Insulation Test of Voltage Transformer
Types of Voltage Transformers
Voltage transformers currently operating in power systems can be classified into three types based on insulation structure: electromagnetic voltage transformers, cascade-type voltage transformers, and capacitive voltage transformers. Electromagnetic voltage transformers include three types—oil-immersed single-phase, oil-immersed three-phase five-limb, and cast resin types—and are commonly used in voltage levels up to 35 kV. Cascade-type voltage transformers are typically employed in voltage levels ranging from 35 kV to 220 kV. Capacitive voltage transformers are generally used in voltage levels of 110 kV and above. They feature a rational insulation design, high insulation strength, and the ability to fully utilize the coupling capacitors required for carrier communication.
The purpose of measuring insulation resistance is to detect whether the insulation of a voltage transformer has overall moisture absorption or aging defects. During measurement, a 2500V insulation resistance tester should be used for the primary winding, and either a 1000V or 2500V tester for the secondary winding; the non-tested side should be grounded. The test data obtained can be compared with previous test results for analysis and judgment. Under normal conditions, the insulation resistance of the primary winding should not be less than 60% of its factory value or previous test values; the secondary winding should generally not be lower than 10 MΩ.
When measuring, factors such as air humidity, dirt on the transformer surface, and ambient temperature should also be considered to avoid affecting the measured insulation resistance data.
Measuring the dielectric loss factor tanδ of a bushing
For voltage transformers rated 35 kV and above, measuring the dielectric loss factor of the primary winding can quickly detect defects such as insulation moisture, degradation, and bushing insulation damage.
Measurement of tanδ for cascade-type voltage transformers
The main methods for measuring the tanδ of cascade-type voltage transformers include: conventional test method, self-excitation method, terminal shielding method, and terminal voltage application method.
There are two possible outcomes when measuring the tanδ of a cascade-type voltage transformer using conventional test methods. The first is that the tanδ exceeds the specified value, which may be caused by internal defects in the transformer or by external porcelain sleeves and secondary terminal boards—typically, the latter is more likely to be the cause. During testing, attention should also be paid to the condition of the outer porcelain surface and relative humidity. The second outcome is a tanδ below the specified value, indicating good insulation between windings and between windings and ground.
When measuring the tanδ between windings and from winding to ground for cascade-type voltage transformers rated 110 kV and above using the self-excitation method, a low voltage is applied to the secondary winding of the transformer under test. By utilizing its inherent inductive coupling, a relatively high test voltage can be generated on the high-voltage winding. It should be noted that when measuring tanδ of cascade-type voltage transformers with the self-excitation method, not only external electric field interference exists, but also interference between power sources and effects of stray impedance. As a result, measurement data tend to be scattered and errors are relatively large. Like conventional methods, the self-excitation method cannot accurately measure the dielectric loss of insulation supports, and thus is rarely used in field applications.
The terminal shielding method can eliminate measurement errors caused by moisture, cracks, or dirt on the secondary terminal board, and accurately reflects the internal insulation condition of the transformer. However, its drawback is that part of the capacitance between the primary winding and ground will be shielded and thus cannot be measured.
The terminal voltage application method primarily measures the tanδ between the primary and secondary windings of a current transformer. However, similar to the conventional method, its measurement results are affected by the secondary terminal board, leading to inaccurate detection of moisture in the winding end insulation.
Measurement of tanδ for Capacitive Voltage Transformers
The tanδ of capacitive voltage transformers is generally measured using the QS1 bridge with a positive connection. However, due to the structural characteristics of capacitive voltage transformers, accurate measurement requires appropriate modification of the test wiring.
AC Withstand Voltage Test
There are two methods for applying AC withstand voltage to electromagnetic voltage transformers. One method involves externally applying power frequency test voltage, which is suitable for AC withstand voltage tests on transformers with rated voltages of 36 kV and below. The wiring method for the withstand voltage test is the same as that used in AC withstand voltage testing of transformers.
For voltage transformers with ratings above 35 kV, the insulation level at the end of the primary winding is relatively low, approximately 5 kV. Therefore, it cannot withstand the same test voltage as the beginning end. An induced voltage withstand test method should be adopted—by grounding the end of the primary winding and applying voltage to one secondary winding, thereby inducing the required test voltage in the primary winding. A no-load test should be performed before and after the induced voltage withstand test to ensure that there is no turn-to-turn short circuit in the primary winding.
Post time: Jul-02-2026