By Distribution Automation Training Center Generator protection © ABB Oy September 16, 2011 | Slide 1 Contents Why Generator protection is important Four quadrant operation of AC machines Type of faults and failures R Generator protection for electrical faults Generator protection tripping methods Protection Application © ABB Oy September 16, 2011 | Slide 2 Why generator protection is important • The typical electrical power system consists of Turbine, Generator, Transformer, Transmission system, Distribution system and Load M G Generation Transmission Distribution Load • The heart of an electrical power system is generator • With no generators there would be no electric power • Prolonged fault in the generator affects rest of the system • Repair of generator is more expensive and time consuming © ABB Oy September 16, 2011 | Slide 3 Why generator protection is important • The requirement of protections is not only dependent on size of the generator but is mainly dependent on it’s necessity / importance to the the power system • The selection and arrangement of protections is influenced to some extent by the method in which the generator is connected to the system in the generating station © ABB Oy September 16, 2011 | Slide 4 Why generator protection is important • The following situations require consideration R R • Protection against faults inside of the protection zone • Effects in the fault location should be minimized • Effects to the network should be minimized. G G Fast, reliable and sensitive operation • Protection against faults outside of the protection zone and against network disturbances R R R R • Limiting the effects (damages) to the generator and to the prime mover to a safe level Selective, stable and reliable operation © ABB Oy September 16, 2011 | Slide 5 Four Quadrant operation of AC machines Q_GENERATION SYNCHRONOUS MOTOR LEADING REGION SYNCHRONOUS GENERATOR LAGGING REGION P_ABSORPTION P_GENERATION SYNCHRONOUS MOTOR LAGGING REGION INDUCTION MOTOR SYNCHRONOUS GENERATOR LEADING REGION INDUCTION GENERATOR Q_ABSORPTION © ABB Oy September 16, 2011 | Slide 6 • Operation Principle of AC Generator • It is a device that converts Mechanical Energy to Alternating Current Electrical Energy • Source of Mechanical Energy • Steam turbine • Water wheel • Wind turbine etc. • Main parts • Stator > Armature > The stationary part of AC Generator • Consists of copper bars / conductors • To generate an Electro Motive Force (EMF) • To carry current crossing the field and creating shaft torque • Rotor > Field > The rotating part of AC Generator • Exciter supplies DC to the rotor to create the magnetic field © ABB Oy September 16, 2011 | Slide 7 • Operation Principle of AC Generator • Electromagnets which produces the field will rotate so that magnetic field sweeps past the armature coils • Works on the principle of electromagnetic induction • it generates voltage that has a frequency proportional to speed of rotor • N = 120 f / P • N = 120 x 50 / 2 = 3000 RPM • N = 120 x 50 / 4 = 1500 RPM © ABB Oy September 16, 2011 | Slide 8 Animation © Motorola, Inc. • Power capability diagram of AC generator © ABB Oy September 16, 2011 | Slide 9 Type of faults and failures • Inside faults and failures • Stator 1. Interturn faults 2. Windings short-circuits 3. Earth-faults 4. Local ‘hot spots’ • Rotor & excitation 5. Earth-faults, interturn faults, short-circuits, breaks 6. Over/under-excitation (causes over / under-voltage, thermal stress, loss-of-synchronism) © ABB Oy September 16, 2011 | Slide 10 Type of faults and failures • Inside faults and failures • Prime mover 7. Over/under-frequency, reverse power, out of step (causes mechanical and thermal stress to the generator and mover • Objectives • Limiting the effects to the generator and network to a safe level. • Fast, Sensible and accurate operation © ABB Oy September 16, 2011 | Slide 11 Type of faults and failures • Outside faults and failures 8. Short –circuits, earth faults, unbalance, overload 9. Disconnection of load, generators or feeders from the network (causes over/ under frequency, over/under voltage) Objective • Limiting the effects to the generator and network to a safe level • Selective and stable operation © ABB Oy September 16, 2011 | Slide 12 R Generator protection for electrical faults 21.............Three phase under impedance protection Backup for system and generator zone phase faults. Requires a time delay for coordination. 24.............Over excitation or Volts / Hz. Protection Protection for generator and it’s associated step-up, aux. transformer 27………..Three phase under voltage protection 32P………Three phase active reverse power protection 32Q………Three phase reactive reverse power protection 37………..Three phase under power / low forward power protection 40………..Under excitation protection 46………..Negative phase sequence protection 49………..Thermal overload protection for generator © ABB Oy September 16, 2011 | Slide 13 R Generator protection for electrical faults 50/51……...Non-directional over current protection 50N/51N…..Non-directional earth fault protection 51V………..Voltage dependent over current protection for system with generator that is directly connected to the bus 59…………Three phase over voltage protection 59BG……..Zero sequence voltage protection. Ground fault protection for an ungrounded system 67…………Three phase directional over current protection 67N……….Directional earth fault protection 81…………Under or over frequency including rate of change of frequency 87G……….Differential protection primarily for phase fault detection 87GN……..Differential protection for sensitive ground fault detection © ABB Oy September 16, 2011 | Slide 14 R Generator protection for electrical faults Additional requirement 25...............Synchro / voltage check function 60…………Fuse failure supervision. Detection of blown potential transformer fuses. CBFP..……Circuit Breaker Failure Protection © ABB Oy September 16, 2011 | Slide 15 Tripping methods • The trip of the circuit breaker is not enough in generator faults • The prime mover (turbine) must be shut down • Magnetization must be removed • Other measures (alarming etc.) • Trip logic varies depending on user’s preferences and capability of prime mover • Common methods for isolating the generator from rest of the network under unacceptable or electrical fault conditions : • Simultaneous tripping • Sequential tripping • Generator tripping © ABB Oy September 16, 2011 | Slide 16 Tripping methods • Factors to consider for choice of tripping method • Severity of fault to the generator • Probability of fault spreading • Amount of resulting over-speed • Importance of removing excitation • Need for maintaining auxiliary power • Need for shutting down the unit • Time required to resynchronise • Effect on the power system © ABB Oy September 16, 2011 | Slide 17 Tripping methods • Simultaneous tripping (Class A Logic) • Fast mode of generator isolation R • Used for internal faults and severe abnormalities in generator protection zone • Tripping of generator breaker, field breaker, shutting down the prime mover by closing the turbine valves carried out at the same time without any time delay © ABB Oy September 16, 2011 | Slide 18 G V DC R Tripping methods • Sequential tripping (Class B Logic) • Primarily used when delayed tripping has not major effect on the generating unit R 1 G 2 V DC • Generally used to trip the generator for problem in R the prime mover and high speed tripping is not the requirement • Turbine trips first followed by tripping of generator and field breaker • Reverse power relay used to ensure that the steam flow is reduced and thus to avoid possible over-speeding condition • Low forward power relay can be used as a back-up to ensure tripping in case if this scheme fails to operate. © ABB Oy September 16, 2011 | Slide 19 2 Tripping methods • Generator tripping (Class C Logic) R 2 • Tripping of generator line breaker • Tripping of generator field breaker G V DC • Turbine continues to run at rated speed • Generally used when there is a possibility to clear the abnormality quickly and reconnection of generator to the system • Tripping generally carried out due to external faults • Unit should be capable of sudden load throw off conditions © ABB Oy September 16, 2011 | Slide 20 R 1 Protection Application • Thermal Overload protection 49 • Nature of the fault • Stator over current or overload • Short circuit • System disturbance accompanied by other generator or line tripping • Coolant circuit failure • Effect of thermal overload • Temperature of the winding increases • Stress on the insulation • Possibility of insulation break through and flashover • Actuating quantity • Current • Temperature © ABB Oy September 16, 2011 | Slide 21 G Protection Application • Insulation • Insulating material is a substance in which the electrical conductivity is negligible and it provides an electric isolation • Useful service life is the length of time (usually in hours) for which an insulating material performs in an adequate or specified fashion • Thermal aging takes place at an elevated temperature. • Temperature class is a designation of the temperature capability of the insulation in electric equipment • Class of insulation © ABB Oy September 16, 2011 | Slide 22 Maximum operation temperature allowed A 105oC B 130oC F 155oC H 180oC Protection Application • Insulation • Insulation life is defined with the base of 20,000 hrs. • Tensile strength reduces after 20,000 hrs. of operation at defined temperature • For operation at reduced temperature, the life of insulation increases • For operation at increased temperature, the life of insulation decreases © ABB Oy September 16, 2011 | Slide 23 Protection Application • Thermal Overload protection criteria • The continuous output capability of a generator is expressed in kilovolt-amperes (kVA) available at the terminal at a specified frequency, voltage and power factor. • For a short time, it is permissible to exceed the continuous output capability • In accordance with ANSI C50.13-1989, the armature winding short-time thermal capability is given by the following: Time (seconds) 10 30 60 120 Armature current (percent) 226 154 130 116 • Stage 1 to be wired for alarm for small overload condition • Operator to take corrective action • Stage 2 to be wired for trip for heavy overload condition • meant for tripping © ABB Oy September 16, 2011 | Slide 24 Protection Application • Thermal Overload protection criteria • Current based protection : • Accuracy of time constants more important for calculation of thermal replica • Thermal overload curve should lie below generator overload capability curve • RTD based protection • Reliable as temperature increase is a function of thermal power • Can work for coolant circuit failure • Need good connection and proper maintenance © ABB Oy September 16, 2011 | Slide 25 Protection Application • Thermal Overload protection criteria © ABB Oy September 16, 2011 | Slide 26 Protection Application • Short circuit protection • Nature of the fault • Stator internal fault • Stator external fault (for example in the machine terminals) • Backup for the network short-circuit • Effects of short circuit • Runaway (especially in close-up short-circuits, the voltage collapses, generator cannot generate power to the network => speed increases rapidly • Loss of stability/mechanical stress when short-circuit protection is tripped => fast operation needed • results in broken coupling, winding burns • Actuating quantity • Current © ABB Oy September 16, 2011 | Slide 27 Protection Application • Short circuit protection criteria • Stator internal fault, does it matter where the CTs are located? a b c R G G G R Protects at internal stator faults © ABB Oy September 16, 2011 | Slide 28 Protects only if CB is closed and other generators or network feed fault current Protection Application • Short circuit protection criteria Blocking 50 • Non-directional over current protection (50/51) • Instantaneous (50) : G • Generally not recommended for Incomer feeder as there are chances of mal-operation CLOSE during fault on outgoing feeder • Can be provided on incomer feeder CLOSE with reverse blocking scheme logic • Pick-up setting : 50 51 Fault 150% or 200% of the generator rated current • Time setting : As per system requirement © ABB Oy September 16, 2011 | Slide 29 50 51 Protection Application • Short circuit protection criteria 51 • Non-directional over current protection (50/51) • IDMT (51) : G • Pick-up setting should be such that it will - not pick up during an emergency overload CLOSE condition - not pick up during starting of highest size CLOSE motor on the bus • Characteristic and time setting : 50 51 Fault • Depends on the system co-ordination requirement • Should be such that it will act - as a back-up to outgoing feeder fault - as a primary protection for bus fault © ABB Oy September 16, 2011 | Slide 30 50 51 Protection Application • Terminal short circuit protection • Realization of protection : • The effect of reactance in an AC System is to cause the initial current to high and then to decay the same to steady state value. • Sub-transient reactance determines the fault current during the first half cycle • Transient reactance determines the fault current after about 6 cycles • 0.5 to 2 seconds it reaches to value of synchronous reactance. • Fault current magnitude could be well below the generator rated current and thus conventional over current relay may not detect the fault current • If the short-circuit occurs at or near machine terminals, the generator voltage can also drop so much that the actual fault current can be less than the rated current © ABB Oy September 16, 2011 | Slide 31 Protection Application • Terminal short circuit protection • Thus arises a need of Voltage controlled over current protection (51V) • In the voltage controlled over current relays, the current pick-up is influenced by the measured voltage • At normal voltage : relative high current pick-up setting Time • At reduced voltage : relative low current pick-up setting Normal voltage : Coordinated with network over current protection Reduced voltage : drops down the pick-up setting and become sensitive Current © ABB Oy September 16, 2011 | Slide 32 Protection Application • Terminal short circuit protection 51V • Voltage controlled over current protection (51V) • Voltage Step Mode : G e.g. If I > = 1 x In ; Voltage limit = 60% Un and K = 0.5 then, Start current for voltage above 60% Un = 1 x In Start current for voltage below 60% Un = 0.5 x 1 x In = 0.5 x In © ABB Oy September 16, 2011 | Slide 33 Protection Application • Terminal short circuit protection • Voltage controlled over current protection (51V) • Voltage Slope Mode : e.g. If I > = 1 x In ; Voltage limit 1 = 80% Un ; Voltage limit 2 = 50% and K = 0.5 then, Start current for voltage above 80% Un = 1 x In Start current for voltage below 50% Un = 0.5 x 1 x In = 0.5 x In © ABB Oy September 16, 2011 | Slide 34 Protection Application • Terminal short circuit protection • Voltage controlled over current protection (51V) • Pick-up setting : • At normal voltage : Criteria similar to non-directional over current protection • At reduced voltage : Should detect the current at reduced voltage and it depends on the fault current calculation • Characteristic and time setting : Depends on the system coordination requirement • Typically provided when the generator is directly connected to the bus © ABB Oy September 16, 2011 | Slide 35 Protection Application • Back-up for short circuit protection 21 • Under impedance protection (21) • Typically provided when generator is connected G to the bus through transformer • the relay measures the impedance by calculating the impedances of the incoming voltages and R currents. • the operation of the underimpedance function block is based on comparing the impedances to the operating characteristic. • relay operates if the measured impedance falls below the set limit i.e. enters the circle. © ABB Oy September 16, 2011 | Slide 36 Protection Application • Back-up for short circuit protection 21 • Under impedance protection (21) • as a result of the origin-centric characteristic, the G operation is non-directional since the angle of the impedance has no effect on the operation but only the magnitude of the impedance determines the operating point. • the voltage should be measured from the generator terminals and the current from the neutral point of the generator. • the function block is usually set to protect the zone between the generator windings and the generator side windings of the step-up transformer. © ABB Oy September 16, 2011 | Slide 37 R Protection Application • Back-up for short circuit protection 21 • Under impedance protection (21) • to prevent mal-operation of the under impedance G protection in case of near-by faults that should not be detected by relay, the settings must be reasonably dimensioned. • a setting equal to 70% of the step-up transformer short-circuit impedance is commonly used. • therefore, if used in directly connected machines where the impedance towards the network is limited only by the lines/busbars, it must be secured that the relay will not cause any selectivity problems. • in general, the voltage-dependent over current protection is recommended for the protection of directly connected machines. © ABB Oy September 16, 2011 | Slide 38 R Protection Application • 100% Stator earth fault protection • 0 – 95 % Stator earth fault protection • 95 – 100 % Stator earth fault protection • The magnitude of stator ground fault current 0 – 95% 95 - 100% decreases as the fault location moves from the stator terminal towards the neutral of the generator • For a ground fault near the neutral, the available NGR phase to ground fault current becomes small regardless of the grounding method © ABB Oy September 16, 2011 | Slide 39 Protection Application • Stator earth fault • Nature of the fault • An insulation break-through between a stator winding and the frame of the machine causes a stator earth fault • Most stator winding faults phase - ground • Winding in close contact with steel slots • Not close to other phase conductors except at end turns • Phase - phase faults less severe • Damages can sometimes be repaired by tapping the damaged insulation • Phase - ground faults very severe • Melting of steel laminations • core damage • repair at manufacturer’s work • long outage © ABB Oy September 16, 2011 | Slide 40 Protection Application • Stator earth fault (0-95%) : • Typically voltage based if ground fault current is limited to < 15 Amp • Voltage relay shall be insensitive to third harmonic quantities • For fault very near to neutral, voltage across relay too low for pick up and thus 100% winding is not 59 GN protected • Hence it is known as 0-95% earth fault protection © ABB Oy September 16, 2011 | Slide 41 Protection Application • Stator earth fault (95-100%) : • First earth fault very near to neutral is not severe • Voltage near neutral is zero • Current is negligible as driving voltage is zero • But if second earth fault occurs at machine terminal and first earth fault is not cleared then ? • fault current is not restricted by NGR or by any other means • high current, may be in kAmp flows instead of few Amps ! • Hence it is necessary to clear the first earth fault near neutral as early as possible which is nothing but 95 – 100% earth fault protection © ABB Oy September 16, 2011 | Slide 42 Protection Application • Stator earth fault (95-100%) : • Basis - presence of third harmonic voltage • V3rd_harmonic at line end & neutral end can vary considerably from maximum to minimum load 59 O/V Tuned for 50Hz • V3rd_harmonic at maximum load V3rd_harmonic at neutral end ---------------------------------V3rd_harmonic at line end 2 x V3rd_harmonic at minimum load constant @ all loads Contact configuration : 27 GN U/V Tuned for 150Hz 27GN 59 Healthy condition : No Trip 27GN 59 Fault near neutral : Trip © ABB Oy September 16, 2011 | Slide 43 27GN 59 Protection Application • Rotor earth fault protection : • Nature of the fault • The excitation field circuit is isolated during normal operation • The field circuit can be exposed to abnormal mechanical or thermal stress and results in a breakdown of insulation between field winding and the rotor iron at a point exposed to excessive stress • Effect of rotor earth fault • Single earth fault • No flow of fault current • But the insulation stressed at the point of fault © ABB Oy September 16, 2011 | Slide 44 Protection Application • Rotor earth fault protection : • Second earth fault • Appears as a rotor winding inter-turn fault • Causes severe magnetic unbalance and unsymmetrical flux distribution • Air gap flux badly distorted • Rotor displaced enough to rub stator • Severe vibration damage to bearings • Protection requirement is to detect earth fault in the excitation circuit © ABB Oy September 16, 2011 | Slide 45 Protection Application • Rotor earth fault protection : • Current injection device REK510 and sensitive earth fault relay : © ABB Oy September 16, 2011 | Slide 46 Protection Application • Zero sequence voltage protection : • Known as Residual over voltage protection : • Nature of the fault G • Earth fault in ungrounded systems wherein there is Y no intentional direct grounding but grounding is through natural capacitance of the system • Effect of the fault • When fault happens, capacitance of the faulty phase is bypassed and system becomes asymmetrical • The absolute value of neutral voltage is then equal to positive sequence phase to earth voltage of the system • The highest potential of the healthy phase during an earth fault increases to the phase-to-phase voltage © ABB Oy September 16, 2011 | Slide 47 59BG Protection Application • Zero sequence voltage protection : • In a healthy ungrounded system, the phase to earth voltages are = 3.81 kV ( e.g. 6.6kV system) thus U1 = UL1-E = 3.81kV U2 = UL2-E = 3.81kV U3 = UL3-E = 3.81kV Uo = 0 • Consider an earth fault in L3 phase, then VT secondary voltage on L1 and L2 phases will be as follows : (e.g. VT ratio = 6.6kV/sq.rt. 3 : 110V/3 ) U1 = UL1-E = 63.5 Volts U2 = UL2-E = 63.5 Volts U3 = UL3-E = 0 Volts © ABB Oy September 16, 2011 | Slide 48 Protection Application • Zero sequence voltage protection : • Uo can be calculated with low of cosine : Uo = Uo = U1 2 U2 2 63.5 Uo = 110 Volts 2 63.5 2 U1 2 U2 Cos 120 o 2 63.5 63.5 Cos120o • The system requires line voltage insulation • Stress on the insulation if fault is not cleared • Effect of the fault • Detection of fault through open delta voltage of VT • Fault can be detected but can not be located • Generally wired for alarm to avoid unwanted tripping due to external faults © ABB Oy September 16, 2011 | Slide 49 Protection Application • Over excitation protection (24) • Nature of the fault • Fundamental voltage – flux relation V = 4.44 x N x x V --- = 4.44 x N x V/f 24 =Kx Measure of flux in the machine • Automatic Voltage Regulator failure • Load rejection when AVR is in manual control • Excessive excitation when generator is off-line • decreasing speed while operator tries to maintain rated stator voltage • increasing of voltage and/or decreasing of frequency © ABB Oy September 16, 2011 | Slide 50 G Protection Application • Over excitation protection (24) • Effects of over excitation • Thermal stress to the iron core by the over-magnetising • Increasing of magnetic flux density • Saturation of iron core • Increasing of leakage flux and spreading of flux to unlaminated parts • serious over heating of metallic parts • localised rapid melting of generator core laminations © ABB Oy September 16, 2011 | Slide 51 Protection Application • Motoring / Reverse power protection (32) • Nature of the fault G G G 32 32 kVAR kW kVAR kW CLOSE CLOSE R 32 kVAR kW CLOSE CLOSE CLOSE CLOSE R M kW kVAR CLOSE R R M R © ABB Oy September 16, 2011 | Slide 52 32 kVAR kW CLOSE kW kVAR G R Protection Application • Motoring / Reverse power protection (32) • Nature of the fault • Motoring is defined as a flow of real power into the generator acting as a motor • Motoring occurs when the energy supply to the prime mover is cut off while the generator is still on line • Generator will act as a synchronous motor and drive the prime mover Q_GENERATION SYNCHRONOUS MOTOR LEADING REGION P_ABSORPTION SYNCHRONOUS MOTOR LAGGING REGION INDUCTION MOTOR SYNCHRONOUS GENERATOR LAGGING REGION P_GENERATION SYNCHRONOUS GENERATOR LEADING REGION INDUCTION GENERATOR Q_ABSORPTION • Is applicable only when generator is operating in parallel with other generating source © ABB Oy September 16, 2011 | Slide 53 Protection Application • Motoring / Reverse power protection (32) • Effect of reverse power • Steam flow through a turbine gives energy for rotation of rotor and carry away the heat of turbine parts • Steam turbines tend to overheat when steam supply is cut off and turbine still rotates • Can cause several thermal stresses in the turbine parts • Realization of the protection • Reverse power protection is for the benefit of the prime mover and not for the generator • Time required for turbine to overheat varies from 30 Seconds to 30 minutes depending on the turbine type © ABB Oy September 16, 2011 | Slide 54 Protection Application • Motoring / Reverse power protection (32) • time delay is essential to avoid false trips due to power swings or during synchronization • time delay should be selected in coordination with allowable turbine motoring time • setting of the relay depends on the prime mover involved, as the power required to motor is the function of the load and losses of the idling prime mover • power required to motor in percent of the rated power is, © ABB Oy September 16, 2011 | Slide 55 • steam turbine 0.5% to 3% • hydro turbine 0.2% to 2% • gas turbine upto 50% • diesel engine upto 25% Protection Application • Low forward power monitoring (37) • Nature of the fault G • Usually indicates a loss of load 37 • a generator operating from the network as a motor kVAR kW can be disconnected by a maximum reverse-power CLOSE relay to protect the prime mover kW kVAR • Realization of the protection CLOSE R • Low forward power interlock relay is provided to prevent the disconnection of the machine from the network, if the prime mover is still driving it, otherwise the machine speed may increase above the permissible level for small loads Reverse power protection © ABB Oy September 16, 2011 | Slide 56 Low forward power monitoring R Protection Application • Under excitation protection (40) 40 40 • Nature of the fault • loss of field • fault in excitation circuit V DC V DC G kVAR kW kVAR kW • voltage regulation system failure G CLOSE CLOSE • accidental tripping of field breaker kW kVAR • high capacitive load CLOSE R kW kVAR CLOSE R • draws reactive power from the network to feed active power Q_GENERATION R SYNCHRONOUS MOTOR LEADING REGION P_ABSORPTION SYNCHRONOUS MOTOR LAGGING REGION INDUCTION MOTOR SYNCHRONOUS GENERATOR LAGGING REGION P_GENERATION SYNCHRONOUS GENERATOR LEADING REGION INDUCTION GENERATOR Q_ABSORPTION © ABB Oy September 16, 2011 | Slide 57 R Protection Application • Under excitation protection (40) • Effect of under excitation • if the generator loses its excitation ( the dc applied to the field winding by the exciter ), then it will draw its excitation from the electrical system by drawing reactive power • prime mover still driving the unit • large slip frequency current induced in rotor leads to rotor overheating • generator looses synchronism and continue running at over synchronous speed • heavy inrush current developed in the stator winding while trying to maintain the synchronism • winding gets over heated and gets damaged • Network voltage collapse as large reactive power is drawn and thus failure of weak electrical systems © ABB Oy September 16, 2011 | Slide 58 Protection Application • Under excitation protection (40) Q_GENERATION • Realization of the protection SYNCHRONOUS GENERATOR LAGGING REGION P_GENERATION • impedance measured at stator terminals • terminal voltage begins to decrease P_ABSORPTION X'd • current tends to increase • change in power factor from lag to lead Xd • drop in impedance • relay characteristics => mho Q_ABSORPTION • Stage 1 can be time delayed to avoid tripping during transient conditions • Stage 2 can be combined with under voltage and with no time delay i.e. instantaneous operation © ABB Oy September 16, 2011 | Slide 59 Protection Application • Negative phase sequence protection (46) 46 • Also known as unbalance protection G • Nature of the fault • Asymmetric fault (2-phase short circuit or earth fault) a b c • Incomplete (1-2 phase) operation of circuit breaker or disconnecting switch a b c • Asymmetric loads (1-phase loads, 3-phase asymmetric loads) © ABB Oy September 16, 2011 | Slide 60 Protection Application • Negative phase sequence protection (46) • Effect of negative phase sequence current • Symmetry of stator current gets disturbed due to asymmetric loading • counter rotating negative phase sequence current is set up • this current induces currents at 2 x rated frequency in, • rotor iron body • wedges • windings • intense rotor heating • © ABB Oy September 16, 2011 | Slide 61 2 = rated current, can melt the rotor ! Protection Application • Negative phase sequence protection (46) • Realization of the protection • Basically a back up protection for uncleared faults / disturbances • Negative sequence current capability of synchronous generator is expressed as I22t = ….. • 1 or 2-stage negative phase-sequence current protection (alarm + trip) • Allowed continuous NPS tolerance must be known (Typically 8..12% with turbogenerator and 20..30% with salient pole machine) • Typical short time tolerances Turbine-generator…….30 sec. Hydraulic generator…..40 sec. Diesel generator………40 sec. Rotating compensator..30 sec. © ABB Oy September 16, 2011 | Slide 62 Protection Application • Under voltage (27) and Over voltage protection (59) : • Nature of the fault • Generally voltage does not depart significantly from pre-set value • Possible reasons of voltage disturbances are, • Fault in voltage regulator • Under-excitation or loss-of-excitation condition under-voltage • Too high inductive or capacitive load because of a trip of load / generator / transmission network • Trip of main breaker with full load and possible over speeding (over-voltage) © ABB Oy September 16, 2011 | Slide 63 G 27 59 Protection Application • Under voltage (27) and Over voltage protection (59) : • Effect of over voltage • Stress caused to the insulator material by over-voltage • Thermal stress of iron core by over-magnetising (over-voltage) • Increasing of magnetic flux density • Saturation of iron core • Increasing of leakage flux and spreading of flux to unlaminated parts • Causes distortion in voltage waveforms • Realization of the protection • Can be considered as a backup protection • main protection by over exciter limiters or overflux protection © ABB Oy September 16, 2011 | Slide 64 Protection Application • Under voltage (27) and Over voltage protection (59) : • Generator manufacturers provides combined voltage and frequency operating ranges of synchronous generators for continuous rated output • 2 stages can be used, 1st stage for alarm and 2nd for trip • alarm stage shall be with time delay to enable primary protection to take care and for operator to take corrective action • trip stage shall be with minimum time delay or fast operation during severe condition © ABB Oy September 16, 2011 | Slide 65 © ABB Oy September 16, 2011 | Slide 66 Protection Application • Over / Under frequency protection (81) : • Nature of the fault • Generally results from full or partial load rejection or from generator overloading • Sudden increase in load => decrease the frequency • Sudden decrease in load => increase the frequency • due to clearing of system faults • sudden disconnection of generator from a grid network • a fault in the generator rotational speed regulator • during starting and stopping the generator © ABB Oy September 16, 2011 | Slide 67 G 81 Protection Application • Over / Under frequency protection (81) : • Effect of over / under frequency • Mechanical and thermal stress to the prime mover • Thermal stress to the iron core by the over excitation (under frequency) • Increasing of magnetic flux density as V/ f is a measure of flux • Saturation of iron core • Increasing of leakage flux and spreading of flux to un-laminated parts • additional heating of the machine • affects auxiliary devices such as pumps, motors as output power reduces with the falling frequency © ABB Oy September 16, 2011 | Slide 68 Protection Application • Over / Under frequency protection (81) : • Realization of the protection • Manufacturers’ data sheet must be checked for the allowable limits • Typical frequency limits for all modern 1500 and 3000 RPM units (STG Units) Lifetime Limit (Minutes)…….. Frequency range (Hz.) Unlimited………………………47.0 to 52.5 90………………………………46.5 – 47.0 (-7% to -6%) 52.5 – 53.0 (5% to 6%) 12………………………………46.0 – 46.5 (-8% to -7%) 53.0 – 53.5 (6% to 7%) 1………………………………45.0 – 46.0 (-10% to -8%) 53.5 – 55.0 (7% to 10%) • Alarm and trip stages as well as frequency base load shedding schemes are set as per the allowable limits © ABB Oy September 16, 2011 | Slide 69 Protection Application • Differential protection (87) : 87 • Nature of the fault • Stator phase to phase fault • Stator phase to ground fault • Depends on the magnitude of earth fault current • Will not respond to : • Turn to turn faults in the winding • Open circuit faults in the winding • Realization of the protection • Differential protection shall be stable for external faults and sensitive for internal faults • Two type of schemes • High impedance scheme and low impedance scheme © ABB Oy September 16, 2011 | Slide 70 G Protection Application • Differential protection (87) : • High impedance scheme : Current balance principle G RStab M R Rstab : Stabilising resistor: external resistor connected in series with relay coil M © ABB Oy September 16, 2011 | Slide 71 : variable resistor: needed if higher voltages are expected Protection Application • Differential protection (87) : • Low impedance scheme G I2 I1 Id Id = I1 – I2 Ib = I1 + I2 2 • Known as stabilized based differential protection • Slope characteristic is the main feature • increases the pick-up value during external fault thus providing a stability • provide sensitive and fast operation during internal fault © ABB Oy September 16, 2011 | Slide 72 Protection Application • Differential protection (87) : • Three phase stabilized based differential protection : Operation Id = I1 – I2 Restraint Ib = I1 + I2 2 © ABB Oy September 16, 2011 | Slide 73 Protection Application • Inadvertent energization (27/50) : • Nature of the fault • accidental energization of a G V DC G V DC 50 50 27 27 OPEN CLOSE generator when it is not running • Effect of an inadvertent energization • with generator at standstill, the machine will act as an induction motor, the field winding & rotor CLOSE R CLOSE R damper circuit acting as a rotor • very high current induced in the rotor and stator circuit • causes rapid over heating and M R © ABB Oy September 16, 2011 | Slide 74 damage Protection Application • Inadvertent energization (27/50) : • Realization of protection • Under voltage before the breaker is close and Over current after the breaker is close When generator is not running and breaker is open : No Trip 27 Time delayed drop off contact 50 Instantaneous When generator is not running and breaker closes accidentally : Trip 27 Time delayed drop off contact 50 Instantaneous When generator is running and condition is healthy : No Trip © ABB Oy September 16, 2011 | Slide 75 27 Time delayed drop off contact 50 Instantaneous Protection Application • Fuse failure supervision • Nature of the fault • failure in a voltage measurement circuitry • Effect of the fault • mal-operation of voltage operated schemes • Realization of the protection • For detection of blown VT secondary fuses • Blocking is provided to voltage operated protection schemes to avoid mal-operation © ABB Oy September 16, 2011 | Slide 76 Protection Application • Synchro check or voltage check function • Nature of the fault • Conditions required for parallel operation of generators • Terminal voltage of generators to be synchronized shall be same - AVR control • Frequency of generators to be synchronized shall be same - Governor control • Phase angle of the voltages shall be same • above criteria should meet at the time of synchronization • Effect of the fault • Power surges • Very high voltage can be developed • Dead short circuit can happen and thus creating a large short circuit current • Mechanical and electrical stress on the system © ABB Oy September 16, 2011 | Slide 77 Protection Application • Synchro check or voltage check function • Realization of the protection • Operating conditions • Live - Live G G V DC • Live - Dead V DC 25 • Essential conditions for Live – Live synchronization • the voltages on both sides of the circuit breaker to be synchronized have the same frequency, are in phase and are of such a magnitude that the concerned busbars or lines can be regarded as live. • Essential conditions for Live – Dead energization • the relay to check the energizing direction. © ABB Oy September 16, 2011 | Slide 78 CLOSE CLOSE OPEN Protection Application CBFP • Circuit Breaker Failure Protection R • Nature of fault • CB fails to clear the fault G G • Mechanical problem in the CB CLOSE CLOSE • Low gas pressure CBFP • Problem in the trip circuit • Effect of fault CLOSE • All the possible damages if fault clearing time is greater than the critical 50 51 Fault clearing time • Realization of the protection • Current sensing after desired fault clearing time • Circuit breaker contact for reference © ABB Oy September 16, 2011 | Slide 79 50 51 © ABB Oy September 16, 2011 | Slide 80
