The most commonly used schemes for bus-zone protection are: 1. Backup Protection for Bus-Bars 2. Frame Leakage or Fault-Bus Protection 3. Differential Overcurrent Protection.
All the protection schemes must also be provided with a check system to ensure that the protection responds only to earth faults occurring within the bus zone and not to extraneous earth faults.
1. Backup Protection for Bus-Bars:
In principle it is the simplest of all to protect the buses with the aid of backup protections of the connected supplying elements which should respond to any fault appearing on the buses.
When no separate bus protection is provided but distance protection is provided for the feeders connected to the bus, it is possible to cover the bus-bars within zone 2 reach of the distance relays.
Referring to Fig. 5.26, the bus A is covered in the second step of distance protection B. Thus, in the event of fault on bus A, the distance protection B will operate. The operating time of the second step can be of the order of 0.4 second. In such a system the protection is slow and there can be unwanted disconnection of all incoming parallel circuits. Distance protection is widely employed for the protection of transmission lines, hence it is quite economical to use the same for bus protection.
Above scheme may be quite satisfactory for small switchgear installations but for large and important installations a separate bus zone protection is provided.
Referring to Fig. 5.26:
(a) The local overcurrent protection at station A provides the primary protection to bus-zone A.
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(b) The remote overcurrent protection or impedance protection at station B provides a backup protection to bus-zone A. In case protection ‘a’ fails protection ‘b’ provides a backup.
(c) Local overcurrent protection of incoming lines or feeders at station B provides primary protection to bus B.
The drawbacks of such protection are:
(i) Delayed action
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(ii) Disconnection of more circuits in case there are two or more incoming lines and
(iii) Exact discrimination not possible.
Bus backup protection may also mean that in case the breaker fails to operate for a fault on the outgoing feeder, then it must be regarded as a bus fault. It should then open all breakers on that bus. Such a backup protection can be provided with appropriate time delay through a timer.
2. Frame Leakage or Fault-Bus Protection:
It is one of the most simple form of protection and is applicable to small size metal clad switchgear. This method consists of insulating the bus-supporting structure and its switchgear from ground, interconnecting all the framework, circuit-breaker tanks etc. and providing a single ground connection through a CT that feeds an overcurrent relay as illustrated schematically in Fig. 5.27. The over-current relay controls a multi-contact auxiliary relay that trips the breakers of all circuits connected to the bus, as illustrated in Fig. 5.28.
Sometimes an impedance is connected in the earth connection to limit the short-circuit current during line-to-earth fault. Also it is necessary to isolate the switchgear framework from lead cable sheaths, cable boxes and the conduit fittings so that when a leakage to the framework occurs, the only path for the leakage current is through the connection from the framework to earth. An external flashover of an entrance bushing will also improperly trip all breakers unless the bushing support is insulated from the rest of the structure and independently grounded.
In case of a sectionalized bus structure, the housing of each section must be insulated from adjoining sections, and separate fault bus-relaying is provided for each section. The fault-bus scheme does not provide overlapping of protective zones around circuit breakers; and consequently supplementary relaying is required to protect the regions between bus sections.
In the schematic arrangement of fault-bus protection illustrated in Fig. 5.27, the metal supporting structure or fault bus is grounded through a CT, the secondary of which is connected to an overcurrent relay. Under normal working conditions, the relay remains inoperative but fault involving a connection between a conductor and grounded supporting structure will result in current flow to ground through the fault bus, causing the relay to operate. The operation of the relay will trip all the breakers connecting equipment to the bus.
Figure 5.28 illustrates the scheme in which the station bus-bars are supplied from a power transformer having star-connected secondary. A CT is also connected in the earth connection of this star-connected secondary. The secondary of CT is connected to the operating coil of the check relay.
From the scheme illustrated in the figure it is obvious that the multi-contact relay is energized only when both the leakage and check relay contacts are closed i.e., there is an earth fault within the protected zone. However, if the earth fault occurs outside the protected zone, the earth leakage current will pass through only the earthed secondary of power transformer and the check relay contacts will close but the frame leakage relay contacts will not close and thus multi-contact trip relay will not operate.
Fault-bus protection is more favourable to indoor than to outdoor installations. Certain existing installations may not be adaptable to such protection, owing to the possibility of other paths for short-circuit current to flow to ground through concrete reinforcing rods or structural steel.
3. Differential Overcurrent Protection:
For the main bus-bars in the power stations, due to their importance in the operating conditions, it is required that the disconnection be without any delay in the case of faults. Hence it is imperative to use a differential current protection without time delay.
The protection is based on simple circulating current principle that under normal operating conditions or under external fault conditions the sum of currents entering into a bus-bar will be equal to the sum of currents leaving the bus-bar. In case the sum of these currents (for a given conductor) is not zero, it must be due to a short circuit either a ground fault or phase-to-phase fault. Hence this protection scheme is applicable to both types of faults i.e., phase-to-phase faults as well as ground faults.
Figure 5.29 shows the application of differential circulating current principle to a bus with four circuits. The CTs are inserted in each phase of the incoming and outgoing feeders of the bus-bar and the secondaries are connected in parallel with due considerations to polarity and phase and the relay operating coil is connected across the pilot wires in such a way that the summation current of secondaries flows through it. All the CTs must be of the same ratio, regardless of the capacities of various circuits. Flow of current in the relay is an indication of a fault within the protected zone and will initiate opening of the breakers of each generator and feeder.
The main drawback of differential overcurrent protection is the difference in the magnetic conditions of the iron-cored CTs which may cause false operation of the relay at the time of an external fault. Even with identical CTs having large iron cores to avoid the saturation with maximum fault currents the dc transient component creates problem due to its slow decay. Biasing of differential relays improves the stability considerably but does not solve the problem completely.
Better discrimination between internal and external faults can be had if high impedance bus differential relay is used in place of usual low impedance relay. High impedance relay is an overcurrent relay with a series resistance. Such a relay remains stable against spill currents due to external faults or CTs inaccuracies.
Another method of protecting bus-bar sections is by means of voltage differential protection, which overcomes the difficulties of iron-cored CTs. In this scheme CTs without iron cores, known as linear couplers are employed so that they have a much larger number of secondary turns than an iron-core CT. In this scheme secondary windings of CTs are connected together in series and the differential relay coil connected across them.
Under normal operating conditions or under external fault conditions, the sum of voltages induced (proportional to the primary currents) in the secondary windings is zero but in the event of an internal fault on the bus-bar, the voltages of the CTs in all source circuits add to cause the flow of current through the secondary windings and the clamp-type differential relay operating coil.
This scheme provides high speed protection for a relatively small net voltage in the differential circuit.