – System 1 Generator With 3 Lines (Pre-Disturbance) The area inside (A), (B) and (C) is larger than the area inside (C) and (D) and is therefore outside of the “equal area criteria” rule and this turbine will go out of step with the system if not tripped. With 2 Lines (Post-Disturbance) The turbine decelerates from (B) to (C). With 1 Line (Post-Disturbance) Angle in degrees 30 60 90 120 150 180

  29. A three phase fault occurs near the generator: • The power supplied by the turbine remains constant during the dynamic period. • The rotor speed increases storing the energy developed by the turbine during the time that the generator output is zero. • The δ between the turbine mechanical power and the electrical power is called the accelerating power.

  30. Loss of Synchronization TWO LINES IN SERVICE Generator at 600 MW Power 90 180 270 360 450 Extended Power Angle Curve during Loss of Synchronism

  31. Loss of Synchronization Seconds after an instability event occurs: The fault accelerates the generator sufficiently to make it unstable. Where the decelerating area is less than the accelerating area. Power 90 180 270 360 450 Acceleration Area Extended Power Angle Curve during Loss of Synchronism

  32. Loss of Synchronization The generator goes through a deceleration area. Once the generator goes beyond the initial deceleration area: The turbine mechanical power becomes greater than the electrical power out of the generator. Deceleration Area Power 90 180 270 360 450 Acceleration Area Extended Power Angle Curve during Loss of Synchronism

  33. Loss of Synchronization The generator begins accelerating again. As it does the electrical power out of the generator goes to zero as the generator rotor angle approaches 180 degrees Deceleration Area Acceleration Area Power 90 180 270 360 450 Acceleration Area Extended Power Angle Curve during Loss of Synchronism

  34. Loss of Synchronization At 180 Degrees: The generator is accelerating very rapidly. Beyond this point the generator electric power reverses and flows into the generator. Deceleration Area Acceleration Area Power 90 180 270 360 450 Acceleration Area Extended Power Angle Curve during Loss of Synchronism

  35. Loss of Synchronization The acceleration continues: Through the negative part of the power angle curve. From point (F) through point (C’) the electrical power into the generator adds to the mechanical power from the turbine and accelerates the rotor assembly to an extremely high rate. Deceleration Area Power (F) (C’) 90 270 360 450 180 Acceleration Area Extended Power Angle Curve during Loss of Synchronism

  36. Loss of Synchronization The acceleration continues: At this point the generator begins to act like an induction generator running ahead of system speed. Deceleration Area Power (F) (C’) 90 180 270 360 450 Governor begins to reduce turbine mechanical power Acceleration Area Extended Power Angle Curve during Loss of Synchronism

  37. Loss of Synchronization Loss of Synchronization: If the generator is not tripped quickly It will begin to run out of step with the system. Deceleration Area Power (F) (C’) 90 180 270 360 450 Governor begins to reduce turbine mechanical power Acceleration Area Extended Power Angle Curve during Loss of Synchronism

  38. Loss of Synchronization Loss of Synchronization: With reduced power from the turbine and with “induction power” flowing out of the generator the generator will experience rapid reversals in “synchronizing” power. Deceleration Area Power (F) (C’) 90 180 270 360 450 Governor begins to reduce turbine mechanical power Acceleration Area Extended Power Angle Curve during Loss of Synchronism

  39. Loss of Synchronization Loss of Synchronization: A generator that experiences these kind of rapid power reversals is said to be “slipping poles”. Deceleration Area Power (F) (C’) 90 180 270 360 450 Governor begins to reduce turbine mechanical power Acceleration Area Extended Power Angle Curve during Loss of Synchronism

  40. Loss of Synchronization Loss of Synchronization: At this point the magnetic bonds between rotor and stator are too weak to keep the generator electrically connected to the system. Deceleration Area Power (F) (C’) 90 180 270 360 450 Governor begins to reduce turbine mechanical power Acceleration Area Extended Power Angle Curve during Loss of Synchronism

  41. Recognizing a Voltage Collapse • Voltage control problems are not new to the utility industry but the problems in the past were primarily associated with the transfer of power from remote generation sites to load centers. • The main symptoms of voltage collapse are – low voltage profiles, heavy reactive power flows, inadequate reactive support, and heavily loaded systems. • The collapse is often precipitated by low-probability single or multiple contingencies.

  42. Recognizing a Voltage Collapse - In high voltage transmission systems, the inductive reactance of a line is typically much greater than the resistance of the line Gen Station A Load Center And Gen • It is very difficult to transfer reactive power long distances. When attempts are made, the reactive losses are often so large that system voltages fall as reactive power reserves are used up.

  43. Recognizing a Voltage Collapse Gen Station A 1.0 % PU 345KV Generation MW Generation MVAR 9000 1150 Load MW Load MVAR Load Center And Gen 6000 1000 1.0 % PU 345KV Generation MW Generation MVAR 3000MW 3000 1150 To transfer 3000MW the lines require 300MVARs Load MW Load MVAR 6000 1000

  44. Recognizing a Voltage Collapse Summer Gen Station A 1.0 % PU 345KV Generation MW Generation MVAR 9000 9600 1150 1262 Load MW Load MVAR Load Center And Gen 6000 1000 1.0 % PU 345KV 3600MW Generation MW Generation MVAR 3000 1200 1150 To transfer 3600MW the lines now require 362MVARs Load MW Load MVAR 6600 6000 1100 1000

  45. Recognizing a Voltage Collapse Summer + in Texas Gen Station A 1.0 % PU 345KV Generation MW Generation MVAR 9000 10,500 9600 1498 1262 Load MW Load MVAR Load Center And Gen 6000 1000 .97 % PU 1.0 % PU 345KV 334.7KV Generation MW Generation MVAR 4500MW 3000 1200 To transfer 4500MW the lines now require 453MVARs Load MW Load MVAR 7500 6600 1245 1100

  46. Recognizing a Voltage Collapse Summer + in Texas + Murphy's Crew is on Gen Station A 1.0 % PU 345KV Generation MW Generation MVAR 10,500 1498 Load MW Load MVAR Load Center And Gen 6000 1000 .97 % PU 334.7KV Generation MW Generation MVAR 3000 1200 Load MW Load MVAR 7500 1245

  47. Recognizing a Voltage Collapse • Transient Instability: A voltage phase angle instability that occurs due to a slow-clearing transmission system fault. Transient instability occurs when a fault on the transmission system near the generating plant is not cleared rapidly enough to avoid a prolonged unbalance between mechanical and electrical output of the generator. • Dynamic Instability Dynamic instability is the condition that occurs when a fast-acting generator AVR control amplifies, rather than damps, some small low frequency oscillations that can occur in a power system.

  48. Dynamic Instability can occur anywhere the load is remote from the generation. • Fast excitation systems are important to improve transient stability, however, a fast-responding excitation system can also contribute a significant amount of negative damping. • Fast excitation systems can reduce the natural damping torque of the system, causing un-damped megawatt oscillations after a disturbance such as a system fault. This type of event can occur if the generator is interconnected to a weak system and loads are far from the generating plant. (October 2006, Gibbons Creek)

  49. October 3rd 2006 College Station Loss of Load Event

  50. Recognizing a Voltage Collapse 1. CCVT on 138 KV Gibbons Creek to Roans Prairie fails

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