What is Partial Discharge?
Partial discharges are "sparks" involving the flow of electrons
and ions when a small volume of gas breaks down. This can occur in voids
within the electrical insulation system itself or adjacent to the insulation
of high voltage stators. The term partial is used since there is a solid
insulation, such as epoxy-mica, in series with the void, which prevents
a complete breakdown. The insulation system and the voids can be considered
to be a series of capacitances with different dielectric constants.
Depending on the size of the void, the dielectric constant, and the
temperature, the stress on the gas within the void may become high enough
for breakdown to occur. In most cases the electric field will not be uniform
and this will tend to lower the breakdown voltage.
Partial discharges are often the result of damage caused by other thermal,
mechanical, electromagnetic and chemical forces acting on the stator winding.
The progressive development of partial discharge activity is the major
symptom of insulation deterioration. These discharges also contribute to
the aging of the machine's dielectric system by eroding away or
deteriorating the insulation system.
Of course, paritla discharges tend to occur near the line ends where
the voltage is highest and not on the side of the neutral point.
Ten reasons why you should use Partial Discharge Analysis (PDA) Technology
1. Averts Catastrophic Failures of Stator Insulation Systems
PDA monitoring significantly reduces forced outage time and the possibility
of generator or motor damage by giving an early warning of stator insulation
system deterioration.
2. Extends the Lifetime of Stator Insulation
PDA readings can identify the early stages of reparable insulation
damage (e.g. slot discharge due to erosion of the semi-conductive coating)
which, if corrected, will extend the time before a machine failure occurs.
3. Maximizes Production Revenue
Once the PDA permanent couplers are installed, no interruption in service
is required to perform PDA monitoring. On-line PDA monitoring of stator
insulation condition can allow you to extend the interval between traditional
off-line insulation tests and visual inspections.
4. Reduces Capital Costs
PDA monitoring eliminates unnecessary premature rewinding of stator
windings.
5. Enables Efficient Maintenance Planning
Using PDA readings, maintenance is based upon the actual condition
of a machine's insulation system rather than intervals of time. Also, priorities
can be accurately established for the order in which the maintenance or
repair of multiple units are performed.
6. Simple and Safe to Perform
PDA monitoring does not require a large external AC power supply and
does not expose the operator to high voltage risk. All the skills necessary
to perform PDA monitoring can be quickly learned and carried out by station
personnel.
7. Gives Specific Information for Assessing Repairs or Rewinds
PDA monitoring can identify the type of the insulation deterioration
(e.g. slot discharge), its possible cause (e.g. loose winding) and its
severity (degenerating, stabilizing or improving). Also, single severe
sources of deterioration can be identified.
8. Confirms Effectiveness of Repairs
Comparison of PDA readings taken before and after repair or rewedging
of the machine allows the user to quickly and easily determine if the repairs
were successful.
9. Provides the Most Complete Diagnosis of Stator Insulation Condition
Since PDA readings are obtained with the machine in normal operation,
all of the thermal, electrical, mechanical and environmental stresses acting
upon the winding are present and their effects incorporated into the readings.
10. Non - destructive - Does Not Damage the Winding
PDA monitoring is a non-destructive test. It is performed with only
the normal thermal, electrical and mechanical stresses which act upon the
winding. This prevents any part of the stator insulation from being subjected
to abnormally high stresses, such as the neutral end of the winding would
experience in DC Hi Pot testing.
Partial discharges occur in various places in a rotating
machine
1. Partial discharge may exist within the main groundwall insulation
as a result of delamination or voids caused by missing or incompletely
cured bonding material.
2. Partial discharge may exist at the copper strand/groundwall insulation
interface as a result of poor bonding.
3. Partial discharges may exist in the endwinding region where adjacent
coils can have full phase to phase voltage between them. Partial discharge
may also exist in the endwinding region as a result of tracking (meaning
of this?).
4. Partial discharge may be present where the coil exits the slot.
Because the outside of the coil in the end winding is not grounded, it
acts like a capacitor with one side not connected. Therefore, it will try
to assume the same potential as the copper. A sharp change in potential
will exist along the surface of the coil between the portion grounded to
the stator core and the ungrounded portion. Stress (semiconductive?)
grading paints eliminate high potential differences which would cause partial
discharge.
5. Within the slot, the coil surface is grounded to the stator core
with a coating of low-resistance conducting paint or tape. Partial discharge
can occur within the slot when contact is lost between the conducting surface
on the coil and the core. This is known as slot discharge. Slot discharge
can seriously burn the surface of the coils and the slot fillers.
In addition to the electrical effects, partial discharge generates ozone
(in
air-cooled (hydro) generator mostly?). Through a series of reactions,
nitrogen based acids are formed at dew point from NOx. These acids will
also attack organic insulations, accelerating the deterioration. In some
cases, severe partial discharges could ignite an explosion. Again
what about hydrogen-cooled turbogenerators? Does some ingress of foreign
particles or gas warrant the use of PD there? |
