BUS BAR PROTECTION

Dr. Hossam A. A. Saleh

 Bus Protection
Introduction: Typical Bus Arrangements
Buses exist wherever two or more circuits are interconnected. The
number of circuits that are connected to a bus varies widely.
Bus faults can result in severe system disturbances, as high fault
current levels are typically available at bus locations and because all
circuits supplying fault current must be opened to isolate the problem.
If there are more than six to eight circuits involved, buses are often
split by a CB (bus tie), or a bus arrangement is used that minimizes the
number of circuits, which must be opened for a bus fault.
The bus CBs usually have disconnect switches on either side to provide
means of isolating them from the system after trouble or for
maintenance. Generally, these switches are operated manually at no
load.
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The buses typically illustrated are
Single bus–single breaker
Double bus with bus tie–single breaker
Main and transfer bus–single breaker
Double bus–single breaker
Double bus–double breaker
Ring bus
Breaker-and-a-half bus
Bus and transformer–single breaker
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The most common causes of bus faults are equipment failures, small-
animal contacts, broken insulators, wind-driven objects, and
contamination.
Differential protection provides sensitive and fast phase and ground-
fault protection and is generally recommended for all buses. Backup is
usually provided by the protection associated with the connecting
circuits. A second differential scheme is sometimes used for very
important buses and may be dictated by redundancy requirements
specified for bulk power systems.
Buses at distribution substations and within industrial complexes are
sometimes protected by less complex, time delayed protection.
 Bus Protection
1. Single Breaker – Single Bus
The single-breaker-bus type is the most basic, simple, and economical
bus design and is used widely, particularly at distribution and lower-
transmission voltages. For this type of bus, differential is easy to
supply as long as suitable CTs are available, with the protective zone
enclosing the entire bus.
This bus arrangement provides no operating flexibility. All bus faults
require opening all circuits connected to the bus. Breaker problems or
maintenance requires that the circuit be removed from service.
However, maintenance may not be too much of a problem if
maintenance on the entire circuit and the protection can be scheduled
together. One set of voltage transformers (VTs) on the bus can supply
voltage for the protection on all the circuits.
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2. Single Buses Connected with Bus Ties
This is an extension of the single bus – single breaker arrangement. It
is used where a large number of circuits exist, especially at lower
voltages, such as for distribution and industrial substations. It provides
flexibility when the substation is fed from two separate power
supplies. One supply that is connected to each bus permits operation
with the bus tie (52T), either open or closed. If one supply is lost, all
circuits can be fed by the other, with 52T closed. Separate differential
zones for each bus are applied. A fault in one bus zone still permits
service to the station by the other bus.
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3. Main and Transfer Buses with Single Breakers
Increased operating flexibility is provided by the addition of a transfer
bus. This bus is protected by a single differential zone (dashed lines). A
bus fault requires tripping all breakers, thereby interrupting all service
connected to the bus.
Normally, the transfer bus is not energized. For any breaker trouble or
maintenance, that circuit is connected to the transfer bus by closing
its normally open (NO) disconnect switch and closing the bus tie (52T)
breaker to continue service. Only one circuit is thus connected to the
transfer bus at any one time. The protection associated with the bus
tie breaker must be suitable and adaptable for the protection of any of
the circuits of the main bus.
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3. Main and Transfer Buses with Single Breakers
This can require different settings, which must be made for each
circuit transferred or operating with compromise protection for the
period of transfer bus operation. This is a disadvantage from a
protection standpoint. In general, it is not desirable to switch or
modify protection systems because the potential for error can result in
no protection or misoperation. The use of microprocessor based relays
can mitigate this problem because multiple setting groups are
available on these types of relays. A setting group can be dedicated to
each circuit and settings applied to specifically match each circuit’s
needs. The required setting group can be enabled automatically by
switch position contacts that identify the specific circuit that is
connected to the transfer bus. One set of VTs on the bus can supply
voltage to all the protection for the several circuits.
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4. Single Breakers – Double Bus
This arrangement provides high flexibility for system operation. The
disadvantage is that it requires complicated switching of the
protection: for both the bus differential and line protection. Two
differential zones for the buses are required.
Let lines 1 and 2 are connected to bus 1, with lines 3 and 4 connected
to bus 2. For this operation, the differential zones are outlined: dashed
for bus 1, and dash–dot for bus 2. As for the previous bus
arrangement, the bus tie protection must be adaptable for the
protection of any of the lines when 52T is substituted for any of the
line circuit breakers. When a line breaker is bypassed and the bus tie
(52T) breaker substituted, using one bus as a transfer bus, the
differential protection on that bus must be removed from service.
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Faults on either buses or associated circuits require tripping of all
circuits connected to the bus at that time. Faults in the bus tie breaker
(52T) must trip both buses and all circuits. VTs for protection are
required for each bus. However, line-side VTs are preferable to avoid
switching if voltage is required for line protection.
Modern microprocessor relays need to be applied to reduce these
complications by using the flexibility of such relays and the
programmable logic, which are provided for such devices.
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5. Double Breakers – Double Bus
This is a very flexible arrangement that requires two CBs per circuit.
Each bus is protected by a separate differential, with zones as
illustrated. The line protection operates from paralleled CTs, and this
provides protection for the bus area between the two zones
overlapping the two CBs. Line protection operates to trip both CBs.
With all DS normally closed (NC) a fault on either of the buses does
not interrupt service on the lines. All switching is done with CBs, and
either bus can be removed for maintenance. Line-side voltage, either
VTs or CCVTs, is necessary if required by the line protection.
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6. Ring Bus
The ring bus arrangement has become quite common, particularly for
higher voltages. High flexibility with a minimum of CBs is obtained.
Each CB serves two lines and must be opened for faults on either line.
The bus section between the CBs becomes part of the line, so that bus
protection is not applicable or required. The interconnection of the CTs
for protection of each line, and line faults must trip two CBs.
If the ring is open for any reason, a fault on a line may separate the
other lines and the bus. This can result in a significant disruption to the
power system network, which must be taken into account from a
system operation and protection coordination standpoint. Line
protection voltage, if required, is obtained from VTs or, more
commonly, at the higher voltages by CCVTs connected to each line.
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7. Breaker – and – Half Bus
This arrangement provides more operating flexibility, but requires
more CBs than the ring bus. This type is also widely used, especially
for larger multi-circuit, higher-voltage systems. Two operating buses
each have separate differential protection. Each line section is
supplied by both buses through two CBs. The center CB serves both
lines; hence, the half designation is given to the center circuit.
The CT interconnections are shown for each line section as dashed
lines. Voltage for line relays must use line-side CCVTs or VTs. Line
faults trip two CBs, but do not cause loss of service to the other lines if
all CBs are normally closed.
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8. Transformer – Bus Combination
This is the single breaker–single bus, with a transformer bank directly
connected to the bus. The advantage is the cost saving of the CB
between the transformer and the bus. It is practical for small stations,
such as distribution, where there is only one transformer to supply
several circuits. Here a fault either in the transformer or on the bus
requires that all service be interrupted, with or without the intervening
CB.
The differential zone includes both the bus and the transformer
(dashed lines). In these applications transformer differential relays
must be used.
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General Summary of Buses
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Differential Protection for Buses
Complete differential protection requires that all circuits connected to
the bus be involved, because it compares the total current entering
the zone with the total current leaving the zone.
The relays and CTs are both important members of a ‘‘team’’ to
provide fast and sensitive tripping for all internal faults, at the same
time, restrain for all faults outside the differential zone.
Two major techniques are in use to avoid possible unequal CT
performance problems: (1) multirestraint current and (2) high-
impedance voltage.
A third system employs air-
core transformers to avoid
the iron-core excitation and
saturation problems.
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1 Multi-restraint Current Differential
Multi-restraint current differential is the most versatile method for
general application using conventional current transformers, but in
general, is more difficult to apply.
Multi-restraint relays are used with a restraint winding connected to
each circuit that is a major source of fault current.
All CTs are connected in wye and to the restraint windings because
there are no phase-shift problems with buses except for the example
shown in Figure 10.8 (Transformer – Bus Combination).
These schemes are designed to restrain correctly for heavy faults just
outside the differential zone, with maximum offset current as long as
the CTs do not saturate for the flow of maximum symmetrical current.
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This can be accomplished by CT ratio selection and by keeping the
secondary burden low. Thus, it is important and recommended that no
other device be connected in the differential circuits.
The restraint windings of the differential relays normally have quite
low impedance; consequently, the major burden encountered is often
that of the leads connecting the CTs to the relays.
This can be kept low by use of large wire, which is also desirable to
minimize physical damage. Accidental breakage or an opening in the
differential circuit can result in incorrect operation and loss of a critical
part of the power system.
Multirestraint bus differential relays do not have ratio taps. These are
not required in most applications, because a common CT ratio can be
normally obtained among the several bus CTs. Otherwise, auxiliary
CTs are required for those that do not match.
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The relays exist with up to six restraint circuits, and can have either
fixed or variable restraint characteristics. Typical sensitivities for
internal faults are on the order of 0.15 A, with operating times of 50–
100 msec.
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2 High-Impedance Voltage Differential
This scheme loads the CTs with a high impedance to force the error
differential current through the CTs instead of the relay-operating coil.
The basic principles are illustrated in Figure 10.9 . For an external fault,
the maximum voltage VR across the differential relay ZR will occur if
the CT on the faulted circuit (1) is completely saturated and the other
CTs (2 and 3) do not saturate. This is the worst case, because in
practice all CTs may not saturate on light external faults or will have
varying degrees of saturation for the heavy faults. An empirical margin
with a safety factor is provided by the manufacturer to modify this
maximum voltage calculation for setting the relay. This calculation is
made for both the maximum symmetrical three-phase and phase-to-
ground faults. The fault currents are different, and the lead resistance
RL (maximum for the various circuits) is RL for three-phase faults and
2RL for phase-to-ground faults.
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CT1 completely saturated

𝑹𝑪𝟏 + 𝑹𝑳𝟏
𝑰𝑹 = 𝑰
𝑹𝟖𝟕 + 𝑹𝑪𝟏 + 𝑹𝑳𝟏 𝑭
𝑹𝑪𝟏 + 𝑹𝑳𝟏
≅ 𝑰𝑭
𝑹𝟖𝟕
𝑽𝑹 = 𝑹𝟖𝟕 𝑰𝑹 = 𝑹𝑪𝟏 + 𝑹𝑳𝟏 𝑰𝑭
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For internal bus faults, as shown in Figure 10.9, the high-impedance
ZR of the bus differential relay forces most of the secondary current
through the CT exciting impedances. Thus, VR will be high to operate
the relay, and is essentially the open-circuit voltage of the CTs. A
varistor or similar protective device across ZR provides circuit
protection by limiting the voltages to a safe level. A tuned circuit
provides maximum sensitivity at rated system frequency, and filters
out the DC transient components. The impedance between the
junction and the relay RLR is negligible compared to the high value of
the relay ZR.
The scheme requires that the total resistance of the CTs and leads to
the junction point (RSþRL) be kept low. Thus, bushing or toroidal
wound CTs, where their secondary impedance is very low, can be used,
and they should be interconnected together as near the CT locations
as possible, preferably equidistant, so that several RL values are
essentially equal and low.
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All CTs should have the same ratio and operate on the full winding.
Operating at CT taps is not recommended, but if necessary, the
windings between the taps must be completely distributed, and the
unused end must be well insulated to avoid high-voltage breakdown
from the autotransformer effect. Auxiliary CTs are not recommended.
If they are required, a detailed analysis or special relays may be
applicable. One type of relay employs added restraint circuits for
applications with widely diverse CTs.
The several limitations outlined are not too difficult to meet with
modern CTs and proper bus design, so this is a very effective and
widely used bus protection system. Typical operating times are in the
order of 20–30 msec, and if a supplementary instantaneous unit is
used for high-current internal faults, times of 8–16 msec are available.
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3 Air-Core Transformer Differential
The major problem in differential schemes results because of the CT
iron, which requires exciting current and saturates at high fault
currents. By elimination of the CT iron, this problem does not exist,
and a simple, fast, reliable bus differential system results. This is
known as the linear coupler differential scheme, and several of these
are in service. Such schemes have not become popular primarily
because existing and conventional iron-core CTs cannot be used in
these schemes and the linear couplers cannot be used for any other
applications.
The linear coupler in appearance is the same as conventional iron-core
CTs and can be mounted on a bushing or can be connected as a
wound-type CT in the primary circuit. It operates as an air-core mutual
reactor where
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where M has been designed to be 0.005 V at 60 Hz. Thus, a secondary

voltage of 5 V is induced for 1000-primary amperes. The linear coupler
secondaries for each circuit on the bus are all connected in series and
to a sensitive relay unit. For an external fault or load, the sum of the
voltage for all current flowing into the bus is equal and opposite to the
voltage that is developed by the current flowing out of the bus. Thus,
the voltage across the relay is essentially zero for no operation.
For an internal fault, with all current flowing into the bus, the linear
coupler secondary voltages add to produce an operating voltage.
Thus, the relay-operating current (IR) is
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where ZR is the relay coil impedance and ZC is the linear coupler
secondary impedance. Typical values of ZC are about 2–20 V for ZR. Lead
impedance is not significant with these values. The relays operate from 2
to 50 mA for high sensitivity. Typical times are 10 msec and less.
The system is quite flexible because the linear coupler secondaries do
not have to be shorted if open-circuited, and circuits can be added or
subtracted with minimum problems. Changing the number of circuits
affects the value of the ZC sum, which is basically offset by a
corresponding change in the VSec sum.
If the primary circuit is subject to high frequencies, such as those that
occur with large capacitors or back-to-back switching of capacitors,
secondary lightning arrestors may be required. Linear couplers
transform all frequencies very efficiently. In linear coupler schemes, it
is important that good connections be maintained in the control
circuitry to prevent excessive voltage drops across contact resistance,
which can affect the performance of such schemes.