High Voltage Bushings Dissipation Factor (Tan Delta or Power Factor) Measurement
The primary function of a bushing is to provide an insulated entrance for an energized conductor into an equipment tank or chamber. Additionally, it can serve as a support structure for other energized components.
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Types of Bushings
Condenser Type:
- Oil-Impregnated Paper (OIP): This design features paper layers that manufacturers impregnate with oil and intersperse with conducting condenser layers. Furthermore, continuous winding interleaves these paper layers.
- Resin-Impregnated Paper (RIP): For this type, paper layers impregnated with resin interleave with conducting condenser layers.
- Resin-Impregnated Synthetic (RIS): This composition utilizes synthetic materials that workers impregnate with resin, interspersed with conducting condenser layers.
- Resin-Bonded Paper (RBP): Consequently, this insulation forms when resin bonds paper layers together, interspersed with conducting condenser layers.
Non-Condenser Type:
- Solid Core: This structure alternates layers of solid and liquid insulation.
- Homogeneous Insulation: This form utilizes a solid mass of insulating material, such as solid porcelain.
- Gas-Filled: Alternatively, this design uses gas as the insulating medium.
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Classification
Specifically, technicians classify bushings based on the presence or absence of:
- Potential Tap (also called “capacitance” or “voltage” tap).
- Dissipation Factor Tap (also called “power factor” tap or “test tap”).
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Condenser Bushing Overview
A condenser bushing essentially consists of a series of concentric capacitors between the center conductor and the ground sleeve or mounting flange. Furthermore, technicians can tap a conducting layer near the ground sleeve and connect it to a terminal, which creates a three-terminal specimen. This tapped bushing functions as a voltage divider. Consequently, Figure 1 illustrates an overview of the condenser type bushing.
- Equal Capacitances (, , and ): These elements ensure an equal voltage distribution from the energized center conductor to the grounded condenser layer and flange.
- Tap Electrode: Operators normally ground this electrode during operation, except for specific designs or bushings that they use with potential devices.
- Capacitance Comparison:
- For bushings with potential taps, is significantly larger than .
- However, for bushings with power-factor taps, and often share the same order of magnitude.
Figure 1: Construction of the condenser type bushing
In this design, developers tap the ground layer of the bushing core and connect it to a miniature bushing on the main bushing’s mounting flange. Next, a screw cap on the miniature bushing housing grounds the tap. When a technician removes the grounding cap, the tap terminal becomes a low-voltage terminal for performing a UST measurement on the main bushing insulation (: conductor to tapped layer).
In some alternative designs, the tapped layer extends into an oil-filled compartment. Consequently, the potential tap floats during operation, and operators insert a special probe through an oil filling hole to contact the tapped layer for measurements.
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Field Testing Procedures for Bushings
Although bushings are relatively simple devices, engineers develop field tests to detect defects, deterioration, contamination, or damage in the insulation. Specifically, the most important tests include:
- Overall Test (Center Conductor to Flange)
- Center Conductor to Tap Test ()
- Tap Insulation Test (Tap to Flange, )
- Hot Collar Test (Collar to Center Conductor)
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Measurement of Insulation Power Factor (Tan Delta) and Capacitance of Bushings
In this test, technicians measure the capacitance and insulation power factor () of the high-voltage capacitive bushings. To perform this test, operators must de-energize the transformer and the high-voltage reactor, and then carry out the following steps:
Surface Cleaning and Preparation:
First, verify that the outer surface of the bushing insulators is clean and dry. Therefore, clean the bushing insulators with a suitable cloth.
Note: Technicians must never perform this test in humid weather or under high moisture conditions.
Measurement Connections:
To measure the capacitance and insulation power factor of a bushing installed on the transformer, you must connect all three phases and the neutral (if present) together. You must make this connection on each winding, meaning:
- First, short-circuit the three phases and neutral on the high-voltage side together.
- Second, short-circuit the three phases and neutral on the low-voltage side together.
- Third, short-circuit the tertiary winding bushings (if present) together.
Moreover, when you test the bushings of one winding, you must ground all other remaining windings.
For example, when a technician tests the U1 bushing of the high-voltage winding:
- The test engineer connects U1 to V1, W1, and N1 (if available).
- Next, the engineer shorts together and grounds the terminals u2, v2, and w2 (and n2, if available).
- Finally, the technician shorts together and grounds the tertiary bushings (if any).
Consequently, you should follow the exact same procedure for testing the bushings of the other windings.
6.1. Test Voltage Guidelines
Tap Insulation Tests:
- Do not exceed 2 kV for potential taps.
- Do not exceed 500 V for dissipation factor taps (test taps).
Overall and Center Conductor to Tap Tests:
In these cases, use a convenient voltage (12 kV) at or below the bushing’s nameplate rating.
Hot Collar Test:
Specifically, perform this test at a test voltage of approximately 10 kV.
6.2 Testing Spare Bushings
When you test a spare bushing, proper handling and setup remain critical to ensure accurate results. First, mount the bushing in a grounded metal rack with no connections to the terminals. Furthermore, avoid testing bushings mounted in wooden crates or placed directly on the floor, because wood or concrete can affect the test results. Finally, ensure that the bushing’s center conductor does not contact any foreign materials (e.g., slings, ropes, or other objects).
Technicians can perform the overall test for spare bushings, with or without taps, by using the GST mode. With this specific test mode, the instrument measures the overall capacitance.
Additionally, you can separately test the main insulation () of bushings equipped with taps by connecting INPUT A to the tap. Specifically, the analyzer measures the insulation using the UST-A mode.
Figure 2: Connecting CAPTAN 12 for the overall test of a spare bushing (GST mode)
Figure 3: Connecting CAPTAN 12 for the main insulation test (C1) of a spare bushing (UST-A mode)
The tap insulation (C2) of bushings equipped with tap, can be measured directly. As illustrated in the figure below, the High Voltage and the INPUT A cable must be interchanged.
Using the GSTg-A mode will bypass the main insulation (C1) and measure the tap insulation (C2).
Figure 4: Connecting CAPTAN 12 for the tap insulation test (C2) of a spare bushing (GSTg-A mode)
6.3. Testing Mounted Bushings:
6.3.1. Center Conductor to Tap, C1
Most high voltage condenser-type bushings have either a potential tap or a power-factor tap, enabling separate testing of the main insulation (C1) without disconnecting the bushing from the equipment.
Measurement: Use the UST-A mode to measure C1, as shown in the figure.
Temperature Correction:
- For Bushings on Transformers: Use the average of the bushing temperature (measured by Thermovision) and ambient air temperature.
- For Bushings on Oil Circuit Breakers: Use the ambient air temperature.
Figure 5: Connecting CAPTAN 12 for main insulation test (C1) of a bushing on a transformer (UST-A mode)
6.3.2. Tap-Insulation Test (Tap to Flange, C2):
Before beginning any measurements, the test engineer must carefully identify the type of tap and its maximum rated voltage. The manufacturer typically specifies the maximum permissible test voltage, which is usually between 500 V and 2 kV. In analogy to the tap insulation test on spare bushings, the C2 insulation is measured by the GSTg-A mode. The connection is shown in picture below. For the capacitance C2 (tap to flange) the dissipation factor is measured but normally not corrected for temperature.
Figure 6: Connecting CAPTAN 12 for tap insulation test (C2) of a bushing on a transformer (GSTg-A mode)
6.4. Hot Collar Test:
The hot collar test is used to evaluate dielectric losses in specific sections of a bushing or pothead by creating localized high voltage stresses.
Test Setup:
- A conductive hot collar band is tightly fitted around the porcelain surface, typically just below the top petticoat.
- A high voltage is applied to the band, generating a high voltage gradient in the insulation directly beneath the collar.
- The hot collar test is performed using the UST-A mode, and the bushing does not need to be disconnected from other components or circuits.
- Ensure the collar band is tightly secured around the porcelain bushing to maintain good contact and prevent partial discharge issues at the interface.
Key Benefits:
- Measures losses in the section directly below the collar.
- Effectively detects issues such as voids in compound-filled bushings or moisture penetration, as the insulation is subjected to a higher voltage gradient than in standard bushing tests.
Figure 7: Connecting CAPTAN 12 for hot collar test of a bushing (UST-A mode)
