25.11. DIRECTIONAL OVERCURRENT (67) PROTECTION |
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and magnitudes. This digital alternative, of course, requires careful attention to relay settings in order to work.
25.11Directional overcurrent (67) protection
While 50 and 51 (instantaneous and time overcurrent) relay functions monitor line current magnitude and guard against excesses, there are applications where the direction of line current is just as relevant as the magnitude. In such cases, we need a protective relay function able to discriminate between current in one direction versus current in the other direction. The ANSI/IEEE number code designation for a directional current-sensing protection is 67.
One such application is generator protection, where an overcurrent relay monitors the amount of current at the point where an electrical power generator connects to a larger network of generators. The problem of directional current monitoring is easiest to understand in the context of a directcurrent (DC) generator and battery circuit, which we will now explore as an introduction to the topic:
Consider a DC generator connected to a secondary-cell (i.e. rechargeable) battery. Here, the voltage polarity never changes, but the direction of current does change depending on whether the generator is acting as a power source (charging the battery) or “motoring” and acting as a power load (discharging the battery):
Generator acting as a source |
Generator "motoring" as a load |
Rline |
Rline |
Gen |
Gen |
Rline |
Rline |
A generator acting as a source (in this case, to charge the battery) is fulfilling its intended function. A generator running as a motor, drawing energy from the battery as a load, is most definitely not fulfilling its intended function. Therefore, we would consider any current in the wrong (generator as load) direction to be excessive, while considerable current in the correct (generator as source) direction would be considered perfectly normal. If we were to install an overcurrent relay in this simple DC system, we would therefore prefer that it be more sensitive to current (i.e. pick up at a lower value) in the “reverse” direction than to current in the “forward” direction.
transformer where a fourth conductor attaches to the center of the Wye winding set may experience line currents on the Wye side that are not seen in the line conductors of the Delta side, and may therefore cause a di erential current relay to operate. This is another reason why connecting CTs di erently than the power transformer windings they sense (i.e. Delta-connected CTs on a power transformer’s Wye side) is a good idea: any zero-sequence currents within the power transformer’s Wye-connected winding will circulate harmlessly through the Delta-connected CT secondaries and never enter the 87 relay. For digitally compensated 87 relay installations where all CTs are Wye-connected, the relay must also be configured to mathematically cancel out any zero-sequence currents on the Wye-connected side of the power transformer.
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CHAPTER 25. ELECTRIC POWER MEASUREMENT AND CONTROL |
Alternating-current (AC) power systems are not that di erent in this regard. In the forwardcurrent direction, the generator acts as a power source, sending electrical power to any loads connected to the generator bus. In the reverse-current direction, the current experiences a phaseshift of 180 degrees from that of the forward-current direction, at which point the generator acts as a load to any other generator(s) on the network. This phase shift is evident if we compare the signal waveforms from PT and CT instrument transformers connected to the generator:
Generator acting as a source |
Generator "motoring" as a load |
CT |
CT |
PT |
PT |
To bus |
To bus |
The fundamental problem we face in designing an AC directional current relay is how to detect this phase shift between forward and reverse current. In our DC generator circuit, a reverse flow of current could easily be detected by monitoring the polarity of voltage drop across a series resistance 53. In an AC circuit, however, the only way to tell if the line current is going the wrong way is if we compare the current waveform against another “reference” waveform (such as line voltage). The di erence in phase shift between forward current and reverse current will be 180 degrees. Thus, an AC directional protective relay requires at least two signal inputs: one representing line current to be monitored, and another serving as a polarizing or reference quantity to be used for phase comparison.
This polarizing quantity may be line voltage, it may be a di erent current in the system, or it may even be a some combination where one signal provides backup in case the other polarizing signal becomes too weak. The challenge of finding a suitable polarizing signal in a power system for a directional relay stems from the fact that voltage and current signal strengths may vary wildly under fault conditions, which is precisely when we need the protective relay to do its job. Consider, for example, using generator line voltage as the polarizing signal to be compared with line current in the 67 relay. Imagine now if that generator su ers a major fault in its windings. Any other generators connected to the same bus will now send power into the faulted generator: a clear case of reverse power flow (into the generator) when we need the directional relay to trip. However, if the fault happens to significantly reduce the line voltage of the failed generator, the directional relay may receive too weak of a polarizing signal to properly operate, and thus may fail to trip the generator’s breaker connecting the failed generator to the bus.
Modern microprocessor-based directional relays have a definite advantage in this regard over legacy electromechanical relay designs, in being able to intelligently select the best polarizing quantity to use during fault conditions. Relays manufactured by Schweitzer Engineering Laboratories having directional protection elements for ground and neutral currents, for example, use a proprietary algorithm called “Best-Choice Ground Directional Element” logic to select from one
53Note the reversal of polarity for the voltage drop across each line resistance in the DC example diagram. A shunt resistor intentionally placed in series with the generator current could fulfill that same directional-sensing role.