The main purpose of feeder automation is to minimize the faulted area and to minimize the outage time. The conventional technique using current-time controlled reclosers, sectionalizers, voltage-time controlled C.B., and reclosing sectionalizers are still widely used in the U.S.A., Japan and many other countries.
The recloser and, sectionalizer scheme, however, can not isolate the load side of faulted section., During loop operation, it causes unnecessary short circuit impact to the network and makes coordination difficult. The voltage-time controlled scheme can be used in the loop network, but it has the disadvantages of long coordination time, frequent operation, close into fault by the LBS, etc.
The trend was to use computers and, communication networks to overcome the problems above, and in this system, the LBS (sectionalizer or reclosing sectionalizer) operate just as a passive remote switch (no active sectionalizer functions).
The concentrated control system has more disadvantages than the distributed control system in terms of reliability, coordination time, investment, etc.
This is the reason why there are continuous efforts to develop distributed control systems likes peer to peer communication control (S&C), and efforts to improve or develop efficient self healing systems like loop systems using dual operation mode reclosers (ABB) and pulsed reclosing systems(S&C).
The VIT self-healing system was developed to fulfill the feeder automation without using communication in a radial network and also in a loop network. The source and load side of a faulted section can be isolated before normal open switch automation close (if this problem is solved, then communication is no onger necessary for feeder automation). The VIT self healing system use the voltage, current, and time based sequential information. As a result, it can isolate the high faulted current, sensitive earth fault current (SEF), and open conductor fault through the coordination of C.B, REC, and sectionalizers.
In a VIT self-healing system, the LBS (not the passive remote switch) does the very important job of isolating high fault current through fault current count, to isolate the SEF and open conductor fault by itself and to isolate the load side of the faulted section.
Of course, the control of recloser and sectionalizer has RTU functions, so it can communicate with the computer, but the feeder automatic itself will be finished without communication. The VIT self healing system is the extreme solution to maximize the functions of the recloser and sectionalizer.
Two kinds of VIT self-healing system are available:, one uses the reclosing sectionalizer scheme, and the other uses the sectionalizer scheme. If the network is long and many sectionalizing points (rural area) are needed, then the VIT reclosing sectionalizer scheme would be considered best. If the network is not so long and 3~5 sectionalizing points are needed, then the VIT sectionalizer scheme would be best.
2. VIT Reclosing Sectionalizer Scheme
2.1 Radial normal close mode, loop mode
– Best suited for long rural network
– Infinite number of switches can be coordinated
– The sectionalizer will trip after fault current count (radial mode) or voltage count (loop mode)
– If the voltage restored, then the close timer starts. If the close timer is completed, then the close and lockout timer starts.
– If the control experiences loss of voltage before completing the close timer, then it goes to lock out while maintaining the open position (load side isolation of faulted section).
– If the control experiences loss of voltage while the lockout timer is running, then it will be open and lockout after The backup device opens (source side isolation of faulted section).
– The normal open switch starts the close timer if one side of the voltage is lost and goes to lockout if the loss of voltage count is completed. If the count is not completed by the time the close timer finishes, then it will close.
2.2 Radial sectionalizer mode
If the radial line is not so long, then the conventional sectionalizer mode can be used as follows.;
– Faulted current count
– Count trip
– Inrush restraint
2.3 High Fault Current
High fault current is always cleared first by the back up C.B or recloser. The reclosing sectionalizer or sectionalizer isolates the faulted section after the back up device clears the fault if the count is completed or experiences loss of voltage before finishing the close timer or lockout timer.
2.4 SEF (Sensitive Earth Fault Current)
According to the settings, the SEF can be directly cleared by the sectionalizer before operation of back up C.B or recloser, at the radial mode or radial sectionalizer mode.
At the loop mode, the SEF should be cleared by the backup device to isolate the load side of the faulted section.
2.5 Open Conductor Fault
The open conductor fault protection time will be longer than the SEF. If the open conductor detects a sensitive earth fault, then it will be cleared by the SEF or directional SEF protection function. (For example, the load side of the open conductor touches the ground and it still is energized through a load side 3-phase transformer).
If the open conductor does not touch the ground, then it will be isolated through unbalance current (source side of open conductor) and unbalance voltage (load side of open conductor).
3. VIT Sectionalizer Scheme
The VIT Sectionalizer doesn’t close into fault to isolate the source and load side of the faulted section, but it can isolate the source and load side of faulted section before the automatic close of a normal open switch.
The C.B. can coordinate with one sectionalizer, and the recloser can coordinate 2 or 3 sectionalizers according to the number of shots for lockout as follows.
– Best suited for urban network
– No close into fault by the load break switch
– The sectionalizer will trip and isolate the source side of faulted section after fault current counting (radial, loop mode)
– The sectionalizer will trip and isolate the load side of the faulted section before normal open switch close through voltage counting (loop mode).
– Offers the fastest speed, and the number of operations will be minimum.
– Radial, loop network
– Automatic setting group change
– SEF, open conductor fault isolation same with reclosing sectionalizer.