Tony Wills - Techical support group
Performing high voltage insulation diagnostic tests (power factor, tan delta) on large capacitive specimens like generators, motors, capacitive voltage transformers and long lengths of cable always presents a challenge. Typically, these tests are performed with the aid of a resonating inductor to neutralise the load presented to the test set by the capacitance of the specimen. Unfortunately, these inductors are expensive, heavy and complicated to operate, which means that an alternative approach is desirable.
Such an alternative is now available and involves making use of an insulation diagnostic analyser like the Megger DELTA4000 that supports variable frequency operation. With this test set it is possible to test four times the size of motor / generator by carrying out the test at 30 Hz with a test voltage of 8 kV, rather than carrying out the usual 60 Hz/12 kV test. And, of course, no resonating inductor is needed.
In addition, it is worth remembering that when performing tan delta tests on large generators and motors, testing individual phases (while grounding the phases that are not being tested) reduces the charging current to one third. Combining the testing of individual phases with frequency reduction and where possible, voltage reduction, reduces the power required from the test set by up to 80% compared with traditional methods – provided, of course, that it is possible to isolate the phases.
Let’s look at an example of what this means in practice. The generator/motor test was set up using a DELTA4000 high voltage insulation diagnostice test set (there was no resonating inductor). The test set is connected to Phase A of the motor/generator, while Phases B and C are grounded. With the frequency set to 30 Hz and the test voltage to 4 kV, the size of the specimen that can be tested is increased to 400 nF (0.4 μF) compared with a maximum of 60 nF (0.06 μF) at 12 kV for testing carried out at 60 Hz. Providing that the phases are tested individually, the total capacitance of all three phases can now be up to 1.2 μF.
The table shows the reduced frequency capabilities of Megger DELTA4000 test sets, giving a good indication of their versatility and their suitability for use in carrying out convenient inductor-free insulation diagnostic tests.
It almost goes without saying that making work easier and safer is always desirable. However, in many instances, the cost of these easier, safer alternatives makes them inaccessible for many electrical engineers. With Megger’s DELTA4000 insulation diagnostic analysers, however, users can access easy to use and safe measurement options with instruments that don’t break the bank!
For more information on the DELTA4000.