8.1 GENERAL
The
general requirements for the testing of all rotating electrical machines,
including generators, are laid down in BS 4999 : 1976, Part 60.
As far
as generators are concerned, the principal requirements are described below.
8.2 MANUFACTURER’S TESTS
Manufacturer’s
tests are given three classifications: ‘Basic’ (formerly called ‘Type Tests’),
‘Duplicate’ and ‘Routine Checks’. Basic
tests are mainly to prove a new design.
They include exhaustive tests to ensure that the design meets the
specification and all other performance requirements. They are normally carried out only on the
‘first of class’ generator, and a Test Certificate is provided on request to
confirm the tests. Basic tests may, on
special request, be repeated on the first machine of a new, large order, but
this is not usual.
Duplicate
tests are for performance. They are
applied to a generator that is of the same design and construction as one
previously made (and in no way altered) and which has already undergone basic
tests. The duplicate tests are to ensure
that the generator is still in accordance with the original design.
Routine
checks are tests to show that each individual generator has been assembled
correctly, is able to withstand the appropriate high-voltage tests and is in
sound working order both electrically and mechanically.
The
three classes of test are listed in Table 1.
TABLE
1 -
MANUFACTURER’S TESTS
Test
|
Basic
|
Duplicate
|
Routine
|
Resistance of windings (cold)
|
X
|
X
|
-
|
No-load losses
|
X
|
X
|
X
|
Temperature rise
|
X
|
-
|
-
|
Tests
for efficiency
|
X
|
-
|
-
|
Momentary overload
|
X
|
-
|
-
|
High
voltage
|
X
|
X
|
X
|
Vibration
|
X
|
-
|
-
|
Short-circuit saturation
|
X
|
X
|
-
|
Short-circuit losses
|
X
|
-
|
-
|
Most of
these tests are self-evident, but the following additional information is given
on the high-voltage test.
For this
test, also called a ‘withstand’ test, a high voltage is applied between the
frame and all the generator stator win dings, with all other conductors, metal
and auxiliary (i.e. heater) circuits bonded to the frame. The actual voltage applied is in accordance
with Table 2 and is sustained for one minute. It may be at any frequency between 25Hz and
100Hz. It is primarily an insulation
test for the generator’s windings and is included also in the routine checks to
ensure that there has been no fault during assembly of any individual machine.
TABLE 2 - HIGH VOLTAGE TESTS
Windings
|
Test Voltage (rms)
|
Generator stator
windings:
<100v br="" kva=""> >100V, <1kva br=""> 1 – 10 000kVA >10 000kVA and <2 br="" nbsp=""> 2 000 - 6 000V 6 000 – 17 000V >17 000V2>1kva>100v> |
500V + twice rated voltage
1 000V + twice rated voltage
1 000V + twice rated voltage, min
500V
1 000V + twice rated voltage
2.5 times rated voltage
3 000V + twice rated voltage
Special agreement
|
Rotor windings
(including related exciters): |
10 times the excitation voltage
minimum 1 500V
maximum 3 500V
|
8.3 ON-SITE TESTS
Any
generator installed on a platform or in an onshore oil installation may be
assumed to have undergone its full routine check tests, and its prototype a
full basic or duplicate test. On-site
tests are therefore only needed to check the original installation and
commissioning, and thereafter to ensure that no deterioration has taken
place. The remainder of this chapter
deals only with tests for the latter purpose.
Deterioration
can occur for many reasons: among them are entry of dampness or water leakage
in the generator or cable-entry boxes, overheating of the windings due to
overloading, or mechanical faults such as vibration or bearing failure.
Both
dampness and overheated windings can cause reduced insulation resistance of the
windings. After drying out, the
generator should be megger tested to ensure that insulation resistance has been
restored. Deterioration can be
progressive, especially when a machine is little used, and a regular programme
of megger testing every generator should be drawn up and the results
logged. After temperature correction
(see para. 8.4), the resistance levels should be plotted, and, if there is
progressive deterioration, this will be immediately apparent.
After
repairs to a generator, a megger test should normally be carried out before
reconnection if the generator or its connections have in any way been
interfered with.
High-voltage
‘withstand’ tests should never be needed on site unless a major overhaul has
been carried out, in which case it would be an engineering or manufacturer’s
concern.
8.4 MEGGER TESTING
‘Megger’
instruments are provided which operate at 250V, 500V or 2 500V, and the
correct one must be used depending on the rated voltage of the generator to be
tested. Normally generators over 415V
and all high-voltage generators would require a 2 500V megger.
When the
megger is connected and the handle wound up, the voltage should continue to be
applied until the needle settles down to a steady value; this might take one
minute or more.
When
testing the insulation resistance of a winding, all other conductors, metalwork
(stator and rotor) and auxiliary circuits such as those for thermistor
protection and heaters should be connected to the frame with light wire (fuse
wire will do), and the test voltage applied between winding and frame. This is to ensure that not only is the
insulation to earth satisfactory for the winding under test, but also that it
is adequate to other circuits and elements which are not normally at earth
potential - e.g. heater elements and circuits.
Where the 3-phase windings are independent and brought out to six
terminals, it is advisable to make a test also between pairs of windings by
removing the star-point links. However,
where generators are star-connected with their star-point internal and
permanently made, inter-phase tests are not possible.
Insulation
resistance is very dependent on temperature, and, in order to compare one
reading with another, it is necessary to reduce the value to a common
temperature. This is usually 40oC. Unlike the resistance of a conductor, which
rises with temperature, the resistance of insulation falls rapidly with
increase of temperature.
The
graph of Figure 8.1 is used to make this correction by means of a ‘temperature
coefficient’.
For example,
if the observed reading (Rt) is 10 megohms when taken at 70oC,
then, using the graph, the temperature coefficient (Kt) is 8.0, and
the corrected reading at 40oC (R40) is then 10 x 8.0 = 80
megohms. It can be seen from this
example that the correction is considerable when the winding is hot at normal
working temperatures.
FIGURE 8.1
INSULATION
RESISTANCE TEMPERATURE COEFFICIENT
The
recommended minimum value of insulation resistance for generators is given in
manufacturers’ literature. As a guide when
precise information is not available, the minimum acceptable value (Rm)
for a generator stator winding is given by:
Rm
= (kV + 1) megohms when corrected to 40oC,
where kV
is the generator’s rated voltage in kilovolts.
Thus:
for 415V or 440V generators
|
Rm 1.4MΩ
|
for 6.6kV generators
|
Rm 7.6MΩ
|
for 11kV generators
|
Rm 12MΩ
|
No comments:
Post a Comment