Time Overcurrent Relays More or less approximates thermal fuse» Allow coordination with fuses Direction of Current nduced Torque Restraining Spring Reset Position Time Dial Setting Disk Basic equation Operating torque T s p Restraining Torque Kd t s = restraining spring torque = applied current p = pick up current K d = disk damping factor = angle of disk rotation (proportional to Time Dial Setting (TDS)
Relay Response t t K p d s t K d p s. Operating torque = Restraining Torque. ntegrate w.r.t. Time trip time K TDS p d s 3. TDS (setting angle), where triptime = t -t Relay Response 4. Finding trip time M A TDS K TDS trip time p s d Where: M = / p A = K d /t s
Standard Curves -- standard formats Reset Time (M < ) tr C TDS M Trip time (M ) tt TDS M A B p Some manufacturers include disk inertia in B US and EC curve parameters Curve A B C P U.S. Moderately inverse (U) 0.004 0.56.08 0.0 U.S. nverse (U) 5.95 0.80 5.95.00 U.S. Very inverse (U3) 3.88 0.0963 3.88.00 U.S. Extremely inverse (U4) 5.67 035 5.67.00 U.S. Short-time inverse (U5) 0.0034 0.006 0.33 0.0.E.C. Class A - Standard inverse (C) 0.4 0.0 3.5 0.0.E.C. Class B Very inverse (C) 3.5 0.0 47.3.00.E.C. Class C Extremely inverse (C3) 80.0 0.0 80.0.00.E.C Long-time inverse (C4) 0.0 0.0 0.0.00.E.C Short-time inverse (C5) 0.05 0.0 4.85 0.04 3
US nverse (U) Characteristic Comparison of curves 4
Extremely nverse Curve and 50E fuse Example Bus # Local Bus # Local Bus #3 Source Vs Z R Z R3 Z3 R4 Local Z4 Faulted Line Want the relay on the faulted line, R4, to be the only relay to trip Max and min fault current (based on ends of faulted line) 3 3 4 5
Example continued The desired coordination can be accomplished by: increasing the time dial settings as one proceeds toward the source.» f relay R is expected to provide backup protection for relay R4,» Then R4, the relay with the greatest source impedance, would be set with the lowest time dial setting Example continued» f MN is defined as the minimum fault current,» Then the pickup current must be set at or below this current but above maximum load current.» Usually with a margin around both» For relays R and R3, the TDS must be set to trip no faster then the next downstream device when the fault current is maximum for an out of zone fault 6
EMTP relay simulation R Bus Gen Source mpedance 30KV Xfmr S Bus Breaker 69KV Breaker 5 5 Fault # C-G Fault # C-G 5 5 50 resistive fault is initiated at 8.3 ms and progresses to a 5 fault at 6 ms as can occur by a tree branch coming in contact with the wire. Example with a trip 7
Comparing relay coordination (light load) Comparing relay coordination (heavy load) 8
Directional Control vs Direction Supervision Reference Signal Phase Current Directional Element (3) 3 DC Bus + 3 Reference Signal Phase Current Directional Element (3) 3 DC Bus + Phase Current Overcurrent Element (50 or 5) 50/ 5 50/5 Phase Current 3 Overcurrent Element (50 or 5) 50/ 5 50/5 5 AC Circuit Breaker 5 AC Circuit Breaker a. Directional Supervision DC Bus - b. Directional Control DC Bus - Directional Step-Time Overcurrent (ANS 67) The directional overcurrent relay can be perceived as a type 50 instantaneous element controlled by a type 3 directional element f the type 67 relay element is to provide backup protection, they use definite time delay for downstream coordination The 67 element requires more attention to detail for coordination than do type 5 relays» The advantage that the stepped time has over the 5 is that the time steps are independently set.» The disadvantage is that overreach errors have a more pronounced affect that often proves difficult to coordinate 9
Directional Step-Time Overcurrent (ANS 67) Bus S 67 5 ncreasing time G F F F3 F4 5 3 4 Overcurrent Elements in Microprocessor Relays Expect the relay to be able to coordinate with fuses and electromechanical relays mplement relay function using the standard curve equations Use digital filters to compute RMS magnitude from measured currents Add directional supervision Take advantage of some calculations difficult to do without microprocessor 0