1. Application
Although the official designation of the 50-element is “instantaneous overcurrent,” a time delay is often added to transform it into a definite-time overcurrent element. A 50-element will operate if the current is greater than the pickup setpoint for longer than the time delay setting.
When the instantaneous overcurrent element is used for phase overcurrent protection, it is labeled with the standard IEEE designation “50.” Ground or neutral instantaneous overcurrent elements can have the designations 50N or 50G depending on the relay manufacturer and/or relay model.
The 50-element can be used independently or in conjunction with time overcurrent (51) functions. When used in a grounding scheme, typically all feeders have identical pickup and time delay settings. The main breaker would have a slightly higher setting and/or longer time delay to ensure that a ground fault on a feeder will be isolated by the feeder breaker before the main breaker operates. An example 50-element ground protection scheme is shown in the following figures.
Principles and Practice
196
© 2012 Valence Electrical Training Services LLC. All rights reserved.
www.valenceonline.com
The 50-element protective curve looks like an “L” on a Time Coordination Curve (TCC, See the Time Coordination Curves (TCC) and Coordination section starting on page 73 for details.) The element will operate if the measured current is on the right side of the vertical line for longer than the time indicated by the horizontal line of the protective curve in Figure 9-2. In this example, a feeder ground fault greater than 10 Amps must last longer than one second before the 50-element will operate. The main breaker protection will operate if any ground fault is greater than 15 Amps for longer than two seconds.
MAIN
50G
PCB2
50G
PCB3
50G
FEEDER 1 FEEDER 2
Figure 9-1: Ground Fault Protection Single-Line Drawing
Main Ground Protection
Feeder Ground Protection
0.10 1.00 10.00
1 10 100
Time in seconds
Current in amperes Time Co-ordination Curve
Main Ground Protection
Feeder Ground Protection
Figure 9-2: Ground Protection TCC
The 50-element can also be applied in conjunction with inverse-time overcurrent elements (51) to better protect equipment during high-current faults. The amount of damage created during a fault can be directly related to the amount and duration of fault current. To limit equipment damage, the relay should operate faster during high fault currents.
The following figures display how the 50-element can enhance equipment protection as well as coordination with other devices. In Figure 9-3, the time overcurrent (51) relay curve intersects the cable damage curve and, therefore, does not provide 100% protection for the cable. The cable is only 100% protected if its damage curve is completely above the protection curve.
Adding a 50-element to the time overcurrent element will provide 100% cable protection as shown in Figure 9-4. However, the addition of the 50-element creates a mis-coordination between the R2 relay and downstream Fuse 1 because the two curves now cross. The relay will operate before the fuse when the relay curve is below and to the left of the fuse curve. This problem can be solved by adding a slight time delay of 0.03 seconds, which will coordinate with the downstream fuse as shown in Figure 9-5.
Chapter 9: Instantaneous Overcurrent (50) Element Testing
© 2012 Chris Werstiuk and Valence Electrical Training Services LLC. All rights reserved. 197
If we wanted to provide the best protection for the cable and fully utilize the available options of most relays, we could add a second 50-element with no intentional time delay set with a pickup setting higher than the maximum fuse current. This is shown in Figure 9-6. Adding another 50-element will cause the relay to trip sooner at higher currents and will hopefully reduce the amount of damage caused by fault.
0.01
Figure 9-3: 50/51 TCC #1
0.01
Figure 9-4: 50/51 TCC #2
0.01
Figure 9-5: 50/51 TCC #3
0.01
Figure 9-6: 50/51 TCC #4
Principles and Practice
206
© 2012 Valence Electrical Training Services LLC. All rights reserved.
www.valenceonline.com
4. Timing Tests
There is often a time delay applied to the 50-element protection even though the 50-element is defined as instantaneous overcurrent protection. Timing tests should always be performed even if time delay is not assigned.
50-element timing tests are performed by applying 110 % of pickup current (or any value above pickup) to the relay and measuring the time between the start of the test and relay operation.
The start command could be an external trigger, a preset time, or a push button on the relay set. The stop command should be an actual output contact from the relay because that is what would happen under real-life conditions.
1 2 3 4 5 6 7
0
TIME IN CYCLES 2A
4A 6A 8.8A8A
TEST IN PROGRESS
PICKUP
Figure 9-14: 50-Element Timing Test
When the 50-element time delay is zero or very small (less than 2 seconds), the actual measured time delay can be longer than expected. There is an inherent delay before the relay can detect a fault plus an additional delay between fault detection and output relay operation.
These delays are very small (less than 5 cycles) and are insignificant with time delays greater than 2 seconds.
The first delay exists because the relay is constantly analyzing the input data to determine if it is valid and this analysis takes a fraction of a cycle. The relay cannot determine the magnitude of the input signal until it has enough of the waveform to analyze and determine the rms or peak current or voltage. The relay is also a computer and computers can only perform one task at a time. If a fault occurs just after the relay processes the line of code that detects that particular fault, the relay has to run through the entire program one more time before the fault is detected.
All of these delays usually require a fair portion of a cycle to complete. The “Operate Time” and
“Timer Accuracy” specifications in the following figure detail this time delay.
Chapter 9: Instantaneous Overcurrent (50) Element Testing
© 2012 Chris Werstiuk and Valence Electrical Training Services LLC. All rights reserved. 207 PHASE / NEUTRAL / GROUND IOC
CURRENT: PHASOR ONLY
Pickup Level: 0.000 pu to 30.000pu in steps of .001 pu Dropout Level: 97% to 98% of Pickup
Level Accuracy: +/- 0.5% of reading or +/- 1% of rated (Whichever is greater) from 0.1 to 2.0 x CT ration +/- 1.5% of reading > 2.0 x CT rating < 2%
Overreach: < 2%
Pickup Delay: 0.00 to 600.00 in steps of 0.01 s Reset Delay: 0.00 to 600.00 in steps of 0.01 s Operate Time: < 20 ms @ 3 x Pickup @ 60Hz
Timing Accuracy: Operate @ 1.5 x Pickup +/- 3% or +/- 4ms (whichever is greater) Figure 9-15: GE D-60 Relay Overcurrent Technical Specifications
The second time delay occurs after the relay has detected the fault and issues the command to operate the output relays. There is another fraction of a cycle delay to evaluate what output contacts should operate and then the actual contact operation can add up to an additional cycle depending on relay manufacturer, model, etc. “Operate Time” in the following figure represents this delay for the specified relay.
FORM-C AND CRITICAL FAILURE RELAY OUTPUTS Make and Carry for 0.2 sec: 10A
Carry Continuous: 6A
Break @ L/R of 40ms: 0.1 ADC max
Operate Time: < 8ms
Contact material: Silver Alloy
Figure 9-16: GE D-60 Relay Output Contact Technical Specifications
Your test-set can also add a small time delay to the test result as shown by the “Accuracy”
specification of the following figure:
MANTA 1710 TIME MEASUREMENT SPECIFICATIONS Auto ranging Scale: 0—99999 sec
Auto ranging Scale: 0—99999 cycles Best Resolution: 0.1ms / 0.1 cycles
Two wire pulse timing mode
Accuracy: 0—9.9999 sec scale: +/-0.5ms +/- 1LS digit all other scales: +/- 0.005% +/- 1 digit
Figure 9-17: Manta Test Systems M-1710 Technical Specifications
Principles and Practice
208
© 2012 Valence Electrical Training Services LLC. All rights reserved.
www.valenceonline.com
What does all this mean? With a time delay of zero, the time test result for a GE D-60 relay, using a Manta M-1710 test-set, could be as much as 32.6ms or 1.956 cycles as shown in the following figure:
MINIMUM TIME TEST RESULT Relay Operate Time: < 20 ms
Relay Timing Accuracy: +/- 4ms Relay Operate Time: < 8 ms
Test-Set : +/-0.5ms
+/- 1LS digit (0.1 ms) 32.6ms or 1.956 cycles
Figure 9-18: 50-Element Minimum Pickup
A) Timing Test Procedure
• Determine which output the 50-element trips and connect the test-set timing input to the relay-output.
• Check the maximum per-phase output of the test-set and use the appropriate connection from Figures 1-7 to 1-11. For example, if the 50-element pickup is 35A and your test-set can only output 25 Amps per phase; use “High Current Connections #1.” If the pickup setting is greater than 50 Amps, use “High Current Connections #2.” If the pickup is higher than 75A (3x25A), you will have to use another test-set or temporarily lower the setting. Remember, setting changes are a last resort.
• Set the fault current 10% higher than the pickup setting. For example, set the fault current at 44.0 Amps for an element with a 40.0 Amp setpoint. Set your test-set to stop when the timing input operates and to record the time delay from test start to stop.
• Apply test current and ensure timing input operates and note the time on your test sheet. Compare the test time to the 50-element timing to ensure timing is correct.
• Review relay targets to ensure the correct element operated.
• Repeat for other two phases.
© 2012 Chris Werstiuk and Valence Electrical Training Services LLC. All rights reserved. 211