Page 391 - Offshore Electrical Engineering Manual

378 CHAPTER 5 Large Motors

Table 7.5.3 High-Voltage DC Tests
Maximum Recommended DC High-
Machine Rating Voltage Tests (V)

Less than 1 kVA (or kW) with rated voltage 500 + (2 × rated voltage)
below 100 V
Less than 1 kVA (or kW) with rated voltage 1000 + (2 × rated voltage)
100 V and above
1 kVA (or kW) and above, but less than 1000 + (2 × rated voltage)
10 MVA (or MW)
10 MVA (or MW) and above, at rated voltages
Up to 2 kV 1000 + (2 × rated voltage)
Above 2 kV and up to 6 kV 2.5 × rated voltage
Above 6 kV and up to 17 kV 3000 + (2 × rated voltage)
Above 17 kV Subject to special agreement

Earths should be applied to each test winding for at least 10 min after completion
of the test and the complete windings for 1 h after final completion of all tests.
BS EN 60034:18-31:2012 permits DC onsite high-voltage tests to be carried out
by agreement between the manufacturer and the purchaser. The test voltages must
not be greater than 1.6 × 0.8 (i.e., 1.28) times the root mean square (RMS) value of
the alternating current voltage specified in BS EN 60034. The values recommended
in Table 7.5.3 are the RMS values quoted and are therefore 80% of the maxima
allowed. These values must still be agreed with the machine manufacturer, however,
before testing is undertaken.

EVALUATION OF MOTOR TEST RESULTS
When the tests described earlier have been carried out, the results must be interpreted
to make a decision on whether or not to accept the machine for operation. An esti-
mation of the machine’s acceptability may be based on a comparison of present and
previous values of insulation resistance and PI, corrected to 40°C in each case.
When the insulation history is not available, recommended minimum values of
PI or of the 1-min insulation resistance value may be used. These values are given
in Table 7.5.4.
The insulation resistance value of one phase of a three-phase winding, with the other
two phases earthed, is approximately twice of that of the entire winding. Therefore, when
the three phases are tested separately, the observed resistance of each phase should be
halved to obtain a value for comparison with the appropriate value in Table 7.5.4(a).
The acceptance criteria in Table 7.5.4 must be modified, however, owing to the
following limitations. First, insulation resistance is not directly related to dielec-
tric strength, and it is therefore impossible to specify the value at which a winding
will fail electrically. Second, the windings of large or slow-speed machines have
extremely large surface areas, with healthy values of insulation resistance less than
the recommended minimum.