Ageing of electric cables in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 232

enough to begin failing in service (breakdown voltage) but without damag-
ing sound insulation (withstand voltage). The Hi-Pot test is easy to perform,
provides trendable data, and does not require access to the entire length of
a cable. However, the cable must be disconnected in order to perform the
test, and the high voltages applied may damage the cable insulation (U.S.
NRC, 2010b).
The partial discharge (PD) test is another insulation quality assessment
method. Partial discharges are small electrical sparks that occur at voids,
gaps, and similar defects within the insulation in medium and high voltage
cables. Over time, these partial discharges will erode the insulation and ulti-
mately break down the cable completely, resulting in embrittlement and
cracking, mechanical damage, and water treeing (U.S. NRC, 2010b). The
lower the PD inception voltage, the greater the degradation of the insula-
tion material (IAEA, 2011).
Measurements of partial discharge are performed in both time and fre-
quency domain by a monitor connected to a cable circuit. If a suffi ciently
high voltage (called the inception voltage) is applied across a cable insula-
tion, an electrical discharge (partial discharge or corona) can occur in small
voids or air gaps in the insulation or between insulation and a ground plane
or shield (U.S. NRC, 2001). The monitor or oscilloscope measures the peak
magnitude of the partial discharge pulse, phase angle, and pulse shapes of
the partial discharge signals acquired. PD test equipment can determine
the location of the voids or gaps by measuring the time lag between direct
and refl ected pulses from the discharge site or by using acoustic emission
monitoring techniques (U.S. NRC, 2001). These measurements can be made
continuously or intermittently and identifi ed on- or off-line (Hashemian,
2010).
The PD test does not require access to the full length of the cable and
enables both the quantifi cation of the severity of insulation defects and
identifi cation of their location in the cable. However, the end terminations
of the cable must be disconnected to perform the test, the test itself requires
highly skilled personnel, and the high testing voltage can weaken or damage
cable insulation (U.S. NRC, 2010b).
Another insulation quality test - AgeAlert TM - is a wireless microsensor
that measures ageing or degradation of electrical insulation. Constructed
out of cable insulation and nano-size conductive particles, it is installed in
multiple locations along cables or embedded in motors and allowed to age
together with the insulation material being monitored (IAEA, 2011). The
parameter that is measured and correlated to cable condition is the resistiv-
ity of the microsensor as a function of its age. Because it is constructed out
of the same material as the cable it measures, it responds to temperature,
humidity and radiation environments much as the insulation does (IAEA,
2011). Under thermal-oxidative conditions, the polymer material becomes

￿ ￿ ￿ ￿ ￿ ￿