Introduction
Capacitor voltage transformers (CVTs) are among the most critical components of power networks, playing a vital role in protection and measurement systems. One of the most important considerations in the operation and maintenance of these devices is the continuous monitoring of their insulation condition.
These transformers are installed inside substations, and any insulation failure in them may lead to high-current short circuits. Such faults can escalate and damage more expensive equipment in the substation, such as power transformers.
Over time, the insulation of CVTs deteriorates due to electrical, mechanical, and thermal stresses. Eventually, the degradation may reach a point where the insulation can no longer withstand these stresses, resulting in dielectric failure.
Insulation aging is generally a slow process. However, with regular monitoring, it is possible to detect early signs of degradation and take corrective action—such as repair or replacement—before a failure occurs and causes a wider system fault.
One of the most effective methods for assessing the insulation condition of CVTs is the dissipation factor (tan delta) test, which evaluates both the capacitive value and the dielectric losses of the insulation system.
Internal Construction and Operating Principle of a CVT
The construction of a CVT consists of several components as shown in the figure below. The first section is the Capacitive Voltage Divider (CVD), composed of a series connection of capacitors, which can be simplified to two capacitors, C1 and C2. Capacitor C2 is larger than C1, and the voltage amplitude reduction is achieved by the ratio .
The second section contains the Electromagnetic Unit (EMU), which includes multiple elements. Initially, a compensating reactor (inductor) is placed to compensate the capacitive effect of the CVD. This reactor is designed so that at the nominal frequency of 50/60 Hz, the inductive reactance exactly cancels out the capacitive reactance of the CVD, thereby stabilizing the voltage transformation.
Following the compensating reactor, the Intermediate Voltage Transformer (IVT) is installed. The IVT is essentially a voltage transformer with a high-voltage primary winding rated for several tens of kilovolts and multiple low-voltage secondary windings, for example, 110 V × √3. As shown in the figure below, the primary winding of the IVT includes a series tap. This tap is factory-adjusted and sealed to guarantee the CVT’s accuracy class and performance specifications.
Finally, on the secondary side, a ferroresonance damping circuit (FSC) is connected. This circuit helps to mitigate ferroresonance phenomena which can cause overvoltages and distortions in the CVT output voltage. More detailed discussions and considerations regarding the FSC will be presented in future application notes.
Capacitance and Dissipation Factor Testing Procedure
For testing the dissipation factor of a CVT, a circuit as shown in the figure below should be used. Important points regarding this test circuit are as follows:
- The CVT must be fully isolated. This means the high-voltage terminal connection to the network should be physically disconnected right above the CVT.
- The burden connected to the secondary terminals must be disconnected. If the burden remains connected during the test, it will cause an increase in the measured dissipation factor.
- The test is performed in the GST (Grounded Secondary Test) mode. The grounding of both the test device and the CVT must be checked to ensure they are properly connected to the substation’s main ground.
- If the end of capacitor C2 (also known as the L-terminal or NHF terminal) is connected to a PLC system, it should be temporarily grounded during the test.
- It should be noted that this test measures the total dissipation factor of the entire CVT. However, the manufacturer only measures and marks the and capacitance of the CVD section on the nameplate. Therefore, the test results obtained on-site are not directly comparable with the factory results.
- The capacitance values marked on the CVT nameplate, which include C1, C2, and their series and parallel combinations, are typical design values and not actual measured values. The real measured capacitance can vary between -5% and +10% relative to the nameplate values. This variation is considered normal and should not be interpreted as insulation degradation unless accompanied by a significant increase in tan δ.
- The higher the CVT voltage rating, the closer the on-site measured dissipatio factor and capacitance values will be to the factory values.
- In high-voltage CVTs, one or more series stacks are used for the CVD. Each stack’s capacitance and are measured separately and marked on the stack’s nameplate. These upper stacks can be tested individually by applying voltage to one terminal and connecting the measuring device to the other terminal. The and capacitance can then be measured in UST mode. This allows on-site measurements to be compared with factory results.
- The and capacitance of a CVT are temperature dependent. Proper correction factors should be obtained from the manufacturer to adjust the measurements accordingly.
- The measured results should be compared with adjacent phases and previous test periods to detect any abnormal changes.
