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Hands on Relay School Transformer Protection Open Lecture Hands on Relay School Transformer Protection Open Lecture

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Hands On Relay School Transformer Protection Open Lecture Hands On Relay School Transformer Protection Open Lecture Class Outline • Transformer protection overview • Review transformer connections • Discuss challenges and methods of current differential Protection • Discuss other protective elements used in transformer protection Scott Cooper [email protected] (727)415-5843 Eastern Regional Manager 204 37th Avenue North #281 Manta Test Systems Saint Petersburg, FL 33704 Transformer Protection Overview Transformer Protection Zones Types of Protection Mechanical Protection • Analysis of Accumulated Gases – Looks for arcing by‐products • Sudden Pressure Relays – Orifice allows for normal thermal expansion/contraction. Arcing causing pressure waves in oil or gas space overwhelming the orifice and actuating the relay. • Thermal – Caused by overload, over excitation, harmonics and geo magnetically induced currents • Hot spot temperature • Top Oil • LTC Overheating Types of Protection Relay Protection • Internal Short Circuit – Phase: 87HS, 87T – Ground: 87HS, 87T, 87GD • System Short Circuit Back Up Protection – Phase and Ground Faults • Buses: 50, 50N, 51, 51N, 46 • Lines: 50, 50N, 51, 51N, 46 Types of Protection Relay Protection • Abnormal Operating Conditions – Open Circuits: 46 – Overexcitation: 24 – Undervoltage: 27 – Abnormal Frequency: 81U – Breaker Failure: 50BF, 50BF‐N Phase Differential Overview • What goes into a “unit” comes out of I + I + I = 0 a “unit” 1 2 3 • Kirchoff’s Law: The sum of the I I 1 UNIT 2 currents entering and leaving a junction is (should be) zero • Straight forward concept, but not that simple in practice with I transformers 3 Phase Differential Overview A host of issues presents itself to decrease security and reliability of transformer differential protection • CT ratio caused current mismatch • Transformation ratio caused current mismatch (fixed taps) • LTC induced current mismatch • Delta‐wye transformation of currents – Vector group and current derivation issues • Zero‐sequence current elimination for external ground faults on wye windings • Inrush phenomena and its resultant current mismatch • Harmonic content availability during inrush period due to point‐on‐wave switching (especially with newer transformers) • Over‐excitation phenomena and its resultant current mismatch • Internal ground fault sensitivity concerns • Switch onto fault concerns • CT saturation, remnance and tolerance Phase Differential Overview‐Transformer Basics Transformer Tap Calculation‐Per Unit Concept Compensation (2) Change in CT Ratio 1:1, Y-Y 4:1, 3Y 1:1, 3Y IA, IB, IC Ia, Ib, Ic IA', IB', IC' Ia', Ib', Ic' IA'*4 = Ia' IB' * 4 = Ib' IC' * 4 = Ic' Phase Differential Overview‐Transformer Basics Transformer Tap Calculation‐Per Unit Concept Compensation (3) Transformer Ratio 2:1, Y-Y 1:1, 3Y 1:1, 3Y IA, IB, IC Ia, Ib, Ic IA', IB', IC' Ia', Ib', Ic' IA' = Ia' / 2 IB' = Ib' / 2 IC' = Ic' / 2 Phase Differential Overview‐Transformer Basics Transformer Tap Calculation‐Per Unit Concept Compensation (2) Change in CT Ratio IA, IB, IC Ia, Ib, Ic IA', IB', IC' Ia', Ib', Ic' There must be an easier way….. Phase Differential Overview‐Transformer Basics Transformer Tap Calculation‐Per Unit Concept 100MVA 100MVA IN OUT Phase Differential Overview‐Transformer Basics Transformer Tap Calculation‐Per Unit Concept Tap Calculation with Wye CTs Tap Calculation with Delta CTs TransformerVA TransformerVA WindingTap = WindingTap = V ∗CTR ∗ 3 L−L VL−L ∗CTR Phase Differential Overview‐Transformer Basics Transformer Tap Calculation‐Per Unit Concept Each measured current is divided by the winding Tap. The result is a percent of rating. These percent of ratings can be compared directly. Phase Differential Overview‐Transformer Basics AB connected delta‐wye transformer Phase Differential Overview‐Transformer Basics • Subtracting Vectors: Subtract from reference phase vector the connected non-polarity vector…in our example Ia-Ib c -b a b • Can be repeated for B & C, or you can assume –120 and –240 displacement from A for B&C respectively •Ib –Ic and Ic –Ia would be the vectors Phase Differential Overview‐Transformer Basics AC connected delta‐wye transformer Ia-Ic Ia Ic-Ib Ia Ia Ic Ib-Ia Ib Ib Ib Ia Ic-Ib Ic Ib-Ia Ia-Ic Ic Ic Ib Phase Differential Overview‐Transformer Basics • Subtracting vectors: Subtract from reference phase vector the connected non- polarity vector…in our example Ia-Ic c a b -c • Can be repeated for B & C, or you can assume –120 and –240 displacement from A for B&C respectively •Ib –Ia and Ic –Ib would be the vectors Phase Differential Overview‐Transformer Basics Angular Displacement Conventions: • ANSI Y‐Y, Δ‐Δ @ 0°; Y‐Δ , Δ‐Y @ X1 lags H1 by 30° – ANSI makes life easy • Euro‐designations use 30° increments of LAG from the X1 bushing to the H1 bushings – Dy11=X1 lags H1 by 11*30°=330° or, H1 leads X1 by 30° – Think of a clock – each hour is 30 degrees 0 11 1 10 2 9 3 Dy1 = X1 lags H1 by 1*30 = 30, or H1 leads X1 by 30 (ANSI std.) 8 4 7 5 6 Phase Differential Overview‐Transformer Basics C c A a b B US Standard Dy Example: • H1 (A) leads X1 (a) by 30 • Currents on “H” bushings are delta quantities Assume 1:1 transformer Phase Differential Overview‐Transformer Basics US Standard Yd Example: •H1 (a) leads X1 (A) by 30 •Currents on “X” bushings are delta quantities C c a B b A Assume 1:1 transformer Phase Differential Overview • Applied with variable percentage slopes to accommodate CT saturation and CT ratio errors • Applied with inrush and over excitation restraints • Set with at least a 20% pick up to accommodate CT performance – Class “C” CT; +/‐ 10% at 20X rated • If unit is LTC, add another +/‐ 10% • May not be sensitive enough for all faults (low level, ground faults near neutral) Phase Differential E‐M Relay Application • CT ratios and tap settings are selected to account for: – Transformer ratios – If delta or wye connected CTs are applied – Delta increases ratio by 1.73 • Delta CTs must be used to filter zero‐ sequence current on all wye transformer windings • Dy transformer connections compensated by yd CT connections to make the currents “apples to apples”. Phase Differential E‐M Relay Application Zero‐sequence elimination: In E‐M relays with wye connected transformers, delta connected CTs are used to remove the ground current. Phase Differential Digital Relay Application Settings compensate for the following: • Transformer ratio • CT ratio • Vector quantities – Which vectors are used – Where the 1.73 factor (√3) is applied • When examining line to line quantities on delta connected transformer windings and CT windings • Zero‐sequence current filtering for wye windings so the differential quantities do not occur from external ground faults Phase Differential Digital Relay Application Angular displacement (IEC and SEL) • *1 IEC (Euro) practice does not have a standard like ANSI *1 • Most common connection is *2 Dy11 (low lead high by 30!) • Obviously observation of *2 angular displacement is extremely important when paralleling transformers! *1 = ANSI std. @ 0° *2 = ANSI std. @ X1 lag H1 by 30°, or “high lead low by 30 ° “ Digital Relay Application All wye CTs shown, most can retrofit legacy delta CT applications Benefits of Wye CTs • Phase segregated line currents – Individual line current oscillography – Currents may be easily used for overcurrent protection and metering – Easier to commission and troubleshoot – Zero sequence elimination performed by calculation Phase Differential Digital Relay Application Zero‐sequence elimination: In digital relays with wye connected transformers and wye connected CTs, ground current must be removed from the differential calculation. •3I0 = [Ia + Ib + Ic] I0 = 1/3 *[Ia + Ib + Ic] •Used where filtering is required, such as wye winding with wye CTs Phase Differential Digital Relay Application 2nd and 4th Harmonics During Inrush Typical Transformer Inrush Waveform Phase Differential Digital Relay Application Harmonically Restrained Differential Element • Inrush Detection and Restraint – Inrush occurs on transformer energizing as the core magnetizes – Sympathy inrush occurs from adjacent transformer(s) energizing, fault removal, allowing the transformer to undergo a low level inrush – Characterized by current into one winding of transformer, and not out of the other winding(s) – This causes the differential element to pickup – Use inrush restraint to block differential element during inrush period Phase Differential Digital Relay Application • Inrush Detection and Restraint – 2nd harmonic restraint has been employed for years – “Gap” detection has also been employed – As transformers are designed to closer tolerances, both 2nd harmonic and low current gaps in waveform have decreased – If 2nd harmonic restraint level is set too low, differential element may be blocked for internal faults with CT saturation (with associated harmonics generated) Phase Differential Digital Relay Application • Inrush Detection and Restraint – 4th harmonic is also generated during inrush – Odd harmonics are not as prevalent as Even harmonics during inrush – Odd harmonics more prevalent during CT saturation – Use 4th harmonic and 2nd harmonic together – M‐3310/M‐3311 relays use RMS sum of the 2nd and 4th harmonic as inrush restraint – Result: Improved security while not sacrificing reliability Phase Differential Digital Relay Application • Overexcitation Restraint – Overexcitation occurs when volts per hertz level rises (V/Hz) – This typically occurs from load rejection and malfunctioning generation AVRs – The
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