[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)
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considerably; furthermore, each material interface further attenuates the propagated signal. Therefore,<br />
acoustic signals can only be detected within a limited distance from the source. Consequently, the<br />
sensitivity for PD inside transformer windings, for example, may be quite low. In typical applications,<br />
many acoustic sensors are carefully distributed around the tested equipment (Eleftherion, 1995;<br />
Bengtsson et al., 1997).<br />
Though not disturbed by the electric network, external influences in the form of rain or wind and<br />
non-PD vibration sources, like loose parts and cooling fans, will generate acoustic signals that interfere<br />
with the PD detection. One way to decrease the external influence is to use acoustic waveguides (Harrold,<br />
1983) that detect signals from inside the transformer tank. This solution is typically only considered for<br />
permanent monitoring of important transformers. As an alternative, phase-position analysis can be used<br />
to reject these disturbances (Bengtsson et al., 1997).<br />
A transformer generates disturbing acoustic signals in the form of core noise, which can extend<br />
up to the 50 to 100-kHz region. To diminish this disturbance, acoustic sensors with sensitivity in<br />
the 150-kHz range are usually employed (Eleftherion, 1995). Such sensors may, however, have less<br />
sensitivity to PD signals as well (Bengtsson et al., 1997). The properties of these signals are such<br />
that it is relatively easy to distinguish them from PD signals; thus, their main effect is to limit<br />
sensitivity.<br />
Regarding the electric multisensor systems discussed here, there are a few descriptions of combined<br />
electric and acoustic PD monitoring systems for transformers in the literature (Wang et al., 1997). Rather<br />
elaborate software must be employed to utilize the potential sensitivity of these systems. If both the<br />
acoustic and the electric parts are designed with the considerations above in mind and an effective<br />
software constructed, systems like this will become effective yet costly.<br />
3.13.3.1.4 Oil Temperatures<br />
Overheating or overloading can cause transformer failures. Continuous measurement of the top-oil<br />
temperature is an important factor in maximizing the service life. Top-oil temperature, ambient temperature,<br />
load (current), fan/pump operations, and direct readings of winding temperatures (if available)<br />
can be combined in algorithms to determine hottest-spot temperature and manage the overall temperature<br />
conditions of the transformer.<br />
3.13.3.1.5 Winding Temperatures<br />
There is a direct correlation between winding temperature and normally expected service life of a<br />
transformer. The hottest-spot temperature of the winding is one of a number of limiting factors for the<br />
load capability of transformers. Insulation materials lose their mechanical strength with prolonged<br />
exposure to excessive heat. This can result in tearing and displacement of the paper and dielectric<br />
breakdown, resulting in premature failures. Conventional winding temperature measurements are not<br />
typically direct; the hot spot is indirectly calculated from oil temperature and load current measurements<br />
using a widely recommended and described test method (Domun, 1994; Duval and Lamarre, 1977; Feser<br />
et al., 1993; Fox, 1983; IEEE, 1995). Fiber-optic temperature sensors can be installed in the winding only<br />
when the transformer is manufactured, rebuilt, or refurbished. Two sensor types are available: optical<br />
fibers that measure the temperature at one point, and distributed optical fibers that measure the temperature<br />
along the length of the winding. Since a distributed fiber-optic temperature sensor is capable<br />
of measuring the temperature along the fiber as a function of distance, it can replace a large number of<br />
discrete sensors and allow a real-time measurement of the temperature distribution.<br />
3.13.3.1.6 Load Current and Voltage<br />
Maximum loading of transformers is restricted by the temperature to which the transformer and its<br />
accessories can be exposed without excessive loss of life. Continuous on-line monitoring of current and<br />
voltage coupled with temperature measurements can provide a means to gauge thermal performance.<br />
Load current and voltage monitoring can also automatically track the loading peaks of the transformer,<br />
increase the accuracy of simulated computer load-flow programs, provide individual load profiles to<br />
assist in distribution-system planning, and aid in dynamically loading the transformer.<br />
FIGURE 3.13.2 <strong>Power</strong>-factor graphical representation.<br />
3.13.3.1.7 Insulation <strong>Power</strong> Factor<br />
The dielectric loss in any insulation system is the power dissipated by the insulation when an ac voltage<br />
is applied. All electrical insulation has a measurable quantity of dielectric loss, regardless of condition.<br />
Good insulation usually has a very low loss. Normal aging of an insulating material causes the dielectric<br />
loss to increase. Contamination of insulation by moisture or chemical substances can cause losses to be<br />
higher than normal. Physical damage from electrical stress or other outside forces also affects the level<br />
of losses.<br />
When an ac voltage is applied to insulation, the leakage current flowing through the insulation has<br />
two components, one resistive and the other capacitive. This is depicted in Figure 3.13.2. The power<br />
factor is a dimensionless ratio of the resistive current (I R ) to total current (I T ) flowing through the<br />
insulation and is given by the cosine of the angle depicted in Figure 3.13.2. The dissipation factor, also<br />
known as tan delta, is a dimensionless ratio of the resistive current to the reactive current flowing through<br />
the insulation and is the tangent of the angle in Figure 3.13.2. By convention, these factors are usually<br />
expressed in percent. Due to the fact that theta is expected to be large, usually approaching 90 degrees,<br />
and delta is commensurately small, the power factor and dissipation factor are often considered to be<br />
essentially equal.<br />
3.13.3.1.8 Pump/Fan Operation<br />
The most frequent failure mode of the cooling system is the failure of pumps and fans. The objective of<br />
continuous on-line analysis of pumps and fans is to determine if they are on when they are supposed to<br />
be on and are off when they are supposed to be off. This is accomplished by measuring the currents<br />
drawn by pumps and fans and correlating them with the measurement of the temperature that controls<br />
the cooling system. This can also be accomplished by measuring pump/fan current and top-oil temperature.<br />
Mode of operation is verified based on current level. Normal operational modes indicate rotation<br />
of fan blades and correct rotation of pump impeller. Abnormal operational modes are usually the result<br />
of improper control wiring to those devices.<br />
Pump failures due to malfunctioning bearings could be a source of metallic particles, and such particles<br />
could be a potential dielectric hazard. Sensors that detect bearing wear are available. The ultrasonic<br />
sensors are embedded in the pump bearings and measure the bearing length, thus determining whether<br />
metal loss is occurring.<br />
Furthermore, continuous on-line analysis should take into account that:<br />
• The temperature that controls the cooling system can differ from the temperature measured by<br />
the diagnostic system.<br />
• The initial monitoring parameters are set for the cooling stages based on the original transformer<br />
design. Any modifications to the cooling sequences or upgrades must be noted, since this will<br />
change the monitoring system output.<br />
• The sensitivity of the diagnostic system is influenced by the number of motors that are measured<br />
by each current sensor.<br />
© 2004 by CRC Press LLC<br />
© 2004 by CRC Press LLC