Basis Stator Earth Fault ProtectionEffects of Arc Burning on Stator Core Laminations . used in older plants .Methods of Neutral-Point Connection (1) Isolated or high ohmic earthed Advantage: Disadvantage: Small fault currents High transient overvoltage for intermittent earth faults (2.5 – 3.5Uph-E) Higher costs < 10 A Very seldom.5 ) Uph-E < 10 A Standard application Compensated or reactive earthed Advantage: Disadvantage: Small fault currents at the fault location Transient overvoltage (<2. better measuring conditions for the protection Great damages to generators. Leakage zero sequence currents G Disadvantage: Application at low voltage generators .400 A Application in industrial plants Solidly (effective) earthed Advantage: Low transient voltage. 95 to 98% protective range of SEF protection Great damages to generators at longer fault duration Disadvantage: < 200 .Methods of Neutral-Point Connection (2) Low ohmic earthed Advantage: Lower transient overvoltage. E = UL21 • UL.E M M UL1.E = UL31 UE = -UL1. phase-earth-voltage • no displacement voltage (UE = 0) UL2.E = 0 • UL.Voltages in Case of an Earth Fault L1 L2 L3 Voltage without earth fault UL3.E .E voltage decreases in the faulty phase (min → 0) UL2.E Voltages at the point of an earth fault at phase L1 UL3..E voltage in the both “healthy” phases are increase (max → phase-to-phase) • UE displacement-voltage (can be measured at star point to earth) ..E • ULx.E UL1. earth fault current in one phase ZE .E IE .. UE/ZE IE/3 .Currents in the Case of an Earth Fault Model: IE/3 Equivalent voltage UE at the point of fault L1 L2 L3 IE =3 . IE/3 = 3 ..3 IE = = 3U0 1 IC. jωCE jωCE 3 UE ZE UL2. earth impedance at one phase ~ UE ~ ~ ZE IE Vector diagram: earth fault in phase L1 UL3...2 = 3U0 .E IC 3 L1 L2 L3 3U0 =3UE IC. E UL2. UE UL1.E UE UE IE= 3 UE ZE At faults close to the star-point the displacement voltage and the earth currents become small .Correlation from Earth fault Location Displacement voltage UE (U0) and earth current IE (3I0) as a function of the fault location of an earth fault in the machine winding.E ZE UL1.E UL2. the displacement voltage caused by an earth fault. can only be measured at the generator .Generators Connected via an Unit Transformer G G G • generator is galvanic isolated • under the assumption of an ideal transformer. if solidly earthing is open) R= ( ) RPrim ü TR 2 3 .Elimination of the Disturbance High Voltage Earth Fault Problem: grid earth faults cause disturbances Solution: attenuation by means of due to the coupling capacitance a load resistor between the two transformer windings CK CG CL CTr UEO ≈ UN 3 üTR = UGen 3 100 3 V 500 3 V Earthing transformer UR (Limb transformation ratio) Note: At solidly earthed transformer the UE0 is appr. 80% of UN/√3 (Safety margin. prim CE.Unit Connection . Rprim UEO Example: UEO = 220 kV 3 R=5Ω üTR = 735Ω 3 2 ⇒ ⇒ U R.4 RPrim = R ⋅ CG+CL+CTr neglected UEO CK Rprim üTr displacement voltage on the high voltage side three phase coupling capacitance primary load resistor earthing transformer ratio equivalent circuit UC UEO CK UR.prim = 293V .sek ≈ 24V ⇒ 24V ⇒ 4.prim ≈ Rprim 1 Rprim + jω C K CK = 10 nF UN.5 kV üTr = 36.Influence on the Coupling Capacitance disturbance voltage UR.83% disturbance 500V influence UR.G = 10. Unit Connection with Neutral Transformer This design is very often used outside Germany. Rsek= Rprim ü2TR Design of Rprim so that the fault current is < 10A .500V) is selected in order to avoid very small load resistors. mostly on small generators Generator Unit transformer R ÜTR = UGen 3 UR UR A high secondary nominal voltage UR (250V . Generator Connected Directly to the Grid G G G M • machines are galvanic connected • displacement voltage caused by an earth fault can be measured in all locations . 3I0> Ohmic current boost IR 3I0 UL1 UL2 IC ICL2 ICL1 .Directional Stator Earth Fault Protection L1 L2 L3 Network L1 L2 CE IC + IR Earthing transformer L3 3U0 3i0 • Direction (√3U0. 3I0) √3u0 • DFT √3U0>. Direct Connection G1 IMess G2 IMess .Fault Currents . Fault Currents .Direct Connection with Earthing Transformer G1 IMeas G2 Earthing transformer Ohmic current RB IMeas . Toroidal and Holmgreen Connection Toroidal Current Transformer IE 3 per phase • magnetic addition of earth currents.Earth Current Detection . • principle is sensitive I´E Holmgreen connection (separate cores) IE 3 per phase L1 L2 L3 I´E L1 L2 L3 IE Holmgreen connection (common neutral return connector Sensitivity is limited Problem: Large CT ratio leads to small currents on the secondary side 3~ ∆I (IE = 3 I0) . Directional Stator Earth Fault Protection .Characteristic . Logic .Directional Stator Earth Fault Protection .