[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

This may be more useful when operating a CT at higher ambient, where there is a need for the
maximum rating factor. For example, if the CT has an actual rating of 2.67 based on temperature-rise
data but is only rated 2.0, and if it is desired to use the CT at 50C ambient with the stated rating factor
2.0, then this unit should work within its insulation rating and within its stated accuracy class.
If a higher-temperature-class insulation system is provided, then the rise must be in compliance with
that class per Table 2.6.8. In some cases, the temperature class is selected for the environment rather than
the actual temperature rise of the CT.
2.6.4.3 Open-Circuit Conditions
The CT functions best with the minimum burden possible, which would be its own internal impedance.
This can only be accomplished by applying a short circuit across the secondary terminals. Since the core
mmf acts like a shunt, with no load connected to its secondary, the mmf becomes the primary current,
thus driving the CT into hard saturation. With no load on the secondary to control the voltage, the
winding develops an extremely high peak voltage. This voltage can be in the thousands, or even tens of
thousands, of volts. This situation puts the winding under incredible stress, ultimately leading to failure.
This could result in damage to other equipment or present a hazard to personnel. It is for this reason
that the secondary circuit should never be open. It must always have a load connected. If it is installed
to the primary but not in use, then the terminals should be shorted until it is to be used. Most manufacturers
ship CTs with a shorting strap or wire across the secondary terminals. The CT winding must
be able to withstand 3500 V peak for 1 min under open-circuit conditions. If the voltage can exceed this
level, then it is recommended that overvoltage protection be used.
2.6.4.4 Overvoltage Protection
Under load, the CT voltage is limited. The level of this voltage depends on the turns and core crosssectional
area. The user must evaluate the limits of the burdens connected to ensure equipment safety.
Sometimes protective devices are used on the secondary side to maintain safe levels of voltage. These
devices are also incorporated to protect the CT during an open-circuit condition. In metering applications,
it is possible for such a device to introduce a direct current (dc) across the winding that could
saturate the core or leave it in some state of residual flux. In high-voltage equipment, arrestors may be
used to protect the primary winding from high voltage-spikes produced by switching transients or
lightning.
2.6.4.5 Residual Magnetism
Residual magnetism, residual flux, or remanence refers to the amount of stored, or trapped, flux in the
core. This can be introduced during heavy saturation or with the presence of some dc component. Figure
2.6.16 shows a typical B-H curve for silicon-iron driven into hard saturation. The point at which the
curve crosses zero force, identified by +B res , represents the residual flux. If at some point the CT is
disconnected from the source, this flux will remain in the magnetic core until another source becomes
present. If a fault current drove the CT into saturation, when the supply current resumed normal levels,
the core would contain some residual component. Residual flux does not gradually decay but remains
constant once steady-state equilibrium has been reached. Under normal conditions, the minor B-H loop
must be high enough to remove the residual component. If it is not, then it will remain present until
another fault occurrence takes place. The effective result could be a reduction of the saturation flux.
However, if a transient of opposite polarity occurs, saturation is reduced with the assistance of the residual.
Conversely, the magnitude of residual is also based on the polarity of the transient and the phase
relationship of the flux and current. Whatever the outcome, the result could cause a delayed response to
the connected relay.
It has been observed that in a tape-wound toroidal core, as much as 85% of saturation flux could be
left in the core as residual component. The best way to remove residual flux is to demagnetize the core.
This is not always practical. The user could select a CT with a relay class that is twice that required. This
may not eliminate residual flux, but it will certainly reduce the magnitude. The use of hot-rolled steel
may inherently reduce the residual component to 40 to 50% of saturation flux. Another way of reducing
residual magnetism is to use an air-gapped core. Normally, the introduction of a gap that is, say, 0.01%
of core circumference could limit the residual flux to about 10% of the saturation flux. Referring to
Figure 2.6.16, a typical B-H loop for an air-gapped core is shown. The drawback is significantly higher
exciting current and lower saturation levels, as can be seen in Figure 2.6.3. To overcome the high exciting
current, the core would be made larger. That — coupled with the gap — would increase its overall cost.
This type of core construction is often referred to as a linearized core.
2.6.4.6 CT Connections
As previously mentioned, some devices are sensitive to the direction of current flow. It is often critical
in three-phase schemes to maintain proper phase shifting. Residually connected CTs in three-phase
ground-fault scheme (Figure 2.6.17A) sum to zero when the phases are balanced. Reversed polarity
of a CT could cause a ground-fault relay to trip under a normal balanced condition. Another scheme
to detect zero-sequence faults uses one CT to simultaneously monitor all leads and neutral (Figure
2.6.17C). In differential protection schemes (Figure 2.6.17B), current-source phase and magnitude
are compared. Reverse polarity of a CT could effectively double the phase current flowing into the
relay, thus causing a nuisance tripping of a relay. When two CTs are driving a three-phase ammeter
through a switch, a reversed CT could show 1.73 times the monitored current flowing in the
unmonitored circuit.
FIGURE 2.6.16 Typical B-H curve for Si-Fe steel.
FIGURE 2.6.17 (a) Overcurrent and ground-fault protection scheme. (b) Differential protection scheme.
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC