CHAPTER 2 - LITERATURE REVIEW

This paper that was published by Newagepublisher.com titled Auto Re-Closing review that the transient faults which are most frequent in occurrence do not cause permanent damage to the system as they are transitory in nature. These faults disappear if the line is disconnected from the system momentarily in order to allow the arc to extinguish. After the arc path has become sufficiently deionized, the line can be reclosed to restore normal service. The type of fault could be a flashover across an insulator. Reclosing could also achieve the same thing with semi-permanent faults but with a delayed action, e.g., a small tree branch falling on the line, in which case the cause of the fault would not be removed by the immediate tripping of the circuit breaker but could be burnt away during a time delayed trip and thus the line reclosed to restore normal service. Now should the fault be permanent, reclosing if no use, as the fault still remains on reclosing and the fault has to be attended personally. It simply means that if the fault does not disappear after the first trip and closure, double or triple-shot reclosing is used in some cases before pulling the line out of service.

Experience shows that nearly 80% of the faults are cleared after the first trip, 10% stay in for the second reclosed which is made after a time delay, 3% require the third reclosed and about 7% are permanent faults which are not cleared and result in lockout of the reclosing relay. The breakers at the two ends trip simultaneously on occurrence of the fault when a line is fed from both ends. The generators at the two ends of the line will drift apart in phase and the breakers must be reclosed before the generators drift too far apart for synchronism to be maintained. This, the reclosed will increases the stability limit [7].

In present day power systems, automatic reclosing finds wide application. It therefore follows the effects of fault clearance and subsequent reclosed, hence it is often necessary to operate sequentially several items of switchgear. Recently logical design principles have been applied for the control of auto reclosed switching sequences in large substations. The obvious advantages are the continuous supply except for the short duration when tripping and reclosed operations are being performed, this renders the substation unattended. The success of rapid reclosed to a large extent depends on the speed of operation of the protections. This is because high speed protection decreases the amount of damage incurred and thus increases the probability of successful operation [7].

In some cases application of automatic reclosing enables us to use very simple but high speed protections of the lines. During the instantaneous protection being applied, indiscriminate tripping of several circuit breakers is possible but the provision of auto-reclose makes it a selective operation. It is essential that the system dead time be kept to a few cycles so that the generators do not drift apart. High speed protection such as pilot wire carrier or distance must be used to obtain operating times of one or two cycles. It is therefore desired that the reclosed to be of the single shot type. High speed reclosed in high voltage circuits improves the stability to a considerable extent on single-circuit ties. [7]

Overcurrent protection with in turn cause automatic reclosed is enables to create high-speed protection of every section against transient as well as permanent faults. This is achieved by installing high speed nonselective overcurrent protections at all head parts of the sections. The pickup value for all such protections is selected for faults inside the full length of the given section and not for faults beyond the step down transformers if it is exists in different zones of protection. Any nonselective operation of faults outside the given section is corrected by automatic reclosing. During the protection of line by composite overcurrent protection, the first stage of the protection can be employed as high speed nonselective overcurrent protection. Practically, the automatic reclosed circuit immediately ties subjected faults to single circuit faults. The continuity through the healthy circuit prevents the generators from drifting apart so fast and the stability limit is therefore increased. However, when the faults occur simultaneously on both the circuits the stability limit also will be increases [7].

Line will be opened after fault incidence, independent of the fault type and are reclosed after a predetermined time period following the opening of initial circuit breaker. For a single circuit interconnectors between two power systems, the opening of all the three phases of the circuit breaker makes the generators in each group start to drift apart in relation to each other, since no interchange of synchronizing power can take place. [7]

On the other hand single-phase auto-reclosed is one in which only the faulted phase is opened during the presence of the single-phase fault and reclosed after a controlled delay period. For multiphase faults, all three phases that are opened and reclosed are not attempted. In case of the single-phase faults, synchronizing power can still be interchanged through the healthy phases. In the case of the single-phase auto-reclosing, each phase of the circuit breaker has to be segregated and provided with its own closing and tripping mechanism. Also it is necessary to fit phase selecting relays that will detect and select the faulty phase. Thus, single-phase auto-reclosing is more complex and expensive as compared to three-phase auto-reclosing. [7]

During the usage of single-phase auto-reclosed, the faulty phase must be reenergized for a longer interval of time. In the case of three-phase auto-reclose, owing to the capacitive coupling between the faulty phase and the healthy conductors must be done to increase the duration of the arc. The advantage claimed for single-phase reclosing is that (system with transformer neutrals grounded solidly at each substation), the interruption of one phase to clear a ground fault causes negligible interference with the load. This is because the interrupted phase current now flows in the ground through neutral points until the fault current is cleared and the faulted phase reclosed. The main drawback is it has longer deionizing time which can cause interference with communication circuits and in certain cases the operation of earth relays in double circuit lines owing to the flow of zero sequence currents [7].

Another paper is from Charles J.Nochumson from website www.eaton.com. This paper review on the application of new technologies in power circuit breaker with high interrupting capacity and short time rating. It brief on the description of an available classes of low voltage circuit breakers.

The Low Voltage Power Circuit Breakers (LVPCBs) are rated and tested to ANSI C37 standards. They are mainly used in Low Voltage Metal Enclosed Draw out Switchgear and built based on ANSI C37 standards. LVPCBs offer field maintainability of main contacts, operating mechanism, and arc chute replacement. LVPCBs are tested in the enclosure and are rated for 100% applications. They offer two steps for stored energy closing mechanisms, which have typical opening or closing times of three to five cycles. For electrical operation, they are typically combined with an internal motor to change the breaker closing springs and solenoids to release the spring stored energy for closing and opening of the breaker.

It was typically applied in systems with selective coordination, synchronizing schemes and automatic transfer scheme applications. Molded Case Circuit Breakers (MCCBs) are tested according to UL489. They are typically applied in Low Voltage Switchboards, Motor Control Centers and Panel boards. MCCB current carrying parts, mechanism and trip devices are completely placed within a molded case, and are not designed to be field maintainable. MCCBs as standard are tested in open air and are de-rated to 80% when placed in an enclosure. Optional MCCBs are available which have been UL tested for applications at 100% in the enclosure. Molded case circuit breakers have over-center toggle operating mechanisms.

For electrical operation, they are typically combined with external motor operating mechanisms, which move the breaker handle. The external motor operator is not fast enough for synchronizing or automatic transfer scheme applications. MCCBs are also available in current-limiting type. Insulated Case Circuit Breakers (ICCBs) also is rated and tested in according to UL489. They are typically applied as mains in Low Voltage Switchboards, Motor Control Centers and some transfer switches. Similar to the MCCB, the ICCB is generally placed in a sealed molded case, and it is not designed to be fully field maintainable. ICCBs are found as either 80% rated devices or 100% rated devices when mounted in the proper enclosure. ICCBs utilize two step stored energy mechanisms, similar to LVPCBs. ICCBs are normally not fast enough to qualify as current-limiting type.