[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

Phase Conductor
Model of CT Insulation
Ir
Ic
I total = Ir + Ic
FIGURE 3.13.3 Schematic representation of relative tan delta measurements.
Measurement Impedance
Installation can require factory modifications, depending on the type of sensor that is installed (Boisseau
and Tantin, 1993). Typically, the hydrogen sensor is located in an area where the oil is stagnant, especially
during periods of low ambient temperatures. This arrangement results in poor accuracy for low hydrogenconcentration
levels. For significant hydrogen concentrations (above 300 ppm) in stagnant oil, the
accuracy has been determined to be acceptable (Cummings et al., 1988). These constraints may not apply
to the thermal conductivity detection (TCD) technology. In this case, the sensor is located externally to
the apparatus and utilizes active oil circulation through the monitor while also providing continuous
moisture-level monitoring.
3.13.3.2.2.3 On-Line Partial Discharge Measurements — On-line partial-discharge measurement techniques
that were discussed in the section on power transformers (Section 3.13.3.1) are also applicable to
instrument transformers.
3.13.3.2.2.4 Pressure — Due to partial-discharge activity inside the tank, gases can be formed, which
increases the pressure after the gases saturate the oil. A threshold-pressure switch can be used to perform
this measurement. The operation of this sensor is possible with an inflatable bellows that is placed between
the expansion device and the enclosure. The installation of the device typically requires factory modification.
In some applications, pressure sensors take a considerable amount of time (on the order of
months) to detect any significant pressure change. The sensitivity of this type of measurement is less
than that of hydrogen and partial-discharge sensors (Boisseau and Tantin, 1993). Pressure sensors are
also available that mount on the drain valve (Cummings et al., 1988).
3.13.3.3 Bushings
Bushings are subjected to high dielectric and thermal stresses, and bushing failures are one of the leading
causes of forced outages and transformer failures. The methods of detecting deterioration of the bushing
insulation have been well understood for decades, and conventional off-line diagnostics are very effective
at discovering problems. The challenge facing a maintenance engineer is that some problems have
gestation time (i.e., going from good condition to failure) that is shorter than typical routine test intervals.
Since on-line monitoring of power-factor and capacitance can be performed continuously, and with the
same sensitivity as the off-line measurement, deciding whether to apply an on-line system is reduced to
an economic exercise of weighing the direct and strategic benefits with the cost.
FIGURE 3.13.4 Bushing sum-current measurements.
3.13.3.3.1 Failure Mechanisms Associated with Bushings
The two most common bushing failure mechanisms are moisture contamination and partial discharge.
Moisture usually enters the bushing via deterioration of gasket material or cracks in terminal connections,
resulting in an increase in the dielectric loss and insulation power factor. The presence of tracking over
the surface or burn-through of the condenser core is typically associated with partial discharge. The first
indication of this type of problem is an initial increase in power factor. As the deterioration progresses,
increases in capacitance will be observed.
3.13.3.3.2 On-Line Bushing Power-Factor and Capacitance Measurements
Measurement of power factor and capacitance is a useful and reliable diagnostic indicator. The sumcurrent
method is a very sensitive method for obtaining these parameters on-line. The basic principle
of the sum-current method is based on the fact that the sums of the voltage and current phasors are zero
in a symmetrical three-phase system. Therefore, analysis of bushing condition can be performed by
adding the current phasors from the capacitance or power-factor taps, as depicted in Figure 3.13.4. If
the bushings are identical and system voltages are perfectly balanced, the sum current, I S , will equal zero.
In reality, bushings are never identical, and system voltages are never perfectly balanced. As a result,
the sum current is a nonzero value and is unique for each set of bushings. The initial sum current can
be learned, and the condition of the bushings can be determined by evaluating changes in the sumcurrent
phasor. By using software techniques and an expert system to analyze changes to the sum current,
changes in either the capacitance or power factor of any of the bushings being monitored can be detected,
as shown in Figure 3.13.5.
Figure 3.13.5a depicts a change that is purely resistive, i.e., only the in-phase component of current is
changing. It is due to a change in C 1 insulation power factor, and it results in the current phasor change
I A from I 0 A to I A . The change in current is in phase with A-phase line voltage, V A , and it is equal to
I . This is then evidence of a power factor increase for the A-phase bushing.
Figure 3.13.5b depicts a change that is purely capacitive, i.e., only a quadrature component of current
is changing. In this case, the change is due to a change in C 1 insulation capacitance, and it results in the
current phasor change I A from I 0 A to I A . The change in current leads the voltage V A by 90˚, and it is
equal to I .
Expert systems are also used to determine whether the sum-current change is related to actual bushing
deterioration or changes in environmental conditions such as fluctuations in system voltages, changes
in bushing or ambient temperature, and changes in surface conditions (Lachman, 1999).
3.13.3.4 Load Tap Changers
High maintenance costs for load tap changers (LTC) result from several causes. The main reasons include:
misalignment of contacts, poor design of the contacts, high loads, excessive number of tap changes,
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC