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. It may also serve as a support structure for other energized components.
1. Types of Bushings:
Condenser Type:
- Oil-Impregnated Paper (OIP): Insulation consisting of paper layers impregnated with oil, interspersed with conducting (condenser) layers. The paper layers are continuously wound and interleaved.
- Resin-Impregnated Paper (RIP): Insulation made of paper layers impregnated with resin, interspersed with conducting (condenser) layers.
- Resin-Impregnated Synthetic (RIS): Insulation composed of synthetic materials impregnated with resin, interspersed with conducting (condenser) layers.
- Resin-Bonded Paper (RBP): Insulation formed by bonding paper layers with resin, interspersed with conducting (condenser) layers.
- Non-Condenser Type:
- Solid Core: Alternating layers of solid and liquid insulation.
- Homogeneous Insulation: A solid mass of insulating material (e.g., solid porcelain).
- Gas-Filled: Uses gas as the insulating medium.
2. Additional Features:
• Outdoor Bushings: The primary insulation is housed in a weatherproof enclosure, typically made of porcelain or silicone. The space between the primary insulation and the weather shed is often filled with insulating oil, plastic, foam, or other compounds.
• Insulating Medium: Some bushings use gases like SF6 as the insulating medium between the center conductor and the outer weather shed.
3. Classification:
Bushings may be classified 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”).
4. 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. A conducting layer near the ground sleeve can be tapped and connected to a terminal, creating a three-terminal specimen. This tapped bushing functions as a voltage divider. An overview of the condenser type bushing is shown in Fig 1.
• Equal Capacitances (C1a, C1b, and C1c): These ensure an equal voltage distribution from the energized center conductor to the grounded condenser layer and flange.
• Tap Electrode: Normally grounded during operation, except for specific designs or bushings used with potential devices.
• Capacitance Comparison:
- For bushings with potential taps, C2 is significantly larger than C1.
- For bushings with power-factor taps, C1 and C2 are often of the same order of magnitude.
Figure 1: Construction of the condenser type bushing
In this design, the ground layer of the bushing core is tapped and connected to a miniature bushing on the main bushing’s mounting flange. The tap is grounded via a screw cap on the miniature bushing housing. When the grounding cap is removed, the tap terminal becomes a low-voltage terminal for performing a UST measurement on the main bushing insulation (C1: conductor to tapped layer). In some designs, the tapped layer is extended into an oil-filled compartment. The potential tap floats during operation, and a special probe is inserted through an oil filling hole to contact the tapped layer for measurements.
5. Field Testing Procedures for Bushings:
Bushings are relatively simple devices, and field tests have been developed to detect defects, deterioration, contamination, or damage in the insulation. The most important tests include:
- Overall Test (Center Conductor to Flange)
- Center Conductor to Tap Test (C1)
- Tap Insulation Test (Tap to Flange, C2)
- Hot Collar Test (Collar to Center Conductor)
6. Measurement of Insulation Power Factor (Tan Delta) and Capacitance of Bushings:
In this test, the capacitance and insulation power factor (tan δ) of the high-voltage capacitive bushings are measured. To perform this test, the transformer and the high-voltage reactor must be de-energized, and the following steps should be carried out:
- Surface Cleaning and Preparation:
- First, ensure that the outer surface of the bushing insulators is clean and dry. Therefore, clean the bushing insulators with a suitable cloth.
Note: This test must not be performed 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, all three phases and the neutral (if present) must be connected together. This connection must be made on each winding, meaning:
- The three phases and neutral on the high-voltage side should be short-circuited together.
- The three phases and neutral on the low-voltage side should also be short-circuited together.
- The tertiary winding bushings (if present) must also be shorted together.
Moreover, when testing the bushings of one winding, all other windings must be grounded.
For example, when testing the U1 bushing of the high-voltage winding:
- U1 is connected to V1, W1, and N1 (if available).
- The terminals u2, v2, and w2 (and n2, if available) are shorted together and grounded.
- The tertiary bushings (if any) are also shorted together and grounded.
The same procedure should be followed 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:
- Use a convenient voltage (12 kV) at or below the bushing’s nameplate rating.
- Hot Collar Test:
- Perform at a test voltage of approximately 10 kV.
- 6.2 Testing Spare Bushings:
When testing a spare bushing, proper handling and setup are critical to ensure accurate results. Mount the bushing in a grounded metal rack with no connections to the terminals. Avoid testing bushings mounted in wooden crates or placed directly on the floor, as wood or concrete can affect the test results. Ensure the bushing’s center conductor is not in contact with any foreign materials (e.g., slings, ropes, or other objects).
- The overall test for spare bushings, with or without taps, can be performed with the GST mode. With this test mode the overall capacitance is measured.
- The main insulation (C1) of bushings equipped with taps can be tested separately (INPUT A to tap). The C1 insulation is measured 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)
6.3.1. 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.
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.
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)
