Mr. Stephen Ho, MM - PD IMD Oct 2011 MTR Protection System Overview For DUAT Lines
MTR 33kV Power Distribution System Transformer, cable, switchgear, bus-bar, rectifier all need protection devices.
MTR 1.5kV DC Traction Power Supply DC Protection need to grade with train service loading
MTR Power System Protection Objectives : • Discriminateand isolatethe • faulty power system apparatus • Minimise damage to equipment • Reduce the extent of personnel injuries • Prevent power supply interruption to • other healthy loads.
Protection Philosophy • Reliability • Selectivity • Speed • Sensitivity • Simplicity
Development of Protection Relay • Electro-mechanical • Static • Digital / Numerical • (Protection/ Control/ • Metering)
Key System ParametersFor Protection Design • Vector Group of Transformers (e.g. Yyd0, Dyn11) • Earthing (solid earth), Transformer Tap (Max/ Min) • Tx Impedance voltage : 5%, 1.5kVA • Fault Level : 3-phase, 25kA at 33kV • 20kA at 11kV • 40kA at 415V • 143kA peak at 1.5kV DC
Protection elements • Current & Voltage transformers • Auxiliary DC Power Supply • Multi-cores and panel wirings • Protection Relay /auxiliary / Trip / Timer Relay • CB Trip Coil • Local Indicators / Remote alarms
Protection Scheme Used at MTR Unit Protection – Clearly define protected zone • Transformer Differential Prot. • Pilot Wire Prot. • Bus-zone protection • LV Restricted earth fault Prot. • Frame Leakage Protection • Directional OC Prot
Protection Scheme Used at MTR Non-Unit Protection – Not Clearly define protected zone • Over-current & Earth Fault Prot. • Directional Over-current & earth fault Prot. • High set Over-current Prot. • Standby earth fault Prot.
33kV Power System Protection • Cable Feeder : Pilot wire protection (Main) • IDMTL OC / Directional OC • (Back-up) • Bus-bar : High Impedance Differential • Protection
Pilot Wire Pilot Wire Protection Substation B Substation B Substation A Substation A Unit Protection (Cable Feeder) Relay Pilot Wire Pilot Wire Relay CT CT Substation B Substation A Substation A
Unit Protection (Cable Feeder) 33kV Cable Fault Substation A Substation B Clear within 140ms
Principle of Translay “S” Protection Relay (LAR and TKL) • Use summation transformers with a neutral section that is tapped, to provide alternative earth fault sensitivities. • Phase comparators tuned to the power frequency are used for measurement and a restraint circuit gives a high level of stability for through faults and transient charging currents. • Internal faults give simultaneous tripping of relays at both ends of the line, providing rapid fault clearance irrespective of whether the fault current is fed from both line ends or from only one line end. Typical static circulating current feeder unit protection circuit diagram
Directional Over Current Relay applied at MTR Transformer side fault Feeder side fault • When there have a fault at either transformer side. Current will flow into the fault side from health side. Directional relay will detect the reverse current to trip the CB to clear the fault at a shorten time. Feeder 33/11kV R R R 33kV/ 11kV • When there have a fault at the transmission line . Current will flow into the fault side from health bar. Directional relay will detect the reverse current to trip the CB to clear the fault at a shorten time.
Principles of Directional Relay • Detect the phase angle between current flow (Ia) and voltage (Vbc) whether it is forward or reverse. If fault falls within the operating zone, trip signal will send out. EHV side
Directional relay outlook Over current relay “MCGG52” + Directional relay “P127” with OC & EF Applied in TKL and new extensions Directional relay with O/C “CDD” (single phase only) Applied in URL Directional relay “METI” without over current elements Applied in LAL METI
Incomers Zone 2 Feeders Feeders Unit Protection (Bus-bar) Zone 1
Incomers Feeders Feeders Unit Protection (Bus-bar ) Zone 2 Zone 1 Fault at Zone-2
33kV Bus Zone Protection Principle • Circulating current arrangement was applied. • This scheme can protect both phase to phase and phase to earth fault. • Operational principles: • If Ig is equal to -Ih, means that it is a through fault, relay will not operate. • If Ig + Ih = If = non-zero, means that the fault is in between protective zone • (Bus-bar), relay will send out trip signal. Ig Ih Phase and earth fault circulating current scheme using three-element relay
Relay and Accessories of Bus-zone Protection Tx LV winding Metrosil Connection Diagram MFAC Relay Shunt Resistor
33kV Bus-zone Protection Relay at MTR system Bus Zone Relay “MFAC” used at LAR & TKL Bus Zone Relay “FAC” used at URL
11kV Bus Bar Protection Principle • Frame-Earth Leakage Protection is used in 11kV bus-bar systems in MTR. • Current type relay is used to detect the current flow. • Both of Frame earth path and transformer neutral return path have detected current flow. Then, trip signal will be sent out to all CBs within the zone. Otherwise, no signal given out. • Only protect the phase to earth fault. Single zone frame earth protection
Relay used at 11kV bus-bar protection systems URL - CAG Relay LAR - MCGG22 Relay LAR - MFAG Relay
Power Transformer Protection (33kV AC) • 33/11kV Dist Tx : Bias Differential Prot (Main) • LV Restricted E/F (Main) • Buchholz Surge Prot (Main) • Winding/ Oil Temp High Trip • (Back-up) • HV Over-current Prot (Back-up) • LV Standby EF Prot (Back-up)
Transformer Substation A Substation B Transformer Protection
Transformer RE/F Substation A Substation B Transformer Protection When the Tx. Star winding has a fault, The R E/F relay will be operated to trip the two breakers.
Fault Point Theory of Transformer differential Protection • It is a circulating current system, red arrow is indicated the direction of secondary current flow. • Normally, no current will flow into the relay except the fault in between both current transformers. • If there have a fault in between both CT, current will flow into to pick up the relay as figure 2. • In MTRC power systems, only Distribution Transformers is applied. Figure 1. Normal operation current flow in between both CT Figure 2. Internal Fault in the transformer
Inrush current when start-up the transformer • Magnetizing inrush current containing a higher harmonic component flows, causing mal-operation of relay. • Restraint second harmonic to avoid mal-operation by inrush current. • Contents and current profile as below of inrush current. Typical Magnetizing inrush waveform Relative strength of Harmonics in Typical Magnetizing-Inrush and Internal Fault Currents
Bias Restraint characteristic of Static Relay MBCH • In order to avoid unwanted operation due to spill current and yet maintain high sensitivity for internal faults, when the difference current may be relatively small, the variable percentage bias restraint characteristic as below. • Normally, setting is set at 20% ~40% of rated current depending on max tap and CT mismatch. Percentage bias characteristics Actual Bias Characteristics
Numerical Differential Relay (P631) • Phase shift compensation, ratio correction and zero sequence filter was applied by software. • Used star-connected line CT’s on all winding of transformers CT winding. • Need no interposing CT. all winding connected directly. • P631 was used at ADM, KBD and PAF. P631 outline and test plugs Connection diagram
Transformer LV Restricted Earth Fault • It is a unit protection scheme with circulating current system. • The residual current of three line current transformers is balanced against the output of a current transformer in the neutral conductor. • A large percentage of the winding can be covered when neutral is solidly earthed. • Providing high speed protection against earth fault for the whole transformer with relatively simple equipment. • It is applied at Distribution and Service Transformers in MTR power systems with high impedance type . Restricted Earth Fault Connection Diagram
Outlook of REF applied in MTR • Starting from LAR, we used the high impedance relay to provide high speed and stabe protection. • In URL, low impedance type relay was used . The three phase current and the neutral current become the bias inputs to a differential element. Metrosil CAG relay FAG relay Shunt Resistor High impedance relay and its components Current type (low impedance) relay
TYPICAL RECTIFIER SUBSTATION CONFIGURATION AC33kV 33kV / 2 x 586V RECTIFIER DC1500V DOWN TRACK OVERHEAD LINE UP TRACK
33kV Rectifier Transformer Protection • Rectifier Tx : High Set Over-current Prot (Main) • Rectifier Over-current Prot • (Back-up) • IDMT OC/ EF Prot (Back-up)
Purpose of rectifier protection • The Protection of a rectifier differs from that of conventional current maximum application. • A large number of rectifiers can withstand relatively long periods of over-charge without incurring damage, generally 150% for 2 hours and 300% for 1 minute. Connection Diagram
Typical settings for the TMS area: Light industrial service TMS=0.025 Medium duty service TMS=0.1 Heavy duty traction TMS=0.8 The high set is typically set at 8 times rated current as this ensures HV AC protection will discriminate with fault from HV and LV sides. Rectifier Protection Characteristics Operating Curve for Rectifier Curve
Types of Rectifier Protection Relays Midos Relay (MCTD) Micom Relay (P122)
11 kV Circuit / Transformer Protection • Cable Feeder : IDMT OC/EF Prot (Main) • Bus-bar : Frame Earth Leakage Prot • 11kV/415V • Service Tx : IDMT/ Inst OCEF Prot (Main) • LV REF Prot (Main) • Buchholz surge Prot (Main) • Winding/ Oil Temp High Trip • (Back-up)
LV Circuit Protection (415V/ 660V ) • Cable Feeder/ Bus-section : IDMT OC/EF (Main) • Bus-bar : LV Standby EF Prot.
Over Current Relay application • Normally, Earth Fault relay applied at chiller system, 3 phase with earth fault used at LV board systems and 2 phase with earth is used at 11kv side with balance load systems (e.g. service transformers). 3 phase with Earth Fault CT Connection 2 phase with Earth Fault CT Connection Earth Fault CT Connection Earth Fault Relay (CDG) 3 ph & E/F Relay (Micom 121) 2 Ph & E/F Relay (MCGG52)
Others O/C Relay used in MTR system Micom Relay (P122 URL new relay alternative) Midos Relay (MCGG52 LAR used) Micom relay P127 (Directional relay used at KTT new 33kV board) Numerical Relay (KCEG used at TKL & DRL)
Discrimination by Time • Breaker nearest to the fault opened first. • Time interval between relay time setting must be long enough to ensure that the upstream relay do not operate before the circuit breaker. Radial System with time discrimination
Time Grading Principle for IDMT protection Grading Formula : Grading Margin – = 0.1*(T1 + T2) +0.1 +0.06 + SM (sec) T1 : Upstream Protection Operation Time T2 : Downstream Protection Operation Time SM : safety margin Apply in Radial- Fed Circuits Margin : Relay/ CT error + Overshoot time + CB Op. Time for Downstream KCGG 142 got additional 40ms for its pick-up element
Numerical relays (For New Extension) • What is Numerical Relay? • - Microprocessor to replace analog circuit used in static relays to implement relay functions • What is major difference between numerical relay and EM/static relay? • - Numerical Relay operate on digital input, multi-characteristic curve and has memory, which can reproduce fault incidences for post-fault analysis. • How about Data Processing and Trip Decision Making ? • Microprocessor to implement protection algorithm and tripping logic • Minimum Operation Time 20~40ms.