Power transformers are required to be designed for long term operation with good performance and reliability as they are one of the most important pieces of equipment in a power system. Therefore they should be designed to withstand all types of transients during its life in service. Most transients are caused by natural phenomena like lightning atmospheric discharge or caused by the system switching operations or disturbances.
It is much too complicated and complex to represent such stresses by tests as the transients are basically dependent on system characteristics and the geographic conditions where the transformer is connected and installed.
International committees have established workgroups consisting of Manufactures, Utilities, Generators and Academic Societies. These workgroups have published standards e.g. IEEE and IEC to provide a standardised testing methodology.
These documents describe the controlled conditions to be simulated at manufacturer’s test field. They specify which tests must be performed on the transformer and the conditions for acceptance. They are known as dielectric tests.
The type and level of the test is a function of the insulation level of the transformer specified by a customer.
The dielectric tests and the application of appropriate protection devices defined by insulation coordination study have the aim to assure a reliable operation of the transformer without failure during its life in service.
Therefore the transformer’s insulation design like core and coil assembly, and leads are submitted to Final Acceptance Tests at manufactures facilities to prove its ability to withstand different kinds of dielectric stresses. Once approved, the transformer is expected to operate in service with reliability for the long-term withstanding possible transients from the system.
This article presents an overview about the dielectric stresses on the main insulation of medium power transformers of 40 MVA designed with solid-liquid insulation. The insulation levels are given in Table 1.
Separate AC source withstand voltage test (Applied Voltage Test)
The main proposal of this test is to check the major insulation between windings and each winding to ground. Each circuit is tested separately. For a two winding transformer, two tests are required.
When the voltage source is connected to primary windings, the secondary windings are grounded and tank as well. Then the test is performed on secondary windings with the voltage source connected and the primary windings grounded.
During this test an AC source is applied with approximately 2 times the rated voltage of on the transformer under test. Primary and secondary transformer voltage circuit is tested separately.
When tested all bushings of the same circuit (primary or secondary) are connected together, including neutral in a wye connection. The bushings from the other circuit are directly grounded.
Figure 1. shows the equipotential lines during an applied voltage test of 230kV on the primary windings (high voltage)of a three phase wye-delta regulating transformer as an example. Figure 1a shows a sketch of the test circuit. The Figure 1b shows the equipotential lines in a core window cut section between neighboring phases.
The more the equipotential lines are concentrated the higher is the electrical field as shown between LV and HV windings and between the top of HV winding and upper yoke as well.
Between phases of regulating winding R there are no equipotential lines because both windings have the same potential. This is a typical stress that occurs during the applied voltage test.
Lightning Impulse Test
Impulse test system is necessary to produce the wave shape 1.2/50 µs as required by the standards. The generator is a very large piece of equipment composed by an RLC circuit in modular stages. The impulse generator is set up in various series and parallel configurations to achieve the wave form 1.2/50 µs and the required impulse voltage level.
The test is applied individually to all terminals of the transformers. When one terminal is tested, the other bushings are solid grounded, grounded via resistances or grounded via a shunt for fault detection purposes. This is done for each terminal with the specified lightning impulse level.
Figure 2. shows the equipotential lines distribution during a lightning voltage test on high voltage terminal of a three phase wye-delta regulating transformer, same example of the Figure 1.
As the neutral bushing is grounded and connected directly on regulating winding, the the regulating winding assumes the potential zero.
The Figure 02b shows the voltage distribution at the peak of the voltage. Observe that the equipotential lines have different distribution in comparison with the applied voltage test. Now the stress between the HV and R windings are higher.
During the lightning impulse test, transient voltages appears within the windings but this is not addressed in this article.
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