3.4 EPOXY RESIN RICH MICA PAPER,
HEATED PRESS CURED
With reference to Table 14, the case of load related slot
discharge detected by directional couplers in 1981 for a 17
kV 160 MVA synchronous condenser operating in
hydrogen at 200 kPa (gauge) at Dorsey remains an
important example [31]. It was found that this horizontal
stator had 308 mm long diagonal split wedges in the slots.
These had fallen out around 12 o’clock allowing the
associated stator bars to move approximately 10 mm in the
direction of the air gap. At full load the radial bar force is
estimated to be 1.6 kN/m of slot length.
The rewinds of three of the 11 kV units up-rated from
32.5 MVA to 43 MVA at Seven Sisters were associated
with the installation of new turbines with draft tube water
depression to facilitate operation as synchronous
condensers.
4 DISCUSSION
Decisions to rewind a stator have been based on the
condition and thermal limitations of the winding as opposed
to the years of service. Older open ventilated units may be
prone to blockage of the cooling ducts in the stator core.
In-stator single coil dissipation-factor tip-up tests proved
useful for determining the condition of asphalt bonded
mica. The use of the electromagnetic (TVA) probe test for
partial discharge is appropriate for commissioning and
periodic off-line maintenance tests provided that the end
connections have been fully insulated and other safety
precautions are addressed [12], [15].
CRTV/RTV coatings used to provide an interference fit
of coils and bars in the stator slot have proven to be durable
over a service period of more than 30 years provided that
physical tolerances were correct upon installation.
Bypassing a faulted stator coil is often the quickest means
of returning a machine to service. However circulating
currents between parallel circuits may be excessive when
one coil represents much more than 5% of the coils in the
circuit.