For the distribution system to function satisfactorily, faults on any
part of it must be isolated or disconnected from the rest of the system as
quickly as possible; indeed, if possible, they should be prevented from
happening. The principal devices to accomplish this include fuses, automatic
sectionalizers, reclosers, circuit breakers, and lightning or surge
arresters. Success, however, depends on their coordination so that their
operations do not conflict with each other.
Fuses
Time-Current Characteristic
A fuse consists basically of a metallic element that melts when “excessive”
current flows through it. The magnitude of the excessive current
will vary inversely with its duration. This time-current characteristic
is determined not only by the type of metal used and its dimensions
(including its configuration), but also on the type of its enclosure and
holder. The latter not only affect the melting time, but, in addition, affect
the arc clearing time. The clearing time of the fuse, then, is the sum
of the melting time and the arc clearing time.
Fuse Coordination
The number, rating, and type of the interrupting devices shown
in Figure 4-3 depend on the system voltage, normal current, maximum
fault current, the sections and equipment connected to them, and other
local conditions. The devices are usually located at branch intersections
and at other key points. When two or more such devices are employed
in a circuit, they will be coordinated so that only the faulted portion will
be de-energized. In Figure 4-28 fuse D must clear before sectionalizer C,
and C must clear before recloser B. Likewise, fuse G must clear before
F, F before E, and both E and B before A. At the transformer locations,
fuse M must clear before D, and N before G. All of these devices must be
coordinated; i.e., their ratings should provide for carrying normal load
currents and for responding correctly to a fault.
Fault current will flow from the source to the fault through the
various devices in its path. The magnitude of this fault current will depend
on the impedance (resistance for dc circuits) between the source
and the point of fault, or roughly, on the distance between them. When
a fault is distant from the source, the impedance of this part of the circuit
is high and the fault current is low; when the fault is close to the source,
the fault current is high.
At the coordinating point farthest from the source, therefore, the
fuse will have the lowest rating consistent with the maximum normal
load at this point; at the other coordinating points along the path of the
current the fuses will have increased ratings as they are closer to the
source. These are indicated in Figure 4-15 and Table 4-3. The characteristics
of these fuses must also coordinate with those of other protective
devices in the same path and with those of the circuit breaker at the
source.
Repeater Fuses
Line fuses are sometimes installed in groups of two or three (per
phase), known as repeater fuses, having a time delay between each two
fuse units. When a fault occurs, the first fuse will blow and the second
fuse will be mechanically placed in the circuit by the opening of the first;
if the fault persists, the second fuse will blow; if there is a third fuse, the
process is repeated. If the fault is permanent, all of the fuses will blow
and the faulted part of the circuit will be de-energized. New fuses must
be installed to restore the line to normal.
Where capacitors are applied to feeders for power factor correction,
fuses chosen to protect the line from the bank (and vice versa) must also
coordinate with sectionalizing and other devices in the circuit back to the
source.
Transformer Fuses
Fuses on the primary side of distribution transformers serve to disconnect
the transformer from the circuit not only in the event of a fault
in the transformer or on the secondary, but also when the normal load
on the transformer becomes so high that failure is imminent. Fuses on
the secondary side protect the transformer from faults or overloads on
the secondary circuit it serves.
The characteristics of a primary fuse are a compromise between
protection from a fault and protection from overload, yet the fuse also
has to coordinate with other fuses on the line. One attempt at a solution
is the completely self-protected (CSP) transformer, in which the primary
fuse, with characteristics based only on protection against fault, is situated
within the transformer tank (and, to differentiate, is called a link)
while overload protection is accomplished by low-voltage circuit breakers
(instead of fuses) on the secondary side of the transformer that are
also situated within the tank. The circuit breakers, once open, however,
must be reclosed manually.
Fuses are provided on the line side of the protectors on low-voltage
secondary networks. These are backup protection in the event the protector
fails to open during back feed from the network into the primary
when it is faulted or deliberately grounded.
Secondary fuses, known as limiters, are also provided at the juncture
of secondary mains to isolate faulted sections of the secondary
mains and to prevent the spread of burning in conductors (usually in
cables) where sufficient fault current does not exist to burn them clear
in a small portion of the mains.
Automatic Line Sectionalizers
Automatic line sectionalizers are connected on the distribution
feeder in series with line and sectionalizing fuses; they are also in series
with and electrically farther from the source than reclosers or circuit
breakers with reclosing cycles. These devices are decreasing in usage,
but many exist on distribution systems.
When a fault occurs on the circuit beyond the sectionalizer, the
fault current initiates a fault-counting relay that is coordinated with the
characteristics of the fuses and other devices. Each time the circuit is deenergized
(from reclosers or circuit breakers), the relay moves toward
the trip position; just before the final operation that will lock out the
recloser or circuit breaker if the fault persists, the sectionalizer will trip
(while no fault current is flowing) and open the circuit at that point,
removing the fault and permitting the circuit breaker or recloser to close
and reset into its normal position; service is thus restored to the rest of
the circuit up to the location of the sectionalizer. If the fault is of a temporary
nature and is cleared before the reclosing devices complete their
operations, the sectionalizer will reset to its normal position after the
circuit is reenergized.
Sectionalizers are rated on continuous current-carrying capacity,
minimum tripping and counting current, and maximum momentary
fault current, as well as for maximum system voltage, load-break current,
and impulse voltage or basic insulation level (BIL).
More than one sectionalizer can be connected in series with a reclosing
device. The sectionalizer nearest the reclosing device can be set
to operate after (say) three operations while the more remote one is set
for (say) two such operations.
Sectionalizers are relatively low-cost devices; they are not required
to interrupt fault current although fault current flows through them.
They may be operated manually and are considered the same as loadbreak
switches.
Reclosers
Reclosers are essentially circuit breakers of lower capacity, both as
to normal current and interrupting duty. They are usually installed on
major branches of distribution feeders in series with other sectionalizing
devices; they perform the same function as repeater fuses connected in
the circuit or circuit breakers at the substation.
Circuit Breakers—Relays
Where the fault current is beyond the ability of a fuse or recloser to
interrupt it safely, or where repeated operation within a short period of
time makes it more economical, a circuit breaker is used. The ability of
circuit breakers has been touched upon earlier; their time-current characteristics,
however, are dependent on the protective relays associated with
them and must be coordinated with those of down-line reclosers, fuses,
and other protective devices.
Overcurrent Relays
Overcurrent relays close their contacts to actuate the circuit that
causes the circuit breaker to open or close when the current flowing in
them reaches a predetermined value.
Instantaneous
Without time delay deliberately added, the relay will close its
contacts “instantaneously,” i.e., in a relatively short time, in the nature
of 0.5 to perhaps 20 cycles. To prevent frequent operation of the breaker
from transient, nonpersistent conditions, undesirably high settings may
be applied to the relay.
Directional Relays
Directional relays are essentially overcurrent relays to which an
element similar to a wattmeter is added, both sets of contacts being in
series. The overcurrent element will operate to close its contacts regardless
of the direction of flow of power in the line; the wattmeter element
will tend to turn in one direction under normal flow of power and in
the reverse direction when power flows in the opposite direction. Hence,
both sets of contacts must be closed and power flowing in a given direction
before the relay will operate. Both elements may be combined into
one so that only a single set of contacts is required.
Differential Relays
Differential relays operate on the difference between the current entering
the line or equipment being protected and the current leaving it.
As long as the incoming current and the outgoing current are essentially
equal, the relay will not operate. A fault within the line or equipment,
however, will disturb this equilibrium, and the relay will operate to trip
the supply circuit breaker or breakers on both sides of the line or equipment
being protected. This type of relay is used to protect buses, transformers,
and regulators at the substation. Since the voltages at which
these operate may be high, current transformers installed on both sides
of the equipment, with proper ratios in the case of transformers, supply
the currents to the relay.
Surge or Lightning Arresters
The function of a surge or lightning arrester is to limit the voltage
stresses on the insulation of the equipment being protected by permitting
surges in voltage to drain to ground before damage occurs. The
surges in voltage generally are caused by lightning (either by direct
stroke or by induction from a nearby stroke) or by switching.
Arresters consist of two basic components: a spark gap and a nonlinear
resistance element (for a valve type) or an expulsion chamber (for
an expulsion type). When a surge occurs, the spark gap breaks down
or sparks over, and permits current to flow through the resistance (or
chamber) element to ground. Since the arrester at this point presents a
low-impedance path, a large current, referred to as 60-cycle follow current,
flows through the arrester. The nonlinear resistance, at the higher voltages,
will tend to restrict this current and eventually cause it to cease to flow;
here, the magnitude of the follow current is independent of the system
capacity.
The expulsion chamber will confine the arc, build up pressures
that eventually blow out the arc, and cause the follow current to cease to
flow; here, the follow current is a function of the system capacity and the
expulsion chamber must be suitably designed. After each such operation,
the arrester must be capable of repeating this operating cycle.
Insulation Coordination
It must be kept in mind that while the arrester is operating, the
surge voltage is also “attacking” the insulation of the line or equipment
it is protecting; the arrester, however, drains the high voltage to ground,
reducing its magnitude, before sufficient time has elapsed to damage the
insulation of the line or equipment.
Insulation characteristics, therefore, can be expressed as functions
of voltage and the time it is impressed. This is usually shown as a volttime
curve, known as the impulse level, and represents the voltage and
its duration the equipment can withstand.
The arrester also has a volt-time curve that indicates the voltage
and time at which the spark gap begins to break down and permit the
passage of the surge to ground.
The insulation characteristic of the line or equipment being protected
must be at a higher voltage level than the volt-time characteristic of the
arrester protecting it; indeed, a sufficient voltage differential must be provided
to ensure safe and positive protection. Figure 4-23 illustrates typical
curves and their relationship. While the impulse level of the line or equipment
must be high enough that the arrester provides adequate protection,
it should be as low as practical to hold down insulation costs.
Basic Insulation Level (BIL)
The coordination of insulation requires the establishment of a
minimum level above which are the components of a system and below
which are the protected devices associated with those components. A
joint committee of electrical engineers, utilities, and manufacturers adopted
basic insulation levels which define the impulse voltages capable
of being withstood by insulation of various insulation classes: “Basic
impulse insulation levels are reference levels expressed in impulse crest
voltages with a standard wave not longer than 1.5 by 40 microseconds.
Apparatus insulation as demonstrated by suitable tests shall be equal to
or greater than the basic insulation level.”
The standard 1.5- by 40-μs wave selected simulates lightning
surges, which are more prevalent than switching surges, and are more
readily reproduced in the laboratory.
Arrester Connection
Arresters should be placed as close to the equipment to be protected,
and the lengths of the connections to the line and to the ground
should be kept as short, as possible. That is because these connections
offer relatively high-impedance paths to voltage surges, so that large
currents flowing through them could cause a voltage drop in them
which, added to the surge voltage, could impose additional stress on
the insulation of the equipment being protected. Moreover, on longer
lines, such surges can be “reflected,” essentially doubling the value of
the surge voltage.
Short leads and minimum distance between the arrester and equipment
protected are desirable for all arrester applications. Further, if the
equipment being protected has a ground, that ground and the arrester
ground should be interconnected to relieve any potential stress that may
develop from the voltage drop across the ground impedance.
Arresters should be connected to the primary side of distribution
transformers and to capacitors, underground risers, and other
equipment; at certain points on long primary lines; and at reclosers in
substations. One arrester should be connected to each phase. For station
circuit breakers, transformers, outdoor regulators, and reclosers
situated on primary lines, arresters should preferably be connected to
both incoming and outgoing sides of such equipment. Voltage ratings
of arresters should take cognizance of whether the systems are delta or
wye, grounded or ungrounded, and of the voltage distortions resulting
from an accidental ground on one phase.
