Basics - Time overcurrent protection, abbreviated with ANSI device number 51, is THE relaying and protection scheme. What I mean is: If we (as a society) had to choose just one way to protect our equipment, 51 protection would be the answer.
It essentially mimics the behavior of fuses (the very first form of protection used in power systems). If I have a small overload the overcurrent device takes a long time to trip. If I have a short circuit then the device trips very quickly. This behavior is really nice, since it ensures equipment isn't damaged by overcurrent, while also keeping our system in service as much as possible!
Figure 1: A Schematic Representation of 51 Protection
Medium voltage switchgear, like the example in Figure 2 below, often use time overcurrent protection to ensure that feeder circuits are protected. 51 is a reliable protection for things like transformers, loads, and buses, so you'll see it show up everywhere.
Figure 2: Medium Voltage Switchgear (Photo Courtesy Eaton)
Pickup - In order to specify the protection of our circuits and equipment, we need to set a "pickup" value for our 51 protection. Essentially, this is the value at which the time overcurrent curve actually begins to take effect. Below the pickup value, the system can operate continuously with no fear of tripping.
How do we set the pickup value, though? Well, there are a couple of things to consider.
The pickup needs to be set above the load current of the circuit.
The pickup needs to be high enough that we avoid nuisance tripping offline.
The pickup value should be coordinated with the ampacity/current ratings of the equipment being fed.
We can meet this design as follows:
To avoid nuisance tripping, our 51 pickup needs to be at least as large as our full load current with any possible measurement errors from our CTs and relays accounted for. Generally, a little bit of margin above this limit is advised as well to ensure satisfactory operation. In practice, something like 10% is recommended for the combined margin and error with modern equipment. For low voltage fuses and molded case circuit breakers 25% margin is the standard set by the NEC.
Curves - The long time pickup setting is only one part of 51 protection. We also have to define the equation for the curve. In theory, we can define any kind of curve. In reality, the IEC and IEEE define standard curves that are used almost universally for relay settings. In the United States, these curves have designation like U1, U2, U3, or U4 that correspond to the level of "inverse-ness" in the graph (how quickly the relay trips on overload).
Figure 3 below shows the Standard US Trip Curves (U1-U5) and their associated pickup time vs. pickup current. The pickup current is expressed as multiples of the long-time setting. For reference, U4 has the following trip times:
~16 cycles at 5x pickup
~6 cycles at 10x pickup
~3 cycles at 20x pickup
Figure 3: Standard US Trip Curves
Properly setting time overcurrent protection for coordination amongst devices requires taking a close look at all the curves involved and modifying any settings as needed. Protection is necessary and coordination is desired. This means that selective coordination of breakers (making sure the right breaker, and only the right breaker, trips) is of secondary importance to protecting equipment and personnel.
Achieving selective coordination means making sure that there is sufficient time delay between curves at the same current value (assuming the same accuracies for CTs and relays).
For example, consider the following typical delays:
MV Circuit Breakers: 3-5 cycles
Relay Pickup Time: 1-3 cycles
Design Margin: 2-3 cycles
This adds up to a total operating time delay of roughly 6-11 cycles. Without at least this much delay, we can't guarantee that the right breaker alone trips.
Comparison with Instantaneous Overcurrent Protection - Time overcurrent protection (51) is often supplemented with instantaneous overcurrent protection (50). 50 protection relies on setting a value of current that results in immediate circuit breaker operation. Generally, we need to set our 50 protection to be above any transient currents (like motor inrush, transformer energization, or through-faults that will be cleared by other breakers). Otherwise, there could be nuisance tripping with undesirable results.
50 protection can be combined with 51 to form a single trip curve, an example of which is seen in Figure 4.
Figure 4: A Combined Trip Curve with 51 and 50 Protections
50 protection is incredibly valuable when applied correctly. Adding instantaneous overcurrent to your trip curve may help with coordination. Perhaps most interestingly, with the use of "maintenance switches" plant personnel can temporarily adjust 50 settings downward so instantaneous trip occurs on very small overcurrents. This can help reduce arc flash hazards for maintenance personnel.
Figure 5: Maintenance Switches on a Switchgear (Photo Courtesy Eaton)
Summary - Overcurrent protection is what keeps our power systems working. Electrical engineers, designers, electricians, maintenance personnel, and operators all need to understand how 50 and 51 protection work to keep a plant running successfully. So, next time you're planning a facility, pay extra special attention to those functions!