[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

voltage in each winding simultaneously without saturating the core. If a winding is permanently grounded
on one end, the applied-voltage test cannot be performed. In this case, many ANSI product standards
specify that the induced primary test voltage be raised to 1000 plus 3.46 times the rated winding voltage
(Bean et al., 1959).
2.2.12.2.3 Impulse Test
Distribution lines are routinely disturbed by voltage surges caused by lightning strokes and switching
transients. A standard 1.2 50-s impulse wave with a peak equal to the BIL (basic impulse insulation
level) of the primary system (60 to 150 kV) is applied to verify that each transformer will withstand these
surges when in service.
2.2.13 Transformer Protection
Distribution transformers require some fusing or other protective devices to prevent premature failure
while in service. Circuit breakers at the substation or fusing at feeder taps or riser poles may afford some
protection for individual transformers, but the most effective protection will be at, near, or within each
transformer.
2.2.13.1 Goals of Protection
Transformer-protection devices that limit excessive currents or prevent excessive voltages are intended
to achieve the following:
• Minimize damage to the transformer due to overloads
• Prevent transformer damage caused by secondary short circuits
• Prevent damage caused by faults within the transformer
• Minimize the possibility of damage to other property or injury to personnel
• Limit the extent or duration of service interruptions or disturbances on the remainder of the system
The selection of protection methods and equipment is an economic decision and may not always
succeed in complete achievement of all of the goals listed above. For example, the presence of a primary
fuse may not prevent longtime overloads that could cause transformer burnout.
2.2.13.2 Separate Protection
Distribution transformers may have fused cutouts on the same pole to protect an overhead transformer
or on a nearby pole to protect a pad-mounted transformer. Sometimes a separate pad-mounted cabinet
is used to house protection for larger pad-mounted and submersible transformers.
2.2.13.3 Internal Protection
When protection means are located within the transformer, the device can react to oil temperature as
well as primary current. The most common internal protective devices are described below.
2.2.13.3.1 Protective Links
Distribution transformers that have no other protection are often supplied with a small high-voltageexpulsion
fuse. The protective link is sized to melt at from six to ten times the rated current of the
transformer. Thus it will not protect against longtime overloads and will permit short-time overloads
that may occur during inrush or cold-load-pickup phenomena. For this reason, they are often referred
to as “fault-sensing” links. Depending on the system voltage, protective links can safely interrupt faults
of 1000 to 3000 A. Internal protective links are about the size of a small cigar.
2.2.13.3.2 Dual-Sensing or Eutectic Links
High-voltage fuses made from a low-melting-point tin alloy melt at 145˚C and thus protect a transformer
by detecting the combination of overload current and high oil temperature. A eutectic link, therefore,
prevents longtime overloads but allows high inrush and cold-load-pickup currents. A similar device called
a “dual element” fuse uses two sections of conductor that respond separately to current and oil temperature
with slightly better coordination characteristics.
2.2.13.3.3 Current-Limiting Fuses
Current-limiting fuses can be used if the fault current available on the primary system exceeds the
interrupting ratings of protective links. Current-limiting fuses can typically interrupt 40,000- to 50,000-
A faults and do so in less than one half of a cycle. The interruption of a high-current internal fault in
such a short time will prevent severe damage to the transformer and avoid damage to surrounding
property or hazard to personnel that might otherwise occur. Full-range current-limiting fuses can be
installed in small air switches or in dry-well canisters that extend within a transformer tank. Currentlimiting
fuses cannot prevent longtime overloads, but they can open on a secondary short circuit, so the
fuse must be easily replaceable. Current-limiting fuses are considerably larger than expulsion fuses.
2.2.13.3.4 Bayonets
Pad-mounts and submersibles may use a primary link (expulsion fuse) that is mounted internally in the
transformer oil but that can be withdrawn for inspection of the fuse element or to interrupt the primary
feed. This device is called a bayonet and consists of a probe with a cartridge on the end that contains
the replaceable fuse element. Fuses for bayonets may be either fault sensing or dual sensing.
2.2.13.3.5 Combination of Bayonet and Partial-Range Current-Limiting Fuses
The most common method of protection for pad-mounted distribution transformers is the coordinated
combination of a bayonet fuse (usually dual sensing) and a partial-range current-limiting fuse (PRCL).
The PRCL only responds to a high fault current, while the bayonet fuse is only capable of interrupting
low fault currents. These fuses must be coordinated in such a way that any secondary fault will melt the
bayonet fuse. Fault currents above the bolted secondary fault level are assumed to be due to internal
faults. Thus the PRCL, which is mounted inside the tank, will operate only when the transformer has
failed and must be removed from service.
2.2.13.4 Coordination of Protection
As applied to overcurrent protection for distribution transformers, the term coordination means two
things:
1. A fuse must be appropriately sized for the transformer. A fuse that is too large will not prevent
short-circuit currents that can damage the transformer coils. A fuse that is too small may open
due to normal inrush currents when the transformer is energized or may open due to short-time
overload currents that the transformer is capable of handling.
2. Transformer protection must fit appropriately with other protection means located upstream,
downstream, or within the transformer. For example, a secondary oil circuit breaker should be
coordinated with a primary fuse so that any short circuit on the transformer secondary will open
the breaker before the primary fuse melts.
Where two fuses are used to protect a transformer, there are two methods of achieving coordination of
the pair: “matched melt” and “time-current-curve crossover coordination” (TCCCC).
2.2.13.4.1 Matched Melt
An example of matched-melt coordination is where a cutout with an expulsion fuse and a backup currentlimiting
fuse are used to protect an overhead transformer. The two fuses are sized so that the expulsion
fuse always melts before or at the same time as the current-limiting fuse. This permits the current-limiting
fuse to help clear the fault if necessary, and the cutout provides a visible indication that the fault has
occurred.
2.2.13.4.2 TCCC Coordination of Bayonet and Partial-Range Current-Limiting Fuses
TCCCC is much more common for pad-mounted and self-protected transformers, where the fuses are
not visible. The TCCCC method is described as follows:
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC