If there is a cascade connection of a ‘one-directional unit’ and one overcurrent unit then it is called a Directional Overcurrent Protection relay. If the polarity of the current is suitable, then the directional unit picks up. If the current magnitude is above the pickup value and also the overcurrent unit has above the picks up value then the trip coil is energized, and the circuit breaker(CB) tripping is ensured. ‘AND’ logic that is used for the programmed in the numerical relay.
Necessary Condition For Directional Overcurrent Protection
The figure shows a system that is radial but it has two sources connected to it. If relays for protection of transmission lines are installed only at one end say towards source A end, it is observed from the figure that after the opening of the relay in red, the fault current will continue to be fed from a source B. Hence, relays are also installed at other ends of the transmission line to detect the fault and disconnect transmission line from the other end as well to protect the other equipment from damage.
A similar situation will exist even for a single source system if parallel paths exist as shown in the given below figure. Hence, If the transmission line system has multiple paths to source then relays required at both ends. However, installing relays at both ends does not provide a complete relaying solution for the protection of fault current. To understand the reason, consider the action of the red relay in as shown in the above figure with respect to two likely faults F1 and F2.
If the fault is at F1 then it is the responsibility of red relays to open the line connect to it. If the fault is at F2, then it is the green relay responsibility that it should trip the transmission line. However, it is completely sure that for fault F2, the circled green relay may trip after the circled red relay which opens to disconnect the feed from the source B, it is due to that both relays are subjected to the same fault current.
The circled green relay can not compete with a circled red relay to clear the fault. To overcome this problem the relay element has to be provided with additional discrimination features to distinguish between faults that it should respond to, and others that it should not respond to. Further, this selectivity will not be sufficient if it is based upon the magnitude of pick up current.
From the above figure, it is apparent that such discrimination will hold between relay sequences
R_1\longrightarrow R_3\longrightarrow R_5\quad and\quad R_6\longrightarrow R_4\longrightarrow R_2
Let’s consider two possible fault locations with respect to relay R3 as shown in given below figure.
The relay R2 should operate if the fault is at F1 because it is on primary side feeder but not behind i.e. at F2. With the polarity of CT connection as shown in below given figure, it is apparent that for fault F2 current I1 seen by the relay lags voltage by 90 degrees.
Fundamental Principle of Directional Overcurrent
If the relay detects any fault and current lags VR =Vp , then permit relay should be tripped. The discrimination based on the principle of a phase angle comparison between a set of phasors, one of which is used as the reference called as directional discrimination principle. Relays with this principle are called directional relays.
For example, overcurrent relays can be converted into directional by adding above discrimination logic to well-known overcurrent logic. These types of relays are called directional overcurrent relays. They are used in the distribution system or sub-transmission system where the ring is the main particular form is used to provide more reliability of service. The cost of this relaying scheme is higher than the non-directional overcurrent due to the additional cost of VT.