Whenever faults occur in power system large currents flow. Especially, if the fault is a dead short circuit at the terminals or bus bars enormous currents flow damaging the equipment and its components. To limit the flow of large currents under these circumstances current limiting reactors are used. These reactors are large coils covered for high self-inductance.
They are also so located that the effect of the fault does not affect other parts of the system and is thus localized. From time to time new generating units are added to an existing system to augment the capacity. When this happens, the fault current level increases and it may become necessary to change the switch gear. With proper use of reactors addition of generating units does not necessitate changes in existing switch gear.
These reactors are built with non-magnetic core so that saturation of core with consequent reduction in inductance and increased short circuit currents is avoided. Alternatively, it is possible to use iron core with air-gaps included in the magnetic core so that saturation is avoided.
The above classification is based on the location of the reactors. Reactors may be connected in series with the generator in series with each feeder or to the bus bars.
Disadvantages:
(a) In the event of a fault occurring on a feeder, the voltage at the remaining healthy feeders also may loose synchronism requiring resynchronization later.
(b) There is a constant voltage drop in the reactors and also power loss, even during normal operation. Since modern generators are designed to with stand dead short circuit at their terminals, generator reactors are now-a-days not used except for old units in operation.
Disadvantages:
(a) Voltage drop and power loss still occurs in the reactor for a feeder fault. However, the voltage drop occurs only in that particular feeder reactor.
(b) Feeder reactors do not offer any protection for bus bar faults. Never the less, bus-bar faults occur very rarely.
As series reactors inhererbly create voltage drop, system voltage regulation will be impaired. Hence they are to be used only in special case such as for short feeders of large cross-section.
Another advantage is that additional generation may be connected to the system without requiring changes in the existing reactors.
The only disadvantage is that this systems requires an additional bus-bar system, the tie-bar.
They are also so located that the effect of the fault does not affect other parts of the system and is thus localized. From time to time new generating units are added to an existing system to augment the capacity. When this happens, the fault current level increases and it may become necessary to change the switch gear. With proper use of reactors addition of generating units does not necessitate changes in existing switch gear.
CONSTRUCTION OF REACTORS
These reactors are built with non-magnetic core so that saturation of core with consequent reduction in inductance and increased short circuit currents is avoided. Alternatively, it is possible to use iron core with air-gaps included in the magnetic core so that saturation is avoided.
CLASSIFICATION OF REACTORS
(i) Generator reactors (ii) Feeder reactors (iii) Bus-bar reactorsThe above classification is based on the location of the reactors. Reactors may be connected in series with the generator in series with each feeder or to the bus bars.
(I) GENERATOR REACTORS
The reactors are located in series with each of the generators as shown in Figure 1 so that current flowing into a fault F from the generator is limited. |
| Figure: 1 |
(a) In the event of a fault occurring on a feeder, the voltage at the remaining healthy feeders also may loose synchronism requiring resynchronization later.
(b) There is a constant voltage drop in the reactors and also power loss, even during normal operation. Since modern generators are designed to with stand dead short circuit at their terminals, generator reactors are now-a-days not used except for old units in operation.
(II) FEEDER REACTORS
In this method of protection each feeder is equipped with a series reactor as shown in Figure 2. In the event of a fault on any feeder the fault current drawn is restricted by the reactor.
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| Figure: 2 |
(a) Voltage drop and power loss still occurs in the reactor for a feeder fault. However, the voltage drop occurs only in that particular feeder reactor.
(b) Feeder reactors do not offer any protection for bus bar faults. Never the less, bus-bar faults occur very rarely.
As series reactors inhererbly create voltage drop, system voltage regulation will be impaired. Hence they are to be used only in special case such as for short feeders of large cross-section.
(III) BUS BAR REACTORS
In both the above methods, the reactors carry full load current under normal operation. The consequent disadvantage of constant voltage drops and power loss can be avoided by dividing the bus bars into sections and inter connect the sections through protective reactors. There are two ways of doing this.(a) RING SYSTEM:
In this method each feeder is fed by one generator. Very little power flows across the reactors during normal operation. Hence, the voltage drop and power loss are negligible. If a fault occurs on any feeder, only the generator to which the feeder is connected will feed the fault and other generators are required to feed the fault through the reactor.(b) TIE BAR SYSTEM:
This is an improvement over the ring system. This is shown in Figure 3. Current fed into a fault has to pass through two reactors in series between sections.Another advantage is that additional generation may be connected to the system without requiring changes in the existing reactors.
The only disadvantage is that this systems requires an additional bus-bar system, the tie-bar.
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| Figure: 3 |
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