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# Loss of excitation

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### Loss of excitation

1. 1. M.M. MuraliMurali MohanMohan DyDy.. SuptdSuptd (O)(O)
2. 2. Excitation:Excitation:-- What it is ?What it is ? Why is required ?Why is required ? How it will be provided ?How it will be provided ? When it will be lost ?When it will be lost ? What will happen if it lost ?What will happen if it lost ? What are the Protection Schemes available?What are the Protection Schemes available? LOSS OF EXCITATION - CONTENTS
3. 3. • Generator is a rotating machine which converts Mechanical Energy from Prime mover into Electrical energy. Basic Principle:Basic Principle: • Basically Generator works on Faraday’s laws of Electro Magnetic Induction. Ist Law: Whenever a conductor placed in a rotating magnetic field an EMF will be induced in that conductor. IInd Law: The magnitude of the Induced EMF is Directly Proportional to the rate of change of flux linkages. E = K.L dØ/dt L = Length of the magnetic flux lines K = Constant dØ/dt = Rate of change of flux linkages GENERATOR
4. 4. Creating the required Magnetic Field strength in the Rotor winding of the Generator by giving D.C supply which when cut by conductors produces Voltage. EXCITATION – WHAT IT IS ? The system which is used to Supply, Control & Monitoring of the D.C supply is called the ExcitationExcitation systemsystem.
5. 5. 1) Basic requirement for the generation of Magnetic FieldMagnetic Field in the air gap between the Rotor and the Stator. 2) Results in the creation of the ““RotatingRotating Magnetic fieldMagnetic field”” in the air gap. 3) To Regulate the Terminal Voltage.Terminal Voltage. 4) To control Reactive Power flowReactive Power flow and facilitate the sharing of reactive load between the machines operated parallel in the grid. 5) Enabling the Maximum utilization of Machine Capability. EXCITATION – WHY IS REQUIRED ?
6. 6. WHAT IS REACTIVE POWER ? • In AC power networks, while Active Power corresponds to useful work, Reactive Power supports voltage magnitudes that are controlled for system Reliability, Voltage stability, and operational acceptability. • Reactive power is essential to “move Active Power” through the Transmission and Distribution system to the customer. • Reactive power is required to “maintain the Voltage” to deliver Active Power (Watts) through transmission lines. ContdContd……..
7. 7. ContdContd…….. (Apparent Power (S))2 = (Active Power (P))2 + (Reactive Power (Q))2 Reactive PowerReactive Power Limitations:Limitations: • Reactive power does not travel very far. • Usually necessary to produce it close to the location where it is needed.
8. 8. Analogy of understanding Reactive PowerAnalogy of understanding Reactive Power Power Factor = Active power / Apparent power = Kw / kVA = Active Power / (Active Power + Reactive Power) = Kw / (Kw + kVAr) The higher kVAr indicates Low Power Factor and vice versa. True Power (MW) ReactivePower(MVAR) Apparent Pow er(M VA) Power factor
9. 9. REACTIVE POWER SOURCES AND SINKS
10. 10. EXCITATION – HOW IT WILL BE PROVIDED ? Excitation systemExcitation system D.C Excitation (up to 110MW) Static Excitation System (200MW) Brush less Excitation system (500MW) Stage- 1 Stage- 2&3
11. 11. 1) Field Open Circuit (Field Current is zero). 2) Field Short Circuit (Field Current is too high). 3) AVR Control Failure. 4) Accidental Tripping of Field Breaker. 5) Loss of supply to the Main Exciter. 6) Poor brush contact in the Exciter. 7) Field Circuit Breaker latch failure. 8) Slip Ring flash over. EXCITATION – WHEN IT WILL BE LOST ?
12. 12. • The Generator delivers both Real and Reactive Power to the grid. The Real power comes from the Turbine while the Reactive power is due to FieldField ExcitationExcitation.. • When Field Excitation is lostField Excitation is lost while the Mechanical Power remains intact, it would attempt to remain synchronized by running as an InductionInduction GeneratorGenerator.. • As an Induction Generator, the machine speeds up slightly above the synchronous speed and drawsdraws ExcitationExcitation from the grid. ContdContd…….. EXCITATION–WHAT WILL HAPPEN IF IT LOST ?
13. 13. GeneratorGenerator GridGrid Mechanical Input Speed = Ns Pm Qe Pe Generator Pe jQe Voltage = V ratedField Current = I f Before Loss of ExcitationBefore Loss of Excitation GeneratorGenerator GridGrid Mechanical Input Speed > Ns Pm Q LOE Pe Generator Pe jQ LOE Voltage = V LOEField Current I f = Zero Loss of Excitation conditionLoss of Excitation condition
14. 14. • Operation as an Induction GeneratorInduction Generator necessitates the flow of ““Slip frequencySlip frequency”” current in the rotor, the current flowing in the Damper Winding and also in the slot wedges and surface of the solid rotor body. Now there are Two Possibilities:Now there are Two Possibilities:-- • Either the grid is able to meet the reactive power demand Fully oror meet it Partially. • If the grid is able to fully satisfy this demand for reactive power, the machine continuous to deliver active power of ‘PPeeMWMW’’ but draws reactive power of ‘QQLOELOE’’ MVA and there is no risk of instability. ContdContd……..
15. 15. • However, the Generator is not designed as an Induction Machine, so “Abnormal Heatingbnormal Heating”” of the Rotor and overloading of Stator winding will take place. • If the Grid able to meet the Reactive Power demand partially then this would be reflected by fall of a Generator Terminal VoltageTerminal Voltage. The Generator would be under excited. • There are certain limits on the degree to which a Generator can be operated within the Under Excited mode. Reduced Excitation weakensweakens the magnetic coupling between the Rotor and Stator. ContdContd……..
16. 16. If the coupling becomes too weak, the Turbine output cannotcannot be fully converted into Electrical form (Pa = Pm-Pe). This leads to acceleration of Rotor, resulting into increased ‘‘δδ’’.. Increased Rotor AngleRotor Angle force the Generator to lose Synchronism. • Therefore, the operation in case of loss of excitation must be quickly detected and checked. ContdContd……..
17. 17. • If a generator is operating at full loadfull load when it loses excitation, it will reach a speed of 2% to 5%2% to 5% above normal. • This over speed condition will be harmful to Steam Turbine driven GeneratorsSteam Turbine driven Generators. • If a Generator is operating at reduced loading ((<< 30%),30%), the machine speed may only be 0.1% to 0.2%0.1% to 0.2% above normal. ContdContd……..
18. 18. • When Excitation is lost, rotor current (If), Internal voltage (E) and terminal voltage (Vt) falls. • Due to reduced voltage, Stator current increases for the same ‘Pe’. • As V/I ratio become smaller, the Generator Positive Sequence ImpedancePositive Sequence Impedance (Z+) as measured at its terminals will reduce and enter the 4th Quadrant of the R-X plane.
19. 19. MVAR - MVAR - MW MW Machine acts as an Induction Generator Machine acts as an Induction Motor Machine acts as an Synchronous Generator Machine acts as an Synchronous Motor P Q P Q P Q P Q + jX - jX + R- R Q-IQ-II Q-III Q-IV O  POWER FLOW DIRECTION AND POWER FACTORPOWER FLOW DIRECTION AND POWER FACTOR
20. 20. Generator Active and Reactive Power after LOE
21. 21. Voltage Drop and Rotor acceleration During LOE faultVoltage Drop and Rotor acceleration During LOE fault
22. 22. Typical Generator Capability Curve
23. 23. • The simplest method by which loss of excitation can be detected is to monitor Field currentField current of the Generator. • If the filed current falls below a threshold, a loss of field signal can be raised. • A complicating factor in this protection is the Slip Frequency CurrentSlip Frequency Current induced in the event of loss of excitation and running as an Induction Generator. LOSS OF EXCITATION – PROTECTION SCHEMES ContdContd……..
24. 24. • The quantity which changes most when a Generator loses Field ExcitationField Excitation is the ImpedanceImpedance measured at the Stator terminals. • On loss of excitation, the terminal voltage begins to decrease and the current begins to increase, resulting in “Decrease of ImpedanceDecrease of Impedance””.. • The Loss of Excitation can be unambiguously detected by a Mho relayMho relay located at the Generator terminals. • In 1949, a Single Phase “Offset Mho Relay” was introduced for the high speed detection of ““Loss ofLoss of ExcitationExcitation”” in Synchronous Generators. ContdContd……..
25. 25. There are Five LOE protection schemes used today, namely, 1) R-X Scheme with Single and Double Relay Scheme (Based on Generator terminal Impedance measurement). 2) R-X with Directional element Scheme (-do-). 3) G-B Scheme (Based on Generator terminal Admittance measurement). 4) P-Q Scheme (Based on Generator Active and Reactive power output). 5) U-I Scheme (Based on the measurement of Phase Angle difference between Phase Voltage and Current). However, R-X Schemes is widely used in Power Systems. ContdContd……..
26. 26. • The diameter of the circle set equal to the ““Synchronous ReactanceSynchronous Reactance”” (Xd) and Offset will be set equal to one - half of the ““Transient ReactanceTransient Reactance”” (X’d/2). • This circle is operation zone for LOE relay. • As viewed from the machine terminals the Relay will operate for any impedance phasor that terminates inside the circular characteristic. ContdContd…….. IMPEDANCE MEASUREMENT (SINGLE ELEMENT)
27. 27. • In normal operation condition, the Generator generates Active and Reactive Power to the system which means both ‘R’ and ‘X’ are positive and the Terminal Impedance is located in the First Quadrant in R-X plane. • When the Excitation is lost, the Generator starts to draws Reactive power from the system and ‘X’ becomes Negative from the LOE relay point of view. • As a result, the Terminal Impedance loci in R-X plane moves to the ‘Forth Quadrant’ and the endpoint of terminal Impedance ranges between the sub Transient Reactance and Synchronous Direst Axis reactance.
28. 28. - R + X + R - X Xd X’d/2 Relay Operating Characteristic When the measured Impedance falls into the operating region, the relay function will be picked up and after a certain Time Delay to enhance the security for power swing, A trip signal will be sent to the GeneratorGenerator Breaker.Breaker.
29. 29. Typical impedance loci on loss of Excitation X R Xd X’d/2 - R - X III III IV Rated Load Medium Load Low load Locus of Apparent Impedance Time Increasing Time = 0 Trip Initially it’s a Motoring Action when Excitation fails After motoring action Machine starts to work As Induction Generator
30. 30. • To limit system voltage, the Generators may have to operate Under Excited and absorb VARS from the power system. • It is important that the Generator be able to do so within its capabilities as defined by the Generator Capability Curve. • The Generator Under Excitation Limiter (UEL) must be set to maintain operation within the capability curve. • The Loss Of Field Relay must be set to allow the Generator to operate within its Under Excited Capability.
31. 31. Impedance measurement (Double Element) • This protection scheme applies Two offset MhoTwo offset Mho ImpedanceImpedance circles by using the Generator Terminal side Voltages and Stator Currents as input signals. • The Offset-mho relay in the impedance plane has two circles with a diameter of Direst Axis Transient Reactance X’d and a Negative offset of X’d /2 for the Outer Circle. • And the diameter of ‘1.0’ (pu) and a Negative offset of X’d /2 for the Inner Circle. • Zones 1 and 2 are for detecting LOE with full load and light load. The typical time delays for Zone - 1 & Zone - 2 are about 0.1 s & 0.5–0.6 s.
32. 32. ZoneZone -- 22 Xd X’d/2 - R + R - X + X 1.0 pu Heavy load Light load Min Exciter Limiter MachineMachine capabilitycapability Steady stateSteady state Stability limitStability limit ZoneZone--11 Machine operatingMachine operating Limit in Leading PFLimit in Leading PF ZoneZone -- 2 setting crosses2 setting crosses Steady state stability limitSteady state stability limit 131300
33. 33. R-X with Directional Element Scheme • It’s a combination of Two Impedance elements, a Directional unit and an Under voltage unit applied at the Generator Terminals. • The Zone - 2 element is set to coordinate with the Steady State Stability Limit. The top of the Zone - 2 circle (positive offset) is set at the System Impedance in front of the Generator. • It will detect reduced or Loss of Excitation condition, raise an alarm and if the abnormality persists, Trips the Generator.
34. 34. 1.1Xd - R + R - X + X - Xd/2 Zone-1 Zone-2 XTG+Xmin SG1 Heavy Load Light Load Impedance locus During loss of field Directional Element Two zone Loss of field scheme with Directional unitTwo zone Loss of field scheme with Directional unit Min Exciter Limiter Machine capabilityMachine capability
35. 35. Stage – 1 Relay details + X + R- R - X Z2 Z1 Z1 = 2.17 Ω Z2 = 12.25 Ω CT Sec = 5A, PT Sec = 110V Make – ALSTOM, Type = YCGF Model – YCGF11AF1A Stage 2&3 Relay details + X + R- R - X Z2 Z1 Z1 = 3.0 Ω Z2 = 14.88 Ω CT Sec = 5A, PT Sec = 110V Make : English Electric, Type = YCGF Model – YCGF11AF1A5 Xd X’d/2 Xd X’d/2
36. 36. Typical Relay setting calculations Information required:Information required:-- PT Ratio : 22000 : 110 = 200 : 1 CT Ratio : 20000 : 5 = 4000 : 1 Transient Reactance (X’d) : 0.30 Ω (0.16 to 0.45 Ω) Synchronous Reactance (Xd) : 2.50 Ω (2.0 to 3.90 Ω) Generator Rating : 588 MVA Generator Voltage : 21.0 KV Calculation:Calculation:-- T = CT Ratio / PT Ratio : 4000 / 200 = 20 Base Ω (Pri) = KV2/MVA : 21 X 21 / 588 = 0.75 Ω Base Ω (Sec) = T X Base ohms (Pri) : 20 x 0.75 = 15 Ω
37. 37. X’d (Sec) = X’d x Base Ω (sec) : 0.30 x 15 = 4.50 Ω. Desired offset = X’d/2 : 4.50 / 2 = 2.250 Ω. Xd (Sec) = Xd (pu) x Base Ω (sec) : 2.50 x 15 = 37.50 Ω Diameter of circle = 37.50 Ω Offset setting = 2.250 Ω
38. 38. Stage – 1 Relay details CT Ratio - 8500 / 5A PT Ratio - 18.7KV / 110V Diameter = 12.25 Ω Offset ZR =2.17 Ω K1 = 0.91, K2 = 2.5 K3 = 0.5, K4 = 2.0 K5 = 13.4 Timer SettingTimer Setting Trip = 2 sec - 2A/40G Reset = 10 sec - 2B/40G VTIGM setting = 80V VAA21:- Time delay on reset = 200 m sec. ( fixed ) Z1 = K3+K4 = K2 Ω Z2 = K1 x K5 Ω TypeType - YCGF ModelModel – YCGF11AF1A Stage - 2 Relay details CT Ratio - 20,000 / 5A PT Ratio - 22KV / 110V Diameter setting - ZF = 14.88 Ω Offset setting - ZR= 3 Ω K1 = 0.8, K2 = 3.0 K3 = 1.0, K4 = 2.0 K5 = 18.6 Timer Setting:Timer Setting: Trip = 2 sec - 2A / 40G Reset = 2 sec - 2B / 40G VTIGM setting = 80V VAA21 = 200 mA Z1 = K3+K4 = K2 Ω Z2 = K1 x K5 Ω TypeType - YCGF ModelModel – YCGF11AF1A5
39. 39. 87 G EHG 37 GA 37 GB 32 GA 32 GB TESTING 87 GT 21 G 40 GA 40 GB 98 G 46 G 50 GDM DR AVR EHG 51 NGT 87 T EM 51 UT 87 UT 64 RUT 51 NUT 400 KV Bus -I 400 KV Bus -II CORE-5 CORE-4 CORE-3 CORE-2 CORE-1 87 HV METERING VT3 VT1VT2 GT TRANS Y/∆ CORE-1 CORE-2 METERING GENERATOR 100% STATOR E/F (64G1) & INTER TURN PROTN (95G) CORE-1 CORE-2 CORE-3 CORE-4 CORE-5 CORE-6 CORE-7 CORE-8 UAT ∆/Y 400KV TEE PROT1/2 LBB LBB B/B PROTN B/B PROTN B/B PROTN B/B PROTN SPARE TEE PRT DIFF 1/2 400 KV Bus -I 400 KV Bus -II 400KV CVT VT1:- 64G2,59G,81G,27G,99GT,64G1,98G,21,40G VT2:- AVR / EHG / SYNC VT3:- PERFORMANCE TEST / AVR /EHG / LOW FRWD /REV POWER RELAYS Typical Generator Protection scheme (500MW)
40. 40. ANSI/IEEE Standard Device Numbers 1 - Master Element 2 - Time Delay Starting or Closing Relay 3 - Checking or Interlocking Relay 4 - Master Contactor 5 - Stopping Device 6 - Starting Circuit Breaker 7 – Rate of Change Relay 8 - Control Power Disconnecting Device 9 - Reversing Device 10 - Unit Sequence Switch 11 – Multifunction Device 12 – Over speed Device 13 - Synchronous-speed Device 14 – Under speed Device 15 - Speed or Frequency-Matching Device 16 – Data Communications Device 20 - Elect. operated valve (SV) 21 - Distance Relay 23 - Temperature Control Device 24 – Volts per Hertz Relay 25 – Synchronizing Check Device 26 - Apparatus Thermal Device 27 – Under voltage Relay 30 - Annunciator Relay 32 - Directional Power Relay 36 - Polarizing Voltage Devices 37 - Undercurrent Relay 38 - Bearing Protective Device 39 - Mechanical Conduction Monitor 40 –Field failure Relay 41 - Field Circuit Breaker 42 - Running Circuit Breaker 43 - Selector Device 46 –Phase- Bal. Current Relay
41. 41. 47 - Phase-Bal. Voltage Relay 48 - Incomplete-Sequence Relay 49 - Transformer Thermal Relay 50 - Instantaneous Over current 51 - AC Time Over current Relay 52 - AC Circuit Breaker 53 – Field Excitation Relay 55 - Power Factor Relay 56 - Field Application Relay 59 – Over voltage Relay 60 - Voltage or Cur. Balance Relay 62 – Time-Delay Stopping / Opening Relay 63 - Pressure Switch 64 - Ground Detector Relay 65 - Governor 66 – Notching or jogging device 67 - AC Directional OC Relay 68 - Blocking or “out of step” Relay 69 - Permissive Control Device 74 - Alarm Relay 75 - Position Changing Mechanism 76 - DC Over current Relay 78 - Phase-Angle Measuring Relay 79 - AC-Reclosing Relay 81 - Frequency Relay 83 - Automatic Selective Control or Transfer Relay 84 - Operating Mechanism 85 – Pilot Communications, Carrier or Pilot Wire Relay 86 - Lockout Relay 87 - Differential Protective Relay 89 - Line Switch 90 - Regulating Device 91 - Voltage Directional Relay 92 - Voltage and Power Directional Relay 94 - Tripping or Trip-Free Relay
42. 42. Induction Generator • An Induction Generator or Asynchronous Generator is a type of AC Electrical Generator that uses the principles of Induction motors to produce power. • Induction Generators and motors produce electrical power when their rotor is turned faster than the SynchronousSynchronous SpeedSpeed.. • In Generator operation, a Prime mover (Turbine) drives the rotor above the synchronous speed. The stator flux still induces currents in the rotor, but since the opposing rotor flux is now cutting the stator coils, an active current is produced in stator coils and the motor now operates as a Generator, sending power back to the Electrical Grid.
43. 43. • The overall reactance of the Armature winding is the sum of its Leakage Reactance plus Fictitious Reactance, which is known as Synchronous ReactanceSynchronous Reactance (Xd). • The Impedance of armature winding is obtained by combining its Resistance and its Synchronous Reactance. It is called Synchronous ImpedanceSynchronous Impedance ‘‘ZsZs’’.. Synchronous Reactance and Impedance
44. 44. • Synchronous Reactance determines steady- state current. However, when a sudden change from steady state occurs, such as short circuit, other reactance's come into play. This happens because the flux in the machine cannot change immediately. • Sub-Transient Reactance determines maximum instantaneous current. It lasts up to about 6 cycles. • Transient Reactance is a longer lasting reactance determining current up to as much as 5 seconds.
45. 45. • Zero Sequence Reactance determines neutral currents in grounding studies. It is also a factor in determining neutral currents when third harmonics are encountered. • Negative Phase Sequence Reactance is used in calculating line-to-line faults. • Transient Reactance (X’d):- It is One of the Five reactance figures frequently used by engineers when comparing Generator capability with load requirement, or when comparing one Generator with another.
46. 46. a)Sub transient Reactance = X’’d b)Transient Reactance = X’d c) Synchronous Reactance = Xd Total Short Circuit Current
47. 47. 21 G GENERATOR BACK UP IMPEDANCE PROTECTION 40 G A / B FIELD FAILURE PROTECTION 46 G NEGATIVE SEQUENCE PROTECTION DR DIGITAL FAULT & DISTRUBENCE RECORDER 98 G POLE SLIPPING PROTECTION 37 GA / GB - 32GA / GB LOW FORWARD / REVERSE POWER RELAYS 87 G GENERATOR DIFFERENTIAL PROTECTION 87 GT OVER ALL DIFFERENTIAL PROTECTION 87 T GENERATOR TRANSFORMER DIFFERENTIAL PROTECTION 87 UT UNIT AUXILLIARY TRANSFORMER DIFFERENTIAL PROTECTION 64 RUT UNIT AUXILLIARY TRANSFORMER RISTRICTED EF PROTECTION 51 NUT UNIT AUXILLIARY TRANSFORMER EARTH FAULT PROTECTION 51 UT UNIT AUXILLIARY TRANSFORMER OVER CURRENT PROTECTION 50 Z BREAKER FAILURE PROTECTION 87 HV TRANSFORMER HV WINDING + OVER HANGE DIFFERENTIAL PROTECTION 51 NGT GENERATOR TRANSFORMER BACK UP EARTH FAULT PROTECTION 99 GT GENERATOR TRANSFORMER OVER FLUXING PROTECTION 64 G2 95% STATOR EARTH FAULT PROTECTION 81 G UNDER FREQUENCY PROTECTION 51 G OVER VOLTAGE PROTECTION 27 G UNDER VOLTAGE PROTECTION RELAY NUMBERS AND THEIR UNIVERSAL NOMENCLATURERELAY NUMBERS AND THEIR UNIVERSAL NOMENCLATURE
48. 48. • Synchronous machine maintains constant flux. When DC field current gets reduced (under excited), to strengthen main field, it absorb reactive power (draw current from AC supply mains). • In reverse, when DC field current gets increased (over excited), to weaken main field, it deliver reactive power to the bus bar. • All these are controlled by magnetizing and demagnetizing effect of Armature Reaction Excitation
49. 49. • Generator Active Power output equation: Eq Us Pe = ----------- Sin δ Xs Where ‘Pe’ = Active Power output to the system. ‘Eq’ = Gen int. vol. behind the d-axis Synch Reactance. ‘Us’ = Equivalent System Voltage. ‘Xs’ = Direct axis Synch Reactance. ‘δ’ = Angle between Eq and Us. Pe α Eq, Us Sin δ.
50. 50. • As the Generator internal Voltage Eq is a function of Field Voltage, the Generator Active Power output is a function of Field Voltage as well. • When the generator operates at δ=90◦, any increase of Mechanical Power or decrease of Electrical Power will lead to Generator Loss of Synchronism. Generator Active PowerGenerator Active Power Vs Load AngleVs Load Angle