SECTION 7 SERVICE AND MAINTENANCE
As part of routine maintenance procedures, periodic attention
to winding condition (particularly when generators have been
idle for a long period) and bearings is recommended. (Refer
to subsections 7.1 and 7.2 respectively).
When generators are fitted with air filters regular inspection
and filter maintenance is required. (Refer to subsection 7.3).
7.1 WINDING CONDITION
Service and fault finding procedures
present hazards which can result in severe
personal injury or death. Only personnel
qualified to perform electrical and
mechanical service should carry out these
procedures. Ensure engine starting
circuits are disabled before commencing
service or maintenance procedures.
Isolate any anti-condensation heater
supply.
Guidance of Typical Insulation Resistance [IR]
Values
The following is offered as general information about IR
values and is aimed at providing guidance about the typical IR
values for generators from new through to the point of
refurbishment.
New Machines
The generators Insulation Resistance, along with many other
critical factors, will have been measured during the alternator
manufacturing process. The generator will have been
transported with an appropriate packaging suitable for the
method of delivery to the Generating Set assemblers works.
Where we expect it to be stored in a suitable location
protected from adverse environmental conditions.
However, absolute assurance that the generator will arrive at
the Gen-set production line with IR values still at the factory
test levels of above 100 MΩ cannot be guaranteed.
At Generating Set Manufacturers Works
The generator should have been transported and stored such
that it will be delivered to the assembly area in a clean dry
condition. If held in appropriate storage conditions the
generator IR value should typically be 25 MΩ.
If the unused/new generators IR values fall below 10 MΩ then
a drying out procedure should be implemented by one of the
processes outlined below before being despatched to the end
customer’s site. Some investigation should be undertaken into
the storage conditions of the generator while on site.
Generators in Service
Whilst It is known that a generator will give reliable service
with an IR value of just 1.0 MΩ. For a relatively new generator
to be so low it must have been subjected to inappropriate
operating or storage conditions.
Any temporarily reduction in IR values can be restored to
expected values by following one of the drying out
procedures.
7.1.1 WINDING CONDITION ASSESSMENT
The AVR should be disconnected and the
Resistance Temperature Detector (R.T.D.)
leads grounded during this test.
The condition of the windings can be assessed by
measurement of insulation resistance [IR] between phase to
phase, and phase to earth.
Measurement of winding insulation should be carried out : -
1. As part of a periodic maintenance plan.
2. After prolonged periods of shutdown.
3. When low insulation is suspected, e.g. damp or wet
windings.
Care should be taken when dealing with windings that are
suspected of being excessively damp or dirty. The initial
measurement of the [IR] Insulation Resistance should be
established using a low voltage (500V) megger type
instrument. If manually powered the handle should initially be
turned slowly so that the full test voltage will not be applied,
and only applied for long enough to very quickly assess the
situation if low values are suspected or immediately indicated.
Full megger tests or any other form of high voltage test should
not be applied until the windings have been dried out and if
necessary cleaned.
Procedure for Insulation Testing
Disconnect all electronic components, AVR, electronic
protection equipment etc. Ground the [RTD's] Resistance
Temperature Detection devices if fitted. Short out the diodes
on the rotating diode assembly. Be aware of all components
connected to the system under test that could cause false
readings or be damaged by the test voltage.
Carry out the insulation test in accordance with the ‘operating
instructions for the test equipment.
The measured value of insulation resistance for all windings
to earth and phase to phase should be compared with the
guidance given above for the various 'life stages' of a
generator. The minimum acceptable value must be greater
than 1.0 MΩ.
If low winding insulation is confirmed use one or more of
the methods, given below, for drying the winding should
be carried out.
1. M ETHODS OF DRYING OUT GENERATORS
Cold Run
Consider a good condition generator that has not been run for
some time, and has been standing in damp, humid conditions.
Warnin
g
!
Caution !