Testing
Type tests are performed by manufacturers in order to confirm that a design meets its specifications, and production tests are done in order to check that each machine as it is manufactured conforms to performance and safety standards.
These tests usually include the following
● full-load tests to measure the temperature rise of the machine windings and insulation. The temperature rise is calculated from the change in resistance of the stator windings.
● tests to determine the excitation current required to deliver a given output voltage. These are done for open-circuit conditions and also for various load currents and power factors. The resulting curves are usually known as ‘saturation’ curves.
● short-circuit tests to determine the current that can be driven by the generator into a short-circuit fault in the connected load.
● transient short-circuit tests to determine the subtransient and transient reactances and time constants.
● overspeed tests to confirm that the rotor does not distort or disintegrate. This test is normally performed at 150 per cent of rated speed, at full rated temperature.
● insulation tests to confirm that the insulation system is capable of withstanding operating and transient conditions. Typical requirements are that stator windings can withstand ac voltages of 1 kV plus twice rated voltage to the frame. Similarly between stator high-voltage and low-voltage windings, rotor windings are also tested to frame but with voltages typically ten times that of the rated rotor voltage but not less than 1.5 kV.
● impulse tests are often used by manufacturers to confirm correct connection of coils and/or shorted turns. This test relies on applying a high-voltage impulse and comparing the waveforms of the transient. Shorted turns will be indicated by travelling wave reflections and non-standard waveforms.
The results from these tests are used to calculate the data described in section 5.7, an example of which has been shown in Table 5.3.
It is now necessary for any generator manufactured or imported into the European Union that relevant EU Directives are met through certification. Strictly it is the manufacturer of the generator set, including the engine and all controls, that is responsible for this certification, but many manufacturers of generators and AVRs will assist in the tests.