Time-current Coordination
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1 Time-current Coordination Time that is controlled by current magnitude permits discriminating faults at one location from another. There are three variables available to discriminate faults, the time dial setting (TDS), the pickup current and the degree of curve inversness. Using the circuit shown in Figure 5.15, the following procedure can be used to determine these three time overcurrent settings for the type 51relay at Bus S. This example assumes that the relay is to provide backup protection for the fuse and hence must see the faults at the load under all conditions. Coordination is most difficult when the source impedance is high compared to the line impedance and there is a large range of possible source impedance values. Es2 Es1 ZS2 Bus S V S Bus R Bus T V R V T Relay S I S 52 ZL1 I R ZL2 F1 F2b F2 F3 Load CTR = 6:5 PTR = 345:12 ZLoad MAX = 359Ω Wye Secondary Ohms 1 = 3 Ω 8 = 9 Ω 8 ZS2 1 = 1 Ω 8 ZS2 = 3 Ω 8 Primary Ohms ZL1 1 = Ω 45.6 ZL1 = Ω 77.5 ZL2 1 = Ω 45.6 ZL2 = Ω 77.5 Figure Single line diagram for fuse - TOC relay time coordination Step 1. Determine the maximum and minimum three phase, phase to phase and phase to ground fault currents for the fault locations identified in Figure The minimum fault current is when source Es2 is not connected and the maximum fault current is when both Es1 and Es2 are connected. The results are presented in Table 5.3. Since the source impedance changes by a factor of four depending upon whether source Es2 is connected or not, it is not surprising to see the fault currents change by a factor of three to four as well. The data in Table 5.3 verifies that there is little difference in fault current for the three fault locations when the source impedance is high. Table 5.3. Fault currents needed for coordination of system shown in Figure 5.15 for zero fault resistance Fault Type Source Z F1 Amps Sec. F2 Amps Sec. F3 Amps Sec. Max Φ Min Φ-Φ Max Min
2 27 Φ-G Max Min Zload MAX = 359Ω primary = 13.3Ω secondary Step 2. Plot the currents for faults at F2 and F3 on a fuse time-current chart to determine the minimum and maximum fuse operating times for faults at these locations. The currents listed in Table 5.3 have been normalized in Table 5.4 so that relay and fuse operate times are based on same multiples of pickup current Figure 5.16 and Figure 5.17 show the time current points for the four fault scenarios. Table 5.4. Currents listed in Table 5.3 converted to multiples of pickup current for the 6A fuse Fault Type Source Z F1 p.u. Amps F2 p.u. Amps F3 p.u. Amps 3Φ Max Min Φ-Φ Max Min Φ-G Max Min Time - Seconds 1 T2 1. Extreme inverse curve T1.1 6A Maximum Minimum Multiples of Pickup Current I F2 min I F2 max Figure minimum and maximum operate times for a fault a F2
3 Time - Seconds 1 T4 1. Extreme inverse curve T3.1 6A Maximum Minimum Multiples of Pickup Current I F3 min I F3 max Figure minimum and maximum operate times for a fault a F3 Step 3. Since the Type 51 relay at Bus S must see the entire length of the line based upon our assumption that the relay is to provide backup protection for the fuse, the relay pickup current must be set more sensitive (below) the minimum fault current for a fault at F3. For this example, the minimum pick is 2.29 p.u. or 137A. Step 4. Select a relay with a time-current curve that parallels the fuse time-current characteristics in the region below I F3 max. Step 5 Set the relay operate time to be above the fuse operate time plus the fixed CTI (Coordination Time Interval) for maximum faults at F2 using the TDS. Only when the line impedance is significant when compared to the source impedance is discrimination possible. Since, for this example, the minimum currents at F1 and F2 are nearly the same, little time difference is available to discriminate faults on the upstream side of Bus R from faults to be cleared by the fuse. For the relay at Bus S, faults at F2 are indistinguishable from those at F2b. The penalty for the coordinated backup protection is slow operations the entire length of the Bus S to Bus R line. The constraint of the minimum current fuse operate time limits the maximum relay operate time to 2.7 seconds at F2, hence requires a TDS setting of 3. (See Appendix 11.3 Figure 11.4) If the dynamic range of the source impedance is comparative small (less than perhaps two to one) and the line impedance is significant as compared to the source impedance, faults can be more easily discriminated. Hence, a type 5 relay could be used to protect the first 8% of the line between Bus S and Bus R with instantaneous operation and let the type 51 relay protect the last 2% and provide backup protection for the fuse Minimum TOC Pickup Settings
4 272 The protection objective is the set the relay pickup as sensitive as possible so to operate as fast as possible with out violating coordination principles. Pickup settings of the most downstream fuse or relay are set in accordance with sections and with the aid of a load flow and fault study programs. The maximum and minimum fault currents are determined for each pickup setting protected by a relay or fuse from fault studies or traditional loading. In the absence of better information, the lesser of line rating or transformer rating can be used. The pickup relay value can be set by computing maximum relay fault current divided by the CTR times the minimum downstream fault current fuse or relay as was described in step three above. For electromechanical relays, the relay had tap settings that acted as a secondary current transformer. These electromechanical relays restrict tap values to a discrete set while microprocessor relays allow fractional tap values. The pickup value must be set sufficiently sensitive to see faults at the end of the protected line and yet not pick up for load. This is not always possible for systems with weak sources. In such cases, load encroachment blocking becomes necessary. (See section ) Consider the time-current coordination of the system shown in Figure Relay 5 requires only an instantaneous element set according to section because there are no downstream devices to coordinate with. The minimum pickup is Relay 4 is also set according to section with the difference being that Load 4 must now be considered TDS Setting Time dials are set to create the necessary coordination time interval, CTI, to guarantee that the relay sees past the start of the next protected line segment but does not operate faster than the primary protection of the next line segment. Factors that influence an acceptable CTI are the maximum breaker fault clearing time, the relay disk over-travels (caused by inertia in electromechanical relays), and a safety margin to account for instrumentation errors, loads, and breaker-failure operate times. Typically the CTI of.3 seconds is used although this may vary from.2 to.5 seconds. This should be used unless there is justification otherwise Inverse Settings Degrees of inversness can also aid in discriminating faults by time. In general, use relays with similar inverse characteristics for easier coordination. Flatter inverse curves (weakly inverse, moderately inverse) are appropriate for the following conditions: 1. No coordination is required or the relay is the farthest downstream. 2. The line is short or there is little current difference for faults close to the relay and faults at the ends of the line. 3. Instantaneous operations provide good coverage. Relays with Inverse characteristics provide faster clearing times than relays with vary or extreme inverse characteristics when the minimum fault current is significantly greater than the maximum load current. Relays with the later two characteristics are used when: 1. The fault current is much greater when the fault is closer to the relay compared to when the fault is at the far end. This is the case when the line impedance is large compared to the source impedance. 2. When the short-term load is high or load encroachment is a potential problem. 3. When coordinating relays with downstream fuses and reclosers. Modern relays permit implementing unique curves that follow no standards for applications where none of the standard curves provide the desired protection. However, doing so repeatedly without good justification can complicate relay coordination efforts in the future.
5 Time-current Coordination Example Consider the system illustrated in Figure 5.18 where the task is to coordinate a single time overcurrent relay with the fuse F1 that has characteristics similar to those shown in Figure 4.6 or Appendix The calculations for this example are included on the Text CD as 51-.mcd. One must first determine the load and fault currents at the boundary conditions. For this example, the maximum relay current is three-phase fault at Bus S and the minimum relay current is a phase fault current at Bus T. The results of the fault current calculations are shown in Table 5.5. The maximum per phase load current at Bus R is 5A, a 65E fuses is selected since that is the conventional size fuse greater than 125% of the maximum load current. The fault current at Bus R and Bus T determine the basis for the coordination time needed for the phase and ground relay units. For a fault at Bus R, the maximum fault current occurs for a three phase-to-phase fault. Since the relay must be set sufficiently to see faults at Bus T which, for this example, is 9 secondary amps. According to the requirements set by Chapter , the relay pickup current can be set as low 1.5 time the maximum load current or secondary amps. Doing so makes the minimum fault current 2.88 multiples of the relay current and 216 primary amps for the fuse. From the fuse time current graph shown in Appendix 11.4, the fuse melt time is 8 seconds. Thus, to coordinate relay ground fault times with the fuse requires that the relay wait the 8 second fuse operate time. The CTI can be ignored for this case as there is no breaker-operate time to consider. Hence for the extremely inverse curve, the TDS must be set above 1. The second point of the relay coordination, the relay can operate+ no faster that the fuse for three-phase faults at F2. The fuse current for is case is 3,34 primary amps and, according to the maximum melt time, is.4 seconds. 334 amps primary is 44.5 multiples of the pickup current that was set to secondary amps in the preceding paragraph. According to the extremely inverse time-current curve shown in Figure 11.4, the TDS could be set as low as 1.. However, the constrain placed by the ground fault current at Bus T, the relay operate time for three-phase faults at Bus R is limited to.4 seconds. Es2 Es1 ZS2 Bus R 5A Bus S V V R S Load Bus T V T Relay S I S 52 F1 ZL1 F2b F2 I R ZL2 F3 5A Load CTR = 12:5 PTR = 345:12 Secondary Ohms 1 = 1 Ω 8 = 3 Ω 8 ZS2 1 = 1 Ω 8 ZS2 = 3 Ω 8 Primaryary Ohms ZL1 1 = Ω 45.6 ZL1 = Ω 77.5 ZL2 1 = 3.27 Ω 45.6 ZL2 = 5.93 Ω 77.5 Figure Single line diagram for time coordination Example Table 5.5. Relay currents in multiples of secondary full-load amps (5A) for the system shown in Figure 5.18 with no load Fault Type Source Z F1 sec. amps F2 sec. amps F3 sec. amps
6 274 3Φ Φ-Φ Φ-G Max Min Max Min Max Min The relay ground elements must be coordinated with the fuse as well. The coordination problems become significant when the ground fault current is on the same order of magnitude as the load current. Such is the case with this example where the phase current for a ground fault at Bus 3 is only 135A as compared to the load maximum current at Bus 1 of 1A. For a Bus 3 ground fault, the maximum fuse operate time is 2.5 seconds while for Bus 2 ground faults, the fuse operate time is still only 1.3 seconds. Even though the relay can be set to operate much faster, it must wait at least 1.6 seconds before clearing faults at Bus 2 on the source side of F1 in order to maintain the coordination. The problem is caused by fuse insensitivity to zero sequence currents. Solutions to this problem are presented in section Aforementioned shortcomings of fuses not withstanding, in the correct application they can provide reliable and cost effective protection.
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